Eocene Green River Research Articles

Tracking drainage basin evolution, continental tectonics, and climate change: Implications from osmium isotopes of lacustrine systems

Organic matter in the lacustrine Eocene Green River Formation in two cores separated by 16 km in the Uinta Basin record similar centimeter-scale initial 187Os/188Os (denoted as Osi) stratigraphic profiles. This demonstrates that the Os isotopic composition of lacustrine organic matter is spatially homogeneous and in this case documents variation on the time scale of thousands of years. The Osi stratigraphy developed here indicates temporal change in the Os isotopic composition of lake water which is regulated by the Os isotopic composition of inflowing chemical weathering products from the surrounding drainage basin. Although observed Osi variability is just above the level of analytical uncertainty, it can be modeled with less than a 6% change in water input from radiogenic to nonradiogenic sources. This highlights the sensitivity of this isotopic tracer. Changes in Os isotope composition may reflect changes in relative discharge of rivers with different Os isotope composition, changes in the footprint of the drainage area to include or exclude different rock types, or unroofing. Thus, this study suggests that Os isotope stratigraphy in lacustrine settings can be used to link basin stratigraphy to changes in the drainage basin which are driven by climatic, tectonic, and geomorphic processes.The similarity in Os isotope stratigraphy between the two cores documented here highlights its utility for detailed chronostratigraphic correlation within and between lacustrine basins. A conservative sedimentation rate suggests that Osi in these lacustrine deposits changed at a rate of ~0.01 per kiloyear, two orders of magnitude faster than has been reported in marine organic-rich sedimentary rocks (Ravizza, 2007). Furthermore, linear regression of 187Re/188Os and 187Os/188Os ratios of all samples yields a Model 3 depositional age determination of 48.3 ± 2.8 Ma (2σ, MSWD = 8.2) which statistically indistinguishable from a weighted mean laser fusion 40Ar/39Ar sanidine age of 49.58 ± 0.32 Ma (Smith and Carroll, 2015) of a temporally adjacent volcanic ash.

Biogenically induced bedded chert formation in the alkaline palaeo-lake of the Green River Formation

Rhythmically bedded cherts are observed in both pelagic marine and lacustrine deposits, but the formation mechanism in the latter remains highly uncertain. Our study of alternating chert–dolomite beds in the Eocene Green River Formation, Utah, USA reveals dense accumulations of organic-matter spheres (30–50 μm diameter) of probable algal cyst origin in the chert layers, and centennial- to millennial-scale periodicities in chert layer deposition. A positive correlation between the degree of degradation of the organic spheres and Si distribution implies decomposition of algal organic matter lead to precipitation of lacustrine chert. As both alkalinity and dissolved silica were likely high in the palaeo-lake waters of the Green River Formation, we hypothesize that decomposition of algal organic matter lowered the pH of sediment pore waters and caused silica precipitation. We propose a formation model in which the initial abundance of algal organic matter in sediment varies with productivity at the lake surface, and the decomposition of this algal matter controls the extent of silica precipitation in sediment. The formation of rhythmically bedded chert–dolomite may be linked to centennial- to millennial-scale climatic/environmental factors that modulate algal productivity, which are possibly tied to solar activity cycles known to have similar periodicities.

When “evaporites” are not formed by evaporation: The role of temperature and pCO2 on saline deposits of the Eocene Green River Formation, Colorado, USA

