Seismic Sequence Stratigraphy(From Sloss to Vail)

1) Krumbein and Sloss: Stratigraphy and sedimentation. 2) Andrew D. Mail: The Geology of Stratigraphic sequence. 3) Hakuyu Okada: The evolution of Sedimentology

SEQUENCE STRATIGRAPHY

 Deals with the genetic interpretation of lateral and vertical facies changes of a basin fill in a chronostratigraphic framework.    ‘A depositional sequence is a stratigraphic unit composed of a relatively conformable succession of genetically related strata bounded at its top and base by unconformities or their correlative conformities’ ( Mitchum & Vail,1977)

 

CONCEPTUAL CONTRAST BETWEEN SEQUENCE CONCEPTUAL CONTRAST BETWEEN SEQUENCE STRATIGRAPHY & LITHOSTRATIGRAPHYSTRATIGRAPHY & LITHOSTRATIGRAPHY

Using the center of the marine basin as the point of reference, any change that shifts the boundary between marine and nonmarine deposition, or the boundary between deposition and erosion, outward from the basin center is a process of transgression. The converse is regression. 

Changes in sea-level or base level cause marked displacement of sedimentary environment and the deposits formed within them. The effects of these displacements are most distinct near the basin margins. 

Transgression and Regression

Units A to I are time stratigraphic units identified by well established biostratigraphic zones, sandstones, shales and limestones represent strandline, nearshore and offshore environments respectively offlap relationships and F – I represents transgression and overlap relationship.

Sr. No.

Overlap Sr. No.

Offlap

01. Displacement of nearshore environments progressively away from marine point of reference.

01. Displacement towards marine point of reference.

02. Offshore lithosome overlie nearshore (fine grained above coarse)

02. Nearshore lithosome overlie offshore.

03. Lithosome becomes younger away from marine point of reference.

03. Lithosome becomes younger towards marine point of reference.

04. Depositional limits successively more distant from marine point of reference.

04. Depositional limits successively closer to marine point of reference.

05. Covered and protected from erosion by younger units.

05. Exposed to erosion, and commonly not preserved.

06. Pinchout common. 06. Truncation common.

Most long term changes in sea level that have produced sequences separated by unconformities have resulted from changes in the rate at which new lithosphere has formed along mid ocean ridges. Mid ocean ridges are great swelling of the seafloor where new lithosphere rises up and remain swollen from heat rising within it. The lithosphere cools and shrinks as it moves away from the ridge axis. Consequently, the seafloor descends on either side of a ridge. Lithospheric plates have been more active at some times than others in the course of Earth’s history. At times of intense plate tectonic activity, rates of spreading tended to be high and so much heat has flowed to the ridges that individual ridges have stood relatively tall. The large total volume of ridges has displaced ocean water, pushing sea level upward and causing broad continental areas to flood. At such times, marine deposition on continents has formed sequences. When plate tectonic activity has become less intense again, total ridge volume shrunk over millions of years and sea level has declined correspondingly. Unconformities have then formed as seas have receded from continents, producing sequence boundaries.

Changes in the volume of mid oceanic ridges have moved sea level upland down at rates on the order of ten meters per million years.

Expansion and contraction of continental glaciers have caused much more rapid and dramatic changes in sea level. Many times during Ice Age, sea level has fallen by as much as 100 meters within few thousand years when glaciers have expanded over the land, “locking up” water and thus removing it from global water cycle.

The advent of high quality offshore seismic data led to a new way of looking at sedimentary sequences, termed Sequence Stratigraphy. The seismic sequence stratigraphic analysis is carried out in logical series of steps.

The fundamental unit of Sequence Stratigraphy is the depositional sequence. This is defined as “a stratigraphic unit composed of a relatively conformable succession of genetically related strata bounded at its top & base by unconformities or their correlative conformities”, (Vail et. al, 1977)

In order to construct stratigraphy based on acoustic data, key reflector structures showing the successional relationships must be recognized on seismic reflection profiles. This is a fundamental requirement for establishing a stratigraphy based on the contact relationships between strata as seismic reflectors.  

Seismic reflection profile data contain details of bedding surfaces, unconformity surfaces, and the continuity of lithofacies, bedding and bed thickness. The basic patterns of these features are shown by the continuity of reflection and the boundary shapes of each reflection surface.  

Well stratified sediments show a continuous reflection with well-traceable parallel structures, where as massive sediments show chaotic or contorted reflections and / or no reflections. The basic styles of reflections are represented mainly by onlap, downlap, toplap and offlap.

develop during transition from onlap to offlap. They typically develop above flooding surfaces, as basin margin depositional systems begins to prograde seaward. The dipping, prograding unit is called clinoforms, & they lapout down ward onto the downlap surface as lateral progradation takes place takes place. The word lapout is used as a general term for all these types of stratigraphic terminations. The broad characterization of the stratigraphic units may be determined from their seismic facies. The styles of seismic facies reflection patterns are best seen in sections parallel to depositional dip.

The boundaries of depositional sequences may be associated with onlap, toplap, downlap or truncation. The onlap takes place at the base of the succession, & the offlap occurs at or near the top. These architectural characteristics record the lateral shift in depositional environments in response to base level change & subsidence downlap surfaces

Onlap is the contact relation between the bottom profile of a basin and the horizontal basin-filling strata. The younger layers successively extend more widely than the distribution of the lower layers. Such a structure is characteristic of the lower surface of layers of basin-fill sediments. It is also important to accept that onlap reflects a relative rise of sea level.

