Multiproxy Approach of the K-T and Chicxulub Ejecta Layers Along the Brazos River, Texas, USA

Details Multiproxy Approach of the K-T and Chicxulub Ejecta Layers Along the Brazos River, Texas, USA Multiproxy Approach of the K-T and Chicxulub Ejecta Layers Along the Brazos River, Texas, USA

May 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agusm.p43a..04a&link_type=abstract

American Geophysical Union, Fall Meeting 2007, abstract #P43A-04

Mathematics

Logic

4944 Micropaleontology (0459, 3030), 6022 Impact Phenomena (5420, 8136), 9610 Cretaceous, 1050 Marine Geochemistry (4835, 4845, 4850), 1051 Sedimentary Geochemistry

Scientific paper

We report the results of preliminary investigations of four K-T boundary sections, which are located in small tributaries (Cottonmouth and Darting Minnow creeks) of the Brazos River and the recently drilled Mullinax-1 core. The study is based on high-resolution sampling, sedimentological observations, biostratigraphy, bulk rock and clay mineralogy, major and trace elements geochemistry and granulometry. The Cottonmouth Creek exposure is characterized by Late Maastrichtian dark grey fossiliferous claystone, interrupted by laterally variable channel fill storm deposits, which previously have been erroneously interpreted as impact tsunami deposits. These deposits consist of a basal shell hash (10cm), followed by glauconitic sand with altered impact spherules (10cm), laminated sandstones, and 4 to 5 hummocky cross-bedded sandstone layers separated by burrowed erosion surfaces that mark repeated colonization of the ocean floor between storm events. Above and below these storm events are dark grey fossiliferous claystones of the late Maastrichtian zone CF1, which spans the last 300,000 years of the Cretaceous. The observed sedimentary succession correspond therefore to incised valley infillings linked to a sea-level drop with a possible emersion, followed by a transgression which culminates at the K-T boundary. More specifically, the storms beds overlying the sequence boundary would correspond to late LST sediments which infill the incised valley, the overlying Maastrichtian claystone corresponds to the Early TST with a maximum at KTB (MFS). The K-T boundary is 40 cm and 90 cm above the storm deposits in the outcrop and Mullinax-1 core respectively. In the Mullinax-1 core, high resolution granulometric analyses of this interval reveal the event bed as repeated thinning upwards sequences, from the spherule- and glauconite-rich sandstones with HCS to fine laminated carbonated sandstones and finally thick bedded mudstone. But the last thinning upwards sequence is separated from the K-T boundary by at least 20 to cm of normal hemipelagic claystone showing calcite, phyllosilicates, TOC, isotope and granulometric values similar to the pre-event sediments and reflect therefore normal sedimentary conditions. The Chicxulub spherule ejecta in the glauconitic sand near the base of the storm beds is reworked from an older original ejecta layer, as indicated by abundant reworked fossil shells. This is similar to the reworked spherule layers at the base of the siliciclastic deposits throughout NE Mexico, where the original layer is within marls up to 5 m below (base of CF1) and predating the K-T by 300,000 years. The original ejecta layer in Cottonmouth Creek lies 60 cm below the basal unconformity of the storm beds and within claystones near the base of zone CF1. This layer consists of a prominent 3-4 cm thick yellow clay of pure and well-crystallized smectite (Cheto Mg-smectite) that possibly represents the alteration product of Chicxulub impact glass. Glass altered smectite spherules are commonly present and present the same geochemical composition as glass and spherules weathered to smectite from Haiti and NE-Mexico. Similar Cheto smectite layers have been documented from ejecta spherule deposits in Central America and the Caribbean. The Brazos results confirm that the Chicxulub impact predates the K-T boundary by about 300,000 years, as earlier observed based on impact glass spherule layers in northeastern Mexico and the suevite breccia from the Yaxcopoil-1 core in Yucatan.

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