Wind River Formation Research Articles

The damselfly palaeofauna from the Eocene of Wyoming and Colorado, USA (Insecta, Odonata, Zygoptera)

AbstractA new family, five new genera, and nine new species of fossil damselflies (Insecta, Odonata, Zygoptera, Calopterygida) from the USA are described, seven from the Eocene Fossil Lake deposits and one from Lake Uinta deposits, both from the Green River Formation, and an additional specimen from the Wind River Formation of Wyoming and Colorado. Namely, Carlea eocenica gen. et sp. nov. (in Carleidae fam. nov.), Labandeiraia riveri sp. nov., Labandeiraia browni sp. nov., Eodysphaea magnifica gen. et sp. nov., Litheuphaea sp. cf. coloradensis Petrulevičius et al., 2007, Zacallites cockerelli sp. nov., Dysagrion integrum sp. nov., Tenebragrion shermani gen. et sp. nov., Tynskysagrion brookeae gen. et sp. nov., and Oreodysagrion tenebris gen. et sp. nov. Epallagoidea and Amphipterygoidea are most common while Calopterygoidea, Coenagrionoidae and Lestoidea damselflies are less diverse. Genera of zygopteran Dysagrionidae are known from Europe and North America, further supporting the hypothesis of Palaeogene terrestrial interchange. Representatives of Epallagoidea and Amphipterygoidea in the Green River Formation confirm that warm conditions occurred at the time of deposition.

Endemism in Wyoming plant and insect herbivore communities during the early Eocene hothouse

AbstractThe warm, equable, and ice-free early Eocene Epoch permits investigation of ecosystem function and macro-ecological patterns during a very different climate regime than exists today. It also provides insight into what the future may entail, as anthropogenic CO2release drives Earth toward a comparable hothouse condition. Studying plant–insect herbivore food webs during hothouse intervals is warranted, because these account for the majority of nonmicrobial terrestrial biodiversity. Here, we report new plant and insect herbivore damage census data from two floodplain sites in the Wind River Basin of central Wyoming, one in the Aycross Formation (50–48.25 Ma) at the basin edge (WRE) and the second in the Wind River Formation in the interior of the basin (WRI). The WRI site is in stratigraphic proximity to a volcanic ash that is newly dated to 52.416 ± 0.016/0.028/0.063 (2σ). We compare the Wind River Basin assemblages to published data from a 52.65 Ma floodplain flora in the neighboring Bighorn (BH) Basin and find that only 5.6% of plant taxa occur at all three sites and approximately 10% occur in both basins. The dissimilar floras support distinct suites of insect herbivores, as recorded by leaf damage. The relatively low-diversity BH flora has the highest diversity of insect damage, contrary to hypotheses that insect herbivore diversity tracks floral diversity. The distinctiveness of the WRE flora is likely due to its younger age and cooler reconstructed paleotemperature, but these factors are nearly identical for the WRI and BH floras. Site-specific microenvironmental factors that cannot be measured easily in deep time may account for these differences. Alternatively, the Owl Creek Mountains between the two basins may have provided a formidable barrier to the thermophilic organisms that inhabited the basin interiors, supporting Janzen's hypothesis that mountain passes appear higher in tropical environments.

Impact to Underground Sources of Drinking Water and Domestic Wells from Production Well Stimulation and Completion Practices in the Pavillion, Wyoming, Field.

A comprehensive analysis of all publicly available data and reports was conducted to evaluate impact to Underground Sources of Drinking Water (USDWs) as a result of acid stimulation and hydraulic fracturing in the Pavillion, WY, Field. Although injection of stimulation fluids into USDWs in the Pavillion Field was documented by EPA, potential impact to USDWs at the depths of stimulation as a result of this activity was not previously evaluated. Concentrations of major ions in produced water samples outside expected levels in the Wind River Formation, leakoff of stimulation fluids into formation media, and likely loss of zonal isolation during stimulation at several production wells, indicates that impact to USDWs has occurred. Detection of organic compounds used for well stimulation in samples from two monitoring wells installed by EPA, plus anomalies in major ion concentrations in water from one of these monitoring wells, provide additional evidence of impact to USDWs and indicate upward solute migration to depths of current groundwater use. Detections of diesel range organics and other organic compounds in domestic wells <600 m from unlined pits used prior to the mid-1990s to dispose diesel-fuel based drilling mud and production fluids suggest impact to domestic wells as a result of legacy pit disposal practices.

