Abstract

The intertidal zone in Craig Bay extends 1200 m offshore making it the broadest non-estuarine tidal flat on the east coast of Vancouver Island, British Columbia, Canada. Superimposed on the low gradient foreshore (1:360) are up to fourteen low amplitude (<=0.6 m) ridges with an average wavelength of 86.3 m. These are permanent fixtures of the intertidal zone exhibiting considerable stability in both form and position. They are generally symmetrical in cross section and in no instance is a landward slip face evident. The ridges have developed in medium-to-fine sand atop a coarse deltaic platform. The thickness of this surficial sand unit is often equal only to the height of the ridges, and the gravel sustrate is frequently exposed in the depressions. The morphology, sediments, and hydraulic environment of this rhythmic foreshore were examined to provide insight into the factors controlling ridge development and maintenance. Although the maximum tidal range exceeds 5.0 m, tidal currents are of limited importance; only in the major runnels are velocities sufficient to initiate sediment transport. Waves within the Strait of Georgia are locally generated (H (sub 1/3) <= 3.0 m, T (sub 1/3) <= 7.0 s) and appear to the the prime agent responsible for the formation and maintenance of this variant of ridge and runnel morphology. The dominant bedform across the intertidal zone is the wave-formed ripple, although current ripples may locally predominate. Small-scale cross-stratification, which is associated with these bedforms, is the most common primary physical sedimentary structure in the surficial sand unit. However, a large proportion of the cores were structureless. This is attributed to bioturbation by sand dollars ( Dendraster excentricus ) and to a lesser extent to air entrapment and water escape.

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