Counter point bar deposits: lithofacies and reservoir significance in the meandering modern Peace River and ancient McMurray Formation, Alberta, Canada

Counter point bar deposits in the meandering Peace River, North‐central Alberta, Wood Buffalo National Park, are distinct from point bar deposits in terms of morphology, lithofacies and reservoir potential for fluids. Previously referred to as the distal‐most parts of point bars, point bar tails and concave bank‐bench deposits, counter point bar deposits have concave morphological scroll patterns rather than convex as with point bars. The Peace is a large river (bankfull discharge 11 700 m³ sec^?1, width 375 to 700 m, depth 15 m, gradient 0·00004 or 4 cm km^?1) in which counter point bar deposits are dominated by silt (80% to 90%), which contrasts with sand‐dominant (90% to 100%) point bar deposits. Beginning at the meander inflection (transition from convex to concave), counter point bar deposit stratigraphy thickens as a wedge‐like architecture in the distal direction until the deposit is nearly as thick as the point bar deposits. The low permeability silt‐dominant lithofacies in counter point bar deposits will limit reservoir extent and movement of fluids in both modern and ancient subsurface fluvial deposits. In the exploration and extraction of bitumen and heavy oil in subsurface fluvial rocks, identification and mapping of reservoir potential of point bar deposits and counter point bar deposits is now possible in the fluvial‐dominated tidal estuarine Lower Cretaceous Middle McMurray Formation, North‐east Alberta. Recent geophysical advances have facilitated imaging of some ancient buried point bar deposits and counter point bar deposits which, on the basis of morphological shape of sedimentary bodies observed from seismic amplitude, can be interpreted and mapped as depositional elements or blocks that contain associated sandstone or siltstone dominant lithofacies, respectively. As counter point bar deposits exhibit poor permeability and thus limit reservoir potential for water, natural gas, light crude, heavy oil and bitumen, counter point bar deposits should be avoided in resource developments. Geophysical imaging, interpretation and mapping of point bar deposit and counter point bar deposit elements provide new opportunities to improve recovery of bitumen and heavy oil and reduce development costs in subsurface cyclic steam stimulation and steam‐assisted gravity drainage projects by not drilling into counter point bar deposits.     

Petrogenesis of Garnet Lherzolite and Megacrystalline Nodules from the Thumb, Navajo Volcanic Field

Garnet Iherzolite and megacrystalline nodules from The Thumbcomprise the deepest mantle sample recovered from the diatremesand intrusions of the Four Corners area. Discrepancies betweenvarious geothermometers applied to these nodules are believedto reflect zoning of Fe in garnets as well as problems in calibrationof the geothermometers. Because the pyroxenes are not zoned,the pyroxene solvus method probably provides more reliable temperatureestimates for this nodule suite than methods based on partitioningof Fe and Mg with garnet. The preferred T-P range of the nodules,950–1230 °C and 32–40 kb, is thought to reflecta localized perturbation of the ambient geotherm by a diapiricor igneous intrusion. Garnet Iherzolites with coarse texture show a trend from fertileto refractory compositions consistent with variable depletionby partial melting. These nodules are interpreted as previouslydepleted mantle rocks, most of which were relatively littleaffected by the thermal perturbation. Garnet Iherzolites withsheared texture are enriched in Fe and Ti relative to the coarseIherzolites and are suggested to have formed by deformationand metasomatism of the coarse lherzolites during intrusionof magma related to the thermal perturbation. The texturallydiverse ‘megacrystalline’ nodules are interpretedas precipitates from this liquid. They are similar to discretenodules from kimberlite pipes, but have unique characteristicsof their own. The above relationships are thought to recordthe process of deep-seated intrusion and partial crystallizationof evolved liquids, possibly related to the host lamprophyre,shortly before the time of eruption.     

