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Detrital zircon from three coarse-grained marine siliciclastic rocks was analyzed for U–Pb, Lu–Hf, and trace element compositions to constrain the timing of deposition and sediment provenance of the Cunningham Lake formation (formerly siliciclastic unit of the Sitlika assemblage) in north-central British Columbia. This strategy tests previously proposed sedimentary linkages between the Cache Creek terrane and the westerly rocks of the Stikine terrane. All three samples indicate maximum depositional ages at ca. 205–202 Ma (Rhaetian). The samples contain a predominant ca. 225–215 Ma detrital population, sourced from proximal contemporaneous volcanic arcs, and minor Permian to Middle Triassic and Carboniferous arc-derived detrital populations. The absence of Precambrian grains is consistent with the strongly suprachondritic zircon compositions (εHf(t) = +7 to +20), and indicates exclusively juvenile sources for the Cunningham Lake formation. Late Triassic sources of zircon are not known in the Cache Creek terrane and, except within western Stikine terrane, are uncommon among the Intermontane terranes that amalgamated with the Cache Creek terrane during Late Triassic–Early Jurassic. The Stikine suite (ca. 230–214 Ma) and coeval volcanic rocks in western Stikinia are the most probable sources of Late Triassic detritus for the Cunningham Lake formation. Stikinia's Paleozoic basement is the probable source of Carboniferous detrital zircon. Volcanic arc–backarc complexes in the Cache Creek terrane are the most likely sources of Permian to Middle Triassic detritus in the Intermontane terranes. Accordingly, the siliciclastic rocks of the Cunningham Lake formation represent an overlap sedimentary succession that links Stikinia to the Cache Creek terrane by the latest Triassic. [ABSTRACT FROM AUTHOR]

New bedrock mapping completed as part of the Nechako NATMAP Project indicates that the area between Babine and Takla lakes in central British Columbia is underlain by rocks of the Early Permian Asitka, Late Triassic Takla, and Early to Middle Jurassic Hazelton volcanic-arc assemblages of the Stikine Terrane. These are cut by large composite stocks of quartz diorite, granodiorite, and quartz monzonite previously mapped as the Late Triassic to Early Jurassic Topley intrusions. New U/Pb (n = 6) and laser [sup 40] Ar/[sup 39] Ar (n = 10) isotopic age dates reported in this paper suggest there are two distinct ages of plutons: the Topley intrusive suite with isotopic ages between 218 and 193 Ma; and, east of Babine Lake, the new Spike Peak intrusive suite with isotopic ages ranging from 179 to 166 Ma. West of the main plutonic belt is a thick volcanic succession of subaerial, porphyritic andesite flows, volcanic breccias, and rhyolitic ash-flow tuffs that have isotopic ages between 185 and 174 Ma. These rocks are assigned to the Saddle Hill Formation of the Hazelton Group. The plutonic roots of this proximal arc assemblage are most likely the coeval and compositionally similar plutons of the Spike Peak intrusive suite that have been unroofed in the area east of the Takla Fault. Major oxide and trace element data support the interpretation that the Topley and Spike Peak granitic rocks formed in a juvenile volcanic-arc environment and that magmatism is related to melts generated above a long-lived subduction zone of unknown orientation.Une nouvelle cartographie effectuée dans le cadre du projet NATMAP Nechako indique que la région entre les lacs Babine et Takla au centre de la Colombie-Britannique est recouverte de roches des assemblages d'arcs volcaniques du terrane de Stikine, soit Asitka du Permien précoce, Takla du Trias tardif et Hazelton du Jurassique précoce à moyen. Ces assemblages sont recoupés par des petits massifs intrusifs composites de diorite quartzique, de granodiorite et de monzonite quartzique qui avaient auparavant été cartographiés en tant que les intrusions Topley, du Trias tardif au Jurassique précoce. De nouvelles datations U-Pb (n = 6) et isotopes laser [sup 40] Ar/[sup 39] Ar (n = 10) présentées dans cet article suggèrent deux âges distincts de plutons, soit la suite intrusive de Topley avec des âges isotopiques entre 218 et 193 Ma et, à l'est du lac Babine, la nouvelle suite intrusive de Spike Peak dont les âges isotopiques varient de 179 à 166 Ma. À l'ouest de la ceinture plutonique principale se trouve une séquence volcanique épaisse d'écoulements subaériens d'andésite porphyrique, des brèches volcanique et des tufs d'écoulement de cendre rhyolitique dont les âges isotopiques varient entre 185 et 174 Ma. Ces roches sont assignées à la Formation de Saddle Hill du Groupe Hazelton. Les racines plutoniques de cet assemblage d'arc proximal sont fort probablement les plutons contemporains et à composition similaire de la suite intrusive de Spike Peak qui ont été décapés dans la région à l'est de la faille de Takla. Les données sur les principaux oxydes et les éléments traces corroborent l'interprétation que les roches granitiques de Topley et de Spike Peak se sont formées dans un environnement d'arc volcanique juvénile et que le magmatisme est relié aux fusions qui se sont produites au-dessus d'une ligne de subduction de longue durée mais dont l'orientation est inconnue.[Traduit par la Rédaction] [ABSTRACT FROM AUTHOR]

