Architecture and evolution of a fjord-head delta, western Vancouver Island, British Columbia

JOURNAL OF QUATERNARY SCIENCE (2004) 19(5) 497–511Copyright � 2004 John Wiley & Sons, Ltd.Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jqs.844

Architecture and evolution of a fjord-head delta,western Vancouver Island, British ColumbiaJEFFREY E. GUTSELL,1 JOHN J. CLAGUE,2* MELVYN E. BEST,3 PETER T. BOBROWSKY4 and IAN HUTCHINSON5

1 Gartner Lee Limited, Whitehorse, YT, Canada2 Department of Earth Sciences, Simon Fraser University, Burnaby, BC, Canada V5A 1S63 Bemex Consulting International, Victoria, BC, Canada4 Geological Survey of Canada, Ottawa, ON, Canada5 Simon Fraser University, Burnaby, BC, Canada

Gutsell, J. E., Clague, J. J., Best, M. E., Bobrowsky, P. T. and Hutchinson, I. 2004. Architecture and evolution of a fjord-head delta, western Vancouver Island,British Columbia. J. Quaternary Sci., Vol. 19 pp. 497–511. ISSN 0267-8179.

Received 15 September 2003; Revised 28 April 2004; Accepted 8 February 2004

ABSTRACT: The architectural framework and Holocene evolution of the Zeballos fjord-head deltaon west-central Vancouver Island was established through a multidisciplinary field-based study. TheZeballos delta is a composite feature, consisting of an elevated, incised, late Pleistocene delta and aninset Holocene delta graded to present sea level. Both deltas have a classic Gilbert-type tripartitearchitecture, with nearly flat topset and bottomset units and an inclined foreset unit. Time domainelectromagnetic (TDEM) and ground-penetrating radar (GPR) surveys, borehole data, and gravelpit exposures provided information on the internal form, lithologies and substrate of both deltas. Bothsets of deltaic deposits coarsen upward from silt in the bottomset unit to gravel in the topset unit. TheTDEM survey revealed a highly irregular, buried bedrock surface, ranging from 20 m to 190 m indepth, and it delineated saltwater intrusion into the deltaic sediments.

Late Quaternary sea-level change at Zeballos was inferred from delta morphology and the GPRsurvey. The elevated, late Pleistocene delta was constructed when the sea was about 21 m higherrelative to the land than it is today. It was dissected when sea-level fell rapidly as a result of gla-cio-isostatic rebound. Relative sea-level reached a position about 20 m below the present datum dur-ing the early Holocene. Foreset beds that overlap and progressively climb in a seaward direction andtopset beds that thicken to 26 m landward imply that the delta aggraded and prograded into ZeballosInlet during the middle and late Holocene transgression. Sea-level may have risen above the presentdatum during the middle Holocene, creating a delta plain at about 4 m a.s.l. Remnants of this surfaceare preserved along the valley margins. Copyright � 2004 John Wiley & Sons, Ltd.

KEYWORDS: delta; fjord; ground-penetrating radar; Quaternary; British Columbia.

Introduction

Fjord-head deltas are found in ‘deep, high-latitude estuaries,which have been (or are presently being) excavated, or modi-fied by land-based ice’ (Syvitski et al., 1987, p. 3). They occurin Alaska, Canada, Greenland, Norway, Antarctica, and south-ern Chile (Syvitski et al., 1987).

Fjord-head deltas can be divided into two groups. High-latitude fjord-head deltas are fed by streams and rivers flowingfrom glaciers and occur in areas of permafrost. They support asparse plant cover and consequently contain shallow distribu-tary channels. Temperate fjord-head deltas occur in areas of

cool wet climate and have abundant vegetation and few deepnarrow distributaries.

The combination of high sediment flux and confinement insteep-sided valleys promotes rapid seaward progradation andaccretion of fjord-head deltas, which are characterized by atripartite architecture of flat topset and bottomset units andan inclined foreset unit (Syvitski and Farrow, 1983). SuchGilbert-type deltas form where large amounts of bedload mate-rial are transported to a river mouth with deep water offshore.

