Pioneer Mountains Research Articles

A cosmogenic 10Be moraine chronology of arid, alpine Late Pleistocene glaciation in the Pioneer Mountains of Montana, USA

We test the hypothesis that glacier systems, located in continental regions proximal to the Laurentide Ice Sheet (LIS), had local ice maxima considerably earlier than the LIS maximum and thus before the insolation minima at ∼21 ka. Ranges located in the northwest US exhibit earlier deglaciation timing between ∼23 and 22 ka, except for the Yellowstone region where younger time-transgressive ages complicate regional interpretations and the northern Montana ice cap where late glacial ages have recently been produced. Constraining the glacial history of more ice sheet-proximal alpine glaciers provides insight into whether the contrasting maximum-ice times in the northern Rocky Mountains were caused by regional climatic differences, such as anticyclonic wind patterns driven by the presence of the LIS. In the Pioneer Mountains of Montana, we measured in situ cosmogenic 10Be in 35 boulders on moraines marking the maximum Late Pleistocene positions of alpine glaciers from three valleys. The 10Be samples produced a range of ages, spanning pre Bull Lake to the last glaciation (i.e., Pinedale/Marine Isotope Stage (MIS) 2). We find an average exposure age for initial deglaciation of 18.2 ± 0.9 during the local Last Glacial Maximum, indicative of synchronous retreat in the Pioneer Mountains. The similarity of initial deglaciation timing of the Pioneer Mountain glaciers with the northwestern Yellowstone glacial system and northern MT ice cap suggests that topography more proximal to the LIS margin maintained full ice extent longer. Our findings, in context of previous work, suggest that in the case of the Pioneer Mountains their more proximal location to the ice margin may have delayed onset of deglaciation by greater exposure to local cooling from katabatic winds and/or additional moisture sourced from large ice-marginal glacial lakes, hence the lack of earlier deglacial ages like those found further to the west and east of the northern Rocky Mountain cordillera.

Experimental river noise alters arthropod abundance

Anthropogenic noise has received considerable recent attention, but we know little about the role that sources of natural noise have on wildlife abundance and distributions. Rivers and streams represent an ancient source of natural noise that is widespread and covers much of Earth. We sought to understand the role that whitewater river noise plays on arthropod abundance in riparian habitats across a desert landscape. For two summers, we continuously broadcasted whitewater river noise and spectrally‐altered river noise (shifted upwards in frequency, but maintaining the same temporal profile) to experimentally tease apart the effects of two characteristics of noise – sound levels and background spectral frequency – on arthropod abundances. We used five types of trapping methods, placed across 20 sites within the Pioneer Mountains of Idaho, USA, to collect and identify 151 992 specimens to the order level. We built Bayesian generalized linear mixed‐effects models with noise characteristics and other habitat variables such as riparian vegetation, elevation, temperature, and moonlight. Of the 42 models we built (one for each order‐trap type combination), 26 (62%) indicated a substantial response to at least one noise variable – sound pressure level, background spectral frequency, or an interaction between the two. Fourteen of 17 (82%) arthropod orders responded to noise in some capacity: Araneae, Coleoptera, Collembola, Dermaptera, Hemiptera, Hymenoptera, Lepidoptera, Neuroptera, Opiliones, Orthoptera, Plecoptera, Raphidioptera, Thysanoptera and Trichoptera. Only three groups appeared to be unaffected, Acari, Archaeognatha and Diptera. Results from this study suggest that the natural acoustic environment can shape arthropod abundances both directly and indirectly (via predator–prey relationships). Future work should further examine the role that the indirect effects of noise play in food webs. Natural noise should be considered an important ecological niche axis, especially as we continue to alter natural acoustic environments and replace them with anthropogenic ones.

