Uranium-trend dating of Quaternary deposits in the Nevada Test Site area, Nevada and California

Abstract

The uranium-trend dating method has been used to estimate the ages of alluvium, colluvium, altered volcanic ash, and eolian deposits in the Nevada Test Site area. For dating of deposits of 5000 to 800,000 years age, the open-system technique consists of determining a linear trend from analyses of four to ten channel samples collected at different depths in a depositional unit, or in the soil profile formed in a depositional unit. The concentrations of {sup 238}U, {sup 234}U, {sup 230}Th, and {sup 232}Th are accurately determined for each sample where analyses are made on subsamples of the less-than-2-mm-size fraction. Isotopic concentrations are determined by alpha spectrometry utilizing radioisotope dilution techniques. Analyses of 36 sample suites are included in this report; U-trend dates were determined on 31 of these suites establishing the age ranges for deposition of four stratigraphic units at the Nevada Test Site. Median ages for these deposits indicate ages of 40 +- 15 Ka for Q2a sediments, 170 +- 40 Ka for Q2b sediments, 270 +- 50 Ka for the younger Q2c stratigraphic unit and 440 +- 60 Ka for the older Q2c unit. Q2s stratigraphic units range in age from about 200 to 500 Ka. Uranium-trend agesmore » of laminar carbonate deposits indicate the time of strong calcium carbonate development rather than the time of deposition of their older host sediments. 19 refs., 35 figs., 5 tabs.« less