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.

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.

In the Appalachian orogen, the Neoproterozoic(?)–Lower Cambrian Chilhowee Group represents the initial drift-facies deposits along and across the eastern Laurentian continental margin following rifting. In the Southern Appalachians, this group forms thrust sheets along the west flank of the Talladega–Blue Ridge belt. Where the base is unfaulted, it lies depositionally above Ocoee Supergroup rift-facies rocks or Grenville basement. Regionally, the Chilhowee grades up into the Lower Cambrian Shady Dolomite, the initial deposits of the marginwide Cambrian–Ordovician carbonate bank. Sequences more interior to the orogen, including the Kahatchee Mountain Group (Talladega belt), the Nantahala and Brasstown Formations (western Blue Ridge), and the Hollis Quartzite (Pine Mountain belt), are considered to be correlative with the Chilhowee based upon similarities in lithostratigraphic sequence, sequence stratigraphy, sandstone provenance, and paleocurrent studies. Assuming an autochthonous Pine Mountain window, palinspastic restorations of foreland thrusts suggest that the Chilhowee Group restores essentially astride that window, and Chilhowee-equivalent units in the Talladega–Blue Ridge belts, in turn, restore farther southeast. This places the respective sequences southeastward in the order of increasing thickness and depth to basement from the base of the carbonate bank facies, with units restored farthest southeast having the most distal marine characteristics. Retro-deformation of thrust belt structures and the Pine Mountain cover sequence restores the Kahatchee Mountain Group at least to the subsurface position of the Wiggins-Suwannee suture, the southeastern limit of Laurentian continental crust, indicating that this group's basement was subducted beneath Gondwanan or peri-Gondwanan crust, and that the basement of even more outboard Laurentian sequences (e.g., eastern Blue Ridge) was overridden even farther.

There is no completely accepted definition of a mélange, and the papers in this volume reflect this fact. In our regional assessment, the term mélange is principally used for a technically fragmented and mixed body of rock. A different term, olistostrome, is used for a chaotic and mixed rock body that formed by sedimentary processes such as slumping or gravity sliding. The term olistostromal mélange is used here if sedimentary processes and tectonic deformation were both involved in the fragmentation and mixing. In some cases there is evidence that these were effectively concurrent. Four main belts of Paleozoic mélanges and olistostromes have been recorded in the Appalachians of the northeastern United States and mainland Canada. These include: (1) olistostromes and olistostromal mélanges along Logan’s line and the Taconic allochthons, which are related to thrusting during the Taconian orogeny; (2) mélanges associated with ophiolite fragments along the Baie Verte–Brompton line, which are thought to represent a Taconian suture; (3) mélanges containing ophiolite fragments along the Hurricane Mountain mélange belt (Boone and Boudette, this volume), which are thought to represent a Penobscottian terrane boundary; and (4) Acadian mélanges and olistostromes such as the Silurian Deadman Harbour mélange, an olistostromal mélange that probably formed at the front of an Acadian overthrust. In addition, Precambrian olistostromes have been recognized in southeastern New England and in the Green Head Formation of New Brunswick. These may have originated along normal faults on the rifted continental shelf of Gondwanaland, or they may relate to an earlier Pan-African cycle. These Precambrian olistostromes, therefore, accompany the extensional development of the Iapetus Ocean, whereas the Paleozoic olistostromes and mélanges mark its progressive closure. Five main sets of Paleozoic mélanges and olistostromes have been recorded in the central and southern Appalachians. These are: (1) a composite mélange-olistostrome belt in the Piedmont of Maryland and Virginia that includes olistostromes such as the Sykesville Formation (of unknown age) and mélanges such as those of the Morgan Run Formation (of debated age) and Mine Run Complex, some of which contain possible ophiolite fragments; (2) mélanges in the Blue Ridge Province from Virginia to Alabama, which contain possible ophiolite fragments; (3) Silurian or Early Devonian olistostromes of the Lay Dam Formation in the Talladega slate belt of Alabama; (4) mélanges, including the Falls Lake and Juliette mélanges in the Carolinas and Georgia, that border the Carolina terrane as defined by Secor and others (1983); and (5) broken formations and mélanges along major faults such as the Pulaski and Brevard, which are mostly related to Alleghanian thrusting. In addition, there is good evidence in the Ocoee Supergroup of the Blue Ridge Province for Late Proterozoic olistostromes related to the initiation of grabens prior to the opening of the Iapetus Ocean.

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.

For many years, the question regarding what happens to the rocks of the Talladega belt in the vicinity of their apparent northeastern terminus near Cartersville has been the subject of controversy. This has coincided with the debate over the age and correlation of metasedimentary rocks that overlie the billion-year-old Corbin gneiss complex to the east of Cartersville. Both of these problems are interrelated, and the resolution of each is dependent on the other. Stratigraphic relationships in the polydeformed rocks exposed in the Salem Church anticlinorium east of Cartersville indicate that the rocks unconformably overlying the Corbin gneiss complex are lithostratigraphic equivalents of the lowermost Ocoee Supergroup. These lithologies can be traced southwestward to the area east of Emerson where the Talladega belt has been presumed to end. Here, it is evident from studying the small- and large-scale structural features that folding has played an important role in the structural and stratigraphic complications that occur. Our mapping suggests that although part of the Ocoee Supergroup does disappear southwest of Cartersville because of folding, other parts of the Ocoee continue on to the southwest and into the Talladega belt. In the Talladega belt of Alabama, rock units such as the Heflin Phyllite, Abel Gap Formation, and Lay Dam Formation are lithologically similar but may be much younger than parts of the lowermost Ocoee Supergroup sequence present in Georgia. Other rock units of the Talladega belt in Alabama also resemble parts of the Ocoee sequence, but they too are not directly relatable to the Ocoee. AH long-range correlations can be considered only speculative until detailed mapping in western Georgia and eastern Alabama is completed. However, there is evidence to suggest that at least part of the Talladega belt is Precambrian in age and was deposited synchronously with the Ocoee Supergroup.

