Abstract The allochthonous Talladega belt of eastern-northeastern Alabama and northwestern Georgia is a northeast striking, fault bounded block of lower greenschist facies metasedimentary and metaigneous rocks that formed along the margin of Laurentia at or outboard of the seaward edge of the Alabama promontory. Bounded by metamorphic rocks of the higher grade Neoproterozoic(?) to Carboniferous eastern Blue Ridge on the southeast and unmetamorphosed to anchimetamorphic Paleozoic rocks of the Appalachian foreland on the northwest, the Talladega belt includes shelf facies rocks of the latest Neoproterozoic/earliest Cambrian Kahatchee Mountain Group, Cambrian-Ordovician Sylacauga Marble Group, and the latest Silurian(?) to uppermost Devonian/earliest Mississippian Talladega Group. Along the southeastern flank of these metasedimentary sequences, a Middle Ordovician back-arc terrane (Hillabee Greenstone) was tectonically emplaced along a cryptic pre-metamorphic thrust fault (Hillabee thrust) and subsequently dismembered with units of the upper Talladega Group along the post-metamorphic Hollins Line fault system. Importantly, strata within the Talladega belt are critical for understanding the tectonic evolution of the southern Appalachian orogen when coupled with the geologic history of adjacent terranes. Rocks of the lower Talladega Group, the Lay Dam Formation, suggest latest Silurian–earliest Devonian tectonism that is only now being recognized in other areas of the southern Appalachians. Additionally, correlation between the Middle Ordovician Hillabee Greenstone and similar bimodal metavolcanic suites in the Alabama eastern Blue Ridge and equivalent Dahlonega Gold belt of Georgia and North Carolina suggests the presence of an extensive back-arc volcanic system on the Laurentian plate just outboard of the continental margin during the Ordovician and has significant implications for models of southern Appalachian Taconic orogenesis.

Talladega “Series” rock units in Polk and Paulding Counties have been mapped through the Cartersville area along the southeastern fault boundary of the “Corbin granite complex,” and correlated with rock units of the Great Smoky Group in Cherokee County. The Talladega “Series” in this area is bounded on the northwest by the Emerson (Cartersville) fault, along which the Talladega was thrust over (1) Valley and Ridge rocks ranging in age from Early Cambrian (Chilhowee Group) to Mississippian (Fort Payne Chert), (2) the Ocoee Supergroup, and (3) the “Corbin granite complex.” The Emerson fault also overrode the Great Smoky fault, which separates Ocoee from Chilhowee rocks in the Cartersville area. The thrust fault along the southeastern boundary of the Talladega “Series” is only one of numerous closely spaced imbricate faults within and southeast of the Talladega. The presence of closely spaced thrusts within the Talladega, the association of Hillabee Chlorite Schist-“Pumpkinvine Creek” lithologies with many of these faults, and the fact that detailed mapping has not been completed between the Rockmart-Yorkville area and the Alabama state line emphasize the need for caution in projecting correlations and interpretations across this area.

The Talladega slate belt in eastern Alabama represents a crystalline thrust sheet composed of low-grade metasediments. Three major lithologic sequences comprise the Talladega slate belt: (1) the Kahatchee Mountain Group, (2) the Sylacauga Marble Group, and (3) the Talladega Group. The contact relationships between the Sylacauga Marble Group and the Talladega Group indicate that the phyllites and slates of the Talladega Group rest unconformably on marbles of the Sylacauga Marble Group. Previous workers have identified specific stratigraphic sequences within the Talladega Group both in the north-central portion of the Talladega slate belt and in the southern portion of the belt. Little work has been carried out in the south-central portion of the Talladega slate belt, a region intermediate between areas to the northeast and southwest where the regional stratigraphy has been defined. To the northwest in Cleburne and Clay Counties, Alabama, the Talladega Group has been broken down into the Heflin Phyllite, the Able Gap Formation, and the Chulafinnee Schist. To the southwest in Chilton County, Alabama, similar units have been mapped as the Lay Dam Formation, the Butting Ram Sandstone, and the Jemison Chert. These units have not been mapped through this intermediate south-central region of the Talladega slate belt because of the absence by faulting of a major sandstone unit, the Cheaha Quartzite, which has been used for regional correlation. Another prominent unit, the Jemison Chert, which outcrops to the southeast of the Cheaha Quartzite, continues across this region and was used to correlate the regional stratigraphy from the northeast with that in the southwest. Detailed mapping has shown that a small slice of paper-thin quartzites of the Jemison Chert interval has overridden the Cheaha Quartzite. The geometric relationships between these two units, the differing petrologic character of these ridge-forming lithologies, the duplication of the Jemison Chert interval, and the emplacement of this imbricate slice of Jemison, in addition to structural fabric data, suggest that this termination of the Cheaha Quartzite is fault related.

The Hillabee Greenstone is a mafic metavolcanic sequence at the stratigraphic top of the Talladega Group in the Northern Alabama Piedmont. The Hollins Line fault forms the upper contact of the Hillabee, and an undetermined amount of the Greenstone has been removed by faulting. The igneous protolith consisted of tholeiitic rocks (ash and lava) with minor calc-alkaline dacites. Volcanism was arc-related, based on geochemical evidence and the associated lithologies. Greenschist facies metamorphism and alteration have resulted in changes in the chemistry and mineralogy of the rocks. However, a variety of remnant igneous textures can be observed, and through statistical means, samples that have experienced minimal alteration can be identified, thereby allowing some of the igneous evolution of the Hillabee to be deduced. The chemistry and mineralogy (norm and mode) indicate tholeiitic fractionation representative of relatively shallow crustal depths with typical enrichment in Fe, V, Ti, and quartz and depletion of Mg, Cr, Ni, Co, and olivine. The dacites are associated with more highly fractionated basalts but do not appear to be a direct product of fractionation from the basalts. Some mafic rocks are typically gabbroic in texture and are interpreted to represent lava flows. Geochemical and field evidence indicates that the exposed Hillabee Greenstone represents an immature arc; plagioclase and olivine were highly fractionated prior to extrusion, and early stages of fractionation are not represented in surface exposures. Temporally earlier volcanics may occur downdip to the southeast toward the volcanic source; the exposed Hillabee Greenstone represents volcanics deposited somewhat distally from the volcanic source on the edge of the basin.