IS THE SERPENT MOUND IMPACT STRUCTURE LARGER THAN FIRST PROPOSED?

The Serpent Mound impact crater is located 9.7 km north of Peebles, Ohio in Adams, Highland, and Pike Counties. The impact occurred in sedimentary strata ranging from Cambrian to Silurian (and possibly as late as Mississippian in age). Serpent Mound (not to be confused with the PaleoIndian effigy) is a complex crater made up of a central uplift, transition zone and a ring graben (all within a 4 km radius of the center). Previous research has failed to observe a modified crater rim, an important morphologic feature of complex craters. Such rims are produced during uplift and collapse of the transient crater in structures larger than 3-5 km. Collapse along normal arcuate faults results in a terraced profile that is no longer reflected in the flat-lying topography immediately outside of the ring graben. The diameter of Serpent Mound’s central uplift suggests that the final crater diameter is much larger than the previously-reported (8 km). Based on the known morphometric relationship of central uplift (D­_cu) to final crater diameter (D) of: D­_cu­ = .22D, where D­_cu ranges from 2-2.4 km for Serpent Mound, the final crater diameter are predicted to be between 9-11 km. This modeled crater diameter corresponds to the radial distance from the center of the structure to a suspiciously arcuate and concentric portion of the Allegheny Escarpment east of the crater. Any preserved evidence for a terraced crater rim would there-fore lie between the ring graben and the arcuate portion of this escarpment; however, exposures in this area are limited. We examined this area for disturbed strata using the contact between the Late Devonian Ohio Shale and Silurian carbonates as an indicator of structural offsets from normal faulting in a modified crater rim. The easternmost exposure of this contact was present east of the escarpment (outside the crater) at 220m above sea level. At 6 sites E and SE of the crater, we observed downward displacement of the contact up to 35m. Along a 2 km SE-to-NW traverse towards the crater, the contact varied from 224 to 251 to 243m above sea level respectively. Total relief of the contact is more than 3 times greater then known erosional unconformities. This relief may however represent the structural offset of the (poorly-exposed) modified rim of the Serpent Mound impact structure, supporting a larger crater diameter.