ALONG-STRIKE VARIATION IN GEOMETRY OF THE LATE CRETACEOUS WESTERN INTERIOR BASIN TRACKS MULTIPLE GEODYNAMIC SUBSIDENCE MECHANISMS

Accommodation in North America during the Late Cretaceous was controlled by surface and subsurface loads related to contractional deformation and mantle circulation, respectively. Surface loads formed wedge-shaped flexural basins associated with either movement on laterally extensive ramp-flat thrusts of the Sevier thrust belt (Early Cretaceous–Eocene) or exhumation of isolated uplifts along crustal-scale ramps starting in the Late Cretaceous. Initiation of exhumation along crustal-scale ramps (a.k.a., “Laramide” deformation) is attributed to shallowing of the subduction angle of the Farallon plate. Numerical models of flat subduction indicate that it is accompanied by a change in the locus and magnitude of dynamic subsidence and forms broad bowl-shaped basins. Hence the locus, magnitude, and geometry of subsidence reflect the geodynamic mechanism(s) of subsidence.

Deposition of Upper Cretaceous strata overlaps the timing of shallowing of subduction, providing an opportunity to examine the effects of this transition on basin development. We integrate subsurface public petroleum well data with new and published stratigraphic correlations to develop subsidence maps of upper Albian–upper Maastrichtian strata spanning basins adjacent to the Rockies in Utah, Wyoming, and Montana. We use subsidence and isopach maps to constrain the timing and spatial extent of changes in subsidence and basin geometry. Maps reveal both long-wavelength subsidence attributed to dynamic subsidence and localized subsidence associated with flexure due to Sevier or Laramide loads. Consistent with numerical models and previous basin analysis, long-wavelength subsidence is first documented in the Coniacian to Campanian (~88 to 83 Ma), and the long-wavelength depocenter migrates eastward. By the Campanian, flexural subsidence ceases adjacent to the Sevier thrust front in Wyoming, whereas flexural subsidence in western Montana is amplified. At the same time, localized Laramide flexural basins also develop throughout Wyoming and into southern Montana. These results support previous models that depict a warped or fragmented Farallon slab where the northward extent of flat subduction coincides with the boundary between the region marked by Laramide deformation and no subsidence adjacent to the thrust belt and that characterized by limited Laramide deformation and enhanced Campanian subsidence adjacent to the thrust belt.