CHARACTERIZATION AND DETAILED DESCRIPTION OF THE NOMLAKI TUFF, NORTHERN CALIFORNIA

The Nomlaki Tuff is exposed in the Cascade foothills and northern Sacramento Valley, California, and was deposited at 3.27 Ma (Knott & Sarna-Wojcicki, 2001). Emplacement of the Nomlaki Tuff was coeval with deposition of early debris flows in the Tuscan Formation and sediments in the Tehama Formation, and is used as an important chronostratigraphic marker in those deposits. Nomlaki Tuff exposures are distributed over ~ 5200 km2 with an average thickness of ~7 m (Anderson & Russell, 1939), which suggests an eruption of at least 35 km3. Exposures of the Nomlaki Tuff are welded at Bear Creek Falls (~37 km west of Lassen Peak) but are not welded at sites further south and west, suggesting the source area was located closer to Bear Creek Falls. The type locality of the Nomlaki Tuff is near Paskenta (~55 km SW of Bear Creek Falls); indicating pyroclastic flows traveled at least 55 km from the source, consistent with a large eruption. Currently, an eruptive center has not been identified other than an early suggestion that it was located 20 km west of Lassen Peak (Russell, 1932). There is no topographic or geologic evidence of a vent in that vicinity. The Latour Volcanic center, about 25 km NW of Lassen Peak is the appropriate age and may be the source area. Lack of evidence for a vent and indication of a large volume eruption has inspired new work to correlate reported exposures of the Nomlaki Tuff in Northern California. Preliminary work revealed the need for a definitive description of deposits for consistent identification. We use pumice mineralogy to confirm Nomlaki Tuff exposures. Pumices in outcrops identified as Nomlaki Tuff are white to gray, with plagioclase, pyroxene, and minor hornblende and quartz phenocrysts. Lithics are larger and more abundant close to the Bear Creek Falls (near-vent) site and smaller and less abundant at the Paskenta (distal) site, detailed study of their lithology and distribution will help constrain the vent location. With these constraints, we have used field relations, lithic populations, and lack of primary pyroclastic deposit textures to confirm that two areas previously mapped as Nomlaki Tuff are not actually Nomlaki Tuff. Thin section and XRF analyses will help characterize additional localities in Northern California, improving the reliability of use of the Nomlaki Tuff as a chronostratigraphic marker.