Concurrent growth of uplifts with dissimilar orientations in the southern Green River Basin, Wyoming: Implications for Paleocene-Eocene patterns of foreland shortening

Abstract

The diverse orientations of Precambrian-involved and fault-bounded uplifts in the Rocky Mountain foreland region seem difficult to reconcile with the inferred Paleocene-Eocene shortening direction and plate convergence. Some have hypothesized that the shortening direction rotated as much as 90° over time, whereas others have hypothesized a relatively stable strain field involving concurrent oblique slip on faults with varied strikes.The stratigraphic thicknesses of the synorogenic Paleocene-Eocene Fort Union, Wasatch, and Green River Formations in the southern Green River Basin of Wyoming are used here to define the pattern of basin subsidence and to interpret the timing of movement on two contractional structures with contrasting orientations. Isopach maps and cross sections of three sequential time-stratigraphic intervals show thinning around the north-south-oriented Rock Springs uplift and regional southward thickening of these three stratigraphic intervals. The southward thickening evidently resulted from flexural subsidence in response to loading from the adjacent east-west-oriented Uinta uplift. Stratigraphic thinning in the vicinity of the Rock Springs uplift demonstrates that growth of the underlying blind thrust system produced an area of comparatively low accommodation space within the subsiding Green River Basin.The concurrent growth of the Rock Springs and Uinta uplifts does not support a rotation of shortening direction from east-west to north-south. The margin of a Neoproterozoic rift basin may have controlled the orientation of the thrust faults bounding the north flank of the Uinta uplift, and Late Cretaceous-throughEocene contraction caused oblique slip on these faults during structural inversion. A north-south-striking blind reverse fault underlies the west flank of the Rock Springs uplift, but Phanerozoic strata cover the Precambrian core of the uplift, so the structure within the Archean basement rocks is unknown. Despite contrasting strikes and different structural histories, the Uinta and Rock Springs uplifts responded concurrently to northeast-southwest-to east-northeast-west-southwest-directed shortening.

Southwestward weakening of Wyoming lithosphere during the Laramide orogeny

“…The wavelength and magnitude of the lithospheric deflection, which is represented by the width and depth of the foreland basin, are dependent on lithospheric stiffness and size (width and height) of the mountain load [Turcotte and Schubert, 1982;Watts, 2001]. Once the influence of sediment loading on basin subsidence is accounted for, the thickness and spatial variations of the preserved strata in the foreland basin can be used to model lithospheric stiffness and corresponding size of the mountain load [e.g., Jordan, 1981 [Keefer, 1965;Curry, 1971;Johnson and Andersen, 2009;Finn et al, 2010] ( Figure 2). This is consistent with flexural subsidence caused by the tectonic loading of the Laramide ranges [e.g., Hagen et al, 1985;Fan and Carrapa, 2014].…”

“…Isopach maps used in 2-D flexural subsidence modeling, modified fromKeefer [1965],Curry [1971],Johnson andAndersen [2009], andFinn et al [2010]. Unit of contour intervals is meter.…”

Late Cretaceous–early Eocene Laramide uplift, exhumation, and basin subsidence in Wyoming: Crustal responses to flat slab subduction

“…Lower Eocene strata in the Wind River Basin thicken toward the Owl Creek and Bighorn Mountains to the northeast of the basin, with a greater sedimentation rate in the lower Eocene than in the Paleocene [ Keefer , ]. Paleocene strata in the northeastern Green River Basin thicken toward the Wind River Range [ Carroll et al ., ], and lower Eocene strata in the southern Green River Basin thicken toward the Uinta Mountains [ Johnson and Andersen , , and references therein]. These observations suggest that although the segmentation of the Laramide intermontane basins was initiated during the latest Cretaceous [ Dickinson et al ., ], topographic loading of the Bighorn, Owl Creek, Wind River, and Uinta Mountains generated local depocenters mainly during the late Paleocene–early Eocene.…”

“…Isopachs of the late Paleocene–early Eocene strata in the Powder River, Bighorn, Wind River, and Green River basins. Contours are in 600 m. The isopachs of the Bighorn Basin and Powder River Basin include undifferentiated Fort Union Formation and Willwood/Wasatch Formation; isopach of the Wind River Basin is for the lower Eocene Wind River Formation, isopach of the Green River Basin is for the Paleocene–middle Eocene Fort Union, Wasatch, and Green River formations [ Keefer , ; Parker and Jones , ; Fox and Higley , ; Hoy and Ridgway , ; Johnson and Andersen , ]. See Figure for the abbreviations.…”

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“…30-45 Ma (e.g., Dickinson et al, 1988;Gries, 1983;Bird, 1998;Cross, 1986). Although there are differences between different workers in inferred slip azimuths and timing on individual structures (e.g., Bird, 1998;Gries, 1983;Johnson and Anderson, 2009), the overall orientation of shortening across the orogen is northeast-southwest (Bird, 1998;Erslev, 1993). Total shortening estimates have ranged from 43 to 120 km across the Wyoming part of the orogen (Bird, 1998;Chapin and Cather, 1983).…”