Uinta Mountains Research Articles

Detrital zircon provenance of Pennsylvanian to Permian sandstones from the Wyoming craton and Wood River Basin, Idaho, U.S.A.

Pennsylvanian rocks of the northern U.S. Rocky Mountains are mature quartzose sandstones. This paper uses detrital zircon geochronology on seven samples from the Wood River Formation, Tensleep Sandstone, and Weber Sandstone to determine if these sandstones have a common provenance, representing eastern Laurentian and Appalachian sand reworked within shallow-marine and eolian environments from the Wyoming craton westward to the Pioneer thrust plate of south-central Idaho. Our work suggests that this continental sand blanket was mixed with local sources on the south in the Yavapai-Mazatzal provinces of the Ancestral Rocky Mountains and in samples from the western Cordilleran thrust belt in south-central Idaho. In total, these Pennsylvanian sandstones contain a broad spectrum of detrital zircon U-Pb ages including, from old to young: A) minor Archean-age (3300–2550 Ma) populations; B) Paleoproterozoic (2000–1600 Ma), Mesoproterozoic (1470–1350 Ma), and major “Grenvillian” (1250–950 Ma) populations; and C) Cryogenian- to Ediacaran-age (665–565 Ma) and minor Paleozoic (495–410 Ma) populations. We interpret these detrital zircon ages to represent provenance mainly from the Appalachian mountain belt of eastern North America; however, central Appalachian versus northern Appalachian derivation is not clearly distinguished. The Weber Sandstone from the north flank of the Uinta Mountains in northeast Utah contains a strong 1700–1640 Ma age population derived from the Yavapai-Mazatzal provinces in the adjacent Ancestral Rocky Mountains. The shallow-marine Hailey Member of the Wood River Formation in south-central Idaho yields a population of >1800 Ma detrital zircons reworked from the uplifted Mississippian Copper Basin Formation. Both the Hailey and Wilson Creek Members of the Wood River Formation contain unique 640–490 Ma grains that may represent provenance from the Big Creek-Beaverhead plutonic belt of east-central Idaho and/or eastern Klamath terrane in the Klamath Mountains of northwest California and southwest Oregon. These new data support published models for Pennsylvanian–Permian transport of siliciclastic sediment with sources mainly from the North American craton, north of the Ancestral Rocky Mountains, into continental margin basins.

A quantitative midge-based reconstruction of mean July air temperature from a high-elevation site in central Colorado, USA, for MIS 6 and 5

Sediments recovered from the Ziegler Reservoir fossil site (ZRFS) in Snowmass Village, Colorado (USA) were analyzed for subfossil chironomids (or midges). The midge stratigraphy spans ~140–77ka, which includes the end of Marine Oxygen Isotope Stage (MIS) 6 and all of MIS 5. Notable shifts in midge assemblages occurred during two discrete intervals: the transition from MIS 6 to MIS 5e and midway through MIS 5a. A regional calibration set, incorporating lakes from the Colorado Rockies, Sierra Nevada, and Uinta Mountains, was used to develop a midge-based mean July air temperature (MJAT) inference model (r2jack=0.61, RMSEP=0.97°C). Model results indicate that the transition from MIS 6 to MIS 5e at the ZRFS was characterized by an increase in MJAT from ~9.0 to 10.5°C. The results also indicate that temperatures gradually increased through MIS 5 before reaching a maximum of 13.3°C during MIS 5a. This study represents the first set of quantitative, midge-based MJAT estimates in the continental U.S. that spans the entirety of MIS 5. Overall, our results suggest that conditions in the Colorado Rockies throughout MIS 5 were cooler than today, as the upper limit of the reconstructed temperatures is ~2°C below modern July air temperatures.

