Sequence Stratigraphy
Sequence stratigraphy is the subdivision of the stratigraphic record on
the basis of bounding discontinuities.
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).
Depositional Sequence
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
Photo by W. W. Little
Photo by W. W. Little
Types of Discontinuities
The stratigraphic record consists various scales of bedding separated by
bounding surfaces (discontinuities) that represent “gaps” in the
sedimentary record. Photo by W. W. Little
Bedding Planes
Bedding planes are surfaces
between beds and bedsets that
represent breaks between episodic
depositional events, such as floods,
storms, and turbidity flows. Photo by W. W. Little
Flooding Surfaces
r -w a t er facies
Deepe
er -w a te r facies
Shallow
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. Photo by W. W. Little
Shoreface Ravinement Surface
Both transgressive and regressive events can develop erosional surfaces
associated with shoreface erosion by wave base.
Sequence Boundaries
Landward facies
Basinward facies
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. Photo by W. W. Little
Photo by W. W. Little
Photo by W. W. Little
Possible futu
re flooding su
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Sequence bo
undary
Photo by W. W. Little
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.
Bedset Terminations
Upper Bounding Surface
Concordant Erosion
Toplap
Onlap
Offlap
Onlap
Downlap
Lower Bounding Surface
Bedsets, defined by discontinuities, terminate against other
bedsets and are defined by the angular relationship between
the two.
Types of Terminations
Overlying Surface
Erosion Toplap Concordant
Underlying Surface
Onlap Downlap Concordant
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.
Parasequence Flooding surface
Shallowing upward
Flooding su
rface
A parasequence is a relatively conformable succession of genetically-
related beds or bedsets bounded by marine flooding surfaces or their
correlative surfaces.
Marine Flooding Surface
Marine Flooding Surface
Marine Flooding Surface
Vertical Trends within a
Parasequence
Coarsening-upward Parasequence Fining-upward Parasequence
Though parasequences represent progradational pulses of deposition,
internally they can either coarsen- or fine-upward, depending upon the
depositional system within which they form.
Role of Accommodation Space
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.
Systems Tracts
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.
Walther’s Law
Shoaling-upward Deposit
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.
Lithostratigraphy & Allostratigraphy
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All
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Based on
Discontinuities
Lithostratigraphy & Sequence Stratigraphy
Time Significance of Bounding Surfaces
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.
Parasequence Set Stacking Patterns
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).
Base-level & Sediment Supply
Transgression
The type of stacking pattern
is controlled largely by the
Aggradation relative balance between
rates of accommodation
production (base-level
fluctuation) and basin filling
Regression (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.
Forced regression
Sediment supply exceeds Sediment supply exceeds accomodation
accomodation production and facies production as accommodation is lost.
shift basinward. Facies shift basinward as landward
areas erode.
Accommodation production exceeds
Sediment supply equal to accomodation
sediment supply and facies shift landward.
production and facies stack vertically.
Role of Graded Profile
Progradation during a stillstand or rising base-level lengthens the
graded profile, resulting in both aggradation (mostly proximal areas)
and progradation (distal regions).
Relative Base-level and Accommodation Space
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: Maximum rise (highstand)
B: Maximum rate of fall
C: 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 Tract
A 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 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.
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: TST
formation 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
Sequences in Measured Sections
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ycle cle
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C Cy
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Te Te
t ng
or Lo
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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.
Sequence Stratigraphy & Eustasy
SB High-frequency
mfs
TS Sequence Stratigraphy
“Sequence Stratigraphy – Basics”
C. G. St. C. Kendall
