Piret Plink-Björklund

Tidal Deposits of the Campanian Western Interior Seaway, Wyoming, Utah and Colorado, USA

The large-scale effects of tidal waves entering the Cretaceous Western Interior Seaway from the Gulf of Mexico have previously been modeled, but the field evidence for tides in the Campanian succession has never been assembled. Tidal deposits in deltaic, estuarine and barrier-lagoon successions along the southwestern margin of the seaway, in Utah, Colorado and Wyoming are documented. Tidal currents dominated the distal, subaqueous segments of many regressive deltaic transects (setting 1), and tidal influence was strong during the transgressive backstepping (setting 2) of shorelines. Marked tidal influence in setting 2 was likely due to increased tidal constriction and coastline irregularity after valley incision as well as possible tidal resonance with the increase of shelf width accompanying sea-level rise. In the regressive deltaic setting the common basinward cross-shelf trend from wave- to tide-dominated probably resulted from tidal amplification as sea level fell (albeit few tens of meters). The seaway narrowed and possibly became restricted to the north during lowstand periods, enhancing the counter-clockwise, Coriolis-driven current gyre in the southern half of the basin. In addition, there is notable increase in tidal influence along all of the 77.5–75 Ma shorelines, irrespective of sea-level stand. These more embayed shorelines (contrasting with straight wave shorelines before and after) are likely due to irregular widespread shallowing around embryonic, subaqueous basement-involved topography, as the seascape adjusted to a slight basinward tilt (as opposed to the earlier backtilt of the foreland basin) and a much more irregular, shallow bathymetry during the Sevier-Laramide transition.