Abstract

The Talladega slate belt, the frontal crystalline thrust sheet of the Blue Ridge allochthon in Alabama, contains a Silurian(?)- Lower Devonian stratigraphic sequence not preserved elsewhere in the region that has important bearing on the tectonic evolution of the southern Appalachian orogen. A metamorphosed upper Precambrian(?) to Lower Cambrian clastic and Lower Cambrian to Lower Ordovician carbonate sequence in the lower part of the Talladega belt represents the Appalachian miogeocline (western margin of Iapetus), which progressed from a rifted basin into a stable carbonate trailing margin platform. A second major metclastic sequence of Silurian(?)-Early Devonian age (Talladega Group) developed as a successor basin unconformably atop the carbonate shelf facies following a period of uplift, tilting, erosion and subsidence of the Cambro-Ordovician carbonate platform. The unconformity at the base of the sequence bevels through the older rocks at a low angle, regionally cutting out the entire 3.5 km thick carbonate bank facies, and locally placing the Talladega Group directly above the lower clastic sequence. Significant deformation predating the unconformity is not apparent. The base of the successor basin is represented by a thick (2-3 km) submarine fan-like debris apron containing a proximal olistostromal facies of bouldery mudstones which grades distally into and is intercalated with turbidite-like sequences and various deep water sandstone facies. Clasts within the olistostromes represent the various clastic and carbonate units below the basal unconformity, as well as a suite of Precambrian basement lithologies which include granite and felsic gneiss, probably derived from the crystalline basement of the miogeocline. Overlying the fan system is a shallow marine to lagoonal sequence of sparsely fossiliferous Lower Devonian metasandstone and conglomerate, black slate, metachert and siliceous argillite. These rocks are succeeded by a thick (~2-5 km) sequence of bimodal metavolcanic rocks. The Silurian(?)-Lower Devonian successor basin and associated rocks are interpreted to have formed as a result of back- arc extension during the onset of Acadian subduction beneath the southeastern margin of Laurentia.