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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]

U-Pb age spectra of detrital zircons in samples from the Paleogene Colton Formation in the Uinta Basin of northeastern Utah and the Late Cretaceous McCoy Mountains Formation of southwestern Arizona (United States) are statistically indistinguishable. This finding refutes previous inferences that arkosic detritus of the Colton was derived from cratonic basement exposed by Laramide tectonism, and instead establishes the Cordilleran magmatic arc (which also provided sediment to the McCoy Mountains Formation) as the primary source. Given the existence of a north-south-trending drainage divide in eastern Nevada and the north-northeast direction of Laramide paleoflow throughout Arizona and southern Utah, we infer that a large river system headed in the arc of the Mojave region flowed northeast ~700 km to the Uinta Basin. Named after its source area, this Paleogene California River would have been equal in scale but opposite in direction to the modern Green River-Colorado River system, and the timing and causes of the subsequent drainage reversal are important constraints on the tectonic evolution of the Cordillera and the Colorado Plateau. © 2010 Geological Society of America.

Isotopic and elemental records of authigenic calcite from lacustrine deposits in the intraforeland basins of Utah were analyzed in an effort to reconstruct the regional paleoclimate, paleohydrology, and paleotopography of the early Cenozoic central North American Cordillera. Isotopic profiles for Paleogene Lakes Uinta, Flagstaff, and Claron show relatively large oxygen isotopic shifts that are diachronous among basins with an ∼7% decrease in δ18 Ocalcite values at ca. 45 Ma in Lake Flagstaff, an ∼5% decrease in δ18Ocalcite values between ca. 42 and 35 Ma in Lake Claron, and an ∼6% decrease in δ18Ocalcite values between ca. 44 and 43 Ma in Lake Uinta. We interpret these negative oxygen isotopic shifts to be the combined result of increased hypsometric mean elevation of basin catchments and related freshening associated with basin infilling. The basins studied also have undergone periods of intense evaporation during periods of hydrologic closure, which, for example, produced an ∼7% increase in δ18Ocalcite values in Lake Uinta beginning at ca. 51 Ma. Hydrologic closure in the Uinta Basin probably resulted from growth of local topography that diverted previously substantial inflows from low elevation regions within the foreland. This study adds to the growing body of evidence that suggests a pattern of along-strike variations in the timing of topographic development and dissection of the Cordilleran landscape during the early Cenozoic. We favor an interpretation that calls for the middle Eocene rearrangement of regional drainage patterns such that intraforeland basins that once received waters from far-flung foreland river systems became dominated by inflows of low δ18O waters from catchments with higher hypsometric mean elevations that drained the adjacent hinterland and/or basin-bounding uplifts. This drainage reorganization is analogous but subsequent to the large-scale integration of catchments in the northern Cordillera that has been recognized on the basis of isotopic and sedimentological evidence in Montana and Idaho at ca. 50-47 Ma, with rivers flowing southeast into Lake Gosiute at ca. 49 Ma and then for a time reaching Lake Uinta, causing a prominent highstand in that lake at 48.6 Ma. The negative oxygen isotopic shifts presented herein, which occurred between ca. 45 and ca. 35 Ma in the intraforeland basins of Utah, may reflect the north-to-south progression of drainage integration in the Cordillera as magmatism and related topography swept southward through the hinterland and increased the hypsometric mean of catchments that fed subjacent intraforeland basins. © 2008 Geological Society of America.

Using multiple isotope systems, we examine the complex effects of drainage reorganization in the Laramide Foreland in the context of stable isotope paleoaltimetry. Strontium, oxygen and carbon isotopic data from lacustrine carbonates formed in the southwestern Uinta Basin, Utah between the Late Cretaceous and late Middle Eocene reveal a two stage expansion in the drainage basin of Lake Uinta beginning at ~ 53 Ma culminating in the Mahogany highstand at 48.6 Ma. A marked increase in 87Sr/86Sr ratios of samples from the Main Body of the Green River Formation is interpreted as the result of water overflowing the Greater Green River Basin in Wyoming and entering Lake Uinta from the east via the Piceance Creek Basin of northwestern Colorado. This large new source of water caused a rapid expansion of Lake Uinta and was accompanied by a significant and rapid increase in the O isotope record of carbonate samples by ~ 6‰. The periodic overspilling of Lake Gosiute probably became continuous at ~ 49 Ma, when the lake captured low-δ18O water from the Challis and Absaroka Volcanic Fields to the north. However, evaporation in the Greater Green River and Piceance Creek Basins meant that the waters entering Lake Uinta were still enriched in 18O. By ~ 46 Ma, inflows from the Greater Green River Basin ceased, resulting in a lowstand of Lake Uinta and the deposition of bedded evaporites in the Saline Facies of the Green River Formation. We thus show that basin development and lake hydrology in the Laramide foreland were characterized by large-scale changes in Cordilleran drainage patterns, capable of confounding paleoaltimetry studies premised on too few isotopic systems, samples or localities. In the case of the North American Cordillera of the Paleogene, we further demonstrate the likelihood that (1) topographic evolution of distal source areas strongly influenced the isotopic records of intraforeland basins and (2) a pattern of drainage integration between the hinterland and foreland may correlate in space and time with the southward sweep of hinterland magmatism. © 2008 Elsevier B.V. All rights reserved.

