A-Type Subduction (Ampferer)........................................................................................................................Subduction de type A

Subducção de tipo-A / Subducción tipo A (Ampferer) / A-Typ Subduktion (Ampferer) / A型俯冲 / Субдукция типа А (Ампферер) / Subduzione tipo A (Ampferer) /

When a continental lithospheric plate (sialic composition) plunges under another plate forming a megasuture. In depth, it appears the subducting lithospheric plate can be, partly, consumed. Due to the compressional tectonic regime (σ₁ horizontal), created by the subduction zone, the sediments of the overriding lithospheric plate are shortened by folds, thrusts and strike slip faults.

See : « Continental Collision »
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« Lithostratigraphy »
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« Supercontinent »

An A-type subduction called, also, Ampferer subduction (named after the Austrian geoscientist Otto Ampferer), is a movement of a continental lithospheric plate under another plate in a collision zone*, with separation of part or all of the upper crust from of the lower crust and mantle. Denser material undergoes subduction. The less denser material can overlap the continental crust on the overriding plate. In A-type subduction, which corresponds, often, to a delamination**, the subsiding or descending plate is, always, continental. In contrast, in a B-type or Benioff subduction (named after the American geoscientist Hugo Benioff), the subducting plate is, always, oceanic. An A-type subduction creates a shortening along few hundred kilometers. A B-type subduction can recycle thousands of kilometers of oceanic crust and upper mantle. This block diagram illustrates, schematically, the A-type subduction zone associated with the collision between the northern divergent margin of the Tethys Sea*** and the Gondwana small supercontinent. The Mesozoic margin forms the Jura mountains, whose acme culminates at 1,720 m. The Jura Mountains are located northward of the Alps, in France, Switzerland and Germany. This margin behaves as the subducting plate. It plunges under the Alps folded belt, which extend from Austria and Slovenia, at East, through northern Italy, Switzerland, Liechtenstein and southern Germany, to south-east France and Monaco.  The Alps act as the overriding plate. In the Mesozoic subducting lithospheric plate, the sedimentary shortening is accomplished by folds and reverse faults. In depth, they disappear on a significant saliferous interval located, more or less, at 800 meters below surface. In the overriding lithospheric plate, the shortening is made, mainly, by large thrusts that flatten at different levels. The shortening in the overriding plate is more important than in the subducting plate. This suggests part of the subducting plate is absorbed in depth by the asthenosphere. However, so far, there is any corroboration test of such a conjecture.  A-type subduction zones are placed where part of the continental crust is, deeply, buried beneath a megasuture (Earth's mobile area, formed by folded and faulted mountain belts, testifying the complexity of accretionary and deformation phases suffered by the geological bodies in the regions where compressional tectonic regimens were predominant). This geological situation implies, in the sedimentary cover, the formation, at large scale, of detachment surfaces, folds and reverse faults. Initially, the large majority of tectonic plate theory supporters did not accept this type of subduction. The relative small density of the continental crust is reluctant to major burials. Molnar and Gray (1979) show that even the lower continental crust can enter into subduction and be individualized from the upper crust. Presently, the majority of geoscientists think A-Type subduction zones may exist. Likewise, Bird et al. (1975) have shown certain thermal and mechanical conditions. It can produce laminations within the lithosphere allowing the denser lower crust to plunge into the asthenosphere causing continental drift and isostasy (gravitational equilibrium and its changes, between the lithosphere and Earth's asthenosphere). The asthenosphere is a ductile part of the Earth's upper mantle extending from the lithosphere to the lower mantle by more than 700 km. It is made up of ductile materials, solid  upper mantle and molten or semi-molten rocks function of the depth and/or proximity of magma chambers. The isostasy results from the fluctuation of the tectonic plates on the denser material, i.e., on the asthenosphere. The equilibrium depends on their relative densities and the weight of the plate. Such equilibrium implies that an increase in the weight of the plate (thickening or deposition of sediments, water or ice on its surface) leads to its sinking. Inversely, there is uplift or rebound, when the weight decreases.

(*) The term collision here used is abusive. In the current language, it translates a change of kinetic energy into potential energy ("deformation") energy, which is not the case in plate tectonics theory. In plate tectonics, kinetic energy does not play any role. Do not forget there are just to three types of energy: (i) Kinetic energy, that is, the ability to do work through the movement ; (ii) Potential energy, i.e., the ability to do work through the position and (iii) Electromagnetic radiation energy, i.e., such as the one transported from the Sun to Earth and which heats us or aliments photosynthesis. Although we often meet terms such as electrical energy, chemical energy, nuclear energy, deformation energy such things do not really exist. They are only practical abbreviations for specific combinations of the kinetic energy and potential energy. The electrical energy, for instance, is only the potential energy of negative charge electrons in the presence of positive charges. (Atkins, Peters, 2003)

(**) Delamination refers to the loss and sinking of the portion of the lowermost lithosphere from the tectonic plate  to which it was attached. Delamination occurs when the lower continental crust and mantle lithosphere break away from the upper continental crust. There are two conditions that need to be met in order for delamination to proceed: (i) The lower lithosphere must be denser than the asthenosphere and (ii) The intrusion of more buoyant asthenosphere making contact with the crust (replacing of dense lower lithosphere must occur).

(***) The sea, between the NE Gondwana small supercontinent and the SE Laurasia continent, which replaced Paleo-Tethys (formerly Tethys Sea of Suess, that is to say, the Paleozoic Ocean, between the Gondwana small supercontinent, Central Europe, Iberia, China and Central Asia). The Tethys Sea began to form in the south of the Paleo-Ttthys Sea. The Cimmerian plate was individualized (Permian / Triassic). The expansion of the Tethys Sea forced the Cimmerian plate to collide with the Laurasia continent. Finally, the Tethys Sea completely replace the Paleo-Tethys Sea. In Greek mythology, Tethys is the mother of Achilles and one of the daughters of Neree and Doris (sister of Lycomedes of King of Scyros).

Abandoned Delta (Dry delta, Sub-delta).....................................................................................Delta Abandonné (Sous-delta)

Delta abandonado/ Delta abandonado / Unterdelta / 被抛弃的三角洲 /Заброшенная (сухая) дельта / Delta abbandonato /

When the upper part of the delta plain is no longer subject to fluvial influence due to the delta seaward progradation and shifting of the main water-course.

See: « Delta »
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« Delta Front »
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« Progradation »

The Ganges-Brahmaputra Delta Building* called, also, Ganges, Sunderban or Bengal Delta Building, is located in Asia, where the Ganges and Brahmaputra rivers discharge into the Bay of Bengal. With an area of about 100,000 km², it is the largest delta in the world. Approximately two-thirds of the delta belong to Bangladesh. The rest constitutes the state of West Bengal of India. The Ganges delta building is the floodplain of three great rivers: (i) Ganges ; (ii) Brahmaputra and (iii) Meghna. Together, these three rivers drain a basin of about 1.72 million km² on the southern side of the Himalayas. This figure shows a small part of the abandoned delta, that is to say, the upper part of the deltaic plain which, due the delta progradation, is no more disturbed by the influence of fluvial and tidal deltas**. During the Pleistocene, before the junction of the rivers, the deltaic activity of the Ganges and Brahmaputra rivers shifted several times. Currently, the active sedimentation area is located in the eastern sector. The northern part represents the abandoned delta plain, where traces of abandoned channels dominate the morphology of the region. The shape and abundance of these channels vestiges, which were, probably, the bed of the Ganges River and many of its distributaries***, suggests the existence of an old meander belt, that is to say, an area developed over a valley filled with a floodplain sediments, on which meanders serpentine due a low slope thalweg of the water-courses. During flooding periods, the rivers of the meander belt, which has, recently, been much modified by human activity, overflow the original margins flooding the adjacent areas. The size of the traces of fossil channels is similar to that of the channels currently active along the Ganges and their distributaries. Many of these ancient river channels are now dominated by tidal water. Hooghly and Pusar rivers are perfect examples of ancient beds of the Ganges River, which today are dominated by the tides. Oxbow lakes and meander relics are common in the abandoned delta area. The inner part of the tidal plain was protected by dikes and the old salted lands were converted into agricultural land. This region, which was. originally, an immense mangrove forest ("Sunderbans"), still retains a little of the original delta morphology, although it is, already, well concealed by the tidal drainage network (set of channels that are connected to each other and drain the water from tidal currents, which may be artificial, man-made, or natural). The large tidal channels, well visible south of the abandoned delta area, form bell-shaped estuaries that are deep enough to be used as transport roots. Inland, the estuary channels are very sinuous. They are relatively stable, since lateral migrations are rare. The comparison of old aerial photographs with modern satellite imagery suggests stable channel paths for at least the last 20 years. In the deep offshore, probably in association with the floods of the Ganges/Brahmaputra, submarine slope fans were deposited. Some "gull wing" structures (opposite turbiditic natural levees and the filling of the depression between them) exceed more than 500 m of thickness. The overloading of sediments deposited in direct or indirect association with the Ganges-Brahmaputra river produces a significant subsidence of the seafloor, which, naturally, induces a significant relative sea level rise. Such a marine ingression, has nothing to do with what is, regularly, advertised by the media and particularly by some naive ecologists (verificationists). This relative sea level rise, that is to say, the rise of the sea level referenced to an Earth's surface, whether the seafloor or the top of the continental crust (bottom of the sediments) is not a consequence of anthropogenic global warming and, particularly, due to an increasing of CO₂ in the atmosphere. It is just the consequence of the subsidence of the seafloor due to the weight of the sediments. Values of 10/20 m  of relative sea level rise per year are frequent not only along the coast but also along the Meghna River. There are other delta types such as: (i) Fan delta ; (ii) Arctic delta ; (iii) Arcuate delta; (iv) Atrophied delta; (v) Complex or Polymorphic delta; (vi) Cuspate or Blunt delta ; (vii) Fingers or Digitated delta; (viii) Flow delta; (ix) Closed delta; (x) Gilbert-type delta; (xi) Lava delta; (xi) Lobated delta; (xiii) Tidal delta; (xiv) Submarine delta; (x) Storm delta; (xvi) Wave delta ; (xvii) Ebbing delta, etc.

(*)  No one confuses a floor (where there may be several apartments) with a building. In the same way, no one should confuse a delta with a deltaic building. A delta has a thickness ranging from a few meters to several tens of meters. A delta building (stacking of several deltas), such as the Niger delta building, can reach thousands of meters.

(**) Submarine deltas formed in symmetrical position, at the openings of the lagoon or in the straits, due to the circulation of the tidal currents.

(***) It is important to differentiate distributary and tributary. A distributary is a water-course that branches off and flows away from the main channel (river bifurcation). A tributary is a water-course that flows into a larger one. A distributary flows directly into the sea, which is not the case of a tributary, which flows into larger stream or a lake.

Abandoned Meander (Oxbow lake)........................................................................................................Méandre Abandonné

Meandro abandonado / Meandro abandonado, Galacho / Altwasser (Fluss) / 故道 / Высохшая река / Meandro abbandonato, Oxbow /

Water-body with an U-geometry formed when a meander of a river is cut off (abandoned) from the main stream forming a lake. Synonymous of Abandoned Channel.

See: « Neck, Chute Cutoff (evolution) »
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« Point Bar (seismic) »
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« Meander Belt »

This figure visualizes series of oxbow lakes along the alluvial plain of Kneai River (one of the most popular river for sport fishing in Alaska). During the melting ice period, many of the channels become, again, active. This is perhaps the most likely reason to explain the absence of water-bodies or oxbow lakes in some of them. When an abandoned channel or oxbow becomes active, what happens, more or less, periodically, part of the channel is filled with sediment which, progressively, leaves less room for water and sooner it is abandoned. The alluvial plain meanders (there are also valley-meanders*) by the force of being accentuated, can be cut out from the main current creating abandoned channels. A cut between two meanders can be done in two ways: (i) Overbanking and (ii) Osculation or Tangency. In the first case, the cut between two meanders is realized during a flood period, when the entire floodplain is flooded. In fact, as soon as the water height begins to decrease, the stream, preferably, takes the more rectilinear path than the long and curved path of the meander. The meander is abandoned and may form a meander lake (oxbow lake). This mechanism is impossible in valley-meanders (the sinuosities of the river coincide with the sinuosities of the valley). A cut by tangency or osculation (contact of two curves) is made by simple exaggeration of the curve of the meander. Progressively, the neck of the meander becomes non-existent, flavouring the flow of the stream, directly, to the next meander leaving an abandoned meander in which a lake can form. As a meandering cut brings two points of the stream in contact, which were initially at a certain distance with different altitudes, the occurrence of a waterfall is possible. The existence period of the waterfall is, more or less, large  and function of the hardness of the incised rocks. All meanders migrate downstream, which allows the capture of small streams through the meander and the transformation, over time, of meanders of valley into meanders of alluvial plain.

(*) There are two types of meanders: (i) Alluvial plain meanders, when the river meanders independently of the course of the valley, that is to say, when the meandering stream does not follow the course of the valley in which it flows, as in Mississippi river, in USA ; (ii) Incised or valley meanders, when the valley meanders like the river, at the same scale ,i.e., the valley follows perfectly the meandering watercourse (e.g., Douro river, in Portugal).

Abandonment Shale (Turbidites)............................................................................................................................Argile d'Abandon

Argilito de Abandono / Arcilla de abandono / Argila Ausfallende / 放弃页岩 / Глина, оставшаяся в покинутом русле реки / Argilla abbandono /

Level or levels of argilaceous rocks deposited over the channels fills (or depressions fills between turbidite lobes) and levees of the submarine turbidite complexes, particularly, in submarine slope fans (SSF), when the channel fills become inactive. The presence of argilaceous levels above the submarine slope fans allows, often, the trapping (and retention) of hydrocarbons, particularly, in the stratigraphic and morphological traps by juxtaposition. Overbanking deposits and channels or depressions fillings can be reservoir-rocks.

See: “ Shale ”
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" Slope Fan "
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" Overbanking Deposit (channel) "

This seismic line comes from the Gulf of Mexico, which, in the classification of the sedimentary basins of Bally and Snelson (1980), corresponds to a Mediterranean-type basin, that is to say, to a Pannonian sedimentary basin that undergone a break-up of the lithosphere and subsequent oceanization*. After the break-up of the lithosphere, the seafloor spreading created a marginal sea, in which deep-water depositional systems are, often, predominant, particularly, turbidite submarine fans.  Submarine slope fans (acronym SSF) are, easily, identified by its typical "gull wing" geometry (mounds with opposite downlaps). Within sequence-cycles (stratigraphic cycles induced by 3^rd order eustatic cycles, which have a time duration ranging between 0.5 and 3-5 My), in the most distal parts of the basin, submarine basin floor fans (acronym SBFF) can, plausibly, be recognized (around 3 seconds t.w.t depth on this line). The migration and vertical stacking of the "turbidite channel fillings**, which, in fact, are the fillings of the depressions (areas of non-deposition) between the turbiditic lobes, are emphasized by horizons of abandonment shales (strong seismic amplitude). In this example, the turbidite intervals have few associated seismic reflections suggesting facies (lithology and associated fauna), relatively, homogeneous and shaly. The seismic horizons associated with abandonment shales (signalized by arrows) have a strong amplitude (in all likelihood induced by hardground surfaces), in particular, those fossilizing the fillings between the levees (natural marginal dikes). The shales correspond, almost always, to pelagic shales (not transported by turbidity currents), whose deposition time contrasts, strongly, with the deposition time of the turbidite beds. While turbidite intervals are deposited, almost, instantaneously (in geological time), pelagic abandonment shales, which, separates, usually, them, have a deposition time that can last hundreds or even thousands of years. It can be said: (i) The sedimentation of the turbidite intervals is, mainly, lateral ; (ii) The sedimentary particles (detritus***) are transported by turbidity currents ; (iii) The abandonment shale sedimentation is vertical ; (iv) The clay particles fall, slowly, through the water column to be deposit on seafloor forming a shale (fine-grained clastic rock composed by clay mineral and other undersized particles). In geology and, particularly, in sequential stratigraphy, the concept of completeness of a sedimentary interval is quite important to understand sedimentation rates. When the completeness (ratio of the deposition time to total time of the corresponding sedimentation interval), is 1, which is very rare, the time interval between the lower and upper boundaries is equal to the effective deposition time. When the completeness of a sedimentary interval is 0.5, this means that the effective deposition time is half the time difference between the lower and upper boundaries. The completeness of a turbidite bed is close to 1. The completeness of the pelagic shale layer between two consecutive turbidite beds (layer E in Bouma's terminology), is very small (around 0.1). A sedimentation rate of a give interval, calculated without taking into account the completeness, what is very frequent, has no geological meaning and it can have severe consequences. On this subject, P. M. Sadler, 1981 (Sediment Accumulation Rates and the Completeness of Stratigraphic Sections - The Journal of Geology, Vol. 89, No. 5 , Sep., 1981, pp. 569-584), in a compilation of almost 25,000 sedimentation rates (allocated, in 11 orders of magnitude) has shown: (i) Sedimentation rates are extremely variable ; (ii) Large part of such variation results of the compilation rates determined for different periods of time ; (iii) There is a systematic tendency to a decreasing value of sedimentation rate with an increasing value of time ; (iv) The gradients of such trends vary with the depositional environments ; (v) Although measurement error and compaction contribute to these decreasing values, they are, mainly, the consequence of discontinuous and unstable sedimentation ; (vi) The essential character of instability may be cyclical or random, but effective accumulation is characterized by fluctuations whose magnitude increases with increasing recurrence or repetition interval ; (vii) The median long-term accumulation rate provide a measure of the expected completeness of sedimentary stratigraphic sections on the time scale of the short-term rate. The expected completeness deteriorates as finer time scales are considered.

(*) Geological process by which a deep oceanic basin, overlying the oceanic crust (SIMA), replaces continental or shallow marine basin (SIMA)

(**) A channel is  landform in which a water-course can flow. A channel fill is a sedimentary interval deposited and, generally, filling-up a channel. Subsequently, on a tentative geological interpretation of seismic line there no channels, but just channel fills.

(***) In geology, a detritus is a particle  derived from a rock by weathering and erosion. Detritus become sediments after transport and deposition. In other words, a sediment is a detritus (sedimentary particle) that was deposited by some geological process.

Ablation...............................................................................................................................................................................................................................................Ablation

Ablação / Ablación / Ablation, Abschmelzung / 消融 / Абляция (вымывание) / Ablazione /

Geological process in a glacial sedimentary environment, by which, downdip the snow line, the ice of a glacier disappears, mainly, by evaporation and fusion. In addition to evaporation and fusion, wind erosion and calving (glacio-fracturation) are, also, considered important ablation mechanisms. The term ablation applies, also, to all loss of snow and ice in an iceberg.

See: "Glacier "
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" Snow Line "
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" Snow Field, Firn "

Calving (breaking of a mass of ice, whether a glacier, iceberg or ice platform) and erosion are probably the most important mechanisms of ablation. Ablation seems to be controlled by insulation*, as suggested by the astronomical theory of paleoclimates (Milankovitch cycles**). Evaporation and sublimation (transition from a solid phase to a gaseous phase without passing through the liquid intermediary phase) have always (in geological terms) played a very important role in the melting of ice caps and glaciers. They have allowed, among other geological events, a large part of the northern continents, such as northern Canada and Europe, were recently (geological time) used as a human residence area. Those who think (i) Climate changes ; (ii) Ablation ; (iii) Thinning of glaciers and ice-seas and (iv) Increasing amount of CO₂ in the atmosphere, are a direct and exclusive consequence of human activity, show a total ignorance of Geology. Long before the presence of man on the Earth's surface, climate changes, thinning and thickening of glaciers, and variations of the CO₂ concentration*** in the atmosphere have, always existed, in the geological history. Bedding planes in sedimentary rocks, for instance, correspond to climatic changes, just as terminal and recessional moraines emphasize thinning ("retreat") and thickening ("advance") of the glaciers. Do not forget that a glacier is a stream or flow of ice. It exists only, as long as, there is flow, that is to say, while accumulation and ablation are balanced. When ablation is greater than accumulation, a glacier continues to flow downhill, but decreases in thickness (it becomes slim). Without climatic changes, glacio-eustasy changes would not exist, and so, sequential analysis. The cyclicity of the sedimentary deposits, which is controlled by eustasy, would be absent. The absolute (eustatic) sea level, which is referred to Earth's centre centre  (using satellite measures), is a function of: (i) Tectono-Eustasy (volume variation of ocean basins in association with oceanic expansion following the breakup of supercontinents) ; (ii) Glacio-Eustasy (water volume variation of the oceans function of the amount of ice, assuming that the total amount of water in all its forms is constant since Earth's formation, more or less, 4.5 Ga) ; (iii) Geoidal-Eustasy (distribution of ocean water caused by variations in the terrestrial gravity field and (iv) Steric sea level rise**** , i.e., sea level rise induced by a thermal expansion of the oceans.

(*) Number of hours during a given period (day, month, year, century, etc.) in which there is direct solar radiation, that is to say, coming from the solar disk without suffering neither reflection nor absorption .

(**) The precession, obliquity, and eccentricity cycles are the Milankovitch cycles. They seem to modulate the solar insulation, that is to say, the total energy that Earth receives from the Sun at the top of the atmosphere, as well as, its geographic distribution  (https://www.skepticalscience. com/Milankovitch.html).

(***) In Early Carboniferous, the CO₂ concentration was about 1500 ppm. In Middle Carboniferous it dropped to 320 ppm. The current concentration (2008) is about 380 ppm, or 0.038%. In the last 600 My, only, during Carboniferous and Quaternary, the CO₂ concentration, in atmosphere, was less than 400 ppm. In the geological past, there was always a higher concentration of CO₂ in the atmosphere than today. During Jurassic time, the concentration was 1800 ppm (4 times higher than currently). Concentrations of 7,000 ppm were found in Cambrian. In Late Ordovician, glaciations were abundant, with a CO₂ concentration of 12 times the currently concentration.

(****) Expansion of oceans due to expansion of water molecules. Steric change is one of the major contributors to sea level change. It occurs at places with higher temperatures and/or greater pressure. The increase in volume causes a decrease in density. This would later inhibit the thermohaline circulation, which transports oxygen and nutrients and regulates the global climate (http://www.esi. utexas.edu/files/047-Learning-Module-Expansion.pdf).

Ablation Zone............................................................................................................................................................................................Zone de Ablation

Zona de Ablação / Zona de ablación / Ablationszone / 消融区 / Зона абляции / Zona di ablazione, Bacino di ablazione /

Area of a glacier where the loss of ice by melting, evaporation and sublimation* exceeds the annual snowfall.

See: « Snow Field, Firn »
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« Glacier »
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« Snow Line »

In the Glärnischfirn** mountain glacier, in north-eastern Switzerland, the ablation zone, downdip the accumulation zone, is obvious. In a glacier, the ablation zone ( zone of loss) is the area in which the annual loss of snow by melting, evaporation, calving (glacio-fracturation) and sublimation exceeds the annual gain in snow and ice (in surface). In the great majority of glaciers, fusion is the most important factor, but in others calving, well visible in this photograph, can be very important. In cartography or on aerial photography (or airborne imagery), as shown in this figure, the ablation zone can be identified as the part of the glacier beneath the snow line. Often, the ablation zone contains water-courses resulting from the melting water, which may be: (i) Supraglaciers, that is, flowing over the glacier ; (ii) Intraglaciers, when they flow within the glacier and (iii) Sub-glaciers, when they flow under the glacier. The ablation zone is, also, an area where many sedimentary particles are deposited, in particular, in the glacier edges. The survival of a glacier depends on the balance between ablation (loss) and accumulation (gain) of snow and ice. When ablation exceeds accumulation, the snow line (limit between the ablation and accumulation zones) retrogrades (it gets higher) and, therefore, it is wrongly to said that the glacier retreats. A glacier does not retreat. Its thickness decreases. The contrary is not necessarily true. Many glaciers retrograde with a positive accumulation / ablation balance. What counts for the survival of a glacier is the balance of the glacier mass and not if it retrogrades or advances. A glacier, which is a n ice flow, may apparently retreat, but if its thickness increases, the accumulation/ablation balance is positive. Climate changes that existed since the Earth's formation (around 4.5 Ga) and not just after man uses fossil fuels, can produce temperature variations and significant snowfalls that can break the mass balance of the glaciers.

(*) Sublimation is the transition from solid to gaseous phase without passing through the intermediate liquid phase.

(**) The Glärnischfirn (sometimes Glärnischgletscher) is a 2 km long and up to 1 km wide glacier in the Schwyz Alps in the Swiss canton of Glarus. The glacier lies on the north-west side of the Glärnischmassivs (also Glärnischgruppe) between 2,300 and 2,800 meters altitude and runs slightly progressively rising from west to east. At this eastern edge it goes 2,000 meters almost vertically down to the Klöntalersee. On the southeast side of the Glärnisch there are two smaller glaciers, the Bächifirn and the Guppenfirn, both of which are not connected to the Glärnischfirn (https://de.wikipedia.org/wiki/Glärnischfirn).

Abrasion............................................................................................................................................................................................................................................Abrasion

Abrasão/ Abrasión / Abrasion, Abtragung / 磨蚀 / Абразия (истирание) / Abrasione /

Mechanical scraping of a rock surface by friction*. Abrasion is active between the rock and the sedimentary particles carried by erosive agents (wind, water-courses, glaciers, sea-waves, gravity, etc.). The result of friction is the formation of detritus or rock debris (rock waste disorganized by friction).

