eocene clocks agree coeval 40ar 39ar u pb and astronomical ages from the green river formation u pb ages of zircon from the firehole and analcite ash beds in the eocene green river formation wyoming united states are indistinguishable from 40 ar 39 ar ages of sanidine after adjusting the latter to the astronomically calibrated age of 28 201 ma for the fish canyon sanidine standard six of nine zircon analyses from the firehole ash yield concordant ages in an overlapping cluster and give a weighted mean 238 u 206 pb age of 51 66 0 20 ma full external 2 uncertainty indistinguishable from the recalibrated 40 ar 39 ar age of 51 40 0 25 ma full external 2 uncertainty which is adjusted to eliminate samples with low radiogenic 40 ar significant 238 u 206 pb age scatter likely reflects some combination of minor inheritance pb loss and possibly magma chamber residence seven of eight analyses of analcite ash zircon yield a weighted mean 238 u 206 pb age of 49 23 0 13 ma full external 2 uncertainty whereas the sanidine 40 ar 39 ar age is 49 24 0 18 ma full external 2 uncertainty calibrating green river formation 40 ar 39 ar ages to the 28 201 ma age for fish canyon sanidine permits the first direct comparison of specific green river formation strata to the astronomical solution for early eocene insolation this comparison supports the hypothesis that periods of fluvial deposition coincided with minima in long and short eccentricity and that periods of lake expansion and evaporite deposition correspond to eccentricity maxima

implications of pre eruptive magmatic histories of zircons for u pb geochronology of silicic extrusions zircons in silicic magmas begin to crystallize 10 s to 100 s of thousands of years ka prior to their eruption u pb and u th dating for zircons from 32 young extrusions indicate a range of mean pre eruption ages from 33 to 247 ka with a median of 92 ka explosive large volume widespread eruptions show a narrower range and a younger median of 70 ka these results indicate that the difference between initial zircon growth and eruption age measured by other e g 40ar 39ar techniques is in excess of the achievable analytical uncertainty for eruptions 30 million years ma and likely for eruptions of up to 300 ma protracted pre eruption zircon growth is well established by an increasing number of studies on magma process time scales but this source of eruption age bias has been largely ignored in chronostratigraphic applications of high precision u pb data until intrinsic properties of zircons or their host deposits enable a priori estimates of pre eruption age bias it is prudent to expand the uncertainties attending all u pb zircon ages interpreted as deposition ages in e g chronostratigraphic studies by 0 200 ka

precise dating of biotite in distal volcanic ash isolating subtle alteration using 40ar 39ar laser incremental heating and electron microprobe techniques precise dating of k rich minerals in volcanic rocks via the 40 ar 39 ar geochronometer has become crucial to resolving many geological problems in some cases ash beds containing biotite phenocrysts but lacking sanidine are the only datable horizons at key stratigraphic intervals necessitating comparison of sanidine and biotite ages within the same chronostratigraphic framework to assess the integrity of biotite and evaluate the accuracy of ages obtained from this often problematic mineral incremental heating 40 ar 39 ar experiments were performed on large millimeter sized euhedral biotite crystals from four key eocene tuffs in the green river and wind river basins and are compared with sanidine 40 ar 39 ar ages unaltered biotite crystals with homogenous k compositions from two tuff beds yielded concordant plateau ages that are indistinguishable from cogenetic sanidine ages in contrast biotite crystals from two other tuffs yielded discordant spectra with relatively young initial steps followed by older down stepping apparent ages plateau ages from the discordant experiments are older than the sanidine ages by 1 to 14 integrated total fusion ages from these experiments exhibit scatter toward both younger and older ages that correlate with the degree of spectral discordance electron microprobe transects reveal that biotite crystals yielding discordant age spectra contain 1 10 m thick k depleted 8 k 2 o alteration zones along internal cleavage planes that are absent in biotite crystals yielding concordant age spectra we propose that these altered zones promote open system behavior phenomena such as loss of k or 40 ar 39 ar k recoil into internal k depleted phases and 39 ar k recoil entirely out of internally corroded biotite crystals are all potential mechanisms that can be related to this alteration due to the presence of these multiple potential pathways that promote the gain or loss of isotopes such altered biotite crystals are unsuitable for high resolution 40 ar 39 ar dating comparisons with sanidine ages or calibration of the geomagnetic polarity timescale

