THE SEARCH FOR IMPACT FEATURES IN PROPOSED TARGET ROCKS OF THE “WOODBURY” STRUCTURE IN THE PINE MOUNTAIN BELT OF SOUTHWESTERN GEORGIA

The southern Appalachian Pine Mountain belt of southwestern Georgia includes a notable, nearly circular topographic feature alternately referred to as the “Cove” or “Woodbury” structure. The topographic rim of this 6-7 km diameter feature consists of lower Paleozoic Hollis Quartzite (metasandstone) of the Pine Mountain Group; while the floor consists of Grenville-age (ca. 1.1 Ga) Woodland Gneiss. Previous workers have suggested two competing models for the origin of this structure - a flexural interference fold (i.e. structural dome) vs. hypervelocity impact crater. In the former model, the structure (Cove dome) is one of several domes and basins within the area that formed during Appalachian orogenesis in the late Paleozoic. In the latter model, the structure (Woodbury astrobleme) is interpreted as the remnant of a central peak ring within a larger (35-40 km) impact crater that formed following deposition of the Hollis Quartzite, but prior to Alleghanian regional metamorphism and deformation. Regional geological constraints (e.g. depositional age of the Pine Mountain Group, nature of regional deformation) preclude a pre-metamorphic impact origin for the proposed “Woodbury” structure. However, the structure could be modeled as a younger (i.e. post-metamorphic) and smaller (6-7 km diameter) impact crater based on purported evidence for a hypervelocity impact origin, including suspected planar deformation features (PDFs) from the Hollis Quartzite. Although PDFs rarely present themselves in the uplifted rim of impact craters, they are commonly found within target rocks inside the crater rim. In this project, we report on the search for shock features from the Woodland Gneiss from multiple locations inside the rim of the Cove/Woodbury structure, including ~12 meters of alluvium and ~30 meters of underlying Woodland Gneiss cuttings from a well drilled in the southeastern corner of the structure. To date no evidence for shock features have been identified in either hand sample or thin section. The lack of obvious macroscopic or microscopic shock features from the Woodbury Gneiss within the topographic rim of the structure – the ideal region where such features should be evident – supports interpretation as a flexural interference fold and not as an impact crater.