See: " Erosion "
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" Glacier "
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" Rock Waste "

The large majority of the islets in northern of Berwick offshore (north of Scotland) are sheepback rocks ("roches moutonnées" of the French geoscientists), that is to say, rocks with a morphology similar to the back of a sheep**. This morphology, which is strongly asymmetrical, is the result of an important ablation created by the downdip movement of a huge mass of ice. The asymmetry of the "roches moutonnées" is, easily,, visible in the photograph and scheme shown in this figure. The steepest part underlines the pull out of blocks from the bedrock by the movement of the ice. It indicates the direction and, in part, the sense of movement of the ice mass (sense is specified by two points on a line parallel to a vector, while direction or orientation is specified by the relationship between the vector and given reference lines; two movements  can  have the same direction but opposite sense). The direction of the displacement is easier to deduce by the relatively, continuous slope of the "roche moutonnée". In the sketch of the lower part of this figure, along the back of a "roche moutonnée", which corresponds to bedrock, more or less, well-polished. There are a certain number of structures, resulting from abrasion, which point out the direction (orientation) and the sense of the movement of the glacier: (i) Glacial grooves (grooves or gouges cut into the bedrock by gravel and rocks carried by glacial ice and meltwater), which are found in rocks of many different ages allowing geoscientists to recognize in the geological history, cold and hot periods, i.e., climate changes; (ii) Conchoidal fractures ; (iii) Fractures; (iv) Gouges traces, (v) Crescent glacial grooves, (vi) Glacial traces, etc. These crescent glacial grooves have nothing to do with the tectonic grooves ("lunules tectoniques" of the French geoscientists) described by Wegmann and Schaer (1957) in the Jura, which, very often, form on the bedding planes, particularly in the carbonate stratigraphic intervals. The tectonic grooves underline the layer to layer displacement, particularly, when the rocks are shortened (submitted to a compressive tectonic regime, vertical σ₁). The crescent tectonic grooves are induced by the weight and displacement, in the direction of the apex of the anticlines, of the overlying layer, during a shortening of the rocks (compression). Crescent glacial grooves are associated with an abrasion induced by the weight and flow of an ice mass, in which the mechanical friction of the pebbles and rock fragments carried by the ice leaves striations on the smooth and polished bedrock surfaces. The concavity of crescent glacial grooves is a reliable criterion of the direction of glacial flow. The mapping of the glacial slickensides*** allows, easily, to locate the centre of ice caps. In Telemark, Norway), for instance, where I worked for my PHD, the centre of the old glacial cap can be deduced by the divergence of the glacial striation of the supracrustal rocks. On the other hand, the vertical displacement of these glacial striae point out an important neotectonics (geological and geomorphological processes that are current or recent in geologic time) induced, mainly, by the isostatic uplift following the melting of the ice deposited during the last glaciation.

(*) In physics, friction (or attrition) is an interaction that opposes the relative movement between two systems in contact. Friction can be studied in the same way as other types of force or torque. Its action is characterized by a norm and an orientation, which makes it a vector. The orientation of the friction force (or torque) created on a body is opposed to the displacement of this body relative to its environment. (https://fr.wikipedia.org/wiki/Frottement)

(**) The origin of the expression "moutonnée" and particularly "roche moutonnée " has nothing to do with the morphology of the back of a sheep, but with the smooth and polished morphology of the wigs used by the aristocracy of the eighteenth century. It was the Swiss geologist Horace-Bénédict de Saussure, who for the first time, in 1786, used the expression "rock moutonnée", by analogy with the wigs fashionable at that time in France, which were smoothed with sheep's fat (hence the term "moutonnée") to keep the hair from which they were made.

(***) The term slickensides is  mainly used to designate smooth rock surfaces with parallel grooves or scratches formed by friction during sliding and movement along a fault.

Abrasion Platform (Wave cut platform)...............................................................Plate-forme d'abrasion marine

Plataforma de Abrasão / Plataforma de abrasión / Abrasionplatte / 海蚀台地 / Абразионная платформа / Piattaforma di abrasione /

Slightly sloping rocky surface that extends from the bottom of the cliff (escarpment) to the offshore (some distance from the shore), between the highest and lowest tide levels. An abrasion surface that is shaped by the action of the waves. An abrasion platform or wave cut platform tends to be smooth, with slight undulations or with steps emphasizing the high tide level or the level of the sea reached during the storms.

See: « Rimmed Carbonate Platform »
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« Drowned Shelf »
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« Shoal-Rimmed Platform »

Abrasion platforms are coastal platforms where the abrasion of wave action is the predominant geological process. When an abrasion platform is under development, it is exposed, only, during low-tide, as illustrated on this figure, but there is, always, the possibility that such a platform is hidden under a cover of beach pebbles (which are important agent of abrasion). If the continental shelf is, permanently, exposed, above the high-tide line, it corresponds, probably, to a raised beach platform, which is not considered as a product of abrasion. It is the result of a mechanical scraping of a rocky surface by friction between the rocks and the moving particles during their transport by wind, glaciers, sea-waves, gravity, running water or erosion. After friction, the moving particles dislodge the less consolidated and weaker parts of the rocks, which can, also, be dissolved in the water. The intensity of the abrasion depends on the hardness, concentration, speed and mass of the moving particles. Erosion is the destruction of soil and rocks and their transportation made, usually, by rainwater, wind or by the ice action. Ice acts by expanding the material in which the infiltrated water freeze. Erosion destroys the structures that make up the soil, releasing sand, clay, oxides, humus, etc. The sedimentary particles and rock debris are transported to the lower parts of the ground aggrading* the streams. Abrasion can be considered as one of the mechanical processes of erosion. In forest-covered soils, erosion is quit insignificant and almost non-existent. Erosion is a natural process that is, always, present and important in the formation of landforms. Since man destroys forests, to use the land in agriculture, erosion becomes very severe in the exposed soil and can lead to desertification (transformation of an area into a desert). When the sea coast is rocky, high and steep, i.e.-, when cliffs or escarpments emphasize the coastline, erosion induces the retreat of the coast. Such a retreat happens, more or less, as follows: (a) Waves scrape the base of escarpment, which becomes unstable due to the loss of its basal support ; (b) Such an instability causes fragmentation of the cliff and falling blocks ; (c) When the cliff has cracks, the air, located in the cracks, is compressed when the waves shock and decompressed when the waves recede ; (d) The fissures of the rocks are enlarged and the rocks, progressively, fractured ; (e) This damage causes a concave excavation at the foot of the cliffs, that geoscientists call undercut, groove or notch, which, without support, crumble (break or fall apart into fragments) ; (f) In this way, the cliff recedes, developing an abrasion platform (strip of bedrock between the near-offshore and the cliff), which is discovered at low-tide. In the geological schema illustrated on this figure we can distinguish: (i) Low-tide cliff / ramp ; (ii) Wave-cut /abrasion platform; (iii) Notch / inner edge ; (iv) Modern cliff ; (v) Old wave-cut / abrasion platform ; (vi) Old notch / inner edge ; (vii) Paleo-sea cliff ; (viii) Terrace deposits ; (ix) Alluvial fan ; (x) Old wave-cut / abrasion platform ; (xi) Paleo-sea level II ; (xii) Paleo-sea level I. In addition, three abrasion platforms (2), (5) and (10), induced by absolute or (eustatic) sea level, can be recognized. Eustasy is the result of the joint action of (A) Tectono-Eustasy, which is controlled by changes in the volume of oceanic basins induced by the break-up and assemblage of the supercontinents ; (B) Glacio-Eustasy, which is controlled by changes in ocean water volume due to the amount of ice (glaciations and deglaciations**) ; (iii) Geoidal-Eustasy, which is controlled by the distribution of ocean water caused by variations in the terrestrial gravity field, and (iv) Thermal expansion of the oceans or steric rise of sea level (when the ocean temperature increases, the density of the water decreases and, for a constant mass, the volume of the oceans increases). Taking into account that just glacial changes are both important (> 10 m) and quite fast (<1 My), recent abrasion platforms were, certainly, created by glacio-eustasy.

(*) Aggradation (or alluviation) is the accumulation of debris (sedimentary particles), such as silts, sands, pebbles, boulders, etc., in areas with poor slope of riverbeds, particularly, at the end of their courses.

(**) Transition from full glacial conditions to warm interglacial with change in continental ice volume. Such a decrease in continental ice volume induce a significant rise of absolute or eustatic sea level.  The melting of  "sea pack ice" (formed when water on the surface of the ocean drops to or below the freezing point) or the melting of "ice shelves"  (thick floating ice platform formed where a glacier or ice sheet enter into the sea) induce a falling of the absolute sea level, since ice is less dense than water. An "ice sheet", also known as continental glacier, is just  a continental ice mass greater than 50,000 km². An inlandsis, also known as " polar ice cap" is a very large glacier in the form of an ice sheet covering the mainland, which thickness can reach several thousand meters.

Abrupt Carbonate Platform...........................................................................Plate-forme carbonatée abrupte

Plataforma Carbonatada Abrupta / Plataforma carbonática abrupta / Abrupte Karbonatplattform / 突变碳酸盐台地 / Карбонатная платформа с обрывистыми окраинами / Piattaforma carbonatica scoscesa /

When the edge of the carbonate platform, which, generally, coincides with tthe basin edge*, is well marked. The seaward limit between a carbonate platform and the continental slope is abrupt. An abrupt carbonate platform implies a continental slope, highly, inclined towards the abyssal plain, as it is, often, the case on rimmed carbonate platforms.

See: « Carbonate Platform »
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« Drowned Shelf »
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« Transgressive Interval »

A continental shelf or continental platform, whose water-depth varies between 0 and 200 meters, is a physiographic province forming part of the morphological structure of the continental margin (submarine extension of a continental block). A carbonate shelf or carbonate platform is a sedimentary body with topographic relief composed of calcareous deposits formed "in situ" (autochthonous calcareous deposits). A carbonated platform can match a continental shelf, but, generally, this does not seem to be the case. Most continental shelves have a water-depth greater than the photic zone where photosynthesis and carbonate formation are impossible. Several factors influence the geometry of a carbonated platform: (i) Inherited topography ; (ii) Synsedimentary tectonics ; (iii) Carbonate manufacture ; (iv) Exposure to currents and trade winds*, etc.. The most important factor seems to be the type of carbonate manufacture. In cold water carbonate manufacturing processes, carbonated ramps are preponderant. In topical manufacturing process, rimmed carbonate platforms and mounds of mud with a boundary zone no very well-marked, are preponderant. The best known platform geometries are associated with tropical manufacturing processes. These carbonate platforms can be subdivided into three main sedimentary environments: (i) Reef, i.e., part of the carbonate platform created in situ by sessile organisms ; (ii) Internal Lagoon, i.e., the part of the carbonate platform behind the reef, which is characterized by shallow and calm waters with sediments composed of fragments of reefs and hard parts of organisms or terrigeneous sediments when the reef is epicontinental and (iii) Slope, which corresponds to the outer part of the platform and connects the reef to the basin. The carbonate platform slope environment acts as a sink for the excess carbonate. Most of the carbonate produced in the lagoon and in the reef is transported by various processes and accumulated on the platform slope, which can be or not a continental slope. Most geoscientists consider five categories of carbonate platforms: A) Rimmed Platforms ; B) Ramp-type Carbonate Platforms ; C) Epiric (epeíric) or Epicontinental Platforms ; D) Isolated Platforms and E) Drowned Platforms.  The rimmed platforms are characterized by the presence of reefs or shoal limestone sands (carbonated sand-bank covered by shallow sea water) on the edge of the platform and clay sands in the lagoon or in open platform. Rimmed platforms are formed in calm waters and its extent varies between 10 and 100 km. In ramp-type carbonate platforms, the carbonated sands of the shoreline pass, at the base of the ramp, to clay sands and deep-water muds. In ramp-type platforms, which can reach 100 km, reefs are rare. The epiric platforms are characterized by the presence of tidal-flats and protected lagoons, which width can reach 10,000 km. In isolated platforms, the facies (lithology) is controlled by the orientation of the dominant winds. Isolated platforms have reefs and sandy bodies (as the rimmed platforms) in the windward margin (facing the side where the wind blows), but in the leeward margin (in the direction where the wind blows), the sediments are more muddy. Isolated platforms can be 100 km wide. The drowned platforms are under the photic zone, where there is not enough light for photosynthesis (use of carbon dioxide and water to obtain glucose through the energy of sunlight). In the geological scheme illustrated in this figure, the platforms connected to the continent are divided into two large families: (A) Ramp-type Platforms and (B) Platform with Marked Edge (topographic rupture). In ramp-type platforms two sub-types can be considered: (A.1) Monoclinal Ramp-type Platform and (A.2) Ramp-type Platforms with slight distal edge. In the platforms with marked edge there are also two subtypes: (B.1) Non-Rimmed Platforms and (B.2) Rimmed Platforms. It is in rimmed platforms that the designation of abrupt carbonated platform is more frequent. The tentative geological interpretations of Canvas auto-traces of the two seismic lines of the Australia offshore illustrate Early or Initial Miocene abrupt carbonate platforms developed during a graded marine ingression (successive accelerated relative sea level rises). These abrupt carbonate platforms were developed in highstand geological conditions, with the sea level higher than the basin edge. In such a conditions, at the level of a sequence-cycle, the basin has a shelf, whose extension increase,s progressively, as relative sea level rise. Middle Miocene prograding carbonates, probably, deposited under lowstand geological conditions (sea level lower than the basin edge, which correspond, in these conditions, roughly, at least seismically, to the shoreline) cover and fossilize the Early Miocene Carbonates.

(*) Displacement of the cold air from the high zones to the low pressure zones, which due to the Coriolis effect, in the inter-tropical areas blow towards East, in Northern Hemisphere, and in reverse direction in the Southern Hemisphere.

(**) In a sequence-cycle, the basin edge corresponds, roughly, to the seawrd of the coastal plain. The position of the basin edge depends on the geological conditions. In highstand, it is the seaward limit of the shelf (continntal platform), except during the 2^nd phase of development of the highstand prograding wedge (HPW), since the basin has no shlef. In lowstand geological conditions (no shelf), the basin edge is the last continental edge of the previous sequence-cycle.

Absolute Sea Level (Eustatic).......................................................................................Niveau de la mer absolu (Eustatique)

Nível do mar absoluto / Nivel absoluto del mar (eustatico) / Absoluter Meeresspiegel (eustatisch) / 绝对海平面 / Абсолютный уровень моря (эвстатической) / Livelloil del mare assoluto (eustatici) /

Sea level referenced to a fixed point, which is, generally, the Earth's centre*. The absolute (eustatic) sea level rises or falls, mainly, function of the volume of the oceanic basins and the volume of the ice sheets and ice caps (glaciers included). The water volume, under all its forms (solid, liquid and gaseous), is considered constant since the Earth's formation, around 4.5 Ga (10⁹ years ago). Synonym with Eustatic Sea Level.

See: « Fair Weather Wave Base »

(*) Using high-quality satellite radar near-global and homogeneous measurement of sea level is possible, thereby overcoming the inhomogeneous spatial sampling from coastal and island tide gauges. However, clarifying rates of global sea-level change requires continuous satellite operations over many years and careful control of biases within and between missions. To date, the TOPEX/POSEIDON satellite-altimeter mission, with its (near) global coverage from 66°N to 66°S (almost all of the ice-free oceans) from late 1992 to the present, has proved to be of most value in producing direct estimates of sea-level change . The present data allow global-average sea level to be estimated to a precision of several millimeters every 10 days, with the absolute accuracy limited by systematic errors. The most recent estimates of global-average sea level rise based on the short (since 1992) TOPEX/POSEIDON time series range from 2.1 mm y−1 to 3.1 mm y−1. The alimeter record for the 1990s indicates a rate of sea-level rise above the average for the twentieth century. It is not yet clear if this is a result of an increase in the rate of sea-level rise, systematic differences between the tide-gauge and altimeter data sets or the shortness of the record. (Church, J.A., J.M. Gregory, N.J. White, S.M. Platten, and J.X. Mitrovica. 2011. Understanding and projecting sea level change. Oceanography 24(2):130–143, https://doi.org/10.5670/oceanog.2011.33)

Absolute Zero.............................................................................................................................................................................................................Zéro absolu

Zero absoluto / Cero absoluto / Absoluter Nullpunkt / 绝对零度 / Абсолютный ноль / Zero assoluto /

The lowest possible temperature at which all molecular motion ceases. The absolute zero or 0°K (Kelvin) is equal -273.15° C (Celsius) or -459° F (Fahrenheit).

See: « Big Bang (theory) »
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« Astronomic Theory of Paleoclimate »
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« Glaciation »

The absolute zero or zero kelvin (0° K), corresponds to the temperature of -273.15 ° C or -459.67 ° F. The absolute zero is a concept in which a body would not contain any energy. However, the laws of thermodynamics show that the temperature can never be exactly equal to zero Kelvin or -273.15 ° C. This is the same principle that guarantees that no system has an efficiency of 100%, although temperatures close to 0 ° K or -273.12 ° C can be reached. Some objects can be cooled down to this point. For a body reach the absolute zero, it can not contain any energy. At extremely low temperatures, in the vicinity of absolute zero, the material exhibits many extraordinary properties, including: (i) Super-conductivity (intrinsic characteristic of certain materials which, when cooled at extremely low temperatures, can conduct electric current without resistance or losses) ; (ii) Super-fluidity (anomalous state of quantum liquids which are at a very low temperature, such that this state has no or negligible viscosity (super-fluidity) and transmits the heat in an abnormally high manner and (iii) Condensation of Bose-Einstein (phase of matter formed by bosons* at a temperature very close to absolute zero, which a large fraction of atoms reaches the lowest quantum state). In order to study such phenomena, scientists have been working on getting lower temperatures. Up to 2004, the lowest temperature obtained for a Bose-Einstein condensate was 450 pK, or 4.5 × 10^-12 K (pico-Kelvin**). The lowest temperature already obtained was 250 pK, during a nuclear magnetic sorting experiment in the Laboratory of Low Temperatures of the Helsinki University of Technology. Recently, the Boomerang nebula, with a temperature around -272 ° Celsius is considered to be the coldest place in the Universe. This nebula, which is located in the constellation Centaurus (one of the largest constellations in the celestial hemisphere South with a, clearly, visible asterism***, whose feet are formed by two bright stars: Alpha and Beta Centauri, also known by the Arabic names of Wazn and Hadar) is about 5,000 light-years from Earth.

(*) In particle physics, a boson is one of the two basic types of elementary particles of nature (the other type are fermions). Examples of bosons include fundamental particles such as photons, gluons, W and Z bosons (the four gauge bosons, force carriers in standard model), the Higgs boson, and the graviton of quantum gravity; composed particles (e.g. mesons and stable nuclei of even mass number such as deuterium with one proton and one neutron, mass number = 2, helium-4 or lead-208, and some quasi particles (pairs of Cooper, plasmons and phonons). (https://es.wikipedia.org/wiki/Bosón)

(**) Pico, symbol p, is the prefix of the International System of Units (SI) which represents 10^-12 times the unit (i.e.,, one thousand-billionth, or one millionth of a millionth); One pico-kelvin or 1 pK = 10^-12 K.

(***)  Asterism is a pattern or group of stars that are recognised in the sky during the night. 

Abundance Zone (Acme)....................................................................................................................................................Zone d'abondance

Zona de Abundância / Zona de abundancia / Fläche des Überflusses / 富足的地区 / Зона обилия / Zona di abbondanza /

Area of a stratigraphic interval limited between the first and last appearance of a given taxon (in a given area), where the fossil taxon reaches a maximum level of abundance. Synonym with Acme Zone or Peak Zone.

See: « Acme »

Abyssal.......................................................................................................................................................................................................................................................Abyssal

Abissal / Abisal / Abyssal / 深海的 / Глубоководный / Abissale /

Sedimentary environment or sea-floor morphology characterized by a water-depth, generally, higher than 3500 meters.

See: " Bathyal "
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" Sedimentary Environment "
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" Shelfal Accommodation "

Seaward, function of water-depth, as illustrated in this figure, several marine sedimentary environments can be recognized: (i) Littoral ; (ii) Neritic ; (iii) Bathyal ; (iv) Abyssal and (v) Hadal. For most geoscientists, the littoral environment is limited between the high level of the high-tide and the minimum level of the low-tide. The limit between the bathyal and abyssal environments varies according the geoscientists. In the chart shown in the lower left corner of this figure, the global spatial distribution of the abyssal environment, whose water-depth is here assumed to range from 3500 to 5000 meters. The water-depth can not be, correctly, determined on conventional seismic lines (time seismic lines), but only on their depth versions, whose acquisition requires special reprocessing and, above all, an accurate knowledge of interval velocities (reason why depth converted seismic lines are expensive and, generally, not available). An abrupt variation of the water-depth, which is, often. visible in the offshore seismic lines, as illustrated above in a Canvas auto-trace of a East Borneo offshore (Makassar Strait) regional seismic line. Abrupt variations of water-depth induce important seismic pitfalls, which modifies, substantially the sea-floor morphology and the geometry of the underlying reflectors. This can be seen by comparing the auto-trace of the time version (top) with the auto-trace of the depth version (below) illustrated in this figure. Seismic waves travel more slowly into the water than through sediments. The velocities of seismic waves within water range between 1.4 and 1.5 km/s, while within Tertiary sandstones, for instance, it ranges between 2.0 and 2.5 km/s*. The different proposed limits for marine environments have no geological or sedimentary implications. Certain environments can be subdivided. The bathyal environment can be subdivided into: (a) Upper Bathyal ; (b) Middle Bathyal and (c) Lower Bathyal. The hadal environment, which is characterized by a water-depth greater than that of the abyssal environment, exists, mainly, in the convergent continental margins, when an oceanic lithospheric plate subdues under a continental lithospheric plate forming B-type subduction zone or Benioff zone (also called Wadati-Benioff zone). It is in this very deep environment that oceanic trenches are located, in which the sea-floor depth can surpass 6 km inducing negative gravitational anomalies. The oceanic trenches are, generally, more or less. parallel to the shoreline**. In sequential stratigraphy, the abyssal environment is, basically, the realm of the lowstand systems tract group (LSTG acronym) and, particularly, of its lower subgroups, that is to say, the submarine basin floor fans (SBFF). Submarine slope fans (SSF), which form the middle subgroup of the lowstand system tract group, may be present, but they deposit, usually, under a smaller water-depth, in the lower part of the bathyal environment (around the lower dip-break of the continental slope). Within a sequence-cycle (SC) (stratigraphic cycle induced by a 3^rd order eustatic cycle, whose time-duration varies between 0.5 and 3-5 My) the distal sediments of younger progradations of the lowstand prograding wedge (LPW) or of the highstand prograding wedge (HPW) are, generally, deposited in the abyssal environment. This is, particularly, true when the geometry of the sedimentary wedges is very progradational. The lowstand prograding wedge (LPW) is the upper sub-group of the lowstand systems tracts group (LSTG). The highstand prograding wedge (HPW) is the upper subgroup of the highstand system tracts (HSTG) of a sequence-cycle. The deepwater sedimentary intervals of the lowstand prograding wedge (LPW) and highstand prograding wedge (HPW) correlate, in time, with updip shallow-water sediments deposited along the same chronostratigraphic line*** . Roughly, it can be said, that along a chronostratigraphic line a sequence-paracycle, formed by one or more depositional systems, is deposited during the stability period of the relative sea level (local sea level referenced to any fixed point of the terrestrial surface, which can be the sea-floor or the top of continental crust, i.e., the bottom of sediments) that takes place after the relative sea level rise, i.e., after a marine ingression.

(*) The velocity of the seismic waves ranges between 2.0 and 6.0 km/s in limestones (function of the age) ; between 2.5 and 6.5 km/s in dolomites ; between 4.5 and 5.0 km/s in salt deposits ; between 4 and 6.5 km/s in anhydrite deposits ; between 2.0 and 3.5 km/s in gypse deposits ; between 5.5 and 6.0 km/s in granite ; between 1.3 and 1.4 km/s in the petroleum and around 0.3 km/s in air.

(**) The shoreline is the limit between land and sea, which is, constantly, subjected to changes induced by the abrasive action of the waves, coastal and tidal currents and, also, by variations in relative sea level (not to be confused with absolute sea level).

(***) Time line over which different, synchronous and genetically linked depositional systems are deposited, that is, if one depositional system is not deposited, the others, generally, also are not deposited either.

Abyssal Dome.....................................................................................................................................................................................................Dôme abyssal

Doma abissal / Domo abisal / Abyss-Kuppel / 深海穹顶 / Абиссальный купол / Dome abissale /

A prominent and isolated landform of the abyssal plain with a form of a dome. Its origine is, generally, volcanic.

See: " Deep Sea-floor "
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" Abyssal Hill "
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" Tephrachronology "

In this map of the Azores triple point*, constructed from data of D.T. Sandwell, it is easy to recognize the major morphologies existing in the abyssal plain, in particular, the abyssal domes or hills. This figure, clearly, illustrates the morphology of the abyssal plain, which is not dependent of absolute (or eustatic) sea level* changes. The vast majority of geoscientists admit the hypothesis that the amount of water (in all its forms) is constant since the Earth's formation (around 4.5 Ga, i.e., 10⁹ years). When the volume of the oceanic basins diminishes, which is the case during the dispersion of the continents individualized by the break-up of supercontinents (mainly Protopangea and Pangea), the absolute sea level rises flooding, partially, the borders of the continents. On the contrary, since the continents begin to approach each other, the absolute sea level falls as the volume of oceanic basins increases. The maximum volume of oceanic basins is reached when the entire continental crust is clump together in a small group of lithosphere plates forming a supercontinent, such as Pangea. The relationship between the volume of oceanic basins and oceanization (formation of new oceanic crust) is easy to understand. The larger the oceanic mountains (new oceanic crust) the smaller the ocean basins. Just imagine an aquarium in deformable material with a certain amount of water. If the base of the aquarium is deformed by a piston, the water level in the aquarium will rise, because, for the same amount of water, the volume of the aquarium has decreased due to the deformation imposed by the piston. When sea-floor spreading is rapid, the new oceanic crust, which forms the oceanic ridges, does not have enough time to cool (get denser) and its morphology is very pronounced. The volume of the oceanic basins decreases and the absolute sea level rises. On the contrary, when the sea-floor spreading is slow, the new oceanic crust has time enough to cool. The cooling increases its density and the sea-floor morphology is less pronounced. The volume of the oceanic basins increases and the absolute sea level falls. As Earth is finite and, more or less, round, oceanization, i.e., sea-floor spreading has a counterpart, that geoscientists name subduction (geological process in which a lithospheric plate moves under another and is forced or sinks due to gravity into the  Earth's mantle).

(*) In geology, at the triple point or triple junction is the point where the boundaries (ridge, trench or transform fault) of three lithospheric plates meet, which is described according to the types of plate margin that meet at them (e.g. Transform-Transform-Trench, Ridge-Ridge-Ridge).

(**) The sea level can be absolute (eustatic) and relative (local). The absolute sea level is referenced to the Earth's centre using, generally, satellite measures. The relative sea level is referenced to any point on the Earth's surface, which may be the top of the continental crust (base of the sediments) or the sea floor. The relative sea level is the result of the combined action of absolute (eustatic) sea level and tectonics (uplift or subsidence of the sea-floor depending on the predominant tectonic regime).

