Universidade Fernando Pessoa

Porto, Portugal

Systemic Stratigraphy Seminar

EXERCISES

The solutions proposed for each exercise are just tentative geological interpretation of the seismic line taking into account (i) the global and regional geological setting of the area where the seismic line was shot and (2) the geometrical relationships between the seismic reflectors and the surfaces defined by them.

Exercise n°1

All geometrical relationships between the seismic reflectors and between them and the disconformities (unconformities or paraconformities) can be recognized on this seismic line.

The geometrical relationships, as well as the internal configuration of the uppermost seismic intervals suggest a coastal environment with a meander belt. A point bar, the abandonment shales and the clay plugs are clearly depicted. The geometrical relationships associated with them do not characterize unconformities. Erosion and deposition are coeval and so they preclude unconformities. In other words, the discontinuities associated are not boundaries of stratigraphic cycles. An unconformity (truncation/downlap) is associated with the scoured surface of the meander belt; it represents an important erosional hiatus. Another unconformity ( truncation) is recognized around 0.4 seconds depth (t.w.t.).

Exercise n°2

This seismic line characterizes the foredeep basin of Perija, in onshore Venezuela (west of Maracaibo lake).

A major unconformity defined by truncation (below) and onlap (above) is easily pick out. It corresponds to the stratigraphic cycle boundary SB. 39.5 Ma, and it can be followed in all seismic lines of the area. However, this angular unconformity can not be followed in continuity, since the interface (acoustical impedance profile) change laterally. Below this unconformity, a downlap surface can also be identified. It indicates the cretaceous maximum flooding surface (MFS 91.5 Ma), i.e. the upper limit of the La Luna formation, where the marine potential source rocks deposited. The strong markers overlying toplap of the progradations of the Colon formation (Upper Cretaceous) marks the upper cretaceous unconformity (SB. 68 Ma). The compressional tectonic regime associated with the foredeep basin is responsible for the thrust faults visible on the left part of the line.

Exercise n°3

This seismic line comes from the same area of the previously one, i.e. from the foredeep basin of the Perija, in onshore Venezuela. This foredeep basin is very particular since it is overlying a mesozoic back-arc basin, which contains the generating petroleum subsystem. The major unconformity corresponds to SB. 39.5 Ma, which is defined by truncation (below) and apparent downlap (above). The apparent downlap is a consequence of the sedimentary shortening associated with the foredeep basin. In fact, it correspond to a slightly inverted onlap. Below the unconformity, the MFS. 91.5 Ma can only be recognized by the stratigraphic signature since the geometrical relationships are indiscernible. However, the Colon formation (Upper Cretaceous) is easily identify; it is isopachous, and bounded by two strong amplitude markers.

Exercise n°4

This seismic lines comes from offshore Indonesia (offshore north Java). It illustrates a back-arc basin without oceanisation, i.e. without break-up of the continental crustal and formation of a marginal sea. This type of sedimentary basin is composed by two tectono-sedimentary basin: (i) basin type-rift, associated with a rifting tectonic phase characterized by a differential subsidence and linked with the lengthening of the continental crust, and (ii) a cratonic basin developed during a sag phase created by regional subsidence. Several unconformities can be distinguished. They are generally characterized by truncation (below) an downlap (above) geometrical relationships. However, one of them is strongly angular. Near of the antiform structure, it is characterized by truncation (below) and concordant (above) geometrical relationships. Nevertheless, away from the apex of the antiform, the angularity of the unconformity diminishes gradually and it changes laterally to a paraconformity. This angular unconformity or the correlative paraconformity dates the age of the tectonic inversion, which is one of the more characteristic features of this type of episutural basin.

