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At geologic timescales, a significant proportion of the sediment eroded from continents is transferred from rivers to littoral cells, then through submarine canyons to deep ocean basins. Accumulation and occasional discharge of sediment from canyon headwall regions is a likely driver for the sediment gravity flows believed to be responsible for much of this mass transport, but the responsible mechanisms and event timescales in canyon heads are imprecisely defined, largely due to the drowning of canyon heads since the last glaciation and the resulting dearth of modern depositional environments to observe. We investigated a littoral cell adjacent to Mattole submarine canyon in Northern California to better understand the influence of bathymetry and coast shape on sediment trajectories and hypothesize that canyon heads promote littoral sediment accumulation due to bathymetric sheltering of the adjacent coast from waves and convergence of littoral cells exposed to varying wave direction. The uppermost gullies of Mattole Canyon extend to the littoral cell, and a train of sediment waves within the uppermost thalweg of Mattole Canyon suggests that gravity-driven flows of shore-derived sediment may have recently occurred. Mattole Canyon and nearby Mendocino Canyon flow into the deep Mendocino Channel, which has the highest observed Late Holocene sediment accumulation rates in Northern California. We documented the beach slope, grain size, and sediment provenance along 15 km of coast adjacent to Mattole Canyon and conducted numerical wave modeling to infer the dominant direction and magnitude of sediment transport, relative wave sheltering by bathymetry, and estimated sediment mobility in the vicinity of the canyon head over a multi-year period. South of the canyon headwall, in the vicinity of the Mattole River outlet, coarser sediment rich in metasandstone clasts and steeper beach slopes coincide with decadal-scale beach accretion. North of the canyon head, sediments are generally finer, beaches are flatter, clast lithologies include more quartz and mudstone grains, and there is decadal-scale net erosion. Wave modeling suggests the Mattole headwall region is subject to sustained sheltering from waves due to canyon bathymetry, littoral sediment convergence above the canyon head for much of a typical year, and occasional bed entrainment of sediment in the canyon's uppermost gullies by large waves. Larger winter waves from the northwest likely result in net southerly drift north of the canyon head, with a high likelihood of preferentially transporting fine-grained, more quartz-rich sediment towards the Mattole headwall. Smaller summer swells from the west and south likely results in net northerly transport of metasandstone clast-rich Mattole River-derived sediment towards the canyon head. The imbalance in seasonality in wave conditions, coast shape, and sediment delivery by the Mattole River and other coastal creeks is broadly consistent with the spatial patterns in grain size, mineralogy, slope, and beach width that we observe in the vicinity of the canyon, and the net delivery of sediment to the Mattole Canyon head. An approximate mass balance of sediment flux from coastal streams feeding Mattole River littoral cell suggests that sediment volumes likely accumulate in the canyon headwall region that could supply several density flow events in a typical decade. These findings highlight the importance of the connectivity of the canyon to the nearshore region for the processes and products in canyons and fans. We hypothesize that feedbacks between canyon bathymetry, sediment accumulation, and mass evacuating flows promotes the long-term persistence of canyon systems. [ABSTRACT FROM AUTHOR]

U-Pb ages of zircon from the Firehole and Analcite ash beds in the Eocene Green River Formation (Wyoming, United States) are indistinguishable from [sup.40]Ar/[sup.39]Ar ages of sanidine after adjusting the latter to the astronomically calibrated age of 28.201 Ma for the Fish Canyon sanidine standard. Six of nine zircon analyses from the Firehole ash yield concordant ages in an overlapping cluster and give a weighted mean [sup.238]U-[sup.206]Pb age of 51.66 [+ or -] 0.20 Ma (full external 2[sigma] uncertainty), indistinguishable from the recalibrated [sup.40]Ar/[sup.39]Ar age of 51.40 [+ or -] 0.25 Ma (full external 2[sigma] uncertainty), which is adjusted to eliminate samples with low radiogenic [sup.40][Ar.sup.*]. Significant [sup.238]U-[sup.206]Pb age scatter likely reflects some combination of minor inheritance, Pb loss, and possibly magma chamber residence. Seven of eight analyses of Analcite ash zircon yield a weighted mean [sup.238]U-[sup.206]Pb age of 49.23 [+ or -] 0.13 Ma (full external 2[sigma] uncertainty), whereas the sanidine [sup.40]Ar/[sup.39]Ar age is 49.24 [+ or -] 0.18 Ma (full external 2[sigma] uncertainty). Calibrating Green River Formation [sup.40]Ar/[sup.39]Ar ages to the 28.201 Ma age for Fish Canyon sanidine permits the first direct comparison of specific Green River Formation strata to the astronomical solution for Early Eocene insolation. This comparison supports the hypothesis that periods of fluvial deposition coincided with minima in long and short eccentricity, and that periods of lake expansion and evaporite deposition correspond to eccentricity maxima. doi: 10.1130/G30630.1

