Tectonic setting of olistostromal units and associated rocks in the Talladega slate belt, Alabama Appalachians

Olistostromal deposits are extensive in the Silurian(?) to Lower Devonian Lay Dam Formation in the Talladega slate belt of central Alabama. These rocks form part of a thick (2 to 3 km) clastic sequence deposited unconformably above the upper Precambrian(?) to Lower Ordovician Appalachian miogeocline displaying a rifted to passive margin, clastic and carbonate bank facies. The Talladega belt is a far-traveled Alleghanian thrust sheet metamorphosed to lower greenschist facies during the Acadian orogeny and thrust above the foreland fold-and-thrust belt. The olistostromes are commonly several hundred meters thick and extend laterally for tens of kilometers. They are unsorted, unbedded, polymictic, matrix-supported and matrix-dominated units containing clasts of diverse provenance, including sedimentary clasts (carbonate rocks, sandstone, chert, and shale) and igneous and high-grade metamorphic clasts (granite, granitic gneiss, anorthosite, gabbroic gneiss, and garnet mica schist). The source of the clastic sequence was uplifted along fault scarps to the south, and included a Grenville basement terrane and its cover of clastic and carbonate sedimentary rocks, probably equivalent to that found unconformably below the Lay Dam Formation. Rapid erosion (caused by differential uplift and extreme relief of the source area), and relatively short transport distances to the Lay Dam basin resulted in little modification of the Lay Dam"s chemically and mechanically unstable mineral and rock fragment suite. The basin thus became characterized by heterogeneous, mineralogically unstable rock fragment compositions. The olistostromes and associated rocks are interpreted as resedimented deposits formed in relatively deep water by gravity-flow mechanisms, such as debris flow and turbidity currents, in a submarine fan-like environment. These rocks are overlain by shallow-water sequences, including the Butting Ram and Cheaha sandstones and the Jemison Chert. The Lay Dam basin is interpreted as an ensialic foreland successor basin formed in response to back-arc extension during initial stages of the Acadian orogeny.