West-central Colorado Research Articles

Abstract: Aggradation and Channel Avulsion: A study of the Eocene Shire Member of the Wasatch Formation, West-Central Colorado

Abstract: Aggradation and Channel Avulsion: A study of the Eocene Shire Member of the Wasatch Formation, West-Central Colorado

Mycoplasmas in wild turkeys living in association with domestic fowl.

One hundred and nineteen Merriam's wild turkeys (Meleagris gallopavo merriami) and 31 domestic chickens coexisting on a ranch in west-central Colorado (USA) were surveyed for mycoplasmosis by serologic and cultural methods. Although no clinical signs were apparent in any wild turkeys tested, 51 (43%) had positive rapid plate agglutination (RPA) reactions for M. gallisepticum (MG) and/or M. synoviae (MS); 37% of 56 adults and 48% of 63 subadults were classified as positive reactors to MG and/or MS. No turkeys tested in 1992 (n = 61) and 17 (29%) of 58 turkeys tested in 1993 were RPA-positive for M. meleagridis (MM). Hemagglutination inhibition (HI) test results were negative for MG, MS and MM as were most enzyme-linked immunosorbent assay (ELISA) test reactions (MG = 99%, MS = 93%, MM = 87%). Immunoblotting showed mild to moderate reactivity to MG proteins in 49% of 41 samples tested. Most chickens were strongly positive for MS by RPA (81%), HI (58%) and ELISA (87%); 48% also were positive for MG by RPA but all were MG-negative by HI and ELISA. No pathogenic mycoplasmas were isolated from either group of birds. Mycoplasma gallopavonis was commonly identified from the wild turkeys, and M. gallinaceum was isolated from both the chickens and wild turkeys. In a transmission study conducted in 1994, disease-free domestic turkeys failed to seroconvert when co-housed with wild turkeys from this population that were RPA-positive for MG. Collectively, the results of this study were inconclusive regarding the status of pathogenic mycoplasmas within this wild turkey population.

Mesozoic-Cenozoic burial, uplift, and erosion history of the west-central Colorado Plateau

Apatite fission-track data show that Permian strata forming the Colorado Plateau surface at the Grand Canyon underwent burial temperatures of ∼90-100 °C in the Late Cretaceous, indicating that ∼2.7-4.5 km of Mesozoic strata covered the area at that time. This is similar to the ∼2.5-3.5 km thickness of Mesozoic section preserved to the north in southern Utah and confirms that those strata once extended south over a much broader area of the Colorado Plateau. Cooling from maximum burial temperatures began about 75 Ma, indicating that Laramide-age erosion removed ∼1.3-4.5 km of Mesozoic strata from the area. This erosion was probably caused by topographic relief created by Laramide reactivation of monoclines and reverse faults. Permian strata in the Waterpocket monocline in southern Utah underwent burial temperatures of ∼90 °C in the Late Cretaceous, consistent with the idea that the Permian to Upper Cretaceous section exposed in the monocline formerly extended south over the Grand Canyon region.

Spatial patterns of atmospheric pollen dispersal in the Colorado Rocky Mountains, USA

Atmospheric pollen dispersal in the southern Rocky Mountains was sampled with 18 Tauber traps located between 2470 and 3780 m elevation. A three year study provided data on both pollen frequencies and pollen accumulation (grains cm −2 yr −1) for six distinct vegetation types. Pollen spectra from the shrub steppe below the forest belt is characterized by Artemisia, Gramineae, Compositae, and other non-arboreal pollen taxa. These pollen types are readily carried upslope to higher elevations. Pollen from sub-alpine conifers, Picea and Abies, is not transported far by the wind, and indicates the presence of nearby host trees. Pinus pollen, readily distributed by the wind, is found in all traps, but only exceeds 20% where the trees are present, and in tundra vegetation. While pollen from Pseudotsuga and Populus trees occurs in low frequencies and low influx rates, it is relatively abundant in montane forest stands dominated by these trees. Although pollen grains are transported both upslope and downslope, most pollen transport is upslope. At least 50% of the pollen deposited in alpine tundra, 20% in the sub-alpine forest, and 7% in the montane forest, comes from plants growing at lower elevations. Pollen spectra deposited in steppe vegetation are the least distorted by pollen from plants growing in other vegetation types, and characterize the broad valleys of sagebrush steppe that makes up the regional vegetation. Spectra in alpine tundra are dominated by pollen from plants growing at lower elevations and at long distances from the collection site. In a comparison of paired collectors, Tauber traps with aerodynamically designed lids were more effective in open vegetation where they collected substantially more pollen than traps without the lids. Tauber lids appear to have less effect on pollen accumulation rates in forested stands, probablu due to the filtering of pollen by trees. Annual pollen accumulation rates in west-central Colorado range between 1000 and 5000 grains cm −2 yr −1. Mean rates of modern pollen influx (in grains cm −2 yr −1) are 1100 in alpine tundra, 2700 in the sub-alpine forest, 3400 in montane forest types, and 2200 in the shrub steppe. These pollen accumulation values reported for the mountains of Colorado are comparable to pollen accumulation rates from other localities in the western United States.

