SystStratSeminar2

Universidade Fernando Pessoa

Porto, Portugal

Systemic Stratigraphy Seminar

Depositional Model

In the sand-shale depositional model illustrated herebelow, P. Vail and coauthors (1977) considered sedimentary building blocks which they termed sequence or sequence cycles:

"succession of genetically related strata (or associated seismic reflections) bounded by unconformities or their correlative conformities deposited during a 3rd order cycle of sea level change between two relative falls of sea level (Mtchum et al., 1977)"

Fig. 1.1- This depositional model was proposed by P. Vail and co-authors for sand-shale facies. Three sedimentary intervals bounded by unconformities are easily recognized. They have been named sequence cycles, i.e. stratigraphic cycles associated with 3rd order eustatic cycles. The intermediate interval (6 to 21) comprises all intervening strata. The others are incomplete. In the lower interval (1-5), only the regressive highstand deposits (highstand sysytems tract, HST) are present. On the contrary, in the upper interval (22-29), the regressive lowstand deposits (lowstand prograding wedge (LPW) ofthe lowstand systems tract (LST)and the transgressive deposits of the transgressive systems tract (TST) are present.

In this model, Exxon's explorationists assumed:

1- Eustasy is the main factor driving the cyclicity of sediment deposits.

2- Subsidence and Terrigeneous Influx rates are smaller than Sealevel Changes, i.e. Eustasy.

3- Eustasy, Subsidence, Accommodation, Terrigeneous Influx and Climate are the major geological parameter affecting the stratal patterns.

4- Terrigeneous Influx is constant in Time and Space.

5- Subsidence increases gradually and linearly basinward.

6- Sedimentary intervals have high completeness.

7- There is no erosion during relative sealevel falls.

8- The time interval between each chronostratigraphic line is 100 k years.

9- In geological time, the sedimentary building blocks, i.e. the sequence cycles are instantaneous and catastrophic depositional events.Their time interval ranges between 0.5 Ma and 3.0 M years.

-In mathemically terms, an instantaneous geologic event, in the Phanerozoic time scale (600 M y), represents a time change of 1/100 of the total time, i.e. more or less 6 M years.

This model has been criticized by several geologists. They think this model is too theorethical but also build-up with too much geological assumptions. Nevertheless, as suggested by P. Bak, we can say that to look for details is fascinating and interesting but unfortunately we learn mostly from generalities.

Stratal Implications

a) Stratal surfaces typically represent a relatively small time-gap.

b) If the time-gap (hiatus) is large, the surface is called unconformity.

- The term large is relative. It depends on the time scale considered.

- The term unconformity is mainly used when the time-gap corresponds to an erosional hiatus

- A time-gap associated to a non-depositional hiatus is often termed disconformity.

c) Stratal surfaces may represent different amounts of time from place to place, i.e. the hiatus changes laterally.

d) Stratal surfaces represent at least some small unit of time common to the surface over its entire extent.

e) The concept of stratal surfaces is completely dependent on the time scale and rock under consideration.

Geometrical Relationships

Three major geometrical relationships between stratal surfaces, or associated seismic reflections, can be recognized:

(i) Onlap........ (ii) Toplap.......... (iii) Downlap

Onlap

An onlap can be defined as a base-discordant relation in which initially (deposition-time) horizontal strata terminate progressively against an initially incllined surface, or in whixh initially inclined strata terminate progressively updip against a surface of greater initial inclination.

Onlap can be:

- Proximal,

when it corresponds to a landward onlap.

- Coastal,

when it concernes coastal plain sediments.

- Marine,

when it concerns marine sediments.

- Apparent,

when it is induced by tectonics (ex: tilted downlap).

Onlapping sediments are characterized by:

- Aggradation,

which corresponds to the vertical component between two consecutive onlap.

- Encroachment

which corresponds to the horizontal component between two consecutive onlap.

Fig. 2.2- The seismic lines can be interpreted in geological terms since we assume that seismic reflectors correspond roughly to chronostratigraphic lines, i. e. stratal surfaces. So, on this seimic line of North Sea, cratonic sediments are onlappping underlying type-rift basin sediments. These geometrical relationships define an onlap surface which fossilizes the unconformity (erosional surface) between the type-rift basin and the cratonic basin. Taking into account the amplitude of the aggradation and the encroachment of the onlapping sediments, these cratonic sediments were deposited under important water depth in association with gravity currents (turbidites).

Toplap

Termination of strata against an overlying surface, i.e. it occurs along the upper interval boundary. The term toplap is used when it results of non-deposition process (sedimentary bypass) with only minor erosion. When this geometrical relationship is induce by erosion we will call it truncation.