Geochemistry of the J-M Reef of the Stillwater Complex, Minneapolis Adit Area II. Silicate Mineral Chemistry and Petrogenesis

The platinum group element (PGE) rich J-M Reef of the StillwaterComplex is associated with the reappearance of olivine and chromiteas cumulus phases within a sequence of gabbroic and noriticcumulates. The Olivine-Bearing Subzone (OBZ I) which hosts theReef is interpreted as the result of fresh influxes of magmainto the Stillwater chamber. Cumulus pyroxenes from the Norite Subzone (No II) overlyingOBZ I are enriched in Fe over Mg, depleted in Cr and have similarNi contents relative to pyroxenes from the underlying GNo I.The most primitive pyroxene compositions are found in PBc andBc layers within OBZ I, and the most evolved as oikocrysts inOBZ I anorthositic layers. Plagioclase An content correlatesclosely with pyroxene MgO/FeO, except within OBZ I where a verticalAn/En trend is observed. These observations may be synthesized into a model involvingreplenishment of the chamber by high pressure differentiatesof a liquid P, representing the parental magma to the UltramaficSeries of the Stillwater. Liquid P crystallizes olivine at thepressure of the Stillwater chamber, but owing to migration ofthe olivine-orthopyroxene phase boundary is saturated with bronziteat higher pressures. The spectrum of cumulus mineralogy andmineral compositions within and above OBZ I may be generatedby a series of magma influxes having compositions derived byvarious degrees of high pressure fractionation from liquid P. The characteristic textures of the heterogenous plagioclase-olivinecumulates of OBZ I may be explained by mixing of P-type replenishingliquid with a fractionated resident liquid containing suspendedplagioclase crystals. Anorthositic layers within OBZ I formedfrom influxes of liquid derived by approximately 30 per centfractionation of P. Model density calculations indicate that the liquids shouldhave entered the chamber as a serious of buoyant plumes. Lateralvariations in OBZ I stratigraphy arise from pulses of differentsize spreading to varying distances from the feeder axis. TheReef formed from a single layer transgressing earlier faciestransitions.     

Petrology of Upper Amphibolite Facies Marbles from the East Humboldt Range, Nevada, USA; Evidence for High-Temperature, Retrograde, Hydrous Volatile Fluxes at Mid-Crustal Levels

Petrographic and petrologic investigations of upper amphibolitefacies metacarbonates from the East Humboldt Range core complex,Nevada, provide important constraints on P-T-X_CO2 conditionsand fluid flow during metamorphism. Three marble assemblagesare observed [(1)dol+cc+bt+cpx+q+ruscapkspamph; (2) cc+bt+cpx+plag+q+sphscapksp;(3) cc+cpx+plag+q+sphscapksp], all of which equilibrated withrelatively CO₂-rich fluid compositions, at P-T conditions of6 kbar and 600–750C. The most recent equilibration eventis recorded in some calcsilicate gneisses where retrograde amphiboleand epidotegarnet replace clinopyroxene and plagioclase, respectively.This is attributed to infiltration of H₂O-rich fluids at and/orafter peak metamorphic temperatures, which continued as therocks were cooled and rapidly uplifted after a Tertiary extension-relatedheating event. Likely sources for the retrograde fluids are the abundant pegmatiticleucogranites in the area. Volumetric fluid-rock ratios of 0.02–0.4are required to generate the retrograde assemblage, and observedleucogranite proportions are more than adequate to provide therequired volume of fluid. Estimates of retrograde fluid fluxesrange from 0.25 to 510² cm³/cm² for a transient temperaturegradient of 5C/m, to 310³ to 710⁴ cm³/cm² for a temperaturegradient of 35C/km. These gradients are characteristic of askarn-type contact metamorphic environment and a regional crustalgeotherm, respectively. They imply different time-scales andlength-scales for the retrograde fluid flow system, with theformer more akin to a contact metamorphic setting with local,meter-scale retrograde fluid flow, and the latter to a regionalmetamorphic setting with regionally high mid- and lower-crustaltemperatures and fluid flow throughout a significant thicknessof the middle crust. Higher gradients are considered more likelygiven the proximity of leucogranites to retrogressed calc-silicategneisses, and the resultant relatively small fluxes are consistentwith a magmatic source. The length-scale of reaction within the retrograde fluid flowsystem was of the order of meters to hundreds of meters andinvolved both pervasive and (later) fracture-controlled down-temperatureflow. Retrograde fluid flow in this terrance, as well as othersdominated by magmatic volatiles, is in the form of multiplediscrete bursts of fluid released in a discontinuous mannerpotentially over long periods of time (1–10 Ma) with locallyvariable thermal gradients along the flow path.     