Properties of modern dust accumulating in the Uinta Mountains, Utah, USA, and implications for the regional dust system of the Rocky Mountains

ABSTRACTA growing body of work is illuminating the importance of dust as a component of soil development and biogeochemical cycling in alpine environments of the Rocky Mountains. Nonetheless, important questions remain about the dust system in this region due to a paucity of focused studies and direct measurements. This project involved deployment of modified marble dust traps in the alpine zone of the Uinta Mountains of Utah to trap modern atmospheric dust over a two year period. Results indicate that dust accumulation rates are similar to values previously reported for the Wind River Range of Wyoming, but less than values for south‐western Colorado, suggesting a south‐to‐north decrease in regional dust flux. The overall mean grain size of Uinta dust is similar to values reported by prior studies in Colorado, indicating a general uniformity in grain size distribution. Uinta dust is dominated by quartz, plagioclase, K‐feldspar, and illite with trace amounts of kaolinite, chlorite, and amphibole. In contrast, only quartz and K‐feldspar are present in the Uinta bedrock, confirming an exotic origin for the dust arriving in the alpine zone. Exotic mineralogies have also been reported in dust from other mountain ranges in the western United States, indicating that eolian delivery of allochthonous minerals is a widespread process. Geochemical analysis reveals that Bi, Na, P, Zn, Sn, Cu, Cd, Ba, Ni, W, Sb, Pb, and Tl in Uinta dust are elevated 10 to > 80‐times above their abundances in local bedrock. These results support reports of anthropogenic elemental enrichment in lake sediments from Colorado and Utah. Patterns of elemental loading suggest that certain elements are delivered by winds arriving from different directions, raising the possibility that some elements could be associated with unique source areas. Copyright © 2014 John Wiley & Sons, Ltd.

Snowpack regimes of the Western United States

AbstractSnow accumulation and melt patterns play a significant role in the water, energy, carbon, and nutrient cycles in the montane environments of the Western United States. Recent studies have illustrated that changes in the snow/rainfall apportionments and snow accumulation and melt patterns may occur as a consequence of changes in climate in the region. In order to understand how these changes may affect the snow regimes of the region, the current characteristics of the snow accumulation and melt patterns must be identified. Here we characterize the snow water equivalent (SWE) curve formed by the daily SWE values at 766 snow pillow stations in the Western United States, focusing on several metrics of the yearly SWE curves and the relationships between the different metrics. The metrics are the initial snow accumulation and snow disappearance dates, the peak snow accumulation and date of peak, the length of the snow accumulation season, the length of the snowmelt season, and the snow accumulation and snowmelt slopes. Three snow regimes emerge from these results: a maritime, an intermountain, and a continental regime. The maritime regime is characterized by higher maximum snow accumulations reaching 300 cm and shorter accumulation periods of less than 220 days. Conversely, the continental regime is characterized by lower maximum accumulations below 200 cm and longer accumulation periods reaching over 260 days. The intermountain regime lies in between. The regions that show the characteristics of the maritime regime include the Cascade Mountains, the Klamath Mountains, and the Sierra Nevada Mountains. The intermountain regime includes the Eastern Cascades slopes and foothills, the Blue Mountains, Northern and Central basins and ranges, the Columbia Mountains/Northern Rockies, the Idaho Batholith, and the Canadian Rockies. Lastly, the continental regime includes the Middle and Southern Rockies, and the Wasatch and Uinta Mountains. The implications of snow regime classification are discussed in the context of possible changes in accumulation and melt patterns associated with regional warming.

Influence of bedding dip on glacial erosional landforms, uinta mountains, usa

We explore the relationships amongst bedding dip, basin aspect, and glacial landforms using field observations and analyses of the northwestern inta ountains of tah. We examine basins on opposing sides of three ice divides in which quartzite beds of the Mount Watson Formation maintain a near constant dip. These areas provide contrasting relationships between ice flow and bedding dip directions while holding rock type and climate constant. We map the occurrence of three glacial erosional landforms: cliffs showing evidence of quarrying, scoured surfaces polished by abrasion, and overdeepenings. Cliffs and overdeepenings are more common in basins where bedding dips up‐basin, while scoured surfaces are more prevalent where bedding dips down‐basin. The significance of jointing in controlling glacial erosional forms is well established and we propose that bedding, as well as joints, dictates the geometry of quarried blocks and influences the spatial patterns of process dominance. Where bedding dips up‐basin, the geometry of pre‐existing weaknesses favours quarrying creating both cliffs and overdeepenings. In contrast, where bedding dips down‐basin, block geometry does not favour the creation of overdeepenings via quarrying and exposed bedding planes are subjected to glacial abrasion, producing scoured surfaces.

