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.

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.

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.

Abstract Independent researchers working in the Talladega belt, Ashland-Wedowee-Emuckfaw belt, and Opelika Complex of Alabama, as well as the Dahlonega gold belt and western Inner Piedmont of Alabama, Georgia, and the Carolinas, have mapped stratigraphic sequences unique to each region. Although historically considered distinct terranes of disparate origin, a synthesis of data suggests that each includes lithologic units that formed in an Ordovician back-arc basin (Wedowee-Emuckfaw-Dahlonega basin—WEDB). Rocks in these terranes include varying proportions of metamorphosed mafic and bimodal volcanic rock suites interlayered with deep-water metasedimentary rock sequences. Metavolcanic rocks yield ages that are Early–Middle Ordovician (480–460 Ma) and interlayered metasedimentary units are populated with both Grenville and Early–Middle Ordovician detrital zircons. Metamafic rocks display geochemical trends ranging from mid-oceanic-ridge basalt to arc affinity, similar to modern back-arc basalts. The collective data set limits formation of the WEDB to a suprasubduction system built on and adjacent to upper Neoproterozoic–lower Paleozoic rocks of the passive Laurentian margin at the trailing edge of Iapetus, specifically in a continental margin back-arc setting. Overwhelmingly, the geologic history of the southern Appalachians, including rocks of the WEDB described here, indicates that the Ordovician Taconic orogeny in the southern Appalachians developed in an accretionary orogenic setting instead of the traditional collisional orogenic setting attributed to subduction of the Laurentian margin beneath an exotic or peri-Laurentian arc. Well-studied Cenozoic accretionary orogens provide excellent analogs for Taconic orogenesis, and an accretionary orogenic model for the southern Appalachian Taconic orogeny can account for aspects of Ordovician tectonics not easily explained through collisional orogenesis.