Activity Patterns and Foraging Behavior of American Pikas (Ochotona princeps) Differ between Craters of the Moon and Alpine Talus in Idaho

Range contractions in the Great Basin over the last century suggest that American pikas (Ochotona princeps) might be highly sensitive to climate change. However, documentation of pikas residing at relatively warm, low-elevation sites has recently shed new light on the possible resilience of pika populations to warmer ambient conditions when they have access to cooler microhabitats for thermoregulation. To provide insight into the possible behavioral mechanisms of adaptation to warmer habitats, we investigated activity patterns, foraging behavior, and space use of a population of pikas living in an atypical, warmer habitat at Craters of the Moon National Monument and Preserve, an extensive lava flow surrounded by high desert grassland and sagebrush communities in southern Idaho. We compared pika behavior at this site to that of a population in a more typical alpine habitat at Grays Peak in the nearby Pioneer Mountains in Idaho. Specifically, we evaluated and compared activity patterns and foraging behavior of pikas in relation to temperature, time of day, and month. Pikas at Craters of the Moon were less active aboveground during all times of the day from late May until mid-August, compared to those at Grays Peak. Surface temperatures were warmer across the entire season at Craters of the Moon than at Grays Peak, and pikas exhibited a crepuscular activity pattern that coincided with variations in surface temperature at Craters of the Moon. Pikas were the least active during the midday, when the surface temperatures were highest and the insulating effect of the lava tubes (i.e., the difference between surface and crevice temperatures) was most pronounced. Pikas at Craters of the Moon spent less time haying and displayed fewer territorial behaviors than pikas at Grays Peak, but both groups filled a similar number of hay piles. The vegetation community was less diverse and sparser at Craters of the Moon than at Grays Peak, and consequently, the vegetation that was consumed and cached reflected these differences. Our results expand the body of literature about American pikas at their environmental limits, and this study is the first step in identifying the unique suite of behaviors that pikas use to persist in a seemingly inhospitable environment at Craters of the Moon.

U-Pb zircon ages of the Wildhorse gneiss, Pioneer Mountains, south-central Idaho, and tectonic implications

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 ortho­gneiss, with Neoarchean U-Pb magmatic zircon ages of 2.60–2.67 Ga. The ortho­gneiss 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.

Origin of Mountain Passes across Continental Divide Segments Surrounding the Southwest Montana Big Hole and Beaverhead River Drainage Basins, USA

The evolution of southwest Montana’s Big Hole and Beaverhead River drainage basins is determined from topographic map evidence related to mountain passes crossing what are today high altitude drainage divides including North America’s east-west Continental Divide. Map evidence, such as orientations of valleys leading away from mountain passes (and saddles) and barbed tributaries found along the downstream drainage routes, is used to reconstruct flow directions of streams and rivers that once crossed the present-day high mountain divides. Large south-oriented anastomosing complexes of diverging and converging channels are interpreted to have eroded what are today closely spaced passes and saddles now notched into high mountain ridges. Water in those south-oriented channels is interpreted to have flowed across emerging mountains and subsiding basins. Headward erosion of deeper southeast-oriented valleys, assisted by crustal warping, concentrated south-oriented water in fewer and deeper valleys as the water flowed from southwest Montana into what are today Idaho and the Snake River drainage basin. Headward erosion of the Big Hole River valley between the emerging Anaconda and Pioneer Mountains, also assisted by crustal warping, reversed all Big Hole Basin drainage so as to create the north-, east-, and south-oriented Big Hole River drainage route. A final and even more major reversal of flow in the present-day north-oriented Montana Missouri River valley, with the assistance of additional crustal warping, next ended all remaining flow to Idaho and the Snake River drainage basin and reversed and captured all drainage in the present-day north-oriented Big Hole, Beaverhead, and Red Rock River drainage basins. The observed map evidence indicates that prior to the final flow reversal events, large volumes of south-oriented water flowed across southwest Montana’s Big Hole and Beaverhead River drainage basins.

