GEOLOGIC MAPPING AND 3D VISUALIZATION OF THE SOUTHERN HALF OF CHAMBLEE QUADRANGLE, GEORGIA

This project provides a high-resolution, geologic map of the southern portion of the 7.5-minute Chamblee Quadrangle of Atlanta, Georgia, at a 1:24,000 scale, an improvement upon the 1:100,000 scale map produced by Higgins and others in 2003. The report aims to resolve geologic uncertainties regarding fault zones and geologic contact boundaries across Dekalb, Fulton, and Gwinnett county. Digital elevation modeling (DEM) derived from Fulton and Dekalb county LiDAR data, Geographic Information Systems (GIS) file digitization based on previously published work, and petrographic analysis of thin sections from procured hand specimens was implemented to support field mapping efforts. Our mapped area, which spans 31.1 square miles, is located within the Inner Piedmont region, below the southern Appalachian Mountains. The complex terranes within the Inner Piedmont province include the medium- to high-grade metasedimentary, Laurentian-sourced Tugaloo terrane, and igneous bodies emplaced by a series of deformational events associated with the Neo-Acadian orogeny (400-360 Ma).

Falling on the eastern side of the nearly 700 km long Brevard Fault zone, geologic units located within our mapped area include: Cambrian Aluminous Schist (Ꞓas), identified by its abundant kyanite, garnet, and staurolite minerals; Mid-Ordovician to Late Protozoic aged plagioclase-rich gneiss (OZm) and Powers Ferry Member (OZsp), a biotite gneiss with extensive feldspar-rich intrusions; Permian to Mid-Proterozoic Long Island Creek Gneiss (PYI), a layered blastomylonitic gneiss; Permian to Upper Ordovician Button schist (POb) exhibiting S-C mylonitic and fish-scale texture; Devonian Lithonia Gneiss (DI), a complex of metagranites and granitic gneisses; Devonian Lithonia Gneiss (OZa) containing fine- to medium-grained amphibolite; and lastly, Mid-Ordovician to Late Proterozoic, finely layered granitic gneiss (Ozcm). Our research supports prior work proposing the Tugaloo terrane was located northward of Laurentia during continental accretion before moving southward along transform faults. Current and comprehensive details of Atlanta’s geology are important in the development of public policy, planning, and health. To ensure informed decision-making, we will ensure digital, public access to our findings and documents.