Debris flood risk assessment for Mosquito Creek, British Columbia, Canada

Mosquito Creek drains a 15.5 km² watershed on the North Shore Mountains north of Vancouver, British Columbia, Canada, and flows through the densely urbanized District and then City of North Vancouver. Previous studies determined that the creek is subject to debris floods (hyperconcentrated flows). The National Research Council of Canada is applying multi-hazard risk assessment procedures for various regions in B.C. and chose Mosquito Creek as one of its target areas. As part of its natural hazard management plan, the District of North Vancouver (DNV) requested an assessment of debris flood hazards and associated risk to life. Using a combination of empirical methods, dendrochronology and some judgment, BGC Engineering Inc. assessed debris flood hazard extent, velocity and depth for estimated 100-, 200-, 500- and 2,500-year debris flow return periods. Based on the results from the hazard assessment, risk for individuals and groups living within the hazard area, including residential homes and a fire hall, was estimated. Compared to risk tolerance criteria accepted on an interim basis by the DNV, we estimate that societal risk exceeds tolerable standards and that individual risk exceeds tolerable standards for 10 homes. The results from the risk to loss of life study have prompted DNV to implement a series of risk reduction measures including installation of a debris containment net and watershed restoration measures.     

DRASTIC-Fm: a modified vulnerability mapping method for structurally controlled aquifers in the southern Gulf Islands,British Columbia,Canada

DRASTIC, the methodology for mapping the intrinsic vulnerability of aquifers, is modified to incorporate the structural characteristics of fractured bedrock aquifers. In these aquifers, groundwater flow is predominantly through fractures, with large-scale fracture zones and faults acting as primary conduits for flow at the regional scale. The methodology is applied to the southern Gulf Islands region of southwestern British Columbia, Canada. Bedrock geology maps, soil maps, structural measurements, mapped lineaments, water-well information and topographic data, assembled within a comprehensive GIS database, form the basis for assigning traditional DRASTIC indices, while adding the structural indices necessary for capturing the importance of regional structural elements in recharge and well capture zone determinations.     

Paragenetic evolution in Peejay Field,British Columbia,Canada

Triassic sediments from the Halfway Formation of Peejay Field in British Columbia, Canada, are described here with special reference to the mineral alterations during diagenesis. These sediments were deposited along the irregular coastline during the middle Triassic transgression. The vertical lithologic succession is complex and considerably altered. The major lithologies concerned are silty sandstones, dolarenite, dololutite, sporadic coquinoid, and accumulated shell debris largely composed of pelecypods, which have been diagenetically altered to dolomite. Evaporites were either introduced through solution and precipitation or by replacement. In certain sandstone units, intense corrosion to complete replacement of clastic quartz as well as matrix by dolomite is observed. The introduction or removal of minerals in the sediments during diagenesis may result from physico-chemical changes in the equilibrium between sediments and the interstitial fluid caused by burial. Hence, the study of diagenetic minerals and paragenesis may be an important tool for interpreting the environment of deposition and the postdepositional history.

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Zusammenfassung Triassische Sedimente der Halfway-Formation des Peejay Fields in British-Columbia, Canada, werden unter Berücksichtigung der Mineralumwandlungen während der Diagenese beschrieben. Diese Sedimente wurden während der Mittel-Triassischen Transgression entlang der unregelmäßigen Küstenlinie abgelagert. Die lithologische Folge ist komplex und beträchtlich verändert. Die Hauptgesteinsfolge besteht aus tonigen Sandsteinen, Dolareniten, Dololutiten, sporadischen Koguinoiden und Schalentrümmern, die meistens von Pelecypoden stammen und diagenetisch in Dolomit umgewandelt worden sind. Evaporite entstanden entweder durch Lösungszufuhr und Ablagerung oder Verdrängung. In einigen Sandsteinhorizonten weisen sowohl klastische Quarzkörner als auch die Grundmasse intensive Korrosionserscheinungen auf, die bis zur vollständigen Verdrängung durch Dolomit führen können. Das Eindringen oder Verschwinden von Mineralien in den Sedimenten während der Diagenese kann durch physikochemische Änderungen des Gleichgewichts zwischen Sedimenten und der miteingeschlossenen Porenlösung durch Sedimentüberdeckung verursacht werden. Somit kann die Untersuchung diagenetischer Mineralien und ihrer Paragenese ein wichtiges Mittel zur Deutung des Ablagerungsmilieus und der Geschichte nach der Ablagerung sein.

