Constraints on the depth of generation and emplacement of a magmatic epidote‐bearing quartz diorite pluton in the Coast Plutonic Complex,British Columbia

Petrology and P–T estimates indicate that a magmatic epidote‐bearing quartz diorite pluton from Mt. Gamsby, Coast Plutonic Complex, British Columbia, was sourced at pressures below ～1.4 GPa and cooled nearly isobarically at ～0.9 GPa. The P–T path indicates that the magma was within the stability field of magmatic epidote early and remained there upon final crystallization. The pluton formed and crystallized at depths greater than ～30 km. REE data indicate that garnet was absent in the melting region and did not fractionate during crystallization. This suggests that the crust was less than or equal to ～55 km thick at 188 Ma during the early phases of magmatism in the Coast Plutonic Complex. Late Cretaceous contractional deformation and early Tertiary extension exhumed the rocks to upper crustal levels. Textures of magmatic epidote and other magmatic phases, combined with REE data, can be important for constraining the P–T path followed by magmas.     

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     

Extensive deformations of rock slopes in southern Coast Mountains, southwest British Columbia, Canada

Extensive deformations of mountain slopes occur in crystalline intrusive rocks in the southern Coast Mountains of British Columbia. Typical morphological evidence of slope movement includes extensive systems of tension cracks, grabens, and antislope scarps (collectively referred to here as linears). These landforms involve displacements along penetrative joints observed in surface exposures. Kinematic tests on rock-structural data indicate that the observed patterns of linears are generally consistent with the feasible gravitational movements along the dominant discontinuities. Most sites indicate sliding as the most likely initial mode of movement, followed by or accompanied by toppling and toppling-induced sliding movements, and do not support the view that linears are the traces of active faults.     

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.     

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.     

Municipal flood hazard mapping: the case of British Columbia,Canada

Historical responses to flood hazards have stimulated development in hazardous areas. Scholars recommend an alternative approach to reducing flood losses that combines flood hazard mapping with land use planning to identify and direct development away from flood-prone areas. Creating flood hazard maps to inform municipal land use planning is an expensive and complex process that can require resources not always available at the municipal government level. Senior levels of government in some countries have addressed deficiencies in municipal capacity by assuming an active role in producing municipal flood hazard maps. In other countries, however, senior governments do not contribute to municipal flood hazard mapping. Despite a large body of research on the importance of municipal land use planning for addressing flood hazards, little is known about the extent of flood hazard information that is available to municipalities that do not receive outside assistance from senior governments for flood hazard mapping. We assess the status of flood hazard maps in British Columbia, where municipalities do not receive outside assistance in creating the maps. Our analysis shows that these maps are generally outdated and/or lacking a variety of features that are critical for supporting effective land use planning. We recommend that senior levels of government play an active role in providing municipalities with (1) detailed and current information regarding flood hazards in their jurisdiction and (2) compelling incentives to utilize this information.     

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.     

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.     

Physical partitioning of mercury in till: an example from central British Columbia, Canada

The physical partitioning of Hg into different grain size fractions of till is predominantly controlled by the primary bedrock mineralogy, the distance of glacial transport, and the relative stability of cinnabar (HgS) in the soil weathering environment. At sites located short distances down-ice from bedrock cinnabar mineralization, the highest Hg concentrations in unoxidized till were measured in the sand- and granule-sized fractions reflecting the abundance of cinnabar in those size ranges. Similar partitioning was measured in oxidized till as cinnabar was found to be relatively resistant to postglacial weathering. Discrete clay-sized cinnabar grains obtained from the unoxidized till were viewed under the scanning electron microscope and suggest that the terminal grade of cinnabar is in the clay-sized range. In till collected from areas barren of cinnabar mineralization, the highest Hg levels were found in the clay-sized fraction which is attributed to the high adsorption of Hg by clay minerals.     

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.     

Numerical modeling of hydrothermal fluid flow coupled with mass transport: An example from the Devonian Wabamun Group, northeast British Columbia, Canada

Several researchers have suggested that upward and lateral hydrothermal fluid flow was responsible for dolomitization of various Devonian and Mississippian reservoirs in the Western Canada Sedimentary Basin (WCSB) based on sedimentological, geochemical and diagenetic evidence. In this study, a numerical model was applied to investigate hydrothermal fluid flow in the Wabamun Group, Parkland field, northeastern British Columbia, Canada. Our numerical results indicate that faults play the most critical role in controlling hydrothermal fluid flow. They provide a pathway connecting the basement of the basin and overlying sedimentary layers. Upwelling fluid flow via faults may bring reactants and heat from underlying strata to shallow formations for diagenetic reactions (e.g. dolomitization and/or chertification) or forming ore deposits. Salinity distribution of formation water, permeability configuration of host rock and regional fluid flow are also important factors affecting hot and brine fluid flow and accompanying heat and mass distribution.     

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.     

Biologically induced mineralization of dypingite by cyanobacteria from an alkaline wetland near Atlin,British Columbia,Canada

Background  

This study provides experimental evidence for biologically induced precipitation of magnesium carbonates, specifically dypingite (Mg₅(CO₃)₄(OH)₂·5H₂O), by cyanobacteria from an alkaline wetland near Atlin, British Columbia. This wetland is part of a larger hydromagnesite (Mg₅(CO₃)₄(OH)₂·4H₂O) playa. Abiotic and biotic processes for magnesium carbonate precipitation in this environment are compared.     

