Abstract: Regressive-Transgressive Successions in the Upper Cretaceous Western Interior Foreland Basin: Succession Hierarchy, Shoreline Trajectory and Paleogeographic Considerations in Correlation

CRABAUGH, JEFF P., and RONALD J. STEEL

Recent revisions in the Late Cretaceous chronostratigraphic framework of the Western Interior Seaway have led to new syntheses of stratigraphic relationships. Some of these studies conclude that stratal stacking patterns are widely variable when comparing clastic units in one subbasin to coeval units in other subbasins. However, the present compilation, and our ongoing field studies in the Mesaverde Group of Colorado and Wyoming, suggest the following points are relevant to the debate.

Firstly, the degree of lateral variability in the timing of regressive-transgressive successions observed from subbasin-to-subbasin is strongly scale dependent. When comparing overall regressive-to-transgressive trends within the entire thickness of formation-scale units in different subbasins, temporal equivalency of patterns is often lacking. However, time-correlations are stronger when comparing regressive-transgressive successions at a level more detailed than formation scale.

Secondly, widely used methods of determining the shoreline position at maximum regression need re-evaluation: using the location of the lateral transition from nonmarine to marine facies, where shoreline progradational trajectory has been flat or below horizontal, can lead to an underestimate of final shoreline position by many 10's of kilometers.

Variability in stratal stacking patterns is attributable to areal variability in accommodation/ sediment-supply ratio. Shifts in the marine delivery-point of large rivers from one subbasin to another, can affect the relative accommodation/ sediment-supply ratios between subbasins, even when subbasins are experiencing similar conditions of tectonic subsidence and eustasy.

AAPG Search and Discovery Article #90946©1997 AAPG Rocky Mountain Section Meeting, Denver, Colorado