New paleontologic evidence constraining the age and paleotectonic setting of the Talladega slate belt, southern Appalachians

Abstract

The Talladega slate belt in the southern Appalachian orogen of Alabama and Georgia is a thick sequence of lower greenschist-facies metaclastic, metacarbonate, and metavolcanic rocks thrust above miogeoclinal rocks of the foreland and overthrust by higher grade metamorphic rocks of the eastern Blue Ridge terrane(s). The age assignments and tectonic affinities of this sequence have been highly controversial. Recent fossil discoveries in key stratigraphic units, however, combined with confirmed fossil occurrences in the Jemison Chert, have established a firm correlation with the Appalachian foreland, thus stratigraphically linking the Talladega belt with Laurentia. The lower predominantly clastic sequence, tectonically bounded at its base by the frontal Blue Ridge thrust system, grades upward into a 3.5-km-thick marble sequence. The basal carbonate unit (Jumbo Dolomite) contains Early Cambrian archaeocyathids, and these fossils, in addition to the stratigraphic position and carbonate lithofacies, establish correlation of this unit with the Lower Cambrian Shady Dolomite. The uppermost unit in the marble sequence (Gantts Quarry Formation) contains Early Ordovician (middle to late Canadian; = early to middle Arenigian) conodonts that confirm correlation of this unit with the Newala Limestone and Kingsport and Mascot Formations of the Appalachian foreland. The carbonate platform sequence is unconformably overlain by a thick clastic sequence that accumulated in a deep successor basin; the Lay Dam Formation is a marine fanlike deposit at the base of this sequence. Conodont molds from the top of the Lay Dam and fossils from the stratigraphically higher Jemison Chert indicate a Silurian to Early Devonian age for the Lay Dam Formation. Paleontologic data and the stratigraphic and structural setting indicate that the Talladega slate belt is the most distally preserved and relatively complete fragment of the Appalachian miogeocline; thus, the tectonic evolution of the Talladega belt is crucial to understanding the western margin of Iapetus. Linkage of the Talladega slate belt rocks with those of the western Blue Ridge to the northeast suggests that the latter once contained a thick Cambrian to Devonian cover sequence which subsequently has been mostly removed.