The Foreslope Hills: large-scale, fine-grained sediment waves in the Strait of Georgia, British Columbia

Marine Geology 192 (2002) 275^295 www.elsevier.com/locate/margeo

The Foreslope Hills: large-scale, ¢ne-grained sediment waves in the Strait of Georgia, British Columbia D.C. Mosher a;
, R.E. Thomson b a

Natural Resources Canada, Geological Survey of Canada^Atlantic, Bedford Institute of Oceanography, 1 Challenger Drive, P.O.Box 1002, Dartmouth, NS, Canada B2Y 4A2 b Department of Fisheries and Oceans, Institute of Ocean Sciences, Sidney, BC, Canada V8L 4B2 Received 30 January 2001; received in revised form 5 December 2001; accepted 2 August 2002

Abstract The Foreslope Hills are a set of 20-m-high, s 5-km-long ridges at 230^350-m water depth in the south-central Strait of Georgia, occupying a s 60-km2 area at the base of the Fraser River delta foreslope. They are composed of clay, silt and fine sand. Previous authors have interpreted them as: (1) a single mass-slide deposit, (2) mud diapirs, (3) in situ rotational failures, and (4) creep deformation features, but collaborative evidence for these interpretations is lacking. Seafloor surface morphological renders derived from recent multibeam sonar bathymetry data show linear, evenly spaced symmetrical ridges with ridge bifurcation, resembling wave ripples but on a much larger scale. Detailed multichannel and single-channel seismic reflection data show sinusoidal internal reflections suggesting upslope migration. The ridges occur within the uppermost portion of the Holocene section in a very high sedimentation rate environment. Consequently, they are probably less than 3000 years old. Moored current meter records for the region show reversing currents, largely oriented normal to the ridge crests, averaging 10 cm/s with a maximum of 50 cm/s. This evidence, combined with increasing documentation of large-scale sedimentary bedforms in slope and rise environments, suggests that the Foreslope Hills should be re-interpreted as sediment waves. Upslope migration is typical of many large-scale, fine-grained sediment waves. The preferred physical interpretation for the creation and maintenance of the Foreslope Hills as sediment waves is the lee-wave model, requiring a phase shift in densitystratified flow as it passes over the wavy topography. Maximum measured bottom currents are easily strong enough to entrain silt and clay delivered to the site by turbidity currents or by sediment plume fallout from Fraser River outflow. Either strong flood tides or severe biannual density intrusions, due to their intense stratification, may be susceptible to lee wave formation and the creation of sediment waves, leading to the observed pattern of upslope migration. 9 2002 Elsevier Science B.V. All rights reserved. Keywords: sedimentology; mud waves; bedforms; seismic stratigraphy; multibeam sonar

1. Introduction * Corresponding author. Fax: +1-902-426-4104. E-mail addresses: [email protected] (D.C. Mosher), [email protected] (R.E. Thomson).

The Foreslope Hills are an ambiguous set of sea£oor morphologic features found in the central Strait of Georgia, British Columbia (Fig. 1) in

0025-3227 / 02 / $ ^ see front matter 9 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 5 - 3 2 2 7 ( 0 2 ) 0 0 5 5 9 - 5

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250^300-m water depth. They are a set of ridges, 20 m high and greater than 5 km long, formed over a 60-km2 area of the lower prodelta slope of the Fraser River delta. They were identi¢ed in bathymetric surveys as early as the 1920s. Interpretations of their origin range from a sediment slide deposit to creep failure to mud diapirs (Mathews and Shepard, 1962; Terzaghi, 1962; Shepard, 1967; Ti⁄n et al., 1971; Luternauer and Finn, 1983; Hamilton and Wigen, 1987; Hart, 1993). Recent detailed sea£oor morphologic renders, derived from multibeam bathymetric sonar data of the Foreslope Hills, are strongly suggestive of large sedimentary bedforms, however. These data in combination with detailed high-resolution seismic re£ection data and observations of strong, near-bottom currents are used to attempt to re-interpret the mode of origin of the Foreslope Hills. 1.1. Location and geologic setting Latest episodes of sediment delivery to the Strait of Georgia are largely a result of Pleistocene glaciations followed by Fraser River out£ow since deglaciation (Wisconsinan) (Clague et al., 1998). The total drainage basin for the Fraser River is an area in excess of 234 000 km2 , with a mean annual discharge of 3400 m3 /s (Roberts and Murty, 1989). The river has constructed a subaerial delta in excess of 1000 km2 with a similar area o¡shore (Clague et al., 1983, 1991, 1998; Mosher and Hamilton, 1998). Roberts and Murty (1989) estimate that the delta has prograded 5.4 km in the last 2250 years. Fig. 1 shows the modern regional surface morphology of the southern Strait of Georgia. The foreslope of the Fraser River delta is about 200 km2 in area, extending from water depths of 10 m to more than 250 m, with slopes up to 23‡, but averaging 2^3‡ (Mosher and Hamilton, 1998). The southern Strait of Georgia includes water

