Wind River Mountains Research Articles

Structural analysis of the Casper Mountain fault zone and area, Wyoming surrounding area, Wyoming: Implications for Laramide kinematics and structural inheritance across the Wyoming Province

Casper Mountain is an E–W trending anticlinal structure that is bound on the north by the oblique-slip Casper Mountain fault. The fault is postulated to reflect preexisting Precambrian structure/fabrics that were reactivated and/or guided deformation during the Laramide orogeny. A structural analysis of the fault zone and surrounding area was conducted to confirm this hypothesis, and to garner insight into both Precambrian origins and Laramide kinematics. Surface and subsurface data for structural analysis was collected and synthesized from numerous published sources along the proposed deformation corridor that roughly coincides with the Oregon Trail structural belt of central Wyoming. The Casper Mountain fault zone is characterized by an E–W rectilinear zone of en échelon, steeply inclined faults. The Casper Mountain fault strikes E–W with smaller faults in the zone striking N65°E. Folds trend to the WNW and are left-stepping. Foliations in Precambrian rocks of Casper Mountain are oriented subparallel to the Casper Mountain fault. The North Granite Mountains fault zone is located due west of Casper Mountain and is similarly oriented E–W with associated faults striking NE, NW/SE, and ENE/WSW, off the dominant master fault. Curvilinear, left-stepping, en échelon folds trend to the northwest and are truncated on the south by the North Granite Mountains fault. Faults in basement rocks of the Popo Agie Primitive Area of the central Wind River Mountains are characterized by moderate to high-angle faults striking E–W, NNW, and NE that coincide with mapped surface lineaments and fabric data. Fabric data suggest that Laramide deformation along the Casper Mountain fault was guided by Precambrian anisotropies. Surface and subsurface mapping of the fault zone and the deformation corridor to the west indicate that the Casper Mountain and North Granite Mountains faults are part of a basement-rooted system (wrench fault) that likely extends westward into the Popo Agie Primitive Area. Here, the steeply inclined (75–90°) proposed master fault is exposed within a WNW-striking corridor of faults that sinistrally offset steeply dipping, NE-striking Proterozoic diabase dikes. The dikes likely intruded older faults that are antithetic to the WNW-striking faults. Other faults strike to the NNW and have shallower dips of 45–65°. These three directions of anisotropy (WNW, NE, and NNW) are proposed to have formed from SW–NE-directed subduction along a long-lived, Neoarchean, active continental margin.

Source regions contributing to excess reactive nitrogen deposition in the Greater Yellowstone Area (GYA) of the United States

Abstract. Research has shown that excess reactive nitrogen (Nr) deposition in the Greater Yellowstone Area (GYA) of the United States has passed critical load (CL) thresholds and is adversely affecting sensitive ecosystems in this area. To better understand the sources causing excess Nr deposition, the Comprehensive Air Quality Model with Extensions (CAMx), using Western Air Quality Study (WAQS) emission and meteorology inputs, was used to simulate Nr deposition in the GYA. CAMx's Particulate Source Apportionment Technology (PSAT) was employed to estimate contributions from agriculture (AG), oil and gas (OG), fire (Fire), and other (Other) source sectors from 27 regions, including the model boundary conditions (BCs) to the simulated Nr for 2011. The BCs were outside the conterminous United States and thought to represent international anthropogenic and natural contributions. Emissions from the AG and Other source sectors are predominantly from reduced N and oxidized N compounds, respectively. The model evaluation revealed a systematic underestimation in ammonia (NH3) concentrations by 65 % and overestimation in nitric acid concentrations by 108 %. The measured inorganic N wet deposition at National Trends Network sites in the GYA was overestimated by 31 %–49 %, due at least partially to an overestimation of precipitation. These uncertainties appear to result in an overestimation of distant source regions including California and BCs and an underestimation of closer agricultural source regions including the Snake River valley. Due to these large uncertainties, the relative contributions from the modeled sources and their general patterns are the most reliable results. Source apportionment results showed that the AG sector was the single largest contributor to the GYA total Nr deposition, contributing 34 % on an annual basis. A total of 74 % of the AG contributions originated from the Idaho Snake River valley, with Wyoming, California, and northern Utah contributing another 7 %, 5 %, and 4 %, respectively. Contributions from the OG sector were small at about 1 % over the GYA, except in the southern Wind River Mountain Range during winter where they accounted for more than 10 %, with 46 % of these contributions coming from OG activities in Wyoming. Wild and prescribed fires contributed 18 % of the total Nr deposition, with fires within the GYA having the highest impact. The Other source category was the largest winter contributor (44 %) with high contributions from California, Wyoming, and northern Utah.