Abstract Halite precipitates in the Dead Sea during winter but re-dissolves above the thermocline upon summer warming, “focusing” halite deposition below the thermocline (Sirota et al., 2016, 2017, 2018). Here we develop an “evaporite focusing” model for evaporites (nahcolite + halite) preserved in a restricted area of the Eocene Green River Formation in the Piceance Creek Basin of Colorado, USA. Nahcolite solubility is dependent on partial pressure of carbon dioxide (pCO2) as well as temperature (T), so these models covary with both T and pCO2. In the lake that filled the Piceance Creek Basin, halite, nahcolite or mixtures of both could have precipitated during winter cooling, depending on the CO2 content in different parts of the lake. Preservation of these minerals occurs below the thermocline (>∼25 m) in deeper portions of the basin. Our modeling addresses both: (1) the restriction of evaporites in the Piceance Creek Basin to the center of the basin without recourse to later dissolution and (2) the variable mineralogy of the evaporites without recourse to changes in lake water chemistry. T from 20 to 30 °C and pCO2 between 1800 and 2800 ppm are reasonable estimates for the conditions in the Piceance Creek Basin paleolake. Other evaporites occur in the center of basins but do not extend out to the edges of the basin. Evaporite focusing caused by summer-winter T changes in the solubility of the minerals should be considered for such deposits and variable pCO2 within the evaporating brines also needs to be considered if pCO2 sensitive minerals are found.

The First Mantidfly (Neuroptera: Mantispidae) from the Early Eocene Green River Formation

Protonolima mantispinoformis gen. et sp. nov. (Neuroptera: Mantispidae) is described from the early Eocene Green River Formation. It likely belongs to Calomantispinae, on the basis of the very distal origin of posterior radius and the basal crossvein between media and cubitus (1m-cu) connecting radius + media and anterior cubitus; if so, it would be the only fossil representative of the subfamily. Protonolima gen. nov. is probably most closely related to the extant American genus Nolima.

The Aspen paleoriver: Linking Eocene magmatism to the world’s largest Na-carbonate evaporite (Wyoming, USA)

Abstract Deposition of trona, nahcolite, and other Na-carbonate evaporite minerals in lakes is commonly closely associated with active volcanism, suggesting that the excess alkalinity required for their formation may arise from fluid-rock interactions involving hydrothermal waters that contain magmatic CO2. Paradoxically, the world’s largest Na-carbonate occurrence, contained within the Eocene Green River Formation in Wyoming, USA, was not associated with nearby active magmatism. Magmatism was active ∼200 km southeast in the Colorado Mineral Belt, however, suggesting that a river draining this area could have supplied excess alkalinity to Eocene lakes. Sedimentologic studies in southwestern Wyoming, along the course of the hypothesized Aspen paleoriver, document fluvial and deltaic sandstone with generally northwest-directed paleocurrent indicators. Sandstone framework grain compositions and detrital zircon ages are consistent with derivation from the Colorado Mineral Belt and its host rocks. These results provide the first confirmation of a fluvial connection to downstream Eocene lakes, and indicate that lake deposits may offer a unique perspective on upstream magmatic and hydrothermal histories.

Climate impact on fluvial-lake system evolution, Eocene Green River Formation, Uinta Basin, Utah, USA

AbstractIn light of a modern understanding of early Eocene greenhouse climate fluctuations and new highly seasonal fluvial system faces models, the role of climate in the evolution of one classically-cited continental, terminal lake system is re-examined. Detailed stratigraphic description and elemental abundance data from fifteen cores and seven outcrop regions of the Green River Formation were used to construct a ∼150 km cross section across the Uinta Basin, Utah, USA. Lake Uinta in the Uinta Basin is divided into five lake phases: (1) post-Paleocene Eocene Thermal Maximum, (2) peak Eocene hyperthermal, (3) waning hyperthermal, Early Eocene Climatic Optimum (EECO), (4) post-hyperthermal, and (5) post-EECO regimes, based primarily on climatically driven changes in fluvial style in combination with sedimentary indicators of lacustrine carbonate deposition, organic matter preservation, salinity, and lake depth. Basinwide siliciclastic dominated intervals were deposited by highly seasonal fluvial systems and record negative organic carbon isotope excursions associated with early Eocene abrupt, transient global warming (hyperthermal) events. Carbonate dominated or organic rich intervals record stable, less seasonal climate periods between hyperthermals, with lower siliciclastic sediment supply allowing the development of carbonate and organic matter preservation. The stratigraphic progression from alternating organic rich and lean zones to the overlying organic rich Mahogany and R8 zones represents the global transition out of the pulsed early Eocene hyperthermal climate regime to a time of sediment starvation and lake stratification, sequestering sedimentary organic carbon. This study provides a novel approach to terrestrial paleoclimate reconstruction that relies largely on unique sedimentary indicators and secondarily on isotopic proxy records within the context of a large basin-wide sedimentologic and stratigraphic data set, thus setting the stage for future detailed geochemical terrestrial paleoclimate proxy development.

