Christopher D. Dean, Daniel Collins, Marijn van Cappelle, Alexandros Avdis, Gary J Hampson

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.