The rugged west coast of British Columbia is indented bynumerous fjords, which have been created from Tertiary rivervalleys by Pleistocene glacial erosion. Sediment-laden streamshave built deltas at the heads of these fjords over the past13 000 yr, following the disappearance of the late Pleistoceneice sheet that covered this region. Although common, thesedeltas have not been studied extensively (Syvitski and Farrow,1983). Kostaschuk and McCann (1983), Kostaschuk (1985),Syvitski et al. (1988), Bornhold and Prior (1990) and Ren et al.

* Correspondence to: J. J. Clague, Department of Earth Sciences, Simon FraserUniversity, Burnaby, BC, Canada V5A 1S6. E-mail: [email protected]

(1996) documented patterns of sedimentation at and beyondthe fronts of fjord-head deltas on the British Columbia main-land coast. None of these workers, however, investigated thethree-dimensional architecture and evolution of the deltas.Nor have they studied the impacts of late Quaternary sea-level change on delta architecture. Since deglaciation11 000–13 000 yr ago, sea-level on some parts of the BritishColumbia coast has ranged from up to 200 m above to severaltens of metres below the present datum (Mathews et al., 1970;Clague et al., 1982). These large and rapid sea-level changesmust have strongly influenced the evolution of deltas in thisregion.

The primary objectives of this paper are to: (i) describe thethree-dimensional architecture of the Zeballos fjord-headdelta, Vancouver Island, British Columbia; (ii) show how thisdelta evolved from the late Pleistocene to present; and (iii)demonstrate that sea-level controls the evolution of this and,by analogy, other fjord-head deltas. We highlight the impor-tance of using an integrated geological and geophysicalapproach to unravel the stratigraphy and facies architectureof a complex delta. Zeballos delta was chosen for studybecause it is accessible by road and is representative of fjord-head deltas on Vancouver Island.

Study site

The Zeballos River delta is located at the head of Zeballos Inleton the central-west coast of Vancouver Island, approximately300 km northwest of Vancouver (Fig. 1). The study site includesthe floodplain, distributary channels, fringing marsh, and sandflats of the modern delta (Fig. 2), as well as remnants of an ele-vated late Pleistocene delta (Figs 3, 4). The tidal range at the

head of the inlet is 4.5 m. Zeballos River discharge ranges from5 to 1200 m3 s�1, with highest flows during autumn rainstormsand during late spring and summer snowmelt.

At the height of the Fraser Glaciation about 16 000 yr ago(Porter and Swanson, 1998), most or all of Vancouver Islandwas covered by the Cordilleran ice sheet. Glacial striae indi-cate that ice flowed in a southwesterly direction over themountainous spine of Vancouver Island to the Pacific Oceanand was at least 700 m thick at Zeballos (Howes, 1981a).Deglaciation of north-central Vancouver Island began shortlyafter 16 000 yr ago (Hebda, 1983) and was complete by11 000–12 000 yr ago (Clague, 1981).

Sea-level fell rapidly following deglaciation because regio-nal glacio-isostatic rebound exceeded the rise in sea-levelcaused by melting of Pleistocene ice sheets (Fig. 5; Clagueet al., 1982; Friele and Hutchinson, 1993). Soon after the be-ginning of the Holocene, sea-level at Tofino, which is 120 kmsoutheast of Zeballos, was lower than the present datum(Bobrowsky and Clague, 1992). Sea-level subsequently rose3–4 m above present before falling during the late Holocene(Friele and Hutchinson, 1993).

Methods

The Zeballos delta was mapped using aerial photographs and adetailed digital topographic map developed for this project.Ground-penetrating radar (GPR) and time-domain electromag-netic (TDEM) techniques were used to image the subsurface ofthe delta. The GPR provided information on the shallow struc-ture of the delta, whereas TDEM yielded data on subsurfacesediment types and on the total thickness of the deltaicsequence. Logs of two shallow boreholes and exposures in a

Figure 1 Map of southwestern British Columbia showing the location of the study area

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Copyright � 2004 John Wiley & Sons, Ltd. J. Quaternary Sci., Vol. 19(5) 497–511 (2004)

gravel pit provided some ground-truth for the geophysicalinterpretations.

