Mountain Bighorn Research Articles

Insight into the Evolution of the Eastern Margin of the Wyoming Craton from Complex, Laterally Variable Shear Wave Splitting

Abstract Dense seismic arrays such as EarthScope’s Transportable Array (TA) have enabled high-resolution seismic observations that show the structure of cratonic lithosphere is more heterogeneous and complex than previously assumed. In this study, we pair TA data with data from the Bighorn Arch Seismic Experiment and the Crust and lithosphere Investigation of the Easternmost expression of the Laramide Orogeny (CIELO) to provide unprecedented detail on the seismic anisotropic structure of the eastern margin of the Wyoming Craton, where several orogens emerged from nominally strong cratonic lithosphere during the Laramide Orogeny. In this study, we use the splitting of teleseismic shear waves to characterize fabrics associated with deformation in the Earth’s crust and mantle. We constrain distinct anisotropic domains in the study area, and forward modeling shows that each of these domains can be explained by a single layer of anisotropy. Most significantly, we find a fast direction in the southern part of the Powder River Basin, which we refer to as the Thunder Basin Block (TBB), that deviates from absolute plate motion (APM). This change in splitting behavior coincides with changes in other modeled geophysical observations, such as active source P-wave velocity models, potential field modeling, and seismic attenuation analysis, which all show a significant change moving from the Bighorn Mountains to the TBB. We argue that these results correspond to structure predating the Laramide Orogeny, and most likely indicate a Neoarchean boundary preserved within the lithosphere.

Geochemical and geochronologic evidence for a contiguous northeastern Wyoming Province

The extent and nature of the Wyoming Province, an Archean craton in southwestern Laurentia, are poorly understood due to limited exposure between spatially isolated basement-cored uplifts. This lack of exposure has led to debate about whether the northeastern Wyoming Province is underlain by contiguous Archean crust or Proterozoic rocks and suture zone associated with the Trans-Hudson orogeny. To assess these models we analyzed samples recovered from drill cores in the buried northeastern Wyoming Province that straddle the proposed Proterozoic suture. Whole-rock geochemical and Nd-Pb-O isotopic data suggest the rocks formed by partial melting of > 3.5 Ga hydrated mafic to tonalitic sources similar to elsewhere in the northern Wyoming Province. Zircon U-Pb dates record Mesoarchean magmatism and metamorphism (3.0–2.8 Ga). Published whole-rock Rb-Sr, hornblende and biotite K-Ar, and apatite fission track dates suggest these rocks have not been heated above 300 °C since 2.5–2.1 Ga. Geophysical potential field data are consistent across northeastern Wyoming contrasting with major discontinuities associated with documented Proterozoic orogens on all other margins of the Wyoming Province. Hence, geochemical, isotopic, and geochronologic data along with geophysical imaging can be most simply interpreted in terms of continuous Archean crust in the northeastern Wyoming Province. A geophysically-imaged reflector east of the Bighorn Mountains may juxtapose Archean terranes with similar ages and sources, similar to the boundary between the Montana metasedimentary terrane and Beartooth-Bighorn magmatic zone in the northwestern Wyoming Province. This work emphasizes the value of integrating geologic and geophysical constraints to constrain Archean provinces and their tectonic evolution.

Comparison of circulating and excreted metals and of autoimmunity between two Great Plains Tribal communities

Metals contaminants of the environment from mine waste have been implicated as contributing agents in autoimmune disease. The current study compares metals and autoimmunity in two Tribal communities residing in the Black Hills and the Bighorn Mountains geographical regions that are scattered with extant hard rock mines. With documented drinking water contamination in both communities, in vivo levels of more than half of the measured serum and urine metals differed between the two communities and were substantially different from their national median values. Serum autoantibodies associated with systemic autoimmune disease were rare or at low-level, but antibodies to denatured (single-stranded) DNA and thyroid-specific autoantibodies were commonly elevated, especially in women. A three-tier statistical modeling process was carried out to examine individual metals exposure as predictors of autoantibody levels. For the most part only weak positive associations between individual metals and systemic autoantibodies were found, although univariate quantile regression analysis showed positive statistical associations of serum lead and antimony with anti-chromatin and anti-histone autoantibodies. Using age and gender-adjusted multivariable statistical models, metals did not predict anti-thyroglobulin or -thyroid peroxidase significantly and metals were generally negative predictors of the other autoantibodies. Overall these results suggest that elevated levels of environmental metals and metalloids in these communities may result in suppression of autoantibodies associated with systemic autoimmune disease.

