J. Vaughn Barrie, R.G. Currie and R. Kung

Geological Survey of Canada Open File Number 3572

Swath bathymetry of southern Roberts Bank highlighting the failure complex, present day submarine distributary channels and relict channels (from Currie and Mosher, 1996). larger image [GIF, 322.9 kb, 889 X 673, notice]

The Fraser River Delta is a Holocene, river dominated depositional feature, built into the deep (>300 m) waters of the Strait of Georgia on Canada's Pacific coast. The delta provides much of the coastal terrain on which the port city of Vancouver and adjacent municipalities have developed. Consequently, the delta has been modified to provide ferry, coal and dredge spoil dumpsite. The confinement of the Fraser River to its present channels began during the early part of this century. Prior to this, distributary channel switching/avulsion occurred regularly. The annual river load (approximately 17.3 X 106 tonnes) is 65% silt and clay, and 35% sand, transported primarily during the spring and summer freshet. Today, most of the sand is removed from the system by dredging and the mud is transported in a plume past the intertidal estuary and northwards into the basin by the dominant flood tidal flow (tide range of 3 to 5 m). The remainder of the sand is deposited as a tractive load from the main channel of the Fraser River into active submarine channel systems asymmetrically about the river mouth. The asymmetry is a result of jetty construction on one side of the main channel. Two causeways to the south of the main channel and one to the north that cross the intertidal zone to the delta foreslope also act as barriers to the dominant northward tidal sediment transport, causing estuarine and localized seabed erosion. Here, the presence of the causeways results in tidal flow separation with clockwise back eddies forming gyres. On the delta foreslope, off the southern causeways, an eroded submarine distributary channel failure complex (Hart and Barrie, 1995) has been exposed by enhanced tidal flows that scour the seabed and form northward migrating subaqueous dunes. The erosion increases the delta slope and, consequently, risk of slope failure. This complex developed when the river mouth discharged in this region (1,000 - 3,500 14C yrs BP) and migrated freely across the delta plain

[Click on an image thumbnail to view a larger image, notice]

% sand	mean grain size
% silt	standard deviation
% clay	Sedimentation rate for the Fraser Delta slope and prodelta based on the depth of the 1964 peak of 137Cs fallout from Hart et al., 1998
Seistec high resolution sub-bottom profile and vibrocore (TUL93-02) taken within Roberts Bank Failure Complex.	Seistec high resolution sub-bottom profile of a portion of the Roberts Bank Failure Complex. A vibrocore (TUL93-07) demonstrates the facies stratigraphy within and overlying a submarine buried paleochannel.
Huntec DTS sub-bottom profile and interpretation within the subaqueous dune field on Roberts Bank. Facies stratigraphy within a submarine buried paleochannel and the overlying subaqueous dune can be observed in core PAR 91-03.

Currie, R.G. and Mosher, D.C. (1996). Swath bathymetric surveys in the Strait of Georgia, British Columbia. Current Research 1996-E, Geological survey of Canada, 33-40.

Evoy, R.W.. Moslow, T.F., Patterson, R.T. and Luternauer, J.L. (1993). Patterns and variability in sediment accumulation rates, Fraser River delta foreslope, British Columbia, Canada. Geo-Marine Letters, 13, 212-218.

Hart, B.S., and Barrie, J.V. (1995). Environmental Geology of the Fraser Delta, Vancouver. Geoscience Canada, 22, 172-183.

Hart, B.S., Barrie, J.V. and Hamilton, T.S. (1998). Sedimentation rates and patterns on a deep water delta (Fraser Delta, Canada): Integration of high-resolution seismic stratigraphy, core lithofacies, and 137Cs fallout stratigraphy. Journal of Sedimentary Research, 68(4), 556-568.

Mosher, D.C. and Hamilton, T.S., (1998). Morphology, structure and stratigraphy of the offshore Fraser delta and adjacent Strait of Georgia. in Geology and Natural Hazards of the Fraser River Delta, British Columbia, (ed.) J.J. Clague, J.L. Luternauer, and D.C. Mosher, Geological Survey of Canada, Bulletin 525, p.147-160.