LGC field course in the Book Cliffs, UT: Presentation 1 of 14 (Principles of Sequence Stratigraphy)

William W. Little

Principles of Sequence Stratigraphy

© 2015 by W.W. Little Geological Consulting, LLCPhoto by W. W. Little

Unless otherwise noted:

• Photos and figures by William W. Little• Drafting by Daniel W. Little• Many credited figures obtained from the SEPM Strata website

© 2015 by W.W. Little Geological Consulting, LLCPhoto by W. W. Little

Stratigraphy is the study of temporal relationships in sedimentary rock bodies and reflects changes in the balance between rates at which space is produced and filled. Stratigraphy can be considered the history of past geological events, particularly base-level fluctuations, and adds the dimension of time to sedimentology.

Stratigraphy

Photo by W. W. Little

Accommodation space is the volume below base level (more properly the graded profile) and above the basin floor available for sediment accumulation and can increase or decrease on global (eustatic), regional (e.g. foreland subsidence), or local (e.g. delta lobe switching) scales.

Basic Concepts: Accommodation Space

Basic Concepts: Preservation Potential

Eustatic sea level rises and falls within a limited range, meaning that most space produced will subsequently be destroyed.

+

=

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.

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

A: HighstandB: Maximum rate of fallC: LowstandD: Maximum rate of rise

For long-term preservation, space production must include tectonic subsidence.

Sedimentation rate reflects the volume of material eroded from a source region and transported to a basin over a specified period of time. Sediment supply can increase or decrease depending upon rates of uplift in the source region, changes to the nature of materials exposed at the surface, and climatic conditions.

Basic Concepts: Sediment Supply

Effects of Variable Accommodation and Sediment Supply

The relationship between accommodation production (A) and sedimentation rates (S) determines the stacking pattern of progradational “wedges.”

S > A = Forward steppingS = A = Vertical stackingS < A = Backstepping

Basic Concepts: Base Level

Base level is the horizon below which continents cannot erode and above which basins cannot fill (or, more correctly, to which both will grade).

Base Level: Graded Profile

A stream naturally adjusts its gradient so as to barely transport the available load, producing a profile that flattens toward the basin. This profile can be modified as it is lengthened or shortened by base level fluctuations that create and destroy accommodation space and by uplift and erosion in the source region that affect sediment supply.

Eustasy (global base level)

From SEPM Strata (sepmstrata.org)

Base-level fluctuations that can be identified to have occurred simultaneously in many localities across the earth have been combined in terms of time, direction, and magnitude to establish a global sea level curve.

Regional/local Base Level

Base-level fluctuations for a particular locality can vary from the global (eustatic) standard due to tectonic uplift/subsidence, sediment compaction, and variations in sedimentation rates.

Tectonic subsidence provides the same effect as a eustatic rise in base level, in that it produces new accommodation space.

Relative Base Level

Because of the common difficulty of distinguishing between global, regional, and local effects, we often simply refer to relative base-level.


Important Terms: Rise vs. Fall

Base level rise and fall refer to the vertical movement of a plane with respect to the basin floor.

Important Terms:Transgression vs. Regression

Transgression and regression refer to the lateral movement of a line forming the boundary between land and sea. It is important to note that regression can occur during either a base level fall (forced) or a base level rise (normal), depending upon the relative rates of rise and sedimentation. Transgressions occur only during a base level rise.

Regression

Transgression

Land

Sea

Important Terms: Progradation, Retrogradation, and Aggradation

All deposition is progradational, meaning that a volume of sediment always builds in a basinward direction; however, progradational wedges (parasequences) can stack in progradational, aggradational, or retrogradational patterns (sets).

Retrogradation


Basic Concepts: Deposition is Progradational (Walther’s Law)

Because sediment always moves basinward, facies that accumulate in more proximal environments prograde over facies in more distal environments, creating a vertical facies succession that represents environments which were once adjacent.

Important Terms: Parasequence

A parasequence is what was once referred to as a progradational or clastic wedge. It represents a single progradational episode that observes Walther’s Law internally but is bound above and below by discontinuities, primarily flooding surfaces.


From SEPM Strata (sepmstrata.org)Photo by W. W. Little


Shoreface Parasequence

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

ing

upw

ard


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 ParasequenceFrom SEPM Strata (sepmstrata.org)

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.

Parasequence Stacking Patterns

Forced regression

Transgression

Regression

Aggradation

Important Terms: Systems Tracts

A systems tract consists of coeval depositional systems within a given depositional basin. Systems tracts within a sequence stratigraphic framework are dependent upon the direction and rate of base level change with respect to sedimentation rates and are identified by stacking pattern, position within a sequence, and nature of bounding surfaces.