Terrestrial paleoenvironmental reconstructions indicate transient peak warming during the early Eocene climatic optimum

Major changes in climate and ecology occurred during the early Eocene climatic optimum, sometime between 52 and 50 Ma. Recent work suggests that the timing and duration of the event are characterized by different responses in the marine and terrestrial realms, and that traditional causal mechanisms may not adequately explain such differences. We applied high-resolution paleopedology, geochemical analysis, and phytolith biostratigraphy techniques to paleosol suites within the well-described Wind River Formation of western Wyoming, USA. This multiproxy record indicates a short (<1 m.y.) peak period of carbon isotopic enrichment (up to 2‰ higher) and elevated pCO2, high temperatures (up to 8 °C higher), increased precipitation (up to 500 mm yr –1 higher), and shifts in fl oral composition (up to 10%). Terrestrial climatic and ecological changes of this kind during the early Eocene climatic optimum are consistent with changes in contemporaneous records that have been ascribed to high atmospheric pCO2, but a transient peak interval suggests that the cause of high atmospheric pCO2 during the early Eocene was likely not increased volcanism or decreased silicate weathering, which operate on longer timescales. Instead, terrestrial records from across western North America agree that early Eocene climatic optimum changes may have been caused by other sources, such as a combination of increased ventilation of oceanic carbon and increased petroleum generation in sedimentary basins. The climatic and environmental changes exhibited by this and other North American terrestrial records also defi ne a pattern of regional response that is relevant for understanding the impacts of global climate change events.

Sedimentology, detrital zircon geochronology, and stable isotope geochemistry of the lower Eocene strata in the Wind River Basin, central Wyoming

Shallow subduction of the Farallon plate beneath the western United States has been commonly accepted as the tectonic cause for the Laramide deformation during Late Cretaceous through Eocene time. However, it remains unclear how shallow subduction would produce the individual Laramide structures. Critical information about the timing of individual Laramide uplifts, their paleoelevations at the time of uplift, and the temporal relationships among Laramide uplifts have yet to be documented at regional scale to address the question and evaluate competing tectonic models. The Wind River Basin in central Wyoming is fi lled with sedimentary strata that record changes of paleogeog raphy and paleoelevation during Laramide deformation. We conducted a multidisciplinary study of the sedimentology, detrital zircon geochronology, and stable isotopic geochemistry of the lower Eocene Indian Meadows and Wind River formations in the northwestern corner of the Wind River Basin in order to improve understanding of the timing and process of basin evolution, source terrane unroofi ng, and changes in paleoelevation and paleoclimate. Depositional environments changed from alluvial fans during deposition of the Indian Meadows Formation to low-sinuosity braided river systems during deposition of the Wind River Formation. Paleocurrent directions changed from southwestward to mainly eastward through time. Conglomerate and sandstone compositions suggest that the Washakie and/or western Owl Creek ranges to the north of the basin experienced rapid unroofi ng ca. 55.5‐54.5 Ma, producing a trend of predominantly Mesozoic clasts giving way to Precambrian basement clasts upsection. Rapid source terrane unroofi ng is also suggested by the upsection changes in detrital zircon U-Pb ages. Detrital zircons in the upper Wind River Formation show age distributions similar to those of modern sands derived from the Wind River Range, with up to ~20% of zircons derived from the Archean basement rocks in the Wind River Range, indicating that the range was largely exhumed by ca. 53‐51 Ma. The rise of these ranges by 51 Ma formed a confi ned valley in the northwestern part of the basin, and promoted development of a meandering fl uvial system in the center of the basin. The modern paleodrainage confi guration was essentially established by early Eocene time. Carbon isotope data from paleosols and modern soil carbonate show that the soil CO 2 respiration rate during the early Eocene was higher than at present, from which a more humid Eocene paleoclimate is inferred. Atmosphere pCO 2 estimated from paleosol carbon isotope values decreased from 2050 ± 450 ppmV to 900 ± 450 ppmV in the early Eocene, consistent with results from previous studies. Oxygen isotope data from paleosol and fl uvial cement carbonates show that the paleoelevation of the Wind River Basin was comparable to that of the modern Great Plains (~500 m above sea level), and that local relief between the Washakie and Wind River ranges and the basin fl oor was 2.3 ± 0.8 km. Up to 1 km of post-Laramide regional net uplift is required to form the present landscape in central Wyoming.

