Sedimentary environments and postglacial history of ...

Sedimentary environments and postglacial history of the Fraser Delta and lower Fraser Valley, British Columbia JOHNJ. CLAGUE Terrain Sciences Division, Geological Survey of Canada, 100 West Pender St., Vancouver, B.C., Canada V6B lR8

Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by Peking University on 06/04/13 For personal use only.

JOHNL. LUTERNAUER Geological Survey of Canada, Pac$c Geoscience Centre, 9860 West Saanich Rd., Sidney, B . C . , Canada V8L 4B2 AND

RICHARD J . HEBDA Archaeology Division, British Columbia Provincial Museum, Parliament Buildings, Victoria, B . C . , Canada V8V 1x4 Received January 12, 1983 Revision accepted March 18, 1983 The Fraser River delta, which is about 1000 km2 in area above low tide level, has been built into the Strait of Georgia in southwestern British Columbia during the Holocene. Present-day sedimentary environments, including foreslope, tidal flat, river channel, floodplain, and bog, also existed earlier during the delta's development. Borehole data reveal a succession of sedimentary environments related to Holocene progradation of the delta south and west of New Westminster. At each site, marine basin and distal foreslope sediments are overlain by proximal foreslope materials, which in turn are overlain by coarser intertidal platform and channel deposits capped by floodplain and bog sediments. Initial growth of the Fraser Delta was preceded both by deglaciation of the region and by the rapid westward extension of the Fraser River floodplain down a partially submerged, glacially scoured trough east of New Westminster. Irregularities on the trough floor were covered by fluvial, deltaic, marine, and lacustrine sediments as the floodplain extended westward. About 10 000 years ago, the Fraser River began to empty directly into the Strait of Georgia through a gap in the Pleistocene uplands at New Westminster. A delta was constructed south and west from this site as the sea dropped below its present level relative to the land. Deltaic progradation continued after sea level stabilized at about - 12m elevation after 8000 years BP. A marine transgression between 7000-7500 and 5000-5500 years ago inundated parts of the Fraser proto-delta and temporarily inhibited its seaward advance. This transgression ended with the sea perhaps 1 or 2 m below its present position, whereupon a large area of the delta became emergent and large bogs began to form. During the remainder of the Holocene, the Fraser Delta grew westward, but apparently not southward, under a regime of relatively stable sea levels. Le delta de la rivikre Fraser, dont la superficie au-dessus de la ligne de make basse est d'environ 1000 km2, s'est dCveloppC durant 1'Holockne dans le dCtroit de GCorgie, dans le sud-ouest dans la Colombie-Britannique. Les milieux ~Cdimentaires actuellement presents sont ceux d'avant-talus, d'estran, de chenal de rivikre, de plaine d'inondation et de marecage, lesquels 1 existaient antkrieurement durant 1'Cpisode de formation du delta. L'examen des forages rCvklent une succession de milieux I ~Cdimentairesen relation lvec la progression du delta a 1'Holockne au sud eta l'ouest de New Westminster. A chaque endroit, les sCdiments distaux de l'avant-talus et du bassin marin sont recouverts par des matkriaux proximaux de l'avant-talus, lesquels sont a leur tour recouverts par des dCp6ts de plate-forme intertidale et de chenal de rivikre surmontCs de sCdiments de plaine d'inondation et de madrage. La croissance initiale du delta du Fraser fut prCctdCe par une dkglaciation de la region et par une extension rapide vers l'ouest de la plaine d'inondation de la rivikre Fraser qui Ctait abaissCe, partiellement submergCe et surcreusCe par les glaciers jusqu'a l'est de New Westminster. Les irrCgularitCs du fond de la vallCe furent recouvertes par des skdiments fluviaux, deltaiques, marins et lacustres au fur et a mesure qu'avancait la plaine d'inondation vers l'ouest. I1 y a environ 10 000 annees, la rivikre Fraser a commencC a se dCverser directement dans le dCtroit de GCorgie au travers une cluse taillee dans les terrains ClevCs plCistocenes a New Westminster. Au sud et a l'ouest de ce site, s'Cdifia un delta en m2me temps que la mer s'abaissait jusque sous son niveau actuel relatif a la c6te. L'avancCe deltaique s'est poursuivie au-dela de la stabilisation du niveau de la mer a environ - 12 m d'C1Cvation aprks 8000 anntes avant le prksent. Entre 7000-7500 et 5000-5500 annCes passtes, une transgression marine a inondC des parties du delta du Fraser et a emp2cht temporairement sa progression sur la mer. Cette transgression a pris fin lorsque le niveau de la mer a atteint 1 ou 2 m sous sa position actuelle, aprks quoi une grande partie du delta fut CmergCe et de vastes markcages commenckrent a se former. Durant llHolockne, la delta du Fraser croissait en direction ouest, mais non vers le sud, selon toute apparence, sous un rCgime de niveaux de mer relativement stables. [Traduit par le journal]

~

Can. J. Earth Sci. 20, 1314-1326 (1983)

3400 rn3/s, is the largest river reaching the west coast of Introduction The Fraser River, with a length of about 1370krn, a Canada (Mathews and Shepard 1962). It breaches the drainage basin area of over 234000 krn2 in south and mountainous spine of western British Columbia and central British Columbia, and a mean discharge of discharges into the Strait of Georgia, a semi-enclosed


CLAGUE ET AL.