depths in excess of 350 m. Channels, gullies, and sea valleys incise the delta slope (Fig. 1). A network of tributary submarine channels is incised into the delta slope just seaward of Sand Heads. The channels coalesce into a single steep-walled valley farther down the slope. The valley is a curvilinear feature up to 60 m deep, 100 m wide, and 6 km long, with walls as steep as 45‡. The valley £oor contains debris £ow deposits (Hart et al., 1992) and is the main conduit for modern sediment delivery to the delta foreslope and proximal prodelta environment from the Fraser River. Although the river has £owed continuously westward into the strait during the last 5000 years, considerable channel meandering and switching is known to have occurred, even in historic times. Dikes and jetties, however, now ¢x channel locations. Channel dredging and aggregate mining in the river has signi¢cantly reduced modern sediment delivery volumes. The estimated modern annual sediment load of the Fraser River is 17.3 million tonnes (McLean and Tassone, 1991). An average of 2.7 million m3 (6 million tonnes) of sand is dredged from the main channel each year (data from 1989^1994) (Hay and Co. Consultants Ltd., unpublished data, 1995). Williams and Hamilton (1994) estimated that sedimentation rates on the tidal marsh were an average of 51% lower during the period from 1964 to 1981 than from 1954 to 1964. Kostaschuk et al. (1998) and Luternauer et al. (1998) provide a thorough review of the modern Fraser River sediment dynamics. Hart et al. (1998) calculated modern sedimentation rates on the delta foreslope as 9 10 cm/yr. The Foreslope Hills lie at the base of the foreslope and modern sedimentation rates in this region are on the order of 5 cm/yr (Hart et al., 1998). 1.2. Physical oceanographic setting The Strait of Georgia is a partially mixed estu-

Fig. 1. Sun-shaded surface morphological map of the southern Strait of Georgia, onshore and o¡shore. The image has been generated with multibeam sonar bathymetric data for submarine regions. SOG 3 is the position of the current meter deployment. The oval outlines the Foreslope Hills and the box indicates the location of Fig. 2. Areas enclosed in white polygon lines are land and other black regions are unsurveyed, either due to shallow water depths or lack of data.

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SOG3 N

6 g. Fi

8 g. Fi

N

W

W

Fig. 2. Map showing seismic re£ection data used in this study of the Foreslope Hills region. The entire area is covered with multibeam sonar bathymetry. Seismic pro¢les include multichannel and single channel seismic re£ection data. Locations of seismic lines in Figs. 6 and 8 are indicated as is the position of SOG 3.

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279

123°20' W

123°25' W

m

200 m

0 25

0m 30

Se

ey all V a

A

49°5'N

Creep(?) Fi 8 g.

A'

0m 15

D

D' 20 0m

C C' g. Fi

30 0

m

6

B B'

0

1

2

Kilometres

3 49°00'N

Fig. 3. Sun-shaded relief image of the Foreslope Hills and the Sand Heads sea valley, generated with multibeam sonar bathymetric data. The image is a composite of the 1994 data overlying the 2000 and 2001 data. The arti¢cial sun angle is at an altitude of 50‡ from horizontal with an azimuth of 320‡. Vertical pro¢les are shown in Fig. 4. Locations of seismic lines in Figs. 6 and 8 are indicated.

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Water Depth (m)

Water Depth (m)

Water Depth (m)

Water Depth (m)

265 270

A

A'

275 280 285 290 295 220 230 240 250 260 270 280 290 300

230 240 250 260 270 280 290 300 310 230 240 250 260

B'

B

C'

C

D'

D

270 280 290 300 0

1000

2000 3000 Distance (m)

4000

5000

Fig. 4. Depth pro¢les across the Foreslope Hills. The letters on the pro¢les are keyed to their locations shown in Fig. 3. The mean wave height is 12 m (maximum 20 m) and wavelength is about 700 m. Vertical lines are placed through the waveforms to assist in observation of the symmetry of most of the ridges (not to be confused with asymmetry in the trend of the pro¢les).

ary characterised by moderately strong tidal currents, seasonally varying strati¢cation, and significant, late-summer deep-water density intrusions. Although separated from the open ocean by a series of channels and shallow sills, the strait remains dynamically coupled to oceanic processes taking place over the continental margins of British Columbia and Washington State. Recent reviews of the physical oceanography of the region (LeBlond, 1983; Crean et al., 1988; Thomson, 1994) de¢ne four principal factors a¡ecting oceanic properties within the strait: (1) tides, (2) freshwater runo¡, (3) winds, and (4) coastal ocean variability. Currents within the strait primarily consist of oscillating tidal £ows superimposed on

more slowly varying estuarine currents generated by river runo¡ and turbulent mixing. Spatial and temporal modi¢cations to this basic £ow structure are brought about by surface winds and the northward intrusion of density currents. Further modi¢cations to the £ow result from topographic ‘steering’, back eddying near coastal promontories, inertial e¡ects, and non-linear interactions of energetic oscillating £ows (tidal recti¢cation). The generation of both coherent and turbulentlike eddies, as well as wave-like phenomena such as internal tides, provide yet another level of complexity to the £ow structure in the strait. Tidal motions in the Strait of Georgia are predominantly at diurnal (K1, O1) and semidiurnal

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Fig. 5. Contours of distribution of mean grain size (in phi units) of surface sediment from grab samples around the Foreslope Hills. The mean grain size ranges from coarse to very ¢ne silt (4.0^8.0 phi). The black dots designate grab sample locations. White triangles designate core site locations.

