Paul Karl Link

Detrital zircon provenance of Mesoproterozoic to Cambrian arenites in the western United States and northwestern Mexico

U-Pb isotopic dating of detrital zircon from supracrustal Proterozoic and Cambrian arenites from the western United States and northern Mexico reveal three main age groups, 1.90 to 1.62 Ga, 1.45 to 1.40 Ga, and 1.2 to 1.0 Ga. Small amounts of zircons with ages of 3.1 to 2.5 Ga, 1.57 Ga, 1.32 Ga, 1.26 Ga, 0.7 Ga, and 0.5 Ga are also present. Detrital zircons ranging in age from 1.90 to 1.62 Ga and from 1.45 to 1.40 Ga are considered to have been derived from Proterozoic crystalline basement rocks of these known ages, and probably in part from reworked Proterozoic supracrustal sedimentary rocks, of the western United States. The 1.2 to 1.0 Ga detrital zircon ages from California, Arizona, and Sonora are characterized by distinct spikes (1.11 Ga, in particular) in the age-probability plots. These spikes are interpreted to indicate the influx of zircon from major silicic volcanic fields. Igneous rocks such as the Pikes Peak Granite (1.093 Ga) of Colorado, and the Aibo Granite (1.110 Ga) of Sonora, Mexico, may represent the deeply eroded roots of such volcanic fields. Samples from farther north along the Cordilleran margin that contain abundant 1.2‐1.0 Ga detrital zircons do not show spikes in the age distribution, but rather ages spread out across the entire 1.2‐1.0 Ga range. These age spectra resemble those for detrital zircons from the Grenville province, which is considered their source. Less common detrital zircons had a variety of sources. Zircons ranging in age from 3.36 to 2.31 Ga were apparently derived from inland parts of the North American continent from Wyoming to Canada. Zircons of about 1.577 Ga are highly unusual and may have had an exotic source; they may have come from Australia and been deposited in North America when Australia and North America were juxtaposed as part of the hypothetical Rodinian supercontinent. Detrital zircon of ;1.320 Ga apparently had the same source as that for tuff (1.320 Ga) in the Pioneer Shale of the Apache Group in Arizona. Detrital zircons of about 1.26 Ga in the Apache Group and Troy Quartzite appear to be related to local, approximately coeval volcanic fields. Zircons of about 0.7 Ga may have had a source in igneous rocks related to rifting of the Proterozoic supercontinent of Rodinia, and 0.5 Ga zircons a source in relatively small areas of granitic rocks of this known, or inferred, age in Oklahoma, Texas, New Mexico, and Colorado.

U-Pb SHRIMP ages of Neoproterozoic (Sturtian) glaciogenic Pocatello Formation, southeastern Idaho

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.

Maximum depositional age and provenance of the Uinta Mountain Group and Big Cottonwood Formation, northern Utah: Paleogeography of rifting western Laurentia

U-Pb detrital zircon analyses provide a new maximum depositional age constraint on the Uinta Mountain Group (UMG) and correlative Big Cottonwood Formation (BCF) of Utah, and significantly enhance our insights on the mid-Neoproterozoic paleogeographic and tectonic setting of western Laurentia. A sandstone interval of the Outlaw Trail formation with a youngest population (n = 4) of detrital zircons, from a sampling of 128 detrital zircon grains, yields a concordia age of 766 ± 5 Ma. This defines a maximum age for deposition of the lower-middle Uinta Mountain Group in the eastern Uinta Mountains and indicates that the group is no older than middle Neoproterozoic in age (i.e., Cryogenian). These data support a long-proposed correlation with the Chuar Group of Grand Canyon (youngest age 742 Ma ± 6 Ma), which, like the Uinta Mountain Group and Big Cottonwood Formation, records nonmagmatic intracratonic extension. This suggests a ∼742 to ≤766 Ma extensional phase in Utah and Arizona that preceded the regional rift episode (∼670–720 Ma), which led to development of the Cordilleran passive margin. This is likely an intracratonic response to an early rift phase of Rodinia. Further, because the Chuar Group and the Uinta Mountain Group–Big Cottonwood Formation strata record intracratonic marine deposition, this correlation suggests a regional ∼740–770 Ma transgression onto western Laurentia.

Miocene to holocene landscape evolution of the western Snake River Plain region, Idaho : Using the SHRIMP detrital zircon provenance record to track eastward migration of the Yellowstone hotspot

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 fi rst directed radially away from the tumescent Yellowstone highland, then subsequently reversed their fl ow 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. Specifi cally, 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 River Plain, suggesting that lowering of the regional Snake River Plain base level, rather than crustal subsidence, drove stream capture. By the late Pliocene (3 Ma), Middle Jurassic detrital zircons are recorded in the Glenns Ferry Formation and Tuana Gravel of the central Snake River Plain, suggesting that surface subsidence reversed the fl ow direction of paleo‐Salmon Falls Creek from southward into Nevada to northward toward Idaho. Miocene strata of the western Snake River Plain lack recycled Proterozoic detrital zircons that are ubiquitous in sedimentary rocks of the central and southeast Idaho thrust belts. Such detrital zircons appear on the central and western Snake River Plain in early Pliocene to Holocene (4‐0 Ma) deposits. This records capture of drainage from the eastern Snake River Plain. The Yellowstone hotspot controlled the east-migrating continental divide, in the wake of which formed the western-draining, and progressively eastward-collecting, Snake River system.

