THE ROLE OF FOLD-THRUST BELT TRANSVERSE ZONES AND CONGLOMERATE COMPLEXES IN DISTAL SANDSTONE TONGUE DEVELOPMENT
Enigmatic marine sandstone tongues (traversing >600 km) locally occupy distal (eastern) zones of the North American Cordilleran foreland basin. The process by which these sandstone tongues crossed such long distances, and how they relate to stages of foreland basin development are unresolved problems. However, the anomalous extent of these sandstone tongues suggests that their genesis is important to understanding foreland basins. This investigation utilizes growth strata, extensive outcrops and subsurface data to elucidate the relationship between Upper Cretaceous thrust-belt-controlled conglomerates and sandstone tongue progradation.
Preliminary analysis of new field data reveals 3 syntectonic unconformity-bound packages in conglomerate-rich growth strata exposed in the Wasatch Plateau. At least two of these packages, the Castlegate- and Bluecastle Tongue- equivalent strata, were correlated to clastic wedge progradation in the lower Castlegate and Cozzette Mbr of the Illes Fm.. However, there are stark differences in the amount of progradation between these two units. The lower, Castlegate-equivalent strata correlate with moderate (<200 km) clastic wedge progradation; whereas the upper, Bluecastle Tongue-equivalent strata correlate with much more extensive (>500 km) progradation. Nascent conclusions are: 1) moderate clastic progradation in the Cordilleran Foreland basin is coeval with uplift in the fold and thrust belt, and 2) more extensive (>500 km) progradation may require another mechanism such as slowing of subsidence despite minor thrust motions, increased sediment supply or eustatic fluctuation. Future work will constrain this stratigraphic framework, focus on the exact connectivity of depositional systems, and asses the relative contributions of other stratigraphic drivers.