A delta plain sheet sand in the Fraser River delta, British Columbia, Canada

Quaternary International, Vol. 20, pp. 27-38, 1993. Copyright ~) 1994 INQUAJElsevierScience Ltd Printed in Great Britain. All fights reserved. 1040-6182/94 $26.00

Pergamon

1040-6182(94) E0011-R

A DELTA PLAIN SHEET SAND IN THE FRASER RIVER DELTA, BRITISH COLUMBIA, CANADA

P.A. Monahan,* J.L. L u t e r n a u e r t and J.V. Barrie:~ *School of Earth and Ocean Sciences, University of Victoria, Victoria, B.C., V8W 2Y2, Canada tGeological Survey of Canada, Cordilleran Division, Vancouver, B.C., V6B 1R8, Canada ,Geological Survey of Canada, Pacific Geoscience Centre, Sidney, B.C., V8L 4B2, Canada Borehole data in the Fraser River delta demonstrate that a nearly continuous sand unit, generally 8 to 20 m thick, underlies the surficial silts and sands of most of the delta plain. The sand unit consists of one or more sharp-based fining-upward sequences and is interpreted to be a complex of distributary channel deposits. Distributary channel migration and the resulting accumulation of channel sands has occurred primarily in a tidal flat environment, due to the interaction of tidal and fluvial processes and the high proportion of sand in the sediment load. This sand unit thus provides a model for sand-rich, river and tide dominated delta plains. Jetties now maintain the position of the Main Channel of the Fraser River where it crosses the tidal flats, and prevent channel migration and further storage of sand in the lower delta plain. As a result, more of the available sand is being transported directly to the river mouth, where failures transport the sand downslope. Consequently, these structures contribute to instability at the river mouth.

1000 km 2 (Milliman, 1980). It is the largest river on the west coast of Canada, with a mean discharge of 3400 m3s-1, and peak flows during the spring freshet of between 5000 and 15,000 m3sec -1 (Church et al., McLean and Tassone, 1991). This mean annual total sediment load is 17.3 million tonnes, of which 35% is sand (McLean and Tassone, 1991). The river divides into four distributary channels where it crosses the delta plain. At the river mouth, the mean tidal range is 3.1 m and the large tidal range is 4.8 m (Thomson, 1981). The delta includes the following surficial environments (as used by Wright (1985) and Potsma (1990)) which are, from east to west (Fig. 1; Luternauer and Murray, 1973; Clague et al., 1983, 1991): (1) an upper delta plain that extends 23 km west from a gap in the Pleistocene uplands at New Westminster, is mantled by floodplain silt and peat, and is now protected by dykes; (2) a lower delta plain consisting of dominantly sandy tidal flats that are up to 9 km wide; (3) a subaqueous delta plain, that is mantled by sand and is up to 2 km wide, and extends to the first major break in slope at a depth of up to 9 m below lowest low tide; and (4) a delta front and slope that descend at an average of 1.5° to depths of up to 300 m in the Strait of Georgia, and are dominantly silty to the north and sandy to the south of the Main Channel. Sediments of the modern Fraser River delta are primarily Holocene. They reach a maximum known thickness of 216 m (Johnston, 1921) and overlie Late Pleistocene glaciogenic deposits. Since deltaic sedimentation commenced approximately 9000 years ago, local relative sea level has risen by 13 m (Clague et al., 1983; Williams and Roberts, 1989, 1990).

