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Evolution of the Terrestrial Biota Across the Cretaceous-Tertiary Boundary in the North American Western Interior

CLEMENS, W. A., and ARENS, Nan Crystal, Museum of Paleontology and Dept. of Integrative Biology, Berkeley, CA 94720-4780

The Hell Creek and Tullock formations exposed in Garfield and McCone counties, northeastern Montana, are the source of an increasingly detailed and diverse record of evolution of the terrestrial biota across the K\T boundary. The extensive paleontological collections and associated geological data in the UC Museum of Paleontology continue to provide a basis for new research projects addressing questions of pattern and process of biotic change across the K\T boundary.

In the latest Cretaceous northeastern Montana was part of the broad eastern coastal floodplain of the regressing Western Interior Sea. Gleyed soils of the Hell Creek Formation developed on a vegetated floodplain with a high and seasonally fluctuating water table. Weakly-developed soil horizons suggest rapid accumulation of sediments, which offers the potential for high temporal resolution of biotic and environmental change before and surrounding the K/T boundary.

Correlations based on palynological, vertebrate paleontological, geological (impact produced detritus), and radiometric (40Ar/ 39Ar) data demonstrate the lithostratigraphic boundary between the Hell Creek and overlying Tullock formations is time transgressive. Some of the uppermost strata of Hell Creek facies are of Paleocene age. Evidence of both long-term (evolutionary or geological scale) and short-term (ecological scale) environmental changes implicates a complex of physical and biotic factors in the K\T mass extinction.

A rise in water table toward the end of the Cretaceous promoted a change from clastic to peat accumulation on the floodplains. An extensive system of deeply incised streams eroded Cretaceous floodplain deposits and channel fill sands accreted vertically and laterally to create a complex system of cross-cutting relationships. Tephra suitable for 40Ar/ 39Ar age determinations, probably derived from a volcanic center to the west, were preserved in the coal swamps of the Tullock and calibrate the record of the first million years of terrestrial biotic change in the Paleocene.

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