Detrital zircon record of mid-Paleozoic convergent margin activity in the northern U.S. Rocky Mountains: Implications for the Antler orogeny and early evolution of the North American Cordillera

The passive to convergent margin transition along western Laurentia drove early development of the North American Cordillera and culminated with the Late Devonian–Mississippian Antler orogeny and emplacement of the Roberts Mountain allochthon in the western United States. New detrital zircon studies in the Pioneer Mountains, east-central Idaho, were conducted to investigate the stratigraphic evidence of this transition and test models for mid-Paleozoic tectonics and paleogeography. Ordovician to Lower Devonian passive margin strata of the Roberts Mountain allochthon and adjacent North American parautochthon contain ca. 1850, 1920, 2080, and 2700 Ma detrital zircons that indicate provenance from the Peace River Arch region of northwestern Laurentia. These detrital zircons are much older than the depositional ages of their host rocks and probably record long-term sediment recycling processes along the Cordilleran margin. Upper Devonian strata, including Frasnian turbidites of the Roberts Mountain allochthon, document the incursion of 450–430 Ma and 1650–930 Ma detrital zircons from an unknown source to the west. Detrital zircon Hf isotope results suggest that the western source was an early Paleozoic arc built on Proterozoic crust, with the Eastern Klamath, Northern Sierra, and Quesnellia terranes as likely candidates. Lower Mississippian syntectonic strata filled a rapidly subsiding, releasing bend basin that was associated with sinistral-oblique plate convergence and reworking of lower Paleozoic rocks in east-central Idaho. The available detrital zircon and stratigraphic data are most consistent with noncollisional models for the Antler orogeny, including scenarios that feature the north to south, time-transgressive juxtaposition of Baltican- and Caledonian-affinity terranes along the Cordilleran margin.

Chapter 38 Neoproterozoic strata of southeastern Idaho and Utah: record of Cryogenian rifting and glaciation

Abstract Neoproterozoic strata in southeastern Idaho and Utah include the <766 Ma Uinta Mountain Group and Big Cottonwood Formation (Fm.) deposited in an east-trending rift basin and, to the west, the lower part of a westward-thickening rift to passive-margin succession that initiated c. 720 Ma. The latter contains a lower diamictite and volcanic succession, with a complex stratigraphic interval of Cryogenian marine glacial deposits (Pocatello and Mineral Fork formations and correlatives). This is overlain by a mostly terrigenous succession of <667 Ma strata assigned to the upper member of the Pocatello Fm. and Brigham Group in southeastern Idaho, to the Kelley Canyon Fm. and Brigham Group in northern and western Utah, and to the McCoy Creek Group and Prospect Mountain Quartzite in adjacent Nevada. Although the Brigham Group and correlative deposits contain no direct evidence for glaciation, widely developed, though stratigraphically restricted, incised valleys, with erosional relief from a few metres to as much as 160 m, are inferred to represent subsequent times of Cryogenian glacially lowered sea level. Overall interpretations of the stratigraphy and sedimentology of these rocks have changed little in the past 10–15 years. The most important recent advances relate to U–Pb geochronology. In strata that lie unconformably below demonstrable glacial deposits, the lower Uinta Mountain Group (formerly thought to be c. 900 Ma) contains populations of detrital zircons as young as 766±5 Ma. Cryogenian magmatism north of the Snake River Plain in central Idaho is recognized near House Mountain, east of Boise at c. 725±5 Ma, in the Pioneer Mountains Core Complex at about 695 Ma, and in central and east-central Idaho at 685–650 Ma. Clasts interpreted to be from the rift-related Bannock Volcanic Member of the Pocatello Fm. are dated at 717±4 Ma and 701±4 Ma. The overlying diamictite-bearing Scout Mountain Member contains a mafic lapilli tuff near the base (686±4 Ma) and a reworked fallout tuff near the top (667±5 Ma). Strongly negative C-isotope data have been obtained from some of the carbonate rocks, although the latter constitute only a small fraction of the succession. Palaeomagnetic data are available only for the Uinta Mountain Group, and suggest an equatorial palaeolatitude.

The Pliocene Lost River found to west: Detrital zircon evidence of drainage disruption along a subsiding hotspot track

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 sinks in the Big Lost Trough, and the water resurfaces at Thousand Springs, Idaho, about 195 km to the southwest. Holocene to latest Pliocene samples from drill core in the Big Lost Trough reveal interplay between the glacio-fluvial outwash of the voluminous Big Lost River system and the relatively minor Little Lost River system. A mixed provenance signature is recognized in fine-grained sands deposited in a highstand of a Pleistocene pluvial-lake system.

