cenozoic topographic and climatic response to changing tectonic boundary conditions in western north america abstract this study presents an oxygen isotopic record from the paleocene to the pliocene based on the analysis of predominantly paleosol carbonate from intermontane basins in southwestern montana and eastern idaho 18 o values of calcite decrease by 7 to 10 between 50 and 47 ma in southwestern montana and idaho most likely as a result of an increase in elevation of 2 5 to 3 5 km this rise in elevation is roughly contemporaneous with the emplacement of the nearby challis volcanics and the formation of metamorphic core complexes in the hinterland of the sevier thrust belt moreover when compared to previous oxygen isotopic studies that show oxygen isotopic shifts of similar magnitude occurring later in the late eocene to early oligocene in northeastern nevada and late oligocene to miocene in southern nevada the results of this study add to a growing body of evidence for a spatial and temporal migration of high surface elevations from north to south in the great basin of western united states this surface uplift history supports tectonic models calling for north to south removal of the farallon slab or delamination of the mantle lithosphere

paleoaltimetry from stable isotope compositions of fossils stable isotope systematics of phosphatic vertebrate and carbonate invertebrate fossils are reviewed emphasizing external vs biological controls on isotope compositions and their variation external controls include elevation and isotopic lapse rates temperature atmospheric circulation patterns aridity and changes to precipitation seasonality e g development of monsoons biological controls include temperature regulation and temperature dependent biological activity water balance behavior and rates of hard part secretion and maturation some key factors include what sources of water an animal samples how those sources respond isotopically to elevation and the isotopic sensitivity of a biologic tissue to environmental changes general research design criteria are enumerated including numbers of fossils and analyses required as well as uncertainties in the interpretability of isotopic shifts isotope zoning from fossil teeth and shells from the indian foreland demonstrate comparable seasonal monsoon signals at 11 ma vs today implying a high and broad plateau at that time mean isotope compositions of fossil teeth from the lee of the cascade range central oregon show a pronounced decrease in 18 o initiating 7 ma signaling topographic rise associated with impingement of basin and range extension on the arc mean compositions and zoning in fossil shells from the northern rocky mountains indicate high elevations and topographic relief in the late cretaceous and early paleogene paleoaltimetry will principally benefit from better estimates of temperature because temperature lapse rates are less variable than isotopic lapse rates and because the combination of temperature and isotopic studies may help distinguish between local isotopic changes and distal ones caused by catchment or rain shadow effects

laramide exhumation of the bighorn mountains wyoming an apatite u th he thermochronology study we report u th he apatite ages from the crystalline core of the bighorn mountains and compare the exhumation history derived from those ages with the exhumation history determined from sedimentary rocks in the adjacent powder river and bighorn basins our u th he apatite ages range from 62 to 369 ma and represent a pre laramide he partial retention zone that was deformed and uplifted at ca 65 5 ma the geometry of the he partial retention zone indicates that the basement in the bighorn mountains is deformed into a doubly plunging anticline the preservation of a pre laramide partial retention zone in the upper few hundred meters of precambrian basement indicates that in general the temperature at the cambrian unconformity did not exceed the apatite he closure temperature this is difficult to reconcile with evidence from adjacent basins for thick sequences of sedimentary rocks 3 4 km prior to 65 ma and normal modern geothermal gradients either the range was never deeply buried 2 3 km its geothermal gradient has been low 20 c since at least the mesozoic or our apatites have higher u th he closure temperatures 80 90 c than those measured for other apatites

provenance of xigaze fore arc basin clastic rocks cretaceous south tibet the xigaze group of south tibet was deposited in a fore arc basin produced by northward subduction of the neotethys under eurasia during middle cretaceous time clasts of andesite from the gangdese magmatic belt to the north and limestone clasts from a shelf rimming the basin to the north are the near exclusive detrital components in the xigaze group in the eastern part of the studied region and in older albian deposits in the west younger cenomanian coniacian deposits from western and central parts of the xigaze group show increasing amounts of tonalite pebbles in conglomerates and quartz sand these trends reflect differential arc uplift and erosional dissection with higher rates in the west some arenites show evidence for sediment contribution from an additional source probably from an orogen to the north of the lhasa block the presence of this orogen indicates that crustal thickening of tibet is not restricted to cenozoic times the middle cretaceous gangdese belt apparently lacked acid volcanic rocks this suggests a low degree of crustal contamination and or differentiation of primitive magma probably because the crust beneath the arc area was thin a thin crust is in accordance with a low elevation of the arc deduced from continuous carbonate production on and marine excursions beyond the arc magma eruption rates of the arc estimated from the xigaze basin sediment volume were probably low 7 5 km 3 km arc segment per million years but within the range of modern arcs the varying sediment compositions of the xigaze group may be typical of fore arc basin deposits in front of continental magmatic arcs the lack of a correlation of trends in sediment composition with fining upward sedimentary supercycles in the xigaze group is taken as evidence in favor of a eustatic cycle control

importance of clay in iron transport and sediment reddening evidence from reduction features of the abo formation new mexico u s a abstract petrographic and geochemical data from reduction features in the abo formation permian of central new mexico provide insight into the source of iron for hematite precipitation in red beds reduction features in the abo formation include isolated reduction spots as well as zones of reduction associated with fractures and coarser grained beds the reduction features contain less iron and more copper than the host rock mean values in ppm host rock fe 6456 cu 9 reduced fe 3730 cu 113 hematite relics are present in reduced areas associated with fractures and reduced bedding but are absent in reduction spots hematite apparently never formed in the reduction spots because of localized reducing conditions present at the time of pervasive reddening reduced fractures and bedding most likely result from migration of reducing pore fluids flowing through more permeable areas and postdate hematite precipitation iron bearing sand and silt grains which are typically regarded to be the principal source of iron for hematite precipitation in red beds are uncommon average 1 in the abo formation where present these grains are not significantly altered in either red host rock or in the white reduction features therefore detrital iron bearing clay size material is the primary source of iron in the abo formation although compositionally variable the clay rich areas contain a mean of 8 4 feo total in the host rock and a mean of 4 6 feo total in reduction spots chemical and mineralogical data from clay rich areas suggest that ferric oxyhydroxides associated with the clays are a more important iron source than iron from the clay crystal structure low whole rock iron content of the abo formation relative to other red bed units is consistent with clay and associated ferric oxyhydroxides being the principal iron source the abo formation may represent a clay dominated end member in a continuum of red beds in which the iron needed for reddening is derived from clays and coarser iron bearing framework grains

40ar 39ar dating and magnetostratigraphic correlation of the terrestrial cretaceous paleogene boundary and puercan mammal age hell creek tullock formations eastern montana the recovery and subsequent prolific radiation of mammals in the northern western interior of north america following the cretaceous paleogene k p boundary is well documented in rocks attributed to the puercan land mammal age the most complete puercan record is that of the tullock formation which crops out widely in garfield and mccone counties eastern montana the tullock formation overlies the dinosaur bearing hell creek formation and consists of a stratigraphic series of channel and overbank deposits from which well preserved puercan faunas have been collected these channel deposits are typically bracketed by widespread coal beds the irz and z coals mark the base of the puercan at the k p boundary as defined by the highest appearing local occurrences of in situ dinosaur fossils the highest stratigraphic occurrence of cretaceous pollen and an anomalously high concentration of the element iridium ir the irz coal is overlain sequentially by the z hfz y w and u coals fossil vertebrates

sedimentology detrital zircon geochronology and stable isotope geochemistry of the lower eocene strata in the wind river basin central wyoming shallow subduction of the farallon plate beneath the western united states has been commonly accepted as the tectonic cause for the laramide deformation during late cretaceous through eocene time however it remains unclear how shallow subduction would produce the individual laramide structures critical information about the timing of individual laramide uplifts their paleoelevations at the time of uplift and the temporal relationships among laramide uplifts have yet to be documented at regional scale to address the question and evaluate competing tectonic models the wind river basin in central wyoming is fi lled with sedimentary strata that record changes of paleogeog raphy and paleoelevation during laramide deformation we conducted a multidisciplinary study of the sedimentology detrital zircon geochronology and stable isotopic geochemistry of the lower eocene indian meadows and wind river formations in the northwestern corner of the wind river basin in order to improve understanding of the timing and process of basin evolution source terrane unroofi ng and changes in paleoelevation and paleoclimate depositional environments changed from alluvial fans during deposition of the indian meadows formation to low sinuosity braided river systems during deposition of the wind river formation paleocurrent directions changed from southwestward to mainly eastward through time conglomerate and sandstone compositions suggest that the washakie and or western owl creek ranges to the north of the basin experienced rapid unroofi ng ca 55 5 54 5 ma producing a trend of predominantly mesozoic clasts giving way to precambrian basement clasts upsection rapid source terrane unroofi ng is also suggested by the upsection changes in detrital zircon u pb ages detrital zircons in the upper wind river formation show age distributions similar to those of modern sands derived from the wind river range with up to 20 of zircons derived from the archean basement rocks in the wind river range indicating that the range was largely exhumed by ca 53 51 ma the rise of these ranges by 51 ma formed a confi ned valley in the northwestern part of the basin and promoted development of a meandering fl uvial system in the center of the basin the modern paleodrainage confi guration was essentially established by early eocene time carbon isotope data from paleosols and modern soil carbonate show that the soil co 2 respiration rate during the early eocene was higher than at present from which a more humid eocene paleoclimate is inferred atmosphere pco 2 estimated from paleosol carbon isotope values decreased from 2050 450 ppmv to 900 450 ppmv in the early eocene consistent with results from previous studies oxygen isotope data from paleosol and fl uvial cement carbonates show that the paleoelevation of the wind river basin was comparable to that of the modern great plains 500 m above sea level and that local relief between the washakie and wind river ranges and the basin fl oor was 2 3 0 8 km up to 1 km of post laramide regional net uplift is required to form the present landscape in central wyoming

field test of autogenic control on alluvial stratigraphy ferris formation upper cretaceous paleogene wyoming internally generated autogenic sedimentary processes can obscure signals of tectonic movements climate conditions or sea level fl uctuations in alluvial basins the stratigraphic effects of autogenic dynamics have been considered negligible on time scales of 10 4 10 6 yr however recent physical and numerical models have shown that over basin fi lling time scales self organization in sedimentary systems can produce stratigraphic patterns similar to those resulting from changing basin boundary conditions here we present the fi rst fi eld based test for autogenic control on stratigraphy in an ancient alluvial basin the ferris formation upper cretaceous paleogene hanna basin wyoming is composed of clusters of closely spaced channel deposits separated by intervals dominated by overbank material a pattern that has been proposed as an example of a potentially autogenic stratigraphy in order to evaluate controls on ferris formation stratigraphy spatial patterns of key channel properties were analyzed these variables including sand body dimensions paleofl ow depth maximum clast size paleocurrent direction and sediment provenance were chosen because they can change in response to climate tectonic or sea level forcing and are commonly measurable in ancient alluvial successions no clear statistical trends were detected in the measured variables which suggests that external forces did not likely control stratigraphic organization in the unit this study demonstrates that autogenic dynamics in natural fi eld scale systems can produce organized stratigraphic patterns over much larger spatial and temporal scales than is typically presumed this fi nding emphasizes the need for further understanding of autogenic stratigraphy and greater care interpreting climate tectonic and eustatic changes from alluvial basins

