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Depositional Environments of Pottsville Sandstones and Conglomerates in Alabama

Suleyman Demirpolat

The Pennsylvanian Pottsville Formation in the Cahaba Basin, Alabama, is structurally in the Cahaba synclinorium in the Appalachian fold and thrust belt. The formation consists of a sequence of conglomerates, sandstones, siltstones, mudstones, and coals, more than 3 km thick. The sandstones are well cemented (with silica) and/or square textured.

Polymictic conglomerates are present in the middle and upper parts (Straven and post-Straven). The conglomerates contain pebbles and cobbles of quartz, metamorphic rocks, and sedimentary rocks up to 25 cm in diameter, and thus differ from conglomeratic sandstone in the Shades Sandstone Member, which contains only small and well-rounded white quartz pebbles. These first and second cycle (or multicycle), sheared, pocked, and half-round pebbles were cut by faulting and ground against each other after deposition. Sheared and pocked pebbles typically develop where sand matrix is not abundant; single pebbles "floating" in sandstone rarely show these fractures. Over 2,000 measured pebble imbrications indicate that the conglomerates are not fluvial deposits. Primary modes of inclination are t the northwest and south-southeast. Secondary modes are to the south-southwest. Primary structures and fossils indicate that sandstone members are in large part marine and probably were deposited on a beach and/or migrating shoals. Cross-beds suggest primary and secondary transport directions to the northwest and southwest, respectively. Study of Pennsylvanian paleowind patterns (relative to the continental position and orientation) and of symmetric and asymmetric ripple marks suggests that paleotransport was to the northwest and that the shoreline was oriented northeast-southwest.

Crossplots of neutron vs. density logs yield an average of 4-5% porosity for these quartzites.

AAPG Search and Discovery Article #91031©1988 AAPG Eastern Section, Charleston, West Virginia, 13-16 September 1988.