     

Petrology of metapelites in the Bugaboo aureole,British Columbia,Canada

Contact metamorphism of greenschist facies Neoproterozoic turbidites by the Cretaceous Bugaboo Batholith in southeastern British Columbia has resulted in a well‐developed contact aureole. The aureole is about 1 km wide and can be divided into three main zones: (i) spotted phyllite zone, extending from the first appearance of spots of cordierite or andalusite to the last occurrence of primary chlorite; (ii) cordierite + andalusite + biotite zone, comprising hornfelses or schists with abundant porphyroblasts of cordierite and andalusite and, at higher grades, fibrolitic sillimanite; and (iii) K‐feldspar zone, characterized by hornfelses and schists that, in the inner part of this zone, are variably migmatitic. Four parts of the aureole were examined, three of which are characterized by schists, and one of which (Cobalt Lake area) is characterized by hornfelses and has exceptional exposure and comparatively unaltered rocks. Petrographic, modal, mineral‐compositional and whole rock‐compositional data were collected from the Cobalt Lake transect, allowing the prograde reaction sequence to be inferred. Notable features of the aureole at Cobalt Lake include: initial development of andalusite and plagioclase at the expense of paragonite‐rich white mica; a narrow interval across which cordierite, andalusite and biotite increase markedly at the expense of chlorite; gradual development of andalusite and biotite at the expense of cordierite and muscovite upgrade of chlorite consumption; and near‐simultaneous development of andalusite + K‐feldspar and sillimanite, the latter indicating a pressure of contact metamorphism of ~3 kbar. In other parts of the aureole, the development of sillimanite downgrade of the initial development of K‐feldspar suggests slightly higher pressures of contact metamorphism. Lack of correspondence between the observed sequence of reactions in the aureole and those predicted thermodynamically suggests that modifications to some of the thermodynamic data or activity–composition models may be required. Textural features in the aureole suggest the influence of kinetic factors on metamorphic recrystallization, including: (i) deformation‐catalysed reaction in the schists compared to the hornfelses, as indicated by different mineral‐growth sequences inferred from microstructures, and (ii) heating rate‐controlled recrystallization, as indicated by the decrease in grain size of hornfelses with increasing metamorphic grade.     

Geometry of a convergent zone, Central British Columbia, Canada

The Canadian Cordillera is separable into two major northerly trending tectonic units—the Pacific Orogen and the Columbian Orogen, with the latter further separated into the Omineca Crystalline Belt and the easterly Rocky Mountain Fold and Thrust Belt. Synkinematic metamorphism of Jurassic age within the Omineca Belt is thought to be associated with accretion of westerly terranes of the Pacific Orogen—more specifically the Quesnellia terrane—that was thrust easterly over the Omineca Belt towards the craton. Mylonitic rocks mark the margin between these two belts and this margin is well-exposed near Crooked Lake, central British Columbia.Structural analysis across the zone of convergence between these two terranes indicates that the cratonic basement and the accreted cover sequences have several phases of deformation and metamorphism in common. The initial common phase of deformation, wherein convergence is accomplished, is characterized by easterly verging folds that are superposed by a second common phase having westerly verging folds that deform the zone of convergence and control the present regional map pattern. A final common phase of deformation produced easterly verging folds.Change in vergence direction is interpreted as resulting from change in direction of transport related to subduction process: first obduction of Quesnellia onto the Omineca craton, followed later by easterly subduction of an oceanic Quesnellia below the craton.All evidence of transport direction(s) points to convergence occurring at very high angles to the zone of convergence. There appears to be no evidence of transport parallel with the strike of the zone. If transport has taken place parallel to the strike of the zone, then this transport occurred before convergence or evidence of this motion has been destroyed during the convergence.     

