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ABSTRACT: Seismic Expression and Depositional Setting of the Late Oligocene-Early Miocene Carbonate Unit, Offshore Alabama


A late Oligocene-early Miocene carbonate unit (Heterostegina Zone Limestone) is widely distributed across the northern Gulf of Mexico. An Interpretation of this unit in the offshore Alabama using a 2D grid of seismic data tied to well control reveals its distribution and facies association. It is characterized by a set of high amplitude reflections, which are more prominent toward the top of the interval. Two large well-developed mounds (20x10 km), along with the inter-mound area of more subtle relief, form an east-west trending, semi-continuous reefal rim of carbonate system. This rimmed margin became established just seaward of the previous clinoform breakpoint. Regionally, landward of the reef/bank rim is the broad intrashelf, non-frame carbonate facies, and seaward of the belt is the deeper shelf carbonate facies. The mounds appear as very high relief (400 feet) features on the seismic section, and one of the mounds is on top of a paleao-uplift related to salt movement.

The main clastic sediment input into the Gulf Basin shifted to the west off Texas and western Louisiana during the Oligocene. Also, the late Oligocene were characterized by basin-wide decrease in clastic sediment supply providing relatively clear and clean waters in the east for the development of carbonates. This condition was terminated by increasing suspended sediment influx during late early Miocene as a new episode of clastic input in Louisiana spilled east onto the area, combined with a continued rapid rise of sea level and flooding event.

AAPG Search and Discovery Article #90941©1997 GCAGS 47th Annual Meeting, New Orleans, Louisiana