FLEXURAL PARTITIONING OF THE LATE ALBIAN-CENOMANIAN CORDILLERAN FORELAND-BASIN SYSTEM, UTAH, WYOMING, AND COLORADO

Late Albian-Cenomanian (~100-93.5 Ma) age stratigraphic units in Utah, Wyoming and Colorado display regional thickness variations that delineate the geometry of the coeval Cordilleranforeland-basin system. These strata, whichconsistoffluvial,tidal,andmarine deposits reach a maximum thickness of ~1200 m along a northeast trending zone between northeastern Utah and southwestern Wyoming. This zone represents the location of the foredeep depozone that formed proximal to the frontal portions of the coeval southern Cordilleran fold and thrust belt to the west. In east-central Utah, correlative strata range in thickness from ~75 m to >10 m and are entirely missing in some localities. This zone of stratigraphic thinning/absence parallels the trend of the foredeep and represents the location of the forebulge depozone. In Colorado, Upper Albian- Cenomanian rocks thicken to >100 meters. This region corresponds to the location of the back-bulge depozone that formed between the forebulge and the craton to the east. In each depozone, Albian-Cenomanian strata display varying stratigraphic relationships likely produced by the combined effects of thrust load-driven flexural subsidence/forebulge uplift, eustatic fluctuations, and the generation of subduction-related dynamic topography.

Application of 2-D lithospheric flexural models that incorporate balanced-restored cross sections of the Cordilleran fold-thrust belt permit the reconstruction of foreland-basin system deposystem evolution through time. Preliminary results suggest an overall decrease in the magnitude of thrust loading in the southern portions of the basin compared to areas in the north. This interpretation is supported by structural reconstructions of the thrust belt loads which interpret greater shortening and topographic loading in proximal-medial positions of the orogenic wedge in northern and central Utah compared to those in the south. Additionally, the presence of marine deposits across the entire foreland basin system by Late Cenomanian time requires a component of dynamic subsidence on the order of 30-50 meters in order to inundate both the forebulge and proximal foredeep/wedgetop depozones of the foreland basin system.