A NEW INTERPRETATION OF THE BLUE RIDGE IN WESTERN GEORGIA AND ALABAMA

Structural and tectonostratigraphic relations indicate that the Blue Ridge in western Georgia and Alabama consists of large nappes comprised of folded thrust sheets. Geologic mapping shows that rather than joining another fault or décollement at depth, the terrane-bounding Allatoona fault also crops out to the S.E. where it bounds the Allatoona allochthon and frames the Dog River window at the northwestern edge of the Brevard zone where western Blue Ridge rocks of the Great Smoky and Chilhowee Groups are exposed from beneath the allochthon. Geologic mapping shows that the Emuckfaw and Bill Arp Formations in the Dog River window and the Lay Dam and Bill Arp Formations in the Talladega belt are all the same unit (the ELB).

The structural configuration in western Georgia consists of the Villa Rica nappe, where the ELB in the core is overlain and underlain by Chilhowee Group rocks and the synformal upper limb is occupied by the Allatoona allochthon comprised of seafloor and lesser amounts of island arc rocks at higher metamorphic grade than the western Blue Ridge rocks. To the S.W. the Villa Rica nappe, is underlain by the Wedowee nappe, cored with Wedowee Group, western Blue Ridge, with Allatoona allochthon (Hillabee Greenstone) above and below the Wedowee. Farther S.W. the structurally higher Mad Indian nappe places high metamorphic grade rocks correlative with the ELB over lower grade rocks belonging mostly to the Wedowee Group.

The age of the ELB is set as pre-Ordovician by radiometric ages of metamorphosed granitoids whose protoliths intruded it at ~460 Ma and ~490 Ma. In Alabama the higher grade ELB has been thrust upon lower grade Lower Ordovician Valley and Ridge rocks, also indicating that the ELB is older than Ordovician. The Talladega belt is divided into the eastern and western Talladega belts. The western is barely metamorphosed (anchizone?), barely deformed, fossiliferous, and unrelated to the eastern, which is at higher grade (chlorite and biotite zones), more highly deformed, and unfossiliferous. A fault beneath the I-65 Coastal Plain outlier has probably juxtaposed the eastern and western Talladega belts.

Radiometric, structural, and tectonostratigraphic data indicate that a large part of the Talladega belt is pre-Ordovician. The most likely correlation of the ELB is with the Great Smoky Group, Ocoee Supergroup.