Search

The gneiss complex of Wildhorse Creek (Wildhorse gneiss) forms the central component of the lowest structural plate in the Pioneer metamorphic core complex of south-central Idaho. The oldest rock in the complex is a felsic ortho-gneiss, with Neoarchean U-Pb magmatic zircon ages of 2.60-2.67 Ga. The ortho-gneiss overlaps in age and is interpreted to be part of the Grouse Creek block of the Albion Mountains to the south. This Archean metagranitoid is structurally interleaved with paragneiss containing quartzite and calc-silicate rock. Structurally below the orthogneiss, some quartzites have multiple concordant populations of detrital-zircon grains as young as ca. 1700 Ma, while others have no zircon grains younger than ca. 2500 Ma. Structurally above the Archean gneiss is a heterogeneous paragneiss that contains calc-silicate and quartzitic rocks with detrital zircons as young as ca. 1460 Ma. Amphibolite in this unit contains zircons dated at ca. 1850 Ma, indicating that this rock can be no older than that and implying considerable structural complexity. The upper part of the Wildhorse gneiss contains metaquartzites bearing zircons as young as ca. 1400 Ma. The protolith of this paragneiss is interpreted as the southernmost exposures of the Lemhi subbasin of the Mesoproterozoic Belt Supergroup. The upper Wildhorse gneiss includes ca. 695 Ma intrusive orthogneiss that is coeval with Neoproterozoic rift-related volcanic or intrusive rocks near Pocatello, House Mountain, and Edwardsburg, Idaho. This Cryogenian meta-intrusive rock is the likely source of the 650-710 Ma detrital-zircon population in the Big Lost River that drains the core complex. Initial eHf values from 675 Ma zircons are between 3.4 and -2.4, suggesting the granitoids had a mixed source in both continental crust and juvenile mantle.

Stratigraphic, geochronologic, and geochemical patterns of Neoproterozoic to Cambrian sedimentary and volcanic rocks in Utah, Nevada, and SE Idaho record a dynamically evolving landscape along the North American Cordillera margin, which included: (1) initial development of intracratonic basins with deposition of siliciclastic strata of the Uinta Mountain Group from ~. 770 to 740. Ma; (2) early rifting and volcanism along a N-S (present day geographic coordinates) basin system with deposition of diamictite-bearing strata of the Perry Canyon and related formations from ~. 720 to 660. Ma; (3) early, broad subsidence with deposition of mature siliciclastic strata of the lower Brigham and McCoy Creek groups from ~. 660 to 580. Ma; (4) final rifting, volcanism, and transition to drift with deposition of variably immature siliciclastic strata of the Prospect Mountain and correlative formations from ~. 570 to 520. Ma; and (5) regional subsidence along a passive margin with deposition of Middle Cambrian to Devonian carbonate-rich strata. The Uinta Mountain Group comprises fluvial to marine, feldspathic to quartzose sandstone, conglomerate, and mudstone, with detrital zircon (DZ) patterns recording a mix of local basement sources to the N and distal Laurentian sources to the SE. The lower Perry Canyon and related formations contain variably feldspathic sandstone, quartz-pebble diamictite deposited during an older glacial episode, and mudstone, with DZ patterns recording a mix of distal sources, local basement sources, and sediment recycling during early rifting. The upper Perry Canyon and related formations contain mafic volcanic rocks, polymict diamictite deposited during a younger glacial episode, volcaniclastic wacke, and mudstone, with DZ patterns recording local basement sources along an evolving rift margin and felsic volcanism from ~. 700 to 670. Ma. Mafic volcanic rocks and trachyte to rhyolite clasts in diamictite have geochemical signatures typical of continental ri

The Cryogenian Perry Canyon Formation, formally named herein with a type section located in northern Utah, consists of a 0.3- to 1.5-km-thick succession of diamictite- and volcanic-bearing strata that record glacial events and early rifting along western

