Abstract

U–Pb, Rb–Sr, and K–Ar methods were employed to estimate the age of a thin crystal tuff bed in the middle Albian Hulcross Formation at Hudson's Hope, northeastern British Columbia. Ten different gravity fractions of altered biotite define an Rb–Sr isochron that passes through the plotted Rb–Sr data point for cogenetic plagioclase. The biotite–plagioclase isochron yields an age of 102.4 ± 1.0 (2σ × MSWD1/2) Ma (MSWD is mean square weighted deviate) and an initial ratio of 0.71311 ± 0.00030. Different size fractions of clear prismatic zircons define a U–Pb discordia line that intersects the concordia at graphic (2σ) Ma. The four heaviest gravity fractions of biotite give a K–Ar age of 106.0 ± 1.8 (2σ) Ma. The most probable age of deposition of the tuff bed is estimated to be very close to or a little more than 102 Ma, but does not exceed 106 Ma.

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