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(M2, S2, N2) periods. The amplitudes of these currents diminish with distance from the narrow tidal channels connecting the strait to the ocean and are strongly modulated at fortnightly and monthly periods. This ‘beating’ of frequency components gives rise to the pronounced spring^neap tidal cycle in the strait. Water column strati¢cation, which a¡ects both the estuarine circulation and density-driven £ows in the strait, is determined primarily by the Fraser River watershed which typically reaches a maximum discharge of around 10 000 m3 /s during spring freshet (snowmelt) conditions and a minimum of around 1000 m3 /s in late winter. The Fraser River accounts for about 80% of the mean annual freshwater discharge of 4400 m3 /s into the Strait of Georgia. The estuarine component of the circulation is characterised by a net (daily-averaged) seaward out£ow of low salinity water within the upper portion of the water column ( 6 50 m depth) and a net landward in£ow of high salinity water within the underlying portion of the water column. Crean et al. (1988) argue that the bottom water intruding northward tends to favour the eastern side of the strait. As suggested by Karsten et al. (1995), this intruding, northward £owing density current would be con¢ned to the seaward slope of the Fraser River delta foreslope region and be subject to baroclinic instability causing the formation of mesoscale turbulent eddies.

2. Methods The Foreslope Hills have been surveyed with a combination of multibeam sonar bathymetric and seismic re£ection tools. Multibeam sonar bathymetry (shown in Fig. 1) consists of data acquired in 1994 with a Simrad EM-100 and in 2000 and 2001 with an EM-1002 system. Both systems operate at

95 kHz. The EM-100 is a 32-beam system mounted on a remotely operated semi-submersible vehicle (DOLPHIN). The EM-1002 is a 111beam, hull-mounted system on the CCGS R.B. Young. Each acoustic beam size is 2.5‡ by 3.0‡, meaning that in 100-m water depth, each beam ‘footprint’ is about 5 m across, giving some indication of the maximum horizontal resolution of which the system is capable. Details of the 1994 survey are provided in Currie and Mosher (1996). Seismic re£ection data consist of a variety of types collected during a number of Geological Survey of Canada (GSC) expeditions between 1982 and 1995. For this study, a grid of seismic re£ection lines with 500-m line spacing, totalling 213 line-km, was run over the Foreslope Hills (Fig. 2) during expedition PGC95001 (Mosher et al., 1995). This survey used a high-resolution Huntec deep tow sparker and a 0.65-l (40 in3 ) sleeve gun and multichannel receiver. The sparker unit was towed between 30 and 60 m below the sea surface and was ¢red at 4 kJ output every 1.5 m. Data were received on a 5-m-long, 10-element, single channel towed array. Data were digitally sampled at 40 Ws. Sleeve gun signals were received on an Innovative Transducers ST-5, 24-channel hydrophone array. Each channel consisted of a group of 3 hydrophones, with a channel separation of 8 m. Data were sampled at 250 Ws over a 1-s window length with a Geometric Strataview. The sleeve gun was ¢red every 16 m, which yielded a 6-fold stack in the data. Seismic re£ectors were digitally picked with seismic interpretation software. An additional 125 line-km of single channel 0.08-l (5 in3 ) air gun data were collected over the study region in 1991 (PGC91008) (Hart, 1993). Physical oceanographic data for this study are based on Aanderaa RCM4 current meter measurements at mooring station SOG3 (Figs. 1 and

Fig. 6. Line 7D (see Figs. 2 and 3 for location). The high-resolution seismic pro¢le at the top (A) shows little subbottom information because of the presence of interstitial gas, although the in¢ll and drape is distinctive. Some internal structure in the ‘Hills’ is obvious in the lower section (B) because of the greater energy and lower frequency content of the seismic source and because multichannel stacking increases the signal to noise ratio. The ‘A’ horizon is a regionally correlatable re£ector throughout the Strait of George and forms the base of Unit 4. It is a major disconformity that Mosher and Hamilton (1998) attribute to a distinctive change from glacial-dominated sedimentation to modern Fraser River sedimentation. The ‘B’ horizon is a local horizon correlatable under most of the Foreslope Hills.

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2). This station was occupied from May 1996 to May 1999. Depth from sea surface to sea£oor at the station during mean high tide was 344 m, with ¢ve instruments deployed in a vertical con¢guration at 324, 296, 221, 146, and 44 m below sea level. All data were recorded at 30-min intervals and smoothed with a ¢lter to 1-h values.

