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Late Cretaceous and Paleocene Rocks in the Denver Basin: Palynological Perspectives
FLEMING, R. Farley, Department of Earth and Space Sciences, Denver Museum of Nature and Science, 2001 Colorado Boulevard, Denver, Colorado 80205-5798

Palynological studies in the Denver Basin Project are focused on developing a biostratigraphic framework for the Denver Basin that is based on documenting the stratigraphic position of critical horizons. These horizons include the Cretaceous-Tertiary (K-T) boundary, which is marked by extinction of certain species of fossil pollen, and the D1/D2 transition, which is marked by the presence of a prominent paleosol.

The palynological K-T boundary has been located in two cores within the basin and an outcrop section near Strasburg, Colorado. Like other K-T boundary sections in the Western Interior of North America, the K-T boundary in these sections is marked by the extinction of species of Proteacidites and Aquilapollenites. In the Castle Pines core, taken along the western margin of the basin, the K-T boundary was located at a depth of about 1410 feet. Although reworked palynomorphs (fossil pollen and spores) are common in assemblages from the Castle Pines core, the approximate position of the boundary could be determined. The Kiowa core was taken near the center of the basin and the K-T boundary was located at a depth of about 878 feet. Assemblages from the Kiowa core are relatively free of reworked palynomorphs. Paleomagnetic data from these two cores support the positions of the K-T boundary determined on the basis of palynology.

Using the position of the K-T boundary in the two wells and the geometry of the Denver Basin, the approximate position of the K-T boundary was projected to the surface along the eastern margin of the Denver Basin. This projection facilitated the discovery of a surface K-T boundary section at the Plains Conservation Center on West Bijou Creek. At this locality, the K-T boundary has been located palynologically within a 2-3-centimeter interval. The uppermost Cretaceous and lowermost Tertiary samples from the outcrop section bracket a layer that is possibly the K-T boundary claystone. Samples have been submitted for iridium analyses and results are pending.

The age of the D1/D2 transition is critical in unraveling the geologic history of the Denver Basin. Palynological assemblages recovered from D1 in the Kiowa core indicate the presence of palynostratigraphic Zones P1and P2, and possibly P3. Basal D2 beds in the Kiowa core produced Platycarya platycaryoides, which is the key indicator of the early Eocene Platycarya platycaryoides Zone. The identification of Eocene fossils in this cores supports other reports of Eocene rocks in the Denver Basin. Assemblages from D1 and D2 in the Castle Pines core are sparse due to the coarse-grained nature of the rocks and definitive age determinations have not yet been made. Outcrops along the western margin of the basin have been examined for rocks that are likely to contain fossil pollen, but none have been found so far. It is possible that more suitable settings for preservation of fossil pollen may exist along the eastern margin of the basin, although good exposures are not common.