Abyssal Guyot (Tablemound)........................................................................................................................................................Mesa abyssale, Guyot

Mesa abyssal / Mesa abisal / Guyot / 海底平頂山 / Подводная гора с плоской вершиной / Guyot (rilievo sottomarino di origine vulcanica) /

Volcano, seamount or submarine volcanic effusion with a morphology, relatively, flat rising from the ocean floor. Synonym with Tablemound

See: « Mesa »
&
« Volcano »
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« Abyssal Plain »

Contrary to a "mesa" which, in sedimentology, is a structure resulting from the differential erosion of a stable platform, where alternating resistant and soft horizons to erosive agents, an abyssal guyot is an intrusive volcanic structure whose top is, more or less, flat. For several geoscientists a submarine landform to be considered a guyot or tablemount, it must stand at least 914 m tall.  However, there are many undersea mounts that can range from just less than 91 m to around 914 m. Very large oceanic volcanic constructions, hundreds of km across, are called oceanic plateaus. In this model of thesea floor of the equatorial Atlantic Ocean, made by the German expedition of the Meteoro, between 1925 and 1929, several abyssal guyots are proposed, in particular, between the Brazilian and Argentinean abyssal basins. It is important to differentiate the submarine hills from the abyssal guyots, sometimes also called submarine plateaus. A submarine hill is a mountain that rises from the ocean floor that does not does reach the surface of the sea, which means that an submarine hill never forms an island. Seamounts, typically, are extinct volcanoes that form very rapidly and rise, generally, from the ocean floor between 1,000 and 4,000 meters. The abyssal guyots are, in general, small volcanic provinces, i.e., small volcanic plateaus. The Kerguelen volcanic plateau, located in the Indian Ocean, can be considered as a large volcanic table. It is, approximately, 3,000 km from the SW Australia and is about three times the size of Japan. This table or small abyssal plateau extends for more than 2,200 km in the direction NW-SE, under a very high water depth. Abyssal guyots correspond to hot spots (place on the Earth's surface that undergone an active volcanism over a, relatively, long geological period). The Kerguelen's guyot*, with the small abyssal tables of the South Atlantic Ocean, are associated with the Gondwana break-up that occurred at about 130 Ma. As lithospheric plates with their continents and oceanic floor, pass over a hot spot, the asssociated volcanoes, eject on the oceanic or continental crust a large amount of volcanic material forming either plateaus or abyssal guyots. The plate tectonics paradigm is, by far, the best explanation of most abyssal guyots.

(*) The Kerguelen Archipelago is located between 48° 35' and 49° 54' South Latitude and between 68° 43 'and 70° 35' East Longitude, at a distance of approximately: (i) 2,000 km from the coast of Antarctica; (ii) 3,400 km from Reunion Island and  (iii) 4,800 km from Australia.

Abyssal Hill (Oceanic hill)..............................................................................................................................................................Colline océanique

Colina Oceânica / Colina oceánica / Abyss-Hügel / 深海丘陵 / Океанический холм / Collina abissale /

Small submarine hill, topographically, well defined, with a height that varies from a few meters to hundreds of meters, rising from the floor of  an abyssal plain (between 3,000 and 6,000 m of water-depth). Abyssal hills occur, preferably, downdip of the abyssal plains. They form, often, the base of the continental slope. Isolated or clusters of abyssal hills are, quite often, fossilized by a thick sedimentary cover. The great majority of oceanic hills is included in the family of the abyssal hills.

See: « Deep Sea-floor »
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« Continental Rise, Glacis »
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« Basin Floor Fan »

An abyssal hill is a small hill rising from the ground of an abyssal plain. Abyssal hills are the most abundant geomorphic structures on Earth. They cover more than 30% of the ocean floor. The abyssal hills have, relatively, sharp edges and rise to heights of a few hundred meters with a width that can range from a few hundred meters to several kilometers. In the high resolution swath* bathymetric chart, illustrated in this figure, the oceanic hills cover a surface of about 900 km². Oceanic hills, although covered, generally, by pelagic sediments, are probably of similar composition and origin as the extrusive volcanic prominences visible on the flanks of mid-oceanic ridge* (MOR) and in certain continental rises (submarine features, between continental slopes and abyssal plains, found all around the world, representing the final stage in the boundary between continents and the deepest part of the ocean, https://en.wikipedia.org/wiki/Continental_rise). Most geoscientists think that many oceanic hills on the sea-floor are buried beneath the sediments that cover the abyssal plains. In the Atlantic Ocean, there are long provinces of oceanic hills parallel to both flanks of mid-oceanic ridges, which may correspond to the ancient ridges, which means that in this case the oceanic hills are abyssal hills. Pacific Ocean has less terrestrial continental influx than Atlantic Ocean. Numerous oceanic trenches separate the ocean floor from the continent. They prevents the transportation of the sedimentary particles (detritus) to the deep parts of ocean basins. However, it can be said that 80 to 85% of Pacific Ocean abyssal plain is occupied by oceanic hills that may be ancient mid-oceanic ridges (MOR) or not. Although these marine topographic forms occupy around 30% of abyssal surfaces, their origin remains a source of debate. Recent investigations suggest that the oceanic hills on the flanks Pacific mid-oceanic ridge are formed by horsts and grabens** that widen, progressively, with time, in association with an extensive tectonic regime (lengthening, normal faults). For some geoscientists, in most cases, the differentiation between oceanic and abyssal hills is, purely, semantic and has no geological significance. The submarine mountains, as certain geoscientists say, are, usually, extinct volcanoes with a conical relief almost always isolated, reaching more than 1,000 meters of height, but never attaining the surface. Mid-oceanic ridges (MOR) or submarine ridges, as they are often termed, are immersed mountain ranges that are about 60,000 km long, making the longest mountain range on the planet. Abyssal hills form part of the oceanic crust, which is about 6 km thick and composed of several layers, not including the overlying sediment. The topmost layer, about 500 m thick, includes lavas  made of basaltic rocks rich in plagioclase and pyroxene. Oceanic crust differs from the continental crust (layer of granitic, sedimentary and metamorphic rocks forming the continents and shallow areas along its coasts, known as continental shelves. Oceanic crust is thinner (5-15 km), denser, younger and has a different chemical composition. The continental crust, which cover about 40% of the Earth's surface, has a thickness ranging between 20 and 80 km. The small density of the continental crust is, probably, the reason why it has a positive topographical relief in relation to the sea level. Continental crust is, mainly, formed by silico-aluminous granitoid rocks (a variety of igneous rock similar to granite composed mainly of feldspar and quartz), while oceanic crust is predominantly composed of basic, plutonic and volcanic rocks (sometimes subaerial lavas).

(*) Strip of the Earth’s surface from which geographic data are collected by a moving vehicle (car or boat).

(*3) A large chain of submerged mountains situated, generally, in the middle of the oceans and resulting from the slow pull apart of tectonic plates. It average height of mid-ocean ridges ranges between 2,000 to 3,000 meters above the surrounding ocean floor. In the central part of MOR there is a rift, i.e., an axial depression running parallel the ridge. Along the rift, lava, from the ascending magma of the sublithospheric mantle, is emitted.

(**3) In geology, geomorphology and physical geography, a horst designates one or more faulted blocks that have remained stable or have risen, relatively, to adjacent structural depressions. Relative uplift or horsts result from the combination of conjugated normal faults. A horst is bounded by its geological counterpart, that is, by grabens, which are sinking structures, equally, induced by conjugated normal faults.

Abyssal Knoll................................................................................................................................................................................................Colline abyssale

Colina Submarina / Colina abisal (submarina) / Abyssisch Hügeln / 深海丘陵 / Абиссальный холм / Collina abissale /

Low relief on the ocean floor, generally, located on the oceanic basins isolated by the ancient mid-oceanic ridges and oceanic slopes. The height of an abyssal knoll can reach several hundred meters. Its diameter can, sometimes, exceed tens of kilometers. Approximately 85% of the Pacific Ocean floor and 50% of the Atlantic Ocean floor correspond to abyssal knolls or abyssal hills. Most geoscientists consider abyssal hills and abyssal knolls as synonymous.

See: « Deep Sea-floor »
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« Continental Rise, Glacis »
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« Basin Floor Fan »

The abyssal plain (abyssal basin of certain geoscientists) designates the deepest part of the ocean, usually, between 2,000 and 6,000 meters deep. It extends from the base of the continental slope to the sides of the oceanic ridges. This plain is, sometimes, interrupted by knoll or hills (with heights between 200 meters and 1,000 meters) or even by submarine mountains, of volcanic origin with elevations greater than 1,000 meters, which can give rise to oceanic islands. Although the abyssal plain is, always, below the photic zone, marine life can be abundant (blind fish, giant octopus, algae, decapod crustaceans, etc.), in particular, near the deep oceanic springs (hot water and nutrient rich) and, particularly, near the rift of the mid-ocean ridge (MOR). In this sketch, in the ocean floor, which is limited between the mid-oceanic ridge and the base of the continental slope, abyssal or submarine hills are easily identifiable, particularly, in the abyssal plain. As can be seen, the mid-oceanic ridge, which forms part of the great immersed mountain range. This mountain range is around 60,000 km long and is, currently, the longest mountainous chain on Earth. As illustrated, it is displaced, laterally, by transform faults* which are the boundary type between lithospheric plates, in which there is no formation or consumption of lithosphere. These faults, which are just active between the rift of the different segments of the mid-oceanic ridge are, partially, hidden due to the deposition of turbidite and pelagic deposits. This scheme follows the conjecture advanced by most of the geoscientists, that abyssal hills correspond, roughly, to the ancient mid-oceanic ridges. Undoubtedly, as sea-floor spreading progresses, the new oceanic crust expelled along the rift of the mid-oceanic ridge pulls, continentward, the older mid-oceanic ridges which, gradually, begin to cool down. This cooling causes the sinking of the ridges as their density increases. On the other hand, this scheme shows that the topography of the sea-floor decreases (increasing of bathymetry) from the mid-oceanic ridge (highest value) to the abyssal plain. The sea-floor morphology varies, constantly, not only function of the sea-floor spreading, but also as a function of its velocity. Faster is the sea-floor spreading more pronounced is the topography of the sea-floor. These variations of the sea-floor topography modify the volume of oceanic basins, which forces the absolute (eustatic) sea level (referenced to the Earth's centre) to rise or fall creating 1^st order eustatic cycles. The conjecture that the amount of water in all its forms is constant since Earth's formation (about 4.5 Ga) has not yet been refuted. At the hierarchical level of the continental encroachment cycles (stratigraphic cycle induced by a 1^st order eustatic cycle), when the absolute (eustatic) sea level falls, globally, coastal deposits prograde forming first order sedimentary regressions. On the contrary, when the absolute sea level rises, the coastal deposits move continentward (retrogradation coastal deposits) forming a 1^st order sedimentary transgression. However, this is not true at the hierarchical level of sequence-cycles. Excluding submarine basin floor and slope fans, which can be deposited during significant relative sea level fall, all other sub-groups of systems tracts are deposited in association with relative sea level rises (here we are talking about relative sea level, i.e., a local sea level referenced to a point of the Earth's surface, which may be the sea-floor or the base of the sediment , and not about the absolute or eustatic sea level). Within a sequence-cycle, when the relative sea level rises in acceleration (marine ingressions each time more important), sedimentary transgressions** are deposited (set of increasingly important marine ingressions and increasingly smaller sedimentary regressions). When the relative sea level rises, in deceleration, increasingly important sedimentary regressions are deposited, which collectively form a 3^rd order sedimentary regression.

(*) A transform fault between two lithospheric plates does not destroyed or created lithospheric material. A transform fault has a relative movement, predominantly, horizontal, either sinistral (lateral leftward) or dextral (lateral rightward). These faults terminate abruptly and are connected, at both ends, to other faults, rifts or subduction zones. Most of the transform faults are, partly, hidden in the deep ocean by pelagic sediments. They can, also, occur in onshore, such as the San Andreas Fault in California.

(**) One could also speak of a third-order sedimentary transgression.

Abyssal Plain (Deep sea Rise)..........................................................................................................................................................Plaine abyssale

Planície  Abissal / Planicie abisal / Abyssisch Ebene / 深海平原 / Абиссальная равнина / Pianura abissale /

Region of the ocean floor (sea-floor) at the base of the continental slope with a declivity less than 1:1000*. An abyssal plain is, generally, covered with turbidite and pelagic deposits which, partially, mask the original topography.

See: « Deep Sea-floor »
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« Abyssal »
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« Shelfal Accommodation »

In an extensional geological setting (tectonic regime characterized by an ellipsoid of the effective stresses** with the largest axis σ₁ vertical), the deep sea-floor is, generally, limited between mid-oceanic ridge (MOR) or oceanic dorsal (most recent centre of oceanic expansion) and the base of the continental slope (continental rise). In a compressional geological setting (tectonic regime characterized by an ellipsoid of the effective stresses with the largest axis σ₁ horizontal), the deep sea-floor, as illustrated in the upper sketch, is most often limited between an oceanic trench (zone limiting of two lithospheric plates, where one of them plates plunges under the other) and the continental rise. The deep sea-floor corresponds almost always to the abyssal plain, on which abyssal hills and knolls are, easily, identified. The abyssal plain has very large dimensions and is, always, formed by new oceanic crust, created at the expansion centres (sea-floor spreading) located either in the rift of the mid-oceanic ridge or along the transform faults, which may be recognize at the extremities of the rift segments. Transform faults are one of the three types of limits between lithospheric plates, different from the others two, since there is no production or consumption of lithosphere. Do not confuse the rift of a mid-oceanic ridge (MOR) with a rift-type basin of the Bally and Snelson basin classification.  The rift of a middle oceanic ridge is the zone where the lithosphere breaks and spread out in two opposite directions, as new oceanic crust is forms. A rift-type basin corresponds to a lengthening of a supercontinent before its breakup. Transform faults, which form a very particular type of faults. They are only active between the rifts of different segments of the mid-oceanic ridge. They are, partially, masked by the deposition of turbidite and pelagic sediments on the abyssal plain. As oceanic crust of the sea-floor ages, it becomes denser and heavier, and most often it goes into subduction, that is to say, it dives under a lighter sector of oceanic crust or under the continental crust which is much less dense. Such a dive creates a B-type subduction zone (descending or subducting plate is oceanic), which is recognized without difficulty by the bathymetry of the associated oceanic trench, as illustrated in scheme in this figure. The oceanic crust descending along the subduction zone, is, progressively, assimilated by the asthenosphere (slightly rigid upper area of the terrestrial mantle that lies below the lithosphere, between about 80 and 200 km deep, above the isotherm 1250° C). As a consequence an important volcanic arc is formed on the overriding lithospheric plate, which limits the upper part of the subduction zone. In this way, inexorably, the oceanic mountains (oceanic ridges) associated with seafloor spreading and the mid-oceanic ridge go into subduction and disappear reducing, substantially, the extension of the abyssal plain, between the continental rise and the oceanic ridge. Over time, the entire abyssal plain disappears and the continent transported on the lithospheric plate, collides with the volcanic arc closing, completely, the sea or ocean that existed between them. Due, in part to their great extent, abyssal plains are considered as a great reservoir of biodiversity. They also exert a significant influence on the carbon cycle of the ocean, calcium carbonate dissolution and atmospheric CO₂ concentrations (periods of 0.1-1 ky). The structure and function of abyssal ecosystems are, strongly, influenced by the flow rate of nutrients toward the seafloor and by the composition of the material installed there. Factors such as climate changes, fishing practices and ocean fertilization should have a substantial effect on primary production patterns in the euphotic zone, which certainly has a significant impact on the flow of organic material to the abyssal plain and thus a profound effect on the structure, function and diversity of abysmal ecosystems (http://www.wikiwand.com/en/ Abyssal_plain).

(*) Downward inclination of the hypotenuses of a triangle, in which one of the cathetus is 1000 time longer that the other.

(**) It is the association of the effective stresses that deforms the sediments and not the tectonic vector (σ_t). At a given point, the effective stresses are: (i) Geostatic pressure (σ_g), i.e., the weight of the sedimentary column, which can be represented by a biaxial ellipsoid ; (ii) Hydrostatic pressure or pore pressure (σ_p), i.e., the weight of the water column maintained in the pores of the rocks and which can be represented by a uniaxial ellipsoid, i.e., by a sphere and (iii) Tectonic vector (σ_t), which corresponds to a tectonic force acting, more or less, parallel to the Earth's surface. The combination of the geostatic pressure, hydrostatic pressures and the tectonic vector can be represented by a triaxial ellipsoid (ellipsoid of the effective stresses), in which the main axis is σ₁, the mean axis σ₂ and the smaller axis σ₃.

Abyssal Red Clay...................................................................................................................................................................................Argile rouge abyssale

Argila vermelha abissal / Arcilla roja abisal / Abyssich rot Ton / 深海红粘土 / глубинное красной глины / Abyssale argilla rossa /

Terrigeneous sediment with about 30-40% of clay minerals and less than 30% of calcium carbonate, enriched by iron oxides. This type of sediment, deposited on great abyssal bottoms (more or less, 5,000 meters), does not contain any organic matter. Lithification of this type of sediment forms a claystone (nonfissile mud-rock). Clay is a particle, whose diameter is less than 0.005 millimetre, however, this term is used, by certain geoscientists, to denominate a rock composed essentially of clay particles.

See : " Controlling Parameter"

Abyssal Trench..............................................................................................................................................................................................Fosse abyssale

Fossa abissal / Fosa abisal / abyssisch Gräben / 深海海沟 / Глубоководный котлован / Fossa abissale /

Arched and narrow depression of the sea-floor ocean much deeper than the adjacent ocean bottom. The oceanic trenches are induced by B-type subduction (Benioff zones). They constitute areas of negative gravitational anomalies. Synonym with Oceanic Trench.

See: « B-Type Subduction (Benioff) »
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« Abyssal »
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« Deep Seafloor »

In the geological cross-section and, particularly, on the tentative geological interpretation of a Canvas auto-trace of a detail of a regional seismic line crossing the Northern offshore of Lombok island (Indonesia), the abyssal trench is, perfectly, defined (see the location of the seismic detail in the geological cross-section, at the southern end of the accretionary prism). In the regional geological section (on top), the abyssal trench marks the upper part of a subduction zone (B-type subduction, in which an oceanic lithospheric plate, i.e., a dense and cold lithospheric plate plunges under a much less dense continental plate). In this subduction zone, the subducting or plunging plate is the Indian lithospheric plate, while the SE Indonesia plate is the overriding plate. In detail, the abyssal trench is defined between the oceanic crust of the subducting lithospheric plate and the accretionary prism of the overriding plate (buoyant continental plate). It is characterized by a great water-depth. On the Canvas auto-trace of a regional seismic line (CGG line n° 103), the thrust faults recognized in the accretionary prism underline a compressional tectonic regime (shortening) created by the friction between the two lithospheric plates. This tectonic regime is characterized by a horizontal σ₁ (main axis of the ellipsoid of the effective stresses), σ₂ horizontal and parallel to the direction of the abyssal trench and σ₃ vertical. Some of the pelagic sediments deposited on the oceanic crust are swallowed along the subduction zone, i.e., the area where two lithospheric plates converge. In a B-type subduction zones (or Benioff zone), the descending or subducting plate is oceanic. The overriding plate can be continental or oceanic. In an A-type subduction zone (Ampferer zone), both plates are continental. When oceanic crust is formed, in the rift of mid-oceanic ridge (MOR), it is young, warm and with low density. As the sea-floor spreading progresses, the oceanic crust cools, contracts, and becomes denser, which facilitates its sinking into the underlying warmer sub-lithospheric mantle. As in A-type subduction zones the two lithospheric plates are continental with, more or less, the same density, the plunging of the descending plate (subducting plate) is more difficult than in a B-type subduction. This causes a significant shortening of both plates with the formation of a folded mountains belt. In this geological section, the Neogene volcanic arc (there are volcanic arches of other ages in this area) is well visible. It separates the Plio-Pleistocene fore-arc basin (external to the volcanic arc) from the Neogene back-arc basin. The position of the volcanic arc, in relation to the abyssal trench, is dependent on the angle or inclination of the subduction plane. When the dip of the subduction plane is important, the volcanic arc (active volcanoes of the overriding plate) is, relatively, close to the abyssal trench. On the contrary, when the dip of the subduction plane is small, the distance between the volcanic arc and the abyssal trench (more or less, stationary) is much greater. Northward of the volcanic arch, towards the continental crust, a Paleocene back-arc basin develops, in which two tectonic sedimentary phases can be evidenced. The first phase corresponds to a lengthening of the continental crust (rifting phase), during which the subsidence is differential (formation and filling of half-grabens, often called, also, rift-ype basins). The second phase corresponds to a gradual sinking (cratonic or sag phase of certain geoscientists), mainly with a thermal subsidence (thermal equilibrium). As all back-arc basin are located within megasutures (in this case the Mesozoic/Cenozoic megasuture), during the final period of its evolution it is, sooner or later, submitted to a compressive tectonic regime, which creates significant tectonic inversions*, in which structural traps can be interesting for petroleum exploration, if the migration of hydrocarbons is post-inversion.

(*) A process where normal faults (extensional tectonic  regimes, lengthening) are reactivated by compressional tectonic regimes (positive inversion) or where reverse faults induced by compressive tectonic regime (shortening) are reactivated by an extensive tectonic regime (negative inversion).  The positive inversion are much more evident than negative inversion since they imply an uplift (shortening) of formerly low-lying areas.

Accelerated Relative Sea Level Rise...............................................Montée du niveau Relative de la Mer en Accélération

Subida do Nível do Mar Relativo em Aceleração / Crecida en aceleración, Subida relativa (del nivel del mar) en aceleración / Erhöhung der Beschleunigung (NN) / 加速上升（海平面）/ Ускоренный подъём уровня моря / Aumento in accelerazione (livello del mare) /

When the accommodation or space available for sediments (combined action of eustasy and tectonics) increases, more or less, regularly, as for instance 3, 5, 8, 10 meters. In an accelerated relative sea level rise, marine ingressions are increasingly important. It is in association with this type of relative sea level rise that, within a sequence-cycle, the sequence-paracycles of the lowstand prograding wedge (LPW) and the transgressive interval (TI) are deposited. Deposition occurs during the stability periods of relative sea level between each ingression increment.

See: « Third Order Eustatic Cycle »

Accelerated Relative Sea Level Fall.............................................................Chute en accélération du Relative de la Mer

Descida do nível do mar eelativo em aceleração / Descenso en aceleración (nivel del mar) / Beschleunigte Meeresspiegel fallen / 加速海平面下降 / Ускоренное снижение (уровня моря) / Caduta di livello del mare in accelerazione /

When the accommodation or space available for sediments (combined action of eustasy and tectonics) decreases, more or less, regularly, as for instance, 13, 8, 4, 2 meters. In an accelerated relative sea level fall, marine ingressions are decreasingly important. In the sea level curve of a 3^rd order eustatic cycle, four segments can be considered: (i) Falling in Deceleration Segment with deposition of the submarine slope fan (SSF) and submarine basin floor fan (SBFF), until the sea level does not descend further (1^st derivative is negative and 2^nd derivative positive or, in other words, the function, i.e., the relative sea level curve is decreasing and concave) ; (ii) Rise in Acceleration Segment with deposition of the lowstand prograding wedge (LPW) and the transgressive interval (TI), till the inflection point that marks the maximum rate of relative sea level rise (1^st and 2^nd derivatives are positives, i.e., the function is increasing and its geometry concave) ; (iii) Rise in Deceleration Segment with deposition of the highstand prograding wedge (HPW) till the point where relative sea level no longer rises (derivative is zero), the 1^st derivative is positive, but the 2^nd derivative is negative, i.e., the function is increasing and its geometry convex ; (iv) Falling in Acceleration Segment with deposition of the 2^sd phase of the highstand prograding wedge (HPW). Sometimes, an additional segment is found in association with the bordering prograding wedge (BPW) above a II-type unconformity, till the point of inflection (1^st derivative maximal) that marks the end of the eustatic cycle.

See: « Third Order Eustatic Cycle »

Accommodation (Sediments)..........................................................................Espace disponible pour les sédiments

Espaço disponível / Espacio disponible (para sedimentos) / Unterkunft, verfügbar Raum (für Sediments) / 住宿, 空间（泥沙) / Свободное пространство (для геологических отложений) / Spazio disponibile (per i sedimenti), Alloggio /

Space available for sediments between the sea-floor and sea level. The accommodation is a function of eustasy (variations of absolute or eustatic sea level) and tectonics. Variations in accommodation are induced by variations in relative sea level, i.e., combined action of eustasy and tectonics (subsidence or uplift of the sea-floor function of the predominant tectonic regime).

See: « Accommodation (marine) »
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« Paleobathymetry »
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« Relative Sea Level Change »

The space available for sediment or accommodation, as some geoscientists say, just makes sense the upstream basin edge (shelfal accommodation). Seaward of the basin edge, the water-depth is, always, greater than 200 m (there is always enough water-depth the deposition of the deep-water depositional systems). Relative sea level changes do not have a large effect on deep-water deposition, except when the relative sea level falls below the basin edge (turbidite depositional systems). Do not forget that there are two types of sea level: (i) Relative Sea Level, which is a local sea level, referenced to the base of the sediment (top of the continental crust) or to the sea-floor and (ii) Absolute Sea Level or Eustatic Sea Level, which is supposed to be global, is referenced to the Earth's centre. The relative sea level is the result of the combination of absolute (eustatic) sea level and tectonics (subsidence or uplift of the sea-floor). Submarine basin floor fans (SBFF) and submarine slope fans (SSF) are the subgroups of systems tracts deposited during significant relative sea level falls. In contrast, upstream of the basin edge, which may or may not coincide with the continental edge (coincides with the depositional coastal break when the basin has a shelf), relative sea level changes control the deposition. An increase in available space (accommodation) induces deposition and a decrease induces erosion. It is important to consider whether the basin has a shelf or not. In the case of a basin with a shelf, i.e., when the depositional coastal break (more or less the shoreline) is upstream of the continental edge (which in this case coincides with the basin edge), a relative sea level rise increases the available space and thus there will be deposition. If the terrigeneous influx is sufficient, all of the space created will be filled during the period of stability of relative sea level that occurs after each marine ingression and the water-depth will remain constant. If the sedimentary supply is not enough to fill the entire space created by the increments of the relative sea level rise (marine ingressions or eustatic paracycles), there will be deposition, but the water-depth will increase, since only a part of the available space created is filled. When the basin has no shelf, the shoreline coincides, roughly, with the upper limit of the continental slope (continental edge), which may or may not coincide with the basin edge(it coincides with the basin edge under highstand geological conditions). Under lowstand geological conditions, the basin edge is, largely, upstream of the continental edge as well as of the shoreline. In this case, several hypotheses are possible: (a) If the available space, created by the relative sea level rise, is completely filled, the water column (water-depth) is zero ; (b) If the available space (accommodation) is not completely filled, a continental shelf is formed with the beginning of transgressive interval deposition ; (c) If the available space is insufficient to accommodate all the sedimentary supply, a part of the sedimentary particles will be deposited on the upper sector of the continental slope and the trigger of turbidite currents is possible. The cyclicity of the available space is induced by eustasy. The rate of eustatic variations is much higher than the rates of tectonic variations. The accommodation magnitude is, mainly, given by tectonics (subsidence or uplift of the sea-floor). As shown in the photograph of this figure, within a sequence cycle, to have deposition (except for turbidite deposits) the available space must always increase. It increases in acceleration during the lowstand prograding wedge (LPW) and during a transgressive interval (TI) and in deceleration during the highstand prograding wedge (HPW). As long as the available space for the sediments decreases, significantly, an unconformity (erosional surface) is formed. As illustrated in this figure, each sequence-cycle is represented just by the highstand systems tracts group* (HSTG), i.e., by the systems tracts forming the sub-groups known as transgressive interval (TI) and highstand prograding wedge (HPW).

