Flat-pebble conglomerate: its multiple origins and relationship to metre-scale depositional cycles

Carbonate flat‐pebble conglomerate is an important component of Precambrian to lower Palaeozoic strata, but its origins remain enigmatic. The Upper Cambrian to Lower Ordovician strata of the Snowy Range Formation in northern Wyoming and southern Montana contain abundant flat‐pebble conglomerate beds in shallow‐water cyclic and non‐cyclic strata. Several origins of flat‐pebble conglomerate are inferred for these strata. In one case, all stages of development of flat‐pebble conglomerate are captured within storm‐dominated shoreface deposits of hummocky cross‐stratified (HCS) fine carbonate grainstone. A variety of synsedimentary deformation structures records the transition from mildly deformed in situ stratification to buckled beds of partially disarticulated bedding to fully developed flat‐pebble conglomerate. These features resulted from failure of a shoreface and subsequent brittle and ductile deformation of compacted to early cemented deposits. Failure was induced by either storm or seismic waves, and many beds failed along discrete slide scar surfaces. Centimetre‐scale laminae within thick amalgamated HCS beds were planes of weakness that led to the development of platy clasts within partly disarticulated and rotated bedding of the buckled beds. In some cases, buckled masses accelerated downslope until they exceeded their internal friction, completely disarticulated into clasts and transformed into a mass flow of individual cm‐ to dm‐scale clasts. This transition was accompanied by the addition of sand‐sized echinoderm‐rich debris from local sources, which slightly lowered friction by reducing clast–clast interactions. The resulting dominantly horizontal clast orientations suggest transport by dense, viscous flow dominated by laminar shear. These flows generally came to rest on the lower shoreface, although in some cases they continued a limited distance beyond fairweather wave base and were interbedded with shale and grainstone beds. The clasts in these beds show no evidence of extensive reworking (i.e. not well rounded) or condensation (i.e. no rinds or coatings). A second type of flat‐pebble conglomerate bed occurs at the top of upward‐coarsening, metre‐scale cycles. The flat‐pebble conglomerate beds cap these shoaling cycles and represent either lowstand deposits or, in some cases, may represent transgressive lags. The clasts are well rounded, display borings and have iron‐rich coatings. The matrix to these beds locally includes glauconite. These beds were considerably reworked and represent condensed deposits. Thrombolites occur above the flat‐pebble beds and record microbial growth before initial transgression at the cycle boundaries. A third type of flat‐pebble conglomerate bed occurs within unusual metre‐scale, shale‐dominated, asymmetric, subaqueous cycles in Shoshone Canyon, Wyoming. Flat‐pebble beds in these cycles consist solely of clasts of carbonate nodules identical to those that are in situ within underlying shale beds. These deeper water cycles can be interpreted as either upward‐shoaling or ‐deepening cycles. The flat‐pebble conglomerate beds record winnowing and reworking of shale and carbonate nodules to lags, during either lowstand or the first stages of transgression.     

Cordilleran front range structural features in northwest Montana interpreted from vintage seismic reflection data

Industry seismic reflection data spanning the Rocky Mountain Cordillera front ranges of northwest Montana were reprocessed and interpreted in this study. Five seismic profiles represent 160 km of deep reflection data collected in 1983 that span the eastern Purcell anticlinorium, Rocky Mountain Trench (RMT), Rocky Mountain Basal Décollement (RMBD), and Lewis thrust. The data were reprocessed using modern techniques including refraction statics, pre-stack time migration (PSTM), and pre- and post-stack depth migration. Results indicate the RMBD is 8–13 km below the Earth's surface and dip 3–10° west. Evidence for the autochthonous Mesoproterozoic Belt and basal Cambrian rocks beneath the RMBD is present in all of the profiles and appears to extend east of the RMT. The Lewis thrust was identified in the seismic profiles and appears to sole into the RMBD east of the RMT. The RMT fault system has a dip displacement of 3–4 km and forms a half graben filled with 1 km of unconsolidated Tertiary sedimentary deposits. The RMT and adjacent Flathead fault systems are interpreted to be structurally linked and may represent a synthetic, en echelon fault system.     