INTRODUCTION The Fraser River delta is the largest on the west coast of Canada. It includes the southern parts of Greater Vancouver as well as key industrial and transportation facilities and it is in the part of Canada with the greatest seismic risk (Milne et al., 1978). Consequently, recent geologic investigations of this area have focused on the effects of human activities on deltaic processes and on the seismic hazards that could affect the region (Luternauer et al., 1993). Although a large volume of geotechnical borehole data has been collected in this area, these data have not been systematically applied to geologic investigations. Cone penetration tests (CPTs) are particularly useful, because they provide a continuous record of the physical properties of the sediments penetrated. The objective of our current investigation is to integrate these geotechnical data with data obtained by the Geological Survey of Canada (GSC), in order to develop a subsurface facies model and to provide a long term perspective of the sedimentary processes operating on the delta. This paper reports an initial result - - that distributary channel san6 deposits form a nearly continuous composite sand unit underlying most of the subaerial delta plain and thus are much more extensive than previously recognized - - and comments on some of the implications of this conclusion. Some of the data used in this analysis have been reported previously (Monahan et al., 1993).

SETTING The Fraser River flows into the Strait of Georgia, where it has built a delta with a subaerial extent of 27

28

P . A . Monahan et al

49.2"

49.1"

49.0"

123.2"

PLEISTOCENE UPLANDS

123.0"

I "?• ~DI.IJI~BIA)"LALTA.

SITES OR LOCALITIES WITH

III~FrlSH

TEST HOLE DATA >2Ore "~ UPPER DELTA DYKED DELTA PLAIN PLAIN

~

PEAT BOG

TIDAL FLATS -LOWER DELTA PLAIN SUBAQUEOUS DELTA PLAIN DELTA FRONT AND SLOPE JETTIES

D • • o + ~)

GSC BOREHOLES CPT DATA OTHER GEOTECHNICAL BOREHOLES WATER WELLS OIL AND GAS TESTS OTHER COREHOLES

0 t

MAP PACIFIC~'~AREA ~~

ocE,.

......

!

U.S.A.

lOkm I

i

i

l

I

i

,

i

j

I

FIG. 1. Location and sedimentary setting of the Fraser River delta, showing the location of borehole data and of sites referred to in other figures. Surficial geology from Armstrong and Hicock (1976a,b) and Luternauer and Murray (1973).

PREVIOUS

WORK

The surficial sediments of the Fraser Delta have been described by Johnston (1921), Mathews and Shepard (1962), Luternauer and Murray (1973), Luternauer (1980), Clague et al. (1983) and Hart et al. (1992a, b). The basic chronology of the growth of the delta was initially determined by Clague et al. (1983). They incorporated subsurface data into their interpretations, but no CPTs were available at the time to facilitate correlations and facies interpretations, and only generalized stratigraphic sections were generated. Roberts et al. (1985) reported on a shallow coring program that focused on the southern part of the delta. Williams (1988) and Williams and Roberts (1989,

1990) investigated the uppermost topset deposits in Lulu lsland in detail in a series of shallow boreholes, and documented the chronology of the mid to late Holocene sea level rise. Between 1985 and 1987, the GSC and Simon Fraser University drilled ten boreholes and acquired 45 km of seismic reflection data in the southernmost part of the delta (Luternauer et al., 1986, 1991; Jol and Roberts, 1988, 1992; Pullan et al., 1989; Clague et al.. 1991; Patterson and Cameron, 1991). These authors documented the chronology of delta progradation in that area. Although several of the earlier investigations report the presence of distributary channel deposits locally (Clague et al., 1983, 1991; Roberts et al., 1985), they did not recognize the existence of a widespread distributary channel sand complex.

29

A Delta Plain Sheet Sand

In recent years, the GSC has extended its drilling programs to other areas of the delta (Patterson and Cameron, 1991; Patterson and Luternauer, 1993; Monahan et al., 1993). The GSC has also acquired CPT data at selected sites to assess the seismic hazard in the region (Finn et al., 1989; Hunter et al., 1991; Woeller et al., 1993a,b). Watts et al. (1992) used CPT data to correlate stratigraphic units in the delta in an assessment of the liquefaction potential of the Greater Vancouver area, demonstrating the utility of this data for regional stratigraphic correlation.

