PREHISTORIC DEBRIS AVALANCHES FROM MOUNT CAYLEY VOLCANO, BRITISH-COLUMBIA

An investigation of diamicton units exposed in an extensive accumulation of volcanic debris in the Squamish valley, west of Mount Cayley volcano, has yielded evidence for at least three major debris avalanches, initiated by the collapse of the western flank of Mount Cayley in the mid-Holocene. Radiocarbon ages obtained from tree fragments contained in the deposits indicate that the events took place at 4800, 1100, and 500 BP. All three debris avalanches dammed the Squamish River and formed temporary lakes upstream of the debris. Failure of the cone took place after considerable dissection of the original edifice had exposed weak pyroclastic materials at the base of the steep upper slope of the volcano. No evidence of older debris avalanches from Mount Cayley has been discovered. Smaller scale debris avalanches probably have been common, and at least two have occurred in historic time (1963 and 1984). Debris avalanches from Mount Cayley and the effects of a possible damming of the Squamish River are major geomorphic hazards to public safety and economic development in the Squamish valley.