refinement of eocene lapse rates fossil leaf altimetry and north american cordilleran surface elevation estimates abstract estimates of continental paleoelevation using proxy methods are essential for understanding the geodynamic climatic and geomorphoric evolution of ancient orogens fossil leaf paleoaltimetry one of the few quantitative proxy approaches uses fossil leaf traits to quantify differences in temperature or moist enthalpy between coeval coastal and inland sites along latitudes these environmental differences are converted to elevation differences using their rates of change with elevation lapse rate here we evaluate the uncertainty associated with this method using the eocene north american cordillera as a case study to do so we develop a series of paleoclimate simulations for the early 55 49 ma and middle eocene 49 40 ma period using a range of elevation scenarios for the western north american cordillera simulated eocene lapse rates over western north america are 5 c km and 9 8 kj km close to moist adiabatic rates but significantly different from modern rates further using linear lapse rates underestimates high altitude 3 km temperature variability and loss of moist enthalpy induced by non linear circulation changes in response to increasing surface elevation ignoring these changes leads to kilometer scale biases in elevation estimates in addition to these biases we demonstrate that previous elevation estimates of the western cordillera are affected by local climate variability at coastal fossil leaf sites of up to 8 c in temperature and 20 kj in moist enthalpy a factor which further contributes to elevation overestimates of 1 km for early eocene floras located in the laramide foreland basins and underestimates of 1 km for late middle eocene floras in the southern cordillera we suggest a new approach for estimating past elevations by comparing proxy reconstructions directly with simulated distributions of temperature and moist enthalpy under a range of elevation scenarios using this method we estimate mean elevations for the north american cordillera of 2 km using proxy temperatures and 3 km using proxy moist enthalpy this discrepancy is likely related to an inconsistency between the proxy data for temperature and moist enthalpy the combination of temperature and moist enthalpy estimates implies a warm and dry environment which is inconsistent with geological evidence for humid conditions our study emphasizes that lapse rates in warm climate were likely much different than today s and highlights the advantages of using climate models to understand past climate states and regional circulation patterns to derive more accurate paleoelevation estimates using proxy temperature and moist enthalpy

tectonic significance of paleobotanically estimated climate and altitude of the late eocene erosion surface colorado erosion beveled the laramide front range uplift in colorado to a surface of low relief by the end of the eocene this study uses j a wolfe s new multivariate climate analysis techniques to determine the paleoelevation of this regional surface by examining the overlying 35 ma florissant flora a multiple regression model explaining 93 3 of the variance in mean annual temperature was developed using wolfe s dataset of 31 leaf physiognomic character states for 86 modern vegetation sites these character states were scored on 29 species collected from one facies of the florissant lake beds the paleotemperature estimate of mean annual temperature 10 7 plus minus 1 5c derived from these data when combined with sea level temperature and terrestrial lapse rate implies a late eocene paleoelevation of 2 4 2 7 km pliocene uplift is thus not required to explain the present elevation of 2 5 km it is unclear when and why the southern rocky mountains achieved this elevation magmatic crustal thickening can explain the late eocene high elevation of the southern rockies but neither this mechanism nor compressive thickening explains why the great plains which are tied to the florissant elevation by the wall mountain tuff were also high this paleoelevation estimate indicates that regional surfacesmore of planation could be formed at high elevation in the eocene probably because of peculiarities of the eocene climate less

eocene oligocene proto cascades topography revealed by clumped δ47 and oxygen isotope δ18o geochemistry chumstick basin wa usa the topography of the present day washington cascades impacts atmospheric circulation and precipitation patterns in the pacific northwest introducing a pronounced orographic rain shadow in the lee of the mountain range the temporal development of cascade topography however remains largely unconstrained for the early cenozoic based on coupled carbonate clumped isotope 47 and oxygen isotope 18o measurements we reconstruct 18o values of eocene groundwater 18owater in the chumstick basin central washington today located in the cascade rain shadow 47 paleo thermometry indicates a systematic change in basin burial temperatures from 110 c to 70 c depending on burial depth in the basin these data are in good agreement with low t thermochronological and vitrinite reflectance data and further constrain the basin burial and exhumation history in concert with field observations microstructural analysis and 18o values of the analyzed carbonates we suggest that the 47 temperatures and 18o values reflect open system carbonate cement recrystallization in meteoric derived groundwaters during early burial diagenesis assuming open system behavior reconstructed mean 18owater values of 7 middle eocene to 9 late eocene early oligocene are consistent with a low elevation origin of the corresponding meteoric waters that permeated the sandstone conglomerate members of the eocene sedimentary units in light of the paleogeographic setting of the chumstick basin the reconstructed 18owater values agree well with pacific derived moisture that did not experience strong rainout the absence of a rain shadow effect therefore permits only moderate eocene oligocene elevations at least for the southern part of the washington proto cascades

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

empirical relationships between elevation and the stable isotope composition of precipitation and surface waters considerations for studies of paleoelevation change a compilation of 68 studies from throughout many of the world s mountain belts reveals an empirically consistent and linear relationship between change in elevation and change in the isotopic composition of precipitation along altitudinal transects the isotopic composition of precipitation decreases linearly with increasing elevation in most regions of the world except in the himalayas and at elevations 5000 m there are no significant differences in isotopic lapse rates from most regions of the world 0 28 permil 100 m except at the extreme latitudes where isotopic lapse rates are higher given information on past changes in the isotopic composition of precipitation preserved in pedogenic or authigenic minerals this global isotopic lapse rate can be used to place numerical constraints on the topo graphic development of some ancient mountain belts or plateaus there are many complicating factors that can confound interpretation of paleoel evation change based on stable isotopes and many of these are unique to specific mountain belts or time periods relevant to all stable isotope based paleoelevation change studies is the temperature dependent isotope fractionation between a pedo genic or authigenic mineral and the water from which it forms in cases where isotopic proxy minerals are sampled from localities where temperature will change simulta neously with elevation change the apparent change in the isotopic composition of precipitation may be dampened by several permil this suggests that samples taken from the rainshadow side of an emerging orographic barrier may be more likely to preserve isotopic changes resulting from mountain uplift than samples taken from atop a rising mountain range or plateau

water utilization of the cretaceous mussentuchit member local vertebrate fauna cedar mountain formation utah usa using oxygen isotopic composition of phosphate abstract while the oxygen isotopic composition of pedogenic carbonate has successfully been used to address the effects of global climate change on the hydrologic cycle detailed regional paleohydrologic studies are lacking since the hydrologic cycle can vary extensively on local or regional scales due to events such as such as mountain building and since pedogenic carbonates calcite form in a narrow moisture regime other proxies such as vertebrate remains must be used to decipher local versus regional variations in paleohydrology in this study the oxygen isotopic composition 18op of phosphatic remains from a diverse set of vertebrate fossils fish turtles crocodiles dinosaurs and micro mammals from the mussentuchit member mm of the cedar mountain formation utah usa aptian to cenomanian are analyzed in order to determine differences among the available water reservoirs and water utilization of each taxon calculated changes in water reservoir 18ow over time are then used to determine the effects of the incursion of the western interior seaway wis and the sevier mountains on paleohydrology during the mm time calculation of 18ow from the results of isotopic analysis of phosphate oxygen suggests that turtles and crocodiles serve as another proxy for meteoric water 18o that can be used as a measure of average local precipitation 18ow similar to pedogenic calcite pedogenic calcites can be slightly biased toward higher values however due to their formation during evaporative conditions turtles and crocodiles can be used in place of pedogenic calcite in environments that are not conducive to pedogenic carbonate formation remains of fish with rounded tooth morphology have 18op values that predict temperatures consistent with other estimates of mean annual temperature for this latitude and time the 18op of ganoid scales and teeth with pointed morphology however indicates that these skeletal materials were precipitated from water that is 18o enriched due to migration to either evaporatively enriched water or 18o enriched estuarine waters of the western interior seaway wis another possibility that cannot be discounted and assuming all morphological remains are from the same taxon is that the pointed teeth and ganoid scales precipitated at different temperatures than rounded teeth mammal and herbivorous dinosaur 18op suggests they primarily drank isotopically depleted river water co existence of crocodiles turtles and mammals allows for calculation of relative humidity from site to site and these calculations suggest humidity averaged 58 and ranged between 42 and 76 the 18ow values estimated from semi aquatic taxa and pedogenic calcite suggest dominance of wis derived moisture during their growth herbivorous dinosaurs particularly indicate that altitude and catchment effects from the sevier mountains are seemingly important for river water 18ow in the fall through early spring these data suggest that temporal changes in the isotopic composition of the mm fauna are produced by the small scale regressive transgressive cycles of the wis

emergence of the canadian rockies and adjacent plains a comparison of physiography between end of laramide time and the present day abstract the landscape of the canadian rockies in southern alberta is not a direct result of constructional processes that is the ridges and peaks have not been pushed into the positions in which we see them today tectonic activity provided original elevation but not mountains at the end of laramide time what are now the front ranges and foothills of the rockies comprised a high elevation upland of relatively low relief the present mountain physiography is the result of 55 60 million years of post orogenic differential erosion in which more resistant rocks have been left at higher elevations than less resistant rocks the canadian rockies and the foothills are developed in a thin skinned thrust and fold belt created during the laramide orogeny the adjacent interior plains cut across foreland basin sediments derived from the mountains the mountains currently consist of large parts of ridges of well indurated paleozoic and locally proterozoic rock alternating with valleys developed in soft mesozoic clastic rock in the foothills where the soft mesozoic rock is at the surface relief is subdued but ridges of more resistant sandstone rise above shaley lowlands the plains are relatively flat but also contain erosional outliers of higher paleo plains surfaces numerous lines of evidence suggest that the mountains and foothills have lost several kilometers of overburden since the end of the laramide orogeny while the western plains have lost at least 2 km requiring that the local relief of the mountains and foothills that we see is erosional in origin local physiography is adjusted to lithology the mountains have high relief because the exposed sub mesozoic rocks can hold up high steep slopes whereas the foothills have low relief because the underlying cretaceous rocks cannot hold up high steep slopes the east facing escarpment at the mountain front is a fault line scarp along a low angle thrust mesozoic rocks involved in the deformation originally extended all the way across the thrust and fold belt and physiography of the belt at the end of laramide time 60 55 ma depended mainly on whether mesozoic or paleozoic proterozoic rocks were exposed at the surface at that time a reconstruction using critical taper theory generally agrees with reconstructions from earlier stratigraphic and paleothermometry studies what are now the front ranges at the eastern edge of the rocky mountains were mostly or perhaps entirely covered with mesozoic rocks and despite that high elevation had a hilly not mountainous character the main ranges in the central rocky mountains were in part stripped of mesozoic cover by then and more mountainous treeline was higher then and the thrust belt may have been largely or entirely vegetated generation of modern relief in the front ranges including the escarpment at the mountain front had to await stripping of mesozoic rocks and incision of rivers into harder substrates in post laramide time the interior plains are an erosional surface that was cut 1 to 3 km below the aggradational top of the foreland basin sediments although some of the present low local relief of the plains results from weakness of underlying cretaceous tertiary rocks the low relief is probably largely related to the process of denudation

