Source rock potential of Lower to Middle Jurassic black clastic sequences of the Intermontane belt1This article is one of a series of papers published in this Special Issue on the theme of New insights in Cordilleran Intermontane geoscience: reducing exploration risk in the mountain pine beetle-affected area, British Columbia.

Abstract

Lower to Middle Jurassic clastic sequences are widespread within the interior of the Canadian Cordillera. These successions cap waning Jurassic volcanism in many Cordilleran arc terranes and are succeeded by clastic sequences of the Intermontane basins. Fine-grained, carbonaceous lithologies, which locally contain elevated levels of organic carbon, characterize these clastic successions. These include sections of the Spatsizi Formation (Abou Member) and Smithers Formation in northern and western Bowser basin, respectively, Ashcroft Formation and equivalent strata in central Quesnellia, and Last Creek Formation and Junction Creek rocks below the Tyaughton–Methow basin. These rocks locally contain metre-thick sections with total organic carbon (TOC) levels >5 wt.% and others have thicknesses approaching 100 m with TOC between 3 and 5 wt.%. Thermal maturation levels are high in many sections, suggesting original organic contents were greater and that these rocks may have been excellent source beds. Associated bitumen in these successions, together with Mesozoic oil in some overlying Intermontane clastic rocks, also suggests these sequences may have been effective oil source rocks. TOC levels, thermal maturity, and thickness of some sections are comparable with shale gas sequences being exploited elsewhere in North America. TOC concentrations within these rocks, together with other elemental abundances, indicate anoxic conditions during deposition. The age of these clastic rocks brackets the global Toarcian anoxic event and that of other organic-rich sequences in North America. Elemental abundances suggest predominantly volcanic-arc complexes as source terranes, although continental signatures are suggested by rocks in western Quesnellia.