Revisions to the fossil sporophyte record of Marsilea

Abstract The fossil record of Marsilea is challenging to assess, due in part to unreliable reports and conflicting opinions regarding the proper application of the names Marsilea and Marsileaceaephyllum to fossil leaves and leaflets similar to those of modern Marsilea. Specimens examined for this study include material assigned to Marsileaceaephyllum johnhallii, purportedly the oldest fossil record of a Marsilea-like sporophyte from the Lower Cretaceous of the Dakota Formation, Kansas, U.S.A.; leaves and leaf whorls of the extinct aquatic angiosperm Fortuna from several Late Cretaceous and Paleocene localities in western North America; and leaves and leaflets resembling Marsilea from the Eocene Green River Formation, Colorado and Utah, U.S.A. Literature on the fossil record of Marsilea was also reviewed. As a result, several taxonomic changes are proposed. Marsileaceaephyllum johnhallii is reinterpreted as an aquatic angiosperm that shares some architectural features with the genus Fortuna, although Marsileaceaephyllum is here maintained as a distinct genus with an emended diagnosis; under this reinterpretation, the name Marsileaceaephyllum can no longer be applied to sporophyte organs with affinities to Marsileaceae. Three valid fossil Marsilea species are recognized on the basis of sporophyte material that includes characteristic quadrifoliolate leaves and reticulate-veined leaflets: Marsilea campanica (J. Kvaček & Herman) Hermsen, comb. nov., from the Upper Cretaceous Grünbach Formation, Austria; Marsilea mascogos Estrada-Ruiz et al., from the Upper Cretaceous Olmos Formation, Mexico; and Marsilea sprungerorum Hermsen, sp. nov., from the Eocene Green River Formation, U.S.A. The species are distinguished from one another based on leaflet dimensions. Leaves from the Eocene Wasatch Formation, U.S.A., are transferred from Marsileaceaephyllum back to Marsilea, although not assigned to a fossil species. Finally, an occurrence of Marsilea from the Oligocene of Ethiopia is reassigned to Salvinia. A critical evaluation of the fossil record of Marsilea thus indicates that (1) the oldest fossil marsileaceous sporophytes bearing Marsilea-like leaves are from the Campanian; (2) only four credible records of sporophyte material attributable to Marsilea are known; and (3) the oldest dispersed Marsilea spores are known from the Oligocene.

Distribution of mineral phases in the Eocene Green River Formation, Piceance Basin, Colorado—Implications for the evolution of Lake Uinta

The mineralogy of the Eocene Green River Formation in the Piceance Basin, Colorado, has been the subject of numerous studies since the 1920s. Most previous work has focused on the resource potential of these lacustrine mudrocks, which in addition to substantial oil shale potential (in-place resources of 353 billion barrels of synthetic crude oil for rocks yielding at least 25 gallons per ton, GPT), includes nahcolite, a currently utilized soda ash resource, and dawsonite, a potential alternative source of aluminum. Another reason to study the mineralogy in this system is that the geographic and stratigraphic distribution of various authigenic minerals may provide insights into the geochemistry and depositional environment of the long-lived Eocene Lake Uinta. In this study, legacy non-quantitative (presence/absence) X-ray diffraction (XRD) data recently published by the U.S. Geological Survey (USGS) for more than 9,000 samples collected from 30 coreholes in the Green River Formation, Piceance Basin were examined. These data were used to better define the stratigraphic and paleogeographic extent of a set of indicator minerals (illite, analcime, albite, dawsonite, and nahcolite) within the Piceance Basin lacustrine strata. This set of minerals was selected based on observations from previous work and variability in their occurrence and co-occurrence within the Piceance Basin. The USGS database has been used to (1) construct maps showing geographic variations in mineral occurrences for 14 stratigraphically defined rich and lean oil shale zones; (2) assess co-occurrences of indicator minerals; and (3) compare occurrence results with quantitative XRD datasets collected on Piceance Basin oil shales. Occurrences of many authigenic minerals (analcime, dawsonite, and nahcolite) varied in the lacustrine strata near and around the depocenter, but others, like quartz, dolomite, and feldspar (potassium + undifferentiated), were widely and consistently present (>90% of samples) across the basin. Shifts in the distribution of indicator mineral occurrences generally coincide with changes identified in previous lake history descriptions and indicate that the water chemistry of Lake Uinta varied significantly going from near-shore to the depocenter and through time.

