Wilkins Peak Member Research Articles

Synoptic reconstruction of a major ancient lake system: Eocene Green River Formation, western United States

Numerous 40Ar/39Ar experiments on sanidine and biotite from 22 ash beds and 3 volcaniclastic sand beds from the Greater Green River, Piceance Creek, and Uinta Basins of Wyoming, Colorado, and Utah constrain ∼8 m.y. of the Eocene Epoch. Multiple analyses were conducted per sample using laser fusion and incremental heating techniques to differentiate inheritance, 40Ar loss, and 39Ar recoil. When considered in conjunction with existing radioisotopic ages and lithostratigraphy, biostratigraphy, and magnetostratigraphy, these new age determinations facilitate temporal correlation of linked Eocene lake basins in the Laramide Rocky Mountain region at a significantly increased level of precision. To compare our results to the geomagnetic polarity time scale and the regional volcanic record, the ages of Eocene magnetic anomalies C24 through C20 were recalibrated using seven 40Ar/39Ar ages. Overall, the ages obtained for this study are consistent with the isochroneity of North American land-mammal ages throughout the study area, and provide precise radioisotopic constraints on several important biostratigraphic boundaries. Applying these new ages, average sediment accumulation rates in the Greater Green River Basin, Wyoming, were approximately three times faster at the center of the basin versus its ramp-like northern margin during deposition of the underfilled Wilkins Peak Member. In contrast, sediment accumulation occurred faster at the edge of the basin during deposition of the balanced filled to overfilled Tipton and Laney Members. Sediment accumulation patterns thus reflect basin-center–focused accumulation rates when the basin was underfilled, and supply-limited accumulation when the basin was balanced filled to overfilled. Sediment accumulation in the Uinta Basin, at Indian Canyon, Utah, was relatively constant at ∼150 mm/k.y. during deposition of over 5 m.y. of both evaporative and fluctuating profundal facies, which likely reflects the basin-margin position of the measured section. The most rapid sediment accumulation for the entire system (>1 m/k.y.) occurred between 49.0 and 47.5 Ma, when volcaniclastic materials from the Absaroka and/or Challis volcanic fields entered the Green River Formation lakes from the north. Our new ages combined with existing paleomagnetic and biostratigraphic control permit the first detailed synoptic comparison of lacustrine depositional environments in all the Green River Formation basins. Coupled with previously published paleocurrent observations, our detailed correlations show that relatively freshwater lakes commonly drained into more saline downstream lakes. The overall character of Eocene lake deposits was therefore governed in part by the geomorphic evolution of drainage patterns in the surrounding Laramide landscape. Freshwater (overfilled) lakes were initially dominant (53.5–52.0 Ma), possibly related to high erosion rates of remnant Cretaceous strata on adjacent uplifts. Expansion of balanced-fill lakes first occurred in all Green River Formation basins at 52.0–51.3 Ma and again between 49.6 and 48.5 Ma. Evaporative (underfilled) lakes occurred in various basins between 51.3 and 45.1 Ma, coincident with the end of the early Eocene climatic optima and subsequent onset of global cooling defined from marine record. However, evaporite intervals in the different depocenters were deposited at different times rather than being confined to a single episode of arid climate. Evaporative terminal sinks were initially located in the Greater Green River and Piceance Creek Basins (51.3–48.9 Ma), then gradually migrated southward to the Uinta Basin (47.1–45.2 Ma). This history is likely related to progressive southward construction of the Absaroka Volcanic Province, which constituted a major topographic and thermal anomaly that contributed to a regional north to south hydrologic gradient. The Greater Green River and Piceance Creek Basins were eventually filled from north to south with Absarokaderived detritus at sedimentation rates 1–2 orders of magnitude greater than the underlying lake deposits.

High-Resolution Stratigraphy of an Underfilled Lake Basin: Wilkins Peak Member, Eocene Green River Formation, Wyoming, U.S.A.

