CORRELATION OF PALYNOSTRATIGRAPHY AND MAGNETOSTRATIGRAPHY IN MAASTRICHTIAN-EOCENE STRATA IN THE DENVER BASIN, COLORADO

Palynology provides data for age determinations of Maastrichtian, Paleocene, and Eocene strata in the Denver Basin. The Denver Museum of Nature & Science’s 2,256-ft Kiowa core is the principal palynostratigraphic reference section for the basin. It is supplemented by the U.S. Geological Survey’s 3120-ft Castle Pines core, which was analyzed palynologically to a depth of 2,000 ft. Strata sampled include the Fox Hills, Laramie, Denver, and Dawson Formations. Reworking of older palynomorphs into younger sediments is a problem in both cores, but particularly in the Castle Pines core, which was situated near the uplifted western margin of the basin. Presence of Cretaceous marine dinocysts in the fluvial basin fill marks such contaminated samples, the ages of which are determined by the youngest terrestrial palynomorphs present. Palynostratigraphic biozones identified in both cores are the upper Maastrichtian Wodehouseia spinata Assemblage Zone; lower to middle Paleocene Zones P1, P2, and P3; and basal Eocene Zone E. Middle to upper Paleocene biozones (and strata) evidently are absent from the basin.

Palynostratigraphic zonations of the Kiowa and Castle Pines cores are correlated with independently established magnetostratigraphic zonations in the same cores. In the Cretaceous parts of both cores, the Wodehouseia spinata Assemblage Zone appears to extend from the lower part of subchron C31n to the upper part of C29r. This result is discordant with data from southern Canada, however, where the range base of W. spinata is near the base of C30n. Either diachroneity in the stratigraphic range of this species or miscorrelation of the palynostratigraphic and magnetostratigraphic zones in the Kiowa core is implied. The K-T boundary is affirmed to be in the upper part of subchron C29r, as it is worldwide. Zone P1 encompasses the upper part of C29r and the lower part of C29n. Zone P2 is equivalent to the upper part of C29n and all of C28r. Zone P3 correlates with most or all of C28n. Reversed magnetic subchrons have not been identified between C31n and C30n in the Kiowa core, or between C28n and C29n in the Castle Pines core, but their positions can be inferred, and palynostratigraphy supports these inferences. Age determinations in the Eocene parts of the cores are less certain, but correlation of Zone E with subchron C24n is indicated.