Mahboob Ahmed

System tract

Definition and main point

Lowstand system tract

Definition and main points

LST stratal stacking patterns and bounding surfaces

Detailed architecture of LST

Wheeler diagram of depositional patterns during a full regressive and transgressive cycle

Fluvial LST

Depositional processes and products of the LST

Coastal LST

Deep marine LST

Grading trends of LST

Thickness, distribution and grain size of LST deposits

Economic Potential of LST

Depositional processes of LST

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

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.

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.

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.

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.

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.

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.

(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.

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.

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.

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.

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.