Abstract

The delta of Fraser River, first investigated in detail by W. A. Johnston in 1919 has been resurveyed and sampled. The slope of the delta front was found to average 1 1/2 degrees but the upper parts off the main channel have inclinations that range mostly from 1 3/4 degrees to 3 1/2 degrees . Gullies occur off the main channel and these are bordered downslope by hills, both features apparently the result of landsliding on the delta front. Near the river mouth the front has advanced an average of 840 ft in 30 years at moderate depth, but appreciably less than this in shallow water. Volume of sediment added annually to the delta front is estimated to be about 700,000X10 6 cu ft; silt predominates but sand is an important constituent. Delta-front sediments are sandy off, and S., of the river mouth, silty on the N. Tidal movements of sediment-laden waters may account for this asymmetric distribution. Within the silty sediment the water content increases regularly with distance from the river mouth, regardless of the depth of the bottom, and the clay:sift ratio increases less markedly. Porosities and liquid and plastic limits of the freshly deposited sediments are high compared with uplifted glaciomarine and buried deltaic sediments of similar texture in the vicinity of the delta. Much exchangeable Ca and Mg remain in the bottom sediment notwithstanding contact with Na-rich sea water. Recent data agree well with Johnston's estimate of 8000 years for the age of the delta. Depth of deltaic sediment is estimated to average almost 400 ft and to range up to at least 700 ft. Environmental conditions and associated deposits on the delta top and front are summarized and the classical distinction of fore-set and bottom-set beds is not readily applicable in this example. Speculation regarding consolidation, based on laboratory and theoretical studies suggests that more than just gravitational pressure plays a part in quick compaction of the sediments of this delta.

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