The Koshak section of the Mangyshlack Peninsula, Kazakhstan, is one of
the most complete Cretaceous-Tertiary (K-T) transitions known from the
boreal Paratethys. Cretaceous species richness is low (11 to 13
species), except for a peak of 20 species near the K/T boundary in the
uppermost Maastrichtian (top 50 cm) that represents the temporary
incursion of low-latitude taxa. This maximum species richness occurred
during climatic warming associated with increased humidity, as suggested
by clay mineral analyses. Biofacies analysis suggests external platform
conditions at this time, followed by a more humid climate, a sea-level
transgression, and deepening basinal facies in the lower Danian Subzone
P1a. Shallower platform conditions resumed in Danian Subzones P1b and
P1c, accompanied by a cooler and probably more arid climate. No abrupt
mass extinction occurred at the Koshak K/T boundary which is marked by
an Ir anomaly, a clay layer and the first appearance of Tertiary
planktic foraminifera. The influx of lower-latitude species ends at or
before the K/T boundary, whereas the majority of the indigenous
Cretaceous assemblage survived into the Danian. These data suggest that
long-term climatic changes may have been the principal factors in the
progressive demise of the Cretaceous planktic foraminifera in the
eastern boreal Paratethys. (C) 1999 Elsevier Science B.V. All rights
reserved.