ABSTRACT

A late Oligocene-earliest Miocene carbonate unit is widely distributed across the northern Gulf of Mexico. In the west, on the Texas and western Louisiana shelves, patch reefs developed on the tops of salt domes, while in the east, on the southeast Louisiana, Mississippi and Alabama shelves, a continuous, widespread carbonate layer as thick as 300-400 meters was deposited. An interpretation of this unit in the offshore Alabama/Mississippi area using a 2-D grid of seismic data (tied to well control), reveals its distribution and facies associations. It is characterized by a set of high amplitude reflections which are

Figure 1. Map showing the distribution of the reef system, carbonate mounds, and associated back-reef and fore-reef facies of the late Oligocene /earliest Miocene carbonate unit, offshore Alabama/Mississippi, based on seismic data provided by Digicon. Heavy lines are locations of Figs. 2, 3, 4. Circles are location of wells. Solid circles with numbers are wells in Fig.5.

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more prominent toward the top of the interval. Two large, well-developed mounds (20 10 km), along with an inter-mound area with more subtle relief, form an east-west trending, semi-continuous carbonate bank/reef system. This reef belt became established just seaward of the previous clinoform breakpoint. Regionally, landward of the reef belt is a broad intrashelf, non-frame carbonate facies, while seaward of the belt is a deeper shelf carbonate facies. The mounds appear as very high relief (120 meters) features on the seismic sections.

The main clastic sediment input into the Gulf basin shifted westward to offshore Texas and western Louisiana, during the Oligocene. The late Oligocene was characterized by a 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 an increase in suspended sediment influx during late-early Miocene, as a new episode of clastic input in Louisiana spilled east into the study area. This also was a time of continued rapid rise of sea level and a flooding event.