In many parts of the world a thin clay or marly unit marks the boundary between Cretaceous and Tertiary rocks. In marine sequences this boundary is defined by the first appearance of typically Paleocene marine plankton in the clay. In continental rocks, the boundary sediment yields the stratigraphically highest occurrence of a Cretaceous assemblage of fossil pollen. Detailed analyses of the marine boundary sediment at Caravaca, Spain, permit a three-fold subdivision: the lowest is apparently a fallout deposit of impact ejecta, preserved as a 0.5-cm lamina of red clay. The main subdivision is a black or dark gray clay or marl, containing reworked extraterrestrial debris, laid down in an oxygen-deficient environment. The uppermost boundary clay is lighter gray in color, transitional in lithology to the overlying Paleocene sediments, which were deposited after the recovery from the terminal Cretaceous convulsive event. The boundary clay unit on land, represented by a section in Raton Basin, New Mexico, consists of a lower white clay, which is apparently a fallout deposit, and an upper carbonaceous shale. Boundary sections elsewhere are similar to those sections. The sedimentology of the boundary sediment records the convulsive environmental changes at/after a terminal Cretaceous event.

Significant changes of the chemistry, temperatures, and plankton fertility in the oceans took place in the first fifty thousand years of the Tertiary. Those changes are recorded by the bulk chemical, the oxygen-isotope, and the carbon-isotope compositions of the oldest Tertiary sediments. Detailed analyses of a Cretaceous/Tertiary section from a South Atlantic drillsite indicated that the extinction of the Cretaceous nannoplankton species took place during the times of the environmental changes. Taking into consideration the various evidences for a terminal Cretaceous large-body impact, we proposed that the impact event was the cause of the changes in ocean environments, which in turn led to the rapid extinction of marine plankton species.

The biostratigraphy of Cretaceous/Tertiary boundary sections from eight localities is summarized and compared in order to define a continuous or complete section. The El Kef, Tunisia, section is apparently the thickest C/T boundary section as yet discribed and contains all the biostratigraphical criteria required to define a complete section; that is, the uppermost Cretaceous Micula prinsii Zone, the “boundary clay” within the lowermost Tertiary Globigerina fringa Zine, followed by the Globigerina eugubina and pseudobulloides Zones. Using these zonations, a correlation of the stable-isotope stratigraphy from the various sections was made. The latest Cretaceous oceans and the earliest Tertiary oceans contained significantly different isotopic signals, which were incorporated into the tests of the calcareous nannofossils. The carbon-isotope signals are apparently global, synchronous, and primarily determined by changes in oceanic fertility, whereas the oxygen-isotope signals are globally induced but modified regionally according to paleogeographic positon and paleocirculation patterns. Further, this combination of biostratigraphy and isotope stratigraphy indicates that Cretaceous nannofossils in the lowest Tertiary sediments, previously thought to be reworked, actually survived the C/T boundary events and continued to reproduce in the earliest Tertiary oceans. These relic species became extinct some tens of thousands of years after the actual C/T boundary, probably as a consequence of the environmental stress following the C/T boundary events rather than as a result of a “catastrophic” extinction coinciding with the C/T boundary.