Abstract Lakes tend to respond noticeably to minor changes in sediment and water balance driven by climatic, tectonic, or geomorphic processes. This unique behavior of lacustrine basins can provide a high-resolution record of geologic processes within the continental setting, far from the globally averaged record of marine strata. The Wilkins Peak Member of the Eocene Green River Formation, in Wyoming, USA, is dominated by aggradation of repetitive sedimentary facies successions recording distinct lacustrine expansions and contractions. These lacustrine consist of up to four successive facies associations: littoral, profundal–sublittoral, palustrine, and salt pan. Because they comprise disparate facies that may never have been simultaneously deposited in the basin, Wilkins Peak Member cycles are non-Waltherian successions that do not readily equate to any established sequence stratigraphic unit. The completeness of the stratigraphic record in the Wilkins Peak Member varies continuously across the basin. At least 126 cycles are present in the ERDA White Mountain #1 core near the basin depocenter, whereas only about one third as many are recognizable 53 km north, nearer to the basin margin. Cycle boundaries terminate northward by gradual amalgamation into palustrine facies, reflecting the interplay between varying lake levels and a south-dipping deposition gradient. Evidence for complete desiccation and hiatuses is commonplace even near the basin center; a truly complete record may not be present anywhere. Based on recent 40Ar/39Ar geochronology of tuffs, the average cycle duration in the White Mountain #1 core is approximately 10,000 years. However, the true average duration is shorter due to the presence of lacunae, and the time required for deposition of the thinnest cycles may have been less than 1000 years. No external driving mechanism is presently known for Eocene cycles of such short duration, raising the possibility that they are instead autogenic cycles related to geomorphic instability of the surrounding landscape. Evaporite deposition corresponded closely in time and space to maximum differential subsidence, suggesting that tectonic influences on drainage patterns and basin accommodation exerted a primary control. Differential accommodation was most pronounced during deposition of the lower Wilkins Peak Member and became progressively less so after that time. Based on structural and stratigraphic observations, a period of increased tectonic uplift commenced concurrent with deposition of the lower Wilkins Peak Member.

High-resolution calibration of Eocene strata: 40Ar/39Ar geochronology of biotite in the Green River Formation

The Sixth tuff of the Wilkins Peak Member of the Green River Formation is an important potential calibration point for magnetic stratigraphy, mammalian evolution, and lacustrine processes. However, two recent 40 Ar/ 39 Ar ages based on laser fusion of biotite from this important bed are not in agreement with one other. Most large, euhedral biotite from the basal 1 cm give concordant age spectra when incrementally heated. However, some crystals yield discordant age spectra and have plateau and integrated ages that are scattered toward both older and younger apparent ages. Electron-probe microanalysis indicates that intergrown alteration phases occur in a small fraction of Sixth tuff biotite crystals, and likely contributed to open-system processes that led to the observed discordance and age scatter. A new age of 49.62 ± 0.10 Ma is obtained from 22 concordant incremental-heating experiments. This age agrees with a new 49.78 ± 0.08 Ma age for the sanidine from the Layered tuff, which is stratigraphically 24 m below the Sixth tuff.

Eocene calibration of geomagnetic polarity time scale reevaluated: Evidence from the Green River Formation of Wyoming

We reevaluate the Eocene geomagnetic polarity time scale on the basis of single-crystal 40 Ar/ 39 Ar ages for air-fall tuffs from the Wilkins Peak Member of the Green River For- mation of Wyoming. Tuff 6 is dated as 49.1 6 0.2 Ma, and tuff 3 is dated as 50.4 6 0.3 Ma (maximum estimate). When combined with published magnetostratigraphic con- straints, these age determinations suggest that the currently accepted age of chron C22r is 1.5-2.5 m.y. too old, which supports a significantly longer duration for the early Eocene, for the early Eocene climatic optimum, and the Wasatchian North American Land Mam- mal Age.

