MODELS OF EPERIC SEAWAY CONNECTIONS: MIDDLE CRETACEOUS OF THE U.S. WESTERN INTERIOR

Mid Cretaceous (Upper Albian-Lower Cenomanian) strata in the U.S. Western Interior between Wyoming and Texas reveal that rocks generally attributed to transgression during the early part of the Greenhorn third-order cycle record three thin sequences of unusually large, regional extent deposited during three lower-order marine cycles. Outcrop and core sections in southeastern Colorado, northeastern New Mexico, and the Oklahoma panhandle show that each sequence records biofacies shifts of over 200 km within vertical sections of less than 20 m that mark ephemeral connections between Boreal and Tethyan realms across eastern Colorado. In each sequence, basal fluvial-estuarine sandstone with nonmarine fossil assemblages passes vertically into a section of marine-influenced shale and sandstone. Biofacies in marine-influenced units show a progressive loss of marine influence up dip. Marine palynomorphs and a diverse Skolithos ichnofacies occur in coastal plain strata up dip of marine shoreface and shelf deposits. Marine fossils become progressively depauperate up dip until only brackish-tolerant ichnofauna, foraminifera, dinoflagellate cysts, acritarchs, and nonmarine palynomorphs remain. The depauperate fauna record intervals when ephemeral biotic connections allowed limited and selective exchange of Tethyan and Boreal fauna. The occurrences of a low diversity agglutinated foraminiferal biota (e.g., Tethyan Ammobaculites and boreal Miliammina), nearshore dinoflagellate cysts, such as Cyclonephelium, Palaeoperidinium and Cribroperidinium, and the acritarch genus Pterospermella in these sediments suggest that the ephemeral connections were so brief that brackish conditions were established instead of fully marine conditions. This observation is supported by isotopic carbon signals and palynofacies analyses. The sequence boundaries separating these thin sequences are unique mappable surfaces over the length of this transition.