History of Snake River Canyon indicated by revised stratigraphy of Snake River Group near Hagerman and King Hill, Idaho

Abstract

A discovery that debris left by the Bonneville Flood (Melon Gravel) overlies McKinney Basalt about 200 feet above the Snake River near King Hill requires that the stratigraphy of the Snake River Group be revised. In former usage, the McKinney Basalt and its immediately older companion, the Wendell Grade Basalt, were considered on the basis of equivocal field relations to be younger than the Melon Gravel and were assigned to the Recent. These lava flows are here reclassified as Pleistocene. The Bancroft Springs Basalt, which consists of both subaerial lava and pillow lava in a former Snake River canyon, was previously separated from the McKinney but is now combined with the McKinney. Accordingly, the name Bancroft Springs Basalt is here abandoned. This revised stratigraphy is first described from geomorphic relations of the McKinney Basalt near King Hill and is then discussed in the light of drainage changes caused by local lava flows during entrenchment of the Snake River. Near King Hill, a former Snake River canyon was completely filled by McKinney Basalt at the place called Bancroft Springs, hut the depth of this lava in the next several miles of the canyon downstream (along a route that approximately coincides with the present canyon) steadily decreased. This ancestral geomorphology is inferred from the former canyon route and, also, from the continuity in gradient of the McKinney lava surface downstream from Bancroft Springs. The drainage history recorded by various lava flows and river deposits of the Snake River Group indicates that the McKinney and Wendell Grade Basalts erupted after the Snake River canyon had reached its present depth of about 500 feet. The Snake River of that time, as far downstream as Bliss, flowed approximately along its present route. The Wood River of that time, however, skirted the north flank of Gooding Butte and joined the ancestral Snake at a junction, now concealed by lava, north of the present canyon about 3 miles west of Bliss. From that place the former Snake River canyon, also now concealed by lava, continued west to Bancroft Springs and thence along a route close to the present canyon to King Hill. To become entrenched in a canyon 500 feet deep, the Snake River downstream from Hagerman became progressively more incised while its upstream route was pushed south in several earlier canyons by intermittent lava flows. Distinctive gravel deposits help to establish the episodes of progressive canyon cutting and to determine the routes of ancestral drainage, including the former position of the Wood River. As canyon cutting continued, springs began to emerge where lavas had filled the earlier canyons. When the Snake River canyon eventually attained its approximate present depth, the Wendell Grade Basalt erupted near Shoshone and, as several tongues, spread west to the canyon rim opposite Hagerman. One tongue crossed the future route of the Wood River, and another covered an upland area of Sand Springs Basalt that had previously reached the canyon floor at Hagerman. The McKinney Basalt then erupted from McKinney Butte northeast of Bliss and spread southward as a subaerial flow, covering part of the Wendell Grade Basalt. It filled the ancestral Wood River canyon and the Snake River canyon of that time west of Bliss as far downstream as King Hill. The resulting dam of lava impounded a deep lake, which extended upstream in the canyon beyond Hagerman. Copious amounts of the McKinney spilled into this temporary lake and produced pillow lava. About 2 miles west of Bliss, pillow lava 500 feet thick completely fills the former canyon and is protected by rimrock of the subaerial McKinney Basalt. From Bliss, the pillow facies extends upstream as far as the McKinney rimrock - about 5 miles. Eruption of the McKinney Basalt diverted the Wood River to a course along the southeast edge of this lava flow. The temporary lake that was dammed by McKinney Basalt west of Bliss spilled along the sou