Revised Quaternary glacial succession and post-LGM recession, southern Wind River Range, Wyoming, USA

We present here a more complete cosmogenic chronology of Pleistocene glacial deposits for the Wind River Range, Wyoming, USA. Fifty-one new and thirty-nine re-calculated 10Be and 26Al exposure ages from Sinks and North Fork canyons, Stough Basin, Cirque of the Towers and the Temple Lake valley allow us to more tightly constrain the timing and sequence of glacial alloformations in the southern portion of the range.Moraines, diamicts and bedrock exposures here have previously been correlated with as many as five Pleistocene and four Holocene glacial events. Exposure ages from Pleistocene alloformations associated with trunk glaciers in Sinks Canyon and North Fork Canyon generally confirm earlier age estimates. Cosmogenic radionuclide (CRN, 10Be and 26Al) ages from moraines and striated bedrock surfaces previously mapped as Pinedale correspond to MIS2, while boulder exposure ages from moraines mapped as Bull Lake correspond generally to MIS5-MIS6. Geomorphic data from a moraine previously mapped as Younger pre-Sacagawea Ridge appears to correspond most closely to the Sacagawea Ridge glacial episode (MIS-16), but the uncertainty of a single 10Be exposure age suggests the unit could be as young as MIS-10 or as old as MIS-18. Boulders from a diamict on Table Mountain previously reported as Older pre-Sacagawea Ridge yield two 10Be exposure ages that suggest the presence of Early Pleistocene glacial activity here possibly older than 1–2 Ma (>MIS-30).Bedrock exposure ages within Sinks Canyon suggest the Pinedale valley glacier had retreated from the floor of Sinks Canyon to above PopoAgie Falls by ca. 15.3 ka. Cirque glaciers in Stough Basin appear to have retreated behind their riegels by ca. 16 ka, which suggests the cirque glaciers were decoupling across their riegels from the valley glaciers below at this time, prior to their readvance to form Lateglacial moraines.New 10Be boulder exposure ages from moraines previously correlated to the Temple Lake and Alice Lake allostratigraphic units in the cirques of Stough Basin and Cirque of the Towers show general equivalence to the stadial event just prior to the onset of the Bølling interstadial (17.5–14.7 ka) and to the Intra-Allerød Cold Period-Younger Dryas stadial phase (13.9–11.7 ka), respectively. From this evidence, the Temple Lake Alloformation of the Wind River Mountains now should correspond to the INTIMATE GS-2.1a (Oldest Dryas) stadial event while the Alice Lake Alloformation should correspond to the INTIMATE GS-2 stadial (IACP-Younger Dryas). Thus, we consider that evidence no longer exists for early-to mid-Holocene glacial events in the southern Wind River Range.

Barrier Wind Formation in the Upper Green River Basin of Sublette County, Wyoming, and Its Relationship to Elevated Ozone Distributions in Winter

