ANALYSIS OF FLORAL DIVERSITY THROUGH THE FIRST 1.4 MA OF THE PALEOCENE, EASTERN DENVER BASIN, COLORADO

Earliest Paleocene deposits above the tightly constrained West Bijou Site K-T boundary, eastern Denver Basin, Colorado contain abundant fossil plant leaves. Studies of the 30m section of Paleocene exposures reveal that this flora is dominated by dicots (74%), also containing monocots, ferns, conifers, and horsetails. The composition of this West Bijou Site flora places it into the FU1 floral zone of the early Paleocene. Analyses of the leaves using physiognomic characters suggest the flora was growing in warm (18.6 ± 2.6 ºC) and wet (150cm/year) conditions. Four localities located at three stratigraphic levels (between 6 and 22 m above the boundary) reveal low diversity assemblages. Statistical analysis of census data from these four leaf localities demonstrates that there is no trend towards increasing alpha diversity in the first ~250 ka of the Paleocene. The West Bijou Site floral assemblage is dominated by six taxa, representing 88% of the total relative abundance. The dominant taxa of the West Bijou Site flora are more similar to other Western Interior basins to the north than to the coeval floras in the western portions of the Denver Basin. Also, these coeval fossil leaf localities located closer to the newly uplifting Rocky Mountain Front tend to have higher species diversity than their more distal eastern Denver Basin counterparts. However, correlating the age of localities from the western side of the Denver Basin out to the eastern side of the basin is difficult. The escarpment of West Bijou Creek in the eastern Denver Basin makes an ideal location for creating a spotty but continuous surface section through the first 1.4 Ma of the Paleocene. Outcrop is locally excellent, and fossils are very abundant in different depositional facies. This surface section in West Bijou Creek can be directly correlated to the nearby Kiowa core, a continuously sampled research core containing a continuous stratigraphic sequence through the synorogenic sediments of the Denver Basin. It has been rigorously analyzed using palynology and magnetostratigraphy, and similar samples collected from the fossil leaf localities will allow for direct correlation. Once tied to the reference Kiowa core, the plant assemblages and diversity characteristics of the eastern Denver Basin floras can be more easily compared those in the western Denver Basin.