Controls on natural levée development in the Columbia River, British Columbia, Canada

Natural levées of the Columbia River near Golden, British Columbia, were investigated to identify the mechanisms that control levée development and morphology. Topographic profiles of 12 levée pairs were surveyed, and measurements of water-surface elevation, flow velocity, flow direction and turbidity were obtained during an average magnitude flood (1·2 years recurrence interval). Sedimentation rates and grain-size distributions were measured from sediment traps placed along levée-to-floodbasin transects. Results show that water and sediment exchange between the channel and floodbasin was mainly by advection. During flooding, local floodbasins behave more as efficient water pathways than water storage features, resulting in down-valley floodbasin flows capable of limiting basinward growth of levées. Levée shape results primarily from two independent factors: (1) maximum channel water stage, which limits levée height; and (2) floodbasin hydraulics, which control width. In the Columbia River, the competence of floodbasin flows results in relatively narrow and steep levées. Natural levées grow under two general conditions of deposition as governed by flood-stage elevation relative to levée-crest elevation: front loading and back loading. During large floods when crests are inundated, front loading preferentially aggrades the proximal portions of levées with sediment directly from the channel, thus increasing levée slope. During average or below-average floods when many levée crests are not overtopped, back loading preferentially aggrades the distal levée areas and floodbasin floor, reducing levée slope. In the study area, a balance between front and back loading sustains these narrow and steep levée shapes for long periods, reflecting an equilibrium between hydraulic regime, floodplain morphology and deposition.     

Oxidation state of mantle xenoliths from British Columbia,Canada

Mössbauer spectra for 17 spinels separated from mantle xenoliths from six different eruptive centers in southern British Columbia, Canada were measured in an effort to accurately determine their Fe³⁺/total Fe ratios, and to examine lateral and vertical variations in oxygen fugacities (f _o2's) calculated for these samples using published thermobarometric methods. Spectra acquired at 298 and 77 K suggest that both Fe²⁺ and Fe³⁺ are tetrahedrally coordinated in lherzolite spinels from this alkaline province. Calculatedf _o2's for spinel lherzolites from British Columbia range from about 0.5 to 1.5 log units below the fayalite-magnetite-quartz (FMQ) oxygen buffer at 15 kbar using the thermobarometric method of O'Neill and Wall. Thesef _o2's are on average more reducing than those reported for the upper mantle beneath the Massif Central and Japanese Arc and fall within the range for fresh MORB glasses and for lherzolite xenoliths from the southwestern United States and Mongolia. Significant variations inf _o2 between samples from different eruptive centers with varying ages are absent, indicating that the oxidation state of the upper mantle was not affected by Cenozoic magmatism within this alkaline province.     