Chemostratigraphy of predominantly siliciclastic Neoproterozoic successions; a case study of the Pocatello Formation and lower Brigham Group, Idaho, USA

Abstract Isotopic chemostratigraphy has proven successful in the correlation of carbonate-rich Neoproterozoic successions. In successions dominated by siliciclastic rocks, chemostratigraphy can be problematic, but if thin carbonates punctuate siliciclastic strata, useful isotopic data may be obtained. The upper Pocatello Formation and lower Brigham Group of southeastern Idaho provide an opportunity to assess the potential and limitations of isotopic chemostratigraphy in overwhelmingly siliciclastic successions. The 5000 m thick succession consists predominantly of siliciclastic lithologies, with only three intervals that contain thin intercalated carbonates. Its depositional age is only broadly constrained by existing biostratigraphic, sequence stratigraphic and geochronometric data. The lowermost carbonates include a cap dolomite atop diamictites and volcanic rocks of the Pocatello Formation. The delta 13C values of these carbonates are distintly negative (-5 to -3), similar to carbonates that overlie Neoproterozoic glaciogenic rocks worldwide. Stratigraphically higher carbonates record a major positive delta 13C excursion to values as high as +8.8 within the carbonate member of the Caddy Canyon Quartzite. The magnitude of this excusion is consistent with post-Sturtian secular variation recorded elsewhere in the North American Cordillera, Australia, Svalbard, Brazil and Nambia, and exceeds the magnitude of any post-Varanger delta 13C excursion documented to date. In most samples, Sr-isotopic abundances have been altered by diagenesis and greenschist facies metamorphism, but a least-altered value of approximately 0.7076 supports a post-Sturtian and pre-Marinoan/Varanger age for upper Pocatello and lower Brigham rocks that lie above the Pocatello diamictite. Thus, even though available chemostratigraphic data are limited, they corroborate correlations of Pocatello Formation diamictites and overlying units with Sturtian glaciogenic rocks and immediately post-Sturtian successions in western North America and elsewhere.

Paleogeographic implications of non-North American sediment in the Mesoproterozoic upper Belt Supergroup and Lemhi Group, Idaho and Montana, USA

A non-North American provenance for the lower Belt Supergroup of North America has been used to support various pre-Rodinian paleogeographic reconstructions. Unlike the lower Belt Supergroup, most upper Belt Supergroup provenance studies have inferred Laurentian sediment sources. We test this hypothesis by analyzing U-Pb and Lu-Hf isotopes on detrital zircons, and whole-rock Nd isotopes from the Missoula (upper Belt Supergroup) and Lemhi Groups, and comparing to possible Laurentian sources. Detrital zircons from 11 sandstones analyzed show dominant ages between 1680 and 1820 Ma. These zircons are predominantly magmatic in paragenesis. Belt Supergroup–aged (1400–1470 Ma) and 2400–2700 Ma populations represent minor components. Lu-Hf isotopic analyses for 1675–1780 Ma Missoula Group and Lemhi Group detrital zircons range from eHf(i) +9 to –12 and +8 and –7, respectively. Belt Supergroup–aged grains from the Bonner Formation, Missoula Group, have eHf(i) values between +5 and –9, exceeding coeval ranges from the Mojave and Yavapai terranes [eHf(i) between +5 and 0]. Whole-rock Nd isotopes from Lemhi Group argillites yield a range in eNd(1400) between +1.1 and –5.9. Immature feldspathic sediment, nearly unimodal detrital zircon spectra, and dissimilar Belt Supergroup–aged zircon Hf signatures suggest that distal portions of the Yavapai and Mojave terranes intruded by A-type magmas were not the source for the Missoula and Lemhi Groups. Instead, a slightly modified Mesoproterozoic proto-SWEAT (southwestern United States and East Antarctica) model can best account for the sedimentologic and isotopic characteristics of the Missoula and Lemhi Groups. An alternative model with a source from southeastern Siberia and the Okhotsk Massif is less preferred.

