DISTRIBUTION AND FORMATION OF AN ANOMALOUS BRECCIA WITHIN THE SERPENT MOUND IMPACT CRATER

The Serpent Mound structure is an 8 km impact crater in southern Ohio. The impact occurred in and affected sedimentary target rock ranging from Cambrian to Silurian (and possibly strata as young as Mississippian) in age. Serpent Mound is a partially-preserved complex crater, with a central uplift, transition zone, and surrounding ring graben. Within the crater lies an anomalous gray, dolomitic polymict breccia with unique characteristics we informally refer to as the Serpent Mound Breccia (SMB). Originally, it was identified in a single (type) location in the western crater. The purpose of this study was to identify and map additional SMB locations and track its lateral distribution, which may allow us to identify a formation/emplacement mechanism. Different breccia generation mechanisms result in a wide variation of lateral distributions. For the 3 most probable mechanisms for the SMB, faulting during rise/collapse of the central uplift, mass wasting from gravitational collapse, or ejecta fallback/resurge, expected distributions range from localized to crater-wide respectively. In order to track the lateral distribution of the SMB, we searched for new potential sites using aerial/satellite imagery, new field exposures, and references from former field investigations. Each potential breccia was compared to the SMB at its type location. Upon positive identification, we mapped each new site and recorded elevations with GPS. GPS elevations and Brunton compass measurements allowed us to determine minimum apparent thicknesses of the SMB. Our team confirmed 8 new exposures in all but the eastern portions of the crater to date, with most of the sites located in the western half. Several other suspected sites are still under investigation. All confirmed and suspected sites lie in the transition zone within the crater. The SMB ranged from 220-246 m above sea level and 3-21m in minimum apparent thickness at individual sites. The widespread distribution of the SMB within the crater does not support an origin by localized fault brecciation or single mass wasting events. It does support generation by a crater-wide geologic process following the impact event, with the most probable mechanism being fallback or resurge of ejecta. Ongoing work continues to address the viability of this hypothesis.