Volcanic and diamictite-bearing strata of the Neoproterozoic Pocatello Formation record middle Cryogenian glaciation and alkaline to subalkaline within-plate magmatism during Rodinia rifting. New mapping along the Oxford Ridge segment of the southern Bannock Range in SE Idaho has resolved stratigraphic relationships between the Scout Mountain and underlying Bannock volcanic members of the Pocatello Formation. Bannock Volcanic Member metabasalt has an upper gradational contact with over 250 m of Scout Mountain Member that includes extrabasinal and volcaniclastic diamictite, in turn overlain by a volcaniclastic unit (the Oxford Mountain tuffite). Previous attempts to date the tuffite include three sets of analyses of the original sample (06PL00) and one resample (04JK09) that yielded sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon concordia ages of ca. 709, 702, and 686 Ma and one isotope dilution-thermal ionization mass spectrometry (ID-TIMS) age of 687.4 ± 1.3 Ma. Several new samples of plagioclase-phyric volcanic sandstone and the tuffite, dated via high-precision (~0.1%) chemical abrasion (CA) ID-TIMS, have multimodal zircon populations with single-crystal ages ranging from as old as 709 Ma to as young as 685 Ma, confirming the epiclastic nature of the deposit. The majority of grains in one sample yielded a 206Pb/238U weighted mean age of 685.5 ± 0.4 Ma, which provides a robust maximum age of deposition. From the type section of the lower Scout Mountain Member, Pocatello Formation at Portneuf Narrows, we report four new SHRIMP maximum depositional ages between 705 ± 5 Ma and 682 ± 6 Ma. A 691 ± 4 Ma (SHRIMP) volcanic clast from the cobble conglomerate member provides a maximum depositional age, and provides a geochronologic correlation with the Oxford Mountain tuffite. The data are interpreted to support a lithostratigraphic correlation between the diamictite on Oxford Mountain and the lower diamictite at Portneuf Narrows and to show that the upper g

SHRIMP analysis of U/Pb ages of detrital zircons in twelve late Miocene to Pleistocene sand samples from six drill cores on the Snake River Plain (SRP), Idaho, suggests that an ancestral Lost River system was drained westward along the northern side of the SRP. Neoproterozoic (650 to 740 Ma, Cryogenian) detrital zircon grains from the Wildhorse Creek drainage of the Pioneer Mountains core complex, with a source in 695 Ma orthogneiss, and which are characteristic of the Big Lost River system, are found in Pliocene sand from cores drilled in the central SRP (near Wendell) and western SRP (at Mountain Home). In addition to these Neoproterozoic grains, fluvial sands sourced from the northern margin of the SRP contain detrital zircons with the following ages: 42 to 52 Ma from the Challis magmatic belt, 80 to 100 Ma from the Atlanta lobe of the Idaho batholith, and mixed Paleozoic and Proterozoic ages (1400 to 2000 Ma). In contrast, sands in the Mountain Home Air Base well (MHAB) that contain 155-Ma Jurassic detrital grains with a source in northern Nevada are interpreted to represent an integrated Snake River, with provenance on the southern, eastern and northern sides of the SRP. We propose that late Pliocene and early Pleistocene construction of basaltic volcanoes and rhyolitic domes of the Axial Volcanic Zone of the eastern SRP and the northwest-trending Arco Volcanic Rift Zone (including the Craters of the Moon volcanic center), disrupted the paleo-Lost River drainage, confining it to the Big Lost Trough, a volcanically dammed basin of internal drainage on the Idaho National Laboratory (INL). After the Axial Volcanic Zone and Arco Volcanic Rift Zone were constructed to form a volcanic eruptive and intrusive highland to the southwest, sediment from the Big Lost River was trapped in the Big Lost Trough instead of being delivered by surface streams to the western SRP. Today, water from drainages north of the SRP enters the Snake River Plain regional aquifer through sink