Stratigraphic complexity in fluvial fans: Lower Eocene Green River Formation, Uinta Basin, USA

AbstractIn this study, measured outcrop sections and geolocated photomosaics are integrated with areal mapping of channel dimensions, degree of amalgamation, calculations of channel‐to‐floodplain ratios and sedimentary facies variability to study and quantify the channel and floodplain deposits in the Sunnyside Delta Interval of the Lower Eocene Green River Formation in the Uinta Basin, Utah. Vertically, sand content and bed thickness increases, due to an increase in the channel‐to‐floodplain ratio, channel size and the degree of channel amalgamation. Laterally, the channel‐to‐floodplain ratio, channel size, the degree of channel amalgamation and the sand content in channel facies decreases in the paleo‐downstream direction. Such vertical and lateral transitions identify the Sunnyside Delta Interval as a fluvial fan (or distributive fluvial system). However, the vertical and lateral transitions occur at multiple spatial scales, demonstrating considerable stratigraphic complexity as compared to the existing facies and architectural models suggested for fluvial megafans and distributive fluvial systems. The smallest‐scale transitions are identified as avulsion‐related packages that form the building blocks of the stratigraphy, whereas the intermediate‐ and largest‐scale transitions are suggested to be related to lobe and whole fan progradation respectively. This documented complexity indicates the significance of self‐organization in building fluvial fan stratigraphy, and demonstrates that changes in the degree of channel amalgamation or in channel‐to‐floodplain ratio are not linked to accommodation changes. On facies scale, an abundance of Froude supercritical‐flow and high‐deposition‐rate facies, in‐channel mud deposits, and in‐channel bioturbation and desiccation indicate deposition in rivers with highly variable discharge. Such discharge conditions suggest seasonally and inter‐annually variable precipitation conditions in the US Western Interior in the Early Eocene.

Eocene Fossil Legume Leaves Referable to the Extant Genus Arcoa (Caesalpinioideae, Leguminosae)

Premise of research. Fossil leaves from the early Eocene Green River Formation of Wyoming and late Eocene Florissant Formation of Colorado have been studied and described here as two species in the...

4D in situ synchrotron X-ray tomographic microscopy and laser-based heating study of oil shale pyrolysis

The comprehensive characterization and analysis of the evolution of micro-fracture networks in oil shales during pyrolysis is important to understand the complex petrophysical changes during hydrocarbon recovery. We used time-resolved X-ray microtomography to perform pore-scale dynamic imaging with a synchrotron light source to capture in 4-D (three-dimensional image + real time) the evolution of fracture initiation, growth, coalescence and closure. A laser-based heating system was used to pyrolyze a sample of Eocene Green River (Mahogany Zone) up to 600 °C with tomograms acquired every 30 s at 1.63 µm computed voxel size and analyzed using Digital Volume Correlation (DVC) for full 3-D strain and deformation maps. At 354 °C the first isolated micro-fractures were observed and by 378 °C, a connected fracture network was formed as the solid organic matter was transformed into volatile hydrocarbon components. With increasing temperature, we observed simultaneous pore space growth and coalescence as well as temporary closure of minor fractures caused by local compressive stresses. This indicates that the evolution of individual fractures not only depends on organic matter composition but also on the dynamic development of neighboring fractures. Our results demonstrate that combining synchrotron X-ray tomography, laser-based heating and DVC provides a powerful methodology for characterizing dynamics of multi-scale physical changes during oil shale pyrolysis to help optimize hydrocarbon recovery.