(*) Within a complete sequence-cycle there are two groups of systems tracts: (i) Highstand Systems Tracts Group (HSTG) ans (ii) Lowstand Systems Tracts Group (LSTG). The highstand systems tracts group is composed by two sub-groups: (i.1) Transgressive Interval (TI), at the base and (i.2) Highstand Prograding Wedge (HPW), at the top. The lowstand systems tracts group is formed by three sub-groups that from the bottom to top are: (ii.1) Submarine Basin Floor Fan (SBFF) ; (ii.2) Submarine Slope Fan (SSF) and (ii.3) Lowstand Prograding Wedge (LPW). Each sub-group of systems tracts is formed by one or several systems tract function of the number of marine ingression (LPW, TI and LPW) and turbiditic currents (SSFF, SFF).

Accommodation (Marine Sedimentation)................................................................................................................Accommodation

Espaço Disponível / Acomodación / Unterkunft / 可用空间 / Жилое помещение (размещение) / Accomodamento

Space available for sediments between the sea-floor and sea level. Variations in available space are induced by relative sea level changes, that is to say, by the combined effects of: (i) Tectonics (subsidence or uplift of the sea-floor) ; (ii) Eustasy (taking into account the sea level variations induced by precession and eccentricity) and (iii) Thickness of deposited sediments, and not just by eustatic variations (absolute or eustatic sea level changes referenced to Earth's centre).

See: « Relative Sea Level Change »
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« Shelfal Accommodation »
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« Stratigraphic Cycle »

The accommodation or available space for the sediments varies with the relative sea level changes. Within a sequence-cycle, seaward of the shoreline, the accommodation is marine. Upstream of the shoreline, which is, more or less, the depositional coastal break of the depositional surface, the accommodation is subaerial (scheme on the upper left). When there is a relative sea level rise, the water-depth increases and, in general, there is deposition (mainly upstream of the depositional coastal break) during the stability period of the relative sea level that occurs after each eustatic paracycle or, in other words, after each marine ingression. On the contrary, when there is a relative sea level fall, the water-depth decreases and, in general, there is erosion or a forced displacement seaward and downward of the coastal deposits (forced sedimentary regression), particularly, landward of the basin edge. Erosion occurs when the relative sea level fall is significant and puts the sea level below of the basin edge, which is the continental edge during the transgressive interval (TI) and during the 1^st phase of the development of the highstand prograding wedge (HPW), i.e., during the highstand systems tracts group (HSTG). A forced regression (deposition of descending depositional systems in the early terminology of P. Vail) occurs when the relative sea level fall is insufficient to exhume the shelf and bring the sea level lower than the basin edge. Within a stratigraphic cycle and, particularly, within a sequence-cycle, one should not confused the basin edge, which corresponds, generally, to the continental edge (or continental break) with the depositional coastal break, which corresponds, roughly, to the shoreline. During the deposition of the lowstand systems tracts subgroups (submarine basin floor fan, submarine slope fan and lowstand prograding wedge) the basin edge is the last continental edge of the preceding sequence-cycle and the sediments are deposit against the continental slope. According to P. Vail, submarine basin floor fan (SBFF) and the submarine slope fan (SSF) deposit, when the relative sea level falls, significantly, and the shelf (if it existed) and the upper part of the continental slope are exhumed. All other sedimentary systems tracts sub-groups forming a sequence-cycle, are deposited during the stability periods of the sea level occurring after each marine ingressions (rises of relative sea level). The sea level, which, can be relative or absolute (eustatic), can rise or fall in acceleration or deceleration. The relative sea level is a local sea level referenced to any point on the Earth's surface which is, generally, the top of the continental crust (base of the sediments) or the seafloor. On the contrary, the absolute (eustatic) sea level is  supposed global and referenced to the Earth's centre. In a sequence-cycle, when the relative sea level rises in acceleration are deposited: (i) The lowstand prograding wedge (LPW) and (ii) The transgressive interval (TI). When the relative sea level rises in deceleration, is deposited the highstand prograding wedge (HPW). In addition to accommodation, the other parameters controlling the depositional systems are: (i) Eustasy (variations of absolute or eustatic sea level) ; (ii) Tectonics (subsidence or uplift of the sea-floor) ; (iii) Climate and (iv) Terrigeneous Supply. Climate is quite important and climatic changes are a reality that exists since the Earth was formed about 4.5 Ga. In a sedimentary stratigraphic section, any alternation between a sand bed or group of beds of sand and a shale or a limestone bed or group of beds is a direct or indirect consequence of a climatic change, which implies, at least, a relative sea level rise (combination of absolute sea level and tectonics). The unconformities (erosional surfaces), which bound the stratigraphic cycles (induced by eustatic cycles) are quite often caused by glacio-eustasy, particularly, when they limits sequence-cycles (induced by 3^rd order eustatic cycles, whose time-duration ranges from 0.5 to 3-5 My). Between the sequence-paracycles (deposited in association with the marine ingressions that characterize eustatic paracycles) there are no unconformities, that is to say, there are no relative sea level fall. Sequence-paracycles are limited by flooding and ravinment surfaces. Within a sequence-cycle, seaward of the continental edge (whether or not it coincides with the basin edge) accommodation (available space for sediments) is sufficient for deposition of submarine fans, sedimentary aprons, pelagic (water-depth greater than 1,000 m) and hemipelagic (water-depth between 200 and 1,000 m) deposits.

Accretion (Lithosphere, Shoreline)....................................................................................................................................................................Accrétion

Acreção / Acreción / Anlagerung, Zuwachs / 增加 / Аккреция (нанос смытой породы) / Accrezione /

Process by which terrestrial material is added to a lithospheric plate or to a continent. The added material may be sedimentary, volcanic (sub-aerial or oceanic) or igneous. Two types of accretion are possible: (i) Accretion associated with the lithospheric plate tectonics and (ii) Accretion associated with the evolution of the coastline (gradual increase of land, on littoral coast, due to river terrigeneous influx, tidal action, wind currents) and accumulation of alluvial deposits.

See: « A-type Subduction »
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« Point Bar (fossil) »
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« Moraine »

On this tentative geological interpretation of a Canvas auto-trace of a Namibia offshore seismic line (Atlantic-type divergent margin deposited over rift-type basins or over a Precambrian or Paleozoic basement), a lateral accretion of volcanic material (subaerial lava flows) followed by a clastic sedimentary accretion of the western African plate are easily recognized. Before the results of wells drilled in this offshore, one of which was located, practically, on the seismic line of this auto-trace, almost all geoscientists thought that the lower seismic interval (in violet on this tentative interpretation), which is characterized by reflectors dipping and thinning seaward ("SDR" is the acronym for Seaward Dipping Reflectors), were interpreted as rift-type basin sediments, i.e., not associated to the lower part of the divergent margin of the Eastern South Atlantic. The results of the wells did not refute that the unconformity BUU (Break-up Unconformity) was induced by the breakup of the small supercontinent Gondwana, which individualized South America and Africa continents. Locally, the BUU corresponds to the interface between the basement (more or less, flattened Paleozoic Folded Belt) and the base of the volcanic interval (SDR), which is basically composed of sub-aerial lavas. At present, the great majority of geoscientists think that the most likely geological history of this area can be summarized as follows: (i) Lengthening of the small supercontinent Gondwana by a normal faults and formation of Early Cretaceous/Late Jurassic rift-type basins, which not only have the geometry of half-grabens, but an eastern vergence as well (rift-type basins are not visible in this auto-trace) ; (ii) Filling of the rift-type basins by non-marine sediments with organic rich lacustrine sediments (potential source-rocks), dipping westward, i.e., contrariwise to the dip of the SDRs ; (iii) Break-up of the lithosphere of the small supercontinent Gondwana and individualization of two lithospheric plates (South America and Africa) accompanied by significant amount the lava effusions flowing toward the margins from the subaerial expansion centres (volcanoes and volcanic dikes located along the breakup zone) ; (iv) Accretion of the margins (Africa and South America) by sub-aerial lavas (sub-aerial volcanic sea-floor spreading), which thinner continentward as they move away from the spreading centres (volcanoes) ; (v) Differential sinking of the margins towards the proto-ocean, formed between them, and beginning of an oceanic accretion (oceanic sea-floor spreading), that is to say, accretion by formation of oceanic crust followed by the deposition of a sedimentary prism. Since the vertical scale of the original seismic line, as well as that of the auto-trace, is in time (t.w.t.), the water-depth must be corrected to obtain a correct geometry of the margin and particularly of the sea-floor. In a depth version of the original seismic line, the margin geometry will be more similar to that of a conventional geological section. The seismic reflectors, especially those from the western part of the seismic line, in a depth auto-trace, will dip much less than is illustrated here, in time. They may even dip eastward, especially, the deeper ones, as one can observe, practically, in all regional depth seismic lines of the divergent margin offshores (whether Atlantic or non-Atlantic type). This is, particularly, true when the water-depth changes, abruptly, toward the abyssal plain. This geometry of Atlantic-type divergent margins (extensional geological setting) and non-Atlantic (compressional tectonic setting), is, often, overlooked by certain geoscientists. Such a negligence can have disastrous implications, particularly, in petroleum exploration: (i) Trap definition ; (ii) Maturation assessment of the organic matter of potential source-rocks ; (iii) Migration direction of hydrocarbons, etc. The proposed tentative interpretation of this auto-trace was made in continental encroachment sub-cycles, induced by 2^nd order eustatic cycles, whose time-duration varies between 3-5 My and 50 My. The continental encroachment sub-cycles considered here are limited by unconformities (SB. 68 Ma, SB. 49,5 Ma, SB. 30 Ma, SB. 10,5 Ma and SB. 5,5 Ma), i.e.,y, by erosional surfaces induced by significant relative sea level falls that put the sea level lower than the basin edge, creating, locally, huge submarine canyons, such as the one visible on unconformity SB. 30 Ma. Within each of the considered continental encroachment sub-cycles, several sequence-cycles may be individualized.

Accretionary Wedge.........................................................................................................................................................Prisme d'accrétion

Prisma de Acreção / Prisma de acresión / Akkretionskeil / 增生楔 / Аккреционный клин / Prisma di accrezione /

Sedimentary wedge (or sedimentary prism), partially, lying on an oceanic trench, over a B-type subduction zone (Benioff subduction), in the seaward border of the overriding plate. The material in the accretionary wedge is, mainly, composed by marine sediments scraped off from the subducting oceanic crust. Erosional debris from the volcanic island arcs formed on the overriding plates can, also, be constituents of the material forming the accretionary wedges.

See : « B-type Subduction Zone (Benioff) »
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« Plate Tectonics Theory »
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« Abyssal Trench »

In a B-type subduction zone (Benioff subduction), the subducting oceanic plate induces accumulation and shortening of marine sediments against the overriding lithospheric plate. However, only when the subduction angle is small, a significant accretionary prism or accretionary wedge is formed. The sediments are shortened (compressed, for some geoscientists) until they form important reverse faults and thrusts* that form a morphological anomaly characteristic of B-type subduction, which can, sometimes, emerge in certain places such as Barbados (Central America). The internal structure of accretionary wedges is similar to that of the superficial folded belts in foreland basins associated with the formations of mountain ranges. A series of thrusts with a polarity or vergence** toward the oceanic trench is formed with the younger thrusts near the trench (shortening in sequence) and a progressive uplift of the thrusts and older structures, near the mountain range. The shape of the accretionary wedge is determined by the rapidity of faulting of the sediments along the basal detachment and inside, which is highly sensitive to the pressure of the pore fluid. Thus, progressively, the reverse faults and thrusts create a sedimentary wedge that reaches its maturity when it has a triangular morphology (in section), which underlines a critical shortening. Once the accretionary wedge reaches critical shortening, it will maintain the geometry and grow only, as a function of shortening, to become larger but similar triangle. Accretionary prisms, as well as, terranes (fragments of crustal material of the subducting plate accreted or suture to the crust of the overriding plate), are not equivalent to tectonic plates, although they are associated with tectonic plates increasing the size of the continents as a result of tectonic collisions***. The material incorporated in an accretionary prism include: (i) Basalts of the ocean floor ; (ii) Pelagic sediments deposited on top of the subducting plate ; (iii) Sediments of the oceanic trench, which are generally rich in turbidites, with material coming from the fore-arc basins, volcanic arcs, mainland, pig-back basins, etc.

(*) Reverse faults are induce by a compressional tectonic regime with an oblong ellipsoid of effective stresses (σ₁ horizontal, σ₂ horizontal and perpendicular to σ₁ and σ₃ vertical). The strike of the reverse faults is always parallel to σ₂. In a reverse fault, the hangingwall moves up and over the footwall. The hangingwall occurs above the fault plane (plane that represents the fracture surface, which limits the two faulted blocks) and the footwall below. Thrusts with a very low angle of dip and a very large total displacement are called  overthrusts or detachments. Large thrusts are found in convergent margins as the associated with A-type subduction (e.g., Alps and Himalayas) and B-type subduction zones (e.g., Western Sumatra and Western South America).

(**) The polarity or vergence designates the direction towards which thrusts occur. - (Jean-François Raoult, Dictionary of Geology, Dunod, 2010, 372)

(***)  Tectonic collisions are associated with both types of subduction zones (A-type and B-type). A subducting plate, particularly, in B-type subduction, is destroyed or partially destroyed, while volcanic arcs and mountains are formed on the overriding plate. In certain cases, a continent on the subducting plate can be sutured with a continent on the overriding plate forming a greater landmass, often, a supercontinent. The term collision, well adapted in everyday language to  express the crash between two vehicles (transformation of kinetic energy into potential energy), is not adapted in plate tectonics, since the kinetic energy does not play any role.

Accumulation Zone (Glacier).........................................................................................................Zone d'accumulation (Glacier)

Zona de Acumulação / Zona de acumulación (glaciar) / Nährgebiet (Eisessig) / 聚集区（冰川） / Зона накопления / Zona di accumulazione (glaciale) /

Area of a glacier where snow and ice accumulate. Area of a glacier, in which snowfalls exceed ice losses by melting, evaporation, calving (glacio-fracturation) and sublimation.

See: « Snow Field, Firn »
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« Glacier »
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« Ablation Zone »

In this photograph (Alean, J., 1982) of the Glärnischfirn Glacier or Glärnischgletscher (north-eastern Switzerland), the accumulation zone, updip of the ablation zone, is well delimited by the equilibrium line or snow line, which separates them. The accumulation zone is the area where snow and ice accumulate. The processes of accumulation, which add snow or ice to a glacier, floating ice (iceberg) or to a snow cover, can be snow falls, avalanches, transport by the wind, freeze over, etc. The accumulation zone contrasts with the ablation zone (or loss zone), which is the area in which the annual loss of melting snow, evaporation, calving* and sublimation** exceeds the annual snow and ice gain (in surface). The survival of a glacier depends on the balance between ablation (loss) and accumulation (deposition) of snow and ice. When ablation exceeds accumulation, the snow line (boundary between the ablation and accumulation zone) retreats (becomes higher) and the glacier thins (avoid the expression retreat, since the glacier being a current of ice flows downslope ever since it exists). Many glaciers retrograde with a positive accumulation/ablation balance (thickening). What counts for the survival of a glacier is the glacier mass balance, not whether it retrogrades up-slope or progradate downslope. A glacier may retrograde, but if its thickness increases, the accumulation/ablation balance is positive. By the same token, the stabilization of the glaciers depends very much on calving, which is a very efficient form of ablation. In Antarctica, for instance, without calving, ice caps*** would have a continuous expansion since accumulation/ablation balance is positive. This is not to say that climatic changes, which exist since the Earth's formation (4,5 Ga) and not only since man uses fossil fuels, does not produce temperature variations and significant snowfalls that disrupt the mass balance of the glaciers.

(*) Breaking of ice chunks from the edge of a glacier. It is a form of ice ablation, mainly, induced by the movement (longitudinal stretching), crevasses formation and expansion of a glacier. Icebergs are formed by ice calving.

(**) Process where ice changes into water vapour without first becoming liquid. This process requires approximately 680 calorie of heat energy for each gram of water converted (http://www.physicalgeography.net/physgeoglos/s.html#sublimation)

(***) Large dome-shape glacier found covering  large expanse of land. Smaller than, ice sheet (covering an area greater than 50,000 km²).

Achafalaya (Deltaic lobe)..................................................................................................................................................Achafalaya (Lobe deltaïque)

Achafalaia / Achafalaya / Achafalaya / Achafalaya / Атчафалая / Achafalaya /

One of the lobes of the Mississippi delta building. During the construction of the Mississppi deltaic building several deltaic lobes developed: (a) Maringoiun ; (b) Teche ; (c) St. Bernard ; (d) Lafourche; (e) Achafalaya and (vi) The current lobe. These lobes are, probably, the consequence of the pendulum effect* described by G. Dally in the Niger delta building.

See: « Biostratigraphy »
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« Biozone »
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« Index Fossil »

The delta lobe of Achafalaya (Mississippi) is a modern delta. It became a sub-aquatic delta, around 1952 and acquired subaerial characteristics, more or less, in 1973. Elongated distributive channels, abandonments and bifurcations of channels are typical of this delta lobe. The present sedimentation environments are similar to those of a sub-delta and those of overbank deposits. A sub-delta is nothing more than a secondary delta induced by crevasse-splays, that is to say, by a sedimentary fan formed when a watercourse breaks its natural levees (marginal dikes) and deposits the sediments in a floodplain. The depositional systems include: (i) Mouth bars ; (ii) Distal bars; (iii) Distributive channels ; (iv) Levees (natural marginal dykes) and (v) Periphytic algae plains (living on the periphyton, which is represented by a thin layer or biofilm of a few millimetres, acting at the interface between the substratum and the surrounding water). These depositional environments are part of the most recent sub-group of highstand sedimentary tracts, which seems to have formed about 1,500 years ago. Each of the deltaic complexes, induced by lateral displacements of the river bed (pendulum effect), covers an area of about 30,000 km² and has an average thickness of about 35 meters. The Achafalaya delta is the youngest of the large deltaic complexes that currently exist on the Louisiana coast. Within this complex, at least 12 sub-deltas formed during the last 4,000 years (Holocene). The Mississippi Delta is a typical example of a set of fingers or digitated deltas, which was built primarily by the alluvium deposited by the Mississippi River upon entering the Gulf of Mexico. It corresponds to an progradational upbuilding of a large number of deltas with an average thickness of 30-50 m. As a skyscraper is a superposition of floors with an average height of about 2,40 meters, a deltaic building is a stacking of deltas whose average thickness is about 30-50 meters. In terms of sequential stratigraphy, a delta is a sedimentary systems tract formed mainly by four depositional systems (lithology and associated fauna), which from upstream to downstream are: (a) Siltes and clays of delta plain ; (b) Sands, sometimes, carbonated from the delta front ; (c) Claystone of the prodelta and (d) Shales and sometimes turbiditic sandy fans (proximal turbidites) in the base of the prodelta. The depositional systems forming a systems tract are synchronous and genetically associated. In other words, if a depositional system is not deposited, generally, the others depositional systems of the systems tract are, also, not deposit. For instance, If the seaward dipping layers of a delta (claystone of the prodelta) are not deposited, any other sedimentary systems will be deposited. A deltaic building is a set of more or less progradational systems tracts of different age which, generally, forming a subgroup of sedimentary systems tracts that may belong to one or more sequence-cycle. The Mississippi deltaic building is characterized by: (1) A weak wave action ; (2) A certain amount of sand transported to the shoreface is dispersed offshore by storms ; (3) A small difference between the low and high-tide ; the tidal range is around 30 cm, but is sufficient to play an important role in the sedimentation, since the inclination of the delta is very small ; (4) A strong subsidence, induced by compaction* of the sediments (30-60 cm/100 years). For 7,000 years, sedimentation processes have moved the shoreline, seaward, between 30 and 80 kilometers. Several times (more or less every 1,000 years), the main path of the Mississippi River changed which created several important delta lobes (each appears to have been initiated by the capture of the main stream by one of its distributive ones): (a ) Maringoiun ; (b) Teche ; (c) St. Bernard ; (d) Lafourche ; (e) Achafalaya and (vi) The present lobe. The pendulum effect may also have been the cause of certain lateral displacements. Such a displacements created local marine ingressions,that should not be confused with the two large marine ingressions that occurred in this region created by glacio-eustasy: (A) The Brackish Ingression, when lakes, bays and lagoons covered alluvial sediments of the deltaic plain and (B) The Marine Ingression, when the longshore of the barrier-bars advanced to the mainland. The first was made, practically, without ravinment, which, in the marine ingression (B) is quite important.

(*) The pendulum effect presupposes that since a watercourse deposits a lobe, in front of its mouth, in general, above it, the available space for the sediments (accommodation) becomes insufficient ; the sedimentary charge is diverted to one of the flanks of the deposited lobe, where more accommodation is available.

(**) The compaction of the sediments can be presented under two aspects: (i) Chemical and (ii) Mechanical. Chemical compaction encompasses the dissolution of minerals under pressure. Mechanical compaction does not encompass chemical processes, but physical aspects such as change in intergranular packing (decrease in porosity and permeability) and deformation or breaking of individual grains.

Acme (Peak, Climax)..............................................................................................................................................................................................................................Acmé

Acme / Acme / Höhepunkt / 頂點 / Высшая точка / Acme

The highest level or degree achieved. The highest stage of development. In topography, the acme is the point of a surface that has the highest elevation in relation to all the immediately adjacent points. In biostratigraphy, the acme (acme zone, abundance zone or peak zone) is the area of a stratigraphic interval, limited between the first and last appearance of a given taxon* (in a given area), where the fossil taxon reach a maximum level of abundance.

See: « Biostratigraphy »
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« Biozone »
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« Index Fossil »

The term acme can be used in different contexts, but always to designate a high level or stage of development (apogee, peak, culmination or climax). Topographically, as illustrated by the Google Earth image of this photograph, it can be said that in Alps mountain folded belt, the "Mont Blanc" peak is the acme of the Alps, as well as the Mount Everest peak (Qomolangma Peak), at the Himalayas folded belt, is the acme of the world. By the same token, it may be said the Mariana Islands oceanic trench, located in the western part of the Pacific Ocean, eastward of the Mariana Islands, is the acme of the depth of the sea. An oceanic trench underlines, on a convergent continental margin, the depression between an oceanic subducting plate and the overriding lithospheric plate whether it is continental or oceanic in nature. The topographic acme of each folded mountain belt is, often, associated with an A-type subduction zone (Ampferer subduction), whereas the acme of the oceanic trenches is, always, associated with a B-type subduction zone (subduction of Benioff). In an A-type subduction zone, the subducting lithospheric plate is, always, continental in nature, while in a B-type subduction zone, the subducting plate is always oceanic. For this reason, the subduction mechanism in an Ampferer (A-type) subduction zone is different of a B-type subduction zone (also called Wadati-Benioff subduction zone). It is very difficult to plunge a not too dense material into a material of equal or greater density. In biostratigraphy (branch of geology that put order in the succession of sedimentary rocks according to their paleontological content), as shown in the lower left corner of this figure, the acme zone of a taxon is the zone of maximum development of the taxon, between the upper and lower limits of the taxon. Each taxon, i.e., each entity grouping all living organisms that have certain well defined taxonomic or diagnostic characteristics in common, has, always, an acme zone (zone of maximum development between the limit of appearance and disappearance of the taxon). For example, the Eurypterids, which are the largest arthropods (phylum** of invertebrate animals possessing rigid exoskeletons and several pairs of articulated appendages, whose numbers vary by class) in Earth history, since they had, more or less, 250 centimetres long (when adults), died out about 250 Ma, at the end of Permian. They had their acme during the Silurian and Devonian. The fossils of these giant scorpions range from Ordovician to Permian. It is permissible to think that they lived roughly between 505 Ma and 248 Ma (time scale of Harland et al., 1982). In the eustatic curve (curve of absolute or eustatic sea level, referenced to the Earth's centre) constructed from the coastal onlaps recognized on regional seismic lines and, in particular, on regional lines of the divergent margins (Atlantic-type or not-Atlantic), it can be said : (i) During the Paleozoic (earliest Phanerozoic 1^st order eustatic cycle), the eustatic acme occurred in Cambrian/Ordovician and (ii) During Meso-Cenozoic, which corresponds to the latest Phanerozoic 1^st order eustatic cycle, the eustatic acme occurred during Cenomanian/Turonian. It was during these periods of highstand (sea level higher than the basin edge) of the post-Pangea continental encroachment cycle, that sea level ingressed or invaded most continents. These marine ingressions moved continentward the shoreline developing, in the distal parts of the shelves, starvation zones. These zones of low sedimentation rate, when influenced by upwelling cold marine currents*** (rich in oxygen and nutrients) favoured the formation and preservation of organic matter. It was in these starvation zones that argillaceous rocks rich in organic matter were deposited, i.e., the main marine source-rocks that generated about 60% of the hydrocarbon world's reserves (bearing in mind that many of the source-rocks were eroded during the acme of Hercynian orogeny).

(*) In biology, a taxon (singular of taxa) is each of the groups or divisions that is used in biological systematics to divide individuals, including gender, family, order, and suborder.

(**) Level of classification or taxonomic rank below Kingdom and above Class.

(***) Upwelling currents are induced by the combined action of  wind, Coriolis effect and Ekman transport, that operate differently in different upwelling types, but  their general effects are the same.

Acme Zone (Apogee).......................................................................................................................................................................Zone de acmé (Apogée)

Zona de Acme / Zona de acme (apogeo) / Acme Zone (Peak) / Acme的区（高峰）/ Зона максимального развития таксона / Acme zona (picco) /

Biozone* containing the maximum abundance of a taxon (name that designates an organism or group of organisms). A set of strata, or stratigraphic interval, characterized by the abundance or development of certain forms, whatever their association or extension. Synonym of Apogee Zone, Peak Zone or Flooding Zone.