reconciling astrochronological and 40ar 39ar ages for the matuyama brunhes boundary and late matuyama chron when five matuyama brunhes m b boundary records from the north atlantic are placed on isotope age models produced by correlation of the 18o record directly or indirectly to an ice volume model the m b boundary lies consistently at the young end of marine isotope stage 19 with a mean age for the midpoint of the reversal of 773 1 ka standard deviation 0 4 kyr 7 kyr younger than the presently accepted astrochronological age for this polarity reversal 780 781 ka two recently proposed revisions of the age of the 40ar 39ar fish canyon sanidine fcs standard to 28 201 0 046 ma and 28 305 0 036 ma would adjust 40ar 39ar ages applicable to the m b boundary and other reversals and excursions back to 1 2 ma to ages older than the new astrochronological ages by 8 24 kyr the variables used to construct the ice volume models cannot account for the discrepancy the fcs standard age that best fits the astrochronological ages is 27 93 ma which is within the uncertainty associated with the commonly used value of 28 02 0 16 ma but younger than the recently proposed fcs ages the edc2 and edc3 age models in the dome c antarctic ice core yield ages of 771 7 ka and 766 4 ka respectively for the 10be flux peak that denotes the paleointensity minimum at the reversal boundary implying that the edc2 rather than edc3 age model is consistent with the observations from marine sediments at least close to the m b boundary

a refined astronomically calibrated 40ar 39ar age for fish canyon sanidine abstract intercalibration between the astronomical and radio isotopic dating methods provides a means to improving accuracy and reducing uncertainty of an integrated multi chronometer geologic timescale here we report a high precision 40 ar 39 ar age for the fish canyon sanidine fcs neutron fluence monitor by multi collector noble gas mass spectrometry through cross calibration with a1 tephra sanidines a1ts of the direct astronomically tuned faneromeni section crete the astronomically intercalibrated 40 ar 39 ar age of fcs of 28 172 0 028 ma 2 external errors is within the uncertainty of but more precise 0 10 than the previous 40 ar 39 ar age determined by intercalibration with astronomically tuned tephras from the melilla basin morocco using this proposed age for fcs combined with measurements using the a1ts as the neutron fluence monitor a weighted mean bishop tuff 40 ar 39 ar sanidine age of 0 7674 0 0022 ma 2 external errors is indistinguishable from the id tims u pb zircon age 0 7671 0 0019 ma the consistency between the astronomically calibrated 40 ar 39 ar sanidine age and u pb zircon age for this quaternary unit suggests that agreement between these two radio isotopic dating techniques is now achievable at better than 0 3 2 in the youngest part of geologic time

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

paleogeographic reconstruction of the eocene idaho river north american cordillera eocene lake gosiute in southwestern wyo ming was progressively fi lled in by volcani clastic sediment between 49 6 and 47 0 ma the source of this material has long been thought to have been the absaroka volcanic province immediately north of the greater green river basin lead isotope compositions of sandstone from this interval however are consistent with derivation from the challis volcanic fi eld the 40 ar 39 ar ages of single de trital k feldspar crystals from greater green river basin sandstones are nearly identical to 40 ar 39 ar eruptive ages for volcanic rocks of the challis volcanic fi eld 49 8 45 5 ma but we also identify mesozoic and proterozoic crystals that are consistent with cooling ages for rocks that were likely exposed in the idaho segment of the north american cordillera during the eocene most of these rocks are traversed by or are up gradient of a major eocene paleovalley in central idaho the sud den appearance of challis derived sediment in the greater green river basin indicates that a major river here named the idaho river connected the interior of the north american cordillera to the greater green river basin this connection requires 500 km to reach from central idaho to the greater green river ba sin the idaho river probably carried detritus that was stripped from distant uplifted moun tains above the active challis volcanic fi eld as far south as the piceance creek basin suggest ing a total length of at least 1000 km middle eocene metamorphic core complexes in the northern cordillera likely produced a major highland in the headwaters of the idaho river which generated river systems that drained both eastward into the wyoming foreland and westward into the oregon coast ranges

