Measures of abundance of extraformational conglomerates and clast composition, taken together, can be used to unravel the unroofing history of an orogenic belt. This study develops a new approach in quantifying coarse detritus as a provenance indicator. 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. The abundance of extraformational conglomerate suggests unroofing of active thrust sheets in the Alleghenian foreland basin, which can be used in deciphering the source of Alleghenian sediments in the southeastern United States. Conglomerate clasts in the upper Pottsville Formation consist mainly of sedimentary lithics (chert, sandstone, and mudstone) and metamorphic lithics (phyllite, schist, and quartzite), with minor volcanic and plutonic lithics. Compositional similarities among clasts in the upper part of the Pottsville Formation in the Cahaba basin and the lithologies of the Valley and Ridge, Piedmont, and Blue Ridge provinces suggest that the sediments were derived from proximal sources in the southern Appalachian provinces. Ternary plots of the sandstone matrix within the conglomerates indicate mostly a recycled orogenic provenance. The textures of the metamorphic clasts suggest a medium- to high-grade metamorphic source terrane. The chemical compositions of garnets (Sp-Alm-Py), including the dominance of almandine-type garnets, also indicate an amphibolite facies provenance. Finally, a whole-rock 40Ar/39Ar age of an andesite clast (∼323 Ma) suggests that andesitic volcanism occurred during the Alleghenian orogeny, and a minimum age of a metatonalite clast (∼295 Ma) suggests the minimum possible depositional age of the Pottsville Formation in the Cahaba synclinorium, indicating that the Cahaba section continued to receive Alleghenian metamorphic detritus later than previously thought.

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