Micropaleontology and sedimentology across the Cretaceous/Tertiary boundary at La Ceiba (Mexico):: impact-generated sediment gravity flows

A micropaleontological and sedimentological study across the Cretaceous/Tertiary boundary-officially Cretaceous/Paleogene (K/P) boundary from the La Ceiba section (Mexico) was performed to examine the K/P planktic foraminiferal biostratigraphy, the sedimentology of a controversial K/P elastic unit, and the benthic and planktic foraminiferal assemblages turnover across this boundary. The elastic unit is stratigraphic ally placed between two pelagic marly units (Mendez and Velasco Formations) and displays a fining-upward gradation similar to a turbidite sequence. This K/P elastic unit contains a basal subunit consisting of calcareous marls rich in millimeter-sized spherules (microtektites) altered to clay minerals, abundant detrital quartz, mica minerals, and shocked quartz. According to the K/P stratotype definition from El Kef (Tunisia), the K/P boundary at La Ceiba must be placed at the base of the elastic (micro spherules) unit since it is equivalent to the base of the boundary clay at El Kef. A short hiatus affects the lower part of the Danian, including the Guembelitria cretacea and Parvularugoglobigerina eugubina biozones and the lower part of the Parasubbotina pseudobulloides biozone. Nearly all commonly recorded Maastrichtian planktic foraminiferal species were found in the uppermost Maastrichtian interval, and there was no support for a gradual mass extinction pattern in the terminal Cretaceous. Benthic foraminiferal assemblages suggest that the La Ceiba section was deposited at lower bathyal depths. Oscillating megatsunami waves and/or a sea-level lowstand cannot explain the nature of the elastic deposits because of the observed deposition paleodepth (more than 1000 in). There is also evidence that the elastic unit was deposited under a W.-h-sedimentation rate in upper flow regimes and that was emplaced as a single-pulse event as turbidites. This datum and other sedimentological features support a sediment gravity flow genesis for the elastic unit. All these results are consistent with the K/P impact theory and the asteroid impact on the Yucatan Peninsula. (C) 2001 Elsevier Science Ltd. All rights reserved.