HEAVY MINERAL CONSTRAINTS ON PROVENANCE OF LOWER PENNSYLVANIAN POTTSVILLE FORMATION, CAHABA SYNCLINORIUM, ALABAMA

The Cahaba Synclinorium, located in north-central Alabama, contains a thick (>2.5 km) synorogenic clastic wedge belonging to the lower Pennsylvanian Pottsville Formation. The Pottsville consists of interbedded sandstone, siltstone, claystone, shale, and numerous coal beds, with orthoquartzite sandstone and conglomerate at the base. This thick sequence was deposited in depositional environments ranging from fluvial-deltaic to shallow marine, located near the paleo-equator.

Modal analysis data reveal succession of quartzose sandstone units by increasingly quartzolithic sandstone units that were derived from a recycled orogenic provenance. Sedimentary lithic fragments dominate over the metamorphic lithic fragments. Volcanic lithic fragments are present in some units. Plagioclase feldspars are more common than potassium feldspars.

This study focuses on abundance and semi-quantitative point count analysis of heavy mineral fractions from the Pottsville Formation of Alabama. Heavy minerals were separated using liquid acetylene tetrabromoethane (specific gravity 2.9). Heavy minerals account for <0.9 to 1.08% of total sample weight, most of which are concentrated in the fine sand (2-3 phi) fractions. The mineral fractions were further classified by magnetic intensities using a Frantz magnetic separator. Most of the grains (88%) fell in the highly magnetic fractions. Mineral identification has predominantly yielded garnet, amphiboles, tourmaline, zircon, spinel, chlorite, and opaque minerals. From the non-opaque mineral varieties, garnet dominated (45%), followed by amphiboles (35%), tourmaline (10%), and zircon, spinel, and chlorite (<10%). Garnets were most likely derived from a metamorphic source. Amphiboles, primarily hornblende, suggest derivation mostly from intermediate and alkaline igneous intrusive rocks, while zircons were probably derived from silicic to intermediate igneous rocks. Tourmaline may point to a granitic and/or granite pegmatitic source.

These heavy mineral data along with modal analysis results, structural cross sections, facies and paleocurrent data of the Pottsville Formation in the Cahaba Synclinorium suggest that the metamorphic belts and previously existing arcs in the Appalachian orogen to the east and southeast were the primary source of sediments.