Brooks Peninsula Research Articles

Geographic variability in the seasonality of euphausiid diel vertical migrations among three locations in coastal British Columbia, Canada

Abstract Diel vertical migration (DVM) is a behaviour observed across zooplankton taxa in marine and limnetic systems worldwide. DVM influences biogeochemical cycling and carbon drawdown in oceanic systems and alters prey availability for zooplanktivorous species. DVM has been well studied among zooplankton, and many exogenous and endogenous triggers as well as adaptive significances have been hypothesized. However, second-order variability in DVM timing, the deviation of DVM times to respective dawn and dusk times throughout the year, is a less-studied phenomenon that can help identify the factors influencing migration timing as well as demonstrate the changes of DVM behaviours within and across systems. Here, we quantified seasonal trends in second-order variability of DVM timing of euphausiids at Brooks Peninsula, Clayoquot Canyon, and Saanich Inlet near Vancouver Island, British Columbia, Canada, over multiple years using upward-facing moored echosounders. We used generalized additive mixed models to characterize this seasonality. DVM timing relative to civil twilight times showed strong seasonality at all locations, with euphausiids remaining near the surface longer than expected in spring and summer, and shorter than expected in winter. Euphausiids spent less time near the surface at Brooks Peninsula and Clayoquot Canyon than at Saanich Inlet throughout the year. Increased primary productivity in Saanich Inlet, which reduced light penetration and hid euphausiids from visual predators, likely drove this difference. Our findings confirm that proper understanding of DVM behaviours must account for seasonal variability due to context-specific oceanographic and ecological parameters. This is particularly pertinent when attempting to model the biogeochemical or predator–prey interactions influenced by DVM behaviours.

Complex Structure in the Nootka Fault Zone Revealed by Double‐Difference Tomography and a New Earthquake Catalog

AbstractWe employ an automatic earthquake detection algorithm to seismic waveforms recorded between the years 2000 and 2020 in southwest British Columbia. 32,121 events which possess at least three paired P‐ and S‐wave arrival times are located, compared to 21,538 seismic events in the existing Geologic Survey of Canada catalog. This augmented catalog is employed for double‐difference seismic tomography across Vancouver Island, with particular focus on the Nootka Fault zone (NFZ). The NFZ is a transform boundary that separates the Juan de Fuca and Explorer plates in a zone of distributed left‐lateral strike‐slip faulting. Tomographic results indicate that a double seismic zone exists within the NFZ that parallels Vp/Vs structure typically observed in subduction zones. Specifically, a dipping high Vp/Vs layer is underlain by reduced Vp/Vs material. Structural complexities are revealed by three Mw > 6 events and their aftershocks. The 2004 Mw 6.3 and 2011 Mw 6.4 events are interpreted to reside southeast of the NFZ within the Juan de Fuca plate at depths as shallow as 20 km, and the 2014 Mw 6.6 event is interpreted to reside within oceanic lithosphere of the NFZ at depths >35 km. Vp/Vs structure further indicates that underthrust oceanic lithosphere of the Explorer plate extends to 20 km depth beneath Brooks Peninsula. Our results support the hypothesis that the NFZ represents a structurally independent slice of oceanic lithosphere that exhibits (a) a more northerly trajectory than typically depicted, and (b) increased plate curvature when compared to either the Juan de Fuca or Explorer plates.

The Northern Terminus of Cascadia Subduction

AbstractThe Juan de Fuca (JDF) plate system is pivoting and breaking apart as a result of resistance to subduction. At the northern end, the Explorer (EXP) plate moves independently of the JDF plate along the Nootka Fault Zone (NFZ), which forms an unstable triple junction with the JDF ridge and the Sovanco Fracture Zone. We trace the subsurface extension of the NFZ using tomography by combining results from a new nearshore study with a previous ocean‐bottom study and other geophysical constraints. Cross sections from the offshore NFZ through central Vancouver Island suggest a strong fold in the subducting lithosphere (∼40°). Near the deformation front, the NFZ is defined by microseismicity that extends into oceanic mantle with low VP/VS (<1.7) values. This anomaly extends to the NE below the continental shelf with a more northerly trajectory than common NFZ depictions through the seismicity concentration off Nootka Island. Tremor locations and receiver functions indicate that this more northerly trajectory persists across Vancouver Island. We propose a revised tectonic evolution for the NFZ that leads to (i) an EXP microplate confined to shallow depths below the convergent margin and which underthrusts North America no farther landward than Brooks Peninsula and (ii) a northern edge of Cascadia subduction that cuts between Brooks Peninsula and Nootka Island. The plate fold offshore Nootka Island and abundant seismicity there and at Brooks Peninsula are ascribed to stress concentrations on either side of the NFZ associated with deformation at the edges of more competent JDF, EXP, and North American plates.

