Brazos River, Texas: Drilling the K-T and Chicxulub Event Strata
Gerta Keller Princeton University
The main objective of this project was to drill the Cretaceus-Tertiary (K-T) transition in the Brazos River area in order to recover a series of cores that span the late Maastrichtian and early Paleocene, including the K-T boundary horizon, the sandstone complex (event strata) and the Chicxulub impact spherule layer. We reasoned that by recovering these three events in stratigraphic order, it could be determined whether there was a cause-and-effect relationship between the Chicxulub impact on Yucatan and the K-T mass extinction. The Brazos River area was chosen because K-T deposition occurred in a shallow shelf environment, sediments are undisturbed by tectonics and sediment deposition is among the most complete worldwide. The Brazos sequences thus held the potential to solve the current controversy over the age of the Chicxulub impact, the biotic effects and whether this impact caused the mass extinction.
DOSECC drilled in two localities and recovered three 75 m long cores spanning from the early through the late Maastrichtian. The cores were sampled and analyzed by an international team of scientists (US, UK, Germany, Switzerland, France, Israel, Egypt). Analysis included sedimentology, mineralogy, stable isotoes, PGE and trace element analysis (ICP-MS), biostratigraphy and quantitative faunal and floral analyses (planktic foraminifera, nannofossils, palynomorphs).
The K-T sequences along the Brazos River of Falls County, Texas, provide the most important and critical information regarding the age and biotic effects of the Chicxulub impact outside of Mexico. New investigations based on outcrops and new cores drilled by DOSECC and funded by the National Science Foundation reveal a complex history of three tectonically undisturbed and stratigraphically well-separated events: the Chicxulub impact spherule ejecta layer, a sea-level lowstand event deposit (sandstone), and the K-T mass extinction (Keller, 2007; Keller et al., 2007).
The newly discovered Chicxulub impact spherule layer is the oldest of the three events and marks the time of the impact about 300,000 years before the K-T boundary (base of zone CF1), consistent with similar observations from NE Mexico and the Chicxulub crater core Yaxcopoil-l. Paleontological analysis reveals that the Chicxulub impact caused no species extinctions or any other significant biotic effects (Keller et al., 2008). The subsequent sea level lowstand resulted in deposition of a sandstone complex, which predates the K-T boundary by about 100,000 years.
Reworked Chicxulub impact spherules and clasts with spherules in this sandstone complex were eroded from the original impact spherule layer. The third event is the K-T boundary mass extinction, which in the new cores is 80 cm above the sandstone complex and clearly unrelated to the Chicxulub impact.
These data reveal that the Chicxulub impact and K-T mass extinction are two separate and unrelated events, and that the biotic effects of this impact have been vastly overestimated. The K-T mass extinction was likely caused by massive volcanism in India, which marks the second catastrophe at the end of the Cretaceous.
NSF EAR Drilling was supported by the US National Science Foundation’s Continental Dynamics Program and Sedimentary Geology and Paleobiology Program under NSF-EAR Grants 0207407, 0447171 and 0750664.
Figure 1 – Gerta Keller and Jerry Baum examine cores from DOSECC drilling along the Brazos River, which recovered an expanded sequence with the K-T boundary 80cm above the sandstone complex with three reworked upward fining impact spherule layers at the base. The original impact spherule layer was discovered 65 cm below.
Keller, G., 2007. K-T mass extinction and Chicxulub impact in Texas. J.South Texas Geology, Geol. Soc. Amer., XLVII (9), p. 15-44.
Keller, G. Adatte, T.. Berner, Z Harting, M. Baum, G. Prauss, M. Tantawy, A. A. and D. Stueben., 2007. Chicxulub Impact Predates K-T Boundary: New Evidence from Texas. Earth and Planet. Sci. Letters 255, 339-356.
Keller, G., Abramovich, S., Berner, Z., Adatte, T., 2008. Biotic effects of the Chicxulub impact, K-T catastrophe and sea-level change in Texas. Paleogeogr., Paleoclimatol., Paleoecol., doi:10.1016/j.palaeo.2008.09.007