In order to better constrain the age and provenance of the Upper Cretaceous Wahweap and Kaiparowits formations in southern Utah, U-Pb SHRIMP ages were obtained for detrital zircons from three sandstone samples, in addition to the first 40Ar-39Ar age for the Wahweap Formation, obtained from a devitrified volcanic ash horizon (bentonite). The ash horizon, located ∼40 m above the base of the Wahweap Formation, yields an age of 80.1 ± 0.3 Ma. The new radiometric data improve upon previous biostratigraphic age estimates for the Wahweap Formation and indicate that the formation was deposited between approximately 81 and 76 Ma. The youngest population of detrital zircons from the base of the Wahweap Formation clusters around 83-82 Ma, while the youngest population in the capping sandstone near the top of the formation is between 77-81 Ma, consistent with the 40Ar-39Ar age. Detrital zircons from the base of the overlying Kaiparowits Formation include a younger population clustering around 77-76 Ma, but are otherwise broadly similar to those in the lower Wahweap. Detrital zircon assemblages suggest the lower Wahweap and Kaiparowits sandstones were primarily deposited by longitudinal stream systems sourced in the Cordilleran magmatic arc in southern California or western Nevada, along with Mesozoic volcanics in southern Arizona. The capping sandstone contains detrital zircons that suggest it was proximately sourced from transverse stream systems that drained eastward out of uplifted Mesozoic quartzose sandstones in the Sevier thrust belt to the west. Revised correlations between the Wahweap Formation and coeval strata and faunas across the Western Interior Basin show that the Wahweap Formation is coeval with Judithian age localities including the type-Judithian Judith River Formation. This suggests that the Aquilan and Judithian North American Land Mammal "ages" are in need of recalibration based on recent acquisition of this and other new radiometric data, as well as new fau

The Late Palaeozoic configuration of Pangaea contributed to a palaeoclimatic extreme that was characterized by both icehouse and monsoonal conditions. This study uses sedimentological, geochemical, and provenance data from silty facies of the Earp and equivalent Supai Formations (Arizona, New Mexico) to shed light on atmospheric circulation and glacial-interglacial climate change in westernmost equatorial Pangaea. Five silt-rich facies comprise both loessite and marine and fluvially reworked loessite. An initial aeolian origin for the silt is indicated by the remarkably invariant grain size and the laterally continuous, sheet-like geometry of beds. The silt-rich facies occur in repetitive facies associations (1-20 m scale) that form mixed continental-marine (loess, marine-reworked loess), shallow-marine, and continental (loess, palaeosol) 'sequences'. Facies repetitions of both mixed continental-marine and shallow-marine sequences reflect a linked glacioeustatic-glacioclimatic control, whereas the continental (loess-palaeosol) couplets reflect a primary glacial-interglacial climatic cyclicity linked to glacioeustasy. Stratigraphic interpretations suggest that aeolian silt flux maximized during glacial to incipient interglacial stages (lowstand to early transgression), and decreased significantly or ceased during interglacials (highstand to early falling stage). Detrital-zircon geochronological data indicate a transition from dominantly north-easterly winds during the Middle Pennsylvanian to north-westerly and south-easterly winds by the Early Permian, which trend is inferred to reflect the onset of monsoonal circulation in western Pangaea. Relative grain-size data support the detrital-zircon data, and exhibit a significant decrease from the Sedona arch/Central Arizona shelf (north) to the Pedregosa basin (south) sections. Whole-rock geochemical data suggest a relatively unweathered source for the silt in the north, and detrital-zircon data indicate significant silt

We report new U-Pb detrital zircon sensitive high-resolution ion microprobe (SHRIMP) age data (702 grains) from 13 samples collected from Miocene to Holocene sedimentary deposits in the western Snake River Plain region. These samples effectively show that modern stream sediments of the Snake River system reliably and repeatedly record the detrital zircon age populations that are present as sources in their drainage basins across the Cordilleran thrust belt and Basin and Range Province. We use this framework and the provenance of Neogene sedimentary rocks in the region to test the effect of the migrating Yellowstone hotspot on regional drainage patterns in southern Idaho since the middle Miocene. Our results indicate that Neogene paleodrainages were first directed radially away from the tumescent Yellowstone highland, then subsequently reversed their flow toward the subsiding Snake River Plain basin. This occurred in east-progressing time-constrained intervals starting at 16 Ma. In northern Nevada, the drainage divide is represented by a northeast-trending, southeast-migrating crest of high topography. Specifically, middle to late Miocene (16-10 Ma) sedimentary deposits of the western Snake River Plain and Oregon-Idaho graben contain early to middle Eocene (52-42 Ma) detrital zircon populations sourced in Challis magmatic rocks north of the Snake River Plain. Middle Jurassic (160 Ma) and middle to late Eocene (42-35 Ma) detrital zircons, sourced from rocks in northern Nevada, are not present. Late Eocene detrital zircons from Nevada are present in two younger than 7 Ma sedimentary units of the Idaho Group along the Oregon-Idaho border. This indicates that by the late Miocene, southeastward headward erosion of the paleo-Owyhee River into the Owyhee Plateau had captured drainage from north-central Nevada and directed it northwestward toward the subsiding western Snake River Plain. The modern Owyhee Plateau is still a topographic high, in contrast to the modern Snake Ri

