Central Wasatch Mountains Research Articles

Finescale Orographic Precipitation Variability and Gap-Filling Radar Potential in Little Cottonwood Canyon, Utah

Abstract Finescale variations in orographic precipitation pose a major challenge for weather prediction, winter road maintenance, and avalanche forecasting and mitigation in mountainous regions. In this investigation, ground-based X-band radar observations collected during intensive observing period 6 (IOP6) of the Storm Chasing Utah Style Study (SCHUSS) are used to provide an example of these variations during a winter storm in the Wasatch Mountains of northern Utah. Emphasis is placed on precipitation features in and around Little Cottonwood Canyon (LCC), which cuts orthogonally eastward into the central Wasatch Mountains. Precipitation during the weakly stratified prefrontal storm stage featured a wavelike barrier-scale reflectivity maximum over the Wasatch Crest and upper LCC that extended weakly westward along the transverse ridges flanking LCC. This precipitation pattern appeared to reflect a veering wind profile, with southwesterly flow over the transverse ridges but cross-barrier westerly flow farther aloft. Sublimation within dry subcloud air further diminished low-level radar reflectivities over lower LCC. In contrast, the cold-frontal stage was associated with stronger reflectivities over lower LCC and the adjoining north- to northwest-facing canyon wall, consistent with shallow, northwesterly upslope flow. These results highlight the finescale precipitation variations that can occur during winter storms in complex terrain and demonstrate the potential for improved analysis and forecasting of precipitation in LCC using a gap-filling radar.

Dust-mediated loading of trace and major elements to Wasatch Mountain snowpack

Depth-integrated snow columns were collected at 12 sites across the central Wasatch Mountains, Utah, during March and April 2010 to determine concentrations of trace elements, major anions and cations, and pH. Sample collection was conducted at or near maximum snow accumulation prior to the onset of melt, and included spring dust events driven by southerly pre-frontal winds. Snow samples were melted in the laboratory and subsampled for analyses on filtered (0.45μm) and unfiltered fractions. All measured elements (Al, As, Ba, Ca, Co, Cr, Cu, Fe, Hg, K, Li, Mg, Mn, Na, Ni, Pb, Sb, Sr, Ti, Tl, U, V, and Zn) and major anions (Cl, NO3, and SO4) displayed significant increases in concentration (for example, factor of 2 to 5 increases for As, Cr, Hg, and Pb) between the six sites sampled in March (prior to dust events) and the six sites sampled in April (after dust events). Acid neutralizing capacity and pH were also elevated in April relative to March snowpack. Comparison of elemental concentration in the particulate (>0.45μm; difference between unfiltered and filtered concentration) and soluble (<0.45μm; filtered concentration) fractions shows that the concentration increase between March and April snowpack for the trace elements is primarily a result of association with dust particles >0.45μm. The results suggest that the majority of trace element loading to the Wasatch snowpack occurs via dust deposition. The major elements were primarily loaded in the <0.45μm fraction, suggesting deposition of soluble dust particles. The overall findings of this paper are similar to other studies regarding the role of dust on nutrient and trace element accumulation in soils and lake sediments, but to our knowledge this is the first study that compares trace element chemistry of seasonal snowpack before and after dust deposition events.

A New Species of Astragalus (Fabaceae) from the Wasatch Mountains of Utah

Abstract Astragalus kelseyae B.L. Corbin, sp. nov. is described as a new species from the central Wasatch Mountains, where it is known from only one small occurrence in Weber County, Utah. It grows in shale talus within the Gambel oak and bigtooth maple shrubland.

Ozone Concentrations in the Central Wasatch Mountains of Utah

In the Salt Lake Valley, a June through August SUM60 value (sum of hourly average ozone concentrations ≥60 parts per billion by volume [ppbv]) of 25,000 ppbv-hr was exceeded in 9 yr between 1978 and 1998. Ozone concentrations in the nearby Central Wasatch Mountains were monitored to determine the potential for vegetation injury. The SUM60 value of 19,000 ppbv-hr in these mountains and peak hourly concentrations >100 ppbv suggests that ozone-sensitive species may be injured. Ozone concentrations in the mountains were greatest during periods of strong upslope winds from the Salt Lake Valley. Both SUM60 values and hourly average concentrations in the Central Wasatch Mountains were strongly correlated with those in the Salt Lake Valley, suggesting that data from valley stations could be used to estimate ozone in the mountains.