3. Results 3.1. Sur¢cial morphology The Foreslope Hills cover an area of about 60 km2 at water depths between 230 and 350 m, at the very base of the Fraser River delta foreslope (Figs. 1 and 3). Multibeam bathymetric data clearly delineate the morphology of the Foreslope Hills. They consist of a series of ridges, up to 5 km long and 20 m high, with wavelengths of 500^ 1200 m (Fig. 4). Most of the ridges are symmetric in pro¢le. The orientation of the ridges changes from NE^SW in the northern part of the area to N^S in the southern part (Figs. 1 and 3). Long linear sedimentary furrows extend northwards beyond the northern limit of the Foreslope Hills (Fig. 1). 3.2. Sur¢cial geology In general, suspended sediment emanating from the Fraser River tends to be transported in a northward direction in the Strait of Georgia and as a result the grain size of sur¢cial sediment tends to ¢ne away from the mouth of the river and away from the delta (Barrie et al., 1999). Sediment transported by bedload or mass-£ow processes down the various foreslope channels of the delta (Hart et al., 1992) tends to accumulate at the base of these channels, creating anomalous

zones of higher grain size. Fig. 5 demonstrates two of these zones at the base of the present day Sand Heads sea valley and the Canoe Passage gully that is now thought to be largely relict. In the area of the Foreslope Hills, sur¢cial sediment consists of clayey silt; the average mean sizes plotting from ¢ne to coarse silt (Fig. 5). There are insu⁄cient samples to determine any relationship of grain size with the complex morphology. Shallow sediment cores (the deepest is 5.8 m) from the Foreslope Hills can be described, in general, as organic rich, sti¡, sandy or silty mud with abundant evidence of gas (cracking due to expansion on recovery). The cores are intensely bioturbated, thus any evidence of sedimentary structures have been obliterated. Cores closer to the delta front and o¡ the ridges of the Foreslope Hills contain massive sandy beds with sharp contacts with surrounding silty mud. One core from a ridge crest but close to the termination of the Fraser River sea valley, contains several thin sand beds at its top, but the remainder of the core is bioturbated silty mud. 3.3. Seismic stratigraphy Mosher and Hamilton (1998) divided the seismic stratigraphy of the Strait of Georgia into four seismo^stratigraphic units. Unit 1 is Late Cretaceous and Tertiary sedimentary bedrock. Unit 2 represents ice and ice proximal deposits (till, diamict and glaciomarine sediments) from Pleistocene glaciations. Unit 3 represents distal glaciomarine sediment from the last Glacial phase (Fraser Stade), and Unit 4 is the uppermost Holocene succession, deposited during progradation of the Fraser River delta. The Foreslope Hills occur within the upper portion of Unit 4 (Fig. 6). There is a maximum of 325 m of post-Glacial sediment in the Strait of Georgia and Unit 4 is a

Fig. 7. Surface elevation and isopach maps of sediment underlying the Foreslope Hills, derived from seismic horizon picks of the grid of seismic survey data shown in Fig. 2. Surface elevation maps (i.e. depth from sea level) to the ‘A’ and ‘B’ horizons show these horizons to be relatively £at-lying, thus the structures of the Foreslope Hills occur completely above them in the stratigraphy. The isopach maps re£ect the distinctive morphology of the Foreslope Hills, as they represent the thickness of sediment from the sea£oor to the ‘A’ and ‘B’ horizons. Travel time to depth conversions were calculated using an interval velocity of 1250 m/s derived from the multichannel RMS stacking velocities. This velocity is low in the upper sediment column because of interstitial gas.

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maximum of 195 m (325 ms) thick (Fig. 7). Highresolution seismic pro¢les of the Foreslope Hills are largely obscured by the interstitial gas (Fig. 6A ; Hart and Hamilton, 1993), but airgun and especially the multichannel re£ection records show the uppermost re£ections under the hills (Figs. 6B and 8). Unfortunately, with increased energy to penetrate the gas, vertical resolution is lost. These records show that the Foreslope Hills do not mimic deeper structure. The regionally extensive re£ector ‘A’, separating seismic Unit 4 from the underlying Unit 3, is relatively £at-lying throughout the Strait of Georgia (Fig. 7). Re£ector horizon ‘B’ is situated above the ‘A’ horizon in the stratigraphic sequence. It is apparent below a portion of the Foreslope Hills. It, too, appears relatively £at, dipping slightly to the west (Fig. 7). Given its position, this re£ector is likely to be middle to late Holocene in age. The thickness of sediment to this horizon is maximally 145 m and minimally 75 m (Fig. 7). Internal re£ectors in the Foreslope Hills are asymmetrical and sinusoidal in shape (Figs. 6 and 8). Bedding thickens towards the southeast in the northern ridges and to the east in the southern ridges. Both directions are upslope. Seismic re£ection records show that the inter-ridge low areas close to the delta foreslope (i.e. to the east) may contain a ¢ll sequence of £at-lying and chaotic re£ectors (Fig. 6). The whole study area may have a drape of relatively transparent sediment (Fig. 6A).

