Characterization, geographic distribution, and number of upper Eocene impact ejecta layers and their correlations with source craters

Multiple impacts occurred during the late Eocene, but the exact number of impact events is unknown. When this study began in 1998, from two to six or more impact spherule layers with a late Eocene age had been proposed. The geographic distribution of each spherule layer was unclear and the types of impact ejecta and their correlation with known impact craters were also in need of further investigation. The objectives of this study were to determine the relationship among upper Eocene impact ejecta from different locations, to determine the upper Eocene impact ejecta distribution patterns, to determine the relationship between the upper Eocene ejecta/spherule layers and the Chesapeake Bay and Popigai impact craters, to correlate the upper Eocene biostratigraphy at different latitudes and different locations, and to determine the possible biologic effect associated with the upper Eocene events. Samples from a number of DSDP/ODP sites, Massignano (Italy), and Barbados were searched for impact ejecta. The data obtained from the petrograhic, mineralogical, and geochemical studies of the impact ejecta and the associated sediment were used to address the above issues.; During this study, impact ejecta (microtektites, cpx spherules, and/or shock-metamorphosed mineral grains) were recovered from six new sites. Microtektites were recovered from Hole 699A with an abundance of ∼ 8/cm2 (>125mum); two microtektites and one cpx spherule were recovered from Hole 703C; microtektites with an abundance of 374/cm2 (>125mum), cpx spherules with an abundance of 9220/cm2 (>125mum), and shocked quartz and coesite were identified at Hole 709C; cpx spherules with an abundance of ∼ 20/cm2 (>125mum) were recovered at Hole 786A; coesite-bearing grains were the only impact ejecta identified at Hole 1073A; microtektites with an abundance of 27/cm2 (>125mum) and cpx spherules with an abundance of 18/cm2 (>125mum) were recovered from Hole 1090B.; Additional data were obtained from seven sites previously known to contain impact ejecta. Some coesite and very few microtektites were recovered from Site 543; shocked zircons containing reidite (a high-pressure ZiSiO4 polymorph) were recovered in the North American microtektite layers at Site 612, Holes 903C and 904A, and Barbados. Microtektites with an abundance of 46/cm2 (>125mum) and cpx spherules with an abundance of 11/cm2 (>125mum) were recovered from Hole 689D. Altered cpx spherules and shocked quartz were recovered from Massignano, Italy. (Abstract shortened by UMI.)...