The major objective of this project is to delineate the shallow (upper 300 m) subsurface structural configuration within the Manson Impact Structure (MIS), Iowa, using state-of-the-art high resolution seismic imaging techniques. The MIS is a bollide impact feature 65.7 Ma in age, and is an important candidate for a source of the Cretaceous/Tertiary-boundary iridium anomaly. The feature may thus have played a central role in one of the best documented examples of mass extinction due to global change in geological history. A detailed knowledge of the structure, stratigraphy, and petrology of the MIS is needed as basic data for the construction of a model of the impact dynamics. However, the exact geometry within the structure is poorly known due to a 30-90 m thick Pleistocene till cover. Seismic reflection lines will provide a direct image of the structure and gross stratigraphy of the impact feature, and will constrain stratigraphic and petrological interpretations inferred from drilling transects, at least in the vicinity beneath the seismic lines. The major seismic line is along an east-west radius of the crater, from the central peak structure to the undisturbed stratigraphic section outside the crater. In addition to delineating the general structure of the central uplift, moat and ring graben, specific objectives include locating the presence (or absence) of faults, defining the depth and extent of the peak pit, and resolving the internal geometry of the impact breccia and melt zones at the edge of the central uplift. This study will provide fundamental geologic knowledge to contribute to an understanding of the global change in the biosphere that may have occurred, at least in part, as a direct result of the bollide impact at Manson. The seismic data will greatly enhance the interpretation of all data sets acquired as a result of the Manson drilling program, and will constrain the shallow subsurface structure and stratigraphy, allowing more accurate models of impact dynamics to be constructed. The seismic reflection data will help to identify targets for future drilling and for subsequent detailed surface geophysical studies such as electromagnetic, magnetic, and microgravity surveys by others.
