RESOLVING THE TECTONIC AND EUSTATIC SIGNALS IN THE CRETACEOUS WESTERN INTERIOR BASIN

Marine retroarc basins such as the Cretaceous western interior basins are ideal sites in which to learn to “read the record of the rocks.” They contain the global sea level record, and as regions of continuous subsidence adjacent to the earth's major mountain chains, they also contain the only real geologic data with which the geodynamic histories of these mountain chains can be elucidated. Sequence stratigraphic principles work surprisingly well in the western interior, in view of such non-Vailian anomalies as a foredeep which is located between the sediment source and its receiving basin, and whose rate of subsidence may exceed the rate of eustatic sea level fall; and a geodynamical system that shifted from thin-skinned to thick skinned tectonics as it advanced onto the craton. Recent studies have shown that the “second order” sea level signal is recognizably that recorded in Tethyan Europe, and the high-frequency cycles yield periods that match the Milankovitch pattern. Unconformities that fit the definition of “sequence-bounding” have long been known although not always understood as such. In the Campanian section of the “Central Wyoming Corridor,” the complementary tectonic signal is best resolved by tracing the section from east to west. The basin's eustasy-dominated eastern margin is characterized by a shelf-slope-basin morphology controlled by eustatic auto-retreat. The eustasy-modified clastic wedges of the central basin alternate between expanded, coal-rich successions of parasequence-like units formed as sedimentation kept pace with relative sea level rise, and high-frequency sequence sets formed during oscillatory sea level fall. In the continental section near the tectonic front, the stratigraphic architecture was shaped primarily by base-level changes reflecting modification of the subsidence rate and was relatively immune to shoreline shifts.