COVER STRATIGRAPHY AND STRUCTURE OF THE FORT MOUNTAIN BASEMENT MASSIF, NORTH GEORGIA BLUE RIDGE

A highly allochthonous Grenville (~1.1 Ga) basement (GB) massif (Fort Mountain Gneiss-FMG) in the Appalachian western Blue Ridge (WBR) of NW Georgia is thrust above Cambrian rocks along the SE margin of the foreland. GB occurs in 3 polyfolded, ~15 km long, SE-dipping lenses surrounded by greenschist facies siliciclastic cover rocks. The GB lenses previously were interpreted as resulting from thrust imbrication, with essentially all GB/cover contacts mapped as thrust faults. Cover units consist of two distinctive Late Proterozoic metasedimentary sequences, the lower (Pinelog Formation-PF) of which occurs only above the eastern GB lens, and consists of meta-arkose. Examination of the PF/GB contact and abundant primary structures in the PF confirm that this contact is a nonconformity. Overlying the PF in sharp contact, and elsewhere (in the absence of the PF) completely surrounding the lower two GB lenses, is a second cover sequence (Padgett Falls Formation-PFF) composed of carbonaceous phyllite and lenses of feldspathic wacke. The PFF everywhere faces away from GB or PF and its lower contact is interpreted as an unconformity that truncates W entirely through the PF to rest directly on GB. A distinctive granite boulder-bearing diamictite occurs along much of the base of the PFF and can be traced around both the NE and SW terminations of the basement lenses. At these terminations, slaty cleavage in the cover rocks and foliation in the porphyroclastic basement gneiss are parallel and strike highly oblique to the GB/cover contact, implying that GB rocks of the FMG are exposed within the cores of a series of NW-verging, shallowly inclined, isoclinal anticlines. The PF pinches out eastward between the PFF and GB, but thickens westward beneath the PFF. However, absence of the PF beneath the PFF on the overturned limb of the E. anticline and on both limbs of the two W. anticlines suggests that the PF was tilted W. along a major pre-PFF E-dipping normal (rift) fault, now located near the hinge of the E. isoclinal anticline.