ciding with the spring freshet, but in the opposite direction of the estuarine out£ow. Bottom currents at SOG3 consist of 20^30 cm/s tidal currents, 10 cm/s estuarine currents, and s 10 cm/s, but seasonally dependent, deep-water intrusions. Tidal currents within 50 m of the sea£oor are predominantly of diurnal and semidiurnal period (Table 1). Major axes of the tidal ellipses are orientated along-topography (roughly along 340^350‡ True) with an order of magnitude weaker £ow across the topography (small minor axis velocities ; seeFig. 9). Mean (vector-averaged) currents for the entire current meter record are around 2 cm/s and are directed to the ESE (Table 1). When the currents are separated into directional sectors (22.5‡ sectors) and then averaged, mean current directions are consistent with the instantaneous maximum £ows (Fig. 9). Over the depth range of 250 to 350 ms, deepwater intrusion takes the form of daily pulses of bottom-trapped, high salinity, low temperature water whose formation is linked to fortnightly and monthly variations in the tidal current. The pulses are gravity currents originating during periods of minimal tidal mixing (neap tides in summer). Deep-water intrusions mainly occur in summer to early fall and are directed to the NNW. The combined £ow, consisting of tidal currents, mean estuarine £ow, and intermittent deep-water intrusions, gives rise to maximum near-bottom currents of 50 cm/s, preferentially orientated to the NNW^SSE (Fig. 9B).

3.4. Physical oceanography 4. Discussion The physical oceanographic parameters used in this study were derived from data collected at station SOG3 (Fig. 9 ; see Figs. 1 and 2 for location). The pattern of tidal oscillatory £ows and estuarine currents can be discerned from Fig. 9 and Table 1. The shallower moorings, particular the mooring at 225 m, show net £ow towards the south and SSE, representing the estuarine £ow from the Fraser River. Currents are particularly strong in this direction through the spring freshet in June and July. The lowest mooring re£ects more the oscillatory currents of the £ood and ebb tides. Particularly strong currents occur coin-

A number of hypotheses have been advanced to explain the genesis of the Foreslope Hills. Mathews and Shepard (1962), Terzaghi (1962), and Hamilton and Wigen (1987) postulated that they are displaced masses resulting from a single, large failure of the slope of the Fraser River delta. By this mechanism, the ‘hills’ would be the result of rotation and signi¢cant transport of coherent blocks of material. Hamilton and Wigen (1987) further suggested that a signi¢cant tsunami might have followed the failure, although there has been no collaborative evidence reported to substantiate

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Fig. 8. A 0.65-l sleeve gun, multichannel seismic re£ection section of Line PGC95001-5D over the Foreslope Hills (see Fig. 2 for location). This pro¢le shows the sinusoidal nature of the internal re£ection horizons suggesting a migration direction to the SE.

such an occurrence (e.g. no evidence of near-shore tsunami deposits). Shepard (1967) speculated that the Foreslope Hills may be mud diapirs, similar in origin to those on the Mississippi delta (see Prior and Coleman, 1984). Ti⁄n et al. (1971) and Luternauer and Finn (1983) concluded that the Foreslope Hills consist of folded sediments deformed by upslope instability. Hart (1993) suggested that the asymmetry in the internal structure of the hills is the result of shear caused by in situ rotational displacement in a downslope direction. He felt the complex represents thick blocks

of base-of-slope strati¢ed muds that have undergone rotation with only moderate downslope movement. Christian et al. (1997, 1998, 1999) suggested a link between delta progradation (driving stress) and creep deformation (yielding in weak sea£oor strata). They acknowledge, however, that the morphology and internal structure of the Foreslope Hills are similar to sedimentary bedforms. Most of the above hypotheses are based principally on qualitative morphologic interpretation, with an incomplete perspective on the detailed

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Table 1 Mean currents and main tidal current constituents derived from hourly records of near-bottom currents at mooring site SOG3 for the summer (S) of 1998 and the winter (W) of 1998/99 (position 49‡6.3 N, 123‡21.0 W; water depth = 350 m) Instrument depth

Mean (Zo)

Main diurnal (K1)

Main semidiurnal (M2)

Speed (cm/s)

Direction toward

Major (cm/s)

Minor (cm/s)

Ellipse orientation

Major Minor (cm/s) (cm/s)

Ellipse orientation

(m)

2.73 2.00 2.73 1.53

114.5 61.1 129.1 93.5

8.43 7.67 9.91 10.43

30.61 30.21 0.02 0.55

342.3 337.2 346.0 347.4

14.13 11.47 11.72 9.35

338.3 350.0 342.9 307.3

296 324 309 337

S S W W

32.63 0.52 31.49 0.45

Direction and orientation of the tidal ellipses is given in ‡ True. Ellipse orientation is the orientation of the major axis of the tidal ellipse

morphology of the Foreslope Hills compared with data resolution provided in this study. There is little other collaborative evidence for these interpretations. There is no evidence of a failure scar on the foreslope to support a massive submarine slide hypothesis, and it is suggested that the internal structure of the hills is too well formed and coherent for them to represent displaced blocks. Imaging of the surface and the subsurface right to bedrock shows no evidence of mud diapirs. The thesis that they represent rotational failure requires backward tilting (asymmetry) in the morphology of the hills ; however, the surface of the hills is very symmetric (Fig. 4). In addition, it is di⁄cult to imagine how only the uppermost portion of the sediment column (in fact, only the uppermost portion of the sur¢cial stratigraphic unit) could be folded without evidence of deformation in the remainder of the sediment column and without evidence of a detachment surface (decollement). Shallow sediment cores are too intensely bioturbated to record any evidence of structural deformation. The most quantitative geotechnical studies, largely addressing slope stability of the Fraser River delta foreslope, were those of Christian et al. (1997, 1998, 1999). They modelled failure criteria for the foreslope and showed that the slope is highly susceptible to £ow-liquefaction failure. No geotechnical measurements were made of the Foreslope Hills, but they calculated driving stress vectors based on gravitational loading and measured slope angles. This analysis showed that principal shear stress vectors are orthogonal with the ridge crests of the Foreslope Hills.