A New Large, Hypercarnivorous Oxyaenid (Mammalia, Creodonta) from the Middle Eocene of the Wind River Formation, Natrona County, Wyoming

Abstract Malfelis badwaterensis, gen.et sp. nov., represented by a nearly complete juvenile skull, isolated I1–2, P1, DP2, and a fragment of the dentary from the same individual (CM 62590), is described from the early middle Eocene (Gardnerbuttean, early Bridgerian) of the Wind River Formation, Wyoming in North America. M. badwaterensis preserves a unique combination of characters that set it apart from all other creodonts including the presence of a P1 and reduced M2, a carnassial complex from DP3–4–M1 with reduced protocones, simplified trenchant P2–3, and M1 with conjoined metacone and paracone, and metastylar blade. M. badwaterensis is referred to the Oxyaenidae based on the well-developed metastylar blade of M1, a reduced M2, and loss of M3. This new creodont was perhaps the largest of all animals from the type locality, which has yielded nearly 80 species of mammals.

Variation in Sedimentology and Architecture of Eocene Alluvial Strata, Wind River and Washakie Basins, Wyoming

Eocene continental, alluvial strata of the Wind River Formation (Wind River Basin) and the Cathedral Bluffs Member of the Wasatch Formation (Washakie basin) provide two examples of Laramide intermontane basin aggradation. These alluvial sediments primarily represent overbank flood deposits marginal to channel complexes. Their sedimentology and architecture, although grossly similar, appear to vary somewhat with proximity to Laramide uplifts. In both cases, repetitive sedimentation on the floodplain produced a succession of depositional couplets, each composed of a light-gray sand overlain by a red clay-rich silt or sand. The lower sands are tabular bodies that, near their distal margins, taper discernibly. They commonly display planar and ripple-drift laminations. Upper clay-rich layers, which are laminated, are also generally tabular. Those floodplain strata depositional proximal to Laramide uplifts show little evidence of scouring prior to deposition of the next, overlying couplet. Most of these sedimentary layers, therefore, are laterally continuous (up to 2 km). This alluvial architecture results in relatively uniform porosity laterally within depositional units but variable porosity stratigraphically through the sequence. In contrast, alluvial sediments deposited farther from the Laramide uplifts have undergone sporadic incision (either during rising flood stage or subsequently) followed by aggradation. As a result, many of thesemore » floodplain couplets are discontinuous laterally and, hence, exhibit large-scale lateral variability in porosity. Both alluvial sequences have undergone similar types and extents of burial diagenesis.« less

Revision of the Wind River faunas, early Eocene of central Wyoming. Part 6. Stratigraphic sections and locality descriptions, upper part of the Wind River formation

Revision of the Wind River faunas, early Eocene of central Wyoming. Part 6. Stratigraphic sections and locality descriptions, upper part of the Wind River formation

Revision of the Wind River faunas, early Eocene of central Wyoming. Part 5. Geology and biostratigraphy of the upper part of the Wind River formation, northeastern Wind River Basin

Revision of the Wind River faunas, early Eocene of central Wyoming. Part 5. Geology and biostratigraphy of the upper part of the Wind River formation, northeastern Wind River Basin

Laramide History of Northwestern Wind River Basin and Washakie Range, Wyoming: ABSTRACT