Interannual variability in primary production and shelf-offshore transport of nutrients along the northeast Pacific Ocean margin

A 33-year hindcast (1979–2011) of a regional circulation-ecosystem model is used to examine interannual variability of primary production and cross-shelf transport of nutrients and phytoplankton along the British Columbia and Washington coast. Linkages between simulated primary production anomalies and forcing are explored through correlations to local upwelling winds, outflow from the Juan de Fuca Strait estuarine circulation, and two indices of Pacific Ocean basin-scale variability, the Pacific Decadal Oscillation (PDO) and the North Pacific Gyre Oscillation (NPGO). The dominant large scale pattern of interannual variability of primary production is correlated to the NPGO but not the PDO. In the shelf region, negative primary production anomalies appear to be primarily associated with El Niño events. Along the northern shelves (between ~48.6 and 53. 9°N), time-series of annual primary production anomalies are significantly correlated with upwelling winds whereas farther south (between 46 and 48.6°N) the anomalies are correlated with nitrate outflow from the Juan de Fuca Strait. Model results indicate that coastal waters off Vancouver Island are rich in nutrients, so phytoplankton growth in this region are less limited by nitrate than on the Washington coast. Off Vancouver Island, surface phytoplankton and nitrate are exported out of the continental shelf by horizontal advection during the upwelling season. In addition, at specific sites along the coast (off Brooks Peninsula and at the southern tip of Haida Gwaii), eddies and jets carry surface nutrient and phytoplankton seaward, increasing primary production offshore.

The First Swordfish (Xiphias gladius) Recorded in Coastal British Columbia

We report the 2nd occurrence of a warm water teleost predator, Swordfish (Xiphias gladius), in Canadian Pacific waters, and the first seen in coastal waters of British Columbia. A Swordfish was observed on 5 September 2017 approximately 20 nm (37 km) from the Brooks Peninsula, Vancouver Island, British Columbia (UTM: Zone 9, 560062 E, 5526365 N, WGS84) in above average water temperatures.

Marine Migration of North American Green Sturgeon

AbstractAn understanding of the distribution of North American green sturgeon Acipenser medirostris in coastal waters is crucial to minimize impacts on this vulnerable species from various fisheries. To determine migratory patterns, we tagged 213 subadult and adult green sturgeon in spawning rivers and summer aggregation areas with uniquely coded ultrasonic pingers and observed their coastal movements with arrays of automated hydrophones deployed along the West Coast of North America from southeast Alaska to Monterey Bay, California. Green sturgeon exhibited an annual migration along the continental shelf from U.S. to Canadian waters in the fall and an apparent return migration in the spring. Peak migration rates exceeded 50 km/d during the springtime southward migration. Large numbers of green sturgeon were detected near Brooks Peninsula on northwest Vancouver Island, British Columbia, during May‐June and October‐November. A single fish was detected in southeast Alaska in December. This pattern of detections suggests that important overwintering grounds may be north of Vancouver Island and south of Cape Spencer, Alaska. A high frequency of detection allowed us to estimate that annual survival of tagged green sturgeon was 0.83 in 2004. The rapid, frequent long‐distance migrations by these fish may make them vulnerable to bycatch in bottom trawl fisheries on the shelf waters of western North America.

