Abstract

The Olympia interglacial sediments in the Interior Plateau region of British Columbia contain several thin, fine-grained rhyolitic and dacitic tephras, which undoubtedly represent the distal portions of widespread air-fall eruptive units. Successful discrimination of these tephras can be made by their mineral assemblage and composition of glass, magnetite and ilmenite, as determined by use of an electron microprobe; positive identification is not possible solely on the basis of field criteria such as coloration, degree of weathering, granulometry, thickness and stratigraphic position.Each tephra layer serves as a valuable time-parallel stratigraphic marker because of its regional extent and distinctive character. Those documented in this study include, in order of increasing age: Rialto Creek tephra, about 20 000 years old; Cherryville tephra, about 25 000 years old; Riggins Road tephra, about 30 000 years old; Duncan Lake tephra, about 34 000 years old; Dufferin Hill and Sweetsbridge tephras are probably close in age to Duncan Lake tephra; Kamloops Lake tephra is slightly older than 34 000 years; Mission Flats tephra is probably older than 35 000 years; Coutlee tephra is more than 37 000 years old; and the exact age of Okanagan Centre tephra is unknown. Such a detailed tephrochronological record should greatly facilitate geochronological and correlation studies of Olympia interglacial sediments in south-central British Columbia and adjacent areas of the Pacific Northwest.No definitive statement can be made at present on the identity of source volcanoes, but it is very likely that the commungtonite-rich tephras are derived from Mount St. Helens in Washington State.

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