Fire history in the eastern Uinta Mountains, Utah, USA

Fire history in the eastern Uinta Mountains, Utah, USA

Sequence stratigraphy and formalization of the Middle Uinta Mountain Group (Neoproterozoic), central Uinta Mountains, Utah: A closer look at the western Laurentian Seaway at ca. 750 Ma

The mid-Neoproterozoic (ca. 750Ma) Uinta Mountain Group (UMG), northeast Utah, USA, records transgressions of an epicontinental sea at least 150 million years before inception of the Laurentian western passive margin. This work refines middle UMG stratigraphy by formalizing three lithostratigraphic formations and interpreting a sequence stratigraphic framework in the central part of the Uinta Mountains. Middle UMG marine and fluvial-deltaic facies associations comprise eight depositional sequences within a kilometer-thick upward-fining composite sequence. These depositional sequences form one progradational–aggradational (“lowstand”) sequence set (PASS) and one aggradational–progradational (“highstand”) sequence set (APSS). Depositional sequences were deposited within a tide- and wave-affected epicontinental seaway that received sediment from a south-flowing fluvial system proximally sourced in the Neoarchean southern Wyoming Province to the north and from a separate system of major, west-flowing trans-Laurentian rivers distally sourced in the Grenville orogen, the mid-continent granite province, and the Yavapai and Mazatzal provinces to the south and east.Middle UMG fluvial-marine paleogeography strengthens the genetic link with the coeval Big Cottonwood and Little Willow formations (Utah), the Chuar Group (Arizona), and upper Pahrump Supergroup (California), which collectively indicate a ca. 770–740Ma epicontinental seaway existed across much of western Laurentia before 700Ma rifting and 600Ma formation of the Cordilleran passive margin. Long-term marine transgression onto continental Laurentia was likely caused by supercontinent breakup, particularly rifting of the Tarim and South China blocks from the remainder of Rodinia. Poorly constrained autocyclic drivers (delta lobe switching, channel avulsion) and allocyclic drivers (climatic fluctuations, local tectonism) controlled higher frequency depositional cycles superimposed on this long-term transgression. Western Laurentia thus records prolonged (750–600Ma) episodic basin formation, mafic volcanism, and faulting before establishment of a continental terrace after 600Ma. The Neoproterozoic western Laurentian epicontinental seaway can be used as a tie-point with other continents to help constrain the original continental configuration of Rodinia.

A lacustrine sedimentary record of Holocene periglacial activity from the Uinta Mountains, Utah, U.S.A.

A lake sediment core from the Uinta Mountains of northern Utah was analyzed to constrain the timing of late Holocene periglacial activity. Records of numerous physical properties were converted to time series spanning the past 5300years using a depth-age model based on four AMS 14C dates. Long-term decreases in organic content and increases in bulk density attest to increasing inputs of clastic sediment. Abundance of mineral P, signaling physical bedrock weathering, reaches maximum values ca. 2900, 2150, and 1400calyr BP, coincident with finer median grain size and a shift toward darker red sediment. These peaks, interpreted as signals of periglacial activity, align with pulses of rock glacier activity in Colorado determined from lichenometry. The youngest peak coincides with lichenometric ages previously determined for periglacial deposits upstream from the lake. A pulse of renewed periglacial activity ca. 400calyr BP represents the Little Ice Age. The late 20th century witnessed extremely high values of organic matter and biogenic silica, and unprecedented low values of C:N, reflecting greatly enhanced in-lake productivity, likely due to disturbance in the watershed.

