Paper No. 5
Presentation Time: 1:30 PM-5:30 PM
POST-DEFORMATIONAL REMAGNETIZATION OF THE WEAUBLEAU-OSCEOLA STRUCTURE, SW MISSOURI
The Weaubleau-Osceola impact structure consists of a 19-km circular fearture that contains deformed Mississippian limestones and a fall-back breccia. The age of the structure is stratigraphically constrained between deposition of the deformed Osagean limestones and the overlying undeformed Pennsylvanian (Desmoinesian) units. Paleomagnetic samples were collected from the tilted Burlington-Keokuk and Sedalia formations inside the structure, the fall-back breccia, and the undeformed Burlington-Keokuk outcrops from outside of the structure. Stepwise thermal and alternating field demagnetization of tilted limestone samples reveals a characteristic remanent magnetization (ChRM) with southeasterly declinations and shallow to moderate positive inclinations that has median destructive field of 20-30 mT and maximum unblocking temperatures of 450°C. The ChRM resides in magnetite, is post folding, and the pole falls near the 309-365 Ma mean pole on the apparent polar wander path. A few of the fall-back breccia samples contain the ChRM but most contain a Modern component residing in hematite. Most of the samples from outside the structure contain a Modern component residing in magnetite, although some contain a poorly defined component that appears similar to the ChRM. A similar component interpreted as a primary chemical remanence has also been reported from undeformed limestones to the southwest. The post-deformational ChRM is not a shock magnetization and is interpreted as a chemical remanent magnetization (CRM) based on the maximum unblocking and burial temperatures. One hypothesis for the origin of the CRM is hydrothermal fluids which were activated as a result of the impact. Preliminary analyses, however, indicate that the rocks contain coeval 87Sr/86Sr values and there is no geochemical for alteration by hydrothermal fluids. The origin of the CRM is enigmatic. It is not clear that the CRM present in the crater is directly related to the impact, as a similar component is found in undeformed limestones outside of the crater. The CRM does appear to be more common inside the deformed structure than in the surrounding rock. We are currently conducting additional studies to determine the mechanism of remagnetization, and to determine if the CRM is related to the impact.