Abstract: The purpose of this paper is to realize the sustainable development of Three Forks Lake and control pollution. It analyzes the water quality of Three Forks Lake in the following aspects: PH, NH3-N, total phosphorus, total nitrogen, permanganate index, transparency, TDS, DO, conductivity. The author did some researches on water environment of Three Forks Lake by means of Cluster analysis and Inverse Distance-weighted Interpolation. The study reveals that most water area of Three Forks Lake is polluted seriously because the water here is having a PH greater than 7 and a greater turbidity, and the water nearer to inland is harder, having more nitrogen, higher permanganate index, less dissolved oxygen. This paper not only theoretically studiess the water quality of Three Forks Lake by cluster analysis, but also get a conclusion of pollution with the combination of the information from Figure 1, Figure 3 and Figure 4, which bascially identical with the authority''s data. [Copyright &y& Elsevier]

The eastern margin of the Kechika Graben in the vicinity of Muncho and Moose lakes, northeastern British Columbia, contains Middle Cambrian rift-related strata herein named the Mount Roosevelt Formation. The formation conformably overlies a quartzite package equivalent to the uppermost clastic unit of the informal late Early Cambrian Gataga group (Bonnia–Olenellus zone) and is conformably overlain by a thick, unnamed carbonate sequence of Middle Cambrian age (Plagiura–Poliella zone). The Mount Roosevelt Formation is subdivided into three members. The lowermost member is characterized by oöid-bearing siltstone and sandstone, interbedded with dolostone, limestone, and hematitic conglomerate. Conformably above this, the middle member is a thick sequence of polymict cobbly pebble conglomerate. The upper member includes karstified dolostone, calcareous-cemented conglomerate and sandstone, and limestone. Collectively the Mount Roosevelt Formation reflects alluvial fan delta progradation into a transgressive marine environment. Deposition occurred in an active fault-controlled basin, located on the eastern margin of the Kechika Graben adjacent to the Muskwa High. Basin initiation in the late Early Cambrian coincided with the reactivation of pre-existing regional faults. High rates of subsidence during the initial phase of extension were accommodated on these faults, which provided a locus for fan delta deposition. Continuing high rates of subsidence limited basinward fan delta development. Deposition of the formation ended with base-level transgression in the early Middle Cambrian that drowned the fan deltas and the adjacent Muskwa High and allowed development of the Kechika Trough above the older graben system. Située à proximité des lacs Muncho et Moose dans le nord-est de la Colombie-Britannique, la bordure est du graben de Kechika contient des strates, ici nommées la Formation de Mount Roosevelt (Cambrien moyen), qui sont reliées à la distension. La formation repose en concordance sur un ensemble de quartzite équivalent à l’unité supérieure, clastique, du groupe de Gataga, informel, datant du Cambrien précoce (zone à Bonnia–Olenellus); elle est recouverte en concordance par une épaisse séquence de carbonate, sans nom, datant du Cambrien moyen (zone à Plagiura–Poliella). La Formation de Mount Roosevelt est subdivisée en trois membres. Le membre inférieur est caractérisé par des siltstones et des grès à oolites, interstratifiés avec des dolomies, des calcaires, et des conglomérats hématitiques. Par-dessus, en concordance, le membre du centre est composé d’une épaisse séquence de conglomérat grenu, hétérogène et caillouteux. Le membre supérieur comprend de la dolomie karstifiée, du conglomérat et du grès à ciment calcaire, et du calcaire. Dans son ensemble, la Formation de Mount Roosevelt est le reflet de la progradation d’un delta alluvionnaire en un environnement marin de transgression. La déposition a eu lieu dans un bassin contrôlé par des failles actives situé sur la bordure est du graben de Kechika, voisin de la zone surélevée de Muskwa. L’amorce du bassin à la fin du Cambrien précoce coïncide avec la réactivation de failles régionales préexistantes. Au cours de la phase initiale d’extension, des taux élevés de subsidence ont été facilités sur ces failles, ce qui a fourni un endroit pour la déposition du delta alluvionnaire. Des taux de subsidence continuellement élevés ont limité le développement du delta alluvionnaire du côté du bassin. La déposition de la formation s’est terminée avec une transgression au niveau de la base au Cambrien moyen précoce, ce qui a ennoyé les deltas alluvionnaires et la zone avoisinante surélevée de Muskwa et a permis le développement de la fosse Kechika par-dessus l’ancien système de graben. [ABSTRACT FROM AUTHOR]

