INTERPRETING EARLY PALEOCENE PLANT DIVERSITY PATTERNS AT THE TIGHTLY CONSTRAINED WEST BIJOU CREEK K-T BOUNDARY SECTION, DENVER BASIN, COLORADO

Tight age control within the West Bijou Creek K-T boundary site, Colorado, provides an unusually well constrained section in which to study patterns of plant biodiversity after the K-T boundary catastrophe. Recent analyses have precisely bracketed the K-T boundary to a 1 cm interval based on the presence of abundant shocked quartz grains immediately above a 3cm clay layer, which has been independently corroborated by palynological analysis. The boundary is also bracketed by two 40Ar/39Ar dates derived from sanidine crystals, hadrosaurian dinosaur teeth 4 meters below, an early-Puercan (Pu1) mammal jaw collected 12m above, distinctly Paleocene megafloras above, and reversely magnetized rocks throughout placing the entire section into polarity subchron C29R. Iridium analyses are underway. 9 Paleocene leaf localities from 8 stratigraphic levels have been studied to determine the characteristics of the flora that survived the K-T catastrophe. This Paleocene flora is dominated by dicots (74%), but also contains monocots, ferns, conifers, and horsetails. The floral composition places it into the FU1 zone of the early Paleocene. Census data from four Paleocene leaf localities at three stratigraphic levels has produced low diversity assemblages. Two localities 6m above the boundary have 9 and 14 taxa present. Two localities higher in the section, 16m & 22m, have equally low numbers of taxa, 15 and 12 taxa respectively. Rarefaction curves suggest that the low species richness of floras directly after the K-T boundary did not increase in the earliest Paleocene, even though this Paleocene section may represent up to 270ka (the duration of subchron C29R based upon marine cyclostratigraphy). A minimum duration for this Paleocene section could be shorter, 187ka, using calculated Denver Basin sedimentation rates. In order to more completely understand the early Paleocene climate of the Denver Basin the mean annual temperature for this flora was estimated in 2 ways, both using Leaf Margin Analysis. Single localities. estimates were calculated first, producing a mean value of 16.7 +/- 6.1 ºC. To test whether a combined estimate gives a more precise estimate, all dicots were used from the 9 Paleocene localities, estimating the temperature to be higher, 18.6ºC, but the error (+/- 2.6ºC) is much lower than for the single localities.