2008 Joint Meeting of The Geological Society of America, Soil Science Society of America, American Society of Agronomy, Crop Science Society of America, Gulf Coast Association of Geological Societies with the Gulf Coast Section of SEPM

Paper No. 4
Presentation Time: 8:00 AM-6:00 PM

Geophysical, structural, and kinematic analysis of the Weaubleau structure: Implications for an oblique impact

EVANS, Kevin R. and MICKUS, Kevin L., Department of Geography, Geology, and Planning, Missouri State University, 901 S. National Avenue, Springfield, MO 65897, kevinevans@missouristate.edu

Oblique impacts are recognized from their crater morphology and patterns of ejecta distribution. In many ancient terrestrial impacts, it is difficult to determine impact trajectories and angles of incidence. Erosion commonly removes shattered crater rims and ejecta blankets. Asymmetrical central uplifts or gravity anomalies are considered controversial evidence. The Weaubleau structure of west-central Missouri shows marked asymmetry in distribution of structural deformation, emplacement of crystalline basement breccia, and Bouguer gravity anomaly, indicating a trajectory from southwest to northeast, but the angle of incidence remains unknown.

The central uplift is a ring-like topographic low in SRTM and DEM imagery, and is coincident with the distribution of a fine-grain carbonate resurge breccia. A Bouguer gravity low is present in the center of the central uplift, where drill core indicates filling with sediment-gravity-flow breccias. A ridge-like gravity high to the northeast appears to be related to the emplacement of crystalline basement breccia at depth on the interior periphery of the central uplift.

Target rocks include a succession (<50 m thick) of middle Mississippian limestone that overlies 3 m of incompetent shale and siltstone. The latter facilitated strain transfer and slip to the northwest. In quarry-wall exposures 3 km northeast of the central uplift (downrange), structural deformation can be divided into two domains: (1) a brittle, upper domain with fracture and injection breccia between megabreccia blocks, and (2) a ductile to brittle lower domain with tight to isoclinal folds, normal and thrust faults, and injection and dilation breccias. L-tectonites indicate subvertical and radial movement away from the central uplift.

In contrast, proximal target rocks 3 km northwest of the central uplift are gently folded. To the southeast, strata are modestly deformed, and to the southwest, Mississippian rocks have been eroded below the sub-Pennsylvanian unconformity on an upthrown block of undeformed Lower Ordovician strata.