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This paper presents reconstructions of Holocene sediment yield from four alpine lakes in southern British Columbia. Sediment yield is reconstructed on the basis of suites of dated and correlated cores from each lake. Facies analysis of the complete set of cores suggests that significant changes in sediment delivery have occurred during the Holocene, specifically a shift to a higher energy sedimentary environment in cooler wetter conditions in the late Holocene. Estimated sediment yields range between 2.3 and 11.2 t km-2 a-1. Maximum Holocene variability is a 115% increase at one site. Highest variability is observed at sites just above treeline suggesting that reduced vegetation cover under cooler conditions is a key control on temporal variability of sediment yield at these sites. The temporal variability observed at Holocene timescales is within the range of natural spatial variability observed between the four closely spaced lakes. Overall, the results suggest that the response of small catchments to climate change is highly contingent upon local catchment conditions, and that at all but the most sensitive sites sediment yield changes associated with moderate climate warming are likely to be of a lesser magnitude than the natural spatial variability. [Copyright &y& Elsevier]

The Quaternary stratigraphy of the Nechako River – Cheslatta Lake area of central British Columbia is described and interpreted to reconstruct the late Quaternary history of the region. Exposures of glacial and nonglacial sediments deposited prior to the last glaciation (Fraser) are limited to three sites. Pollen assemblages from pre-Fraser nonglacial sediments at two of these sites reveal forested conditions around 39 000 BP. During the advance phase of the Fraser Glaciation, glacial lakes were ponded when trunk glaciers blocked some tributary valleys. Early in the glaciation, the drainage was free in easterly draining valleys. Subsequently, the easterly drainage was blocked either locally by sediments and ice or as a result of impoundment of the Fraser River and its tributaries east of the study area. Ice generally moved east and northeast from accumulation zones in the Coast Mountains. Ice flow was influenced by topography. Major late-glacial lakes developed in the Nechako River valley and the Knewstubb Lake region because potential drainage routes were blocked by ice.La stratigraphie du Quaternaire de la région de la rivière Nechako et du lac Cheslatta sise au centre de la Colombie Britannique est décrite et interprétée dans le but de reconstruire l'histoire du Quaternaire tardif de cette région. Seul trois coupes stratigraphiques mettent en évidence des sédiments glaciaires et non-glaciaires mis en place avant la dernière glaciation (Fraser). À deux sites, l'assemblage pollinique des sédiments non-glaciaires prédatant la Glaciation de Fraser reflète un environnement forestier aux environs de 39 000 BP. Lors de l'avancée des glaciers au début de la Glaciation de Fraser, des lacs glaciaires furent retenus dans les vallées secondaires par les glaciers qui occupaient les vallées primaires. Au début de cette glaciation, le drainage vers l'est fut tout d'abord ouvert mais subséquemment, il fut bloqué soit localement par des sédiments et de la glace ou en raison du blocage du fleuve Fraser et ses tributaires à l'est de la région d'étude. Les glaciers se sont écoulés vers l'est et le nord-est à partir de zones d'accumulation centrées sur la Chaîne Côtière. Les écoulements glaciaires furent influencés par la topographie. Des lacs post-glaciaires se sont formés dans la vallée de la rivière Nechako et dans la région du lac Knewstubb suite au blocage du drainage potentiel par la glace.[Traduit par la Rédaction] [ABSTRACT FROM AUTHOR]

