Anomalous clastic wedge development during the Sevier-Laramide transition, North American Cordilleran foreland basin, USA

Abstract

New, high-resolution, regional correlation and isopach maps provide evidence that (1) Laramide-style deformation began as early as ca. 77 Ma in central Utah, and (2) rapid (208 km m.y. –1 ) and extensive (400 km) progradation of a clastic wedge was facilitated by reduced subsidence during the transition from Sevier- to Laramide-style deformation. This study defi nes three Campanian, alluvialto-marine clastic wedges that traversed 200– 400 km eastward across the Utah-Colorado segment of the Cordilleran foreland basin. Wedges A and C are thicker successions with rising-trajectory shoreline stacking patterns (Blackhawk Formation and Lower Castlegate Sandstone, Bluecastle Tongue and Rollins Sandstone) that refl ect relatively slow overall progradation (50–81 km m.y. –1 ) of narrow (10–20 km wide), wave-dominated shorelines. In contrast, wedge B consists of lower-volume successions with a fl at to falling shoreline stacking pattern (Middle Castlegate Sandstone, Sego Sandstone, Neslen Formation, Corcoran and Cozzette Members of the Iles Formation) that suggests rapid progradation (~208 km m.y. –1 ) of embayed (60–80 km wide), mixed-energy (wave- and tide-infl uenced ) shore lines. Wedges A and C prograded 200–250 km in more than ~3 m.y., whereas wedge B prograded 340–400 km in ~2 m.y. (~170 km m.y. –1 ). The anomalously extensive wedge B is unique in the Utah-Colorado segment because of its long extent, rapid progradation rate, dominance of tidally infl uenced facies, long-transit transgressions/regressions, and low-accommodation, nested sequence architecture. Stratigraphic relationships indicate development of wedge B coeval with both Sevier- and Laramide-style deformation in Utah. Assuming a constant sediment supply, the extensive (>300–400 km) “sheet-like” amalgamated wedge (wedge B) may have been caused by (1) reduced subsidence driven by a fl exural interference pattern, whereby two short-wavelength (±200 km) fl exural profi les are superimposed during the uplift of both basement-cored and thin-skinned thrust belt, (2) an increase in, or eastward migration of, dynamic subsidence during tectonic transitions associated with slab fl attening or rollback (i.e., long-wavelength fl exure), or (3) reduced subsidence due to short-wavelength fl exural interference augmented by the reduction, or migration, of long-wavelength fl