Estimating paleobathymetry using numerical tidal modelling in the Cretaceous Western Interior Seaway, USA

The depth of shallow seas exerts a strong control on water-body mixing, sedimentation and biodiversity but evaluating bathymetry in the geological past has traditionally proven challenging. Despite an extensive sedimentologic record and long history of geologic study, accurate estimates of water depth in the Cretaceous Western Interior Seaway, USA, remain uncertain. Herein paleotidal modelling, paleogeographic reconstructions and depositional process regimes are combined to evaluate the basin physiographic controls on tidal sedimentary processes in the region of the 'Utah Bight' of the Western Interior Seaway and thereby constrain paleobathymetric estimates. Paleotidal modelling using a global and astronomically-forced tidal model and paleobathymetric sensitivity tests suggest that tides were only able to transport silt or sand in the 'Utah Bight' if the seaway had a deeper basin center (~400 m) and a deeper southern entrance (>100 m) than previously estimated. This deeper paleobathymetry increased tidal inflow into the basin and enhanced local resonance effects in the 'Utah Bight'. Maximum tidal velocity vectors suggest a dominant south-easterly ebb tide, consistent with paleocurrent measurements from the Sego Sandstone. However, the preservation of tidal cross stratification in very fine- to fine-grained sandstones implies local tidal amplification below the maximum resolution of model meshes. Tidal modelling is shown to be a useful tool for evaluating paleobathymetry in ancient epicontinental seas at scales appropriate to mesh resolution.