Regional-scale paleobathymetry controlled location, but notmagnitude, of tidal dynamics in the Late Cretaceous Western Interior Seaway, USA

Despite extensive outcrop and previous sedimentologic study, the role of tidal processes along sandy, wave- and river-dominated shorelines of the North American Cretaceous Western Interior Seaway remains uncertain, particularly for the extensive mid-Campanian (c. 75-77.5 Ma) tidal deposits of Utah and Colorado, USA. Herein paleotidal modelling, paleogeographic reconstructions, and interpretation of depositional process regimes are combined to evaluate the regional-scale (100-1000km) basin physiographic controls on tidal range and currents along these regressive shorelines in the ‘Utah Bight’, southwestern Western Interior Seaway. Paleotidal modelling using a global and astronomically forced tidal model, combined with paleobathymetric sensitivity tests, indicates the location of stratigraphic units preserving pronounced tidal influence only when the seaway had a deep center (~400 m) and southern entrance (>100 m). Maximum tidal velocity vectors under these conditions suggest a dominant south-easterly ebb tide within the ‘Utah Bight’, consistent with the location and orientation of paleocurrent measurements in regressive, tide-influenced deltaic units. The modelled deep paleobathymetry increased tidal inflow into the basin and enhanced local-scale (10-100 km) resonance effects in the ‘Utah Bight’, where an amphidromic cell was located. However, the preservation of bidirectional, mudstone-draped cross stratification in fine- to medium-grained sandstones requires tides in combination with fluvial currents and/or local tidal amplification below the maximum resolution of model meshes (c. 10 km). These findings suggest that whilst regional-scale controls govern tidal potential within basins, localized physiography exerts an important control on the preservation of tidal signatures in the geologic record.