Structural, eruptive, and intrusive evolution of the Grizzly Peak caldera, Sawatch Range, Colorado

Research Article| September 01, 1991 Structural, eruptive, and intrusive evolution of the Grizzly Peak caldera, Sawatch Range, Colorado C. J. FRIDRICH; C. J. FRIDRICH 1Department of Geology, Stanford University, Stanford, California 94305 Search for other works by this author on: GSW Google Scholar R. P. SMITH; R. P. SMITH 2Idaho National Engineering Laboratory, EG&G Idaho, Inc., P.O. Box 1625, M.S. 2107, Idaho Falls, Idaho 83415 Search for other works by this author on: GSW Google Scholar ED DEWITT; ED DEWITT 3U.S. Geological Survey, M.S. 905, Box 25046, Federal Center, Denver, Colorado 80225 Search for other works by this author on: GSW Google Scholar E. H. MCKEE E. H. MCKEE 4U.S. Geological Survey, M.S. 941, 345 Middlefield Road, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar Author and Article Information C. J. FRIDRICH 1Department of Geology, Stanford University, Stanford, California 94305 R. P. SMITH 2Idaho National Engineering Laboratory, EG&G Idaho, Inc., P.O. Box 1625, M.S. 2107, Idaho Falls, Idaho 83415 ED DEWITT 3U.S. Geological Survey, M.S. 905, Box 25046, Federal Center, Denver, Colorado 80225 E. H. MCKEE 4U.S. Geological Survey, M.S. 941, 345 Middlefield Road, Menlo Park, California 94025 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1991) 103 (9): 1160–1177. https://doi.org/10.1130/0016-7606(1991)103<1160:SEAIEO>2.3.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation C. J. FRIDRICH, R. P. SMITH, ED DEWITT, E. H. MCKEE; Structural, eruptive, and intrusive evolution of the Grizzly Peak caldera, Sawatch Range, Colorado. GSA Bulletin 1991;; 103 (9): 1160–1177. doi: https://doi.org/10.1130/0016-7606(1991)103<1160:SEAIEO>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Volcanic and shallow intrusive features of the deeply dissected 34 Ma Grizzly Peak caldera in west-central Colorado record evolution of the magmatic center from pre-caldera through post-resurgent stages. Pre-caldera dikes, zones of hydrothermally altered rocks, and lava flows formed along a circular swarm of cone-sheet fractures around the site of the future caldera. Early, largely rhyolitic uppercrustal magmatism culminated in the caldera-forming eruption of the Grizzly Peak Tuff. Intracaldera tuff is zoned from high-silica rhyolite at the base to low-silica rhyolite at the eroded top and, further, contains dacite to mafic latite pumice lumps in two heterogeneous tuff layers in the upper third of the preserved section.Half of the erupted tuff ponded in the 17-by 23-km, >600-km3 caldera, filling the asymmetric depression to a compacted thickness locally >2.7 km, including intercalated rock-avalanche megabreccias shed from ring-fault scarps. The asymmetric caldera has an inner ring-fracture zone that separates two structural segments that collapsed to different depths. Caldera fill buried collapse structures as they formed; the inner ring-fracture zone is a growth (or syn-depositional) fault in the single-cooling-unit tuff. Welded-tuff ring dikes are locally exposed at erosion levels below the caldera fill. These dikes are remnants of fissure vents in the outer ring-fracture zone.Following collapse, the Grizzly Peak caldera was uplifted, forming a complexly faulted resurgent dome. Resurgence resulted partly from emplacement of a composite granodiorite laccolith now exposed in the eroded core of the dome. A belt of mafic latite to rhyolite porphyry dikes and small stocks formed across the center of the domed caldera during the waning of the magmatic center. Latite intrusions in this suite represent the penetration of relatively mafic magma to the surface following solidification of the felsic subcaldera batholith. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Non-monotonic chemical and O, Sr, Nd, and Pb isotope zonations and heterogeneity in the mafic- to silicic-composition magma chamber of the Grizzly Peak Tuff, Colorado