Extra-basinal fluid infiltration, mass transfer, and volume strain during folding: Insights from the Idaho-Montana thrust belt

Finite strain and geochemical variations along strain gradients were used to study cleavage development in carbonates in the Lost River Range, Idaho. Deformation accommodating layer-parallel shear was partitioned into thin deforma- tion zones during folding of Willow Creek anticline. Cleavage intensity is strong to very strong in deformation zones and weak in surrounding rocks. Strain magnitudes range from e s0.32 (1.15:0.93:0.73; x, y, z, principal axes of strain) outside, to e s0.64 (1.29:0.80:0.52) inside, deformation zones. We found a positive linear correlation between strain, cleavage development, and negative dilation. Volume loss, at a cubic centimeter scale, ranges from 12 percent to 49 percent. Cleavage selvages are depleted in Ca and 18 O and enriched in other elements relative to microlithons and less deformed protolith carbonates. Mass-balance considerations indicate that cleavage was formed by incongruent pressure solution leading to a passive concentration of less soluble components during Ca loss and metasomatic additions of Si, Al, and K to produce authigenic clay minerals in selvages. Data for the Willow Creek locality and elsewhere in the Lost River Range (Davidson and others, 1998), Pioneer Mountains, and the Tendoy Range (Bebout and others, 2001), show that across the Sevier orogen, fluid infiltration was heterogeneous at centimeter-kilometer scales and resulted from positive feedback between deforma- tion and far-traveled surficial fluids. Metasomatic strain softening enhanced deforma- tion zone development, which generated increased permeability as the thrust belt evolved from a porosity-based closed system to a discontinuity-based open system. Increased fluid infiltration and isotopic exchange was associated with volume loss, increasingly prolate strains, and crystallization of clays, to produce the cleavage selvages. Mass transfer was accommodated by diffusion early and advection later in the deformation history. Mesoscopic structures (deformation zones, faults, veins) focused fluid flow and were kinematically related to larger-scale structures (faults, fault- related folds). The inferred addition of surficial fluids to depths of >7 kilometers in the thrust belt implies a fluid regime involving significant topographically driven recharge. Deformed whole-rocks and microsamples are lower in 18 O than unde- formed samples which have O- and C-isotope compositions similar to those of marine carbonates. Veins are even lower in 18 OV-SMOW, with minimum values of 5 permil reflecting penetration of the crust by ocean-derived precipitation with 18 O near to somewhat lower than 0 permil (range of 7.5 to 2.5‰ calculated for H2 Oi n equilibrium with these veins). The inferred penetration, into the thrust belt, of nearshore meteoric waters is consistent with proximity to the reconstructed Western Interior Seaway. Later fluid infiltration locally lowered the 18 O of carbonates even further, based on the O-isotope compositions of veins related to younger compres- sional deformation and to the onset of crustal-scale extension in the Eocene. This progression toward lowered 18 O of the surficial waters is compatible with the retreat of the seaway during emergence of the thrust wedge and Paleogene extension, uplift, and subaerial volcanism.

Correlation of Upper Cretaceous strata from Lima Peaks area to Madison Range, southwestern Montana and southeastern Idaho, USA

An 40Ar/ 39Ar age of 85.81 Ma±0.22 my was obtained on sanidine from a volcanic procellanite bed near the top of the 2135+m-thick Upper Cretaceous Frontier Formation in the Lima Peaks area of southwestern Montana. This early Santonian age, combined with previously determined age data including a palynological age of Cenomanian for the lower Frontier at Lima Peaks, and a U-Pb isotopic date of about 95 Ma for the base of the Frontier Formation in the eastern Pioneer Mountains north of the Lima Peaks area, provides an age range for the nonmarine formation. In the Madison Range, farther east in southweastern Montana, this age range corresponds to marine strata of not only the Frontier Formation, but also the overlying Cody Shale and Telegraph Creek Formation, a sequence that totals less than 760 m thick. The Upper Cretaceous marine formations of the madison Range are closely zoned by molluscan faunas that are well constrained with radiometric dates. The 40Ar/ 39Ar age of 85.81 Ma±0.22 my at Lima Peaks is bracketed by radiometric dates for the Scaphites depressus—Protexanites bourgeoisianusbiozone and the overlying Clioscaphites saxitonianus—Inoceramus undulatopilcatusbiozone of the Western Interior. Fossils of both of these biozones are present in the Cody Shale and the Telegraph Creek Formation in the Madison Range. The Telegraph Creek contains two units of volcanic ash that are approximate time equivalents of the volcanic procellanite of the Lima Peaks area. Clasts in the conglomerate of the upper part of the Frontier in the Lima Peaks area were shed during the initial stages of uplift of the Blacktail-Snowcrest Highlands which rose to the north. The dated porcellanite lies above the conglomerates and indicates that the uplift was initiated by middle or late Coniacian, 87–88 Ma.

