The Clarno Formation is an Eocene to lower Oligocene volcanic and volcaniclastic formation in central Oregon. It was apparently formed in response to subduction of Pacific Ocean crust under the western margin of North America.
Clarno flow rocks, which constitute at least 50% of the formation, are compositionally intermediate between flow rocks typical of continental margins and those typical of island arcs. The dominant rock type is andesite, in a range from rhyolite to basalt, and the SiO2 frequently distribution is unimodal about a mean of 60%. The principal phenocryst in the basalts and andesites is clinopyroxene. Abundances of lithophile elements (K, Rb, and Ba) in the basalts are very similar to average abundances in calc-alkalic basalts of island arcs. Lithophile abundances in Clarno andesites, however, are generally greater than those of island-arc andesites and less than those of andesites from continental margins. The Clarno Formation may be inferred to have formed on a crust intermediate between oceanic and continental; a thickness of 20 to 30 km can be estimated from K2O contents and also from Rb-Sr relationships.