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 2 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 m yr during the evaporative wilkins peak phase than the freshwater to saline tipton 88 34 m yr and laney 104 18 m 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

magnetic fabric and 3d geometry of the hombreiro sta eulaliapluton implications for the variscan structures of eastern galicia nw spain abstract in this paper the internal structure and 3d geometry of the hombreiro sta eulalia granitic pluton hercynian belt nwspain are described characterization of the internal structure was performed from surface data based on microstructural analysis and anisotropy of magnetic susceptibility studies the geometrical features of the pluton morphology at depth were derived from gravity data the pluton is made up of paramagnetic granites and leucogranites k values ranging between 1 1 and 13 3 10 5 si with well developed magnetic fabrics anisotropy degree p 1 05 for 78 of the sampling localities the results obtained enable us to depict a flat lying magnetic structure with associated n120 e trending magnetic lineations the gravity survey corroborates the subhorizontal laminar geometry of the pluton which has an estimated thickness of 2 km and an almost planar floor the 3d morphology is elongate at depth parallel to the linear internal fabric this would imply that magma emplacement took place along this preferential direction whereas the deepest zones of the pluton trend wnw ese elongate parallel to regional 1 it is almost rootless we present data showing that the hombreiro sta eulalia pluton emplacement was closely related spatially and chronologically along with other granitoid plutons from the lugo dome with two major structures of eastern galicia the basal shear zone of the mondonedo nappe and the extensional vivero fault

strontium isotope record of paleohydrology and continental weathering eocene green river formation wyoming sr isotope ratios in carbonate rich lacustrine strata provide highly resolved and geo graphically specific records of past changes in weathering and regional drainage patterns the eocene green river formation is perhaps the best documented pre quaternary la custrine unit in the world therefore these strata are ideally suited for studying the be havior of sr the 87 sr 86 sr ratios measured for primary carbonate in lake expansion contraction cycles of the laney member are directly linked to changing lake facies four distinct cycles are preserved in a 6 4 m interval of the arco washakie basin no 1 core each represented by a vertical succession of transgressive stromatolite facies containing dolomicritic intraclasts laminated micrite facies deposited during lake highstands and lake marginal dolomicrite facies which represent subsequent lowstands initial 87 sr 86 sr ratios range from 0 711 99 to 0 713 31 the least radiogenic isotope compositions are as sociated with laminated micrite that records increased regional runoff from phanerozoic marine carbonate and tertiary volcanic rocks during lacustrine highstands the most radiogenic isotope compositions are associated with dolomicrite recording a higher pro portion of runoff from basin bounding precambrian uplifts and the exposed lake plain during lacustrine lowstand the observed 87 sr 86 sr variations may also be due in part to changes in differential weathering of the surrounding landscape where drier climates are associated with decreased differential weathering and therefore higher 87 sr 86 sr in runoff

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

resolving milankovitchian controversies the triassic latemar limestone and the eocene green river formation although orbital forcing is commonly proposed as the driver of ancient sedimentary rhythms the lack of adequate independent time control radio isotopic data to calibrate these cycles has stood as a major challenge to evaluation of the hypothesis here i apply a new statistical approach to evaluate cyclicity in two historically important rhythmic units for which orbital forcing has been proposed the triassic latemar limestone dolomites italy and the eocene green river formation wyoming usa a major advance of the new method is its explicit evaluation of the null hypothesis of no orbital signal the null hypothesis can be rejected with a high degree of confi dence in the latemar limestone probability 0 30 and green river formation probability 0 07 the analyses also resolve controversies about the specifi c orbital calibrations at each site both data series reveal the expected precession obliquity and eccentricity orbital components and yield astrochronologies that are consistent with proposed radio isotopic based time scales

