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Description
Eastern Oregon lies close to the boundary of the Western Snake River Plain Province, the Columbia River Basalt Province, and the Basin-and-Range Province. These three coeval provinces contain extensive Miocene-to-younger volcanic rocks; however, the relationship between them is debatable. As a contribution to the understanding of the tectonomagmatic evolution of this region, portions of the Winnemucca Creek and Mosquito Mountain 7.5-minute quadrangles have been mapped at 1:24,000 scale. The map area lies within the Burns quadrangle (1:250,000) and includes the town of Riverside. Mapping, supplemented by forty-two whole-rock analyses, twenty-seven thin-section descriptions, and one Ar-Ar age forms the basis for developing a coherent volcanic stratigraphy and interpreting the geologic evolution. The generalized volcanic stratigraphy of the Bums quadrangle has been subdivided by this study into newly recognized units and the regional stratigraphic position of the entire sequence has been resolved. The volcanic stratigraphy is chemically distinct and younger than the tholeiitic Columbia River and Steens Mountain basalts, which are exposed to the north and south, respectively. The close proximity of the older tholeiites suggests that the volcanic succession was deposited in a basin or graben. The basaltic andesite of Street Canyon, the lowermost unit, is separated from the intracanyon lavas of the andesite of Riverside and the Big Curve flow by the Kool Spring formation, a bimodal sequence of pyroclastic deposits containing interbedded tholeiitic olivine basalt, a unique chemical type in the succession. Two regionally extensive HAOT lavas, the Drinkwater basalt and the Voltage flow, are the youngest flows in the study area. Dramatic differences in jointing and glass content differentiate the andesite of Riverside into two chemically identical flow types: the Sheep Rock and Blaylock Canyon flows. These distinct characteristics are consistent with variable cooling histories which suggest the Sheep Rock flow advanced down wet canyons and the Blaylock Canyon flow advanced down dry canyons. Major-, trace-, and rare-earth element analyses for the mildly alkaline to calc-alkaline basaltic andesite of Street Canyon and andesite of Riverside suggest that they correlate with the Shumway Ranch Basalt, which outcrops to the north and east of the map area. The andesite of Riverside dated at 10.14 ± 0.23 is slightly younger. The transitional to tholeiitic lavas, the olivine basalt of the Kool Spring formation and the Big Curve flow, do not correlate locally. Crystal fractionation is most evident in the transitional to tholeiitic lavas; however, the mildly alkaline to calc-alkaline lavas may have experienced some degree of crystal fractionation. The geologic evolution of the study area is summarized in five progressive stages: (1) extrusion of successive mildly alkaline to calc-alkaline lavas (basaltic andesite of Street Canyon); (2) bimodal volcanism (Kool Spring formation); (3) uplift and erosion; (4) extrusion of intracanyon lavas (andesite of Riverside and Big Curve flow); and (5) sporadic extrusion of HAOT lavas (Drinkwater basalt and Voltage flow).