Despite early interest in Neogene floras, primarily Miocene sites associated with Mio-Pliocene volcanic deposits of the Interior Plateau of British Columbia, few systematic accounts of the Miocene macrofloras of British Columbia - or elsewhere in non-Arctic Canada - have been published since the pioneering studies of J. W. Dawson and his contemporaries in the late 19th century. In this report, the Red Lake macroflora from sediments of the middle Miocene Deadman River Formation exposed in the Red Lake diatomite mine north of Kamloops, British Columbia, is illustrated, and a preliminary assessment presented, along with a brief review of Miocene floras from British Columbia and the U.S. Pacific Northwest. The Red Lake macroflora contains rare Ginkgo leaves, shoots of Cupressaceae (Cupressinocladus, Metasequoia, Taxodium) and shoots and seeds of Pinaceae (Pseudotsuga, Tsuga), maple (Acer) seeds and leaves, Liquidambar (fruit), Trochodendraceae (Zizyphoides auriculata leaves, Nordenskioeldia interglacialis fruits), leaves of 4 species of red and white oaks (Quercus columbiana, Q. prelobata, Q. pseudolyrata, Quercus sp.), leaves of an alder (Alnus harneyana) and birch (Betula thor), chestnut (Castanea spokanensis), beech (Fagus pacifica), sycamore (Platanus dissecta), elm (Ulmus speciosa), leaves of unidentified taxa, fruits of Tilia pedunculata (Malvaceae) and fruits and inflorescences of other unidentified taxa, and leaves of a reed or rush (indet. monocot). The Red Lake middle Miocene climate reconstructed from leaf physiognomy was temperate and mesic, with mean annual temperature ~11-13°C, mild winters (coldest month mean temperature ~3°C), mean annual precipitation 170 -51/+73 cm/yr, and growing season precipitation ~92 cm, with moderate seasonality of precipitation (three wettest months ~51 cm vs. three driest months ~25 cm). The Red Lake flora shows similarities to middle to late Miocene floras from the U.S. Pacific Northwest (i.e., richness in oaks) but is of much lower diversity and lacks key elements common to many of the contemporaneous U.S. Miocene floras (e.g., foliage of Pinaceae esp. Pinus), and is missing taxa detected in the microflora, a pattern likely due to sampling effectiveness at the Red Lake Mine and sampling of different lithofacies for macro- and microfloras. [ABSTRACT FROM AUTHOR]

Recent methodological advances have led to a rapid expansion of evolutionary studies employing three‐dimensional landmark‐based geometric morphometrics (GM). GM methods generally enable researchers to capture and compare complex shape phenotypes, and to quantify their relationship to environmental gradients. However, some recent studies have shown that the common, inexpensive, and relatively rapid two‐dimensional GM methods can distort important information and produce misleading results because they cannot capture variation in the depth (Z) dimension. We use micro‐CT scanned threespine stickleback (Gasterosteus aculeatus Linnaeus, 1758) from six parapatric lake‐stream populations on Vancouver Island, British Columbia, to test whether the loss of the depth dimension in 2D GM studies results in misleading interpretations of parallel evolution. Using joint locations described with 2D or 3D landmarks, we compare results from separate 2D and 3D shape spaces, from a combined 2D‐3D shape space, and from estimates of biomechanical function. We show that, although shape is distorted enough in 2D projections to strongly influence the interpretation of morphological parallelism, estimates of biomechanical function are relatively robust to the loss of the Z dimension. [ABSTRACT FROM AUTHOR]