Map and aerial photograph interpretation

A digital map of the Zeballos delta was produced byMcElhanney Consulting Services Ltd from 1:20 000-scale aerialphotographs taken in 1995 and from an existing 1:20 000-scaleGeographic Data BC topographic map. The digital map has a2-m contour interval from mean sea-level to 30 m above meansea-level.

Ground-penetrating radar

Sixteen GPR transects, totalling 2.4 km, were run on the mod-ern and late Pleistocene deltas (Fig. 6). The six profiles pre-sented in this paper are a subset of these 16 transects, chosento illustrate a range of representative features. They include thetransects along Fair Harbour Road (FH1–4), Parkway Avenue(PW1), and the Zeballos gravel pit (GP1).

During a GPR survey, short pulses of high-frequency electro-magnetic energy are transmitted into the ground by a radar unit

as it is moved over the surface. A portion of the energy isreflected back to a receiver when it encounters a change inthe electric properties of the materials, due to a difference insediment type or an unconformity or other contact (Davisand Annan, 1986; Davis and Annan, 1989; Fisher et al.,1995; Jol et al., 1996). The returning energy is recorded, andthe resulting continuous digital output can then be processedand interpreted to determine stratigraphy, structure, and moist-ure content of the subsurface materials.

The GPR system used in this study is a PulseEKKO 100, madeby Sensors & Software Ltd. (1994). Fifty MHz antennae wereused to achieve a reasonable depth of penetration (up to about40 m), with some loss of spatial resolution. Depths of reflectorsand geological units were estimated through common mid-point analysis.

All GPR profiles were edited and processed using the Sensors& Software programs pulseEKKO Plot, Edit (version 4.22) andTools (version 2.0). The records were then interpreted usingprinciples of radar stratigraphy (Beres and Haeni, 1991; Joland Smith, 1991; Huggenberger, 1993; Smith and Jol, 1997;van Heteren et al., 1998; van Overmeeren, 1998).

Ground-penetrating radar underestimates the true depths ofdipping reflectors and thus yields dip angles that are too low(Ulriksen, 1982; Sensors & Software, 1994; Reynolds, 1997).To correct this problem, we measured dips of inclined reflec-tors directly off radar profiles and then applied Ulriksen’s

Figure 2 Zeballos study site (Province of British Columbia photograph 30BCB95035: 273, June 26, 1995)

FJORD-HEAD DELTA SEDIMENTS, BRITISH COLUMBIA 499


(1982) equation. This equation migrates the angle in question[�¼ sin�1 (tan �u)], where �u is the measured angle on radarprofile and � is the corrected angle.

Time-domain electromagnetics

Twenty time-domain electromagnetic soundings were taken onthe Zeballos delta plain (Fig. 6). The TDEM systems use a cur-rent to induce a time-varying electromagnetic field in theground. The field produces conductivity variations that canbe mapped. The primary factors controlling conductivity arelithology, degree of saturation and the salinity of pore or frac-ture fluids (Kafri et al., 1997).

A Geonics Protem 47 TDEM system (Geonics, 1991) wasused for this study. The system consists of a transmitter that gen-erates a primary electromagnetic field and one or more recei-vers that measure secondary electromagnetic fields generatedin the earth by eddy currents induced by the primary field(Fitterman and Stewart, 1986; Mills et al., 1988; Best et al.,1995, Best et al., 2002). At Zeballos, the TDEM system was

used in the central sounding mode, because that configurationprovides the best accuracy and lateral resolution (Kafri et al.,1997). Voltage-versus-time data for three frequencies (ultra-high, UH; very high, VH; and high, H) were downloaded fromthe TDEM system using Interpex TEMIXGL software. High-quality, relatively noise-free data for each frequency wereselected, averaged and plotted as normalised voltage valuesversus time. The normalised voltage values were then con-verted to apparent resistivities using the late-time normalisedvoltages developed by Fitterman and Stewart (1986) and Stoyer(1990).

The Interpex TEMIXGL software provides two options forinterpreting data using layered Earth models: (i) a smooth resis-tivity model; and (ii) a multilayer resistivity model with sharpboundaries between layers (Interpex, 1994). In this study, thesmooth resistivity model was used for initial estimates of layerthickness and resistivity. These estimates were then input into amultilayer resistivity model. The goal of the modelling is todevelop subsurface resistivity stratification that produces theobserved decay in voltage (Best et al., 1985; Fitterman andStewart, 1986; Mills et al., 1988; Best et al., 2002).