Geotechnical Boreholes A large volume of borehole data has been generated in the course of geotechnical investigations. We obtained geotechnical data primarily from publicly funded projects, either directly from the public agencies that conducted their own geotechnical investigations or from engineering consulting firms (see acknowledgements). To date we have obtained geoteehnical borehole data from 300 sites widely distributed across the delta, 129 of which include CPT data (Fig. 1). A CPT is performed by pushing a standardized cone-tipped rod into unlithified sediment at 2 cm/sec with rig designed for that purpose. The resistance to penetration is measured at the cone tip ('cone bearing': DATABASE AND METHODS 'Qc' or 'Qt'), and the frictional resistance is measured by a friction sleeve immediately above the cone (Figs 2, This study is based primarily on boreholes drilled 3). These two curves can be combined to provide an by the GSC and geotechnical boreholes, in particular, estimate of the sediment type: in general, sand has CPTs. These data are supplemented by published higher cone bearing and lower 'friction ratio' ('Rf: borehole logs (Roberts et al., 1985; Luternauer et the ratio of sleeve friction to cone bearing) values than al., 1986; Williams, 1988; Williams and Roberts, 1989), finer sediments (Figs 2, 3). In addition, instantaneous and the records of wells drilled for water and petroleum pore pressure values can also be recorded to provide information about the volume change characteristics (Fig. 1). and permeability of the sediments ('U': in metres of water). The more permeable and in general coarser G S C Boreholes The GSC has participated in 25 boreholes throughout sands have values that plot on the hydrostatic gradient the delta since 1986, and summary logs have been (Campanella et al., 1983; Robertson and Campanella, published for the twelve drilled up to 1991 (Clague 1983a,b; Robertson, 1990). Measurements are recorded every 2.5 to 5 cm, et al., 1991; Luternauer et al., 1991; Patterson and Cameron, 1991; Patterson and Luternauer, 1993). producing a nearly continuous record in which bed Most have been logged by geophysical methods, and thicknesses, gradational and sharp contacts, and fining most of those drilled since 1990 have been continuously and coarsening upward sequences can be recognized. cored. The boreholes range in depth from 15 to 367 m. Consequently, CPTs can be used for stratigraphic Although these boreholes provide excellent lithologic interpretations in a similar manner as geophysical well and chronologic control, they are sparsely distributed logs. In particular, the 'friction ratio' is similar to the gamma ray log (Fig. 2; Monahan et al., 1993). across the delta.

NATURAL GAMMA

CORE DESCRIPTION

CPS 40

5O

60

70

80

90

O

CONE BEARING Qt (bar) 40 50 120 160

PORE FRICTION PRESSURE RATIO U (Metres Rf (%) of Water) 0 2 4-50 45

SILT interbedded SAND and SILT . . . . . . . . . . . . . m .edium SAND granules at base interbedded fine SAND and SILT

I

medium SAND granules at base interbedded fine SAND and SILT

20

. . . . . . . . . . . . . .

' ~ *

FIG. 2. Comparison of a gamma ray log and core d~scription (left) from a Geological Survey of Canada borehole (FD91-1) with adjoining CPT (right) at the Vancouver International Airport. Fining-upward sequences shown by arrows. See Fig. 1 for location. Gamma ray log courtesy of J.A. Hunter of the Terrain Sciences Division of the GSC.

30

e . A . Monahan et al.

0

% by weight

PORE FRICTION PRESSURE CONE RATIO U (metres BEARING Rf (%) of water) Qt (bar) 40 100 200 0 2 4 0

100

0 FILL SILT

lOm

SAND

-lp12. IJJ a

20m

1

--

SILT

30m

~

clay + silt <.074mm

~

sand 0.074 - O,420mm

~

sand + gravel 0.420mm - 4mm

I ~

gravel >4mm

FIG. 3. Comparison of grain size data (left) from a borehole at the Main Terminal Building, Vancouver International Airport with an adjoining CPT (right). See Fig. 1 for location.