Strong compositional zonation of the 34 Ma Grizzly Peak Tuff in west-central Colorado is attended by non-monotonic trends in O, Sr, Nd, and Pb isotope ratios. Fiamme from the tuff cluster in chemical compositions and petrographic characteristics, indicating the magma chamber was not continuously zoned but consisted of at least seven compositional layers. The most mafic magma erupted (57 wt% SiO2, fiamme group 7) had δ18O= +8.5, initial 87Sr/86Sr=0.7099, eNd, and 206Pb/204Pb=17.80, suggesting that the magma was produced by ∼50% fractional crystallization of basaltic magma that assimilated 20 to 40 wt% Proterozoic crust. Isotopic compositions of more evolved parts of the chamber (up to 77 wt% SiO2, fiamme group 1) depart from the mafic “base-level” composition of fiamme group 7, and reflect late-stage assimilation that occurred largely after compositional layering was established. δ18O values decrease by as much as 1.5‰ from fiamme groups 7 through 4, indicating assimilation of hydrothermally altered roof rocks. δ18O values abruptly inerease by up to 1.5‰ between fiamme groups 4 and 3. This discontinuity is interpreted to reflect evolution in an asymmetric chamber that had a split-level roof, allowing assimilation of wall rocks that varied vertically in degree of hydrothermal alteration. This chamber geometry is also supported by collapse structures in the caldera. Late-stage assimilation of heterogeneous wall rocks is also indicated by variations in Sr, Nd, and Pb isotope ratios. Large Sr isotope disequilibrium exists between some phenocrysts and whole-rock fiamme, and initial 87Sr/86Sr ratios in phenocrysts are as high as 0.7170. e values regularly increase from-13.0 in fiamme group 7 to-11.3 in fiamme group 3, and then decrease to-12.2 in fiamme group 1. 206Pb/204Pb ratios generally increase from 17.80 to 17.94 for fiamme groups 7 through 1. The rhyolitic parts of the Grizzly Peak Tuff have isotopic compositions that could be attributed to a purely crustal melt. It is unlikely, however, that the mafic parts of the tuff were generated by crustal melting, and the compositional and isotopic variations across the entire zonation of the tuff are best explained by fractional crystallization of mantle-derived magmas, accompanied by extensive assimilation of Proterozoic crust.

Occurrence of Cryptosporidium in Home Daycare Centers in West-Central Colorado

Thirty white middle-class 5 yr olds from home daycare centers were examined for Cryptosporidium. Five fecal samples were collected from each child, over a 3-wk period, concentrated by sedimentation, and stained by a modified acid-fast technique. Nine children were positive (30%), 6 of these were asymptomatic (5 of these were under 2 yr old). Two girls and 1 boy, under 2 yr old, had mild diarrhea within 3 wk prior to fecal collection.