Oxygen isotope values from lacustrine carbonate in the Laney Member of the Green River Formation (Wyoming) exhibit a sudden, basinwide, ~6[per thousand] upsection decrease in [delta][sup.18]O at ca. 49 Ma. [sup.40]Ar/[sup.39]Ar geochronology constrains the duration of the isotopic shift to [less than or equal to] ~200,000 years. This change coincides with a sudden change in lake type, from balanced-filled in the lower LaClede Bed to overfilled in the upper LaClede Bed, as well as an increase in the proportion of calcitic (>80% calcite out of total carbonate by X-ray diffraction [XRD]) samples from 32% to 73%. The [delta][sup.18]O shift is correlatable through several locations across the Greater Green River Basin, and also coincides with a previously observed shift to less radiogenic [sup.87]Sr/[sup.86]Sr. Minimum [delta][sup.18]O values observed are the same as values previously reported in aragonitic bivalves from the same unit, indicating that low [delta][sup.18]O in this record is not diagenetic. A simultaneous shift to evaporative conditions in the Uinta Basin to the south indicates that the [delta][sup.18]O shift and lake-type change are not driven by regional climatic cooling and/or humidity increase. We propose that all of these observations resulted from the capture by Lake Gosiute of a river that drained higher elevations in central or north-central Idaho. Mass-balance modeling of Eocene Lake Gosiute indicates that capture of a river with an annual average discharge of ~20 billion [m.sup.3]/a (slightly larger than the modern Snake River) and [delta][sup.18]O of -24[per thousand] standard mean ocean water (SMOW) or lower would be capable of producing the observed change. A more likely alternative is a river with less negative [delta][sup.18]O and greater discharge. For example, if river waters had a [delta][sup.18]O composition of ~-16[per thousand], an estimated river discharge of [greater than or equal to]50 billion [m.sup.3]/a would produce the same effect. doi: 10.1130/B26522.1

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 Permian marks an important, yet poorly understood, tectonic transition in the Tian Shan region of northwestern China between Devonian-Carboniferous continental amalgamation and recurrent Mesozoic-Cenozoic intracontinental orogenic reactivation. The Turpan-Hami basin accommodated up to 3000 m of sediment and is ideally positioned to provide constraints on this transition. New stratigraphic data and mapping indicate that extension dominated Early Permian tectonics in the region, whereas flexural, foreland subsidence controlled Late Permian basin evolution. Lower Permian strata in the northwestern Turpan-Hami basin consist of coarsegrained debris-flow and alluvial-fan deposits interbedded with mafic to intermediate volcanic sills and flows. In contrast, Lower Permian rocks in the north-central and northeastern Turpan-Hami basin unconformably overlie a Late Carboniferous volcanic arc sequence. These Lower Permian strata include possible shallow-marine carbonate rocks and thick volcanic and volcaniclastic rocks that are in turn overlain by littoral- to profundal-lacustrine facies. Above a regional Lower Permian/Upper Permian unconformity, regional sedimentation patterns record the development of a more integrated sedimentary basin. The Upper Permian is entirely nonmarine and can be correlated east-west along the depositional strike of the basin. The lower Upper Permian consists of a broad belt of braided fluvial deposits shed northward. These strata are overlain by fluctuating littoral--and profundal-lacustrine facies and associated fluvial facies. The uppermost Permian is characterized by shallow lakeplain and fluvial environments. The Early Permian association of diffuse volcanism and partitioning of subbasins by normal faulting is consistent with an early phase of lithospheric extension. Local relationships indicate west-east extension in the Turpan-Hami basin along faults oriented normal to Late Devonian-Carboniferous collisional sutures within the Tian Shan. The cause of extension in the wake of Carboniferous orogenesis remains enigmatic. However, the temporal and spatial relationships of the two strain regimes suggest that they are genetically related. Upper Permian stratigraphy and unconformities and local Late Permian-Triassic contractional deformation record foreland-basin development when the Turpan-Hami region became a wedge-top basin with respect to the north Tian Shan fold-and-thrust belt. Flexurally induced Late Permian subsidence is also manifested in the larger Junggar basin to the north, where >4000 m of strata are preserved in the foredeep region. The Turpan-Hami and Junggar basins were depositionally connected for much of the Late Permian when a vast lacustrine system developed across northwestern China. This lacustrine paleogeography was only occasionally interrupted, possibly by structural damming during uplift of the orogenic wedge. Keywords: Asia, Junggar basin, lacustrine, stratigraphy, Tian Shan, Xinjiang.