Morphology of the Explorer–Juan de Fuca slab edge in northern Cascadia: Imaging plate capture at a ridge-trench-transform triple junction

Research Article| November 01, 2008 Morphology of the Explorer–Juan de Fuca slab edge in northern Cascadia: Imaging plate capture at a ridge-trench-transform triple junction P. Audet; P. Audet 11Department of Earth and Ocean Sciences, University of British Columbia, 6339 Stores Road, Vancouver V6T 1Z4, Canada Search for other works by this author on: GSW Google Scholar M. G. Bostock; M. G. Bostock 11Department of Earth and Ocean Sciences, University of British Columbia, 6339 Stores Road, Vancouver V6T 1Z4, Canada Search for other works by this author on: GSW Google Scholar J.-P. Mercier; J.-P. Mercier 11Department of Earth and Ocean Sciences, University of British Columbia, 6339 Stores Road, Vancouver V6T 1Z4, Canada Search for other works by this author on: GSW Google Scholar J. F. Cassidy J. F. Cassidy 22Pacific Geoscience Center, Geological Survey of Canada, Sidney, British Columbia V8L 4B2, Canada Search for other works by this author on: GSW Google Scholar Author and Article Information P. Audet 11Department of Earth and Ocean Sciences, University of British Columbia, 6339 Stores Road, Vancouver V6T 1Z4, Canada M. G. Bostock 11Department of Earth and Ocean Sciences, University of British Columbia, 6339 Stores Road, Vancouver V6T 1Z4, Canada J.-P. Mercier 11Department of Earth and Ocean Sciences, University of British Columbia, 6339 Stores Road, Vancouver V6T 1Z4, Canada J. F. Cassidy 22Pacific Geoscience Center, Geological Survey of Canada, Sidney, British Columbia V8L 4B2, Canada Publisher: Geological Society of America Received: 01 Apr 2008 Revision Received: 30 Jul 2008 Accepted: 06 Aug 2008 First Online: 02 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 © 2008 Geological Society of America Geology (2008) 36 (11): 895–898. https://doi.org/10.1130/G25356A.1 Article history Received: 01 Apr 2008 Revision Received: 30 Jul 2008 Accepted: 06 Aug 2008 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation P. Audet, M. G. Bostock, J.-P. Mercier, J. F. Cassidy; Morphology of the Explorer–Juan de Fuca slab edge in northern Cascadia: Imaging plate capture at a ridge-trench-transform triple junction. Geology 2008;; 36 (11): 895–898. doi: https://doi.org/10.1130/G25356A.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The Explorer plate is a young oceanic microplate that accommodates relative motion between the Pacific, Juan de Fuca, and North America plates near northern Vancouver Island, Canada. The northern limit of Explorer plate–Juan de Fuca subduction and the fate of the slab in northern Cascadia are poorly understood. We use passive teleseismic recordings from an array of POLARIS broadband seismic stations to image crustal and upper mantle structure beneath northern Vancouver Island into the interior of British Columbia. A clear signature of subducted material extends northeast from the Brooks Peninsula at crustal levels, beneath Georgia Strait and the mainland deep into the mantle to 300 km depth. Complexity in slab morphology results from Juan de Fuca ridge subduction and toroidal flow around the slab edge, in agreement with geophysical and geological data. We propose a tectonic model for the Explorer plate in which its separation from the Juan de Fuca plate is caused by the thermomechanical erosion of the slab edge and slab thinning at shallow levels, both of which slow convergence with North America and lead eventually to plate capture. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Current tectonics of northern Cascadia from a decade of GPS measurements

Global Positioning System (GPS) measurements to study regional deformation were initiated in northern Cascadia in the late 1980s and early 1990s. On the basis of a decade of GPS data, we derive a crustal velocity field for NW Washington‐SW British Columbia. The permanent and campaign GPS velocities are defined with respect to North America in the ITRF2000 reference frame. Velocity uncertainties are estimated using a model of time series noise spectra. This new velocity field is the basis for interpretation of the tectonics of the northern Cascadia subduction system. GPS velocities are interpreted in terms of interseismic loading of the megathrust using different coupling models. Our data confirm that the upper part of the megathrust is nearly fully locked. An exponential model for the downdip transition zone gives slightly better agreement with the data compared to the common linear transition. The landward decrease of forearc strain loading is smaller than predicted by any of the current subduction interseismic models. This could be a consequence of a small (0–3 mm/yr) long‐term motion of the southern Vancouver Island forearc, with respect to North America, or of a concentration of interseismic strain across the elastically weaker Cascadia volcanic arc. In northern Vancouver Island, our velocity field supports the existence of an independent Explorer microplate currently underthrusting underneath North America, at least up to Brooks Peninsula. Further north, GPS velocities indicate transient and/or permanent deformation of northernmost Vancouver Island related to the interaction with the Explorer microplate and possibly with the Queen Charlotte transform margin.

