Age and Origin of the Chicxulub Impact and Sandstone Complex, Brazos River, Texas: Evidence from Lithostratigraphy and Sedimentology
Published:January 01, 2011
Thierry Adatte, Gerta Keller, Gerald R. Baum, 2011. "Age and Origin of the Chicxulub Impact and Sandstone Complex, Brazos River, Texas: Evidence from Lithostratigraphy and Sedimentology", The End-Cretaceous Mass Extinction and the Chicxulub Impact in Texas, Gerta Keller, Thierry Adatte
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Multidisciplinary investigations based on the lithology, sedimentology, mineralogy, and biostratigraphy of upper Maastrichtian to lower Danian boundary (KTB) sequences along 3.5 km of the Brazos River in Falls County, Texas, reveal depositional sequences, including an impact-spherule-rich sandstone complex, characteristic of sequence stratigraphic models applied to shallow shelf areas, such as incised valleys, lag conglomerate, storm deposits, and repeated bioturbation. The top of the Corsicana Formation coincides with a channel, which we interpret as an incised valley. The erosion surface marks a major depositional sequence boundary (SB) associated with the latest Maastrichtian sea-level fall. Initial channel deposits consist of coarse shelly glauconitic sand with large lithified clasts containing impact spherules and large bored and encrusted phosphatized concretions, which we interpret to indicate that the Chicxulub impact occurred well prior to the lithification, erosion, and redeposition at the base of the channel. The primary Chicxulub ejecta layer lies about 40–65 cm below the sandstone complex in a 3-cm-thick yellow clay layer that consists of cheto smectite (altered impact glass) interbedded in claystones of the Corsicana Formation. Above the sandstone complex, claystones, and mudstones are burrowed and correspond to a condensed interval interpreted as a maximum flooding surface (MFS). Based on biostratigraphy and the δ13C shift, the KT boundary is up to 1 m (50–100 ky) above the sandstone complex and coincides with increased sediment accumulation during the early Danian sea-level rise (HST). These features are inconsistent with a single catastrophic bolide impact on Yucatán and associated megatsunami deposition as commonly interpreted.
The biostratigraphy and KT characteristic δ13C shift of the Brazos sections indicate that the KTB, the sandstone complex, and the Chicxulub impact occurred as three different stratigraphic events during the late Maastrichtian planktic foraminiferal zone CF1. These are represented by: (1) the Chicxulub impact sequence deposited about 200–300 ky prior to the KTB, (2) the sandstone complex with reworked impact spherules deposited in incised valleys during the latest sea-level fall about 100–150 ky prior to the KTB, and 3) the KTB event during the subsequent HST and following the condensed MFS.
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The End-Cretaceous Mass Extinction and the Chicxulub Impact in Texas
One of the liveliest, contentious, and long-running scientific debates began over three decades ago with the discovery of an iridium anomaly in a thin clay layer at Gubbio, Italy, that led to the hypothesis that a large impact caused the end-Cretaceous mass extinction. For many scientists the discovery of an impact crater near Chicxulub on Yucatan in 1991 all but sealed the impact-kill hypothesis as proven with the impact as sole cause for the mass extinction. Ever since that time evidence to the contrary has generally been interpreted as an impact-tsunami disbturbance. A multi-disciplinary team of reserachers has tested this assertion in new cores and a dozen outcrops along the Brazos River, Texas. In this area undisturbed sediments reveal a complete time stratigraphic sequence containing the primary impact spherule ejecta layer in late Maastrichtian claystones deposited about 200-300 thousand years before the mass extinction.