hypersaline brine diagenesis and evolution in the dead sea lake lisan system israel abstract water soluble salts were extracted from 57 aragonite and detrital laminae sampled from the pleistocene lake lisan sediments the solutions and solids were analysed for ca mg na k sr cl so 4 and bicarbonate average na cl mg ca sr ca mg cl ca cl sr cl k cl k na and k mg ratios calculated for seven stratigraphie elevations in the lisan column indicate that the dead sea rift sediment brine system has remained practically closed for 70000 years the soluble salts reflect both the hydrologie evolution of the lake as well as diagenetic processes mg ca ratios systematically decrease in younger sediments and this decline is accompanied by a 1 5 4 fold sr enrichment in aragonite solubles relative to salts extracted from detrital laminae the major process responsible for these observations is dolomitization of sr rich aragonite by interstitial brine diagenesis was triggered by slow desiccation of the lake s sediments after the lisan waters started to recede due to a change in climate very sharp chemical gradients which developed in the sediments after deposition have been preserved because further diagenesis was quenched by the advancing dry out h 2 o loss by evaporation induced difrusional migration of na and cl from aragonite laminae into adjacent detrital layers this lowered the na cl ratio in the aragonite and raised it in the detritus relative to the initial dead sea na cl ratio of 0 26 the early lisan brine was similar in its chemical composition and total salinity to that of the present dead sea

a strategy for cross calibrating u pb chronology and astrochronology of sedimentary sequences an example from the green river formation wyoming usa abstract astronomical calibration of the geological timescale has been limited until recently by the precision and accuracy of radioisotopic dates especially for pre neogene records uncertainties for radioisotopic dates of older strata were typically much larger than a single precessional cycle and dates were often sparse leading to the practice of orbital tuning of cyclic strata in order to astronomically calibrate the desired interval ideally in order to test the assumptions of astronomical calibration with geochronology it is necessary that the precision of radioisotopic dates be comparable to the period of the cycle being tested the new u pb ca tims chemical abrasion thermal ionization mass spectrometry zircon dates reported here conform to this precision requirement with 2 analytical uncertainties from 11 000 to 52 000 years for seven volcanic ashes from the wilkins peak member of the green river formation the zircon dates have simple distributions with few outliers and allow accurate estimations of the eruption ages with potential inaccuracies of less than precessional cycle the eocene green river formation wyoming usa has long been recognized as a record of cyclicly deposited lacustrine sediments and the abundant intercalated volcanic ashes make it a suitable place to test new approaches to astronomical calibration of cyclic strata the abundance of different types of marker beds including tuffs that are intercalated with the sedimentary cycles guarantee an unambiguous correlation between sampling locations of dated tuffs on the margins of the basin and the basin center where the cyclicity is best developed thus reducing any stratigraphic uncertainties to a fraction of hypothesized precession cycle tuning based orbital age models accepted by the previous geochronology significantly deviate from the new geochronology whereas a previously rejected model that assumes a short eccentricity period of 125 ky is now allowed in order to test possible explanations for the apparent 125 ky period such as changes in orbital periods or gaps in the sedimentary record we present an iterative strategy to select future ashes for dating such that the astronomical calibration testing is optimized we iteratively contrast two ad hoc age models that bracket the linear interpolation between the dated ashes the optimal intervals for further dating are located where the deviations between the models exceed our reported uncertainties we propose that the iterative approach described here should become the standard for establishing a rigorous orbital calibration of the stratigraphic record where sufficient ashes exist