Exercise n°5

This seismic line from offshore Australia illustrates mainly the regressive phase of a continental margin divergent, i.e. the uppermost phase of the continental encroachment cycle post-Pangea. The type-rift basin which underlain the divergent margin are not visible. The margin itself, is composed by two sedimentary phases: (i) the transgressive phase characterized by an aggradational backstepping geometry and (ii) the regressive phase characterized by a progradational forestepping geometry. The limit between these two phase is the major cretaceous downlap surface (MFS 91.5 Ma), which correspond to top of the dark green seismic interval (Middle Cretaceous). In the regressive phase several major relative sea level falls can be identified. Taking into account the stratigraphic signature of the offshore Australia and its calibration proposed and tested by P. Vail, from top to bottom, it is possible to recognize the following unconformities: SB. 1.6 Ma, SB. 2.4 Ma, SB. 3.8 Ma, SB. 4.2 and SB. 5.5 Ma, in the Plio-Pleistocene, and SB. 6.3 Ma and SB. 8.2 Ma, in the Upper Miocene. Downward, the SB. 10.5 Ma and the SB. 25.5 can be identified.

Exercise n°6

On this seismic line, all geometrical relationships are present. In addition, three major unconformities, with important erosional hiatus, can be identified. The downlaps, particularly those in the uppermost seismic interval, point out the direction of the terrigeneous influx (westward). In the second interval (c, d) the chronostratigraphic lines were deformed by salt movements. In fact, originally onlap become apparent downlap, which do not give any information on the direction of sediment transportation. Actually, several geometrical relationships are partially deformed by the halokinesis. Salt flowage causes subsidence and increases locally the space available for sedimentation.

Exercise n°7

On this seismic line, all geometrical relationships are easily recognized. They characterize unconformities, downlap surfaces and detachment surfaces linked with a foredeep basin. Some of the unconformities are predominantly eustatic, whereas others are tectonically enhanced. The sediments were shortened by a compressive tectonic regime created by the decouplage between two continental lithospheric plates which are part of a subduction zone type A. The shortening was done for the most part by thrust faults and associated cylindrical folds on the upthrown block. An obvious detachment surface is recognize between 3.5 (right) and 5.6 seconds (left). It matches a tectonic disharmony, which separates undeformed (below) from deformed sediments (above). The age of the shortening is done by the age of onlap surface which fossilize the shortened sediments.

Exercise n°8

This line illustrates the post-Pangea continental encroachment stratigraphic cycle in the offshore west of Australia. This cycle is linked with the meso-cenozoic eustatic cycle induced by the breakup of Pangea and the ensuing drifting of the continents. Rift-type basins and a divergent margin were formed. A rift-type basin bound by two normal faults (in red) can be foresee below 1.5 seconds. Upwards, in the divergent margin, two sedimentary phases deposited: (i) the transgressive, with its backstepping geometry, and (ii) the regressive forestepping phase. Their internal configuration clearly indicate a continuous eustatic rise during the transgressive phase and a progressively eustatic fall during the regressive phase. The potential source rocks deposited either in the rift-type basin or close to the major downlap surface which individualize the two sedimentary phases. Both potential source rocks are immature.

Exercise n°9

On this line from the offshore Kalimantan (Indonesia), the progradation of the shelf break during the lower section of the divergent margin non-Atlantic is easily followed. Five downward and basinward shifts of the shelf break bring to light five unconformities, i.e. five boundaries of stratigraphic cycles induced by 3rd order or higher eustatic cycles. The successive displacements of the shelf break, indicates a predominant progradation versus aggradation. Such a geometry suggests a relative sea level more or less constant or a slow relative sea level rise. Indeed, a high rate of sea level rise (fast increase of accommodation) favours aggradation, whereas a low rate of sea level rise (low increase of accommodation) favours progradation. The location of potential reservoirs is partially controlled by the ration progradation/aggradation.

Exercise n°10

On this line of the offshore Australia, it is easily to follow the successive displacements of the shelf break during the regressive phase of the post-Pangea continental encroachment stratigraphic cycle. During this phase, the shelf breaks are coincident with the depositional coastal breaks, since the margin had no platform (at least seismically speaking). The geometry of the displacement is mainly progradational (if there are landward displacement of the depositional coastal break they are under seismic resolution). The major unconformity are easily recognized by the downward and basinward shifts of the shelf breaks. In the lower section of the regressive phase, the relative sea level is more or less constant (slightly falling), since progradation is much more striking than aggradation. On the contrary, between 1 and 2.7 seconds (t.w.t.), the ratio progradation/aggradation suggests a faster relative sea level rise. The progradation follows a steep path.

