DECATURVILLE STRUCTURE, CENTRAL MISSOURI: EVIDENCE FOR A PRE-DEVONIAN MARINE IMPACT?

Decaturville (6.6 km dia) is an eroded impact structure, but the depth of erosion is poorly constrained because no capping strata are found above deformed target rocks. As a consequence, the stratigraphic age also is contentious. Previous investigators have shown that the youngest widely exposed strata are folded and faulted beds of Lower Ordovician Jefferson City Dolomite in the moat region surrounding the central uplift. Exotic blocks of Middle Silurian, Upper Ordovician, and Cambrian rock also have been mapped in isolated exposures around the moat. No rocks younger than Middle Silurian have been found in the structure, nor are rocks of Upper Ordovician or Middle Silurian preserved 30 km to the west or 60 km to the south, where Mississippian strata rest unconformably on Lower Ordovician strata, nor are they preserved 55 km to the north, where Lower Devonian carbonates fill paleokarst below the sub-Mississippian unconformity.

In the 1970's, investigators argued that the age of impact was likely post-Pennsylvanian and perhaps as young as Cretaceous. More recently, paleomagnetic studies suggest a Pennsylvanian or Early Permian age; both age ranges are based on the occurrence of a sulfide breccia in the central uplift that is assumed to be a brecciated MVT deposit that was emplaced prior to the impact. The sulfide emplacement clearly pre-dated brecciation, but the age of this mineralization has not been dated. Drill logs show that the sulfides are confined to an isolated area known as the sulfide pit rather than being widely distributed across the structure.

Bulk samples collected for conodonts from a polymict breccia in the roadcut and samples collected from individual clasts have yielded Upper Ordovician (Mohawkian and Cincinnatian) faunas, which is consistent with their having been derived from the Kimmswick Limestone and Leemon Formation. New field studies have also uncovered several blocks of silicified oöid grainstone breccia that also contains loose unbroken oöids in the matrix. The occurrence of exotic Upper Ordovician and Middle Silurian blocks together with presence of marine components in this breccia suggests a pre-Devonian marine impact.