DIGITAL GEOLOGIC MAPPING AND STRUCTURAL RE-EVALUATION OF SOME OF THE MOST SOUTHERN EXPOSURES OF THE BREVARD ZONE, ALABAMA

We present a digital geologic map of some of the most southern exposures of the Brevard zone (BZ) in the 1:24,000 Red Hill Quadrangle, east-central Alabama. Northeast of Alabama the BZ is classically known as a polyphase fault zone along which late-stage, Alleghanian, right-slip, brittle-plastic shears have largely obliterated the earlier, dip-slip movement history. In Alabama, the BZ is defined by rocks and structures associated with the Jackson’s Gap Group (JGG) that are shown on earlier maps to make an uncharacteristic due-S bend before disappearing beneath Mesozoic and younger sedimentary rocks of the Gulf Coastal Plain. We have made three important discoveries in our study area. (1) Retrogressive right-slip shears are not coincident with the general S-strike of the JGG but rather continue straight along their S55ºW trend, some merging with the Alexander City fault zone. (2) The JGG occurs in a major oblique thrust/right-slip duplex. Stacked, repeating panels (~200m thick) of N-striking JGG are stretched and/or cut by NE-striking oblique thrust/right-slip ductile shear zones. The duplex (500-675m thick from roof to floor thrust) is framed by SE-dipping, oblique-thrust/right-slip faults: Katy Creek fault, the traditional boundary between the JGG and the Inner Piedmont, is the roof thrust; and the Abanda fault, the traditional boundary between the BZ and eastern Blue Ridge, is the floor thrust. (3) Panels of JGG comprise an unusual assemblage of quartzite, conglomerate, chert(?), carbonate-bearing pelite, and volcanics(?) that also is of distinctly lower-metamorphic grade with respect to typical Alabama Piedmont rocks. The computer generated maps were developed as follows. Station localities were recorded using a Garmin Rino120® handheld GPS and imported into USGS TOPO!® Alabama, which, in turn, was used to construct base maps and topographic profiles. Structural/fabric data, contacts, and structural traces were digitally overlain using Corel Designer 9.0® and a standard PC and a Wacom® touchscreen monitor and pen. The digitized map and various layers were then printed directly from Corel Designer 9.0® or exported as a Windows® metafile (.wmf) into other drawing software packages and printed on an HP Designjet 5000 plotter in the Department of Geology and Geography, Auburn University.