Most CPTs obtained are between 20 and 40 m in depth, and the deepest is 100 m. They were performed by ConeTec Investigations Ltd, the British Columbia Ministry of Transportation and Highways, the Civil Engineering Department of the University of British Columbia, Hughes Insitu Engineering Ltd and Western Geosystems Inc. Other geotechnical boreholes were in general discontinuously sampled, and include many standard penetration tests (SPTs). The SPT blowcount or 'N' (the number of hammer blows to drive a sampler 300 mm according to a standard procedure) can be related in a general way to the CPT cone bearing (Robertson and Campanella, 1983a). Consequently, these data are used in this study to support the lithologic interpretations based on CPT data and to extend stratigraphic interpretations based on CPT data to areas of poor CPT control, particularly where grain size data are also available. The grain size data were obtained by sieve analysis. Because they were done for engineering pu/poses, the sieves used do not conform to boundaries used in the Wentworth scale, and the sand-silt boundary used is 0.074 mm (Figs 3, 4).

Furthermore, some analyses are based on a single sieve, in order to determine the relative proportion of sand to finer sediment (Fig. 8).

RESULTS The borehole data demonstrate that an extensive unit of sand underlies the surficial deposits of the delta plain. This sand ranges from fine to coarse grained and commonly has a gravel component, particularly near the base of the unit (Figs 3 and 4). Silt clasts are common, and shell debris occurs in some boreholes. The sand is relatively homogeneous, with little structure evident in cores. However, large scale high angle cross-bedding occurs locally. The sand unit is well documented at several sites at the Vancouver International Airport. Figure 2 shows a comparison of a CPT with a gamma ray log and core description from an adjoining GSC borehole. These data show that the sand unit is organized into two sharp-based fining-upward sequences at this site. A gravel component was observed in this core at the

31

A Delta Plain Sheet Sand % by w e i g h t

100

lOm

TD 16.8m

~ ~ ~

sand 0.074 - 0.420mm

R

gravel >4ram

c l a y + silt <.074mm

sand + gravel 0.420mm - 4mm

FIG. 4. Grain size data from borehole log, Sewer Pumphouse, Vancouver International Airport. See Fig. 1 for location.

N

•

4.9 km

•

4.9 km

base of these sequences. At two nearby sites at the airport, CPT and grain size data from geotechnical boreholes demonstrate that the sand unit consists of a similar sharp-based fining-upward sequence (Figs 3 and 4). At these sites too, there is a gravel component in the lower part of the sand. This sand unit, organized into one or more sharpbased fining-upward sequences, is present near the top of the deltaic section in nearly every borehole and log in the upper delta plain (Figs 5 and 6). It thus forms a nearly continuous unit in the delta topset. It is generally 8 to 20 m thick and has a sharp base with several metres of local relief. The topset sand unit overlies fine sand and silt in which seaward dips can be recognized commonly on reflection seismic data (Clague et al., 1991; Pullan et al., 1989) and on CPT correlations (Fig. 7), and which are interpreted to be mainly foreset deposits. The upward fining of the topset sand unit continues into the overlying unit of silt and silty sand across a gradational or interbedded contact. The latter unit is overlain by a unit of organic-rich silt that was deposited in a floodplain environment (Williams and Roberts, 1989). The east-west cross section (Fig. 6) also shows how the sand unit climbs and the overlying deposits thin from east to west, reflecting the rise in sea level as the delta prograded westward (Williams and Roberts, 1989, 1990). In log 6 on this section, an upper sharp-based fining-upward sand unit also occurs between 4 and 12 m. In log 7, the upper and lower sands observed in log 6 are combined in an unbroken sand unit 40 m thick. Gravel was reported in the lower 5 m of the sand in boreholes adjacent to log 7.

•

10.4 km

$

•

Rt(%)

L

.

i .

U

4s

k

--

[

&q

t

m

I

--4

i I t I

Pb

,4g.ex:~

FIG. 5. North-south CPT cross section through Fraser delta deposits along Western margin of upper delta plain showing the continuity of the topset sand unit (stippled). The southernmost log directly offsets GSC borehole 87-1, reported by Luternauer et al. (1991) and Clague et al. (1991). Datum is mean sea level.

