PALEOMAGNETISM OF THE DECATURVILLE IMPACT BRECCIAS AND THE WEAUBLEAU-OSCEOLA STRUCTURE, SOUTHWEST MISSOURI: TESTING THE SERIAL IMPACT HYPOTHESIS

In this study we use paleomagnetic field tests to constrain the timing of two structures in Missouri (USA), the poorly-dated Decaturville impact and the Weaubleau-Osceola structure, a supposed impact. We use the information to evaluate the possibility of a serial impact which has been proposed for these as well as other features along the 38th parallel in North America. Previous studies at Decaturville suggested a Pennsylvanian-Cretaceous age range for the impact. A modified paleomagnetic conglomerate test reveals that the magnetization within a mixed breccia in the Ordovician Jefferson City Dolomite (JCD) is post-deformational and resides in magnetite. The pole (50.1°N, 127.8°E; dp = 2.5°, dm = 4.9°) lies on the Early to Middle Permian portion of the apparent polar wander path. Because of the clasts all have the same magnetization, the breccia and the impact can be no younger than the mid Permian. The remagnetization mechanism was either heat during emplacement or chemical processes caused by fluid flow that may or may not be related to impact. The breccia pole is distinct from an Early Pennsylvanian pole in the JCD outside of the impact structure. Previous studies have documented that impact brecciation occurred after MVT mineralization at Decaturville and dates for the mineralization in nearby MVT deposits range from Pennsylvanian to Triassic. This suggests that the impact could not have been older than Pennsylvanian and along with the pole position, constrains the timing of the impact to the Pennsylvanian-mid Permian. Tilt test results from deformed Mississippian strata at Weaubleau-Osceola indicate a post deformation magnetization with a late Mississippian pole. The magnetization is interpreted as chemical in origin, perhaps related to hydrothermal fluids. The pole and stratigraphic constraints indicate that the deformation event can be no younger than late Mississippian. These results suggest that the Weaubleau-Osceola and Decaturville structures are different ages and could not have been caused by a serial impact.