In-Situ Stresses in Low-Permeability, Nonmarine Rocks

Summary In-situ stress measurements have been performed over a 2,500-ft [760-m] interval in the nonmarine section of the Cretaceous Mesaverde in the Piceance basin in Colorado. These measurements included 52 hydraulic fracture measurements of the minimum in-situ stress and 22 anelastic strain recovery (ASR) measurements. Stress data obtained in sandstones, shales, mudstones, siltstones, and coals show the effect of lithology on the magnitudes of the stresses. Introduction Many oil and gas production and completion activities depend on the in-situ stresses in and around the reservoir rock, and the degree of success of these activities can often be influenced by determination of the stresses and their inclusion in the design process. Some important examples include hydraulic fracture growth, proppant crushing in hydraulic fractures, fracturing during waterflooding, wellbore stability, and the behavior of natural fractures during any process in which the pore pressure and/or stresses are altered. Knowledge of the stress field at depth does not come easily, however. Calculations of the stresses are relatively crude because the three-dimensional stress field depends on a complex history of properties and loading conditions and may include anelastic, plastic, and fracturing episodes. While much of the effect of this past history may be negligible in many cases, stresses may also be locked in sufficiently so that the total history must be known. Additionally, we would also need to know other conditions, such as drained vs. undrained behavior of the pore fluids, whether the tectonic loads should be considered stress or strain boundary conditions (important for layered media), the history of diagenesis, and details of the constitutive model(s) of the bulk rock. In all but the simplest cases, we will have difficulty performing credible stress calculations. The previous discussion suggests that stress measurement is the preferred option, and much research has been performed to develop usable stress measurement techniques that can be applied at depth in a wellbore. These includethe well-known hydraulic fracturing technique;various core analyses, such as ASR, differential strain curve analysis (DSCA), and differential wave velocity analysis;wellbore condition logs (televiewers and calipers) to examine eccentricity and breakouts;hydraulic fracture diagnostics to estimate the stress field orientation; andother ideas. The subject of this paper is two-fold:a compendium of all in-situ stress measurements conducted in nonmarine rocks at the Multiwell Experiment (MWX) in the Piceance basin of Colorado anda critical analysis of the techniques, results, and implicat ions. This study builds on many previous stress measurements and analyses at this site, many of which have been in marine rocks. MWX Site The MWX25 is an experiment funded by the U.S. DOE to characterize low-permeability reservoirs in the western U.S. and to test technology for the production of natural gas from these marginal resources. The site is located in the Piceance basin in west-central Colorado near the town of Rifle. The facility consists of three closely spaced wells (100 to 220 ft [30 to 67 m] apart) painstakingly char-acterized by the recovery and analysis of more than 4,100 ft [ 125m] of core, advanced logging programs, detailed well testing, frequent stress testing, and highly instrumented stimulation experiments. The Mesaverde group, the tight sand target, is between the depths of 4,100 and 8,200 ft [ 1250 and 2500 m] at this location.

Geochemistry and petrogenesis of early Proterozoic amphibolites, west-central Colorado, U.S.A.

Early Proterozoic supracrustal rocks exposed in west-central Colorado include amphibolites intimately associated with felsic volcanics and associated volcaniclastic sediments. Three successions are recognized. The Dubois succession (1770-1760 Ma) is comprised chiefly of amphibolites and felsic volcanics. The amphibolites exhibit low concentrations of light REE and HFSE and are broadly similar in composition to tholeiites from immature island arcs and, in some respects, to NMORB. The Cochetopa succession (1745-1730 Ma) includes large amounts of felsic volcanics and volcaniclastic sediments together with amphibolites that exhibit variable but slight incompatible-element enrichment similar to basalts from evolved island-arc systems and associated back-arc basins. The Black Canyon succession is coeval with the Cochetopa succession and is comprised chiefly of volcaniclastic sediments with minor amphibolite. These amphibolites exhibit significant incompatible-element enrichment similar to basalts from continental-margin arc systems. Geochemical modeling indicates that the amphibolites evolved by closed-system fractional crystallization of depleted (for the Dubois succession) to variably enriched parental basalts (for the Cochetopa and Black Canyon successions), chiefly at shallow depths. Incompatible-element distributions in the amphibolites indicate that mantle sources changed with time from relatively depleted (1770-1760 Ma) to variable but slightly enriched (1745-1730 Ma). A provisional model calls upon the Dubois succession forming in an island-arc system followed by collision with the Archean Wyoming Province at ∼ 1750 Ma. The Cochetopa succession was deposited in an intra-arc basin developed within a new continental-margin arc, and the Black Canyon succession formed in a back-arc basin associated with this arc.

