Eastern Wyoming Research Articles

Strong Physical Contrasts Across Two Mid‐Lithosphere Discontinuities Beneath the Northwestern United States: Evidence for Cratonic Mantle Metasomatism

AbstractMid‐lithosphere discontinuities are seismic interfaces likely located within the lithospheric mantle of stable cratons, which typically represent velocities decreasing with depth. The origins of these interfaces are poorly understood due to the difficulties in both characterizing them seismically and reconciling the observations with thermal‐chemical models of cratons. Metasomatism of the cratonic lithosphere has been reported by numerous geochemical and petrological studies worldwide, yet its seismic signature remains elusive. Here, we identify two distinct mid‐lithosphere discontinuities at ∼87 and ∼117 km depth beneath the eastern Wyoming craton and the southwestern Superior craton by analyzing seismic data recorded by two longstanding stations. Our waveform modeling shows that the shallow and deep interfaces represent isotropic velocity drops of 2%–8% and 4%–9%, respectively, depending on the contributions from changes in radial anisotropy and density. By building a thermal‐chemical model including the regional xenolith thermobarometry constraints and the experimental phase‐equilibrium data of mantle metasomatism, we show that the shallow interface probably represents the metasomatic front, below which hydrous minerals such as amphibole and phlogopite are present, whereas the deep interface may be caused by the onset of carbonated partial melting. The hydrous minerals and melts are products of mantle metasomatism, with CO2‐H2O‐rich siliceous melt as a probable metasomatic reagent. Our results suggest that mantle metasomatism is probably an important cause of mid‐lithosphere discontinuities worldwide, especially near craton boundaries, where the mantle lithosphere may be intensely metasomatized by fluids and melts released by subducting slabs.

Exploring the ceratopsid growth record: A comprehensive osteohistological analysis of Triceratops (Ornithischia: Ceratopsidae) and its implications for growth and ontogeny

Ceratopsids represent one of the most iconic groups of non-avian dinosaurs. These large quadrupedal ornithischians are well-known for their bizarre cranial ornamentations, which are distinctive among different ceratopsids. However, only very little data exist on ceratopsid osteohistology and growth rates. Here, we present a detailed osteohistological analysis on Triceratops horridus preserved in a relatively large bonebed from the Lance Formation (eastern Wyoming, USA) as well as additional Triceratops cf. prorsus specimens from Canada. Deciphering the bone microstructure of this iconic dinosaur allows to better understand the growth and development of ceratopsids. The Triceratops limb elements show a distinct pattern of slower growing parallel-fibred and faster growing woven-parallel bone tissue that serves as basis for the definition of histologic ontogenetic stages (HOS). Lower (i.e., younger) HOS correspond to woven-parallel tissue while higher (i.e., older) HOS correspond to parallel-fibred tissue. The intraskeletal variation in histology is best explained through the Three-Front Model, indicating significant differences in cortical thickness between different limb bones. The Triceratops primary growth record is poorly expressed, and the few growth marks preserved show irregular spacing inconsistent with expected growth patterns. The HOS scheme provides seven stages that correspond to biological age classes and that show a correlation with body size. Our analysis suggests that the taxonomic ambiguity between Torosaurus and Triceratops cannot be solved based purely on histological data, but requires additional taphonomic, taxonomic and histological analyses. This study expands the current ceratopsian histological database and helps to better understand ceratopsid growth patterns.

Grasshopper Species Composition Differs Between Prairie Dog Colonies and Undisturbed Sites in a Sagebrush Grassland

