Petrology of the Cenozoic volcanic rocks of the basal Clarno Formation, central Oregon
Wilson, Patricia McDowell
Rogers, John J. W.
Master of Arts
The Clarno Formation is a volcanic unit of Eoceneearly Oligocène age in central Oregon. It is unconformably underlain by the Cretaceous clastic rocks of the HudspethGable Creek Formations and succeeded by the tuffs of the John Day Formation. The basal 2-3 feet of the Clarno Formation, consisting of flows, lahars, and pyroclastics, was examined near the town of Mitchell. The flow units were sampled for major element analysis. Analyses were performed for twenty-six basal flows by means of x-ray fluorescence and neutron activation. Several chemical parameters were used to delineate the calcalkaline nature of the basal Clarno Formation: AFM diagrams of Clarno data reveal no iron enrichment; K2O contents range from .4% - 3.56%; KgO/NagO ratios are generally lower than .5* Chemical criteria were also used to classify the Clarno Formation as continental, including: the slope of the potash vs. silica variation diagram, the level of KgO at 55% SiOg, the values of T = [(AlgOg - Na2)/Ti2], and the values of the ratio of total iron to magnesium. An attempt is made to relate the geochemistry of the basal Clarno Formation to the general tectonic regime of the developing continental margin of the Cenozoic Northwestern United States.