40Ar/39Ar geochronology of the Eocene Green River Formation, Wyoming

The deposits of Eocene Lake Gosiute that constitute the Green River Formation of Wyoming contain numerous tuff beds that represent isochronous, correlatable stratigraphic markers. Tuff beds selected for 40Ar/39Ar analysis occur within laminated mudstone, are matrix supported, and lack evidence of reworking. These tuffs contain 2%–15% euhedral phenocrysts of quartz, plagioclase, sanidine, biotite, and minor amphibole, pyroxene, and zircon, encased in a matrix of altered glassy ash. Air abrasion and handpicking under refractive- index oils were required to obtain clean, unaltered phenocrysts of sanidine. 40Ar/39Ar age determinations from single-crystal and <1 mg multigrain aliquots of sanidine and biotite allowed the identification and exclusion of xenocrystic contamination. Laser-fusion experiments on phenocrysts from the Rife, Firehole, C Bed, Grey, Main, Sixth, and Analcite tuff beds from the Tipton, Wilkins Peak, and Laney Members yielded weighted-mean ages (±2σ analytical uncertainties) of 51.25 ± 0.31 Ma, 50.70 ± 0.14 Ma, 50.56 ± 0.26 Ma, 50.39 ± 0.13 Ma, 49.96 ± 0.08 Ma, 49.70 ± 0.10 Ma, and 48.94 ± 0.12 Ma, respectively. Ages for sanidine and biotite from the Main tuff are indistinguishable when presumably xenocrystic contaminants are excluded from the age calculation. Moreover, the 40Ar/39Ar ages are consistent with the stratigraphic order of the tuff beds and with provenance in the Absaroka and Challis volcanic fields. Our 40Ar/39Ar-based age model indicates that sediment accumulated three times more rapidly (327 ± 86 μm/yr) during the evaporative Wilkins Peak phase than the freshwater to saline Tipton (88 ± 34 μm/yr) and Laney (104 ± 18 μm/yr) phases. The much lower accumulation rates for the Tipton and Laney Members are permissive of an annual origin for <1-mm-thick laminae and precessional forcing of 1–3-m-thick depositional cycles in these units. However, previously described cycles in the Wilkins Peak Member have average durations that are significantly shorter than the 19–23 k.y. precessional modes. The Green River Formation encompasses an ∼5 m.y. period between ca. 53.5 and 48.5 Ma, spanning magnetic chrons 24n through 21r. The Green River Formation was therefore deposited during the warmest period of the Cenozoic corresponding to the early Eocene climatic optimum as defined by the global marine O isotope record. Deposition of bedded evaporites (trona) of the Wilkins Peak Member began at ca. 51 Ma, or ∼2 m.y. after the onset of the highest inferred temperatures of the climatic optimum. The Bridgerian–Wasatchian faunal turnover occurred subsequently during Wilkins Peak time, at ca. 50.6 Ma. Thus, our 40Ar/39Ar ages strongly suggest that Wilkins Peak evaporite deposition and the turnover from Wasatchian to Bridgerian fauna were not directly caused by the initiation of maximum greenhouse conditions.

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.

Lake basin response to tectonic drainage diversion: Eocene Green River Formation, Wyoming

A previously unidentified major sequence boundary within the Eocene Green River Formation separates fluctuating profundal facies of the Tipton Shale Member from evaporative facies of the Wilkins Peak Member. During deposition of the Tipton Shale Member, rivers entered the basin from the north, across the subdued Wind River Mountains, and deposited the southward prograding deltaic complex of the Farson Sandstone Member. Boulderrich alluvial fan deposits overlie the Farson Sandstone adjacent to the Continental Fault, and correlate basinward to hypersaline lacustrine deposits of the Wilkins Peak Member. The alluvial fan deposits record a period of reverse motion on the Continental Fault and uplift of the southeastern Wind River Range, which diverted drainage away from the greater Green River Basin. This decreased inflow caused Lake Gosiute to shrink, exposing its bed to desiccation and erosion, and contributed to hydrologically-closed conditions and periodic evaporite deposition thereafter. This study is one of the first to demonstrate a direct relationship between movement along a specific basin-bounding structure, and a change in the overall style of lacustrine sedimentation. The identification of similar relationships elsewhere may challenge conventional interpretations of climate as the dominant factor influencing the character of lake deposits, and provide an important, but previously unexploited, approach to interpreting continental deformation and regional drainage organization.

