POSSIBLE MIDDLE CHESTERIAN (MISSISSIPPIAN) MICROBIAL MOUNDS IN NE ALABAMA

Two possible bioherms 0.7 m tall by 0.4 m wide and possessing synoptic relief are exposed along State Highway 72 near Woodville, AL. Shallow marine carbonates of the Monteagle Limestone pass upward through approximately 7 meters of mixed siliciclastic-carbonate strata into shallow marine limestone and dolostone of the Bangor Limestone. The heterogeneous interval above the Monteagle carbonates is roughly equivalent to the Hartselle Sandstone, which pinches out just west of the study site. The exposed Monteagle carbonates are dominated by skeletal grainstone enriched in echinoderm detritus, but includes lenses of laminated to low angle-cross laminated oolite. Mud-packstone interlayers are comparatively rare. Two intervals marked by prominent color-variation, argillite lenses, and dolomitized brecciated clasts, are interpreted as exposure surfaces in the uppermost Monteagle. This is overlain by dark gray shale, lenticular to channelized skeletal grainstone, and brown to black bituminous, fossiliferous silty shale. Brachiopods and crinoid detritus are common. Some limestone is enriched in well sorted, well rounded, gravel-sized dolomite intraclasts, likely derived from erosion of exposure surfaces. Dolomite clasts to 5 cm in diameter occur at the base of a skeletal grainstone channel in this interval. A small bioherm is correlative with an exposure surface in the Monteagle carbonate and is selectively dolomitized. It contains fenestrae, but no other obvious microbial structures. A second mound caps a grainstone unit in the transition zone, and is sharply overlain by dark gray shale. It is composed of thin seams of mud-packstone interbedded with dolomitic and siliciclastic sediment. The carbonate layers may be microbial. This outcrop adds to the scanty record of Mississippian bioherms in Alabama.