Investigating eutrophication in high elevation lakes in the Uinta Mountains, Utah (USA) using a multiproxy approach

Investigating eutrophication in high elevation lakes in the Uinta Mountains, Utah (USA) using a multiproxy approach

Fingerprinting of glacial silt in lake sediments yields continuous records of alpine glaciation (35–15 ka), western USA

Fingerprinting glacial silt in last glacial-age sediments from Upper Klamath Lake (UKL) and Bear Lake (BL) provides continuous radiocarbon-dated records of glaciation for the southeastern Cascade Range and northwestern Uinta Mountains, respectively. Comparing of these records to cosmogenic exposure ages from moraines suggests that variations in glacial flour largely reflect glacial extent. The two areas are at similar latitudes and yield similar records of glacial growth and recession, even though UKL lies less than 200km from the ocean and BL is in the continental interior. As sea level began to fall prior to the global Last Glacial Maximum (LGM), existing glaciers in the UKL area expanded. Near the beginning of the global LGM (26.5ka), the BL record indicates onset of glaciation and UKL-area glaciers underwent further expansion. Both records indicate that local glaciers reached their maximum extents near the end of the global LGM, remained near their maxima for ~1000yr, and underwent two stages of retreat separated by a short period of expansion.

Constraining the timing and provenance of the Neoproterozoic Little Willow and Big Cottonwood Formations, Utah: Expanding the sedimentary record for early rifting of Rodinia

U–Pb ages of detrital zircon spectra indicate that the Little Willow Formation in the Wasatch Range, Utah, is not a part of a Paleoproterozoic basement complex (∼1700–1800Ma) as previously thought, but is a metamorphosed part of the Big Cottonwood Formation (∼750Ma). The youngest detrital zircon grains in the metamorphic Little Willow and unmetamorphic Big Cottonwood Formations are 750–850Ma. These young zircons form a small, but persistent population possibly from Rodinia rift-related magmatism. The majority of the zircons are Grenville-age with other smaller populations derived from the Laurentian anorogenic granites, Mazatzal/Yavapai terranes, and Wyoming Craton. The distribution of new U–Pb detrital zircon ages from the Little Willow Formation has a high statistical probability of similarity to the detrital zircon spectra previously reported from both the Big Cottonwood Formation and the Uinta Mountain Group. Based on these similarities, we propose that the Little Willow Formation represents some of the earliest sediment shed into the Uinta rift basin during the earliest-known phases of Rodina break-up in western Laurentia. The Little Willow is not, therefore of Paleoproterozoic or Archean age as is shown on existing geologic maps.Lu–Hf isotopes in detrital zircons from the Little Willow and Big Cottonwood Formations compared with potential source regions provide evidence that the sediment could have been derived from eastern Laurentia and thus requiring a transcontinental river to transport the sediment ∼2000km. The U–Pb and Lu–Hf values from the Little Willow and Big Cottonwood Formations correspond well with the ∼1.0Ga Grenvillian basement, 1.4Ga A-type Granites intruding the Central and Western Yavapai, evolved portion of the 1.7–1.8Ga Santaquin Complex, and the 1.6–3.0Ga Farmington Canyon Complex (Wyoming Craton).

Physical, Chemical, and Thermal Properties of Soils across a Forest-Meadow Ecotone in the Uinta Mountains, Northeastern Utah, U.S.A.

Physical, chemical and thermal properties of five pedons in the Uinta Mountains of northeastern Utah were studied to determine how soils and microclimates vary across the ecotone between subalpine forest and dry meadow. All five profiles have developed in the same parent materials, from base upward: basal till, glaciofluvial sediment, and coarse supraglacial debris with infilled eolian silt. Temperatures were measured at the soil surface and at depths of 10 and 50 cm for 420 days. Meadow soils (Typic Humicryepts) are warmer, have Bw horizons, and contain more Ca, organic carbon, and clay in the A horizon. Forest soils (Inceptic Haplocryalfs) have Bt horizons, higher B/A horizon clay ratios, are more acidic, and contain more exchangeable Mg and total exchangeable cations than meadow soils. Although subzero temperatures were never recorded at the surface of the forest soil in the summers of 1998 and 1999, those at the surface of the meadow soil fell below 0 °C on ∼35% of the nights. Mean 4:00 a.m. temperatures in the meadow during the summer are significantly colder than those in the forest (2.2 °C vs. 5.2 °C). The meadow formed following an initial forest clearing event, possibly fire or an insect infestation. Since that time, forest encroachment has been slow because seedling establishment in the meadow is inhibited by a combination of frequent summer freezing events, moisture stresses resulting from textural discontinuities between soil parent materials, and competition between tree seedlings and meadow vegetation.

