FAULTY FOUNDATIONS: EARLY BREAKUP OF THE CORDILLERAN FORELAND (Invited Presentation)

Anomalous features of Upper Cretaceous basin fill in southern Utah challenge existing tectonic and depositional models of the Cordilleran foreland basin. Extreme thickness variations, net to gross changes, and facies distributions of fluvial deposits of the Naturita (formerly Dakota) and Straight Cliffs Formations are documented across the Southwestern High Plateaus. Regional correlations demonstrate abrupt, step-wise thickening, and a punctuated increase in overall grain size of key intervals from west to east (i.e., proximal to distal relative to the fold-thrust belt). These trends are opposite of those predicted in foreland basin systems. Except for in the most proximal sections, fluvial drainage systems were oriented subparallel to the fold-thrust belt at this latitude, based on paleocurrent and provenance data. Combined, these results suggest that modern plateau-bounding faults had an earlier expression in the Cretaceous (by Cenomanian time) and influenced the position of the main axial river system by creating paleotopography and isolated sub-basins. Laramide-style tectonism (e.g., basement-involved faults) is already cited as a driver for sub-basin development in latest Cretaceous-Tertiary time, but new data suggest that this part of the foredeep was ‘broken’ into distinct subbasins from its earliest stages. Regional implications of these data indicate that thin and thick-skinned thrusting were simultaneous in southern Utah and were likely influenced by inherited Proterozoic basement heterogeneities that dominate the edge of the Colorado Plateau. More broadly, this new interpretation suggests that tectonic models for this area should be reevaluated, and that such revisions could have larger-scale geodynamic implications for foreland basin evolution and flat-slab subduction models.