EVIDENCE FOR A LARGE LOW-ANGLE OBLIQUE IMPACT CRATER IN WEST-CENTRAL GEORGIA TO EAST-CENTRAL ALABAMA: THE EARLY NEOPROTEROZOIC WOODBURY-MANCHESTER STRUCTURE

Recent analyses of KAGUYA data (Nature Comm., 2020) concluded that the Earth-Moon system experienced an intense asteroid storm approximately 800 Ma, producing at least 8 lunar craters larger than 20 km in diameter, including Copernicus (93 km). These findings suggest the probability that the Earth may have been hit during the same interval by enough mass to produce at least one Chicxulub-scale impact structure among many other smaller ones. Yet the authors note the absence of large Neoproterozoic craters in the terrestrial record.

We previously have presented petrographic and geochemical evidence of a large impact structure preserved in the Pine Mountain Terrane of west-central Georgia (Meteoritics & Planet. Sci. Abstracts; 2010; Lunar Planet. Sci. Conf., 2009, 2016; Geol. Soc. Am. Abstracts with Program, 2016). U-Pb geochronology of zircons in charnockitic melt rocks demonstrate that the impact occurred between 800 and 900 Ma, around the same time as Copernicus. Recent field work and petrographic analysis of quartzite blocks entrained in the charnockite confirm the impact origin of the melt. They contain abundant quartz grains exhibiting multiple sets of decorated planar deformation features (PDFs) diagnostic of shock between about 7 and 25 GPa. The blocks also establish a connection between the melt and the system of uplifted Grenvillian quartzite ridges that bisect the Terrane. These ridges also contain shatter cones, shatter cleavages, and multiple-striated surfaces indicative of somewhat lower pressure shockwaves, approximately 3 to 7 GPa, and very high strain rates.

An oblong structure of at least 200 km long and 70 km wide is required to encompass the total area of melting and deformation. Although the northern third of the structure appears to have been displaced by the Towaliga fault and the southwestern edge of the structure may have been modified by a number of shear zones, we have not identified evidence to suggest major disruption of the overall architecture due to Paleozoic compression. The central collapse pit (The Cove) remains especially undisturbed and is surrounded by scroll-like transpressional ridges like those found at some other oblique impact structures.

Preliminary calculations suggest that the Woodbury-Manchester Impact Structure was created by an impactor more than 25 km in diameter.