Sedimentary facies of the Eocene Green River Formation reflect a rapid increase in water supply to Lake Gosiute ca. 49 Ma, marked by a stratigraphic fill-to-spill surface. Deposits below this surface constitute repetitive lacustrine expansion-desiccation cycles, whereas those above consist of continuous profundal lacustrine mudstone, grading upward into volcaniclastic deltaic sandstone. Above the fill-to-spill surface, calcitic mudstone [[delta].sup.18]O decreases from ~+26[per thousand] to +20[per thousand] over an interval representing ~100 k.y. We interpret this shift to have resulted from capture of a foreland river (or rivers) that drained higher topography north of Lake Gosiute, most likely in north-central Idaho. Accurate paleoelevation estimates derived from stable isotopic records in intermontane basins thus may require detailed knowledge of regional drainage systems. Keywords: Green River Formation, drainage, lacustrine, carbonate, Laramide, paleoaltimetry.

Precise dating of K-rich minerals in volcanic rocks via the [sup.40]Ar/[sup.39]Ar geochronometer has become crucial to resolving many geological problems. In some cases, ash beds containing biotite phenocrysts but lacking sanidine are the only datable horizons at key stratigraphic intervals, necessitating comparison of sanidine and biotite ages within the same chronostratigraphic framework. To assess the integrity of biotite and evaluate the accuracy of ages obtained from this often problematic mineral, incremental heating [sup.40]Ar/[sup.39]Ar experiments were performed on large, millimeter-sized, euhedral biotite crystals from four key Eocene tuffs in the Green River and Wind River Basins and are compared with sanidine [sup.40]Ar/[sup.39]Ar ages. Unaltered biotite crystals with homogenous K-compositions from two tuff beds yielded concordant plateau ages that are indistinguishable from cogenetic sanidine ages. In contrast, biotite crystals from two other tuffs yielded discordant spectra, with relatively young initial steps followed by older, down-stepping apparent ages. Plateau ages from the discordant experiments are older than the sanidine ages by I to 14%. Integrated (total fusion) ages from these experiments exhibit scatter toward both younger and older ages that correlate with the degree of spectral discordance. Electron microprobe transects reveal that biotite crystals yielding discordant age spectra contain 1-10 [micro]m thick K-depleted ( Keywords: Biotite, sanidine, [sup.40]Ar/[sup.39]Ar, recoil, Bridger Formation, Wagon Bed Formation, Wind River Formation, EPMA, tephra, turf, volcanic ash

