Columbia River Basalt Group 

(Miocene) 

(U.S.A. Pacific Northwest and southern and central Canadian Cordillera, unit QTvm)

Consists chiefly of thick flows of olivine tholeiite erupted as flood basalt, between 16.5 and 6 Ma, and 
minor interbeds of fluvial and lacustrine sedimentary rocks. Basalt in group has an estimated volume of 
210 km2 (Swanson and Wright, 1978). Group forms most of unit QTvm in Washington and northern 
Oregon (Hooper and Conrey, 1989; Hooper 1982; Wells and others, 1989).

In southern Canadian Cordillera, group consists of Chilcotin volcanic field of late Tertiary, mainly 
Oligocene(?), Miocene, and Pliocene age (16 to 2 Ma). Mainly subaerial, rarely subaqueous, olivine basalt 
flows, intercalated pyroclastic rocks, and local gabbroic and basaltic intrusions (Bevier, 1983; Mathews, 
1988). Volcanic field also locally contains younger volcanic centers of intermediate to felsic composition, 
such as the plume-related(?) Anaheim volcanic rocks, and the rift-related Ediza volcanic rocks (Wheeler 
and McFeely, 1991). Columbia Group interpreted as a back-arc basin tectonically paired to the Cascade 
volcanic-plutonic belt. REFERENCES: Swanson and Wright, 1978; Bevier, 1983; Mathews, 1988; 
Hooper and Conrey, 1989; Hooper 1982; Wells and others, 1989; Wheeler and McFeely, 1991