fine tuning the calibration of the early to middle eocene geomagnetic polarity time scale paleomagnetism of radioisotopically dated tuffs from laramide foreland basins age calibration of the early to middle eocene geomagnetic polarity time scale remains highly uncertain due to conflicting magnetostratigraphic radioisotopic and astrochronologic results in this study new paleomagnetic polarity determinations of 29 ash fall tuffs preserved in strata of five laramide foreland basins were used in conjunction with previously published 40 ar 39 ar ages from the same tuffs to evaluate eight different calibration models for the early to middle eocene part of the geomagnetic polarity time scale reliable paleomagnetic information was recovered from 23 tuffs of which 17 showed normal polarity and six showed reversed polarity after comparison of the models with the paleomagnetic and radioisotopic data from the tuffs and an array of independent chronostratigraphic observations the new willwood model is herein selected as the best alternative to the current geomagnetic polarity time scale calibration for the early to middle eocene three important implications are apparent in our proposed model first the early eocene is shortened by 0 6 m y and the middle eocene is lengthened by 0 8 m y compared with the 2004 geomagnetic polarity time scale also the early eocene climatic optimum is estimated to have lasted from 52 9 to 50 7 ma 1 m y longer than previously suggested and overlapping in time with the inferred age of the wasatchian bridgerian faunal transition our new model agrees with a previous astronomical model when it is tied to the oldest proposed age for the paleocene eocene thermal maximum at 56 33 ma

zircon dating of oceanic crustal accretion most of earth s present day crust formed at mid ocean ridges high precision uranium lead dating of zircons in gabbros from the vema fracture zone on the mid atlantic ridge reveals that the crust there grew in a highly regular pattern characterized by shallow melt delivery combined with results from previous dating studies this finding suggests that two distinct modes of crustal accretion occur along slow spreading ridges individual samples record a zircon date range of 90 000 to 235 000 years which is interpreted to reflect the time scale of zircon crystallization in oceanic plutonic rocks

spectral analysis of the lower eocene wilkins peak member green river formation wyoming support for milankovitch cyclicity abstract this study is the first to employ spectral analysis to examine meter scale sedimentary cyclicity in the wilkins peak member of the lower eocene green river formation of wyoming generally regarded as the classic example for orbital forcing of lacustrine sediments at eccentricity and precession time scales this long standing interpretation was recently contested with a much shorter duration 10 ky inferred for the dominant cyclicity earlier work lacked adequate age control or spectral analysis or both our analysis is based upon an evaluation in the frequency domain of oil yield values from four boreholes accuracy estimation for suggested orbital interpretations and comparison to independent geochronology cored intervals 266 364 m thick represent a span of 1 2 1 7 m y with temporal resolution of 3 5 ky 1 m for oil yield values variations in spectral power with depth within the original records are interpreted to reflect changes in the rate of sediment accumulation these changes are corrected prior to testing the orbital forcing hypothesis by using two methods 1 a minimal adjustment three segments accounting for the dominant changes of spectral frequency with depth and 2 correlating the published definitions of precessional cycles in these records to a 21 ky cosine curve orbital age models resulting from the two tuning methods are compared to available chronology and the tuned records are tested for the expected spectral peaks from orbitally forced records we conclude that the dominant cyclicity of the wilkins peak member is orbitally forced orbital age models overlap 40 ar 39 ar ages and inferred periods include long and short eccentricity weak obliquity and precession eccentricity is resolved in the analyzed records but the expected 95 and 125 ky periods are not resolved controlling the range of possible tuning periods and the accuracy of orbital age models sub milankovitch variability exists and can be resolved to a minimum period of 3 5 ky by the analyzed records however it cannot be characterized fully with the available chronology or by the previously calculated mean cycle duration