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

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

Erosion of Laramide-style uplifts in the western United States exerted an important first-order influence on Paleogene sedimentation by controlling sediment supply rates to adjacent closed basins. During the latest Cretaceous through Paleocene, these uplifts exposed thick intervals of mud-rich Upper Cretaceous foreland basin fill, which was quickly eroded and redeposited. Cretaceous sedimentary lithologies dominate Paleocene conglomerate clast compositions, and the volume of eroded foreland basin strata is approximately twice the volume of preserved Paleocene basin fill. As a result of this sediment oversupply, clastic alluvial and paludal facies dominate Paleocene strata, and are associated with relatively shallow and ephemeral freshwater lake facies. In contrast, large, long-lived, carbonate-producing lakes occupied several of the basins during the Eocene. Basementderived clasts (granite, quartzite, and other metamorphic rocks) simultaneously became abundant in lower Eocene conglomerate. We propose that Eocene lakes developed primarily due to exposure of erosion-resistant lithologies within cores of Laramide uplifts. The resultant decrease in erosion rate starved adjacent basins of sediment, allowing the widespread and prolonged deposition of organic-rich lacustrine mudstone. These observations suggest that geomorphic evolution of the surrounding landscape should be considered as a potentially important influence on sedimentation in many other interior basins, in addition to more conventionally interpreted tectonic and climatic controls. Keywords: Fort Union Formation, Green River Formation, Paleocene, Eocene, lacustrine.

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 [sup.40]Ar/[sup.39]Ar 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. [sup.40]Ar/[sup.39]Ar age determinations from single-crystal and Our [sup.40]Ar/[sup.39]Ar-based age model indicates that sediment accumulated three times more rapidly (327 [+ or -] 86 [micro]m/yr) during the evaporative Wilkins Peak phase than the freshwater to saline Tipton (88 [+ or -] 34 [micro]m/ yr) and Laney (104 [+ or -] 18 [micro]m/yr) phases. The much lower accumulation rates for the Tipton and Laney Members are permissive of an annual origin for 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 [sup.40]Ar/[sup.39]Ar 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. Keywords: Green River Formation, sedimentation rates, lacustrine, [sup.40]Ar/[sup.39]Ar, Wasatchian-Bridgerian, Early Eocene Climate Optimum.

Cyclic strata exposed in the Inyo Mountains of eastern California contain a continuous 6 m.y. record of deep marine deposition that spans the Pennsylvanian-Permian boundary. To better understand the geologic evolution of southwest Laurentia and the role of glacially driven eustasy in upper Paleozoic stratigraphy, we measured two detailed ~600 m composite stratigraphic sections of the Keeler Canyon Formation and collected a handheld spectral gamma ray log. Post-depositional deformation complicates field relationships, but 1:5000 scale mapping of faults and folds permits assembly of two continuous sections. Measured strata alternate at the 5-20 m scale between intervals of fine-grained laminated marlstone and intervals of mixed carbonate and siliciclastic turbidites and debrites. Based on facies characteristics and a prominent marker horizon, we reassign the Pennsylvanian-Permian age upper Salt Tram unit of the upper Keeler Canyon Formation to a new Estelle Member. We estimate sediment accumulation rates within the Keeler Canyon Formation using published conodont biostratigraphy and correlative U-Pb geochronology from Eastern Europe combined with spectral analysis and timescale optimization using the Astrochron R package. Evolutive harmonic analysis of gamma ray-derived element concentrations reveals prominent bundled periodicities that are consistent with both long and short eccentricity cycles. Average sediment accumulation rates calculated using the time scale optimization function of Astrochron suggest a gradual increase from 40-60 m/m.y. to ~120 m/m.y. during the late Pennsylvanian and then a minima of ~50 m/m.y. near the Pennsylvanian-Permian boundary, which is followed by an increase to ~175 m/m.y. into the Early Permian. This trend in sediment accumulation rates and subsequent Permian contractile deformation are compatible with flexural subsidence in a SW-migrating foreland basin related to the southern part of the late Antler orogenic system. [ABSTRACT FROM AUTHOR]

