Sequence Stratigraphy System Tracts system

Sequence Stratigraphy System Tracts

1. GeoHikingClub-2020

2. Models of Sequence Stratigraphy
Exxon Model 1977
Bilal ul Haq Model 1980
Galloway Model 1989
Malcolm Rider Model 2002
Dr Nadeem Ahmed Model 2003
Smewing Model 2005
Catuneanu Model 2006
Ashton Embray model 2002 & 2009.
GeoHikingClub-2020

3. GeoHikingClub-2020

4. Figure 2.7 Generalized trend of
peat accumulation during the
various stages of a base-level
cycle, in response to changes in
accommodation. See text for
discussion. No temporal scale is
implied for the relative duration
of systems tracts. Abbreviations:
TST—transgressive systems
tract; RST—regressive systems
tract; HST—highstand systems
tract; FSST—falling-stage
systems tract (early lowstand
system); LST—lowstand systems
tract; MFS—maximum flooding
surface; BSFR—basal surface of
forced regression; CC—
correlative conformity (sensu
Hunt and Tucker, 1992); MRS—
maximum regressive surface.
Catuneanu Model 2006
GeoHikingClub-2020

5. GeoHikingClub-2020

6. Dr Nadeem Ahmed Model 2003
GeoHikingClub-2020

7. GeoHikingClub-2020

8. GeoHikingClub-2020

9. 3. System Tracts
Introduction
System Tracts
Lowstand system tract
Definition and stacking patterns
Economic potential
Transgressive system tract
Definition and Stacking Patterns
Economic Potential
Highstand system tract
Definition and stacking patterns
Economic potential
Shelf Margin system tract
Definition and stacking patterns
GeoHikingClub-2020

10. Introduction
The concept of systems tract was introduced to define a linkage of
contemporaneous depositional systems, forming the subdivision
of a sequence(Brown and Fisher, 1977).
Systems tracts are interpreted based on
Stratal stacking patterns,
Position within the sequence and
Types of bounding surfaces, and are assigned particular
positions along an inferred curve of base-level changes at the
shoreline.
Systems tracts are ‘genetic stratigraphic units (Galloway 2004)
(Fig. 3.1).
GeoHikingClub-2020

11. Introduction
Each systems tract is defined by a specific type of stratal stacking
pattern, closely associated with a type of shoreline shift (i.e.,
forced regression, normal regression, or transgression), and
represents ‘a specific sedimentary response to the interaction
between
Sediment flux,
Physiography,
Environmental energy, and
Changes in accommodation’(Posamentier and Allen, 1999).
GeoHikingClub-2020

12. Figure3.1Showingthedepositionalhierarchyofthemainsystemtracts.
GeoHikingClub-2020

13. FIGURE 1.7 Timing of system tracts and sequence boundaries for the sequence models currently
in use (modified from Catuneanu, 2002). Abbreviations: LST—lowstand systems tract; TST—
transgressive systems tract; HST—highstand systems tract; FSST—falling-stage systems tract;
RST—regressive systems tract; T–R—transgressive-regressive.
GeoHikingClub-2020

14. The early Exxonsequence model accounts for the subdivision of
depositional sequences into four component systems tracts, as
first presented by Vail (1987) and subsequently elaborated by
Posamentier and Vail (1988) and Posamentier et al. (1988).
These are the
Lowstand
Transgressive
Highstand
Shelfmarginsystems tracts.
These systems tracts were first defined relative to a curve of
eustaticfluctuations (Posamentier et al., 1988; Posamentier and
Vail, 1988), which was subsequently replaced with a curve of
relative sea-level(base-level) changes (Hunt and Tucker, 1992;
Posamentier and James, 1993).
GeoHikingClub-2020

15. The lowstandand the shelf-marginsystem tracts are similar
concepts, as being both related to the same portion of the
reference sea-level curve (the stage of fall-early rise), so they were
used interchangeably as part of a depositional sequence (Vail,
1987; Posamentier and Vail, 1988; Vail et al., 1991).
A sequence composed of lowstand, transgressive and highstand
system tracts was defined as a ‘type-1’ sequence,
Whereas a combination of shelf-margin, transgressive and
highstand systems tracts was said to have formed a ‘type-2’
sequence(Posamentier and Vail, 1988).
GeoHikingClub-2020

