THE SHUMAKER CREEK MEMBER AND BASALT OF POWATKA: TWO HIGH-BA WANAPUM BASALT FLOWS, COLUMBIA RIVER BASALT GROUP

The Shumaker Creek Member and the Basalt of Powatka are small volume flows (≈25 to 30 km³) that overlie the Frenchman Springs Member, Wanapum Basalt, in southeastern Washington and northeastern Oregon. Swanson et al. (1979, USGS Bull. 1457-G) informally named the basalt of Shumaker Creek; subsequently, Hooper et al. (1995, Can. J. Earth Sci., v. 32, p. 410-423) elevated it to member status. Wright et al. (1979, USGS OFR 79-711) identified a 23-meter wide Shumaker Creek dike in Joseph Creek canyon, northeastern Oregon. Shumaker Creek lavas occur in two lobes extending northward from the crest of the Blue Mountains in southeastern Washington. These two lobes appear to have flowed around a topographic high associated with older N₂ Grande Ronde basalt vents. A third lobe extended a short distance into the northeastern Troy Basin in Oregon.

Ross (1978, Ph.D. thesis, Idaho) first described and informally named the Basalt of Powatka for exposures near Troy OR. Swanson et al. (1981, USGS OFR 81-797) recognized that it was present throughout much of the Troy Basin and extended as far south as Elgin OR and as far west as the crest of the Blue Mountains near Tollgate OR. The eruptive source for the Powatka has not been recognized. Recent fieldwork near Troy OR, integrated with geochemical analyses, established that the Basalt of Powatka is younger than the Shumaker Creek Member. Field relationships suggest that the older Shumaker Creek lavas probably formed a topographic barrier that limited the eastward movement of the younger Powatka flows in the eastern Troy Basin.

The Shumaker Creek and Powatka lavas are basaltic andesites; both are notable for their elevated P₂O₅ and Ba (P₂O₅: ≈1.005 and ≈1.229 wt%; Ba: ≈1200 and ≈1085 ppm, respectively) and lower TiO₂ (≈2.45 and ≈2.60 wt%, respectively) content compared to the voluminous Frenchman Springs, Roza, and Priest Rapids Members of the Wanapum Basalt. Incompatible element ratios (i.e., Zr/Nb, Zr/Y) and normalized trace element patterns for the Shumaker Creek and Powatka lavas are similar to those observed in the voluminous Wanapum members, suggesting derivation from similar source materials; however, the higher Ba concentrations at similar incompatible element ratio values suggest that they may have formed by lower degrees of partial melting.