32

P.A.

M o n a h a n et al. 5

w

uwwwm

3

2

1 • Qt (bet) =Rff%) F

4.3kin

•

3.7kin

Ot (bar)

F~(%)

Oe,L (MPa)=e Rff%)• =

a.=,~

6 F R A S E R RIVER

4

Qc •

5,4kin

Qe • (MPs) Rff%)

1.~,,

(MI~)RK%)

Qc

'

~

a

I I

p,m,

-

g

=

ii

Pt.lmrltXlli

FIG. 7. C P T cross section at a site on eastern Sea Island, showing apparent dip of 7 ° in silt and sand. Note the lower contact of the sand unit has the appearance of an angular unconformity. See Fig. 1 for location. Logs from LeClair (1988).

SW

CPT6 CONE BEARING Qt (bar) 0

60

120

180

240

~

25m

~

FRICTION RATIO Rf (%) 0

0

5

CPT1

NE

CONE BEARING ( ~ (bar)

FRICTION RATIO Rf (%)

60

0

120

180 240

5

E " - - 2 0

r

I,n

p. ~-

._

uJ o

_

~

=

SILT

r D-

and~ SAND

~

--~

~

40

40 r r-b-

;_

~--. =--

p m.,-

r P p

=.--

60,

, . . L

60 Refusal @ 6 1 m (probably P l e i s t o c e n e )

FIG. 6. East-west C P T cross section through Fraser delta deposits across upper delta plain showing the continuity of the topset sand unit (stippled)• The easternmost log is a g a m m a ray log. D a t u m is m e a n sea level.

A Delta Plain Sheet Sand

33 --

2.11 k m _ _

Line A W

Datum

a)

(

S~fk)or ~

Zero Tide ...................................................................... ~ . l (

-

" "~'~'~.

;"=

i

'

7

Rf ( % )

,oo 5 ,~oo o

'

~ ~ _ ~ " ~ -

.

-

E

~.

~~_____L_~__.L i

~::::iiiiii

::----:~

""

" "~'="~--

......... 4 - ......... + - - " ~ a - - ~ " ~ .

)

---'- ~ o

Qt (Bar)

~

. . . . . . ]~ Zero T~le

-

Jiii~iiiiii!i

'~, !iiiil

:~~ii

~-

a=~

-20--

....

0

,

,

Line B --

__ 2.8 km

2.5 km __ N

N

Datum

Z e r o T i d e ...................................

(MPa)

S

-

N

%,,

%Silt/~md

;-

$eafk)or

-10

• ...~....

__ 2.5 km

%Siltl~lnd

°

__:__--_~. '

,

-----_ ~-_---__--_---

-20

o

i_ !J

40

,i

LEGEND

%Silt

%8a1~1

-:---

40

I 0

o

40

~:::::5

t; FIG. 8. Cross sections through Fraser delta deposits across the tidal flats using standard penetration test and grain size data from boreholes. For each borehole, the blowcount (N) is shown on the left, and the sand--silt ratio is shown on the right. The datum for these cross sections is zero tide, 3 m below mean sea level. (A) East-west cross section across the tidal flats to the upper delta plain showing the presence of the topset sand unit (coarse stippling) on the eastern part of the tidal flats only. The eastern and westernmost logs are CPTs. (B) North-south along the western margin of the tidal flats showing the absence of the topset sand unit seen elsewhere on the delta plain.

A l t h o u g h little C P T control is available o n the tidal flats, S P T and grain size data f r o m geotechnical b o r e h o l e s in this area can be correlated with CPTs. T h e s e data d e m o n s t r a t e that the topset sand unit that underlies the u p p e r delta plain can be recognized in b o r e h o l e s in the eastern part o f the tidal fiats (Fig. 8, A). H o w e v e r , this sand unit is not present in b o r e h o l e s n e a r the o u t e r margin o f the tidal flats, such as the western margin o f S t u r g e o n B a n k , w h e r e sand and silty sand are i n t e r b e d d e d with and in a general w a y gradationally overlie s a n d y silt and silt (Fig. 8, B).