16. GeoHikingClub-2020

17. 3.2 System Tracts
Lowstand system tract
Transgressive system tract
Highstand system tract
Shelf Margin system tract
GeoHikingClub-2020

18. Lowstandsystem tract
Definition and stacking patterns
The lowstand systems tract, when defined as restricted to all
sedimentary deposits accumulated during the stage of early-rise
normal regression (sensu Hunt and Tucker, 1992), is bounded by
the subaerial unconformity and its marine correlative conformity
at the base (Type-1 Sequence Boundary), and by the maximum
regressive surface at the top (Transgressive Surface) (Figs. 3.2).
Where the continental shelf is still partly submerged at the onset
of base-level rise, following forced regression, the basal composite
boundary of the lowstand systems tract may also include the
youngest portion of the regressive surface of marine erosion (Fig.
2.12).
GeoHikingClub-2020

19. Consequently, depositional processes and stacking patterns are
dominated by low-rate aggradationand progradationacross the
entire sedimentary basin.
As accommodation is made available by the rising base level, this
‘lowstand wedge’ is generally expected to include the entire suite
of depositional systems, from fluvial to coastal, shallow-marine
and deep marine (Fig. 4.5).
Sediment mass balance calculations indicate, however, that the
grading trends observed within shallow-marine successions do
not correlate with the grading trends that characterize the age
equivalent deep-water deposits.
GeoHikingClub-2020

20. As a result, the lowstand sediments of the basin-floor submarine
fan complex are overall finer grainedrelative to the underlying late
forced regressive deposits.
The maximum grain sizeof the sediment transported by gravity
flowsduring the lowstand normal regression is also expected to
decrease with time, due to the gradual lowering in fluvial slope
gradients and related competence following the onset of base-
level rise.
The increase with time in the rate of base-level rise also
contributes to the overall fining-upwardfluvial profile, as it
creates more accommodation for floodplain deposition and
increases the ratio between floodplain and channel
sedimentation.
GeoHikingClub-2020

21. •Figure 3.4 Subaerialunconformity (red line) on a dip-oriented, 2-D seismic transect (location shown on the 3-D illuminated
surface) (De Soto Canyon area, Gulf of Mexico; image courtesy of H.W. Posamentier). Red arrows indicate truncation of
underlying forced regressive shallow-marine strata. The deep-water forced regressive deposits downlapthe prograding
continental slope (yellow arrows). Thinner yellow lines provide a sense of the overall stratalstacking patterns. Note that the
subaerialunconformity is associated with offlap, decrease in elevation in a basinwarddirection, and irregular topographic relief
(differential erosion). The basinwardtermination of the subaerialunconformity indicates the shoreline position at the end of
forced regression. The subaerialunconformity is onlapped(fluvial onlap; green arrow) and overlain by a topsetof lowstand
normal regressive strata. The white arrow indicates the shoreline trajectory during the subsequent lowstandnormal
regression. For scale, the channel on the 3-D illuminated surface is approximately 1.8 km wide, and 275 m deep at shelf edge.
The illuminated surface is taken at the base of forced regressive deposits. Abbreviations: FR–forced regressive deposits;
NR–normal regressive deposits.
GeoHikingClub-2020

22. Economic potential
Petroleum plays
Rising base level during the lowstand normal regression provides
accommodation across the entire basin, from fluvial to marine
environments.
Sediment budget observations indicate a concentration of the
coarsest river-borne sediment within fluvial and coastal
depositional systems, which arguably form the best reservoirs,
with the highest sand/mud ratio, of the lowstand systems tract.
The trapping of sand within aggrading fluvial to shallow-marine
systems following the onset of base-level rise results in a net
decrease in the volume of sediment available for deep-water
gravity flows, and also in a lowering of the sand/mud ratio in
submarine fans.
Shelf-edge deltas and correlative strandplains continue to
prograde the upper slope, with the development of a topset in
response to coastal aggradation (Fig. 3.2).
GeoHikingClub-2020

