Chicxulub Impact Predates K-T Boundary in Texas and Caused no Mass Extinction

Details Chicxulub Impact Predates K-T Boundary in Texas and Caused no Mass Extinction Chicxulub Impact Predates K-T Boundary in Texas and Caused no Mass Extinction

May 2005

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agusm.p11a..03f&link_type=abstract

American Geophysical Union, Spring Meeting 2005, abstract #P11A-03

Physics

1219 Local Gravity Anomalies And Crustal Structure, 1517 Magnetic Anomaly Modeling, 1519 Magnetic Mineralogy And Petrology, 1520 Magnetostratigraphy <!: : : : : : : : : : >, 1630 Impact Phenomena, 3030 Micropaleontology, 3662 Meteorites, 4863 Sedimentation, 6022 Impact Phenomena

Scientific paper

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.

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