49326920 11-20-08 Sequence Stratigraphy

8/4/2019 49326920 11-20-08 Sequence Stratigraphy

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Sequence Stratigraphy

Important because it places strata into apredictable, cronostratrigraphic

framework and relates them toaccommodation space


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Accommodation space equation

T+E=S+W

T: Tectonic subsidence

E=rate of Eustatic sea level rise

S: sedimentation rate

W: water depth

Space is

created by

changing in tectonicsubsidence and Eustasy

Space is filled withwater and sediments


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Hierarchy

1) Depositional sequence (bounded by sequence

boundary)

2) System tracts: a linkage of contemporaneous

depositional systems

3) Parasequence set (stacking patterns: progradational,

aggradational, retrogradational)

4) Parasequence (bounded by flooding surfaces)

5) Flooding surface


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Depositional system

A 3D assemblage of lithofacies geneticallylinked and coexisting today (fluvial, delataic,

barrier-island)


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Depositional sequence

A relative conformable succession ofgenetically related strata bouned by

unconformities (i.e. sequence boundary) or

related conformities.

Important: every depositional sequence is therecord on one cycle of relative sea level

Important: every Depositional sequence isbounded above and below by unconformities

or correlative conformities.


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System tract (subdivision of the depositional system)Genetically associated stratigraphic units that were deposited during specific

phases of the relative sea-level cycle (Posamentier, et al, 1988) and

represented in the rock record as three-dimensional facies assemblages. Theyare defined on the basis of bounding surfaces, position within a sequence,

and parasequence stacking pattern (Van Wagoner et al., 1988)

Sequence boundary

HST

TST

TS

LST

Sequence boundary


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The key vertical succession in all

depositional sequences:

From bottom to top:

1) Sequence boundary

2) Low Stand system

Track (LST)3) Transgressive surface

4) Transgressive system

track (TST)

5) Maximum flooding

surface

6) Highs stand system

track (HST)


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Origin and scale of Parasequences

Because shallow water facies within a parasequence will pinch out laterally in adowndip direction and deeper water facies within a parasequence will pinch out inan updip direction, the facies composition of a single parasequence changespredictably updip and downdip. Thus, a single parasequence will be composed ofdeeper water facies downdip and shallower water facies updip, as would beexpected.


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Parasequences

Example of siliciclastic wave

dominated shoreline:

1) bioturbated offshore

mudstones,

2) pass through the storm beds of

the transition zone or lower

shoreface,

3) continue through the trough

crossbedding of the shoreface,

4) pass upwards into the seaward

inclined laminae of theforeshore,

5) and be capped by a backshore

or coastal plain coal bed.


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System tracks and parasequences

are arranged into parasequence sets

(stacking pattern)

A succession of genetically related

parasequences that form a distinctive stackingpattern (example: progradatioal,

retrogradational, aggradational)


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Stacking patterns

Sedimentation>accomodation


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Stacking patterns

Sedimentation=accomodation


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Stacking patterns

Sedimentation


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96_0118_13

Rate of sediment supply Rate ofaccommodation

AggradationalParasequence

ProgradationalParasequence

Sedim

ent sup

ply=

Accomm

odation

Increasing rate ofsediment supply

Sediment bypass

Sequence Boundary

RegionalIncision

0

Increasingra

teo

faccommod

ati

on

Losso

faccommoda

tion

Accommodation

vs

Sediment Supply

(modified from Shanley and McCabe,1996)


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Hierarchy of surfaces

Sequence boundary (Depositional sequence)

Transgressive surface (System tract)

Maximum flooding surface (Parasequence)


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Seismic sequence

A depositional sequence is a relatively conformablesuccession of seismic reflectors bounded at its top and base

by sequence boundaries (unconformities and their correlative

conformities) (Vail, et al., 1977). Within this package of

reflectors it is sometimes possible to identify reflectorgeometries representing a succession of genetically linked

deposition systems (Systems Tracts) which are interpreted to

have been deposited between eustatic-fall inflection points

(Posamentier, et al., 1988).


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Base discordantonlap

downlap


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Type of truncations

(progradationalsequence; regression=

r.sea level fall)

(retrogradationalsequence;

Transgression=r. sea

level rise)


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Erosional Sequence Boundary(Cretaceous Mancos Shale near Woodside, Utah)

Tidal influence


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HST

LST

TSTHST

Sequence boundary


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Low Stand system Tract

49326920 11-20-08 Sequence Stratigraphy

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