Figure 3 Digital elevation model, showing the low terraces on the east and west sides of the Zeballos River delta



Boreholes and gravel pit exposures

Geophysical data were supplemented by borehole data at twosites (Fig. 6). The depths of the two boreholes are 9 and 43 m.The borehole logs provided information on lithology, sedimentcolour, unit thickness and water table depth. Exposures in agravel pit were photographed and sketched, and the stratigra-phy and sedimentology of the deposits were documented.Lithostratigraphical units were defined on the basis of lithology,

clast content and size, sorting, degree of rounding of matrixand clasts, sedimentary structures and lithological contactrelationships.

Results

Aerial photograph analysis and mapping revealed that theZeballos delta is a composite feature, comprising a dissected,relict, perched delta and a much larger modern delta (Figs 3and 4). The surface of the former is at an elevation of 17–20 m; the latter grades southward from 4 m a.s.l. at its apex tothe shoreline at the head of Zeballos Inlet.

Electromagnetic survey

Results of the TDEM survey are summarised in Figure 7. Theburied bedrock surface is irregular and ranges in depth from20 to 190 m. We infer the presence of four sediment layersbeneath the delta plain on the basis of resistivity values: (i) ashallow, low resistivity layer (< 400 ohm-m); (ii) an underlyinghigher resistivity layer (> 400 ohm-m); (iii) a conductive layerat depth (< 60–100 ohm-m); and (iv) a very conductive layer,which is present only beneath the Zeballos tidal marsh(ca. 10 ohm-m) (Best et al., 2002). Resistivity and thicknessvalues for each of the layers were determined using a multi-layered Earth model with sharp boundaries.

The modern delta is divided into three zones based on theTDEM survey (Fig. 7). Zone I includes the tidal marsh at thedelta front. It is underlain by very conductive materials, prob-ably gravel and sand saturated with saline groundwater. Thenorthern boundary of this zone marks the limit of saltwaterintrusion into the delta. Zone II includes much of the deltanorth of zone I and is characterised by an upper unit of low-resistivity gravel and sand. The low resistivity values may be

Figure 4 Location and extent of the elevated late Pleistocene deltaand the Holocene delta

Figure 5 Generalised sea-level curve for Zeballos, based on this study and data from other sites on western Vancouver Island. Inset shows sea-levelcurve for the Tofino area (Friele and Hutchinson, 1993)



due to the high porosity of the sediments. Zone III, found onlyat sites 27-2 and 27-3 (Fig. 7), lacks the upper low-resistivitygravel and sand unit, possibly because both sites are disturbed(gravel pit, campground) and some sediment has beenremoved. Alternatively, the well-drained sediments at the twosites may have contained little water at the time of the survey.

The elevated upper delta

The upper delta is the remnant of a formerly more extensivefeature that has been incised by Zeballos River. It is located

near the apex of the modern delta, west of Zeballos River.The uppermost part of the preserved delta plain is 20 m abovemean sea-level and slopes gently to the south and southwest. Alower surface, 17 m above mean sea-level, is inset into themain delta plain. The delta is well below the regional post-glacial marine limit of 46 m a.s.l. inferred by Howes (1981a).Although not directly dated, it must be of late Pleistocene agebecause it was built at the close of the last glaciation when thecrust was glacio-isostatically depressed and thus relative sea-level was higher than the present datum. Other elevated deltasat similar elevation on the west coast of Vancouver Island dateto about 11 000–13 000 14C yr ago (Howes, 1981a).

Figure 6 Locations of ground-penetrating radar (GPR) transects, domain electromagnetic (TDEM) soundings, gravel pit exposures and boreholes



A gravel pit (Fig. 5) provides several good exposures of thesediments that constitute this elevated delta. The sedimentsare mainly parallel-bedded, gravelly sand and clast-supported,pebble–cobble gravel dipping, on average, 25 � to the south-west (Fig. 8). They are the foresets of a high-energy, Gilbert-type delta. The uppermost metre or two of sediment has beencleared from the area around the gravel pit, but in a few placesthe foresets are sharply overlain by up to 2 m of horizontallystratified, clast-supported, pebble–cobble gravel and sand,interpreted to be the delta topsets.

