GEOLOGIC MAP OF THE NORTHWEST ATLANTA 7.5-MINUTE QUADRANGLE, COBB AND FULTON COUNTIES, GEORGIA

Detailed geologic mapping at 1:24,000-scale of the Northwest Atlanta, Georgia, quadrangle was conducted as part of a geologic mapping program associated with four separate tunnels in the metro-Atlanta area. Surface mapping was supplemented with nearly 65 km of underground mapping in the tunnels. Underground mapping was conducted in the Chattahoochee, Nancy Creek, Atlanta West CSO, and City of Atlanta Raw Water Tunnels. The Chattahoochee Tunnel was excavated perpendicular to the Brevard Shear Zone (BSZ) and transects sixteen different mappable lithologies. The Nancy Creek Tunnel was excavated parallel and beneath the surface expression of the BSZ in an area of intense mylonitization. The Atlanta West CSO and City of Atlanta Raw Water Tunnels were excavated perpendicular to the BSZ and transect the Katy Creek Fault.

The main tectonic feature of the NW Atlanta quadrangle is the Katy Creek Fault which separates Dadeville Complex from Brevard Zone lithologies. Overall, movement on the Katy Creek Fault appears to be dextral-reverse. The hanging wall of the Katy Creek fault is comprised of the Clairmont Mélange, sillimanite schist, and sillimanite gneiss. Strike of foliation in Clairmont is highly variable and dips are generally less than 14 degrees. The footwall of the Katy Creek Fault is characterized by highly sheared rocks of the BSZ. These include button schist, mylonite, ultra-mylonite, phyllonite, augen gneiss, quartzite, biotite gneiss, and garnet schist. The foliation in the BSZ is very consistent and persists up to 13 km northwest (perpendicular) of the zone of intense shearing.

The Ben Hill Granite intrudes across the Katy Creek Fault and stitches together Clairmont and BSZ lithologies. Higgins and Atkins (1981) indicate that the Ben Hill Granite age is ~325 Ma. Preliminary age dates of mica growth in the BSZ of Alabama by Poole (per com) suggest that the last shearing movement occurred ~317 Ma. This, coupled with the age of the Ben Hill and the lack of shear fabrics in the granite, constrains emplacement of the Clairmont thrust sheet along the Katy Creek Fault. Based on these ages and the contact relations along the northern boundary (Katy Creek Fault) and eastern & western boundaries (intrusive), the granite represents a syn- to post-tectonic granite that was emplaced during late-stage movement along the Katy Creek Fault.