SERPENT MOUND: A LATE SILURIAN-MIDDLE DEVONIAN OR POST-MISSISSIPPIAN-AGED IMPACT?

The Serpent Mound structure is an 8 km complex impact crater located in southern Ohio north of the town of Peebles (39°2' N, 83°24' W). The impact resulted in the deformation of Cambrian through Middle Silurian-aged (at minimum) sedimentary strata. Previous workers have proposed that Late Devonian Ohio Shale and superimposed Mississippian strata of the so-called ring graben were also present at the time of impact as evidenced by their downward displacement from normal stratigraphic positions. An examination of more recent data sets (field exposures and drill cores) however, indicates that this offset is much more complicated than previous thought. The base of the late Devonian is either uplifted, not displaced, or lowered below its normal stratigraphic position (outside of the crater) from one exposure to another. This, coupled with the presence of a polymict dolomite breccia within the crater comprised of Silurian-aged clasts that may represent ejecta, suggests the impact occurred sometime in the interval following the Middle Silurian and prior to the Late Devonian.

For this work we are examining polymict fault breccias generated within the central uplift of the Serpent Mound impact structure. Based on color, grain size, and general appearance, other workers have identified these fault breccia clasts as belonging to the Late Devonian Ohio Shale. The presence of Late Devonian (and by association Mississippian) strata in these fault breccias would confirm a post-Mississippian impact. Their absence may support a pre-Late Devonian impact. Shale clasts from polymict breccias at field exposures and a drill core (3274) from the central peak have been collected. Initial comparisons of these clasts to typical Ohio Shale have called into question their classification as Ohio Shale. Ongoing petrographic and XRD analyses are providing insight into their identity and will provide additional information about the timing of the Serpent Mound impact event.