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FIG.1. Satellite image of the Fraser Delta and surrounding region (compare with Fig. 2). ERTS image E-1385-18365-5.

marine basin located between Vancouver Island and the mainland. Here, it has constructed a delta with a combined intertidal and supratidal area of about 1000km2 during the 10 000 - 11 000 years since the disappearance of the late Pleistocene Cordilleran Ice Sheet (Figs. 1,2). The Fraser Delta is important in a geologic sense because it serves as a model for high-energy, sand-rich estuarine systems. It also provides valuable information on deltaic processes in formerly glaciated areas and contains a detailed record of Holocene environmental changes in southwestern British Columbia. In addition, the Fraser Delta is a highly valued and intensely used area: its agricultural soils are among the most productive in Canada; it is an important link in the Fraser River salmon fishery and supports the largest population of wintering waterfowl in the country; it also has been a site of explosive urbanization associated with the growth of satellite communities to Vancouver. Although contemporary sedimentary processes and sediments on the Fraser Delta have been described and discussed by numerous individuals (Luternauer 1980; Luternauer and Finn, in press; and references therein), the Holocene evolution of the delta has been the subject of only limited comment (Johnston 1921; Mathews and Shepard 1962; Blunden 1973, 1975). A major objective

of this paper, therefore, is to summarize the geologic history of the Fraser Delta from the close of the last glaciation to the present. A second interrelated objective is to formulate a simplified framework of Holocene sedimentary environments by which the evolution of the delta can be traced. To meet these objectives, we have reviewed available information bearing on the distribution, character, environment of deposition, and age of surface sediments of the Fraser Delta and lower Fraser River valley, and have examined and interpreted logs of over 1500 boreholes drilled for foundation studies in this region.

Setting The Fraser Delta is located at the western edge of the Fraser Lowland, a triangular area bounded on the west by the Strait of Georgia, on the north by the Coast Mountains, and on the south and southeast by the Cascade Mountains (Fig. 2). The Fraser Lowland consists of flat-topped and gently rolling hills and plateaus, most of which are below 150 m in elevation and which are separated by wide valleys. The largest of these valleys extends east across the lowland from the apex of the Fraser Delta at New Westminster and is occupied by the Fraser River.

FIG.2. h a t i o n map, Fraser Delta and vicinity. Dotted line encloses the delta, which is here defined to include not only the active foresIope and tidal flats, but also the neady flat, dyked surface south and west of New Westminskr. The latter surface is underlain by relatively thin fluvial and organic sediments, which, in turn, ovelie thick deltaic sediments.


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CLAGUE ET AL.


The Fraser Lowland is part of a major structural trough that has subsided episodically since late Cretaceous time and into which more than 4 km of sediments eroded from the adjacent mountains has been deposited (Mathews 1972). The present physiography of the region was produced largely during late Tertiary and Quaternary time when rivers and glaciers dissected the Coast and Cascade mountains, shedding detritus into nearby lowlying areas and the sea (Mathews 1972; Ryder 1981). The subaerial portion of the Fraser Delta, which ranges from about 1 to 5 m above mean sea level, extends 15-23 km west and south from a narrow gap in the Pleistocene uplands at New Westminster to meet the sea along a perimeter of about 40km. Twenty-seven kilometres of this perimeter adjacent to the four main distributary channels of the Fraser River faces west onto the Strait of Georgia; about 13 km faces south into Boundary Bay. These two sectors are separated by Point Roberts Peninsula, an upland of Pleistocene sediments and formerly an island. Very gently sloping tidal flats extend up to 9 km from the landward edge of the delta to the foreslope. The western foreslope, in most places, is inclined from one to a few degrees towards the marine basins of the Strait of Georgia, and terminates at about 300m water depth 5-10km seaward of the tidal flats. The southern foreslope is ill defined; it slopes more gently than the western foreslope and terminates in much shallower water (ca. 30 m). The Fraser Delta has been built by a sand-dominated river that transports most of its load (10-30 x lo6 t/year) during freshet in late spring and early summer (Milliman 1980). More than half the sediment discharged during this 2-3 month period is sand. Throughout the remainder of the year, the river carries mainly silt and clay, and both water flow and sediment concentrations are much lower than during freshet. Tides influence the tr&sport of sediment in the estuary of the Fraser River. The maximum tidal range is almost 5 m at the mouth of the river, decreasing both landward and with increasing river flow (Ages and Woollard 1976). Mixed semidiurnal tides change the river level at New Westminster by up to 2.3 m during winter months and up to 0.8 m during freshet (Thomson 1981). This, in combination with the storage and discharge of tidally exchanged water in Pitt Lake (Ashley 1978, 1979), causes impedance and acceleration of river flow and leads to cyclic deposition and reentrainment of sediment on channel floors.

and with the intradelta (tidal flat, river channel, floodplain, aild bog) (Fig. 3, Table 1).