Describing a Newly Excavated Packrat Midden Near Natural Trap Cave, Wyoming to Help Assess Biases in Fossil Microfaunal Accumulation

Microfauna communities resulting from packrat accumulation provide crucial information about how natural communities respond to environmental changes. However, fossil accumulation mechanisms can be complex, making it difficult to interpret these community responses. Natural Trap Cave (NTC) is an open-mouth cave with an 85-foot drop that often lies unseen by passing animals, making it a major location for fossil accumulation. NTC, located in the Bighorn Mountains of Wyoming, has accumulated a paleontological record of the local community’s biota dating back to the Late Pleistocene, 25,000 years ago. The rim of the cave’s mouth is home to packrat middens located directly above the major deposits of microvertebrate (<5 kg) fossils. It is hypothesized that middens along the rim have accumulated seeds, bones, teeth, and bird pellets from the local communities over time and have periodically washed into the cave below. We seek to assess the bias in microvertebrate community composition introduced by packrat accumulation. We identified bones and teeth collected from a 9-section grid of a modern midden located adjacent to the cave, which allow us to see which species dominate the NTC environment. We calculate species evenness, richness, and relative abundances using MNI. To assess local bias resulting from packrat accumulation, we compare the modern midden with live-and camera-trap data of small mammal communities. To assess which NTC fossils result from packrat accumulation, we compare the modern midden with fossil material from within the cave, ranging in age from 2 to 20 ka. Once we understand these biases present in the fossil accumulation, we can associate species’ relative abundances with changes in climate, such as drought, increasing temperatures, and flooding. By knowing how climate has affected NTC species’ abundances through time, we will be prepared to aid in conservation efforts by predicting how species numbers will be affected by future climate change.

Geologic map of the Bald Mountain Quadrangle, northern Bighorn Mountains, Wyoming

Here we present the results of detailed (1:24,000 scale) mapping of the Bald Mountain 7.5 Minute Quadrangle Wyoming, which includes Archean basement rocks of the Laramide Bighorn uplift. These basement rocks are mantled by Paleozoic cratonic strata of the Flathead, Gros Ventre, Gallatin, Bighorn, Madison and Amsden formations. This region was explored for gold associated with the basement rocks, and Th in the basal Flathead sandstone. Isotopic age determinations (LA-ICPMS U-Pb on zircon) of the basement rocks were conducted at the University of Arizona Laserchron Center. Three phases of Archean rocks were defined, all of which range from ∼2880-2890 Ma, and include a prominent, pink, strongly foliated (WNW trending) alkali feldspar granite with pegmatite, a weakly foliated purple granite, and a yellow, poorly exposed adamellite. These rocks were uplifted during the Paleogene Laramide orogeny, and now form a southwest verging breached drape fold over a steeply inclined, basement-cored reverse fault. Dips of footwall strata range from 10–70° to the SW. Hanging wall rocks dip gently to the NE. Quaternary landslide deposits occur along steep slopes in the Gros Ventre Shale and alluvium occurs along the principle streams.

Sediment provenance and stratigraphic correlations of the Paleogene White River Group in the Bighorn Mountains, Wyoming