AbstractMeteorological factors affect the concentrations and distributions of pollutants during episodes of degraded air quality. Over the last 10 years, the upper Green River basin (UGRB) of Sublette County, Wyoming, has experienced numerous wintertime ozone episodes stimulated by emissions from oil and natural gas development operations, resulting in the region being determined to be in marginal nonattainment of the National Ambient Air Quality Standards. Examination of surface wind field patterns in the UGRB using observations from a network of surface monitoring stations for 2011 and 2012, with an emphasis on ozone-episode days, confirms that increased ozone concentrations are most frequently measured on days on which winds are light and variable. Dispersion and dilution of ozone and its precursor pollutants on these days is therefore inefficient, and so these episodes invariably occur within and close by the gas fields. On days that instead experience afternoon southeasterly winds, episodes can often be observed at locations on the northwestern perimeter of the basin remote from pollutant source regions. Simulations using the Weather Research and Forecasting Model, conducted for the case study of 15 February 2011, identify these southeasterlies as barrier winds caused by southwesterly flow at 700 hPa impinging on the Wind River Mountains that flank the UGRB to the northeast. Characterization of the barrier wind and the overall airflow patterns facilitates more accurate future forecasting of the time-dependent geographical distribution of increased concentrations of ozone and other pollutants in the region.

A note on the trophic structure of alpine streams in the Wind River Mountains, Wyoming, USA

A note on the trophic structure of alpine streams in the Wind River Mountains, Wyoming, USA

Soil transport on a moraine foreslope

Recent work has demonstrated that glacial moraine erosion and soil mixing occur at rates high enough to influence chemical weathering processes and the determination of depositional ages. In this study, we complement previous moraine crest studies by investigating a 1.8-m-deep soil profile from the foreslope of a terminal, Bull Lake-age moraine (~ 140 ky) in the Wind River Mountains (Wyoming, USA). Techniques applied include measurement of the grain size distribution, major element concentrations, and cosmogenic 10Be concentrations in quartz. Results suggest a 30-cm-thick moving soil layer overriding a previously eroded, mixed soil layer. Mass balance calculation based on the cosmogenic nuclide concentration in the different soil layers of the slope profile as well as a previously investigated crest profile suggests that one-third of the 30-cm-thick layer is coming from the crest area and two-thirds of the material from the eroding moraine slope. Foreslope denudation rates are 0.015 to 0.018 mm y − 1 based on the 10Be concentration of the lowermost samples and assuming a depositional age of 140 ky. The 10Be concentration of the foreslope surface sample suggests a denudation rate of 0.020 mm y − 1 for the sediment source area. Analysis of major elements in a limited number of surface samples reveals that the immobile element Ti is enriched downslope, whereas more mobile elements are generally depleted downslope. The internal structure of a moraine foreslope is complex, and care must be taken in calculating moraine age and denudation rate with depth profiles.

The coupled release of REE and Pb to the soil labile pool with time by weathering of accessory phases, Wind River Mountains, WY

Rare earth element (REE) distributions and Pb isotope compositions were explored in soils varying in age from ca. 0.4 to ⩾300 ka, developed on moraines in the Wind River Mountains, Wyoming. Soil extracts (0.6 M HCl) were used to examine the soil labile pool while the major element distribution in soil profiles was used to determine the extent of weathering at different soil depths. The results show that the chondrite-normalized REE patterns of the deepest bulk soil within each profile reflects the composition of the moraine till, except for the oldest soil. Up to ca. 12 ka, the soil extract fraction is enriched in light REE, indicating early release of light REE to the soil labile pool while that of the two oldest soils are relatively enriched in heavy REE. In the soil extracts the La/Sm ratio normalized to the deepest soil (La D/Sm D) decreases systematically with soil age. Similarly, the Eu-anomaly in the deepest soil from each profile (Eu D/Eu D*) decreases slightly with soil age in the three young soils; however, Eu D/Eu D* increases with soil age in the older soils. The systematic trends of these two ratios indicate the depletion of light REE in young soils and the enrichment of Eu and heavy REE in the older soils. Based on the Pb isotope ratios, the relative contribution of Pb to the soil labile pool via mineral weathering of U- or Th-rich phases was assessed for the different stages of weathering. The whole-soil profile 208Pb/ 204Pb ratio was found to decrease with soil age and with La D/Sm D, whereas it increased with the Eu D/Eu D* ratio. In each horizon, Pb isotope ratios ( 206Pb/ 204Pb, 207Pb/ 204Pb, and 208Pb/ 204Pb) ratio generally decrease with soil age. In order to overcome possible effects from parent material heterogeneity, the amount of radiogenic Pb as compared to the whole-soil composition was calculated and this was found to decrease systematically with soil age.

