Southwestern Colorado Research Articles

Sublacustrine-fan deposition in the Oligocene Creede Formation, Colorado, U.S.A.

ABSTRACT The Oligocene Creede Formation represents mostly lacustrine deposits of an annular caldera lake, Lake Creede, within the Creede caldera in southwestern Colorado. The preserved record of Lake Creede reflects high sedimentation rates in a high-relief lake basin. The deep-water facies in the Creede Formation provide insight into sediment-gravity-flow and subaqueous-fan processes that are applicable to similar lacustrine and marine settings. The deep-water facies are dominated by sediment-gravity-flow and suspension-lamina deposits. Sediment gravity flows were deposited by four major processes: (1) lithic-rich debris flows generated by subaerial slope failure that deposited coarse-grained beds around the basin margin, (2) intraclast-rich debris flows generated by lacustrine-slope failure that deposited massive coarse-grained beds across the basin, (3) debris flows or high-density turbidity currents that deposited massive sandstone and conglomerate with fine-grained, turbidite caps in fan-delta, sublacustrine-fan, and profundal settings, and (4) low-density flows that deposited fine-grained tuffaceous turbidites. The compositional characteristics of the first two types of sediment-gravity-flow deposits suggest specific modes of initiation (e.g., subaqueous and subaerial slope failure); however, the latter two types were probably generated by a variety of mechanisms. The low-density flows were probably generated through flow transformations of the other flows. Exposures of fan-delta and sublacustrine-fan deposits are observed basinward of known or inferred paleovalleys. In the Antlers Park area, coarse-grained sediment-gravity-flow deposits are arranged into a hierarchy of lens-shaped units that are comparable to features observed in modern lacustrine and marine subaqueous fans along active tectonic margins. In the Wason Ranch area, coarse-grained deposits are arranged into broad lens-shaped units that are most consistent with intermediate to shallow-water deposition at the base of a fan delta, similar to the delta-fed ramp model of Heller and Dickinson (1985) or delta-fed apron of Galloway (1998). These exposures and others provide examples of the range of depositional environments in which coarse-grained deposition can occur in lacustrine basins, especially those influenced by high sedimentation rates. These exposures also provide insight into the sequence stratigraphic significance of sublacustrine-fan deposits in lacustrine versus continental-margin marine settings.

Hydrogeothermal Studies in New Mexico and Implications for Ground-Water Resources

Subsurface temperatures and heat-flow data provide valuable information on both regional and local ground-water flow patterns. In the San Juan Basin of northwestern New Mexico and southwestern Colorado, heat-flow data are used to estimate basin-scale horizontal flow and predict hydraulic conductivities much higher than values measured in pumping tests or laboratory studies. Along the central Rio Grande rift in the Albuquerque Basin, hot water at depth may be present at a number of locations; however, the hydrogeology is such that warm springs are absent. In south-central New Mexico, along the southern Rio Grande rift, a number of warm springs result from tectonic activity and erosion exposing aquifers which allow deeply-circulating ground water to come near surface. In the Roswell Basin of southeastern New Mexico, recharge to the Pecos River deep aquifer is predicted from depths of ~1,000–1,500 m; in addition a ground-water divide appears present near the Mescalero ridge. On the Llano Estacado downward ground-water movement from the surface is suggested to depths of ~1,000–1,500 m. Along the Rio Grande at Bosque del Apache and Canutillo, and along the Pecos River at Artesia, there appear to be dual-flow systems with river water moving downward to depths of ~50–100 m, while upward flow of ground water occurs from much greater depths. This implies separation of the ground-water systems at these locales, an important ramification regarding the recharge of different zones. The deep pumping at Canutillo appears to have exceeded the natural recharge, and this may well be the case at other locales where pumping depths are below river recharge. Shallow pumping at Canutillo appears to have caused a much greater recharge from the river than was the prepumping case.

The Ute Indians of Colorado in the Twentieth Century.

This comparative history of the Southern Ute and Mountain Ute peoples demonstrates how two culturally and historically related tribes, living side by side in southwestern Colorado, have taken very different paths in the modern era. Historian Richard K. Young makes a unique contribution to twentieth-century American Indian studies in his exploration of Colorado's two remaining tribes' divergent responses to federal Indian policies and changing economic and social conditions since passage of the Indian Reorganization Act in 1934.This book, which includes a review of the Utes' precontact and nineteenth-century history, is based on primary research in U. S. and tribal documents, interviews with tribal members, and the few available secondary sources. By examining the Ute experience, Young highlights the dilemmas faced by all tribes with respect to economic development, energy and water resources, cultural identity and adaptation, spiritual life, tribal politics, and the struggle for tribal self-determination.