'

Foreslope Prodelta environments proximal to active distributary mouths are characterized by high sediment influx and by lateral and downslope movement of material by gravity and strong bottom currents. In contrast, sedimentation rates generally are lower in distal slope environments, and energy levels there decrease with increasing water depth. Prodelta sediments are inclined parallel to the submarine slope and consist of laminated and bedded fine sand and sandy to clayey silt (Johnston 1922; Luternauer 1980). The coarsest sediments occur on the upper foreslope in the immediate vicinity of active distributary mouths and over most of the foreslope south of the Main Channel (Fig. 3). Sand is widespread in the latter area either because this part of the Fraser Delta is sediment starved and an area of nondeposition or erosion, or because sand eroded from Point Roberts Peninsula and transported north by currents is accumulating on the southern portion of the western foreslope (Luternauer and Murray 1973; Luternauer 1980; Luternauer and Finn, in press). West and north from the mouth of the Main Channel, slope sediments become finer with increasing water depth and distance from the primary source (Luternauer and Murray 1973; Pharo and Barnes 1976). The foreslope is cut by gullies formed by mass wasting or turbidity currents and maintained by the sliding of accumulated gully-bottom sediments and the flushing action of tidal currents (Mathews and Shepard 1962; Scotton 1977; Shepard and Milliman 1978; Luternauer 1980;Luternauer and Finn, in press). Also present locally on the slope are sand waves generated by strong tidal currents (Luternauer et al. 1978; Luternauer 1980). Tidal j u t s The foreslope is bordered by tidal flats, the most seaward of the intradelta environments. The main part of the intertidal platform slopes about 0.05" and is mantled mainly by fine to medium sand that has been shaped into low swells by wind-generated waves and currents (Medley and Lutemauer 1976; Medley 1978; Luternauer 1980). A discontinuous fringe of marsh, underlain primarily by muddy sediments, occurs at the landward edge of this zone (Fig. 3; Moody 1978;Luternauer 1980; Shepperd 1981). The tidal-flat sediments overlie foreslope deposits and

'The prodelta is the subtidal portion of a delta, sloping gently Surface sediments and present-day down to the floor of the basin in which the delta is advancing. sedimentary environments The intradelta includes the intertidal and supratidal portions of Present sedimentary environments on the Fraser Delta the delta that are landward of the prodelta (Bates and Jackson include those associated with the prodelta (foreslope) 1980).

CAN. J. EARTH SCI. VOL. 20. 1983


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FIG. 3. Sediments and sedimentary environments of the Fraser Delta. The sections are generalizedfrom borehole data. MSL = mean sea level. Distribution of surface sediments from Medley and Luternauer (1976),Luternauer (1980),and Armstrong and Hicock (1980a,b),.

Sphagnum peat

4

1

Woody peat

Sedge peat

Horizontal stratification

Horizontal stratification

Cross-bedding

Overlies swamp and marsh deposits

Shoestring

Truncates and interfingers with floodplain, intertidal, and organic deposits; overlics foreslope deposits Above intertidal deposits

Domed blanket

Blanket

Blanket

Prism

Shape of lithosome

Between foreslope deposits and floodplain or organic deposits

Between marine-basin and intertidal deposits

Position in sedimentary sequence

Alnus. Pinus, Tsuga, Sphagnum. Ericaceae

.

Alnus. Picea. Pinus. Pseudotsuga Tsuga, Cyperaceae. Chenopodiaceae, Gramineae, sundry freshwater aquatics

AIRUS,*piceat ~ i n u . ~ . d Psedotsuga ." T s u ~ u , ~ Cyperaceae,' Chenopodiaceae;f Polypodiaceae Alnus, Picea, Pinus. Pseudotsuga, Tsuga, reworked Tertiary pollen

Alnus, Picea, Pinus, Pseudofsuga, T.ruga, reworked Tertiary pollen

Representative common pollen and sporesa

'Distal to source. doccurs mainly on lower and middle tidal flats. 'Occurs mainly on upper tidal flats. Carex spp. and Scirpus spp. are the'dominant contributors of Cyperaceae pollen. 'Common only in salt marsh. qncludes freqhwater marshes and swamps in which sedge and woody peats accumulate. Organic accumulation in these wetland environments culminates in raised bogs dominated by Sphagnum.

Trom Hebda (1977) and Shepperd (1981). bProximal to source.

Bog

Mud, peaty silt

Floodplaing

J.

Medium-coarse sand

.1

Inclined stratification. cross-bedding, graded bedding, large channel flls, bioturbation structures, soft-sediment deformation

Sedimentary structures

Fine-medium sand,d Horizontal stratification, mudC cross-bedding, bioturbation structures

Fine sand,b mudb,'

Sediments

River channels

Intradelta Tidal flats

Prodelta (foreslope)

Environment

TABLE1. Sediments and depositional environments of the Fraser Delta


r

>

3

FD

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CAN. I. EARTH SCI. VOL. 20, 1983