Combining cosmogenic nuclides and major elements from moraine soil profiles to improve weathering rate estimates

Previous studies of chemical weathering rates for soil developed on glacial moraines generally assumed little or no physical erosion of the soil surface. In this study, we investigate the influence of physical erosion on soil profile weathering rate calculations. The calculation of chemical weathering rates is based on the assumption that soil profiles represent the integrated amount of weathering since the time of moraine deposition. The weathering rate of a surface subjected to denudation is the sum of the weathering loss from the existing soil profile added to the weathering loss in the material removed by denudation, divided by the deposition age. In this study, the amount of weathered material removed since moraine deposition is calculated using the denudation rate estimated from cosmogenic nuclide data and the deposition age of the moraine. Weathering rates accounting for denudation since moraine deposition are compared to weathering rates based on the assumption of no physical erosion and on the assumption of steady-state denudation for the Type Pinedale moraine (∼ 21 ka) and the Bull Lake-age moraine (∼ 140 ka) in the Fremont Lake Area (Wind River Mountains, Wyoming, USA). The total weathering rates accounting for denudation are 8.15 ± 1.05 g (oxide) m − 2 y − 1 for the Type Pinedale moraine and 4.78 ± 0.89 g (oxide) m − 2 y − 1 for the Bull Lake-age moraine, which are ∼ 2 to 4 times higher, respectively, than weathering rates based on the assumption of no physical erosion. The weathering rates based on denudation since moraine deposition are comparable or smaller than weathering rates assuming steady-state denudation. We find the assumption of steady-state denudation is not valid in depositional landscapes with young deposition ages or slow denudation rates. The decrease in weathering rates over time between the Type Pinedale and Bull Lake-age soils that is observed in the case of no physical erosion is decreased when the influence of denudation on the total weathering rates is taken into account. Fresh unweathered material with high reactive mineral surface area is continuously provided to the surface layer by denudation diminishing the effect of decreasing weathering rate over time.

Wood-mediated geomorphic effects of a jökulhlaup in the Wind River Mountains, Wyoming

A jokulhlaup burst from the head of Grasshopper Glacier in Wyoming's Wind River Mountains during early September 2003. Five reaches with distinct sedimentation patterns were delineated along the Dinwoody Creek drainage. This paper focuses on a portion of the jokulhlaup route where erosion of the forested banks created 16 large logjams spaced at longitudinal intervals of tens to hundreds of meters. Aggradation within the main channel upstream from each logjam created local sediment wedges, and the jams facilitated overbank deposition during the jokulhlaup. Field surveys during 2004 and 2006 documented logjam characteristics and associated erosional and depositional features, as well as initial modification of the logjams and flood deposits within the normal seasonal high-flow channel. Overbank deposits have not been altered by flows occurring since 2003. Field measurements supported three hypotheses that (i) logjams present along the forested portions of the jokulhlaup route are larger and more closely spaced than those along adjacent, otherwise comparable stream channels that have not recently experienced a jokulhlaup; (ii) logjams are not randomly located along the jokulhlaup route, but instead reflect specific conditions of channel and valley geometry and flood hydraulics; and (iii) the presence of logjams facilitated significant erosional and depositional effects. This paper documents a sequence of events in which outburst floodwaters enhance bank erosion and recruitment of wood into the channel, and thus the formation of large logjams. These logjams sufficiently deflect flow to create substantial overbank deposition in areas of the valley bottom not commonly accessed by normal snowmelt peak discharges, and through this process promote valley-bottom aggradation and sediment storage. Changes in the occurrence of glacier outburst floods thus have the potential to alter the rate and magnitude of valley-bottom dynamics in these environments, which is particularly relevant given predictions of worldwide global warming and glacial retreat. Processes observed at this field site likely occur in other forested catchments with headwater glaciers.

