BASALT OF HUNTZINGER, ASOTIN MEMBER OF SADDLE MOUNTAINS BASALT, COLUMBIA RIVER BASALT GROUP (CRBG) IDENTIFIED IN WESTERN WASHINGTON

Geologic mapping in the Abernathy Creek and Germany Creek valleys, adjacent to the Columbia River, in western Washington has identified a CRBG flow as the basalt of Huntzinger on the basis of chemical composition (8 analyses), magnetic polarity, stratigraphic position and lithology. Preliminary field mapping indicates that the Huntzinger flow is up to 30 m-thick and it originally covered at least 75 km² in this area. This flow appears to pinch out near the Columbia River and several kilometers west of Abernathy Creek, but its extent to the north, and east of Germany Creek has not been determined. The Huntzinger flow occurs between two fluvial sedimentary interbeds that thicken north of the Columbia River for about 6 kilometers. The lower interbed (up to 60 m-thick) overlies a Sand Hollow flow of the Frenchman Springs Member (Wanapum Basalt). The upper interbed (up to 25 m-thick) underlies the Pomona Member (Saddle Mountains Basalt). Published radiometric ages for Pomona, Huntzinger, and Sand Hollow units are 12, 13, and 15.3 Ma respectively. Interbed and flow thicknesses suggest that this area was subsiding during Saddle Mountain time. This discovery represents the only known occurrence of the Huntzinger flow west of the Cascade Range. The Huntzinger flow followed an ancestral Columbia River canyon through the Cascade Range and spilled out onto a lowland underlain by fluvial sediments accumulated in a structural depression active at least between 12 and 15 m.y ago.