On the role of isostasy in the evolution of normal fault systems

The footwalls of west-dipping normal faults that separate the west-central Colorado Plateau from the Basin and Range province record at least 5-7 km, and perhaps as much as 15-20 km, of west-side-up Neogene uplift, with an axis just 10-20 km west of undeformed plateau strata. The uplift is expressed as folding and steep faulting in pre-Tertiary cratonic and disconformably overlying Neogene strata, forming a basement-cored anticline and coincident topographic high on the western margin of the plateau. The authors interpret the uplift as a nonelastic response of the crust to buoyancy forces accompanying the tectonic denudation of the plateau margin. Profound, isostatically driven deformation of the footwalls of major normal faults may be common in extensional terrains, calling into question several assumptions fundamental to existing models of the evolution of normal fault systems.

Influence of Refuging Consumers (Pikas: Ochotona Princeps) on Subalpine Meadow Vegetation

Talus slopes in western North America frequently are bordered by vegetation that differs in biomass, cover, height, and species composition from vegetation located farther from talus. These areas are grazed by pikas (Ochotona princeps), which nest in that talus. Foraging theory predicts that pikas will produce a gradient of grazing pressure, which could produce the observed vegetational zonation. In a subalpine meadow in west—central Colorado, pikas produced the predicted gradient of grazing pressure, along which plant groups varied clinally in abundance. The effects of pikas on talus—border vegetation were tested by excluding pikas from small plots at several distances from talus. Vegetational cover and species richness increased in exclosures relative to control plots. These changes in vegetation following exclusion of pikas were greatest near talus and decreased with distance, paralleling the gradient of foraging by pikas. The effect of pika exclusion on cushion plants changed quantitatively and qualitatively with distance from talus and with time since grazing was stopped. These patterns were predicted from a model of the interaction of competition and predation along a gradient of grazing pressure. Mechanisms similar to those proposed for pikas are also expected for other small herbivores that forage from fixed nest sites.

Compositional layers in the zoned magma chamber of the Grizzly Peak Tuff

The 34 Ma Grizzly Peak Tuff in west-central Colorado preserves evidence for distinct compositional layers in a high-level magma chamber. Intracaldera tuff consists of a single cooling unit, at least 2.7 km thick, zoned from a high-silica rhyolite base to an eroded low-silica rhyolite top. Two horizons of heterogeneous tuff beneath wedges of caldera-collapse breccia contain fiamme of dacite to mafic latite along with the rhyolitic fiamme that make up the rest of the tuff. Compositional zoning defined by fiamme is a step function rather than a continuous gradient. Seven petrographic groups of fiamme each have distinct compositions separated by compositional gaps. As the same clusters and gaps are found in collections of fiamme from widely separated stratigraphic levels, step-function zoning must be intrinsic to the chamber rather than a consequence of the tapping process. We interpret the seven fiamme groups as seven separately converting layers in a density-stratified magma reservoir tapped by eruption of the Grizzly Peak Tuff. Excluding the thin heterogeneous tuff horizons, ∼3 km of caldera fill is drawn from the three most silicic layers, suggesting that the three rhyolitic layers were each at least 1 km thick in the magma chamber.

Reverse zoning in the resurgent intrusions of the Grizzly Peak cauldron, Sawatch Range, Colorado

The deeply eroded Grizzly Peak cauldron, 17 by 23 km and source of the 34-m.y.-old Grizzly Peak Tuff, is located in the Sawatch Range in west-central Colorado. The Lincoln Gulch composite stock is exposed in the core of a resurgent dome in the northern part of the cauldron and consists of three units. Their forceful emplacement was a cause of cauldron resurgence. The first intrusion lifted its roof along a vertical fault that surrounds the stock. The second resurgent intrusion, a complex body with broad sill-like overhangs, was emplaced within the first intrusion by further piston uplift of the roof. The third intrusion, emplaced in the second, is a small funnel-shaped body. Normal faults in surrounding inter-layered intracaldera tuff and caldera-collapse breccia are radial to the intrusion-cored piston block. Most of the tuff dips away from the resurgent intrusions. Pyroclastic dikes cutting tuff that roofs the stock may have been emplaced by venting of the intrusions. The three resurgent intrusions are concentrically zoned, with the most differentiated material at the margins. From margin to core, compositions range from biotite granodiorite to hornblende-bearing biotite quartz monzodiorite. The intrusions are porphyroaphanitic throughout, but both phenocrysts and groundmass become coarser toward the cores. The compositional patterns are termed reverse zonations because most documented concentrically zoned intrusions become more silicic toward their cores. Crystal settling and marginal accretion of crystalline phases can be eliminated as means of forming reverse zoning because mafic-coreward gradients in element and mineral content are developed inward from contacts of all attitudes. Diffusion-based mechanisms of in situ differentiation are too slow, given the short solidification times for these sill-like bodies. We interpret reverse zonation to result from rearrangement of a vertically graded magma column during emplacement of the intrusions. Progressively more mafic magma from successively deeper levels in a subjacent chamber rose into the cores of the intrusions, displacing more silicic magma toward the margins. This emplacement pattern was preserved by rapid solidification, perhaps due to volatile loss on venting.

