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Miocene Systems Tracts of East and West Cameron OCS Areas, Offshore Western Louisiana, Gulf of Mexico
S. Y. Yang (1), J. S. Watkins (2), M-J. Jiang (3), X. Du (2), B. E. Bradshaw (2)
The conventional seismic stratigraphic sequence model is composed of a relatively conformable succession of genetically related strata bounded at its top and base by unconformities and their correlative conformities, as determined from the nature of discontinuities in seismic profiles. Alternatively, Galloway has suggested the use of a genetic sequence model whose boundaries coincide with condensed sections.
We have studied the Miocene of East and West Cameron OCS areas in light of these two models. This area has been characterized by relatively high sedimentation rates, extensive structural alteration by growth-faults and to a lesser extent through diapirism. Identification of unconformable sequence boundaries using unconformities is difficult because of the scarcity of identifiable baselap and because of expansion at growth-faults. The condensed section is easily recognized, correlated, and mapped using nannofossil, well-log, and seismic data. Thus, our sequences correspond to Galloway's genetic sequence model.
In the East and West Cameron OCS areas, eight condensed sections from the late Early Miocene through the Early Pliocene have been recognized. These are used to define stratigraphic sequences and their corresponding systems tracts. The highstand systems tracts deposited on the shelf are characterized by variable-to-low amplitudes during high sediment influx and moderate-to-low amplitudes during low sediment influx. The highstand systems tracts are characterized by high amplitude and high continuity downdip of the growth-faults, indicating the deposition of turbidites in deep-water environments. Seismic facies of transgressive systems tracts are characterized by high-amplitude, high-frequency, and moderate-continuity. The condensed section exhibits a strong reflection. Lowstand fans and lowstand prograding wedges were deposited on the downthrown side of the fault only when sediment influx was high. Shelf margin deltas characterized by moderate- to low-amplitudes, low-frequency, and thick sand units were deposited on the shelf during high sediment influx. During low sediment influx, lowstand slump deposits, which include thick sands characterized by low amplitude and low continuity, were deposited in the area downdip of the growth-faults.
We suggest designating the classic sequence model as a "Sloss sequence", and the genetic model as a "Frazier sequence". These names honor stratigraphers who were among the first to study each of the models.
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