can be distinguished from them partly on the basis of sedimentary structures and microfossils (Table 1). Foreslope sediments are inclined and locally deformed by mass-movement processes; pollen in these sediments is mainly arboreal and river transported. In contrast, tidal-flat sediments are flat lying and undeformed, although original stratificationgenerally is indistinct due to the activity of burrowing organisms (Swinbanks 1979, 1981; Swinbanks and Murray 1981); these sediments contain Polypodiaceae fern spores, arboreal pollen (dominant in sands of the lower intertidal zone), and Cyperaceae pollen (mainly in muds of the upper intertidal zone). The Cyperaceae pollen is derived from sedges (Carex spp.) and bulrushes (Scirpus spp.) forming both tussocks between tidal channels and extensive dense stands of vegetation. This vegetation and localized mats of blue-green algae, which bloom during summer months, trap fine detritus that is carried to the delta front by the Fraser River and transported landward by rising tides. Mud within the vegetated zone grades into sand in adjacent low areas and tidal channels. Narrow marshes on the southern tidal flats bordering Boundary Bay differ from the more extensive marshes on the western flats adjacent to the distributaries of the Fraser River (Kellerhals and Murray 1969; Moody 1978; Swinbanks 1979; Swinbanks and Murray 1981; Shepperd 1981). The former are dominated by salttolerant plants of the Chenopodiaceae family, and the latter by fresh and brackish water plants, mainly Cyperaceae. The pollen content of the associated sediments differs correspondingly, providing a means of discriminating former marsh environments. Floodplain Inland of the tidal flats is the dyked portion of the intradelta. Part of this area is up to about 1 m below high tide level and therefore would experience periodic inundation by the sea if not dyked. The sediments that underlie much of the dyked intradelta are flat lying to very slightly inclined, sandy to clayey silts of overbank and uppermost intertidal origin (Armstrong 1956,1957; Blunden 1973, 1975; Hebda 1977; Armstrong and Hicock 1980a,b; Shepperd 1981; Styan 1981). These sediments were deposited, in part, in fresh and brackish water marshes and swamps bordering both existing and former distributary channels of the Fraser River. Much of the sediment accumulated during spring floods or at times of very high tides. Historically, on such occasions, low-lying areas of the present subaerial delta have been inundated (J. E. Armstrong, personal communication, 1981). Clastic floodplain sediments overlie and grade seaward into tidal-flat deposits. The two are not easily distinguished on the basis of sedimentologic criteria,

although cross-bedding and bioturbation structures are common in the tidal-flat sediments but rare in overbank deposits (Table 1). Sediments deposited in floodplain marshes and swamps commonly contain, in addition to abundant sedge and tree pollen, small amounts of pollen and spores from freshwater plants such as water plantain (Alisma plantago-aquatica), scouring rush (Equisetum sp.), skunk cabbage (Lysichitum americanum), and buckbean (Menyanthes n-ifoliata); these freshwater plants are not found on the tidal flats. Bogs Much of the eastern part of the intradelta is covered by organic deposits (Johnston 1921; Anrep 1928; Hebda 1977; Styan 1981). These deposits have a maximum thickness of 8 m and lie on a poorly drained substratum close to, and locally below, low tide level. They began to accumulate when the top of the Fraser Delta was built high enough above sea level to avoid regular flooding by the river and the sea. The continued and prolonged growth of plants, first sedges, then shrubs and heath, and finally Sphagnum, further raised the delta surface and restricted clastic sedimentation, eventually leading to the formation of domed peat bogs. River channels Overbank and intertidal muds intertongue with, and are truncated by, coarser sediments associated with the principal distributary channels of the Fraser River. The distributary channels, scoured to depths as great as 22 m below sea level, are floored by sand, generally medium to coarse, in places containing scattered pebbles and gravel lenses (Mathews and Shepard 1962). The continuity of the large peat bogs on the eastern Fraser Delta attests to the stability of distributary channels in this area during late Holocene time. At only one place is there a gap in the peat deposits not now occupied by an active distributary channel. This is on eastern Lulu Island where a former river channel 1-2 km wide crosses organic terrain from the Main Channel to the North Arm of the Fraser River (Fig. 3). The relative stability of the distributary channels on this part of the delta contrasts with the instability on the seaward part, where there have been marked shifts in channel positions historically (Fig. 4; Johnston 1921).

Subsurface sediments and past sedimentary environments Fraser Delta sediments average about 120m in thickness (Mathews and Shepard 1962). One drill hole on the western part of the delta is reported to have penetrated 216 m of sand before reaching bouldery material, presumably of glacial origin (Johnston 1921). Numerous boreholes on the tidal flats to depths as great as 139 m indicate that the subsurface sediments there are mainly interbedded sand, silty sand, and sandy


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FIG.4. Changes in channel position at the mouth of the Main Channel, 1827 to present. Former channel positions have been obtained from old charts and maps. The present channel is indicated by a cross-hatched pattern. Land areas are stippled.

and clayey silt (section A, Fig. 3). Complex lateral and vertical variations in sediment texture probably result from shifts in the positions of distributary channel mouths, changes in current patterns, and sea-level fluctuations during Holocene time. Farther east, on the western portion of the dyked area of the Fraser Delta, boreholes pass through materials that record systematic temporal changes in sedimentary environments related to Holocene growth of the delta. Over much of this part of the delta, a mantle of fine-textured floodplain and upper intertidal sediments, generally several metres thick, overlies sandy sediments similar to those beneath the present tidal flats (section B, Fig. 3). Broad ribbons of channel sand interdigitate with and truncate the floodplain and tidal-flat sediments and in places extend to more than 20 m below the surface. The floodplain and river-channel deposits prograded over the intertidal sediments as the Fraser Delta front advanced seaward. The relatively coarse channel and intertidal deposits, in turn, overlie thick interbedded silt and sand deposited in former delta-slope environments. These sediments, like those on -the present-day foreslope, exhibit pronounced lateral and vertical variations in texture resulting, in part, from shifting distributary channels. The sequence of sediments below the eastern portion of the dyked area is similar to that described above except