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

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.

A discontinuity represents a break in deposition, across which there is a violation of Walther’s Law and can be represented by a large range of scales.

Types of Discontinuities (scale)

Discontinuities can be produced by either a fall or a rise in base level, the former creating an erosional surface and the latter a surface or interval of non- to very slow sedimentation.

Types of Discontinuities (process)


Discontinuities differ in character, mode of formation, and timing from proximal to distal portions of a basin. Proximal unconformities tend to be erosional and form through incision during base level fall. Distal unconformities are more likely to be non-depositional, forming through sediment starvation during base level rise.

Timing of Discontinuities


Sequence Boundaries(Discontinuities formed by a drop in base level)

Sequence boundaries are surfaces bounding depositional sequences. Depending upon whether base-level falls or slowly rises with respect to rates of basin filling, they can be erosional (type 1) or conformable (type 2) and are recognized by the abrupt placement of more landward facies over more basinward facies with missing facies between.

Distal facies

Proximal facies


Flooding Surfaces(Discontinuities formed by a rise in base level)

Flooding surfaces represent relative rises in base-level. They are recognized by deeper-water (basinward) facies abruptly overlying shallower-water (landward) facies with missing facies between.

Proximal faciesDistal facies

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).

• A 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).

Sedimentary deposits related to a single cycle of base level change.

My Definition of a Sequence

Litho- vs. Chronostratigraphy

Sediment fills space to base level, then accumulates through basinward progradation, producing lithostratigraphic units with bounding surfaces that are time transgressive.

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

Walther’s Law


Litho- vs. Chronostratigraphy

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

Litho- vs. Sequence Stratigraphy

Sequence stratigraphy correlates rock bodies on the basis of where they fall within a cycle of base level fluctuation, focusing on bounding allostratigraphic (unconformable) surfaces.

Lithostratigraphy correlates rock bodies on the basis of physical characteristics and stratigraphic position.

Exercise

Correlate these four cores or measured sections.

Lithostratigraphic Solution

Given the similarity in physical characteristics, the most common solution would be to hang the sections at the top of the first prominent sandstone.

Sequence Stratigraphic Solution

An alternative would be to assume an overall progradational stacking pattern common to highstand systems tracts and to hang the section on the first recognizable parasequence.

Elements: 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 (HST) below from the FSST or lowstand systems tract (LST) above.


Sediment bypassFormation of sequence boundary

Elements: Falling-stage Systems Tract

A FSST can form as a downstepping (offlaping) deposit while relative base level falls (forced regression) and the SB is produced; however, because of cannibalization, this systems tract is often missing or poorly developed.


Falling-stage delta


Cannibalization during progressive falling stages

Lowstand Systems Tract

An 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.


LST

HST

SB

Transgressive Surface

The 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.


Possible future transgressive surface

Sequence boundary

Both transgressive and regressive events can develop erosional surfaces associated with reworking at wave base. This commonly occurs as shoreface erosion during development of the transgressive surface.

Shoreface Ravinement Surface

Transgressive Systems Tract

The transgressive systems tract is typically thin in coastal areas, thick in proximal areas, 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.

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 retrogradational to aggradational. Basinward regions are characterized by a lack of sedimentation, producing a starved zone or condensed interval. Typically forms a downlap surface for the 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.

Complete Sequence


Variations


Multiple Cycles

Chronostratigraphic Significance

By plotting time against space, facies migration, discontinuity development, and sea-level history can be reconstructed through use of a Wheeler Diagram.

Whe

eler

Dia

gram


What it looks like in outcrop

Deltaic

Distal shoreface

Medial shoreface

Proximal shoreface

Proximal shoreface

Coastal plainFluvial

Deltaic (LST)

Distal shoreface (TST)

Medial shoreface

Proximal shoreface

Proximal shoreface

Coastal plainFluvial

Marine (HST)

(SB)

(TS)

(MFS)

(HST)


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.

What it looks like in measured section

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

What it looks like in a seismic section

Truncation (base level drop & erosion)

Overlying Surface

Toplap (static base

level)

Concordant (rising base

level)

Underlying Surface

Onlap (rising base level &

shoreline transgression)

Downlap (progradation) Offlap (base level drop & forced regression)

Bedset Terminations

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

Reflector Terminations & Systems Tracts

Bedset terminations can be used to identify systems tracts within seismic sections.

HST

FSST/LSTTST

HST

SB

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

What it looks like in a seismic section

LGC field course in the Book Cliffs, UT: Presentation 1 of 14 (Principles of Sequence Stratigraphy)

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Transcript of LGC field course in the Book Cliffs, UT: Presentation 1 of 14 (Principles of Sequence Stratigraphy)