Eocene strata of the Elko Formation record lacustrine deposition within the Nevada hinterland of the North American Cordillera. We present a detailed geochemical stratigraphy enabled by high‐sampling‐resolution geochronology from lacus trine limestone and interbedded volcanic rocks of the Elko Formation. Two intervals of lacustrine deposition, an early Eocene "Lake Adobe" of limited aerial extent and a laterally extensive middle Eocene "Lake Elko," are separated by ∼5 m.y. of apparent unconformity. Sediments deposited in the apparently short‐lived (49.5–48.5 Ma) early Eocene Lake Adobe exhibit high‐amplitude covariation of δ18O, δ13C and 87Sr/86Sr, which suggests a dynamically changing catchment and precipitation regime. Lake Elko formed during the middle Eocene, and its strata record three geochemically distinct phases, indicating it was a single interconnected water body that became increasingly evaporative over time. The lower Elko Formation (44.0–42.5 Ma) was deposited in a freshwater lake. Middle Elko Formation (42.5–41.2 Ma) lithofacies and geochemistry suggest that an increasingly saline and alkaline Lake Elko experienced salinity stratification‐induced hypolimnion disoxia and burial of 12C‐rich organic matter. The upper Elko Formation (41.2–40.5 Ma) records a shallow final phase of Lake Elko that experienced short residence times and a breakdown in stratification. A sharp decline of 87Sr/86Sr in the upper Elko Formation reflects an increasing aerial extent of low‐87Sr/86Sr volcanic deposits from nearby calderas. Middle Eocene strata record ponding of paleodrainage, increasing hydrologic isolation and volcanism, consistent with progressive north to south removal of the Farallon flat slab and/or delamination of the lower lithospheric mantle of the North American plate. Plain Language Summary: Large lakes formed in the region that is now northeast Nevada between 50 and 40 million years ago. This region was at high elevation and undergoing a transition from compressional to extensional plate tectonic forces at that time. The geologic record shows that lakes formation started locally at 49.5 million years ago with a short‐lived freshwater Lake Adobe. Later, approximately 43.5 million years ago, a broad Lake Elko formed and then became increasingly evaporative over the next 3 million years, likely due to removal of the shallowly subducting Farallon slab from beneath the region. Key Points: Lakes formed in the hinterland of the North American Cordillera during the Eocene during a transition from contraction to extensionLake Elko expanded and became increasingly evaporative across 3.5 Ma from 43.5–40.5 Ma, coincident with an episode of explosive siliceous magmatismLake formation and drainage isolation likely records the progressive removal of the flattened Farallon slab from beneath the Cordillera [ABSTRACT FROM AUTHOR]

The Bighorn basin, Wyoming preserves complete records of middle Paleocene to lower Ecocence continental data and its geochronology is studied by geometric polarity time scale (GPTS). The biostratigraphic revision reveal that Mason Pocket, the primary fauna upon which the Tiffanian land-mammal age was originally based, is correlative to Chron 26and not Chron 25 and the age calibration of the biozone provides a robust framework for testing the synchrony of taxonomic first and last appearances.

U-Pb ages of zircon from the Firehole and Analcite ash beds in the Eocene Green River Formation (Wyoming, United States) are indistinguishable from 40Ar/39Arages of sanidine after adjusting the latter to the astronomically calibrated age of 28.201 Ma for the Fish Canyon sanidine standard. Six of nine zircon analyses from the Firehole ash yield concordant ages in an overlapping cluster and give a weighted mean 238U-206Pb age of 51.66 ± 0.20 Ma (full external 2s uncertainty), indistinguishable from the recalibrated 40Ar/39Ar age of 51.40 ± 0.25 Ma (full external 2s uncertainty), which is adjusted to eliminate samples with low radiogenic 40Ar*. Signifi cant 238U-206Pb age scatter likely reflects some combination of minor inheritance, Pb loss, and possibly magma chamber residence. Seven of eight analyses of Analcite ash zircon yield a weighted mean 238U-206Pb age of 49.23 ± 0.13 Ma (full external 2s uncertainty), whereas the sanidine 40Ar/39Ar age is 49.24 ± 0.18 Ma (full external 2σ uncertainty). Calibrating Green River Formation 40Ar/39Ar ages to the 28.201 Ma age for Fish Canyon sanidine permits the fi rst direct comparison of specifi c Green River Formation strata to the astronomical solution for Early Eocene insolation. This comparison supports the hypothesis that periods of fl uvial deposition coincided with minima in long and short eccentricity, and that periods of lake expansion and evaporite deposition correspond to eccentricity maxima. [ABSTRACT FROM AUTHOR]

