Lake Idaho: New Perspectives Through Basalt Stratigraphy

Margaret D. Jenks, Bill Bonnichsen

Since the earliest geological investigations in Idaho, researchers have speculated on the existence of a large lake in the western Snake River plain. O. C. Marsh, writing in King's 1878 report on the geological exploration of the 40th parallel, suggested, based on fish paleontology, the presence of a large lake covering parts of southern Idaho and Oregon. Recent investigators of sediments and fossils have debated the size of the lake, even suggesting a series of small lakes in a broad river valley. Our mapping of basalt units in the northern Bruneau River canyon suggests that a large, permanent lake indeed existed, and that toward the end of its evolution during the Pliocene may have had a highstand elevation of 3,600-3,800 ft. Lake margin features are preserved by the in ividual basalt units that were changed in character as they flowed into the lake. This change from solid basalt to basalt rubble and boulders enclosed within a dark disaggregated matrix is present in successively younger units that flowed northwestward from volcanoes to the south. Stratigraphic evidence of successively younger flows, emplaced at continually higher elevations, suggests that the lake gradually filled and that the lakeshore transgressed southward. The regressive facies of the lake is preserved in the gravel sequences that are present at the mouths of present-day river canyons, whose ancestral drainages debouched into the slowly draining lake. From the undeformed lake-margin features present throughout the region, Lake Idaho apparently occupied the western Snake River plain epression, and was connected to a series of lakes in eastern Oregon. The configuration of these lakes strongly suggests that this lake system, prior to capture by the Snake River through Hells Canyon, may have drained through the present Grand Ronde River system.

AAPG Search and Discovery Article #91040©1987 AAPG Rocky Mountain Section Meeting, Boise, Idaho, September 13-16, 1987.