Coarse-grained siliciclastic and limestone breccia deposits have been described from over a dozen K/T boundary sections in northeastern Mexico and variously interpreted as the result of an impact-generated mega-tsunami or sea-level low stand. In addition, a K/T boundary sequence in northeastern Brazil has been reinterpreted as the result of two bolide impacts. In both regions, the clastic deposits are presumed to have originated as a result of a bolide impact on Yucatan. We review the age, biostratigraphy, lithology, and depositional environment of these deposits in order to evaluate the time of deposition and their stratigraphic relationship to the K/T boundary event. Our observations indicate that the near-K/T boundary siliciclastic deposits of northeastern Mexico cannot be explained by deposition resulting from a single catastrophic event. They appear to have been deposited by normal sedimentary processes over an extended time period spanning thousands of years. This is indicated by the presence of multiple bioturbation horizons, discrete layers of normal hemipelagic sedimentation within the clastic deposits, and mineralogically and sedimentologically discrete horizons correlatable over 300 km. Such correlations do not support chaotic single-event deposition. Clastic deposition occurred sometime during the last 170 to 200 k.y. of the Maastrichtian but ended sometime before the K/T boundary. This is indicated by the presence of the index species Plummerita hantkeninoides. These deposits stratigraphically correlate with the limeclast breccia of the Poty section in northeastern Brazil, which also predates the K/T boundary. Benthic foraminifers indicate that deposition of the limestone breccia at Poty and siliciclastic sediments in northeastern Mexico coincided with a eustatic sea-level low stand that lowered sea level by 70 to 100 m. The coincidence of clastic deposition with the latest Maastrichtian sea-level low stand suggests that they are causally related. However, since clastic deposition is not necessarily related to sea-level regressions, and the glass-bearing deposits in northeastern Mexico are unique, a pre-K/T boundary event (bolide impact or volcanism) cannot be excluded.
|Original language||English (US)|
|Number of pages||13|
|Journal||Special Paper of the Geological Society of America|
|State||Published - Jan 1 1996|
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