The Chicxulub impact crater and oblique impact
McDonald, Matthew A.
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Was Chicxulub an oblique (<45 degrees) impact? The answer to this question will help our understanding of the environmental consequences of the bolide that struck the Earth 65 ma in the Yucatan Peninsula. Planetary impact events, and impact simulations in the laboratory, show that oblique impacts have clear asymmetric ejecta distributions. However, the subsurface structures of the resultant craters are not well understood. From January 5, 2005 - February 19, 2005, we acquired 1822 km of seismic reflection data onboard the R/V Maurice Ewing imaging the massive (190+ km) Chicxulub impact crater to improve the understanding of subsurface crater structure and assess implications for impact obliquity. Pre-crater stratigraphy outside the central basin of the Chicxulub impact crater includes packets of high amplitude reflectors on the seismic profiles, likely due to the presence of interbedded evaporate and carbonate layers. These reflective horizons are offset downward into the crater marking the post-impact slumping and formation of the terrace zone. A terrace zone forms when the overextended transient crater, which is formed by the initial impact, collapses gravitationally; large amounts of sediment, both inside and outside the transient crater, slump inward as a series of downward stepping blocks. Can the terrace zone tell us something about oblique impact? This thesis is divided into two separate chapters addressing this question, and each will be independently submitted for publication. The first chapter presents the 3-D structure of the terrace zone based on seismic reflection data from the 2005 survey and an earlier survey done in 1996. The second chapter presents an analysis of Venusian craters addressing how oblique impacts relate to peak ring offsets. These craters are structurally comparable to Chicxulub, and the analysis implies that Chicxulub's SE peak ring offset is not related to impact trajectory. The two chapters in this thesis are mutually supportive, and suggest that the effects of oblique impact are overshadowed by other factors such as pre-impact structure and target properties. Therefore, subsurface asymmetries in the terrace zone do not appear to provide information about impact obliquity. The terrace zone, however, may help reconstruct the shape of the transient cavity and explain a concentration of ejecta toward the northwest.