Development of An Anomalous Clastic Wedge in the North American Cordilleran Foreland Basin: Implications for Foreland Basin Dynamics during the Sevier-Laramide Transition

A new high-resolution, 400 km long correlation and biostratigraphically constrained isopach maps provide evidence that an extensive, internally complex clastic-wedge (Wedge B) developed due to interference of basement uplift (San Rafael Swell) on the foreland flexural profile, and was possibly enhanced by reduced dynamic subsidence. Three Campanian, alluvial-to-marine, clastic wedges traversed 250-400 km eastwards across the Utah-Colorado segment of the Cordilleran foreland basin. The upper and lower wedges (Wedges A and C) are thick (600-1000 m) successions with a rising-trajectory shoreline stacking pattern (Blackhawk Formation and lower Castlegate Sandstone; Bluecastle Tongue and Rollins Sandstone) that reflect slower progradation of narrow (10-20 km wide), wave-dominated shorelines. In contrast, Wedge B consists of thinner (200-300 m) successions with a flat-to-falling shoreline stacking pattern (middle Castlegate Sandstone, Sego Sandstone, Neslen Formation, Corcoran and Cozzette Members of the Iles Formation) that reflects rapid progradation of embayed (60-80 km wide), tide-influenced shorelines. Wedges A and C prograded 200-250 km in more than ca. 5 m.y., whereas Wedge B prograded 350-400 km in just ca. 2 m.y. (~170 km m.y.-1). Correlation of thrust-related growth strata to the basin fill, paleocurrents and isopach maps indicate that uplift of the San Rafael Swell began as early as ca. 77 Ma, and that Wedge B developed coeval with both Sevier- and Laramide-style deformation. Uplift of the basement-cored San Rafael Swell in the foredeep may have altered the flexural profile of the foreland basin, reduced net subsidence and accommodation in this area, and catalyzed extensive progradation and development of an internally complex clastic wedge (Wedge B).