MICROSTRUCTURAL ANALYSIS OF CALCITE WITHIN THE WEAUBLEAU STRUCTURE, WEST-CENTRAL MISSOURI

There are currently 114 confirmed impact structures on Earth, and many unconfirmed but suspected impact structures. One such suspected structure is the Weaubleau-Osceola structure located in St. Clair County, western Missouri. Although poorly exposed, the structure may be circular in shape with highly deformed Ordovician and Mississippian strata within center. Undeformed Pennsylvanian strata overlie much of the structure. Macroscopic deformation features recognized within the structure include faults, folds, and various types of breccia. Features indicative of an impact structure include quartz grains with planar deformation features (found within light fractions of resurge breccia samples processed for conodonts) and mixed conodonts that have been reworked stratigraphically up section.

This project examined microstructures within calcite of the affected Mississippian limestone to evaluate the operative calcite deformation mechanisms within a likely impact structure. Five limestone samples were collected from within the structure and one sample was collected several kilometers outside of the structure. Microstructures were observed under petrographic and universal stage microscopes. Mechanical twins and microfractures were found to be the dominant microstructures. Point counting provided twin and fracture densities per sample site. Twin density was found to be highest within a sample collected near the center of the structure, with most other samples displaying low twin density comparable to that found regionally outside of the structure. Similarly, microfracture density was highest within a sample collected near the center of the structure. The increase in fracture and twin densities towards the center of the structure indicates that mechanical twinning, as well as fracturing, is operative at the high strain rates resulting from an impact event.