Samples from 13 basalt flows of the Columbia River Group, which are part of a measured section near Prineville Dam in central Oregon, were analyzed for trace and major elements, using instrumental neutron activation and x-ray fluorescence techniques. The 13 flows are compositionally similar to one another and contain anomalously (for basalt) large amounts of Ba and P. They also have higher Sm, Yb, Lu, and Sr contents, lower Co content, and higher total alkali and K2O for their MgO content than do the Picture Gorge and Yakima Basalt. These 13 flows define a new chemical type in the Columbia River Group, the Prineville chemical type. The Prineville chemical-type flows, interfingering with other basalt types of the Columbia River Group, are found in other areas and therefore will be useful in stratigraphic studies.

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