Interregional model for deposition of Upper Cretaceous pelagic rhythmites, U.S. western interior

Abstract

The Bridge Creek Member of the Greenhorn Limestone and Fort Hays Member of the Niobrara Chalk comprise rhythmic sequences of bioturbated pelagic limestone or chalk beds that alternate with generally shaly interbeds. Within each member individual couplets are time parallel and can be traced with great precision for hundreds of kilometers. Certain of the Bridge Creek beds can be traced from northeastern Kansas to the Four Corners area, a distance exceeding 1000 km. Regional stratigraphic analysis of these remarkable bedding sequences permits integration of concepts generated by several students of pelagic sedimentary sequences into a comprehensive model. In the basin center, Bridge Creek rhythms resulted from dilution cycles wherein a background of pelagic sedimentation was punctuated by episodes of increased siliciclastic influx and water column stratification. Consequent development of oxygen‐depleted bottom waters created high potential for preservation of laminae and organic matter in shaly interbeds. Within a given couplet, predominating influence of western siliciclastic source areas is evident in westward decrease of carbonate content in both bioturbated limestones and shaly interbeds, disappearance of one and then another of the limestone beds, and increase in abundance of the coarse detrital fraction. Toward the east, both bioturbated limestones and shaly interbeds grade progressively into chalky lithotypes. At some point where effects of detrital‐dilution episodes were minimal and the water was probably shallower, stratification of the water column was not possible, and interbeds are not only thinner but also bioturbated. Because of erosion this facies is not preserved in the Bridge Creek interval of Kansas, but it is exemplified there by the Fort Hays Member. Still farther eastward, where detrital influx was negligible, most of the Bridge Creek equivalent comprises a massive, bioturbated chalk sequence that lacks interbeds. This facies is preserved in northwestern Iowa. In condensed sections, the Bridge Creek Member contains considerable skeletal grainstone that bespeaks improved benthic circulation, even during deposition of shaly interbeds. The condensed sections developed on seafloor highs that apparently presaged the Laramide orogeny. The foregoing interpretation of regional variations in the Bridge Creek Member is generally applicable to the less widespread Fort Hays rhythmic sequence, in which simple dilution cycles seem to have predominated.