Peaks of Identity in Colorado's San Juan Mountains

The glaciated ranges of southwestern Colorado constituting the San Juan Mountains are culturally significant to residents and visitors. As certain mountains are imbued with meaning, they become "peaks of identity," tangible and towering symbolic landscapes representing a distinctive set of community and cultural ideals. This paper explores the symbols and themes of San Juan peaks of identity, with a focus on the mountain amenity town of Lake City and nearby Uncompahgre Peak. Uncompahgre's icon dominates mountain representations in Lake City and instantly identifies the community; its symbolism embodies aesthetics of form and elevation and the sanctity of hallowed ground. Mountain symbolism in the San Juans is mainly projected through land use and the display of icons and names on signs and government seals. Although the San Juans are sacred to the Utes and Navajos and represent a rich mining heritage, they also symbolize idealized natural scenery, landmarks of home, recreation opportunities, and spiritual renewal. Many San Juan communities identify with mountains in a generic sense, but this article focuses on the traits, variability, and depth of meaning of the mountains that are landscape signatures of community identity.

THE NEARCTIC LEAFHOPPER GENUS AURIDIUS: BIOLOGY, POLYMORPHISM AND NEW SPECIES (RHYNCHOTA: HOMOPTERA: CICADELLIDAE)

AbstractAuridius Oman is revised, and eight new species are described: aurigineus (California), cosmeticus (Montana), melinus (California), safra (Idaho–Oregon), sandaraca (Alberta–Ontario), sulphureus (New Mexico), thapsinus (Arizona–Nevada), and vitellinus (Oregon). Auridius gilvus Hamilton & Ross, 1972 is synonymized with A. auratus (Gillette and Baker 1895). The 12 known species are illustrated and keyed, with notes on host associations, phenology, wing polymorphism, and two new subspecies: A. ordinatus amarillo (southwestern Colorado – New Mexico) and A. ordinatus crocatus (British Columbia – Oregon).

1995 Epizootic of Vesicular Stomatitis (New Jersey Serotype) in the Western United States: an Entomologic Perspective

Entomologic and epizootic data are reviewed concerning the potential for transmission of vesicular stomatitis (VS) virus by insects, including field data from case-positive premises in New Mexico and Colorado during the 1995 outbreak of the New Jersey serotype (VSNJ). As with previous outbreaks of VSNJ in the western United States, the 1995 epizootic illustrated that risk of exposure is seasonal, increasing during warm weather and decreasing with onset of cool weather; virus activity spread from south to north along river valleys of the southwestern and Rocky Mountain states; clinical disease was detected most commonly in horses, but also occurred in cattle and 1 llama; and most infections were subclinical. Overall, 367 case-positive premises were identified during the 1995 outbreak, with foci of virus activity along the Rio Grande River south of Albuquerque, NM, in southwestern Colorado, and along the Colorado River near Grand Junction, CO. The establishment of a 16-km (10-mile) radius zone of restricted animal movement around confirmed positive premises, along with imposition of state and international embargoes, created economic hardship for livestock owners and producers. The importance of defining the role of blood-feeding insects as biological vectors of VSNJ virus relative to risk factors that promote high levels of insect transmission, such as the presence of livestock along western river valleys, blood feeding activity, and frequent transport of animals for recreational purposes, is emphasized as a basis for developing effective disease management.

Importance of mechanical disaggregation in chemical weathering in a cold alpine environment, San Juan Mountains, Colorado

Weathering of welded tuff near the summit of Snowshoe Mountain (3660 m) in southwestern Colorado was studied by analyzing infiltrating waters in the soil and associated solid phases. Infiltrating waters exhibit anomalously high potassium to silica ratios resulting from dissolution of a potassium-rich glass that occurs as a trace phase in the rock. In laboratory experiments using rock from the field site, initial dissolution generated potassium-rich solutions similar to those observed in the field. The anomalous potassium release decreased over time (about 1 month), after which the dominant cation was calcium, with a much lower potassium to silica ratio. The anomalous potassium concentrations observed in the infiltrating soil solutions result from weathering of freshly exposed rock surfaces. Continual mechanical disaggregation of the rock due to segregation freezing exposes fresh glass to weathering and thus maintains the source of potassium for the infiltrating water. The ongoing process of creation of fresh surfaces by physical processes is an important influence on the composition of infiltrating waters in the vadose zone.