Fig. 2.3- On the right part of this seismic line, between 2.0 and 2.7 seconds, a progradational interval is easily recognized. This configuration is composed by a succession of seismic reflectors (chronostratigraphic lines) which terminate against an overlying surface with minor erosion. Such a geometrical relationship, named toplap, indicates that the associated strata lap out landward. Seaward, i.e. downdip, the oblique reflectors flatten and become horizontal defining what geologists call an apparent downlap. The time difference between the toplap and the apparent downlap (roughly 0.2 milliseconds) suggests the oblique reflectors are associated with a deltaic slope, i.e. a prodelta.

Toplap implies that each strata laps out in a landward direction at the top of the unit and the successive terminations progress seaward. On contrary, truncated strata can lap out in any direction.

In addition, downdip of a toplap there is always a downlap or an apparent downlap. Toplap is mainly associated with coastal deposits (coastal toplaps).

Truncation:

Truncation (fig. 2.4) corresponds to a termination of strata, or seismic reflection interpreted as strata, along an unconformity surface due to post-depositional erosional.

Truncation as toplap occurs along the upper interval boundary.

Truncation implies deposition of strata and their subsequent removal to form an unconformity surface.

Fig.2.4- On this seismic line we can recognize two interval. The upper one is composed by parallel reflectors slightly tilted westward. The lower one is also composed by parallel reflectors that they have a much higher westward dip. The strata associated with the lower interval were tilted and then truncated. The time-gap between these two sedimentary intervals is an erosional hiatus.

Downlap

Downlap is a base discordant relation in which initially inclined strata terminated downdip against an initially horizontal or inclined surface (fig. 2.5).

A downlap geometrical relationship indicates the direction away from the source of clastic supply.

A seismic downlap is a downlap interpreted on a seismic section, i.e. a relation in which a seismic reflection interpreted as initially inclined strata terminates downdip against a reflection discontinuity interpreted as an initially inclined or horizontal sedimentary discontinuity.

A sedimentary discontinuity can be:

- a flooding surface,

- a complex transgressive backstepping surface,

- an unconformity or

- paraconformity (conformity with an important hiatus)

There are two kind of apparent downlap:

(i) Those created by tectonics (particularly by halokinesis or shalokinesis).

- They correspond very often to tilted onlap due the salt or shale flowage.

(ii) Those induced by seismic resolution.

- Seismic reflections, representing units of inclined or tangential strata, can terminate downdip, but the strata themselves actually flatten and continue as units, which are so thin that they fall below the seismic resolution.

Fig. 2.5- On this seismic line, some oblique reflector terminate in downlap and other in apparent downlap. These geometrical relationships indicate the seaward direction and the subsequently the direction away from the source terrigeneous influx. In this example, these geometrical relationships are associated with the toe of continental slope progradation, since the amplitude of the oblique reflectors preclude deltaic deposits. The downdip discontinuities are unconformities or their correlative conformities (sedimentary cycle boundaries).

Stratigraphic Surfaces & Stratigraphic Boundaries

A stratigraphic surface can be considered as a continuous physical boundary. At least three major groups of stratigraphic surfaces can be observed in the field or on seismic data:

a) Stratal Surface

Bedding planes and chronostratigraphic seismic reflectors are stratal surfaces.

- A seismic reflector represents a more or less thick sedimentary interval. The thickness of this interval depends on seismic resolution; it can ranges between 10 and 100 meters.

b) Discontinuity Surface

Discontinuity surfaces are physical surfaces caused by erosion or by non deposition.

They include:

(i) Unconformities;

They are caused by erosion.

(ii) Paraconformities;

They separate parallel strata with an hiatus of non-deposition.

(iii) Depositional hiatuses;

They can be defined by:

a) Toplap/Downlap............................Sub-aerial/Sub-aqueous

b) Downlap/App. Truncation.........Sub-aqueous/Sub-aqueous

c) Onlap/Conformable......................Sub-aqueous/Sub-aqueous

c) Diachronous Surface

In this group are included the retrogradational transgressive (backstepping) surfaces and on seismic data, the reflectors associated with gas/oil-water plane contact ("bright-spots).

Stratigraphic boundary which separate rocks of significantly different environments or lithology, can be:

(i) Synchronous, i.e. parallel to stratal surfaces.

(ii) Diachronous, i.e. step across stratal surfaces.

Fig. 2.6- On this line, all groups of discontinuity surfaces can be recognized: unconformities, paraconformites and depositional hiatus. Try to recognize them and characterize them using geometrical relationships.