Source Rock Potential of Middle Cretaceous Rocks in Southwestern Montana

The middle Cretaceous in southwestern Montana is composed of a marine and nonmarine succession of predominantly clastic rocks that were deposited along the western margin of the Western Interior Seaway. In places, middle Cretaceous rocks contain appreciable total organic carbon (TOC), such as 5.59% for the Mowry Shale and 8.11% for the Frontier Formation in the Madison Range. Most samples, however, exhibit less than 1.0% TOC. The genetic or hydrocarbon potential (S1+S2) of all the samples analyzed, except one, yield less than 1 mg HC/g rock, strongly indicating poor potential for generating commercial amounts of hydrocarbons. Out of 51 samples analyzed, only one (a Thermopolis Shale sample from the Snowcrest Range) showed a moderate petroleum potenti l of 3.1 mg HC/g rock. Most of the middle Cretaceous samples are thermally immature to marginally mature, with vitrinite reflectance ranging from about 0.4 to 0.6% R0. Maturity is high in the Pioneer Mountains, where vitrinite reflectance averages 3.4% R0, and at Big Sky, Montana, where vitrinite reflectance averages 2.5% R0. At both localities, high R0 values are due to local heat sources, such as the Pioneer batholith in the Pioneer Mountains.

Correlation Chart of Lower and Lower-Upper Cretaceous Blackleaf Formation, Eastern Pioneer Mountains, Southwestern Montana, to Drummond, Central-Western Montana

Correlation Chart of Lower and Lower-Upper Cretaceous Blackleaf Formation, Eastern Pioneer Mountains, Southwestern Montana, to Drummond, Central-Western Montana

Correlation of Lower and Upper Cretaceous Blackleaf Formation, Lima Peaks Area to Eastern Pioneer Mountains, Southwestern Montana Foreland Basin: ABSTRACT

Correlation of Lower and Upper Cretaceous Blackleaf Formation, Lima Peaks Area to Eastern Pioneer Mountains, Southwestern Montana Foreland Basin: ABSTRACT

Cretaceous Fluvio-Lacustrine Facies of Upper Kootenai Formation and Flood Member of Blackleaf Formation, Southwestern Montana: ABSTRACT

The Flood Member of the Albian Blackleaf Formation is exposed in the eastern foothills of the Pioneer Mountains, southwestern Montana. Two localities were studied along a 10-mi north-south outcrop belt: Frying Pan Gulch (south) and Apex (north). Strata are 625 ft thick and composed of mixed siliciclastics and carbonates; they are grouped into coarse and fine-grained facies (CGF and FGF) sequences. The Flood Member is underlain conformably by the upper carbonate member of the Kootenai Formation.