RECLAMATION OF WASTE ROCK MATERIAL AT THE SUMMITVILLE MINE SUPERFUND SITE USING ORGANIC MATTER AND TOPSOIL TREATMENTS

The Summitville Mine was a high elevation (3,500 m) open-pit gold mine located in southwestern Colorado. The mine was abandoned in 1992 leaving approximately 200 ha of disturbed area comprised partially of two large waste rock piles. Reclamation of waste rock material is challenging due to extreme climatic conditions in conjunction with a high acid-production potential and low organic matter concentration of the material. In addition, stockpiled topsoil at the site is acidic and may be biologically inactive due to long-term storage, and therefore sufficient plant growth medium may be limited. The purpose of this study was to determine the effect of organic amendments (mushroom compost vs. biosolids) and topsoil (stockpiled vs. nonstockpiled) on aboveground biomass, herbaceous cover, and trace element uptake. An on-site field study was established in 1995 to identify the most effective combination of treatments for successful reclamation of waste rock material. Incorporation of organic matter increased total aboveground production and cover, with mushroom compost being more effective than biosolids, but did not show significant trends relative to trace element uptake. The use of topsoil did not show a significant response relative to aboveground production, cover, and trace element uptake. This study shows that waste rock materialsmore » can be directly revegetated if properly neutralized, fertilized, and amended with organic matter. Additionally, stockpiled topsoil was equivalent in plant growth to non-stockpiled topsoil when neutralized with lime.« less

Erratum: Floodwater Farming, Discontinuous Ephemeral Streams, and Puebloan Abandonment in Southwestern Colorado

Erratum: Floodwater Farming, Discontinuous Ephemeral Streams, and Puebloan Abandonment in Southwestern Colorado

Screening Assessment of Dissolved Zinc from Inactive Mines in a Mountain Watershed

A screening-level assessment of dissolved zinc from inactive and abandoned metal mines in the Cement Creek Basin was performed. The basin is part of the Upper Animas River Basin in the San Juan Mountains of southwestern Colorado. Stream discharge and dissolved zinc concentrations were measured at 49 stations below nonpoint sources including tailings and waste rock, point sources such as adits, and background areas. One measurement was made at a station during three flow events: storm runoff, peak snowmelt runoff, and baseflow. The highest concentrations occurred in the upper part of the basin immediately downstream from nonpoint and point source discharges, especially in the North Fork of Cement Creek. The mean concentration in Cement Creek was highest during baseflow (1350 μg l-1) and lowest during snowmelt (796 μg l-1). Most exceedances of national acute and chronic criteria for brown trout were chronic criteria exceedances in the upper part of the basin. Subareas with the greatest extent of nonpoint source areas in the upper part of the watershed, especially those contributing to Upper Cement Creek and the North Fork of Cement Creek, generally had the highest loadings and unit area loadings. The greatest loadings from all subareas to Cement Creek occurred during snowmelt (219 000 g day-1 and 17 400 kg for the snowmelt season). The highest unit area loadings from all subareas also occurred during snowmelt (190 g ha-1 day-1 and 15 147 g ha-1 for the snowmelt season). Loadings from subareas with extensive nonpoint source areas were always much greater than those from point sources and background areas.

Peak to Peak in Paleoindian Time: Occupation of Southwest Colorado

One-hundred and sixty-six Paleoindian projectile points from suiface contexts in southwest Colorado are documented, analyzed, and evaluated in terms of existing models of Paleoindian adaptation to the Rocky Mountain environment. The southwestern Colorado projectile points—morphologically and in terms of raw material use—appear to share much in common with mountain traditions defined for other Rocky Mountain regions, and there is some basis for pursuing the notion that the roots of this mountain tradition may lie to the west, in the Great Basin.

Floodwater Farming, Discontinuous Ephemeral Streams, and Puebloan Abandonment in Southwestern Colorado

Geoarchaeological study on the southern piedmont of Sleeping Ute Mountain in southwestern Colorado indicates the presence of discontinuous ephemeral streams that were the foci of episodic Puebloan occupation between A.D. 600s and 1280. Characterized by arroyos, discontinuous ephemeral streams contain alternating aggrading and degrading reaches and are well suited for ak chin floodwater agriculture. Episodic Puebloan abandonment of the southern piedmont correlates with periods of drought but does not appear to be linked to stream entrenchment. We question a priori assumptions of droughts correlated to stream entrenchment and urge caution in the use of drought-arroyo models for settlement shifts in alluvial flood plains without supporting stratigraphic or geomorphic evidence.