that: (1) peat up to 8 m thick directly underlies the surface and overlies channel, floodplain, and intertidal deposits; and (2) delta-top beds tend to be thicker and extend to greater depths than farther west (section C, Fig. 3). These differences are thought to be related to differences in the age of various parts of the delta and to changes in sea level during its formation. Thick peat is absent from the surface of the western delta, in part because the latter is younger than the delta surface to the east. Likewise, thinning of floodplain and intertidal sediments towards the west may be due to the decreasing age of the delta platform in this direction. Deposition of floodplain and intertidal sediments commenced on the eastern Fraser Delta during early Holocene time when the sea probably was as much as 12 m lower relative to the land than at present (see following section); these sediments are about 15 m thick in this area. In contrast, similar, but much thinner, sediments on the western intradelta were deposited after 5000 years ago when sea level was within 1 or 2 m of its present position. In summary, the succession of sedimentary environments recorded from bottom to top in boreholes is the same as the succession of present-day environments on the Fraser Delta, proceeding from foreslope through tidal flats to supratidal delta (Fig. 5). Sediments at the base of the deltaic pile accumulated in environments presently found on the lower foreslope and in marine

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CAN. J. EARTH SCI. VOL. 20, 1983 M a r i n e basin Foreslope

sw /, /(

1C

T i d a l flat

I

Floodplain

,

Bog

Nf


1

FIG. 5. Fraser Delta profile and generalized section showing relationships of sedimentary environments.

basins beyond. These are overlain successively by sediments deposited in upper foreslope, intertidal, river-channel, floodplain, and bog environments.

Evolution of the lower Fraser River valley and delta At the close of the last glaciation, the piedmont glacier covering the Fraser Lowland downwasted and receded northwest up the Strait of Georgia and east across the Fraser Lowland. As lowland areas were freed of their ice cover, they were invaded by the sea (Armstrong 1981). Large deltas and submarine outwash fans were constructed as sediment-charged meltwater poured into the sea from decaying glaciers. By shortly before 11 000 years ago, much of the Fraser Lowland had emerged from the sea, and marine waters became restricted to relatively narrow arms in the low valleys now occupied by the Fraser River, its tributaries, and a few other streams in the area (Fig. 6; Clague et al. 1982). At the same time, large amounts of meltwater and sediment were funnelled into the Fraser River valley from the snout of the piedmont glacier, which at that time was located in the Mission City area (Arrnstrong 1981). Coarse sediment transported by this meltwater was deposited on the shallow submerged floor of the valley, and an outwash train, probably in contact with remnant ice masses and graded to sea levels 10-20 m higher than present, rapidly prograded westward. Remnants of this outwash train occur as terraces underlain by glacial-deltaic and glaciofluvial sediments in the Fraser River valley between Pitt Meadows and Glen Valley. Apparently, outwash did not accumulate in the valley west or east of these localities. The Pitt Meadows area probably corresponds to the delta front of the outwash train when it became inactive due to glacier recession or when sea level fell below about 10m elevation. Glen Valley may have been near the snout of the piedmont glacier when the outwash train was active or near a large mass of residual stagnant ice separate from the main piedmont complex. In fact, many isolated

masses of stagnant ice may have persisted west of the active ice front until almost 11 000 years ago, giving rise to the sporadic distribution of outwash between Pitt Meadows and Glen Valley. About 11 000 - 11 300 years ago, glacier ice disappeared from the eastern Fraser Lowland and from the Fraser Canyon to the northeast (Mathewes et al. 1972; Armstrong 1981; Clague 1981). In the Fraser River valley, large settling basins vacated by glacier ice (e .g ., the lowland at Matsqui) were filled with fluvial, deltaic, marine, and lacustrine sediments, and the outwash train between Glen Valley and Pitt Meadows was incised. As a result, a floodplain extended itself westward down the valley at approximately the present elevation of the Fraser River. When the sea attained its present level 11 000 years ago or shortly thereafter, the active delta front of this floodplain probably was east of Pitt Meadows. The delta of the Fraser River southwest of New Westminster was not in existence at this time; instead, the area was part of the Strait of Georgia, Point Roberts Peninsula was an island, and the sea was in direct contact with the Surrey and Burrard uplands in the vicinity of New Westminster. The sea also extended east of New Westminster through a gap in the uplands into a fjord now occupied by Pitt Lake and the Pitt River floodplain (Fig. 6). Stagnating and disintegrating remnants of the Cordilleran Ice Sheet supplied meltwater and sediment to the Fraser River until perhaps as late as 10000 years ago, when ice completely disappeared from lowland valleys and plateaus in the British Columbia interior (Fulton 1971). During deglaciation, prodigious quantities of sediment were transported by meltwater streams and rivers as unvegetated drift-covered slopes were eroded (Church and Ryder 1972). Initially, most river systems aggraded their floodplains because of their inability to transport the sediment supplied to them. However, the supply of sediment soon decreased as slopes were stabilized by vegetation. Rivers then incised their earlier deposits, partly to attain grade with the sea, which was


CLAGUE ET AL.

r

l

GLACIER ICE WATER

I

I

LAND FLUVIAL, GLACIOFLUVIAL, AND ORGANIC DEPOSITS

- HANEY L - LADNER MC - MISSION CITY NW - NEW WESTMINSTER PL - PITT LAKE

H

V

-VANCOUVER

WR

- WHITE

-

ROCK

FIG.6. Late Quaternary geologic evolution of the Fraser Delta and lower Fraser River valley. Dates are approximate, and ice margins hypothetical.

falling relative to the land at this time, and partly in response to changing hydraulic regimen due to a reduced supply of sediment. As a result of these factors, the Fraser River transported much larger amounts of sediment to the Fraser Lowland at the close of the Pleistocene than it does today, and the Fraser River floodplain rapidly prograded westward.

