GEOLOGIC MAP OF THE TUGALOO TERRAIN IN THE VICINITY OF THE BREVARD SHEAR ZONE, NORTHERN-HALF OF THE CHAMBLEE QUADRANGLE, GEORGIA

A detailed geologic map of the northern-half portion of the Chamblee quadrangle in metropolitan Atlanta, Georgia was developed at an improved 1:24,000 scale to remedy geologic uncertainties of regional delineations, fault zones, fracturing, and structural locations based on geologic maps published in 1976 (1:500,000 scale) and 2003 (1:100,000 scale) across the tri-county area of Dekalb, Fulton, and Gwinnett counties. Unmanned aerial vehicle (UAV) drone surveying, digital elevation modeling, GIS file digitization, photogrammetry, and petrographic analyses were employed to support field mapping efforts. The mapped area is 31.3 square miles and lies in the complex Piedmont province of Georgia. It is comprised of igneous and metamorphic units and part of the Tugaloo terrane that was emplaced by transform faults around 400 to 360 Ma during the Neo-Acadian orogeny.

In the study area, the Sandy Springs group is located NW of a large shear zone (Brevard Fault) and is chiefly composed of a metamorphosed succession of sedimentary rocks. Field mapping revealed four major units in this rock belt: 1) Middle Ordovician to Late Proterozoic Powers Ferry Member (OZsp) consisting of a biotite gneiss schist with interbedded amphibolite, 2) Cambrian Chattahoochee Palisades Quartzite (Ccp) which is a massive and vitreous white-blueish feldspathic quartzite, 3) Cambrian Aluminous Schist (Cas) found with abundant kyanite, garnet, and staurolite minerals, and the 4) Cambrian Aluminous Quartzite (Caq) which is a thinly bedded unit of muscovite-bearing quartzite. The phyllitic Button schist unit (POb) of Permian to Upper Ordovician age was found along the Brevard Fault zone and can be identified by its distinctive fish-scale texture. Our findings align with previous work that suggests the Tugaloo terrane was situated further northward of Laurentia during continental accretion before moving southward along transform faults. The resolution of our map will allow for targeted investigations into the emplacement of the Tugaloo terrane and the processes that shaped the eastern North American margin between the Cambrian to Permian. This map could also benefit infrastructural projects in one of the fastest-growing metropolitan regions in the country.