Geophysical studies of crustal structure in the Rocky Mountain region: A review

The Rocky Mountains have fascinated the geological community for over 100 years, but crustal-scale geophysical studies are relatively rare in this region. However, a knowledge of crustal structure is essential if we are to fully understand the region's tectonic history. Thus, we have compiled and synthesized existing information on crustal structure in order to provide as complete a picture as possible at this time. We have focused on Wyoming, Colorado, and New Mexico where there are enough data to make useful correlations with geologic features. The Rocky Mountain region includes the crest of a broad uplift on which the Southern Rocky Mountains are located. In turn, the Southern Rocky Mountains are bisected by the Rio Grande rift. In the Rio Grande rift, distinct crustal thinning (at least 5 km relative to adjacent areas) can be documented from Albuquerque, New Mexico southward. The area of thinned crust widens southward, as does the physiographic expression of the rift. The thinnest crust documented to date (about 28 km) is found west of El Paso, Texas. In contrast with East Africa, the crustal thinning is gradual from the rift valley to the shoulders, perhaps reflecting the back-arc thermal regime that existed prior to rifting. The thickest crust in the region (about 53 km) appears to be associated with both the Southern Rockies in Colorado and the topographically lower Great Plains in Colorado and New Mexico. This lack of correlation between topography and crustal thickness implies that the mantle is playing a major role in the attainment of isostatic balance in this area. Magmatic modification of the crust during rifting appears to have been minor. However, the modification due to the voluminous mid-Tertiary magmatism in the Datil–Mogollon volcanic field (southwestern New Mexico) and San Juan volcanic field (southwestern Colorado) is substantial. In the Datil–Mogollon field, a batholith that accounts for about one fifth of the crustal thickness has been detected in the upper crust, and a feature of similar dimensions is indicated in the San Juan region. There is evidence of the crust thinning northward from Colorado into Wyoming, which could be a relic of Archean rifting of the southern margin of the Wyoming craton.

A Case Study Approach to Teaching Environmental Mineralogy

Recent student interest in environmental geology encouraged me to design a new course entitled Environmental Mineralogy as a way to teach advanced topics of applied mineralogy. Rather than try to cover all topics that might be included in Environmental Mineralogy, I concentrated on a few illustrative subjects and investigated, with the students, a case study related to each topic. The first topic, silicosis, was illustrated by the 1930–31 Hawk's Nest incident in which many workers died from acute silicosis contracted while excavating a five-kilometer tunnel through sandstone. The second topic, asbestos-related disease, was illustrated by the two-week delay in opening New York City schools in 1993 because of fear that children were in danger from asbestos in their schools. The third topic concerned pollution from metal mining, and the case study was the Summitville gold mine in southwestern Colorado. The fourth topic was minerals and mineraloids proposed for isolating radioactive waste at the Yucca Mountain site. Each student presented an additional topic related to minerals and the environment. The students learned that minerals can cause health and environmental problems, and that minerals can also contribute to the solution of environmental problems. From the case studies, the students learned about the complexities of environmental debates and the vested interests of different constituencies.

Paleomagnetism of the Middle Proterozoic Electra Lake Gabbro, Needle Mountains, southwestern Colorado

The Electra Lake Gabbro is a small 1.435 Ga pluton that intrudes 1.7 to 1.6 Ga gneisses and schists of the Needle Mountains in southwestern Colorado. Paleomagnetic samples were collected from the main phases of the gabbro, diabase dikes, granite, and alaskite dikes that cut the gabbro and from a partially melted zone in gneiss along the southern margin of the pluton. Gabbro, diabase, and some melt zone samples have a single‐polarity characteristic magnetization of northeast declination (D) and moderate negative inclination (I). Demagnetization behavior and rock magnetic characteristics indicate that the remanence is carried by nearly pure magnetite. After correction for the minor west dip of overlying Paleozoic strata, we obtain a mean direction of D = 32.1°, I = −41.9° ( k: = 94, α95 = 3.3°, N = 21 sites) and a paleomagnetic pole at 21.1°S, 221.1°E, (K = 89, A95 = 3.4°). This pole is similar to poles from the Middle Proterozoic Belt Supergroup but is located at a higher southerly latitude than poles from other 1.47–1.44 Ga plutons from North America, most of which plot at equatorial latitudes. The reason for this discrepancy is not clear but may result from a combination of factors, including unrecognized tilting of the gabbro, the failure of this relatively small pluton to fully average paleosecular variation, and uncertainties in the overall reliability of other 1.5–1.4 Ga poles of the North American apparent polar wander path.

