THE ORIGIN OF A TEPHRA-LIKE BED NEAR MOUNT-CAYLEY VOLCANO, SOUTHWESTERN BRITISH-COLUMBIA, CANADA

A distinct white bed 1-2 cm thick is present within backwater deposits at the upstream end of the Turbid Creek debris fan, southwestern Brit ish Columbia, Canada. This white bed and the enclosing backwater depos its accumulated within an impoundment of Squamish River caused by a la rge ca. 4800 BP debris avalanche originating from Mount Cayley, a Plio cene -Pleistocene volcano. The white bed is composed of silt and clay detritus and resembles tephra. About 5% of the grains exhibit optical characteristics consistent with volcanic glass. The remainder, however , are mineral and lithic particles (mainly cristobalite and Na-feldspa r, with minor quartz, and trace amphibolite, mica group, and chlorite) . The white bed is interpreted to be dust derived from the debris aval anche and washed into the impoundment, based upon (i) the high proport ion of mineral and lithic grains within the bed, (ii) the contemporane ity of the bed and the debris avalanche, (iii) the lack of Holocene vo lcanic activity at Mount Cayley, and (iv) the significant age differen ce between the bed and known tephras in southwestern British Columbia.