Abstract

Trace- and major-element analyses and paleomagnetic measurements have been used in conjunction with field studies, mainly in central Oregon, to investigate the relation between the Picture Gorge and Yakima basalts as well as other more recently recognized basalt types in the Columbia River Group. Picture Gorge and Yakima basalts are rarely seen in contact, as they were separated at the time of eruption by the Blue Mountains anticline. Nearly all areas previously mapped as Yakima basalt on the south side of the anticline are in fact Picture Gorge basalt. Locally, however, a few flows of Picture Gorge basalt crossed the divide; in the Butte Creek area, Yakima and Picture Gorge flows are interbedded, thus providing a link for correlating the two units. Stratigraphic evidence thus suggests that the upper part of the Picture Gorge basalt was erupted at the same time as the lower part of the Yakima basalt.

Miocene basalts in western Oregon have been compared with the Columbia River Group east of the Cascade Range. The main area mapped as Columbia River Group in the lower Willamette valley has the composition of Yakima basalt, but Miocene intrusions near Eugene and flows in the Sardine Formation are compositionally distinct from the chemical types recognized farther east.

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