Detrital zircon ages for tuffaceous sandstones and conglomerates of the White River Group provide insights on Paleogene basin evolution, magmatic activity, and paleodrainage throughout the Laramide broken foreland basin system of the northern Rocky Mountains in the western United States. Nonmarine deposits of the upper Eocene-Oligocene White River Group are preserved irregularly across northern Wyoming and western South Dakota. Residual Laramide uplifts and active magmatic centers supplied clastic and volcaniclastic sediment to broad, low-relief valleys beginning around 40 Ma. Subhorizontal strata of the White River Group are exposed in the elevated Bighorn Mountains (∼2300 to 2800 m), where sections ∼10-50 m thick rest unconformably on Precambrian-Paleozoic rocks along a surface of moderate to low relief (up to 150 m). U-Pb ages were obtained for detrital and igneous (ash-fall) zircons from seven samples (3 tuffaceous sandstones, 2 conglomerates, 2 sandstones) spanning three localities in the Bighorn Mountains (Darton’s Bluff, Hazelton Road, Freeze Out Point). Each locality contains conglomeratic layers, with clasts of local crystalline basement, and interbedded tuffaceous sandstones. Detrital zircon age spectra for four samples reveal peak ages around 2.9 Ga, matching the age of Archean crystalline basement within Bighorn Mountains, and maximum depositional ages (MDAs) of 27 Ma (sample 20BH15; Oligocene) and 35 Ma (sample FO-2; late Eocene). During the Paleogene, the Bighorn Mountains region received sediment from local crystalline basement and long-distance river transport from igneous and sedimentary sources to the west. The Bighorn Mountains were exhumed and stripped of Phanerozoic cover strata by early Eocene time, suggesting that post-Laramide input from Paleozoic-Mesozoic strata was likely from relict highlands of the Cordilleran (Sevier) fold-thrust belt rather than local Laramide block uplifts. In addition, Cenozoic magmatic provinces in the San Juan Mountains and Great Basin are inferred to have contributed volcaniclastic sediment through both eruptive ash clouds and north- to northeast-flowing fluvial systems that reached northeastern Wyoming. The White River Group preserved in the Bighorn Mountains represents localized late Eocene-Oligocene sediment accumulation atop a Laramide basement high coeval with regional deposition across the adjacent Great Plains. Both regions were supplied sediment from alluvial fans and fluvial drainage networks that tapped Laramide basement uplifts, Cordilleran thrust-belt, and foreland sources, along with Cenozoic igneous centers of the western U.S. interior.

Temperature effects on Mormon cricket Anabrus simplex embryo development, hatching and nymphal growth: Thermal performance curves change with ontogeny

Thermal performance curves for development are an essential tool for population modeling and pest management. More broadly, they characterize how selection on thermal limits or maximum rates change with life stage. The effects of temperature on development of Mormon cricket embryos, and hatching and development of nymphs were measured on a population from the Bighorn Mountains of Wyoming and modeled with four non-linear equations. Taylor's Gaussian curve characterized embryonic development, which was most rapid at 26.9 °C. However, half-grown embryos aestivated at high temperatures, resulting in a significant shift in the optimum temperature to complete embryonic development to 24.1 °C and a reduction in the breadth of the performance curve (thermal breadth). Fully grown embryos hatched fastest at relatively low temperatures (21.8 °C), whereas nymphal development was maximized at relatively high temperatures (35.7 °C). Thermal breadths for nymphal hatching and development were also significantly broader than that for embryonic development. Differences in optimum temperature and thermal breadth of each life stage should be taken into consideration in population modeling, comparisons among populations, and epigenetic studies of acclimation.

Refining Robel Pole Applications for Grazed Idaho Fescue Grasslands in the Big Horn Mountains, Wyoming

Administrative oversight of grazing on public land requires efficient and reliable rangeland monitoring. In a portion of the Bighorn National Forest of Wyoming, the US Forest Service uses the modified Robel pole as a monitoring tool to measure residual herbaceous biomass after grazing. Questions about the vegetation standards have persisted for decades. From 2018 to 2020 we quantified relationships between herbaceous biomass and visual obstruction across spatial, temporal, management, and topoedaphic gradients in 11 grazed pastures selected mutually by stakeholders. Initial standards were developed on the basis of 1 yr of research, and state visual obstruction readings (VORs) were correlated with biomass with an R2 = 0.81, yet our research resulted in R2 values of approximately half (ranging from 0.39 to 0.45) with significantly different slopes 2 of 3 yr. Moreover, the intercepts for each year individually, when averaged, or when combined were always greater than the relationship used to develop initial standards, resulting in 140−564 more kg per ha of biomass at the 5-band standard level, or situations where lower band levels (2.5−3.9) would yield similar biomass levels. In addition to VOR and biomass, slope, aspect, plant and ground cover, weather, and livestock management variables were assessed with information theory models for predicting residual biomass. The top model included VOR, slope, exotic grass, and temperature. This suggests that greater knowledge of the site will increase understanding of biomass production on the site and help to mitigate disagreements among stakeholders by tailoring expectations to variable sites. Finally, the higher intercepts in all of our models may explain the persistent skepticism about the application of the original standard and provide the opportunity to refine standards to better match postgrazing monitoring with resource objectives, particularly if functional group dominance on these sites have changed over time.