zircon crystallization and recycling in the magma chamber of the rhyolitic kos plateau tuff aegean arc in contrast to most large volume silicic magmas in continental arcs which are thought to evolve as open systems with significant assimilation of preexisting crust the kos plateau tuff magma formed dominantly by crystal fractionation of mafic parents deposits from this 60 km3 pyroclastic eruption the largest known in the aegean arc lack xenocrystic zircons secondary ion mass spectrometry sims u pb ages on zircon cores never older than 500 ka and display sr nd whole rock isotopic ratios within the range of european mantle in an area with exposed paleozoic and tertiary continental crust this evidence implies a nearly closed system chemical differentiation consequently the age range provided by zircon sims u th pb dating is a reliable indicator of the duration of assembly and longevity of the silicic magma body above its solidus the age distribution from 160 ka age of eruption by sanidine 40ar 39ar dating smith et al 1996 1 to ca 500 ka combined with textural characteristics high crystal content corrosion of most anhydrous phenocrysts but stability of hydrous phases suggest 1 a protracted residence in the crust as a crystal mush and 2 rejuvenation reduced crystallization and even partial resorption of minerals prior to eruption probably induced by new influx of heat and volatiles this extended evolution chemically isolated from the surrounding crust is a likely consequence of the regional geodynamics because the thinned aegean microplate acts as a refractory container for magmas in the dying aegean subduction zone continent continent subduction 1 ref 32

early eocene carbon isotope excursions and landscape destabilization at eccentricity minima green river formation of wyoming repeated global reorganizations of carbon cycling and biotic oceanic and terrestrial processes occurred during the early eocene and appear to have been paced by cyclic variations in the eccentricity of the earth s orbit the phase relationship s between insolation variation terrestrial paleoclimate and atmospheric pco2 during these events remains enigmatic however due to their poorly constrained timing relative to specific orbital configurations here we use tiered interpolation between radioisotopic ages and paleomagnetic polarity chrons to compare high resolution 13c and lithofacies records from the wilkins peak member of the green river formation of western north america to a fe intensity xrf record from the western atlantic ocean and to numerical solutions for earth s orbital configuration wilkins peak member lithofacies stacking patterns record cyclic geomorphic responses to insolation and climate fluctuations spanning an interval of 1 8 ma previous macrostratigraphic analyses using 40ar 39ar and u pb ash bed ages indicate that these cycles reflect long and short eccentricity modulation of precession hydrologic variance appears to have occurred inversely with intervals of maximum sediment advection with carbonate and evaporite dominated lacustrine modes during eccentricity maxima and siliciclastic dominated alluvial modes during eccentricity minima stable carbon isotope analyses of 126 meters of wilkins peak member strata reveal a regular 5 per mil oscillation between high 13c lacustrine modes and low 13c alluvial modes tiered interpolation between paleomagnetically characterized terrestrial ash beds facilitates the integration of 11 radioisotopic ages with the geomagnetic polarity timescale resulting in significant expansion of chron c23 and shortening of chron c22 relative to timescales based on seafloor magnetic anomaly profiles the new proposed timescale permits direct comparison of terrestrial and marine climate proxy records from the early eocene climatic optimum at ca 250 kyr 250 kyr resolution and reveals prominent 100 kyr and 405 kyr scale oscillations within both records wilkins peak member 13c minima which occurred during low eccentricity alluvial modes likely coincided with global 13c minima scenario 1 but may alternatively reflect productivity driven local effects within lake gosiute scenario 2 if scenario 1 proves accurate early eocene negative 13c hyperthermal excursions occurred during eccentricity minima rather than maxima as formerly believed

encounters with an unearthly mudstone understanding the first mudstone found on mars the sheepbed mudstone forms the base of the strata examined by the curiosity rover in gale crater on mars and is the first bona fide mudstone known on another planet from images and associated data this contribution proposes a holistic interpretation of depositional regime diagenesis and burial history a lake basin probably received sediment pulses from alluvial fans bed cross sections show millimetre to centimetre scale layering due to distal pulses of fluvial sediment injections fine grained hyperpycnites fall out from river plumes and some aeolian supply diagenetic features include mineralized synaeresis cracks and millimetre scale nodules as well as stratiform cementation clay minerals were initially considered due to in situ alteration but bulk rock chemistry and mineralogy suggests that sediments were derived from variably weathered source rocks that likely contained pre existing clay minerals x ray diffraction analyses show contrasting clay mineralogy in closely spaced samples consistent with at least partial detrital supply of clay minerals a significant 30 wt amorphous component is consistent with little post depositional alteration theoretical modelling of diagenetic reactions as well as kinetic considerations suggest that the bulk of diagenetic clay mineral formation occurred comparatively late in diagenesis diagenetic features synaeresis cracks and nodules were previously thought to reflect early diagenetic gas formation but an alternative scenario of synaeresis crack formation via fabric collapse of flocculated clays appears more likely the observed diagenetic features such as solid nodules hollow nodules matrix cement and raised ridges synaeresis cracks can be explained with progressive alteration of olivine glass in conjunction with centrifugal and counter diffusion of reactive species anhydrite filled fractures in the sheepbed mudstone occurred late in diagenesis when fluid pressures built up to exceed lithostatic pressure generating fluid overpressure by burial to facilitate hydraulic fracturing suggests a burial depth of at least 1000 m for the underlying strata that supplied these fluids this article is protected by copyright all rights reserved

