IMPACT GENERATED SOFT-SEDIMENT RESUSPENSION AND CALCAREOUS NANNOFOSSIL MIXING PRODUCED BY THE CHICXULUB AND MANSON IMPACTS

A combination of industry wells and seismic has identified an extensive micritic deposit at the K/Pg boundary in the deep-water Gulf of Mexico with penetrated thicknesses ranging from 10 to 200 m, and seismically estimated thicknesses of over 1000 m. Evaluation of publicly available biostratigraphic data from nine wells found a mixture of Lower Maastrichtian and Upper Campanian calcareous nannofossils with rare uppermost Maastrichtian to Aptian specimens, similar to the Cretaceous / Tertiary "cocktail" assemblage identified in Gulf of Mexico and Caribbean DSDP cores. No succession of bioevents is discernible within the deposit, and preservation is generally poorer than is found in sediments directly underneath the deposit. Examination of calcareous nannofossils from three cores taken within the Upper Campanian impact crater at Manson, Iowa, found that the matrix samples consistently contained a mixture of Lower Campanian to Santonian species, while clasts were predominantly Turonian in age. These results indicate that earthquakes and/or tsunamis associated with marine bolide impacts resuspend unconsolidated sediments into the water column after which they settle atop locally-derived mass-transport deposits containing pelagic components initiated by the first shock waves as well as later, distally-derived deposits containing shallower components. In neritic settings seafloor sediments are less condensed and more consolidated, restricting resuspension to a narrower age range than in basinal settings. This process is responsible for the deposition of seemingly in situ marls atop spherule-rich beds in locations such as the Brazos River, Texas, northeastern Mexico, and the Chicxulub crater. This accounts for the erroneous interpretations of graded beds as in situ marls containing environmentally stressed, "dwarfed" foraminifera, multiple K/Pg boundary impacts, and the miss-dating of the K/Pg boundary Chicxulub impact as occurring 300 k.y. prior to the boundary.