Identifying Twentieth Century Dude Ranches in the Teton Valley Region

The states of Wyoming, Montana, and to a lesser extent Colorado are commonly understood as the industrial heartland of U.S. dude ranching in the twentieth and twenty-first centuries (Borne 1983). Though there were earlier small scale efforts to host easterners on ranches in the West from the 1850s onwards, dude ranching is commonly understood to have begun in 1879 in Medora, North Dakota by the Eaton Brothers (Borne 1983, Rothman 1998). Dude ranching--when outsiders pay to stay on a ranch ­ usually demonstrates most/if not all of the following six characteristics: 1). it embraces of the West's nineteenth century agricultural heritage; 2). it celebrates wild, preserved landscapes; 3). it provides an economic vehicle for ranchers to maintain their cultural heritage, and/or investors and managers to have a piece of the American West; 4). it demonstrates a distinct dude ranch aesthetic (architecture, clothing, food, music, stories, education and landscape); 5). it includes horse­related activities; and 6). it provides a safe and contained regional experience transforming the traveler from "mere' tourist status to that of a liminal space in­between outsider and insider. Since the late nineteenth century Wyoming has developed five centers of dude ranch activity located primarily near mountain ranges, within or close to public lands (National Park Service (NPS) or Forest Service (FS)): 1) Medicine Bow Mountain Range in southeastern Wyoming; 2) Big Horn Mountain Range (eastern and western slopes) in north-central Wyoming; 3) eastern gate region of Yellowstone National Park, northwest Wyoming; 4) Wind River Mountain Range (eastern and western slopes), northwestern Wyoming; and 5) Teton Mountain Range in northwestern Wyoming. My work seeks to establish the extent of dude ranching in Teton Valley.

Tectonic and paleogeographic implications of late Laramide geologic history in the northeastern corner of Wyoming's Hanna Basin

The Hanna Basin of south-central Wyoming has been considered anomalous among other Laramide depocenters of the Rocky Mountain region because of its combination of small size and great thickness of synorogenic strata. Most prior interpretations of the Hanna Basin have assumed a history of subsidence and sedimentary infilling discrete from surrounding basins. In contrast, we summarize new geological and paleontological information from the northeastern corner of the modern Hanna Basin suggesting that, prior to late Paleocene time, the Hanna Basin and nearby Carbon, Pass Creek, Laramie, and Shirley Basins were unified and depositionally continuous with the much larger Green River Basin to the west. Only late in the local expression of the Laramide orogeny (late Paleocene and early Eocene) did this “greater Green River Basin” become subdivided through completion of development of intrabasinal, basement-involved thrust faulting and associated anticlines. We view the present Hanna Basin as only a small, structurally defined remnant of an enormous, ponded basin that extended eastward during most of Paleocene time from the Wyoming–Idaho–Utah thrust belt to the newly uplifted Laramie Mountains. That basin was bounded on the south by the Uinta Mountains and combined Sierra Madre–Medicine Bow Mountains and on the north by the Gros Ventre Range, Wind River Mountains, and Sweetwater arch. When the original configuration of the unified greater Green River Basin is taken into account and combined with palinspastic removal of late Laramide faulting that defines its various margins, this basin at least rivaled dimensions of the Powder River Basin of Wyoming and Montana; the greater Green River Basin greatly exceeded the volume of sedimentary accumulation within the Powder River Basin. Rates of Paleogene basinal erosion associated with structural subdivision of eastern components of the greater Green River Basin were prodigious. Within context of a measured section, we describe and interpret the lithologic nature, depositional settings, source areas, previous geographic extent, and deformational history of the Hanna Formation as seen today in the eastern Hanna Basin. Temporal control is linked to summaries of included mammalian fossils, freshwater molluscs, leaf macrofloras, and palynomorphic assemblages. Unexpectedly early occurrences of certain palynomorphic assemblages suggest that paleoecological controls on distributions of pollen-forming plants during Paleocene time were particularly important. Deformation of strata in the eastern Hanna Basin chiefly reflects a complex interaction between a major, south-directed, low-angle, basement-involved thrust system and the thick and generally incompetent, mudstonerich Hanna Formation. We argue that out-of-the-basin faults, the result of spatial crowding in the basin, are kinematically linked to the basement-involved thrust system through a triangle-zone geometry. This trishear-style of deformation involved north-directed, out-of-the-basin, decollement-type thrusts and a synchronous south-directed, basement-involved, blind-thrust system. Map relationships along the northern margin of the basin indicate that many of the out-of-the-basin faults emplaced younger rocks onto older rocks, seemingly an anomalous structural relationship for contractional deformation. One explanation of these relationships involves synchrony of the: (1) cutting down section of north-directed, out-of-the-basin fault planes; with (2) upturning of basinal strata related to south-directed, basement-involved faulting along the northern margin of the Hanna Basin. Finally, the inherited Precambrian structural grain of the Archean Wyoming province was instrumental in controlling the orientation of many of the Laramide-age, basement-involved structural features. Tectonic heredity, rather than changing stress orientations, probably accounts for the variation in orientations of structural features associated with the northern and eastern margins of the Hanna Basin.