Folding of a detachment and fault – Modified detachment folding along a lateral ramp,southwestern Montana,USA

The inversion of the Middle Proterozoic Belt sedimentary basin during Late Cretaceous thrusting in Montana produced a large eastwardly-convex salient, the southern boundary of which is a 200 km-long oblique to lateral ramp subtended by a detachment between the Belt rocks and Archean basement. A 10 km-long lateral ramp segment exposes the upper levels of the detachment where hanging wall Belt rocks have moved out over the Paleozoic and Mesozoic section. The hanging wall structure consists of a train of high amplitude, faulted, asymmetrical detachment folds. Initial west-east shortening produced layer parallel shortening fabrics and dominantly strike slip faulting followed by symmetrical detachment folding. “Lock-up” of movement on the detachment surface produced regional simple shear and caused the detachment folds to become asymmetrical and faulted. Folding of the detachment surface after lock-up modified the easternmost detachment folds further into a southeast-verging, overturned fold pair with a ramp-related fault along the base of the stretched mutual limb.     

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

An⁴⁰Ar/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. The⁴⁰Ar/³⁹Ar age of 85.81 Ma±0.22 my at Lima Peaks is bracketed by radiometric dates for theScaphites depressus—Protexanites bourgeoisianusbiozone and the overlyingClioscaphites 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.     

Molecular genetic markers and maturity indices in intermontane lacustrine facies: Kishenehn formation, Montana

Molecular markers have considerable promise as thermal maturation indicators in source rocks having a well-defined depositional environment. This occurs in the Kishenehn Formation (northwest Montana and southeast British Columbia), an Oligocene unit comprised of fluvial and lacustrine shales. Up to 3 km of these non-marine sediments were deposited in a 900 sq.km elongated half graben atop the Precambrian rocks of the Lewis Thrust sheet. Specific outcrops of the Coal Creek Member of the Kishenehn Formation are known to contain excellent potential source rocks, including extensive oil shales. The source rock potential and molecular marker geochemistry of most of the major Kishenehn outcrops have now been investigated, and regional results are presented in this paper.source rock analysis of a regional suite of Kishenehn samples indicates excellent petroleum potential (immature Type I kerogen) throughout the basin. Several classes of distinctive diagenetic molecular markers occur within the Coal Creek Member of the Kishenehn Formation, including diasterenes, spirosterenes, methylspirosterenes, B-ring monoaromatic anthrasteroids, ββ-hopanes and moretanes. Molecular markers indicative of non-marine deposition include dehydroabietane (conifer input) and an onocerane recently identified in leaf fossils of Miocene lacustrine beds in adjacent Idaho. Triterpane ratios are significantly more sensitive than vitrinite reflectance methods in assessing thermal maturation in the Kishenehn, and are successfully used in this study to order outcrop locations according to thermal maturity level. The systematic variation of specific markers with increasing thermal maturation suggests that molecular correlation is feasible, and could provide a method of assessing stratigraphic continuity in the basin.     

Coarse bedload transport in a mountain river

Coarse bedload transport dynamics are investigated utilizing hydrodynamic and sediment transport data obtained in an extensively instrumented study reach located in Squaw Creek, Montana, USA. During 1991 and 1992, a number of discrete bedload transport events associated with the daily rise and fall in stream discharge were investigated. Data show that initiation of sediment transport was accompanied by a reduction in bed roughness and by changes in bulk hydraulic parameters. For larger discharges, coarser fractions of the bed material mobilized, and bedload transport rates and average hydraulic parameters stabilized. As discharge reduced, mobile coarse particles became less frequent and deposited fine particles were removed, resulting in an increase in bed roughness. These observations are attributed to the downstream translation of bar sediments during the passage of a hydrograph. Bedload pulses were aperiodic but spatially variable. Flow turbulence and velocity profile data obtained during low flows allowed comparison between average bed shear stress and apparent bed roughness estimates obtained using different approaches. © 1998 John Wiley & Sons, Ltd.     