Essential Oil Yield, Composition, and Bioactivity of Sagebrush Species in the Bighorn Mountains.

Sagebrush (Artemisia spp.) are dominant wild plants in large areas of the U.S., Canada and Mexico, and they include several species and subspecies. The aim was to determine if there are significant differences in essential oil (EO) yield, composition, and biological activity of sagebrush within the Bighorn Mountains, U.S. The EO yield in fresh herbage varied from 0.15 to 1.69% for all species, including 0.25–1.69% in A. tridentata var. vaseyana, 0.64–1.44% in A. tridentata var. tridentata, 1% in A. tridentata var. wyomingensis, 0.8–1.2% in A. longifolia, 0.8–1% in A. cana, and 0.16% in A. ludoviciana. There was significant variability in the EO profile between species, and subspecies. Some EO constituents, such as α-pinene (0–35.5%), camphene (0–21.5%), eucalyptol (0–30.8%), and camphor (0–45.5%), were found in most species and varied with species and subspecies. The antioxidant capacity of the EOs varied between the species and subspecies. None of the sagebrush EOs had significant antimicrobial, antimalarial, antileishmanial activity, or contained podophyllotoxin. Some accessions yielded EO with significant concentrations of compounds including camphor, eucalyptol, cis-thujone, α-pinene, α-necrodol-acetate, fragranol, grandisol, para-cymene, and arthole. Therefore, chemotypes can be selected and possibly introduced into culture and be grown for commercial production of these compounds to meet specific industry needs.

Luminescence ages and new interpretations of the timing and deposition of Quaternary sediments at Natural Trap Cave, Wyoming

Natural Trap Cave, located in the Big Horn Mountains of north-central Wyoming, has a history of trapping and preserving a range of North American fauna that plummeted into the deep vertical entrance. These animal remains were buried and preserved within sediments of the main chamber and, in turn, have helped elucidate the procession of faunal dynamics during the latest glacial cycle. The cave location, south of the Laurentide and Cordilleran Ice Sheets, and proximal to Yellowstone, is at an ideal geographical juncture to provide insights to ecological changes in North America. The sediments that the animals are buried in inform us about transport and deposition both inside and outside of the cave that relate to catchment dynamics. We report on a series of optically stimulated luminescence (OSL) ages derived from samples obtained within the cave during excavation work in 2014 and in 2018. We also examine chronology produced by argon, tephrochronology, fission track, and luminescence techniques that have been used for understanding the infilling of the cave. The cave sediment ages and in situ measured gamma spectroscopy as measured in this study helped resolve an improved chronological age model when combined with previous data.The suite of OSL ages is interpreted through the stratigraphic relationships (and vertebrates contained within) which requires the use of an adequate age model; we use either the central age model or minimum age model where appropriate and with justification. Lowest sediments are dated to ∼150 ka with a hiatus at ∼130 to 52 ka. Above this, sediment deposition and entrainment of paleontological materials are representative of Pleistocene and early Holocene times, between 37 ± 6 ka and 7.6 ± 0.5 ka. The stratigraphic architecture suggests that deposition of materials into the cave is episodic and rapid, followed by quiescent periods where hydrologic scour, heavy overland flow, or possibly a cryo-hydrologic process may have altered unit relationships. Thus, the complementary geochronometers and the characteristics of quartz versus feldspar luminescence signals improve temporal interpretations of these complex deposits. This adapted understanding of mixing also sets the stage for future work with the aim to improve our understanding of ages and sources for ash units within these cave deposits. The three ash units recognized in the cave may represent an in-situ reworking of the same ash or may be representative of previously undocumented eruptions from the Yellowstone Caldera.

An age-depth model and revised stratigraphy of vertebrate-bearing units in Natural Trap Cave, Wyoming

