A PETROGRAPHIC STUDY OF THE WEAUBLEAU-OSCEOLA AND CROOKED CREEK IMPACT STRUCTURES, MISSOURI

Impact structures are unique phenomena caused by high-velocity collisions of extraterrestrial rock/ice bodies with the surface of the earth. Almost 30 are currently recognized in the United States alone (Koeberl and Anderson 1996). The State of Missouri contains three such structures. One of the three, the Weaubleau-Osceola impact structure, has only recently been recognized as an impact structure (Evans et al. 2003). A petrographic study of breccia samples from Weaubleau-Osceola and from the Crooked Creek structure in Missouri was performed in order to contribute to a better understanding of the formational history of both of these structures. The breccia samples from the Weaubleau-Osceola structure examined in this study are of two types: the first is a heterogeneous breccia containing mostly brachiopod and echinoderm hash. The other type is similar, but secondary weathering processes have dissolved some of the fossil hash, making the rock highly porous. In addition to fossil hash, other types of fragments include angular to rounded quartz grains, chert, limestone, broken oolites, and iron oxides (limonite/hematite). The Crooked Creek samples are much different. They consist of chert breccias, dolomite breccias, and dolomite shatter cones. Opaque minerals, angular to well-rounded quartz grains (several with shock features), chert, and pulverized dolomite are characteristic of these samples.