Grasshoppers are major consumers of plant biomass in grassland and shrubland ecosystems. While often considered generalists, grasshopper species have differing habitat preferences and interactions with other consumers in grasslands. There are conflicting accounts of how prairie dog colonies and differences in vegetation impact grasshopper abundance and composition. We conducted a landscape-scale survey of grasshopper communities, plant communities, and prairie dogs in a grassland/shrubland ecosystem in eastern Wyoming. Over the study landscape, spurthroat grasshoppers (Melanoplinae) were associated with lower sagebrush cover and lower cover of C3 perennial graminoids, bandwing grasshoppers (Oedipodinae) were associated with low-lying areas with a high cover of C4 grasses and a low cover of cheatgrass, and slantface grasshoppers (Gomphocerinae) were associated with low vegetation height. Prairie dogs, presumably because of their effects on vegetation, had different impacts on different groups of grasshoppers. Melanoplinae grasshoppers, the Wyoming toothpick grasshopper (Paropomala wyomingensis Thomas), and grasshoppers with early-season phenology were associated with prairie dog colonies. However, because some species of grasshoppers were positively and others negatively associated with prairie dogs, the net effect of prairie dogs on total grasshopper biomass was neutral. Thus, to determine the role of grasshoppers in prairie ecosystems, it will be important to determine whether there is functional equivalence of grasshopper species in consuming plant biomass and as food for vertebrates.

Stable isotope record of Triceratops from a mass accumulation (Lance Formation, Wyoming, USA) provides insights into Triceratops behaviour and ecology

Our understanding of Late Cretaceous dinosaur ecosystems from North America has considerably improved through stable isotope analyses on fossil bones and teeth. Oxygen and carbon stable isotopic compositions of structurally-bound carbonate in these fossil apatites are commonly used to infer variations of ingested water and food sources, which are in turn related to environmental and climatic conditions. Incremental isotopic records potentially provide insights into seasonality and migratory behaviour. So far, these reconstructions are based on vertebrate remains from spatiotemporally diverse datasets. Here, we present oxygen (δ18O) and carbon (δ13C) isotopic records from a large, spatially and temporally well-constrained, Triceratops bonebed from the Upper Maastrichtian Lance Formation (eastern Wyoming. USA). These isotopic compositions allow to elucidate the palaeoecology of these large herbivores and their ecosystem in detail, as well as their habitat use, diet and possible migration. The δ18O signature from incrementally sampled Triceratops teeth reveal relatively low intra-tooth variation (average 1.3 ‰), comparable to contemporaneous dinosaur species as well as modern herbivorous mammals. Average δ13C values (−5.4 ‰) are somewhat higher than for modern C3 plant grazers, and hint towards complex interactions during carbon uptake by non-avian herbivorous dinosaurs. Calculated δ18O of drinking water (−14.8 ‰) combined with the local sedimentology of fine-grained siliciclastic deposits with high total organic and low carbonate contents strongly suggest a freshwater environment. Additionally, the combined average δ18O and δ13C Triceratops isotope signatures indicate a living environment intermediate between inland forests and coastal floodplains, expanding on earlier theories of ornithischian niche partitioning. Our robust dataset provides meaningful tests of habitat and palaeoecological hypotheses for Triceratops, and highlights the application of spatiotemporally well-constrained fossil remains.

Historical forests of the Black Hills, South Dakota, USA, determined using General Land Office surveys

Forests in the western United States generally have increased in tree density since Euro-American settlement, particularly through increases in fire-sensitive species, such as spruces, firs, and junipers. Like most areas, the Black Hills region in western South Dakota and eastern Wyoming was logged for forest products and underwent agricultural conversion before historical forests were documented. To supplement historical reconstructions and accounts, we compared tree composition and densities (diameters ≥12.7 cm at 1.37 m above ground height) from historical General Land Office (GLO) records (years 1878 to 1915) and current Forest Inventory and Analysis (FIA) tree surveys (years 2011 to 2016) in the Black Hills Highlands of South Dakota. For composition, ponderosa pine (Pinus ponderosa P. Lawson & C. Lawson) decreased from 95% to 86% of all trees, with a consequent increase specifically of white spruce (Picea glauca (Moench) Voss) from 1.5% to 6.7% of all trees. Ponderosa pine currently is smaller in mean diameter by 7.4 cm, while white spruce is larger in mean diameter by 2.4 cm than historically. When the 35% of historical survey points without recorded trees were excluded, historical tree densities indicated an overall forested structure of savannas and open woodlands with tree densities ranging from 66 trees ha–1 to 162 trees ha–1. However, historical forests of the Black Hills incorporated dense stands. Tree densities have increased two- to more than four-fold, to 311 trees ha–1 currently. These comparisons provide another source of information, paralleling changes documented in surface fire-dependent pine and oak forests throughout the United States, of transitions in forest composition and structure since Euro-American settlement.