A 2200-yr record of hydrologic variability from Foy Lake, Montana, USA, inferred from diatom and geochemical data

A 2200-yr long, high-resolution (∼5 yr) record of drought variability in northwest Montana is inferred from diatoms and δ¹⁸O values of bio-induced carbonate preserved in a varved lacustrine core from Foy Lake. A previously developed model of the diatom response to lake-level fluctuations is used to constrain estimates of paleolake levels derived from the diatom data. High-frequency (decadal) fluctuations in the de-trended δ¹⁸O record mirror variations in wet/dry cycles inferred from Banff tree-rings, demonstrating the sensitivity of the oxygen-isotope values to changes in regional moisture balance. Low frequency (multi-centennial) isotopic changes may be associated with shifts in the seasonal distribution of precipitation. From 200 B.C. to A.D. 800, both diatom and isotope records indicate that climate was dry and lake level low, with poor diatom preservation and high organic carbon: nitrogen ratios. Subsequently, lake level rose slightly, although the climate was drier and more stable than modern conditions. At A.D. 1200, lake level increased to approximately 6 m below present elevation, after which the lake fluctuated between this elevation and full stage, with particularly cool and/or wetter conditions after 1700. The hydrologic balance of the lake shifted abruptly at 1894 because of the establishment of a lumber mill at the lake's outlet. Spectral analysis of the δ¹⁸O data indicates that severe droughts occurred with multi-decadal (50 to 70 yr) frequency.     

U.K.-U.S. WORKSHOP ON INTERFACING MICRO- AND MACRO-MODELS OF SPATIAL BEHAVIOR

Five semiarid Montana ghost towns abandoned for more than 45 years were studied to understand better the nature of soil and vegetation recovery following severe human impacts. Discriminant analysis was used to interpret and classify variation among land-use intensity groups. Recovery at the five towns was strongly linked to the degree of the initial soil disturbance, vegetation type, and precipitation. Recovery of the vegetation to ambient conditions was far from complete in all but one town.     

Plant functional traits and microbes vary with position on striped periglacial patterned ground at Glacier National Park,Montana

The retreating snowfields and glaciers of Glacier National Park, Montana, USA, present alpine plants with changes in habitat and hydrology. The adjacent and relic periglacial patterned ground consists of solifluction terraces of green, vegetation-rich stripes alternating with sparsely vegetated brown stripes. We established georeferenced transects on striped periglacial patterned ground for long-term monitoring and data collection on species distribution and plant functional traits at Siyeh Pass and at Piegan Pass at Glacier National Park. We documented species distribution and calculated the relative percent cover (RPC) of qualitative functional traits and used 16S rRNA from soil samples to characterize microbial distribution on green and brown stripes. Plant species distribution varied significantly and there were key differences in microbial distribution between the green and brown stripes. The rare arctic-alpine plants Draba macounii, Papaver pygmaeum, and Sagina nivalis were restricted to brown stripes, where the RPC of xeromorphic taprooted species was significantly higher at the leading edge of the Siyeh Pass snowfield. Brown stripes had a higher percentage of the thermophilic bacteria Thermacetogenium and Thermoflavimicrobium. Green stripes were co-dominated by the adventitiously-rooted dwarf shrubs Salix arctica and the possibly N-fixing Dryas octopetala. Green stripes were inhabited by Krummholz and seedlings of Abies lasiocarpa and Pinus albicaulus. Prosthecobacter, a hydrophilic bacterial genus, was more abundant on the green stripes, which had 6,524 bacterial sequences in comparison to the 1,183 sequences from the brown stripes. While further research can determine which functional traits are critical for these plants, knowledge of the current distribution of plant species and their functional traits can be used in predictive models of the responses of alpine plants to disappearing snowfields and glaciers. This research is important in conservation of rare arctic-alpine species on periglacial patterned ground.

     

UTOPIA,DYSTOPIA, AND SUBLIME APOCALYPSE IN MONTANA'S CHURCH UNIVERSAL AND TRIUMPHANT

ABSTRACT. The search for a perfected place on earth is rarely easy or forgiving. Utopias and dystopias demonstrate the practical frailties of attempts to build better human associations, whether as literary efforts or in actual, on‐the‐ground experiments. With a proprietary interest in evaluating the dimensions of place making, geographers can lay claim to be frontline critics of efforts that attempt to craft a better world. The catalog of personality cultists, charismatically murderous leaders, and apocalyptic mass death is long. The Church Universal and Triumphant, currently of Paradise Valley, Montana, is examined as the home for twenty‐five years of an ongoing effort to “improve” lives. If its dilemmas and apparent failures are in keeping with the history of past utopian ventures, geographical lessons remain to be learned about the costs of planning for perfection.