Holocene Progradation Research Articles

Liquefaction features on the Fraser delta: evidence for prehistoric earthquakes?

Sand dykes and sills and vented sand boils on the Fraser River delta and adjacent Serpentine River floodplain in southwestern British Columbia record one or more major liquefaction events. The source of the dykes is a shallow subsurface saturated sand unit that was deposited in foreslope and distributary-channel environments during Holocene progradation of the Fraser delta. The dykes cut steeply through a crust of delta topset muds and locally flatten out as sills in overlying peaty sediments. At two sites, evidence was found for venting of sand onto a subaerial or intertidal surface. Liquefaction and upward movement of sand and water locally deformed the intruded sediments, causing some subsidence and uplift of the delta surface. All observed liquefaction features are younger than ca. 3500 BP, and at least some are younger than 2400 BP. It is not clear, however, whether they formed during one or several separate events. A review of possible causes suggests that the liquefaction features probably are seismically generated; they thus may provide the first direct evidence for moderate to large, prehistoric earthquakes in the Vancouver metropolitan area.

Postglacial deltaic sediments, southern Fraser River delta, British Columbia

The Fraser River delta, the largest delta on the west coast of Canada, has been built into the Strait of Georgia during the Holocene. Drill-hole and seismic reflection records reveal a succession of sedimentary units deposited during early Holocene progradation of the delta. These overlie an irregular surface developed on Pleistocene drift. Mud and silt, similar to sediments presently accumulating off the mouth of Fraser River in the southern Strait of Georgia, are conformably overlain by a thick unit of sandy foreset beds, dipping gently to the south-southwest into Boundary Bay and deposited in a foreslope environment. The foreset unit is sharply overlain by a much thinner topset sequence comprising silt and sand deposited in intertidal, fluvial-channel, and overbank environments, and peat deposited in swamps and bogs. Fifteen accelerator mass spectrometry radiocarbon dates on shell and wood indicate that most of the deltaic sediments south of the Main Channel of Fraser River were deposited between ca. 7500 and 5000 BP. By 5000 BP the locus of sedimentation had shifted from the south, into Boundary Bay, to the west and southwest, into the Strait of Georgia proper.

Sedimentary environments and postglacial history of the Fraser Delta and lower Fraser Valley, British Columbia

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 −12 m 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.