Paper No. 7
Presentation Time: 9:45 AM
MIXED-AGE ECHINODERMS, CONODONTS, AND OTHER FOSSILS FROM A MID-MISSISSIPPIAN IMPACT RESURGE BRECCIA, ST. CLAIR COUNTY, MISSOURI
The Weaubleau Structure (~ 8 km in diameter) is aligned W-E with two confirmed impact structures along the 38th parallel in Missouri. Strata from the middle Osagean (mid Mississippian) Burlington Ls. all the way to basement were shattered by a meteorite impact. These units occur in core in quasi-stratigraphic order, and granite clasts occur 70 m deep in the core. The top 9 m of breccia is also exposed in an adjacent roadcut. Exposed breccia is mostly echinoderm fragments and angular lithoclasts (chert, mudstone) that form a graded, unbedded unit that probably is a marine resurge deposit. Later this breccia was karsted and then overlain by mid-Pennsylvanian clastics, now partly eroded away. Lower Ordovician, Kinderhookian, and Osagean conodonts are mixed together in the 9 m of exposed breccia. The latest Osagean to early Meramecian conodonts Gnathodus texanus and Taphrognathus varians occur throughout this exposed breccia and indicate the oldest possible age of impact. Individual echinoderm plates, columnals, and calyxes that individually weather out of the breccia are like those in the local Burlington Ls. and Keokuk Ls. Typical upper Burlington fossils include the crinoids Aorocrinus parvus, Macrocrinus gemmiformis, M. verneuilianus, Uperocrinus hageri, U. pyriformis, and the blastoids Globoblastus norwoodi, Poroblastus sp., and Schizoblastus sayi. Fossils typical of the Keokuk include echinoid spines 22 mm long and the blastoid Pentremites conoideus. Some crinoids, including Actinocrinites plates, Dorycrinus spines, and Platycrinites radial plates and oval columnals, are typical of both Burlington and Keokuk. No Meramecian echinoderms are known. Horn corals are up to 45 mm long but have not been identified. Although the lower part of the Burlington was shattered by impact, excellent preservation of echinoderms typical of the upper Burlington and Keokuk indicates those units must have been uncemented, loose crinoidal sediment during impact. Clasts derived by shattering Lower Ordovician, Kinderhookian, and lower Osagean strata were reworked by resurge currents with loose crinoidal sediment to form the unbedded breccia. Logic suggests that the age of impact could not be much younger than latest Osagean or earliest Meramecian, or else the crinoidal sediments would have been cemented into hard limestone.