Sequence Stratigraphy(Part-III)
2- SURFACES
i) Sequence boundaries
A sequence boundary is a surface that separates older sequences from younger ones. Commonly it is an unconformity (indicating subaerial exposure), but in limited cases a correlative conformable surface. In other words, the sequences are enveloped by the sequence boundaries (SB). These boundaries are the product of a fall in sea level that erodes the subaerially exposed sediment surface of the earlier sequence or sequences.
Two distinct types of sequence boundary are recognized. Type-1 sequence boundaries equate to those formed during a forced regression whereas Type-2 sequence boundaries are those formed during a normal regression. Type 1 and Type 2 unconformities can bound the same sequence at different localities and are the products of different rates of sedimentation and accommodation space for the same time interval.
ii) Transgressive Surface (TS)
This is a marine-flooding surface that forms the first significant flooding surface in a sequence. In most siliciclastic and some carbonate successions, it marks the onset of the period when the rate of creation of accommodation space is greater than the rate of sediment supply. It forms the base of the retrogradational parasequence stacking patterns of the Transgressive Systems Tract (TST). In areas of high sediment supply, e.g. on rimmed carbonate platforms, the rate of sediment supply may keep pace with the rate of relative sea-level rise and thus the TS will mark a change from a progradational to an aggradational parasequence stacking patterns.
A transgressive surface is often characterized by the presence of a surface marked by consolidated muds of firmgrounds or hardgrounds that are cemented by carbonates. Both surfaces are often penetrated by either burrowing or boring organisms.
If the rate of sediment supply is low over the transgressive surface this may merge landward with the maximum flooding surface. When TS extends over LST valley fill, the response on the resistivity log curve may show a small local increase in resistivity followed by a low value. This increase in resistivity is in response to the carbonate cementation of the hardground, while the low is associated with deposition of transgressive shales.
iii) Maximum Flooding Surface (mfs)
A surface of deposition at the time the shoreline is at its maximum landward position (i.e. the time of maximum transgression. It separates the Transgressive and Highstand Systems Tract. Seismically, it is often expressed as a downlap surface. An mfs is often characterized by the presence of radioactive and often organic rich shales, glauconite, and hardgrounds. An mfs can often be the only portion of a sedimentary cycle, which is rich in fauna.
iv) Marine Flooding Surface
It is a surface separating younger from older strata, across which there is evidence of an abrupt increase in water depth. It may also display evidence of minor submarine erosion. It forms in response to an increase in water depth.
v) Regressive Surface of Erosion
It is a subaqueous surface of marine erosion formed during a relative sea-level fall. As sea level falls, wave base and the upper shoreface zone of current transport drops, too, and planes off the seafloor sediments that formerly lay below wave base and the upper shoreface currents. Such a fall usually is followed by the superposition of coarser-grained upper shoreface deposits sharply overlying finer-grained lower shoreface or shelf deposits associated with the following transgression.
Condensed section
A thin marine stratigraphic interval characterized by very slow depositional rates (<1-10 mm/yr). It consists of fine-grained sedimentary rocks such as hemipelagic and pelagic sediments. These are deposited on the middle to outer shelf, slope, and basin floor during a period of maximum relative sea-level rise and maximum transgression of the shoreline. It first begins to form in more distal slope and basinal environments, and as the shoreline backsteps landward, gradually expands in its coverage to include not only the basin but all of the slope and part of the shelf as well.
Commonly the upper layer of the Transgressive Systems Tract is a condensed section which is associated with the mfs where it is overlain by the downlapping Highstand Systems Tract. Sometimes the transgressive surface marking the base of the Trangressive Systems Tract is immediately overlain by a condensed section that is in turn capped by the mfs.
vi) Ravinement Erosional Surface
It is a time transgressive or diachronous subaqueous erosional surface resulting from nearshore marine and shoreline erosion associated with a sea-level rise. This erosional surface parallels the migration of the shoreface "razor" across previously deposited coastal deposits. Burrows in this surface are often filled by sediments deposited during a sea-level rise.
