Study of microfossils, particularly Foraminifera, from the part of the Cretaceous Mancos Shale in northwestern Colorado that is equivalent to the Niobrara Formation of the High Plains region has determined that these fossils can be used to establish biostratigraphic subdivisions and interpret paleogeographic conditions. The microfossils used in the investigation came from two surface sections and one well in northwestern Colorado and one surface section in northeastern Colorado.

The microfauna contains several species in common with beds of the same age in the Gulf Coast and Great Plains areas, and also contains forms which have been reported from equivalent beds in Alberta and the Arctic Slope of Alaska. The microfossils are predominantly calcareous and represent a limited number of planktonic and benthonic forms. Samples taken throughout each of the sections were studied and plotted systematically to determine the stratigraphic distribution of the microfossils.

Concentrations of larger numbers of fossils were found in the lower part of the Niobrara-equivalent strata. This lower part is characterized in both northwestern and northeastern Colorado by a variety of benthonic calcareous foraminifers, notably species of Globorotalites and Bifarina, and Pleurostomella austiniana. It is termed informally the Globorotalites interval. Further study may make possible the subdivision of this interval on the basis of the stratigraphic distribution of Globorotalites and Bifarina. The microfauna of the upper part of the section is more limited and is characterized particularly by some species of Gavelinella, notably G. talaria. This upper part is termed the Gavelinella interval. Throughout the Niobrara-equivalent section the planktonic species Heterohelix lobulosa and Hedbergella loetterlei are common, and in most samples the planktonic forms predominate.

Interpretation of the environment of deposition of the northwestern Colorado beds has been based on (1) the ratio of planktonic to benthonic foraminifers, (2) modern analogues of foraminiferal genera found as fossils, (3) size and abundance of microfossils, and (4) variety of genera and species.

The evidence indicates that large volumes of clay-size clastic sediment were supplied to northwestern Colorado during the deposition of the Niobrara-equivalent part of the Mancos Shale. This fine-grained clastic material diluted the population of foraminifers found as fossils in the sediment (in comparison with the living population) and may have restricted the variety of genera and species.

The absence of keeled foraminifers in Niobrara-equivalent beds in northwestern Colorado indicates colder water temperatures (probably less than 20° C) in this region than in areas on the south and east at the same time. The colder water temperatures, as well as the distribution of fine-grained clastic sediment, probably resulted from a combination of a southward-directed, nearshore circulation system along the western boundary of the western interior seaway and the possible upwelling of deeper, colder water. Ratios of planktonic to benthonic Foraminifera and modern analogues of some genera indicate that the seaway was probably on the order of 200 m deep in northwestern Colorado during deposition of the Niobrara-equivalent beds.