23. Equally good reservoirs may form in coastal, shallow-water
and deep-waterenvironments during the lowstandnormal
regression of the shoreline.
The change in the type of dominant gravity flows that
manifest during the lowstandtime, from high-density
turbidity currents at the end of base-level fall/onset of base-
level rise to low-density turbidity currents, has important
consequences for the lithology, morphology and location of
deep-water reservoirs within the basin (Figs. 4.4 and 4.5).
The main risks for the exploration of lowstandreservoirs
relate to charge, seals, and sourcerocks, especially toward the
basin margins.
GeoHikingClub-2020

24. Coal Resources
The lowstand systems tract is defined by high sediment supply in
an overall low accommodation setting, and therefore
environmental conditions are generally unfavourable for peat
accumulation (Fig. 2.7).
As the rates of base-level rise increase with time during the
lowstand stage, gradually more accommodation becomes available
to the overbank environment, and so chances of peat
accumulation and subsequent coal development tend to improve
toward the top of the lowstand systems tract (Fig. 3.6).
GeoHikingClub-2020

25. Transgressive system tract
Definition and stacking patterns
The transgressive systems tract is bounded by the maximum
regressive surface (transgressive surface) at the base, and by the
maximum flooding surface at the top.
This systems tract forms during the stage of base-level rise when
the rates of rise outpace the sedimentation rates at the shoreline
(Fig. 2.5).
It can be recognized by its overall fining-upward profileswithin
both marine and nonmarine successions.
As the rates of creation of accommodation are highest during
shoreline transgression (Fig. 2.5).
The transgressive systems tract is commonly expected to include
the entire range of depositional systems along the dip of a
sedimentary basin, from fluvial to coastal, shallow-marine and
deep-marine (Fig. 3.2).
GeoHikingClub-2020

26. Figure 3.6 Note that the transgressive systems tract may consist of
two distinct wedges, one on the continental shelf and one in the
deep-water environment, separated by an area of sediment bypass
or erosion around the shelf edge.
GeoHikingClub-2020

27. As a result, the transgressive systems tract tends to be composed
of two distinct wedges separated by an area of non-deposition
around the shelf edge;
One on the continental shelf consisting of fluvial to shallow-
marine deposits.
One in the deep-water setting consisting of gravity flow
deposits and pelagic sediments (Figs. 2.19 and 3.2).
Both these wedges shift toward the basin margin during
transgression, following the general retrogradational trend, by
onlappingthe landscape and the seascape, respectively, in a
landward direction.
Within the deep-water portion of the basin, the transgressive
deposits are often seen onlapping the continental slope, forming a
transgressive slope apron associated with marineonlap
(Galloway, 1989; Figs. 2.4, 2.19 and 3.2).
GeoHikingClub-2020

28. Coastal onlapis also an important type of stratal termination,
diagnostic for transgression, forming within the continental shelf
based transgressive wedge by the shift of shoreface facies on top of
the landward-expanding wave-ravinement surface (Figs. 2.4, 2.19
and 3.2).
The fluvial portion of the transgressive systems tract commonly
shows evidence of tidal influences (Shanley et al., 1992; Shanley
and McCabe, 1993), and is characterized by an overall fining-
upward vertical profile.
As accommodation is generated rapidlyduring transgression, and
the water table rises in parallel with the base level, the fluvial
portion of the transgressive systems tract often includes well
developed coal seams (Fig. 2.20).
GeoHikingClub-2020

29. The transgressive fluvial deposits may form a significant portion
of incised-valley fills,or may aggrade in the inter-fluve areas of
former incised valleys.
Where incised valleysare inherited from previous stages of base-
level fall and are not entirely filled by lowstand deposits, their
down-stream portions are commonly converted into estuaries at
the onset of transgression (Dalrymple et al., 1994).
Where not reworked by the tidal-ravinement surface, the contact
between lowstand fluvial and the earliest (stratigraphically
lowest) overlying estuarine facies is represented by the maximum
regressive surface(T.S).
GeoHikingClub-2020

30. In this setting, the maximum regressive surface (T.S) is relatively
easy to map in outcrop or core, at the abrupt change from coarse
fluvial sandand gravel (lowstand deposits) to the overlying
estuarine facies comprising finer-grainedand more varied
lithologies with abundant tidal structures such as clay drapes and
flasers.
This contrast between lowstand fluvial and overlying transgressive
estuarine facies may also be strong enough to be seen in well logs,
at the contact between ‘clean’ and blocky sand and the younger,
more interbedded and finer-grained lithologies(Fig. 2.15).
GeoHikingClub-2020