Great Plains Population of the Dwarf Shrew (Sorex nanus), with a Record from Nebraska

A diminutive shrew from Sioux County, Nebraska, is shown to be the first state record for the dwarf shrew, Sorex nanus. This specimen appears to be a member of a dwarf shrew population occurring on the Great Plains in the vicinity of the Black Hills in western South Dakota, eastern Wyoming, and southeastern Montana, but not within the Black Hills proper. Other populations of this species occur in montane environments in the Rocky Mountains, from Montana south through Wyoming, Colorado, New Mexico, Utah, and some isolated mountains in Arizona. The arid grassland habitats inhabited by the Great Plains population are strikingly different from the alpine and subalpine rockslides, coniferous forests, montane streams, and sagebrush communities at elevations generally above 1400 m occupied by other populations. Future researchers should examine genetic and ecological relationships among these populations of the dwarf shrew.

Toward Clarifying the Wyoming Ranges of the Vireo gilvus Complex

The Warbling Vireo (Vireo gilvus), generally recognized as one polytypic species, is widely distributed across North America, but differences in morphology, song, genetics, and ecology suggest the western and eastern populations may represent two species. Understanding their distributions enables tracking of range changes and other factors that might affect the conservation of populations. Therefore, I studied museum specimens, specimen data, and identifications provided by investigators recording songs to help clarify the ranges of the two taxa in Wyoming. Of 18 specimens in the U.S. National Museum collected from 1858 to 1930, I identify 15 as the western species V. swainsoni. These are spread over most of Wyoming, east to Crook and Albany counties. Only three represent the eastern species V. gilvus, two from Greybull, Big Horn Co., and one from Cheyenne. Whether the overlap represents sympatry of breeding populations in eastern Wyoming or overlap in migration remains to be determined.

The value of CO2-Bulk energy storage with wind in transmission-constrained electric power systems

High-voltage direct current (HVDC) transmission infrastructure can transmit electricity from regions with high-quality variable wind and solar resources to those with high electricity demand. In these situations, bulk energy storage (BES) could beneficially increase the utilization of HVDC transmission capacity. Here, we investigate that benefit for an emerging BES approach that uses geologically stored CO2 and sedimentary basin geothermal resources to time-shift variable electricity production. For a realistic case study of a 1 GW wind farm in Eastern Wyoming selling electricity to Los Angeles, California (U.S.A.), our results suggest that a generic CO2-BES design can increase the utilization of the HVDC transmission capacity, thereby increasing total revenue across combinations of electricity prices, wind conditions, and geothermal heat depletion. The CO2-BES facility could extract geothermal heat, dispatch geothermally generated electricity, and time-shift wind-generated electricity. With CO2-BES, total revenue always increases and the optimal HVDC transmission capacity increases in some combinations. To be profitable, the facility needs a modest $7.78/tCO2 to $10.20/tCO2, because its cost exceeds the increase in revenue. This last result highlights the need for further research to understand how to design a CO2-BES facility that is tailored to the geologic setting and its intended role in the energy system.

Anthropogenic and Natural Disturbance Differentially Affect Sagebrush Bird Habitat Use