Spatial patterns of recreation impact on experimental campsites

Management of camping impacts in protected areas worldwide is limited by inadequate understanding of spatial patterns of impact and attention to spatial management strategies. Spatial patterns of campsite impact were studied in two subalpine plant communities in the Wind River Mountains, Wyoming, USA (a forest and a meadow). Response to chronic disturbance and recovery from acute disturbance were both assessed. Previously undisturbed sites were camped on at intensities of one and four nights/year, for either one or three consecutive years. Recovery was followed for three years on sites camped on for one year. Percent bare ground, assessed in 49 contiguous 1 m 2 quadrats, increased with increasing use frequency, particularly on the forest sites. Magnitude of impact varied spatially within campsites, with impact decreasing as distance from the center of the campsite increased. On the more fragile forest sites, this radial impact pattern developed rapidly and remained after three years of recovery. Concentration of camping activities around a centrally located small cooking stove was the apparent cause of this pattern. Maximum variation in magnitude of impact occurred at intermediate levels of campsite use and disturbance. The magnitude, variability and spatial pattern of impact varied with the spatial scale of analysis. Generally, results of these controlled experiments are consistent with earlier studies of campsites and validate the management implications derived from those studies. Even where campers use low-impact techniques, low levels of camping use can cause substantial impact and it is important to concentrate use. On resistant sites, however, it is possible that low levels of use can be sustained with minimal resultant impact.

Impacts of camping on vegetation: response and recovery following acute and chronic disturbance.

Experiments with controlled levels of recreational camping were conducted on previously undisturbed sites in two different plant communities in the subalpine zone of the Wind River Mountains, Wyoming, USA. The plant communities were coniferous forest with understory dominated by the low shrub Vaccinium scoparium and a riparian meadow of intermixed grasses and forbs, of which Deschampsia cespitosa was most abundant. Sites were camped on at intensities of either one or four nights per year, for either one (acute disturbance) or three consecutive years (chronic disturbance). Recovery was followed for three years on sites camped on for one year and for one year on sites camped on for three years. Reductions in vegetation cover and vegetation height were much more pronounced on sites in the forest than on sites in the meadow. In both plant communities, increases in vegetation impact were not proportional to increases in either years of camping or nights per year of camping. Close to the center of campsites, near-maximum levels of impact occurred after the first year of camping on forested sites and after the second year on meadow sites. Meadow sites recovered completely within a year, at the camping intensities employed in the experiments. Forest sites, even those camped on for just one night, did not recover completely within three years. Differences between acute and chronic disturbance were not pronounced.