structural and sedimentological development of footwall growth synclines along an intraforeland uplift east central bighorn mountains wyoming structural sedimentological and provenance data from paleogene synorogenic deposits of the east central flank of the bighorn mountains provide new information about the development of footwall growth synclines the evolution of fault related folds and the erosional unroofing history of intraforeland uplifts three conglomerate units the upper conglomerate member of the fort union formation and the kingsbury and moncrief members of the wasatch formation are incorporated within an asymmetric east verging growth syncline in the footwall of the main range bounding thrust system three stages of footwall deformation are recorded within these conglomerates analysis of mapped progressive unconformities retrodeformed balanced cross sections and conglomerate clast composition data define these stages as part of a continuum of deformation associated with the development of footwall growth synclines development of an anticline syncline pair marked the earliest stage of growth syncline formation stage i rotation of the shared fold limb resulted in amplification of the growth syncline fine grained synorogenic sediment derived from easily eroded mesozoic mudstone bypassed the growth syncline during this stage by the end of lebo shale deposition an average of 12 1 of shortening and 6 46 km of uplift had occurred along the range margin continued growth syncline development was marked by the deposition of the kingsbury conglomerate the kingsbury conglomerate was derived from resistant middle and lower paleozoic carbonate strata in the uplifted source terrane intraformational unconformities recording as much as 55 of bed rotation were developed within the kingsbury conglomerate as fold limb rotation occurred coeval with deposition cross sections indicate that during this early stage of fault related folding an average of 16 9 shortening and 8 12 km of uplift occurred along the eastern flank of the bighorn mountains end of stage i the intermediate stage stage ii of footwall growth syncline development involved partial truncation of the growth syncline by the advancing thrust faults and deposition of the moncrief conglomerate the lower portion of the moncrief conglomerate was rotated basinward in the developing growth syncline the final stage of deformation stage iii was dominated by the thrust faulting of middle and lower paleozoic strata eastward over steeply dipping mesozoic strata and rotated eocene synorogenic conglomerate during this stage of deformation the moncrief conglomerate was deformed as the initially blind thrusts propagated into the near surface conglomerate deposits truncated the entire footwall syncline and overrode the synorogenic conglomerate package cross sections in areas where this final stage of deformation is well developed indicate that an average of 24 1 shortening and 9 7 km of uplift had occurred along the eastern margin of the bighorn mountains the caliber of synorogenic deposition in the powder river basin was linked directly to the lithologic composition of the bighorn mountains approximately half of the 3 6 km thick source stratigraphic section of the eastern bighorn mountains was eroded prior to accumulation of conglomerate the majority of this eroded material was derived from mesozoic mudstone and poorly indurated sandstone that were incapable of generating coarse detritus the first paleogene conglomerates deposited along the east central bighorn mountains therefore do not represent the initiation of laramide uplift but instead represent the exposure of coarse clast forming rock types from the lower half of the hanging wall stratigraphic section i e the mississippian madison limestone and ordovician bighorn dolomite

widespread basement erosion during the late paleocene early eocene in the laramide rocky mountains inferred from 87sr 86sr ratios of freshwater bivalve fossils reconstruction of the timing and pattern of exhumation of basement involved uplifts during the laramide is essential to the understanding of the mechanisms of crustal shortening and thickening in the region we use reconstructions of the 87 sr 86 sr ratios of late cretaceous early cenozoic river water from fossil shells in six basins of the rocky mountains contextualized by modern river water sr chemistry studies to trace the erosion of precambrian basement cores in the laramide ranges the 87 sr 86 sr ratios and sr concentration of modern river water in the rocky mountains are controlled by river bedrock lithology weathering of precambrian silicate rocks in the cores of laramide ranges produces high 87 sr 86 sr ratios in highland rivers weathering of paleozoic and mesozoic carbonates along the basin margins reduces the 87 sr 86 sr ratios and increases sr concentration of rivers as they flow basinward lowland rivers that have headwaters in precambrian basement mostly have 87 sr 86 sr ratios 0 711 whereas rivers confined to or with very long reaches in basins have 87 sr 86 sr ratios between 0 709 and 0 711 river water 18 o values do not change in response to changes in catchment elevation our results from fossil shells show that carbonate weathering dominated the river water sr chemistry in the alberta foreland williston and crazy mountains basins and that proterozoic low grade metamorphic carbonates in the belt purcell supergroup were not exposed in the canadian rocky mountains during late cretaceous early paleocene time silicate weathering influenced the sr chemistry of surface waters in the powder river basin during the late paleocene and the washakie basin during the early eocene suggesting precambrian silicate basement rock was extensively exhumed and eroded the observation that precambrian basement was eroded earlier around the powder river basin than around the washakie basin is consistent with a previous study that suggests laramide ranges in northeast wyoming reached high elevation earlier than the ranges adjacent to the sevier thrust belt widespread basement erosion in late paleocene early eocene time was controlled by tectonic exhumation and may have been intensified by the wet and warm global climate

u pb and re os geochronological evidence for the jurassic porphyry metallogenic event of the xiongcun district in the gangdese porphyry copper belt southern tibet prc abstract two large copper gold porphyry deposits no i and no ii were discovered in the last decade within the xiongcun district which is located along the south margin of the gangdese porphyry copper belt these two deposits are hosted by two porphyries that were emplaced into the early jurassic volcano sedimentary rock sequences of the xiongcun formation the porphyries are both quartz diorite bodies with one of these distinguished by its large quartz eyes our new molybdenite re os and zircon u pb dates for no i and no ii deposits and host intrusions reveal the presence of two middle jurassic ore forming episodes in the xiongcun district the no ii deposit is related to the 181 175 ma quartz diorite porphyry and formed ca 172 6 2 1 ma the no i deposit is related to the 167 161 ma quartz diorite porphyry with large quartz eyes and formed ca 161 5 2 7 ma geochronological and geochemical features of the jurassic ore bearing porphyries in the xiongcun district indicate that the porphyry copper gold mineralization formed in an island arc setting which is related to the northward subduction of the neo tethys oceanic plate these observations in combination with geochronological data reported at gangdese porphyry copper belt indicated four major metallogenic events existed in the gangdese porphyry copper belt at 172 161 ma 65 48 ma 40 23 ma and 20 12 ma we have identified two mineralization episodes comprise middle jurassic metallogenic event 172 161 ma in the xiongcun district which provides new constrains for the middle jurassic metallogenic event in the gangdese porphyry copper belt

the cenozoic climatic and topographic evolution of the western north american cordillera herein we present oxygen isotope records from cretaceous to recent terrestrial sediments in the western north american cordillera the purpose of this analysis is to use oxygen isotope records to understand the coupled surface elevation and climate histories of this region through the cenozoic to do this we constructed 18o maps of surface waters for time intervals that trace the development of topography of western north america these maps are based on 4861 oxygen isotope analyses from both published 4478 and new 383 data we determined the 18o values of surface waters using paleotemperatures derived previously from floral assemblages and the appropriate isotope fractionation factors these data suggest that in the late cretaceous to early eocene the sevier hinterland formed a plateau of unknown height around 50 ma a topographic wave developed in british columbia and eastern washington that swept southward reaching northeastern nevada at 40 to 38 ma and southern nevada 23 ma this southward encroachment of an eocene plateau sweep caused reorganization of drainage patterns such that the intraforeland basins of wyoming and utah drainages extended deep within the sevier hinterland as the wave swept southward the landscape within the sevier hinterland developed into a rugged and high mountain range with the hypsometric mean elevation of 4 km and relief of 1 5 km this eocene highland was bordered on the west by a high sierra nevada ramp and on the east by the intraforeland basins that captured water draining these growing highlands the spatial and temporal evolution of this highland correlates with the timing of volcanism and extension these observations support tectonic models that call for north to south removal of the farallon slab or piecemeal removal of mantle lithosphere the isotopic data show that prior to growth of this highland the north american monsoon nam penetrated much farther north in the paleocene eocene than today the combined effects of global cooling increasing latitudinal temperature gradients and the generation of the orographic barrier created by the growing north to south highland produced a southward migration of the nam front by the oligocene the hydrologic regime that we observe today was in place it has been modified since then as a result of basin and range extension and collapse of the highlands in the mid miocene this collapse allowed the nam to penetrate farther north into the great basin of nevada and utah

elevation or alteration evaluation of isotopic constraints on paleoaltitudes surrounding the eocene green river basin low oxygen isotope values 16 relative to peedee belemnite standard obtained from microbial carbonates of the green river formation have been interpreted as evidence for snowmelt and high elevations surrounding this early eocene lake basin however low values from microbial precipitates could also represent a diagenetic overprint we investigate these alternate hypotheses by measuring the oxygen isotope composition of altered and unaltered freshwater bivalves from the basin we analyzed subannual samples from 3 individuals and bulk samples from 54 individuals subannual samples exhibit clear seasonal fluctuations in oxygen isotope values these fluctuations are large enough to require seasonal shifts in temperature and or the oxygen isotope value of the lake water however the lowest value of any unaltered sample either bulk or high resolution corresponds to lake water 18 o of 12 relative to standard mean ocean water and is not low enough to require the addition of snowmelt to the lake a few bulk samples exhibit very low oxygen isotope values which would seem to suggest snowmelt however these samples also show clear evidence of calcite in x ray diffraction patterns demonstrating the presence of diagenetic precipitates given that 1 diagenetic alteration is a plausible explanation of very low 18 o values 2 alteration has not been examined in microbial carbonates and 3 unaltered aragonitic bivalves provide no independent evidence for low 18 o values for lake waters we conclude that robust isotopic evidence for high elevations surrounding the green river basin during the eocene is currently lacking

stable isotope based paleoaltimetry theory and validation paleoaltimetry is the quantitative estimate of the surface height above mean sea level of ancient landforms atmospheric thermodynamic modeling of the behavior of 18o relative to 16o during condensation from water vapor establish the systematic relationship that exists between 18o p and elevation where 18o p is the difference between a low altitude preferably sea level 18o p and a potentially high elevation sample comparison of model predictions with observations suggests that the model captures the first order behavior of 18o p during condensation and precipitation in orographic settings the actual relationship between 18o p and elevation depends sensitively on climate and specifically starting temperature and to a lesser degree relative humidity thermodynamic modeling allows the 18o p and elevation relationship to be explicitly calculated for any given starting climate state this makes the theoretical approach significantly more appropriate than empirically calibrated approaches based typically on quite limited samples presently available in most orographic settings today paleoaltimetry archives derive their isotopic compositions from surface and or ground water and hence it is important to understand the systematic differences between these reservoirs and precipitation surface waters and ground waters integrate not just the change in isotopic composition with altitude but also variations in hypsometry within the drainage basin and precipitation amount as functions of elevation thus these archives should reflect the precipitation amount weighted hypsometric mean elevation of the paleo drainage basin from which they derive their waters analysis of modern data from the himalayan region supports this expectation foreland basin rivers are such integrators and it is shown using an example from the siwaliks that the rivers draining the front of the himalayas in the past had isotopic compositions comparable with modern rivers draining the himalayan front suggesting little net change in himalayan hypsometry over the past 11 million years