Unconformities

Unconformities are discontinuous stratigraphic surfaces.

Fig. 2.7- On this seismic line several unconformities can be identified. They truncate the underlying sediments and they are fossilized by the onlaps of the overlyng strata. In addition, two major downlap surface are easily recognized.

Unconformities time-gaps may represent:

a) simply prolonged periods of subaerial exposures with minimal erosion, possibly with local valley or channel downcutting,

b) periods of uplift and major subaerial erosion of strata,

c) submarine erosion by turbidites, slump or submarine currents.

Submarine erosion induced by gravity or turbiditic currents is generally relativelly local. The associated unconformities with such an erosion are mainly located in deep water environments (slope and abyssal plain). These unconformities do not have equivalent in shallow water environments. Consequently, they are not boundaries stratigraphic cycles boundaries but when the cycles are incomplete, i.e. only composed by deep water strata.

Uplif and Erosion are characterized by:

(i) Onlap above the unconformity and

(ii) Truncation below the unconformity.

Valley or Channel are also characterized by:

(i) Onlap above the unconformity and

(ii) Truncation below the unconformity.

Very often, geologists confound valley with valley fill and channel with channel fill:

- A valley is a morphology which corresponds to any low-lying land bordered by higher ground.

- A valley fill, or valley infilling, name the more or less unconsolidated sediment deposited by any agent so as to fill or partially fill the valley.

- A channel is the bed where a natural body of water flows or may flow.

- A channel fill or channel inffilling name the sediments deposited in a stream channel especially in an abandoned cutoff channel or where the transporting capacity of the stram is insufficient to remove material supplied to it (Bates & Jackson, in Glossary of Geology, Second Edition, American Geological Institut, 1980).

Depositional Coastal Break

Fig. 2.8- The displacement of the depositional coastal break is clearly illustrated by the red dots. Relative sealevel falls create downward and basinward shifts of the coastal break which emphasize the unconformities bounding the sequence cycles. There are two basinward shifts of the coastal break: (i) between the chronostratigraphic lines 5 and 6, and between the lines 21 and 22.

The Depositional Coastal Break or Depositional Shoreline Break corresponds to the lower level of erosion of the waves when the sea is quite, i.e. around 10-20 meters below sealevel. The coastal break can be correspond also to the shelf break ( limit between the platform and the upper slope. Actually, at the end of a regression the coastal break and the shelf break are coincident, since the basin has not platform. then, as early as a transgression takes place, the coastal break migrates landward creating a platform. The backstepping of the coastal break increases the water depth on platform creating at same time starved conditions on the distal area near the shelf break.

Fig. 2.9- On this seismic line of the offshore Kalimantan, between 3 and 5 seconds, the depositional coastal break and the shelf break are very often coincident. This interval is mainly regressive. It a global forstepping geometry. Actually, the progradation of the depositional coastal break is not continuous. Three downward and basinward shifts of the shelf break induce by relative sealevel falls are clearly recognized. They characterize three unconformities. These unconformities are mainly induced by eustasy since the geometry of the coastal plain is horizontal. On the other hand, between each unconformity, the coastal break (= shelf break) shows progradational (in blue) and retrogradational (in green) displacements .

Landward of the depositional coastal break, all accommodation, i.e. all space available for the sediments is filled. In other words, a relative sea level rise of 15 meters increases the accommodation of 15 meters and 15 meters of sediments will be deposited landward of the depositional coastal break.

Seaward of the depositional coastal break, the space available is partly filled or not filled at all. A sea level rise of 20 meters increases the accommodation of 20 meters. If 5 m of sediments are deposited seaward of the shelf break, the water depth increases 15 meters. We can say that seaward of the coastal break, accommodation is equal to the sediment thickness plus the water depth.

In the same sense, we can say that seaward of the shelf break the accommodation is big enough to allow sedimentation without relative sea level rise, i.e. sedimentation can occur even during relative sealevel fall (ex: turbidites).

The recognition of the downward and basinward shifts of the coastal break, is an important step to the identification of the unconformities. This is particularly true on seismic interpretation. Actually, on seismic lines, landward of the shelf break and on the abyssal plain, the horizontal geometry of the reflectors is not favorable to identify the unconformities. Practically, the unconformities are recognized nearby the shelf break, by the downward and basinward shifts, and then extrapolated landwards and seaward.

Paleo-Water Depth

Taking into account that landward of the depositional coastal break, which, as we said before, can coincide with the shelf break (when the basin does not have platform), all accommodation is filled by sediments, the water depth is zero. Thus, seaward of the coastal break, the paleo-water depth along the chronostratigraphic lines can be calculated using the coastal plain are datum.

