KILMICHAEL STRUCTURE (SUSPECTED IMPACT FEATURE), MISSISSIPPI

Kilmichael structure, a suspected impact feature in the coastal plain of central Mississippi, is a 10-km diameter circular feature with remarkable faults and stratigraphic deformation. Drilling to -234.5 m at the center of this circular structure revealed a stratigraphic succession of rock types consistent with an impact-produced structure. This sequence, from top down, is: (1) post-impact laminated marine sediments; (2) conglomeratic aqueous wash-back or resurge deposits of mixed provenance; (3) interbedded impact breccias target rock blocks (i.e., surge-back deposits); (4) large, deformed and rotated blocks of the Upper Cretaceous Ripley Formation; and (5) interbedded impact breccias and target rock blocks, mainly Upper Cretaceous chalks. This sequence is similar to drilled sequences from near crater center at Wetumpka impact structure, Alabama, and Chesapeake Bay crater, Virginia. The stratigraphic age of this structure is probably late early to early late Paleocene, based on fossil age of material from the youngest recognizable target rock block drilled in the core hole. The target area was in a marginal marine to shallow marine setting at time of impact and the drilled stratigraphy is consistent with other marine target impacts. Unfortunately, at present, no definitive shocked materials and geochemical traces of the impactor are known from this structure.