HR: 09:10h
AN: P11A-03 INVITED     [Abstracts]
TI: Chicxulub Impact Predates K-T Boundary in Texas and Caused no Mass Extinction
AU: * Keller, G
EM: gkeller@princeton.edu
AF: Princeton University, Department of Geosciences, Princeton, NJ 08544 United States
AU: Adatte, T
EM: Thierry.Adatte@unine.ch
AF: University of Neuchatel, Geological Institute, Neuchatel, CH-2007 Switzerland
AU: Berner, Z
EM: zsolt.berner@img.uni-karlsruhe.de
AF: University of Karlsruhe, Institute for Mineralogy and Geochemistry, Karlsruhe, 76128 Germany
AU: Baum, G
EM: GBaum@dnr.state.md.us
AF: Maryland Geological Survey, Environmental Geology & Mineral Resources, Baltimore, MD 21218 United States
AU: Stueben, D
EM: doris.stueben@img.uni-karlsruhe.de
AF: University of Karlsruhe, Institute for Mineralogy and Geochemistry, Karlsruhe, 76128 Germany
AB: In the Chicxulub crater and throughout NE Mexico the impact breccia and spherule ejecta layer, respectively, predate the K-T boundary by about 300,000 years (Keller et al., 2003, 2004). The stratigraphic separation between the K-T boundary and the Chicxulub impact ejecta varies from 50 cm in the Chicxulub crater, to over 14 m in NE Mexico, with the variation due to erosion, non-deposition and paloetopography. New studies from drilling and exposures along the Brazos River, Texas, confirm these findings based on biostratigraphy, paleomagnetic stratigraphy, geochemistry, stable isotopes, and faunal assemblages. In this area, the spherule ejecta is reworked near the base of a series of `event beds' representing variable storm deposits separated by repeated colonization of the ocean floor by invertebrates. The base of these storm beds overlies an undulating erosion surface of latest Maastrichtian claystone. The original spherule ejecta layer appears to be within the underlying claystone, in the lower part of chron 29R and near the base of biozone CF1, which marks the last 300,000 years of the Cretaceous. Above the `event beds' latest Maastrichtian claystone sedimentation continues up to the K-T boundary, which is characterized by a sharp (1.4 ng/g) iridium anomaly that marks the K-T as a second major impact. The distance between the top of the `event beds' and the K-T boundary varies from 20 cm to 1.6 m depending on local tectonics and erosion. Evaluation of the biotic effects of the Chicxulub and K-T impacts upon planktic foraminifera, which suffered most severely of all marine organisms, reveals no species extinctions associated with the Chicxulub impact and no significant species population changes, except for species dwarfing as a result of increased biotic stress. These Brazos results confirm the 65.3 Ma age for the Chicxulub impact determined from NE Mexico and the crater core Yaxcopoil-1. They also show that the Chicxulub impact did not cause a mass extinction, but the 65.0 Ma K-T impact did.
UR: http://geoweb.princeton.edu/people/faculty/keller/chicxulub.html
DE: 1630 Impact phenomena
DE: 3030 Micropaleontology
DE: 3662 Meteorites
DE: 4863 Sedimentation
DE: 6022 Impact phenomena
SC: Planetary Sciences [P]
MN: 2005 Joint Assembly