INTERPRETATION T h e topset sand unit is interpreted to be a c o m p l e x o f channel deposits. T h e organization o f sand into sharpbased fining-upward sequences is typical of channel deposits (e.g. R e a d i n g , 1986), and the sharp irregular base indicates an erosional contact. In the Fraser delta, the principal channel e n v i r o n m e n t s are distributary and tidal channels. H o w e v e r , coarse sand and gravel size material, which are present in the topset sand unit, occur in distributary channels and not normally in o t h e r

34

P.A. Monahan et al.

environments in the Fraser River delta (Clague et a l . , 1983). Furthermore, the overall thickness of the topset sand unit is consistent with the depth of distributary channels, and not with tidal channels. For example, the sand unit at the top of log 6 in Fig. 6 can be confidently attributed to a distributary channel, since the CPT was conducted on a bar surface beside an active distributary channel, the depth of which corresponds to the base of the sand. The topset sand unit contrasts with sediments that occur along most of the present delta front and that grade downward from sand on the outer tidal fiats to sandy silt and silt on the delta slope (Fig. 8: Luternauer and Murray, 1973; Hart et a l . , 1992b). Consequently, the sand unit is formed by processes operating on the delta plain itself, and not primarily along its margins. In contrast, tidal channels increase in size towards the margins of the tidal fiats (Luternauer, 1980). Thus, the delta topset sands are interpreted to be distributary channel rather than tidal channel deposits. The borehole data demonstrate that the surficial silts and sands of the upper delta plain and eastern tidal fiats are underlain by a nearly continuous composite unit of distributary channel sands. Consequently, the entire delta plain, except for parts of the western tidal fiats and the subaqueous delta plain, has been reworked by distributary channel migration, which has eroded the original tidal fiat, subaqueous delta plain and in most places the uppermost delta front deposits.

DISCUSSION Historical records show that channel migration has been more intense where the distributaries cross the tidal fiats. Fig. 9 shows the migration of the outer part of the Main Channel during the 100 year period prior to construction of the jetties which now control it (Johnston, 1921; Luternauer and Finn, 1983; Clague e t a l . , 1983). In particular, migrating meanders reworked 10 km 2 of the tidal fiats between 1896 and 1912 (Johnston, 1921). Stratigraphic sequences interpreted to represent channel migration in a tidal fiat environment have been recognized in boreholes. They consist of channel sand that contains some shell debris, and an overlying unit of burrowed silt and sand with shell debris that is interpreted to have been deposited in a tidal fiat environment. Examples have been observed in recent GSC and other boreholes near Ladner (GSC boreholes FD92-4 and FD92-11, Fig. 1). Williams (1988) recovered intertidal foraminifera from silt and silty sand overlying the topset sand unit in some of the cores he examined. Migration and abandonment of distributary channels in the upper delta plain historically has been less extensive than on the tidal fiats (Johnston, 1921), and has been largely inferred from geological and geomorphological evidence. The linear gap in the peat

123"15'

123"10'

123"15'

123010'

49*05'

FIG. 9. 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 and the north side is bounded by a jetty. Land areas are stippled. From Luternauer and Finn (1983) and Clague et al. (1983).

A Delta Plain Sheet Sand

L

I

lOcm

....I

FIG. 10. Photograph of split core from a borehole in central Lulu Island (GSC borehole FD92-3). Note interbedding of medium grained sands and laminated sands and silts. Between 11.0 and 11.2 m. See Fig. 1 for location.

