PATTERNS OF POST K-T BOUNDARY PLANT DIVERSITY AND RATES OF RECOVERY IN THE DENVER BASIN, COLORADO

A highly fossiliferous K-T boundary locality was recently discovered in the eastern portion of the Denver Basin, Colorado in West Bijou Creek. Precise bracketing of the boundary to 4cm based on palynology is supported by magnetostratigraphy, vertebrate fossils, and radiometric dating of multiple ash layers. Paleomagnetic samples are reversed in the 30m of Paleocene exposure, constraining the Paleocene portion in this area to a maximum of 300,000 yr. Localities with well-preserved plant megafossils are common in the Paleocene section, and occur in continuous layers at several stratigraphic levels. These localities have been extensively sampled and studied to determine patterns of biodiversity during the latest Cretaceous and earliest Paleocene. Megafossils from this locality are used to assess the rate at which Paleocene plant diversity levels recovered from the K-T extinction event, and how lateral variation of diversity on a floodplain affects interpretations of rates of recovery. A dense sampling strategy provides control on the original heterogeneity of the flora both in time and across the landscape. Fossils from each locality were collected by census, a method that counts every fossil leaf specimen found in the locality during excavation. This eliminates the bias introduced by the collector regarding the numbers of each taxa collected, as well as bias introduced by more intensive sampling at different localities. Censusing fossil leaf localities produces quantitative data on the relative abundance of taxa. This provides species dominance information that can be compared to other localities within the Paleocene at West Bijou Creek, and within the Denver Basin. This method also provides information on the percentages of insect damage types on the leaves, as well as leaf size data. Census data from fossil leaf localities document low diversity assemblages that are typical of basal Paleocene localities of the Great Plains. A locality 6 meters above the boundary has 9 taxa present, with 350 counted specimens. Rarefaction curves are flat for this locality at 9 species with only 300 specimens collected.