Microprobe analysis of biotites - A method of correlating tuff beds in the Green River Formation, Colorado and Utah
Quantitative electron microprobe analyses of biotite grains for iron, magnesium, and titanium from tuff beds in the lacustrine Green River Formation (Eocene) of Colorado and Utah provide a tentative method of identification and a permissive stratigraphic correlation of tuffs. Tuff beds that have been identified and correlated by stratigraphic means were sampled at five localities in Colorado and Utah to determine if microprobe analyses of biotite could be used as a method of correlation. Although most of the original phenocrysts and glass shards of these pyroclastic beds have undergone extensive postdepositional alteration, biotite seems to have been unaffected. The iron, magnesium, and titanium contents of the biotite do not uniquely characterize individual tuff beds, but when the proportions of these elements are compared in a continuous stratigraphic sequence of beds, correlation of individual tuffs or groups of tuffs is possible over areas exceeding several hundred square miles. The method may be used for detailed correlation of pyroclastic beds where stratigraphic, faunal, and radiometric methods have been unsatisfactory.