Petrofacies and Detrital Geochronology of the Upper Pottsville Conglomerate Magnafacies, Cahaba Synclinorium, Southern Appalachian Fold and Thrust Belt, Alabama

Abstract

Measures of abundance of extraformational conglomerates and clast composition, taken together, can be used to unravel the unroofing history of an orogenic belt. An attempt has been made in this study to develop a new approach to assess sediment provenance based on coarse detritus. Known as the “conglomerate measures” in the Cahaba synclinorium, the uppermost Pottsville Formation consists of conglomerates and subordinate sandstone, shale, and coal. These siliciclastic sediments were deposited during the Alleghenian orogeny primarily in braidplain-anastomosis environments in Alabama and Mississippi. Clasts in conglomerates of the upper Pottsville Formation consist mainly of sedimentary lithic fragments (chert, sandstone, and mudstone), metasedimentary lithic fragments (phyllite, schist, quartzite), and volcanic and plutonic rock fragments. Compositional similarities between clasts in the upper part of the Pottsville in the Cahaba basin and lithologies of Valley and Ridge, Piedmont, and Blue Ridge provinces suggest that the sediments were derived from proximal sources in the southern Appalachian provinces. Composite modal analysis and ternary plots of sandstone composition in conglomerate matrix in the Cahaba basin indicate mostly a ‘recycled orogenic’ provenance. Matrix compositions also indicate that the source of the upper Pottsville sediments was to the east and southeast in the evolving Appalachians. Heavy mineral assemblages in sandstones suggest a medium- to high-grade metamorphic source terrane in the Piedmont. Chemical compositions of garnets (Sp-Alm-Py) indicate an amphibolitic facies provenance and the dominance of almandine garnets suggest that a medium- to high-grade metamorphic source supplied a large quantity of sediment to the Cahaba basin. A whole rock 40Ar/39Ar age of an andesite clast (~323 Ma) suggests andesitic volcanism during Alleghenian orogeny. Minimum age of a metatonalite clast (~295 Ma) suggests minimum depositional age of the Pottsville Formation in the Cahaba synclinorium, which indicate that the Cahaba section continued to receive Alleghenian metamorphic detritus later than previously thought.