Four samples ( TL5b, TL11h, TL11i, and TL11v) from the pristine collection of the Tagish Lake meteorite, an ungrouped C2 chondrite, were studied to characterize and understand its alteration history using EPMA, XRD, and TEM. We determined that samples TL11h and TL11i have a relatively smaller proportion of amorphous silicate material than sample TL5b, which experienced low-temperature hydrous parent-body alteration conditions to preserve this indigenous material. The data suggest that lithic fragments of TL11i experienced higher degrees of aqueous alteration than the rest of the matrix, based on its low porosity and high abundance of coarse- and fine-grained sheet silicates, suggesting that TL11i was present in an area of the parent body where alteration and brecciation were more extensive. We identified a coronal, 'flower'-like, microstructure consisting of a fine-grained serpentine core and coarse-grained saponite-serpentine radial arrays, suggesting varied fluid chemistry and crystallization time scales. We also observed pentlandite with different morphologies: an exsolved morphology formed under nebular conditions; a nonexsolved pentlandite along grain boundaries; a 'bulls-eye' sulfide morphology and rims around highly altered chondrules that probably formed by multiple precipitation episodes during low-temperature aqueous alteration (≥100 °C) on the parent body. On the basis of petrologic and mineralogic observations, we conclude that the Tagish Lake parent body initially contained a heterogeneous mixture of anhydrous precursor minerals of nebular and presolar origin. These materials were subjected to secondary, nonpervasive parent-body alteration, and the samples studied herein represent different stages of that hydrous alteration, i.e., TL5b (the least altered) < TL11h < TL11i (the most altered). Sample TL11v encompasses the petrologic characteristics of the other three specimens. [ABSTRACT FROM AUTHOR]

The ecological theory of adaptive radiation states that differences in ecological circumstances among local populations are the cause of divergence that leads to speciation. The role of parasites in contributing to divergence has seldom been considered, despite their ubiquity and known selective effects. The potential for parasites to contribute to divergence between closely related taxa was examined by quantifying the variation in parasite burdens between sympatric three-spined stickleback species ( Gasterosteus aculeatus complex) in two lakes in coastal British Columbia, Canada. In doing so the relative importance of geographical differences between lakes and trophic or microhabitat differences between species within lakes were evaluated. The entire metazoan parasite burdens of a total of 255 limnetic and benthic sticklebacks in Paxton and Priest lakes were assayed over five time points between spring and autumn. Despite their sympatric distributions, there were large differences in parasite burdens between benthic and limnetic sticklebacks within lakes and these were consistent across both lakes. In particular, limnetics suffered greater burdens of the parasites Schistocephalus solidus and Diplostomum scudderi and benthics had much higher burdens of parasitic glochidia (mollusc larvae). Parasite burdens also differed quantitatively between lakes, but in general such differences were less pronounced than those between the stickleback species. The documented differences in parasite burdens between stickleback species have potential to contribute to divergent selection on life history, immunological and secondary sexual characters that could contribute to reproductive isolation between the species. [ABSTRACT FROM AUTHOR]

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The Laurentide Ice Sheet reached the Canadian Cordillera during the last glacial maximum in northeastern British Columbia and adjacent Northwest Territories and all regional drainage to unglaciated areas in the north was dammed by the ice. Converging ice-flow patterns near the mountain front suggest that the Laurentide Ice Sheet likely coalesced with the Cordilleran Ice Sheet during the last glaciation. With deglaciation, the ice masses separated, but earlier ice retreat in the south meant that meltwater pooled between the mountain front and the Laurentide margin. The level of the flooding was controlled by persistent ice cover on the southern Franklin Mountains. Glacial Lake Liard formed when the Laurentide Ice Sheet retreated east of the southern Liard Range and, at its maximum extent, may have impounded water at least as far south as the Fort Nelson River. Deglaciation of the plains was marked by local variations in ice flow caused by a thin ice sheet becoming more affected by the topography and forming lobes in places. These lobes caused diversions in local drainage readily traced by abandoned meltwater channels. Radiocarbon ages from adjacent areas suggest the relative chronology of deglaciation presented here occurred between 13 and 11 ka BP. L’Inlandsis laurentidien a atteint la Cordillère canadienne durant le dernier maximum glaciaire dans le nord-est de la Colombie-Britannique et les Territoires du Nord-Ouest adjacents et tout le drainage régional vers des secteurs non glaciés au nord était retenu par la glace. Des patrons d’écoulement glaciaire convergents à proximité du front des montagnes suggèrent que l’Inlandsis laurentidien se soit probablement uni avec la glace de la Cordillère au cours de la dernière glaciation. Lors de la déglaciation, les masses de glace se sont séparées. Cependant, le retrait antérieur au sud signifiait une accumulation de l’eau de fonte entre le front des montagnes et la bordure laurentidienne. Le niveau d’inondation était contrôlé par la surface gelée persistante sur les monts Franklin Sud. Le lac glaciaire Liard s’est formé lorsque l’Inlandsis laurentidien a reculé à l’est de la chaîne Liard sud et, à son étendue maximale, il pourrait avoir endigué de l’eau au moins aussi au sud que la rivière Fort Nelson. La déglaciation des plaines a été marquée par des variations locales de l’écoulement glaciaire causées par le fait qu’une couche de glace mince est plus affectée par la topographie et forme des lobes à certains endroits. Ces lobes ont dévié le drainage local facilement tracé par les chenaux abandonnés d’eau de fonte. Les âges radiocarbone de secteurs adjacents suggèrent que la chronologie relative de déglaciation présentée ici ait eu lieu entre 13 et 11 ka avant le présent. [ABSTRACT FROM AUTHOR]