A complex history of Laramide tectonism, erosion, and deposition is recorded in lower Cenozoic rocks of the northwestern Wind River basin and Washakie Range, Wyoming. These rocks include the Indian Meadows Formation (lower Eocene), Wind River Formation (middle to uppermost lower Eocene), and Aycross Formation (middle Eocene), separated by unconformities. Major structures developed during latest Cretaceous through earliest Eocene time, prior to deposition of preserved Tertiary rocks. During this interval, the Washakie Range arched upward and thrust southwestward over the basin axis and erosion stripped more than 5,000 ft (1,500 m) of upper Mesozoic rocks from the area, forming the basal Tertiary unconformity. During the Eocene, the Indian Meadows Formation accumulated as alluvial fans spread from deep mountain valleys southward across the basin. Large Paleozoic masses slid southward off the oversteepened range front. Near the end of Indian Meadows deposition, a new fold arched and thrust southward, depressing and downfaulting the older range front and disturbing rocks in the basin. During the middle early Eocene, the Wind River Formation accumulated as new alluvial fans spread southward from canyons of the rejuvenated range. Erosion had breached the Precambrian core of the range, flooding downstream areas with granitic debris. During the early middle Eocene, the range collapsed along normal faults. The Aycross Formation accumulated in flood plains in the basin and in broad valleys that developed along normal fault traces in the range. During this time, the Absaroka volcanic field to the north became the dominant local source. By the end of Aycross deposition, the basin and all but the highest parts of the collapsed, deeply eroded range were buried by volcaniclastic debris. End_of_Article - Last_Page 955------------

A new species ofOrohippus(Perissodactyla, Equidae) from the early Eocene of Wyoming

ABSTRACT Orohippus proteros n. sp. is the earliest and most primitive species of Orohippus known. It was collected from the Lost Cabin Member of the Wind River Formation, along with a fauna which is clearly of early Eocene age. Known only from the type specimen, O. proteros is distinguished from all contemporary hyracotheriines by the presence of a small hypocone on P4. Orohippus proteros is more primitive than all other members of the genus and later hyracotheres in the small size of the P4 hypocone relative to the protocone and in the subtriangular shape of P4. Orohippus is viewed as a monophyletic taxon with O. proteros as the ancestral species.

Geochemical and Sedimentologic Problems of Uranium Deposits of Texas Gulf Coastal Plain: DISCUSSION

Huang (1978) stated that the mobility and accumulation of uranium in host rocks are controlled by several factors, including the hydrologic factor. Some of his statements are misleading as indicated by study of ancient stream channels in Wyoming which has led to increased discoveries of uranium deposits. Because it is believed that the transportation mechanism for uranium in paleoaquifer host rocks is groundwater, the flow of the uranium-bearing solution is a function of the infiltration or recharge into the aquifer, not necessarily of the rainfall. Huang stated that the time of accumulation depends upon the dip of the host rock. The flow of a groundwater system is largely independent of the dip of the beds as stated by Daray's (1856) law: Q = KA delta h/delta l h. Dividing this equation by the area, A, will result in the flux or average velocity of a particle. Data was included for the Wind River Formation in the Gas Hills Uranium district of Wyoming. some data differs from Huang. According the Lohman equation, groundwater velocity would be three times greater in the Texas example than in the northern Wyoming area, presuming the same porosity, groundwater gradient, and flow-path length. (DP)

Lepidopteran Leaf Mine from the Early Eocene Wind River Formation of Northwestern Wyoming

A lepidopteran mine, probably of Phyllocnistis, on a leaflet impression of Cedrela (Meliaceae) discovered in late early Eocene strata near Dubois, Wyoming, is the earliest record of leaf mining and of the Phyllocnistidae. Considerable prior evolution of the mining habit, antiquity of the Cedrela-Phyllocnistis relationship, and subtropical climatic conditions are indicated.

