2002 Denver Annual Meeting (October 27-30, 2002)

Paper No. 5
Presentation Time: 8:00 AM-12:00 PM


CRAWFORD, Ralph F., The Geologic Mapping Institute, 1297 Briardale Lane, Atlanta, GA 30306 and HIGGINS, Michael W., The Geologic Mapping Institute, 162 Spring Drive, Roswell, GA 30075-4849, crawford@sprintmail.com

The geologic map of the Atlanta quadrangle has been compiled into ESRI’s ArcView 8.2. The quadrangle spans from the eastern Valley and Ridge allochthon in its northwestern corner, across the Great Smoky fault and the Blue Ridge allochthon, and across the Brevard fault zone and into the Piedmont, which underlies the southeastern half of the quadrangle. Southeast of the Brevard fault zone the Atlanta area is underlain by a large, recumbent, refolded, isoclinal fold nappe. Farther east, are nappes of Devonian(?) Lithonia Gneiss, overthrust by quartzite and aluminous schist of the Sandy Springs Group, which also borders the northwestern side of the Brevard zone. Northwest of the Brevard fault zone, a large area of the western Blue Ridge is structurally overlain by the Allatoona allochthon, consisting of metamorphosed ocean-floor rocks, including garnetiferous amphibolite, metaplagiogranite, metagabbro, metapyroxenite, metalliferous quartzites (magnetite quartzite, gondite, and banded iron formation) interpreted to have been black-smoker deposits, metalliferous schists, and metamorphosed ultramafic rocks. The Allatoona allochthon has been thrust upon the western Blue Ridge rocks on the Allatoona thrust fault, which bounds the allochthon on the northwest and southeast. The Dog River window, framed by the Allatoona fault, exposes western Blue Ridge rocks below the Allatoona allochthon. The time of thrusting on the Allatoona fault is dated at about 430 M.A. (Early Silurian) by Rb-Sr isochron and Pb-U zircon ages from the Austell Gneiss, which has been overthrust by the fault and mylonitized adjacent to the fault, and is interpreted to have been intruded during faulting. The Mulberry Rock Gneiss, which underlies the northwestern trace of the Allatoona fault in the Mulberry Rock Culmination is interpreted to be correlative with the Austell Gneiss. The Keaton Creek synform and Caney Creek antiform in the southwestern part of the Dog River window are best interpreted as occupying parts of the upper and lower limbs of a large, recumbent, refolded, isoclinal fold that makes up the Blue Ridge in western Georgia.