Lacustrine Green River Formation Research Articles

The second European representative of the epallagid genus Labandeiraia in the lowermost Eocene Oise amber (Odonata, Zygoptera)

Among the calopteran damselflies, the family Epallagidae was clearly dominating the diversity of the Holarctic Cenozoic (11 described species in eight extinct genera), while the Calopterygidae were extremely rare at the same time (six Cenozoic species in two extant and one fossil genera) (Fossilworks database at http://www.fossilworks.org/cgi-bin/bridge.pl?action=home, consulted 21/10/2022). They mainly belonged to the extinct subfamily Eodichrominae Cockerell, 1923, and their most diverse genus was the early Eocene genus Labandeiraia Petrulevičius et al., 2007, known from compression fossils from the lacustrine Green River Formation (USA) and the marine Fur Formation in Denmark (Petrulevičius et al., 2007; Bechly et al., 2020).

Spatial and temporal cycle variations in the Eocene Lacustrine Green River Formation, Piceance Creek Basin, Colorado

Lithological cycles in the lacustrine Green River Formation of the Piceance Creek Basin, Colorado, vary spatially and temporally, and are challenging to correlate across the basin. In the center of the basin, lacustrine deposits contain bedded evaporites and kerogen-rich carbonate mudstone, whereas basin margin deposits contain alluvial sandstone, kerogen-poor mudstone, stromatolite, and mud cracks. Because coarsening upward cycles identified at the basin center differ significantly from cycles at the basin margin, exactly how cycle boundaries extend from one location in the basin to another is commonly unclear. In this investigation, eleven lithofacies are identified based on rock composition, sedimentary textures, physical sedimentary structures, biogenic structures, and chemical features. Lithofacies are grouped into four facies associations: (1) Alluvial Facies Association; (2) Shoreline Facies Association; (3) Shallow Lake Facies Association; and (4) Deep Lake Facies Association. Lithological cycles in the Green River Formation vary temporally and spatially, with each stratigraphic section preserving a record of deposition in a particular area of the basin as the Piceance lake evolved. To address the temporal and spatial variation of the cycles, multiple “type” cycles are described for each location in the study area. Facies associations and temporal stacking of cycles suggest that the Piceance lake was initially a deep, freshwater lake, following the Long Point transgression, as lake level rose above the Douglas Creek Arch to fill both the Uinta and Piceance Creek basins. Lake levels then stabilized but later began a period of increased fluctuation. Following basin closure, the Piceance lake became hypersaline under evaporative concentration conditions, precipitating nahcolite and halite at the basin center, with exposure of mudflats at the basin margin. The lake would remain this way until the Mahogany transgression deepened and diluted the lake. Together, facies associations and “type” cycles described in the Green River Formation lacustrine deposits reveal a record of lateral and vertical facies changes that occurred in the Piceance lake during its evolution.

The role of organic matter diversity on the Re-Os systematics of organic-rich sedimentary units: Insights into the controls of isochron age determinations from the lacustrine Green River Formation