The Taseko Lakes area yields the most diverse and well-preserved late Hettangian ammonite assemblage in British Columbia. In total, 27 ammonite taxa are described, of which Fergusonites hendersonae, Eolytoceras constrictum and Pseudaetomoceras victoriense are new. Based on this study, Phylloceras cf. asperaense Hillebrandt, Togaticeras togatum (Neumayr), T. aff. goisernense Rakús, Eolytoceras aff. guexi Taylor, Discamphiceras aff. silberlingi Guex, Schlotheimia cf. cuevitensis Hillebrandt and Franziceras aff. graylockense Taylor are newly recognized in the North American Mineralense Zone whereas Nevadaphyllites psilomorphus (Neumayr), N. aff. compressus Guex, Fergusonites hendersonae sp. nov., Eolytoceras chongi Hillebrandt, E. constrictum sp. nov., Paracaloceras spp. A and B, P.? spp., Metophioceras? sp. and Schreinbachites? aff. laqueoides (Hyatt) are newly recognized in the Rursicostatum Zone. Discamphiceras silberlingi, D. sp. nov., Alsatites aff. bipartitus (Hillebrandt), Alsatites sp., Pseudaetomoceras victoriense sp. nov. and Paracaloceras aff. varaense Hillebrandt are newly recognized in both the Mineralense and Rursicostatum zones. The Taseko Lakes fauna allows clear correlations with contemporaneous strata in other areas of North America as well as with the Canadensis Zone and probably the Arcuatum horizon of the South American succession. Broad correlations are also possible with Hettangian or lowest Sinemurian strata in New Zealand and the eastern Tethys. Furthermore, the Taseko Lakes fauna permits correlation of the Mineralense and Rursicostatum zones with the circum-Mediterranean Marmoreum Zone. Finally, this fauna suggests that the Mineralense Zone and the lower to middle portion of the Rursicostatum Zone in Taseko Lakes are equivalent to the Complanata Subzone whereas the upper portion of the Rursicostatum Zone correlates with the Depressa Subzone of the north-west European Angulata Zone. [ABSTRACT FROM AUTHOR]

Copyright of Canadian Journal of Earth Sciences is the property of Canadian Science Publishing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

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Three classic sections of Middle and Late Triassic fossiliferous limestones cropping out around Williston Lake in British Columbia, Canada, were sampled for paleomagnetic study. The objective was to test the suitability of these units for detailed magnetobiostratigraphic study with the aim of improving the reference Triassic geomagnetic polarity time scale. The Williston Lake characteristic magnetizations differ, however, from any Triassic North America cratonic reference directions. A satisfactory agreement is found instead with Cretaceous – early Cenozoic North America cratonic reference directions. The exclusive occurrence of normal polarity suggests that remagnetization likely occurred during the Cretaceous long normal superchron. Remagnetizations may have been triggered by connate brines, which moved along aquifers of porous sandstones and carbonates in the early stages of Laramide folding.Trois sections fossilifères classiques de calcaires (Trias moyen et tardif), affleurant autour du lac Williston en Colombie Britannique, ont été échantillonnées pour une étude paléomagnétique. L'objectif était de s'assurer de la pertinence de ces unités pour une étude magnéto-bio-stratigraphique détaillée visant à améliorer l'échelle référentielle de temps de la polarité géomagnétique au Trias. Les magnétisations caractéristiques du lac Williston diffèrent toutefois de toute direction cratonique référentielle au Trias en Amérique du Nord. Toutefois, une corrélation satisfaisante est établie avec les directions cratoniques référentielles du Crétacé–Cénozoïque précoce en Amérique du Nord. L'occurrence exclusive d'une polarité normale suggère qu'il y eut probablement une remagnétisation au cours du long superchron normal du Crétacé. Les remagnétisations peuvent avoir été déclenchées par des saumures connées qui se sont déplacées le long d'aquifères de grès et de carbonates poreux dans les premières phases du plissement Laramide.[Traduit par la Rédaction] [ABSTRACT FROM AUTHOR]

