THE CHALLENGES OF INVESTIGATING POTENTIAL IMPACT STRUCTURES IN ANCIENT TECTONIC TERRANES: A CASE STUDY FROM WOODBURY, GEORGIA, USA
In seeking to expose this part of the impact record, it is important for us to be open to the possibility that traditional signatures may need to be evaluated through complex P-T-t paths; yet we must equally guard against creating ad hoc criteria. Likewise it is imperative that we not unravel fifty years of impact petrology to buttress endogenic hypotheses against revision, succumbing to ad hocproposals that one fault is the sole exception that created shock metamorphism in the absence of an impact (especially when endogenic and impact models may not be mutually exclusive).
As a case study of the methods, difficulties, and successes (?) in addressing this problem, we present the culmination of a decade of work on Woodbury, a probable Neoproterozoic impact structure in the Pine Mountain Terrane of west-central Georgia. We present new data showing that some “explosion cones” found in megaclasts of peraluminous granite within a 1.1 Ga charnockite (itself containing relict shocked quartz and feldspar) are ancient features and have characteristics consistent with shatter cones. And we show that that the granite clasts do not contain evidence of high-grade metamorphism assumed by structural geologists for the entire region. We propose that Alleghanian docking of a continental fragment containing a buried impact structure can best explain the combined observations of workers with terrestrial and extraterrestrial biases.