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Palynology of Strata Exposed During Construction of Denver International Airport
NICHOLS, D.J., U.S. Geological Survey, MS 939, Box 25046, Denver, CO 80225

During construction of the Denver International Airport (DIA), an artificial outcrop exposure of unprecedented extent was created. Through the courtesy of the DIA construction staff and in cooperation with the Denver Museum, samples were collected from several localities in the Denver Formation at the DIA site. The samples yielded fossil pollen and spores that provide insight into the age of the rocks underlying DIA and the nature of the ancient vegetation and climate of the Denver Basin.

Samples were obtained from outcrop exposures, cores, and auger holes. Outcrop samples were collected from the east wall of the excavation for the west parking garage at the terminal building; from near the base of the embankment on the east side of the runway area, at a locality from which fossil leaves were also collected; and from a deep excavation at the intersection of the railway tunnel and Concourse B, where a thick coal bed was exposed. Core samples were collected from bore holes drilled by Shepherd Miller Inc. at the site of a new radar facility built after the opening of the airport. Auger hole samples, which consisted of cuttings whose exact stratigraphic position is somewhat uncertain, came from below the level of the excavation in the primary construction area.

Assemblages of fossil pollen and spores obtained from all outcrop and core samples are similar. Commonly occurring species are pollen of palms (Arecipites tenuiexinous) and pollen of platanaceous angiosperms (Tricolpites spp.); both of these groups are represented by abundant fossil leaves from the DIA site and suggest a tropical forest vegetation. Also present in the palynological assemblages are spores of sphagnum moss (Stereisporites spp.) and ferns (species of Cyathidites, Gleicheniidites, Laevigatosporites, and Reticuloidosporites). Samples from the thick coal bed at Concourse B, which was subsequently buried by continued construction activities, indicate that sphagnum moss and ferns inhabited the mire in which the coal formed. Accumulation of peat in this coal-forming mire was interrupted periodically by volcanic ash falls, and 47 thin beds of volcanic ash were counted in the coal bed. The biostratigraphically important pollen species Momipites inaequalis and M. tenuipolus are present in the outcrop and core assemblages. These species are characteristic of Zones P1 and P2 of the regional palynostratigraphic zonation, and they indicate early Paleocene age for these strata.

Samples from the auger holes evidently are of latest Cretaceous age. Assemblages from these samples include pollen of the characteristically Cretaceous species Proteacidites retusus. Evidently the Cretaceous-Tertiary boundary lies at some undetermined level just below the excavated surface at DIA.

Certain pollen species present as rare components of assemblages from DIA samples (especially Psilastephanocolpites sp. and Thomsonipollis magnificus) show that palynologically the Denver Basin is closely similar to the Raton Basin of southern Colorado and northern New Mexico. Evidently the floras that inhabited these basins in early Paleocene time differed somewhat from those farther north.