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

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 [sup 40]Ar/[sup 39]Ar 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 refractiveindex oils were required to obtain clean, unaltered phenocrysts of sanidine. [sup 40]Ar/[sup 39]Ar age determinations from single-crystal and < 1 mg multigrain aliquots of sanidine and biotite allowed the identification and exclusion of xenocrystic contamination. Laserfusion experiments on phenocrysts from the Rife, Firehole, C Bed, Grey, Main, Sixth, and Analcite tuff beds from the Tipton, Wilkins Peak, and Laney Members yielded weighted-mean ages (±2σ analytical uncertainties) of 51.25 ± 0.31 Ma, 50.70 ± 0.14 Ma, 50.56 ± 0.26 Ma, 50.39 ± 0.13 Ma, 49.96 ± 0.08 Ma, 49.70 ± 0.10 Ma, and 48.94 ± 0.12 Ma, respectively. Ages for sanidine and biotite from the Main tuff are indistinguishable when presumably xenocrystic contaminants are excluded from the age calculation. Moreover, the [sup 40]Ar/[sup 39]Ar ages are consistent with the stratigraphic order of the tuff beds and with provenance in the Absaroka and Challis volcanic fields. Our [sup 40]Ar/[sup 39]Ar-based age model indicates that sediment accumulated three times more rapidly (327 ± 86 µm/yr) during the evaporative Wilkins Peak phase than the freshwater to saline Tipton (88 ± 34 µm/ yr) and Laney (104 ± 18 µm/yr) phases. The much lower accumulation rates for the Tipton and Laney Members are permissive of an annual origin for < 1-mm-thick laminae and precessional forcing of... [ABSTRACT FROM AUTHOR]

Weathering, erosion, and sediment transport in modern landscapes may be investigated via direct observation of attributes such as elevation, relief, bedrock lithology, climate, drainage organization, watershed extent, and others. Studies of ancient landscape evolution lack this synoptic perspective, however, and instead must rely more heavily on downstream records of fluvial deposits. Provenance analysis based on detrital grain ages has greatly enhanced the utility of such records but has often focused broadly on regional to continental scales. This approach may overlook important details of localized watersheds, which could lead to significant misinterpretation of past sediment dispersal patterns. The present study, therefore, explores the impact of geographic and stratigraphic sampling density on detrital zircon provenance, based on a high-density investigation of U-Pb ages (N = 23, n = 4905) obtained from a narrow chronostratigraphic range (~2 m.y.) within a relatively small (~25,000 km²) area of an Eocene nonmarine sedimentary basin. Based on multidimensional scaling and DZmix modeling, these strata comprise seven distinct, approximately isochronous detrital zircon (DZ) chronofacies, defined as ". . . a group of sedimentary rocks that contains a specified suite of detrital zircon age populations" (Lawton et al., 2010). Four of these DZ chronofacies reflect long-distance transport from extrabasinal source areas. DZ chronofacies CO-1 and CO-2 are interpreted to derive from a primary sediment source in central Colorado (USA), corroborating previously proposed long-distance sediment transport via the Aspen paleoriver. DZ chronofacies ID-1 and ID-2 are interpreted to have been delivered to the basin from central Idaho by the Idaho paleoriver. In contrast, DZ chronofacies UT-1 and UT-2 are interpreted to reflect local drainage from the Uinta Uplift south of the basin, and DZ chronofacies WY-1 is interpreted to have been sourced from the Rawlins, Granite, and Sierra Madre uplifts to the north and east via the Toya Puki paleoriver. Lateral transitions between different DZ chronofacies in some cases occur over distances as little as 5 km, implying that depositional systems carrying sand from disparate watersheds directly competed to fill available basin accommodation. The results of this study reveal a high degree of complexity of Eocene rivers that converged on the Greater Green River Basin, indicating that their deposits contain a rich record of finescale landscape evolution across much of the Laramide foreland and Cordilleran orogen. These results illustrate the need for adequate sample density when assessing basin-scale provenance and offer a cautionary consideration for researchers using sandstone (and incorporated authigenic cement) in other nonmarine basins as the basis for paleoaltimetry or detrital thermochronology studies. [ABSTRACT FROM AUTHOR]