ABSTRACT: Nonmarine Sequence Stratigraphic Interpretation of the Wilkins Peak Member, Eocene Green River Formation, Wyoming

ABSTRACT: Nonmarine Sequence Stratigraphic Interpretation of the Wilkins Peak Member, Eocene Green River Formation, Wyoming

Lake-type controls on petroleum source rock potential in nonmarine basins

Based on numerous empirical observations of lacustrine basin strata, we propose a three-fold classification of lacustrine facies associations that accounts for the most important features of lacustrine petroleum source rocks and provides a predictive framework for exploration in nonmarine basins where lacustrine facies are incompletely delineated. (1) The fluvial-lacustrine facies association is characterized by freshwater lacustrine mudstones interbedded with fluvial-deltaic deposits, commonly including coal. Shoreline progradation dominates basin fill, resulting in the stacking of indistinctly expressed cycles up to 10 m thick. In map view, the deposits may be regionally widespread but laterally discontinuous and contain strong facies contrasts. Transported terrestrial organic matter contributes to mixed type I-III kerogens that generate waxy oil (type I kerogen is hydrogen rich and oil prone; type III kerogen is hydrogen poor and mainly gas prone). The Luman Tongue of the Green River Formation (Wyoming) and the Honyanchi Formation (Junggar basin, China) provide examples of this facies association, which is also present in the Songliao basin of northeastern China, the Central Sumatra basin, and the Cretaceous Doba/Doseo basins in west-central Africa. (2) The fluctuating profundal facies association represents a combination of progradational and aggradational basin fill and includes some of the world's richest source rocks. Deposits are regionally extensive in map view, having relatively homogenous source facies containing oil-prone, type I kerogen. Examples include the Laney Member of the Green River Formation (Wyoming), the Lucaogou Formation (Junggar basin, China), the Bucomazi Formation (offshore west Africa), and the Lagoa Feia Formation (Campos basin, Brazil). (3) The evaporative facies association represents dominantly aggradational fill related to desiccation cycles in saline to hypersaline lakes and may include evaporite and eolianite deposits. Sublittoral organic-rich mudstone facies are relatively thin but may be (Begin page 1034) quite rich and widespread. The highest organic enrichment coincides with the deepest lake stages. Low input of land plant organic matter results in minimal lateral contrasts in organic content. In some cases a distinctive type I-S (sulfur-rich) kerogen may generate oil at thermal maturities as low as 0.45% vitrinite reflectance equivalent. Examples include the Wilkins Peak Member of the Green River Formation (Wyoming), the Jingjingzigou Formation (Junggar basin, China), the Jianghan and Qaidam basins (China), and the Blanca Lila Formation (Argentina).

ABSTRACT: Sediment and Water Provenance and Controls on Lacustrine Sedimentation, Wilkins Peak Member, Green River Basin, WY

ABSTRACT: Sediment and Water Provenance and Controls on Lacustrine Sedimentation, Wilkins Peak Member, Green River Basin, WY

Cyclicity in the Green River Formation (Lacustrine Eocene) of Wyoming

ABSTRACT Basinal facies of the Green River Formation have two main modes, lacustrine and playa. The lacustrine mode (Tipton and Laney members) accumulated mainly varved oil shale. Here annual cycles are recorded as varves. Variations in verve thickness demonstrate El Nino (ENSO)-type and sunspot cycles (Ripepe et al., this volume). Milankovitch-scale cycles are not obvious in lithic variations, but gamma ray logs record 1) precessional variations with a mean period (varve-timed) of 19.5 ka, and 2) a bundling of these in the ca. 100 ka eccentricity cycle. In the playa mode (Wilkins Peak Member), the lithic succession oil shale-trona-dolomitic marlstone records the precessional drying up of a lake and is again bundled in sets of 5, by the 100 ka eccentricity rhythm. The Tipton Member persisted f r 450 ka, the Wilkins Peak Member for ca. 1 Ma.

