The lithospheric folding model applied to the Bighorn uplift during the Laramide orogeny

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.

Rise of the Colorado Plateau: A Synthesis of Paleoelevation Constraints From the Region and a Path Forward Using Temperature-Based Elevation Proxies

The Colorado Plateau’s complex landscape has motivated over a century of debate, key to which is understanding the timing and processes of surface uplift of the greater Colorado Plateau region, and its interactions with erosion, drainage reorganization, and landscape evolution. Here, we evaluate what is known about the surface uplift history from prior paleoelevation estimates from the region by synthesizing and evaluating estimates 1) in context inferred from geologic, geomorphic, and thermochronologic constraints, and 2) in light of recent isotopic and paleobotanical proxy method advancements. Altogether, existing data and estimates suggest that half-modern surface elevations were attained by the end of the Laramide orogeny (∼40 Ma), and near-modern surface elevations by the mid-Miocene (∼16 Ma). However, our analysis of paleoelevation proxy methods highlights the need to improve proxy estimates from carbonate and floral archives including the ∼6–16 Ma Bidahochi and ∼34 Ma Florissant Formations and explore understudied (with respect to paleoelevation) Laramide basin deposits to fill knowledge gaps. We argue that there are opportunities to leverage recent advancements in temperature-based paleoaltimetry to refine the surface uplift history; for instance, via systematic comparison of clumped isotope and paleobotanical thermometry methods applied to lacustrine carbonates that span the region in both space and time, and by use of paleoclimate model mediated lapse rates in paleoelevation reconstruction.

Diapiric relamination of the Orocopia Schist (southwestern U.S.) during low-angle subduction

The Orocopia Schist and related schists are sediments subducted during the Laramide orogeny and are thought to have been underplated as a laterally extensive layer at the base of the crust in the southwestern United States Cordillera. This concept is hard to reconcile with the existence of continental mantle lithosphere in southeastern California and western Arizona. Analytical solutions and numerical modeling suggest that the Orocopia Schist may have ascended through the mantle lithosphere as sediment diapirs or subsolidus crustal plumes to become emplaced in the middle to lower crust. Modeled time-temperature cooling paths are consistent with the exhumation history of the Orocopia Schist and explain an initial period of rapid cooling shortly after peak metamorphism. The Orocopia Schist represents a potential example of relaminated sediment observable at the surface.

A new stratigraphic framework and constraints for the position of the Paleocene–Eocene boundary in the rapidly subsiding Hanna Basin, Wyoming

The Paleocene–Eocene strata of the rapidly subsiding Hanna Basin give insights in sedimentation patterns and regional paleogeography during the Laramide orogeny and across the climatic event at the Paleocene–Eocene Thermal Maximum (PETM). Abundant coalbeds and carbonaceous shales of the fluvial, paludal, and lacustrine strata of the Hanna Formation offer a different depositional setting than PETM sections described in the nearby Piceance and Bighorn Basins, and the uniquely high sediment accumulation rates give an expanded and near-complete record across this interval. Stratigraphic sections were measured for an ∼1250 m interval spanning the Paleocene–Eocene boundary across the northeastern syncline of the basin, documenting depositional changes between axial fluvial sandstones, basin margin, paludal, floodplain, and lacustrine deposits. Leaf macrofossils, palynology, mollusks, δ13C isotopes of bulk organic matter, and zircon sample locations were integrated within the stratigraphic framework and refined the position of the PETM. As observed in other basins of the same age, an interval of coarse, amalgamated sandstones occurs as a response to the PETM. Although this pulse of relatively coarser sediment appears related to climate change at the PETM, it must be noted that several very similar sandstone bodies occur with the Hanna Formation. These sandstones occur in regular intervals and have an apparent cyclic pattern; however, age control is not sufficient yet to address the origin of the cyclicity. Signs of increased ponding and lake expansion upward in the section appear to be a response to basin isolation by emerging Laramide uplifts.

Zircon geochronology and paleomagnetism of an Archean harzburgite intrusion, eastern Bighorn Mountains, Wyoming

A harzburgite intrusion, which is part of the trailside mafic complex) intrudes ~2900-2950 Ma gneisses in the hanging wall of the Laramide Bighorn uplift west of Buffalo, Wyoming. The harzburgite is composed of pristine orthopyroxene (bronzite), clinopyroxene, serpentine after olivine and accessory magnetite-serpentinite seams, and strike-slip striated shear zones. The harzburgite is crosscut by a hydrothermally altered wehrlite dike (N20°E, 90°, 1 meter wide) with no zircons recovered. Zircons from the harzburgite reveal two ages: 1) a younger set that has a concordia upper intercept age of 2908±6 Ma and a weighted mean age of 2909.5±6.1 Ma; and 2) an older set that has a concordia upper intercept age of 2934.1±8.9 Ma and a weighted mean age 2940.5±5.8 Ma. Anisotropy of magnetic susceptibility (AMS) was used as a proxy for magmatic intrusion and the harzburgite preserves a sub-horizontal Kmax fabric (n=18) suggesting lateral intrusion. Alternating Field (AF) demagnetization for the harzburgite yielded a paleopole of 177.7 longitude, -14.4 latitude. The AF paleopole for the wehrlite dike has a vertical (90°) inclination suggesting intrusion at high latitude. The wehrlite dike preserves a Kmax fabric (n=19) that plots along the great circle of the dike and is difficult to interpret. The harzburgite has a two-component magnetization preserved that indicates a younger Cretaceous chemical overprint that may indicate a 90° clockwise vertical axis rotation of the Clear Creek thrust hanging wall, a range-bounding east-directed thrust fault that accommodated uplift of Bighorn Mountains during the Eocene Laramide Orogeny.