By about 10 500 years ago, the floodplain probably was continuous east of Pitt Meadows (Fig. 6). All of the glacially eroded basins on the floor of the valley to the east were now completely filled with sediment, and the Fraser River emptied into a fjord extending northeast to present Pitt Lake. A delta then was built across and up Pitt fjord, isolating Pitt Lake from the sea. Shortly

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CAN. J. EARTH SCI. VOL. 20, 1983


T E P H R A (a. 6600

6------1

ALLUVIUM

FIG.7. Stratigraphy of a core at Pitt Meadows Auport. Sediments above the peat are estuarine and fluvial in origin and probably were deposited in response to a relative rise in sea level, which resulted in a transgression of the Fraser Delta.

thereafter, the Fraser River extended its floodplain west to New Westminster and began to empty into the Strait of Georgia proper. While these events were occurring, the level of the sea continued to fall relative to the land (Clague et al. 1982). By 7000-8000 years ago, when the sea was at its lowest position, the front of the Fraser Delta probably was far to the southwest of New Westminster. The main evidence for both lower sea levels and the position of the Fraser Delta front during early Holocene time is the presence of terrestrial organic and fluvial sediments below present sea level beneath the floodplain of the Fraser River and its tributaries (Mathews et al. 1970; Clague et al. 1982). Five radiocarbon dates on peat at about - 10 to - 11 m elevation in this area range from 7300 + 120to 8360 + 170 years BP (S-99 and GSC-225, respectively). The peat represents organic accumulations on an alluvial surface graded to a sea level in the Strait of Georgia about 12 m lower than at present. This alluvial surface probably was similar in form to the present Fraser River floodplain and extended downstream at least as far as Port Mann, and probably much farther. Although direct evidence of this buried alluvial surface has not been found west of Port Mann, a radiocarbon date of 6400 197 years BP (WAT-369) on intertidal sediments at - 1 to -2 m elevation on Lulu Island indicates that the outer edge of the tidal flats was more than 15 km southwest of New Westminster 6400 radiocarbon years ago, and that a substantial proto-delta

*

was in existence prior to this time. Other evidence for a fairly extensive terrestrial or shallow-water platform during early to middle Holocene time includes the presence of: (1) 6600 year old Mazarna tephra at shallow depth beneath eastern Lulu Island (Blunden 1973, 1975); (2) 6790 135 BP (GSC-395) marine shells, 15 m below the surface of the delta, 6 km south of Ladner (Dyck et al. 1966); and (3) 6600 + 90 BP (GSC-2714) wood, 3 m below the sea floor on the western foreslope (?), 2 km northwest of Point Grey. Unfortunately, the latter two sites are only 300 m and 2km, respectively, from the edges of Pleistocene uplands. Thus, the dated sediments possibly are not deltaic deposits sensu stricto, but rather may be littoral materials deposited upon a shallow eroded platform fringing the uplands. The period of low sea levels during the early Holocene was followed by a marine transgression. Rising seas triggered aggradation of the Fraser River floodplain and caused a marine incursion onto part of the Fraser Delta. At Pitt Meadows Airport, peat occurring at - 10m elevation anddated 77 10 + 80yearsBP (GSC-3099) is overlain by 11.5 m of silt and sand (Fig. 7). The clastic sediments above the peat contain fresh and brackish water diatoms (Geological Survey of Canada, Diatom Report No. 81 -7),and probably were deposited in an estuarine environment in response to a rise in the level of the sea relative to the land. A 2 cm thick bed of Mazama tephra is interbedded with silt 5 m above the

*

CLAGUE ET AL.


1

peat, indicating that the sea probably rose about 5 m relative to the land between 7700 and 6600 years BP. This relatively rapid sea-level rise continued until about 5000-5500 years BP (Clague et al. 1982), at which time the Fraser River floodplain again became stable, and organic sedimentation commenced over nearly the entire eastern portion of the Fraser Delta. Sediments from the base (0-2 m below mean sea level) of the large peat bogs that cover most of the eastern delta have yielded radiocarbon dates ranging from 4650 k 80 to 5510 + 80 years BP (GSC-3045 and GSC-3066, respectively). Paleoecological analyses have demonstrated that these basal sediments likely were deposited in the intertidal zone at about the same elevation as similar sediments being deposited today at the mouth of the Fraser River (Hebda 1977). This suggests that the sea rose to within 2 m of its present level in this area before 5000 years ago. The data also indicate that the entire eastern portion of the Fraser Delta became emergent about 5000-5500 years ago, and that an intradelta platform extending at least 15 km southwest from New Westminster formed prior to this time. The seaward edge of this platform about 5000 years ago was west of Burns and Lulu Island bogs. Most or all of the southern delta front at Boundary Bay was already in existence at this time. In fact, the Fraser River did not enter the eastern half of Boundary Bay after 5000 years BP, as evidenced by the absence of river sediments in the continuous late Holocene organic sequence at Bums Bog, located directly between Boundary Bay and the Fraser River (Hebda 1977). Westward progradation of the Fraser Delta during the last 5000 years has been accompanied by comparatively stable sea levels. The sea probably has not fluctuated more than 2 m from its present position during this period. As the delta grew, there were major changes in channel patterns near distributary mouths comparable to those of historic time documented by Johnston (1921) and shown in Fig. 4. Because much of the channel instability at any given time was at the mouth of the river rather than inland, the zone in which channels shifted frequently migrated seaward in step with the advancing delta front. The Fraser Delta continues to prograde westward into the Strait of Georgia, although normal patterns of sedimentation have been profoundly modified since the late 1800's. Dykes along the distributary channels and at the landward edge of the tidal flats protect farm land and communities from inundation by catastrophic floods and extreme high tides, but also prevent accretion of overbank deposits, a natural prehistoric phenomenon. Dredging of the Fraser River distributaries in order to maintain navigable channels has reduced the amount of sand reaching the front of the delta, and channelization has decreased the likelihood of future channel shifts.