See: « Fossil »
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« Maximum Faunal Abundance Peak »
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« Condensed Section »

On this tentative geological interpretation of a Canvas auto-trace of a seismic line from West Australia offshore, the seismic horizon underlined by the red arrows corresponds to a stratigraphic biozone (general term that the geoscientists use to denominate any type of biostratigraphic unit) of the type apogee (acme zone). The exploration well, drilled not far from this line, corroborated such an interpretation. The interface between the lower green aggradational seismic interval (with parallel internal configuration) and the above violet progradational intervals (with oblique/tangential internal configuration) stands for a major downlap surface. Most likely, such a downlap surface marks the limit between the transgressive and regressive phases of  post-Pangea continental encroachment cycle induced by the last Phanerozoic 1^st order eustatic cycle. It corresponds, likely, to the Cenomanian/Turonian  maximum flooding surface (MFS 91.5Ma), which is the peak of the Cretaceous marine ingression (post-Pangea absolute sea level acme). In such highstand geological setting, the production and preservation of organic matter is paramount. Organic rich intervals (some are condensed section) deposited in association with this major downlap surface (DS 91.5 Ma) are often apogee biozones and can be considered as potential source-rocks*. In addition to the apogee biozones, which are valuable chronostratigraphic position index, there are other types of biozones, among which we can mention: (i) Biozones of Association (cenozones) - stratigraphic intervals characterized by a natural set typical of all fossil forms present or of a certain type or types of present forms ; (ii) Biozones of Extension (both vertical and horizontal) - a set of strata representing the stratigraphic extension of a certain element of the total set of fossil forms present (taxon extension zone, zone of coincidence extension, lineage or phylogenetic zone) ; (iii) Biozones of Interval - stratigraphic intervals between different biohorizons, etc. A biozone of interval, by definition, is neither a biozone of extension of a given taxon, nor a biozone of coincidence of several taxa, nor has distinct fossil associations or peculiar biostratigraphic characteristics. The biozone or acme zone represents the apogee or general maximum development (maximum abundance or frequency of presences), but not the total extent of a specie, genus or other taxon. When trying to define what is the maximum development, i.e., when trying to define the acme limits of a biozone, a problem is created. The maximum development may mean, for instance, an abundance of specimen of a taxon, but also the number of species of a genus, etc. The apogee zone takes the name of its taxon whose zone of maximum development zone it comprises, such as, for example, apogee zone of Didymograptus (Hedberg, H.D., 1980). Didymograptus is an extinct genus of graptolites living, mainly, in Lower Ordovician northern ocean and characterized by its typical tuning fork shape.

(*) A potential source-rocks when buried enough can become a source-rock, if its organic matter reaches maturation.

(**) A biozone is a stratigraphic unit defined by the biological content of sediments (or biostratigraphy). It is a general term for any category of biostratigraphic unit.

Acoustical Impedance................................................................................................................................................Impédance (Acoustique)

Impedância acústica / Impedancia (acústica) / Schallkennimpedanz / 声阻抗 / Акустический импеданс / Impedenza acustica /

Product of the velocity* of seismic waves by the density of the medium where they propagate. A seismic reflection is induced by a variation of the acoustic impedance. Sound waves** travel in all directions. Only waves traveling towards the centre of the Earth can be reflected by the structures (interfaces) underlying the explosions, which geophysicists produce on the surface.

See: « Reflection Coefficient »
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« Seismic Reflection »
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« Reflection Seismic »

Snell's law expresses the relation between the incidence and refraction angles for a wave that reaches an interface between two media with different impedances. Snell's law can be derived from Fermat's principle (the path taken between two points by a ray of light is the path that can be traversed in the least time***). Snell's law is used to determine the direction of light rays through refractive media with varying indices of refraction. The indices of refraction of the media, labelled n₁, n₂ and so on, are used to represent the factor by which a light ray's speed decreases when traveling through a refractive medium, such as glass or water, as opposed to its velocity in a vacuum. Snell's law has a limiting condition: the wave must be continuous through the interface, i.e., the phase of the incident wave must be constant in all plane. Snell's law is formulated as: n₁sinθ₁ = n₂sinθ₂, where θ₁ and θ₂ are the angles, measured relative to the perpendicular of the interface, of the incident wave and refraction, respectively. This law controls all reflections within the critical angle, from which refraction occurs. A seismic reflection is a function of the reflection coefficient (R = {(v₂.d₂) (v₁.d₁)} / {v₂.d₂) (v₁.d₁)}, which corresponds to the relation between the reflected and incident wave amplitude (reflectivity). The relation between the reflected and incident is the square of the reflection coefficient. The greater the difference between the impedance of the intervals that define the interface, the greater is the the reflection amplitude. When the impedance of the upper interval and that of the lower interval are the same, there is no reflection associated with the sedimentary interface. To have a seismic reflection it is necessary to have a contrast of acoustic impedance. However, several times, we have observed seismic reflections (discontinuous, i.e., with a polarity that varies laterally) between two adjacent intervals with the same acoustic impedance, when all electrical logs of an exploration well crossing the interface (spontaneous potential, gamma ray, sonic, neutron, density) are similar. An exhaustive interpretation of all electrical logs of the wells, particularly the dipmeter, suggested the presence of an tectonically enhanced unconformity (angular unconformity) at the level of the interface. This probably means that a significant change in the structural behaviour of the seismic intervals defining the interface can create a seismic refection even if there is no acoustical impedance associated with.

(*) The velocity of a seismic wave is a scalar quantity. Its speed is the magnitude of its velocity, without taking into account de direction. In other words, in physics what is called the speed of an object (of a particle, for instance) is the rapidity with which the object moves without regard to direction.

(**) Sound is a vibration  that propagates as an audible  wave of pressure, through a transmission medium such as a gas, liquid or solid. In the absence of matter, the vibration can not spread from one place to another. The sound can not be broadcast in a vacuum.

(***) In 1650, Fermat discovered a way to explain reflection and refraction as the consequence of one single principle.  It is called the principle of least time or Fermat's principle: All possible paths the light might take  bouncing off a mirror or pass through a piece of glass, on its way from point A to B, light takes the path which requires the shortest time or, in a more accurate statement, any hypothetical small change in the actual path of a light ray would only result in a second order change in the optical path length. The first order change in the optical path length would be zero (http://electron6.phys.utk.edu/optics421/modules /m1/Fermat's%20principle.htm).

Actualism................................................................................................................................................................................................................................Actualisme

Actualismo / Actualismo / Aktualismus / Actualism / Актуализм (актуалистический метод) / Attualismo /

Hypothesis or conjecture according which the geological processes that existed in the Past are still working in Present. This hypothesis, also known as Uniformitarianism, is often summarized by the adage: "The Present is the key to the Past". Actualism, which is incompatible with certain religious beliefs, in particular, with Creationism, is opposed to Catastrophism. Synonym with Uniformitarianism.

See : « Catastrophism (Principle) »
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« Geological Principle »
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« Uniformitarianism Principle »

The classic concept of a young Earth, advanced and defended by certain religious communities, for whom the Earth is old only about 6,000 years, since God created Adam in paradise 4004 years before Christ, was, totally, refuted by a series of scientific methods, that is to say, by testable methods using the Karl Popper falsification criterion (1934), which is not a criterion of truth, but of scientificity*. The discovery of Geological Time or Deep Timer (as expressed by McPheee, 1980) and the Earth's age (more or less 4.5 Ga) provoked an immense restriction on the importance of man and allowed geoscientists to conceive the Earth's history on a, totally, different way. With the immensity of geological time, processes and mechanisms such as erosion (abrasion of rocks and soils by wind, water and ice), sedimentation (accumulation of rocks detritus), sediment shortening (more or less sedimentary compression), etc. could, perfectly, shaped the Earth and explain its present landforms. This hypothesis defended and summarized by C. Lyell (Scottish geologist, 1797–1875) who popularised the revolutionary work of James Hutton) by the celebrated slogan "The Present is the key to the Past", which contains the basic idea of Actualism, i.e., the hypothesis that the Earth was gradually shaped by processes and forces that still act today. This theory was first, clearly, expressed in 1749 by the count de Buffon** (Georges-Louis Leclerc De Buffon,1707-1788), who proposed an age for the Earth  of about 75, 000 years, and was developed by James Hutton. Actualism is illustrated in this figure. The meanders, oxbow, oxbow lakes, point bars, etc., which are now observed in most alluvial plains are described by models*** that can be, satisfactorily, used to describe ancient geological objects such as those illustrated in photography at the lower left corner. Actualism contrasts with Catastrophism that say Earth was shaped by unique and catastrophic events, such as the Diluvium or a collision with an asteroid.

(*)  Scientificity for Karl Popper is something that can not be true, that has to be ambiguous and that implies, always, a refutation. Thomas Samuel Kuhn considers scientificity as the best possible approximation of truth, whereas for Boaventura Sousa Santos (http://www. pagination/?baa=7&cat=135&doc=10200&IDML=2) it is something that is put at the service of humanity and can help to make sense of each other's lives. For certain geoscientists scientificity is the description of reality, while for others it may be the fantastic theories resulting from observation and correlation with certain facts. Scientificity is the modality with which science proceeds to cognize reality. It must be objective, reliable, verifiable and sharable. It consists , on the one hand, in acquiring experiences by experimental observation, and on the other, in formulating hypotheses and theories whose efficacy is tested by experimenting. (https://wigging-global.com/dictionary/ scientificity/48719)

(**) Georges-Louis Leclerc (1707-1788), better known as the Earl of Buffon, was a precursor not only to Lamarck and Darwin, who considered him to be one of the first to study, scientifically, the origin of the species, but also a Lyell precursor. Buffon refused. always, to use the Diluvium ("Deluge"), which he considered a simple catastrophe to explain the Earth's landforms, which for him can only be explained by currently observable causes. Such an attitude means that Buffon was an adept of the actualism as it is now said (https://halshs.archives-ouvertes.fr/hal-00945668/document). In the same way, he said in the study of natural history there are two very dangerous obstacles: (i) Absence of a method and (ii) Desire to refer everything to a particular system (https: // www mundodasmensagens. com / sentence / Tm8gZXN0 ¨/). Buffon can, also, be considered as a precursor of certain ideas of K. Popper ("Theory precedes Observation").

(***) The basic idea of a model, whether geological or climatic, is always the same: in the absence of a complete understanding of a phenomenon or object, an approximate satisfactory description is attempted.

Adiabatic (Process)..........................................................................................................................................................................Adiabatique (Processus)

Adiabático / Adiabático / Adiabatische / 絕熱的 / Адиабатический / Adiabatico /

Process in which no heat is gained or lost by a system. When a gas or rock is compressed under adiabatic conditions, its pressure and temperature increase without gain or loss of heat. The adiabatic cooling of air, as it rises in the atmosphere, is, probably, one of the major causes of cloud formation.

See: « Cloud »
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« Thermodynamic Laws »
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« Thermal Flux »

In thermodynamics* (branch of physics in which we study the interaction between heat ** and other manifestations of energy), an adiabatic process is an isocaloric process in which no heat is transferred or leaves of the fluid in question. This means that adiabatic temperature changes occur, for instance, when the pressure of a gas varies without any heat being added or retired. In the same way, adiabatic heating occurs when the pressure of a gas is increased due to the work of a piston. Diesel engines are based on adiabatic heating during compression, which raises the temperature, sufficiently, for the fuel to ignite. Adiabatic heating is known in the Earth's atmosphere when a mass of air descends downhill along the slope of a mountain, whether it is a katabatic wind (wind that transports denser air from a high topographic point to a lower point due to the gravity) or a foehn (a type dry alpine wind blowing in the leeward of the mountainous system of the Alps, that is, along the slope opposite the side where the wind comes). A foehn wind is the residue of a rain wind, which is formed by adiabatic warming of the air, which has lost most of its moisture on the windward slopes (slopes on the side where the wind blows). An adiabatic heating *** occurs whenever the mass pressure of air or any other substance decreases due to the work that it exerts. As can be deduced from this figure, an adiabatic cooling occurs in the Earth's atmosphere when a current of air moves from the bottom up, either by orographic lift (displacement of the air that encounters an obstacle of the landform that forces it to rise) or by mountain winds which can cause clouds to form if the air cools below the dew point****. For a given temperature, the air can contain a maximum of steam. This maximum amount increases if the air temperature increases. At sea level, if the air temperature is 0° C, the air may contain a maximum of 4 grams of steam per kilogram of dry air, whereas if the temperature is 30° C, it may contain up to 27 g/kg without condensation. This means that an adiabatic fall of the air temperature causes the condensation of excess water vapour, which is often the case with night cooling, particularly in starry nights (cloudless) when the loss of energy by emission of infrared radiation is more important. In B-type subduction zones (Benioff), when in the overriding lithospheric plate, the magma rises to the surface. It suffers an adiabatic cooling before the eruption, since its temperature decreases due to the change of pressure of the system without any exchange with the outside of the system.

(*) Thermodynamics is a phenomenological theory (set of concepts, abstractions of observable phenomena and quantifiable properties, together with scientific laws that express the relations between observations and said concepts) constructed from deductive reasoning, which study real systems without modelling and which follows an experimental method.

(**) Do not confuse heat with temperature. Heat designates the exchange of energy between bodies. Temperature characterizes the shaking of molecules of a body. Heat or heat energy is characterized by the transfer of thermal energy flowing from a warmer body (with higher temperature) to a colder body (with a lower temperature). Temperature is a physical quantity that characterizes the kinetic energy (movement or agitation) of the molecules and the thermal state of a body (hot or cold).

(***) If we consider an air particle that moves vertically, as it is transported at an ever-increasing altitude, its pressure decreases and its volume automatically increases. Such a volume increase implies a work (air particle considered has to move the outside air to dilate), which implies a decrease in temperature, which for a dry air (without water vapour) is about 1° C/100 m is called the adiabatic temperature gradient. If instead we imagine a particle of descending air, the pressure will increase, its volume will decrease and its temperature will increase to the opposite gradient.

(****) Temperature at which air must be cooled to become saturated with water vapor. When further cooled, the airborne water vapor will condense  to form liquid water (dew). When air cools to its dew point through contact with a surface that is colder than the air, water will condense on the surface. (https://en.wikipedia.org/ wiki/Dew_point)

Aeolianite.......................................................................................................................................................................................................................................Éolianite

Eolianito / Eolianita / Aeolianite / Eolianite （岩沉积风沙过程） / Эолианит / Aeolianite (roccia depositati dagli processi eolici) /

General term used for all products of wind deposition. Any rock formed by lithification of sandy sediments, deposited by wind. Some geoscientists use, also, this term to describe coastal limestones formed from biogenic carbonate sediments that form coastal dunes by wind and then form aeolianites by lithification.

See: « Diagenesis »
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« Coastal non-Marine Deposit »
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« Karstification »

Theoretically an aeolianite is any rock formed by lithification of sediments deposited by wind processes, i.e., by wind. Many geoscientists use the term aeolianite for the most common form, which is a limestone composed of shallow-water carbonate sediments of marine biogenic origin, formed in coastal dunes by the wind, and later lithificated. Any sedimentary particle carried by the wind, such as silt, clay or volcanic ash, may be included in an aeolianite. Conventionally, the term aeolianite is restricted to dune sand, although thinner sedimentary particles may be present. Sand of the dunes is, partially, cemented by internal carbonate precipitation by percolation of the groundwater. The carbonate ratios vary, but are, typically, greater than 50% and often more than 90%. The carbonate comes from the remains of shells, corals, bryozoans*, etc., that live on the sea-floor. Although aeolianites, which some geoscientists, also, call rocky dunes, occur in almost every part of the world, they are much more frequent between latitudes 20° and 40°, both in Northern and Southern Hemispheres. They are not too frequent in equatorial zones and, practically, non-existent near the poles. There is no apparent difference in distribution between the hemispheres. If the extent and thickness of the deposits are taken into account, it is in the southern hemisphere that most of the aeolianites are located. The favourable conditions for the formation of aeolianites are: (i) A warm climate, conducive to the production of carbonate by marine shallow-water animals, such as the production of shellfish from marine molluscs ; (ii) Continental winds to transform beach sediments into dunes ; (iii) A relatively low onshore topography, i.e., an absence of cliffs, to allow the formation of dune systems ; (iv) Poor rainfall to allow rapid lithification and (v) Tectonic stability. The world's largest aeolianite deposits are located on the South and West coasts of Australia. On the west coast, there are more than 800 kilometers of aeolianite cliffs, which in some places may be more than 150 meters thick. These cliffs are, locally, known as the Limestone Formation of Tamala. They correspond to an alternation of dunes and shallow-water sediments. In the late Pleistocene aeolianites, occurring in the SW Alentejo, in Portugal (http://www.lneg.pt/iedt/units/16/pages/26/30/95), in which footprints and tracks of mammals and birds were found. Carbonated aeolianites, also of Pleistocene age, occur along the Atlantic coast of Portugal (Malhão and Pessegueiro Beach, near Vila Nova de Milfontes) as well as on the Algarve coast between Sagres and Armação de Pêra. The aeolianite outcrops of the Fonte de Areia on the island of Porto Santo are, particularly, well known (https://www. geocaching.com/geocache/GC4TZA6_fonte-de-areia?Guid=0623342e-c741-4cc1-b435-83ccd6e36173): "Fonte da Areia is located north of the island and was formed after the Tertiary Era subsequently the formation of a coral barrier around the island. With the absolute (eustatic) sea level fall during the Quaternary glaciations, the winds took care of wearing the corals, transporting to the island the sands with which they built the sand dunes of the Fonte de Areia and the nice beach that occupies almost all the southern littoral. These sedimentary formations at Fonte da Areia are represented by calcoarenitic aeolianites, silto-argillaceous paleosols, beach deposits, limestone crusts, fluvial and slope deposits. The calcoarenitic aeolianites deposits of the Aeolianitic Formation, can reach thicknesses of the order of 40 to 50 m. The calcoarenites present a whitish-yellow colour being constituted by fragments of calcareous algae and exoskeletons of marine and terrestrial organisms. The consolidation of the organogenic carbonate sands in calcoarenites was related to the action of the rain-water that, percolating by infiltration in the upper levels of the very porous sediment, is enriched in calcium bicarbonate due to the dissolution until they reach the saturation at deeper levels, precipitating the calcium carbonate that functions as cement that joins the sand particles".

(*) Small colonial invertebrates, common in the sea, but which can also occur in fresh water. Colonies of bryozoans grow from a single individual, which after a free larval phase attaches to a solid substrate, and begins its asexual reproduction by budding. In this way, every colony of bryozoans is formed by clones of the first animal, which is called ancestrula. Individuals within the colony are called zooids, each zooid lives within a capsule secreted by the organism (zooecium), that can be of different forms, and interconnected of different manners, depending on its morphology. (http://www.planetainvertebrados.com.br/index.asp?pagina= especies_ ver&id_categoria)

Age (Geological)..................................................................................................................................................................................................................................................Âge

Idade / Edad / Alter / 年龄 / Возраст / Età /

Geological time division lower than an Epoch. Valanginian is an Epoch of Neocomian. It belongs to Cretaceous Period, which belongs to Mesozoic Era, that is part of Phanerozoic Eon.

See: « Geological Time »
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« Eon »
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« Geological Time Scale »

Geological time is divided into Eons, Eras, Periods, Epochs and Ages. On this scale, due to lack of space, only a few Epochs and Ages are represented. In the last column, the time from the beginning of each Period to date is given in percentage. The geological time from the beginning of Ordovician to Today represents about 13% of the total geological time (Archean, 100%). This column underlines, perfectly, what geoscientists know of Earth's history represents just the "tip of the iceberg". Precambrian represents 80% of the history of our planet. As illustrated, the stratigraphic equivalents, i.e., the rocks of the temporal divisions are, respectively: Enothems, Erathems, Systems, Series and Stages. Therefore, geoscientists should not say, for instance: "the rocks of this sedimentary interval were deposited during the Upper Cretaceous", but rather "the rocks of this sedimentary interval were deposited during the Late Cretaceous". The Upper Cretaceous is a geological system (group of rocks) and not a geological period (time interval of Earth's history). By the same token, one should not confuse Ages and Stages. The Early Valanginian* (geological age) is the geological period during which the rocks of the Lower Valanginian (geological stage) were deposited. According to the chronostratigraphic chart of the Mesozoic and Cenozoic (Hardenbol J. et al., 1998), in the Cenozoic, there are 20 ages, whereas in Mesozoic there are 30 and in Cretaceous, only, 12. In Cretaceous, from the most recent to the oldest, the ages were denominated: (i) Maastrichtian (65,0 - 71,3 Ma) ; (ii) Campanian (71.3 - 83.5 Ma) ; (iii) Santonian (83.5 - 85.8 Ma) ; (iv) Coniacian (85.8 - 89.0 Ma) ; (v) Turonian (89.0 - 93.5 Ma) ; (vi) Cenomanian (93.5 - 98.9 Ma) ; (vii) Albian (98.9 - 112.2 Ma) ; (viii) Aptian (112.2 - 121.0 Ma) ; (ix) Barremian (121.0-127.0 Ma) ; (x) Hauterivian (127.0 - 132.0 Ma) ; (xi) Valanginian (132.0 -137.0 Ma) and (xii) Berrisian (137.0 - 144 Ma). The time limiting these different ages are referred to the magnetostratigraphy (Cande and Kent, 1992, 1995; Gradstein et al., 1994).

(*) Quite often the geological  time or rock units are name after the typical areas. Valanginian, which base is defined by the first appearance of Calpionellid species "Calpionellites darderi" and the top by the first appearance of the ammonite genus Acanthodiscus, was named and defined, in 1855, by Édoard Desor, in the Cretaceous outcrops of the Valangin (small village north of Neuchâtel) in the Jura Mountains of Switzerland. 

Aggradation (Alluviation) ...........................................................................................................................................................................Aggradation

Acumulação / Agradación / Ablagerung / 沉積 / Аградация (намыв) / Aggradazione

Abandonment and deposition of materials transported by geodynamic agents. Elevation of land surface due to deposition of sedimentary particles, which became sediments. Sedimentary accumulation occurs in areas where the terrigeneous influx is greater than the amount of material the system is capable of carrying. The sedimentary accumulation is made by onlapping (preponderant vertical accumulation) or by downlapping (preponderant lateral accumulation). When in a lateral accumulation, progradations are oblique, practically, there is no upbuilding (vertical construction). This is not the case when progradations are sigmoid (S-shaped upside down). Synonym of Alluvium Accumulation.

See: « Deposition (clastics) »
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« Fluvial Deposition »
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« Relative Sea Level Rise »

The concept of aggradation is illustrated in this figure, either by the photograph, which represents a river during the dry season or by the geological schemes of a aggradation simulation in a glacial environment. Theoretically, to have sediment aggradation, i.e., to have deposition, it is necessary, generally, to create or increase the available space for sediments (marine or subaerial accommodation). In the case, illustrated in the photograph, the accumulation of different sedimentary intervals (three are, more or less, evident) was during flooding periods of a water-course (river, stream, brook, etc.). The water level of the stream has risen, the available space has increased, which allowed the deposition of fluvial mud. Other types of fluvial accumulation are possible either in the floodplain, as illustrated in the sketch (stages 1 and 2), either in the bed or in the meanders of the streams. In the first case, a flood period is required for the water, which is laden with sediments, to overflow the bed and the natural levees (natural marginal dikes) to deposit the sedimentary particles in the floodplain. Deposition on the bed of a stream implies, for a given current carrying capacity, that the size of the particles carried by the stream is sufficient to prevent all transport by entrainment or saltation (transport of particles that jump from point to point, uplifted and driven by wind or water in swirling currents or with intermediate energy flow variation between the one that allows suspension and the one that promotes dragging). The accumulation in the meander is due to a simple loss of the transport competence of the current in the concave part of the meander, where the flow velocity is smaller. However, in turbidite systems and in particular in deep turbiditic systems, this is not true. In a sequence-cycle, there is sufficient available space for sedimentation seaward of the continental edge (water depth > 200 m). In marine depositional systems and, particularly, upstream of the continental edge (it can corresponds or not to the basin edge), which may correspond, more or less, to the shoreline, aggradation requires, always, the creation or increasing of available space for sediments (accommodation). Such a creation or increasing is due to a relative rise in sea level rise, that is to say, by a marine ingression (eustatic paracycle), in response to the combined action of eustasy* (absolute or eustatic sea level) and tectonics (subsidence or uplift of the sea-floor). The available space thus created may be the result of: (i) An absolute (eustatic) sea level rise ; (ii) A sinking of the sea-floor or (iii) A combination of (i) and (ii). The available space created, landward of the basin edge, i.e., on the shelf (marine or shelfal accommodation) or on the coastal plain (subaerial accommodation), depending on the terrigeneous influx, may be fully filled or not. In nonmarine depositional systems, the principle of creating available space is, more or less, the same, but the mechanisms are different. In a marine aggradation, a e relative sea level rise (marine ingression) is required to create or increase the available space for the sediments. Later the available space is, partially or totally, filled during the stability period of the relative sea level that takes place after a marine ingression, and which precedes a new the relative sea level rise. A marine ingression moves the shoreline to the mainland. However, during the equilibrium or stillstand phase of relative sea level, sedimentation resumes by  a gradual seaward shifting the shoreline until a new marine ingression occurs. Put another way, after each marine ingression, a sedimentary regression develops until a new ingression moves the shoreline back to the mainland. A simulation of aggradation in a glacial environment, in which dry and wet periods alternate, is illustrated in the left part of this figure. Water from glacial ice creates alluvial fans, delta and lakes. During the first dry period, erosion creates sedimentary detritus** that are transported, downstream, to an area that was formerly rich in water. During the wet season, water-courses transport the sedimentary particles downstream, building deltas (particularly, Gilbert deltas) at the bottom of a lake. In the subsequent dry period (lower simulation), the water disappears. The alluvial fan (Gilbert-type delta) and the bottom of the lake are higher due to the aggradation induced by deposition of the sedimentary particles transported during the previous humid period.

(*) The term eustatism is used to express the variations of the absolute or eustatic sea level and not for the curve of the relative sea level changes. Eustasy translates, at planetary level (?), a change in the sea level referred to the Earth' s centre, caused, mainly, by: (i) Changes in the volume of water in the global ocean (glacio-eustasy) ; (ii) Changes in the global volume of the ocean basins (tectono-eustasy) ; (iii) Distribution of ocean water caused by variations in the earth's gravity field (geoidal-eustasy) and (iv) Increase in the temperature of the oceans (steric sea level rise or thermal expansion of the oceans).

(**) Sedimentary detritus or particles transported by watercourses become sediments when deposited.

Aggradation (Building up of sediments).........................................................................................................................................Aggradation

Agradação / Agradación / Ablagerung / 沉積 / Аградация (намыв) / Aggradazione /

General term that expresses the sedimentation at the Earth's surface. Synonym with Accumulation and, sometimes, with Alluviation.