They interpret from this correspondence that the Foreslope Hills are in response to gravitational loads imposed by progradation of the delta, coupled with downslope creep of the lower slope sediments. Indeed, creep-style deformation has been reported on the Fraser delta foreslope (Currie and Mosher, 1996; Mosher and Hamilton, 1998) and is visible in multibeam imagery (Figs. 1 and 3), just south of the Fraser River sea valley. The morphologic expression of creep failure at this site is a series of contour parallel ridges that are an order of magnitude smaller than the Foreslope Hills and quite di¡erent in their morphology. The orientation of the Foreslope Hills ridges is not contour parallel. The stress vectors of Christian et al. (1997, 1998, 1999) are based exclusively on slope angle measurements derived from bathymetry; no direct measures of stress have been acquired. The same bathymetry can be used to explain the formation and positioning of the Foreslope Hills as sedimentary bedforms, since the bathymetry controls the direction and intensity of the local hydrodynamic regime. The recent acquisition of multibeam bathymetric data is crucial to the interpretation of the Foreslope Hills (Fig. 3). Sur¢cial morphologic images, generated with these data, are strongly suggestive of sediment waves (as reported by Mosher and Hamilton, 1998), with long, linear, evenly spaced, symmetrical ridges with ridge bifurcation, in some cases. In the northern portion, ENE^ WSW ridge orientation is perpendicular to mean £ow directions. To the south, ridge orientation is almost N^S. Current £ow directions are not

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Fig. 9. Summary of moored current meter data collected at SOG 3 (see Fig. 2 for location). (A) Time series of vectors representing daily-averaged current velocities and directions for a full year (1996) at three mooring depths. The highlighted zone shows a strong density intrusion spike in the deep mooring and strong shallower currents in the opposite direction due to the spring freshet. (B) Rose diagram of current velocities within 22.5‡ sectors of the compass. The white ‘spider web’ represents maximum velocities, the grey-shaded region represents daily averaged velocities and the solid lines represent percent time that currents are in that preferred direction (i.e. within that 22.5‡ sector).

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known locally in this southern portion, but are likely complex as the £ows de£ect around the Fraser River delta. Beyond the northern edge of the Foreslope Hills, multibeam sonar data reveal a series of linear sea£oor furrows perfectly aligned with the principal bottom water current direction (Fig. 1). These sedimentary furrows are further indications of bottom currents impacting the sea£oor sediments, as the bottom boundary £ow develops a secondary helical £ow (Flood, 1983). Sedimentary furrows have been recognised in deep-water environments (e.g. Bryant et al., 2000) and Flood (1983) estimates minimum currents of 5^20 cm/s are needed to generate the helical-type £ow required. The internal structure of the Foreslope Hills has been imaged with seismic re£ection pro¢les that show sinusoidal re£ectors on the upslope side. In situ gas and limitations in vertical resolution of the seismic systems, however, make it dif¢cult to image details of the internal structure. In addition, shallow sediment cores are not helpful as bioturbation and gas disturbance has destroyed any evidence of sedimentary structures. Fig. 10 shows a 2-D synthetic seismogram resulting from a model where horizons are sinusoidal in shape with thinning on one side of the hill and thickening on the opposite site. The synthetic seismic record shows that individual horizons are not resolved on the thinning side and are resolved on the opposite side. Such a pro¢le could explain seismic images over the Foreslope Hills. Hart (1993) contended that the shape of these re£ections, i.e. lack of re£ections on one side of the hill, was determined by in situ rotational shear due to failure. In the context of the synthetic seismogram, these sinusoidal re£ectors are interpreted as depositional beds on the upslope £ank of the bedform. The lack of re£ectors on the opposite, or downslope side, is interpreted as either non-deposition or erosion. This sedimentary sequence indicates that if there is net migration of the bedform it is likely to be in the upslope direction. The Foreslope Hills have many similarities in scale, composition, structure, morphology and environmental setting with sediment waves found in a number of continental slope and rise settings.

