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Barrier Island Depositional Systems in Black Warrior Basin, Lower Pennsylvanian (Pottsville) in Northwestern Alabama

Christopher A. Haas, Robert A. Gastaldo

The basal Pennsylvanian lower Pottsville Formation in the Black Warrior basin of northwestern Alabama is part of a southwestward-thickening wedge of terrigenous sediments consisting of orthoquartzitic sandstone, siltstone, and shales with discontinuous coals. The predominant facies include well-sorted, structureless or low-angled cross-bedded sandstones. Common subfacies are poorly sorted, medium-angled cross-bedded orthoquartzite with shale clasts and coal stringers, and fine-grained, massive to cross-bedded subgraywacke sandstone. The shale facies are dark gray, silty, carbonaceous, and burrowed. These facies contain scattered concentrations of marine and plant fossils. Numerous authors have proposed a barrier-island/back-barrier/prodelta depositional system based prima ily on sandstone lithologies, geometry, and paleocurrent data. However, detailed description of identifiable litho-types and sedimentary structures within this system have been neglected.

The present study delineates each lower Pottsville lithofacies, to confirm or refute a barrier-island model. Preliminary interpretation of lithofacies using lithologic criteria, sedimentary structures, and fossil assemblages confirms a barrier depositional system.

Exposures along I-65 in southern Cullman County are interpreted to represent lagoonal deposits based on the high percentage of mud-sized material, massive and structureless washover sandstone beds, and highly rippled interbedded sandstones and silty shales that contain microcross-stratification. Exposures in northern Cullman County are interpreted to represent tidal channel-fill deposits, flood tidal sequences, and possible foreshore sandstone deposits. Tidal channel-fill deposits are recognized by coarse sandstone textures with pebble lags, large-scale cross-bedding, and their geometry. Flood tidal sequences are recognized by stacked cross-bedded sets and additional sedimentary structures. Foreshore deposits are interpreted based on the orientation of low-angle planar bedding.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.