The Stratigraphy and Petrology of the Frontier Formation of Wyoming

Abstract The Frontier formation (Upper Cretaceous) of Wyoming is composed of two interfingered clastic lithosomes: one of coarse detritus which was deposited under predominantly non-marine conditions in the foredeep marginal basins of the Cordillera and spread eastward; the other composed of marine shales that were concurrently under deposition in the shelf areas of the present Great Plains. On the basis of correlations herein presented, the formation is divided into two superimposed units. The lower division includes those strata which lie between the top of the Mowry shale and the top of the transgressive phase of the First Wall Creek sandstone. These beds, which are present over most of Wyoming, are of Cenomanian and Turonian ages. The upper division, of Lower Coniacian age, comprises a geographically restricted belt of strata in western Wyoming and along the Wind River Mountains. The interpretation of the petrography of the three main sandstone intervals of the lower division of the Frontier shows that source areas in central Idaho were predominantly sedimentary rocks, with lesser amounts of volcanic and low rank metamorphic rocks. The detrital particles show little effect of intense or prolonged fluvial transport, but, within the depositional locus of Wyoming, the mineralogy of the clastic fraction becomes differentiated away from the principal distributary systems. Environments of deposition varied from non-marine in the west to marine in the east and migrated laterally in response to marine regression and transgression. Thus, the formation exhibits a cyclical depositional history in which the better developed sandstone intervals show a vertical gradation in depositional environments from paludal at the base, to fluvial in their mid-sections, to marine at the top. Diagenesis has superimposed upon the clastic mineralogy a mineral paragenesis which seems to cross depositional environmental boundaries. The reasons for this are thought to be (1) the lateral and vertical gradation between environments; (2) the distribution of pyroclastic material both vertically and areally within a sandstone; and (3) the marine nature of the under and overlying shales which controlled the chemical system of burial.