Uranium-Lead Ages of the Uranium Deposits of the Gas Hills and Shirley Basin, Wyoming

The uranium-lead ages for two massive uraninite samples from two deposits, one each in the Gas Hills and Shirley Basin districts, are both 22 + or - 3 m.y. as determined by 206 Pb/ 238 U and 207 Pb/ 235 U. These are sufficiently concordant to confirm an early Miocene time of mineralization for both deposits. Uranium-lead ages for several ore-grade samples from an upper sandstone layer of the Shirley Basin indicate 24 + or - 5 m.y. The ore samples show less concordancy in age than the massive uraninite samples and are believed to give a less reliable age because of a large common lead correction.Radioactive secular equilibrium between 238 U and 234 U was determined by alpha spectrometry to be > 99 percent for the massive uraninite samples; equilibrium between 238 U and 230 Th (ionium) was established by a direct method of isotopic mass ratios. A method of indexing the common lead correction using 208 Pb is used with these young samples which contain negligible 232 Th.Both the Gas Hills and Shirley Basin uranium deposits, located about 85 miles (137 km) apart, are in the Wind River Formation. The radioactive equilibrium found in the massive uraninite samples supports the reliability of the age found for these two samples. The loss of equilibrium in the ore-grade samples, along with the less reliable lead age, indicates that some local migration of elements occurred surrounding the roll at the Shirley Basin. Apparently this has not affected the lead-uranium system sufficiently to invalidate a date of 22 m.y. for the emplacement of both deposits and the conclusion that ore deposition stopped at about that time.

Uranium Deposits of Gas Hills, Wyoming: ABSTRACT

The Gas Hills uranium district is in central Wyoming along the southeastern margin of the Wind River basin. The initial discovery was made by Neil E. McNeice in the fall of 1953. Earliest development of the district's ore reserves was quite slow, but accelerated when the larger, more experienced mining firms became active in the area. Surface drilling proved to be the most effective tool for finding and developing the uranium ore reserves. New drilling techniques were needed to obtain samples of the below-water-table ores. Many methods were tried, but frozen core drilling and bucket augering proved to be the most reliable for obtaining accurate samples. Since the initial discovery, the Gas Hills uranium district has produced about 12% of the United States total production. The present land surface is characterized by barren, subdued, rolling hills. These are traversed locally by steeply dipping hogback ridges of older, more resistant rocks, which are the flanks of truncated, northward plunging folds formed by crustal disturbances prior to the deposition of the Wind River Formation. A steep erosional escarpment that rises abruptly above the north sloping basin floor bisects the region and divides the surface drainage between tributaries of the Wind River on the north and tributaries of the Sweetwater River on the south. Volcanism occurred during late Eocene time, as evidenced by relic vents at the southern end of the Rattlesnake Hills, and by local volcanic debris in the middle and upper Eocene rocks. Sedimentary rocks exposed in the Gas Hills uranium district include sandstone, limestone, dolomite, shale, and tuffaceous sandstone, mudstone, and shale. They range in age from Cambrian to Miocene and have a composite thickness of over 14,000 ft. The source beds for the uranium deposits are arkosic sandstones interstratified with lenticular mudstones and shales. Two distinct types of sandstone are present in the Wind River Formation. The youngest is yellowish-orange to yellowish-gray arkose, derived primarily from Precambrian gneissoid and granitoid rocks; it contains little clay, abundant calcium carbonate, and limonite cement, and is host for all uranium deposits of the district. The second type of sandstone is pale yellowish-gray to pale olive, derived from areas of schists of Precambrian age; it contains abundant clay matrix. There are four types of uranium deposits in the district, the most important being the solution-front deposits. They were formed along the margins of highly altered, tabular sand beds that are enclosed by overlying and underlying fine-grained siltstone, claystone, and carbonaceous mudstone beds. Solution fronts can be followed for long distances and individual ore bodies along them may reach thousands of feet in length. The solution fronts are ideally crescentic or C-shaped when viewed in cross section, with thin mineralization forming the tips of the crescents. The uranium minerals occur as earthy brown to black coating on, and interstitial fillings between, the quartz-sand grains. The primary uranium ore minerals are coffinite and uraninite. The three other types of deposits include transitional-bedded, oxidized, and residual-remnant deposits. Several quite large transitional bedded deposits have been mined, but the oxidized and residual-remnant deposits are commonly small and difficult to mine. Ground waters, trapped by the southward tilting of the Tertiary rocks during late Miocene time, became stagnant. These waters dissolved uranium and other elements from the enclosing rocks, and after erosion had exposed the highest beds of the Wind River Formation, the mineral-rich solutions gained egress from the enclosing sand aquifers on the north and the solution-front ore deposits began to form. End_of_Article - Last_Page 905------------