Excavations at Natural Trap Cave (NTC) began in the mid-1970's and quickly yielded evidence of a diverse steppe fauna that existed during the Last Glacial Maximum (MIS 2) along the western slope of the Bighorn Mountains in north central Wyoming. NTC is a karst pit cave with a 24.5 m vertical drop into a roughly 43 m wide bell-shaped ‘Main Chamber’ and during the course of early excavations a plugged entrance to a fossil-bearing ‘Lower Chamber’ was discovered. Stratigraphic relationships below the entrance to the Main Chamber of the cave were originally studied in the mid-1970's, but were never formally published. Although stratigraphy, taphonomy, and depositional circumstances were briefly discussed over the following years, little has been done to correlate the numerous stratigraphic schemes used by various authors. In this study, four stratigraphic sections were measured and analyzed to establish an easily modifiable lithostratigraphic system of nomenclature. We provide the first correlations of all stratigraphic nomenclature used throughout excavations at NTC to facilitate comparisons with current and previous collections and publications. By leveraging more than 100 radioisotopic dates we developed an age-depth model and chronostratigraphic framework to further interrogate spatiotemporal relationships between strata, paleoenvironmental proxies, and fossil assemblages. Deposition is shown to be discontinuous; sediment accumulation in the study area is restricted to the buildup through peak penultimate and Last Glacial maxima. More recent (<10 ka) Holocene deposits unconformably cover the eroded surface of underlying Pleistocene strata. There is active reworking of sediments with transport and deposition of reactivated material within the Lower Chamber. We note that the two hiatuses coincide with interglacial periods and may reflect changing depositional circumstances within the cave such as extended periods of non-deposition, erosion, or bypass (possibly leading to deposition in the Lower Chamber). Contrary to previous reports, we demonstrate that it is unlikely a prominent snow cone existed or contributed to the pattern of sediment and fossil distribution within the study area, furthermore, we do not observe a continuous Pleistocene-Holocene transition in the study area. Further stratigraphic work will be needed to better understand the interrelationship between Main and Lower chamber deposits and the evolution of sediment accumulation in NTC.

Active-Source Interferometry in Marine and Terrestrial Environments: Importance of Directionality and Stationary Phase

ABSTRACT We utilize active-source seismic interferometry with dense seismic arrays both offshore and onland to explore the utility of this method to create virtual sources and reveal body-wave reflections in these two different environments. We first utilize data from an ocean-bottom cable (OBC) array in the Gulf of Mexico with equal numbers of sources (160 airgun shots) and receivers (160 ocean-bottom four-component sensors). We next use data from a geophone array across the Bighorn Mountains of Wyoming with many receivers (1300 vertical-component geophones) but a small number of sources (14 borehole active-source shots). We find that the OBC virtual source results, which produce strong reflections from sub-seafloor structures, are far superior to the onland results which lack usable reflections, and we explore reasons for these differences through a set of selective stacking approaches. We present techniques to account for the direction the seismic waves travel (directionality) and stationary phase and show that improvements can be made when incorporating these corrections. Although interferometric methods are based on assumptions of large numbers of widely distributed actual sources, we find that selective exclusion of potentially problematic source–receiver pairs can yield improved results. These geometric adjustments to active-source interferometry methods have utility for dense-nodal-array surveys that are now common in academic studies, but that often suffer from sparse source geometry.

Lesson Learned in the Bighorn Mountains

Lesson Learned in the Bighorn Mountains

Evidence for Pleistocene gene flow through the ice-free corridor from extinct horses and camels from Natural Trap Cave, Wyoming

Natural Trap Cave (Bighorn Mountains, Wyoming) preserves an abundance of fossil remains from extinct Late Pleistocene fauna and is situated near a past migration route that likely connected populations in Eastern Beringia and the contiguous US—the ice-free corridor between the Cordilleran and Laurentide icesheets. Some palaeontological evidence supports a correspondingly high affinity between fauna recorded in Natural Trap Cave and Eastern Beringia versus elsewhere in the contiguous US, but this hypothesis has not yet been extensively tested using genetic data. In the present study, we analysed 16 horse specimens and one camel specimen from Natural Trap Cave. Of the horse specimens we analysed, we obtained 10 unique and previously unreported mitochondrial haplotypes belonging to two distinct (extinct) genetic clades—two haplotypes corresponded to a caballine horse (Equus sp.) and eight corresponded to the stilt-legged horse (Haringtonhippus francisci). With only one exception, it appears these newly sequenced individuals all shared a common ancestor more recently with Eastern Beringian individuals than with others from the contiguous US. In addition, mitochondrial data from a specimen assigned to Camelops sp. revealed that it shares a closer affinity with specimens from the Yukon Territory than those from Idaho or Nevada, though all appear to belong to a single species (“yesterday's camel”; Camelops cf. hesternus). Together, these results are consistent with a high level of genetic connectivity between horse and camel populations in the Bighorn Mountains and Eastern Beringia during the Pleistocene.