Two GPR lines were run at the Zeballos gravel pit—one onthe delta top just above the northeast face of the pit and a sec-ond about 7 m lower, on the floor of the pit. Two radar facieswere identified (Fig. 9). Radar facies A consists of continuousparallel reflectors dipping 16–25 �. The reflectors extend from12 m above mean sea-level to approximately 12 m below meansea-level. The apparent dip of the reflectors decreases where

they pass through the water table (Fig. 9), because the refractiveindex of saturated sediment is higher than that of dry sedimentof the same type. The contact between facies A and facies B isgradational and is marked by a change from continuous,strongly dipping reflectors to semicontinuous, more gently dip-ping reflectors. Facies B consists of gently inclined, subparallel,semicontinuous reflectors that become increasingly discontin-uous at depth.

The modern delta

The lower, modern Holocene delta covers an area of approxi-mately 1 km2. Most of its surface is graded to present sea-leveland slopes about 1 � seaward. Terraces 4–6 m above mean sea-level border the valley walls to the west and east (Fig. 3). The

Figure 7 Interpretation of TDEM survey. Topographic contour interval is 10 m



western terrace is about 1 km long and up to 500 m wide. Theeastern terrace is about 500 m long and 50 m wide.

The seaward edge of the delta plain is straight, with only aslight indentation at its western edge. From here, the delta slopeextends south from the level of the lowest tide to a depth of100 m in Zeballos Inlet. The upper portion of the slope, to adepth of 50 m, slopes about 21�; below this, the average slopeof the delta front is 6�.

Fair Harbour transects 1–4 extend in a southwest–northeastdirection along the western side of the delta (Fig. 5). Threeradar facies (A, C and D) were identified in the profiles (Figs10 and 11). The uppermost radar facies (C) consists of contin-uous, parallel, horizontal reflectors in FH1; parallel to subpar-allel, horizontal, continuous to semicontinuous reflectors inFH2 and FH3; and wavy and hummocky, parallel, semicontin-uous reflectors in FH4. Facies C thickens progressively fromsouthwest to northeast, from 8 m in FH1 to 24 m in FH4. Thecontact between facies C and D in FH1 is an onlap boundary.The contact between facies C and underlying facies A in FH1,FH2 and FH3 is marked by a continuous, wavy, flat-lyingreflector.

Radar facies A compromises continuous to semicontinuous,dipping, parallel reflectors in profiles FH1 and FH2, and semi-continuous to continuous, parallel to subparallel, horizontalreflectors in FH3 (Figs 10 and 11). Reflector dip angles decreasetowards the northeast, averaging 32� in FH1 and 23� in FH2.The minimum thickness of facies A is 24 m in FH1 and 21 min FH2 and FH3; the radar signal is lost below these depths.Facies A overlaps facies D in FH3 along a prominent, continu-ous, irregular reflector.

Radar facies D is characterised by abundant hyperbolicreflectors (van Heteren et al., 1998). It is present at the surface

near the southwest end of FH1 and drops in a northeasterlydirection to a depth of about 32 m near the middle of thetransect, where the signal is lost (Fig. 10). The same facies ispresent in FH3 and FH4 at depths ranging from 13 to 21 m(Fig. 11).

The GPR transect PW1 (Fig. 12) crosses the Holocene deltain an east–west direction and provides a view of the subsurfaceof the delta perpendicular to the Fair Harbour transects. Tworadar facies, C and E, are present in this profile. Facies C isup to 26 m thick and is characterised by wavy and hummocky,parallel to subparallel, horizontal, semicontinuous reflectors.The 26-m thickness is a minimum because sediments belowfacies C are almost completely free of reflections. Facies E isup to 9 m thick and has a prograded fill pattern (Mitchumet al., 1977; Sangree and Widmier, 1979) of semicontinuous,subparallel reflectors. The contact between facies C and E islocally indistinct, but is marked elsewhere by a semicontinuousreflector with a concave shape. Borehole 77-2 is located atthe east end of PW1 adjacent to Zeballos River (Fig. 5). Itrecords 9 m of sandy cobble gravel (Thurber Consultants Ltd,1977).

