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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 46 (1962)

Issue: 8. (August)

First Page: 1416

Last Page: 1438

Title: Sedimentation of Fraser River Delta, British Columbia

Author(s): W. H. Mathews (2), F. P. Shepard (3)

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 1½° but the upper parts off the main channel have inclinations that range mostly from 1 3/4° to 3½°. 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 feet 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 700,000 × 106 cubic feet; silt predominates but sand is an important constituent. Delta-front sedimen s are sandy off, and south, of the river mouth, silty on the north. 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: silt 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 calcium and magnesium remain in the bottom sediment notwithstanding contact with sodium-rich sea water.

Recent data agree well with Johnston's estimate of 8,000 years for the age of the delta. Depth of deltaic sediment is estimated to average almost 400 feet and to range up to at least 700 feet. 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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