NEW INSIGHT INTO THE BREVARD ZONE IN GEORGIA: RESULTS FROM DETAILED SURFACE AND SUBSURFACE GEOLOGIC MAPPING OF THREE LARGE TUNNELS IN THE METRO-ATLANTA AREAOF GEORGIA

Detailed geologic mapping along three tunnel alignments combined with subsurface mapping of nearly three miles of one tunnel have provided new insight into the physical characteristics of the Brevard Zone in the metro-Atlanta area. Exposures within these tunnels show extreme lithologic heterogeneity and structural complexity within the Brevard that have not been previously documented within this zone in metro-Atlanta. Lithologies and structures identified during surface mapping are reliably projected to tunnel depth for the tunnels that were excavated perpendicular to the Brevard. Parallel to the Brevard, these features are less reliably projected to tunnel depth due the structural complexity of the zone.

The Chattahoochee Tunnel was excavated perpendicular to the Brevard and transects sixteen different mappable lithologies. These lithologies range in textural complexity from ultramylonites to highly sheared schist and gneiss. Locally, retrograde mineral assemblages are present but not ubiquitous. Current work on the Brevard defines the zone based on retrograde mineral assemblages; however, exposures within this tunnel and along the alignment at ground surface illustrate greater textural variability as opposed to mineralogic variability.

The Nancy Creek Tunnel was excavated parallel and beneath the surface expression of the Brevard in an area of intense mylonitization. Surface mapping indicates three distinct mineralogical varieties of mylonite. These represent mylonitized biotite gneiss, metagraywacke and granite. Surface mapping and published maps of this area indicate strong northeast-striking southeast-dipping structures; however, detailed subsurface mapping revealed the presence of very complex, polydeformed, southeast-verging, overturned folds overprinted by late brittle faults.

Currently, the Atlanta West CSO Tunnel is being excavated perpendicular to the Brevard. This tunnel begins in mylonites similar to the Nancy Creek Tunnel; over a short horizontal distance, the tunnel transects highly deformed rock units of the Clairmont Melange. Lithologies within the Clairmont show no textural or mineralogical effects of structural deformation related to the Brevard as observed in the other tunnels, such as folding, shearing and associated retrograde mineral assemblages.