Origin of Lacustrine Rocks of Wilkins Peak Member, Wyoming

Wilkins Peak Member of the Green River Formation, with its interbeds of trona, oil shale, and dolomitic mudstone, contrasts markedly with the predominantly oil-shale and calcitic mudstone sequences of the remainder of the succession. Much discussion has centered on the origin of these lacustrine rocks. A perennial meromictic (stratified) lake model has been replaced in recent years by a playa-lake model. The subsurface stratigraphy, however, was not well known when the models were developed. Paleogeographic reconstruction indicates that the region was subjected to variations in hydrographic conditions mainly in response to climatic fluctuations. During humid subtropical periods, widespread during the deposition of the Tipton Shale Member and Laney Member, the lakes were p edominantly open meromictic systems with rivers entering the basin from the north. These environmental conditions were modified during deposition of the Wilkins Peak Member as the climate became more arid, and along the southern margin of the basin, shallow, saline-alkaline playa lakes formed in response to these changes. The widespread occurrence in the subsurface of fluvial clastic sequences of the Wasatch Formation associated with the Wilkins Peak Member would indicate that the playa lakes were frequently flooded and freshened by southern drainage systems. Therefore, the lakes were continuously undergoing modifications under the influence of climatic and possibly tectonic processes. A composite model is supported that combines aspects of both the meromictic and playa-lake models. Beca se of these complexities, it is unlikely that one modern lake can serve as an analog for ancient Lake Gosiute.

Palynology and Oil Shale Genesis in Fossil Butte Member of Eocene Green River Formation, Wyoming: ABSTRACT

The palynoflora of the Fossil Butte Member of the Green River Formation was studied to relate the flora and climate to the genesis of oil shale in ancient Fossil Lake. Samples were collected from measured sections representing open and marginal lacustrine environments. These samples were analyzed using standard palynologic and visual kerogen techniques. Evidence from the palynoflora suggests that during deposition of the Fossil Butte Member, the climate was in transition between humid, subtropical to a cooler, drier, warm temperate one with moderate fluctuations during various episodes of deposition. Additional evidence indicates that moist lowlands and flood plains existed around Fossil Lake, with upland forests on the surrounding ridges and mountains. Streams originating in the highlands supplied water to Fossil Lake and the surrounding vegetation. The age of the palynoflora along with K-Ar age determinations indicate that deposition of oil shale in Fossil basin was contemporaneous with deposition of the evaporite facies of the Wilkins Peak Member in the Green River basin. This suggests that factors affecting oil shale deposition i the two basins were locally, as well as regionally, controlled. The dominance of amorphous kerogen in the kerogen samples suggests an algal origin for the majority of the oil shales. The palynoflora, the kerogen type, and the stratigraphic relationships indicate that the deposition of oil shale in Fossil basin was due to local climatic and environmental conditions. End_of_Article - Last_Page 1208------------

Depositional subenvironments in an arid closed basin; the Wilkins Peak Member of the Green River Formation (Eocene), Wyoming, U.S.A.

ABSTRACTThe Wilkins Peak Member of the Green River Formation is a non‐marine dolomitic deposit that formed in an arid, hydrologically closed basin. A transect from the margin to the centre of the Wilkins Peak basin was examined in outcrop exposures in the south‐eastern corner of the basin and was supplemented with published core descriptions. The rocks were divided into seven facies based upon lithology, layering style and sedimentary fabrics: (1) AF facies—quartzose conglomerates and sandstones representing sandy alluvial fans coming from the Uinta Mountains. (2) SF facies—quartzose, peloidal, dolomitic sandstones which represent sheet flood deposits extending basinward from the alluvial fans. (3) DM facies—irregularly laminated dolomitic mudstones with abundant mudcracks which represent sheet flood deposits on a subaerially exposed mudflat. (4) EL facies—dolomitic mudstones with regular lamination and fewer but larger mudcracks representing sporadically exposed lake margins or shallow ponds. (5) PL facies—finely laminated oil shales and dolomitic mudstones with rare mudcracks which represent shallow perennial lakes. (6) SM facies—trona or halite deposits with dolomitic mudstone partings or dolomitic mudstones and sandstones disrupted by evaporites of varied morphology. These represent a central salt pan and the surrounding saline mudflat. (7) AS facies—arkosic sandstones, siltstones and mudstones which represent braided river and more basinal deltaic plain deposits.The five dolomitic facies form poorly defined cycles representing the intersection of sheet floods with a lake which transgressed and regressed in response to climatic variations. There are at least forty 3–4 m thick cycles in the Wilkins Peak and one large‐scale cycle comprising the whole member, which mimics the facies changes of the smaller cycles. This is compatible with the transitions of the Wilkins Peak Member to the Tipton and Laney members as climatic responses and not shifts in drainage. The sheet flood sandstones, actively aggrading subaerial mudflats, and bedded evaporites of the Wilkins Peak are apparently unique within the Green River Formation. They reflect the unusual provenance and hydrology of the Wilkins Peak and suggest chemical and sedimentary models established for the other Green River members will not be applicable.