Fig. 2.10- On a chronostratigraphic line, the proximal break marks the depositional coastal break whereas the distal indicates the toe of the continental slope. Landward of the shelf break, the water depth is zero. Taking the coastal plain as reference, it is possible to calculated the paleo-water depth seaward of the shelf break.

Fig. 2.11- In this seismic example, the paleo-water depth of the continental slope can be calculated (t.w.t.), since the depositional coastal break is easily recognized. Two paleo-water depth, 0.2 and 0.4 seconds (t.w.t.) were calculated.

Accommodation

Accommodation is the space available to the sediments. Variation in sedimentary accommodation reflect the space made available for sediment accumulation through a combination of subsidence and sealevel rise or fall.

Whether this space is entirely filled depends on the rate of sediment supply to the basin and where the space avalaible is related to the depositional coastal break.

The key to sequence stratigraphic interpretation of well data lies in the applicationof the accommodation concept to the interpretation of depositional environments.

Fig. 2.12- This figure shows that the space available for sediments is completely fill (if the sediment supply is big enough) landward of the depositional shelf break. However, seaward of the shelf break, the new space created is equal to the sediment filling plus the water depth.

P. Vail identifies the variations in sedimentary accommodation in several steps:

1) Locate the marine shale wedge:

- In siliciclastic section this wedge typically occurs on the shelf, where it is overlain and underlain by coarser sediments and extents laterally into the slope and basin.

- In carbonate section, this wedge may be shales, marls or calcareous muds, and it is typically overlain by coarser clastic carbonates or by evaporites and underlain by transgressive carbonates.

2) In sediments deposited on the shelf, locate overall fining and thinning upward pattern at the base of the marine shale wedge:

- In siliciclastic these often form a bell shaped log pattern and reflect the retrogradation of successive parasequences as rapid sealevel rises forces the shoreline away from the basin and across the shelf.

- In carbonates, these variations may be less visible on logs and may require sample and core studies to recognize the fining upward pattern and the increase in depositional water depth.

3) In sediments deposited on the shelf, locate the overall coarsening and thickening upward patterns at the top of the marine shale wedge:

- In siliciclastic this often forms a funnel shaped pattern at the top of the marine shale wedge, representing progressive progradation of delta-front sands, followed by deltaic and fluvial sand-shale sections representing decreasing marine conditions.

- Similar shallowing upwards facies pattern can be observed in carbonates trough core and sample studies, aided by biostratigraphy.

Bayline

The bayline is the line of demarcation between the areas characterized by subaerial and marine acommodation, i.e. between the alluvial plain and coastal plain, or between fluvial and marine sediments. Note that coastal plain sediments are considered as marin deposits.

Fig. 2.12- The bayline is the frontier between fluvial (alluvial plain) and marine sedimentation (coastal plain).

Fig. 2.13- The displacements of the depositional coastal break are easily recognized. On the lower part of the section, the depositional coastal break coincide with the shelf break. On the upper part, following a RSL rise, the depositional coastal break is individualized from the shelf break and displaced progressively landward. The break on the unconformity (in red) which limits the regressive from the transgressive interval, is the bayline.

Fig. 2.13- On this line it is easy to recognize several breaks along an unconformity. The first break (near the continent) is associated to the bayline, the second one corresponds to the depositional coastal break, whereas the third corresponds to the shelf break.

Sealevel Curve versus Tectonic Signal

Eustatic signals can be recognized as worldwide events. The amplitude of the events cannot be determined, but assuming an eustatic sealevel behavior, then tectonic tectonic behavior can be predicted or vice-versa.

Fig. 2.14- Geological section used to test the role of the sealevel curve and the tectonic signal.

The simulations results highlight the importance of taking care in selecting the sealevel model to be used in the simulation. At the same time, it is critical that, once a sealevel model has been selected, no matter what that model, it must be used consistently from basin to basin. Such models can be used to maker extremely accurate predictions of sedimentary geometry.

Fig. 2.15- Simulation avoiding tectonic signal (subsidence), only sealevel changes have been used for sedimentation.

This is because sealevel is only part of the accommodation for sedimentary fill, which also depends upon the residual of tectonics. The amplitude exhibited by sealevel events of Haq and others (1987) are undoubtedly model-dependent, and they may be very wrong or partly wrong.

Fig. 2.15- Simulation avoiding eustatic sealevel, only the tectonic signal (subsidence) was used for sedimentation. This result is very similiar to that obtain on simulation 1.

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