The study of Cretaceous rocks of the Western Interior basin has resulted in significant contributions to sequence stratigraphy. Much of the landmark work has focused on the excellent exposures of clastic wedges along the western margin of the seaway. This reflects, in part, the difficulty in recognizing progradational, aggradational, and retrogradational stratal patterns and erosional surfaces in the fine-grained facies of the central and eastern parts of the basin. Yet the ability to trace such patterns from the western foredeep to the cratonic margin on the east offers a means to evaluate hypotheses for the cause of relative sea level changes in the Western Interior (tectonic vs. eustatic). This talk will (1) summarize predicted patterns of sedimentation that would be expected given tectonic, eustatic, and combined drivers of sea level change, (2) present results from a high-resolution stratigraphic study of Cenomanian-Turonian strata across the basin, and (3) compare these results to the predicted patterns in order to better constrain the interpretation of relative sea level changes.

The new high-resolution data base is composed of lithologic, sedimentologic, petrographic, geochemical, and paleontologic data from over 50 core and outcrop sites throughout the basin. The data include trends in sediment composition and grain size reflecting mode of deposition, geochemical parameters reflecting condensation (e.g., organic carbon, trace elements), and stratigraphic patterns reflecting sediment supply to accommodation ratios (e.g., evidence of erosion followed by onlap in the fine-grained mudrocks) that together allow a detailed reconstruction of relative sea level. Tracking three sea level fall events through a longer-term cycle of sea level change facilitates a hierarchical classification of processes. Recent Ar-Ar dating of bentonites through the study interval provides an extremely detailed temporal framework for the study. The data indicate that multiple factors played a role in controlling relative sea level changes, but the relative importance of the different drivers changed through time in connection with the evolution of the basin.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah