Baranoski, et al., report that the Serpent Mound disturbance is an area of geologically complex rocks in southern Ohio near the junction of Adams, Highland, and Pike Counties. The disturbance takes its name from, but should not to be confused with, the Serpent Mound effigy mound, a major archaeological site located in the southwest portion of the subject geologic structure. The Serpent Mound geologic disturbance is a circular structure about 8 km (5 miles) in diameter and consists of three structural elements: a central uplift, a transition zone, and a ring graben. The disturbed area contains highly faulted and folded sedimentary rocks of Ordovician through Mississippian age. The origin of the structure generally has been attributed to either an exogenic event, such as a meteorite impact, or an endogenic event, such as explosive release of highly pressurized gas trapped beneath the Earth’s surface.
Paleomagnetic data suggest that the disturbance formed prior to the Late Permian (256 million years ago). Core DGS 3274, drilled in the central uplift, contains 903 meters of severely deformed (fractured, faulted, and brecciated) to mildly deformed sedimentary rocks ranging in age from Late Cambrian (?) to Late Devonian. Shatter cones and shock metamorphism in the form of planar deformation features in quartz, as well as a small iridium anomaly found in a breccia from this core, provide convincing evidence for a meteorite-impact origin of the Serpent Mound disturbance. Core DGS 3275, drilled in the transition zone, penetrates 629 meters of severely deformed to undeformed sedimentary rocks ranging in age from Late Cambrian (?) to Middle Silurian. Core DGS 3275 has fewer breccias and less deformation than core DGS 3274, but does contain shatter cones and most of the types of small-scale structures reported in DGS 3274. No systematic search for iridium or shock metamorphic features was performed on core DGS 3275. Core and seismic data indicate that, in the central uplift, strata above the Ordovician-age Carntown unit of the Black River Group (equivalent to drillers’ “Gull River” limestone) are highly disturbed and discontinuous. Cores DGS 3274 and 3275 also show complex cross-cutting structural relationships and mineralization. These features indicate relative timing of structural events and support the seismic interpretation of multiple episodes of deformation. These younger rocks in the central uplift, in some cases, have been displaced a maximum of 275 meters above their expected level, when compared to similar strata outside the disturbance. Conversely, the Carntown unit and the underlying Wells Creek Formation can be traced across the Serpent Mound disturbance on seismic profiles and appear to be displaced downward at least 276 meters below their expected elevations. Seismic data indicate that Paleozoic rocks below the Carntown are thinner than expected for the region, which may partially account for the Carntown unit and older rocks being lower than expected. Much of the depression below the Serpent Mound disturbance, not accounted for by thinning, may be the result of movement along pre-existing Precambrian faults, which were reactivated after the formation of the Carntown unit.
Although the Carntown unit and the Wells Creek Formation can be traced across the Serpent Mound disturbance on seismic profiles, both units are broken by faults extending upward from the Precambrian. Evidence suggests that post-Carntown faulting may have occurred before the formation of the Serpent Mound disturbance as well as after its formation. A localized, oblong, positive magnetic anomaly is slightly offset in relation to the surface outline of the Serpent Mound disturbance. This magnetic anomaly is on trend with the northwest-trending contours of regional total magnetic intensity maps. A localized gravity anomaly beneath the central uplift appears to be related to the impact. The core and geophysical evidence suggests that a meteorite impact occurred on or near a pre-existing geologic structure.
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