The range of 187Re/188Os values measured from samples of five organic-rich lacustrine mudstones units in the Eocene Green River Formation in the easternmost Uinta Basin covaries with organic matter diversity driven by changing water column conditions. A set of samples from the Douglas Creek Member has the highest pristane/phytane ratio and lowest β-carotane/n-C30 ratio compared to overlying units, indicating deposition in an oxic-anoxic environment with low salinity that would have allowed for the accumulation of a diverse assemblage of aquatic organisms. These samples define the broadest 187Re/188Os range of 1504. In contrast, samples from the R6 and Mahogany zones possess lower pristane/phytane ratios and higher β-carotane/n-C30 ratios indicating deposition in a more restricted lacustrine environment with elevated salinities and alkalinities that would have limited aquatic organic matter diversity. The R6 and Mahogany zones have the narrowest range of 187Re/188Os values measured in this study of 254.9 and 154.6, respectively. As noted by previous workers, these results suggest that organic matter diversity plays a primary role in determining the range of 187Re/188Os ratios in a sample set, and in turn the uncertainty of Re-Os age determinations from organic-rich sedimentary rocks.The Re-Os data from the R3 zone and R6 zone yield ages of 49.7 ± 3.4 Ma and 42.0 ± 18 Ma, respectively, which are statistically indistinguishable based on 2σ uncertainty from three previously reported Re-Os age determinations and those provided by 40Ar/39Ar geochronology of interbedded volcanic ash beds. Although the age uncertainty is high, these findings further highlight the importance of Re-Os geochronology in lacustrine basins, particularly those with thick mudstone successions that lack volcanic ash layers, reliable biostratigraphy, or magnetostratigraphic control. In these cases, even ages with large uncertainties can be useful to constrain burial history and thermal history models.Together, the initial 187Os/188Os ratios of five sets of samples analyzed from the Uinta Basin define the largest Os isotope stratigraphic record from any lacustrine basin compiled to date and record a shift from a value of 1.40 to 1.48 between the R3 and R4 zones in the lower part of the Parachute Creek Member. This small shift may signify a change in the chemical weathering products that entered the lake preserved 20 to 50 m above the contact between the Douglas Creek and the lower Parachute Creek members during a period when the basin transitioned from a shallow lake with mostly open hydrology to an alkaline lake with more frequent basin restrictions.

Evaluating aromatization of solid bitumen generated in the presence and absence of water: Implications for solid bitumen reflectance as a thermal proxy

Geological models for petroleum generation suggest thermal conversion of oil-prone sedimentary organic matter in the presence of water promotes increased liquid saturate yield, whereas absence of water causes formation of an aromatic, cross-linked solid bitumen residue. To test the influence of hydrogen from water, organic-rich (22 wt% total organic carbon, TOC) mudrock samples from the Eocene lacustrine Green River Formation Mahogany zone oil shale were pyrolyzed under hydrous and anhydrous conditions in closed system batch reactors at temperatures between 300 and 370 °C for 72 h. Pre- and post-pyrolysis samples were characterized using petrographic approaches including optical microscopy, reflectance, Raman spectroscopy, and scanning electron and transmission electron microscopy to quantify differences in relative appearance, abundance, and composition of solid bitumen newly generated during the pyrolysis experiments. Petrographic analyses were supplemented by geochemical screening measurements (TOC content and programmed temperature pyrolysis). Results show post-hydrous pyrolysis residues contain lower TOC, are comprised of solid bitumen with greater aromaticity, and have textures indicative of lower viscosities, relative to anhydrous residues from the same temperature pyrolysis conditions. These observations suggest solid bitumen forming from thermal conversion of oil-prone sedimentary organic matter under anhydrous conditions may be less aromatic, although more cross-linked, than solid bitumen forming under hydrous conditions at the same time-temperature combination. To explain these results, we suggest that a radical disproportionation mechanism is favored in the presence of hydrogen donated from water, and that this disproportionation promotes aromatization in the solid residue with concomitant expulsion of saturated hydrocarbons.

Lake basin closure and episodic inflow as recorded by radiogenic Sr isotopes: Eocene Green River Formation in the Piceance Creek Basin, Colorado

Lacustrine basins are excellent archives of lake evolution, and deposits record the uplift and weathering histories of the surrounding terrain. The application of Sr isotopes has been tested in several lacustrine basins, both modern and ancient, based on the premise that lakes are well mixed, and shifting Sr isotopes may suggest changes in lake provenance. In the Eocene lacustrine Green River Formation in the Piceance Creek Basin of Colorado, Sr isotope analysis of carbonate mudstones indicates that radiogenic Sr in the center of the Piceance lake decreased during the evolution of the lake, from 52.8–48.4 Ma. Because deposition in the basin center occurred away from the influence of episodic alluvial inflow at the basin margin, Sr isotope evolution in the Piceance lake after basin closure is recorded in the John Savage #24-1 core deposits, not the Anvil Points deposits. Sr isotope analysis of carbonate mudstones at Anvil Points below 55 m shows fluctuating radiogenic Sr values that record episodic inflow from the White River Uplift. This inflow is responsible for the difference in radiogenic Sr trends recorded between the basin center and margin. Above 55 m, fluctuating Sr isotope values at Anvil Points record episodic inflow from the White River Uplift, without inflow of Paleozoic and Mesozoic carbonates. The boundary at 55 m records the hydrologic closure of the Uinta and Piceance lakes around 52 Ma, when lake level lowered beneath the basin sill and the lakes were no longer connected across the Douglas Creek Arch. A significant increase in radiogenic Sr across the 55-m-boundary records this transition from open to closed hydrology, reflecting a loss of dissolved Sr sourced from Paleozoic and Mesozoic carbonates.

