Abstract

Foraminiferal data from seven stratigraphic sections between the Utah-Wyoming thrust belt and western Kansas record 13 correlatable paleoecologic events between Coniacian and early Campanian time, and provide a framework for interpreting tectonic timing and regional sea-level changes. At least three events are synchronous within available biostratigraphic resolution. We infer that the events are regionally isochronous and result from short-term (<1 m.y.) sea-level fluctuations that affected the entire Western Interior seaway. Paleodepth of water and compaction-corrected thickness data allow high-resolution subsidence analysis for synchronous intervals, which span ∼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 the sections, we interpret asymmetric subsidence to record loading by Sevier orogenic belt thrust plates and attendant sedimentation. Accelerated differential subsidence in the westernmost sections occurred during paleoecologic intervals 7 and 8, coincident with coarse clastic progradation 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 (Paxton-Pavant thrust) and Wyoming (Crawford-Meade thrust) sectors of the thrust belt. Differential subsidence on the scale of 200 m in sections only 54 km apart in central Utah suggests local faulting and partitioning in the proximal part of the foreland basin during regional flexure.

That the 13 paleoecologic events are observed across the foreland basin indicates that from western Kansas to the Sevier thrust belt the rate of change of sea level was sufficiently rapid to exceed the rate of sea-floor sedimentation in the case of sea-level rise, and the maximum rate of basin subsidence in the case of sea-level fall. The recognition of these events suggests that correlatable sea-level changes that are found within and beyond the flexural wavelength of foreland basins can be used to distinguish tectonic events from regional sea-level changes. The recognition of relatively short-term subsidence events indicates that tectonic events can occur in foreland basins on time scales similar to third-order (or possibly higher) eustatic sea-level changes. Although this study examines only data from the U.S. Western Interior, the technique of using high-resolution subsidence analysis to identify asymmetric subsidence promises to aid in identifying the tectonic component of subsidence in other foreland basin settings.

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