A FUSED TUFF IN THE TERTIARY GRANDE RONDE BASALT, COLUMBIA RIVER BASALT GROUP, WEST-CENTRAL IDAHO

A previously undocumented 1.6 m thick rhyolitic fused tuff was identified between two flows of Grande Ronde basalt within the Clearwater Embayment of west-central Idaho. The fused tuff is vitric, variably vesicular, and partly spherulitic with distorted black and dark gray layering. It represents a partially reworked airfall ash that rests on the Buck Springs Member (R1) and is overlain by the Downey Gulch Member (N1). Accurate identification of this ash will aid in the construction of a precise geochronology for the lower part of the Grande Ronde in the Clearwater Embayment. The tuff is a peralkaline, high-silica rhyolite with chemical affinities to rocks associated with McDermitt caldera, approximately 500 km to the SSW, and we tentatively ascribe it to that source.

Cross-sectional geometry of the fused tuff and associated volcanic units in outcrop is suggestive of a paleochannel in a sedimentary interbed. The sequence of emplacement of the volcanic units in the sedimentary interbed began with the erosion of a paleochannel into the interbed and upper portion of the underlying Buck Spring flow. The tephra was then deposited in the bottom of the paleochannel. The upper 0.8 m of the air-fall tephra was reworked and mixed with coarse-grained sand. A basaltic andesite flow was emplaced over the tephra, heating and fusing it into a coherent mass. Emplacement of the basaltic andesite caused secondary mobilization of the fusing tephra resulting in the distorted layering (flow-banding) observed in the fused tuff. The basaltic andesite also partly mixed with the rhyolitic liquid to produce a thin zone of dacite.