CONODONTS AND OTHER FOSSILS FROM CORES IN THE RESURGE BRECCIA AT THE WEAUBLEAU IMPACT STRUCTURE, ST. CLAIR COUNTY, MISSOURI

The Weaubleau Structure (~5 miles in diameter) is an impact structure in west-central Missouri that displays several kinds of breccia. Carbonate resurge breccia, exposed in road cuts and a quarry, has abundant lithoclasts (chert, clay mudstone, limestone, dolomite) and bioclasts (conodonts, crinoid columnals and plates, blastoids, brachiopods, corals). Conodonts and intact crinoid and blastoid calyxes from breccia outcrops plus brachiopods, bryozoa, and trilobites from overlying chert residuum indicate a time of impact near the Osagean-Meramecian boundary. The Missouri Department of Transportation drilled five 2-inch-diameter cores into the breccia; the deepest core, 318 ft, is near the center of the structure. The resurge breccia is not bedded, and abundant clasts are like those found on outcrop. A burrowed sandstone clast 5.75 in long occurs 300 ft deep in the core. Intervals of coarse- and fine-grained breccia appear to be randomly arranged vertically, and some intervals appear to be thin, graded turbidites that record slumps from the sides of the impact crater. Above the breccia is 55 ft of chert residuum and Pennsylvanian gray shale. The core was split, and 1-ft samples collected at 20-ft intervals were dissolved to recover conodonts. Most samples were productive, and Early Ordovician and Early and Middle Mississippian (Kinderhookian, Osagean) conodonts occur at random levels in the core. Taxa include many in breccia outcrops. Early Ordovician taxa are those known in the Jefferson City-Cotter Dolomite, including Colaptoconus quadraplicatus. Mississippian genera include Gnathodus (G. texanus is the most common conodont), Hindeodus, Polygnathus, Pseudopolygnathus, Siphonodella, and Taphrognathus. Two conodont elements, probably from the Upper Devonian Chattanooga Shale, were obtained in outcrop breccia samples, but the core yielded no Devonian fossils. Silicified crinoid stems and plates occur at the bottom and at several other levels in the core; probably these were derived from unconsolidated Burlington Limestone sediment. Faunas and clasts suggest that resurge and turbidity currents combined loose bottom sediments and fallback clasts to produce a heterogeneous breccia that has a mixed-age fauna. The base of the gray shale produced two typical Pennsylvanian pectiniform conodont elements.