Stratification and Mean Flow Effects on Diurnal Tidal Currents off Vancouver Island

A series of numerical experiments with a three-dimensional, baroclinic model were conducted to study the influences of density stratification and wind-driven currents on the K1 tide over the continental shelf off Vancouver Island. The region is one of anomalously large diurnal tidal currents due to the generation of coastally trapped waves at the entrance to Juan de Fuca Strait. Model results are compared with data obtained from a number of moorings deployed over the shelf, including a line extending for over 300 km in the alongshore direction. The results show that inclusion of stratification significantly improves the representation of K1 currents in the model, particularly with respect to the alongshore phase propagation of the clockwise and counterclockwise rotary components. With homogeneous water, the coastally trapped waves propagate relatively slowly and are dissipated before reaching Brooks Peninsula. In contrast, the ambient stratification permits coastally trapped motions to propagate beyond Brooks Peninsula, in agreement with observations. The seasonal variability of the K1 currents is also examined from long records obtained at a number of sites over the shelf. The observations indicate a seasonal modulation in the phase of the rotary components that increases in amplitude with distance from the generation region of the shelf waves. Attempts to model the observed summer/winter phase difference by including a seasonal-mean wind stress are discussed.

The 1978 Brooks Peninsula, Vancouver Island earthquakes

AbstractA pair of Mw = 5.5 earthquakes occurred beneath the Brooks Peninsula on the northwest coast of Vancouver Island on 2 June and 25 July 1978. These are the largest and best-recorded earthquakes to date in the vicinity of northern Vancouver Island and the adjacent coast margin. A detailed study of these earthquakes was undertaken to examine the contemporary tectonics of this region, specifically the poorly understood interaction between the Explorer plate and the North American plate at the northern end of the Cascadia subduction zone. Both earthquakes were followed by well-defined aftershock sequences. A four-station temporary seismograph array deployed on the Brooks Peninsula following the 2 June mainshock allowed for accurate aftershock locations. This earthquake was located at 50.15° N, 127.84° W, based on the center of a 9-km-diameter circular region of aftershocks. The 25 July earthquake was located 4 to 7 km to the northeast of the June epicenter based on waveform comparison of the two events. Both earthquakes occurred at 15 to 16 km depth. The focal mechanisms as determined from body-wave modeling are nearly identical and show left-lateral strike-slip motion along a shallow north-dipping, east-west-striking fault. The focal mechanism and depth of these two earthquakes indicates that they were not megathrust events on the Explorer/North America plate boundary, but rather that they occurred within the North American plate, 5 to 10 km above the megathrust. The northeast-directed pressure axes for these earthquakes suggests coupling across the Explorer-North America segment of the Cascadia subduction zone, consistent with contemporary convergence of the Explorer Plate with the North American plate in a northeast-southwest direction.

Three-Dimensional Model Simulations of Tides and Buoyancy Currents along the West Coast of Vancouver Island

A three-dimensional finite element model is used to calculate the barotropic tides and seasonal buoyancy flows off the western and northern coasts of Vancouver Island. The model buoyancy currents and the harmonics of eight tidal constituents are compared with those from previous models, and those from tide gauge and current meter observations. The rms differences between observed and calculated sea level tidal amplitudes are within 2.3 cm for all constituents, whereas the rms differences between observed and calculated phases are, with the exception of Q1, within 3.58. The model currents are more accurate than those from previous models. Of particular interest are the diurnal continental shelf waves. It is shown that these waves are generated through the conservation of potental vorticity arising when the strong diurnal tidal currents in Juan de Fuca Strait encounter the abrupt topography near the entrance to the strait. These waves do not appear to propagate beyond Brooks Peninsula, a large promontory cutting across the continental shelf. A power budget analysis reveals that the reason for this is not the blocking effect of the peninsula but rather there is little energy left in the waves when they reach that point. This energy loss is primarily through frictional dissipation in a series of trapped eddies along the shelf break. The location of these eddies varies with the forcing frequency and appears to be related to the spacing of canyons. It is also demonstrated that the strong diurnal currents observed over the shallow banks in Queen Charlotte Sound to the north of Brooks Peninsula do not arise from the oscillatory diurnal flows in Queen Charlotte Strait. Unlike the case for Juan de Fuca Strait, the region offshore of Queen Charlotte Strait does not support diurnal coastally trapped waves. Seasonal changes in the wavelengths of the Vancouver Island shelf waves are shown to arise through an advective interaction (Doppler shift) with the buoyancy-driven Vancouver Island coastal current and the wind-driven shelf break current.

