ABSTRACT The Bighorn uplift, Wyoming, developed in the Rocky Mountain foreland during the 75–55 Ma Laramide orogeny. It is one of many crystalline-cored uplifts that resulted from low-amplitude, large-wavelength folding of Phanerozoic strata and the basement nonconformity (Great Unconformity) across Wyoming and eastward into the High Plains region, where arch-like structures exist in the subsurface. Results of broadband and passive-active seismic studies by the Bighorn EarthScope project illuminated the deeper crustal structure. The seismic data show that there is substantial Moho relief beneath the surface exposure of the basement arch, with a greater Moho depth west of the Bighorn uplift and shallower Moho depth east of the uplift. A comparable amount of Moho relief is observed for the Wind River uplift, west of the Bighorn range, from a Consortium for Continental Reflection Profiling (COCORP) profile and teleseismic receiver function analysis of EarthScope Transportable Array seismic data. The amplitude and spacing of crystalline-cored uplifts, together with geological and geophysical data, are here examined within the framework of a lithospheric folding model. Lithospheric folding is the concept of low-amplitude, large-wavelength (150–600 km) folds affecting the entire lithosphere; these folds develop in response to an end load that induces a buckling instability. The buckling instability focuses initial fold development, with faults developing subsequently as shortening progresses. Scaled physical models and numerical models that undergo layer-parallel shortening induced by end loads determine that the wavelength of major uplifts in the upper crust occurs at approximately one third the wavelength of folds in the upper mantle for strong lithospheres. This distinction arises because surface uplifts occur where there is distinct curvature upon the Moho, and the vergence of surface uplifts can be synthetic or antithetic to the Moho curvature. In the case of the Bighorn uplift, the surface uplift is antithetic to the Moho curvature, which is likely a consequence of structural inheritance and the influence of a preexisting Proterozoic suture upon the surface uplift. The lithospheric folding model accommodates most of the geological observations and geophysical data for the Bighorn uplift. An alternative model, involving a crustal detachment at the orogen scale, is inconsistent with the absence of subhorizontal seismic reflectors that would arise from a throughgoing, low-angle detachment fault and other regional constraints. We conclude that the Bighorn uplift—and possibly other Laramide arch-like structures—is best understood as a product of lithospheric folding associated with a horizontal end load imposed upon the continental margin to the west.

ABSTRACT Rattlesnake Mountain is a Laramide uplift cored by Archean gneiss that formed by offset along two reverse faults with opposing dips, the result being an asymmetric anticline with a drape fold of Cambrian–Cretaceous sediments. Rattlesnake Mountain was uplifted ca. 57 Ma and was a structural buttress that impeded motion of upper-plate blocks of the catastrophic Heart Mountain slide (49.19 Ma). North of Pat O’Hara Mountain anticline, Rattlesnake Mountain anticline has a central graben that formed ca. 52 Ma (U-Pb age on vein calcite in normal faults) into which O- and C-depleted fluids propagated upward with hydrocarbons. The graben is defined by down-dropped Triassic Chugwater shales atop the anticline that facilitated motion of Heart Mountain slide blocks of Paleozoic limestones dolomite (i.e., the Ordovician Bighorn Dolomite and Mississippian Madison Limestone) onto, and over, Rattlesnake Mountain into the Bighorn Basin. Heart Mountain fault gouge was also injected downward into the bounding Rattlesnake Mountain graben normal faults (U-Pb age ca. 48.8 ± 5 Ma), based on O and C isotopes; there is no anisotropy of magnetic susceptibility fabric present. Calcite veins parallel to graben normal faults precipitated from meteoric waters (recorded by O and C isotopes) heated by the uplifting Rattlesnake Mountain anticline and crystallized at 57 °C (fluid inclusions) in the presence of oil. Calcite twinning strain results from graben injectites and calcite veins are different; we also documented a random layer-parallel shortening strain pattern for the Heart Mountain slide blocks in the ramp region ( n = 4; west) and on the land surface ( n = 5; atop Rattlesnake Mountain). We observed an absence of any twinning strain overprint (low negative expected values) in the allochthonous upper-plate blocks and in autochthonous carbonates directly below the Heart Mountain slide surface, again indicating rapid motion including horizontal rotation about vertical axes of the upper-plate Heart Mountain slide blocks during the Eocene.

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.