raising the colorado plateau shallow marine rocks exposed on the 2 km high 45 km thick colorado plateau in the western united states indicate that it was near sea level during much of the phanerozoic isostatic calculations however illuminate the difficulty in maintaining a 45 km thick crust at or near sea level we propose that an isostatically balanced 30 km thick proto colorado plateau crust was thickened during the late cretaceous to early tertiary by intracrustal flow out of an overthickened sevier orogenic hinterland this plateau would have been supported by a thick 70 km crustal root which is proposed to have been the source region for hot and weak mid crustal material that flowed eastward from the plateau toward the low elevation proto colorado plateau

come a little bit closer a high resolution climate study of the early paleogene laramide foreland the early paleogene greenhouse climate has long captured the attention of researchers however due to limited data and the coarse spatial resolution of general circulation model gcm results it has proven difficult to investigate early paleogene climate at the level of detail characteristic of the actual climate system we present a high resolution regional climate modeling study of the north american laramide foreland our simulation depicts early paleogene climate dynamics in western north america with unprecedented detail increased horizontal resolution improves matches between modeled temperatures and data in some basins however large scale cold biases found in prior gcm simulations persist our study provides insight into the existence of snow and perennial ice in the laramide highlands and northwestern cordillera and describes the summer monsoon along the rocky mountain front in great detail monsoonal precipitation initiates in the southeastern rockies in may penetrates north and west as temperatures warm and peaks at 1 cm day along the southern and central mountain front basins farther to the north and west are drier

environmental change across a terrestrial cretaceous paleogene boundary section in eastern montana usa constrained by carbonate clumped isotope paleothermometry the cretaceous paleogene k pg mass extinction has been attributed to the impact of a large bolide at the end of the cretaceous period although other potential causes have also been proposed most notably climate change caused by deccan traps india flood volcanism reconstructing paleoclimate particularly in terrestrial settings has been hindered by a lack of reliable proxies the recent development of carbonate clumped isotope paleothermometry has contributed to temperature reconstructions using geochemical proxies in terrestrial settings we employ this method along with new stratigraphic constraints in the hell creek cretaceous and overlying fort union paleogene formations montana usa to examine changes in temperature leading to and across the k pg boundary we demonstrate that well preserved ca 66 ma aragonitic bivalves serve as suitable paleoclimate archives although there are limitations in the stratigraphic availability of fossil bivalves for clumped isotope analysis we record an apparent 8 c decrease in summer temperatures over the last 300 k y of the cretaceous that corresponds broadly with patterns observed in other paleotemperature proxies this observed decrease plausibly could be explained by an absolute temperature decrease or by other environmental stresses on the organisms but in either case suggests changing living conditions over the interval previously documented declines in vertebrate and invertebrate biodiversity occur over the same stratigraphic interval at this study location these results are consistent with published models of the end cretaceous mass extinction in which destabilized ecosystems become more susceptible to an abrupt event like a bolide impact

stable isotope stratigraphy and paleoclimatology of the paleogene bighorn basin wyoming usa abstract climatic warming from the late paleocene into the early eocene had profound effects on atmospheric and marine circulation marine thermal gradients and benthic biota in addition marine carbon isotope values decreased substantially in both surface and deep waters because carbon is rapidly exchanged between reservoirs at the earth s surface such as marine surface water the atmosphere land plants and materials that obtain carbon from plants e g soil minerals and herbivores carbon isotope fluctuations provide time lines linking the marine and continental records we analyzed the carbon isotope composition of paleosol carbonates and mammalian tooth enamel from stratigraphic sections in northwestern wyoming carbon isotope correlation demonstrates that a short interval of extreme high latitude warming coincided precisely with the first appearance of several important modern mammalian orders in addition oxygen isotope analyses of fossils and paleosol carbonates provide information about climatic conditions on land in the paleogene we reconstructed the oxygen isotope composition of local meteoric water using biogenic minerals paleocene eocene meteoric water was significantly 18 o depleted indicating substantial loss of water vapor during its transport to the region soil temperature was calculated as a proxy for mean annual temperature assuming oxygen isotope equilibrium between soil carbonate and meteoric water calculated temperatures were plausible 10 25 c but highly variable prohibiting high resolution analysis of local temperature variations in response to global climatic warming

spatiotemporal distribution of river water stable isotope compositions and variability of lapse rate in the central rocky mountains controlling factors and implications for paleoelevation reconstruction abstract stable isotope based paleoaltimetry is the most widely used approach for paleoelevation reconstruction interpretations of stable isotope data in continental interiors however are undermined by surface water isotope compositions that are influenced by multiple factors here we present a stable isotope dataset of modern river water samples collected over two summers and one spring from the central rocky mountains rockies and the adjacent great plains by examining the spatial and temporal variations of river water 18 o d and d excess values and their relationships with climatic and geographic parameters as well as through back trajectory analysis of moisture sources we elucidate the influences of elevation and climatic parameters on the spatiotemporal variation of river water isotopic values in the bighorn river drainage a typical intermontane drainage in the central rockies the isotopic difference between highland and lowland rivers is small which we attribute to highland precipitation that dominates lowland river discharge in the north platte river drainage across the central rockies and great plains the river water 18 o values show poor correlation with elevation west of 105 w central rockies but increase as elevation decrease east of 105 w in the western great plains this eastward increase across the western great plains leads to an average oxygen isotope lapse rate of 2 3 km which we interpret as being caused primarily by condensation temperature controlled isotopic fractionation at various elevations and secondarily by evaporation in the upper reaches of streams that contribute to the north platte river plus direct contribution of moisture from the gulf of mexico in the great plains in this continental interior setting multiple moisture sources including recycled continental moisture contribute to surface water and evaporation influences river water isotope values to various degrees depending on the relative humidity within an individual river catchment these results suggest that paleoclimate and atmospheric circulation pattern must be carefully evaluated when applying stable isotope based paleoaltimetry in continental interiors our findings have implications for paleoelevation reconstruction in the study area including that 1 within the central rockies the isotopic difference of river water and unevaporated basinal precipitation can be used to infer paleorelief of the laramide ranges with respect to the basin floors 2 along a regional transect crossing the central rockies and great plains the modern isotope lapse rate of the north platte river drainage can be used to constrain the paleorelief between the two regions in semi arid climate

middle cenozoic uplift and concomitant drying in the central rocky mountains and adjacent great plains when and how the central rocky mountains rockies of western north america gained modern topography remain controversial questions we reconstruct the middle and late cenozoic topography along a transect that extends from the great plains of western nebraska across the central rockies in wyoming based on reconstructed surface water d values from volcanic glass d values our data show gradual increases of surface water d values in both the central rockies and adjacent great plains during middle and late cenozoic time and the establishment of a similar to present surface water d gradient between the central rockies and western great plains before earliest oligocene time these observations suggest that the region underwent differential uplift to form relief similar to that of today before earliest oligocene time this uplift has caused regional drying and the gradual increase of surface water d values over the past 35 m y when placed in the context of other paleoaltimetry studies our work suggests that along our transect the central rockies and adjacent great plains underwent uplift during the late eocene and have not undergone any large magnitude 500 m uplift since that time

paleobotanical evidence of eocene and oligocene paleoaltitudes in midlatitude western north america comparisons of the physiognomy of leaves from modern vegetation of known climates with that of eocene and oligocene fossil leaf assemblages from middle latitudes of western north america indicate paleoaltitudes comparable or higher than those at present using canonical correspondence analysis a multivariate statistical approach that includes nonlinear relationships between characters and environmental parameters we relate leaf physiognomy of fossil samples to modern vegetation of known environmental parameters including moist enthalpy of the atmosphere a thermodynamically conserved quantity of the atmosphere that varies with altitude by estimating enthalpy values for both lowland leaf assemblages and coeval interior leaf assemblages altitudes for the interior assemblages can be estimated to 890 m in northeastern washington and southern british columbia the high eocene altitudes indicate subsidence or collapse of an area undergoing crustal extension and may reflect an immediately preceding period of uplift that triggered the extension similarly other areas that have undergone cenozoic crustal extension appear to have been at least as high as they are at present three sites from the rocky mountains also indicate elevations at least as high as at present and therefore suggest subsidence either resulting from cooling of a hot upper mantle or erosion and isostatic compensation of surrounding terrain high altitudes during eocene and oligocene time in western north america appear to have been normal even in areas such as the green river basin and therefore cast doubt on the commonly inferred late cenozoic uplift of that region

molybdenite re os and albite 40ar 39ar dating of cu au mo and magnetite porphyry systems in the yangtze river valley and metallogenic implications abstract the area of the middle lower yangtze river valley eastern china extending from wuhan hubei province to western zhenjiang jiangsu province hosts an important belt of cu au mo and fe deposits there are two styles of mineralization i e skarn porphyry stratabound cu au mo fe deposits and magnetite porphyry deposits in several nne trending cretaceous fault bound volcanic basins the origin of both deposit systems is much debated we dated 11 molybdenite samples from five skarn porphyry cu au mo deposits and 5 molybdenite samples from the datuanshan stratabound cu au mo deposit by icp ms re os isotope analysis nine samples from the same set were additionally analyzed by ntims on re os results from the two methods are almost identical the re os model ages of 16 molybdenite samples range from 134 7 2 3 to 143 7 1 6 ma 2 the model ages of the five samples from the datuanshan stratabound deposit vary from 138 0 3 2 to 140 8 2 0 ma with a mean of 139 3 2 6 ma their isochron age is 139 1 2 7 ma with an initial os ratio of 0 7 8 1 mswd 0 29 these data indicate that the porphyry skarn systems and the stratabound deposits have the same age and suggest an origin within the same metallogenic system albite 40 ar 39 ar dating of the magnetite porphyry deposits indicates that they formed at 123 to 125 ma i e 10 20 ma later both mineralization styles characterize transitional geodynamic regimes i e the period around 140 ma when the main ns trending compressional regime changed to an ew trending lithospheric extensional regime and the period of 125 115 ma of dramatic ew trending lithospheric extension

a new approach to stable isotope based paleoaltimetry implications for paleoaltimetry and paleohypsometry of the high himalaya since the late miocene the change in oxygen isotopic composition of precipitation is modeled using equilibrium fractionation during rayleigh distillation linked to the thermodynamics of atmospheric ascent and water vapor condensation the primary controllers of the vertical variation in isotopic composition with elevation are the low elevation temperature and relative humidity as these control the vertical distribution of condensation an empirical fit of precipitation versus model condensation based on alpine stations is derived this fit is represented in the model as the weighted mean composition of condensation within a 1000 m thick air parcel 1500 500 m above the ground surface and is used for all other regions comparison of model versus observed modern precipitation reveals a close fit particularly of more highly elevated sites comparison of modern waters in the himalayas and southern tibet with model predictions particularly as revealed by comparison of observed and predicted hypsometry provides additional support to the validity of the model finally application of this model to estimates of paleo waters in the himalayas and southern tibet reveals that this region had already achieved its present hypsometry by the late miocene about 10 ma ago fl 2001 published by elsevier science b v

