Llinas, Juan Carlos¹ 
(1) University of Alabama, Tuscaloosa, AL

ABSTRACT: Controlling Factors on the Occurrence of Microbial Buildups in the Upper Jurassic Smackover Formation, Eastern Gulf Coastal Plain

Numerous studies of the Smackover Formation have shown the presence of diverse types of reefs following a trend proximal to the updip limit of Smackover deposition from Arkansas to the Florida panhandle. In Alabama, this trend comprises the Manila and Conecuh Sub-basins, which were restricted embayments rimmed by the elevations of the Choctaw, Conecuh and Pensacola Ridges. Study of well cores in the area indicates that Smackover sediments were accumulated in a shallow carbonate marine environment with arid climatic conditions. Isolation from open waters generated a marine environment with depleted oxygen levels, high concentration of organic material and low energy. The dry climate resulted in minimum erosion, low sediment background, and high evaporation rates that increased the salinity and calcium carbonate concentration in the water. These abnormal conditions, hostile for common reef-builders, favored the establishment of microbial reefs formed by cyanobacteria and other microbes that developed on the flanks and crests of local paleohighs. The association with basement highs is a clear evidence of the control that paleotopography exerted in the distribution of the microbial buildups. In Vocation Field, located on the southern margin of the Manila Sub-basin, and Appleton-NW Appleton Fields localized on the western side of the Conecuh Sub-basin, microbial reefs reached thicknesses over 150 feet. In these oil fields, microbial structures consist of boundstones with varied thrombolite textures, developed on top of basement rocks. The reefs developed during a transgressive phase, filling up rapidly the accommodation space generated by the combination of a eustatic sea level rise and thermal subsidence.The demise of the reefs occurred during the following regressive phase.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.