High-Resolution Simulations and Microphysical Validation of an Orographic Precipitation Event over the Wasatch Mountains during IPEX IOP3

Abstract This paper investigates the kinematic flow and precipitation evolution of a winter storm over and upstream of the Wasatch Mountains [Intermountain Precipitation Experiment third intensive observing period (IPEX IOP3)] using a multiply nested version of the fifth-generation Pennsylvania State University (PSU)––National Center for Atmospheric Research (NCAR) Mesoscale Model (MM5). Validation using in situ aircraft data, radiosondes, ground-based radar, and surface observations showed that the MM5, which featured four domains with 36-, 12-, 4-, and 1.33-km grid spacing, realistically simulated the observed partial blocking of the 8–12 m s−1 ambient southwesterly flow and development of a convergence zone and enhanced lowland precipitation region upwind of the initial Wasatch slope. The MM5 also properly simulated the advance of this convergence zone toward the base of the Wasatch during the passage of a midlevel trough, despite not fully capturing the westerly wind shift accompanying the trough. Accurate simulation of the observed precipitation over the central Wasatch Mountains (within 25% of observed at all stations) required a horizontal grid spacing of 1.33 km. Despite close agreement with the observed surface precipitation, the Reisner2 bulk microphysical scheme produced too much supercooled cloud water and too little snow aloft. A model microphysical budget revealed that the Reisner2 generated over half of the surface precipitation through riming and accretion, rather than snow deposition and aggregation as implied by the observations. Using an intercept for the snow size distribution that allows for greater snow concentrations aloft improved the snow predictions and reduced the cloud water overprediction. Sensitivity studies illustrate that the reduced surface drag of the Great Salt Lake (GSL) enhanced the convergence zone and associated lowland precipitation enhancement upstream of the Wasatch Mountains. The presence of mountain ranges south of the Great Salt Lake appears to have weakened the along-barrier flow and windward convergence, resulting in a slight decrease in windward precipitation enhancement. Diabatic cooling from falling precipitation was also important for maintaining the blocked flow.

Exhumation of the central Wasatch Mountains, Utah: 2. Thermokinematic model of exhumation, erosion, and thermochronometer interpretation

The Wasatch fault is a ∼370 km long normal fault in Utah that marks the boundary between the stable Colorado Plateau to the east and the extending Basin and Range to the west. Understanding the thermokinematic evolution of this fault can provide insights into intracontinental extensional tectonics and deformation processes in other rift zones (e.g., East Africa Rift, Transantarctic Mountains). We explore the thermokinematics of footwall exhumation and erosion in the Cottonwood Intrusive Belt of the central Wasatch Mountains. Emphasis is placed on using low‐temperature thermochronometers to quantify (1) the spatial and temporal variability of exhumation and erosion rates, (2) the geometry of footwall tilt, (3) the fault dip angle, and (4) the magnitude and duration of exhumation. These processes are investigated using two‐dimensional (2‐D) thermal‐kinematic models coupled with cooling‐rate‐dependent kinetic models which predict exhumed apatite fission track (AFT) and (U‐Th)/He ages. The range of model parameters considered includes footwall exhumation and erosion rates at the fault between 0.2 and 2.0 mm yr−1, footwall tilt hinge positions between 15 and 40 km distance from the fault, a single planar normal fault with dip angles of 45° and 60°, and exhumation magnitudes of up to 15 km at the fault. Simulations include the formation of a low thermal conductivity sedimentary basin and erosion of heat‐producing layers. Erosion maintains a constant topographic profile. The kinematic and exhumation history of the Wasatch Mountains is investigated by comparing model predicted thermochronometer ages to observed AFT, ZFT, and (U‐Th)/He ages. Predicted and observed ages are compared using a reduced chi‐square analysis to determine a best fit kinematic model for the Wasatch Mountains. The preferred model includes exhumation occurring on either a 45° or 60° dipping fault, a footwall hinge located a minimum of 20–25 km from the fault, and a step decrease (deceleration) in the footwall exhumation rate at the fault from 1.2 to 0.8 mm yr−1 at around 5 Ma. The model also suggests an exhumation duration of ∼12 Myr ± 2 Myr).