Soft-bodied fossil of a lizard from the Parachute Creek Member, Green River Formation (Eocene), Utah

A rare specimen of soft tissue preservation of a lizard from the Parachute Creek Member of the Eocene Green River Formation, Uinta Basin, Utah, is described. The preservation is unusual in that it is a miner­alized body lacking the skeleton. This, and other small boneless vertebrate specimens also from the Para­chute Creek, indicate occasional demineralizing conditions in Lake Uinta, but not apparently in the other two lakes of the Green River Formation—Fossil Lake and Lake Gosuite.

Phosphatized tungsten-metabolizing coccoid microbes interpreted from oil shale of an Eocene lake, Green River Formation, Utah, USA.

Microscopic globular structures have been observed in some beds of oil shale from eastern Utah. These beds comprise carbonate-dominated mud that is interlaminated with variably thick and continuous organic-rich layers. Collectively they are enriched in phosphorus, REEs, and actinides. The beds are considered of lacustrine origin and assigned to the Eocene Green River Formation. The globules themselves are of microcrystalline carbonate fluorapatite (μCFA), often contain concentric internal structures, and usually group together in clusters of up to 80, possibly more. Detailed SEM and microprobe analyses have revealed tungsten (W) to be almost exclusively associated with the globular clusters found within the more organic-rich laminae, often at concentrations of over 200ppm, two orders of magnitude above shale standards. The globular structures are present in freshly cut sections where they occasionally grade into a μCFA matrix cement. This, together with the draping of the clusters by stringers of organic matter that would have accumulated in the Eocene lake, confirms that the structures are not a contaminant. The limited range of sizes and globular shapes is consistent with the morphology of coccoidal bacteria: Concentric internal structures may represent remnants of the nucleoid and cell wall. Paired concentric structures may indicate cell division (reproduction) processes were occurring until mineralization. The phosphate mineralization itself may have been promoted by release of phosphate from the stressed cells, bringing porewaters to supersaturation, or by the cells acting as nucleation sites. The recording of trace amounts of W almost exclusively in globular clusters preserved in the most organic-rich stringers (anoxia prone) further suggests facultative use of W-enzymes in a microbial metabolism. Combined, their context, morphology, and indication of biogenic process are strong evidence that the structures are fossilized (phosphatized) microbes, possibly sulfate-reducing bacteria, or methanogenic archaea.

Systematics and phylogeny of the Zygodactylidae (Aves, Neognathae) with description of a new species from the early Eocene of Wyoming, USA.

Zygodactylidae are an extinct lineage of perching birds characterized by distinct morphologies of the foot and wing elements. Although the clade has a complex taxonomic history, current hypotheses place Zygodactylidae as the sister taxon to Passeriformes (i.e., songbirds). Given the rather sparse fossil record of early passeriforms, the description of zygodactylid taxa is important for inferring potentially ancestral states in the largest radiation of living birds (i.e., the ∼6,000 species of extant passeriforms). Despite the exceptional preservation of many specimens and considerable species diversity in Zygodactylidae, the relationships among species have not been previously evaluated in a phylogenetic context. Herein, we review the fossil record of Zygodactylidae from North America and describe five new well-preserved fossils from the early Eocene Green River Formation of Wyoming. Two specimens are identified as representing a new species and the first records of the taxon Zygodactylus outside Europe. Anatomical comparisons with previously named taxa and the results of phylogenetic analysis including newly described specimens and previously named zygodactylid taxa provide the first hypothesis of the species-level relationships among zygodactylids. The monophyly of Zygodactylidae is supported in these new analyses. However, the monophyly of Primozygodactylus and the taxonomic distinction between Zygodactylus and Eozygodactylus remain unresolved and would likely benefit from the description of additional specimens.