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Finally, man-made structures on the tidal flats, such as causeways, bulk-loading facilities, and a ferry terminal, have altered natural sediment dispersal patterns at the delta front.

Acknowledgments We are grateful to S. Lichti-Federovich (Geological Survey of Canada) for providing diatom analyses, to D. Lister (B.C. Ministry of Transportation and Highways) for supplying borehole records, and to M. C. Roberts (Simon Fraser University) for assisting with drilling on the Fraser Delta and in the Fraser River valley. P. McLaren (Geological Survey of Canada) and W. H. Mathews (University of British Columbia) reviewed a draft of the paper and contributed to its improvement. AGES,A., and WOOLLARD, A. 1976. The tides in the Fraser estuary. Canada Department of Environment, Pacific Marine Science Report 76-5, 100 p. ANREP,A. 1928. Peat bogs for the manufacture of peat litter and peat mull in southwest British Columbia. Geological Survey of Canada, Summary Report for 1927, Part A, pp. 53-61. ARMSTRONG, J. E. 1956. Surficial geology of Vancouver area, British Columbia. Geological Survey of Canada, Paper 55-40, 16 p. 1957. Surficial geology of New Westminster maparea, British Columbia. Geological Survey of Canada, Paper 57-5, 25 p. 1981. Post-Vashon Wisconsin glaciation, Fraser Lowland, British Columbia. Geological Survey of Canada, Bulletin 322, 34 p. ARMSTRONG, J. E., and HICOCK,S. R. 1980a. Surficial geology, New Westminster, British Columbia. Geological Survey of Canada, Map 1484A. 1980b. Surfical [sic] geology, Vancouver, British Columbia. Geological Survey of Canada, Map 1486A. ASHLEY, G. M. 1978. Bedforms in the Pitt River. In Fluvial sedimentology. Edited by A. D. Miall. Canadian Society of Petroleum Geologists, Memoir 5 , pp. 89- 104. 1979. Sedimentology of a tidal lake, Pitt Lake, British Columbia, Canada. In Moraines and varves. Edited by Ch. Schliichter. A. A. Balkema, Rotterdam, The Netherlands, pp. 327-345. BATES, R. L., and JACKSON, J. A., editors. 1980. Glossary of geology. 2nd ed. American Geological Institute, Falls Church, VA, 749 p. BLUNDEN, R. H. 1973. Urban geology of Richmond, British Columbia. University of British Columbia, Department of Geological Sciences, Report No. 15, 13 p. 1975. Urban geology of Richmond, British Columbiainterpreting a delta landscape. University of British Columbia, Department of Geological Sciences, Adventures in Earth Science Series, No. 15, 35 p. CHURCH, M., and RYDER, J. M. 1972. Paraglacial sedimentation: a consideration of fluvial processes controlled by glaciation. Geological Society of America Bulletin, 83, pp. 3059-3072. CLAGUE, J. J. 1981. Late Quaternary geology and geo-