Timing and synchrony of births in bighorn sheep: implications for reintroduction and conservation

Context Timing (mean birthdate) and synchrony (variance around that date) of births can influence survival of young and growth in ungulate populations. Some restored populations of ungulates may not adjust these life-history characteristics to environments of release sites until several years after release, which may influence success of reintroductions. Aims We quantified timing and synchrony of births from 2005 to 2007 in four populations of reintroduced bighorn sheep (Ovis canadensis) occupying two ecoregions (Central Basin and Range and Wasatch and Uinta Mountains) in Utah, USA, to investigate whether bighorns would adjust these life-history characteristics to environmental conditions of the two ecoregions. We also compared timing and synchrony of births for bighorns in their source herd (Antelope Island) with bighorns in an ecologically similar release site (Stansbury Mountains) during 2006 and 2007. Methods We relocated female bighorns to record birthdates of young, and observed groups of collared bighorns to quantify use of elevation by those ungulates. We also calculated the initiation, rate and timing of peak green-up by ecoregion, using the normalised difference vegetation index. Key results We quantified 274 birthdates, and although only separated by 57 km, bighorn populations occupying the Central Basin and Range Mountains gave birth an average of 29 days earlier than did those on the Wasatch and Uinta Mountains, which corresponded with the initiation of vegetation green-up. Additionally, bighorn sheep on the Stansbury Mountains (ecologically similar release site) gave birth at similar times as did bighorns on Antelope Island (source area). Conclusions Populations of bighorn sheep that were reintroduced into adjacent ecoregions adjusted timing of births to environments and green-up of vegetation in restoration areas. Timing and synchrony of births for reintroduced bighorn sheep in an ecologically similar release site were the same as those of their source area. Implications Consideration should be given to the adjustment of timing and synchrony of births when reintroducing bighorns, especially when animals are released into different ecoregions. Also, biologists should select release sites that are ecologically similar to source areas, thereby reducing potential negative effects of animals adjusting timing and synchrony of births to environmental conditions of restoration areas.

A reassessment of Mojavia and a new Cheyenne Belt alignment in the eastern Great Basin

A separate basement terrane, Mojavia or the Mojave province with characteristic 2.0–2.3 Ga model ages, has been proposed to underlie much of the western U.S. Its existence, posited on patterns of Pb and Nd isotope data, has been propagated in the literature for more than two decades. New and compiled U/Pb geochronology shows there is no direct evidence of >2.0 Ga juvenile basement rocks exposed within Mojavia of the eastern Great Basin. Pb and Nd model ages, by contrast, vary from Archean to Neoproterozoic with large variations exhibited, commonly within small regions. Archean ages are concentrated northward, suggesting the influence of sediment shed southward from the Wyoming province onto Paleoproterozoic basement terranes. In places, including the Farmington Canyon complex, sediment has been tectonically reprocessed and now is preserved as high-grade metamorphic rock in accretionary melanges. There is no strong evidence that Mojavian basement exists outside of the Mojave Desert region proper. A statistical evaluation of common Pb and Sr isotopes in Phanerozoic igneous rocks shows distinct differences north and south of one proposed boundary between Mojavia and the Yavapai province. However, close examination suggests that variation in these parameters could be produced by the influence of Archean material shed from the Wyoming province rather than representing a distinct difference in the age and isotopic character of the basement. Statistical discrimination of candidate terranes and terrane boundaries may be valuable for their recognition, but such differences alone do not prove their existence. The northern boundary of Mojavia with the Wyoming province in the eastern Great Basin has been given the same name, the Cheyenne Belt, as the exposed suture be tween Paleoproterozoic and Archean basement in southern Wyoming. We reassess the location of this boundary in the Great Basin. New age controls on key basement outcrops in the Uinta Mountains and Farmington Canyon complex that were previously considered Archean indicate that these rocks are Paleoproterozoic in age (∼1.7 Ga). Thus, the Cheyenne Belt has traditionally been placed too far south; it must lie in a poorly defined location north of these localities.

