BOUGUER GRAVITY AND MAGNETIC ANOMALIES ASSOCIATED WITH THE WEAUBLEAU-OSCEOLA IMPACT STRUCTURE, MISSOURI

Recent discovery of the Weaubleau-Osceola meteorite impact structure in southeastern St. Clair County, Missouri has been confirmed by field and laboratory studies. This feature was first recognized from digital elevation models because of its distinctive circular drainage pattern: this pattern marks the location of a ring moat with an uplifted outer rim, marking the extent of the tectonic rim. The point-of-impact is marked by a 7-km diameter ring with nearly 400 m of uplift, located somewhat eccentrically to the larger structure. The center is approximately 3 km south of the village of Vista.

Evidence for a meteoritic origin of the structure includes highly deformed carbonates that are overlain by undeformed but karstified polymict carbonate breccia. The upper part of this breccia is interpreted as a fall-back component, and it contains a large number of quartz grains and granules that have multiple directions of planar fractures and decorated shock lamellae. It also contains mixed conodont faunas of early Ordovician and early to middle Mississippian ages. Shallow core drilling show that at approximately 220 ft (67 m) depth in the ring uplift, granite and carbonate breccia with possible melt material are encountered. Undeformed Pennsylvanian sediments partially cover this succession.

To determine the three-dimensional extent of the structure, a gravity and magnetic analysis was conducted. The existing gravity data are sparse in St. Clair County, so we collected an additional 300 stations within and surrounding the structure. These data were processed into Bouguer gravity anomalies and merged with the existing data. The Bouguer gravity anomaly map indicates a complex pattern of high-frequency minima and maxima with two low-amplitude gravity minima at the suspected point of impact and to the northeast. A gravity maximum coincides with the ring uplift. A residual gravity anomaly map removes the regional northwest-trending anomalies to highlight the minima thought to be related to the impact site. Aeromagnetic data confirm the finding from the gravity data.