THE SERPENT MOUND BRECCIA (SMB): A UNIQUE POLYMICT CARBONATE BRECCIA DEPOSITED IN THE SERPENT MOUND IMPACT STRUCTURE, SOUTHEASTERN OHIO

The Serpent Mound structure is a deformed circular area (8 km diameter) within the Interior Lowlands Plateaus Physiographic Province of southern Ohio at the intersection of Highland, Pike, and Adams Counties. Serpent Mound (not to be confused with the Paleoindian site) is a complex impact crater formed by the collision of an asteroid or comet with Earth, thought to have occurred between 256- 330 Ma.

One outstanding issue with Serpent Mound crater is an enigmatic breccia that appears to be intrinsic to the crater itself. This breccia, informally known as the Serpent Mound Breccia (SMB) is a matrix-supported polymict breccia with a unique weathering profile and appearance. It is moderately-to poorly-cemented, subhorizontally oriented throughout the crater, and in at least 2 locations, displays an overall fining-upward sequence. We initially examined multiple hypotheses to account for its origin; including fault propagation, mass wasting, or fallback/resurge ejecta from the impact event itself.

Field observations revealed a lateral distribution inconsistent with the brecciation by impact related fault movements or post-impact mass wasting. Geochemical analyses (XRF/XRD) are also unsupportive of these two hypotheses. The bulk composition and mineralogy of the SMB additionally do not represent the geologic units expected to have been comminuted during faulting, or all of the upslope (Ordovician-Silurian) strata that would have collapsed from the nearby central uplift during impact-related mass wasting.

The strong correlation between the bulk composition of the SMB and Middle Silurian dolostones is inconsistent with fallback of ejecta in a subaerial impact, which would excavate well into Upper Ordovician target rock. However, this correlation may be consistent with resurge and concentration of ejecta into a shallow crater formed after a marine impact. If the SMB does represent ejecta resurge deposits, the absence of a Late Devonian to Mississippian component from the SMB supports a post-Middle Silurian to pre-Late Devonian impact event, in contrast to the previously proposed post-Mississippian age.