The use of sagebrush ( Artemisia) as a biogeochemical indicator of base-metal deposits in Precambrian rocks of west-central Colorado

The efficacy of sagebrush ( Artemisia) as a biogeochemical indicator of base-metal mineralization in stratabound Precambrian ore deposits in west-central Colorado was investigated by collecting new (mostly flowering) growth from several sagebrush shrubs over and near five such deposits in three different areas. These are the Sedalia mine and two mines in the Turret district near Salida, and two mines in the Cochetopa district southeast of Gunnison. Two species were used, A. tridentata and A. frigida, depending on the area. Sagebrush clippings were separated into two subsamples consisting of (1) stems, and (2) leaves and blossoms stripped from the stems. These subsamples were ashed separately and the ash analyzed with an emission spectrograph for 30 elements. There appear to be no appreciable differences in the analyses of the two subsamples, indicating that composite samples would provide adequate information for further investigations. Eight of these elements, Ag, Bi, Cu, Pb, Sn, Y, Zn, and Zr, are present in notably higher concentrations in the ash of samples growing over mineralized ground than in that of control samples growing over barren ground. Although the distribution pattern, and the number of these anomalous elements, differs at each of the five localities, three of them, silver, copper, and lead, show good contrast and close association with subjacent mineralization in all five study areas.

