ABSTRACT

The Cretaceous-Tertiary boundary is exposed in a series of outcrops near and along the Brazos River near Eloise in Falls County. At the best known exposure, some 400 m downstream from the Texas highway 413 bridge over the Brazos River, the precise location of the boundary, based on a detailed study of nannofossils, is in a mudstone interval approximately 15 cm above a 50 cm-thick prominent sandstone-mudstone-siltstone complex that has been used traditionally to designate the boundary. The complex consists of a basal friable sand resting on a scoured surface, a hard, rippled calcareous sandstone, a thin, soft mudstone and a prominent hard chalk or siltstone. This same complex is exposed about one mile downstream at the mouth of a gully, where the thickness and the character of the members of the complex are very similar. Toward the head of the same gully the sandstone complex is again exposed but there the rippled sandstone member is substantially thicker and more massive, although the other members of the complex retain their thickness and character. At a short lateral distance downstream from the mouth of the gully, the sandstone complex becomes increasingly clayey and disappears quickly. The entire complex has been interpreted variously as a storm deposit or shelf turbidite. These, however, do not appear to offer an entirely satisfactory explanation of the depositional conditions.

At all the sections in which the sandstone complex is present, the Cretaceous-Tertiary boundary is slightly above the complex. In all of the sections deposition across the boundary is continuous although the various biostratigraphic units may vary in thickness in the sections. A most reasonable explanation of the Cretaceous-Tertiary transition as expressed in these sections is that deposition of the sandstone complex - probably by several, related processes - occurred in latest Cretaceous time. The biologically catastrophic boundary event occurred very rapidly but somewhat later and probably was not directly related to deposition of the sandstone complex.