Geological Survey Professional Paper 644—F History of Snake River Canyon Indicated by Revised Stratigraphy of Snake River Group Near Hagerman and King Hill, Idaho

Surveys between the years 1954 and 1961 established that the McKinney Basalt erupted from McKinney Butte 8 miles northwest of Gooding and that the Wendell Grade Basalt came from Notch Butte 4 miles south of Shoshone (Malde and others, 1963). As these events were then understood, the McKinney Basalt flowed south toward the Snake River, then west in the upland north of the canyon rim, and finally cascaded from the upland to present river level near King Hill; the Wendell Grade Basalt flowed west and reached the canyon rim at several places near Hagerman (fig. 3). Exposures 5 miles northeast of Hagerman along the Malad River (which is the local name for the lower reach of the Big Wood River) showed that the McKinney overlies the Wendell Grade. Because both the McKinney and the Wendell Grade lavas retain rough surfaces that are only partly mantled by windblown surficial material (fig. 4)—in contrast to contiguous older lavas for which the original surface roughness has been almost entirely obliterated—and because of erroneous interpretations explained below, the McKinney and Wendell Grade were assigned to the Recent.

FIGURE 3.—Generalized geologic map of area between Hagerman and King Hill. Geology modified from Malde, Powers. and Marshall (1963). Base from U.S. Geological Survey Twin Falls quadrangle. 1955-63, and Halley quadrangle, 1955-62. (click on image for an enlargement in a new window)

FIGURE 4.—Pressure ridges of McKinney Basalt, which are partly covered with surficial material. View is north toward McKinney Butte from the railroad 5-1/2 miles east of Bliss.

A discovery made in 1962 by George Stone, who was then collecting petrographic samples with Howard Powers, showed that the McKinney Basalt, and hence the Wendell Grade Basalt, is older than the Melon Gravel and is therefore of Pleistocene age, in spite of its youthful appearance. The decisive geology was found along the Union Pacific Railroad 4 miles south east of King Hill at a site 200 feet above the Snake River. There, the Bonneville Flood scoured McKinney Basalt to make the intricate anomalous topography known as scabland (fig. 5), and it left well-rounded boulders of Melon Gravel on the eroded McKinney, some of which were more than 5 feet long (fig. 6). Prior to this discovery, I had recognized stream erosion of nearby McKinney Basalt no higher than 125 feet above the river and thought that this erosion could have been produced by overflow where the McKinney had dammed the Snake. My other previous interpretations about the relation of the McKinney Basalt to Melon Gravel, which I will now summarize, were also wrong. I had erred in two places.

FIGURE 5.—Scabland surface of McKinney Basalt about 200 feet above the Snake River 4 miles southeast of King Hill. Part of the basalt was removed by the Bonneville Flood, and the remaining lava was extensively corroded, rounded, and polished.

FIGURE 6.—Basalt boulder of Melon Gravel more than 5 feet long on McKinney Basalt 175 feet above the Snake River 4 miles southeast of King Hill. Another boulder, nearly buried by surficial sand, is at the right.

First, along the Snake River 3 miles southeast of King Hill, the McKinney Basalt overlies basaltic cobbles and boulders formerly interpreted to be Melon Gravel (Malde and Powers, 1962, p. 1217)—the only local gravel deposit then known to consist solely of basaltic debris. The length of outcrop of this gravel amounts to less than a mile, much of it being obscured by talus from the overlying basalt, but sections of the gravel several feet thick are locally well exposed where protected by an overhang of lava (fig. 7). I now discern subtle differences between this deposit and typical Melon Gravel. The basaltic gravel under the McKinney is subangular, rather well sorted, more cobbly than bouldery, and lacks interstitial sand. In contrast, the nearby Melon Gravel is rounded, poorly sorted, more bouldery than cobbly, and contains much basaltic sand. Because of the unequivocal evidence found by Stone and Powers (that is, boulders of Melon Gravel lying on McKinney Basalt), this gravel below the McKinney must be older than the Melon Gravel.

FIGURE 7.—Basaltic gravel under McKinney Basalt on right bank of the Snake River 3 miles southeast of King Hill. Pieces of the gravel are frozen in the base of the lava.

Second, the terminus of the McKinney Basalt near King Hill, in the area known as The Pasture (fig. 8), was formerly interpreted as several small tongues of lava that had descended shallow troughs in the Melon Gravel (Malde and Powers, 1962, p. 1217), but the physiography is now understood as subdued bars of Melon Gravel on the McKinney.

FIGURE 8.—Generalized geologic map of area along Snake River southeast of King Hill showing inferred route of former canyon of Snake River, which is filled with McKinney Basalt. Cross sections (A-A' to K-K') are shown in figure 10. (click on image for an enlargement in a new window)

Observations by Powers (in Malde and Powers, 1962, p. 1217), about the stratigraphic position of the Wendell Grade Basalt with respect to Melon Gravel, in the light of present knowledge, were also deceptive. Powers found a place 2 miles east of Hagerman where Wendell Grade Basalt had cascaded from the canyon rim onto landslide debris. Because this landslide closely resembles another landslide 2-3 miles northwest that overlaps Melon Gravel (Malde, 1968, fig. 19), Powers decided that the Wendell Grade Basalt was also younger than Melon Gravel. From present evidence, this conclusion is false. The McKinney Basalt clearly overlies Wendell Grade Basalt on the Malad River, as previously mentioned, and both are older than the Melon Gravel.