The Wyoming carbon underground storage project: Geologic characterization of the Moxa Arch and Rock Springs Uplift

The Wyoming carbon underground storage project: Geologic characterization of the Moxa Arch and Rock Springs Uplift

Chapter 38 Neoproterozoic strata of southeastern Idaho and Utah: record of Cryogenian rifting and glaciation

Abstract Neoproterozoic strata in southeastern Idaho and Utah include the <766 Ma Uinta Mountain Group and Big Cottonwood Formation (Fm.) deposited in an east-trending rift basin and, to the west, the lower part of a westward-thickening rift to passive-margin succession that initiated c. 720 Ma. The latter contains a lower diamictite and volcanic succession, with a complex stratigraphic interval of Cryogenian marine glacial deposits (Pocatello and Mineral Fork formations and correlatives). This is overlain by a mostly terrigenous succession of <667 Ma strata assigned to the upper member of the Pocatello Fm. and Brigham Group in southeastern Idaho, to the Kelley Canyon Fm. and Brigham Group in northern and western Utah, and to the McCoy Creek Group and Prospect Mountain Quartzite in adjacent Nevada. Although the Brigham Group and correlative deposits contain no direct evidence for glaciation, widely developed, though stratigraphically restricted, incised valleys, with erosional relief from a few metres to as much as 160 m, are inferred to represent subsequent times of Cryogenian glacially lowered sea level. Overall interpretations of the stratigraphy and sedimentology of these rocks have changed little in the past 10–15 years. The most important recent advances relate to U–Pb geochronology. In strata that lie unconformably below demonstrable glacial deposits, the lower Uinta Mountain Group (formerly thought to be c. 900 Ma) contains populations of detrital zircons as young as 766±5 Ma. Cryogenian magmatism north of the Snake River Plain in central Idaho is recognized near House Mountain, east of Boise at c. 725±5 Ma, in the Pioneer Mountains Core Complex at about 695 Ma, and in central and east-central Idaho at 685–650 Ma. Clasts interpreted to be from the rift-related Bannock Volcanic Member of the Pocatello Fm. are dated at 717±4 Ma and 701±4 Ma. The overlying diamictite-bearing Scout Mountain Member contains a mafic lapilli tuff near the base (686±4 Ma) and a reworked fallout tuff near the top (667±5 Ma). Strongly negative C-isotope data have been obtained from some of the carbonate rocks, although the latter constitute only a small fraction of the succession. Palaeomagnetic data are available only for the Uinta Mountain Group, and suggest an equatorial palaeolatitude.

A ∼13,000-year paleolimnological record from the Uinta Mountains, Utah, inferred from diatoms and loss-on-ignition analysis

Paleolimnological research in mountainous regions of the Western United States provide baseline understanding of how these lake systems will respond to ongoing climate change. Fossil diatom assemblage and loss-on-ignition data were investigated from a ∼13,000-year lake sediment core from the Uinta Mountains, northeastern Utah, USA. Results indicate the presence of three major zones of environmental change: 1) from ∼12.9–10.9 cal ka, modern lake formation began, temperatures were cool, and water was turbid; 2) from ∼10.9–3.5 cal ka conditions were warmer-than-present, and the lake stratified during the summer; and 3) beginning approximately 3.5 cal ka, temperatures decreased relative to the previous zone and the lake appears to have become increasingly alkaline. Shifts in diatom taxa relative abundances within the middle zone (ca. 8.2–6.5 and 5.0–3.5 cal ka) are probably caused by precipitation-driven decreases in alkalinity and increases in lake depth. This record reflects evolution of the lake system and suggests that currently rising temperatures will reduce ice cover and enhance lake stratification and internal nutrient cycling. Any future changes in precipitation patterns may also impact lake ecology through shifts in alkalinity and/or depth.