ABSTRACTNorth American sagebrush (Artemisia spp.)‐obligate birds are experiencing steep population declines due in part to increased disturbance, mainly human‐caused, across their range. At the eastern edge of the sagebrush steppe, this issue may potentially be exacerbated because of natural disturbance by black‐tailed prairie dogs (Cynomys ludovicianus). Our goal was to compare local and landscape models of habitat use by greater sage‐grouse (Centrocercus urophasianus), Brewer's sparrow (Spizella breweri), and sage thrasher (Oreoscoptes montanus) with models including effects of natural (i.e., prairie dog) and anthropogenic disturbance. We used a combination of field data collection, and state and national datasets for the Thunder Basin National Grassland, eastern Wyoming, USA, to understand the factors that influence lek attendance by sage‐grouse and habitat use by 2 passerines in this system. For all 3 species, models including big sagebrush (Artemisia tridentata) cover at local and landscape scales were the most competitive among univariate models, supporting the paradigm that sagebrush is key for these species. Models including anthropogenic disturbance (well density, road density) explained more variation than models of prairie dog disturbance alone for 2 of the 3 species, but long‐term disturbance by prairie dogs did reduce abundance of Brewer's sparrows. Although long‐term prairie dog disturbance has the potential to reduce sagebrush cover for sagebrush‐obligate birds, such events are likely rare because outbreaks of plague (Yersina pestis) and lethal control on borders with private land reduce prairie dog disturbance. Conversely, anthropogenic disturbance is slated to increase in this system, suggesting potentially accelerated declines for sagebrush birds into the future. © 2020 The Wildlife Society.

Over 13,000 elements from a single bonebed help elucidate disarticulation and transport of an Edmontosaurusthanatocoenosis.

Over twenty years of work on the Hanson Ranch (HR) Bonebed in the Lance Formation of eastern Wyoming has yielded over 13,000 individual elements primarily of the hadrosaurid dinosaur Edmontosaurus annectens. The fossil bones are found normally-graded within a fine-grained (claystone to siltstone) bed that varies from one to two meters in thickness. Almost all specimens exhibit exquisite preservation (i.e., little to no abrasion, weathering, and breakage), but they are disarticulated which, combined with our sedimentological observations, suggests that the bones were remobilized and buried after a period of initial decay and disarticulation of Edmontosaurus carcasses. Because of the large number of recovered fossil elements, we have been able to gain deeper insight into Edmontosaurus biostratigraphy including disarticulation and transport of skeletal elements. The most common postcranial elements in the bonebed are pubes, ischia, scapulae, ribs, and limb bones. By contrast, vertebrae, ilia, and chevrons are rare. The most common craniomandibular bones include dentaries, nasals, quadrates, and jugals, whereas the premaxillae, predentaries, and braincase bones are underrepresented. Thus, overall, chondrocranial and axial elements, as well as distal elements of the limbs, are distinctly underrepresented. We hypothesize that following decay and disarticulation, hydraulic winnowing removed the articulated sections (e.g., vertebral columns) and the small, distal-most elements before, or at the same time, the remaining bones were swept up in a subaqueous debris flow that generated the deposit. Comparison of the HR Bonebed with other widely dispersed Upper Cretaceous hadrosaurid-dominated bonebeds reveals many shared attributes, which suggests similar processes at work in the formation of these bonebeds across space and time. This in turn reflects similar behavior by populations of these species around the world resulting in similar modes of death, becoming interred in similar depositional settings.

Survival and Cause-Specific Mortality of Neonate Elk in a Unique Predator Environment in the Black Hills of South Dakota and Wyoming, U.S.A.

We conducted a 2 y study of survival and cause-specific mortality of elk (Cervus elaphus) calves to determine the current status of elk occupying the southwestern region of the Black Hills National Forest, South Dakota and eastern Wyoming. We captured and fit 71 calves ≤10 d of age with expandable radiocollars during summer 2012 (n = 37) and 2013 (n = 34). We used known-fate analysis in Program MARK to estimate summer (15 May–25 Sep.) and annual (12 mo) survival for elk calves. The top model for summer survival was {S1–2wk,3–20wk} indicating mortality during 1–2 wk of age and 3–20 wk of age best explained survival; overall probability of surviving 20 wk was 0.79 (95% CI = 0.68–0.88). For annual (12 mo) survival, model {Sbirthweight} had the lowest AICc value indicating birth weight of elk calves best explained survival as heavier born calves had a higher probability of survival. The overall probability a calf survived to 12 mo of age was 0.75 (95% CI = 0.61–0.84). Cougar (Puma concolor) predation accounted for 81% of mortalities; remaining mortalities were from starvation (6.3%, n = 1) and unknown causes (6.3%, n = 1). Our results document high survival for calves likely due to productive habitats, an ecosystem-specific predator guild, and high alternative prey.