Completion of Glen Canyon Dam in 1963 resulted in complete elimination of sediment delivery from the upstream Colorado River basin to Grand Canyon and nearly complete control of spring snowmelt floods responsible for creating channel and bar morphology. Management of the river ecosystem in Grand Canyon National Park now relies on dam-release floods to redistribute tributary-derived sediment accumulated on the channel bed to higher-elevation sandbars. Here, we used multivariate mixing analysis of sediment elemental compositions to evaluate the extent to which flood deposits derive from tributary-supplied sand compared to reworked, relict predam sediment. The concentrations of seven major and trace elements (Fe, Ca, K, Ti, Rb, Sr, and Zr) were measured in very fine-, fine-, and medium-grained sand from flood deposits using X-ray fluorescence and interpreted using a Bayesian mixing model to characterize the proportion of sand originating from the Paria River, the only major tributary within the study reach. Flood deposits from the 2013 and 2014 controlled floods contained 69% ± 16% and 84% ± 20% Paria River-derived material, respectively, with substantial variation among sites. Based on a sand mass balance, we calculated that under decreasing storage conditions since 1963, ~77%-83% of the annual Paria River sand flux needs to be retained within the mass of active sand stored in Marble Canyon each year to reach the observed concentration of Paria River sand at sample locations. This finding suggests that the use of controlled floods may continue to be effective for sandbar maintenance, provided sand inputs from the Paria River do not decline. [ABSTRACT FROM AUTHOR]

An abstract of the article "Precise dating of biotite in distal volcanic ash: Isolating subtle alteration using 40Ar/39Ar laser incremental heating and electron microprobe techniques," by M. Elliot Smith, Brad S. Singer, Alan R. Carroli, and John H. Fournelle is presented.

Basins in orogenic hinterlands are directly coupled to crustal thickening and extension through landscape processes and preserve rec ords of deformation that are unavailable in footwall rocks. Following prolonged late Meso zoic--early Cenozoic crustal thickening and plateau construction, the hinterland of the Sevier orogen of western North America underwent late Cenozoic extension and formation of metamorphic core complexes. While the North American Cordillera is one of Earth's best-studied orogens, estimates for the spatial and temporal patterns of initial extensional faulting differ greatly and thus limit understanding of potential drivers for deformation. We employed (U-Th)/(He-Pb) double dating of detrital zircon and (U-Th)/He thermo chronology of detrital apatite from precisely dated Paleogene terrestrial strata to quantify the timing and magnitude of exhumation and explore the linkages between tectonic unroofing and basin evolution in northeastern Nevada. We determined sediment provenance and lag time evolution (i.e., the time between cooling and deposition, which is a measure of upper-crustal exhumation) during an 8 m.y. time span of deposition within the Eocene Elko Basin. Fluvial strata deposited between 49 and 45 Ma yielded Precambrian (U-Th)/He zircon cooling ages (ZHe) with 105-740 m.y. lag times dominated by unreset detrital ages, suggesting limited exhumation and Proterozoic through early Eocene sediment burial (<4--6 km) across the region. Minimum nonvolcanic detrital ZHe lag times decreased to <100 m.y. in 45-43 Ma strata and to <10 m.y. in 43-41 Ma strata, illustrating progressive and rapid hinterland unroofing in Eocene time. Detrital apatite (U-Th)/He ages present in ca. 44 and 39 Ma strata record Eocene cooling ages with 1--20 m.y. lag times. These data reflect acceleration of basement exhumation rates by >1 km/m.y., indicative of rapid, large-magnitude extensional faulting and metamorphic core complex formation. Contemporaneous with this acceleration of hinterland exhumation, syntectonic freshwater lakes developed in the hanging wall of the Ruby Mountains--East Humboldt Range metamorphic core complex at ca. 43 Ma. Volcanism driven by Farallon slab removal migrated southward across northeastern Nevada, resulting in voluminous rhyolitic eruptions at 41.5 and 40.1 Ma, and marking the abrupt end of fluvial and lacustrine deposition across much of the Elko Basin. Thermal and rheologic weakening of the lithosphere and/or partial slab removal likely initiated extensional deformation, rapidly unroofing deeper crustal levels. We attribute the observed acceleration in exhumation, expansion of sedimentary basins, and migrating volcanism across the middle Eocene to record the thermal and isostatic effects of Farallon slab rollback and subsequent removal of the lowermost mantle lithosphere. [ABSTRACT FROM AUTHOR]