Lake basin response to tectonic drainage diversion: Eocene Green River Formation, Wyoming

A previously unidentified major sequence boundary within the Eocene Green River Formation separates fluctuating profundal facies of the Tipton Shale Member from evaporative facies of the Wilkins Peak Member. During deposition of the Tipton Shale Member, rivers entered the basin from the north, across the subdued Wind River Mountains, and deposited the southward prograding deltaic complex of the Farson Sandstone Member. Boulderrich alluvial fan deposits overlie the Farson Sandstone adjacent to the Continental Fault, and correlate basinward to hypersaline lacustrine deposits of the Wilkins Peak Member. The alluvial fan deposits record a period of reverse motion on the Continental Fault and uplift of the southeastern Wind River Range, which diverted drainage away from the greater Green River Basin. This decreased inflow caused Lake Gosiute to shrink, exposing its bed to desiccation and erosion, and contributed to hydrologically-closed conditions and periodic evaporite deposition thereafter. This study is one of the first to demonstrate a direct relationship between movement along a specific basin-bounding structure, and a change in the overall style of lacustrine sedimentation. The identification of similar relationships elsewhere may challenge conventional interpretations of climate as the dominant factor influencing the character of lake deposits, and provide an important, but previously unexploited, approach to interpreting continental deformation and regional drainage organization.

Trampling Disturbance of High-Elevation Vegetation, Wind River Mountains, Wyoming, U.S.A.

Trampling experiments were conducted in five high-elevation plant communities in the Wind River Mountains. In one experiment (simulating infrequent acute disturbance), plots were trampled once and recovery was followed for 3 yr. In another experiment (simulating chronic disturbance), plots were trampled for three successive years and recovery was followed for one year. Yearly trampling intensities were 0 to 500 passes (up to 800 and 1000 in two communities). Structural responses (reductions in cover and height) were more pronounced than compositional responses. Low levels of trampling caused substantial reductions in cover and height, but rates of change decreased as trampling intensity increased. The ability of different plant communities to tolerate trampling disturbance varied by at least an order of magnitude, suggesting that impacts can be reduced by directing use to more tolerant plant communities. Moreover, tolerance can be gauged on the basis of readily observable plant characteristics. Plant communities with groundcover dominated by chamaephytes (primarily low, woody shrubs) or by erect, caulescent forbs were less resistant than those with groundcovers dominated by turf-forming or caespitose graminoids, or by forbs with matted, caespitose or rosette growth forms. Plant communities with woody, chamaephytic groundcover were less resilient than other plant communities.

Trampling Disturbance of High-Elevation Vegetation, Wind River Mountains, Wyoming, U.S.A.

Trampling experiments were conducted in five high-elevation plant communities in the Wind River Mountains. In one experiment (simulating infrequent acute disturbance), plots were trampled once and recovery was followed for 3 yr. In another experiment (simulating chronic disturbance), plots were trampled for three successive years and recovery was followed for one year. Yearly trampling intensities were 0 to 500 passes (up to 800 and 1000 in two communities). Structural responses (reductions in cover and height) were more pronounced than compositional responses. Low levels of trampling caused substantial reductions in cover and height, but rates of change decreased as trampling intensity increased. The ability of different plant communities to tolerate trampling disturbance varied by at least an order of magnitude, suggesting that impacts can be reduced by directing use to more tolerant plant communities. Moreover, tolerance can be gauged on the basis of readily observable plant characteristics. Plant communities with groundcover dominated by chamaephytes (primarily low, woody shrubs) or by erect, caulescent forbs were less resistant than those with groundcovers dominated by turf-forming or caespitose graminoids, or by forbs with matted, caespitose or rosette growth forms. Plant communities with woody, chamaephytic groundcover were less resilient than other plant communities.

