Palynological Zonation of the Upper Cretaceous Frontier Formation Near Dubois, Wyoming

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Abstract
The Upper Cretaceous Frontier Formation in northwestern Wyoming consists of alternating sandstone and shale, interbedded with coal, lignite, bentonite and tuff. The formation varies in thickness from about 600 feet to more than 2000 feet. At Little Horse Creek, in the northwestern end of the Wind River Basin, the formation is 752 feet thick, and is overlain by the marine Cody Shale and underlain by the Mowry siliceous shale of Lower Cretaceous age. The Frontier sediments are sequentially and laterally variable; they contain few diagnostic species of marine fossils or vegetative plant fossils. However, the presence of abundant spores, pollen grains, and microplanktonic organisms provides a means of zoning the formation. Three approaches to zonation of the Frontier Formation are discussed: 1—recognition of cyclic lithologic units: 2—delineation of marine and nonmarine zones based on the ratio of spores and pollen to microplankton; and 3—time zonation based on range and occurrence of species of spores. The formation is a regressive unit compared with older and younger rocks, but, within itself, it becomes more marine upward. It consists of cycles of sediments, probably regressive, many of which are incomplete. Palynology aids the identification of units within a cycle and allows distinction of marine from nonmarine zones. Key species of spores are used to distinguish among zones. Distribution and abundance of species indicate their probable value for correlation of the formation outside the Little Horse Creek area.

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