Depositional system and lake-stage control on microbialite morphology, Green River Formation, eastern Uinta Basin, Colorado and Utah, U.S.A.

ABSTRACT Lake-margin lacustrine carbonates of the Green River Formation, in the eastern Uinta basin of Colorado and Utah, occur interbedded with fluvial and shoreline-parallel sandstone and shale. Microbial bindstones were deposited in a saline-alkaline lake during and after the Early Eocene Climate Optimum (EECO) (52–50 million years ago) that is characterized by global hot-house conditions, elevated atmospheric CO2, and highly fluctuating climate conditions. The stratigraphic architecture, chemostratigraphy, and morphology of the microbialites and other associated carbonate beds can be related to these climatic conditions. Three facies associations are recognized in the carbonate units across the lake margin from upper littoral to lower sublittoral environments: facies association 1, delta proximal non-microbial carbonates, characterized by quartzose bioclastic, peloidal, intraclastic packstones and grainstones–rudstones, quartose peloid wackestones and sandy oil shale; facies association 2, microbialite associated non-microbial carbonates, composed of ostracod, ooilitic, peloidal packstones–grainstones and intraclastic packstones, grainstones and rudstones; and facies association 3, microbial carbonates, consisting of diverse forms of stromatolitic and thrombolitic lithofacies. Multiple scales of carbonate cyclicity are suggested by shifts of δ18O and δ13C stable isotopes and deepening-upward microbialite facies. High-frequency cycles, on the order of 1 to 5 m thickness, are characterized by positive shifts in stable isotopes and interpreted deepening trends from littoral to lower sublittoral conditions. Large-scale trends, on the order of tens to hundreds of meters thickness record long-term lake changes, including: 1) sparse microbialite deposition during initial fresh conditions in lake stage 1, with low macro-structure diversity and light δ18O and δ13C isotope values; 2) transitional lake stage 2 corresponding to moderate macro-structural diversity, large meter-scale biostromal and biohermal buildups, and a positive shift in δ18O and δ13C isotope values that suggest increasing saline and alkaline conditions; 3) a highly fluctuating lake stage 3 that contains the highest microbialite macro-structural diversity and marks the interval of heaviest δ18O and δ13C isotope values, suggesting the greatest lake restriction, and the highest salinity and alkalinity conditions; and 4) a rising lake stage 4 that marks the lowest microbialite macro-structure diversity and a reversal in trend of δ18O and δ13C isotope values, that indicate deepening and freshening conditions.

Winter Ozone Pollution in Utah’s Uinta Basin is Attenuating

High concentrations of ground-level ozone have been observed during wintertime in the Uinta Basin of western Utah, USA, beginning in 2010. We analyze existing ozone and ozone precursor concentration data from 38 sites over 11 winter seasons and conclude that there has been a statistically significant (p < 0.02) decline in ozone concentration over the previous decade. Daily exceedances of the National Ambient Air Quality Standard for ozone (70 ppb) have been trending downward at the rate of nearly four per year. Ozone and NOx concentrations have been trending downward at the rates of about 3 and 0.3 ppb per year, respectively. Concentrations of organics in 2018 were at about 30% of their values in 2012 or 2013. Several markers, annual ozone exceedance counts and median ozone and NOx concentrations, were at their largest values in the period 2010 to 2013 and have never recovered since then. We attribute the decline to (1) weakening global demand for oil and natural gas and (2) more stringent pollution regulations and controls, both of which have occurred over the previous decade. We also see evidence of ozone titration when snow cover is absent.

High Ethylene and Propylene in an Area Dominated by Oil Production

We measured the spatial distribution and composition of ozone-forming hydrocarbons, alcohols, and carbonyls in Utah’s Uinta Basin during the winter months of 2019 and 2020. The Uinta Basin contains about 10,000 producing oil and gas wells. Snow cover and the region’s unique topography (i.e., a large basin entirely surrounded by mountains) promote strong, multi-day temperature inversion episodes that concentrate pollution and lead to wintertime ozone production. Indeed, organic compound concentrations were about eight times higher during inversion episodes than during snow-free springtime conditions. We examined spatial associations between wintertime concentrations of organics and oil and gas sources in the region, and we found that concentrations of highly reactive alkenes were higher in areas with dense oil production than in areas with dense gas production. Total alkene+acetylene concentrations were 267 (42, 1146; lower and upper 95% confidence limits) µg m−3 at locations with 340 or more producing oil wells within 10 km (i.e., 75th percentile) versus 12 (9, 23) µg m−3 at locations with 15 or fewer oil wells (i.e., 25th percentile). Twenty-eight percent of the potential for organic compounds to produce ozone was due to alkenes in areas with dense oil production. Spatial correlations and organic compound ratios indicated that the most likely source of excess alkenes in oil-producing areas was natural gas-fueled engines, especially lean-burning (i.e., high air:fuel ratio) artificial lift engines.