For example, Howe (1996) shows sediment waves in the Rockall Trough that appear identical in form and structure to the Foreslope Hills. Sediment waves are usually interpreted as depositional bedforms produced either by turbidity currents or bottom currents. One of the fairly consistent and puzzling features of the Foreslope Hills and many other ¢ne-grained sediment waves is that they demonstrate a consistent pattern of upstream or oblique migration (see Normark et al., 1980; in press). Under simple £ow conditions it would be expected that the strongest £ow would be on the upstream side of the bedform, producing deposition on the downstream side, giving rise to an overall downstream migration. The downstream direction is expected to be downslope. Wynn et al. (2000) reviewed two physical models to explain deep-water sediment waves with apparent upslope migration: (1) an antidune model, and (2) a lee-wave model. Case 1 is by analogy with £uvial antidunes (see McCave and Tucholke, 1986) produced by internal standing waves at a density contrast between two water masses. Normark et al. (1980) use this model to describe sediment waves of a turbidity current origin on the Monterey fan levee. In this case, the density differences are produced by downslope, ¢ne-grained, over-bank turbidity £ows. Wave migration results from the di¡erential deposition along the standing wave. The antidune model is less favoured to describe the Foreslope Hills. Turbidity currents are suspected to exist (McKenna et al., 1992; Evoy et al., 1993; Chillarige, 1995), emanating from the mouth of the Fraser River. In the region of the Fraser River sea valley, ridges of the Foreslope Hills are oriented parallel to the sea valley, i.e. to the downslope direction of £ow. If the hills were antidunes generated by turbidity currents then they should be roughly perpendicular to the £ow direction. There are strong velocity and density contrasts in water masses of the strait, with out£ow of freshwater from the Fraser River over deep Paci¢c saline water. This contrast could generate internal standing waves as required in the antidune model. Standing waves generated on the western side of the strait have been noted, but only where there are high current accelera-

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Fig. 10. (A) High resolution boomer pro¢le across sediment waves in the Rockall Trough (from Howe, 1996). These features are believed to be analogous to those of the Foreslope Hills. (B) Input for the 2-D synthetic seismogram (C) modelled after the structure shown in (A) and after the interpreted structure of the Foreslope Hills. High resolution boomer pro¢les across the Foreslope Hills gave little structural information because of the interstitial gas (see Fig. 7). A 0.65-l sleeve gun signature is used as the source function for generation of the synthetic seismogram, to model the acoustic response. It shows that it is impossible to resolve the thin beds on the thin-bedded side of the bedforms, hence they appear as a single complex re£ection. On the opposite slope, however, the thicker beds are resolved.

tions due to restricted passages. In general, the interface between water masses is too shallow to a¡ect the deep basins. Flood (1988) described the lee-wave model for the formation of sediment waves. In a simple uni-

directional, density-strati¢ed £ow, phase-shifts in the streamlines are caused by the sinusoidal topography as the current £ows over that topography. Depending on the wavelength of the features, the density gradient, the Coriolis e¡ect

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and the velocity of the £ow, internal waves develop on the density gradient, which have a maximum bed velocity at 1/4 wavelength and minimum velocity at 3/4 wavelength. This velocity pattern results in less deposition or possibly erosion on the downstream £ank of a wave and more deposition on the upstream £ank. Without a unique combination of these parameters, streamline displacement is exponential with no phase shift. In these cases, sediment accumulates in the troughs. The £ow regime for the formation of bedforms that migrate upstream in a strati¢ed £uid is: f 9kU9N where: f ¼ 26sina ¼ 2U0:7291U1034 sinas31 is the Coriolis parameter for latitude a and earth rotation rate 6, k = 2Z/L is the wavenumber of the bedforms (wavelength, L) and N is the Brunt^Va«isa«la (buoyancy) frequency : N ¼ ððgDbÞ=ðbo DzÞÞ1=2 for the vertical density gradient Db/Dz. For the latitude of the Strait of Georgia moorings, a = 49‡, fW1.1U1034 s31 , and N for the water depths deeper than 200 m ranges from 0 to 27.6U1034 s31 , with typical values of 6.9U1034 s31 . Thus, a conservative estimate is 1.1U1034 s31 9 kU100 9 6.9U1034 s31 . For normal £ow velocities (U100 = 0.1 m/s), bedform wavelengths would be estimated as 0.91U103 m 6 L 6 5.7U103 m, while for the extreme case during deep-water intrusion (U100 = 0.5 m/s) 0.23U103 m 9 L 9 5.7U103 m. The observed wavelengths of the Foreslope Hills are between 0.8U103 m and 1.2U103 m (Figs. 3 and 4). This lee-wave model is preferred to describe the formation of the Foreslope Hills, although it may be an over-simpli¢cation. Their formation is probably a complex interaction of turbidity currents with bottom morphology, suspension plume fallout and deep-water tidal £ood currents. Sediment is delivered to the foreslope and prodelta of