Postglacial sediment yield to Chilliwack Lake,British Columbia,Canada

Tunnicliffe, J., Church, M. & Enkin, R. J. 2012 (January): Postglacial sediment yield to Chilliwack Lake, British Columbia, Canada. Boreas, Vol. 41, pp. 84–101. 10.1111/j.1502‐3885.2011.00219.x. ISSN 0300‐9483. Seismic records and evidence from sediment cores at Chilliwack Lake provide the basis for a long‐term (postglacial) sediment budget for a 324‐km² Cordilleran catchment. Chilliwack Lake (11.8 km² surface area), situated in the North Cascade Mountains, near Chilliwack, British Columbia, was formed behind a valley‐wide recessional moraine in the final phase of post‐Fraser alpine glaciation. Seismic surveys highlight the postglacial lacustrine record, which is underlain by a thick layer of sediments related to deglacial sedimentation. Sediment cores provide details of grain‐size fining from the delta to the distal lake basin. The cores also show a record of intermittent fire and debris flows. Magnetic measurements of lake sediments provide information on grain size, as well as a dating framework. The total postglacial lake‐floor deposit volume is estimated to be 397 ± 27 × 10⁶ m³. Including estimates of fan and delta deposition, the specific postglacial yield to the lake is calculated to be ～86 ± 13 Mg km² a^?1. The sediment volume in the uppermost (Holocene) lacustrine layer is 128 ± 9 × 10⁶ m³, representing ～41 ± 4 Mg km² a^?1 in the Holocene. Compared with other Cordilleran lakes of similar size, particularly those with glacial cover in the watershed, Chilliwack Lake has experienced relatively modest rates of sediment accumulation. This study provides an important contribution to a growing database of long‐term (postglacial) sediment yield data for major Cordilleran lakes, essential for advancing our understanding of the pace of landscape evolution in formerly glaciated mountainous regions.     

Influence of tectonic structures on the Hope Slide, British Columbia, Canada

The Hope Slide, which occurred on January 9, 1965, involved an estimated 47-Mm³ of meta-volcanics and intrusive rocks. Previous workers reported the presence of tectonic structures (faults and shear zones) along the failure surface at the Hope Slide. These tectonic features were investigated in detail to assess their effects on rock-mass quality and the related implications for slope stability. This paper integrates basic field and laboratory concepts from structural and engineering geology. Subdividing the failure area into structural domains allowed distinct discontinuity sets to be associated with specific tectonic structures. The Geological Strength Index (GSI) was used to estimate the rock-mass damage related to the tectonic structures. Low GSI values were seen to outline tectonic damage zones. Point-load tests were used to characterise the compressive strength of rocks adjacent to the tectonic structures. Strength anisotropy, tentatively attributed to damage caused by a large shear zone, was observed in greenstone samples. Seepage zones along the failure surface were observed preferentially along shallow discontinuities that dipped downslope and in rock masses of good quality (GSI > 40). An alternative morphology of the slope failure is proposed by distinguishing between the extent of the surficial damage due to the rock-slope failure and the zone of failed material (depletion zone). For the first time, a kinematic mechanism for the Hope Slide is proposed, based on a preliminary 3-dimensional block model. A pre-1965 DEM was produced from estimates of material lost and gained as reported by previous workers. The pre-1965 DEM revealed that the tectonic structures recognised during fieldwork bounded the material that failed in the 1965 event.     

Petrology and Cooling Rates of the Valhalla Complex, British Columbia, Canada

Rocks from the Valhalla metamorphic core complex, British Columbia,Canada, have experienced granulite facies metamorphism at conditionsof 82030C, 81 kbar. Peak metamorphism was accompanied bydehydration melting of muscovite, but not biotite, followedby minor back reaction of garnet $ K-feldspar $ H₂0= sillimanite$ biotite$plagioclase. At conditions very near those of thepeak, extensive shearing produced s-c (schistositS-cisaillement)fabrics, ribbon quartz and grain size reduction of garnet atseveral locations. Gamet-biotite Fe-Mg exchange thermometryyields temperatures that range from 580 to 1051C Low temperaturesare calculated from biotite modified dominantly by Fe–Mgexchange with garnet; high temperatures are calculated fromFerich biotites produced from the above retrograde reaction.Geothermometry is useless in these rocks to estimate peak temperaturea priori, but is very useful to help constrain the complex reactionhistory of biotites. Geochronology on monazite, zircon, allanite,titanite, hornblende, muscovite, biotite and apatite has beenused to constrain the timing of the metamorphic peak at 67–72Ma and the average cooling rate to 24 6C/Ma. Diffusion modelingof Fe–Mg exchange between biotite inclusions and hostgarnet yields cooling rates of either 3–80C/Ma or 200–2500C/Ma, depending on the choice of diffusion coefficients. Theformer value is consistent with the average cooling rate of24 C/Ma for the complex determined from geochronology, butthe faster rate cannot be ruled out and may indicate initialvery rapid cooling by thrusting of the complex onto cooler basementIt is suggested that cooling rates determined from geochronologicvs petrologic methods may not be directly comparable becausepetrologic methods sample near-peak nutamorphic cooling rateswhereas geochronologic methods sample post-peak to ambient coolingrates. KEY WORDS: geothermometry; geochronology; garnet diffusion; cooling rates; Valhalla complex ^*Corresponding author. Telephone: (518) 276-6103, Fax: (518) 276-8627. e-mail: spear{at}harold.geo.rpi.edu HTTP: //www.geo.rpi.edu/facstaff/spear/valhalla/valhalla.html     