The Talladega belt in Alabama and Georgia is the northwesternmost belt of the Appalachian Piedmont metamorphic province. It contains low-rank metasediments and metavolcanics that have been thrust faulted onto Paleozoic sediments of the Valley and Ridge province via the Cartersville-Talladega fault system. The age of several formations in the southwestern part of the Talladega belt in Alabama has been determined to be Devonian, but controversy exists concerning the age of much of the rest of the belt. Another major problem has been the age and structure relationships of the Talladega belt to the Precambrian and Lower Cambrian rocks of the Blue Ridge province on strike with the Talladega belt to the northeast. In the Borden Springs area, Cleburne County, Alabama, nappes of the Lower Cambrian Weisner and Shady Formations rest on younger Paleozoic rocks immediately northwest of the Talladega belt. A sequence composed mainly of slates and quartzites characterized by graded beds lies between the Talladega belt and the nappes of Weisner and Shady. Distinctive lithologies within this sequence are found also within the Talladega belt near Borden Springs and also near the southwest end of the belt in Alabama within metasediments immediately below the Jumbo Dolomite of the Sylacauga Marble Group. Although the slate-quartzite sequence has been interpreted in recent years as being Ordovician to Devonian, detailed mapping in the Borden Springs area indicates that it is correlative with the Early Cambrian Weisner and Shady, although somewhat different in sedimentary aspect from Weisner and Shady in nappes to the west. Therefore, the Talladega belt may contain rocks at least as old as Early Cambrian and may be at least partly equivalent in age to rocks of the Blue Ridge province. The slate-quartzite sequence lies northwest, west, and southeast of an anticlinal region of younger Paleozoic sediments in western Georgia, over which it was thrust faulted. It thus forms an imbricated nappe sequence rooted, if at all, beneath the Piedmont province to the southeast.

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.

A major carbonate sequence occurs within the lower part of the Talladega slate belt in Chilton, Coosa, and Talladega Counties. The carbonate units are overlain by a major regional unconformity known as the pre-Lay Dam Formation unconformity. The carbonate sequence below the unconformity is represented in different areas by the Jumbo Dolomite, the Marble Valley carbonates, and the Sylacauga marbles. At the type location in Chilton County, the Jumbo is a 67-m-thick, predominantly thickly bedded dolostone. The contact with the underlying slates of the Wash Creek Slate (Mount Zion Formation) is an interlayered zone of dolostone and fine, commonly graphitic, clastic rock. This zone grades upward into a dolostone that contains a few pelitic layers. Near the base, the Jumbo contains intraclasts and recrystallized fragments up to 6 cm in length. Rounded quartz grains are disseminated in the lower section of the Jumbo. Near the middle of the massively bedded dolostone is a layer that contains intraclasts of massive and laminated carbonate as much as 12 cm in length. Just below the unconformity in the type section, the upper part of the Jumbo contains laminations of fine-grained clastic rock. Along strike to the northeast and southwest the unconformity appears to have erosional relief. Less than 1 km west of the type location the unconformity truncates the carbonate sequence completely. To the northeast the pre-Lay Dam Formation unconformity appears to rise in the section in the Marble Valley carbonates and the Sylacauga marbles, exposing a very thick carbonate sequence. Stratigraphic and structural relationships of the Jumbo, Marble Valley, and Sylacauga marbles are not yet resolved. Initial data indicate that the Jumbo occurs stratigraphically below and to the northwest of the Marble Valley carbonates, suggesting that the Jumbo is the oldest carbonate unit in the sequence.

The age of the Erin Shale, Clay County, Alabama, has been variously interpreted based primarily upon the acceptance or rejection of the original Carboniferous age assignment. The failure to confirm the in-place occurrence of the reported Pennsylvanian megafossils, coupled with both regional and detailed mapping, has led to the recent placement of the Erin Shale in a stratigraphically equivalent position with the Lay Dam Formation of inferred Early Devonian age. An investigation of the previously illustrated Carboniferous fossils reportedly collected from the Erin Shale and suites of rock specimens collected during the present study has added new pieces to the Erin puzzle but has not resolved the age asignment problem. Petrographic and x-ray diffraction powder analyses of the adhering matrix on Lepidostrobus hobbsii D. White and phyllites collected from the Erin outcrop belt indicate that these rocks are similar if not indistinguishable. Identifiable coal fragments exist in the Erin as do poorly preserved, unassignable fossil plant fragments. Much additional work is needed to fully define the fossiliferous nature of the Erin.

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.