LOCATING THE SOURCE OF THE NOMLAKI TUFF

The Nomlaki Tuff was deposited throughout the western United States with proximal pyroclastic deposits in the northern Sacramento Valley, California. Emplacement occurred during a major Pliocene dacitic eruption located somewhere in the southern Cascades near the Lassen Volcanic Center (LVC). The exact location of the Nomlaki Tuff’s source is unknown, however, several widespread source areas have been suggested. The areas considered by previous workers range from the Yana Volcanic Center south of the LVC to Snow Mountain located 40 km NW of LVC. This work aims to further constrain the location of the source of the Nomlaki Tuff. Field characteristics, including, pumice size, clast sorting, and welding intensities can be used to determine the proximity of various field sites to the original source. Multiple sites located in the northern Sacramento Valley and Cascade foothills were measured to assign site proximity and direction back to the source. Deposits interpreted to be proximal to the source are located east of the Sacramento Valley and western-most deposits (in the Sacramento Valley) are interpreted to be distal ash flows. Nomlaki Tuff outcrops interpreted to be most proximal to the source are within the Bear Creek drainage, 5 km north of Shingletown. There, large pumice (<241 cm) and lithic (<150 cm) fragments are poorly sorted in welded tuff with columnar jointing. All other measured outcrops of Nomlaki Tuff are interpreted to be further from the source, suggesting the source vent is located east of the Bear Creek drainage, perhaps even within several kilometers based on the degree of welding. This location eliminates the Lassen Volcanic Center as well as other previously suggested locations further south. The Latour Volcanic Center (LTVC) is of appropriate age and location to be the source of the Nomlaki Tuff, so it may be the best candidate for the source vent. Several dacite lava flows of Pliocene age exist within the LTVC near Buck Butte. Correlation between dacitic lithics in the Nomlaki Tuff deposit and Pliocene lava flows of the LTVC can be used to ascertain whether the two are related.