Chapter 19: The Peñasquito Gold-(Silver-Lead-Zinc) Deposit, Zacatecas, Mexico

Peñasquito is an Au-Ag-Zn-Pb deposit and currently the principal Au-producing mine in Mexico. It is the most recent major discovery in the historically important Concepción del Oro mining district. Current Au reserves plus historic production at Peñasquito stand at 12.67 Moz, in addition to 527 Moz Ag, 3,600 lb Pb, and 8,000 lb Zn in remaining proven and probable reserves. Mineralization is centered on the Peñasco and Brecha Azul diatreme breccias, which cut an Upper Jurassic to Upper Cretaceous marine carbonate-dominated sedimentary sequence, which underwent folding during the Laramide orogeny. The diatreme breccias and associated mineralization are associated with early Oligocene quartz-feldspar porphyries dated at 34.4 ± 0.4 to 33.7 ± 0.4 Ma and thus 3 to 10 m.y. younger than the other skarn and polymetallic deposits known in the district. The Peñasco diatreme is about 1 km in diameter and hosts epithermal-style disseminated mineralization, whereas the contiguous Cretaceous carbonaceous and calcareous siltstone and interbedded sandstone of the Caracol Formation is the principal host for stockwork and manto-type, massive base metal sulfide mineralization. Skarn-type mineralization is Cu-Zn rich, extends to the current depth of drilling some 2 km below the premine surface, and is hosted by the Jurassic-Cretaceous sequence beneath the Caracol Formation. In addition, weakly developed stockwork Mo (±Cu) mineralization has also been intersected by drilling at depths of nearly 2 km.

Stratigraphic relationships along the monoclinal eastern base of Bald Ridge and northwestern edge of Wyoming’s Bighorn Basin, U.S.A.

ABSTRACT This geologic study is focused on a less than 5 square-mile (ca. 13 km2) tract of public land in northwestern Wyoming, 8 miles (12.9 km) south-southwest of the small town of Clark in Park County. The study area is south of Clarks Fork of Yellowstone River along the eastern base of the topographic feature called Bald Ridge, also known structurally as Dead Indian monocline. Since the Middle Eocene, the study area has been along the northwestern margin of the Bighorn Basin. Prior to that time, the study area existed near the west–east center of the basin. Bald Ridge became elevated late in the Laramide orogeny (no older than the Early Eocene) through east-directed faulting of basement rocks via the extensive Line Creek–Oregon Basin thrust system. As that active faulting occurred, the overlying Phanerozoic strata (Lower Cambrian through Lower Eocene) responded with numerous west-directed, out-of-the-basin thrusts as a new western-basin margin developed along the eastern realm of the newly born Absaroka volcanic field. Most of that deformation occurred after deposition of uppermost levels of the Lower Eocene Willwood Formation. The key purpose of the present paper was to improve the accuracy of mapping of the Jurassic into Eocene stratigraphy along the newly restricted, northwestern edge of Wyoming’s Bighorn Basin. The stratigraphic column in a north–south band along the eastern flank of the Beartooth Mountains and continuing southward into the present study area was markedly deformed and deeply eroded late during the Laramide orogeny. The present small, more southerly study area is structurally and erosionally simpler than its more northerly equivalent. Thus, its study adds important geological information to the history of the northern Cody Arch, a convex-westward string of related basement-involved uplifts extending southward to southwest of the city of Cody. Progressively steepening eastward dips of strata characterize a west-to-east transect from the summit of Bald Ridge (capped by the shallowly dipping, Mississippian Madison Limestone) to the western edge of strongly overturned outcrops of the Eocene Willwood Formation. The Upper Cretaceous Meeteetse Formation is the stratigraphic horizon at which the dips attain vertical or slightly overturned orientations. All consequential faults within the newly mapped area are thrusts, and they show generally westward (out-of-the-basin) displacements. Despite those west-directed displacements, their primary cause was tectonic shortening at depth below Bald Ridge that was directed to the northeast or east-northeast. During the Laramide orogeny, certain thrust planes within the east-dipping Phanerozoic rock column cut down-section stratigraphically (but uphill relative to Earth’s surface) and thereby placed younger strata upon older. The cumulative result, as recognized at several levels within the present area of study, was marked thinning of the total section. For example, surface exposures of the mostly Paleocene Fort Union Formation, 4,000 feet (1,219 m) thick only 7 miles (11.3 km) to the east, was completely eliminated from the local surface stratigraphy by that means. The northern end of Bald Ridge is formed by the highly asymmetric Canyon Mouth anticline. That structure differs strongly in the attitude of its hinge line from the general east-northeast dip of strata cloaking Bald Ridge. The Canyon Mouth anticline’s hinge line plunges steeply to the southeast, and dips on its northeastern flanks are vertical to partly overturned. Surprisingly, hinge lines and flanks of all other anticlinal/synclinal structures recognized within the present map area share those same orientations with Canyon Mouth anticline. These consistent but unexpected differences in orientation from unfolded strata may represent very late events in the history of Laramide strain vectors across the study area. Working in northern parts of this study area, an independent group determining radiometric ages of detrital-zircon grains reported close agreements in age with their host localities in the Early Cretaceous Mowry Shale and Frontier Formation. However, under the present paper’s interpretation of the local stratigraphy, the other workers misidentified formational hosts for all three samplings. That resulted in age-determination errors of depositional history within the Upper Cretaceous section of as much as 28.8 million years.