Engineering geomorphological interpretation of the Mitchell Creek Landslide,British Columbia,Canada

The recent assessment of the Mitchell Creek Landslide (MCL) in northern British Columbia is a good case history of engineering geomorphological analysis of a large landslide. It was completed using historic aerial photographs, with approximately 20-year time intervals dating back to the mid-twentieth century and field investigations completed between 2008 and 2014. The large bedrock slide initiated between 1956 and 1972 and continues to experience ongoing annual movements. Significant glacial downwasting and retreat has been observed in the photographic record, and it is hypothesized that alpine glaciation has contributed to development of the MCL. This paper documents four aspects of the engineering geomorphological assessment completed at the MCL: (i) topographic evolution, (ii) slope morphology, (iii) deformation features, and (iv) displacement behavior. Four distinct geomorphic zones have been defined at the MCL based on these analyses, controlled by different failure mechanisms. The extents of these zones have changed little over the documented history of the landslide, and rates of movement estimated from aerial photography have been consistent over the last 60 years. Retreat of the Mitchell Valley Glacier appears to have played an important role in landslide initiation, as the ice mass receded the kinematic freedom of the slope increased. This study of the initiation and development of the MCL demonstrates the capabilities of a multi-faceted approach to engineering geomorphology. The combination of historical aerial photographs with digital photogrammetric modeling and point cloud analysis techniques, and geomorphological mapping, allows for development of a robust understanding of landslide behavior.     

Juggling through Ghanaian urbanisation: flood hazard mapping of Kumasi

More recently, driven by rapid and unguided urbanisation and climate change, Ghanaian cities are increasingly becoming hotspots for severe flood-related events. This paper reviews urbanisation dynamics in Ghanaian cities, and maps flood hazard zones and access to flood relief services in Kumasi, drawing insight from multi-criteria analysis and spatial network analysis using ArcGIS 10.2. Findings indicate that flood hazard zones in Kumasi have been created by natural (e.g., climate change) and anthropogenic (e.g., urbanisation) factors, and the interaction thereof. While one would have expected the natural factors to guide, direct and steer the patterns of urban development from flood hazard zones, the GIS analysis shows that anthropogenic factors, particularly urbanisation, are increasingly concentrating population and physical structures in areas liable to flooding in the urban environment. This situation is compounded by rapid land cover/use changes and widespread haphazard development across the city. Regrettably, findings show that urban residents living in flood hazard zones in Kumasi are also geographically disadvantaged in terms of access to emergency services compared to those living in well-planned neighbourhoods.     

Geochemistry of coals from the Elk Valley coalfield, British Columbia, Canada

The Elk Valley coalfield of British Columbia is one of the major coal producing areas in Canada. The coals are of Cretaceous and Jurassic–Cretaceous age and range in rank from high-volatile to low-volatile bituminous (%Ro_max: 0.8–1.6). Coal seams from outcrops and active mines in this coalfield were analysed for rank and maceral composition using reflected light microscopy, for geochemistry using AAS, INAA, and ICPES, and also by proximate and ultimate analyses. The Elk Valley coal seams contain low average concentrations of hazardous elements such as As, Mo, Pb, and Se. However, there are seams that contain relatively high concentrations of some of these elements, such as 8 mg/kg and 108 mg/kg of arsenic. When the geochemistry of coal seams is compared within the different parts of the coalfield, the elemental composition amongst the seams from various sections located in the central area of the coalfield is similar. Coal seams in the northern area of the coalfield have different geochemistry than coal seams in other areas of the coalfield; seams in the northern area have much higher As, Br, Cr, Cu, Fe, K, and Na content, but contain less Ca.     

