Cores drilled into the central peak region of the K-T boundary-age Manson impact structure have encountered three major impact breccias. The shallowest is a sedimentary clast breccia with rare crystalline, melted or shocked clasts. This is underlain by a crystalline clast breccia with a glassy matrix followed by a crystalline clast breccia with a sandy matrix. Shock-metamorphosed and shock-melted materials are common in both crystalline breccias. This study will investigate the petrology of these breccias and related breccias encountered by new drilling, and will develop models for their emplacement and subsequent deformation. It will be conducted by University of Iowa personnel in conjunction with the Iowa Department of Natural Resources-Geological Survey Bureau (IDNR-GSB). This study promises to enhance understanding of cratering processes at the Manson site as well as provide information about the nature of ejecta that may have been incorporated into K-T boundary sediments worldwide. Thin-sections used in this study will be cut from core allocated to the IDNR- GBS and petrographically examined to determine clast types, sizes, and shapes as well as the nature of matrix materials. Optically obscure clasts and matrix materials will be identified using SEM- EDS and XRD techniques. Changes in clast and matrix abundances with depth will be quantified by point-counting. Electron microprobe analysis will be used to characterize changes in mineral and glass phase compositions with depth. These data will be stored, manipulated and graphically displayed using standard soft- ware running on IBM-PD2 computers. It will be made available to other Manson researchers when it is compiled.