Stratigraphy and Preliminary Biostratigraphy of the Flagstaff Rim Area, Natrona County, Wyoming

ABSTRACT Entry, Robert J. Stratigraphy and Preliminary Biostratigraphy of the Flagstaff Rim Area, Natrona County, Wyoming. Smithsonian Contributions to Paleo­biology, number 18, 43 pages, 19 figures + frontispiece, 1973.—About 750 feet of sediments of the early Oligocene (Chadronian) White River Formation are exposed along Flagstaff Rim in south-central Natrona County, Wyoming. About 4,000 specimens of fossil vertebrates have been collected from these outcrops. The White River Formation unconformably overlies rocks ranging in age from Precambrian to medial or late Eocene. The lithology of the White River For­mation is predominantly claystone and conglomerate in the lower part of the section, changing to predominantly tuffaceous siltstone and conglomeratic chan­nel sandstones in the upper part. Four stratigraphic sections are described. A geo­logic map of about 40 square miles illustrates the areal limits of the White River Formation and its relationships to underlying and overlying formations. Several distinct and easily recognizable volcanic ash beds occur at intervals within the White River sequence. These serve as convenient markers for precise strati­graphic zonation of fossils and have also provided minerals for potassium-argon dating. Dates obtained range from 35.7 to 31.6 million years. A boulder conglomerate unit, previously considered to be the basal unit of the White River Formation and/or part of the Wind River Formation is shown to be a distinct, and probably unnamed, unit, and should not be assigned to either of these formations. It unconformably overlies the Wind River Formation and is separated from the White River Formation by an erosional disconformity with several hundred feet of relief. This information allows new interpretations of the structure of the area and adds a previously unrecognized episode of depo­sition and erosion to the history of the area. The most common fossil in the White River sequence is the artiodactyl genus Leptomeryx, which is represented by two morphologically distinct lineages. One lineage is provisionally divided into two and the other into three size groups that are believed to represent different species. The local stratigraphic ranges of the different groups do not overlap. In each lineage, the size increases higher in the section. None of the groups are definitely assigned to named species, pending studies to determine the validity and limits of the named species. Preliminary analysis of other elements of the fauna shows that there is recog­nizable change through time within individual lineages and that the faunal composition as a whole changes through time, within the local sequence. When the entire fauna is analyzed in detail, it should be possible to establish local range zones of the fossil species and, by their use, to gain greater temporal resolution within Chadronian time than has previously been possible.

The mammalian fauna of the Lost Cabin member, Wind River formation (lower Eocene) of Wyoming

The mammalian fauna of the Lost Cabin member, Wind River formation (lower Eocene) of Wyoming

Tertiary Wind River Formation--Uranium Resources and Geology: ABSTRACT

Sedimentary rocks of the Wind River Formation of early Eocene age and its equivalents (Battle Springs and Wasatch Formations) are the host rock for at End_Page 1898------------------------------ least 95% of the uranium reserves in Wyoming. Wyoming reserves were estimated by the AEC at 53,270 tons of U3O8 as of January 1, 1967. Distribution of reserves in tons of U3O8 together with production to January 1, 1967 are estimated as follows: Gas Hills, 19,560± tons reserve, 17,300 tons produced; Shirley basin, 27,000± tons reserve, 3,450 tons produced; Crooks Gap, 4,000± tons reserve, 2,400 tons produced; other Tertiary basins including Powder River, 1,000± tons reserve, 1,100 tons produced. Wind River and equivalent rocks crop out in broad areas within the intermontane Tertiary basins of central Wyoming. The sediments that comprise these lower Eocene beds were derived from mountain ranges that were uplifted during Late Cretaceous through earliest Eocene times. Two facies predominate within the major basins. A coarse arkosic sandstone and conglomerate facies with interbedded siltstone dominates near the mountain fronts. Major uranium deposits occur in this facies. Farther out in the basins the coarse-grained sediments grade into or interfinger with a variegated fine-grained facies. Economic concentrations of uranium occur near the margins of tongues of altered sandstone within the coarse-grained facies and are classified as roll-type deposits. Character of alteration differs from basin to basin but has been recognized to some degree in all basins. Alteration consists of the oxidation products produced by mineralized ground water passing through a transmissive sandstone bed. Geometrically the deposits are tongue-shape in plan and crescent-shape in vertical section with the concave side toward the altered sandstone. Deposition of uranium occurred at the front of an advancing aqueous chemical system which moved through the host sandstone bed. The oxygenated water rich in uranium, selenium, and other trace elements moved along the hydrologic gradient and oxidized and leached various minerals, including uranium, as it progressed. Precipitation of the uranium and associated elements occurred at a point within the aquifer where the pH and Eh of the system dropped sharply. The change in chemical environment was the result of the presence of H2S, probably produced by anaerobic bacteria. End_of_Article - Last_Page 1899------------