Exhumation of the central Wasatch Mountains, Utah: 1. Patterns and timing of exhumation deduced from low‐temperature thermochronology data

The Wasatch Mountains are often cited as an example of normal fault growth and footwall flexure. They represent a tilted footwall at the edge of the Basin and Range extensional province, a major rift basin. Thus understanding the detailed spatial and elevation changes in coupled thermochronometer data, and how these changes can be interpreted, may aid in the analysis of thermochronometer data from other extensional regions around the world. We present a dense data set from the Cottonwood Intrusive Belt (CIB) of the Wasatch that includes apatite fission track (AFT), zircon fission track (ZFT), and apatite (U‐Th)/He ages. ZFT, AFT, and apatite (U‐Th)/He ages are 10, 5, and 3 Ma, respectively, adjacent to the Wasatch fault. AFT and (U‐Th)/He ages increase slightly with distance east of the fault until about 15–20 km, where a more abrupt increase in these ages occurs at or near the Silver Fork‐Superior fault zone. ZFT and AFT ages are concordant with 31–38 Ma pluton emplacement ages on the eastern side of range. Modeling of the data leads to the following interpretation: (1) Early cooling and ∼3–4 km of exhumation for the middle and eastern parts of the range occurred in the late Oligocene‐middle Miocene. (2) Beginning at 10–12 Ma, the locus of exhumation shifted westward toward the present range front, where the rocks cooled from &gt;200°C in the last 10–12 Myr. Our data and interpretations are consistent with a model in which the locus of faulting and exhumation shifted opposite the direction of tilt, similar to that predicted by rolling‐hinge extensional models. However, this westward shift and rapid Miocene to recent exhumation may be a local effect superimposed on lower fault displacement and exhumation rates elsewhere along the Wasatch.

Potential effects of ozone, climate, and spruce budworm on Douglas-fir growth in the Wasatch Mountains

This study assessed the potential for ozone injury to vegetation in the central Wasatch Mountains of Utah by determining if Douglas-fir (Pseudotsuga menziesii var. glauca (Beissn.) Franco) experienced reduced growth since the onset of high ozone concentrations. Dendrochronological techniques were used to model Douglas-fir growth in six central Wasatch Mountain stands, where ozone concentrations suggest the possibility of injury to vegetation, and in four Bear River stands, where ozone concentrations are lower than injury thresholds. Previous-year growth, temperature, and Palmer's Z index variables explained 53–70% of the variance in annual growth. Reduced Douglas-fir growth, not explained by the model or insect and disease records, occurred after 1970 in the central Wasatch Mountains but not in the Bear River Mountains. Douglas-fir growth in the central Wasatch Mountains was slightly negatively correlated with the previous-year ozone concentrations in nearby Salt Lake valley. Between 1962 and 1973, a fluctuating budworm population due to varying spring precipitation was suggested to have replaced precipitation as the factor limiting tree growth, causing an anomalous negative relationship between precipitation and Douglas-fir growth. Because of a suite of possible factors (e.g., undetected climatic variables, fungal pathogens, competitive interactions) influencing Douglas-fir growth, ozone may be one factor in a larger stressor complex reducing growth.

