A CASE OF MISTAKEN IDENTITY: THE "WOODBURY STRUCTURE" OF SOUTH CENTRAL GEORGIA

The “Cove” in the Pine Mountain belt of Georgia in the southern Appalachian Piedmont is a circular topographic feature, ~6 km in diameter, defined by the Hollis Quartzite of the Pine Mountain Group. Resistant quartzite exposed along the rim dips away from the center of this structure, with underlying units of the Sparks-Halawaka Schist and Grenville-aged Woodland Gneiss discontinuously exposed within the interior. The Cove Dome has historically been interpreted as resulting from interference folding of Grenville basement and overlying cover rocks within a much larger recumbent fold nappe. An alternative interpretation for the Cove, however, suggests it is an impact crater (Woodbury astrobleme) which formed along the Early Paleozoic margin of North America (Laurentia) prior to Appalachian orogenesis. Evidence presented as supporting an impact model include macro- and microstructural features identified as shatter cones, planar deformation features, pseudotachylite veins, spherulites, breccia dikes, and shocked zircon surrounded by disordered quartz with a Raman spectral signature suggesting the presence of coesite. Because a number of these suspected impact features were found outside of the proposed “Woodbury astrobleme”, it has been suggested the Cove is part of a much larger impact structure, the “Woodbury structure” (>35 km diameter). Critical elements of the local and regional geology, however, make it unlikely that the Cove is an astrobleme as proposed. P-T conditions in rocks of the middle-upper amphibolite facies Pine Mountain Group exceed 6 kb and 500°C. These conditions are well above those at which quartz undergoes plastic deformation, making it unlikely that the macro- and microstructural features reported would be preserved during protracted Appalachian orogenesis. Similarly, it is improbable that a circular impact feature would be preserved during isoclinal folding of units within the region. Although the reported “impact” features could be attributed to a post-deformation impact crater, geologic data gathered from outcrop mapping and drilling within the Cove are more supportive of the original structural dome model. Detailed fracture analysis, geologic mapping, and petrography indicate that the Cove Dome is the result of tectonic, not impact, processes.