Origin of the cretaceous bridge creek cycles in the western interior, United States

Abstract

Alternating beds of burrowed limestone and laminated, organic carbon-rich marlstone in the Bridge Creek Member of the Greenhorn Formation were deposited during maximum transgression of the Cenomanian-Turonian sea in the Western Interior of the United States. One view of the origin of the Bridge Creek limestone-marlstone rhythms holds that they represent dilution cycles produced by alternating climates within the Western Interior, the marlstones recording wet episodes when high runoff produced a sediment-laden brackish lid on the seaway and the limestones recording dry episodes when runoff was low and the water column well-mixed. A second view holds that the rhythms represent productivity cycles, the limestones representing times of high productivity of calcareous planktonic skeletons and the marlstones representing times of low productivity. In this view, the rhythms were produced by alternations in vertical circulation that characterized much of the world ocean at the time. The productivity origin of the rhythms is supported by the presence of abundant calcispheres in the limestone beds, the concentration of clay-size quartz in many of the limestone beds, the comparatively high δ 18O and low δ 13C in limestone beds, the similarity in proportion of clay minerals in marlstone and limestone beds, the pelagic depositional rate of the Bridge Creek Member, and by the discovery that Bridge Creek deposition began with a marlstone and not a limestone bed. The dilution model is not consistent with these features and it has not been supported by modeling of paleoclimates and lithofacies that were predicted from it, nor by macrofaunal distributions within the seaway. The dilution model is not consistent with the occurrence of diverse and abundant planktonic foraminifera in both marlstone and limestone beds, which indicate that near-normal marine salinities prevailed during deposition of both. Bed-by-bed analysis of the Bridge Creek Member may provide an analog for events in the Tethys that are poorly recorded in deposits from the deep sea itself.