Relationships between Thrust-Belt-Controlled Conglomerates and Extensive Sandstone Tongues in the Cordilleran Foreland Basin, U.S.A.

Jennifer Aschoff¹, Ron Steel¹, and Mark Kirschbaum^2
1 University of Texas at Austin, Austin, TX
² U.S. Geological Survey, Denver, CO

Enigmatic alluvial-to-marine sandstone tongues transit great distances (>500 km) across the shallow Cordilleran foreland basin. The process by which the tongues prograded great distances, and how they relate to fold-thrust belt kinematics and foreland basin stages, may be crucial to understanding foreland basin systems. We show how proximal growth stratal packages and syntectonic unconformities correlate with anomalous marine sandstone tongues. This correlation elucidates the relationship between Upper Cretaceous thrust-belt-controlled conglomerates and sandstone tongue progradation. Utilizing recently-discovered outcrops, petrographic analysis and published data it is possible to establish a detailed up- to down-dip sequence-stratigraphic correlation from Central Utah to Colorado. Analysis of these data reveals 4 syntectonic unconformity-bound packages in conglomerate-rich growth strata exposed in the Wasatch Plateau. Our work suggests that the upper and lower growth strata packages link basinwards to high-volume, wave-dominated stratigraphic successions (Blackhawk and Rollins Sandstones) that reflect extensive, shoreline progradation with a rising shoreline trajectory. In contrast, the middle two growth stratal packages link to low-volume, tide-influenced successions (Castlegate, Sego, Corcoran, Cozette and Bluecastle) that reflect flat-to-falling shoreline trajectories during repeated fall and rise of base level. As such, the style of shoreline progradation, as well as the amount of progradation is related to thrust kinematics in the Sevier fold and thrust belt. Nascent conclusions are: (1) there is a genetic linkage between transverse-zone conglomerate complexes and distally-extending marine sandstone tongues, and (2) the magnitude and style of clastic-wedge progradation in the Cordilleran Foreland basin are sensitive to thrust-belt development.