A procedure for making objective preliminary assessments of outburst flood hazard from moraine-dammed lakes in southwestern British Columbia

Existing methods of evaluating the hazard posed by moraine-dammed lakes are unsystematic, subjective, and depend on the expertise and biases of the geoscientist. In this paper, we provide a framework for making objective preliminary assessments of outburst flood hazard in southwestern British Columbia. Our procedure relies on remote sensing methods and requires only limited knowledge of glacial processes so that evaluations of outburst flood hazard can be incorporated into routine hazard assessments of glaciated regions. We describe objective approaches, which incorporate existing empirical relations applicable to the study region, for estimating outburst peak discharge, maximum volume, maximum travel distance, maximum area of inundation, and probability. Outburst flood hazard is greatest for moderately large lakes that are impounded by large, narrow, ice-free moraine dams composed of sedimentary rock debris and drain into steep, sediment-filled gullies above major river valleys. We demonstrate the application of the procedure using three case studies and show that flood hazard varies, especially with major changes in lake level. Our assessment scheme yields reproducible results and enables engineers and geoscientists to prioritize potentially hazardous lakes for more detailed field investigation.     

Upstream control of river anastomosis by sediment overloading,upper Columbia River,British Columbia,Canada

Anastomosing rivers, systems of multiple interconnected channels that enclose floodbasins, constitute a major category of rivers for which various sedimentary facies models have been developed. While the sedimentary products of anastomosing rivers are relatively well‐known, their genesis is still debated. A rapidly growing number of ancient alluvial successions being interpreted as of anastomosing river origin, including important hydrocarbon reservoirs, urge the development of robust models for the genesis of anastomosis, to facilitate better interpretation of ancient depositional settings and controls. The upper Columbia River, British Columbia, Canada, is the most‐studied anastomosing river and has played a key role in the development of an anastomosing river facies model. Two hypotheses for the origin of upper Columbia River anastomosis include the following: (i) downstream control by aggrading cross‐valley alluvial fans; and (ii) upstream control by excessive bedload input from tributaries. Both upstream and downstream control may force aggradation and avulsions in the upper Columbia River. In order to test both hypotheses, long‐term (millennia‐scale) floodplain sedimentation rates and avulsion frequencies are calculated using ¹⁴C‐dated deeply buried organic floodplain material from cross‐valley borehole transects. The results indicate a downstream decrease in floodplain sedimentation rate and avulsion frequency along the anastomosed reach, which is consistent with dominant upstream control by sediment overloading. The data here link recent avulsion activity to increased sediment supply during the Little Ice Age (ca 1100 to 1950 ad ). This link is supported by data showing that sediment supply to the upper Columbia study reach fluctuated in response to Holocene glacial advances and retreats in the hinterland. Upstream control of anastomosis has considerable implications for the reconstruction of the setting of interpreted ancient anastomosing systems. The present research underscores that anastomosing systems typically occur in relatively proximal settings with abundant sediment supplied to low‐gradient floodplains, a situation commonly found in intermontane and foreland basins.     