Bird tracks from the Green River Formation (Eocene) of Utah: ichnotaxonomy, diversity, community structure and convergence

ABSTRACT Abundant well-preserved bird tracks from lacustrine Green River Formation (Eocene) deposits in Utah, are diverse and paleoecologically significant, but remain poorly known. Three of the four previously reported morphotypes (A-D) are named here for the first time with two representing new ichnotaxa. In order of increased footprint length (L) the ichnotaxa are: cf. Avipeda phoenix (L = 2.0 cm); Quadridigitus semimembranus ichnogen. et ichnosp. nov. (L = ~3.5 cm); Jindongornipes falkbuckleyi ichnosp. nov. (L ~ 8.0 cm); and previously named, web-footed Presbyornithiformipes feduccii (L = 9.5 cm). Collectively these avian ichnotaxa, associated invertebrate traces including ubiquitous nematode trails (Cochlichnus), and other rarer, small tetrapod tracks represent the “shorebird” ichnofacies or ichnocoenosis. However, the local track assemblages from the Uinta Basin do not correspond to the avian body fossil record from multiple Green River basins in the larger region. Thus, both records are essential for a cumulative picture. The Green River ichnotaxa are morphologically similar to those from lake basins in the Korean Cretaceous, and modern shorebird assemblages. As modern shorebirds are not known from Cretaceous or Eocene body fossils, the most plausible explanation is community convergence, among taxonomically disparate, but ecologically convergent trackmakers. Community convergence concepts help refine the ichnofacies paradigm.

Insolation forcing of sub-lacustrine debris flows – Could monsoon intensification have played a role? Eocene lacustrine Green River Formation, Piceance Creek Basin, Colorado

Mudstone breccias deposited by debris flows in the lacustrine Green River Formation of the Piceance Creek Basin, Colorado, may not represent random and unpredictable events as previously thought. Spectral analysis of mudstone breccia beds in the Wolfridge Minerals John Savage #24-1 basin center core reveals a record of short eccentricity (94.58 kyr), obliquity (50.78 kyr and 39.26 kyr), and precession (21.85 kyr and 18.56 kyr). Of particular interest to this investigation are the records of eccentricity and precession, together which indicate that insolation forcing of monsoons occurred every ~18–21 kyr, and that every ~94 kyr, insolation maxima exceeded a threshold of 528 W/m2, resulting in more significant monsoon amplification. Strengthened monsoon activity would have led to an increase in precipitation and runoff delivered to the lake. This increase in inflow resulted in lake-level rise which, in turn, decreased basin slope stability and triggered periodic sub-lacustrine debris flows that deposited the mudstone breccias.

Diagenetic sequestration of rare earths and actinides in phosphatic oil shale from the lacustrine Green River Formation (Eocene), Utah, USA: an SEM and LA-ICP-MS study

