The document discusses the lowstand systems tract (LST), defining it as deposits that accumulate after the onset of relative sea-level rise during a period of early rise and normal regression. The LST includes fluvial, coastal, shallow marine, and deep marine deposits characterized by progradation or retrogradation. Key points covered include the depositional processes and products of each environment within the LST, as well as the economic potential of LST deposits for reservoirs and placer deposits.

1. Mahboob Ahmed

2. Depositional
System tract processes of LST
 Definition and main point  Fluvial LST
 Lowstand system tract  Depositional processes and
 Definition and main points products of the LST
 LST stratal stacking patterns  Coastal LST
and bounding surfaces  Deep marine LST
 Detailed architecture of LST
 Grading trends of LST
 Wheeler diagram of
depositional patterns during  Thickness, distribution
a full regressive and and grain size of LST
transgressive cycle deposits
 Economic Potential of LST

3.  The term was first used by Brown and Fisher (1977) to
represent contemporaneous depositional systems.
 Genetically associated stratigraphic units that were
deposited during specific phases of the relative sea-level
cycle (Posamentier , et al, 1988).
 These units are represented in the rock record as three-
dimensional facies assemblages.
 A component unit of a sequence which is bound by
sequence-stratigraphic surfaces.
 Systems tracts are defined on the basis of :
1. types of bounding surfaces
2. stratal geometry
3. position within sequence

4.  Includes deposits that accumulate after the onset of relative
a sea-level rise.
 In LST all sedimentary deposits accumulated during the
stage of early rise normal regression.
 LST bounded by the SU and its correlative conformity at
the base and by the MRS at the top.
 This systems tract lies directly on the upper surface of the
Falling Stage Systems tract.
 stacking patterns are dominated by low rate aggradation
and progradational (deltaic or fluvial facies) or
retrogradational (submarine fans) across the entire
sedimentary basin.

5.  Lst Deposited basin ward of shelf margin and overlies type
1 sequence boundary.
 accommodation is made available by the rising base level.
 LST include the entire suite of depositional systems, from
fluvial to coastal, shallow-marine and deepmarine.
 Lowstand deposits essentially consist of the coarsest
sedimentary unit of both the nonmarine and marine
deposits.
 Non marine strata of LST forms fining upward profile in
lower part and in upper part of shallow marine succession
LST forms coarsening upward profile.

9.  During the early rise normal regression sediments of LST
are more evenly distributed between the fluvial, coastal,
and deep-water systems.
 Fluvial LST
 Typical examples of Lowstand fluvial deposits are
amalgamated channel fills in which sand accumulate
within incised valleys.
 The ‘Lowstand prism’ gradually expands landward via
fluvial aggradation and onlap.
 Within the Lowstand successions of amalgamated channel
fills, paleosols may be present, reflecting syndepositional
conditions of limited accommodation on floodplains.

10.  low- and high-accommodation systems tracts may provide
a more realistic approach to describing fluvial deposits in a
sequence stratigraphic framework.
 Due to topographic irregularities at the stratigraphic level
of the subaerial unconformity, the nonmarine portion of
the Lowstand systems tract may display a discontinuous
geometry of depositional sequence.
 The increase with time in the rate of base-level rise also
contributes to the overall fining-upward fluvial profile, as it
creates more accommodation for floodplain deposition.
 The contact between Lowstand fluvial and the overlying
estuarine facies is the maximum regressive surface.

12.  Prograding shoreline and shoreface delta front includes in
coastal LST.
 Coastal aggradation during Lowstand normal regression triggers
a decrease in slope gradient in the downstream portion of fluvial
systems.
 in a low-gradient shelf-type setting with high sediment supply
are generates large area of fluvial aggradation.
 Steep topography (e.g., in a high-gradient ramp setting, such
 as a continental slope or a fault-bounded basin margin) with low
sediment supply formed small size area of fluvial aggradation.
 The preservation potential of coastal and adjacent Lowstand
fluvial strata may be low due to subsequent transgressive
ravinement erosion.

13.  Lowstand sediments of the basin-floor submarine fan all
are overall fine-grained deposits.
 Sand/mud ratio of the sediment load transported by
turbidity Currents in deeper part.
 High-density turbidity currents formed by late stage of
forced regression.
 The deep-water portion of the Lowstand systems tract is
dominated by low-density turbidities.
 The transition from high-density to low-density turbidities
at the onset of base-level rise during early normal
regression.
 Aggradation in fluvial to shallow-marine environments
reduces the amount of sediment supply to the deep basin,
and the turbidity currents are dominantly of low density.

14.  (4) due to decreasing slope
gradients and associated
fluvial competence.
 (5) due to the progradation
of delta front/shoreface
facies over finer
prodelta/shelf sediments.
 (9) transition from high-
density to low-density
Turbidities.

16.  Petroleum Plays
 Rising base level during the Lowstand normal regression
provides accommodation across the entire basin, from
fluvial to marine environments.
 Highest sand/mud ratio, of the Lowstand systems tract are
best use for reservoir rock.
 The petroleum plays of the Lowstand systems tract occur in
terms of origin and syndepositional processes, ranging
from fluvial to coastal, shallow- and deep-marine systems.
 Equally good reservoirs may form in coastal, shallow-water
and deep-water environments during the Lowstand normal
regression of the shoreline.

17.  onset of base-level rise the low-density turbidity currents,
has important for the lithology, morphology and location
of deep-water reservoirs within the basin.

18.  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.
 Environmental conditions of Lowstand fluvial system are
generally unfavorable for peat accumulation.

19.  No unconformities form during the Lowstand normal
regression.
 but the Lowstand systems tract is closely associated with
all three types of unconformity related placer deposits.
 placer deposits represented by lag deposits associated with
subaerial unconformities or regressive surfaces of marine
erosion.
 Lowstand deposits are particularly prone to ‘reef’ facies
which develops in the case of gravel-bed fluvial system
during the early stage of normal regression.
 depositional reefs causes the mineralization and
precipitation of placer deposits.