Sequence Stratigraphy

West Siberia Examples

Sequence stratigraphy highlights the role of allogenic (or external) controls on patterns of deposition, as opposed to authogenic controls that operate within depositional environments.

Eustasy (changes in sea level)

Subsidence (changes in basin tectonics)

Sediment supply (changes in climate and hinterland tectonics)

Accommodation refers to the space available for deposition (closely connected to relative sea level in shallow marine environments);however, application of this concept to subaerial environments is problematic.

An increase of accommodation is necessary to build and preserve a thick stratigraphic succession; this requires eustatic sea-level rise and/or basin subsidence (i.e., relative sea-level rise), as well as sufficient sediment supply.

The subtle balance between relative sea-level change and sediment supply controls whether aggradation, regression (progradation), forced regression, or transgression (retrogradation) will occur.

Sequence stratigraphy is based on the application of the systematic subdivision of the section by well defined surfaces.

These surfaces are used to provide a frame work to the interpretation of the depositional settings of the sedimentary section.

This interpretation is then is used to predict the extent and character of the component sedimentary facies.

Schematic Aggradational Facies Geometries Schematic Progradational (Regression) Facies Geometries
Schematic Forced Regression Facies Geometries Schematic Transgressional Facies Geometries

 


Depositional Sequences and Sequence Stratigraphic Surfaces

A depositional sequence is a stratigraphic unit bounded at its top and base by unconformities or their correlative conformities, and typically embodies a continuum of depositional environments, from updip (continental) to downdip (deep marine).

A relative sea-level fall on the order of tens of meters or more will lead to a basinward shift of the shoreline and an associated basinward shift of depositional environments; commonly (but not always) this will be accompanied by subaerial exposure, erosion, and formation of a widespread unconformity known as a sequence boundary.

Sequence boundaries are the key stratigraphic surfaces that separate successive sequences.

Sequence stratigraphy is based on the application of the systematic subdivision of the section by well defined surfaces.

These surfaces are used to provide a frame work to the interpretation of the depositional settings of the sedimentary section.

This interpretation is then is used to predict the extent and character of the component sedimentary facies.

Seismic reflectors are often believed to approximate isochronous surfaces that may be relevant in a sequence-stratigraphic context.

 

   

 

Geometric relationships of Seismic reflectors

 


Sequence Boundary (SB)

Envelope Sequence

Significant erosional unconformity & correlative disconformity formed by drop in base level (often, but not always,sea level).

Erodes subaerially exposed sediment surface of earlier sequence or sequences.

Diachronous boundary between underlying Highstand System Tract (HST) & overlying Falling Stage System Tract (FSST) or Early Lowstand System Tract (ELST).

Erode surface of downstepping sediments deposited during accompanying forced regression associated with sea level fall.

 

Characteristics of Sequence Boundary (SB) from Seismic

Defined by erosion or truncation of underlying reflectors or the correlative conformity.

Can be inferred from onlapping reflectors overlying a surface.

Should the upper surface of a Falling Stage System Tract be eroded when a shoreline is forced seaward (a forced regression!), by a drop in sea level (or base level), then the interpretation of a sequence boundary is ambiguous.

Characteristics of Sequence Boundary (SB) from seismic

 

Characteristics of Sequence Boundary (SB) from Well Logs, Core and Outcrop

Defined by erosion or incision of underlying flooding surfaces (mfs and TS).

Inferred from interruption in the lateral continuity of these surfaces.

 


Transgressive Surface (TS)

Product of rise in base level (sea level?)

Lies at boundary between underlying Late Lowstand System Tract (LLST) and the overlying Transgressive System Tract (TST).

Marine-flooding surface that forms first significant flooding surface in a sequence - often marking base of most prominent onlap.

Formed when rate of creation of accommodation space is greater that the rate of sediment supply.

In rimmed carbonate platforms, the rate of sediment supply may keep pace with the rate of relative sea-level rise and thus TS marks change from a progradational to an aggradational parasequence stacking patterns.

 

Characteristics of Transgressive Surface (TS) from Well Logs, Core and Outcrop

 

 


Maximum Flooding Surface (mfs)

 

 

 

 

Characteristics of Maximum Flooding Surface (mfs) from Seismic

Often expressed as a downlap surface

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Characteristics of Maximum Flooding Surfaces from Well Logs, Core and Outcrops

Defined by organic, often radioactive (big kicks on gamma ray logs), black shales.

Inferred from presence of finest grain size.

Inferred from presence of condensed faunal association.

Makes up condensed section and are usually thin.

Marine shelf and basinal sediments associated with this surface are the product of slow rates of deposition of pelagic-hemipelagic sediments.

Not infrequently overlain by coarser sediments (often sand sized)

Product of maximum flooding or transgression of shelf or stillstand in base level (sea level?).

Lies at the boundary between the underlying Transgressive System Tract (TST) and the overlying High stand System Tract (HST).

Often expressed as a downlap surface

 

 


Systems Tracts and Parasequences Systems Tracts and Parasequences

Systems tracts are the building blocks of sequences, and different types of systems tracts represent different limbs of a relative sea-level curve:

Falling-stage (forced regressive) systems tract

Lowstand systems tract (LST) Transgressive systems tract (TST)

Highstand systems tract (HST)

The various systems tracts are characterized by their position within a sequence, by shallowing or deepening upward facies successions, or by parasequence stacking patterns.

Parasequences are lower order stratal units separated by (marine) flooding surfaces; they are commonly autogenic and not necessarily the result of smaller- scale relative sea-level fluctuations.

 

Effects of Base-Level Fluctuation

Generally, relative sea-level fall leads to reduced deposition and formation of sequence boundaries in updip areas, and increased deposition in downdip settings (e.g., submarine fans).

Relative sea-level rise will lead to trapping of sediment in the updip areas (e.g., coastal plains) and reduced transfer of sediment to the deep sea.

Clastic Environments

Relative sea-level fall in clastic environments commonly leads to fluvial incision into offshore (shelf) deposits, usually associated with soil formation (paleovalleys with interfluves).

Relative sea-level rise causes filling of paleovalleys, commonly with estuarine or even shallow marine deposits.

Submarine fans and associated high aggradation rates in the deep sea occur especially during late highstand and lowstand, when sediments are less easily trapped updip of the shelf break.

Carbonate Environments

Relative sea-level fall in carbonate environments can lead to the development of karstic surfaces (dissolution of limestones) or evaporites (e.g., sabkhas), depending on the climate.

Highstands generally expand the area of the carbonate factory (drowning of shelves) and vertical construction of reefs, as well as accumulation of other carbonates is enhanced.

Extreme rates of relative sea-level rise can lead to the drowning of carbonate platforms.

 

Idealised Parasequence stacking with changing relative sea-level

 

Idealised Cross-section of System Tracts

 

 

 

 

 

Clastic Sequence Stratigraphic Hierarchies

 

Clastic Sequence Stratigraphic Hierarchies

 

Clastic Sequence Stratigraphic Hierarchies

 

Clastic Sequence Stratigraphic Hierarchies

 

Fluvial Shallow-Marine Deep Marine

 

Idealised Facies Sequences (Clastic) : Littoral/Shore-face a) Regression b) Transgression

 


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Material from a number of sources, published on the WWW.

Kendall, U. of South Carolina

Mitchum et al, 1977

Tornquist, U. of Illinois, Chicago