ABSTRACT

Was the transitory Cretaceous-Tertiary (K-T) boundary event by itself the instrument of destruction of much of the Earth's biota, including the dinosaurs, or was the Earth's biota already critically stressed? The boundary event occurred during the Laramide Orogeny and during a time of sea level regression. A general trend toward an increasingly wet environment, starting in the latest Maastrichtian, is recorded by the occurrence of gleyed soils and coal immediately below the boundary and by pervasive Paleocene coal-bearing and lacustrine sediments.

The initial evidence for a physically identifiable datum at the K-T boundary was the recognition of a geochemical (iridium) anomaly within a claystone intercalated between paleontologically defined Maastrichtian and Paleocene limestones at Gubbio, Italy. Subsequent studies have demonstrated the association of a worldwide boundary claystone containing anomalous elemental abundances, shocked quartz, soot, microspherules, and isotopic anomalies involving carbon, oxygen, and nitrogen, confirming a universal datum compatible with a paleontologically based K-T boundary. The above features and plant and animal extinctions support the hypothesis that one or more catastrophic events occurred at the end of the Cretaceous. Possible physical changes in the environment, postulated to result from the causal or associated events, include acid rain, an increased amount of pyrotoxins, the destruction of the ozone layer, and a greenhouse effect from increased atmospheric water and carbon dioxide. These are familiar environmental factors within a modern context.

Paleontology continues to direct the search for this unique boundary horizon. Once found, it provides a physically definable time line, formed by a geologically instantaneous event, against which regional and international biological events can be referenced. In this way, it is a useful stratigraphic datum for developing an understanding of the degree of abruptness of extinction events, the reasons for their selectiveness, and the recovery rate and resilience of the biota following a biological crisis. Microfossils such as the pollen and spores of terrestrial plants provide the high stratigraphic resolution necessary to address such aspects of change.

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