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The numerous biotic, climatic, and tectonic events of the Devonian cannot be correlated and investigated without a wellcalibrated time scale. Here, we updated the calibration of the Devonian time scale using a Bayesian age-depth model that incorporates radioisotopic ages and astrochronology durations. We used existing radioisotopic ages collected and harmonized in the last two geologic time scale compilations, as well as new U-Pb zircon ages from Emsian {Hercules I K-bentonite, Wetteldorf, Germany: 394.290 ± 0.097(0.21)[0.47] Ma} and Eifelian K-bentonites {Tioga B and Tioga F K-bentonites, Fayette, New York, USA: 390.82 ± 0.18(0.26)[0.48] Ma and 390.14 ± 0.14(0.23)[0.47] Ma, respectively}. We anchored floating astrochronology stage durations on radioisotopic ages and chained astrochronologic constraints and uncertainty together to extrapolate conditioning age likelihoods up or down the geologic time scale, which is a new method for integrating astrochronology into age-depth modeling. The modeling results in similar ages and durations for Devonian stages regardless of starting biostratigraphic scaling assumptions. We produced a set of rescaled biostratigraphic zonations, and a new numerical calibration of Devonian stage boundary ages with robust uncertainty estimates, which allow us to evaluate future targets for Devonian time scale research. These methods are broadly applicable for time scale work and provide a template for an integrated stratigraphic approach to time scale modeling. [ABSTRACT FROM AUTHOR]

Accurate age-depth models for proxy records are crucial for inferring changes to the environment through space and time, yet traditional methods of constructing these models assume unrealistically small age uncertainties and do not account for many geologic complexities. Here we modify an existing Bayesian age-depth model to foster its application for deep time U-Pb and 40Ar/39Ar geochronology. More flexible input likelihood functions and use of an adaptive proposal algorithm in the Markov Chain Monte Carlo engine better account for the age variability often observed in magmatic crystal populations, whose dispersion can reflect inheritance, crystal residence times and daughter isotope loss. We illustrate this approach by calculating an age-depth model with a contiguous and realistic uncertainty envelope for the Miocene Santa Cruz Formation (early Miocene; Burdigalian), Argentina. The model is calibrated using new, high-precision isotope dilution U-Pb zircon ages for stratigraphically located interbedded tuffs, whose weighted mean ages range from ca. 16.78 ± 0.03 Ma to 17.62 ± 0.03 Ma. We document how the Bayesian age-depth model objectively reallocates probability across the posterior ages of dated horizons, and thus produces better estimates of relative ages among strata and variations in sedimentation rate. We also present a simple method to propagate age-depth model uncertainties onto stratigraphic proxy data using a Monte Carlo technique. This approach allows us to estimate robust uncertainties on isotope composition through time, important for comparisons of terrestrial systems to other proxy records. [ABSTRACT FROM AUTHOR]

Understanding the time scales of magmatic differentiation, storage, and eruption of large-volume silicic magmas is a primary goal of igneous petrology. Within the Huckleberry Ridge Tuff (HRT; Idaho, USA), representing the earliest and largest caldera-forming eruption associated with Yellowstone volcanic activity, zircon morphological zoning patterns coupled to strongly correlated changes in Ti-in-zircon thermometry and trace element indicators of progressive differentiation provide a proxy record for the evolution of the HRT member B magma body. Tandem in situ and isotope dilution U-Pb dating of single zircon crystals demonstrates an absence of pre-Pleistocene xenocrysts, but reveals the presence of antecrysts recycled from pre-caldera rhyolites in the HRT magma. The petrochronologic interpretation of autocrystic zircon thermal, chemical, and temporal characteristics suggests that HRT member B differentiated over ~10 k.y. prior to eruption at 2.0794 ± 0.0046 Ma as defined by new astronomically calibrated, single-crystal total fusion 40Ar/39Ar sanidine analyses. This refined eruption age demonstrates that the transitional polarity preserved by HRT member B does not record the Reunion subchron, but rather a separate, younger geomagnetic event. Our novel approach places the thermal and chemical regime of silicic magmas within a temporal context and demonstrates the rapid evolution of a large volume of silicic magma. [ABSTRACT FROM AUTHOR]