Discussion

Stacked delta model

Aerial photograph interpretation, gravel pit exposures and theground-penetrating radar survey indicate the presence of twodeltas at Zeballos (Fig. 13). The higher delta was constructedat the mouth of the Zeballos canyon during or shortly afterdeglaciation, when sea-level was higher, relative to the land,than it is today owing to glacio-isostatic depression of the crust(Fig. 13; Howes, 1981b; Clague et al., 1982). Howes (1981b)suggested that the late-glacial marine limit at the heads of inletson west-central Vancouver Island is as high as 46 m a.s.l., butno evidence exists for shorelines higher than 21 m a.s.l. at thehead of Zeballos Inlet.

The lower delta is inset into the late Pleistocene delta(Fig. 13). It formed during the Holocene when sea-level wasvariously below, at, and perhaps up to a few metres higher thanpresent. The GPR records show the effects of sea-level changeon the internal structure of the Holocene delta, as discussedbelow.

Evolution of the deltas

Elevated upper delta

At the end of the last glaciation, large amounts of sedimentwere carried by Zeballos River into Zeballos Inlet. Sand andgravel were deposited at the delta front as foreset and topsetbeds, whereas clay and silt were transported into the basinand deposited as horizontal and gently inclined bottomsetbeds.

The highest foreset beds in the gravel pit at the edge of thelate Pleistocene delta are 20 m above mean sea-level. Perhaps1–2 m of sediment have been removed from the surface at thegravel pit, thus the water plane to which the delta was graded isprobably no higher than 21 m. The exposures in the gravel pitreveal a classic Gilbert-type foreset sequence of alternatinggravel and sand layers (Fig. 8). Sedimentary structures are typi-cal of Gilbert foresets—clast- and matrix-supported beds, both

Figure 8 Sand and gravel foreset beds in the Zeballos gravel pit(northeast exposure, Fig. 6)



graded and ungraded, with localised pebble and cobble imbri-cation (Nemec, 1990). The sand layers may record low dis-charges at the delta front, and gravel layers indicate higherflow or small slumps on the delta slope. Syndepositionalslumping is indicated by undulating, eroded, lower contactsof the coarse gravel layers and scour-and-fill features. Slumpsand sediment avalanches, which produce these structures,are common at steep delta fronts (Prior and Wiseman, 1981;Prior and Bornhold, 1988, 1989; Bornhold and Prior, 1990;Nemec, 1990).

The GPR survey at the gravel pit provides additional infor-mation on the structure of the late Pleistocene delta. Radarfacies A (Fig. 9) consists of oblique, prograding reflectionconfigurations, indicative of dipping sand and gravel beds(Sangree and Widmier, 1979; Beres and Haeni, 1991;Huggenberger, 1993; van Heteren et al., 1998; Pelpola andHickin, 2004). The high amplitude of the reflectors suggests

an alternation of sand and gravel, and the deep penetrationby the GPR signal indicates a lack of silt (van Heteren et al.,1998).

Facies B in Fig. 9 is interpreted to be bottomset beds of thelate Pleistocene delta. It is characterised by subparallel, semi-continuous reflectors indicative of silt and sand beds (Beresand Haeni, 1991; Huggenberger, 1993; van Heteren, et al.,1998).

The minimum thickness of the foreset unit, determined fromthe GPR survey, is 24 m. The TDEM survey indicates 46 m ofhigh resistivity gravel and sand, which are interpreted to beforesets beds (38 m in the sounding plus 8 m to the elevationof GP1 survey), plus an additional 5 m of conductive sand, siltand clay, interpreted to be bottomsets (Fig. 7). A problem arisesin interpreting the position of the bottomset beds in profileGP1, at 24 m below the gravel pit surface (Fig. 9). What weterm bottomset beds in the GPR profile may, in fact, be

Figure 9 The GPR transect in the Zeballos gravel pit. (a) Uninterpreted profile. (b) Interpreted profile. Note the change in reflectors at the water table



shallow-dipping sandy foreset beds deposited on the lower partof the delta slope.

