Trace-Element Geochemistry of Brazos Sections, Texas, U.S.A.

TitleTrace-Element Geochemistry of Brazos Sections, Texas, U.S.A.
Publication TypeBook Chapter
Year of Publication2011
AuthorsGertsch, B, Keller, G, Adatte, T, Bartels, D
Book TitleThe End-Cretaceous Mass Extinction and the Chicxulub Impact in Texas
VolumeNo. 100
EditionSEPM, Special Publication
PublisherSEPM (Society for Sedimentary Geology)
KeywordsBrazos, K-T boundary, Texas, Trace Elements

A geochemical study of major (MEs) and trace (TEs) elements across the Cretaceous-Tertiary (KT) transition was carried out on 2 sections of the Brazos area to investigate signals of the Chicxulub impact, as well as redox conditions and weathering processes in these shallow-water environments. Results show that ME and TE patterns are primarily affected by the type of lithology, including claystones, mudstones, a 3 cm thick yellow clay layer, a sandstone complex with 2-3 spherule-rich layers and alternating hummocky cross-bedded and laminated sandstone layers. In the yellow clay layer, TEs (Mn, Ni, Cr, Na) concentrations are comparable to the spherules-rich layers and geochemistry of spherules. Relative abundance of MEs (Al, Ca, Fe, Mg) in both spherule-rich layers and the yellow clay layer shows also a good correlation. This indicates that the yellow clay layer is likely an alteration product of a spherule-rich layer. No other evidence of the Chicxulub impact could be determined based on MEs and TEs values. In the claystone/mudstone intervals, TE concentrations are constant and slightly enriched in redox-sensitive TE (Mo, U, V), which indicates that dysoxic conditions prevailed. Occurrence of rare large pyrite framboids (30-50 μm) below the sandstone complex confirms that redox conditions were dysoxic in the shallow-water Brazos environments. High values for Al and weathering indices show high detrital input dominated by chemical weathering. These results reveal that persistent high stress conditions and high continental runoff prevailed through the late Maastrichtian-early Danian transition. No significant geochemical and environmental change due to the Chicxulub impact is detected. Sudden increases in trace elements (Co, Cr, Ni) possibly related to an impact, are observed only in reworked intervals within the sandstone complex.  PDF