THE WOODBURY-MANCHESTER IMPACT STRUCTURE IN SOUTHEASTERN NORTH AMERICA: IMPLICATIONS FOR THE EARLY NEOPROTEROZOIC EARTH-MOON ASTEROID/COMET SHOWER AND SNOWBALL EARTH

The Earth-Moon system appears to have experienced an asteroid or comet storm during the Early Neoproterozoic that produced many large impact structures on the Moon including the 93-km-wide Copernicus Crater that formed c. 800 Ma. From the observed lunar flux, Terada et al. (Nature Comm., 2020) calculated that Earth should have accumulated at least one or two Chicxulub-scale (150-200 km) craters at that time. Our refined U-Pb ages of zircons crystallized in a charnockitic melt rock, which contains shocked rock fragments of Grenvillean quartzite, granite, and biotite schist (all containing planar deformation features indicative of shock pressures exceeding 10-20 GPa), demonstrate that the Woodbury-Manchester impact structure (WMIS) in west-central Georgia formed at approximately 800 Ma. The minimum modeled original dimensions of the structure, sufficient to incorporate the mapped extent of the melt, is at least 150 kilometers. Consequently, we suggest that the WMIS was formed during the Early Neoproterozoic storm.

Field and petrographic analyses of quartzite ridges that extend medially through the region of melt rocks have identified shatter cones, shatter cleavages, feather features, planar fracturing, and injected shear melts that occur in a map pattern that suggests that the ridges were uplifted in response to a very low-angle collision or successive closely-spaced impacts. The structure, including the medial ridge complex, is similar in size and morphology to the lunar crater Schiller and the Martian crater Hale, which have been argued to have formed by similar processes. The minimum size of the impacting body is calculated to be about 25 kilometers. The structure likely was preserved mostly intact beneath the Appalachian detachment and has been exhumed since the end of the Paleozoic.

Arnscheidt and Rothman (Proc. Royal Soc. A, 2020) showed that Earth's decline into Cryogenian cooling most likely was formed by a sudden dramatic reduction in solar insolation. Therefore, they postulate a volcanic trigger for the last Snowball Earth. Alternatively, we propose that the WMIS (especially if it represents a large low-angle oblique strike), in connection with the expected increase in interplanetary dust during an asteroid or comet storm (including the contribution from successive large lunar impacts), could be capable of starting the collapse.