GEOLOGY OF THE BREWERSVILLE 7.5-MINUTE QUADRANGLE, GREENE AND SUMTER COUNTIES, ALABAMA: MAPPING AND DETRITAL ZIRCON U-PB RESULTS FROM THE USGS EDMAP PROGRAM

The Gulf Coastal Plain remains unexplored at the 1:24,000-scale in Alabama. In west-central Alabama, Late Cretaceous through Paleogene, shallow-marine strata crop out in an arcuate belt that influences the region’s agriculture and economics. Therefore, we mapped the Brewersville 7.5-minute quadrangle to understand lithological distributions as well as the sedimentary age and provenance of the clastic Ripley Formation. Formal units mapped include, from oldest to youngest, the Demopolis Chalk, the Ripley Formation, the Prairie Bluff Chalk, and the Porters Creek Formation. Units strike in a NW-SE direction, dip gently (5-10°) to the southwest, and contain conjugate fracture sets that indicate an overall vertical sigma-one stress, presumably associated with Mesozoic-Cenozoic regional extension. We report the location of three new Cretaceous-Paleogene (K-Pg) outcrops between the Prairie Bluff Chalk and the Porters Creek Formation identified during mapping.

Detrital zircon U-Pb geochronology results from the Ripley Formation prohibit a maximum depositional age estimate because of the lack of syn-depositional grains. Previously reported calcareous nannofossil age determination (reaffirmed here) for the Demopolis Chalk brackets the Ripley Formation to Maastrichtian (< 72 Ma). Detrital U-Pb ages yield Paleoproterozoic, Mesoproterozoic, Neoproterozoic, Devonian, Mississippian, and Pennsylvanian populations, which suggests a southern Appalachian Piedmont sediment source. We interpret these results to indicate that along the western Gulf Coastal Plain the Ripley Formation was sourced by the paleo-Alabama river only and did not receive any detritus from the north or west.