Cambios ambientales cretácicos en Asia oriental y meridional: El paso Cenomaniense-Turoniense en el Tibet meridional

Abstract

Fossil assemblages consist on foraminifera and rare ostracodes; all samples have been quantitativelystudied. Planktonic foraminifera allow us to characterise the Rotalipora cushmani Whiteinella archaeocretacea and Helvetoglobotruncana helvetica zones. Dominant species belong to genera Rotalipora and Dorothia. Planispiral rounded and tapered rounded morphotypes are dominant in benthic assemblages from the upper part of the Rotalipora cushmani Zone, which shows an oxygen deficiency in the bottom waters. The scenery is completely different during the W. archaeocretacea Zone. All species of the genus Rotalipora became extinguished and other species temporally disappeared; both diversity and abundance are very low in these samples. The keeled/non-keeled planktonic foraminifera ratio felt from 1 00% to a mere 2%, then increasing until similar figures at the top of this zone. Genus Ammodiscus and other agglutinant foraminifera species are the most frequent specimens. This is probably caused by very low oxygen bottom waters. During the H. helvetica Zone, biotic recovery is well settled with the appearance of Helvetoglobotruncana helvetica (Bolli) and Marginotruncana spp. in the plankton, but it is not so clear in the benthos, which remains poor and with low diversity assemblages