New mapping, mineralogical, and geochemical studies help characterize late Tertiary primitive, alkaline, sodic basanite, alkali olivine basalt, transitional basalt, and diabase in the Nechako River, Whitesail Lake, and McLeod Lake map areas of central British Columbia and distinguish the Miocene Cheslatta Lake suite. The suite encompasses scattered erosional remnants of topographically distinct, columnar-jointed, olivine-phyric basalt and diabase volcanic necks, dykes, and associated lava flows north of the Anahim volcanic belt and west of the Pinchi Fault. Volcanic centres at Alasla Mountain and at Cutoff Creek, near Cheslatta Lake, are proposed as type areas. Olivine, plagioclase, and pyroxene phenocrysts, megacrysts, and (or) xenocrysts; common ultramafic xenoliths; and rare but significant plutonic and metamorphic xenoliths are characteristic. Basanite, transitional basalt, and alkali olivine basalt groundmass contain plagioclase, clinopyroxene, Fe-Ti oxides, feldspathoid, olivine, and apatite. The Cheslatta Lake suite is characterized by its alkaline character, olivine-rich (>10 wt.%) normative mineralogy, and silica-undersaturated nature (>1 wt.% normative nepheline; hypersthene-normative rocks are uncommon). Mg numbers vary between 72–42. Some samples encompass near-primitive mantle melt compositions. Cheslatta Lake suite rocks in the Nechako River area are distinguished from the underlying Eocene Endako and stratigraphically higher Neogene Chilcotin groups basaltic andesite lavas within the study area, and from the Chilcotin Group basalt in the type area south of the Anahim volcanic belt, by form, preserved thickness, phenocryst–xenocryst mineralogy, amygdule abundance, included xenoliths, isotopic age, and major and incompatible, high field strength, and rare-earth trace element contents.Une nouvelle cartographie et de nouvelles études minéralogiques et géochimiques aident à caractériser la basanite sodique, alcaline et primitive du Tertiaire tardif, le basalte alcalin à olivine, le basalte de transition et la diabase dans les régions cartographiques de la rivière Nechako et des lacs Whitesail et McLeod du centre de la Colombie-Britannique et aident à distinguer la suite de Cheslatta Lake, datant du Miocène. La suite comprend des lambeaux épars de l'érosion de roches topographiquement distinctes et à structure prismée; ces roches comprennent des basaltes à olivine porphyrique, des « necks » de diabase volcanique, des dykes et des coulées de lave associées, au nord de la ceinture volcanique Anahim et à l'ouest de la faille Pinchi. Nous proposons les centres volcaniques du mont Alasla et du ruisseau Cutoff, près du lac Cheslatta, comme régions types. Les roches suivantes sont caractéristiques des phénocristaux, des mégacristaux et (ou) des xénocristaux d'olivine, de plagioclase et de pyroxène; des xénolites ultramafiques communs et de rares mais significatifs xénolites plutoniques et métamorphiques. La matrice rocheuse de basanite, de basalte de transition et de basalte alcalin à olivine comprend du plagioclase, du clinopyroxène, des oxydes de Fe–Ti, des feldspathoïdes, de l'olivine et de l'apatite. La suite de Cheslatta Lake est caractérisée par son caractère alcalin, sa minéralogie normative riche en olivine (>10 % en poids) et sa sous-saturation en silice (les roches à > 1 % en poids de néphéline normative, hypersthène normatif sont peu communes). Les nombres pour le Mg varient entre 72 et 42. Quelques échantillons présentent des compositions du manteau quasi-primitif en fusion. Dans la région de la rivière Nechako, les roches de la suite de Cheslatta Lake se distinguent des roches sous-jacentes Endako, datant de l'Éocène, et des laves basaltiques à andésite des groupes Chilcotin, datant du Néogène, stratigraphiquement plus hautes ainsi que du basalte du Groupe Chilcotin dans la région type au sud de la ceinture volcanique Anahim par les points suivants la forme, la conservation de l'épaisseur, la minéralogie des phénocristaux–xénocristaux, l'abondance d'éléments amygdaloïdes, les xénolites inclus, l'âge isotopique ainsi que, significatif et incompatible, une grande intensité du champ et des contenus trace en éléments des terres rares.[Traduit par la Rédaction] [ABSTRACT FROM AUTHOR]