The Cheam rock avalanche,Fraser Valley,British Columbia,Canada

The Cheam rock avalanche, which occurred about 5,000 years ago in the lower Fraser Valley, British Columbia, is the largest known catastrophic landslide in western Canada (175×10⁶ m³). A photo-draped digital elevation model of the rock avalanche reveals two morphologically distinct areas, an eastern area of arcuate hummocky ridges separated by flat-floored depressions and a lower western area with a subdued, gently rolling surface. Debris is up to 30 m thick and consists of rubbly, clast- and matrix-supported diamicton derived from local argillaceous metasedimentary rocks. Failure was probably caused by high pore water pressures on a thrust fault that daylights in the source area. Plastic deformation of sediment beneath the rock avalanche debris suggests that liquefaction occurred due to undrained loading when the debris struck the Cheam terrace. Liquefaction also explains the morphology and travel distance of the western debris lobe. The coincidence of well-sorted sands (the Popkum Series soil) with the rock avalanche debris indicates that significant amounts of water flowed over the surface of the landslide just after it came to rest. Stó:lõ Nation oral history suggests that the debris may have buried a village, causing the first known landslide fatalities in Canada.     

Avulsions, channel evolution and floodplain sedimentation rates of the anastomosing upper Columbia River, British Columbia, Canada

Ages of channels of the anastomosing upper Columbia River, south‐eastern British Columbia, Canada, were investigated in a cross‐valley transect by ¹⁴C dating of subsurface floodplain organic material from beneath levees. The avulsion history within the transect was deduced from these data, and morphological stages in channel development were recognized. Additionally, floodplain sedimentation rates were established. The new data demonstrate that the upper Columbia River is a long‐lived, dynamic anastomosing system. Results show that anastomosis at the study location has persisted since at least 2700 cal. years BP, with avulsions occurring frequently. At least nine channels have formed in the studied cross‐valley transect within the past 3000 years. Channel lifetimes from formation to abandonment appear to be highly variable, ranging from approximately 800 to 3000 years. Log jams provoking avulsions and/or silting up of old channels are proposed as reasons for this variability. Long‐term average floodplain sedimentation rates appear to be significantly lower than previously proposed by Smith (1983, Int. Assoc. Sedimentol. Spec. Publ., 6, 155–168). A long‐term (4550 years) average of 1·75 mm year^?1 (after compaction) was based on ¹⁴C dates, while a short‐term sedimentation rate of 0·8 mm was determined for a single, relatively small, seasonal flood in 1994 using sediment traps. However, short‐term sedimentation rates vary considerably over the floodplain, with levees aggrading up to four times faster than floodbasins. Channels of the upper Columbia River anastomosed reach follow a consistent pattern in their development, with each stage being characterized by different morphology and processes. Channel evolution comprises the following succession: (1) avulsion stage, in which a crevasse splay channel deepens by scour and levee sedimentation; (2) widening and deepening stage, in which bank slumping and bed scouring dominates; (3) infilling stage, in which either channel narrowing (bank accretion) or channel shallowing (bed accretion) takes place; and (4) abandonment stage, in which the residual (remnant) channel is filled exclusively by silt, clay and organic material. Vertical stacking (super‐ imposition) of active channels on recent channel‐fill sand bodies is a notable feature of the upper Columbia River, which suggests that reoccupation of residual channels is a common process.     

Multiple paleosols of the late Albian Boulder Creek Formation, British Columbia, Canada

Fifteen lithified paleosols, closely spaced in vertical sequence, occur in the top 90 m of the late Albian Boulder Creek Formation in the foothills of northeastern British Columbia, Canada. The paleosols have well-developed profiles 0·5 to 1·5 m thick, including A, B and C horizons. The paleosols are characterized by their grey colour, cutans, vertical roots, peds, spherulitic siderite and absence of sedimentary structures. The paleosols formed during a period when one or more basin wide unconformities occurred as a result of either eustatic sea level fluctuations or local tectonic events. These unconformities represent the terrestrial record of a lowered base level which caused valley incision and decreased rates of sedimentation on the incised flood plain. The climate was humid to subhumid. Overall, the environment in which these soils developed was of low relief and subject to little erosion. The water table was high for part of the year but there is also evidence of periodic drying and oxidation of organic debris. Cumulatively, the paleosols in this interval may represent 150000 yr of non-deposition.     

