Abstract
The Nomlaki Tuff, a widespread Pliocene marker bed is exposed near the base of the Tuscan and Tehama Formations. An eruption 3.27 Ma from the Yana Volcanic Center in Northern California was responsible for distributing this ash layer. Distal air-fall tuff exposures are found in Utah, New Mexico, and Death Valley California, while the maximum exposed welded and unwelded ash-flow tuffs are found in the Northern Sacramento Valley. Analyses of pumice were done at the University of California Davis Microprobe Laboratory on the Cameca SX 100. Stratigraphic deposits display an inverted stratigraphy. The more evolved pumice, defined as chemo-type 1, erupted first and was deposited stratigraphically lower than the second, more primitive eruptive unit, chemo-type 2. The zoned magma chamber model is the most plausible scenario explaining the geochemical patterns of the eruptive unit. Further evidence to support this model is the Eruption Temperature. Chemo-type 1 deposits had an average temperature of 849ºC, while chemo-type 2 deposits had an average temperature 957ºC. Geothermometric (Powell, and Powell, 1977) equilibrium temperatures were based on the calculated molecular fractions of ulvospinel and ilmenite from Carmichael (1967), Anderson (1968), Lindsley and Spencer (1982), and Stormer (1983). The zoned magma chamber model explains the similar Nomlaki-like glass compositions from the distal outcrops in Death Valley California documented by Knott (2006) as the different eruptive phases observed at the proximal locations.