The upper Green River Formation of the eastern Uinta Basin of Utah comprises variably organic-rich mudrock deposited in a stratified, alkaline lake that fluctuated in size and in hydrology from open to evaporitic. Beds of oil shale, representing periods of deep lake, suboxic-anoxic deposition include examples with anomalously high phosphate and toxic trace-metal levels, as determined by whole rock X-ray diffraction, scanning electron microscopy (SEM), and both solution and laser-ablation (LA) inductively coupled plasma-mass spectrometry (ICP-MS). The SEM identifies an early, pre-compaction, diagenetic succession of euhedral rhombic dolomite and high-Mg-calcite, nodular pyrite, blocky low-Mg calcite, and microcrystalline carbonate fluorapatite (CFA). LA-ICP-MS indicates the CFA intervals have distinct lower and upper margins where P content rises from ~0.079 to ~3.2% across 2–3 cm of section. Enrichments of rare earth elements (REEs) and actinides (Th, U) show a complex correlation with the phosphatic margins. Light REEs average a tenfold enrichment at the outer margins with progressively heavier REEs exhibiting up to 20-fold enrichment toward the inner margin (Lu from 0.15 to 2.63 ppm). Actinide abundances also increase tenfold (U from ~5 to ~80 ppm) toward the inner margin. There is greatest similarity between Th and the Ho distributions and between U and Lu. This is a similar distribution to what elsewhere has been found in fossil bone and teeth. The microcrystalline CFA is interpreted to have precipitated (or recrystallized from a precursor phosphate) when the rate of dissolved P being generated from organic-matter breakdown exceeded P diffusion rates into the water column, allowing for P supersaturation. As the crystallites grew, diffusion of REE and actinide ions and complexes from surrounding porewaters into the concreting phosphate occurred only until pores were occluded and permeability dropped. As with fossil bone, at the outer margin of the phosphate interval, this allowed for fractionation and preferential substitution of Ca cations in the CFA lattice by light REEs of similar ionic radius. Heavier REEs and actinides of smaller ionic radius diffused further into margin of the phosphate interval.

Re–Os geochronology of the lacustrine Green River Formation: Insights into direct depositional dating of lacustrine successions, Re–Os systematics and paleocontinental weathering

Lacustrine sedimentary successions provide exceptionally high-resolution records of continental geological processes, responding to tectonic, climatic and magmatic influences. These successions are therefore essential for correlating geological and climatic phenomena across continents and furthermore the globe. Producing accurate geochronological frameworks within lacustrine strata is challenging because the stratigraphy is often bereft of biostratigraphy and directly dateable tuff horizons. The rhenium–osmium (Re–Os) geochronometer is a well-established tool for determining precise and accurate depositional ages of marine organic-rich rocks. Lake systems with stratified water columns are predisposed to the preservation of organic-rich rocks and thus should permit direct Re–Os geochronology of lacustrine strata. We present Re–Os systematics from one of the world's best documented lacustrine systems, the Eocene Green River Formation, providing accurate Re–Os depositional dates that are supported by Ar–Ar and U–Pb ages of intercalated tuff horizons. Precision of the Green River Formation Re–Os dates is controlled by the variation in initial 187Os/188Os and the range of 187Re/188Os ratios, as also documented in marine systems. Controls on uptake and fractionation of Re and Os are considered to relate mainly to depositional setting and the type of organic matter deposited, with the need to further understand the chelating precursors of Re and Os in organic matter highlighted. In addition to geochronology, the Re–Os data records the 187Os/188Os composition of lake water (1.41–1.54) at the time of deposition, giving an insight into continental runoff derived from weathering of the geological hinterland of the Green River Formation. Such insights enable us to evaluate fluctuations in continental climatic, tectonic and magmatic processes and provide the ability for chemostratigraphic correlation combined with direct depositional dates. Furthermore, initial 187Os/188Os values can be used as a diagnostic tool to distinguish between lacustrine and marine depositional settings when compared to known oceanic 187Os/188Os values.

10 k.y. depositional cyclicity in the early Eocene: Stratigraphic and 40Ar/39Ar evidence from the lacustrine Green River Formation

Ar/ 39 Ar dating of sanidine from two interbedded tuffs reveals that the maximum average duration of depositional cycles in the Wilkins Peak Member, Green River For- mation, was ;10 k.y., marking the first time that subprecessional cycles have been rec- ognized in lacustrine strata. The origin of these cycles is uncertain, but may relate to a nonlinear climatic response to orbital forcing of insolation. Alternatively, regional tectonic and geomorphic controls on drainage stability may have promoted autocyclic delivery of sediment to the lake. Owing to an interaction between basin-floor relief and varying am- plitudes of lake expansion, only one-third of the cycles identified near the basin center are present near the basin margin. This spatial variability in the temporal completeness of the stratigraphic record is not apparent from examination of individual localities, indi- cating that studies based on time-series analysis from other lacustrine systems may need reevaluation.

