"ROUND ROCKS" AND OTHER ENIGMATIC FEATURES OF WEAUBLEAU STRUCTURE, ST. CLAIR COUNTY, MISSOURI, USA

The mid-Mississippian (latest Osagean or earliest Meramecian epoch) Weaubleau structure contains abundant planar deformation features (PDFs) in fine quartz grains, which indicate exposure to shock pressure. The structure is interpreted to have formed from an oblique meteorite impact. Deformation is heterogeneously distributed across a 19-km-diameter circular drainage feature, interpreted as the tectonic rim. An 8-km-diameter circular area of extensive brecciation, located eccentrically in the southwestern part of the structure, is interpreted as the main impact area (MIA). The field of deformation is fan-shaped away from the MIA. Deformation dies out with distance away from the MIA and with increasing depth. The eccentricity of the tectonic rim and MIA indicate a trajectory from southwest to northeast, which in a mid-Mississippian paleogeographic orientation would have been from west to east. The structure is remarkably well preserved below Pennsylvania strata that partly cover upland areas. The depth of deformation is unknown but likely exceeds 0.5 km because granitic material has been uplifted minimally 350 m in the east-central part of the MIA.

Impact structures are intrinsically anomalous features, and Weaubleau is no exception. Eight distinctive breccia or conglomeratic facies are present. Four anomalous diagenetic features are found in the structure: (1) "round rocks", chert concretions that typically are cored by siltstone or chert clasts, are found in the uppermost impact breccia; (2) sub-millimeter octahedra of hematite after magnetite are disseminated in uppermost breccia; (3) rare, conically shaped, radial calcitic cement clusters after aragonite, which resemble shatter cones, are found both in both the main impact area and down-trajectory area; and (4) a small prospect of transparent tabular barite is found on the perimeter of the MIA near the town of Vista. Distinctive structural features include: (1) oblique stylolites, (2) LS-tectonites, (3) broken folds with thickened axial sections, and (4) thrust faults. None of the diagenetic or structural features is diagnostic, but they characterize a panoply of processes that occur during the excavation and modification phases of impact cratering.