Holocene delta

Radar facies C is characterised by high-amplitude, continuoushorizontal reflectors (Figs 10–12). These characteristics,together with the good energy penetration within the unit, sug-gest that radar facies C comprises horizontally stratified sandand gravel with little fine-grained material (Mitchum et al.,1977; Sangree and Widmier, 1979; Beres and Haeni, 1991;Huggenberger, 1993; van Heteren et al., 1998). We interpretthis facies to be the topset unit of the modern delta. Facies Cprogressively thickens along GPR transects FH1–4 to a maxi-mum of 24 m beneath the northern, older part of the delta (Figs10 and 11), and to 26 m at PW1 near the centre of the deltaPW1 (Fig. 12). The thickening is probably the result of sea-levelrise, as discussed below. The central location on the delta oftransect PW1, in an area where slow compaction of the sedi-ment pile may have facilitated greater aggradation (Barrell,1912), may explain the minor difference in maximum thicknessof the topset unit at FH4 and PW1.

The GPR profiles provide evidence for channel abandon-ment and filling as the delta grew. An example is facies E inGPR profile PW1 (Fig. 10), which is the fill of an abandonedchannel within the topset unit. Deep radar penetration andstrong reflectors indicate that the fill consists of gravel, asopposed to silt and sand.

Facies A (Figs 10 and 11) is interpreted to be foreset beds onthe basis of the dip of the reflectors in the direction of deltaprogradation. The radar configurations, their strength anddeep penetration are characteristic of bedded sand and gravel(Sangree and Widmier, 1979; Beres and Haeni, 1991;Huggenberger, 1993; van Heteren et al., 1998). The topset–foreset contact climbs seaward, evidence of deposition duringa time of rising sea-level.

The average dip of the foresets along the Fair Harbour trans-ects (FH1–4) increases to the south from 23� to 32�. The GPRprofile near the delta front is parallel to the southerly dip of themost distal foreset beds, whereas farther north the transectsmay intersect older foreset beds that have a component ofdip towards the west. The TDEM results indicate that the topsetand foreset units are, collectively, more than 80 m thickbeneath much of the delta plain and may locally exceed150 m in thickness (Fig. 7).

Topset and foreset beds onlap facies D (Figs 10 and 11),which we interpret to be bedrock on the basis of its prominentirregular upper contact, reflection-free lower section, and theTDEM survey (van Heteren et al., 1998). The irregularity ofthe buried bedrock surface is evident in the Fair HarbourGPR profiles (Figs 10 and 11) and in the TDEM soundings(Fig. 7).

Effects of sea-level change on delta evolution

During or immediately after deglaciation, large amounts ofsediment were eroded from valley walls and carried intoZeballos Inlet, where they were deposited on a delta gradedto a sea-level position about 20 m higher than today. Becausethe rate of isostatic rebound exceeded the rate of eustatic sea-level rise, the sea fell relative to the land and Zeballos Riverincised the delta. A slightly lower delta plain at the Zeballosgravel pit formed during this incision.

The sea continued to fall relative to the land after deglacia-tion, leaving the late Pleistocene delta as an incised, relict land-form (Fig. 3). Sea-level fell below the present datum around10 000–11 000 14C yr BP and reached its lowest level around8000–9000 14C yr BP (Clague et al., 1982; Friele and Hutchin-son, 1993) (Fig. 5). As sea-level fell, a new, lower delta pro-graded into Zeballos Inlet. The evolution of this delta can beinferred from geomorphology and the GPR profiles.

The Fair Harbour GPR profiles show that the topset sequence(facies C) thickens to the north towards the apex of the moderndelta (Figs 10 and 11). This thickening is the result of sea-levelrise during early and middle Holocene time (Bobrowsky andClague, 1992; Friele and Huchinson, 1993). Zeballos Riverkept pace with the rising sea by aggrading its floodplain. Evi-dence for this includes the thick topset deposits and the south-ward overlap of foreset beds in GPR profiles FH1 and FH2 (Fig.10). The topset–foreset contact is about 22 m below sea-levelbeneath the northern part of the delta. The same contact atthe seaward edge of the delta is 2–3 m below sea-level. Theseobservations suggest that sea-level was about nearly 20 mbelow the present datum in the early Holocene.

