sequence stratigraphy - principles

Sequence stratigraphy is the subdivision of the stratigraphic record on the basis of bounding discontinuities.

Sequence Stratigraphy

Formal Definitions of a Sequence• A relatively conformable succession of genetically related strata

bounded at their upper surface and base by unconformities and their correlative conformities (Vail, et al., 1977).

• Sequence is composed of a succession of genetically linked deposition systems (systems tracts) and is interpreted to be deposited between eustatic-fall inflection points (Posamentier, et al., 1988).

• Study of rock relationships within a time-stratigraphic framework of repetitive, genetically related strata bounded by surfaces of erosion or non-deposition, or their correlative conformities (Posamentier et al., 1988; Van Wagoner et al., 1988).

• The sequences and the system tracts they enclose are subdivided and/or bounded by a variety of "key" surfaces that bound or envelope these discrete geometric bodies of sediment. They mark changes in depositional regime "thresholds" across that boundary (Kendall).

A depositional sequence is defined as a relatively conformable succession of genetically-related (according to Walther’s Law) strata bound by unconformities and correlative conformities. Boundaries are diachronous, though the sequence represents an isochronous event; therefore, sequences have chronostratigraphic significance.

Depositional Sequence

Depositional Sequence

Photo by W. W. Little

Photo by W. W. Little

Photo by W. W. Little

The stratigraphic record consists various scales of bedding separated by bounding surfaces (discontinuities) that represent “gaps” in the sedimentary record.

Types of Discontinuities

Photo by W. W. Little

Bedding planes are surfaces between beds and bedsets that represent breaks between episodic depositional events, such as floods, storms, and turbidity flows.

Bedding Planes

Photo by W. W. Little

Flooding Surfaces

Flooding surfaces bound parasequences and represent relative rises in base-level. They can be recognized by deeper-water (basinward) facies abruptly overlying shallower-water (landward) facies and often involve shoreface erosion, forming a ravinement surface.

Shallower-water faciesDeeper-water facies

Both transgressive and regressive events can develop erosional surfaces associated with shoreface erosion by wave base.

Shoreface Ravinement Surface

Photo by W. W. Little

Sequence Boundaries

Sequence boundaries are surfaces bounding depositional sequences. Depending upon the relative rate of base-level fall with respect to basin filling, they can be erosional (type 1) or conformable (type 2) and are recognized by placement of more landward facies over more basinward facies.

Basinward facies

Landward facies

Photo by W. W. Little

Photo by W. W. Little

Photo by W. W. Little

Possible future flooding surface

Sequence boundary

Subsurface (seismic) Expression

Seismic sections record changes in impedance across discontinuities; therefore, unless disrupted by structures, patterns within a seismic profile reflect parts of a stratigraphic sequence.

Upper Bounding Surface

Toplap Concordant Erosion

Lower Bounding Surface

Onlap

Onlap

Downlap

Offlap

Bedset Terminations

Bedsets, defined by discontinuities, terminate against other bedsets and are defined by the angular relationship between the two.

Erosion

Overlying Surface

Toplap Concordant

Underlying Surface

Onlap Downlap Concordant

Types of Terminations

Bedset terminations are named according to their angular relationship with underlying and overlying bounding surfaces.

Reflector Terminations & Systems Tracts

Discontinuities & Chronostratigraphy

Lithostratigraphic cross-section showing space/space relationships.

Wheeler diagram showing time/space relationships.

By plotting time against space, facies migration, discontinuity development, and sea-level history can be reconstructed.

Litho- vs. Chronostratigraphy

Lithostratigraphic units (formations, members, groups) are time transgressive and are different ages in different places.

Effects of Changing Accommodation on the Stratigraphic Record

Base-level curves are based primarily on facies changes (FUS/CUS) and lapping relationships at bed terminations. Offlap and toplap typify progradation. Onlap represents retrogradation. Concordance demonstrates aggradation. Downlap can be developed during any of the three.

A parasequence is a relatively conformable succession of genetically-related beds or bedsets bounded by marine flooding surfaces or their correlative surfaces.

Parasequence

Flooding surface

Flooding surface

Sha

llow

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upw

ard

Marine Flooding Surface

Marine Flooding Surface

Marine Flooding Surface

Though parasequences represent progradational pulses of deposition, internally they can either coarsen- or fine-upward, depending upon the depositional system within which they form.

Vertical Trends within a Parasequence

Coarsening-upward Parasequence Fining-upward Parasequence

Flooding surfaces and sequence boundaries are produced in response to rises and falls, respectively, in base-level. Lateral facies shifts accompany vertical base-level fluctuations, affecting the character of systems tracts.

Role of Accommodation Space

A systems tract is a three-dimensional assemblage of genetically-related (according to Walther’s Law) depositional systems. Systems tracts migrate and change character in response to the direction and rate of base-level fluctuation. These changes are recorded by geometrical relationships between bounding surfaces.

