Regional stratigraphy and age of Chilcotin Group basalts, south-central British Columbia

Abstract

Mio-Pliocene Chilcotin Group basalt flows form a 50 000 km2 lava plateau in south-central British Columbia. Two periods of eruptive activity at 2–3 and 6–10 Ma are indicated by a compilation of available age data, including 10 new K–Ar age determinations, and basalts from these two periods are chemically indistinguishable. The Chilcotin Group consists of thin, crudely columnar-jointed pahoehoe flows, some thick, tiered flows, pillow lava and pillow breccia, and rare silicic tephra layers. The presence of many vesicle sheets and cylinders and collapsed pahoehoe toes suggests that the basalts were volatile-rich. Known vents (gabbro and basalt plugs) for the basalt flows form a northwest trend along the axis of the lava plateau. The plateau appears to have formed from the overlap of many low-profile shield volcanoes and is similar in morphology to other plains basalts such as the Snake River Plain and parts of Iceland. Glaciation has stripped off an unknown volume of the flat-lying basalt flows.