Sr isotope ratios in carbonate-rich lacustrine strata provide highly resolved and geographically specific records of past changes in weathering and regional drainage patterns. The Eocene Green River Formation is perhaps the best documented pre-Quaternary lacustrine unit in the world; therefore, these strata are ideally suited for studying the behavior of Sr. The [sup.87]Sr/[sup.86]Sr ratios measured for primary carbonate in lake expansion-contraction cycles of the Laney Member are directly linked to changing lake facies. Four distinct cycles are preserved in a 6.4 m interval of the Arco Washakie Basin No. 1 core, each represented by a vertical succession of transgressive stromatolite facies containing dolomicritic intraclasts, laminated micrite facies deposited during lake highstands, and lake-marginal dolomicrite facies, which represent subsequent lowstands. Initial [sup.87]Sr/[sup.86]Sr ratios range from 0.711 99 to 0.713 31. The least radiogenic isotope compositions are associated with laminated micrite that records increased regional runoff from Phanerozoic marine carbonate and Tertiary volcanic rocks during lacustrine highstands. The most radiogenic isotope compositions are associated with dolomicrite, recording a higher proportion of runoff from basin-bounding Precambrian uplifts and the exposed lake plain during lacustrine lowstand. The observed [sup.87]Sr/[sup.86]Sr variations may also be due in part to changes in differential weathering of the surrounding landscape, where drier climates are associated with decreased differential weathering and, therefore, higher [sup.87]Sr/[sup.86]Sr in runoff. Keywords: strontium, Laney Member, lake, cycle, weathering.

The Late Permian Phosphoria Formation of the western United States contains 1.7 x10 (super.12)t of phosphate and locally as much as 32.9% total organic carbon. The mineral wealth of the Phosphoria Formation is generally attributed to Late Permian upwelling along the western margin of the continent. Unlike in most modern environments, upwelling in the Late Permian occurred proximal to a broad, shallow, evaporative sea rather than an abrupt continental margin. Evaporite deposits that are contemporaneous with the organic- and phosphate-rich strata are present within the Phosphoria and adjacent basins. Elevated concentrations of the bio-marker gammacerane are ubiquitous in the organic-rich facies of the Phosphoria Formation, which suggests that the basin was salinity stratified. Depositional environments incorporating both upwelling and salinity stratification are unusual and poorly documented in modern settings. We infer the presence of salinity stratification in the Phosphoria basin and propose a paleoceanographic model for the Phosphoria sea during the deposition of organic- and phosphate-rich sediment. From the western margin of the continent, upwelling water flowed into the evaporative Phosphoria sea, replacing and overriding hypersaline bottom water that drained westward. This paleoceanographic model of the Phosphoria sea may help to explain the distribution and preservation of organic- and phosphate-rich strata in the Phosphoria Formation.

Previously described organic-rich shale facies of the Meade Peak Member of the Phosphoria Formation at Soda Springs, Idaho, consist of well-sorted, laminated siltstone containing little or no clay. The planar-parallel fabric preserved in many of the siltstone beds suggests suspension settling onto the outer shelf or upper slope, most likely under low oxygen conditions. Evidence for transport of silt to the depositional site by sediment gravity flows is absent, as are sandstone beds. We propose that Meade Peak siltstone facies record subaqueous deposition of windborne silt that was transported southward from central Montana. As such, they provide the first direct geologic evidence of a wind regime favorable for marine upwelling, the process commonly postulated to have localized the deposition of Permian phosphatic sediments. They may also provide indirect evidence of upwind eolian sand transport at that time.