Tuscan Formation
Tuscan Formation
late Pliocene
Consists of a succession of volcaniclastic debris flow deposits interbedded with lesser volcaniclastic conglomerate, sandstone, and siltstone, and minor tuff, sporadically blanketing the northwestern map area.
Clasts are dominantly volcanic (e.g. andesite and basaltic andesite), and include material sourced from the Maidu, Yana, and Butt Mountain volcanic centers, however, lower strata also contain variable proportions of metamorphic basement rock types (Lydon, 1968; Helley and Harwood, 1985; Poletski, 2010). Debris flow deposits are typically poorly sorted or chaotic, often reversely graded, with angular to subrounded clasts typically up to 20 cm and locally up to 3 m; they are volumetrically dominant and generally regarded as volcanogenic (i.e. lahars) due to presence of cooling fractures and wood casts (e.g., Lydon, 1968; Helley and Harwood, 1985; Clynne and Muffler, 2010). Volcaniclastic conglomerates, volcanic lithic sandstones, and siltstones are predominantly fluvial, but transitional (e.g. hyperconcentrated flow) deposits are also present.
Pyroclastic deposits are volumetrically minor but important for stratigraphic age control, and include the Nomlaki Tuff and Ishi Tuff. The Nomlaki Tuff is a dacitic pumice to pumice lapilli tuff (e.g., Helley and Harwood, 1985), which has an Ar-Ar age of 3.339 ± 0.016 Ma (Sarna-Wojcicki and others, 2021) and a magnetostratigraphically estimated age of 3.27 ± 0.04 Ma (Henry and Perkins, 2001). The Ishi Tuff is a fine-grained, pumiceous, pyroclastic fall deposit, commonly reworked, which has a zircon fission-track age of ~2.7 Ma (Helley and Harwood, 1985). Some have reported the presence of block-and-ash flows (e.g. Blake and others, 2000), but others reported no evidence of their presence (e.g. Poletski, 2010).
The Nomlaki Tuff is present near the base of the sequence and constrains the lower age limit. The Ishi Tuff is present approximately midway up the section (Helley and Harwood, 1985). Lava flows as old as 2.4 Ma sourced from the Maidu volcanic center unconformably overlie the Tuscan Formation and constrain its age in this region to >2.4 Ma (Clynne and Muffler, 2010). Thus, the age of the Tuscan Formation spans approximately ~600 kyr and may extend into the earliest Pleistocene.