A 36Cl age determination for Mystery Creek rock avalanche and its implications in the context of hazard assessment, British Columbia, Canada

The Sea to Sky Corridor has experienced hundreds of historic and prehistoric landslides. The most common types of historical landslides are rock falls and debris flows, which are relatively small in volume but can be damaging. These types of failures are more common in the southern part of the corridor, between Horseshoe Bay and Porteau, where infrastructure has been built in close proximity to steep slopes. Farther north, fewer landslides have been reported historically, but those that have been recorded are usually large and date to prehistoric time (e.g., Cheekye fan and Mystery Creek rock avalanche). As part of a Geological Survey of Canada surficial geology and landslide inventory mapping study, Mystery Creek rock avalanche, near Whistler, British Columbia, was sampled for ³⁶Cl dating. Samples were collected from three large flat boulders of quartz diorite in the rock avalanche deposit to test a correlation with the previously reported radiocarbon age of 800 ± 100 years BP on charcoal. One sample revealed a mean age of 2,400 years and the other two, 4,300 and 4,800 years, respectively. These new results point to four possible interpretations: (1) Mystery Creek landslide is about 800 years old; (2) Based on the overlapping 2σ uncertainties, the rock avalanche took place between 2,200 and 3,600 years ago; (3) The rock avalanche deposit is 2,400 years old and the other two blocks are too old; and (4) The rock avalanche is between 4,300 and 4,800 years old. Although there is strength in numbers and it is likely that the age varies between 4,300 and 4,800 years, we favor the second interpretation where the age range is broader and statistically significant for all three samples. Moreover, at this time, we favor discounting the radiocarbon age based on a greater number of samples analyzed for ³⁶Cl analysis and lack of detailed information on the charcoal sampling. The causes and triggers of the Mystery Creek rock avalanche remain unknown, but direct glacial debuttressing can be ruled out. Some of the causes are likely a combination of the regional tectonic setting which produced preferential planes of weakness reflected in the trend of major faults, headscarp, and reverse scarps. Yearly cycles of freezing and thawing are considered a plausible cause based on present-day climate records. Finally, a large earthquake still remains a possible trigger because of the active tectonic setting and the presence of potentially contemporaneous landslides in the same area. Mystery Creek rock avalanche and other historic and pre-historic landslides contributed to validation of a heuristic rock fall/rock slide/rock avalanche susceptibility mapping study, in which their headscarps correlated well with medium-high to high susceptibility zones. In terms of hazard assessment, Mystery Creek rock avalanche, although pre-historic in age, occurred in present-day climatic and geological conditions. This poses a threat to infrastructure such as the Sea to Sky Highway, railway, and power line.     

The Metamorphism of the Blue River Ultramafic Body, Cassiar, British Columbia, Canada

The Blue River ultramafic body is an ‘Alpine’-typeperidotite tectonically emplaced within spilitic volcanic rocksin northern British Columbia. The intrusive margins were shearedand serpentinized to a lizardite-chrysotile plus brucite assemblageduring emplacement, prior to thermal metamorphism in the aureoleof a younger batholith. Relatively anhydrous peridotite andhydrous serpentinite were both affected by thermal metamorphism.The body has been subdivided into units defined by the mineralassemblages observed in meta-peridotite and meta-serpentiniteabove and below the isograd for the advent of the mineral talc.Isograds were also established for prograde metamorphic olivine,tremolite, and enstatite. The intrusive was subjected to two metamorphic processes, oxidationand dehydration. The nucleation of metamorphic olivine in weaklymetamorphosed serpentinite was erratic, and turbid porphyroblastcores are enriched in Fe and Mn. The dehydration reaction isthought to have been metastable. Above the talc isograd, serpentine, in both peridotite and serpentinite,reacted with original spinel to form ferritchromit and chlorite.The chlorite becomes progressively more aluminous with increasein grade. The oxidation process inhibited dehydration in meta-peridotiteas a stable chlorite was formed. The process also served toreduce the Fe content of the silicate system, as shown by thecomposition of the olivine generated from excess serpentinein high grade meta-serpentinite.