Downlap is the style where the down-end of inclined strata contacts the horizontal surface of older sediments, showing a convergent relation between the progradational strata and the basement.

Toplap is a style in which the uppermost part of inclined strata is suddenly cut and covered by horizontal strata. This is the structure of unconformity. Toplap follows minor erosion or non-deposition and major erosion with large relief giving an erosional truncation. Offlap is a pattern of reflectors where the upper end of inclined strata migrates towards the central part of a basin and younger beds are, therefore, progressively further away from the basin margin. This phenomenon suggests a regression with lowering sea-level.

Parallel or subparallel reflections indicate uniform rates of deposition. Divergent reflections result from differential subsidence rates such as in a half graben or across a shelf margin hinge zone. Prograding clinoform reflection are particularly common on continental margins, where they reflect deltaic or continental slope outgrowth. Variations in the patterns of progradation reflect different combinations of depositional energy, subsidence rates, sediment supply, water depth & sea level position. Sigmoidal clinoform have low depositional dip, (less than 10) whereas oblique clinoforms may show depositional dips upto 100.

Parallel oblique clinoform show no topsets. This implies shallow water depths. Complex sigmoid oblique clinoform patterns implies periods of sea level. Still stand with the development of truncated topsets alternating with periods of sea-level rise, Hummocky clinoform patterns are generally considered as strata forming small, interfingering clinoform lobes building into shallow water such as offlapping lobes of delta undergoing distributary switching. Shingled clinoforms reflect offlaping sediment bodies on continental shelf. Chaotic reflection may reflect slumped or contorted sediment masses or those with abundant channels or cut & fill structures. Disrupted reflection are usually caused by faults. A marine flooding surface is a surface that separates older from younger strata, across which there is evidence of abrupt increase in water depth. These are readily recognizable in the stratigraphic record.

The term parasequence encompass “a relatively conformable succession of genetically related beds bounded by marine flooding surfaces & their correlative surfaces. Parasequences are progradational & therefore the beds within parasequences shoal upward.” A Depositional System is “a three dimensional assemblage of lithofacies, genetically linked by active (modern) or inferred (ancient) processes & environments.” A System Tract is “a linkage of contemporaneous depositional system.” Each is defined objectively by stratal geometries at bounding surfaces, position within the sequence, & internal parasequence stacking pattern. Each is interpreted to be associated with a specific segment of the eustatic curve. (i.e. eustatic lowland).

The Exon sequence model contains four basic system tracts.  

The Low Stand System Tract (LST) develops on the continental slope & basin floor at times of low relative sea level. It may contain several components, including a) slope fans b) basin floor fans c) lowstand wedge consisting of aggradational fill of incised valleys & progradational wedge which may downlap onto the basin floor fan.

The top of the low stand system tract is marked by transgressive surface, above which is a Transgressive System Tract (TST). This may be thin succession of marine shales, a basin floor gravel lap, or retrogradational succession of shelf deposits, including marine shale & sandstone or platform (subtidal - supritidal) carbonates.

The top of the transgressive system tract correspond to the maximum flooding surface, above which is a Highstand Systems Tract (HST). This forms the top of the stratigraphic sequence. It consist of shelf to nonmarine, deposits arranged in successive facies succession. Clinoform architectures are characteristics.  

Shelf Margin Systems Tract (SMST) may be deposited at times of slow fall in relative sea level, when sea level does not drop below the edge of the continental shelf. It consists of shelf & slope clastics or carbonates arranged in progradational geometries & bounded at the top by transgressive surface.

The sequence boundaries are drawn at the unconformity surfaces. Two types of unconformity are recognized. 

A Type 1 unconformity develops where sea level fall is rapid, more rapid than tectonic subsidence. The coastline may move out to near the shelf sedge, the development of incised fluival valleys on the shelf takes place, clastic detritus is transported down these fluvial systems to the base of the continental slope, forming lowstand system tracts. High stand deposits below the unconformity may be deeply eroded.

A Type 2 unconformity develops when relative sea level falls slowly, resulting a gradual seaward shifting of facies tract, but only minor subaerial exposure & erosion. A shelf margin system tract is formed under these conditions. Type 2 unconformity are more difficult to identify as they are not characterized by deep erosion or major facies shift.

According to Vail et. al (1977) cycles of sea level change, can be correlated around the world, indicating that they are not a response to local tectonic events but the result of global or eustatic sea level changes.

According to them seismic surveys can be used to erect a sequence Stratigraphy with a hierarchy of cycles on four different scales & these cycles can be correlated universally because seismic reflectors are chronostratigraphic horizons.

The first order cycles are believed to have a span of at least 50 m.y., & to be driven by break up of continental plates. It is a result of assembly of supercontinents by sea floor spreading & their subsequent rifting and dispersal. The complete cycle takes 200 – 500 m.y. The second order cycles, with span of 3 to 50 m.y. are also believed tobe driven by plate movements. They are attributed to volume changes in oceanic spreading centers. The third order cycle are on a scale of 5,00,000 (0.5 m.y.) years to 3 m.y. & are believed to be driven by long term tectonic processes, & short term climatic changes. These are the parasequences, are believed to span 10,000 to 50,000 years & are supposed to be driven by climatic cyclic events.

Some of the fundamental assumptions of sequence Stratigraphy are open to question, notably the tetra cyclic nature of global transgressions and regressions, and the synchronicity of seismic reflectors. The main contribution of sequence Stratigraphy to petroleum exploration is that it has drawn attention to the fact that high stands of the sea favor the deposition of source rocks and drops in sea level leads to transport of sand off the continental shelf, to be deposited as submarine fans on the basin floor.

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