ISOTHERMAL PLATEAU FISSION-TRACK AGE AND REVISED DISTRIBUTION OF THE WIDESPREAD MIDPLEISTOCENE ROCKLAND TEPHRA IN WEST-CENTRAL UNITED-STATES

A tephra bed in southeastern Idaho is correlated to Rockland tephra, a widespread silicic air-fall unit previously identified in northern California and western Nevada. Correlation of proximal Rockland tephra and its distal correlative is based on glass chemistry determined by electron microprobe and neutron activation analysis. An age estimate of proximal Rockland tephra is precisely determined by application of the isothermal plateau fission-track (ITPFT) technique to its hydrated glass shards. A weighted mean age of 0.47 +/- 0.04 Ma compares well with age estimates based on co-existing mineral phases using the external detector fission-track and additive-dose thermoluminescence techniques. The presence of Rockland tephra 800 km east of its presumed eruptive source will improve the development of detailed chronologies and correlation of Pleistocene sedimentary sequences located in coalescent intermontane basins of west-central United States.