Brazos River

In March 2005, DOSECC drilled the Cretaceous-Tertiary (K-T) boundary sequences in two localities along the Brazos River, Falls County, Texas. The project was organized by Professor Gerta Keller (PI), Geosciences Department, Princeton University, and funded by NSF-EAR. The Brazos River area was chosen to test the current controversy over the age of the Chicxulub impact: whether this impact was the K-T killer as commonly believed or predated the K-T boundary by 300 k.y., as suggested by the new Chicxulub crater core Yaxcopoil-1 (DOSECC) and outcrops in NE Mexico (Keller et al., 2003, 2004).

The Brazos River area provides a simple and inexpensive test of these results by drilling a couple of 100 feet deep holes across the K-T boundary. This area was chosen because of the complete stratigraphic sequences, which are comparable to the K-T boundary stratotype section at El Kef, Tunisia, the location about 1000 km from the impact crater, and the presence of both the K-T Ir anomaly and “event deposit” with Chicxulub impact ejecta spherules in widely separated stratigraphic intervals. These attributes mark the Brazos River area as the most important K-T impact locality outside Mexico and critical to resolving the current controversy regarding the age of the Chicxulub impact and its potential kill-effect.

Drilling of three 75-100 feet deep holes was done with DOSECC’s CS-500 rig and was completed in four days. The highly professional and incredibly hard-working two-man drilling crew (Vance Hiatt and Donald Bagley) did a fantastic job and succeeded with nearly 100% core recovery.

The cores recovered the K-T boundary in undisturbed dark laminated mudstones 90 cm above the 30 cm thick “event deposit”, which consists of reworked Chicxulub impact spherules, glauconite, shell hash and mudclasts at the base followed by bioturbated laminated and hummocky sandstones. Between the event deposit and the K-T boundary are upward fining silty mudstones (first 10 cm) followed by 80 cm monotonous laminated shales and mudstone with shells and burrows infilled with pyrite. The stratigraphic separation of the K-T boundary and event deposit provides the most promising test for the K-T and Chicxulub impact events. Initial results were presented at the May 23-25, 2005 AGU, Baltimore, MD.

Gerta Keller, Princeton University