Sea level cycle phase and systems tracts
|Exploring for Oil and Gas Traps|
|Series||Treatise in Petroleum Geology|
|Part||Critical elements of the petroleum system|
|Chapter||Sedimentary basin analysis|
|Author||John M. Armentrout|
Depositional cycles can be subdivided into systems tracts, each representing a specific phase of relative sea level, e.g., highstand, falling (regressive), lowstand, and rising (transgressive). Nonmarine systems tracts can be related to rise and fall in lake level or water table level, which may or may not be synchronized with sea level change. [See Wheeler for a discussion of base level.] Identifying each cycle phase of a depositional sequence and mapping the contained facies provides a paleogeographic map for a relatively short time interval. Such high-resolution maps provide useful predictions for hydrocarbon prospecting. This subsection discusses the concept of sea level cycle phase, identification of cycle phase, construction of a cycle chart, and how sea level cycles of different duration interact.
Phases of a sea level cycle
Each cycle can be subdivided into four phases of relative sea level change:
Cycle phase & sedimentation
Deposition or erosion of sediments depends on the interaction of cycle phase and the creation of accommodation space. The sediments comprising a depositional sequence are deposited during falling, lowstand, rising, and highstand phases of a sea level cycle. Erosion, which forms a critical element of the boundaries of a depositional sequence, generally occurs during falling sea level and lowstands. Within the basin depocenter, the sequence boundary consists of a conformity that correlates with the erosional unconformity along the basin margin.
Systems tracts are composed of all deposits accumulating during one phase of relative sea level cycle and preserved between specific primary chronostratigraphic surfaces. Erosion usually dominates the falling phase of a sea level cycle, and the deposited sediments are most often assigned to the lowstand systems tract.
Lowstand systems tracts
The lowstand systems tract occurs between the basal sequence boundary and the transgressive surface. Lowstand systems are thickest toward basin centers because much of the basin margin is undergoing erosion. Lowstand systems with shelf-to-slope geometries may have basin center gravity-flow deposits due to sediment bypass of the slope and thick shelf-edge deltaic systems prograding into deep water. Fluvially dominated depositional systems are common.
Transgressive systems tracts
The transgressive systems tract encompasses those deposits between the transgressive surface and maximum flooding surface. Transgressive systems tracts show landwardstepping depositional patterns and basin margin onlap due to relative rise in sea level, forcing sediment accumulation toward the basin margin. The basin center is likely to become progressively more sediment starved, and coastal depositional systems may show a strong tidal influence.
Highstand systems tracts
The highstand systems tract is between the maximum flooding surface and the overlying sequence boundary. Highstand systems tracts show a progradational stacking pattern due to sediment supply exceeding the accommodation space. Progradation results in basinward downlapping onto the maximum flooding surface. Basin centers may still be sediment starved if shelves are broad. Coastal depositional systems tend to be wave to fluvially dominated, thin, and widespread. Definition and further discussion on identifying characteristics of each of the surface types and systems tracts can be found in Posamentier and Vail, Loutit et al., Van Wagoner et al., and Armentrout. 
- Sea level cycle phase
- Determining sea level cycle order
- Systems tracts identification
- Systems tracts and trap types
- Identifying sea level cycle phase with biostratigraphy
- Biofacies and changing sea level
- Constructing age model charts
- Superimposed sea level cycles
- Sequence stratigraphy
- Wheeler, H., E., 1964, Base level, lithosphere surface, and time stratigraphy: GSA Bulletin, vol. 75, p. 599–610., 10., 1130/0016-7606(1964)75[599:BLSAT]2., 0., CO;2
- Vail, P., R., 1987, Seismic stratigraphy interpretation procedure, in Bally, A., W., ed., Atlas of Seismic Stratigraphy: AAPG Studies in Geology 27, p. 1–10.
- Jervey, M., T., 1988, Quantitative geologic modeling of siliciclastic rock sequences and their seismic expression: SEPM Special Publication 42, p. 47–69.
- Posamentier, H., W., Vail, P., R., 1988, Eustatic controls on clastic deposition II—sequence and systems tract models: SEPM Special Publication 42, p. 125–154.
- Armentrout, J., M., 1991, Paleontological constraints on depositional modeling: examples of integration of biostratigraphy and seismic stratigraphy, Pliocene–Pleistocene, Gulf of Mexico, in Weimer, P., Link, M., H., eds., Seismic Facies and Sedimentary Processes of Submarine Fans and Turbidite Systems: New York, Springer-Verlag, p. 137–170.
- Armentrout, J., M., 1996, High-resolution sequence biostratigraphy: examples from the Gulf of Mexico Plio–Pleistocene, in Howell, J., Aiken, J., eds., High Resolution Sequence Stratigraphy: Innovations and Applications: The Geological Society of London Special Publication 104, p. 65–86.
- Brown, L., F., Fisher, W., L., 1977, Seismic-stratigraphic interpretation of depositional systems: examples from Brazilian rift and pull-apart basins, in Payton, C., E., ed., Seismic Stratigraphy—Applications to Hydrocarbon Exploration: AAPG Memoir 26, p. 213–248.
- Loutit, T., S., Hardenbol, J., Vail, P., R., Baum, G., R., 1988, Condensed sections: the key to age determination and correlation of continental margin sequences: SEPM Special Publication 42, p. 183–213.
- Mitchum, R., M., Jr., Van Wagoner, J., C., 1990, High-frequency sequences and eustatic cycles in the Gulf of Mexico basin: Proceedings, Gulf Coast Section SEPM 11th Annual Research conference, p. 257–267.
- Armentrout, J., M., Malacek, S., J., Mathur, V., R., Neuder, G., L., Ragan, G., M., 1996, Intraslope basin reservoirs deposited by gravity-driven processes: South Ship Shoal and Ewing Banks areas, offshore Louisiana, in Pacht, J., A., Sheriff, R., E., Perkins, B., F., eds., Stratigraphic Analysis: Utilizing Advanced Geophysical, Wireline, and Borehole Technology for Petroleum Exploration and Production: Proceedings, Gulf Coast Section SEPM 17th Annual Research conference, p. 7–18.