The Cougar Creek Complex is a compositionally diverse intrusive suite constructed of variably deformed and nondeformed dikes and plutons interpreted as the roots of the Wallowa arc terrane in the Blue Mountains Province, Oregon-Idaho. New high-precision U-Pb zircon ages, geochemical data, and structural analysis defi ne two compositionally and temporally distinct cycles of magmatism, and two episodes of contractional deformation in the Cougar Creek Complex. From Middle Permian to Early Triassic time (265.4 ± 0.2 Ma to 248.8 ± 0.1 Ma), the Wallowa arc was dominated by silicic arc magmatism. During the Early to Middle Triassic (248.8 ± 0.1 Ma to 229.4 ± 0.1 Ma), an apparent hiatus in intrusive activity suggests a cessation or lull in arc magmatism, or a shift of the arc axis. This lull in volcanic activity coincides with D1 deformation, uplift, and erosion of the Wallowa arc. In the Late Triassic (229.4 ± 0.1 Ma to 229.1 ± 0.5 Ma), voluminous mafi c to intermediate magmatism, exhibiting trace-element characteristics of a subduction-modifi ed, strongly depleted mantle source, was initiated, and this represents a change in "normal" arc magmatism. We interpret the sequence of D1 deformation, an apparent gap in igneous activity, exhumation of the Wallowa arc, and a switch from "normal" silicic arc magmatism to depleted, mid-ocean-ridge basalt--like mafic volcanism as a response to spreading-ridge subduction and subsequent slab window magmatism beneath the overriding Wallowa arc. Late Triassic D2 synmagmatic left-lateral mylonitic shear zones in the Cougar Creek Complex illustrate strain partitioning in an intrusive, transpressional arc axis environment undergoing sinistral-oblique subduction. Synchronous U-Pb crystallization ages, 40Ar/39Ar cooling ages, the abundance of plutonic clasts within conglomerate units, and unimodal Late Triassic detrital zircon ages for volcanic sandstone units suggest uplift and erosion of the Wallowa arc during the Late Triassic; this is inferred to be related to the upwelling of buoyant asthenosphere through an opening slab window beneath the Wallowa arc. We propose that Late Triassic mafic igneous rocks from the Wallowa terrane and Wrangellia resulted from the subduction of a spreading ridge and slab window magmatism beneath a late Paleozoic arc, thus linking the Late Triassic geologic evolution of two fundamental components of the Cordilleran orogen. [ABSTRACT FROM AUTHOR]

A quantitative biostratigraphic and radiometric calibration is presented for the Pennsylvanian through Early Permian global time scale, based upon high-precision, isotope dilution--thermal ionization mass spectrometer (ID-TIMS) U-Pb zircon ages for interstratified ash beds in the parastratotype sections of the southern Urals of Russia. Twenty-four ash-bed ages in three outer ramp and basinal sections of the Pre- Uralian foredeep bracket the biotic definitions of global stages and regional substages from the base of the Upper Pennsylvanian Kasimovian Stage to the base of the Lower Permian Artinskian Stage; four additional ash-bed ages in two sections of the eastern slope of the Urals constrain the global Bashkirian and Serpukhovian Stages. Quantitative stratigraphic methods (CONOP9) are applied to a compilation of over 2000 bioevents in 22 stratigraphic sections supplemented by our dated volcanic horizons to refine the Pennsylvanian--Early Permian global time scale. Significant shifts in the duration of several stages are demonstrated, ranging from one to six million years, compared with prior estimates. The unprecedented density of radiometric calibration points for the Pennsylvanian--Permian transition provides a high-resolution (~0.1-Ma) global chronostratigraphic standard for testing and improving biostratigraphic correlations across Euramerica. We integrate radiometric ages, biostratigraphic correlation, and cyclostratigraphic tuning of major cyclothems to the long-period (404-ka) eccentricity cycle to elucidate the tempo, magnitude, and forcing of eustatic changes and cyclothemic deposition associated with the waxing and waning of Gondwanan ice sheets, and establish a pan-Euramerican chronostratigraphic framework for most of Pennsylvanian and Early Permian time. [ABSTRACT FROM AUTHOR]