Assessing biomarker syngeneity: An in situ approach using monoclonal antibodies

Lipid biomarkers preserved in ancient rocks have the potential to reveal much about ancient ecosystems. However, establishing that the compounds of interest are syngenetic has proven to be an analytically challenging task. Traditional biomarker analyses rely on extraction of large quantities of powdered rock, making the association of molecules with sedimentary fabrics difficult, if not impossible. As an alternative approach, here we show that monoclonal antibodies that bind specifically to geolipids can be used as molecular probes for in situ detection and localization of such compounds. Monoclonal antibodies that bind to squalene and cross-react with the biomarker squalane were evaluated for labeling sediment-associated hydrocarbons. The anti-squalene antibodies were shown by dot immunoblotting with composed standards to cross-react also with other isoprenoids, such as phytol and its diagenetic products, suggesting reactivity towards acyclic isoprenoids. Then, the anti-squalene antibodies were shown to react with naturally occurring crude oils and, via an immunofluorescence-labeling approach, to bind to isolated organic-rich laminae in rocks from the Eocene Green River Formation known to contain squalane among other linear isoprenoids. These results suggest that squalane, or structurally similar organic biomarkers that cross-react with the antibodies, are confined to discrete organo-sedimentary fabrics within those rocks, providing evidence for their syngeneity. Depending on the specificity and sensitivity of the antibody/geolipid pair, an in situ antibody detection approach may be useful for establishing biomarker syngeneity in older rocks.

A fossil caddisfly (Insecta: Trichoptera) from the Eocene of Colorado.

A new genus and species of fossil caddisfly (Insecta: Trichoptera) from the Lower Eocene (Ypresian) Green River Formation of Colorado is described. Litholimnephilops yinani gen. et sp. nov. is the first adult caddisfly to be described from the Green River Formation, and is characterized by large adult body size, presence of ocelli, dark leg spines, and a lack of terminal crossveins in the anterior anastomosis region of the forewings. Terminal genitalia are not visible in the preserved specimen. Familial placement is uncertain, though similarities with the families Limnephilidae and Phryganeidae are observed.

Brackish to hypersaline facies in lacustrine carbonates: Purbeck Limestone Group, Upper Jurassic\u2013Lower Cretaceous, Wessex Basin, Dorset, UK

Sedimentary facies and stratigraphic architecture of non-marine carbonates are controlled by a range of environmental parameters, such as climate, hydrology and tectonic setting, but the few published facies models do not account for this variability. Outcrop and petrographic observations from the Mupe Member of the Purbeck Limestone Group (Upper Jurassic–Lower Cretaceous) in Dorset, southern England, are the basis for new depositional models of non-marine microbialites and associated carbonates in an extensional basin. Ten facies are defined, described and grouped into five facies associations. The Mupe Member is characterised by accumulation of in situ microbial mounds developed around tree remains preserved as moulds and silicified wood. Mounds occur within three stratigraphic units, separated by three palaeosoils, characterised by less-porous, bedded, inter-mound packstone–grainstone that commonly onlap mound margins. Mounds are developed mainly in the shallowest areas of the lake, as indicated by their shapes, facies relationships and association with palaeosoils. These microbial mounds are compared to modern (Laguna Bacalar, Mexico and Great Salt Lake, Utah, USA) and ancient (Eocene Green River Formation, Uinta Basin, Utah, USA) analogues to assess their value as palaeoenvironmental indicators. Facies transitions indicate an earlier, brackish-water lake and a later hypersaline lake for the Mupe Member, both within a semi-arid climate setting in an extensional basin. The fact that the microbialites are covered by evaporitic strata, together with sedimentological, palaeontological and stable isotope data, suggest that there was a sharp change from through-flowing brackish-water, to a closed hypersaline, lacustrine system.