New discoveries of the Vedde Ash in southern Sweden and Scotland

The Vedde Ash (c. 10 300 ¹⁴ C BP) provides a key time-parallel marker horizon within the Younger Dryas chronozone or GS-1 event of the GRIP stratigraphy. Until recently, the known distribution of wind-blown Vedde Ash outside Iceland was restricted to the west coast of Norway, off-shore sequences close to the Outer Hebrides and the Greenland summit GRIP ice core. The first discoveries of the Vedde Ash in Scotland were reported in 1997, following the development of a new technique for extracting rhyolitic micro-tephra particles from minerogenic deposits. Here we report on the discovery of the Vedde Ash at additional sites in Scotland and at sites in southern Sweden. The concentration of tephra particles in sediments is highest in sites in western Norway, but is also relatively high in sites in southwestern Sweden, suggesting that the main ash cloud travelled eastwards from its volcanic source of Katla, in southern Iceland. Electron microprobe analyses do not indicate any clear geochemical evolution within the samples reported here.     

Cordilleran Ice Sheet lobal interactions and glaciotectonic superposition through stadial maxima along a mountain front in southwestern British Columbia, Canada

Glaciotectonic structures in subglacial till and substrate, as well as stone fabric, provenance and surface features in till, indicate that complex interactions of late Wisconsinan glacial lobes occurred along a mountain front in the western Fraser Lowland of southwestern British Columbia. Tills of this study represent subglacial deposition through the maxima of two stades in the Fraser Glaciation, the Coquitlam and the Vashon. Through each stadial maximum, temperate glacial ice was grounded and commonly overrode proglacial outwash while superimposing deformations in subglacial till during three phases: (1) pre-maximum glacier flow down valleys and into lowland piedmont ice, (2) coalescent piedmont ice during stadial maxima when flow was westward along the mountain front and across valley mouths, and (3) post-maximum glacier flow down valleys into lowland piedmont ice but prior to general deglaciation. Valley glaciers appear to have shifted flow directions during phases 1 and 3. During stadial maxima (phase 2), Fraser Lowland piedmont ice may have been part of an outlet glacier-ice stream complex that terminated in salt water over the continental shelf.     

Low‐dimensional representation of error covariance

Ensemble and reduced‐rank approaches to prediction and assimilation rely on low‐dimensional approximations of the estimation error covariances. Here stability properties of the forecast/analysis cycle for linear, time‐independent systems are used to identify factors that cause the steady‐state analysis error covariance to admit a low‐dimensional representation. A useful measure of forecast/analysis cycle stability is the bound matrix , a function of the dynamics, observation operator and assimilation method. Upper and lower estimates for the steady‐state analysis error covariance matrix eigenvalues are derived from the bound matrix. The estimates generalize to time‐dependent systems. If much of the steady‐state analysis error variance is due to a few dominant modes, the leading eigenvectors of the bound matrix approximate those of the steady‐state analysis error covariance matrix. The analytical results are illustrated in two numerical examples where the Kalman filter is carried to steady state. The first example uses the dynamics of a generalized advection equation exhibiting non‐modal transient growth. Failure to observe growing modes leads to increased steady‐state analysis error variances. Leading eigenvectors of the steady‐state analysis error covariance matrix are well approximated by leading eigenvectors of the bound matrix. The second example uses the dynamics of a damped baroclinic wave model. The leading eigenvectors of a lowest‐order approximation of the bound matrix are shown to approximate well the leading eigenvectors of the steady‐state analysis error covariance matrix.