See: « Onlap »
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« Progradation »
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« Submarine Fan »

In Sequence (or sequential) Stratigraphy, aggradation describes, mainly, the stacking of sedimentary intervals deposited during stability periods of of the relative sea level following a marine ingression. The relative sea level rise can be in acceleration (transgressive interval, TI and lowstand prograding wedge, LPW) or in deceleration (highstand prograding wedge, HPW). The aggradation of deep-water deposits (submarine basin floor fans, SBFF, and submarine slope fans, SSF) is, often, associated with relative sea level falls. Seaward of the continental edge, the available space for sediments (accommodation) is, largely, sufficient for deposition without any relative sea level rise. The North Sea, from where the seismic line of the Canvas auto-trace, illustrated in this figure comes, corresponds to what can be called an aborted divergent margin. The lengthening zone of the substratum (characterized by a differential subsidence), along which rift-type basins developed, moved westward before the break-up of the lithosphere of the small supercontinent Eurasia. This means in the North Sea, there was no sea-floor spreading (oceanic expansion). Above the rift-type basins, since Late Jurassic to Present, a cratonic basin formed, characterized by a thermal subsidence, whose eastern margin was deformed by glacio-isostasy (regional uplift of the continental crust, in response to the melting of large layers of ice that once sank it). On this tentative geological interpretation of a Canvas auto-trace of a North Sea seismic line, the sediments of  the Cenozoic cratonic basin (regional thermal subsidence), fossilize the A unconformity that correlates, laterally (in the Atlantic divergent margin, i.e., in the eastern Rockwall Bank), with the break-up of the Eurasian lithosphere (small supercontinent of the Pangea supercontinent). This unconformity, misnamed, often, BUU (acronym of break-up nonconformity) limits the top of a buried hill (paleo-high) of a Mesozoic rift-type basin (another unconformity is, also, well visible within the cratonic basin). The A unconformity (erosional surface) was induced by the combined action of eustasy (changes of absolute or eustatic sea level) and tectonics (subsidence or uplift of the sea-floor), that is to say, by a relative sea level fall*. This unconformity separates the rift-type basins from the cratonic basin. Although this A unconformity was, mainly, induced by a relative sea level fall, locally, it was tectonically enhanced and has become what many geoscientists (especially, the structuralists) call an angular unconformity. The geoscientists adepts of the sequential stratigraphy, in order to point out that this type of unconformity is not induced by tectonics, call it "enhanced unconformity", as it is induced, fundamentally, by a relative sea level fall. The tectonics only reinforce, locally, the geometric relationships between the reflectors that it separates. Laterally, it (enhanced unconformity) changes to a cryptic unconformity (difficult to recognize, which is the main characteristic of the unconformities in the sequential stratigraphy). In the field and on the seismic lines, as shown above, an unconformity is underscored by a seismic surface defined by onlaps (green lightning arrows) of overlain sediments (cratonic basin) and by toplaps of the underlying sediments (rift-type basin). Probably, the onlaps are marine. The sedimentary environment, at the base of the cratonic basin, is a deep-water environment. However, the depositional water-depth decreases as and when aggradation occurs. In the rift-type basin, reflector terminations (lapouts), which correspond, more or less, to the sedimentary interfaces (chronostratigraphic lines) ending against the unconformity, are toplaps (by truncation). In general, on the seismic lines, there is any reflector that can be followed in continuity in association with an unconformity, particularly, with a tectonically enhanced unconformity (angular unconformity), since the acoustical impedance profile changes laterally. The geoscientist on this tentative geological interpretation, underlined in continuity the theoretical seismic surface defined by the reflector terminations that characterize the erosional surface separating the rift-type basin from the cratonic basin. Within the cratonic basin, the aggradation can be calculated (in time) by the onlap terminations. It is here, more or less, 2 seconds (t.w.t., that is to say, double time), which in depth corresponds, approximately, to 2,600 meters (for an interval velocity of about 2,600 m/s). It is for this reason that these onlaps are considered to be marine onlaps.

(*) The sea level can be (i) Absolute (eustatic) when referenced to the Earth's centre or (ii) Relative (local) when referenced to any point on the Earth's surface, which may be the top of the continental crust (base of the sediments) or the sea-floor. A relative sea level rise can not be confused with an absolute sea level rise, once the relative sea level is the result of the combined action of absolute (eustatic) sea level with tectonics (uplift or subsidence).

Aggradational Offlap...............................................................................................................................Progradation aggradante

Progradação Agradante / Progradación agradante / Progradation aggradante (mit Verlandungszonen) / 加积前积 / Намывное несогласное регрессивное налегание / Progradazione aggradante /

When the sedimentary aggradation (upbuilding) is less significant than the seaward displacement (outbuilding) of the depositional coastal break (roughly the shoreline). The basin edge is, more or less, coincident with the depositional coastal break, when the basin has no shelf (lowstand geological conditions).

See: « Depositional Dip »
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« Progradation »
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« Offlap-Break »

In the upper stratigraphic intervals of the post-Pangea divergent continental margins, regressive episodes are predominant, as illustrated on this tentative geological interpretation of a Canvas auto-trace of a detail of an Australia offshore seismic line. Regressive episodes, generally, exhibit a progradational geometry, with downlaps oriented along the de direction of the sedimentary supply (terrigeneous influx) . Overall, on this tentative interpretation, the coastal deposits were displaced seaward and upward. Progradational or regressive intervals are associated with an absolute or eustatic sea level* fall. The absolute sea level is the supposed global sea level referenced to Earth's centre. It should not be confused with the the relative level**. The absolute sea level fall began in the Cenomanian/Turonian, i.e., at the major downlap surface 91.5 Ma. Assuming the water volume (in all forms: solid, liquid and gaseous) is constant since the Earth's formation (more or less 4.5 Ga), one can say that since Cenomanian/Turonian, the water volume of the post-Pangea ocean basins began to increase. Such a increasing is due to the volume decrease of the ocean ridges and to the subduction of the dense oceanic crust along the subduction zones of Benioff-Wadati***, which produced, necessarily, a continuous absolute sea level fall. Individually, each progradational interval, particularly, within a sequence-cycles (stratigraphic cycle induced by 3rd order eustatic cycle, whose time-duration varies between 0.5 My and 3-5-My), which form continental encroachment sub-cycles (limited between unconformities whose age difference varies between 3-5 My and 50 My) is associated with a relative sea level rise in deceleration. Such a sea level rise is, probably, associated with changes in the tectonic subsidence (sediment thickness, plus, water-depth, minus, effect of isostatic compensation, due to sedimentary overload and increased compaction effect). During the transgressive intervals, the relative sea level rise is in acceleration, The marine ingressions are, increasingly, important and sedimentary regressions, increasingly, smaller. In this tentative interpretation, it is easy to see each increment of relative sea level rise, which displaces the shoreline continentward, is followed by a stability period of the relative sea level, during which deposition occurs. This implies a displacement of the shoreline, which emphasizes the basin edge, which coincides with the continental edge, since the basin (at the level of the sequence-cycle) has no shelf. During most regressive intervals, the basin has no platform. The basin edge moves seaward (progradation, outbuilding), but also upwards (aggradation, upbuilding). The aggradation of the basin edge (which in this case coincides, more or less, with the shoreline) never reaches the amplitude of the horizontal displacement seaward. If aggradation is higher than the seismic resolution, the aggradational progradations are well visible and the aggradation is easily calculated. These progradations exhibit a typical sigmoid geometry. The more progradations are aggradational the more the geometry of the chronostratigraphic lines looks like the geometry of a stretched and inverted S. This type of progradations contrasts with oblique progradations, in which aggradation is, practically, zero or lower than the seismic resolution.

(*) The absolute or eustatic sea level is a function of: (i) Tectono-Eustasy, that is controlled by the volume variation of the ocean basins in association with oceanic expansion (sea floor spreading) following the break up of the supercontinents ; (ii) Glacio-Eustasy, which is controlled by the water volume of water in the oceans as a function of the amount of ice (assuming that the amount of water in all its forms is constant since the formation of the Earth around 4.5 Ga) ; (iii) Geoidal-Eustasy, which is controlled by the distribution of ocean water caused by variations in Earth's gravity field (where gravity is stronger than normal, sea level is thrown to the Earth's centre) and (iv) Stereic sea level rise or thermal expansion of the oceans, which is controlled by rising ocean temperatures (if the temperature increases, the density of the water decreases and, for a constant mass, the volume increases).

(**) The relative sea level is the local sea level referenced to any fixed point on the Earth's surface, which can be the base of the sediments (top of the continental crust) or the sea-floor, and is the result of the combined action of the absolute (eustatic) sea level and tectonics (subsidence of the sea-floor when extensional regimes are predominant or uplift, when compressional tectonic regimes are predominant).

(***) In honour of the seismologists Hugo Benioff and Kiyoo Wadati.

Albedo.............................................................................................................................................................................................................................................................Albédo

Albédo / Albedo / Albedo / 反照率 /Альбедо (коэффициент отражательной способности) / Albedo /

Ratio between the amount of electromagnetic energy reflected by a surface and the amount of incident energy. In the particular case of solar energy received by the Earth, the albedo is the ratio of reflected solar energy to that received by the Earth's surface. The albedo ranges from 0 to 1. It is zero for a very black earth surface and 1 for an ideal mirror-type earth surface.

See: « Milankovitch Cycle »
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« Equinoxial Precession »
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« Feedback »

This map represents the albedo measurements, i.e., the ratio of the solar energy reflected and received by the Earth, made from MODIS*, which was placed on board the NASA satellite "Terra and Aqua". Ocean areas have been excluded and there are not enough data in the blank areas (Antarctica and North Greenland). The reddish tones correspond to albedo values between 0.0 and 0.4. Smaller is the albedo lighter are the red tones. Areas covered by snow and desert areas have a much more important albedo than forest regions. They reflect much more the energy received from the Sun. Areas with strong albedo exert a positive feedback**, which should be taken into account in the problems of so-called "global warming". The more ice (or snow) on the Earth's surface, the more solar energy is reflected into the atmosphere, which causes a cooling, which in turn induces more snowfall, which increases the snow-covered areas ***, which increases the albedo, which, in turn, decreases the temperature and so on. This is what happens during the glacial periods. In contrast, during inter-glacial periods, a decrease in the land surface covered by snow and ice causes an increase in temperature which, in turn, decreases snowfall, which decreases the albedo thus causing an increase in temperature and so on. Therefore, when one of the factors controlling the insolation (see Astronomical theory of the Paleoclimates) or a displacement of the continents toward the poles (Plate tectonics), produces a global cooling, the positive feedback mechanism begins to work. The Earth’s surface reflects on average about 30% of the light received from the Sun, but that this value varies from about 8% (cultivated areas) to 90% (fresh snow) and that certain clouds (such as the cumulus) can reflect 80% of the light before it reaches Earth. Clouds are made of small droplets and crystals that develop in altitude when ascending air currents, brought about by convergence and convection, associated with the frontal or orographic (induced by the presence of mountains) forcings.

(*) MODIS is the acronym for "Moderate Solutions Engine Macromedia"), which is an instrument made up of a series of scientific observation instruments that was launched into Earth's orbit by Flash in 1999, aboard Terra satellite (EOS AM), and in 2002 onboard the AIR satellite. MODIS, which weighs about 274 kg. It captures data in 36 spectral bands in wave lengths from 0.4 μm to 14.4 μm, mapping the entire Earth within one to two days. With its low spatial resolution but high temporal resolution, MODIS data is useful for tracking landscape changes over time. Examples of such applications include: (i) Vegetation monitoring ; (ii) Long-term land cover variations (e.g., rates of deforestation) ; (iii) Snow cover patterns ; (iv) Flooding ; (v) Marine incidents ; (vi) Variation of the water level of the main lakes ; (viii) Detection and mapping of forest fires, etc.

(**) One of the retroaction or feedback mechanisms by which the effects or outputs of a system cause cumulative effects on input. In contrast, in a negative feedback the output effects or outputs cause effects on the input.

(***) Snow formation (J-P Vigneau, 2005) corresponds to the particular situation in which the temperature of the different air layers is negative at all levels and when the positive temperature layers above the substrate are too thin to ensure melting. More the air is cold, plus snowflakes are frequent: (i) on vane for very low temperatures ; (ii) pulverized for temperatures above -10° C. Snow with floats loaded with water, called heavy snow (density 0,5), is explained by a blockage of the melting due to the presence of a very low layer at a temperature close to zero .

(****) Forcings refers to disturbances or disarrangements in the Earth's energy balance, which cause changes in temperature. These forcing are fluctuating and thus decisive in the assessment and prediction of global warming (https://www.futura-sciences.com/planete/dossiers/ climatologie-rechauffement-climatique-question-forcages-1117/page/2/).

Albic (Horizon).....................................................................................................................................................................................................................................Albique

Álbico / Álbico / Albic / 白浆 / Альбский (горизонт) / Albico /

Horizon of a soil with a light colour, in which the clay and free iron oxides were removed, or in which the oxides were separated. The light colour of the horizon is determined by the colour of the sand and silt particles that form it and not by the colour of the coatings of these particles. In general, the limits of an albic horizon, which is little structured, are very well marked.

See: « Soil »
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« Solifluction »
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« Permafrost »

An albic horizon of a soil (mixture of mineral fragments, organic matter, air and water that forms the Earth's surface in many regions and supports the growth of plants) is a, more or less, white horizon with little or no clay and iron oxides in the sand particles. This horizon, also called E horizon, that forms, especially in forest areas, has a light colour and is leached. It is located under O horizon or A horizon, always above B horizon. Its  identification in the field is on Munsell soil colours (colour space created by A., H., Munsel  in the beginning of 20th century and adopted by the United States Department of Agriculture (USDA) as the official colour system for soil  research in the 1930) that specifies colours  based on three properties of colour : (i) Hue (measured by degrees around horizontal circles) ; (ii) Value (lightness) measured vertically on the core cylinder from 0 (black) to 10 (white) and (iii) Chroma (colour purity) measured radially outward from the neutral (gray) vertical axis. The presence of coatings around sand and silt grains can be determined using an optical microscope for analysing thin sections. Uncoated grains usually show a very thin rim at their surface. Coatings may be of an organic nature, consist of iron oxides, or both, and are dark coloured under translucent light. Iron coatings become reddish in colour under reflected light, while organic coatings remain brownish-black The main horizons that can be distinguished in a soil are: (i) O Horizon, with an average thickness of about 5 cm ; (ii) B Horizon, whose average thickness is about 25 cm ; (iii) A Horizon, whose average thickness is about 80 cm and (iv) C Horizon, which, in general, have an average thickness of about 120 cm. O horizon has more than 20-30% organic matter, a dark colour and is often saturated with water for long periods of time. A horizon, which is immediately below O horizon, is characterized by a significant obliteration of the original rock structure, by a humic accumulation of organic matter that is intimately mixed with the mineral fraction, and by the properties resulting from agriculture and grazing. B horizon is characterized by a total obliteration of the original rocky texture and presents one or more following events: (i) Illuvial concentration* ; (ii) Removal of carbonates ; (iii) Residual concentration of sesquioxides (oxides containing three oxygen atoms with two radical atoms or another element) ; (iv) Coatings of sesquioxides ; (v) Mineral changes and (vi) Loss of resistance. C horizon, sometimes, called regolith, is little affected by the pedological processes**. This horizon has none of the properties of the other horizons. It is a mineral horizon, which may have been modified, even if, in general, there is no evidence of pedogenesis. The relationships of the albic horizon with the other diagnostic horizons can be summarized as follows (http://www.fao.org/ docrep/w8594e/w8594e06.htm #albic%20horizon): (A) Albic horizon is, normally, overlain by humus-enriched surface horizons (mollic, umbric or ochric horizons) but may be at the surface due to erosion or artificial removal of the surface layer ; (ii) It can be considered as an extreme type of eluvial horizon, and usually occur in association with illuvial horizons such as an argic (a subsurface horizon which has a distinctly higher clay content than the overlying horizon), nitric (a dense subsurface horizon with a higher clay content than the overlying horizon) or spodic (dark coloured subsurface horizon which contains illuvial amorphous substances composed of organic matter and aluminium, with or without iron) horizon, which they overlie ; (iii) In sandy materials albic horizons can reach considerable thickness, up to several metres, especially in humid tropical regions, and associated diagnostic horizons may be hard to establish.

(*) Illuviation is the process of accumulation in a horizon of the soil of elements coming from another horizon. Most of the time, the illuviation corresponds to the descent of materials from A horizon to B horizon. The illuviation can be: (i) (i) Mechanical, when percolation of rainwater reaches a horizon of drier soil, and water entrains thin materials downwardly through the capillary action of microchannels and (ii) Chemical when the soluble constituents are deposited due to differences in soil chemistry, in particular pH and redox or oxidation potential (electron transfer reactions between the reducer, which supplies the electron and the receiving oxidant) (https://es.wikipedia. org/wiki/Iluviación). The eluviation or leaching is the transport in the soil, by a lateral or ascending stream of water, of thin sediments dissolved or in suspension, when the rain exceeds the evaporation.

(**) Pedogenesis is a set of physical, chemical and biological processes responsible for the transformation of rocks into a soil and after for its evolution.

Algae.......................................................................................................................................................................................................................................................................Algue

Alga / Alga / Alge / 藻类 / Водоросли / Alga /

Organisms that can belong to different groups of living beings. Traditionally, algae were considered as simple plants and sometimes related to higher plants. At present, algae are a large and diversified paraphyletic group* from simple autotrophic organisms, ranging from unicellular to multicellular forms.

See: « Brown Algae »
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« Photosynthesis »
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« Autotrophic (organism) »

Algae form a polyphyletic set. Its members are scattered among different kinship groups. Among them we can consider: (i) Prokaryotes or Prokaryontes and (ii) Eukaryotes or Eukaryontes. In prokaryotes group only cyanobacteria are considered as algae. Traditionally, cyanobacteria are called blue-green algae, which is what literally means their ancient systematic name cyanophytes (as opposed to phaeophyceae or brown algae). The other groups of prokaryotes (which perform non-oxygenic photosynthesis forms) are not considered as algae, but rather as bacteria **. Many groups of eukaryotes, all belonging to the Protista kingdom (a diverse group of eukaryotes that includes most of the organisms that do not fit into either Animalia or Plantae kingdom) are considered algae. In most cases the eukaryotes belong to the same clade (a group of organisms originated from a single exclusive ancestor) or to the same evolutionary branch with heterotrophic forms, which have traditionally been described as protozoa or as false mushrooms. These include: a) Euglenoids or euglenophytes: unicellular forms of fresh water with green-plasts, related to kinetoplastea (group that includes both the unicellular heterotrophic forms of the same environments and the protists that produce the sleep disease) ; b) Dinoflagellates: unicellular protists, most of which have different colour plasters, derived from endosymbiosis of other unicellular algae ; c) Chromophytes: a very heterogeneous clade of protists that includes among its members some of the most important aquatic photosynthesizers; d) Haptophytes: unicellular whose carbonate scales contribute in an important way to the oceanic sedimentation ; (e) Cryptophytes: unicellular flagellate forms of cold waters, mainly marine ; (f) Glaucophytes: unicellular freshwater protists characterized by cyanelas that are plastid (cytoplasm-specific cells, double-membrane organelle) with characteristics typical of cyanobacteria ; g) Rhodophytes: red algae and g) Chlorophytes: green algae.

(*) Taxon that includes a group of descendants of a common ancestor in which several descendants are included, but not all of them.

(**) The cyanobacteria, which were the first forms of life on Earth, between 3.3 and 1.5 Ga, were developed in dome-shaped structures called stromatolites consisting of alternating levels of organic and inorganic matter. Structures of this type can be observed on several coastlines, particularly in the Bahamas and Australia. Many geoscientists consider cyanobacteria, which perform photosynthesis, were responsible not only for the reddish colouring of the earth's surface but also for the first major biotic crisis. They admit that the oxygen released by the cyanobacteria was fixed by the iron of the rocks, which probably gave a reddish coloration to the pristine terrestrial surface and that from a certain moment, the oxidation of the rocks, allowed the accumulation of oxygen not only in the atmosphere, but also in the oceans, which certainly triggered a major biotic crisis.

(***) The oldest rocks on Earth, formed 4.0 Ga, that is say, about 500 million years after the formation of the solar system already contained evidence of bacterial life. Bacteria ruled the Earth for the first two billion years of its existence. It was during this time that many of the basic biochemical reactions on which life is based developed. By far the greatest innovation has been the photosynthesis by which carbon dioxide (CO₂) and water (OH₂) are reduced in carbohydrates (C₆H₁₂O₆) using sunlight as a source of energy releasing oxygen into the atmosphere (Cesare Emiliani, 1995).

Allochem (Carbonates)..................................................................................................................................................................................................................Allochème

Aloquímico / Aloquímico / Allochem / Allochem（碳酸盐）/ Аллогенная / Allochem (carbonati) /

Term introduced by Folk, in 1959, mainly, to describe the grains that can be recognized in the carbonated rocks. The term allochem is now used, more generally, to denote one or more varieties of roughly organized large carbonate aggregates forming the granular structure of most mechanically deposited carbonates. Allochems, which contrast with interstitial material, such as calcite of a calcareous cement or matrix, include intra-clasts, oolites, fragments of fossils, etc.

See: « Diagenesis »
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« Calcite »
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« Dolomitization »

In this photograph of a thin section, the original allochems (ooids, that is to say, spherical particles less than 2 mm in diameter, similar to fish eggs, resulting from the precipitation of concentric layers of CaCO₃ around a core) are, perfectly, visible. The dolomitization of calcite is partial. In this diagenesis process, the ooids were partially replaced by euhedral dolomite. Currently, and generally, any fragment greater than 1/2 mm may be considered as an allochem. Oocysts (a sporozoan zygote undergoing sporogenous development), peloids (humus and minerals formed for many years by geological, biological, chemical and physical processes), fossils and pre-existing carbonate fragments are the most common alchems. All fragments that underwent chemical transformations are also considered allochems. Likewise, when the aragonite shells are dissolved and replaced by calcite, it can be said that they are an allochem, since they are always distinguished from the matrix in which they are found, whether this is a micrite (limestone) or (crystalline interstitial calcite). Certain geoscientists consider allochems as precipitates of a solution within the deposition basin due mainly to biological activity or that were transported as solids to the basin. Thus, the rest of the skeleton of certain organisms (whole shells or starter) or any other hard part of a plant or animal, oolites (or ooids), pellets (small balls or balls), sand grains, etc., may be considered allochem. The erosive, transport, sedimentation and biological processes associated with the formation of sedimentary rocks produce a constituent components. The main components are: (i) Terrigeneous or Clastic components: loose crystals, fragments of crystals or fragments of preexistent rocks by processes of alteration and disintegration ; its morphology and size are, directly, related to the transport suffered from the area of origin to the deposit area; (ii) Orthochem components (inside components): materials formed by direct chemical or biochemical precipitation in the sedimentation area itself, during or immediately after deposition (inside components, the microcrystalline binding materials of matrix or cement) and (iii) Allochem components (outside components): materials of chemical or biochemical origin formed in the sedimentation basin itself, but are incorporated in the sediment as clasts. These materials were able to undergo light transport within the basin, but their origin is closely related to that of the sedimentary rock where it is found. In the left part of this figure the four main types of allochems are illustrated: (a) Intraclasts ; (b) Oolites or Ooids ; (c) Fossils and (d) Pellets. Intraclasts are irregularly shaped grains that form by syndepositional erosion  of partially lithificated sediment. Oolites or Ooids are sedimentary rock made up of ooids (ooliths) that are cemented together. Fossils are preserved remains any once-living thing from a past geological age. Pellets* are small spherical to ovoid or rod-shaped grains that are common component of many limestones.

(*) Pellets differ from oolites  and intraclasts, which are also found in limestones. They differ from oolites in that pellets lack the radial or concentric structures that characterize oolites. They differ from intraclasts in that pellets lack the complex internal structure, which is typical of intraclasts. In addition, pellets, quite unlike intraclasts, are characterized by a remarkable uniformity of shape, extremely good sorting, and small size (https://en.wikipedia.org/wiki/ Pellets_(petrology).

Allochthone, Allochthigeneous (Material)........................................................................................................Allochtone

Alóctone / Alóctono / Allochthon / Allochtone / Аллохтон / Allocthono

Material that did not originate in the place where it is. This type of material is, particularly, abundant in turbidite deposits, either in submarine basin floor fans (SBFF) or in submarine slope fans (SSF). In a submarine basin floor fan, only the sediments of the pelagic layer ("E" layer in Bouma's sequence) are settle "in situ". Synonym with Allogene.

See: « Turbidite »
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« Halokinesis »
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« Compensatory Subsidence »

On this tentative geological interpretation of a Canvas auto-trace of a detail of a deep offshore seismic line from the Gulf of Mexico (USA), it is easy to distinguish the allochthonous salt from the autochthonous salt. The autochthonous salt, although laterally drained, is in the original stratigraphic position of deposition, while the allochthonous salt is not. The two saliferous horizons communicate by a vertical salt structure but, often, they are separated by a, more or less, vertical salt weld. There may be several levels of allochthony. In certain sedimentary basins with an significant salt interval, there are salt domes (salt diapirs) of second and third generation. Salt domes rooted in a first allochthonous salt level (1^st order level) are domes or diapirs of second generation. Salt domes rooted in a second-order allochthonous salt level are third-generation salt domes. On this tentative interpretation, the salt structure, that slightly deformed the seafloor, may be considered either as a 1^st order salt dome, since it is rooted in the autochthonous salt level, or as a small level of allochthonous salt. In any case, since the upper part of a 1^st order dome flows laterally, forming an important salt overhang, as is the case on this tentative interpretation, the salt is no longer in its original stratigraphic position and must be considered as an allochthonous salt layer. An unconformity, locally, tectonically enhanced (angular unconformity for certain geoscientists) divides the interval post autochthonous salt into two large stratigraphic intervals: (i) The upper interval (in yellow on this tentative interpretation) was, locally, affected by a tectonic regime created by vertical salt flow (diapirism). Such a local tectonic regime is characterized by a vertical σ₁ (main axis of the effective stress ellipsoid, that is to say, the result of the combination of the geostatic pressure σ_g, hydrostatic pressure or pore pressure σ_p, and the tectonic vector, σ_t) and σ₂ and σ₃ horizontal and equal ; (ii) The lower interval, coloured in green, on this tentative interpretation, was shortened by a compressional regime (σ₁horizontal, more or less oriented West-East, σ₂ horizontal and perpendicular to σ₁ and vertical σ₃) created in response to extension (sedimentary lengthening) occurring upstream of this seismic line, i.e., in conventional offshore (depth of water less than 200 m) and onshore. The shortening of this interval was done by cylindrical folds and reverse faults (well visible on this tentative interpretation), while the lengthening of the upper sedimentary interval was, naturally, produced by normal faults. The normal faults are not represented on this tentative interpretation, since the vertical displacement of the faulted blocks is inferior to the seismic resolution, which on this line is, more or less, 20-40 meters. Geoscientists in charge of tentative geological interpretations are obliged to take them into account, since sediments can just be lengthened by normal faults (there is no other way to lengthen sediments). Within the yellow interval, the antiform structure overlying the salt dome can only be explained by a sedimentary lengthening, which can just be done by normal faults. The uplift of the base of the salt (as well as the underlying horizons) under the vertical salt structure connecting the allochthonous to the autochthonous salt corresponds to a seismic pitfall (an unforeseen or unexpected or surprising difficulty). The velocity* of seismic waves within the salt is much faster than through sediments. This means that the seismic waves spend less time when they cross a salt interval to reach the same depth. In a well-processed depth version of the original seismic line of this auto-trace, the base of the salt interval is, more or less, subhorizontal. This empirical or contingent truth** can be used by geoscientists to test their tentative geological interpretations. If the base of the salt interval or associated tectonic disharmony is, more or less, rectilinear (subhorizontal or slightly inclined to the basin), the base of all salt diapiric structures should also obey this conjecture. Taking into account this conjecture, what is often interpreted as salt dome, roughly, cylindrical becomes a drop of salt, more or less, disconnected from the autochthonous salt level. In other words, on this tentative geological interpretation it is not impossible that the allochthonous salt is disconnected from the autochthonous salt.

