DETRITAL RECORD OF THE LOWER PENNSYLVANIAN POTTSVILLE FORMATION, CAHABA SYNCLINORIUM, ALABAMA

The Cahaba Synclinorium in north-central Alabama contains a thick (>2.5 km) synorogenic clastic wedge that is assigned to the Pottsville Formation. The Pottsville consists of interbedded sandstone, siltstone, claystone, shale, and numerous economic coal beds, with orthoquartzite sandstone and conglomerate at the base. This thick succession of siliciclastic rocks was deposited near the paleo-equator in depositional environments ranging from fluvial-deltaic to shallow marine. Modal and heavy-mineral analyses of Pottsville sandstone provide information on provenance.

Modal analysis data reveal succession of quartzose sandstone units by increasingly quartzolithic sandstone units. Sedimentary lithic fragments dominate over the metamorphic lithic fragments. Volcanic lithic fragments are also present in some units. Plagioclase feldspars are more common than potassium feldspars.

Heavy minerals in the Pottsville Formation account for <0.9 to 1.08% of total sample weight, most of which concentrate in the fine sand (2-3Φ) fraction. Heavy minerals predominantly are highly magnetic (85%). Mineral grains include subangular to rounded garnet, chlorite/chloritoid, augite, biotite, amphibole, epidote, tourmaline, diopside, pyroxene, apatite, zircon, and opaque minerals. Opaque minerals dominate in the Pottsville sands, followed by garnets.

Pottsville sandstone samples plot in the "recycled orogenic" provenance field of the QtFL ternary diagram (Dickinson, 1985). The heavy minerals suggest derivation from multiple metamorphic and metasedimentary source terranes. Volcanic source rocks have also contributed sediment to the Pottsville Formation. The quartzose nature of the older units suggests a high degree of marine reworking coupled with the deep chemical weathering that is characteristic of the equatorial zone.

These detrital data along with existing structural cross sections, facies analyses, and paleocurrent analyses of the Pottsville Formation in the Cahaba Synclinorium suggest that the Alleghenian orogen to the east and southeast was the primary source of sediment.