Detection of Preble's Meadow Jumping Mice (Zapus hudsonius preblei) Following Low-Severity Fire

Wildfire activity in the western United States has increased in recent decades and is predicted to continue increasing. As wildfires cause landscape-level habitat modifications, understanding how wildlife respond to these disturbances is crucial. The Preble's meadow jumping mouse (Zapus hudsonius preblei) is listed as federally threatened, but little information is available on population responses to wildfire. We conducted live-capture surveys prior to and following a natural low-severity fire in eastern Wyoming. Although we did not detect jumping mice in the year following wildfire, we documented a breeding population both immediately prior to and 2 years following fire. Captures of other small mammals varied greatly, with >7 times as many captures 2 years postfire compared to other years. Understanding the effects of disturbance on population persistence is vital not only to ensure conservation objectives are met for this threatened species, but also to evaluate impacts of changes in fire activity to ecosystems overall.

Adding realism to the Agglomeration Bonus: How endogenous land returns affect habitat fragmentation

The Agglomeration Bonus has been shown to be a potentially successful policy to reunite fragmented habitat and increase conservation enrollment in laboratory testbed experiments. Yet, one key criticism has been that land prices have been assumed exogenous and fixed in these experiments. In the field, voluntary conservation enrollment by landowners will likely affect the value of the surrounding land. This endogeneity of land values suggests a discrepancy between results from laboratory experiments and true landowner decisions under a given policy. We address this concern by using an experimental design that accounts for the endogenous effect on surrounding land values from habitat conservation based on estimated returns from an actual landscape in eastern Wyoming as a case study. We show that without incorporating endogenous land values, traditional laboratory experiments likely will underestimate the amount of habitat fragmentation resulting from basic conservation policies without Agglomeration Bonuses. We also find that a low-cost Agglomeration Bonus can work to reunite this fragmented habitat, even under endogenous land value conditions. Our research indicates that through voluntary conservation decisions by private landowners, a more cost effective Bonus scheme can create contiguous habitat across privately held land, even when incorporating realistic endogenous land values.

Evaluation of the Wyoming Weather Modification Pilot Project (WWMPP) Using Two Approaches: Traditional Statistics and Ensemble Modeling

AbstractThe Wyoming Weather Modification Pilot Project randomized cloud seeding experiment was a crossover statistical experiment conducted over two mountain ranges in eastern Wyoming and lasted for 6 years (2008–13). The goal of the experiment was to determine if cloud seeding of orographic barriers could increase snowfall and snowpack. The experimental design included triply redundant snow gauges deployed in a target–control configuration, covariate snow gauges to account for precipitation variability, and ground-based seeding with silver iodide (AgI). The outcomes of this experiment are evaluated with the statistical–physical experiment design and with ensemble modeling. The root regression ratio (RRR) applied to 118 experimental units provided insufficient statistical evidence (p value of 0.28) to reject the null hypothesis that there was no effect from ground-based cloud seeding. Ensemble modeling estimates of the impact of ground-based seeding provide an alternate evaluation of the 6-yr experiment. The results of the model ensemble approach with and without seeding estimated a mean enhancement of precipitation of 5%, with an inner-quartile range of 3%–7%. Estimating the impact on annual precipitation over these mountain ranges requires results from another study that indicated that approximately 30% of the annual precipitation results from clouds identified as seedable within the seeding experiment. Thus the seeding impact is on the order of 1.5% of the annual precipitation, compared to 1% for the statistical–physical experiment, which was not sufficient to reject the null hypothesis. These results provide an estimate of the impact of ground-based cloud seeding in the Sierra Madre and Medicine Bow Mountains in Wyoming that accounts for uncertainties in both initial conditions and model physics.