Origin of Uraniferous Phosphatic Beds in Wilkins Peak Member of Green River Formation, Wyoming

The distribution of uranium and phosphorus was studied in four drill cores from the Wilkins Peak Member of the Green River Formation in Wyoming. Of the studied occurrences of anomalously high uranium concentrations, 13% were associated with localized organic matter, and the remainder were associated with stratiform phosphate-rich beds. The uranium probably substitutes for calcium in apatite in these beds. It is proposed that the apatite forms by replacement of calcite during times of flooding of the normally highly saline lake. The flood waters bring in phosphorus and cause a decrease in both pH and ratio of bicarbonate to phosphate, which favors the replacement. Uranium is incorporated in the apatite as the apatite forms or soon after. No special source, other than weathering of volcanic ash, is required for the phosphorus or the uranium. The uraniferous phosphatic beds do not appear to have any economic potential at the present time. Misleadingly high concentrations of both uranium and phosphorus are observed in outcrop samples as a result of selective leaching of other components.

Unreliable paleomagnetic results from the Wilkins Peak Member of the Eocene Green River Formation, Wyoming

A paleomagnetic investigation of in‐situ and exploration drill core samples of the Wilkins Peak Member of the Eocene Green River Formation in the Green River Basin of Wyoming yielded poor and unreliable paleomagnetic results. X‐ray and thermomagnetic analysis of samples from the study shows that pyrrhotite is the dominant magnetic mineral in the samples; pyrrhotite is known to be a common authigenic phase in Green River Basin sediments. Two earlier studies of the Green River Formation suggest unexpected behavior of the Eocene magnetic field; magnetite was identified in one sample of the former study. Similar lithologies have been sampled by all three studies. Because magnetic mineralogy may vary from authigenic to primary within one rock type, it is important for future paleomagnetic studies of the Green River Formation to carefully investigate the mineralogy and timing of remanence acquisition.

Physical Evidence for Saline Cycles of Deposition in Eocene Lake Gosiute in Southwest Wyoming: ABSTRACT

The Wilkins Peak Member, the saline unit of the Green River Formation in southwest Wyoming, is more than 985 ft (300 m) thick and contains more than 35 beds of trona or trona with halite. The trona and halite were deposited in the deepest part of the basin of Lake Gosiute, during arid periods of the Eocene Epoch, by the periodic evaporation and contraction of the lake waters. Alternating with the arid periods were more humid periods, when the lake expanded and less saline sediments were deposited across and beyond the previously deposited salt beds. The water-level fluctuations resulted in a concentric pattern of End_Page 1698------------------------------ lithofacies within the Wilkins Peak Member. The innermost of the cyclic lithofacies, which includes bedded evaporites, occupies the depositional center of the ancient lake basin and consists of repetitious ascending sequences of (1) dark-brown oil shale, (2) white to brown trona and halite, and (3) gray or green dolomitic mudstone. A second lithofacies that encircles the first, but does not contain bedded evaporites, shows in vertical section cyclic sequences of (1) gray siltstone, (2) dark-brown oil shale, (3) tan or light-brown oil shale, and (4) gray or green dolomitic mudstone. The oil shale and siltstone in these cycles were deposited as lake-bottom muds and along ephemeral shorelines during periods of lake expansion and water-freshening; the trona and halite and gray and green mudstone were deposited in salt pans and on m d flats during periods of lake contraction and water-salting. A third lithofacies is present at the outermost margins of the lake basin, but the cyclic sequences there, consisting mostly of gray and green mudstone and some thin interbedded tan and brown oil shale, are largely obscured by irregularly interbedded tan algal limestone, oolites, and thin beds of dolomite and sandstone. Floodplain deposits of interbedded red and gray sandstone and mudstone are present nearly everywhere between the outer edges of the lake basin and surrounding mountains. The number of major contractions of Lake Gosiute during deposition of the Wilkins Peak Member can be determined by counting the number of oil shale beds involved in cyclic sequences near the depocenter of the basin. Seventy-three oil shale beds, and hence 73 saline cycles, were counted in a hole cored in the Blacks Fork area by the U.S. Energy Research and Development Administration. Histograms of oil shale beds in drill holes and outcrops in other parts of the basin support 70 to 75 as the probable total number of saline cycles. The time intervals of the saline cycles can be roughly estimated by using potassium-argon dating methods for biotites in tuff, and by counting oil-shale varves. From these data, the shortest cycle in which salines were deposited is believed to have lasted less than 1,500 years and the longest cycle more than 100,000 years. End_of_Article - Last_Page 1699------------

