Physics – Geophysics
Scientific paper
May 2005
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agusm.p12a..01t&link_type=abstract
American Geophysical Union, Spring Meeting 2005, abstract #P12A-01
Physics
Geophysics
3000 Marine Geology And Geophysics, 3022 Marine Sediments: Processes And Transport, 3675 Sedimentary Petrology, 5420 Impact Phenomena (Includes Cratering)
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. The study is based on high-resolution sampling, sedimentological observations, biostratigraphy, bulk rock and clay mineralogy, 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 K-T boundary is 40 cm above the storm deposits. Granulometric analyses of this interval reveal no size grading due to suspension settling from storm or tsunami waves, but rather indicate normal hemipelagic sedimentation. 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 siliclastic 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. We may have discovered the original ejecta layer in Cottonmouth Creek 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. The presence of a significant amount of gypsum may derive from remobilization of Chicxulub sulfate bearing spherules. Similar Cheto smectite layers have been documented from ejecta spherule deposits in Central America and the Caribbean. The Brazos studies confirm that the Chicxulub impact predates the KT 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.
Gerta K.
Thierry Amedee
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