A CONSTRAINED DIAMETER FOR THE SERPEANT MOUND COMPLEX CRATER

The Serpent Mound structure is a heavily eroded, roughly circular structure of relatively intense deformation encompassed by flat, glaciated sediments in southwestern Ohio. The presence of shatter cones, high-pressure polymorph coesite, and planar deformation features in quartz have established the Serpent Mound structure as the only confirmed impact crater in Ohio. It exhibits morphological features typical of modified complex craters, including a central peak and ring graben.

The Serpent Mound structure was described in detail by Reidel (1975) and estimated to have a diameter of 7-8km. Milam (2010), however, noted that for all complex craters there is a known morphometric relationships between the central peak diameter D_cp, and rim-to-rim diameter D, such that:

D_cp = (0.23 + 0.03)D (from Pike 1985)

Using Milam’s (2010) minimum and maximum values for D_cp in the equation reported above yields a range of 10-25km for D, approximately 1.5 to 3 times larger than the initial 7-8km estimate. Additionally, subsurface deformation observed in core samples and the conspicuously circular shape of local topography both imply a rim-to-rim diameter of 10-14km.

Glacial erosion has eradicated the eastern half of the crater and fluvial erosion has obscured much of the western half. The 7-8km estimate was based on the occurrence of concentric normal faults outside the ring graben. However, it is likely that these faults extend beyond a 7-8km and still represent part of the crater.

Concentric normal faults are formed during crater rim collapse in the modification stage of impact. The crater rim collapses inwardly along these faults, producing terraces of inwardly rotated blocks between faults. Within these blocks, the beds are stratigraphically displaced downward.

To more accurately constrain the diameter of the structure, transects will be completed along the eastern half of the crater outbound from the central uplift for approximately 7.5km. Each transect will be used to construct a geologic profile, using the contact between Middle Silurian Carbonates and overlying Ohio Shale as datum to identify vertical displacement. Vertically displaced beds along normal faults may be interpreted as collapsed portions of the crater rim, and used to more accurately constrain the overall diameter of the structure.