Continental signature of a ridge‐trench‐triple junction: Northern Vancouver Island

South of the Juan de Fuca‐Pacific‐North America triple junction located off northern Vancouver Island there is subduction, and to the north there is oblique convergence along the Queen Charlotte transform fault. In the mid‐Tertiary, periods of extension, including crustal thinning and volcanism, formed Queen Charlotte Basin. Heat flow on northern Vancouver Island (67 mW/m2) is the same as in Queen Charlotte Basin to the north, but it decreases abruptly to the south over a distance of 70 km to an average value of 46 mW/m2. Compared to the rest of the island, the northern island has more subdued topography, a lower mean elevation, slightly higher Bouguer gravity values, and lower seismicity. Late Tertiary volcanics, which occur only on the northern part of the island, geochemically resemble the Tertiary Masset Formation which formed during periods of extension in Queen Charlotte Basin. These data are consistent with a Tertiary history including periods of extension for northern Vancouver Island, similar to Queen Charlotte Basin, and distinguish it from the rest of Vancouver Island. The southern boundary of this northern block extends northeastward from Brooks Peninsula, approximately above the inferred northern edge of the subducted Juan de Fuca plate and parallel to a series of linear magnetic anomalies associated with a northeasterly trending fault zone. The abrupt transition in observed heat flow is most likely associated with the edge of the subducted plate, which produces large temperature differences at depths of 30–40 km. The time constant for these thermal effects to reach the surface conductively indicates that this boundary has been stable over tens of millions of years, and therefore the location of the subducting plate's northern edge must have been stationary or moving very slowly over this period. Consequently it is likely that the triple junction has been located on average for the last 40 Myr off Brooks peninsula.

Summer sea surface temperature variability off Vancouver Island from satellite data

Satellite‐sensed advanced very high resolution radiometer (AVHRR) sea surface temperature (SST) data over eight summers (1984–1991) were used to analyze the summer SST patterns of variability off the west coast of Vancouver Island. Empirical orthogonal function (EOF) analysis of the spatial variance for 133 nearly cloud‐free summer images was performed. The first EOF mode, which resembled the mean of all images, showed a strong cool water band located at the northwest corner of Vancouver Island, a cool tongue extending seaward from the Strait of Juan de Fuca and a warm patch off Barkley Sound. The second mode revealed topographically controlled upwelling: cool water over the shelf region with its seaward boundary roughly following the 200‐m depth contour, plus a cold eddy located just north of the Juan de Fuca Canyon. The third mode displayed cool water extending southwestward off Brooks Peninsula, while the fourth mode showed a cool water plume extending off Cape Scott at the northern tip of Vancouver Island. These four modes accounted for 33, 12, 10, and 5% of the SST variance, respectively. The temporal amplitude of these EOF modes revealed how the SST features changed as summer progressed. From these images, we also constructed an overall seasonal coolness index, which revealed the summers of 1986 and 1991 to have the coolest coastal water, with both summers immediately preceding an El Niño.

The flow of a coastal current past a blunt headland

Abstract The effect of an abrupt headland on a barotropic oceanic boundary current with variable bottom topography is investigated. The objective is to explore with a very simple model some of the observed features of flow past Brooks Peninsula, an obstacle to boundary currents on the west coast of Vancouver Island. It is shown that the seasonal variation in the background current field causes a large change in the response to the headland. The difference is both quantitative and qualitative and results from the ability of southward alongshore flows to support topographic Rossby lee waves. As a result of the presence of the lee waves a strong offshore flow occurs just downstream of the Peninsula and this ejects water from the continental shelf into deep water producing features reminiscent of the so‐called “squirts” and “jets”.