ABSTRACT: Packaging an Outcrop Analog: An Example from the Lacustrine Green River Formation, Northeast Uinta Basin

ABSTRACT: Packaging an Outcrop Analog: An Example from the Lacustrine Green River Formation, Northeast Uinta Basin

High-ResoIution Stratigraphy of the Green River Formation, NE Uinta Basin: Implications for Red Wash Reservoir Compartmentalization: ABSTRACT

An extensive subsurface data base from the Greater Red Wash area and outcrops 4-8 miles from main producing areas are used to construct a high-resolution correlation framework for the lowerpart of the Eocene lacustrine Green River Formation. An orthogonal grid of 29 cross-sections and 11 cores comprise the subsurface database. The outcrop database consists of 12 measured sections (7,500 ft) along an oblique depositional dip profile at Raven Ridge. Outcrop gamma ray profiles collected for 4600 ft of the measured sections allow for a good outcrop to subsurface tie and provide a calibration set for interpreting subsurface log patterns. The correlation framework is based on the identification of a hierarchical set (4 scales) of base-level cycles (changes in accommodation to sediment supply ratio). Cycle stacking patterns (lakeward-stepping, vertically-stacked and landward-stepping) provide the chronostratigraphic framework to properly evaluate depositional facies. For mapping reservoir zones, medium-scale maximum flooding surfaces (base-level rise-to-fall turnarounds) are the most useful correlation surfaces. To better understand depositional facies and heterogeneity within cycles or reservoir zones, base-level fall-to-rise surfaces are correlated in outcrop sections. This allows for the evaluation of sediment volume partitioning and facies differentiation between base-level fall and base-level rise parts of cycles at all scales.more » Facies indicate a steep-gradient (high-subsidence), high-energy, storm-dominated, lacustrine shoreface profile. Best reservoirs are stacked shoreface cycles (25-35 ft thick) and occur during long-term base-level fall. Reservoir facies are restricted to upper and middle shoreface, through and hummocky cross-stratified sandstones. Another important reservoir facies is massive dispersed-pebble sandstone interpreted as high-density sediment gravity flows.« less

Trona resources in southwest Wyoming

Bedded trona (Na2CO3·NaHCO3·2H2O) in the lacustrine Green River Formation of Eocene age in the Green River Basin, southwest Wyoming, constitutes the largest known resource of natural sodium carbonate in the world. In this study, 116 gigatons (Gt) of trona ore are estimated to be present in 22 beds, ranging from 1.2 to 11 meters (m) in thickness. Of this total, 69 Gt of trona ore are estimated to be in beds containing less than 2 percent halite and 47 Gt in beds containing 2 or more percent halite. These 22 beds underlie areas of about 130 to more than 2,000 km2 at depths ranging from about 200 m to more than 900 m below the surface. The total resource of trona ore in the basin for which drilling information is available is estimated to be about 135 Gt.

Lacustrine Hummocky Cross-Stratification in Green River Formation, Southern Uinta Basin, Utah: ABSTRACT

Hummocky cross-stratification (HCS) is a common sedimentary structure in marine strata but has only been reported in a few lake deposits. However, recent field investigations in the southern Uinta basin show that HCS occurs within several different depositional environments and lithologies in the lacustrine Green River Formation (Eocene). HCS is well developed within the open-lacustrine informal S2 marker unit of the Parachute Creek Member, where it exhibits many of the characteristics common in marine HCS: numerous low-angle, second-order truncations; antiforms (hummocks); association with planar laminations and ripples; gently curved parallel to nearly parallel laminations; fanning of laminations into swales; and scattered dip directions of truncation surfaces and laminae. Hummock spacing ranges from 20 cm to approximately 2.75 m. Hummocks produced by laminae that thicken toward the hummock crest indicate that at least some of the HCS was produced by bed forms molded by the flow, and not by a random scour-and-drape mechanism. Common laminae downlap at set boundaries, and asymmetric hummocks indicate that at least some of the hummocks were subject to minor migration during growth.

