CORRELATION OF LAVA PREVIOUSLY CALLED BANCROFT SPRINGS BASALT
Study since 1962 indicates that lava which fills a former canyon of the Snake River at Bancroft Springs, 5 miles south-southeast of King Hill (fig. 8), and which also forms the north rim of the present canyon as far upstream as the mouth of Malad Canyon, is McKinney Basalt as originally defined by Stearns (1936). The name, Bancroft Springs Basalt, which was previously applied to this lava (Malde and Powers, 1962, p. 1216), is hereby abandoned for reasons given in this discussion. Also, pillow lava between Bliss and Malad Canyon, which was formerly included in the Bancroft Springs Basalt, is here identified as a facies of the McKinney. This new assignment of the pillow lava is discussed in the next section, and the assignment is then incorporated in subsequent remarks on the river history. Paleomagnetic measurements compatible with this correlation are discussed in a separate section by Allan Cox.
Assignment of the canyon-filling lava at Bancroft Springs to the McKinney Basalt stems from my liberation from faulty concepts about the age of the McKinney and from a better comprehension of certain features of the ancestral physiography.
The canyon-filling lava forms an imposing basalt cliff 300 feet high at Bancroft Springs and is obviously older than the present canyon (fig. 9). The lava is also older than the Melon Gravel, which in places litters the lava surface with boulders as large as 15 feet. (See also, Malde, 1968, fig. 21.) Until George Stone and Howard Powers determined that a downstream lobe of McKinney Basalt that cascaded from the upland is also older than the Melon Gravel, I thought that this lobe represented an eruption younger than the lava at Bancroft Springs and that it reached the Snake River after the present canyon was cut. (The downstream lobe occupies the area called The Pasture in fig. 8.) This idea evidently was a misconception. I now infer that the lava at Bancroft Springs and the downstream lobe were formed concurrently by the eruption of McKinney Basalt and that both are older than the present canyon. The correlation of these bodies of lava cannot be proved from available outcrops because any possible link between them along the canyon is concealed by Melon Gravel. Nevertheless, their former connection can be inferred from the route of the ancestral canyon and, also, from the continuity of the surface gradient of the lava that extends downstream from Bancroft Springs. These features can be traced in a series of geologic sections drawn across the path of the ancestral canyon (fig. 10).
In section AA', the former canyon is shown to be completely filled with McKinney Basalt, which at one time undoubtedly extended across the area of the present canyon to the small remnant of lava on the southwest side. The former canyon, with a floor at an altitude of about 2,575 feet, is intersected by the present canyon at Bancroft Springs (section DD') and probably continues northwest into an area covered by Melon Gravel. Sections FF' through II' show the possible presence of concealed remnants of the canyon-filling lava and indicate the inferred gradient of the former canyon floor. Beyond section II', the former canyon is again intersected by the present Snake River, and the McKinney Basalt is underlain by basaltic gravel, as previously mentioned. Finally, the former canyon approximately coincides with the present canyon near section KK' where the McKinney reaches present river level.
The surface gradient of lava between Bancroft Springs and The Pasture can be measured from the cross sections and from the geologic map. For convenience, the altitudes and the indicated gradients are given in table 1. Because the original surface of the lava is preserved, except for a few feet of local stripping by flood erosion, the present altitudes closely indicate the original gradient. North of section DD', the lava surface descends rather uniformly at a gradient of about 2 percent, and the projected surface corresponds with the surface of the McKinney Basalt at section GG'. Thereafter, the surface gradient descends more gradually.
TABLE 1.Altitudes and gradients of surface of McKinney Basalt southeast of King Hill
In summary, the former canyon was deeply filled by McKinney Basalt at Bancroft Springs and was filled only to shallow depth downstream at The Pasture; the lava surface probably was graded continuously between these two localities. The differences in thickness of the McKinney Basalt between Bancroft Springs and The Pasture appear to be a consequence of the steep gradient of the lava surface. At The Pasture, McKinney Basalt in the canyon was joined by a lava cascade that spilled simultaneously from the adjacent upland.
Last Updated: 28-Mar-2006