- UW Department of Geology and Geophysics
A 50-m-thick section of the Denver Formation (D1 sequence of Raynolds) exposed at the West Bijou Site of the Plains Conservation Center, Arapahoe County, Colorado, contains the first complete Cretaceous-Tertiary (K-T) boundary section in the Denver Basin. The 3-cm-thick boundary claystone coincides with a 21 percent palynological extinction, contains iridium and shocked-mineral anomalies, and is immediately overlain by a 7-cm-thick, fern-spore anomaly interval. The entire 50-m section is of reversed magnetic polarity and is interpreted to be subchron C29r because of paleontological data, the presence of a tuff with an 40 Ar/39 Ar date of 65.73 ± 0.13 Ma, and by correlation to the Kiowa cored well. A diagnostic basal Puercan (Pu1) mammal jaw was found 12 m above the boundry clay, and ceratopsian and hadrosaurian dinosaur fragments occur 4 m below the boundary clay. Estimates of the basin sedimentation rate derived from the duration of C29r based on marine cyclostratigraphy suggest that the 28 m of basal Paleocene strata represent approximately 200,000 years. Abundant fossil leaves found within the Paleocene sedimentary rocks at the West Bijou Site K-T boundary section allow assessment of early Paleocene patterns of plant diversity. Nine leaf localities at eight stratigraphic levels in the basal 22 m of the Paleocene section were sampled and analyzed to better understand the flora that survived the global K-T catastrophe. The Paleocene flora of this site is taxonomically dominated by dicotyledonous angiosperms (74%), with lesser numbers of monocotyledons (10%), ferns and allies (11%), and conifers (5%). By number of specimens, angiosperms comprise greater than 95 percent of the flora. Within the sampled section, there were no recognizable directional trends in diversity or abundance, suggesting that earliest Paleocene vegetation was stable, although not particularly diverse. The West Bijou Megaflora is strikingly similar in composition and relative abundance to basal Paleocene floras from the northern Great Plains. This K-T boundary disaster-recovery flora, also known as the FUI flora, is shown to have been widespread, ranging from central Colorado to southern Canada, a distance of at least 1,100 km. The West Bijou Site megaflora is markedly different from coeval floras along the western margin of the Denver Basin, supporting the hypothesis that orography and elevation provided greater influence on basal Paleocene floral composition and diversity than did latitude.