Major Petroleum-Impregnated Rock Deposits of Western Colorado Plateau: ABSTRACT

Petroleum-impregnated rock deposits occur widely in rocks of Permian to Tertiary age in the western Colorado Plateau of Arizona and Utah. Although over 60 occurrences and deposits have been recognized to date, only six, all in Utah, are estimated to contain over one billion bbl of petroleum in place. Two giant deposits occur in marine and marginal marine strata in the dissected plateau region of southeastern Utah. The largest of these, and the largest single known deposit in the United States, is the Tar Sand Triangle. It underlies approximately 225 sq mi (585 sq km), and is estimated to contain as much as 16 billion bbl of petroleum, principally in the White Rim Sandstone (Permian). The Circle Cliffs deposit contains approximately 1.3 billion bbl of petroleum in siltstone and sandstone of the Moenkopi Formation (Triassic). The deposit underlies an area of approximately 28 sq mi (73 sq km) on both sides of a breached anticline. Several similar, but smaller deposits occur in the Moenkopi Formation to the north, in the Capitol Reef anticline, and in the San Rafael uplift. The remaining giant deposits are located in the Uinta basin, and all contain petroleum which probably originated in the lacustrine Green River Formation (Eocene). These deposits include P. R. Spring, Hill Creek, Sunnyside, Asphalt Ridge, and Asphalt Ridge Northwest. The total amount of petroleum contained in these deposits in the Tar Sand Triangle and Uinta basin approaches a resource base of 29 billion bbl. The deposits are variable, and each offers a variety of technologic and economic challenges. End_of_Article - Last_Page 686------------

Aegirine of possible authigenic origin in Middle Devonian sediments in Caithness, Scotland

SummaryAn occurrence of aegirine in Middle Devonian lacustrine beds in Caithness is described. The mineral occurs as euhedral to subhedral prismatic crystals within a finely laminated siltstone. Microprobe analyses reveal a remarkably ‘pure’ composition consisting almost entirely of Na, Fe3+, Si, and O. X-ray diffractometry yielded the unit cell data: a 9·657 Å, b 8·800 Å, c 5·296 Å, and β 107·37°. Comparison with other natural aegirine compositions shows marked differences from those of igneous or metamorphic origin but a close similarity to a unique occurrence of authigenic aegirine in the lacustrine Green River Formation of Wyoming, U.S.A. Consideration of the nature of the Caithness occurrence, and comparison of the Devonian Orcadian basin with the Eocene Green River basin indicate similar geological environments. It is suggested that the mineral formed in hypersaline conditions in which sodium may have been derived from contemporaneous alkaline volcanism within the Orcadian basin.

Transported fossil biota of the green river formation, Utah

Fossils of the lacustrine Green River Formation (Early Eocene) of Colorado, Utah, and Wyoming consist of few indigenous and many transported elements, among those that were obviously produced in environments different from those of final burial are adult and larval insects, gymnosperm and angiosperm leaves, feathers, and patches of moulted lizard skin. Of these the insect larvae are most abundant and difficult to explain. The relatively undamaged condition of delicate organisms seems to rule out extensive water transport. It is concluded that strong winds acting either to carry specimens through the air or to drive them over the water surface delivered many of these organic objects to the place of burial and fossilization.

A biogenic-chemical stratified lake model for the origin of oil shale of the Green River Formation: An alternative to the playa-lake model