Sedimentologic Framework of Green River Formation, Wyoming: ABSTRACT

During deposition of the Green River Formation, ancient Lake Gosiute began as a freshwater lake, evolved to a saline, alkaline lake, and ended as a freshwater lake. This evolution reflects the change from a closed-basin to an open-basin hydrologic regime; as a result, sedimentation in the Lake Gosiute system was strongly influenced by the relation between evaporation and inflow of water into the basin. In the Green River Formation, stratification sequences, sedimentary structures, and mineralogy of lithofacies provide important insights into the evolution of the system and the competing factors that determined the type of sediment accumualated in the lake and fringing environments. Carbonate sedimentation was strongly influenced by lacustrine transgressions and regression across a low topographic gradient. Terrigenous rocks reflect progradation of beach and deltaic shorelines during wetter climatic intervals when detritus that was produced and stored in upland areas during preceding drier intervals was transported to the lake. Hydrochemistry of lake Gosiute during the deposition of the Wilkins Peak member was controlled by ground water discharge; during the deposition of the Tipton and Laney Members it was controlled largely by surface water. Calcite precipitated as a result of mixing calcium-rich inflow and saline-alkaline lake waters. Dolomite formed as a result of periodic flooding and drying of the playa fringe (carbonate mud flat), where carbonate muds were saturated with saline-alkaline lake waters and underwent evaporitive pumping. Some surface waters were preconcentrated by dissolution of efflorescent crusts in alluvial plain and mud-flat sediments. Trona and halite precipitated from brine pools as the lake shrank during periods of intense aridity. During himid periods the lake expanded and oil shal was deposited. End_of_Article - Last_Page 1000------------

Sedimentary Subenvironments of Wilkins Peak Member of Green River Formation (Eocene), Wyoming: ABSTRACT

The Wilkins Peak Member of the Green River Formation is a nonmarine, closed-basin, dolomitic carbonate deposit that intertongues with siliciclastic deposits at the basin edges. A transect from the basin margin to the basin center reveals six major subfacies. (1) The alluvial-fan subfacies is poorly sorted boulder conglomerates, cross-bedded gravels, and horizontally laminated grits and sands. These are interpreted, respectively, as fan-apex incised channels; mid-fan channel-bar deposits; and fan-toe, shallow, braided channels. (2) Sand-flat subfacies consists of wedge-shaped sheets of dolomitic sands extending tens of kilometers into the basin center and changing from Bouma-like graded units (20 to 30 cm thick) near the basin edge to horizontally laminated or coarse gra ed beds (1 to 10 cm thick) toward the basin center. These are interpreted as having been deposited by decelerating sheetfloods. (3) Dry-mud-flat subfacies includes densely mud-cracked and graded dolomitic mudstone laminites and thin beds. These are interpreted as subaerial mud flats in which sheetflooding and in-basin, shallow debris flows were the important depositing mechanisms. (4) Nonsaline-ephemeral-lake subfacies is dolomitic mudstones with laminations composed of pinch-and-swell sand, a thick mud cap, and sparse deep mud cracks. These are interpreted as sporadically exposed, shallow-lake margins or isolated shallow ponds. (5) Perennial-lake subfacies is oil shales or finely laminated dolomitic mudstones that are rarely cracked. These are interpreted as having been deposited in a s allow but persistent lake. (6) Saline-ephemeral-lake subfacies consists of either dolomitic mudstones and oil shales disrupted by intrasediment salt-crystal molds, or massive trona and/or halite beds containing mud partings. These are interpreted as brine-soaked lake and mud-flat deposits, or deposits in very shallow brine pools. These subfacies occur as asymmetric cyclic sequences 3 to 4 m thick that are interpreted as random sheetflood deposits superimposed on transgressive and regressive beds laid down in a shallow central lake which occasionally dried up. The small-scale lacustrine cyclic sequences probably provide delicate indications of minor climatic changes. These sequences not only provide a facies model for other deposits but also a possible criterion for predicting the large-scale geometry of less well-exposed basins. End_of_Article - Last_Page 530------------

