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Geology of Wreck Bay, Vancouver Island

University of British Columbia

Wreck Bay is located on the west coast of Vancouver Island at 49°00'N, I25°38'W. It is roughly crescent shaped with a small cuspate foreland named Sand Point in the middle, and measures 2½ miles (2.17 kilometers) between the enclosing headlands of Quisitis and Wya Points. Rocks exposed along the coast are indurated, unmetamorphosed, impure sandstones and mudstones of late Jurassic to early Cretaceous age. They were derived from the hinterland northeast of Wreck Bay, and were rapidly deposited into a trough which extended parallel to the present-day coastline. The contact between these sediments and the source rocks is thought to lie beneath a thick cover of Pleistocene material which now overlies the Estevan Coastal Plain; the southwestern edge of the paleotrough, from seismic evidence, appears to lie 5 - 6 miles (4.35 - 5.22 kilometers) seaward from the present-day coastline. Infilling of both sides of this paleotrough with Pleistocene and Recent sediments has resulted in a narrow, arcuate, present-day trough on the continental shelf adjacent to Wreck Bay. The Pleistocene sediments, consisting of cohesive grey clay and glaciofluvial outwash, were also derived from the mountainous hinterland to the northeast, and recent sediments derived therefrom are dispersed across the bay and inner shelf. Boulders and gravel freed from the retrograding sea cliff behind the beach have settled to the base of wave erosion in the bay, and this coarse "mat" is covered by a thin veneer of very well sorted fine sand which becomes progressively finer further away from shore. A nearshore surface current transports clay, silt and some of the sand southeastwards to Wya Point and the offshore trough. During the summer, breaker heights in the bay vary from 0.75 - 4.00 feet (0.23 - 1.27 meters), and it is calculated that during winter storms, wave heights exceed 19 feet (5.75 meters). The foreshore in summer consists of fine, light-coloured sand, and slopes gently seaward at less than 2.6°. Profile changes on the foreshore result from three controlling factors: the breaker height, the breaker incident angle, and the position of the water table on the beach. The direction of littoral drift near the middle of the beach changes with tide level, but generally it is towards Sand Point and very strong; near Quisitis and Wya Points it is weak, and consistently away from them; elsewhere, it is weak and variable in direction. Transverse profiles were found to be most sensitive to tidal range where the brisker incident angle was small and consistent; they were virtually insensitive where the breaker incident angle was small and variable. In winter, the foreshore is generally less steep than in summer, and near Sand Point the surface material of the beach is reduced to coarse gravel as sand is carried out to the middle of the bay; northwest and southeast from here, the beach surface consists of dark-coloured medium sand; adjacent to the two headlands, the light-coloured fine sand of summer remains. Profile changes in winter are determined by breaker heights only, the other two controlling factors becoming insignificant. Runnels, or incipient beach cusps, tend to form wherever littoral drift is not too strong, and their spacing is apparently related to the thickness of the swash wedge. The cliffbase along the northwest half of Wreck Bay very closely approximates a log-spiral curve in plan due to the angular relationship between prevailing wave fronts and the coastline; the southeast half, however, does not, because a complex wave pattern is created in the lee of islands located in the middle of the bay. The value of gold contained in the backshore near Lost Shoe Creek is calculated to be $10,650. An offshore placer deposit at 20 fathoms (36,6 meters) depth is indicated by a great increase in the amount of magnetite and other heavy minerals there, together with the fact that a small mode of very fine sand, which contains most of the heavy minerals onshore, reappears in samples collected from this bathymetric level.

Text

2011-05-11

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http://hdl.handle.net/2429/34468

Geological Sciences

University of British Columbia

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