enso and sunspot cycles in varved eocene oil shales from image analysis abstract on the premise that sequential changes in varve thickness offer a proxy for climatic variations we investigated varve thickness in three core segments from the distal lacustrine oil shales tipton and laney members of the green river formation by means of an image analysis program of two strong bimodal periodicities one at 4 8 5 6 years is interpreted as an el nino type enso phenomenon of atmospheric dynamics while the other at 10 4 14 7 years is interpreted as the sunspot cycle originally recognized in this formation by bradley 1929 1931 weaker periodicities may exist at ca 8 and 33 years the latter also recognized by bradley taken in conjunction with the work of bradley 1929 1931 and of crowley et al 1986 this suggests that some but not all of the oil shale of the green river formation is truly varved and can be used to infer climatic time series automated instrumental scans have greatly extended the possibilities of varve studies as approaches to climatic variations in time but careful selection of facies in their geological framework is necessary

paleogeographic record of eocene farallon slab rollback beneath western north america eocene terrestrial strata in western north america are now dated with sufficient precision to examine the hypothesis that rollback of the farallon flat slab created a trenchward migrating wave of dynamic and thermal topography we use patterns of accumulation and unconformity lake type stratigraphy and physical and isotopic provenance tools to track its migration across the laramide foreland between 53 ma and 47 ma hydrologic ponding and coincident unconformity development began at 53 ma culminating at 51 8 ma in the hydrologic closure of an 900 000 km2 area between the cordilleran and rocky mountain divides four subsequent stream diversions and a southwest expanding regional unconformity record trenchward migration of uplift from 50 to 47 ma volcaniclastic detritus from magmatic centers coincident with the uplifted region progressively filled the green river formation lakes from north to south these findings are consistent with recent numerical and conceptual models of slab rollback that predict initial dynamic subsidence above the slab hinge followed by uplift and volcanism triggered by influx of asthenosphere beneath the overriding plate based on the surface record rollback proceeded systematically at a rate of 6 cm yr across the wyoming craton from 53 to 47 ma resulting in removal of the entire shatsky conjugate oceanic plateau from the base of the north american lithosphere

tectonically induced change in lake evolution recorded by seismites in the eocene green river formation wyoming seismogenic sedimentary structures seismites provide an opportunity to understand the effects of syndepositional tectonics on lacustrine sedimentation the lowermost wilkins peak member eocene green river formation wyoming usa contains laterally extensive intervals displaying folds load structures mixed brittle ductile deformation microfaults breccias and sedimentary dykes which formed intrastratally in a low energy sublittoral lacustrine environment they developed in situ by liquefaction and plastic or brittle behaviour of semi lithified sediment or by remobilization and injection of granular material their morphological attributes and lateral extent zonation in deformation intensity and the depositional setting all imply deformation triggered by earthquake induced stresses in shallow buried sediment their stratigraphic distribution coincides with a shift in hydrological conditions from a balanced filled to an underfilled lake type independent of climate change this study is the first to ascribe seismites to a pulse of tectonic activity that caused a synchronous change in lacustrine evolution

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 cycles 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

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