Abstract

A large (ca. 5 × 106 m3) landslide occurred on the west flank of Mount Cayley in the southern Coast Mountains of British Columbia in 1963. Failure commenced when a large block of poorly consolidated tuff breccia and columnar-jointed dacite was detached from the subvolcanic basement and slid into the valley of Dusty Creek, a small tributary of Turbid Creek. As the detached block accelerated, it quickly fragmented into an aggregate consisting of angular clasts up to several metres across, partially supported by a matrix of fine comminuted rock material. The landslide debris moved about 1 km down Dusty Creek as a wedge-shaped mass up to 70 m thick, banking up on turns and attaining a maximum velocity of 15–20 m/s. The debris mass thinned as it spread across the broader, flatter valley of Turbid Creek, and was deposited as an irregular blanket with a maximum thickness of 65 m along a 1 km length of this valley. As a result of the landslide, Turbid and Dusty Creeks were blocked, and lakes formed behind the debris. These debris dams were soon overtopped and rapidly breached, causing floods and probably debris flows to sweep down Turbid Creek valley far beyond the terminus of the landslide.From an analysis of the annual rings of slide-damaged trees, it is concluded that the landslide probably occurred in July 1963. Although the largest earthquake of 1963 and a moderately intense rainstorm also occurred during this month, there were much larger earthquakes and storms in this area on many previous occasions, and these did not cause large slope failures. Thus, it appears that the stability of the slope at the head of Dusty Creek gradually deteriorated over a long period of time until a relatively minor event, such as a small earthquake or storm, triggered the failure.The main contributing factors to this landslide are geologic and include the presence of: (1) hydrothermally altered faults and fractures in poorly lithified pyroclastic rocks and in jointed volcanic flows; (2) an outward-sloping unconformity separating the Quaternary volcanic sequence from older basement rocks; and (3) fractured glassy selvages surrounding small intrusions in the base of the volcanic pile.Deposits of one or more landslides that predate the 1963 event also occur in Turbid Creek valley. These older deposits are present over a much larger area than the 1963 slide deposits and probably were emplaced by highly mobile debris flows with high water content.

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