Download - 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