EVIDENCE FOR FLOW INFLATION AND INTRAFLOW CHEMICAL VARIATION OF A FRENCHMAN SPRINGS BASALT OF THE COLUMBIA RIVER BASALT PROVINCE

East of Wallula Gap, Swanson et al. (1971) mapped a dike of the Ice Harbor Member of the Columbia River basalts. Subsequent quarrying revealed a Umatilla flow cut by a ~16 m wide exposure of fault breccia whose further margin is bordered by discontinuous, nearly glassy basalt that apparently grades into a 17 m high exposure of basalt characterized by vivid rhythmic banding created by color horizons in the upper 9 m. Two hypotheses may explain this exposure: the banded unit is a near- surface dike and the upper 9 m is the remnant of a lava pond. In this case, the thin glassy basalt represents a chill margin. Alternatively, the banded unit is an inflated flow cut by a fault and both the fault breccia and the banded unit were later intruded by a dikelet. Chemical compositions and vesicle distributions obtained from a vertical transect within the central banded unit support the second hypothesis. The banded basalt has major and trace element characteristics that place it within the Frenchman Springs Member of the CRB. The thin glassy basalt appears to be a dikelet of the Goose Island Member that intruded the breccia. The vertical transect of the bottom 8 m of the exposed Frenchman Springs unit shows rhythmic vesicle layering with multiple horizons of small, spherical vesicles (<2.0 mm dia.) and elongated larger vesicles (dia. >10 mm). From 4.5 to 5.6 m above the base of the quarry this banded Frenchman Springs outcrop shows internal chemical variation in SiO2, MgO, FeO, K2O, and Ba, Zr, and Y as well as other elements. The nature of the chemical variation and multiple vesicle horizons is consistent with this being an inflated flow.