In current studies by the U.S. Geological Survey along the Snake River from Hagerman to King Hill (fig. 1), one of the difficult stratigraphic problems in dividing the youngest rocks has been to determine the position of two local lava flows, the McKinney Basalt and its immediate predecessor the Wendell Grade Basalt, with, respect to the Melon Gravel that was deposited by the catastrophic late Pleistocene Bonneville Flood. Until recently, these lava flows were interpreted on the basis of equivocal field relations to be younger than the Melon Gravel and were assigned to the Recent (Malde and Powers, 1962, p. 1217). Now, however, the McKinney and Wendell Grade Basalts are known to be older than the Melon Gravel and are accordingly dated as Pleistocene. Moreover, the McKinney is now believed to include lava previously classified separately as Bancroft Springs Basalt (Malde and Powers, 1962, p. 1216). (The revised age of the McKinney Basalt was briefly mentioned by Malde, 1964, footnote, p. 204.) This report explains the geologic and paleomagnetic evidence by which this revised stratigraphy has been learned and then applies this knowledge to a new analysis of late Pleistocene history along this segment of the Snake River.
The analysis of the river history deals with the entrenchment of the Snake River canyon and with the inferred routes of ancestral canyons that are recorded by various lava flows and river deposits of the Snake River Group. Thick deposits of pillow lava a few miles downstream from Hagerman are explained as a facies of the McKinney Basalt that accumulated in a temporary lake 500 feet deep; the lake was impounded by another part of the McKinney Basalt in the Snake River canyon of that time. This pillow lava, though previously known to be older than the Melon Gravel (part of the Bancroft Springs Basalt as used by Malde and Powers, 1962, p. 1216), has not been heretofore explained from a knowledge of the canyon history.
The former and present classifications of the Snake River Group are compared in figure 2.
My work along the Snake River has been shared by Howard A. Powers, who first opened my eyes to the role played by local lava flows in modifying river history. He did much of the geologic mapping on which my present understanding of the Snake River Plain depends. We have spent so much time together discussing the perplexing matters that led to this report, both at the outcrops and in the office, that many of the ideas presented here are as much his as mine. Even so, especially because the features to be described stem mainly from geomorphology rather than volcanology, I take responsibility for whatever faulty deductions are found in these pages.
Last Updated: 28-Mar-2006