Sea-level on the west coast of Vancouver Island rose 3–4 mabove its present level in the middle Holocene (Friele andHutchinson, 1993), after which it gradually fell (Fig. 5). This fallmay be responsible for minor incision of the surface of themodern delta by Zeballos River, which produced the terracesat 4–6 m above mean sea-level along the east and west sides ofthe valley (Fig. 10). At present, the Zeballos River does notcarry as much sediment as it did during deglaciation, but it iscontinuing to build its delta out into Zeballos Inlet.

Limitations of GPR and TDEM surveys

The interpretation of GPR profiles is difficult and somewhatsubjective. Incorrect interpretations can arise from misidentifi-cation of background noise as stratigraphical reflectors or byinterpreting each reflector as a separate bed (Reynolds,1997). In addition, a GPR profile is not a true geologicalcross-section because, as Cagnoli and Russell (2000) pointout, the radar antennae receive energy from a circular zone,not a line. Some features thus may be located at the sides of,rather than directly below, the antennae. The science of radarstratigraphy is relatively new, and there is not yet a comprehen-sive catalogue of radar facies. Haeni (1988), Huggenberger(1993), and van Heteren et al. (1998) focus on specific environ-ments, but many environments have not been characterised.Applying a specific reflection configuration and its interpretedfacies to an environment may also lead to misinterpretation. AsJol and Bristow (2003) note, radar facies are not unique to a sin-gle environment, and some radar facies have been ascribed todifferent depositional environments by different researchers.Another problem centres on determining accurate depths fromwave velocities. Each type of sediment has a unique dielectricconstant and electric conductivity that determine the velocityof electromagnetic waves passing through the material. A con-stant velocity, determined from the common midpoint analysis,was used in this study. This velocity may overestimate orunderestimate the depths of some deeper units.

The TDEM survey method also has some limitations, perhapsthe most important of which are the assumptions that unitboundaries are sharp and that Earth is layered. Boundaries ofunits at Zeballos, except that between bedrock and unconsoli-dated sediments, are gradational or interfingering, not sharp.Furthermore, most units have inclined or irregular contacts.



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Figure 12 Parkway Avenue transect 1 (PW1, Fig. 6). (a) Uninterpreted profile. (b) Interpreted profile. Note the infilled channel in the topset unit

Figure 13 Schematic diagram showing relationship between the late Pleistocene and Holocene deltas



Another problem is that some materials have a wide range ofresistivities. Glacially derived gravel and sand, for example,has resistivities ranging from about 600 to 10 000 ohm-m(Palacky, 1987).

Conclusions

This study used an integrated geophysical approach to deter-mine the facies architecture, stratigraphy and evolution of theZeballos delta. Ground-penetrating radar and time-domainelectromagnetic surveys provided information on the stratigra-phy and architecture of the sediments underlying the Zeballosdelta plain. Geophysical interpretations were tested againststratigraphical and lithological data obtained from boreholesand exposures.

The GPR survey, in combination with aerial photo-graph interpretation, delineated two Gilbert-style deltas atZeballos—a dissected late Pleistocene delta with a surfaceabout 21 m above mean sea-level, and a larger Holocene deltanear present sea-level. Each delta consists of topset, foreset,and bottomset units. Overlapping foresets beds and a thick top-set unit in the modern delta imply deposition controlled by ris-ing sea-level during middle and late Holocene time. Sea-levelmay have risen above the present datum during the middleHolocene, creating a delta plain at about 4 m a.s.l. The TDEMsurvey provided information on the type and thickness of thedeltaic sediments. It also identified a highly irregular bedrocksurface beneath the delta, with depths ranging from 20 to190 m, and a zone of saltwater intrusion at the delta front.Our study shows that GPR and TDEM are valuable tools in stu-dies of fjord-head deltas.

Acknowledgements McElhanney Consulting Services Ltd preparedthe map used in this study. We thank Pauline Favero for assistance withfield work, Channa Pelpola for help with the ground-penetrating radarsurvey, and Gail Ashley, Eric Leonard and Stephen Robinson for helpfulreviews that resulted in an improved paper. The study was supported bythe Natural Sciences and Engineering Research Council of Canada,Geological Survey of Canada and Simon Fraser University.

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Architecture and evolution of a fjord-head delta, western Vancouver Island, British Columbia

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