The Duluth Complex (Minnesota, USA) is one of the largest mafic intrusive complexes on Earth. It was emplaced as the Midcontinent Rift developed in Laurentia's interior during an interval of magmatism and extension from ca. 1109 to 1084 Ma. This duration of magmatic activity is more protracted than is typical for large igneous provinces interpreted to have formed from decompression melting of upwelling mantle plumes. While the overall duration was protracted, there were intervals of more voluminous magmatism. New 206Pb/238U zircon dates for the anorthositic and layered series of the Duluth Complex constrain these units to have been emplaced ca. 1096 Ma in <1 m.y. (duration of 500 ± 260 k.y.). Comparison of paleomagnetic data from these units with Laurentia's apparent polar wander path supports this interpretation. This rapid emplacement bears similarities to the geologically short duration of well-dated large igneous provinces. These data support hypotheses that call upon the co-location of lithospheric extension and anomalously hot upwelling mantle. This rapid magmatic pulse occurred >10 m.y. after initial magmatism following >20° of latitudinal plate motion. A likely scenario is one in which upwelling mantle encountered the base of Laurentian lithosphere and flowed via "upside-down drainage" to locally thinned lithosphere of the Midcontinent Rift. [ABSTRACT FROM AUTHOR]

We present chemostratigraphy, biostratigraphy, and geochronology from a succession that spans the Ediacaran-Cambrian boundary in Sonora, Mexico. A sandy hematite-rich dolostone bed, which occurs 20 m above carbonates that record the nadir of the basal Cambrian carbon isotope excursion within the La Ciénega Formation, yielded a maximum depositional age of 539.40 ± 0.23 Ma using U-Pb chemical abrasion-isotope dilution-thermal ionization mass spectrometry on a population of sharply faceted volcanic zircon crystals. This bed, interpreted to contain reworked tuffaceous material, is above the last occurrences of late Ediacaran body fossils and below the first occurrence of the Cambrian trace fossil Treptichnus pedum, and so the age calibrates key markers of the Ediacaran-Cambrian boundary. The temporal coincidence of rift-related flood basalt volcanism in southern Laurentia (>250,000 km3 of basalt), a negative carbon isotope excursion, and biological turnover is consistent with a mechanistic link between the eruption of a large igneous province and end-Ediacaran extinction. [ABSTRACT FROM AUTHOR]

Well-preserved strata of the late Tonian Chuar Group exposed in the Grand Canyon host fossil evidence for the development of eukaryotic predation, the presence of unique biomarkers, and large changes in C, S, and Mo isotope chemostratigraphy. Despite the importance of this critical succession, few radioisotopic age constraints are available to place these records into a global context. Here, we couple high-precision U-Pb chemical abrasion-isotope dilution-thermal ionization mass spectrometry (CA-ID-TIMS) on zircon crystals with the rhenium-osmium (Re-Os) sedimentary and sulfide geochronometer to refine the temporal framework of this pivotal interval of Earth history. Zircons recovered from a tuff within the uppermost Walcott Member of the Kwagunt Formation yield a weighted mean 206Pb-238U age of 729.0 ± 0.9 Ma (mean square of weighted deviates [MSWD] = 0.86), differing significantly from the previous air-abrasion upper-intercept age of 742 ± 6 Ma on zircons from this same horizon. Organic-rich carbonates from the Carbon Canyon Member of the Galeros Formation yield a model 1 Re-Os age of 757.0 ± 6.8 Ma (MSWD = 0.47, n = 8), and an initial Os isotope (187Os/188Os [Osi]) composition of 1.13 ± 0.02. The radiogenic Osi value from this horizon suggests that the basin was restricted from the open ocean during de-position of the Carbon Canyon Member, in agreement with sedimentological and stratigraphic data. The Re-Os geochronology of marcasite (FeS2) nodules from the Awatubi Member of the Kwagunt Formation yield a model 1 age of 751.0 ± 7.6 Ma (MSWD = 0.37, n = 5), with an Osi of 0.44 ± 0.01. This Re-Os date is interpreted to constrain the growth of the marcasite nodules in the Awatubi Member during deposition. The formation of sulfides and the less radiogenic Osi value are consistent with an influx of sulfate-laden seawater to the basin during depOsition of the Kwagunt Formation. Attempts to apply the Re-Os geochronometer to the Walcott and Tanner Members of the Chuar Group failed to yield meaningful ages, despite elevated Re enrichments (>20 ng/g). The Re-Os data from these units yielded negative Osi values, which suggest disturbance to the Re-Os system. The low Os abundances (typically <100 pg/g) relative to the amount expected based on the elevated Re abundances suggest leaching of Os due to oxidative weathering on geologically recent time scales. Finally, the Carbon Canyon Member provides a useful case study for quantifying how input uncertainties in the Re-Os geochronometer propagate into the resulting age uncertainty, and we discuss the protocols that will yield the best improvement in age precision for future studies. The U-Pb and Re-Os geochronology presented here illustrates the power of coupling these systems and the importance of recent improvements in both methods. Our analysis suggests that for our data, the most efficient way of reducing uncertainties in the presented Re-Os dates is through improved precision of measured Os values. [ABSTRACT FROM AUTHOR]

