Potassium-Argon Dating of Some Lower Tertiary Strata in British Columbia

Twenty potassium-argon dates were determined on biotite, sanidine, plagioclase, hornblende and glass shards separated from volcanic rocks at eight localities in British Columbia. Fifteen of the dates fall within the range 47 to 52 m.y. (millions of years), confirming the middle Eocene age obtained for the strata by Rouse and Mathews (1961). A 21 m.y. date yielded by a single sample of glass shards is low because of the susceptibility of glass to argon loss. Dates of 56 to 126 m.y. from the remaining samples probably reflect contamination by reworked older rock fragments. Samples of plagioclase and hornblende yielded dates in agreement with other data and are thus useful in dating materials where sanidine or biotite is not available. Multiple mineral separates, sample purity and lack of contamination are necessary requisites for reliable dates.