ABSTRACT

Recent mapping and laboratory studies indicate at least seven major ash-flow sheets on the northern and southeastern margins of the eastern Snake River Plain. Four of these sheets are located on the northern margin of the Plain between Howe Peak in the southern Lost River Range and Yellowstone National Park; three sheets are on the southeastern margin between Idaho Falls and Yellowstone National Park Preliminary correlations indicate that some of these sheets have similar caldera sources.

The lowest major ash-flow sheets on the northern margin of the eastern Snake River are the tuff of Howe Peak exposed in the southern Lost River and Lemhi Ranges and the tuff of Edie Ranch exposed in the southern Beaverhead and Centennial Mountains. These units are similar stratigraphically and petrographically as well as having similar magnetic polarities, but the distribution patterns of the tuffs indicate that they are distinct and have unique sources. The source for the tuff of Howe Peak is believed to be buried on the plain east of the Lost River Range whereas the tuff of Spring Creek, the lowest major ash-flow sheet on the southeastern margin of the plain, erupted from the Rexburg caldera complex approximately 6.3 m.y. ago. Preliminary mapping and laboratory studies indicate that the tuff of Edie Ranch may be equivalent to the tuff of Spring Creek and may represent a distal facies of an extensive ash-flow sheet The tuff of Howe Peak is unique and does not correlate with the tuffs of Spring Creek or Edie Ranch.

The middle major stratigraphic unit on the northern margin of the eastern Snake River Plain is the tuff of Blue Creek which is interpreted to be equivalent to the tuff of Elkhorn Spring on the southeastern margin. Petrographic characteristics and magnetic polarity of these ash-flow sheets are similar. Volcanic facies relationships indicate that the tuff of Blue Creek represents the near-source facies of a regionally extensive, large-volume ash-flow sheet; the tuff of Elkhorn Spring is part of this sheet but is a distal facies which was ponded within the Rexburg caldera complex. Facies relations suggest a buried caldera south and east of the Lemhi Range and south of the Beaverhead Mountains as source for these units.

The youngest major ash-flow sheets on the eastern Snake River Plain are the tuff of Kilgore exposed on the northern margin of the plain, and the tuff of Heise, exposed on the southeastern margin. The tuff of Kilgore and the tuff of Heise are interpreted as equivalent units on the basis of similar petrographic characteristics, magnetic polarities, and facies relations. Preliminary mapping indicates that the source for these units is a buried caldera located near Kilgore, Idaho.