ABSTRACT We report the results of 167 calcite twinning strain analyses (131 limestones and 36 calcite veins, n = 7368 twin measurements) from the Teton–Gros Ventre (west; n = 21), Wind River ( n = 43), Beartooth ( n = 32), Bighorn ( n = 32), and Black Hills (east; n = 11) Laramide uplifts. Country rock limestones record only a layer-parallel shortening (LPS) strain fabric in many orientations across the region. Synorogenic veins record both vein-parallel shortening (VPS) and vein-normal shortening (VNS) fabrics in many orientations. Twinning strain overprints were not observed in the limestone or vein samples in the supracrustal sedimentary veneer (i.e., drape folds), thereby suggesting that the deformation and uplift of Archean crystalline rocks that form Laramide structures were dominated by offset on faults in the Archean crystalline basement and associated shortening in the midcrust. The twinning strains in the pre-Sevier Jurassic Sundance Formation, in the frontal Prospect thrust of the Sevier belt, and in the distal (eastern) foreland preserve an LPS oriented approximately E-W. This LPS fabric is rotated in unique orientations in Laramide uplifts, suggesting that all but the Bighorn Mountains were uplifted by oblique-slip faults. Detailed field and twinning strain studies of drape folds identified second-order complexities, including: layer-parallel slip through the fold axis (Clarks Fork anticline), attenuation of the sedimentary section and fold axis rotation (Rattlesnake Mountain), rotation of the fold axis and LPS fabric (Derby Dome), and vertical rotations of the LPS fabric about a horizontal axis with 35% attenuation of the sedimentary section (eastern Bighorns). Regional cross sections (E-W) across the Laramide province have an excess of sedimentary veneer rocks that balance with displacement on a detachment at 30 km depth and perhaps along the Moho discontinuity at 40 km depth. Crustal volumes in the Wyoming Province balance when deformation in the western hinterland is included.

ABSTRACT The Bighorn Basin (Wyoming, USA) contains some of the most extensively exposed and studied nonmarine early Paleogene strata in the world. Over a century of research has produced a highly resolved record of early Paleogene terrestrial climatic and biotic change as well as extensive documentation of spatiotemporal variability in basin-scale stratigraphy. The basin also offers the opportunity to integrate these data with the uplift and erosional history of the adjacent Laramide ranges. Herein, we provide a comprehensive provenance analysis of the early Paleogene Fort Union and Willwood Formations in the Bighorn Basin from paleocurrent measurements ( n > 550 measurements), sandstone compositions ( n = 76 thin sections), and U-Pb detrital zircon geochronology ( n = 2631 new and compiled age determinations) obtained from fluvial sand bodies distributed widely across the basin. Broadly, we observed data consistent with (1) erosion of Mesozoic strata from the Bighorn and Owl Creek Mountains and transport into the eastern and southern basin; (2) erosion of Paleozoic sedimentary cover and crystalline basement from the Beartooth Mountains eastward into the northern Bighorn Basin; (3) conglomeratic fluxes of sediment from the Teton Range or Sevier fold-and-thrust belt to the southwestern Bighorn Basin; and (4) potential sediment provision to the basin via the Absaroka Basin that was ultimately derived from more distal sources in the Tobacco Root Mountains and Madison Range. Similar to previous studies, we found evidence for a system of transverse rivers contributing water and sediment to an axial river system that drained north into southern Montana during both the Paleocene and Eocene. Within our paleodrainage and provenance reconstruction, the basin-scale patterns in stratigraphy within the Fort Union and Willwood Formations appear to have been largely driven by catchment size and the lithologies eroded from the associated highlands. Mudrock-dominated strata in the eastern and southeastern Bighorn Basin were caused by comparably smaller catchment areas and the finer-grained siliciclastic strata eroded from nearby ranges. The conglomeratic and sand-dominated strata of the southwestern area of the Bighorn Basin were caused by large, braided fluvial systems with catchments that extended into the Sevier thrust belt, where more resistant source lithologies, including Neoproterozoic quartzites, were eroded. The northernmost early Paleogene strata represent the coalescence of these fluvial systems as well as rivers and catchments that extended into southwestern Montana that contained more resistant, crystalline lithologies. These factors generated the thick, laterally extensive fluvial sand bodies common in that area of the basin. When combined with provenance patterns in adjacent Laramide basins, our data indicate asymmetric unroofing histories on either side of the Bighorn and Owl Creek Mountains. The Powder River Basin to the east of the Bighorn Mountains displays a clear Precambrian crystalline provenance, and the Wind River Basin to the south of the Owl Creek Mountains displays provenance similarities to Lower Paleozoic strata, in contrast to provenance in the Bighorn Basin, which indicates less substantial unroofing. We infer that the differing unroofing histories are due to the dominant vergence direction of the underlying basement reverse faults. Overall, this provenance pattern persisted until ca. 50 Ma, when more proximal igneous and volcaniclastic units associated with the Absaroka and Challis volcanics became major sediment sources and the Idaho River system became the dominant transport system in the area.