The region in which the showing occurs is within the Inter- montane Belt, underlain dominantly by Lower to Middle Jurassic volcanic and sedimentary rocks of the Hazelton Group. These as- semblages are overlain by the Upper Cretaceous to Lower Tertiary Ootsa Lake Group and Miocene plateau basalt. Intruding Lower Jurassic rocks of the Hazelton Group in the northeastern part of the map sheet is a belt of granodiorite, diorite and quartz diorite plutons of the Lower Jurassic Topley intrusive suite. Felsic plutons of probable Cretaceous age intrude both Lower and Middle Jurassic Hazelton strata.

The Ootsa Lake Group, in the southern part of the mapsheet, is preserved in a fault angle depression now partly covered by Tsacha Lake. In this area the Ootsa Lake Group is predominantly a volcanic assemblage. In this assemblage sedimentary horizons are intercalated with felsic tuffs. The Tsacha Lake showing consists of diatomite and pumice hosted within these sediments.