Evidence for Stress Localization Caused by Lithospheric Heterogeneity From Seismic Attenuation

AbstractThe Wyoming Craton underwent tectonic modifications during the Laramide Orogeny, which resulted in a series of basement‐cored uplifts that built the modern‐day Rockies. The easternmost surface expression of this orogeny ‐ the Black Hills in South Dakota ‐ is separated from the main trend of the Rocky Mountains by the southern half of the Powder River Basin, which we refer to as the Thunder Basin. Seismic tomography studies reveal a high‐velocity anomaly which extends to a depth of ∼300 km below the basin and may represent a lithospheric keel. We constrain seismic attenuation to investigate the hypothesis that variations in lithospheric thickness resulted in the localization of stress and therefore deformation. We utilize data from the CIELO seismic array, a linear array that extends from east of the Black Hills across the Thunder Basin and westward into the Owl Creek Mountains, the BASE FlexArray deployment centered on the Bighorn Mountains, and the EarthScope Transportable Array. We analyze seismograms from deep teleseismic events and compare waveforms in the time‐domain to characterize lateral variations in attenuation. Bayesian inversion results reveal high attenuation in the Black Hills and Bighorn Mountains and low attenuation in the Thunder and Bighorn Basins. Scattering is rejected as a confounding factor because of a strong anticorrelation between attenuation and the amplitude of P wave codas. The results support the hypothesis that lateral variations in lithospheric strength, as evidenced by our seismic attenuation measurements, played an important role in the localization of deformation and orogenesis during the Laramide Orogeny.

Rayleigh Wave Propagation in the Bighorn Mountains Region, Wyoming

ABSTRACT Short-period Rayleigh waves, Rg, provide strong constraints on the depth of shallow seismic events and are of interest for monitoring small explosions. Characterizing the seismic sources that generate Rg requires an understanding of how shallow crustal structure affects Rayleigh wave propagation. In support of these efforts, this study utilizes observed waveforms from small shallow explosions recorded on temporary seismic network deployments in the Bighorn region, Wyoming. We study regional near-surface geology by measuring changes in surface-wave amplitude and polarization during propagation through basins, foothills, and mountains. We develop additional insight by carrying out surface-wave eigenfunction analyses and numerical-wave simulations, which together reproduce many characteristics seen in the observed waveforms. Our results show how sedimentary basins in the Bighorn region allow for amplified prograde-polarized higher-mode and retrograde-polarized fundamental-mode Rayleigh waves, whereas adjacent mountains only support retrograde motion. These different modes provide distinct constraints on the Earth structure and source characteristics, potentially enabling targeted inversions in future studies. Our findings provide insight into Rg propagation through complex near-surface geology, improving our understanding of shallow propagation and source effects that are relevant to explosion monitoring efforts.

Teleseismic P-Wave Coda Autocorrelation Imaging of Crustal and Basin Structure, Bighorn Mountains Region, Wyoming, U.S.A.

ABSTRACT We demonstrate successful crustal imaging via teleseismic P-wave coda autocorrelation, using data recorded on a 261 station array of vertical-component high-frequency geophones in the area of the Bighorn Mountains, Wyoming, U.S.A. We autocorrelate the P-wave coda of 30 teleseismic events and use phase-weighted stacking to yield seismic profiles comparable to low-passed versions of those produced via controlled-source vertical seismic reflection. Our process recovers reflections from the bottoms of the Bighorn and Powder River basins that flank the Bighorn Mountains. We also identify a mid-crustal reflector that aligns with a region of increased reflectivity, previously interpreted as a Precambrian province boundary. Our results demonstrate the utility of crustal imaging with teleseismic P-wave coda energy using modern large-array seismic data, and they corroborate previous interpretations of crustal structures in the study area.