bogs in eastern Lulu Island has been interpreted to be an abandoned former distributary (Fig. 1; Clague et al., 1983); sloughs beside the major channels indicate that some channel migration has occurred, and locally channel sand overlies floodplain silt (log 6, Fig. 6). Upper delta plain channel sand deposits have been interpreted in boreholes based on the absence of shell debris and the presence of unburrowed and laminated silt interbedded with sand at the top of the topset sand unit (Fig. 10). Such sedimentary structures were observed in a shallow trench dug by us on a bar beside an active distributary channel at the head of the delta (Fig. 11). However, channel migration in an upper delta plain environment has probably never been as intense as channel migration in a tidal flat environment; the sediments originally deposited at the prograding outer margin of the lower delta plain have been eroded by channel migration in all but the westernmost part of the present delta plain, whereas thick floodplain silt is preserved in the eastern delta topset (Figs 6 and 8). The extensive channel migration required to produce a nearly continuous channel sand deposit most likely

35

resulted from the interaction of tidal and fluvial processes and the high proportion of sand in the sediment load. Sediment transport and deposition in the distfibutary channels is influenced by tidal movement of a salt wedge that intrudes beneath the fresh river water. It can reach upstream as far as the head of the delta during low water periods, but is restricted to the tidal flats in the Main Channel during the freshet, when most sand transport occurs (Milliman, 1980). Kostaschuk et al. (Kostaschuk and Luternauer, 1989; Kostaschuk et al., 1989a, 1992b) have shown that suspended bed material is rapidly deposited as the salt wedge migrates upstream from the river mouth on a rising tide, and is then more slowly resuspended as the tide falls. Furthermore, in the tidal flats the bed material load (i.e. episodically transported in suspension or along the bed) in the Main Channel includes all of the sand load, whereas upstream from the delta the bed material load excludes the very fine sand component (Kostaschuk and Luternauer, 1989). These observations indicate that the cyclic interruption of sand transport and the time lag in sand resuspension reduce the capacity of the fiver to transport its naturally occurring sand load as it crosses the tidal flats. This interpretation is consistent with our conclusion that the lower delta plain has been a site of net channel sand accumulation. At present, sediment accumulation in the Main Channel occurs primarily near the river mouth (Kostaschuk and Luternauer, 1987; Kostaschuk and Atwood, 1990). However, prior to jetty construction the cyclic variations in fluid and sediment transport would have forced the channel to adjust its dimensions and pattern (Schumm, 1977), resulting in rapid bank edge erosion, channel migration, and the deposition of distfibutary channel sand. Furthermore, channel migration was facilitated by the sandy nature of the original tidal flat, subaqueous delta plain and some of the delta front deposits which were removed by this process, and of the channel deposits which were reworked by continuing channel migration. As the river flow shifted from one major distributary to another, channel sands were deposited in other areas on the lower delta plain, thus forming a nearly continuous composite unit of sand that underlies all but the youngest part of the delta plain. The evidence presented earlier for abandoned distributary channels on the upper delta plain, in particular the abandoned channel in eastern Lulu Island, demonstrates that other distributaries have been important in the past. Thus, the sheet-like channel sand complex of the Fraser delta, with its erosional and diachronous base and occasional marine fossils, is a useful model for sand-rich river and tide dominated delta plains. It contrasts with the sheet-like sand units of most large deltas described, which generally coarsen upward and were formed in a delta front setting (Elliott, 1986; Wright, 1985). This sand unit was deposited during a continuously rising sea level (Williams and Roberts, 1989, 1990) and is generally time-equivalent to the underlying foreset deposits, although the erosional base

36

P.A. Monahan et al.

FIG. 11. Photograph of a trench in a beach beside an active distributary channel at the head of the delta. Note the interbedding of coarse sand and laminated silts and sands. Depth of trench about 0.5 m. Located at site of GSC borehole FD92-5, and log 6 Fig. 6. See Fig, 1 for location.

