Geology Reference
In-Depth Information
1 7
Tidal Deposits of the Campanian
Western Interior Seaway,
Wyoming, Utah
and Colorado, USA
Ronald J. Steel , Piret Plink-Bjorklund ,
and Jennifer Aschoff
Abstract
The large-scale effects of tidal waves entering the Cretaceous Western Interior
Seaway from the Gulf of Mexico have previously been modeled, but the fi eld evi-
dence 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 infl uence was strong during the transgressive back-
stepping (setting 2) of shorelines. Marked tidal infl uence 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 amplifi cation 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 infl uence 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.
17.1
Background
17.1.1 Historical Recognition
There has been a signifi cant change of view regarding
the infl uence of tides in the Late Cretaceous Western
Interior Seaway (WIS) (Fig.
17.1
) of North America
since Shaw (
1964
) advocated 'tideless' epeiric seas.
Thanks to improved recognition criteria, new fi eld data