Ravinement surfaces are commonly ascribed to the transgressive movement of the landward margin of the Transgressive Systems Tract. However, these erosional surfaces tend to occur
wherever the landward edge of the sea rises over an underlying sedimentary surface. Thus if the Late Lowstand Systems Tract has a subaerial landward margin it will have an updip ravinement surface associated with it.
wherever the landward edge of the sea rises over an underlying sedimentary surface. Thus if the Late Lowstand Systems Tract has a subaerial landward margin it will have an updip ravinement surface associated with it.
In outcrops and wells ravinement surfaces are commonly equated with the transgressive surface. However, as the attached movie shows, these erosional surfaces are time transgressive and tend to occur wherever the landward edge of the sea rises over an underlying sedimentary surface. They only match the transgressive surface when it tops the shelf margin.
vii) Clinoform, Undaform and Fondoform Surfaces
(1) Undaform stands for any surface underlying an unda environment.
(2) Clinoform stands for any surface underlying a clino environment.
(3) Fondoform stands for any surface underlying a fondo environment.
So clinoform surface can be used for the sloping depositional surface that is commonly associated with strata prograding into deep water.
So clinoform surface can be used for the sloping depositional surface that is commonly associated with strata prograding into deep water.
In 1951, it was the proposed that the depositional settings of sediment accumulation on the shelf, slope, and bottom be called as:
(1) Unda for shallow water overlying the shelf,
(2) Clino for the deeper water overlying the slope, and
(3) Fondo for the deepest water covering the bottom of the basin.
Basin-floor Fan
It is a portion of the lowstand Systems Tract characterized by deposition of submarine fans on the lower slope or basin floor. Fan formation is associated with the erosion of canyons into the slope and the incision of fluvial valleys into the shelf. Siliciclastic sediment bypasses the shelf and slope through the valleys and canyons to feed the basin-floor fan. The basin-floor fan may be deposited at the mouth of a canyon or widely separated from it, or a canyon may not be evident.
Hiatus
A cessation in deposition of sediments during which no strata form or an erosional surface forms on the underlying strata; a gap in the rock record. This period might be marked by development of lithified sediment (hardground) or burrowed surface characteristic of periods when sea level was relatively low. A disconformity can result from a hiatus.
Hardground
A horizon cemented by precipitation of calcite just below the sea floor. Local concretions form first in a hardground and can be surrounded by burrows of organisms until the cement is well developed.
Stacking Patterns
Strike
A direction of line formed by the intersection of a plane, such as a dipping bed, with a horizontal surface
Azimuth
The angle between the vertical projection of a line of interest onto a horizontal surface and true north or magnetic north measured in a horizontal plane, typically measured clockwise from north.
Apparent Dip
The angle that a plane makes with the horizontal measured in any randomly oriented section rather than perpendicular to strike.
Two-dimensional (2-D) survey
Seismic data or a group of seismic lines acquired individually such that there are significant gaps (commonly 1 km or more) between adjacent lines. A 2 D survey typically contains numerous lines acquired orthogonally to the strike of geological structures (such as faults and folds) with a minimum of lines acquired parallel to geological structures to allow line-to-line tying of the seismic data and interpretation and mapping of structures.
Pinch out 1
A type of stratigraphic trap. The termination by thinning or tapering out ("pinching out") of a reservoir against a nonporous sealing rock creates a favorable geometry to trap hydrocarbons, particularly if the adjacent sealing rock is a source rock such as shale.
Pinch out 2
A reduction in bed thickness resulting from onlapping stratigraphic sequences
Seismic Stratigraphy
•Seismic stratigraphy is based on the principle that seismic reflectors follow stratal patterns and approximate isochrons (time lines)
•Reflection terminations provide the data used to identify sequence-stratigraphic surfaces, systems tracts, and their internal stacking patterns
•Technological developments have been prolific:
•Vertical resolution improved to a few tens of meters
•Widespread use of 3D seismic
•Seismic data should preferably always be interpreted in conjunction with well log or core data
•A better understanding of stratigraphic sequences can be obtained by the construction of chronostratigraphic charts (‘Wheeler diagrams’); these can subsequently be used to infer coastal-onlap curves
•Variations in sediment supply can produce stratal patterns that are very similar to those formed by RSL change (except for forced regression); in addition, variations in sediment supply can cause stratigraphic surfaces at different locations to be out of phase
•In principle, sequence-stratigraphic concepts could be applied with some modifications to sedimentary successions that are entirely controlled by climate change and/or tectonics (outside the realm of RSL control).
