The palynologically defined Cretaceous-Tertiary boundary in the western interior of North America occurs at the top of an iridium-rich clay layer. The boundary is characterized by the abrupt disappearance of certain pollen species, immediately followed by a pronounced, geologically brief change in the ratio of fern spores to angiosperm pollen. The occurrence of these changes at two widely separated sites implies continentwide disruption of the terrestrial ecosystem, probably caused by a major catastrophic event at the end of the period.

An iridium abundance anomaly, with concentrations up to 5000 parts per trillion over a background level of 4 to 20 parts per trillion, has been located in sedimentary rocks laid down under freshwater swamp conditions in the Raton Basin of northeastern New Mexico. The anomaly occurs at the base of a coal bed, at the same stratigraphic position at which several well-known species of Cretaceous-age pollen became extinct.

T he fossil record demonstrates that mass extinction across the Cretaceous-Tertiary (K-T) boundary is more severe in the marine than the terrestrial realm. We hypothesize that terrestrial ecosystems were able to recover faster than their marine counterparts. To test this hypothesis, we measured sedimentary 13 C as a tracer for global carbon cycle changes and compared it with palaeovegetational changes reconstructed from palynomorphs and cuticles across the K-T boundary at Sugarite, New Mexico, USA. Different patterns of perturbation and timescales of recovery of isotopic and palaeobotanical records indicate that the 13 C excursion reflects the longer recovery time of marine versus terrestrial ecosystems.

Upper Cretaceous and lower Tertiary rocks exposed in the Raton basin, New Mexico and Colorado, contain a thin, indium-rich, kaolinitic clay bed that marks the palynologically defined Cretaceous-Tertiary boundary. The clay bed is presumed to have been deposited as the result of a catastrophic event, possibly the impact of an asteroid, at the end of the Cretaceous; it is preserved in sediments laid down in the quiet waters of ponds and coal-forming swamps. Occurring in a conformable sequence of nonmarine rocks, this extraordinary marker bed enables observation of depositional conditions of an instant of geologic time throughout a wide area in the basin. Investigations made at various sites where the boundary clay is present indicate that component elements of a dynamically aggrading fluvial system existed at the close of the Cretaceous. This depositional system was characterized by meandering rivers, broad floodplains, and levees. The levees were breached locally by crevasse splays that periodically invaded the floodplains and associated poorly drained and well-drained swamps.