Chemical analyses, correlations, and ages of upper Pliocene and Pleistocene ash layers of east-central and Southern California

Abstract

Eight widespread Pleistocene ash layers of east-central and southern California are characterized and correlated on the basis of chemical composition of volcanic glass (determined by neutron activation, electron probe, and energy-dispersive X-ray fluorescence analysis), stratigraphic criteria, and petrographic characteristics. Irt order of increasing age, these are the Lava Creek B ash bed (formerly referred to as the Pearlette type 0 ash bed; about 0.6 m.y.), the Bishop ash bed (0.73 m.y.), the Glass Mountain-D ash bed (estimated to be about 0.8-0.9 m.y.), the Glass Mountain-G ash bed (estimated to be about 1.0-1.1 m.y.), the Bailey ash (1.2 m.y.), the middle white ash of the Manix basin (estimated to be about 1.9 m.y.), the Huckleberry Ridge ash bed (formerly referred to as the Pearlette type B ash bed; about 1.9 m.y.), and the lowermost gray ash of the South Mountain area (Huckleberry Ridge? ash bed; estimated to be about 1.9 m.y.). Three chemical types of the Bishop ash are recognized and assigned to an informal Bishop ash-bed group. The Friant Pumice Member (of the Turlock Lake Formation) of east-central San Joaquin Valley is one of the members of this group. The apparent 0.1-m.y. difference in K-Ar ages of the Friant Pumice Member (0.6 m.y.) and the Bishop Tuff (about 0.7 m.y.) may be due to factors other than differences in the true ages of these tephra units; alternatively, there may be two ash layers that have very similar glass chemistry and are here recognized as belonging to the Friant Pumice Member, but differ in age by as much as 0.1 m.y. The Huckleberry Ridge ash in the Manix basin is correlated with the same ash in Meade County, Kansas, over a distance of about 1,500 km. Results of chemical analyses permit correlation of sedimentary strata deposited in diverse environments: marine, fluvial, and lacustrine. Chemical characteristics of ash layers also permit identification of source areas from which tephra was erupted; the main eruptive centers were the Long Valley-Glass Mountain area of east-central California, the Coso volcanic field of southeastern California, and the Yellowstone area of northwestern Wyoming and eastern Idaho. Systematic chemical depletion trends with time provide a possible independent method of estimating ages of ash layers of the Long Valley-Glass Mountain family.