NEW DIGITAL GEOLOGIC MAP OF THE ATLANTA, GA 1-DEGREE X 30-MINUTE QUADRANGLE, VER 1.1

During the past three years much new geologic mapping by the authors in the Atlanta 1-degree quadrangle has led to significant revisions to the Open-File map of 1998. The geologic map has been compiled into an extensive database in ESRI’s ArcView 3.2 and 8.1. The Atlanta quadrangle spans from the Valley and Ridge province in its northwestern corner, across the western Blue Ridge province, and across the Brevard fault zone, which separates the Blue Ridge from the Piedmont province. The southeastern half of the quadrangle is underlain by rocks of the Piedmont province. In the Atlanta quadrangle a large area of the western Blue Ridge province is structurally overlain by the western Piedmont allochthon, which contains a suite of ocean-floor rocks, broken formation, and melange. The western Piedmont allochthon has been thrust upon the parautochthonous western Blue Ridge rocks on the Allatoona thrust fault. The time of thrusting on the Allatoona fault is dated at about 430 M.A. 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, but is interpreted to have been intruded during fault movement. The allochthon and parautochthon were later multiply folded together. The Dog River window exposes parautochthonous, Laurentian margin, western Blue Ridge rocks below the Allatoona fault and the western Piedmont allochthon. The Crawfish Creek synform and Happy Valley antiform in the southwestern part of the Dog River window expose a nearly complete sequence of parautochthonous western Blue Ridge rocks, including rocks of the Chilhowee Group, and the overlying western Blue Ridge allochthon. Southeast of the Brevard fault zone large east-trending folds cross what was previously thought to be a large synform (“Newnan-Tucker synform”), but is now known to be part of complex fold interference patterns. Along the eastern side of the quadrangle large outcrop areas of Lithonia Gneiss are structurally overlain by nearly flat-lying Sandy Springs Group rocks in nappes that have been thrust upon the Lithonia and other rocks in the area. Northwest of the Brevard fault zone, these same Sandy Springs Group rocks are steeply dipping. The Sandy Springs allochthons were emplaced before being intruded by the Carboniferous, ~320 M.A. Stone Mountain Granite.