Spatial and Temporal Evolution of Coastal Depositional Systems and Regional Depositional Process Regimes: Campanian Western Interior Seaway, U.S.A.

This paper provides a critical review and regional synthesis of Late Cretaceous shallow-marine deposits along part of the western margin of the Western Interior Seaway of North America, which contains the most extensively documented outcrop-based studies of siliciclastic coastal depositional systems in the world. The results of this synthesis are presented in the form of paleogeographic maps (covering present-day New Mexico, Utah, Colorado, and Wyoming, USA) for five timeslices in the Campanian. These maps are used to evaluate the spatial and temporal evolution of regional depositional process regimes along a large (> 1000 m) stretch of coastline. The evolution of regional depositional process regimes is linked to tectonic and paleoceanographic controls on the Western Interior Seaway, which enables the results of this synthesis to be applied to prediction of depositional process regimes in other, less intensively studied basins. Six gross depositional environments have been mapped for each timeslice: (1) alluvial to coastal-plain sandstones; (2) coastal-plain coals, mudstones, and sandstones; (3) shoreline sandstones; (4) marine mudstones; (5) gravity-flow siltstones and sandstones; and (6) marine marls and chalk. Shoreline sandstones in each timeslice are interpreted in further detail using documented evidence for the three principal classes of depositional process (wave, tidal, and fluvial) and published reconstructions of coastal morphology, which is widely considered to reflect depositional process regime. Based on these interpretations, shoreline sandstones are assigned to five categories of depositional process regime: (1) regressive wave-dominated shorefaces and delta fronts; (2) regressive river-dominated delta fronts; (3) regressive mixed tide- and wave-influenced delta fronts; (4) regressive tide-dominated delta fronts; and (5) transgressive barrier islands, back-barrier lagoons, and estuaries. The accuracy of and uncertainty in classification of depositional process regime are critically evaluated. Additionally, stratal thickness and sediment routing pathways have been interpreted in order to assess the impact of tectonic and paleoceanographic controls on spatial and temporal changes. In all of the evaluated shallow-marine successions, thin tide-influenced intervals were deposited in back-barrier and lagoon systems associated with net-transgressive shorefaces and estuaries. However, it is notable that all preserved tide-dominated and tide-influenced regressive deltaic systems are located along the northern margin of the “Utah Bight,” a tectonically related embayment that had a pronounced expression during the middle Campanian. The southern margin of the embayment, and coastlines to the north, are conspicuously more wave-dominated, which supports their exposure to a larger wave fetch. In contrast, the northern margin of the embayment was relatively wave-protected. It is concluded that tidal range was amplified due to resonance of the principal semidiurnal tide in the strongly embayed geometry of the middle Campanian Utah Bight.