(*) Here, it will be correct  to say the speed of  seismic waves, since the velocity of a seismic wave is a scalar quantity. Its speed is the magnitude of its velocity, without taking into account de direction. In physics what is called the speed of an object (seismic wave, for instance) is the rapidity with which the object moves without regard to direction.

(**) Contingent propositions depend on facts, whereas analytic proposition  are true without regard to any facts. A fact is a reality that cannot be logically disputed or rejected. If you say "rocks are hard" you don't care how great your reasoning skills are, if you take a rock you feel its weight. However, when you say this, you are not speaking a truth, you are speaking a fact. If you say "rocks are not hard," you are not lying, you are incorrect. Facts are concrete realities that no amount of reasoning will change. When you acknowledges a fact, you are doing just that. Facts are not discovered, facts are not created, facts are simply acknowledged. A truth on the other hand, is almost the opposite. Truths are those things that are not simply acknowledged, but must be discovered, or created. If you say "God exists," and you possess strong reasoning for the affirmative of that statement, then God really does exist, that is a reality. However, if another individual possesses strong reasoning for the negative, and because of this reasoning they believe that God does not exist, then that is also a reality. If we were to debate our ideologies, and you reasoning appeared stronger than theirs, they may choose to adopt your belief that God does exist. If they do, then the existence of God is just as true as the nonexistence of God which they believed a week ago. Truths, as opposed to fact, are much more fluid and malleable than their empirical counterparts. (https://philosophy.stackexchange.com/ questions/ 8053/what-is-the-difference-between-fact-and-truth)

Allocyclic (Mechanism).........................................................................................................................................................................................Allocyclique

Alocíclico / Alocíclico / Allocyclic / Allocyclic / Аллоциклический / Alociclico

Depositional process resulting from changes in the energy source or in the terrigeneous supply in a sedimentary system in which the events responsible (uplift, subsidence, climatic changes, relative sea level changes, etc.) are external to the system.

See: « Deposition (carbonates) »
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« Milankovitch Cycle »
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« Relative Sea Level Change »

An allocyclic mechanism contrasts with autocyclic mechanism, in which the responsible events belong to the depositional system, such as lateral displacement of delta depocenters (pendulum effect), deviation or avulsion of the turbidite currents, formation of meanders, deposit of point bars, etc. Since the term cycle refers to recurring events, which may or may not be periodic, the same is true of allocyclic and autocyclic mechanisms that may be periodic or not. The building blocks of sequential stratigraphy, tie., the sequence-cycles, are induced by, more or less, periodic allocyclic mechanisms. The duration of each eustatic cycle, which induces a sequence-cycle, ranges between 0.5 and 3-5 My. The photograph in the lower left corner of this figure illustrates an outcropping of the Oligocene-Miocene limestones of the Abrakurrie formation (South Australia), in which the events responsible for the deposition process are, mainly, external to the sedimentary system. This is particularly true for the sedimentation cycles associated with the wave base action which, as shown, defines on the platform a carbonate depositional zone and an erosional zone with a levelling surface (applanation surface). The term levelling surface is related to, more or less, continental portions characterized by a flat or gently wavy relief (landform), modelled by the action of subaerial erosion (wave base action) which truncates, without distinction, geological structures of different nature and strength. The wave base limit, that is to say, the depth of the erosional action of the sea-waves, which varies according to the state of the sea (calm, agitated or very agitated), is dependent on the geological context of the basin and relative sea level changes. Allocyclic processes occur, mainly, in highstand geological conditions (when within a sequence-cycle, the sea level is above the basin edge) and, particularly, during the deposit of the lower sub-group of the highstand systems tracts group (HSTG), i.e., during the transgressive interval (TI). In a sequence-cycle, when the first marine flood,i.e., when the relative sea level covers, for the first time, the coastal plain of the lowstand prograding wedge (LPW), the shoreline (more or less, equivalent on seismic lines of depositional coastal break) moves continentward. It is what geologists call a marine ingression (or marine transgression). The edge of the lowstand prograding wedge (LPW) becomes the new basin edge, since a shelf is formed. At the end of each eustatic paracycle, that is to say, at the end of each marine ingression, which during the transgressive interval (TI) are increasingly important and separated by more or less long periods of stability of the relative sea level, geological conditions of low sedimentation rate (starved basin) become predominant in the distal part of the shelf. During sedimentary regressions, progressively less important, induced by the marine ingressions, in the distal part of the shelf, condensed stratigraphic sections are deposited. They are sometimes capped by hardgrounds. In highstand geological conditions (significant water-depth), the limit of the wave base action may not reach the sea-floor. In this case the hardground, which caps in the distal part of the basin, the condensed stratigraphic section, is not eroded. On the contrary, as illustrated in this figure, in lowstand geological conditions (relative small water-depth), it can not be excluded that the wave base action will destroy, partially or totally, the hardground, as well as the condensed stratigraphic section. A succession of accelerated marine ingressions and increasingly smaller sedimentary regressions can create, in the distal part of the basin, a stacking of condensed stratigraphic sections bounded by hardground surfaces, as shown in this figure. After the first accelerating marine ingression, which creates or increases the available space for the sediments, a stability period of the relative sea level occurs, during which the sediments are deposited. The terrigeneous supply or the formation of carbonate material causes the shoreline to move slowly seaward as the sediments deposit until the next eustatic paracycle take place (without relative sea level fall between them). Thus, is formed the first sequence-paracycle of the transgressive interval (TI), i.e., the first sedimentary regression of the transgressions (set of increasingly important marine ingressions and the associated progressively smaller regressions, which globally have a retrogradational geometry*). The following eustatic paracycle, once again, shifts the shoreline landward (new marine ingression), increasing the accommodation and the spatial extent of the shelf exaggerating the starved basin conditions in the distal part of the shelf. After the new marine ingression, the shoreline progrades, again, seaward as sediments are deposited during the new stability period of relative sea level, etc., etc. In other words, after each eustatic paracycle, a sequence-paracycle is deposited.

(*) It was C. Emiliani who, in 1991, called the set of increasingly important marine ingressions and the increasingly smaller associated sedimentary regressions, which is characterized by a global retrogradational geometry, "transgressions" and not transgression as many geoscientists say. This means, that sedimentary transgressions are, in reality, a stacking of smaller and smaller sedimentary regressions. In most languages, a transgression is simply the ingression of sea-water onto the continent with no reference to sedimentary deposits. The sea can ingress the continent, but the sediments do not. Sediments, with rare exceptions prograde, always, seaward.

Allogene (River).........................................................................................................................................................................................................................Allogène

Alógeno / Alógeno / Nicht bodenständig / 同种异体 / Аллоген / Allogenico (fiume) /

A river or surface water-course, flowing on a karsified landform, fed by a source coming from nokarsified terrain.

See: « Karst »
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« Swallow Hollow »
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« Resurgence »

Many geoscientists consider the term allogene is synonymous of allochthone or allochthigeneous. All of them designate materials that did not originate in the place where they are. However, in sequential stratigraphy, by tradition or habit, the term allochthone is, more often, used to describe rocks. Thus, geoscientist say, for instance, allochthonous salt as opposed to autochthonous salt. The allochthonous salt is not in its original stratigraphic position, whereas the autochthonous salt is. The term allochthigeneous is, preferably, used for minerals and sedimentary particles. So, turbidite layers are, mainly composed, of allochthigeneous sedimentary particles. Therefore, the fossils contained therein being transported do not allow a good dating, much less a determination of the sedimentary environment where they have deposited. It is interesting to note on this subject that the reservoir-rocks of the famous Frigg gas field (North Sea) were, during the early years, considered as delta sands, until some geoscientists realize that all fossils, in which such a interpretation was based, were transported. The Frigg finger delta building was overnight transformed into a stacking of submarine basin floor fan, i.e., into a deep-water turbidite system. The term allogene is most, commonly, used to designate surface water currents that have a distant origin from the environment in which they are found. It is, often, used in karst regions to designate currents that pass through or have a swallow hollow (disappearance in a karst subsoil of a surface watercourse) in the karst zone, but whose source is located in another river basin. As illustrated in this figure (Guangxi, Zhuangzu, China) the river that crosses the karsified zone has its origin in another sedimentary basin located hundreds of kilometers northward. Therefore, it can be considered as an allogene river. Likewise, most of the underground rivers circulating in the karst conducts are allogenic currents, although many of them have a resurgence (karst spring, fed at least partially by losses of allogenic surface currents) in the karsified zone.

Allogenic (Process)..............................................................................................................................................................................Allogénique (Processus)

Alogénico / Alogénico (proceso) / Allogener (Prozess) / 自旋回（碳酸盐岩）/ Аллогенных (процесс) / Allogenico (processo) /

Process or mechanism that is not part of the sedimentary system itself, such as, in a fluvial system, the size, shape of the channel, or the formation of meanders, etc.

See: « Autocycle »

Allostratigraphic Unit...............................................................................................Unité stratigraphique discordante

Unidade estratigráfica discordante / Unidad estratigráfica discordante / Widersprüchliche stratigraphischen Einheit / 不和谐的地层单位 / Несогласная стратиграфическая единица / Unità stratigrafica discordanti /

Discordant interval limited by unconformities (type I or II). It is important not to confuse discordant stratigraphic intervals with sequence-cycles. A sequence-cycle is limited by unconformities in the upper part of the basin. In the deep parts of the basin, it is not defined by unconformities but by their correlative paraconformities.

See: « Depositional Sequence »
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« Biostratigraphic Unit »
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« Chronostratigraphic Unit »

Strata can be subdivided, depending on their physical characteristics, independent of the fossils they contain and their spatial relations, in three main types of stratigraphic units: (i) Lithostratigraphic ; (ii) Biostratigraphic and (iii) Chronostratigraphic. A lithostratigraphic unit can be formed by sedimentary rocks, igneous, metamorphic and in some cases by an alternation of these types of rocks. A lithostratigraphic unit corresponds to a three dimensions rocky body, since the concept of lithostratigraphic unit is applied vertically and laterally. A biostratigraphic unit is a set of rocks or strata unified by their fossils or paleontological characteristics, which differentiate them from adjacent rocks or strata. A biostratigraphic unit can be defined by: (a) The presence of fossils ; (b) The type of fossils it contains or just by the fossils of a certain type ; (c) A taxon, which is a taxonomic unit, associated, essentially, with a scientific classification system indicating a unit at any level ; (d) A particular association of fossils ; (e) The distribution of taxon or fossil taxa ; (f) The abundance of certain species of fossils ; (g) The morphological characteristics of the fossils ; (h) The way of life and habitat of fossilized organisms ; (i) The stages of evolutionary development of fossils ; (j) The variations of other factors related to the fossils that the strata contains, etc. A chronostratigraphic unit measures the geological time, whose scale is based on the time-series of the main geological events that have occurred on Earth since its formation (± 4.6 Ga). The main chronostratigraphic units are Era, Period, Epoch, Age and Sub-Age, which correspond, respectively, to the chronostratigraphic units, Group or Erathem, System, Series, Stage or Sub-Stage. This tentative geological interpretation of a Canvas auto-trace of a Turkey offshore (Black Sea) regional offshore seismic line was made in stratigraphic cycles. The boundaries between these cycles are unconformities (erosional surfaces) or their deep-water correlative paraconformities that correlate, upstream, with the unconformities. The intervals considered are allostratigraphic stratigraphic units. They are individualized by unconformities or their correlative paraconformities. The study of the allostratigraphic units, also called discordant stratigraphic units, is the field of allostratigraphy, which involves the stratigraphic interpretation, the correlations and the cartography using discontinuities and surfaces with a chronostratigraphic value (unconformities, omission surfaces, ravinment surfaces, flooding surfaces, etc.) to subdivide a sedimentary section. The allostratigraphy subdivides the stratigraphy on the basis of unconformities and other disconformities. It is a way of defining and naming successions of rocks without putting a particular emphasis on the discontinuity type that should be used as a boundary basis. The allostratigraphic units (allomembers of certain geoscientists) represent intervals, lithologically, heterogeneous that may include beds that were, previously, included in different geological formations. The limits between the allostratigraphic or discordant units, which cross conventional lithostratigraphic boundaries, illustrate, better, the genetic relationships between the different lithostratigraphic units. Allostratigraphic units not only can include stratigraphic cycles (continental encroachment cycles, continental encroachment subcycles, sequence-cycles and high frequency cycles, i.e., induced by 4th and 5th order eustatic cycles with a time-duration less than 0.5 My), but also the genetic stratigraphy units (limited by the marine flooding surfaces) proposed by Galloway (1989). Summarizing: (i) Allostratigraphy represents a genetic way of defining and naming sequences of rocks bounded by unconformities and emphasizes cartography ; (ii) Sequential stratigraphy represents another way of interpreting successions of rocks in the context of the cyclicity of sea level changes (absolute or eustatic and relative sea level), which allows to predict ages and lithologies. Age is based on the analysis of the sequence-cycles to extract the eustatic signal that is correlated with the global eustatic curve. The lithology is predicted by the analysis of the sedimentary systems tracts that are lateral associations synchronous and genetically of depositional systems (lithology and fauna, more or less typical), linked together (if a depositional system of a systems tract is not deposited, generally, the others do not deposit either).

Allostratigraphy...............................................................................................................................................................................Allostratigraphie

Alostratigrafia / Aloestratigrafia / Allostratigraphy / Allostratigraphy / Аллостратиграфия / Allostratigrafia /

Study of the sedimentary rocks defined and identified from the unconformities that limit them and that can be mapped. The allostratigraphy allows the cartography of the sedimentary rocks on the base of deposition time (sedimentary systems tracts in sequential stratigraphy). Practically, synonymous, with Sequential Stratigraphy.

See: « Sequence Stratigraphy »
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 « Unconformity »
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« Systems Tract »

In this figure two tentative geological interpretations of a geological section are represented. Although the absence of vertical and horizontal scale does not allow a plausible geological interpretation, as far as, all geological interpretation of a geological section or seismic line is dependent, among other things, of scale, it can be said that the upper tentative is allostratigraphic. It is based on unconformities and biostratigraphy. The different sedimentary packages were individualized by the erosional surfaces delimiting them. In sequential stratigraphy, erosional surfaces are supposed to be induced by significant relative sea level falls (result of the combined action of absolute or eustatic sea level, which is supposed to be global and referenced to the Earth's centre, and tectonics) that displace basinward and downward the coastal onlaps. Such displacements are the result of exhumation of the shelf (if the basin had a shelf) and the upper part of the continental slope. Then, from the identification of these erosional surfaces, which the geoscientists call unconformities, they were dated by biostratigraphy, that is, by the fossils found in each of the different sedimentary packages, since they correspond, in geological terms, to time lines. In this particular case, the geoscientist also mapped, within each sedimentary package (stratigraphic cycles), the different depositional systems, i.e., the different facies or lithologies that compose them. The lower tentative interpretation is not in time, but in facies (lithology and fauna in a particular sedimentary environment). The geoscientist has just identified and mapped the different lithologies (facies), which from the continent seaward correspond, probably, to: (i) Silts and shales (coloured in dark brown) ; (ii) Silts (light pink); (iii) Fine sand (in yellow) and (iv) Deep shales (in light brown). If the vertical scale is of the order of tens or hundreds of meters, it is most likely that the chronostratigraphic lines correspond to delta progradations and that their different segments correspond from updid to downdip, to the delta plain and delta front (upper subhorizontal delta beds or top-set), prodelta (seaward dipping delta beds) and base of the prodelta (lower sub-horizontal delta beds or bottom-set). However, if the vertical scale is of the order of hundreds to thousands of meters, the chronostratigraphic lines underline continental progradations and their different segments, from continent to deep-water, correspond to the coastal plain/shelf, continental slope and abyssal plain. It is in this sense that many geoscientists say that the geological interpretation and, in particular, the tentative geological interpretation of the seismic lines, is scale dependent. In practice, this means that a seismic line without location and without the scales (horizontal and vertical) is impossible to interpret correctly and therefore its tentative interpretation difficult to refute. The upper tentative interpretation is more exhaustive than the lower. The geoscientist who interpreted it not only highlighted the lithologies, but also the main time lines (unconformities) that individualize the different sedimentary packages. This allow him to propose an explanation of the cyclicity of the deposits, which is not the case of the lower tentative interpretation which is, purely, lithostratigraphic. In order to determine the age of the unconformities, the geoscientist had to recognize within each sedimentary package the sequence-cycles and, within them, the different groups of sedimentary systems tracts (lateral associations of synchronous and genetically linked depositional systems), particularly the submarine basin floor fans (SBFF). The age of an unconformity corresponds to the age of the relative sea level fall responsible for it it, which corresponds, more or less, to the age of the smallest hiatus (without deposition), between the two stratigraphic cycles (eventually, sequence-cycles) limited by the unconformity. The age of the smallest hiatus is, practically, the age of the submarine basin floor fan deposited during the relative sea level fall. However, as within the submarine basin floor fans, the fauna is all transported, only the fauna of the pelagic layers (E bed or E layer in the Bouma's nomenclature), which separate the turbiditic layers, allows a dating difficult to refute. In geological terms, the deposition time of a turbidite layer (A, B, C and D levels of Bouma*) is instantaneous. Summing up, allostratigraphy is based on the cartography of the unconformities (erosional surfaces induced by significant relative sea level falls) and on the biostratigraphy, whereas the lithostratigraphy is based on the lithology without taking into account the biostratigraphy, which orders the lithological units function of the fossils they contain.

(*) From bottom to top, the Bouma sequence is : (i) A level : massive to normally graded, fine- to coarse-grained sandstone, often with pebbles and/or ripup clasts of shale near the base ; dish structures may, often, be present ; an unconformity or a para-conformity is, always, present at the base of A level. (ii) B level: Planar-laminated fine- to medium-grained sandstone, which base has, often sole marks (flute casts, groove casts and parting lineation) ; (iii) C level: ripple-laminated fine-grained sandstone, which, often, are deformed into convolute laminations and flame structures ; (iv) D level: parallel-laminated siltstone. A non-turbiditc level (pelagic interval formed by ungradded mudstone, with a centimetric thickness), considered as E level in Bouma's terminology, is, often, present, between the consecutive turbidite beds.

Allotropy.......................................................................................................................................................................................................................................Allotropie

Alotropia / Alotropia / Allotropie / 同素异形体 / Аллотропия / Allotropia/

Existence of an element in two or more forms in the same state (solid, liquid or gas). The physical properties of the allotropic forms (colour, crystalline form if solid, density, etc.) may differ greatly, but identical chemical compounds may form from different forms.

See: « Isotope »
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« Carbon »
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« Nuclear Fission »

Allotropy, in which various forms are stable, under different conditions and reversibly interconvertible at certain temperatures and pressures, is called Enantiotropy. Typical examples of allotropy are, for instance, diamond/graphite or oxygen/ozone. There are three types of allotropy*: (i) Enantiotropy ; (ii) Monotropy and (iii) Dynamic Allotropy. As an example of enantiotropy there may be mentioned the sulphur, which in the solid state, has in two crystalline forms allotropic: orthorhombic and monoclinic. The orthorhombic form is stable at a temperature below 95.5° C. Below this temperature, the monoclinic form becomes orthorhombic. The monoclinic form is stable between 95.5° C and 119.25° C (melting temperature). All orthorhombic form will become monoclinic between these temperatures. Unlike an enantiotropy, where all allotropic forms are stable at different temperatures, in one monotropy only one form is stable at normal temperatures. As an example of monotropy, carbon may be mentioned. Contrary to what one might think at first glance, the only stable solid allotrope form of carbon is graphite. The diamond converts to graphite at all temperatures, which may seem strange, since the diamond not only has a much greater resistance to chemical attacks, but also a much stronger molecular structure. Therefore, the conversion of the diamond into graphite is very slow. In dynamic allotropy, two allotropy forms exist in equilibrium. However, the ratio of the two forms which are in equilibrium with one another varies with temperature. As an example of this type of allotropy we can cite the two liquid forms of sulphur, in which conversion from one form to another is accompanied by a change in colour and viscosity (resistance to flow). Indeed, sulphur when heated above the melting temperature, the resulting yellowish liquid becomes progressively dark and more viscous up to 180 ° C. It then becomes almost black and becoming less viscous.

(*) The concept of allotropy refers only to the different forms of a chemical element within the same phase or state of matter (solid, liquid, gas). Phase changes of an element are not, by definition, associated with an allotropic change of form (e.g., liquid oxygen and gaseous oxygen are not two allotropic forms). For some chemical elements, allotropic forms may exist in different phases. For example, the two allotropic forms of oxygen, dioxygen (O₂) and ozone (O₃) may exist in the solid, liquid and gas phases (https://fr.wikipedia.org/wiki/Allotropie).

Alluvial (Interval)..........................................................................................................................................................................................................................Alluvial

Aluvial / Aluvial (intervalo) / Alluvialen / 冲积 / Аллювиальный (намывной) / Alluvionale /

Sedimentary interval composed, in general, of alluvium* deposited by a stream (water-course) in an alluvial plain. In sequential stratigraphy, two types of alluvial deposits are considered: (i) Fluvial Alluvial Deposits, which are deposited between the depositional coastal break of the depositional surface (roughly the shoreline) and the bayline (first slope break of the deposition surface from upstream) and (ii) Alluvial Deposits, in strict sense, which are deposited upstream of the bayline (upstream limit the coastal prism of Posamentier and Vail). Fluvial deposits, in which relative sea level changes have, practically, no direct influence, are deposited upstream of the bayline.

See: « Bay-line »
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« Gilbert Delta »
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«Depositional Environment  »

The alluvial term, which comes from the Latin alluvius and means "to wash again", does not apply to underwater deposits deposited in the sea, estuaries, lakes or lagoons. Sediments from a flood are calibrated or semi-calibrated (when sediments have all, roughly, the same size, otherwise they are classified as poorly calibrated sediments). They are, generally, deposited at the base of the slope of a mountain, in the bed of a stream, in a floodplain, or in a delta. The morphology of the alluvial deposits is the same as that of the non-marine fans. The alluvial deposits are, above all, abundant upstream of the bayline, which, in sequential stratigraphy, separates the fluvial deposits (upstream) from the coastal deposits (downstream). The bayline that emphasizes the upstream boundary of the coastal prism (including river and shallow-water deposits) may move upstream when the shoreline progradation is accompanied by aggradation. At a given geological moment, underlined by a chronostratigraphic surface, the bayline corresponds to the first slope break of a deposition surface, from which a current ceases to erode, in order to begin, above all, to deposit. In the provisional equilibrium profile of a water-course (usually a river), the bayline corresponds to the inflection point from which the current reaches a provisional equilibrium. Of course, the position of the bayline changes with the position of the shoreline. A relative sea level fall, which displaces basinward the mouth of the water-course, breaks the provisional equilibrium profile of the stream, and consequently the bayline moves downstream, whereas the water-courses incise their beds to reach a new provisional equilibrium profile. However, there is no consensus on this issue. For some geoscientists, it is not the bayline that underlines the equilibrium of the water-courses, but the shoreline (river mouth). The equilibrium profile of a stream should not be confused with the equilibrium point of a margin, which corresponds to the point at which subsidence and absolute (eustatic) sea level change compensate. In some cases, however, the equilibrium point may coincide with the bayline. When the relative sea level rises or falls, the equilibrium point moves landward or seaward. Many alluviums** can have an economic value. They can contain ores like gold, platinum, diamonds and a whole series of more or less precious stones. The terms alluvial and alluvium should be avoided as possible. They have very broad meanings and so they can give rise to confusion. For many geoscientists, alluvium is a deposit of clastic sediments (sand, gravel, or mud) formed by a fluvial system in the drainage basin and banks, including flood plains and delta areas, with thinner material extravasated from the channels during floods. Clastic sedimentary particles deposited in estuarine area. For some geoscientists, the particles of the terrigeneous supply, directly, worked by the waves in the marine coastal or lacustrines zones are also considered alluviums. On the other hand, alluvium can be synonymous with flooding. The terms alluvium and flooding are, currently, used as synonyms. The term alluvium refers exclusively to a meteorological event, while flooding applies, to any water supply, even of anthropic origin and not always with a catastrophic meaning. The alluviums can be catastrophic, when caused by atmospheric conditions that provoke torrential rains during long periods of time. They are particularly devastating and dangerous phenomena such as those occurring on the island of Madeira in 1803 and on 20 February 2010. They are now part of the list of natural disasters. This figure shows the alluvial fan of Leuk, located in the Rhone valley, in the canton of Valais (Switzerland), about a dozen kilometers NE of the town of Sierre. This alluvial fan is, perfectly, associated with the gravitational flows of the chalk*** horizons of the Valais Alps, between the Schwarzhorn (Black Peak) and the Rhone valley. As it is, clearly, visible in this image, the "Graben" torrent now separates the alluvial fan into two sectors. The village of Leuk (Loesch) is located in the eastern sector of the fan which, unlike the West sector, is highly cultivated and inhabited.

(*) In wikipedia free online encyclopedia we can read that an alluvium  is loose, unconsolidated (not cemented together into a solid rock ) soil or sediments, which has been eroded, reshaped by water in some form, and redeposited in a non-marine setting. Alluvium is typically made up of a variety of materials, including fine particles of silt and clay and larger particles of sand and gravel. When this loose alluvial material is deposited or cemented into a lithological  unit, or lithified, it is called an alluvial deposit.

(**) An alluvium is a deposit of debris (sediments), such as sand, mud, clay, pebbles, silt or gravel, transported by running water. Alluviums can settle in the stream bed or accumulate at the point of slope failure. There are several types of alluvium: (i) fluvial alluvium, which is deposited by a river or river; (ii) fluvio-glacial alluvium deposited by glacier melt-water; (iii) fluvio-marine alluvium accumulating in estuaries.