magnetic polarity stratigraphy and biostratigraphy of middle late paleocene continental deposits of south central montana exposures of the fort union formation on the divide between hunt creek and cub creek in the northern clark s fork basin carbon county montana were selected for magnetostratigraphic study of the transition between the torrejonian and tiffanian land mammal ages paleomagnetic samples were collected from 25 sites within a 160 m thick section of the fort union formation at that location rock magnetic analyses indicate that alternating field demagnetization to peak fields in the 10 to 40 mt interval successfully removed secondary components of natural remanent magnetism nrm characteristic nrm directions define three polarity zones a 50 m thick normal polarity zone bracketed by two reversed polarity zones the cub creek local faunule cc 2 to 3 or ti 1 occurs within the upper portion of the normal polarity zone cub creek local faunules cc 1 cc 3 and eagle quarry all ti 1 occur in the upper reversed polarity zone these data along with faunal and magnetostratigraphic data from the san

paleoceanography of the late cretaceous maastrichtian western interior seaway of north america evidence from sr and o isotopes abstract well preserved fossils of the late cretaceous western interior seaway wis of north america have been analyzed for sr concentration and sr and o isotopes in order to decipher paleosalinities and paleotemperatures the samples are from four biofacies within the seaway late maastrichtian offshore interior pierre shale nearshore interior fox hills formation brackish reduced salinity fox hills formation and freshwater hell creek formation samples were also obtained from the severn formation of maryland considered to be representative of the open ocean all biofacies except the freshwater are demonstrably within the jeletzkytes nebrascensis ammonite zone 87 sr 86 sr ratios show significant and systematic decreases from marine mean 1 s d 0 707839 0 000024 to brackish facies mean 1 s d 0 707677 0 000036 consistent with dilution by freshwater with a lower 87 sr 86 sr ratio than seawater such variation disallows using the 87 sr 86 sr ratios of fossil shell material to assign ages to fossils from the late cretaceous wis without knowledge of the salinity in which the organism grew the sr isotope ratios for scaphitid ammonites within a single biofacies are similar to each other and different from those for scaphites in other biofacies implying that these organisms are restricted in their distribution during life the 87 sr 86 sr values of freshwater unionid mussels range widely and are not compatible with the freshwater endmember 87 sr 86 sr ratio required by the trend in 87 sr 86 sr vs biofacies established from the other samples paleosalinities for the biofacies are estimated to range from 35 in the open marine to a minimum of 20 in the brackish based on the presence of cephalopods in all four facies and the known salinity tolerance of modern cephalopods producing reasonable 87 sr 86 sr values for the freshwater endmember of a 87 sr 86 sr vs 1 sr plot requires a sr concentration 0 2 0 5 that of seawater for the dominant freshwater input to the wis such high sr concentrations relative to seawater are not observed in modern rivers and we suggest that the brackish environment in the wis arose through the mixing of freshwater and seawater in a nearshore aquifer system reactions of the solution with aquifer solids in this subterranean estuary moore mar chem 65 1999 111 125 produced brackish water with the sr concentration and isotopic composition recorded in the brackish biofacies 18 o values of the fossils show decreases from the marine to brackish biofacies consistent with increasing temperatures from 13 to 23 c or if temperatures were relatively constant to a decrease in the 18 o of the water in which the shell formed the latter interpretation is consistent with less than fully marine salinities in the nearshore biofacies but both changes in temperature and the isotopic composition of the water may have occurred in this environment

paleogeographic and paleotectonic setting of laramide sedimentary basins in the central rocky mountain region in the rocky mountain region between central montana and central new mexico sedimentologically isolated nonmarine basins were produced by basement deformation during the laramide orogeny within the area formerly occupied by a broad late cretaceous foreland basin in which laterally continuous marine facies had accumulated previously laramide structures of varying trend and scale reflect heterogeneity of crustal strain caused by shear between the continental lithosphere and an underlying subhorizontal slab of subducted oceanic lithosphere laramide basins include perimeter basins along the cratonic periphery of the arcuate laramide province axial basins along a north south intramontane trend and ponded basins located farther west closer to the overthrust belt twelve specific stratigraphic and sedimentologic criteria for the onset duration and termination of laramide deformation allow the chronology of basin development to be inferred independently for each basin maastrichtian initiation of laramide deformation was approximately synchronous throughout the laramide province but termination of laramide deformation was systematically diachronous from north to south between early and late eocene time widespread eocene erosion surfaces truncate syntectonic laramide sequences and are overlain by largely volcanic and volcaniclastic post laramide strata of eocene age in the north and oligocene age in the south fluvial depositional systems draining toward the great plains were dominant in perimeter and axial basins but ponded basins were occupied at times by large lakes that served as regional sediment traps paleocene drainages from ponded basins also led eastward toward the continental interior but partly interconnected lakes that developed within several ponded basins by mid eocene time were either closed hydrologically or else they drained westward into the tyee paleoriver of the pacific northwest

geochemical model of δ18o of pedogenic calcite versus latitude and its application to cretaceous palaeoclimate abstract in the absence of a systematic global data base on the 18 o composition of modern soil calcite predicted isotopic values are calculated as a function of latitude between 60 and 20 using 18 o of summer and winter meteoric water ambient soil temperature equal to either measured values in asia or mean annual temperature and 0 10 20 and 30 evaporation of soil water the results show overall higher isotopic values for summer versus winter precipitation but steeper positive slopes for the winter precipitation curves where soil temperature is equated to mean annual temperature the summer precipitation curves are nearly flat at high latitudes and have a negative slope at low latitudes in all cases the 18 o values increase with increasing percent evaporation of soil water oxygen isotopic data for early aptian and late maastrichtian cretaceous palaeosol calcite as a function of palaeolatitude of the western interior of north america plot in the modern winter precipitation field which along with vertic features may suggest a winter wet palaeoclimate steeper positive slopes for the cretaceous curves compared to the modern curves may be attributable to greater latitudinal gradients in mean annual temperature and soil temperature in the cretaceous compared to today and or to a greater degree of evaporation in cretaceous soils at low latitudes compared to the modern earth distinguishing between these possibilities will require a more complete understanding of the variables that affect oxygen isotopes in pedogenic calcite as well as better cretaceous terrestrial palaeoclimate proxies with which to compare the isotopic data

tectonic stratigraphic and sedimentologic significance of a regional discontinuity in the upper judith river group belly river wedge of southern alberta saskatchewan and northern montana the lithostratigraphic interval between the taber and lethbridge coal zones in the upper portion of the nonmarine judith river group of southeastern alberta is divisible into two lithostratigraphic units separated by a regionally extensive and diachronous discontinuity the lower unit referred to here as the oldman formation is characterized by very fine grained to fine grained sandstones that contain fewer than 2 volcanic rock fragments sandstone bodies with numerous sets of horizontally stratified sandstone showing little or no evidence of lateral accretion siliceous paleosols ganisters and a relatively high gamma ray signal in the upper half of the formation the oldman formation comprises deposits of a low sinuosity perhaps ephemeral fluvial system that originated in the southern cordillera of canada and northern montana and flowed northeastward perpendicular to the axis of the alberta basin the upper unit is assigned to a new formation the dinosaur park formation and is characterized by

rapid late miocene rise of the bolivian altiplano evidence for removal of mantle lithosphere abstract the oxygen isotopic compositions of carbonates deposited in the northern altiplano demonstrate a rapid change in late miocene time which we attribute to an increase in elevation rainfall samples over an elevation transect adjacent to the altiplano of northern bolivia display a systematic decrease in mean annual 18 o values and define a local 18 o vs altitude gradient r gonfiantini m a roche j c olivry j c fontes g m zuppi the altitude effect on the isotopic composition of tropical rains chem geol 181 2001 147 167 which we apply to paleo meteoric water values determined from carbonates age constraints from magnetostragraphy and paleoelevation estimates suggest surface uplift of 2 5 to 3 5 km occurred between 10 3 ma and 6 8 0 4 ma when the altiplano rose to its current elevation only the removal of dense eclogitic lower crust and mantle lithosphere can generate a change of this magnitude and rapidity this change coincides with an 30 decrease in the rate of convergence between the south america and nazca plates and the propagation of deformation into the eastern subandean zone which we infer to have resulted from the increase in the force per unit length applied by the elevated andean plateau to the surrounding lowlands

skiing in the eocene uinta mountains isotopic evidence in the green river formation for snow melt and large mountains isotopic analysis of lacustrine carbonates from the eocene green river formation suggests that lake waters were derived partly from snow melt this evidence for cool climates is in marked contrast to paleontological and model evidence for mild temperatures in the continental interior oxygen isotope ratios of carbonates frequently reach 12 to nearly 16 peedee belemnite which suggests that lake waters probably had 18 o of 18 o enrichment that typically occurs in modern large saline lakes suggests that the source waters to the green river basin had a 18 o of 3000 m for the mountains encircling the green river basin

late cretaceous early eocene laramide uplift exhumation and basin subsidence in wyoming crustal responses to flat slab subduction low angle subduction of the farallon oceanic plate during the late cretaceous early eocene is generally considered as the main driver forming the high rocky mountains in wyoming and nearby areas how the deformation was transferred from mantle to upper crust over the great duration of deformation 40 myr is still debated here we reconstruct basin subsidence and compile paleoelevation thermochronology and provenance data to assess the timing magnitude and rates of rock uplift during the laramide deformation we reconstruct rock uplift as the sum of surface uplift and erosion constrained by combining paleoelevation and exhumation with regional stratigraphic thickness and chronostratigraphic information the amount and rate of rock uplift of individual laramide ranges was less than 2 4 4 8 km 0 21 0 32 mm yr during the early maastrichtian paleocene stage 1 and increased to more than 3 km 0 38 0 60 mm yr during the late paleocene early eocene stage 2 our quantitative constraints reveal a two stage development of the laramide deformation in wyoming and an increase of rock uplift during stage 2 associated with enhanced intermontane basin subsidence exhumation and uplift during stage 1 is consistent with eastward migration of cordilleran deformation associated with low angle subduction whereas the change in exhumation during stage 2 seems to follow a southwestward trend which requires an alternative explanation we here suggest that the increase of rock uplift rate during the late paleocene early eocene and the southwestward younging trend of uplift may be a response to the rollback and associated retreating delamination of the farallon oceanic slab

investigation of early eocene water vapor transport and paleoelevation using oxygen isotope data from geographically widespread mammal remains the oxygen isotope composition 18 o of apatite from mammalian tooth enamel can be used to infer the 18 o value of ingested water which is in turn related to that of precipitation stored in surface reservoirs therefore the 18 o value of phosphate from fossil tooth enamel can be used to infer the 18 o value of these reservoirs in the past in this paper tooth enamel from a semiaquatic mammal taxon coryphodon collected from five early eocene localities in north america is used to construct patterns in 18 o values of river water for this time period at all localities the 18 o value of river water 18 o r is estimated to have been higher during the early eocene relative to present day north american rivers although the 18 o vs latitude gradient was shallower during the eocene higher 18 o r values are consistent with warmer eocene air masses being able to hold more water vapor and with an increase in the poleward transport of both moisture and latent heat the regular decrease in 18 o r with latitude indicates that global atmospheric circulation patterns and hydrological transport were not much different from those of the present although the shallower 18 o vs latitude gradient during the eocene may reflect regional differences in precipitation evaporation and river recharge at a more regional scale the 18 o value of river water can provide insight into topographic relief during the early eocene in the case of intermontane basins of wyoming differences in average 18 o r values between basins indicate that laramide mountain relief was on the order of 475 m it is suggested that anomalously low 18 o r values reported previously do not provide unambiguous evidence for permanent snow at higher elevations and may instead reflect brief episodes of cooler winters and or altered atmospheric circulation patterns

