Correlation and source direction of Green River Formation Tuffs in San Pete County, Utah

Tuffs in the Eocene Green River Formation from San Pete County in central Utah have been examined in three detailed stratigraphic sections and fourty-one petrographic thin sections. The Green River Formation can be divided into a lower member dominated by mudstone and an upper unit dominated by limestone, both of which are mainly lacustrine in origin. Interbedded throughout the formation are numerous, thin, air-fall tuffs. Primary tuff minerals in order of decreasing abundance are biotite, hornblende, potash feldspar, and quartz with accessory muscovite and sphene. Secondary (diagenetic) minerals include calcite, zeolite, pyrite, and clays. Glass shards are rarely recognizable since virtually all glass has been altered to zeolites and related minerals. Nevertheless, it is believed that the mode of the tuffs was originally dominated by glass particles. Criteria for correlation of tuff beds include petrographic characteristics, hornblende:biotite ratios, the ratio of total primary minerals to combined groundmass and secondary minerals, stratigraphic interval, and thickness. Based on the correlation of three prominent tuff beds, it is concluded that the contact between the upper and lower members of the Green River Formation is distinctly time transgressive, and that the source areas for volcanic detritus is north or northeast of the San Pete Valley.No embarg