Research Article| July 01, 1978 A biogenic-chemical stratified lake model for the origin of oil shale of the Green River Formation: An alternative to the playa-lake model GEORGE A. DESBOROUGH GEORGE A. DESBOROUGH 1U.S. Geological Survey, Denver, Colorado 80225 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1978) 89 (7): 961–971. https://doi.org/10.1130/0016-7606(1978)89<961:ABSLMF>2.0.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation GEORGE A. DESBOROUGH; A biogenic-chemical stratified lake model for the origin of oil shale of the Green River Formation: An alternative to the playa-lake model. GSA Bulletin 1978;; 89 (7): 961–971. doi: https://doi.org/10.1130/0016-7606(1978)89<961:ABSLMF>2.0.CO;2 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 A model is proposed which involves biogenic Mg enrichment, a stratified lake environment, and authigenic growth of minerals that led to the development of oil shale in the lacustrine Green River Formation. The chemistry and mineralogy of Ca-Mg-Fe carbonates and other minerals in oil shale are consistent with an authigenic origin. The higher content of magnesium with respect to calcium in kerogen-rich rocks is probably due to the preferential concentration of magnesium with respect to calcium by blue-green algae whose remains released these cations after accumulation on the lake bottom. These elements were available for incorporation in Ca-Mg-Fe carbonates which crystallized in lake-bottom muds, while degradation of admixed algal material led to the development of kerogen. In modern lacustrine environments, primary and secondary Ca-Mg-Fe carbonate development and stability in terms of geologic time are consistent with authigenic development of the Ca-Mg-Fe carbonates present in Green River Formation oil shale. Iron is a significant component of these rhombohedral carbonate assemblages in oil shale, and thus it limits interpretations of origin of oil shale in the context of CaCO3-MgCO3 equilibria.The variable composition and variety of Ca-Mg-Fe carbonates in oil shale also prohibits interpretations in terms of CaCO3-MgCO3 equilibrium diagrams. Similarly, inferences regarding the chemical composition and structure of these carbonates lead to misinterpretations due either to the lack of adequate chemical data or to misunderstanding of crystallographic parameters.Greater amounts of mineral matter in time-stratigraphic intervals in the depositional center of the Piceance Creek basin, compared to the basin margins, suggest strongly that authigenic mineral development is more important than detrital accumulation of minerals in the richer oil-shale sequences.Systematic consideration of all of the arguments developed to promote the playa-lake model to explain the chemical, mineralogical, and geologic aspects of Green River Formation oil shale leads to the conclusion that the biogenic-chemical model proposed here is more appropriate than a strictly chemical model, because the playa-lake model so far does not consider the influence of any biogenic factors. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Clay Minerals of the Green River Formation (Eocene) of Wyoming

X-ray analyses indicate that illite, smectite, loughlinite (Na20.3MgO. 6SIO2.8H20) and chlorite are present in the < 2/zm fraction of the lacustrine Green River Formation (Eocene) of Wyoming. Material amorphous to X-rays and random mixed-layer structures are present in subordinate amounts. Individual crystals vary from pure, well-ordered structures to poorly crystallized forms. Illite is ubiquitous, occurring in all the samples studied. It is generally associated with other clay minerals but frequently occurs as the only clay mineral found in the 'oil shales'. Smectite is present in all three members of the Green River Formation, but its presence frequently shows an inverse relationship to both analcime and loughlinite. Loughlinite is restricted to the saline facies of the Wilkins Peak Member. Chlorite occurs only in the silty and sandy beds of the Tipton Shale Member. The distribution of random mixed-layer structures and amorphous material is irregular. Several independent lines of evidence favour an in situ origin for many of the clay minerals reported in this investigation. The 'oil shales', which are in most cases varved, kerogenous, dolomitic marls, have a variable clay mineral composition. There is a marked tendency, however, for illite to be the dominant or the only clay mineral present. Apparently the geochemical conditions favouring the accumulation of the oil shales also favoured in situ generation of illite.

'Salt-biscuits'; a special growth structure of NaCl in salt sediments of the Tuz Goelue ('Salt Lake'), Turkey

ABSTRACT X-ray analyses indicate that illite, montmorillonite, and chlorite are abundant in the less-than-2-µ fraction of the Tipton Shale Member of the lacustrine Green River Formation (Eocene). Material amorphous to X-rays and mixed-layer structures are present in subordinate amounts. These minerals have been known to occur in rocks of lacustrine origin, but this investigation shows a general correlation between lithology and clay mineral suite. An illite suite is restricted to the to the argillaceous and analcimic tuffs near the top of the member. A chlorite-illite suite is found only in the coarser clastics developed in the middle of the member, and a montmorillonite-illite suite is restricted to the montmorillonitie tuffs and oil shales at the base of the member. Source rock and dia enetic alteration account for the distributional pattern.