Abstract

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 can be used to establish biostratigraphic subdivisions and interpret paleogeographic conditions. The lower part of the Niobrara-equivalent strata in northern Colorado is termed informally the Globorotalites interval. The upper part of the Niobrara-equivalent strata is called the Gavelinella interval. The absence of keeled foraminifers in Niobrara-equivalent rocks in northwestern Colorado indicates colder water (probably less than 20 degrees C) than in areas on the south and east. The colder water temperatures and the distribution of the fine-grained sediment probably resulted from a southward-directed, nearshore circulation system and an upwelling of deeper, colder water in the western interior seaway.

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