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/Ter tiary boundary-officially Cretaceous/Paleogene (K/P) boundary from the La C eiba section (Mexico) was performed to examine the K/P planktic foraminifer al 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 pelagi c 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 sphe rules (microtektites) altered to clay minerals, abundant detrital quartz, m ica minerals, and shocked quartz. According to the K/P stratotype definitio n 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 b ase of the boundary clay at El Kef. A short hiatus affects the lower part o f the Danian, including the Guembelitria cretacea and Parvularugoglobigerin a 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 d eposited at lower bathyal depths. Oscillating megatsunami waves and/or a se a-level lowstand cannot explain the nature of the elastic deposits because of the observed deposition paleodepth (more than 1000 in). There is also ev idence 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 turb idites. This datum and other sedimentological features support a sediment g ravity 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.