Detrital-zircon age-spectra effectively define provenance in Holocene and Neogene fluvial sands from the Snake River system of the northern Rockies, U.S.A. SHRIMP U-Pb dates have been measured for forty-six samples (about 2700 zircon grains) of fluvial and aeolian sediment. The detrital-zircon age distributions are repeatable and demonstrate predictable longitudinal variation. By lumping multiple samples to attain populations of several hundred grains, we recognize distinctive, provenance-defining zircon-age distributions or "barcodes," for fluvial sedimentary systems of several scales, within the upper and middle Snake River system. Our detrital-zircon studies effectively define the geochronology of the northern Rocky Mountains. The composite detrital-zircon grain distribution of the middle Snake River consists of major populations of Neogene, Eocene, and Cretaceous magmatic grains plus intermediate and small grain populations of multiply recycled Grenville (∼950 to 1300 Ma) grains and Yavapai-Mazatzal province grains (∼1600 to 1800 Ma) recycled through the upper Belt Supergroup and Cretaceous sandstones. A wide range of older Paleoproterozoic and Archean grains are also present. The best-case scenario for using detrital-zircon populations to isolate provenance is when there is a point-source pluton with known age, that is only found in one location or drainage. We find three such zircon age-populations in fluvial sediments downstream from the point-source plutons: Ordovician in the southern Beaverhead Mountains, Jurassic in northern Nevada, and Oligocene in the Albion Mountains core complex of southern Idaho. Large detrital-zircon age-populations derived from regionally well-defined, magmatic or recycled sedimentary, sources also serve to delimit the provenance of Neogene fluvial systems. In the Snake River system, defining populations include those derived from Cretaceous Atlanta lobe of the Idaho batholith (80 to 100 Ma), Eocene Challis Volcanic Group and associ

Three stratigraphically well defined rocks from the glaciogenic Scout Mountain Member, Neoproterozoic Pocatello Formation, southern Idaho, yielded sensitive, high-resolution ion-microprobe (SHRIMP) U-Pb zircon ages that constrain the age of the upper diamictite and its cap carbonate to between ca. 710 and 667 Ma. (1) Zircons from an epiclastic plagioclase-phyric tuff breccia immediately below glaciogenic Scout Mountain Member diamictite on Oxford Mountain, just north of the Utah border, yield a SHRIMP U-Pb concordia age of 709 ± 5 Ma. (2) A porphyritic rhyolite clast from the upper Scout Mountain Member diamictite at Portneuf Narrows, south of Pocatello, yields a concordia age of 717 ± 4 Ma. (3) The simple igneous zircon population from a reworked fallout tuff bed in the uppermost Scout Mountain Member, 20 m above the upper diamictite and its cap carbonate and immediately below a second cap-like carbonate, has a concordia age of 667 ± 5 Ma. These data support previous interpretations that the Scout Mountain Member glaciation scoured nearby volcanic highlands composed of the bimodal Bannock Volcanic Member and suggest that the volcanism was 717 ± 4 Ma. This age is close to, but distinctly older than, ca. 685 Ma U-Pb SHRIMP ages from the lithostratigraphically correlative Edwardsburg Formation in central Idaho. These data imply that the major rifting phase in this part of western Laurentia spanned 717-685 Ma rather than 800-750 Ma, as previously suggested. Further, because the Scout Mountain succession has been correlated with the Sturtian glacial phase on the basis of lithostratigraphy plus C and Sr isotope values in the carbonates, these data suggest that the Sturtian glacial epoch may have lasted until 670 Ma.