Print this pageAAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Tidal Deposits of the Campanian Western Interior Seaway
(1) Jackson School UT Austin, Austin, TX.
(2) Geological engineering, Colorado School of Mines, Golden, CO.
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 tidal processes in the Cretaceous successions has never been assembled. Field data from the southwestern reaches of the seaway in Utah, Colorado and Wyoming indicate that tidal influence was prominent along the Campanian coastlines in two stratigraphic settings: (1) tidal currents strongly influenced or dominated the distal regressive segments of many deltaic cycles (sites where low relative sea level caused the seaway to narrow and possibly be restricted to the north), in contrast to the storm wave-dominated facies of proximal reaches (sea-level highstand sites) of the same deltaic transects;(2) tidal influence was relatively strong during the transgressive development of many shorelines, at most sites across 100km-wide transgressive tracts; thin transgressive veneers as well as thicker estuarine deposits (some in valleys, some not) are documented. Tidal effects in the second setting are well known and may be due to increased tidal prism as sea level rose across a landward-shallowing shelf or because the increase of shelf width with sea-level rise brought the system closer to tidal resonance. In the regressive setting the common cross-shelf trend from wave-dominated to tide-dominated shorelines may possibly have resulted from tidal amplification as the seaway narrowed or became partially restricted to the north during relative lowstand periods. In addition, there is a remarkable increase in tidal influence along all of the 77.5-75Ma shorelines, not restricted to lowstand positions. These generally more embayed shorelines in this period are likely due to irregular but 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.