MUDROCK LITHOFACIES ANALYSIS OF THE ORDOVICIAN ATHENS SHALE, ALABAMA FOLD AND THRUST BELT: PRELIMINARY RESULTS

The Athens Shale was deposited in slope to basinal settings in a depocenter that formed in response to Middle Ordovician Taconic orogenesis and, owing to post-Taconic deformation, is now only intermittently exposed in the Alabama fold and thrust belt. Studies of limited outcrop exposures and a 140-ft core of the Athens Shale have been initiated in attempt to better characterize mudrock lithofacies variability and associated depositional processes. Basal parts of the Athens shale contain common carbonate interbeds that reflect slope instability and mass transport from the western margin of the basin. Transport mechanisms included slumping, debris flows, and turbidity currents. Slumping is reflected by convolute laminae and soft-sediment folding in wackstones and mudstones. Debris-flow deposition is manifest by intervals of irregular nodules of bioclastic limestone surrounded by muddier fossiliferous matrix. Turbidites comprise thin, graded fossiliferous grainstone to packstone beds. Bioclasts in the debrites and turbidites include crinoid, brachiopod, bryozoan, and trilobite fragments. Frequency and thickness of carbonate event beds, mainly distal turbidites, progressively decrease up-section as shales become increasingly dominant. Shales are laminated and variably calcareous (20-40%), carbonaceous, and pyritiferous. Lack of bioturbation and body fossils other than common graptolites indicate consistently oxygen-deficient bottom waters. Laminae commonly reflect mm-scale concentrations of fine silt-sized quartz and carbonate grains that may reflect continued influence by bottom currents. The vertical succession of lithofacies through the Athens Shale is interpreted to record overall deepening of the basin. However, variations in carbonate and organic carbon likely reflect higher-frequency changes in water depth and/or paleoceanographic conditions.