The Neogene Alert Bay Volcanic Belt of northern Vancouver Island, Canada: Descending-plate-edge volcanism in the arc-trench gap

The Alert Bay Volcanic Belt trends northeasterly across northern Vancouver Island, coincident with the trace of the subducted Juan de Fuca—Explorer plate edge. Volcanism began in the west, at Brooks Peninsula, about 8 Ma ago, but occurred in most centers 3.5 ± 1 Ma ago. There is a suggestion of eastward migration of activity and shift from basalt to dacite or rhyolite with time. Most of the volcanism was coincident with a time of rapid changes in the geometry of subduction, as inferred from offshore magnetic patterns, and with a hiatus in mainland, Cascade volcanic arc activity. Geometry and chronometry suggest this is a descending-plate-edge volcanic belt, where disruption of steady-state plate-consumption patterns triggered magma genesis. Chemically the rocks are quite variable, with divergent fractionation trends. One trend resembles that of Mull (Hebrides), with a plagiophyric basalt of transitional alkaline-subalkaline, mildly tholeiitic, and aluminous character which differentiated to clinopyroxene andesite, and eventually to tholeiitic rhyolite and mildly tholeiitic calc-alkaline dacite, both of K-poor magma type. The other trend is like the Cascades, with aluminous, aphyric, calc-alkaline basalt, hornblende and/or hypersthene andesite, and K-poor dacite. This divergent character is also evident in Ba, Rb, Nb, and Zr fractionation trends. Major- and trace-element discriminant diagrams generally identify the basalts as within-plate types. The 87Sr/ 86Sr isotope ratio is relatively low, averaging 0.70325, and shows no trend with rock type or differentiation series. Oxygen in the entire suite is relatively heavy, δ 18O averaging 7.1%. Even the basalts are 18O enriched. Oxygen shows no trend with degree of hydration, rock type, or series. These isotopic and chemical data are compatible with minor crustal contamination of mafic primary magmas, followed by fractional crystallization under different oxidation and hydration conditions.

Diurnal-Period Continental Shelf Waves along Vancouver Island: A Comparison of Observations with Theoretical Models

Abstract Current and sea level data from the Coastal Oceanic Dynamics Experiment conducted of Vancouver Island from May 1979 to September 1980 reveal shelf waves of diurnal period whose motions dominate the flow over the continental shelf. In this paper, current meter data from mooring lines off Brooks Peninsula, Estevan Point and Carmanah Point are compared with theoretical solutions for a combination of free shelf waves and barotropic Kelvin waves over uniform alongshore topography. Use of Brink's scheme for baroclinic shelf waves gives close agreement between the modeled and observed K1-period horizontal currents over the continental margin off central Vancouver Island and accurately predicts vertical velocities and sea levels. The shelf waves am shown to be entirely dominated by the first baroclinic mode. Wavelength derived from the model closely approximate wavelenghts computed from the alongshore change in phase of the observed diurnal current ellipses. The baroclinic model successfully predicts the...

A dwarf coastal variety of maidenhair fern, Adiantum pedatum

A new variety of the maidenhair fern, Adiantum pedatum var. subpumilum W. H. Wagner, is described. Known previously only as a garden plant, a natural population has been discovered on coastal rock cliffs on the Brooks Peninsula of NW Vancouver Island. The basic frond structure is like the typical variety, but the plant is extremely dwarfed, the fronds are only 6–12 cm tall with imbricated pinnules. The venation pattern is much simplified. Horticultural plants of var. subpumilum yield n = 29 pairs of chromosomes, like the typical variety. The diminutive stature of var. subpumilum is genetically fixed.

Sedimentation and Tectonics of Pacific Continental Margin of British Columbia: ABSTRACT