the Fraser River delta through turbidity currents and suspension fallout emanating from the Fraser River (McKenna et al., 1992; Evoy et al., 1993; Chillarige, 1995; Kostaschuk et al., 1998). This material can be entrained during or soon after deposition by deep currents in the Strait of Georgia. Current velocities in deep water here are typically on the order of 10 cm/s with a maximum of about 50 cm/s during density intrusions (Fig. 9). The critical threshold for grain movement of coarse silt (the modal size in the region of the foreslope hills) requires current velocities (at 1 m above the bed) of about 20 cm/s (U
100 = 20 cm/s) (Miller et al., 1977; Yalin and Karahan, 1979). Current velocities in the strait, therefore, can be strong enough to transport the material constituting the Foreslope Hills. These currents show a strong trend to the NNW^SSE at station SOG3, perpendicular to ridge orientations in the vicinity of SOG3. Deep tidal currents are bi-directional, but the strong, periodic density intrusions are unidirectional to the NNW. In order to generate sediment waves demonstrating upslope migration, a strong unidirectional current (to the NNW) is preferred. These density intrusions may be primarily responsible for lee wave formation and construction of the Foreslope Hills, or it may be that even strong £ood currents (NNW) can have this e¡ect because of the stronger density gradient associated with this £ow than the ebb current. The surface shapes of the Foreslope Hills are highly symmetrical. Flood (1988) shows that mud waves with shorter wavelengths and smaller wave heights tend to be more symmetric, even at higher £ow speeds. This symmetry is notable in other deep-water sediment waves of the same size scale and occurring at the same latitude, such as in the Rockall Trough (Howe, 1996). It is di⁄cult to discern that the Foreslope Hills are active features today without more detailed study. Their stratigraphic position indicates they ¢rst formed within the last 2000 to 3000 years. If they are not active, then they certainly have been within the last several hundred years. Sedimentation rates in the region are about 5 cm/yr (Hart et al., 1998), and most of the Foreslope Hills are not buried. Channel detainment, dredging and aggre-

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gate mining in this century, however, has greatly altered sediment delivery (quantities, particle sizes and pathways) to the delta and its foreslope (McLean and Tassone, 1991; Williams and Hamilton, 1994 ; Hay and Co. Consultants Ltd., unpublished data, 1995; Kostaschuk et al., 1998; Luternauer et al., 1998; Barrie et al., 1999). It may be that the system has been altered to a degree that the processes that constructed and maintained these features are no longer present. Sediment in¢ll of the troughs between ridges, in particular those closest to the delta foreslope, is observed (Fig. 7). This in¢ll may not be indicative of inactivity, but more likely that the features closest to the delta were/are being buried by river mouth sedimentation processes (see Hart et al., 1992). The in¢ll material appears in acoustic character as turbidites and debris £ow deposits and is likely re£ected in the sand beds that are apparent at the tops of a few cores closest to the delta front.

5. Summary and conclusions The Foreslope Hills are a set of sea£oor morphologic ridges covering an area in excess of 60 km2 in the central Strait of Georgia, British Columbia. They occur in water depths of 230^350 m at the base of the Fraser River delta foreslope, consisting of 20-m-high, s 5-km-long ridges with wavelengths in the order of 1 km. Ridge orientation changes from about 180‡ in the south to 60‡ in the north. They are highly symmetrical at the surface but their internal structure shows asymmetry with thickening towards the upslope direction. Former interpretations as to their origin have ranged from sediment slide (with varying opinions on the distance of displacement) to creep failure to mud diapirs. Recent multibeam bathymetric and high resolution seismic re£ection data, evidence of strong, density-driven, near-bottom currents, and increasing documentation of large sedimentary bedforms in many slope and rise settings leads to the interpretation that the Foreslope Hills are sediment waves and not mass-failure related. Their dimensions, constituent grain sizes, and position in the Strait of Georgia make them

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an ideal and accessible analogue for the study of deep-water sediment waves. Bottom currents at the site, with mean and maximum velocities of 10 and 50 cm/s, respectively, can entrain and transport sediment of the size classes (mean of ¢ne^coarse silt) found in this region. The preferred physical mechanism invoked to explain the construction of the Foreslope Hills is the lee-wave model. Sediment-laden bottom currents interact with the sea£oor morphology causing undulations in the current £ow, which results in deposition on the upstream side of the bedform and erosion or non-deposition on the downstream side. The Foreslope Hills appear to have formed within the last few thousand years, given their stratigraphic position.

Acknowledgements We wish to express our appreciation to the many people who helped to collect the data that made this paper possible, especially the technical sta¡ at the GSC^Paci¢c. P. Milner and E. Sargent with the Canadian Hydrographic Service and K. Iwanowskia and G. Rathwell with GSC^Paci¢c collected the multibeam data. B. Nichols assisted in the acquisition of the multichannel seismic data and C. Athias conducted the processing. Seismic Micro-Technology supplied the modelling software. We thank David Piper for valuable discussions concerning the genesis of the Foreslope Hills and reviewing the manuscript and Homa Lee, Thiery Mulder and Russell Wynn for critical reviews. This is Geological Survey of Canada Contribution No. 2000297. References Barrie, J.V., Currie, R. Kung, R., 1999. Sur¢cial Sediment Distribution and Human Impact ^ Fraser Delta. Geological Survey of Canada, Open File Rep. 3572, 1 map sheet. Bryant, W., Bean, D., Dellapenna, T., Dunlap, W., Silva, A., 2000. Massive bed-forms, mega-furrows, on the continental rise at the base the Sigsbee Escarpment, Northwest Gulf of Mexico. Am. Assoc. Pet. Geol. Bull. 84, 1673. Chillarige, A.V., 1995. Liquefaction and Seabed Instability in the Fraser River Delta. Ph.D. Thesis, University of Alberta, Edmonton, AB.

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