Long‐Term Relationships Between Ocean Variability and Water Resources in Northeastern Utah1

Tingstad, Abbie H. and Glen M. MacDonald, 2010. Long‐Term Relationships Between Ocean Variability and Water Resources in Northeastern Utah. Journal of the American Water Resources Association (JAWRA) 46(5):987‐1002. DOI: 10.1111/j.1752‐1688.2010.00471.xAbstract: The Uinta Mountains in the northwestern Colorado River Basin are an important source of water for Utah and the western United States. This article examines 20th Century hydrology in the Uinta Mountains region in the context of the previous four to eight centuries as well as possible relationships with Pacific and Atlantic Ocean variability using new tree‐ring based reconstructions for streamflow and snowpack. The 20th Century appears to have been unusually wet compared with previous centuries. Relationships between hydrology in the region and the El Niño‐Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO), and Atlantic Multidecadal Oscillation (AMO) are largely insignificant in instrumental datasets but may have been stronger, although inconsistent, over the longer time spans represented by the paleoclimate records. Impacts of individual modes of sea surface temperature variability may sometimes be enhanced by periods when climate forcing by ENSO, PDO, and/or AMO coincide. Such episodes are associated with deviations from mean hydrology as high as +14% and as low as −18%. The 20th Century could be a misleading benchmark to base water resource estimates upon and flexible water management strategies are necessary to take into account the large range of natural variability observed in the longer‐term hydroclimatology as well as the challenges to predictability due to the apparently complex and inconsistent influence of ocean‐driven variability.

Fault fluid composition from fluid inclusion measurements, Laramide age Uinta thrust fault, Utah

The Laramide age (Late Cretaceous to Early Oligocene) Uinta thrust fault exposed on the northeast flank of the Uinta Mountains provides a fluid conduit for hydrocarbons that enter the Jurassic–Triassic age Nugget Sandstone, chemically reduce the red ferric oxide coloring minerals and bleach the sandstone. Mass spectrometric analyses of gases released from fluid inclusions during crushing of fault-associated calcite veins show fault fluids trapped in inclusions contain substantial CH 4 in addition to CO 2, N 2, Ar, H 2S, and C 2–C 6 hydrocarbons. The abundance of CO 2, CH 4, and other hydrocarbons is used to establish fault fluid redox conditions. Fluid inclusion temperature measurements together with gas analysis show fluids were trapped in inclusions over a range of temperatures and pressures. Temperature and salinity of the Uinta fault fluids determined by fluid inclusion homogenization temperatures corrected for hydrostatic pressure (350–850 bar) and ice melting are 95 to 217 °C and 0 to 11.8 eq. wt.% NaCl (average 3.8%). Vitrinite reflectance temperature estimates are 87 to 152 °C and temperature estimated from overburden thickness and thermal gradient are 142 to 202 °C. Log a H2(aq) ranges from − 5.3 to−5.7 estimated from CO 2/CH 4 equilibria at 160 °C. N 2/Ar ratios in excess of air-saturated water are consistent with derivation of N 2 from thermal decomposition of organic matter, and carbon and oxygen isotopic compositions of vein calcite suggest involvement of formation water. The Nugget Sandstone exposed in the fault footwall on the flanks of the Uinta Mountain uplift has been bleached from the moderate reddish brown color produced by hematite that coated the sand grains during early diagenesis to cream and nearly white colors, similar to bleached outcrops that have been observed in the correlative Navajo Sandstone on the Colorado Plateau. Geochemical modeling suggests that ample CH 4 and other hydrocarbons are available to reduce ferric oxide, bleach the sandstone, and precipitate disseminated pyrite and siderite.