The Salamanca Formation of the San Jorge Basin (Patagonia, Argentina) preserves critical records of Southern Hemisphere Paleo cene biotas, but its age remains poorly resolved, with estimates ranging from Late Cretaceous to middle Paleocene. We report a multi-disciplinary geochronologic study of the Salamanca Formation and overlying Río Chico Group in the western part of the basin . New constraints include (1) an 40Ar/39Ar age determination of 67.31 ±0.55 Ma from a basalt fl ow underlying the Salamanca Formation, (2) micropaleontological results indicating an early Danian age for the base of the Salamanca Formation, (3) laser ablation HR-MC-ICP-MS (high resolutionmulti collector-inductively coupled plasmamass spectrometry) U-Pb ages and a highresolution TIMS (thermal ionization mass spectrometry) age of 61.984 ±0.041(0.074) [0.100] Ma for zircons from volcanic ash beds in the Peñas Coloradas Formation (Río Chico Group), and (4) paleomagnetic results indicating that the Salamanca Formation in this area is entirely of normal polarity, with reversals occurring in the Río Chico Group. Placing these new age constraints in the context of a sequence stratigraphic model for the basin, we correlate the Salamanca Formation in the study area to Chrons C29n and C28n, with the Banco Negro Inferior (BNI), a mature widespread fossiliferous paleosol unit at the top of the Salamanca Formation, corresponding to the top of Chron C28n. The diverse paleo botanical assemblages from this area are here assigned to C28n (64.67- 63.49 Ma), 2-3 million years older than previously thought, adding to growing evidence for rapid Southern Hemisphere fl oral recovery after the Cretaceous-Paleogene extinction. Important Peligran and "Carodnia" zone vertebrate fossil assemblages from coastal BNI and Peñas Coloradas exposures are likely older than previously thought and correlate to the early Torrejonian and early Tiffanian North American Land Mammal Ages, respectively. [ABSTRACT FROM AUTHOR]

Cenozoic South American Land Mammal Ages (SALMAs) have historically been correlated to the geologic time scale using 40Ar/39Ar dating and magnetostratigraphy. At Gran Barranca (68.7°W, 45.7°S)--one of South America's key areas for constraining SALMAs--existing radioisotopic ages have uncertainties of up to 4 m.y. To better constrain the ages of mammalian assemblages, we employed high-precision (±<40 k.y.) U-Pb dating using single zircon crystals. We dated nine tuffs from the Sarmiento Formation containing middle Eocene-early Miocene faunas (Barrancan, Mustersan, Tinguirirican, Deseadan, Colhuehuapian, and "Pinturan"). The new dates span from 39.861 ± 0.037 Ma to 19.041 ± 0.027 Ma. The La Cancha Tuff, occurring within the Tinguirirican faunal level yielded an age of 33.581 ± 0.015 Ma, confirming that the Vera Member contains the only fossiliferous geologic section encompassing the Eocene-Oligocene transition in the Southern Hemisphere. The pre-Deseadan fauna, La Cantera, is ≤ 30.77 Ma, the age of the Colhuehuapian is expanded to 21.1-20.1 Ma, and the Pinturan may be as old as ca. 19 Ma. The new U-Pb dates confirm that atmospheric temperatures and vegetation remained constant across the Eocene-Oligocene transition in Patagonia and that hypsodonty occurred in South American ungulates much earlier than on any other continent. Additionally, refinement of the SALMA boundaries will eventually provide the context necessary to compare faunal transitions across continents, although currently too much data are missing to allow such comparisons. Finally, the new ages provide a high-resolution age model from which hypotheses about rates of environmental and evolutionary change at Gran Barranca can be tested. [ABSTRACT FROM AUTHOR]