has the appearance of an angular unconformity (Fig. 7). An ancient analogue could be easily misinterpreted as an incised valley fill sequence deposited during a lowstand of sea level (Van Wagoner et al., 11987) and to be much younger than the underlying marine deposits. Recognition of this extensive distributary channel sand complex in part explains the widespread occurrence of sands susceptible to liquefaction in the Fraser delta (Clague et al., 1992; Watts et al., 1992). Channel sands tend to be looser than shoreface sands that have been subjected to breaking waves (de Mulder and Westerhoff, 1985). At present, sediment transport through the delta has been modified by dredging and the presence of dykes and jetties. The jetties that confine the Main Channel where it crosses the tidal flats not only increase the capacity of the river to transport sand by channelizing the river flow (Milliman, 1980), but also prevent channel migration and further net accumulation of sand on the lower delta plain. Thus, a greater proportion of the available sand load is now transported to the river mouth, where intermittent failures transport the sand down the delta slope (Kostaschuk and Luternauer, 1987; Kostaschuk et al., 1989c, 1992a; Kostaschuk and Atwood, 1990; Hart et al., 1992a, b; McKenna et al., 1992). In this way, the jetties contribute to instability at the river mouth. Hart et al. (1992b) also concluded that the jetties contribute to river mouth instability because

sand is delivered to the same point on the delta front. The jetties partially offset the effects of dredging, which reduces the amount of sand reaching the delta front. Between 1974 and 1983, dredging of the Lower Fraser River exceeded the estimate long term natural supply of bed material by one million tonnes per year (McLean and Tassone, 1991). During this period however, sand was resupplied by degradation of some reaches of the distributary system and continued to be transported to the river mouth, where failures were reported (McLean and Tassone, 1991; Kostaschuk et al., 1989b; McKenna et al., 1992). Consequently, because artificial channelization of the Main Channel has increased sand transport efficiency and prevents further sand storage in the lower delta plain, continued dredging is required so that instability at the delta front is not increased over naturally occurring conditions.

CONCLUSIONS Distributary channel migration in the Fraser delta has produced a nearly continuous composite sand unit generally 8 to 20 m thick underlying the surficial sands and silts of most of the delta plain. Most of the channel migration (i.e. prior to jetty construction) occurred in a tidal flat environment, due to the interaction of tidal and fluvial processes and to the high proportion of sand in the sediment load. Jetties now prevent channel

A Delta Plain Sheet Sand

migration and further accumulation of sand in the lower delta plain, thereby contributing to instability at the delta front. The results of this study demonstrate that subsurface investigations of a modern delta can be successfully used to highlight important sedimentary processes that operate on a time scale longer than direct scientific observation.

ACKNOWLEDGEMENTS The authors gratefully acknowledge the generous assistance of the following in providing the data that were necessary for this study: the British Columbia Ministry of Transportation and Highways Geotechnical and Materials Engineering Branch, the British Columbia Ministry of Environment Groundwater Section, the British Columbia Ministry of Energy, Mines and Petroleum Resources Petroleum Geology Branch, British Columbia Hydro and Power Authority, British Columbia Railway Company, British Columbia Ferry Corporation, the Vancouver International Airport Authority, Vancouver Port Corporation, Public Works Canada, Greater Vancouver Regional District, the City of Richmond, the Corporation of Delta, School District 38 (Richmond), School District 37 (Delta), the City of Vancouver, B.C. Gas Inc., ConeTec Investigations Ltd, Klohn-Crippen Ltd, Geopacific Consultants Ltd, Macleod Geotechnical Ltd, Cook Pickering and Doyle Ltd, HBT Agra LTD, Triton Consultants Ltd, Golder Associates, the Dominion Company, PBK Engineering Ltd, Delcan Consultants, Delta Christian School, R.G. Campanella of the Department of Civil Engineering of the University of British Columbia, and J.A. Hunter and H.A. Christian of the GSC. The drafting and preparation of the figures was by R. Franklin and B. Sawyer. This paper was reviewed by J. Alcock, B.S. Hart, P.S. Mustard, G.T. Norgard and B. Ward, whose comments are greatly appreciated. The cores for the GSC boreholes were acquired by Sonic Drilling Ltd and Foundex Explorations Ltd. P.A. Monahan is receiving scholarship support from NSERC and the University of Victoria.

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