An upwelling model for the Phosphoria sea: A Permian, ocean-margin sea in the northwest United States

The Permian Phosphoria Formation, a petroleum source rock and world-class phosphate deposit, was deposited in an epicratonic successor basin on the western margin of North America. We calculate the seawater circulation in the basin during deposition of the lower ore zone in the Meade Peak Member from the accumulation rates of carbonate fluorapatite and trace elements. The model gives the exchange rate of water between the Phosphoria sea and the open ocean to the west in terms of an upwelling rate (84 m yr -1 ) and residence time (4.2 yr) of seawater in the basin. These hydrographic properties supported a mean rate of primary productivity of 0.87 g m -2 d -1 of carbon in the uppermost few tens of meters of the water column (the photic zone) and denitrifying redox conditions in the bottom water (below approximately 150 m depth). High rain rates, onto the sea floor, of the organic matter that hosted the phosphate and several trace elements contributed to the accumulation of phosphorite, chert, and black shales and mudstones. Evaporation in the Goose Egg basin to the east of the Phosphoria basin ensured the import of surface seawater from the Phosphoria sea. Budgets of water, salt, phosphate, and oxygen, plus the minor accumulation of the biomarker gammacerane, show that exchange of water between the two basins was limited, possibly by the shallow carbonate platform that separated the two basins.

Geochemistry of Upper Proterozoic rift-related volcanics, northern Utah and southeastern Idaho

Mafic volcanic rocks associated with Upper Proterozoic diamictites are widespread in western North America, and a continental rift setting has been proposed for their origin. We report trace-element and rare-earth-element geochemistry for these volcanics from northern Utah and southeastern Idaho. The analyzed samples are high-Ti within-plate basalts, and they vary from transitional tholelitic-alkaline in the south to alkaline in the north. The geochemistry, together with the geologic setting of the volcanics, indicates a rift setting, but the timing of this rifting and its relationship to final continental separation are still controversial.

Pre- to synglacial rift-related volcanism in the Neoproterozoic (Cryogenian) Pocatello Formation, SE Idaho: New SHRIMP and CA-ID-TIMS constraints

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 tuffi te, 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 206 Pb/ 238 U 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 glaciogenic diamictite in the Portneuf Narrows section is younger than 685 Ma. This 685 Ma age from rift-related rocks that underlie the Brigham Group passive-margin succession provides a maximum age for onset of rift subsidence. Lu-Hf analyses of 685–730 Ma igneous zircons yield enriched initial e Hf values in the range +2 to –17, indicating that they crystallized from magma that incorporated depleted Paleoproterozoic to Archean crustal components of the underlying Farmington Canyon Complex and Wyoming craton.

Stratigraphic, geochronologic, and geochemical record of the Cryogenian Perry Canyon Formation, northern Utah: Implications for Rodinia rifting and snowball Earth glaciation

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 Laurentia. The formation is divided into seven informal members, from bottom to top: (1) arkosic grit deposited as proximal turbidites during onset of rifting; (2) quartzite and grit deposited as medial turbidites; (3) pebbly slate with dropstones deposited during an older glacial episode; (4) slate with interbedded quartzite deposited mostly as distal turbidites; (5) mafic volcanic and intrusive rocks; (6) diamictite with basement and volcanic clasts deposited during younger glaciation; and (7) graywacke with rare carbonate layers. The formation has a basal nonconformity on Archean to Paleoproterozoic basement rocks of the Facer Formation and is overlain by arkosic quartzite of the Maple Canyon Formation and argillite of the Kelley Canyon Formation. Changes in member thicknesses reflect syndepositional faulting. Vertical and lateral changes in provenance and maximum depositional ages were evaluated from detrital zircon (DZ) U-Pb geochronology of five well-exposed stratigraphic sections across the sedimentary basin. DZ patterns for the arkosic grit record local sources during onset of rifting, patterns for the pebbly slate and slate members record distal Laurentian sources and possible sediment recycling, and patterns for diamictite and graywacke record basement sources along the rift margin and reworking of mafic to felsic volcanic rocks. Young zircon grains derived from volcanic rocks give maximum depositional ages of 703 ± 6 Ma and 667 ± 5 Ma for the lower part of the diamictite and graywacke members, respectively, suggesting a late Sturtian age for the younger glaciation. Associated cap carbonates, however, have Marinoan-style features, indicating that global correlations based only on lithology may not be valid. DZ data for the Kelley Canyon Formation record a change to distal Laurentian sources during final stages of early rifting. Mafic volcanic and intrusive rocks are subalkaline to alkaline and have trace-element ratios indicative of a rift setting. Volcanic clasts in diamictite include basalt, trachyte, and rhyolite. Volcanic zircon grains have e Hf = –1 to –21, which record crustal contamination during igneous activity. Rare mafic to ultramafic dikes in the upper part of the section were intruded during waning stages of igneous activity. Early rifting spanned ca. 720 Ma to 660 Ma but was incomplete and followed by deposition of mature quartzite of the overlying lower Brigham Group across a broad basin, indicating that Rodinia rift models showing pre–720 Ma separation of western Laurentia may need revision. Final rifting and transition to drift occurred ca. 550 Ma during deposition of the upper Brigham Group.