Palynology of Maastrichtian and Paleocene rocks, lower Colville River region, North Slope of Alaska

Abstract

Outcrops of Maastrichtian rocks are rare on the North Slope of Alaska, and it is even more unusual to find outcrops of Maastrichtian and Paleocene age in the same vicinity. In general, Late Cretaceous and Paleogene rocks have not been well dated in published papers pertaining to northern Alaska. In this article, we describe palynomorph assemblages from 20 outcrop samples taken from nine localities along the lower Colville River and nearby areas. The latest Cretaceous palynomorph assemblages that are from marine rocks contain late Campanian or Maastrichtian dinoflagellate cysts; however, these cysts and other marine fossils are absent from the Tertiary samples, indicating that the Tertiary strata are nonmarine. Our latest Cretaceous and early Tertiary samples contain abundant spores and pollen grains and can be readily dated by reference to well-known pollen assemblages from the northern Great Plains and northwestern Canada. By dating each sampled outcrop, we can approximate the position of the Cretaceous–Tertiary boundary in the study area. Outcrop samples near the boundary are too widely separated stratigraphically to permit us to determine whether or not an unconformity exists at the boundary in the study area. Using spores, pollen grains, and dinoflagellate cysts, we date the samples below the Cretaceous–Tertiary boundary as being of approximately mid-Maastrichtian age and the samples above the boundary as being undifferentiated Paleocene in age. On the North Slope, as in regions to the east and south, the Maastrichtian pollen assemblages represent a different kind of flora and vegetation than the Paleocene assemblages. The Maastrichtian assemblages have moderately high diversities of angiosperm pollen taxa, most of which were probably insect pollinated. In contrast, the Paleocene assemblages have low diversities of angiosperm taxa, and many of these taxa were probably wind pollinated. Differences between the mid-Maastrichtian and Paleocene assemblages may have been caused at least in part by climatic changes but may also have been caused by the effects of a hypothesized bolide impact at the end of the Maastrichtian for which there is increasing evidence.