FLOODPLAINS TO SEAWAYS: A PALEOGEOGRAPHIC VIEW INTO EARLY CRETACEOUS EVOLUTION OF THE ROCKY MOUNTAIN REGION

Early Cretaceous strata in the Rocky Mountain region of the western U.S. document a time of transition from stable fluvial continental interior deposystems during the Late Jurassic to a dynamic, frequently flooded, foreland basin system of the Late Cretaceous. By leveraging the wealth of research on the structural, stratigraphic, and geochronologic evolution of the Western Interior Basin, we have produced several metadata-rich paleogeographic maps illustrating how this region evolved from the Late Jurassic to the earliest Late Cretaceous. These maps are created through a process which includes peer review of outcrop, metadata and paleoenvironmental maps. Reviewed products are compared to paleobiogeographic data and compiled to ground production of a paleo-satellite map. These collaborative maps showcase the efforts of many in our geoscience community and provide new views of the evolution of Early Cretaceous landscapes.

Maps of seven time intervals were created to illustrate the latest Jurassic (early Tithonian, ~148 Ma) to Cretaceous (early Cenomanian, ~98 Ma) transition. The Late Jurassic system in the Rocky Mountain region is dominated by vast fluvial and floodplain deposits and is well constrained owing to the multitude of fossil-rich sites with associated geochronologic ages. Though not as well resolved as the Jurassic owing to a significant regional hiatus and limited geochronology, Early Cretaceous (Valanginian) landscapes consisted of a series of east-west trending distributive fluvial systems carrying coarse-grained sediment from the advancing Sevier Orogenic Belt to a vast playa-floodplain system, analogous to the eastern flank of the Andes Mountains. Upper Early Cretaceous deposits are increasingly well-constrained owing to the availability of biostratigraphic data and abundant subsurface data from industry exploration and development. By the late Aptian, intermittent incursions by the Arctic lobe of the nascent Western Interior Seaway had begun to flood the northern portion of the region. The Arctic and Tethyan lobes of the seaway connected for the first time in the early Albian before parting again at the end of the Albian, beginning a pattern of cyclical transgression and regression which persisted and defined the Rocky Mountain region throughout the Late Cretaceous.