Abstract

Chesterian (Mississippian) strata in northern Alabama comprise a mixture of shallow marine carbonate and siliciclastic units. Monteagle Limestone consists primarily of medium- to thick-bedded skeletal grainstone consisting of oolitic intervals and thin lenticular layers of green-gray calcareous shale. The Hartselle Sandstone is mostly quartzarenite in north-central Alabama, but toward the northeast, near its stratigraphic terminus 30 km east of Huntsville, Alabama, Hartselle-equivalent strata consist primarily of shale with thin skeletal limestone and calcareous sandstone interbeds. We studied four sections along U.S. Highway 72 that cut through the Monteagle-Hartselle interval in this region in order to resolve the petrography and faunal characteristics of these strata. Monteagle Limestone is dominated by skeletal grainstone enriched in echinoderm and bryozoan debris. Some allochems are oolitically coated. Parallel-laminated to low-angle cross-laminated oolites occur locally. Two intervals are marked by prominent color variation and lenses of green-gray calcareous shale. Dolomitized intervals, some pervasive, are found in all of the outcrops. Portions of the Monteagle contain well-sorted subrounded dolomite clasts, which likely originated from erosion of dolostone during synsedimentary subaerial exposure. The Hartselle-equivalent interval consists of tan/brown to gray-black bituminous and fossiliferous silty shale with interbedded lenticular to channelized skeletal grainstone beds. Brachiopods and crinoids are common in both rock types, but limestone strata also contain bryozoans (including Archimedes and rare rugose corals. Microbial colonies coat many of the skeletal allochems and form one small buildup atop limestone at the base of the Hartselle-equivalent interval. Two smaller mounds were found within dolostone intervals in the Monteagle. Lithofacies relationships between the four sections suggest that the Monteagle Limestone was deposited as a series of carbonate shoals and that calcareous shale might have formed in slightly deeper water intershoal areas. Hartselle-equivalent shale was deposited in shallow water in advance of a prograding siliciclastic wedge derived from the southwest, but interdigitation with carbonate lithofacies was common. Shallow marine carbonate sedimentation (Bangor Limestone) resumed domination on the platform soon after deposition of the Hartselle.