Uranium-trend systematics for dating Quaternary sediments
1985; United States Department of the Interior; Linguagem: Inglês
10.3133/ofr85298
ISSN2332-4899
Autores Tópico(s)Geological and Geochemical Analysis
ResumoA new concept in uranium-series dating called uranium-trend dating has been tested extensively over the past several years to determine the reliability of this technique in estimating the time of deposition of surficial deposits ranging in age from 5,000 to about 800,000 years B.P. The open system dating technique consists of determining a linear trend from analyses of three to ten samples covering various layers or soil horizons formed in a given depositional unit.In each sample an accurate determination of the concentrations of 238 U, 231*U, 230 Th, and 232 Th is required.Whole-rock samples are used and analyses are made on subsamples of less than 2 mm-size fraction.Isotopic concentrations are determined by alpha spectrometry utilizing radioisotope dilution techniques.The analytical results are plotted where ( 238 U-230 Th)/ 238 U vs. ( 231*U-238 U)/ 238 U ideally yields a linear array in which the slope of the line of best fit changes in a systematic way with the increasing age of the deposit.The rate of change of slope is determined by the half-period of uranium flux, F(0), in the local environment.The flux consists of a mobile component of uranium that migrates either in solution or on colloids that through time slowly works its way through void spaces between mineral grains and produces a time-dependent trail of the daughter products, 231*U and 230 Th.An empirical model is based on the distribution of the trail of daughter products in the sediment, and the model compensates for changes in F(0) in response to different lithologies, textures, and climatic regimes.Analyses of units with known ages of deposition are required for time calibration of the empirical model; calibrations at 5,000 years, 12,000 years, 150,000 years, 600,000 years and 730,000 years were provided by correlations with deposits dated by radiocarbon and K-Ar.At best, the uranium-trend ages have a potential estimated accuracy of about ± 10 percent for deposits older than 100,000 years; however, the uncertainty in the slope is strongly dependent on the quality of the linear trend regarding scatter of data points and the length of the slope line.Percentage errors in the ages are not symmetric throughout the range; they are greater both for young ( 600,000 years) deposits.Quaternary deposits from a variety of climatic and depositional environments were analyzed in this investigation.These deposits include alluvium, colluvium, eolian sand, till, loess, zeolitized volcanic ash, and coastal marine sediments.Most of the deposits have been modified by pedogenic processes.Depending on local climatic conditions and lithologic compositions, these soils range from aridosols to spodosols.Some of the soils are buried such as coastal marine terrace paleosols.Owing to the need to explain the methodology and the results of U-trend dating as well as the influence of various geomorphic, climatic, and lithologic factors in various study areas, this report is divided into several chapters.These include an introductory chapter on the methodology of U-trend dating followed by separate chapters with different co-authors that describe the results for (a) time calibration units and deposits in the Rocky Mountain region, (b) alluvial deposits in the San Joaquin and Sacramento Valleys of California, (c) deposits in the Rio Grande and Pecos Valleys of New Mexico, (d) Quaternary deposits on the Nevada Test Site area of Nevada and California, (e) effects of fault disturbance on uranium-trend dating of calcareous alluvium on the Nevada Test Site, (f) marine deposits on the southeastern Atlantic Coastal Plain and (g) marine terrace deposits from southern California and Barbados, West Indies.
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