DISTINGUISHING MIDDLE LATE CRETACEOUS TECTONIC EVENTS FROM REGIONAL SEA-LEVEL CHANGE USING FORAMINIFERAL DATA FROM THE US WESTERN INTERIOR

Foraminiferal data from seven stratigraphic sections between the Utah- Wyoming thrust belt and western Kansas record 13 correlatable paleoeco logic events between Coniacian and early Campanian time, and provide a framework for interpreting tectonic timing and regional sealevel chan ges, At least three events are synchronous within available biostratig raphic resolution. We infer that the events are regionally isochronous and result from short-term (< 1 m.y.) sea-level fluctuations that aff ected the entire Western Interior seaway. Paleodepth of water and comp action-corrected thickness data allow high-resolution subsidence analy sis for synchronous intervals, which span similar to 7.5 m.y. Sections located across 1000 km of the Western Interior show between 200 m of cumulative subsidence to the east and 1800 m to the west. Comparing th e sections, we interpret asymmetric subsidence to record loading by Se vier orogenic belt thrust plates and attendant sedimentation, Accelera ted differential subsidence in the westernmost sections occurred durin g paleoecologic intervals 7 and 8, coincident with coarse elastic prog radation of the Emery Sandstone in Utah and the Shurtliff Sandstone in Wyoming, This pulse of rapid subsidence most likely records discrete isochronous thrusting and sediment-loading events in both the Utah (Pa xton-Pavant thrust) and Wyoming (Crawford-Meade thrust) sectors of the thrust belt. Differential subsidence on the scale of 200 m in section s only 54 lan apart in central Utah suggests Local faulting and partit ioning in the proximal part of the foreland basin during regional flex ure. That the 13 paleoecologic events are observed across the foreland basin indicates that from western Kansas to the Sevier thrust belt th e rate of change of sea level was sufficiently rapid to exceed the rat e of sea-floor sedimentation in the case of sea-level rise, and the ma ximum rate of basin subsidence in the case of sea-level fall, The reco gnition of these events suggests that correlatable sea-level changes t hat are found within and beyond the flexural wavelength of foreland ba sins can be used to distinguish tectonic events from regional sea-leve l changes. The recognition of relatively short-term subsidence events indicates that tectonic events can occur in foreland basins an time sc ales similar to third-order (or possibly higher) eustatic sea-level ch anges. Although this study examines only data from the U.S. Western In terior, the technique of using high-resolution subsidence analysis to identify asymmetric subsidence promises to aid in identifying the tect onic component of subsidence in other foreland basin settings.