Superposition of Cenozoic extension on Mesozoic compressional structures in the Pioneer Mountains metamorphic core complex, central Idaho

Research Article| November 01, 1988 Superposition of Cenozoic extension on Mesozoic compressional structures in the Pioneer Mountains metamorphic core complex, central Idaho ROBERT L. O'NEILL; ROBERT L. O'NEILL 1Department of Geological Sciences, Lehigh University, Bethlehem, Pennsylvania 18015 Search for other works by this author on: GSW Google Scholar TERRY L. PAVLIS TERRY L. PAVLIS 1Department of Geological Sciences, Lehigh University, Bethlehem, Pennsylvania 18015 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1988) 100 (11): 1833–1845. https://doi.org/10.1130/0016-7606(1988)100<1833:SOCEOM>2.3.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation ROBERT L. O'NEILL, TERRY L. PAVLIS; Superposition of Cenozoic extension on Mesozoic compressional structures in the Pioneer Mountains metamorphic core complex, central Idaho. GSA Bulletin 1988;; 100 (11): 1833–1845. doi: https://doi.org/10.1130/0016-7606(1988)100<1833:SOCEOM>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search nav search search input Search input auto suggest search filter All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Part of the Pioneer Mountains of central Idaho is a metamorphic core complex in which Mesozoic(?) fabrics have been over-printed by early Tertiary extension. The earlier deformation (Mesozoic?) affected all pre-Tertiary rocks in the core and resulted in macroscopic, northeast-vergent folding and complex mesoscopic folding of a Precam-brian basement gneiss complex and an overlying upper Proterozoic and lower Paleozoic metasedimentary sequence. A synkinematic granodiorite pluton intruded between the gneiss complex and the metasedimentary sequence records a relatively simple strain indicative of east-northeast-directed ductile thrusting of the hanging wall. The pre-Tertiary structures are probably a deep-seated manifestation of Sevier/Laramide compression. The early Tertiary extension is demonstrated by (1) a conjugate set of ductile shear zones showing normal offset; (2) a lineated "carapace" of mylonitic quartzite at the structurally highest levels of the core; and (3) a brittle, oblique-slip fault system along the northern edge of the core. Kinematic indicators from all three types of Tertiary structures suggest northwest/southeast transport. The lineated mylonitic quartzites, which are cut by the brittle fault system, are interpreted as the basal portion of a large ductile shear zone created during extension which subsequently passed into brittle deformation conditions. Exposure of the metamorphic core apparently occurred prior to Eocene Challis volcanism as shown by quartzite clasts apparently derived from the metasedimentary sequence volcanics within the core, however, suggests that extension continued during volcanism. Neogene Basin and Range extension is super-imposed on the older core-complex-type extension; the two extension directions are nearly 90° apart. The early Tertiary extension is coeval and coaxial with regional Eocene extension observed in northern Idaho, Washington, and British Columbia and may have been caused by strike-slip complications or escape tectonics during rapid Farallon/North American plate convergence. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not currently have access to this article.

Sandstone Diagenesis and Source Rock Analysis of Cretaceous Blackleaf and Frontier Formations in Parts of Beaverhead, Madison, and Gallatin Counties, Montana: ABSTRACT

The mid-Cretaceous Blackleaf and Frontier Formations and equivalent strata in parts of Beaverhead, Madison, and Gallatin Counties, Montana, are composed of a marine and nonmarine sequence of clastic and carbonate rocks deposited along the western margin of the Western Interior seaway. At present seven dry holes have penetrated this sequence in southern Beaverhead County. Sandstones in the formations are diagenetically diverse and exhibit calcite, silica, sericite, zeolite, and clay mineral alteration. Alteration is more pervasive in the upper Blackleaf and lower Frontier Formations because of compositional heterogeneity and the presence of volcanic detritus. Lower Blackleaf alteration is less complex and is mainly due to quartz overgrowths and calcite cements. Porosity in lower Blackleaf quartz arenites in eastern Beaverhead, Madison, and Gallatin Counties has been reduced significantly by quartz overgrowths. Lower Blackleaf selitharenites in Beaverhead County contain calcite pore-filling cement but exhibit the best reservoir rock potential of both formations. Vitrinite reflectance ranges from a maximum of 3.62% for shales in the lower Blackleaf Formation in the Pioneer Mountains in northern Beaverhead County to a minimum of 0.43% for the lower Frontier Formation in the Lima Peaks area to the south. Maximum reflectance values are related to the proximity of Cretaceousmore » and Tertiary intrusive bodies. Reflectance values for all other samples within the study area lie just below or at the base of the oil generation window. Total organic carbon (TOC) values for both formations are higher in the Gallatin Range (maximum value equals 8.1%), but Rock-Eval pyrolysis data indicate generally poor source rock potential. Black shales of the lower Blackleaf and lower Frontier Formations provide the best source rock for generation of hydrocarbons.« less

