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Distribution of the Chilcotin Group basalts, British Columbia Dohaney, Jacqueline Anne Marie

Abstract

The Chilcotin Group basalts (CGB) are Oligocene to Late Pleistocene, stratified olivine-phyric basaltic lavas that overlie a large proportion of the Interior Plateau of British Columbia. The distribution of the CGB is poorly understood regionally; the current distribution is based on compilations of previously published geological maps that employ a diverse set of lithostratigraphic definitions of the Group. Exposure of the basalts is typically poor, but the thickest and most extensive sections are exposed in the valley-margins of major rivers (e.g., the Fraser River). This study collates and interprets spatial data sets and reassesses the distribution of the CGB with the intent of producing a new, more robust distribution of the CGB within the Taseko Lakes (0920) and Bonaparte Lake (092P) map areas, with the goal of better characterizing their geological history and physical volcanology. The new distribution map demonstrates several important observations: (1) the distribution of the CGB is less extensive than previous compilations by up to 48%; this implies that, regionally, the CGB is probably significantly over-estimated; (2) there are abundant, yet not previously identified “windows” through the basalt that expose underlying rock units which may be geologically and economically important; (3) CGB volcanism spanned the Oligocene to the Pleistocene (—30 Ma) and was centered in the central Fraser River area (south of Williams Lake, B.C.) throughout the Pliocene Pleistocene; (4) the CGB was likely erupted from a multitude of small-volume monogenetic vents, rather than a series of long-lived volcanic centres or fissures; and (5) the CGB is thickest where lavas ponded in paleo-valleys, providing a key to mapping the distribution of Neogene channels in the Fraser Basin drainage.

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