Seed Dispersal by Ants in the Rocky Mountains

-Field tests quantifying the behavioral responses of ants to the seeds of twenty Rocky Mountain species were conducted in Gunnison County, Colorado. The results indicate that Claytonia lanceolata Pursh, Corydalis aurea Willd., C. caseana A. Gray., Delphinium nelsoni Greene, and Viola nuttallii Pursh are myrmecochores. The seeds of these species all bear a conspicuous elaiosome and are selectively removed by three species of ants, Formica podzolica Francoeur, Formica canadensis Francoeur, and Myrmica discontinua Weber. These are the first records of ant dispersal from the Rocky Mountains. Comparatively little is known about the ant dispersed (myrmecochorous) plant species of the western United States. Scattered reports suggest that they may be common in some areas, particularly the arid south-west (Berg, 1966; Bullock, 1974; O'Dowd and Hay, 1980; Nesom, 1981) and the moist north-west (Nordhagen, 1959; Berg, 1972). Almost nothing is known about ant dispersal in the Rocky Mountains. Sernander (1906) reported that between 6% and 13% of herbaceous species at high altitudes in Sweden, the French Alps and the north Tyrol were ant dispersed, and speculated that a similar proportion of species might be ant-dispersed in other mountainous regions of the world. Preliminary field experiments with the seeds of Viola nuttallii Pursh strongly suggested that this species is myrmecochorous. This paper reports new information on seed disperal by ants of this and other species. MATERIALS AND METHODS.-The two study sites were located in Gunnison County, in west-central Colorado. One was at an altitude of approximately 3,024m, near the Rocky Mountain Biological Laboratory at Gothic. This is a dry, rocky, subalpine meadow. Areas of little ground cover were mixed with areas of dense vegetation dominated by grasses and false hellebore (Veratrum californicum Durand). There were also small stands of willow (Salix, sp.) and aspen (Populus tremuloides Michx.). The other site was on a steep roadcut near Crested Butte, at an altitude of 2,719m. Vegetation on this site was very sparse and dominated by Corydalis aurea, weeds, and a few grasses. The method for evaluating myrmecochory in a species is detailed in Beattie and Culver (1981) and utilizes both morphological and behavioral criteria. Since ants carry many kinds of objects back to their nests, we looked for morphological adaptations of the seeds which indicate myrmecochory, and specific behavioral responses on the part of the ants. Behavioral responses of undisturbed ant foragers were quantified by the use of bioassays of fresh seeds, in the habitat where the plants were growing. Clumps of five seeds were placed at random in the habitat and observed for periods of up to one hour. Each bioassay was replicated as many times as the supply of fresh material would allow. The responses of ants were classified as follows: ignore-the ant passed close enough to the seed to detect it but exhibited no response at all; antennate-the ant examined the seed with its antennae; pick-up-the ant iesponded by rolling the seed and attempting to pick it up, but the response was not sufficiently intense for the ant to carry it away; remove-the ant removed the seed at least 10 cm. In most cases, an ant which removed a seed 10 cm actually continued to carry it back to the nest. RESULTS.-We bioassayed the seeds of twenty species belonging to fifteen families (Table 1). There was no interaction between ants and the seeds of This content downloaded from 207.46.13.180 on Thu, 08 Sep 2016 06:26:21 UTC All use subject to http://about.jstor.org/terms The Southwestern Naturalist TABLE 1.-Species bioassayed (species marked by an asterisk were ignored in all bioassays). Boraginaceae Mertensia fusiformis Greene Compositae Agoseris aurantiaca (Hook). Greene* Senecio interregemus Nutt.* Taraxacum officinale Wiggars* Cyperaceae Carex egglestonii Mack.* Fumariaceae Corydalis aurea Willd. C. caseana A. Gray Graminae Melica spectabilis Scribn.* Poa interior Rydb.* Hydrophyllaceae Hydrophyllum capitatum Dougl. ex Benth.* Liliaceae Erythronium grandiflorum Pursh Onagraceae Epilobium minutum Lindl. ex Hook.* Polemoniaceae Collomia linearis Nutt.*

Chemical and Biological Limnology of Six West-central Colorado Mountain Ponds and Their Susceptibility to Acid Rain

This paper reports the values of a number of physical and chemical parameters for six high altitude ponds investigated during the summers of 1965 and 1972. Transfers of zooplankton from pond to pond reported here and in the literature suggest that the existence of two different species associations in these ponds is not directly due to the summer water conditions. The ponds were rather acidic and therefore likely to be very susceptible to acid rain, given their low conductivity and buffering capability. There is no evidence of a significant pH decrease between 1965 and 1972.

Geochemistry of the Dubois greenstone succession: An early proterozoic bimodal volcanic association in west-central Colorado

The Dubois greenstone succession, an Early Proterozoic supracrustal succession in west-central Colorado, is composed of fine-grained felsic volcaniclastic sediments, ash-flow tuffs, and tholeiitic flows, sills and dikes. The volcanic rocks comprise a bimodal suite of tholeiite and rhyolite. The tholeiites are divided into two groups: Group I exhibiting nearly flat REE patterns (20–30 × chondrites) and Group II with light REE-enriched patterns (40–70 × chondrites). These groups can be produced by two-stage melting of an undepleted or enriched garnet lherzolite source in which garnet remains in the residue of only the first melting stage which produces Group II magmas. The felsic magmas can be produced by partial melting of a high-grade gneiss in the lower crust in which zircon is entrained in the magma. The Dubois greenstone succession appears to have formed by felsic and mafic subaqueous eruptions in which some ash-flows, upon entering water, underwent phreatomagmatic explosions giving rise to detritus which formed the volcaniclastic sediments. Although tectonic setting cannot be assigned to the succession with a high degree of confidence, an immature back-arc basin developed on sialic crust in consistent with most data.