31. The formation and preservation of transgressive coastal deposits
depends on the rates of
Base-level rise,
Sediment supply,
The wind regime and
The amount of associated wave-ravinement erosion, and
The topographic gradients at the shoreline.
Under restricted detrital supply conditions, the shallow-marine
portion of the transgressive systems tract may also be represented
by carbonate condensed sections(Fig. 2.20).
The overall thicknessof the shallow-water portion of the
transgressive systems tract decreases toward the shelf edge, where
transgressive deposits are commonly missing (Galloway, 1989;
Figs. 2.19 and 3.2).
GeoHikingClub-2020

32. Economic potential
Petroleum plays
On the continental shelf, likely close to the shelf edge, the best
reservoirs are concentrated along the coastline, being represented
by back-steppingbeaches (open shoreline settings), estuary-
mouth complexes, retrograding bayhead deltas or even prograding
deltas (river-mouth settings).
Landward from the shoreline, the potential for petroleum
explorationof the transgressive systems tract is generally
moderate to poor because of the extensive development of fine-
grained floodplain faciesin response to the rapid rates of base-
level rise.
In addition to coastal facies, shelf-sand deposits and deep-water
turbidites may also make good prospects for petroleum
exploration.
GeoHikingClub-2020

33. Economic potential
The main contribution, however, of the transgressive systems tract
to the development of petroleum systems within a sedimentary
basin is the accumulation of source rocks and seal facies, within
most transgressive depositional environments.
Transgressive shallow-marine shales, for example, usually form
regionally extensive covers across continental shelves, which may
serve as reference units for stratigraphic correlation that can be
easily identified on 2-D seismic lines, based on their ‘transparent’
seismic facies(Fig. 3.7).
GeoHikingClub-2020

34. Coal resources
The transgressive systems tract is arguably the best portion of a
stratigraphic sequence for coal exploration.
The time of end-of-shoreline transgression marks the peak for
peat accumulation and subsequent coal development because the
water table is at its highest level relative to the landscape profile,
following a time characterized by a high accommodation to
sediment supply ratio during the transgression of the shoreline
(Fig. 2.6).
Assuming that all favorable conditions are fulfilled, the best
developed coal seams are expected to overlap with the maximum
flooding surface (Hamilton and Tadros, 1994; Fig. 2.6).
GeoHikingClub-2020

35. Highstand system tract
Definition and stacking patterns
The highstand systems tract, defined as, “forms during the late
stage of base-level rise, when the rates of rise drop below the
sedimentation rates, generating a normal regression of the
shoreline (Figs. 2.5)”.
Consequently, depositional trends and stacking patterns are
dominated by a combination of transgression, aggradationand
progradationprocesses (Fig. 3.2).
The highstand systems tract is bounded by the maximum
flooding surface (MFS) at the base, and by a composite surface
(Type-1/Type-2S.B) at the top that includes a portion of the
subaerial unconformity (SB-1 or SB-2).
GeoHikingClub-2020

36. Figure3.8Highstandsystemstract
GeoHikingClub-2020

37. Nevertheless, the bulk of the ‘highstand prism’ consists of
fluvial, coastal, and shoreface deposits, located relatively close to
the basin margin.
Highstand deltas are generally far from the shelf edge, as they
form subsequent to the maximum transgression of the
continental shelf, and develop diagnostic topset packages of
aggrading and prograding delta plain and alluvial plain strata.
There are three main phases of the highstand system tract
formation;
In the early phaseof the highstand, there is an effect of the
transgressive system tract and reterogradation is still in process
and build finning upward sequence.
GeoHikingClub-2020

38. The second phaseof the highstand stage is defined by
relatively high rates of base-level rise, although lower than the
sedimentation rates, which results in a stacking pattern with a
strong aggradational component. Consequently, the ratio
between floodplain and channel fill architectural elements also
tends to be high.
In contrast, the late phaseof the highstand stage is defined by
much lower rates of base level rise, which result in a stacking
pattern with a stronger progradational component, and hence
it is prone to an increase in channel clustering and implicitly in
the ratio between channel fill and floodplain architectural
elements.
GeoHikingClub-2020