Geologic Map of the Park Reservoir Quadrangle, Sheridan County, Wyoming

This project involved the construction of a detailed geologic map of the Park Reservoir, Wyoming 7.5-Minute Quadrangle (Scale 1:24,000). The Quadrangle occurs entirely in the Bighorn National Forest, which is a popular recreation site for thousands of people each year. This research advances the scientific understanding of the geology of the Bighorn Mountains and the Archean geology of the Wyoming Province. Traditional geologic mapping techniques were used in concert with isotopic age determinations. Our goal was to further subdivide the various phases of the 2.8–3.0 Ga Archean rocks based on their rock types, age, and structural features. This research supports the broader efforts of the Wyoming State Geological Survey to complete 1:24,000 scale geologic maps of the state. The northern part of the Bighorn Mountains is composed of the Bighorn batholith, a composite complex of intrusive bodies that were emplaced between 2.96–2.87 Ga. Our mapping of the Park Reservoir Quadrangle has revealed the presence of five different Archean quartzofeldspathic units, two sets of amphibolite and diabase dikes, a small occurrence of the Cambrian Flathead Sandstone, two Quaternary tills, and Quaternary alluvium. The Archean rock units range in age from ca. 2.96–2.75 Ga, the oldest of which are the most ancient rocks yet reported in the Bighorn batholith. All the Archean rocks have subtle but apparent planar fabric elements, which are variable in orientation and are interpreted to represent magmatic flow during emplacement. The Granite Ridge tear fault, which is the northern boundary of the Piney Creek thrust block, is mapped into the Archean core as a mylonite zone. This relationship indicates that the bounding faults of the Piney Creek thrust block were controlled by weak zones within the Precambrian basement rocks.

The osteology and phylogenetic position of the loricatan (Archosauria: Pseudosuchia) Heptasuchus clarki, from the ?Mid-Upper Triassic, southeastern Big Horn Mountains, Central Wyoming (USA).

Loricatan pseudosuchians (known as “rauisuchians”) typically consist of poorly understood fragmentary remains known worldwide from the Middle Triassic to the end of the Triassic Period. Renewed interest and the discovery of more complete specimens recently revolutionized our understanding of the relationships of archosaurs, the origin of Crocodylomorpha, and the paleobiology of these animals. However, there are still few loricatans known from the Middle to early portion of the Late Triassic and the forms that occur during this time are largely known from southern Pangea or Europe. Heptasuchus clarki was the first formally recognized North American “rauisuchian” and was collected from a poorly sampled and disparately fossiliferous sequence of Triassic strata in North America. Exposed along the trend of the Casper Arch flanking the southeastern Big Horn Mountains, the type locality of Heptasuchus clarki occurs within a sequence of red beds above the Alcova Limestone and Crow Mountain formations within the Chugwater Group. The age of the type locality is poorly constrained to the Middle—early Late Triassic and is likely similar to or just older than that of the Popo Agie Formation assemblage from the western portion of Wyoming. The holotype consists of associated cranial elements found in situ, and the referred specimens consist of crania and postcrania. Thus, about 30% of the osteology of the taxon is preserved. All of the pseudosuchian elements collected at the locality appear to belong to Heptasuchus clarki and the taxon is not a chimera as previously hypothesized. Heptasuchus clarki is distinct from all other archosaurs by the presence of large, posteriorly directed flanges on the parabasisphenoid and a distinct, orbit-overhanging postfrontal. Our phylogenetic hypothesis posits a sister-taxon relationship between Heptasuchus clarki and the Ladinian-aged Batrachotomus kupferzellensis from current-day Germany within Loricata. These two taxa share a number of apomorphies from across the skull and their phylogenetic position further supports ‘rauisuchian’ paraphyly. A minimum of three individuals of Heptasuchus are present at the type locality suggesting that a group of individuals died together, similar to other aggregations of loricatans (e.g., Heptasuchus, Batrachotomus, Decuriasuchus, Postosuchus).

An additional example of the ichnogenus Zoophycos in Cambrian strata (Gallatin Formation; Wyoming, USA)

The ichnogenus Zoophycos is relatively rare in Cambrian-age sedimentary rocks, being far more commonly encountered in post-Devonian, and particularly post-Paleozoic, strata. A new occurrence of Zoophycos from the Upper Cambrian Gallatin Formation of the Bighorn Mountains, north-central Wyoming, USA, is reported herein. Material consists of a single specimen preserved within quartz silt representing the upper surface of a graded quartz arenite bed. The burrow is a relatively wide, curved, horizontally oriented lobe with a poorly visible marginal tube but well-developed internal lamellae. Discovery of this specimen contributes to the paleobiogeographic distribution of lower Paleozoic Zoophycos, adds to the literature suggesting that most Cambrian Zoophycos were horizontal and shallow-tier structures, and provides further support to the notion that relatively complex deposit-feeding behavior evolved earlier in the Phanerozoic than had long been recognized.