The geology of the Tuff of Bridge Spring: Southern Nevada and northwestern Arizona

Shirley Ann Morikawa, University of Nevada, Las Vegas


The Tuff of Bridge Spring (TBS) is a regionally-widespread, andesite to rhyolite (59.50 to 74.91 wt. %) ash-flow tuff of mid-Miocene age (ca. 15.2 Ma) that is exposed in the northern Colorado River extensional corridor of southern Nevada and northwestern Arizona. Determination of the areal distribution, geochronology, lithology, geochemistry, and internal stratigraphy of the TBS is important for its establishment as a reliable stratigraphic reference horizon for tectonic reconstructions of the extensional corridor during the middle Miocene. Based on reoccurring patterns of major and trace element variation, the TBS is divided into constant Cr/variable SiO{dollar}\sb2{dollar} and variable Cr/variable SiO{dollar}\sb2{dollar} chemical members. Reconciliation of chemical member assignments and regional stratigraphic relationships allows the division of the TBS into three stratigraphic members. The regionally-extensive nature of a Zr/Ti vs. Ba chemical horizon in the TBS suggests that its chemical signature is magmatic in origin; The presence of linear isotopic arrays of Nd/Rb/Pb plots, regionally-consistent geochemical trends, and disequilibrium textures in feldspars in the TBS suggests it was formed by magma mixing processes which involved the injection of mafic magma into a normally-zoned felsic magma chamber. The Nd/Rb/Pb isotopic signature of the Tuff of Bridge Spring suggests that the TBS may be cogenetic with either the Aztec Wash pluton, Nevada or the Mt. Perkins pluton, Arizona. Comparison of the isotopic signatures of the TBS with tuffs collected from Salt Spring Wash, Arizona, and the Lucy Gray Range, Nevada suggests that these tuffs are not cogenetic with the TBS. Incremental release {dollar}\sp{40}{dollar}Ar/{dollar}\sp{39}{dollar}Ar analysis of the tuff of Dolan Springs (16.09 {dollar}\pm{dollar} 0.15 Ma) suggests it was derived from a different source than the TBS.