Extensional deformation with northwest vergence, Pioneer core complex, central Idaho

The Pioneer Mountains of Idaho contain a detachment fault that separates a lower-plate core of Precambrian and Ordovician metasedimentary rocks and Cretaceous and Eocene plutonic bodies from a surrounding upper plate of Paleozoic sedimentary and Tertiary sedimentary and volcanic units. The detachment system developed during Tertiary extension, and it overprinted Paleozoic and Mesozoic east-directed compressional features. Beneath the detachment, both brittle and ductile (mylonitic) deformation can be observed. Trends of stretching lineations in the mylonite and striations along the detachment surface both cluster around N65°W. Composite planar fabrics (S- and C-surfaces) in the mylonite and development of the mylonite zone on the northwest side of the core both suggest a top-to-the-west sense of shear. Both brittle and ductile deformation affect the Eocene pluton of the core; therefore, the deformation must be Eocene or younger. Displacement is estimated at about 17 km. The extension direction in the Pioneer complex correlates with that of other core complexes in the region north and south of the Snake River Plain; this suggests that the direction of extension in core complexes was controlled by an inherited structural framework which was crustal in scale.

Studies of the Idaho black shale belt: Stratigraphy, depositional environment, and economic geology of the Permian Dollarhide Formation

The Permian Dollarhide Formation, which consists of three informal members and is ∼2,300 m thick, is contained within the central Idaho black siliceous shale belt. The formation is a tectonostratigraphic unit within the Pioneer Mountains thrust stack. Major lithologies in the formation consist of calcareous, carbonaceous, siliceous siltite inter-bedded with fine-grained calcareous sandstone; fine-grained calcareous quartzite; and medium to dark gray, thin-bedded limestone. In the middle member, there are distinctive calcareous quartzites, and the upper member has conspicuous black (carbonaceous), pyritic argillite units. Sedimentary structures contained within the Dollarhide Formation include convolute bedding, cross-bedding, graded beds, flame structures, small-scale diapiric structures, synsedimentary folds, and load casts. It is suggested that the Dollarhide Formation correlates to the upper portion of subunit 6 and to subunit 7 of the Wood River Formation, which was deposited shoreward of the Dollarhide Formation. Petrology suggests deposition near and within an oxygen-minimum zone, rapid sedimentation at times, and, in places, turbidity-flow–emplaced beds. Mature quartz clasts and heavy minerals suggest possible derivation from a successor to the Antler highland to the east, with a sedimentary (Ordovician Kinnikinic Quartzite?) and crystalline basement as sources. Suggested depositional setting of the Dollarhide Formation is slope (lower and middle members) and basin outboard of the slope (upper member), thereby indicating transgression or subsidence during deposition. Trace-element geochemistry of the Dollarhide Formation rocks of the lower and middle members shows that they conform to the general values in black shales, as defined for Ag, Co, Cr, Mn, and Mo. The upper member, however, hosts epigenetic lead-silver-zinc veins and is locally the hanging-wall sequence of a syngenetic barite horizon.