Point bar and floodplain formation of the meandering Beatton River, northeastern British Columbia, Canada

This study examines the morphology, sedimentology and genesis of the point bars and floodplain of the Beatton River. The formation of point bars occurs in distinct stages. An initial point bar platform composed mainly of coarse sediment is formed adjacent to the convex bank of a migrating meander bend, and is the base on which develops a single scroll bar of fine traction and suspended load. With continued sedimentation, the scroll bar grows, eventually supporting vegetation and becoming a floodplain ridge. Scroll bars form with greatest size and frequency in rapidly migrating bends, and the shape of the meander bend appears to determine both the location of the initial bar deposit, and its direction of growth up or downstream. Approximately one-half of the floodplain sediment is derived from suspended load, and the initiation of a scroll bar appears to be due to excessive deposition of suspended load in a zone of flow separation over a point bar platform. The critical flow condition for the initiation of a scroll bar does not occur with the same recurrence interval on different shaped meander bends, however, the average recurrence interval within the study reach is approximately every 30 years. Sedimentation rates on point bars and on the floodplain indicate two relatively distinct stages of floodplain alluviation. The most rapid is for surfaces less than 50 years old, although sediment accumulation still persists on surfaces up to 250 years in age. Although frequently flooded, surfaces older than this accumulate very little sediment. Despite 2–3 m of overbank deposition, the amplitude of floodplain ridges is maintained by secondary currents which sweep sediment from the swales towards the ridge crests.     

Pre-Younger Dryas resurgence of the southwestern margin of the Cordilleran ice sheet, British Columbia, Canada

Clague, J. J., Mathewes, R. W., Guilbault, J.-P., Hutchinson, I. & Ricketts, B. D. 1997 (September): Pre-Younger Dryas resurgence of the southwestern margin of the Cordilleran ice sheet, British Columbia, Canada. Boreas , Vol. 26, pp. 261–278. Oslo. ISSN 0300–9483.
A lobe of the Cordilleran ice sheet readvanced into the central Fvaser Lowland, southwestern British Columbia, Canada, on at least two occasions near the end of the last glaciation. This ice also flowed into the previously deglaciated, lower reaches of mountain valleys adjacent to the Fraser Lowland and into Washington state. The first of these advances occurred before about 11900 BP and ended with glacier retreat and the establishment of lodgepole pine forest on newly deglaciated terrain. Parts of this forest were overridden by ice during a second advance, shortly after 11300 BP. The younger advance is most likely older than the Younger Dryas Chronozone (11000–10000 BP) and may correlate with an intra-Allerad cooling event (the Killarney-Gerzensee oscillation). The older advance may have occurred during the Oldest Dryas or Older Dryas cold period. Non-climatic factors could also be involved, as emergence of the Fraser Lowland before the older advance greatly reduced or eliminated calving at the glacier margin and thus altered the mass balance of the ice lobe.     

Structural and engineering geology of the East Gate Landslide, Purcell Mountains, British Columbia, Canada

The East Gate Landslide is a prehistoric landslide that was reactivated in January 1997. The slope failure took place in the lower greenschist metasedimentary units of the Precambrian Horsethief Creek Group. The Grizzly Creek Thrust is a regional overturned fault that coincides with the location of the headscarp of the East Gate Landslide. Four discontinuity sets were recognised from detailed engineering geological mapping of the headscarp and surrounding area. The main scarp of the section reactivated in 1997 was sub-divided into three structural domains based on its position within the landslide, lithology, and orientation of the discontinuity sets. Limit-equilibrium techniques, finite-difference (FLAC) and distinct-element (UDEC) codes were used to investigate the failure mechanism of the 1997 event. The results of the field observations and numerical models suggest that the 1997 failure involved a complex mechanism incorporating components of rock-slumping, bi-planar, and pseudo-circular failure that was controlled by both the orientation of the discontinuity sets and reduced rock-mass quality due to tectonic deformation.     

Zoning of garnets as an indicator of metamorphic evolution in metapelites of the Yenisei Ridge

The geochemical patterns of major and trace elements in zonal garnets and the mineral inclusions in them formed by progressive and regressive metamorphism of pelites are established. It is shown that an increase in temperature and pressure led to a decrease in the Y and HREE contents in garnets, and the increase in their contents is related to a decrease in the PT-parameters of their formation. A negative correlation between the CaO and REE contents in garnet indicates their isomorphism. The main reason for the sharp increase in the CaO content in garnets during collision metamorphism is mass transfer between the garnet and the plagioclase. The deviations from this situaiton, which are expressed in simultaneous increase in the grossular component in garnet and the anorthite component in plagioclase, are caused by metamorphic reactions related to the epidote decomposition. The mass transfer of major and trace elements between the reacting phases in metamorphic reactions mostly occurred with preservation of the balance of matter. The mirror shape and the character of the REE patterns of the rock-forming minerals relative to the composition of the rock indicate the equilibration of the HREE and Y contents between garnet, the major concentrator of these elements in the rock, and other phases. The balance between the LREEs and HREEs in the rock is achieved by the presence of variable amounts of monazite.     

Evolution of a provenance as revealed by petrographic analyses of Cretaceous formations in the Queen Charlotte Islands, British Columbia, Canada

Two contrasting marine sedimentary facies, the Haida Formation of sandy and argillaceous sediments and the conformably overlying Honna Formation of gravelly sediments were formed within different types of tectonic basins during mid- to Late Cretaceous time. The sediments of both formations were derived from the east. Sandstones from the two formations show characteristics of mature magmatic arc provenance and are classified as lithic and feldspathic arenites. However, the Honna sandstones are more feldspathic and less quartzose than the Haida sandstones. The Honna sandstones have many volcanic rock fragments (VRF) but the Haida sandstones do not. Feldspars of the VRF-rich Honna sandstones, however, do not seem to have been derived from the breakdown of volcanic rocks. The observed petrographic differences between the two formations can be ascribed to a process in which the volcanic cover and the basement rock denudation took place simultaneously in the source area during deposition of the Honna Formation.