Origin and emplacement of igneous rocks in the central Wasatch Mountains, Utah

The calc-alkaline igneous rocks in the central Wasatch Mountains were emplaced between 36–30 Ma. They form a belt comprised of eleven stocks and the Keetley volcanic field aligned along the crustal suture between the Archean Wyoming province and accreted Paleoproterozoic terranes. Magmatism associated with this belt and its westward continuation into the Bingham mining district has been related to mid-Cenozoic extension. These rocks consist of two types of stocks based on texture: a western type, which is coarse grained, and mostly equigranular, and an eastern type (including the Keetley volcanic rocks), which is fine grained and porphyritic. The compositional variation in the western stocks (Little Cottonwood, Alta, and Clayton Peak stocks) forms three distinct compositional groups. The compositional variation in the eastern stocks is similar to the compositional variation in the Alta stock. Major and trace element variations in these rocks resemble those of subduction-related magmas. However, the high K2O contents and low esr values are not consistent with this origin. These magmas formed from melting of mafic igneous rocks. We propose that magmas were generated by decompression melting due to gravitational collapse of the crust that had been thickened during Cretaceous to early Cenozoic deformation. Magmas rose to varying levels in the crust along an east–west lineament. The igneous rocks of the central Wasatch Mountains have eNd(t) similar to most of the Phanerozoic igneous rocks in the miogeocline (MG), but have significantly lower eSr(t). That anomaly has been explained as due to melting of a basement long depleted in Rb (Farmer and DePaolo, 1983, 1984). However, the Wasatch igneous belt rocks are high-potassium, calc-alkaline rocks and all have very similar incompatible trace element patterns, whereas only a few MG rocks are calc-alkaline or high potassium. Furthermore, in the MG rocks incompatible trace element patterns are variable. One possible explanation for the dilemma of long-time depletion of Rb in these high-potassium, calc-alkaline rocks is that the crust may have been recently charged with Rb and K during the Sevier-Laramide event (100–40 Ma) by dehydration of the subducting slab. This event was followed by melting during mid-Cenozoic collapse of the orogen (ca. 40–20 Ma). The source of the magmas was melting of mafic rocks in the lower crust. Some of these magmas ponded, formed magma chambers, and differentiated. Some involved little ponding and erupted directly on the surface in the form of the Keetley volcanic field. Continued melting and extension produced new magmas from a similar crustal source. These magmas were emplaced below a series of pull-apart structures associated with strike-slip displacement along an east–west suture. This suture may have been controlled by the Archean-Proterozoic boundary. Some magma bodies were emplaced quickly to the surface without significant fractionation. Others coalesced and fractionated over a protracted period of time. These magma bodies interacted with crustal rocks, and differentiated to relatively evolved compositions.

Evolution of hydrothermal fluids in the Park Premier Stock, central Wasatch Mountains, Utah

Park Premier is a low-grade gold-rich porphyry copper system located about 3 km east of precious and base metal veins of the Park City district, Utah. The Park Premier stock consists of five small intrusions that were emplaced into coeval volcanic rocks at depths less than 1 km. Two of the intrusions are microaplitic granodiorite porphyries that developed strong hydrothermal alteration, stockwork quartz veins, and low-grade disseminated copper-gold mineralization. Early alteration of these intrusions included biotite replacement of hornblende, deposition of disseminated magnetite and actinolite, local sodic alteration of plagioclase, and formation of (1) quartz + magnetite + actinolite + or - Na plagioclase and (2) quartz + K feldspar + or - magnetite + or - chalcopyrite veins. Mineral compositions, fluid inclusion data, and inferred alteration reactions indicate that early alteration occurred at temperatures > or = 600 degrees C from hydrothermal fluids with unusually high salinities (> or = 82 wt % total salts) and low K/Na molar ratios ( or = 700 degrees C) and low pressure (< or = 200 bars). Quartz + pyrite + or - sericite veins and local alteration of high-temperature minerals to chlorite, sericite, calcite, and clay minerals occurred at much lower temperatures (< or = 270 degrees C) by the influx of dilute (0-3 equiv wt % NaCl), boiling meteoric(?) water.Large areas of the igneous wall rocks of the microaplitic porphyries are altered to a variety of hydrolytic assemblages, including albite + sericite, sericite, kaolinite-dickite, pyrophyllite, and alunite (all + quartz + or - pyrite). Albite + sericite, sericite, and local kaolinite-dickite alteration probably formed from chloride-dominated fluids derived from the microaplitic porphyries. Pyrophyllite and alunite alteration postdate all other types of alteration and formed along major northeast-trending fracture zones from fluids that were probably sulfate rich.The hydrothermal system at Park Premier contains several features that are unusual in other well-described porphyry copper systems but which may be common in gold-rich porphyry systems; they include: (1) early, extremely saline fluids that only migrated locally, (2) abundant magnetite, actinolite, and sodic plagioclase and relatively minor biotite, K feldspar, and sulfide minerals, (3) sparse quartz vein formation at temperatures < 500 degrees C, (4) little interaction of the microaplitic porphyries with dilute fluids until the intrusions cooled to temperatures < or = 270 degrees C, (5) early albitization of wall rocks adjacent to the microaplitic porphyries, and (6) relatively high Au contents and high Au/Cu ratios of the microaplitic porphyries. These differences probably arose primarily from the very shallow depths of emplacement, small size of the intrusions, and low(?) sulfur contents of the magmas.