Within extended orogens, records that reflect the driving processes and dynamics of early extension are often overprinted by subsequent orogenic collapse. The Copper Mountains of northeastern Nevada preserve an exceptional record of hinterland extensional deformation and high-elevation basin formation, but current geochronology and thermochronology are insufficient to relate this to broader structural trends in the region. This extension occurred concurrent with volcanism commonly attributed to Farallon slab removal. We combine thermochronology of both synextensional hanging-wall strata and footwall rocks to comprehensively evaluate the precise timing and style of this deformation. Specifically, we apply (U-Th)/(He-Pb) double dating of minerals extracted from Eocene-Oligocene Copper Basin strata with multi-mineral (U-Th)/He and 40Ar/39Ar thermochronology of rocks sampled across an ~20 km transect of the Copper Mountains. We integrate basement and detrital thermochronology records to comprehensively evaluate the timing and rates of hinterland extension and basin sedimentation. Cooling and U-Pb crystallization ages show the Coffeepot Stock, which spans the width of the Copper Mountains, was emplaced at ca. 109-108 Ma, and then cooled through the 40Ar/39Ar muscovite and biotite closure temperatures by ca. 90 Ma, the zircon (U-Th)/He closure temperature between ca. 90 and 70 Ma, and the apatite (U-Th)/He closure temperature between 43 and 40 Ma. Detrital apatite and zircon (U-Th)/(He-Pb) double dating of late Eocene fluvial and lacustrine strata of the Dead Horse Formation and early Oligocene fluvial strata of the Meadow Fork Formation, both deposited in Copper Basin, shows that Early Cretaceous age detrital grains have a cooling history that is analogous to proximal intrusive rocks of the Coffeepot Stock. At ca. 38 Ma, cooling and depositional ages for Copper Basin strata reveal rapid exhumation of proximal source terranes (cooling rate of ~37 °C/m.y.); in these terranes, 8-12 km of slip along the low-angle Copper Creek normal fault exhumed the Coffeepot Stock in the footwall. Late Eocene-early Oligocene slip along this fault and an upper fault splay, the Meadow Fork fault, created a half graben that accommodated ~1.4 km of volcaniclastic strata, including ~20 m of lacustrine strata that preserve the renowned Copper Basin flora. Single-crystal sanidine 40Ar/39Ar geochronology of interbedded tuffs in Copper Basin constrains the onset of rapid exhumation to 38.0 ± 0.9 Ma, indicating that surface-breaching extensional deformation was coincident with intense proximal volcanism. Coarse-grained syndeformational sediments of the Oligocene Meadow Fork Formation were deposited just prior to formation of an extensive regional Oligocene-Miocene unconformity and represent one of the most complete hinterland stratigraphic records of this time. We interpret this history of rapid late Eocene exhumation across the Copper Mountains, coeval volcanism, and subsequent unconformity formation to reflect dynamic and thermal effects associated with Farallon slab removal. The final phase of extension is recorded by late, high-angle normal faults that cut and rotate the early middle Miocene Jarbidge Rhyolite sequence, deposited unconformably in the hanging wall. These results provide an independent record of episodic Paleogene to Miocene exhumation documented in Cordilleran metamorphic core complexes and establish that substantial extension occurred locally in the hinterland prior to province-wide Miocene extensional break-up. [ABSTRACT FROM AUTHOR]