A triple junction of three lithospheric plates at the British Columbia continental margin has had considerable influence on structure and sedimentation in marginal basins. South of the triple point, compressional forces from subduction of oceanic plate beneath the continent have resulted in deformation mainly by folding and block faulting, but north of the triple point stresses are relieved by strike-slip movement along transform faults with only broad folding taking place in Tertiary sediments. Tofino basin, south of the triple junction, has undergone major uplift, linear en echelon folding, and elongate diapirism on the outer shelf. More than 12,000 ft of Tertiary mudstone and siltstone has been drilled adjacent to such structures by Shell Canada Ltd. The fine clastics in these distal facies are not conducive to petroleum accumulation. However, potentially productive reservoir beds may exist in the proximal turbidite sequences nearer shore. Subsequent uplift and erosion of the latter also may have resulted in clean second-cycle wedges west of the uplift boundary. In the Tofino basin, as in other areas of the west coast, hydrocarbon prospects appear to have the highest potential in stratigraphic traps. North of Brooks Peninsula, structural style is dominated by shelf-edge faulting which, west of Queen Charlotte Islands, is transform movement between the Pacific and North America plates. The Queen Charlotte basin has undergone net subsidence of several thousand feet with late Tertiary nonmarine sediments over Tertiary and Mesozoic volcanic basement and Paleozoic intrusives in the north, and late Tertiary marine sediments over Tertiary volcanic rocks in the south. Sediments reach 15,000 ft in thickness. Permeabilities are reduced by silts and clays, but facies changes between interfingering marine and nonmarine depositional sites should make good stratigraphic traps. Winona basin at the base of the slope is folded only gently and broadly in the north but more highly deformed into prominent sedimentary ridges in the south. Three to six km of sediment fills the deepest point under the base of the slope. The oldest sediments in the flank of the basin are Pliocene. The present deepwater basin undoubtedly has received a high proportion of second-cycle clastic deposits from the uplifted older Tertiary belt. End_of_Article - Last_Page 1460------------

Depositional and Tectonic History of Tertiary Sequence on Continental Margin of British Columbia: ABSTRACT

Sampling and seismic profiling in the Tofino basin west of Vancouver Island have disclosed a thick sequence of Tertiary rocks ranging in age from Eocene to Pliocene. Most of these rocks were deposited in deep-water environments and subsequent uplift has exposed them in many areas. Eocene and Oligocene sediments were deposited in a belt along the present shoreline area off Vancouver Island, whereas Miocene and early Pliocene rocks are present farther seaward. Later Pliocene rocks form a regressive sequence overlapping the older Tertiary in most areas. Several major periods of deformation resulted in faults, folds, and diapirs on the continental shelf. Deformational patterns show a marked change from north to south. North of Brooks Peninsula, including Queen Charlotte Sound and Hecate Strait, sediments are generally undeformed by folding but are truncated by faults along the steep continental slope. The Kyuquot uplift south of Brooks Peninsula exposed Eocene and Oligocene rocks across the shelf. Farther south, Miocene and Pliocene rocks unconformably overlie the uplift. Folding increases southward, culminating in an area of diapirism off Nootka Sound. Elongate diapirs trend parallel or subparallel with the coastline. Tectonic features observed on the shelf and slope probably can be explained best by a consideration of the configuration and projected relative movements across spreading centers and along transform faults off the coast of British Columbia. The juxtaposition of differing tectonic styles may be due to the presence of a triple-point junction which has subsequently migrated north. End_of_Article - Last_Page 431------------

Tectonics and Depositional History of the Continental Margin Off Vancouver Island, British Columbia

Sampling and seismic profiling in the Tofino Basin west of Vancouver Island show there is a thick sequence of Tertiary rocks ranging in age from late Eocene to Pliocene. The rocks are mainly mudstones containing abundant foraminifera indicating a bathyal depositional environment throughout most of the Tertiary. Subsequent uplift has exposed the deep water sediments on the shelf over much of the area. Eocene-Oligocene sediments occur in a belt along the inner shelf, while Miocene and Pliocene rocks lie seaward of this. Pliocene rocks form a regressive sequence overlapping the older Tertiary, with the greatest thickness in the south.At least two major periods of deformation resulted in faulting, folding, and diapirism on the continental shelf. Deformational patterns show a marked change from north to south. North of Brooks Peninsula sediments are undeformed by folding but are truncated by faulting along the steep continental slope. The Kyuquot Uplift south of Brooks Peninsula exposes Eocene-Oligocene sediments across the shelf. Farther south Mio-Pliocene sediments unconformably overlie the uplift. Folding increases southward culminating in an area of diapirism off Nootka Sound. Elongate diapirs trend parallel or subparallel to the coastline.Tectonic features on the shelf and slope appear to be related to present and earlier configurations of nearby offshore spreading centers, plates, and transform faults. Crustal plate movements may have been responsible for the observed shelf and slope deformations.