Geologic setting, depths of emplacement, and regional distribution of fluid inclusions in intrusions of the central Wasatch Mountains, Utah

Nine mid-Tertiary calc-alkaline stocks, a subvolcanic porphyry system, and coeval volcanic rocks are exposed in a 45-km-long east-trending belt across the central Wasatch Mountains, Utah. The intrusions vary systematically from west to east in texture, style of emplacement, extent of contact metamorphism, hydrothermal alteration, and mineralization. Pressure-depth estimates based on metamorphic mineral assemblages, stratigraphic reconstructions, and fluid inclusion data indicate a regular variation in paleodepths ranging from about 11 km on the west to less than 1 km on the east. These data indicate that the central Wasatch Mountains have been tilted down to the east about 20 degrees during the late Cenozoic. Fluid inclusion populations in igneous quartz also vary systematically with paleodepth; high-salinity (halite-saturated) fluid inclusions are present in the eastern porphyry stocks and in the upper parts of the Alta and Clayton Peak stocks in the center of the belt but are absent in the deeper parts of the Alta and Clayton Peak stocks and in the Little Cottonwood stock on the west side of the belt. In the Alta and Clayton Peak stocks, nearly planar high-paleosalinity horizons, presently dipping 15 degrees to 20 degrees east, separate rocks containing high-salinity fluid inclusions (above the high-paleosalinity horizon) from those lacking such fluid inclusions. Comparison of fluid-inclusion populations in igneous and vein quartz in the Alta and Clayton Peak stocks indicates that high-salinity fluids predated most of the vein-forming hydrothermal activity and provide the earliest record of fluids to circulate in these stocks. High-salinity fluids probably formed either by boiling of fluids released during the late stages of crystallization in the parts of the intrusions where pressure was less than about 1,300 bars or by exsolution of immiscible high-salinity brines from the crystallizing magmas. Most hydrothermal mineralization associated with the intrusions, including Ag-Pb-Zn ores in the Park City district, are associated spatially with parts of the intrusions where high-salinity fluids were present. The major exception is the porphyry molybdenum system in the eastern part of the Little Cottonwood stock, which probably was at too great a depth (approximately 7 km) to form high-salinity brines and is dominated by low-salinity CO 2 -rich fluids.

Mesozoic and Early Tertiary Paleostructure and Sedimentology of Central Wasatch Mountains, Uinta Mountains, and Uinta Basin: ABSTRACT

During latest Cretaceous-Eocene time, 5,000 m (16,000 ft) of beds were deposited in central and northeast Utah. In the Late Cretaceous, sediment derived from the Sevier-Laramide thrust belt was transported to the east and southeast. Southerly paleocurrent directions in the base of the Currant Creek Formation (Maestrichtian) raise the possibility that uplift of the Uintas may have begun by then. The thrust belt continued as a major highland during the early Paleocene, and major uplift of the Uintas occurred. By the middle Paleocene there was an extensive lake which regressed during the late Paleocene as uplift of the Uintas continued. Lake Uinta reached its maximum size during the middle Eocene. During the late Eocene, Lake Uinta regressed and, near the end of the epoch, t e lake expired. Major sediment influx was from the east and southeast. Lower (early Duchesnean) and upper (Late Duchesnean) conglomeratic intervals record major episodes of uplift in the Uintas during latest Eocene. Structurally, the Wasatch Mountains are part of a marginal foreland fold and thrust belt. In the northern Wasatch Mountains, pre-Late Cretaceous thrust fault plates were folded in part of a large, ramp-anticline that is cored by allochthonous, crystalline basement. Foreland thrust belt structures in the central Wasatch Mountains were folded about the east-trending Uinta axis as the Uinta Mountains formed. Eastward movement on the Hogsback thrust during the Paleocene was transferred onto the adjacent Uinta axis and Uinta Mountains structure, causing about 20 km (12 mi) of sinistral slip in the western Uinta Mountains. Deformation in the Uinta Mountains continued following cessation of movement on the Hogsback thrust system. A south-dipping fault ramp was located beneath the Uinta Mount ins and extended to depths of 15 to 20 km (9 to 12 mi). End_Page 1351------------------------------ Oblique-slip on this ramp probably resulted in about 20 km (12 mi) of crustal shortening perpendicular to the trend of the mountains. End_of_Article - Last_Page 1352------------