A New Species of Dichobunid Artiodactyl from the Early Eocene of Wyoming

A new species of artiodactyl, Bunophorus sinclairi , from the early Eocene Wind River Formation of Wyoming is described, and a revision is made in the generic description of Bunophorus . This species differs from previously described early Eocene artiodactyls in the possession of a second cusp on P4. The possibility that a new genus should be erected for this species is considered and rejected.

Stable isotopic geology of the gas hills, wyoming, uranium district

A sulfur, carbon, and oxygen isotopic study of more than 300 samples has been made of the sandstone-type uranium deposits of the Gas Hills district, Wyoming. The deposits occur as rolls and blankets in the arkosic, lower Eocene Wind River formation located on the southern edge of the Wind River Basin.Pyrite cement associated with uranium mineralization (mean delta S 34 = --23ppm, + or - 12.3ppm) is isotopically similar to the acid soluble sulfides produced by anaerobic bacteria in nearby springs and is significantly lighter and more variable in delta S 34 composition than undoubted magmatic hydrothermal sulfides. Syngenetic-diagenetic pyrite nodules in barren siltstones have a delta S 34 composition of 3.7ppm, + or -10.6ppm. Isotopic analyses of hydrogen sulfide from neighboring oil fields, pyrite in granitic Precambrian cobbles, and groundwater sulfate from the overlying tuffaceous strata are consistent with, but do not prove, the volcanic leach hypothesis of origin for the sulfur of the epigenetic pyrite.The very light delta S 34 composition of pyrite associated with jordisite and coalified wood, and the enrichment of S 34 in selenium-bearing pyrite and groundwater sulfate of the host Wind River formation suggest limited production of hydrogen sulfide, resulting in isotopically heavy supergene sulfides. The extremely light isotopic composition of pyrite associated with jordisite on the leading edges of roll ore bodies is probably caused by oxidation and re-reduction of previously formed biogenic sulfides.Further evidence of a biogenic origin for these deposits is furnished by the carbon isotopic composition of epigenetic calcite cement (--22ppm, + or -4.0ppm) that is intimately associated with organic material. The similarity of the delta C 13 values of calcite associated with asphalt and the calcite associated with coalified wood implies that the anaerobes derived their energy from both. The heavier isotopic composition of most of the other epigenetic calcites suggests some contamination by sedimentary carbonate during calcite precipitation.The epigenetic calcite cements contain some of the lightest oxygen yet reported for carbonates (mean delta SMOW O 18 = 5.6ppm, + or -6.0ppm) and may indicate that the uranium deposits formed during Pleistocene time.This study shows that syngenetic-diagenetic and epigenetic deposits of bacterial origin have distinguishable sulfur and carbon isotopic characteristics. The extremely light delta S 34 composition of sandstone-type uranium deposits is believed to be primarily due to the reduction of fairly light sulfate by starved anaerobes in an environment that approximates a closed system. Further enrichment in S 32 occurs when these isotopically light sulfides are oxidized to light sulfate which is again reduced to pyrite during the formation of roll ore bodies.