Sedimentation in an Ancient Playa-Lake Complex: The Wilkins Peak Member of the Green River Formation of Wyoming

Research Article| March 01, 1975 Sedimentation in an Ancient Playa-Lake Complex: The Wilkins Peak Member of the Green River Formation of Wyoming HANS P. EUGSTER; HANS P. EUGSTER 1Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, Maryland 21218 Search for other works by this author on: GSW Google Scholar LAWRENCE A. HARDIE LAWRENCE A. HARDIE 1Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, Maryland 21218 Search for other works by this author on: GSW Google Scholar Author and Article Information HANS P. EUGSTER 1Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, Maryland 21218 LAWRENCE A. HARDIE 1Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, Maryland 21218 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1975) 86 (3): 319–334. https://doi.org/10.1130/0016-7606(1975)86<319:SIAAPC>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 HANS P. EUGSTER, LAWRENCE A. HARDIE; Sedimentation in an Ancient Playa-Lake Complex: The Wilkins Peak Member of the Green River Formation of Wyoming. GSA Bulletin 1975;; 86 (3): 319–334. doi: https://doi.org/10.1130/0016-7606(1975)86<319:SIAAPC>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 The Wilkins Peak Member of the Green River Formation of Wyoming has been examined in outcrop with the object of reconstructing its depositional environment. Based on their assemblages of sedimentary structures, seven rock units are described, six of which define depositional subenvironments. These units are (1) flat-pebble conglomerate, (2) lime sandstone, (3) mudstone, (4) oil shale, (5) tronahalite, (6) siliciclastic sandstone, and (7) volcanic tuff. Their respective subenvironments are (1) rapid transgression of a shallow lake, (2) lake shore oscillating over a mud flat (slow transgression), (3) playa mud flats, (4) shallow lake with occasional desiccation, (5) seasonally dry salt lake, (6) braided stream, and (7) not specific.These subenvironment deposits are arranged in depositional cycles. We have observed four types of cycles involving flat-pebble conglomerate (A), oil shale (B), mudstone (C), lime sandstone (D), and also trona. These cycles are I: A-B-C, II: D-B-C, III: D-C, and IV: B-trona-C. Individual cycles have been correlated over distances of up to 24 km.The Wilkins Peak Member is thought to have been deposited in a playa-lake complex, which consisted of a shallow, central playa lake that was surrounded by vast, normally exposed mud flats fringed by alluvial fans. Evaporative concentration of bicarbonate-rich inflow waters led to saturation with respect to calcite, most of which must have been deposited as cement within alluvial fans. Evaporation continued in the capillary zone of the mud flats, precipitating calcite first, then magnesian calcite, and eventually protodolomite. The carbonates accumulated as a soft micritic mud at the fringes of the playa mud flats.During periods of desiccation, the muds were subject to cracking, and the mud-crack polygons contributed sand- and silt-size dolomitic micrite intraclasts that were transported to the central lake by the next storm. When the central lake was large, oil shale accumulated in it, with the organic matter derived from a flocculent ooze consisting of bottom-dwelling blue-green algae and fungi. During dry periods the lake shrank, and trona and halite precipitated in the central portions.An understanding of the Wilkins Peak sediments can be achieved only by considering as inseparable the hydrologic, sedimentary, geochemical, and biologic processes responsible for their formation. 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.