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chronology of British Columbia. Part 2: Summary and discussion of radiocarbon-dated Quaternary history. Geological Survey of Canada, Paper 80-35,41 p. CLAGUE, J., HARPER, J. R., HEBDA,R. J., and H o w ~ sD. , E. 1982. Late Quaternary sea levels and crustal movements, coastal British Columbia. Canadian Journal of Earth Sciences, 19, pp. 597-618. DYCK,W.,LOWDON, J. A.,FYLEs,J. G., ~ ~ ~ B L A W., K JR. E, 1966. Geological Survey of Canada radiocarbon dates V. Radiocarbon, 8, pp. 96- 127. [Reprinted 1966. Geological Survey of Canada, Paper 66-48, 32 p.] , R. J. 1971. Radiocarbon geochronology of southern FULTON British Columbia. Geological Survey of Canada, Paper 71-37, 28 p. HEBDA,R. J. 1977. The paleoecology of a raised bog and associated deltaic sediments of the Fraser River delta. Ph.D. thesis, University of British Columbia, Vancouver, B.C., 202 p. JOHNSTON, W. A. 1921. Sedimentation of the Fraser River delta. Geological Survey of Canada, Memoir 125, 46 p. 1922. The character of stratificationof the sediment in the Recent delta of the Fraser River, British Columbia, Canada. Journal of Geology, 30, pp. 115- 129. KELLERHALS, P., and MURRAY,J. W. 1969. Tidal flats at Boundary Bay, Fraser River delta, British Columbia. Bulletin of Canadian Petroleum Geology, 17, pp. 67-91. [Reprinted 1976. In Holocene tidal sedimentation. Edited by G. deV. Klein. Dowden, Hutchinson &Ross, Stroudsburg, PA, pp. 118-142.1 LUTERNAUER, J. L. 1980. Genesis of morphologic features on the western delta front of the Fraser River, British Columbi+ status of knowledge. In The coastline of Canada. Edited by S . B . McCann . Geological Survey of Canada, Paper 80- 10, pp. 381-396. LUTERNAUER, J. L., and FINN,W. D. L. In press. Stability of the Fraser River delta front. Canadian GeotechnicalJournal. LUTERNAUER, J. L., and MURRAY, J. W. 1973. Sedimentation on the western delta-front of the Fraser River, British Columbia. Canadian Journal of Earth Sciences, 10, pp. 1642-1663. LUTERNAUER, J. L., SWAN,D., and LINDEN,R. H. 1978. Sand waves on the southeastern slope of Roberts Bank, Fraser River delta. British Columbia. In Current research. part A. ~ e o l o ~ i Survey ci of Canada, Paper 78-lA, pp: 351-356. MATHEWES, R. W., BORDEN,C. E., and ROUSE,G. E. 1972. New radiocabon dates from the Yale area of the lower Fraser River canyon, British Columbia. Canadian Journal of Earth Sciences, 9, pp. 1055-1057. MATHEWS, W. H. 1972. Geology of Vancouver area of British Columbia. 24th International Geological Congress, Montreal, P.Q., Guidebook to Field Excursion A05-C05, 47 p. MATHEWS, W. H., and SHEPARD, F. P. 1962. Sedimentation of Fraser River delta, British Columbia (includes "Discussion" by K. Terzaghi). American Association of Petroleum Geologists Bulletin, 46, pp. 1416-1443. [Reprinted 1976. In Modem deltas. American Association of Petroleum Geologists, Reprint Series, No. 18, pp. 94-1 16.1

MATHEWS, W. H., FYLES,J. G., and NASMITH, H. W. 1970. Postglacial crustal movements in southwestern British Columbia and adjacent Washington State. Canadian Journal of Earth Sciences, 7, pp. 690-701. MEDLEY, E. 1978. Dendritic drainage channels and tidalflat erosion, west of Steveston, Fraser River delta, British Columbia. B.A.Sc. thesis, University of British Columbia, Department of Geological Sciences, Vancouver, B.C., 70 p. J. L. 1976. Use of aerial MEDLEY,E., and LUTERNAUER, photographs to map sediment distribution and to identify historical changes on a tidal flat. In Report of activities, part C. Geological Survey of Canada, Paper 76-lC, pp. 293-304. MILLIMAN, J. D. 1980. Sedimentation in the Fraser River and its estuary, southwestern British Columbia (Canada). Estuarine and Coastal Marine Science, 10, pp. 609-633. MOODY,A. I. 1978. Growth and distribution of the vegetation of a southern Fraser Delta marsh. M.Sc. thesis, University of British Columbia, Vancouver, B .C., 153 p. PHARO,C. H., and BARNES,W. C. 1976. Distribution of surficial sediments of the central and southern Strait of Georgia, British Columbia. Canadian Journal of Earth Sciences, 13, pp. 684-696. RYDER,J. M. 1981. Geomorphology of the southern part of the Coast Mountains of British Columbia. Zeitschrift fiir Geomorphologie, 37, pp. 120- 147. SCOTTON, S. 1977. The outer banks of the Fraser River delta, engineering properties and stability considerations. M.A. Sc. thesis, University of British Columbia, Vancouver, B.C., 103 p. SHEPARD,F. P., and MILLIMAN,J. D. 1978. Sea-floor currents on the foreset slope of the Fraser River delta, British Columbia (Canada). Marine Geology, 28, pp. 245251. SHEPPERD, J. E. 1981. Development of a salt marsh on the Fraser Delta at Boundary Bay, British Columbia, Canada. M.Sc. thesis, University of British Columbia, Vancouver, B.C., 99 p. STYAN,W. B. 1981. The sedimentology, petrography and geochemistry of some Fraser Delta peat deposits. M.Sc. thesis, University of British Columbia, Vancouver, B.C., 188 p. SWINBANKS, D. D. 1979. Environmental factors controlling floral zonation and the distribution of burrowing and tube-dwelling organisms on Fraser Delta tidal flats, British Columbia. Ph.D. thesis, University of British Columbia, Vancouver, B .C., 274 p. 1981. Sediment reworking and biogenic formation of clay laminae by Abarenicola pacijica. Journal of Sedimentary Petrology, 51, pp. 1137-1 146. SWINBANKS, D. D., and MURRAY,J. W. 1981. Biosedimentological zonation of Boundary Bay tidal flats, Fraser River delta, British Columbia. Sedimentology, 28, pp. 201-237. THOMSON, R. E. 1981. Oceanography of the British Columbia coast. Canadian Special Publication of Fisheries and Aquatic Sciences 56, 291 p.

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