Abstract

Two pumiceous tephra layers, widespread in meadow topsoils of the southern Sierra Nevada, are correlated on the basis of radiocarbon dates and trace-element analyses with two eruptive centers at the northern and southern ends of the Mono Craters—Inyo craters volcanic chain in eastern California. Pumice and obsidian that were erupted in the northern part of the chain are uniform in trace-element content, whereas those erupted from the southern part are nonuniform and distinctly different, particularly in Sr content. Similar differences are recognized in the two most recent and widespread tephra layers originating from these sites. These tephra layers are the deposits of the most recent explosive eruptions of magma from the Mono Craters and the Inyo craters.

Tephra 1, characterized by sanidine microphenocrysts and a Sr content of about 215 ppm, was erupted 720 ± 60 yr B.P. Its distribution defines a south-trending lobe extending over the Sierra Nevada from the upper San Joaquin drainage area to the Little Kern drainage area. Sr, Rb, and Zr contents of the ash are similar to those of a tephra-ringed obsidian dome at the south end of the Inyo craters.

Tephra 2, characterized by a lack of microphenocrysts and a Sr content of less than 20 ppm, was erupted 1190 ± 80 yr B.P. It is encountered as a fine ash layer in the Sierra Nevada from northernmost Yosemite to Kings Canyon. Its low Sr content indicates geochemical affinity with the Mono Craters. Panum Crater, a tephra-ringed dome at the north end of the chain, appears to be its most likely source vent.

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