The Cretaceous-Paleogene Boundary on the Brazos River, Texas: New Strathigraphic Sections and Revised Interpretations

Malcolm B. Hart¹, Thomas E. Yancey², Andrew D. Leighton¹, Brent Miller², Chengjie Liu³, Christopher W. Smart¹, and Richard J. Twitchett¹
¹School of Geography, Earth & Environmental Sciences, Plymouth University, Drake Circus, Plymouth PL4 8AA, U.K.
²Department of Geology & Geophysics, Texas A&M University, MS 3115, College Station, Texas 77843–3115, U.S.A.
³ExxonMobil Exploration Company, 233 Benmar Dr., Houston, Texas 77060, U.S.A.

The investigation of new sections of the Cretaceous-Paleogene (K/Pg) transition and basal Paleocene in the Brazos River area of Texas provides evidence that the Corsicana Formation sea floor mudstones were significantly eroded by the end- Cretaceous impact disturbances. Erosional relief on the 75–100 m deep sea floor is visible in Cottonmouth Creek and the new River Bank South section as a series of ridges and erosional troughs. Trough lows are filled, in places, with mud-matrix mass flow deposits containing large blocks of Maastrichtian mudstones and transported concretions. These are overlain with granular shelly layers containing spherules and hummocky cross-stratified storm sandstones. Some of the more positive areas of the sea floor remained exposed to shelf waters and were colonized with a thin oyster pavement before burial with mudstones, siltstones, and sandstones of the Kincaid Formation. A return to quiet water conditions during the basal Paleocene is recorded in a 3–6 m section of foraminifera-rich sandstones bounded above and below with zones of carbonate and pyrite concretions, best seen on the newly described River Bank South section. The distinctive yellow-weathering claystone exposed in Cottonmouth Creek and a new locality (River Bank North), north of the Route 413 bridge, are confirmed as volcanic ashes and dated as latest Maastrichtian, thereby removing the necessity for a pre-K/Pg boundary, and pre-extinction, impact event.

AAPG Search and Discovery Article #90158©2012 GCAGS and GC-SEPM 6nd Annual Convention, Austin, Texas, 21-24 October 2012