GEOLOGIC MAPPING AND TEPHROCHRONOLOGY OF SEDIMENTS ASSOCIATED WITH THE MIOCENE WEISER BASALT AND PAYETTE FORMATION IN HOLLAND GULCH, NORTHWESTERN MARGIN OF THE WESTERN SNAKE RIVER PLAIN, IDAHO

The Holland Gulch Quadrangle on the northwestern margin of the western Snake River Plain near Weiser contains the Weiser basalt of Fitzgerald (1981) and one of the type sections for the Payette Formation of Kirkham (1931), and an angular unconformity overlain by Idaho Group sediment. The stratigraphy is complicated by normal faulting, but we have identified a distinctive ash bed within the fossil leaf and lignite-bearing silty sediments of the lower Payette Formation. The distinctive ash occurs within sediment 70 m above the reversed-polarity basalt of Linson Gulch. 12 ash samples were collected from sedimentary sections in the quadrangle. Glass shards from each sample were analyzed by electron microprobe for Si, Ti, Al, Fe, Mn, Mg, Ca, Ba, Na, K, Cl, and O. Ash samples from four localities in and near Linson Gulch appear identical based on the glass shard composition and pattern of shard-to-shard variation revealed by the microprobe analysis on the weight percent of calcium and iron components. The ash composition was statistically compared to all other Neogene fallout and ash-flow tuffs in the University of Utah tephrochronologic database (Perkins, et al, 1998, GSA Bulletin, 110; p 344-360) and is a good match for the 14.93 ± 0.08 Ma Obliterator Ash of Downing (1992), which was Ar-Ar dated at Succor Creek south of the western plain. Other stratigraphic sections that contain the Obliterator Ash are the Bruneau River north of the Nevada-Idaho border, and in Nevada at Stewart Valley and Huntington Creek sections of Perkins et al (1998). The 14.93 Ma date is a surprisingly old age for the Payette Formation that will be checked by Ar-Ar dating in progress on basaltic andesite units associated with the sediment to help establish a geochronology for the Weiser Basalt and Payette Formation in this area.