early eocene biotic and climatic change in interior western north america imprecise correlation of the marine and terrestrial fossil records has been a major obstacle to understanding migration and extinction of continental biotas and early cenozoic climate change new sup 40 ar sup 39 ar data from the willwood formation in the bighorn basin of wyoming establish an age of 52 8 plus minus 0 3 ma for earliest lostcabinian late wasatchian faunas and coeval early eocene floras strata just beneath earliest wasatchian faunas can be correlated with the np9 np10 boundary in marine sedimentary units which has an interpolated age of approximately 55 7 ma this new information allows the authors to estimate the durations of the wasatchian approximately 5 m y and the lostcabinian approximately 2 m y and shows that the continental biotas are coeval with the acme of cenozoic warmth inferred from delta sup 18 o measurements of foraminifera from 58 to 50 ma paleoclimate in the continental interior at about 45 degree n was warm and equable but patterns of temperature change inferred from continental floras do not track precisely the marine delta sup 18 o record

middle to late cenozoic cooling and high topography in the central rocky mountains constraints from clumped isotope geochemistry abstract the timing and mechanism of the formation of the high elevations of the central rocky mountains rockies in the western interior of north america remain controversial we examine the middle and late cenozoic paleorelief between the great plains and the central rockies using clumped isotope geothermometry of carbonate cements of fluvial and eolian sandstones our petrographic observations suggest that the studied carbonate cements were formed primarily during eodiagenesis in near surface conditions and retain primary paleoclimate and paleoelevation signals carbonate clumped isotope temperatures decrease 10 c and calculated water o 18 values decrease 4 from 35 ma to 5 ma in both the central rockies and the western great plains following middle and late cenozoic global cooling trend a persistent longitudinal gradient in temperature and o 18 value existed between the central rockies and the great plains during the oligocene and miocene with temperatures 4 8 c higher and calculated water o 18 values 3 6 higher in the great plains these observations suggest the mean elevation of the central rockies was 1 km higher than the western great plains during the middle and late cenozoic similar to the present day regional relief when placed in the context of other paleoaltimetry studies and geological observations our findings support the hypothesis that the high mean topography of the rockies was developed during the late eocene possibly related to isostatic adjustment or dynamic uplift caused by foundering of lower mantle lithosphere or the farallon slab or both

late paleocene early eocene climate changes in southwestern wyoming paleobotanical analysis the warmest global temperatures of the cenozoic era occurred in early eocene time following a warming trend that started in late paleocene time the greater green river basin of southwestern wyoming is one of the best areas in the rocky mountains for paleobotanical investigation of the paleocene eocene climatic transition intensive sampling has resulted in the recovery of an estimated 189 species of plant macrofossils from the tiffanian clarkforkian wasatchian and bridgerian land mammal ages the leaf morphologies and taxonomic affinities of these fossils were used in combination with other indicators to evaluate paleocene eocene climates following cool humid conditions in the tiffanian the clarkforkian was humid and subtropical and several plant families with modern tropical affinities appeared however as in the tiffanian clarkforkian floras had low diversity and were dominated by a single species in the birch family mean annual temperature mat rose from an estimated 12 c in the tiffanian to 19 c in the clarkforkian while mean annual precipitation map for the tiffanian and clarkforkian is estimated to have been 130 150 cm little fossil plant material is preserved from the latest clarkforkian or the earliest wasatchian which is thought to have contained an interval of cooling and drying followed by renewed warming by the middle wasatchian the time of the cenozoic thermal maximum the inferred mat was about 21 c and the map was near 140 cm a second influx of plant families with tropical affinities appeared in the area and diversity increased significantly but most plant families known from the clarkforkian persisted species turnover from the clarkforkian to the wasatchian was greater than 80 a second turnover of more than 80 of species but not families from the wasatchian to the early bridgerian accompanied drying and increased seasonality of precipitation the early bridgerian mat is inferred to have been near 20 c and the map to have been about 80 cm except for the tiffanian and possibly portions of the early wasatchian paleoclimates during the study interval were predominantly frost free although the moderating influence of the green river lake system has been suggested as a possible explanation for mild eocene winters in wyoming this study shows that virtually frostfree climates existed in the area prior to and independent of significant lake development

fault dating in the canadian rocky mountains evidence for late cretaceous and early eocene orogenic pulses fault rocks from the classic rocky mountain foreland fold and thrust belt in southwestern canada were dated by ar analysis of clay grain size fractions using x ray diffraction quantification of the detrital and authigenic component of each fraction these determinations give ages for individual faults in the area illite age analysis the resulting ages cluster around 72 and 52 ma here called the rundle and mcconnell pulses respectively challenging the traditional view of gradual forward progression of faulting and thrust belt history of the area the recognition of spatially and temporally restricted deformation episodes offers field support for theoretical models of critically stressed wedges which result in geologically reasonable strain rates for the area in addition to regional considerations this study highlights the potential of direct dating of shallow fault rocks for our understanding of upper crustal kinematics and regional tectonic analysis of ancient orogens

paleoelevation reconstruction using pedogenic carbonates paleoelevation reconstruction using stable isotopes although a relatively new science is making a significant contribution to our understanding of the recent growth of the world s major orogens in this review we examine the use of both light stable isotopes of oxygen and the new clumped isotope 47 carbonate thermometer in carbonates from soils globally the oxygen isotopic composition 18 o of rainfall decreases on average by about 2 8 km of elevation gain this effect of elevation will in turn be archived in the 18 o value of soil carbonates and paleoelevation can be reconstructed provided 1 temperature of formation can be estimated 2 the effects of evaporation are small 3 the effects of climate change can be accounted for and 4 the isotopic composition of the carbonate is not diagenetically altered we review data from modern soils to evaluate some of these issues and find that evaporation commonly elevates 18 o values of carbonates in deserts an effect that would lead to underestimates of paleoelevation some assessment of paleoaridity using qualitative indicators or carbon isotopes from soil carbonate is therefore useful in evaluating the oxygen isotope based estimates of paleoelevation sampling from deep 50 cm in paleosols helps reduce the uncertainties arising from seasonal temperature fluctuations and from evaporation the new clumped isotope 47 carbonate thermometer expressed as 47 offers an independent and potentially very powerful approach to paleoelevation reconstruction in contrast to the use of 18 o values nothing need be known about the isotopic composition of water from which carbonate grew in order to estimate of temperature of carbonate formation from 47 values using assumed temperature lapse rates with elevation paleoelevations can thereby be reconstructed case studies from the andes and tibet show how these methods can be used alone or in combination to estimate paleoelevation in both cases the potential for diagenetic alteration of primary carbonate values first has to be assessed clear examples of both preservation and alteration of primary isotopic values are available from deposits of varying ages and burial histories 47 values constitute a relatively straightforward test since any temperature in excess of reasonable surface temperatures points to diagenetic alteration for 18 o values preservation of isotopic heterogeneity between different carbonate phases offers a check on diagenesis results of these case studies show that one area of south central tibet attained elevations comparable to today by the late oligocene whereas 2 7 0 4 km of uplift occurred in the bolivian altiplano during the late miocene

controls on synorogenic alluvial fan architecture beartooth conglomerate palaeocene wyoming and montana late palaeocene uplift of the beartooth range in northwestern wyoming and southwestern montana generated the beartooth conglomerate along the eastern and northeastern flanks of the range systematic unroofing sequences and intraformational unconformities folds and faults in the conglomerate attest to deposition during uplift along the eastern flank at least three ancient alluvial fan systems and a braidplain system can be distinguished on the bases of petrofacies and lithofacies the two southern fans consist of 700 m of sedimentary clast conglomerate and subordinate sandstone dominated by hyperconcentratedflow and stream flow facies the next fan to the north is dominated by plutonic and metamorphic clasts and contains abundant mud matrix supported debris flow facies as well as stream flow facies the northernmost depositional system consists of arkosic channellized fluvial conglomerate and sandstone overbank mudstone and crevasse splay sandstone units palaeocurrent data indicate eastward dispersal away from the beartooth range outstanding exposure of the beartooth conglomerate allows facies to be mapped on lateral photographic mosaics a seven fold hierarchy of bounding surfaces and enclosed lithosomes exists in the beartooth conglomerate first through fourth order surfaces are analogous to first through fourth order surfaces that recently have been documented in sandy fluvial facies with one exception sediment gravity flows are bounded by first order surfaces fifth order surfaces are either erosional e g lateral migration of fanhead trench or accretionary e g aggradation of fan surface during backfilling of trench and construction of lobes on lower fan during entrenchment on upper fan some fifth order surfaces coincide with intraformational angular unconformities and are thus the result of long term fanhead entrenchment following uplift of the upper part of the fan sixth order surfaces bound individual fan packages that are several hundred metres thick and 10 km2 in area the enclosed sixth order lithosomes are distinguishable in terms of petrofacies and lithofacies a single seventh order surface bounds the entire beartooth conglomerate lower order lithosomes are produced by intrinsic processes of fan construction fifth order lithosomes can be attributed to both extrinsic and intrinsic controls sixth and seventh order lithosomes are generated by extrinsic controls

paleocene eocene record of ophiolite obduction and initial india asia collision south central tibet 1 uppermost cretaceous to eocene marine sedimentary sequences occur both to the south and north of the yarlung zangbo suture in south central tibet they consist of indian margin strata of the northern tethyan himalaya and asian margin strata of the gangdese forearc both assemblages are characterized by major changes in depositional environment and sedimentary provenance at 65 ma and an appearance of detrital chromium rich spinel of ophiolite affinity tio2 generally 0 1 wt during the paleocene ophiolitic melange exposed along the suture could have provided a source for detrital spinel the melange occurs in the hanging wall of a north dipping south directed mylonitic shear zone which includes a tectonic sliver of mafic schist amphibole from the schist yields 40ar 39ar ages of 63 ma which we attribute to cooling during slip along the shear zone and southward obduction of the melange melange obduction was coeval with the development of an angular unconformity within the gangdese forearc basin to the north between late maastrichtian time and 62 ma upper paleocene to middle eocene sandstones in the northern tethyan himalaya yield 200 120 ma u pb detrital zircon ages and 190 170 ma 40ar 39ar detrital mica ages these detrital grains were most likely sourced from regions north of the yarlung zangbo suture suggesting that onset of india asia collision in south central tibet is middle eocene or older in age collectively our results support previous suggestions that oceanic rocks were obducted onto the northern margin of india during latest cretaceous earliest tertiary time coeval changes in gangdese forearc sedimentation raise the possibility that this obduction event marks onset of tectonic interaction between india and asia at 65 ma alternatively in concert with the conventional view of eocene collision initiation the obducted oceanic rocks may be of intraoceanic origin while coeval changes in gangdese forearc sedimentation may be a consequence of an increase in the rate of ocean continent convergence following the demise of the intraoceanic subduction zone