(***) Porous sedimentary rock that a variety of white limestone consists essentially of calcium carbonate in the form of calcite, which forms in relatively deep water conditions from the gradual accumulation of tiny calcite plates (coccoliths) dropped by micro - organisms called Coccolithophores (unicellular marine algae that are part of the phytoplankton of the euphotic zone of the more temperate areas of the oceans). Coccolithophores have a carapace made up of calcite scales (coccoliths) that deposit on the seafloor when coccolithophores die. Certain geoscientists consider that the annual deposition of coccoliths on seafloor may exceed 1.5 x 10⁶ tonnes.

Alluvial Delta (Alluvial fan that reaches the sea)......................................................Éventail alluvial(Aboutissant à la mer)

Fan Delta / Delta aluvial, Abanico aluvial / Schwemmkegel (zum Meer, See) / 冲积扇（海） / Веерообразная дельта / Conoide alluvionale (che porta al mare, lago) /

A progradational geological body formed by alluvial sediments deposited, directly, into the sea when the shoreline (more or less the depositional coastal break of the depositional surface) is located near the bayline (line separating river from the paralic/delta deposits that emphasizes the upstream limit of the coastal wedge). Certain geoscientists call these geological bodies Gilbert-Type Deltas.

See: « Alluvial »
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" Delta ”
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" Progradation "

On this tentative geological interpretation of a detail of a seismic line from the northern Angola offshore, located near the Congo canyon, it is possible to associate the progradational filling of the upper part of a submarine canyon to an alluvial delta-type, which certain geoscientists consider as a Gilbert-type delta. The terminations of the reflectors (lapouts*) associated with the filling, i.e., onlaps, toplaps and downlaps are well visible. Likewise, the progradational internal configuration (sigmoid progradations) of the fill is obvious. Most onlaps underline the erosional surface associated with the incision. Others, particularly, those visible within the filling, underline small lateral displacements (pendulum effect) of the depocenters. All these lapouts, strongly, suggest that the upper part of the submarine canyon was exhumed and later filled with sediments of fluvial nature, which means that during the relative sea level fall, responsible for exhumation, the bayline** was very close to the shoreline. The incision (narrow erosion  caused by a river or stream that is far from its base level) of the submarine canyon is associated with an erosional surface, which was induced by a significant relative sea level fall (local sea level referenced either to the sea-floor or the base of the sediments and which is the result of the combined action of the absolute or eustatic sea level, supposed global and referenced to the Earth's centre and tectonics), which created lowstand geological conditions (sea level lower than the basin edge, which may coincide, in certain cases, with the continental-edge or continental border). Lowstand geological conditions were, probably, enhanced by the uplift of the northern coast of Angola, that occurred during the Late Tertiary. The upper part of the canyon was, directly, filled (or almost) by fluvio-alluvial sediments. It is possible to advance the hypothesis that a fluvial stream, with a density, probably, similar to the sea, has drained into the upper part of the canyon already covered with water and that a homopicnal flow (with the same density as the water-body where it enter) has induced the deposition of the basal, frontal (dipping) and upper beds, which characterize most deltas. This is particularly true for Gilbert-type deltas, also called alluvial deltas or alluvial fans, which are progradational geological bodies formed by alluvial sediments, of glacial origin or not, which are deposited, directly, in the sea or in a lake when the shoreline line is situated near the bayline. Theoretically, when the density of a fluvial current is equal to the density of the water of the receiving basin (usually the sea) or, in other words, when the flow of the river is homopicnal, the sediments deposit at the entrance of the sea forming d Gilbert-type deltas, particularly, when the river carries a significant sandy load. If the water density of the river is greater than the density of the water of the receiving basin, the water flow of the river is hyperpicnal and plunges to the bottom of the receiving basin, more or less, as a turbidity current, depositing submarine fans (basin floor or slope fans) since the current begins to decelerate. As can be seen from the photograph shown in this figure, the seaward dipping beds of an alluvial delta (more or less synonymous with Gilbert-type delta) have a depositional slope that can exceed 25°, as is the case, for example. in the Laga delta, in the region of Vizcaya in Spain. In addition to this delta type, other deltas can be identified by their dynamics and the shape of the emergent plain: (i) Abandoned Delta or Sub-Delta ; (ii) Arctic Delta ; (iii) Arcuate Delta ; (iv) Atrophied Delta ; (v) Complex or Polymorphic Delta ; (vi) Cuspate Delta ; (vii) Finger Delta ; (viii) Flow Delta ; (ix) Closed Delta ; ; (x) Lava Delta ; (xi) Lobated Delta ; (xii) Tidal Delta ; (xiii) Submarine Delta ; (xiv) Storm Delta ; (xv) Wave Delta ; (xvii) Ebb Delta.

(**) Lapouts or Seismic reflection terminations, are stratal discontinuities recognized on seismic sections separating, apparently, conformable from non-conformable sedimentary packages, that are the basis of seismic sequentilal stratigraphy.

(*) The bayline was, more or less, defined as follows by Posamentier and Vail (1988): (a) The coastal plain is formed by processes of progradation of the sea-floor, rather than by exhumation ; (b) The sediments that accumulate on the coastal plain during the progradation of the shoreline are part of what is called the coastal wedge (or coastal prism), which includes river and shallow water deposited ; (c) The coastal prism is wedge-shaped and extends to the continent by onlapping over the pre-existing topography ; (d) The upstream limit of the coastal wedge is the bayline, which may move updip when the progradation of the shoreline is accompanied by aggradation ; (e) The bayline is the limit between the coastal plain and the alluvial plain ; (f) Upstream of the bayline, changes in relative sea level have, practically, no influence on depositional systems.

Alluvial Fan (Fan delta)....................................................................................................................................................................Cône de déjection

Cone de Dejecção / Cono aluvial (deyección) / Schwemmkegel / 冲积扇 / Конус выноса / Cono alluvionale /

Sedimentary, nonmarine lobe, composed of a heterogeneous rock mass, relatively, little inclined and deposited by a torrent (especially in semiarid regions).

See:« Alluvial »
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« Bay-line »
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« Depositional Environment  »

An alluvial fan is deposited when: (i) A stream flows out of a narrow valley upstream of a much wider valley ; (ii) A current is tributary and flows into the main stream ; (iii) The constraint of the valley disappears abruptly or (iv) The gradient of the current decreases rapidly. An alluvial fan is steeper near the mouth of the valley. Its highest point points upstream and plunges, convexly, downstream, as the gradient decreases (AGI, 1999). The Death Valley photograph (near the highway) not far from Badwater (USA) illustrates, perfectly, alluvial fan or fan deltas, which, as can be seen, consist of sedimentary material transported by water and deposited in cone-shaped bodies at the base of the slopes. In this particular case, alluvial fans that formed at the mouth of valleys or continental canyons may, in certain respects, be considered as delta equivalents. However, this is not always the case. Alluvial fans may form, also, at the base of coastal cliffs, caused by sea or glacial erosion, in association with, more or less vertical, ditches* formed along the cliffs, as appears to be the case in the islands of the Svalbard archipelago (between Norway and the North Pole). Generally, alluvial fans form on the basis of topographic components where there is a significant rupture of the slope of the terrain. This, of course, implies that coarser sediments are deposited at the apex of the fan (upstream) and that the edges are made of finer material. Alluvial fans, such as the Death Valley (Mojaves desert, California), are typical of arid or semiarid climates, but it can not be said that they are restricted to these types of climate. Alluvial fans also form in humid climates, such as the great alluvial cones of the Koshi River (Nepal). Over time, an alluvial fan is, progressively, destroyed by erosion, upstream and downstream, and it will, eventually, level the terrain.  Alluvial fans are subject to flooding (https://en.wikipedia.org/wiki/Alluvial_fan) and can be even more dangerous than the upstream canyons that feed them, since the convex surfaces force water to spread widely until there is no zone of refuge. If the gradient is steep, active transport of materials down the fan creates a moving substrate that is inhospitable to travel on foot or wheels. But as the gradient diminishes downslope, water comes down from above faster than it can flow away downstream, and may form ponds to hazardous depths.  In 2008 high monsoon  flows breached the embankment, diverting most of the Koshi river into an unprotected ancient channel and across surrounding lands. Over a million people were rendered homeless, about a thousand lost their lives and thousands of hectares of crops were destroyed. In sequence stratigraphy, alluvial fan are deposited upstream of the bayline (limit between the fluvial and continental deposits), or upstream of the coastal wedge* of Posamentier and Vail, (1988) where the changes in relative sea level have no effect on the creation of available space for sediments (accommodation).

(*) Long narrow earth excavation, often, induced by fractures, which are small natural waterways.

(**) Set of sediments that accumulate in the coastal plain during the progradation of the shoreline, which includes fluvial and shallow marine deposits. The coastal prism (Posamentier and Vail, 1988) is wedge-shaped and extends to the continent by onlapping over pre-existing topography. The upstream limit of the coastal prism is the bayline, which can move updip when the shoreline progradation is accompanied by aggradation (upbuilding).

Alluvial Plain..........................................................................................................................................................................................................................Plaine alluviale

Planície Aluvial / Llanura aluvial / Schwemmebene / 冲积平原 / Аллювиальная равнина / Pianura alluvionale /

Slightly seaward dipping horizon or stratigraphic interval or a, more or less, wavy surface created by extensive deposit of alluviums, usually, near a river that, periodically, overflows its banks. An alluvial plain is generally associated with a flooding plain, delta or alluvial fan.

See: « Point Bar »
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« Bay-line »
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« Alluvial »

Alluvial plains are geological formations, characterized by being flat or very little inclined, formed by deposition of the sedimentary particles transported by one or more rivers, creating an alluvial soil constituted of clay, silt and sand. Examples of alluvial plains are the Tagus and Sado plains (Portugal). The Sado flooding plains, which are, more or less, narrow sedimentary strips that are flooded every time the river overflows and are, often, cultivated, form as part of the formation process of alluvial plains. They are very larger areas encompassing the old Sado area that no longer floods due to the elevation of the soil by the accumulation of sediments, or the change of course of the river over thousands or millions of years . The Fertile Crescent, where the invention of agriculture allowed the sedentarization (settling of a nomadic population) of man is the region comprising the present states of Palestine, Israel, Jordan, Kuwait, Lebanon and Cyprus, as well as parts of Syria, Iraq, Egypt, south-eastern Turkey and south-west Iran, which corresponds, geologically, to a more or less continuous set of alluvial plains. This photograph shows a meander belt and the alluvial plain of the Animas River (Colorado, USA), few kilometers upstream the Durango city. In this photograph, the Animas River flows to the left, and the scale is given by the roads (easy to recognize on either side of the alluvial plain). The regional slope of the alluvial plain, which is very small, is leftward, but the presence of numerous natural marginal dykes (fluvial levees) creates, locally, flood plains that can tilt, gently, upstream. This alluvial plain can be interpreted in several ways. It all depends on how the meanders formed. Two main types of meanders can distinguish: i) Valley Meanders or Incised Meanders are those in which the river valley meanders as the river, on the same scale, which means that the meanders are, more or less, permanent and (ii) Alluvial Plain Meanders, which certain French geoscientists mistakenly call free meanders or wandering meanders, are those in which the sinuosities of the river are independent of the valley path and have very different scales. In alluvial plain meanders, the valley is much larger than the river and the meanders are not permanent. The tendency for lowland rivers to form meanders is a form of energy dissipation in periods when flow rates are higher. When flow rates increase, water level rise as well as the bottom erosion of the river. When the flow decreases, the water level is low and there is sediment deposition. The valley meanders are as frequent as those of the alluvial plain. Examples of the former are, for instance, the meanders of the Colorado River, particularly in the Grand Canyon (USA), as well as the meanders of the river Douro (Portugal) or the Mondego River (between Penacova and Coimbra, where the river runs through a narrow valley). As examples of alluvial plain meanders, the meanders of the Mississippi river, in the United States of North America, or of the Amazon river in Brazil are classic examples. Often, as is, probably, the case with the Animas river meanders illustrated in this figure, certain meanders of alluvial plain are valley meanders that have evolved in free meanders by valley calibration. The meanders of alluvial plain form as a consequence of simple and normal evolution of a river that builds up its alluvial plain, progressively, since the meanders tend to exaggerate themselves and to cut each other, either by overflow or osculation (contact of two meander curves). When incised meanders (valley meanders) evolve in meanders of alluvial plain, that means that the valley has been gradually calibrated. The meanders are exaggerated and migrate downstream, since it takes a certain amount of time for the line of higher velocity of the current reaches its maximum displacement towards the concave margin. This migration downstream results in the calibration of the entire valley to the meander dimensions, which transforms the valley meanders into alluvial plain meanders. When a incised valley, induced by a relative sea level fall that broke the river's provisional equilibrium profile, is filled (relative sea level began to rise) during the deposition of the upper part of a lowstand prograding wedge (LPW) of the associated sequence-cycle forming an apparent or false alluvial plain.

Alluvium...............................................................................................................................................................................................................................................Alluvion

Aluvião/ Aluvión / Schwemmland / 冲积 / Аллювий / Alluvione /

Quaternary fluvial deposit, loose and unconsolidated, composed mainly of sand, clay and gravel. Over time, an alluvium is lithificated and transformed into a, more or less, compact sedimentary rock.

See: « Alluvial »
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« Gilbert Delta »
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« Sedimentary Environment »

These photographs illustrate alluvial deposits on the United States onshore. Typically, an alluvium consists of varied sedimentary material, both in size and lithology. Small water-streams may produce an alluvium, but it is, above all, in the floodplains and deltas of the great rivers that alluvial deposits are of considerable size and contain, generally, various ores such as gold, platinum, and precious stones. A stream is a water-course, which channels the floods. The median line, which separates the valley into two, more or less, symmetrical parts is the thalweg. Water-courses have different names depending on size and behaviour. When the bed of a river is too small to contain all the water and the material it carries, the river overflows the bed creating levees (natural marginal dikes) and flood plains, in which alluvial deposits are deposited. Certain geoscientists consider alluviums only the deposits of the Quaternary age. Others use this term in a more general way. This type of deposition is similar to that of deep-water overbanks. Both have the same geometry. The lithology is, generally, finer in deep-water deposits. There is, however, a very significant difference which is. perfectly. visible on the seismic lines. A river implies ,always, a bed where water flows, whereas a turbidite current does not**. A turbidite current flows, generally, on a more or less inclined and bedless surface. As it loses speed***, two lobes are deposited. Between the lobes, there is no deposition. It is the zone of passage of the fastest part of the current, which carries, downstream, the fine sediments before they settle as long as the current loses competence. The next turbidite current utilizes the depression between the lobes and thus the new overbank deposits exaggerate the depression morphology, while the fine sediments settle further downstream. The filling of the depression between the deep overbank deposits is subsequent to the deposition of these, whereas the bed of a river it predates the levees (natural marginal dykes of the river).

(*) The bed or gutter of a stream is the space that can be occupied by a water-course, which can be sub-divided into: (i) Apparent bed, where, normally, water and the material carried by it flow; (ii) Greater bed or flood bed, which corresponds to the area of the valley that can be flooded ; and (iii) Lower bed or drought bed, which is the area occupied by the water course during drought. (https://en.wikipedia.org/ wiki / Drought).

(**) In a river, natural marginal dikes (levees) always are deposited higher than the river bed, while in deep turbidite systems, the first natural turbidite marginal dyke is deposited at the level of the base of the turbiditic current.

(***) The speed is the magnitude of the velocity. It does not take t taking into account de direction. The velocity of a seismic wave is a scalar quantity. In other words, in physics what is called the speed of an object (current, for instance) is the rapidity with which the object moves without regard to direction.

Alluvium Accumulation (Alluviation)............................................................................................................................Alluvionnement

Assoreamento / Acumulación aluvial / Aufschüttung / 冲积层堆积 / Отложение аллювия / Accumulo di alluvione /

Deposition of sedimentary particles (sand, clay, slime, etc.) that accumulate by the action of the currents and gravity, where before the water flowed. Accumulation of alluvial deposits in the bottom of the valleys or in the river mouths with decrease of the water column. Alluviation is often regarded synonymous of Aggradation.

See: « Aggradation »
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« Fluvial Deposition »
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« Debris Flow »

According to several geoscientists, the rates of denudation* during the Paleozoic, inferred from the volume of the preserved sedimentary rocks, suggest an average continental erosion of about 16 meters during a million years (16m/My). Such erosion would have produced a sediment accumulation of about 5 Gt/year. However, it appears that erosion increased irregularly since Paleozoic to Pliocene, as during this period of Tertiary period, the average continental erosion was about 53 m/My with 16 Gt year of deposition. Currently, the sedimentary transport of the rivers seems to be similar to that of the rivers during Late Neogene, with a continental denudation of about 62 m/My and a deposition of about 21 Gt/year. The terrigeneous supply of the rivers and the morphology of hydrographic basins suggests that natural erosion is mainly confined to drainage areas. About 83% of the rivers terrigeneous influx derives from 10% of the highest areas of the Earth. Sub-aerial erosion resulting from human activity (mainly from agriculture and civil construction) has, significantly, increased the rate of denudation, mainly in lowland areas. The disparity between natural terrigeneous influx (± 21 Gt/y) and anthropogenic terrigeneous influx (± 75 Gt/y), inferred from agricultural land losses, explains the thickness and age of the sediments. As illustrated in the figure of the Tagus River (Portugal), the alluviation of channels and alluvial plains, which is, in average, ± 12.6 m/My), is the most important geomorphological process in terms of erosion and deposition. Anthropogenic or anthropic alluviation or siltation (this term refers to the effects, processes or materials that are the result of human activities) is superior to that induced by Pleistocene glaciers and induced by current alpine erosion (fluvio-glacial processes). On the other hand, available data suggest that, since 1961, the area under cultivation has increased globally by about 11%, while the world population has practically doubled. The combined effect of these two factors, that is to say, cultivated area and population growth, is dramatic, as the area cultivated per capita (per person or per head) decreased by about 44% over the same period of time (± 50 years), which on average gives a decrease of plus or minus 1% per year. That is, about 25 times the rate of soil loss predicted by the denudation of farmland induced by man. In a per capita food production context, soil loss due to erosion of agricultural land is insignificant when compared to the impact of population growth. (http://gsabulletin.gsapubs.org /content/119/1-2/140. abstract). In the diagram of this figure, the soil is the surface layer of the earth crust that results from the weathering, that is, the disintegration of the rocks by the action of the physical, chemical and biological agents that act, above all, on the Earth surface. The soil is, in general, composed of organic and mineral materials, its main components being clay, limestone, humus and sand. The loess can be considered as a particular soil, which corresponds to the deposition of fine sedimentary particles (quartz, clay, calcium oxide, limonite, etc.) carried by the wind, usually, unstratified and very fertile. As illustrated, in the geological schemes on the left side of this figure, sedimentation (accumulation of sand, pebbles, mud, etc. in the uneven areas of the river-bed, especially, at the end of its course) is very active in the valleys without vegetation (vegetation present in areas close to water-bodies, i.e., riparian zone), since the vegetation not only filters the water from the rains, but also the roots of the plants prevent the land from the banks of the rivers to crumble. In other words, riparian deforestation facilitates the silting of adjacent rivers, while riparian forestation makes siltation difficult. Currently, they reach rivers each year (which contain about 0.0001% of the total water on Earth) and the oceans more than 24 x 10⁹ tons of humus swept by rain.

(*) Abrade of rock masses on the surface of the Earth's crust caused by the action of destructive and erosive agents, which changes the soil or rock material to disintegrated or dissolved. It can be said that, currently, a continent wears about 135 tonnes per km², corresponding to a reduction of about 1 meter every 22,000 years. In other words, without a tectonic or isostatic uplift, in order for a continent, such as Europe to be at sea level, it would take about 20 My.

Alteration (Weathering).............................................................................................................................................................................................Altération

Alteração / Alteración, Meteorización / Verwitterung, Zersetzung / 风化 / Выветривание (эрозия) / Alterazione /

Decomposition of land surface rocks, soils and minerals by direct contact with the atmosphere. Alteration or weathering occurs in situ, without movement, whereas erosion implies movement of rocks particles and minerals by erosion agents such as water, wind, ice and gravity.

See: « Erosion »
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« Disaggregation, Desintegration »
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« Atmosphere »

Although the alteration facilitates erosion, it is best not to confuse these two geological processes. Erosion implies transport by any of the erosive agents (water, ice, wind), while alteration or weathering does not. Alteration is a simple decomposition and rupture of the rocks on the Earth surface by natural chemical and mechanical processes. Alteration is controlled by the weather (state of the atmosphere) and climate ( average weather conditions for a particular location and over a long period of time.). As these occur at the Earth's surface. Alteration decreases in intensity in depth, although most of it occurs less than one meter deep of the soil and rocks. Do not confuse climate, which includes statistics of temperature, humidity, atmospheric pressure, wind, rain, and other meteorological elements of a given region over a long period (arbitrarily about 30 years), and weather that is the condition of those same elements for periods of less than 1 week. The change may be physical or chemical. Physical alteration corresponds mainly to the disintegration of the rocks by mechanical forces concentrated along the fractures, joints and faults, or by the separation of the rocks in more or less concentric envelopes. The most common processes of physical alteration are: (i) Fracturation by freezing ; (ii) Pressure removal ; (iii) Crystallization of salts ; (iv) Hydration and (v) Insolation (solar heat effect on rocks on earth's surface rocks, solar heat during the day causes the rocks to dilate, which when cooled by night contract and fracture). The chemical change corresponds to the decomposition of the rocks by chemical reactions that occur in general in water and in particular in the water of the soils, which is particularly rich in carbon dioxide (CO₂) produced during the decomposition of the plants. The most frequent processes in the chemical alteration are: (i) Carbonization (dissolution of calcium carbonate by acid water), such as the formation of bicarbonate (HCO_3-) or the formation of a karstic topography, which results from carbonization* of limestone ; (ii) Chelation (a process during which a polyatomic building consisting of one or more cations (positively charged ions), between a binder and a metal cation) is formed, practically corresponding to a bonding of mineral cations and organic molecules produced by plants; (iii) Hydrolysis ** and (iv) Oxidation.

(*) Progressive enrichment of carbon from organic waste, with relative loss of oxygen and hydrogen, mostly from their own carbohydrates and plant tissues, especially lignin (class of complex organic polymers that form important structural materials in the support tissues of vascular plants and some algae) and cellulose (A.M. Galopin de Carvalho, Dictionary of Geology, ISBN 9789727803361).

(**) Chemical bonding breaking of a molecule with the addition of a water molecule. The water molecule breaks down into hydrogen (H +) and hydroxyl (OH-) ions that bind to the two molecules resulting from the break, which can have a positive and negative charge.

Amalgamated Turbidites...................................................................................................................Turbidites amalgamées

Turbiditos Amalgamados / Turbidita amalgamada / Amalgamated Turbidit / 合并浊流沉积 / Объединенный турбидит / Torbiditici amalgamato /

Deposits associated with deep-water submarine basin floor fans (SBFF). The amalgamated turbidites are, often, located in the central part of the turbidite lobes. They are composed of a stacking of deposits produced by consecutive turbidite currents (very separated in time). The thickness of these deposits exceeds several hundred meters. It can be said that amalgamated turbidites are characterized by a very weak presence of clay levels, which contrasts with laminated turbidites. Amalgamated turbidites correspond to type 1 turbidites (or substage 1) of Emiliani Mutti.

See: « Basin Floor Fan »
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“ Slope Fan »
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« Lowstand Systems Tract »

On this tentative geological interpretation of an Canvas auto-trace of a detail of an Adriatic Sea seismic line, the interval immediately overlying the unconformity, located between 2 and 2.3 seconds (double time), which was slightly tectonically enhanced, corresponds to a vertical stacking of E. Mutti type-1 turbidites. This interpretation was corroborated by the results of exploration wells drilled in this region and, particularly, by the results of well A (shown in this figure). They corroborate, also, that the direction of the seismic line ism roughly perpendicular and not parallel, to the flow direction of the turbidite currents. This means that the slope break of the unconformity, perceptible in the western part of this auto-trace, does not correspond to the break of the base of the continental slope and that the turbiditic lobes are, probably, disconnected from the base of the continental slope. In P. Vail's terminology, this interval corresponds to a stacking of submarine basin floor fans (SBFF) associated with a significant relative sea level fall. The relative sea level is a local sea level, referenced to any point on the Earth's surface, which is the result of the combined action of the absolute (eustatic) sea level, supposed to be global and referenced to the Earth's centre, and tectonics (subsidence or uplift of the sea-floor). For the most part, the geological events that are at the origin of these deposits are not the same for  Vail and for Mutti. Vail thinks that turbidite deposits are induced by significant relative sea level falls, which puts the sea level lower than the basin edge (continental edge when, at the level of a sequence-cycle, the basin has a shelf), which means that for him, turbidites are directly associated with unconformities (erosional surfaces that bound the sequence-cycles). At the level of a sequence-cycle (a stratigraphic cycle limited between two unconformities, whose age difference is greater than 0.5 My and lesser than 3-5 My) to have deposition the relative sea relative level has to rise in order to create or increase the available space for sediment (accommodation). For deep-water turbidite depositional systems, this is not true. Downstream of the continental edge (whether the basin has or not a shelf) there is always room for deposition. Mutti does not deny the possibility of turbidite deposits in association with significant relative sea level falls (as suggested by P. Vail). However, he thinks that in the case of unchannelized turbidites and, especially, in amalgamated turbidites, the most likely is that they have been deposited during highstand geological conditions (sea level higher than the basin edge) in association with floods of the great rivers, instabilities and diverse ruptures of the continental edge or upper continental slope. For Mutti, turbiditic currents can, also, be induced by landslides or large-scale ruptures of the basin edge, preferably when it coincides with the continental edge (seismically coincident with the shoreline). This is the case, under highstand geological conditions, during the deposition of the 2^nd phase of development of the highstand prograding wedge (HPW) of a sequence-cycle. When the sea level is lower than the basin edge, which happen, immediately, after an unconformity (significant relative sea level fall), obviously, the basin edge corresponds to the last continental edge of the previous sequence-cycle, since a new sequence-cycle starts with the unconformity. Under these conditions, the origin of the turbidite systems, in the deepest parts of the basin, is, easily, explained by the model proposed by Vail, although the Mutti model can also work. The electrical logs of well A, illustrated in this figure (spontaneous potential and resistivity) but, mainly, the spontaneous potential, strongly, suggest, amalgamated turbidites are limited by erosional surfaces (unconformities). They belong to different sequence-cycles. The different lobes have abrupt contacts (upper and lower), which differentiates them from Type-2 turbidites and, above all, from Type-3 turbidites (Mutti's terminology).

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Last updated: June, 2019