EQUIVALENCE OF PILLOW LAVA NEAR BLISS TO McKINNEY BASALT
Another stratigraphic problem that has been clarified by studies made since 1962 concerns the age and origin of pillow lava that extends discontinuously upstream along the Snake River canyon from below Bliss to the mouth of Malad Canyon. The outcrops of pillow lava reach from present level near Bliss (about 2,650-ft. alt.) to places 500 feet higher and can be accounted for only by lava that entered a deep lake. Such a lake could have formed only behind a lava dam.
Russell (1902, p. 113-116) recognized pillow lava in the canyon at Bliss and compared these outcrops with subaqueous lavas in the Hawaiian Islands. Stearns (1936, p. 439-441), using the now-abandoned name of Bliss Basalt, considered the pillow lava as possibly a subaqueous phase of the McKinney Basalt. (See also, Stearns and others, 1938, p. 76-78). His stratigraphic placement of the McKinney, however, was incorrect. Malde and Powers (1962, p. 1216) assigned the pillow lava to a supposed early lava from McKinney Butte, the Bancroft Springs Basalt, but did not explain the canyon history that accounts for the deep lake in which the pillow lava was deposited.
As determined from petrography and geographic proximity, two possible sources of the pillow lava exist: McKinney Butte and Gooding Butte (fig. 3). The pillow lava, together with lava from both these vents, is fine-grained porphyritic plagioclase-olivine basalt, having plagioclase laths as much as 7 millimeters long that are clustered with olivine crystals 1-2 millimeters across. The petrography of these lavas differs from that of all others nearby. Gooding Butte, however, can be rejected as a potential source, because its time of eruption must have been earlier than the entrenchment of the ancestral Snake River to the depth represented by the pillow lava; the much younger McKinney Butte is therefore the only feasible source. This conclusion is derived from rather detailed knowledge of the canyon history, as described in the next section. Briefly, from an analysis of drainage history based on the effect of local lavas during canyon entrenchment, I infer that a dam of McKinney Basalt filled a former canyon of the Snake River and that the pillow lava accumulated simultaneously by the spilling of McKinney Basalt into the resulting lake. This interpretation can be easily tested by drilling about 3 miles west of Bliss, where the lava dam of McKinney Basalt is inferred to be concealed.
Last Updated: 28-Mar-2006