Holocene depositional history and Anasazi occupation in McElmo Canyon, southwestern Colorado

Holocene depositional history and Anasazi occupation in McElmo Canyon, southwestern Colorado

Holocene depositional history and Anasazi occupation in McElmo Canyon, southwestern Colorado

Holocene depositional history and Anasazi occupation in McElmo Canyon, southwestern Colorado

Stochastic calibration of an orographic precipitation model

In this study a stochastic approach to calibration of an orographic precipitation model (Rhea, 1978) was applied in the Gunnison River Basin of south-western Colorado. The stochastic approach to model calibration was used to determine: (1) the model parameter uncertainty and sensitivity; (2) the grid-cell resolution to run the model (10 or 5 km grids); (3) the model grid rotation increment; and (4) the basin subdivision by elevation band for parameter definition. Results from the stochastic calibration are location and data dependent. Uncertainty, sensitivity and range in the final parameter sets were found to vary by grid-cell resolution and elevation. Ten km grids were found to be a more robust model configuration than 5 km grids. Grid rotation increment, tested using only 10 km grids, indicated increments of less than 10 degrees to be superior. Basin subdivision into two elevation bands was found to produce ‘optimal’ results for both 10 and 5 km grids. © 1998 John Wiley & Sons, Ltd.

RESEARCH: Assessment of Nonpoint Source Pollution from Inactive Mines Using a Watershed-Based Approach

/ A watershed-based approach for screening-level assessment of nonpoint source pollution from inactive and abandoned metal mines was developed and illustrated. The methodology was designed to use limited stream discharge and chemical data from synoptic surveys to derive key information required for targeting impaired waterbodies and critical source areas for detailed investigation and remediation. The approach was formulated based on the required attributes of an assessment methodology, information goals for targeting, attributes of data that are typical of basins with inactive mines, and data analysis methods that were useful for the case study. The methodology is presented as steps in a framework including evaluation of existing data/information and identification of data gaps; definition of assessment information goals for targeting and monitoring design; data collection, management, and analysis; and information reporting and use for targeting. Information generated includes the type and extent of and critical conditions for water-quality impairment, concentrations in and loadings to streams, differences between concentrations in and loadings to streams, and risks of exceeding target concentrations and loadings. Data from the Cement Creek Basin, located in the San Juan Mountains of southwestern Colorado, USA, were used to help develop and illustrate application of the methodology. The required information was derived for Cement Creek and used for preliminary targeting of locations for detailed investigation and remediation. Application of the approach to Cement Creek was successful in terms of cost-effective generation of information and use for targeting.KEY WORDS: Water quality assessment; Nonpoint source pollution; Inactive mines; Watershed

VERIFICATION OF THE RHEA‐OROGRAPHIC‐PRECIPITATION MODEL1

ABSTRACT: Observed April 1 snowpack accumulations within and near the Gunnison River basin in southwestern Colorado are compared with simulations from the Rhea‐orographic‐precipitation model to determine if the model simulates reliable magnitudes and temporal and spatial variability in winter precipitation for the basin. Twenty simulations of the Rhea model were performed using‘optimal’parameter sets determined for 10‐kilometer (km) grids (10‐km by 10‐km grid cells) through stochastic calibration. Comparisons of Rhea‐model simulations of winter precipitation with April 1 snowpack accumulations at 32 snowcourse stations were performed for the years 1972–1990. For most stations and most years the Rhea model reliably simulates the temporal and spatial variability in April 1 snowpack accumulations. However, in general, the Rhea‐model underestimates April 1 snowpack accumulations in the Gunnison River basin area, and the underestimation is greatest for locations that receive the largest amount of snow. A significant portion of the error in Rhea‐model simulations is due to the calibration of the Rhea model using gauge‐catch precipitation measurements which can be as much as 50 percent below actual snowfall accumulations. Additional error in the Rhea‐model simulations is a result of the comparison of gridded precipitation values to observed values measured at points.