Oil and Gas Potential of Idaho Thrust Belt North of Snake River Plain: ABSTRACT

The thrust belt north of the Snake River plain in south-central Idaho has the elements necessary for major oil and gas accumulations: large traps, thick reservoir rocks, top seals, rich oil-source rocks, and in at least parts of the area, a favorable temperature history. Only drilling is lacking. The Utah-Wyoming Sevier thrusting extended north of the Snake River plain into southwestern Montana and south-central Idaho. Styles of thrusting and resulting traps are similar to the Utah-Wyoming portion of the thrust belt. After thrusting, large listric normal faults formed northwesterly trending valleys that were filled with Tertiary sediments. Some companies are looking for traps in thrusted Paleozoic rocks; others are exploring the valleys looking for stratigraphically trapped oil generated from deeply buried Tertiary sediments. Paleozoic strata thicken westward from a normal cratonic sequence consisting of about 5,000 ft (1,524 m) of chiefly carbonate rocks in the east to more than 30,000 ft (9,144 m) of sandstones, conglomerates, shales, and carbonates in the west. Of particular importance as a hydrocarbon source rock was the accumulation of about 3,000 ft (914 m) of organic-rich Mississippian McGowan Creek Shale in a starved basin between the Antler uplift on the west and the craton on the east. Outcrop samples of this shale contain from 0.8-6.3% total organic carbon. Paleotemperatures in much of the area were too hot for preservation of oil according to alteration of conodonts. At least two important exceptions to the generally high paleotemperatures exist. The Tendoy thrust plate in the eastern part of the area has a conodont alteration index of 1, End_Page 869------------------------------ which is immature. Surface samples from the White Knob thrust plate in the Pioneer Mountains west of Arco in the southwestern part of the area have a conodont alteration index of 2 and vitrinite reflectance ranging from 0.45 to 0.64. Where the McGowan Creek Shale on the White Knob plate extends into the subsurface to appropriate depths, it should be generating oil and gas at the present time. Additional sampling and drilling may indicate the existence of other cooler areas. The McGowan Creek Shale acts as a seal on 5,000 ft (1,524 m) of lower Paleozoic dolomite. At least 1,500 ft (457 m) of this dolomite section may have enough porosity to be effective oil or gas pay. End_of_Article - Last_Page 870------------

Geology of North End of East Pioneer Mountains, Beaverhead County, Montana: ABSTRACT

The north end of the Pioneer Mountains is underlain by the following rocks: lower Proterozoic gneiss and amphibolite; middle proterozoic clastic rocks of the Missoula Group (Mount Shields Formation?); basal Cambrian clastic unit; Cambrian-Devonian-Carboniferous shelf sequence; Permian Phosphoria Formation; and Lower Triassic Dinwoody Formation. Jurassic rocks are missing. The Lower Cretaceous Kootenai Formation is of lagoonal to fluviatile facies, overlain by a thick (^approx 2 km, 1 mi) section of fluviatile Colorado Group. An upper member of the Colorado Group has yielded a Campanian-to-Maestrichtian pollen flora. The sedimentary rocks are cut by calc-alkalic plutonic rocks (80 to 65 m.y.B.P.), the oldest of which are synchronous with upper Colorado sedimentation. The y ungest pre-Quaternary rocks are Eocene and Oligocene calc-alkalic lavas and Oligocene pumiceous tuff. The Missoula Group is entirely in thrust sheets that postdate the Colorado, so the thrusting is no older than Campanian, but the thrusts are cut by 72 to 74 m.y.B.P. plutons. The Johnson thrust of Fraser and Waldrop, on the 1972 U.S. Geological Survey Geologic Quadrangle Map 988, is part of this thrust system. A klippe of Missoula Group on Morrison Hill is an erosional remnant, but most of the overthrust rocks are west of the Wise River valley and probably overlie Phanerozoic strata. In addition to the thrust sheets, two families of high-angle faults dominate. One family trends west-northwest and displacements along these faults can be measured in kilometers. The Johnson thrust is interpreted to have been displaced by a sinistral fault of this system along the straight valley of Big Hole River between Seymour Creek and Dewey. The eastward projection of the mountain front at Maiden Rock, just south of Divide, resulted from block displacement along two strands of this fault. This particular fault is of major significance because it marks an abrupt jump in the initial strontium ratios of intrusive rocks, from values typical of the Boulder batholith (0.706 to 0.709) to those of the Pioneer batholith (0.711 to 0.716), indicating that different crustal blocks were juxta osed. The second family of high-angle faults trends north-northeast. Wise River valley is interpreted to be a graben in this system. One important fault in the system is the Fourth of July fault, with downdrop (to west) of several kilometers; the fault may be continuous with the Comet Mountain fault. The west-northwest fault system began at least before the Eocene lava flows, but complex field relations between the two high-angle fault systems indicate their growth must have overlapped in age, possibly through the late Tertiary. End_of_Article - Last_Page 1362------------