Goshen Points in the Northwestern Plains: New Evidence from Montana

ABSTRACTGoshen projectile points are commonly associated with the regions of eastern Wyoming, western South Dakota, and southeastern Montana; however, they have also been identified as isolates and in a few sites in northern Colorado and the Canadian Prairie Provinces. This paper shares an occurrence of an atypical Goshen point made on a flake from southwestern Montana.

Assemblage-level structure in Morrison Formation dinosaurs, Western Interior, USA

­The Upper Jurassic Morrison Formation is both geographically extensive and well-sampled, making it an ideal candidate for biogeographic analysis at both coarse and ‑ner scales. Historically, however, this has not translated into a consensus on patterns of ecological structure and connectivity, particularly with regard to the characteristic dinosaur faunas. Here, we use both traditional (genus richness, alpha and beta diversity) and bipartite network-based (biogeographic connectivity, local endemism, and average occurrence) measures to examine patterns of structure on a per-locality basis. Given the broad geographic range of the formation, we subdivide the Morrison Formation into four discrete regions based roughly on latitude and lithology—north (Montana, South Dakota, and northern Wyoming), west (Utah and western Colorado), east (central and eastern Colorado and southern Wyoming), and south (Arizona, New Mexico and Oklahoma). Further investigation revealed many coeval sites (ca. 152 Ma) in the east and west regions. Presence-absence data were also compared using network analysis to determine the presence and content of discrete subassemblages within the larger region-level assemblages. Based on our results, we favor reconstructions of the Morrison Formation as a ‘mosaic’ type environment over most of its depositional history, with patches of open environments interspersed with more closed, forested regions. ­is is suggested by relatively low rates of local endemism (patches are consistent in plant and animal structure) and connectivity across the majority of the formation, as well as the recovery of three non-overlapping assemblages dominated by dierent guilds of herbivorous dinosaurs.

Diverse Management Strategies Produce Similar Ecological Outcomes on Ranches in Western Great Plains: Social-Ecological Assessment

Experiments investigating grazing systems have often excluded ranch-scale decision making, which has limited our understanding of the processes and consequences of adaptive management. We conducted interviews and vegetation monitoring on 17 ranches in eastern Colorado and eastern Wyoming to investigate rancher decision-making processes and the associated ecological consequences. Management variables investigated were grazing strategy, grazing intensity, planning style, and operation type. Ecological attributes included the relative abundance of plant functional groups and categories of ground cover. We examined the environmental and management correlates of plant species and functional group composition using nonmetric multidimensional scaling and linear mixed models. After accounting for environmental variation across the study region, species composition did not differ between grazing management strategy and planning style. Operation type was significantly correlated with plant community composition. Integrated cow-calf plus yearling operations had greater annual and less key perennial cool-season grass species cover relative to cow-calf−only operations. Integrated cow-calf plus yearling ranches were able to more rapidly restock following drought compared with cow-calf operations. Differences in types of livestock operations contributed to variability in plant species composition across the landscape that may support diverse native faunal species in these rangeland ecosystems. Three broad themes emerged from the interviews: 1) long-term goals, 2) flexibility, and 3) adaptive learning. Stocking-rate decisions appear to be slow, path-dependent choices that are shaped by broader social, economic, and political dynamics. Ranchers described having greater flexibility in altering grazing strategies than ranch-level, long-term, annual stocking rates. These results reflect the complexity of the social-ecological systems ranchers navigate in their adaptive decision-making processes. Ranch decision-making process diversity within these environments precludes development of a single “best” strategy to manage livestock grazing.