Erosion along the Nechako Reservoir and Cheslatta River Spillway has resulted in unusually well-exposed late Quaternary and Holocene stratigraphy. Surficial sediments in the study area are mostly products of Late Wisconsinan glaciation. However, evidence for pre-Late Wisconsinan sedimentation has been found along the shores of the Nechako Reservoir, including till of an older glaciation and organic-bearing, blue-grey, lacustrine sediments of probable Middle Wisconsinan age. Stratigraphic correlation of the lake sediments suggests that an extensive lake system occurred in the region during the Olympia Nonglacial Interval. Late Wisconsinan ice initially moved along major valleys, with glaciofluvial outwash deposited in front of the advancing ice. Advance-phase glaciolacustrine sediments are rare but significant, as slope failures are spatially associated with areas where they are preserved. The distribution of these sediments and associated deltaic deposits indicates that advance-phase glacial lakes occurred up to approximately 855 m asl, at least several metres above the modern reservoir level. Sediments deposited in front of the ice margin were overridden during ice advance and are best preserved in large valleys. At the glacial maximum, ice flowed northeasterly throughout the study region. Crag and tails, flutings, and drumlinoid ridges with a generally consistent northeast trend are the dominant landforms. Till is the most common Pleistocene surficial sediment, covering approximately 80% of the area; large areas of exposed bedrock are rare. Late-glacial glaciofluvial and Holocene fluvial deposits are uncommon and occur mainly along the Cheslatta River valley. De l'érosion le long du réservoir Nechako et du déversoir de la rivière Cheslatta a produit de très beaux affleurements de la stratigraphie du Quaternaire tardif et de l'Holocène. Les sédiments de surface dans la région à l'étude sont surtout des produits de la glaciation du Wisconsinien tardif. Toutefois, des preuves de sédimentation au pré-Wisconsinien tardif ont été retrouvées le long des berges du réservoir Nechako; elles comprennent un till d'une glaciation plus ancienne et des sédiments lacustres gris-bleu contenant de la matière organique, datant probablement du Wisconsinien moyen. La corrélation stratigraphique des sédiments lacustres suggère qu'un système extensif de lacs était en place dans la région durant l'intervalle non glaciaire Olympia. La glace du Wisconsinien tardif a tout d'abord avancé le long des vallées majeures alors que du matériel stratifié fluvio-glaciaire était déposé en avant de la glace qui avançait. Des sédiments glacio-lacustres de phase avancée sont rares mais importants car les ruptures de pentes sont associées aux régions où ils sont préservés. La distribution de ces sédiments et des dépôts deltaïques associés indique que des lacs glaciaires de phase avancée se sont retrouvés à environ 855 m au-dessus du niveau de la mer, au moins plusieurs mètres au-dessus du niveau moderne du réservoir. Lors d'une avancée glaciaire, la glace passe par-dessus les sédiments déposés en avant du front glaciaire et ils sont ainsi mieux conservés dans les grandes vallées. Au maximum glaciaire, la glace coulait vers le nord-est dans toute la région à l'étude. Les formes de terrain dominantes, à direction généralement nord-est, sont les « crag and tail », les rainures glaciaires et les drumlinoïdes. Le till est le sédiment de surface le plus commun du Pléistocène, couvrant environ 80 % de la région; les grandes aires d'affleurements rocheux sont rares. Les dépôts fluvio-glaciaires tardi-glaciaires et les dépôts fluviaux de l'Holocène sont peu communs et se retrouvent surtout le long de la vallée de la rivière Cheslatta.[Traduit par la Rédaction] [ABSTRACT FROM AUTHOR]