GEOLOGIC MAP ILLUSTRATING THE TECTONOSTRATIGRAPHY AND STRUCTURAL GEOLOGY OF A PART OF THE BREVARD ZONE IN GEORGIA: RESULTS FROM DETAILED GEOLOGIC MAPPING IN THE CHATTAHOOCHEE RIVER NATIONAL RECREATION AREA, SANDY SPRINGS, 7.5-MINUTE QUADRANGLE, GEORGIA

As part of the National Park Service Geologic Resources Inventory, detailed geologic mapping of part of the Sandy Springs, GA, 7.5-minute quadrangle was conducted within the Chattahoochee River National Recreation Area (CRNRA) in Cobb and Fulton Counties, Georgia, between Morgan Falls Dam and Paces Mill Park. This detailed geologic mapping revealed a complicated tectonostratigraphy and a complex deformational history within the Brevard Zone. Additionally, this detailed mapping has provided new insight into the physical characteristics of the Brevard Zone in the metro-Atlanta area as recently described by Kath and Crawford (2006).

Exposures within the river corridor show extreme lithologic heterogeneity and structural complexity within the Brevard Zone that has not been previously documented within this area at a 1:24,000-scale. Lithologies range in textural complexity from mylonites to highly sheared schist and gneiss. Locally, retrograde mineral assemblages are present but not ubiquitous. Much previous work on the Brevard Zone, defines the zone based on retrograde mineral assemblages; however, exposures along the river corridor illustrate greater textural variability as opposed to mineralogic variability.

Also, much previous work has suggested that the Chattahoochee River follows the Brevard Zone from near Gainesville, Georgia, to the Alabama-Georgia state line. However, detailed mapping within the CRNRA on the Sandy Spring and northern part of the Northwest Atlanta Quadrangles shows that the Chattahoochee River is mostly sub-parallel (divergence angle between 5 and 10 degrees) to the Brevard Zone in areas where the rocks are less mechanically and chemically stable. Where the rocks are more mechanically and chemically stable, the Chattahoochee River cuts across the Brevard Zone at high angles forming abundant shoals. These overall trends give a stair step pattern to the river as it crosses the Brevard Zone in the study area.

This project was partially supported through the Geological Society of America, GeoCorps America project number 2013018.