Systems Tracts

According to Walther’s Law, absent an unconformity, facies stacked vertically were deposited laterally to one another. Therefore, facies boundaries within a parasequence are diachronous.

Walther’s Law

Shoaling-upward Deposit

Lithostratigraphy & AllostratigraphyLithostratigraphy & Allostratigraphy

Lithostratigraphy

Based onLithology

Allostratigraphy

Based on Discontinuities

Lithostratigraphy & Sequence Stratigraphy

Though diachronous over their lateral extent, bounding surfaces have chronostratigraphic significance, in that everything above is younger than everything below the surface. Because events producing bounding surfaces have identifiable beginning and ending points, they represent isochronous events (e.g. base-level fluctuations). Time relationships are typically shown by Wheeler Diagrams.

Time Significance of Bounding Surfaces

Depending upon the direction and relative rate of base-level fluctuation, sets of parasequences can form patterns that are progradational (basinward- stepping), aggradational (vertical stacking), or retrogradational (landward-stepping).

Parasequence Set Stacking Patterns

The type of stacking pattern is controlled largely by the relative balance between rates of accommodation production (base-level fluctuation) and basin filling (sediment supply). E.g., progradation can occur during either a base-level fall or rise, depending upon the amount of sediment delivered to the basin.

Base-level & Sediment Supply

Forced regression

Transgression

Regression

Aggradation

Sediment supply exceeds accomodation production and facies shift basinward.

Sediment supply exceeds accomodation production as accommodation is lost. Facies shift basinward as landward areas erode.

Accommodation production exceeds sediment supply and facies shift landward.

Sediment supply equal to accomodation production and facies stack vertically.

Progradation during a stillstand or rising base-level lengthens the graded profile, resulting in both aggradation (mostly proximal areas) and progradation (distal regions).

Role of Graded Profile

Relative base-level is the cumulative result of rates and direction of eustatic base-level fluctuation and basin subsidence or uplift, leading to creation or destruction of accommodation space.

Relative Base-level and Accommodation Space

Under constant basin subsidence coupled with eustatic fluctuation, four points of significance to sequence stratigraphy are identified:

A: Maximum rise (highstand)B: Maximum rate of fallC: Maximum fall (lowstand)D: Maximum rate of rise

Sequence Boundary

A sequence boundary (SB) is produced as relative base-level drops. Erosion begins in landward regions and progresses basinward (diachronous) with deposition in more basinal areas, producing the falling-stage systems tract (FSST). The SB separates the highstand systems tract (HSST) below from the FSST or lowstand systems tract (LSST) above.

Formation of Sequence Boundary: SB

Falling-stage Systems TractA FSST can form while relative base level falls and the SB is produced; however, because of cannibalization, this systems tract is often missing or poorly developed. If base-level experiences an absolute fall, a forced regression occurs and depositional units can downstep (offlap) in a basinward direction.

Photo by W. W. Little

Photo by W. W. Little

Photo by W. W. Little

Lowstand Systems Tract

A LST is produced during the early stages of relative base-level rise. Erosion continues in landward areas, but preservation potential is higher than for FSST sediments, as accommodation is produced in a progressively more landward direction. These are characterized by onlap onto FSST deposits and/or the sequence boundary. Parasequence patterns change from progradational to aggradational.

Lowstand Systems Tract: LST

Transgressive SurfaceThe transgressive surface (TS) separates the LST below from the TST above and forms during the maximum rate of relative base-level rise, as basinal accommodation development surpasses sediment supply. Stacking patterns change from aggradational to retrogradational. It is the first significant flooding surface within a sequence and commonly marks the base of the most prominent onlap exhibited by the sequence. Erosion often accompanies formation of the TS.

Transgressive Systems Tract

The transgressive systems tract is typically thin and characterized by a retrogradational parasequence set as landward regions become flooded. This systems tract is bounded by the TS below and the maximum flooding surface (MFS) above.

Transgressive Systems Tract: TSTformation of maximum flooding surface

Maximum Flooding Surface

The MFS forms the boundary between the TST and HST and represents the greatest landward incursion of the sea. Parasequence stacking patterns change from retrogradation to aggradation. Basinward regions are characterized by a lack of sedimentation, produced a starved zone or condensed interval. Typically forms a downlap surface for highstand systems tract (HST) deposits.

Highstand Systems Tract

The HST is found between the MFS and the upper SB. As accommodation development slows, parasequence sets change from aggradational to progradational. Bed terminations are characterized by onlap in proximal regions and downlap in more basinal areas.

Highstand Systems Tract: HST

Complete Sequence

Long Term C

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Short T

erm C

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Recognition of stratigraphic surfaces in measured sections can be used as a means of determining sea-level history for one area and correlating that history to litholigically different strata of another.

Sequences in Measured Sections

Sequence Stratigraphy & Eustasy

High-frequency Sequence Stratigraphy

“Sequence Stratigraphy – Basics”C. G. St. C. Kendall

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