Late quaternary glacial and vegetation changes, Little Cottonwood Canyon area, Wasatch mountains, Utah

Glacial geology and 14C dating in the central Wasatch Mountains indicate: an early canyon-mouth glaciation (Dry Creek till), probably during isotope stage 6; on that till, a paleosol (Majestic Canyon soil) dated at about 26,000 yr B.P.; overriding that soil, a later canyon-mouth glaciation (Bells Canyon till) probably beginning prior to about 19,000 yr B.P.; a midcanyon deglacial pause (Hogum Fork till) prior to 12,300 yr B.P.; an upper-canyon deglacial pause (Devils Castle till) prior to 7500 yr B.P.; and late Holocene periglaciation. Pollen ratios from bog profiles in the mid to upper reaches of the canyon suggest that temperatures cooler than the Holocene average occurred until after about 8000 yr B.P. Warmer and dryer than average conditions were initiated about 8000 to 7500 yr B.P. During the later portion of this Altithermal period conditions became relatively warm and wet. Two subsequent episodes of cooler than average temperatures correspond chronologically to the initial stades of Neoglaciation elsewhere in the Rocky Mountains. However, there is no geomorphic evidence of corresponding glacial activity in the canyon area. Relative moisture during these two periods differs significantly, suggesting that Neoglacial conditions were controlled primarily by changes in summer temperature.

Late Devonian--Early Mississippian Correlations Central Wasatch Mountains, Utah: ABSTRACT

Strata of Late Devonian age have been recognized west of the Wasatch Front (Pinyon Peak Limestone and City Creek Limestone) and a few miles east of the Wasatch Mountains in the western Uinta Mountains (Pinyon Peak Limestone?). These consist commonly of a basal sandstone or shale a few feet thick which grades upward to a dolomite sequence that ranges in thickness from 50 to 300 feet. This succession is in turn overlain by the Madison Limestone (Mississippian). On the west this contact is conformable but on the east it is unconformable. In the Wasatch Mountains, sandstone or shale a few feet thick rests on older rocks (mostly of Cambrian age) and changes upward through about a 3-foot interval into medium to dark gray dolomite about 50-150 feet thick, which in turn is over ain conformably by the Madison Limestone. These pre-Madison rocks were tentatively correlated with the Formation (Devonian) by earlier workers on the basis of stratigraphic position and lithologic character. Subsequent workers of the U. S. Geological Survey have reported corals of Mississippian affinity from the exposures in American Fork Canyon and have thus assigned a Mississippian age to these strata. Restudy of several of the Wasatch Front exposures disclosed well preserved molds of the brachiopod Cyrtospirifer whitneyi(?) in the basal sandstone in the Big Cottonwood Canyon area. This fossil is generally considered to be of Late Devonian age and has been collected from the Pinyon Peak Limestone in the western Uintas and in the areas west of the Wasatch. Thus the Mississippian age assigned to these rocks on the basis of corals is questionable. Moreover, a Late Devonian age is more consistent with a regional stratigraphic correlation on the basis of physical evidence. Therefore the Jefferson (?) of the Wasatch is here correlated with the Pinyon Peak Limestone of the areas east and west. End_of_Article - Last_Page 261------------

Paleozoic Geology of North and West Sides of Uinta Basin, Utah

Paleozoic Geology of North and West Sides of Uinta Basin, Utah