Exercise n°11

This seismic line is representative of the Meso-Cenozoic divergent margin East of USA. From bottom to top, we can recognize an aggradation interval deposited during the transgressive phase of the continental encroachment stratigraphic cycle, and a prograding interval deposited during the regressive phase. The amplitude of the dips of the slope progradations suggests a carbonaceous facies. In the upper part of the line, it is easily to follow the successive displacements of the shelf break during Miocene. The ratio progradation/aggradation suggests a higher rate of sea level rise during the Mesozoic than during the Cenozoic.

Exercise n°12

This line illustrates the vertical superposition of two different sedimentary basins. On the bottom, below the major unconformity identified between 1.4 (on the right) and 1.9 seconds (on the left), it is possible to recognize the Mesozoic Mediterranean-type basin and a above the proximal part of the divergent margin of the Gulf of Mexico. In the Mediterranean-type basin, the geometrical relationships are easily recognized. Several unconformities and major downlap surface can be identified without difficulty. On the these downlap surfaces (top blue interval), which matches an unconformity indicates the location of the more likely marine potential source rocks (MFS. 91.5 Ma). On the left pat of the line, in depth water environments (seaward of the shelf break), the onlapping fill geometry of the seismic markers is linked with the deposition of slope and basin floor fans.

Exercise n°13

This seismic lines is located near frontier between offshore Alabama and offshore Gulf of Mexico. A Mesozoic mediterranean type-basin and a Cenozoic divergent margin are easily identified. The older is considered as a mediterranean type-basin because the lengthening of the substratum (Paleozoic fold belt) was big enough the breakup of the continental crust and to create a marginal sea above the new oceanic crust. At the base of this basin, a salt layer and a Jurassic reef buildup located near the shelf break, can be recognized. A major downlap surface (interface between the green colours) separates the transgressive backstepping phase from the overlying upper cretaceous regressive forestepping phase. Seaward, the upper cretaceous sediments are too condensed to be recognized on seismic data. The more likely marine potential source rocks are closely connected with this downlap surface.

Exercise n°14

The spatial and temporal distribution of the geometrical relationships strongly suggest deepwater environment within the major stratigraphic interval, which is bounded by two unconformities. The geometrical relationships allow the individualization of three internal configurations. The lower one (yellow), has a more or less parallel configuration; it suggests an onlapping fill by basin floor fans. The middle one (blue), has a hummocky or wavy configuration; it suggests a slope fan deposition make up by a stacking of channel-leveed complexes. The upper one (violet) has a progradational configuration which characterizes a lowstand prograding wedge. The trilogy compose the lowstand sedimentary interval of a stratigraphic cycle associated with a eustatic cycle of 3rd order, i.e. a eustatic cycle with a time duration between 0.5 and 3 M years.

Exercise n°15

This seismic line from offshore China illustrates deltaic progradations within the regressive phase of the post-Pangea continental encroachment cycle of the divergent margin of East China. Actually, within a more or less isopachous interval which thickness is not higher than 0.25 seconds, depositional coastal breaks can be individualized on the chronostratigraphic line. This interval is bounded by two unconformities. The lower one is easily identified by truncation (below) and concordance (above). The upper one is a subtle unconformity; in fact, the erosion associated is just recognized by the identification a elated incised valleys. The amplitude of the progradations, or clinoforms, strongly suggest a deltaic environment, i.e. a prodelta. The toplap are associated with a minor erosion, i.e. the hiatus is mainly non-depositional. At the toe of the clinoforms proximal turbidites were probably deposited.

Exercise n°16

This seismic line from the offshore of Gulf of Mexico illustrates the weight of salt tectonics on deformation of original geometrical relationships. Actually, the original onlaps were completely tilted and became apparent downlaps. The reason of such a deformation was the complete lateral salt flowage, i.e. the development of a salt weld, which is responsible of the depocenter visible on right end of the profile. The salt flowage explain also the apparent toplap, that we can recognize on the upper part of seismic interval (yellow) below the unconformity. Above the unconformity, retrogradational (backstepping) seismic markers point out the location of the more likely potential marine source rocks.