POLLINATOR CHOICE AND STABILIZING SELECTION FOR FLOWER COLOR INDELPHINIUM NELSONII

Bright colors are characteristic of the flowers of many higher plants. For the last century these colors have been interpreted as adaptations by which flowers signal their presence to animal pollinators (Darwin, 1876; Muller, 1883; Plateau, 1899; Clements and Long, 1923; Kugler, 1943; Daumer, 1958; Heinrich, 1975; Jones, 1978; Kevan, 1978). If this is correct, the exact color(s) found within populations of single species, and the array of colors represented in an assemblage of sympatric species, may be explicable largely as evolutionary consequences of pollinator foraging behavior. Several plant species in the Rocky Mountains of Colorado exhibit polymorphisms well suited for studies of relationships between flower color, pollinator behavior, and plant fitness. Although flowers of these species are normally brightly colored, we have noticed rare individuals whose flowers are white or pale. Such color changes are likely to have a genetic basis, as we shall discuss later. Species with such flower color polymorphisms include Delphinium nelsonii Greene (Ranunculaceae), Lupinus argenteus Pursh (Leguminosae), Ipomopsis aggregata (Pursh) V. Grant (Polemoniaceae), Iris missouriensis Nutt. (Iridaceae), and Linum lewisii Pursh (Linaceae). The low frequencies of individuals in populations of these species suggest an equilibrium between their production by spontaneous mutation and some form of selection against them (cf. Crow and Kimura, 1970; Harding, 1970). Selection against albinos could take at least two forms. Mutations causing reduced flower pigmentation might be associated with physiological or developmental changes (e. g., reduced ovule number, stigma size, or receptivity) which lower intrinsic fecundity. Alternatively, pollinators might discriminate against albinos and thus exert stabilizing selection for flower color. In this paper we describe experiments and observations with one of the species listed above, Delphinium nelsonii, a small perennial herb whose flowers are normally deep blue. Near the Rocky Mountain Biological Laboratory (RMBL) in west-central Colorado, D. nelsonii is pollinated primarily by broad-tailed hummingbirds (Selasphorus platycercus Swainson) and bumblebee queens (B ombus appositus Cresson, B. flavifrons Cresson, B. californicus F. Smith, and B. nevadensis Cresson) (Waser, 1978; Price and Waser, 1979). We present evidence that paleor white-flowered albinos are rare in populations near the RMBL; that selection against albinos occurs in the form of reduced seed sets relative to normal blueflowered individuals; that intrinsic fecundity of albinos is not lower than that of normal individuals; that both hummingbirds and bumblebees discriminate against albinos in a way sufficient to explain their low seed sets and consequent rarity; and that discrimination may be due to difficulty pollinators experience in extracting nectar from albino flowers.

Thermal Effects on Sedimentary Organic Matter and Recognition and Mapping of Subjacent Igneous Plutons, Ruby Range, Colorado: ABSTRACT

The Ruby Range igenous complex, west-central Colorado, is characterized by mid-Tertiary granodiorite plutons which intrude Paleozoic, Mesozoic, and Tertiary strata. Conventional contact metamorphism is limited to a mile-wide zone adjacent to the Ruby Range axis. Geochemical and related studies, including hydrocarbon levels and types, kerogen composition, visual kerogen, and vitrinite observations, on the organic matter in intruded sediments demonstrate that low-grade thermal metamorphic effects extend about 6 mi (9.6 km) from the Ruby Range intrusive axis. This is an unusually wide thermal aureole and indicates important heat sources in addition to the intrusions along the Ruby Range axis. Peripheral sills and laccoliths caused little thermal effect on the sediments. It i concluded that the thermal aureole adjacent to the Ruby Range is enlarged because of subjacent intrusions. Specifically, it is postulated that the White Rock pluton projects into the subsurface beneath the sedimentary cover. Recognition of organic matter metamorphism in the overlying sediments permits tracing the approximate margin of the pluton from its outcrop area in the northeast for a distance of about 10 mi (16 km) to the southwest along its subjacent trend. Relations between the trend of metamorphic isotherms and topography indicate that the pluton has a steep (stocklike) southeast margin. Significantly, the recently discovered molybdenum deposit beneath Mt. Emmons, near Crested Butte, Colorado, is located approximately along the projected trend of the subjacent extension of the hite Rock pluton. Recognition of very low grades of thermal metamorphism of organic matter may also have application to the location and evaluation of geothermal reservoirs. End_of_Article - Last_Page 893------------

Dirofilaria immitis in West Central Colorado

Dirofilaria immitis in West Central Colorado