39. Progradation therefore accelerates with time during the highstand
stage, in parallel with the decrease in the rates of base-level rise
and the corresponding decrease in the rates of creation of fluvial
and marine accommodation.
The vertical profile of the fluvial highstand deposits may therefore
be described as fining-upward, if one plots the maximum grain
size observed within channel fills, even though the net amount of
sand tends to increase up section.
The fining-upward trend is even more evident in most preserved
stratigraphic sections, as the amalgamated channels at the top of
the highstand systems tract are usually subject to erosionduring
the subsequent fall in base level.
GeoHikingClub-2020

40. The shallow-marine portion of the highstand systems tract displays a
coarsening-upward profile related to the basinward shift of facies, and
includes low-rate prograding and aggrading normal regressive strata.
In the case of a continuous regression, the shallow-marine portion of the
highstand systems tract consists of a single upward coarsening facies
succession (‘parasequence’) that downlaps the maximum flooding
surface.
In the case of the more complex pattern of highstand regression, the
shallow-marine portion of the highstand systems tract includes a
succession of stacked prograding lobes (‘parasequences’), in which each
lobe extends farther seaward relative to the previous one.
This shallow marine architecture is often referred to as a forestepping, or
seaward-stepping pattern of basin fill.
GeoHikingClub-2020

41. Economic potential
Petroleum plays
The best potential reservoirs of the highstand stage tend to be
associated with the shoreline to shorefacedepositional systems,
which concentrate the largestamounts of sand, with the highest
sand/mud ratio.
Both strand-plains (open shorelines) and deltas (river-mouth
settings) prograde and downlapthe maximum flooding surface,
which marks the lower boundary of the highstand normal
regressive package.
At the top, the highstand reservoirs may be truncated by the
subaerial unconformity.
The sand/mud ratioand the reservoir connectivitywithin the
fluvial systems tend to improve upwards, as the decreasing rates
of base-level rise during the highstand normal regression lead to
an increase in the degree of channel amalgamation. GeoHikingClub-2020

42. Economic potential
No significant reservoirs are expected to develop during this stage
in the shelf and deeper-marine settings.
The down-side of the increased degree of fluvial to shallow-
marine sand amalgamation and connectivity toward the top of the
highstand systems tract is the corresponding poorer
representation of source and seal rocks.
It can be concluded that the petroleum play significance of the
highstand systems tract consists in the accumulation of reservoir
faciesmainly within proximal regions (fluvial to coastal and
shoreface environments) and of source and seal faciesmainly
within the distal areas of the basin (shallow-to deep-water
environments).
GeoHikingClub-2020

43. Coal resources
Coal exploration is restricted to the nonmarine portion of the basin,
where the thickest and most regionally extensive coal seams are
generally related to episodes of highest water table relative to the
landscape profile.
Providing that all favorable conditionsrequired for peataccumulation
are met, which involve the interplay of subsidence, vegetation growth
and sediment supply, these most significant coal seams tend to be
associated with maximum flooding surfaces (Hamilton and Tadros,
1994), hence marking the base of the highstand systems tract (Fig. 2.7).
The lower portion of the highstand systems tract, defined by a
predominantly aggradational sedimentation pattern, may still include
well-developed coal seams interbedded with overbank fluvial facies,
above the tidally-influenced transgressive fluvial channel fills.
The upper portion of the highstand systems tract commonly lacks coal
deposits due to insufficient accommodation and the relatively high
sediment input that results in the amalgamation of meander belts.
GeoHikingClub-2020

44. Shelf margin system tract
Definition and stacking patterns
A system tract deposited during a time of less-pronounced relative
sea level fall(i.e. the rate of eustatic fall is slightly less than or equal to
the rate of basin subsidence).
Such that the shelf becomes partially exposed, but the shoreline does
not extend seaward all the way to the offlap break (Fig. 2.10).
Resulting deposit is called a shelf margin system tract which consists of
prograding topsets and clinoforms.
It becomes aggradational, and finely retrogradatinonalupward (with
time).
The type-2sequence boundary is recognized by a downward shift in
coastal onlap, but it does not shift beyond the offlap break (Fig. 2.10).
The shelf margin system tract is recognized most readily on seismic
lines and is very difficult if not impossible, to detect from outcrops,
cores and logs.
GeoHikingClub-2020

45. GeoHikingClub-2020