An integrated 3-D reservoir characterization at Riverton Dome Field, Wyoming

Riverton Dome is a prime example of a mature Rocky Mountain oil and gas field that has benefited from new drilling based on a recent 3-D seismic survey. The field was discovered in 1949 through surface geology and seismic (Hinton, 1957; Borgerding, 1989) and was acquired by Snyder Oil Corporation (SOCO) in 1993. In 1998 SOCO acquired a 38-mi2 3-D seismic survey over Riverton Dome Field with funding from Belco Energy, Tom Brown Incorporated, and the U.S. Department of Energy to determine reservoir distribution, structural configuration, and compartmentalization. The survey was acquired with fractionated bins to image the steep dip on the flanks of the structure as a cost-effective alternative to using a smaller bin size. It was processed independently by two vendors. Results were very similar, providing confidence in the high quality of the data. Geologic maps were integrated with various seismic attributes to define reservoir models and to project reservoir trends beyond the current field. As a result of the Riverton Dome 3-D study, three successful wells were drilled in the Muddy Sandstone and additional opportunities in multiple Upper Cretaceous reservoirs and the Tensleep Sandstone were identified. Riverton Dome, a structurally controlled, mature oil and gas field is on the Wind River Indian Reservation, approximately six miles southeast of the town of Riverton in the southwestern portion of Wyoming's Wind River Basin (Figure 1). A series of thrust-faulted anticlines strikes NNW-SSE, parallel to the large Wind River Mountain thrust to the west. These features formed in the Late Cretaceous-Paleocene during the early stages of the Laramide orogeny (Keefer, 1965, 1970; Gries, 1983). Both Riverton Dome Field and Beaver Creek Field to the south are on the crest of a north-plunging asymmetrical anticline bounded on the west by a steeply dipping thrust fault that originates in basement. The …

Chlorine-36 and cesium-137 in ice-core samples from mid-latitude glacial sites in the Northern Hemisphere

Chlorine-36 ( 36Cl) concentrations, 36Cl/Cl ratios, and 36Cl fluxes in ice-core samples collected from the Upper Fremont Glacier (UFG) in the Wind River Mountain Range, Wyoming, United States, and the Nangpai Gosum Glacier (NGG) in the Himalayan Mountains, Nepal, were determined and compared with published results from the Dye-3 ice-core drilling site on the Greenland Ice Sheet. Cesium-137 ( 137Cs) concentrations in the NGG also were determined. The background fluxes for 36Cl for each glacial site were similar: (1.6±0.3) × 10 −2 atoms/cm 2 s for the UFG samples, (0.7±0.1) × 10 −2 atoms/cm 2 s for the NGG samples, and (0.4±0.1) × 10 −2 atoms/cm 2 s for the Dye-3 samples. The 36Cl fluxes in ice that was deposited as snow during peak atmospheric nuclear weapon test (1957–1958) were (33±1) × 10 −2 atoms/cm 2 s for the UFG site, (291±3) × 10 −2 atoms/cm 2 s for the NGG site, and (124±5) × 10 −2 atoms/cm 2 s for the Dye-3 site. A weapon test period 137Cs concentration of 0.79±0.05 Bq/kg in the NGG ice core also was detected in the same section of ice that contained the largest 36Cl concentration.

Temperature profiles and bathymetry of some high mountain lakes.

The ice cover in high mountain lakes breaks up and disappears in about an hour, in part because it has been divided into fragile vertical spindles, which are individual crystals. Contributing to this process are vertical holes in the ice remaining after particles of dust melt downward as they are warmed by the sun. Temperature profiles of lakes in the Wind River Mountains of Wyoming include those of five lakes in the high wilderness. All are isothermal at or near 5 degrees C below a depth of 20 to 30 m, regardless of elevation or lake depth. Large lakes have a deeper mixed layer than do small ones because of longer fetch and thus more effective wind shear.

Sheep Grazing in National Forest Wilderness

The Bridger Wilderness of Bridger-Teton National Forest takes up most of the west slope of the Wind River Mountains in western Wyoming. Within the wilderness boundary lies the tallest of Wyoming's mountains, Gannett Peak; the Green River, a major tributary of the Colorado, begins here. But Bridger Wilderness is more than big mountains and important headwaters; it is a battleground for old adversaries who have struggled to direct the use of America's wildlands, and who compete to influence the federal agency charged with administering this piece of public land (Figure 1). The livestock industry, environmentalists, and the US Forest Service are the players in this battle. And though this particular battleground is shaped by uniquely American institutions and history, the contest itself is one that can be found around the globe.