OBLIQUE IMPACT AT WEAUBLEAU-OSCEOLA STRUCTURE, MISSOURI

The Weaubeau-Osceola structure is a middle to late Mississippian meteorite impact site. The unusual geometric configuration of structural elements and local strain fabrics indicate that it was oblique impact from the southwest with an estimated angle of incidence of ~15°-30°.

An outer ring, 19 km in diameter, outlines an area of intense to progressively milder deformation. Geologic mapping and logs of water-well cuttings indicate development of a tectonic rim around its perimeter. A 7 km-diameter inner ring in the southwestern part of the outer ring consists of low ridges and broad valleys that developed on a carbonate impact breccia. This ring marks the extent of the transient crater.

The inner and outer rings are nearly coincident on the southwestern margin, where Mississippian strata are faulted and cut out below the sub-Pennsylvanian unconformity. With approximately 60 m of Mississippian strata downthrown to the northeast, the fault records shock loading on the leading edge of the impact.

A broad field of deformation is found northeast and east of the inner ring (target strata are buried to the north). Tight folds in quarry exposures, 3 km outside of the inner ring, indicate lateral movement directed away from the transient crater. Associated thrust faults may have formed during excavation or modification stages of the crater development. Incompetent shale in the Northview Formation served to lubricate movements and forms a significant component of the injection breccia pockets in the quarry. More distally, a bedding plane in the overlying carbonates was a decollement.

Two lengthy orthogonal lineaments are tangential to the inner ring and intersect at the geometric center of the outer ring. These are interpreted as conjugate fractures. Their bisector runs through the center of the inner ring, indicating a principal paleo-stress (s1) component trending to the northeast. Sets of pervasive, penetrative needle-like displacement features in the carbonate target rock, which superficially resemble stylolites, are inclined toward the inner ring.

Literature studies suggest that impacts with incidences >30° tend to form concentrically ringed structures, whereas low-angle impacts (<15°) typically form oval craters. A trajectory for the Weaubleau-Osceola impactor is inferred between these limits.