KT Boundary Chromites Determined to be Terrestrial: Cr Isotopic Evidence for Excavation and Ejection of Mafic/Ultramafic Rocks by the KT Boundary Impact

Evidence for a mantle and/or basaltic component in KT boundary distal ejecta is apparently inconsistent with ejection from Chicxulub Crater since it is located on ∼35 km thick continental crust (DePaolo et al., 1983; Montanari et al., 1983; Hildebrand and Boynton, 1988, 1990). Evidence for mafic/ultramafic target rocks was reinforced by discovery of chromites, some with shock planar deformation features (PDF), in impact layer samples from sites in southern Colorado and eastern Wyoming (Bohor et al., 1990). However, until now it was unclear whether the chromites originated with an impactor or with terrestrial target rocks. To this end, high‐precision 54Cr/52Cr isotope ratios were measured on KT boundary chromites along with known terrestrial chromites. We find a terrestrial 54Cr/52Cr ratio in KT boundary chromites from impact layer samples collected at the above sites over the last several years (Fig. 1). Ejected terrestrial chromites suggest the impact sampled terrestrial mafic and/or ultramafic target rocks not known to exist in the Chicxulub target area.

Early Cenozoic drainage reorganization of the United States Western Interior–Gulf of Mexico sediment routing system

Abstract Continental-scale drainages host the world’s largest rivers and offshore sediment accumulations, many of which contain significant petroleum reserves. Rate of sediment supply in these settings may be a signal of external controls (e.g., tectonics) on landscape evolution, yet deciphering these controls remains a major challenge in interpreting the ancient stratigraphic record. Integration of new and published detrital zircon U-Pb ages from the United States Rocky Mountain region and Gulf of Mexico (GOM) sedimentary basin demonstrates profound changes in the U.S. continental drainage divide that controlled the rate of sediment delivery to the northern GOM during Paleocene–Eocene time. Sedimentation rate increased dramatically during deposition of the lower Wilcox Group, reaching approximately three times the Cenozoic average, accompanied by pronounced shoreline regression and delivery of a large volume of sand to the basin floor. We hypothesize that this increase in sediment delivery to the GOM resulted from drainage capture of a significant portion of the Sevier-Laramide structural province (∼900,000 km2) that included the headwaters of the California and Idaho Rivers. Capture of the California River drainage may have occurred in the vicinity of the Hanna Basin of eastern Wyoming that previously emptied northward into a shallow seaway, but was subsequently diverted southward to the Rockdale delta, which accumulated within the Houston embayment during the time of deposition of the lower Wilcox Group. Detrital zircon U-Pb ages from Wilcox samples within the Rockdale delta show a remarkable similarity with contemporaneous Laramide synorogenic units, including enrichment in detritus derived from the Cordilleran arc and basement terranes of western North America relative to older and younger units in the Houston embayment. A subsequent order of magnitude decline in sedimentation rate to the GOM can be partly attributed to well-documented drainage closure (∼800,000 km2) that accompanied lake formation in interior Laramide basins (ca. 53–51.8 Ma). Our results demonstrate that tectonically induced drainage migration in the high-relief segments of continental-scale drainages can have a pronounced effect on the rate of sediment transfer to continental margins.

Heat flow and destabilized cratons: a comparative study of the North China, Siberian, and Wyoming cratons

ABSTRACTHeat has a close relation to cratonic stability. We reconstructed the thermal history of the eastern North China, Wyoming, and Siberian cratons by using xenolith and thermal indicator data, and then the thermal lithospheric thickness in geological time was calculated by combining the crustal structural and thermal parameters. Researches from three cratons demonstrate that heat-flow values of the eastern North China Craton (eNCC) and Wyoming Craton (WC) have increased exceeding 20 mW/m2 in the Meso-Cenozoic and the thermal lithospheric thickness has decreased by >100 km because of the cratonic destabilization. However, the thermal history variation of the Siberian Craton (SC) is small, ~5 mW/m2, the lithosphere has remained stable and it has experienced relatively little lithospheric thinning <100 km. The differences of the cratonic thermal regime are related to the breakup of the Pangea supercontinent. Destabilized cratons on active continental margin are directly affected by plate subduction. Stable cratons in the interior of the supercontinents are directly affected by the upwelling of superplumes and that may result in lithospheric thinning. Differences in the thermal evolutions of cratons provide new geothermal evidence for the different thermal regimes associated with the breakup of the Pangea supercontinent.