Oldest new genus of Myrmeleontidae (Neuroptera) from the Eocene Green River Formation

Epignopholeon sophiae gen. et sp. nov. (Neuroptera: Myrmeleontidae) is described from the early Eocene of the Green River Formation (Colorado, U.S.A.). It represents the oldest confident record of the family. The new genus is remarkable in that tergite 7 of the female is much shorter than its long sternite 7. The preserved wing venation shows that the genus belongs to the subfamily Myrmeleontinae, and most probably to the tribe Gnopholeontini. The discovery of this species is consistent with estimations of relatively dry and warm conditions during deposition of the upper Parachute Creek Member of the Green River Formation.

Highly Seasonal and Perennial Fluvial Facies: Implications For Climatic Control On the Douglas Creek and Parachute Creek Members, Green River Formation, Southeastern Uinta Basin, Utah, U.S.A.

Abstract: The early to middle Eocene Green River Formation consists of continental strata deposited in Laramide ponded basins in Utah, Colorado, and Wyoming. This study (1) documents fluvial and lacustrine strata from the Douglas Creek and Parachute Creek members of the middle Green River Formation, southeastern Uinta Basin, Utah, and (2) uses new interpretations of the link between climate and fluvial sedimentary expression to interpret the terrestrial evolution of early Eocene climate. The stratigraphy was analyzed via outcrops along a 10 km transect in Main Canyon on the Tavaputs Plateau, and is divided into three distinct, stratigraphically separated depositional settings: (1) the lowermost Interval 1 is dominated by amalgamated sandstone channels that contain 70–100% upper-flow-regime sedimentary structures. The channels are interpreted to represent fluvial deposits controlled by a highly seasonal climate, where most deposition was limited to seasonal flooding events. (2) Interval 2 is dominated by alternating siliciclastic and carbonate lacustrine deposits, interpreted as local pulsed fluvial siliciclastic input into shallow Lake Uinta, and periods of fluvial quiescence represented by littoral carbonate deposition. (3) The uppermost Interval 3 is dominated by erosionally-based, trough cross bedded sandstone channels interbedded with littoral lacustrine and deltaic deposits. The Interval 3 sandstone channels are interpreted as perennial fluvial deposits with relatively little variation in annual discharge, akin to modern humid-temperate fluvial systems. The stratigraphic transition from seasonally controlled (Interval 1) to perennial (Interval 3) fluvial deposits is interpreted to represent a fundamental shift in Eocene climate, from the peak hyperthermal regime of the Early Eocene Climatic Optimum (EECO) to a more stable post-EECO climate.

Ichneumonid parasitoid wasps from the Early Eocene Green River Formation: five new species and a revision of the known fauna (Hymenoptera, Ichneumonidae)

The parasitoid wasp family Ichneumonidae is one of the most species-rich groups of organisms, but its fossil record remains very poorly studied, which impedes inferences of the origin of its diversity. We here describe two new fossil genera and five new species of Ichneumonidae from the Eocene Green River Formation: Carinibus molestus gen. et sp. nov., Ichninsum appendicrassum gen. et sp. nov., Mesoclistus? yamataroti sp. nov., Scambus? mandibularis sp. nov., and Scambus? parachuti sp. nov. The newly described Mesoclistus? yamataroti represents the first record of the subfamily Acaenitinae from this fossil locality. In addition, we revise the ten previously described fossil ichneumonids from the Green River Formation, following a conservative approach when re-assessing their taxonomic positions: we keep the current placement of six revised fossils, but express the uncertainty in genus-assignment according to open nomenclature rules: Eclytus? lutatus Scudder, Glypta? transversalis Scudder, Pimpla? eocenica Cockerell, Phygadeuon? petrifactellus Cockerell, Plectiscidea? lanhami Cockerell and Rhyssa? juvenis Scudder. We exclude three fossil genera from their current subfamilies and place them within Ichneumonidae incertae subfamiliae: Eopimpla Cockerell, Lithotorus Scudder and Tilgidopsis Cockerell. Furthermore, we move Tryphon amasidis Cockerell and LeVeque to the new genus Trymectus gen. nov. In the light of these revisions, we discuss the importance of careful taxonomic placement of fossils and difficulties in ichneumonid palaeontology caused by host-related homoplasies and a lack of knowledge about the age of the recent subfamilies.