•The global sea-level curve for the Mesozoic and Cenozoic (inferred from coastal-onlap curves) contains first, second, and third-order eustatic cycles that are supposed to be globally synchronous, but it is a highly questionable generalization
•Conceptual problems: spatially variable RSL change due to differential isostatic and tectonic movements undermines the notion of a globally uniform control
•Dating problems: correlation is primarily based on biostratigraphy that typically has a resolving power comparable to the period of third-order cycles
Recommended procedures for performing seismic sequence analysis include:
1. Identifying the unconformities in the area of interest. Unconformities are recognized as surfaces onto which reflectors converge.
2. Mark these terminations with arrows.
3. Draw the unconformity surface between the onlapping and downlapping reflections above; and the truncating and toplapping reflections below.
4. Extend the unconformity surface over the complete section. If the boundary becomes conformable, trace its position across the section by visually correlating the reflections.
5. Continue identifying the unconformities on all the remaining seismic sections for the basin.
6. Make sure the interpretation ties correctly among all the lines.
7. Identify the type of unconformity:
a. Sequence boundary: this is characterized by regional onlap above and truncation below.
b. Downlap surface: this is characterized by regional downlap.
Recommended color codes:
Red: Reflection patterns and reflection terminations.
Green: Downlap surfaces
Blue: Transgressive surfaces
Other colors: Sequence boundaries
If using only black and white:
Thin solid lines ( ) : Reflection patterns
Dashed lines (---------): Downlap surfaces
Dotted lines ( ………. ): Transgressive surfaces
Sequence stratigraphic application to outcrops
Although sequence stratigraphy was originally designed for seismic sections, sequence principles can be readily applied to outcrop, core, and well logs. The first step in this approach is to interpret individual beds in terms of depositional events, including an evaluation of the shear stress in the environment, the type of flow (currents, waves, tides, and combined flow), bioturbation and trace fossils, etc. This information is critical for the next step, to recognize bedsets, that is groups of beds that record similar depositional processes, and to interpret those bedsets as facies, the records of particular depositional environments. These steps are critical because errors at this point may cause errors in interpretations of relative depth, which in turn affects the recognition of parasequences and stacking patterns. Solid facies work is essential for a solid sequence analysis.
From successions of facies in an outcrop, shallowing-upward successions can be recognized as well as flooding surfaces such that parasequences can be delimited. Vertical trends in the range of water depths present in successive parasequences can be used to identify stacking patterns and to recognize surfaces that mark the turnarounds from one parasequence set to the next. Potential sequence boundaries should be identified at this step based on one or more of the following criteria: clearly defined erosional truncation, direct evidence of subaerial exposure, or abrupt basinward shifts of facies. Likewise, potential condensed sections should be recognized on the basis of unusual burrowed surfaces, abundant diagenetic materials, fossil concentrations, closely spaced bentonite beds, or radioactive shales. Condensed sections may, but do not necessarily, lie along the maximum flooding surface.
From the recognition of parasequence sets and potential sequence boundaries and condensed sections, systems tracts and major stratal surfaces (sequence boundary, transgressive surface, and maximum flooding surface) can be recognized. It is important to stress that not all of these surfaces or systems tracts may be present within any given sequence in an outcrop. The absence of one or more surfaces or systems tracts may provide important clues as to the relative position of the outcrop within the basin. For example, lowstand systems tracts are commonly absent in updip areas where the transgressive surface and sequence boundary are merged as one surface. In such areas, significant portions of the highstand systems tract may have eroded away and the sequence boundary is marked by the beginning of retrogradational stacking. In downdip areas, the transgressive and highstand systems tracts may be thin and relatively mud-rich, whereas the lowstand systems tract may be characterized by the abrupt appearance of thick sandy facies. Many more variations are possible and many basins are characterized by a typical pattern of sequence architecture.
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