EARLY PALEOCENE FLORAL RECOVERY IN WEST BIJOU CREEK, EASTERN DENVER BASIN, COLORADO

Fossil plants preserved in numerous localities along the West Bijou Creek escarpment in the Eastern Denver Basin, Colorado allow for a reconstruction of floral change through the first 1.5 million years of the Paleocene. This surface section serves as an independent test of the hypothesis that the Denver Basin’s Paleocene rainforests arose as a result of higher rainfall on the western margin of the basin during Laramide uplift. Exposures are spotty, but a composite surface section has been created with the locally excellent outcrops. The section has been correlated to the nearby Kiowa Core using stratigraphy, pollen, and pmag. The base of the 200-m-thick study section starts at the tightly constrained West Bijou Site K-T boundary and ends at the top of the D1 sequence (Denver Formation), stretching for 17 km to the south. Four Paleocene localities collected in the first 22 meters above the boundary were censused in 2001. These localities are low in diversity (~13 taxa per 400 specimen quarry) and are dominated by only 6 morphotypes (88%). The West Bijou Site flora represents the southern extent of the FUI disaster recovery flora that dispersed after the K-T boundary cataclysm, and shares very few taxa with coeval floras from the western Denver Basin. The four plant localities collected in 2004 were chosen based upon lithology and their stratigraphic position in the 200-m-thick Paleocene section, starting at the transition from pollen Zone P1 to P2 (35 m level). The lower two localities are in a siltstone lake facies, while the upper two are in a channel sandstone facies. Despite the taphonomic bias associated with these different facies, a trend in diversity and composition is apparent. Diversity levels in the lower two localities is equal to or slightly higher than the previously collected P1 localities directly above the West Bijou Site K-T boundary, but the dominant species in the flora are identical. Preliminary analysis of the highest locality (Zone P3), Cold Coyote Canyon, suggests that within ~1.3 Ma of the K-T boundary the flora has completely changed. The leaves are larger and have a higher percentage of entire margins, suggesting a warmer and wetter climate. Although this may be due to facies bias, the locality is noticeably more diverse and the flora now includes species that are present in sites on the western margin of the basin.