quantifying sediment supply to continental margins application to the paleogene wilcox group gulf of mexico sediment supply to the ocean influences basin margin growth and reflects upstream landscape evolution including patterns of sediment routing denudation and tectono climatic perturbations in source areas constraining sediment supply is useful for inputs to stratigraphic forward models and for predictions of reservoir presence and quality because of the importance of sediment supply geoscientists have developed various methods to estimate it here we apply monte carlo simulation mcs to the bqart model that is used to describe an empirical relationship between river catchment paleogeography climate and sediment load we calculate a range of sediment supply from north american source areas to the gulf of mexico that suggests an overall decrease in median sediment supply from the late paleocene to the early eocene from 404 514 to 144 204 million tons per year depending on the published paleogeographic model that we used to guide our selection of input variables comparison of these estimates with downstream sediment records shows that the subsurface depositional rates are within the 10th 90th percentile range of this bqart mcs uncertainty model the 50th percentile values of bqart mcs results are overall larger than the published wilcox sediment volume which indicates that the size of wilcox deep water fans might be underestimated we use source of change analysis to show the influence of each river catchment input of the bqart model on change in sediment supply from the late paleocene to the early eocene integration of empirical based methods such as bqart with physics based experimental and modeling approaches might provide better constrains on sediment supply and deposition in frontier areas of oil and gas exploration

flat latitudinal gradient in paleocene mammal richness suggests decoupling of climate and biodiversity mammal faunas from western north america exhibit no significant change in species richness with latitude during the torrejonian ca 63 60 ma and tiffanian ca 60 58 ma north american land mammal ages in contrast to a strong richness gradient in modern mammalian faunas of the same region today the latitudinal gradient in oxygen isotope composition of mammalian bioapatite from the paleocene faunas is similar to that of modern meteoric and surface waters suggesting that the temperature gradient in the paleocene was similar to the modern one the flat richness gradient in the middle paleocene indicates either different responses to climatic gradients of faunas dominated by extinct clades of placental mammals or distinct ecological processes during the paleocene diversification of mammals following the end cretaceous mass extinction

chemical weathering and lithologic controls of water chemistry in a high elevation river system clark s fork of the yellowstone river wyoming and montana seasonal analyses of surface water geochemistry were conducted in the clark s fork of the yellowstone watershed to determine whole rock weathering rates the clark s fork of the yellowstone is a high elevation catchment with distinct bedrock lithologies using dissolved solute concentrations and stream flow data we calculated cation denudation rates of 119 g m 2 yr 1 65 900 eq ha 1 yr 1 for carbonate rich sedimentary rocks 16 6 g m 2 yr 1 8200 eq ha 1 yr 1 for andesitic volcanics and 9 8 g m 2 yr 1 5300 eq ha 1 yr 1 for granitic gneisses ca na ratios indicate that chemical weathering of disseminated calcite in granitic rocks contributes to the total solute load in these subcatchments removal of this c 2100 eq ha 1 yr 1

re os isotopic dating of molybdenites in the xiaoliugou w mo deposit in the northern qilian mountains and its geological significance abstract the xiaoliugou is a large scale w mo deposit newly explored in the northern qilian mountains caledonian fold belt northwestern china it is a vein and skarn type deposit genetically associated with the xiaoliugou caledonian granodiorite stock re os isotopic dating for the molybdenite in the deposit yields an isochron age of 462 13 ma 2 and model ages of 436 to 496 ma these results suggest that w mo mineralization occurred before the collision of plates and mineralized substances mainly originated from the crust

evolution of early cenozoic marine temperatures the equator to high southern latitude sea surface and vertical temperature gradients are reconstructed from oxygen isotope values of planktonic and benthic foraminifers for the following five time intervals late paleocene early eocene early middle eocene late eocene and early oligocene paleotemperatures are calculated using standard oxygen isotope temperature equations with adjustments to account for 1 variations in sea water 18o related to changes in global ice volume over time and 2 latitudinal gradients in surface water 18o these reconstructions indicate that sea surface temperatures sst of the southern oceans in the early eocene were as high as 15 c whereas temperatures during the late paleocene and early middle eocene reached maximum levels of 10 12 c by the late eocene and early oligocene high latitude sst had declined to 6 and 4 c respectively for most of the early paleogene low latitude sub tropical temperatures remained constant and well within the range of holocene temperatures 24 ndash 25 c but by the late eocene and early oligocene declined to values in the range of 18 to 22 c the late paleogene apparent decline in tropical temperatures however might be artificial because of dissolution of near surface foraminifera tests which biased sediment assemblages toward deeper dwelling foraminifera moreover according to recent plate reconstructions it appears that the majority of sites upon which the late eocene and early oligocene tropical temperatures were previously established were located either in or near regions likely to have been influenced by upwelling global deepwater temperature on average paralleled southern ocean sst for most of the paleogene we speculate based on the overall timing and character of marine sea surface temperature variation during the paleogene that some combination of both higher levels of greenhouse gases and increased heat transport was responsible for the exceptional high latitude warmth of the early eocene

late paleocene high laramide ranges in northeast wyoming oxygen isotope study of ancient river water abstract the distribution and initial timing of the establishment of high surface elevations in the rocky mountains during the early cenozoic remain controversial despite the importance of these data in testing tectonic models for this region we track the timing and distribution of high elevation in the rocky mountains during the late cretaceous early eocene by examining annual and seasonal 18o values of the ancient river water which are calculated from the 18o values of well preserved freshwater bivalve shells in the powder river basin of the eastern laramide province the 18o values of the ancient river water vary between 23 0 and 8 0 smow in both seasonal and annual records in the late paleocene early eocene the large variation suggests that the ancient rivers were fed yearly or seasonally by snowmelt from highlands of 4 5 1 3 km this can be explained by the existence of the bighorn mountains and black hills with a drainage pattern similar to the present in northeast wyoming the 18o values of ancient river water along the front of the sevier thrust belt generally follow a trend from lower values in north 14 2 1 4 in the early paleocene crazy mountains basin to higher values in south 11 1 0 8 in the late paleocene bighorn basin and 7 1 1 6 in the early eocene washakie basin the variations within each basin are relatively small these rivers most likely rise in the sevier thrust belt and may reflect highland elevation of 1 2 km the 18o values in the alberta foreland and williston basin are very low 20 5 in the late cretaceous indicating the rivers were fed by snowmelt from the canadian rocky mountains of 4 3 1 0 km high the attainment of high elevation in the eastern laramide province prior to the western province could be explained by southwestward progression of back thrusts soled into an earlier east directed master detachment which may be formed by the westward rollback of subducted shallow slab

geochronology and mammalian biostratigraphy of middle and upper paleocene continental strata bighorn basin wyoming the bighorn basin of northwestern wyoming preserves one of the most complete records of middle paleocene to lower eocene continental biota the geochronology of this important interval depends partly on numerical calibration of the geomagnetic polarity time scale gpts but the middle and late paleocene parts of the gpts have been poorly constrained radioisotopically a new volcanic ash from the northern bighorn basin provides the first radioisotopic age for upper middle paleocene strata upper selandian and is an important calibration point for the base of polarity chron c26n and the selandian thanetian stage boundary in the gpts sanidine from the ash has a weighted mean age of 59 00 0 30 2 ma based on twenty three 40 ar 39 ar bulk laser fusion analyses the ash corroborates the revised age estimate for the base of chron c26n in the most recent geomagnetic polarity time scale gpts 04 and confirms that c26n is older than depicted in the 1995 gpts we place paleocene mammalian biozones in the bighorn basin into a refined geochronologic framework these biozones are the primary basis for biostratigraphic correlation of middle and upper paleocene continental deposits in north america new paleomagnetic data provide synchronous tie points for correlation among three stratigraphic sections in the northern bighorn basin and to the gpts paleomagnetic correlation study of new fossil material review of known faunas and refined stratigra phy enable us to reevaluate several biozones we define three new zones based on first occurrences of taxa the phenacolemur ti 4b probathyopsis ti 5a revised and copecion cf 3 revised zones the phenacolemur zone includes some faunas previously placed in the plesiadapis churchilli and plesiadapis fodinatus zones ti 4 and ti 5a respectively the probathyopsis zone replaces the p fodinatus zone and the copecion zone replaces the phenacodus ectocion acme zone cf 3 seven of the biozones considered here are subdivisions of the tiffanian land mammalage thetiffanianwasoriginallytypifiedbythemasonpocketfaunainsouthwest ern colorado but it is now much better known from faunas in the northern bighorn basin biostratigraphic evidence and new paleomagnetic data from the bighorn basin indicate that mason pocket is considerably older than was previously recognized and occurs in chron c26r rather than c25r the geochronologic framework presented here helps to constrain the temporal ranges of species that occur in the middle and late tiffanian and it allows the bighorn basin record to be better compared to other faunal floral and paleoclimate records both regionally and globally

depositional environment sediment provenance and oxygen isotope paleoaltimetry of the early paleogene greater green river basin southwestern wyoming u s a depositional process sediment provenance and paleotopography are critical to paleogeographic reconstruction during the coeval laramide and sevier orogenies in the cordillera orogenic system here we conduct lithofacies analysis sediment provenance and paleoelevation studies in the fluvial lower paleogene fort union and the main body of the wasatch formations in the center of the greater green river basin in order to reconstruct paleogeography the depositional environment changed from low energy floodplains and associated meandering river during the paleocene to braided rivers during the earliest eocene the lowermost eocene strata have more precambrian basement derived feldspar grains compared to the paleocene strata this change in sediment provenance was associated with the change to dominant northwestward paleoflow direction suggesting major uplift of the laramide uinta mountains during the earliest eocene the 18o values of authigenic carbonates were stable during the early paleocene but decreased gradually from 10 permil during the late paleocene to 18 permil during the earliest eocene suggesting the lack of highland precipitation in the basin catchment during the early paleocene and significant surface uplift and arrival of highland precipitation from the uinta mountains during the earliest eocene the reconstructed paleoelevation of the uinta mountains was at least 3 km possibly greater than 3 5 km and the paleoelevation of the greater green river basin floor was at most 1 km and the paleorelief between the greater green river basin and uinta mountains was 2 km during the earliest eocene our record of the earliest eocene establishment of high topography of the uinta mountains through renewed uplift is 4 to 6 myr older than previously thought if the carbonate cements in the fort union formation record the isotope compositions of surface water from the sevier orogen their high 18o values likely suggest that the sevier orogen had rugged topography during the paleocene

prolonged high relief in the northern cordilleran orogenic front during middle and late eocene extension based on stable isotope paleoaltimetry the paleoelevation and size of the north america cordilleran orogen during the late cretaceous paleogene contractional and subsequent extensional tectonics remain enigmatic we present new estimates of paleorelief of the northern cordilleran orogenic front during the middle and late eocene using oxygen isotope compositions of unaltered molluscan fossils and paleosol carbonates in the kishenehn basin bounded by several mountains ranges to the east the kishenehn basin was a half graben developed during middle eocene to early miocene collapse of the cordilleran orogen these mollusk taxa include three sympatric groups with affinities to wet tropical semi arid subtropical and temperate environments our reconstructed surface water 18o values vary between 19 8 and 6 3 vsmow during the middle and late eocene the large differences in paleoenvironments and surface water 18o values suggest that the catchment of the kishenehn basin was at variable elevation the estimated paleorelief between the basin and the surrounding mountains based on both rayleigh condensation model and predictions of eocene precipitation isotope values using an isotope enabled global climate model is 4 km and the basin floor was 1 5 km high this high topography and high relief paleogeography suggest that the cordilleran orogenic front reached an elevation of at least 4 km and the crust thickness may have reached more than 55 km before eocene gravitational collapse we attribute the maintenance of high eocene topography to the combination of an inherited thick crust thermal uplift caused by mantle upwelling and isostatic uplift caused by removing lower lithosphere or oceanic slab