Maximum depositional age and provenance of the Uinta Mountain Group and Big Cottonwood Formation, northern Utah: Paleogeography of rifting western Laurentia

Research Article| September 01, 2010 Maximum depositional age and provenance of the Uinta Mountain Group and Big Cottonwood Formation, northern Utah: Paleogeography of rifting western Laurentia Carol M. Dehler; Carol M. Dehler † 1Department of Geology, Utah State University, Logan, Utah 84322, USA †E-mail: carol.dehler@usu.edu Search for other works by this author on: GSW Google Scholar C. Mark Fanning; C. Mark Fanning 2Research School of Earth Sciences, Australian National University, Canberra 0200, Australia Search for other works by this author on: GSW Google Scholar Paul K. Link; Paul K. Link 3Department of Geosciences, Idaho State University, Pocatello, Idaho 83209, USA Search for other works by this author on: GSW Google Scholar Esther M. Kingsbury; Esther M. Kingsbury 3Department of Geosciences, Idaho State University, Pocatello, Idaho 83209, USA Search for other works by this author on: GSW Google Scholar Dan Rybczynski Dan Rybczynski 1Department of Geology, Utah State University, Logan, Utah 84322, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Carol M. Dehler † 1Department of Geology, Utah State University, Logan, Utah 84322, USA C. Mark Fanning 2Research School of Earth Sciences, Australian National University, Canberra 0200, Australia Paul K. Link 3Department of Geosciences, Idaho State University, Pocatello, Idaho 83209, USA Esther M. Kingsbury 3Department of Geosciences, Idaho State University, Pocatello, Idaho 83209, USA Dan Rybczynski 1Department of Geology, Utah State University, Logan, Utah 84322, USA †E-mail: carol.dehler@usu.edu Publisher: Geological Society of America Received: 22 May 2009 Revision Received: 10 Nov 2009 Accepted: 12 Nov 2009 First Online: 08 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 © 2010 Geological Society of America GSA Bulletin (2010) 122 (9-10): 1686–1699. https://doi.org/10.1130/B30094.1 Article history Received: 22 May 2009 Revision Received: 10 Nov 2009 Accepted: 12 Nov 2009 First Online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Carol M. Dehler, C. Mark Fanning, Paul K. Link, Esther M. Kingsbury, Dan Rybczynski; Maximum depositional age and provenance of the Uinta Mountain Group and Big Cottonwood Formation, northern Utah: Paleogeography of rifting western Laurentia. GSA Bulletin 2010;; 122 (9-10): 1686–1699. doi: https://doi.org/10.1130/B30094.1 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 U-Pb detrital zircon analyses provide a new maximum depositional age constraint on the Uinta Mountain Group (UMG) and correlative Big Cottonwood Formation (BCF) of Utah, and significantly enhance our insights on the mid-Neoproterozoic paleogeographic and tectonic setting of western Laurentia. A sandstone interval of the Outlaw Trail formation with a youngest population (n = 4) of detrital zircons, from a sampling of 128 detrital zircon grains, yields a concordia age of 766 ± 5 Ma. This defines a maximum age for deposition of the lower-middle Uinta Mountain Group in the eastern Uinta Mountains and indicates that the group is no older than middle Neoproterozoic in age (i.e., Cryogenian). These data support a long-proposed correlation with the Chuar Group of Grand Canyon (youngest age 742 Ma ± 6 Ma), which, like the Uinta Mountain Group and Big Cottonwood Formation, records nonmagmatic intracratonic extension. This suggests a ∼742 to ≤766 Ma extensional phase in Utah and Arizona that preceded the regional rift episode (∼670–720 Ma), which led to development of the Cordilleran passive margin. This is likely an intracratonic response to an early rift phase of Rodinia. Further, because the Chuar Group and the Uinta Mountain Group–Big Cottonwood Formation strata record intracratonic marine deposition, this correlation suggests a regional ∼740–770 Ma transgression onto western Laurentia. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.