Mass Transport of Gases across the Air–Water Interface: Implications for Aldehyde Emissions in the Uinta Basin, Utah, USA

When they dissolve in water, aldehydes become hydrated to gem-diols: R−COH+H2O↔RCH(OH)2. Such reactions can complicate air–water transport models. Because of a persistent belief that the gem-diols do not exist in the vapor phase, typical models do not allow them to pass through the air–water interface, but in fact, they do. Therefore, transport models that allow both molecular forms to exist in both phases and to pass through the interface are needed. Such a model is presented here as a generalization of Whitman’s two-film model. Since Whitman’s model has fallen into disuse, justification of its use is also given. There are hypothetical instances for which the flux predicted by the current model is significantly larger than the flux predicted when models forbid the diol form from passing through the interface. However, for formaldehyde and acetaldehyde, the difference is about 6% and 2%, respectively.

Additional records and stratigraphic distribution of the middle Eocene carettochelyid turtle Anosteira pulchra from the Uinta Formation of Utah, North America

Background Anosteira pulchra is one of two species of the obligately-aquatic freshwater clade Carettochelyidae (pig-nosed turtles) from the Eocene of North America. Anosteira pulchra is typically rare in collections, and their distribution is poorly documented. The Uinta Formation [Fm.] contains a diverse assemblage of turtles from the Uintan North American Land Mammal Age. Whereas turtles are abundantly preserved in the Uinta Fm., A. pulchra has been reported only from a few specimens in the Uinta C Member. Methods We describe new records of Anosteira pulchra from the Uinta Basin and analyze the distribution of 95 specimens from multiple repositories in the previously published stratigraphic framework of the middle and upper Uinta Fm. Results Here we report the first records of the species from the Uinta B interval, document it from multiple levels within the stratigraphic section and examine its uncommon appearance in only approximately 5% of localities where turtles have been systematically collected. This study details and extends the range of A. pulchra in the Uinta Fm. and demonstrates the presence of the taxon in significantly lower stratigraphic layers. These newly described fossils include previously unknown elements and associated trace fossils, with new anatomical information presented. This study provides insight into the taxonomy of Anosteira spp. in the middle Eocene, and suggests the presence of a single species, though no synonymy is defined here due to limits in Bridger material.

Lacustrine cyclicity in the early Eocene Green River Formation, Uinta Basin, Utah: Evidence from X-ray fluorescence core scanning

ABSTRACT The Green River Formation preserves an extraordinary archive of terrestrial paleoclimate during the Early Eocene Climatic Optimum (EECO; ∼ 53–50 Ma), expressing multiple scales of sedimentary cyclicity previously interpreted to reflect annual to Milankovitch-scale forcing. Here we utilize X-ray fluorescence (XRF) core scanning and micro X-ray fluorescence (micro-XRF) scanning in combination with radioisotopic age data to evaluate a rock core record of laminated oil shale and carbonate mudstone from Utah's Uinta Basin, with the parallel objectives of elucidating the paleo-environmental significance of the sedimentary rhythms, testing a range of forcing hypotheses, and evaluating potential linkages between high- and low-frequency forcing. This new assessment reveals that the ∼ 100-μm-scale laminae—the most fundamental rhythm of the Green River Formation—are most strongly expressed by variations in abundance of iron and sulfur. We propose that these variations reflect changes in redox state, consistent with annual stratification of the lake. In contrast to previous studies, no support was found for ENSO or sunspot cycles. However, millimeter- to centimeter-scale rhythms—temporally constrained to the decadal to centennial scale—are strongly expressed as alternations in the abundance of silicate- versus carbonate-associated elements (e.g., Al and Si vs. Ca), suggesting changes in precipitation and sediment delivery to the paleo-lake. Variations also occur at the meter scale, defining an approximate 4 m cycle interpreted to reflect precession. We also identify punctuated intervals, associated principally with one phase of the proposed precession cycle, where Si disconnects from the silicate input. We propose an alternative authigenic or biogenic Si source for these intervals, which reflects periods of enhanced productivity. This result reveals how long-term astronomical forcings can influence short-term processes, yielding insight into decadal- to millennial-scale terrestrial climate change in the Eocene greenhouse earth.

Evidence for Wide Dispersal in a Stem Galliform Clade from a New Small-Sized Middle Eocene Pangalliform (Aves: Paraortygidae) from the Uinta Basin of Utah (USA)

A new bird coracoid from the Uinta Formation in the Uinta Basin in Utah (USA) records the presence of the only known pangalliform from the middle Eocene of North America, occurring in a >15 million year gap in their history. This fossil represents a new taxon, informally termed the Uintan paraortygid, which is also currently the best-supported record of the extinct Paraortygidae in North America (and among the oldest records of the group in the world). The specimen exhibits a derived enlarged procoracoid prominence with a small procoracoid process, and concave elliptical scapular cotyle that are shared with the middle Eocene paraortygids, Xorazmortyx and Scopelortyx; however, the Uintan paraortygid also has a possibly autapomorphic (pneumatic) fossa adjacent to the scapular cotyle. The similarity in body size and morphology among these widely distributed early paraortygids suggests phylogenetic affinity among them. Given their occurrence in the United States, Uzbekistan, and Namibia during the middle Eocene, these birds likely were good fliers with an increased ability to disperse; and probably had a flexible biology or diet allowing them to occupy a diversity of habitats from coasts and forests to semi-arid savannah-like habitats. The problematic early records of Odontophoridae need to be reexamined as potential members of Paraortygidae and associates of these small-bodied taxa.