Exercise n°17

On this seismic line the influence of the salt tectonic on deformation of the original geometrical relationships is clearly denoted. Original onlaps became progressively apparent downlap as the salt withdraw. In this sense, the westward thickening of the green stratigraphic interval, i.e. in opposite direction of the salt flowage, is meaningful. The salt withdraw created a confined accommodation which gave birth not only to the depocenter on the right end of the line, but to the yellow depocenter as well. All unconformities, but the upper one, were deformed by the salt movements (halokinesis). All observed tectonic inversions are extensional and associated with the salt tectonics.

Exercise n°18

This seismic line characterizes the proximal area of the offshore Angola, nearby the Ambriz structural high. Around 10 kilometres landward of the end of this line the basement outcrops. The geometrical relationships and the internal configuration of the lowermost stratigraphic interval strongly suggests a salt weld at its base. We can say that the geometrical relationships linked to this interval were deformed by the flowage of the evaporites which were initially underlying. The green intervals represent the transgressive phase of the post-Pangea continental encroachment cycle. However, their retrogradational geometry is not too obvious. On the contrary, the progradational, or forestepping, geometry of the sediments linked to the regressive phase is easily recognized, as well as the basal deep water sediments. The occurrence of an onlap filling geometry downward and basinward of the progradation geometry, shows the importance of the by-pass geological event during the regressive phase.

Exercise n°19

This seismic line clearly illustrates the compressional tectonic regime of the back-arc basins. Actually, these basins are located within the megasuture, where the compressional tectonic regimes are predominant. Taking into account the tectonic inversions, the majority of the geometrical relationships are deformed and some of them are apparent. The major unconformities are angular, and the erosional hiatuses are consequential. They suggest the shortening of the sediments was not a single and instantaneous tectonic episode, but a continuous event with several paroxysmal phases. The last paroxysmal phase can be roughly dated by the age of the overlying sediments. It is important to note the age of an unconformity is given by the age of the linked basin floor fans, i.e. where the hiatus is minor. The normal faults, bordering the rift-type basins created during the rifting phase of the back-arc basin, were reactivated with a reverse movement during the compressional regimes. The tectonic inversion was big enough to fully change the old geometry of the normal faults; in fact any null point is discernible along the fault planes.

Exercise n°20

The sedimentary shortening of the back-arc basins of SE Asia is clearly illustrated on this line. The shortening is here done by thrust-faults with opposite vergence which create typical "fish tail" structure or triangular zones. The majority of these thrust-faults match very often over-pressure horizons, which act as décollement surfaces. On the other hand, it is important to note that the basement not involved. One of the characteristics of this style of shortening is the simplicity of superficial structures and the complexity of the deeper structures. So, on this line, in surface, the shortening was done by a nice symmetric cylindrical anticline with linked uplift, whereas in depth, due to volume problems, it can just be effective by reverse faulting and detachment above an infrastructure or the basement.

Exercise n°21

On this line from the offshore north of Brazil, are illustrated the confined compressional structures developed on the upper-middle continental slope. This structures are associated with a compressional tectonic regime created as a response to the extensional regime developed up-dip. In fact, the faults are listric, i.e. they have a normal movement up-dip and a reverse down-dip, their movement is similar to the movement of a shovel (listron in Greek).

Exercise n°22

This line illustrates the foredeep basin of Maturin (onshore Venezuela). Actually, above a Paleozoic or a Precambrian substratum, it is easy to recognize: (i) several rift-type basin, (ii) divergent margin and (iii) a foredeep basin. In the beginning, during the type-rift basins, the subsidence is differential, then, during the margin, it is thermal, and finally, during the foredeep basin, it is flexural (after 10). In the divergent margin still is possible to identify the aggradational, backstepping, transgressive phase, and the forestepping, progradtional regressive phase. The famous source rocks of Venezuela, i.e. La Luna formation, are liked to the maximum flooding surface of Middle Cretaceous (M.F.S. 91.5 Ma), which separates the transgressive and the regressive phases. This MFS is easily recognized by the abrupt change in geometry of the chronostratigraphic lines.