characteristics and genesis of gangdese porphyry copper deposits in the southern tibetan plateau preliminary geochemical and geochronological results porphyry type cu mo au deposits have been discovered along the gangdese magmatic arc in the southern tibetan plateau extensive field investigations and systematic studies of geochemistry s pb isotopic tracing together with re os and 40ar 39ar isotopic dating indicate that the mineralisation of the copper belt is genetically related to emplacement of late orogenic granitic porphyries during the post collisional crustal relaxation period of the late himalayan epoch these porphyries are petrochemically k enriched and belong to shoshonitic to high k calc alkaline series they display enrichment of large ion lithophile elements lile rb k u th sr pb and depletion of high field strength elements hfse nb ta ti and the heavy rare earth elements hree and y without eu anomalies these characteristics demonstrate that subduction played a dominant role in their petrogenesis and residual garnet was left in the magma sources pb isotope data show a linear correlation in the plumbotectonic framework diagram ranging from orogenic pb in the eastern segment of the copper belt to mantle pb in the western segment these constitute a mixing line of the indian oceanic morb with indian oceanic sediments and suggest that the porphyry magmas were dominantly derived from partial melting of subducted oceanic crusts mixed with a minor quantity of sediments and mantle wedge components the gangdese porphyry copper polymetallic belt has alteration characteristics and zonation typical of porphyry type copper deposits which include potassic alteration k feldspathisation and biotitisation silicification sericitisation and propylitisation mineralisation mainly occurs in strongly altered granitic cataclasite at the exo contact with veinlet disseminated textures the porphyries themselves are weakly mineralised with disseminated pyrite and chalcopyrite the copper deposits contain simple ore mineral associations consisting of chalcopyrite pyrite bornite molybdenite sphalerite and oxidised minerals of malachite covellite and molybdite during supergene oxidation primary ores underwent secondary enrichment to form economic orebodies with cu grade ranging from 1 to 5 ore sulphides of the copper belt display s and pb isotopic compositions identical to the ore bearing porphyries their 34s values vary between 3 8 and 2 4 and are typical of mantle sulphur the 206pb 204pb 207pb 204pb and 208pb 204pb ratios vary in the ranges 18 106 to 18 752 15 501 to 15 638 and 37 394 to 39 058 respectively and yielded radiogenetic lead enriched signatures twelve molybdenite samples from the copper belt yielded isochron ages of 14 76 0 22 ma and 13 99 0 16 ma for the nanmu and chongjiang deposits and model ages of 13 5 to 13 6 ma for the lakang e deposit meanwhile 40ar 39ar isotopic dating of two biotite phenocrysts from the chongjiang and lakang e deposits give plateau ages of 13 5 1 0 ma and 13 42 0 10 ma respectively during the geodynamic evolution of the gangdese collision orogenic belt intrusion of the ore bearing porphyries took place just before the rapid uplift and e w extension of the southern plateau and the ore forming process may have occurred simultaneously with the uplift and extension 14 0 1 ma

late paleogene extensional collapse of the cordilleran foreland fold and thrust belt the cordilleran foreland fold and thrust belt collapsed and spread to the west during a middle eocene to early miocene ca 49 20 ma episode of crustal extension the sedimentary and structural record of this event is preserved in a network of half grabens that extends from southern canada to central utah extensional structures superposed on this allochthonous terrain are rooted to the physical stratigraphy structural relief and sole faults of preexisting thrust fold structures the sole faults dip 3 6 west above an undeformed precambrian crystalline basement and accommodated tectonic transport of a thick up to 20 km eastward tapering hanging wall during regimes of crustal shortening and extension the chronology of tectonism for the foreland fold and thrust belt is established here by dating latest thrusting and initial normal faulting and is best defined where thrusts and normal faults are linked by common detachment surfaces dated movement on two extensionally reactivated thrusts the lewis thrust of northwest montana and southeast british columbia and the medicine butte thrust of southwest wyoming and northeast utah suggests that the hiatus between the end of crustal shortening in the early or early middle eocene and the start of extension in the early middle eocene was brief lateral spreading and extensional basin formation in the cordilleran foreland fold and thrust belt were partly concurrent with formation of metamorphic core complexes and regional magmatism conceptually linking extensional processes that were simultaneously deforming both the hinterland and foreland of the late paleogene cordilleran orogenic wedge is accomplished by applying the extensional wedge coulomb critical taper model the rapid drop in north america pacific plate convergence rate and or steepening of the subducted oceanic slab at ca 50 ma resulted in a large reduction in east west horizontal compressive stress in the cordillera as a result the cordilleran orogenic wedge was left unsupported and it gravitationally collapsed and horizontally spread west until a new equilibrium was established at ca 20 ma subsequently crustal extension and magmatism during the basin and range event ca 17 0 ma overprinted much of this earlier phase of extension

apatite fission track thermochronology of the sierras pampeanas central western argentina implications for the mechanism of plateau uplift in the andes over much of its length the andean orogen is characterized by a generally east vergent geometry and a progressive eastward cratonward migration of individual arc orogenic elements a departure from this model occurs in the sierras pampeanas region of central western argentina where a terrane of active basement uplifts is currently developing to the east of the main cordillera apatite fission track data from uplifted precambrian and phanerozoic basement rocks of the sierras pampeanas constrain the tectonic development of this terrane and indicate that deformation associated with exhumation may have propagated broadly westward since the late miocene two pre andean cooling events during the carboniferous permian ca 300 280 ma and the early jurassic middle jurassic ca 200 174 ma have been identified the onset of andean deformation is represented by a cooling event during the late paleocene middle eocene this cooling was followed by a period of middle miocene late miocene reheating during foreland basin style sedimentation exhumation and possible westward migration of the exhumation front in the sierras pampeanas commenced during the late miocene pliocene to the east of the dominantly east vergent precordillera fold and thrust belt the apparent convergence of deformation in these two terranes and the progressive closure of miocene pliocene intermontane basins in the sierras pampeanas may reflect the early stages of andean plateau uplift the relative timing of plateau development along strike suggests that lateral thickening of the orogen is progressing southward at least from the latitude of central bolivia ca 20 s furthermore the time space coincidence between basement uplift and flattening of the subducted slab beneath the sierras pampeanas suggests that a relationship exists between westward displacement of the terrane and the dynamics of plate interaction

paleoelevation estimates for the northern and central proto basin and range from carbonate clumped isotope thermometry 1 quantitative paleoelevation histories can help explain both why and how widespread cenozoic extension occurred in the basin and range province of western north america we present new estimates of preextensional paleoelevations for the northern and central basin and range using clumped isotope 47 thermometry of lacustrine carbonates collected from each region comparison of carbonate 47 derived mean annual air temperature maat estimates 16 c 20 c for the late cretaceous eocene sheep pass basin of east central nevada with published maat estimates for the eocene coastal northern sierra nevada 20 c 25 c suggests that the early paleogene sheep pass basin had a paleoelevation of 2 km such a modest paleoelevation suggests that either 1 the proto northern basin and range did not attain maximum paleoelevations of 3 4 km until the late eocene early oligocene or 2 the sheep pass basin was a local high relief 1 km setting contained within a 3 km orogenic highland nevadaplano similarity of 47 derived maat estimates 17 c 24 c for carbonates from the central basin and range and the near sea level southern sierra nevada bena basin indicate that middle miocene paleoelevations in the death valley region were 1 5 km these fairly low paleoelevations are incompatible with preextensional crustal thicknesses 52 km and indicate that mean elevation change was minor 500 m and lithospheric mass was not conserved during 100 neogene extension of the central basin and range but was instead likely compensated by synextensional magmatic additions to the crust

oxygen isotope evidence for high altitude snow in the laramide rocky mountains of north america during the late cretaceous and paleogene the 18 o values of river waters in late cretaceous and paleogene basins of alberta montana wyoming and colorado are calculated from the isotopic composition of unaltered aragonite mollusk fossils the isotopic composition of these river waters has significant implications for the uplift and climate history of the mountains surrounding the basins our data show that the 18 o values of river water varied dramatically ranging from 23 to 5 relative to vienna standard mean ocean water with an uncertainty of 2 close stratigraphic association of river deposits with very different isotopic values indicates two different water sources rainfall in the warm low elevation basins and snowfall in surrounding mountains the 18 o difference between basinal and highland precipitation suggests that local relief was similar to today9s on the order of 2 5 3 km seasonal variation in shell 18 o values also indicates that there are times when large snow fields in the maastrichtian and paleocene mountains of montana and wyoming survived summer melting and were multiannual features

late cretaceous to early tertiary magnetostratigraphy of a continental sequence red deer valley alberta canada the red deer valley exposes a conformable 350 m thick late campanian to early paleocene fluvial deltaic coal bearing sequence named the edmonton group systematic sampling and paleomagnetic analysis of the dominant lithologies mudstones sandstones and shales were carried out over five sections correlated by coal seams to provide some 500 sites stratigraphic levels primary detrital remanent magnetization directions after alternating field cleaning produce a polarity reversal pattern correlatable to sea floor magnetic anomalies 29 33 the campanian maastrichtian boundary based upon the first appearance of wodehouseia and the maastrichtian paleocene cretaceous tertiary boundary based upon an ir anomaly associated with dinosaur and cretaceous palynofloral extinctions correlate closely magnetostratigraphically with the marine foraminiferal section at gubbio italy radiometric dating of bentonites has indicated the ages of these boundaries to be approximately 71 and 64 ma respectively

high and dry in central tibet during the late oligocene the time at which the tibetan plateau rose to its present high elevation remains controversial with estimates ranging from 40 ma to more recent than 7 ma new stable isotope analyses of modern and accurately dated ancient paleosol carbonate in the nima basin of central tibet point to an arid climate and high paleoelevation 4 5 5 km comparable to today s setting by 26 ma oxygen isotope values of ancient 26 ma soil carbonate are both very negative and indistinguishable after modest corrections for changes in global climate from the lowest least evaporated oxygen isotope values of modern soil carbonates in the area substantial enrichments in oxygen 18 in paleolacustrine carbonates as well as high carbon isotope values from paleosol carbonates indicate considerable lake evaporation and low soil respiration rates respectively and both are consistent with the present arid climate of the nima area blockage of tropical moisture by the himalaya and perhaps the gangdese shan probably has contributed strongly to the aridity and very negative oxygen isotope values of soil carbonate and surface water in the nima area since at least the late oligocene the maintenance of high elevation since at least 26 ma suggests that any flow of lower crust from beneath central tibet must have been balanced by coeval northward insertion of indian crust beneath the plateau