IMPACT ORIGIN OF THE WEAUBLEAU-OSCEOLA STRUCTURE IN SOUTHWESTERN MISSOURI

Recent discovery of a 19-km-diameter circular drainage feature encompassing the Weaubleau structure sensu Beveridge (1951) in southeastern St. Clair County, Missouri has prompted renaming of this feature as the Weaubleau-Osceola structure. The new name more accurately reflects the extent of the deformation and a paradigm shift in interpretation of its origin. Field and laboratory studies now provide evidence that the deformation was caused by a meteorite impact. The circular drainage is a ring moat, and an uplifted outer rim marks the margin of the transient crater with a diameter of approximately 27 km.

Within the circular structure, highly deformed carbonates of the Pierson-Burlington-Keokuk formations undivided (Osagean) are overlain by undeformed polymict carbonate breccia and by younger Pennsylvanian conglomerate (Graydon Formation, Atokan) and sandstone of the Cherokee Group (Desmoinesian). Exposures of deformed carbonates in 10-25 m high quarry walls near the center of the structure indicate a downward transition from brittle to ductile deformation; three structural domains include heavily fractured and sheared rock overlying tightly folded, thrust-faulted strata, which overlie beds with more open folds. The latter also contain zones of injection breccia that were derived from fluidized siltstone and shale of the Northview Formation (Kinderhookian).

Macroscopic and microscopic features also support an impact hypothesis. Clusters of needle-like “stylolites” are common in the deformed carbonates; these commonly have conical geometries very similar to shatter cones. Other sets of stylolites have multiple orientations that indicate oblique pressure fronts with up to ~35° variation in direction. Quartz grains from the superposed fall-back (?) breccia contain multiple sets of planar fractures and (unindexed) shock lamellae.

Age constraints of the impact range from latest Osagean to Atokan, but karstificiation of the target rock (late Osagean) and impact breccia and development of extensive Pennsylvanian paleo-valleys suggest that a middle to late Mississippian age is likely.