Ione Formation
Ione Formation
Eocene
Predominantly finer-grained fluvial and deltaic, estuarine, and minor shallow marine deposits that crop out along the western valley margin approximately from Oroville to Fresno. Consists of interbedded sandstone and clay, with lesser pebbly sandstone and gravel interbeds. Exhibits moderate to high compositional maturity; sandstones are quartzose to arkosic; clays are predominantly kaolinitic; gravel clasts are predominantly vein quartz and quartzite. Compositionally maturity decreases up-section. Attains maximum recorded thickness of ~190 m near type section (Creely and Force, 2007).
Generally interpreted to be time-equivalent and interfingering with the Tertiary pre-volcanic gravels (map unit pvg), with the Tertiary gravels and Ione Formation representing more proximal and distal (respectively) deposits of the same system (e.g., Lindgren, 1911; Allen, 1929; Jenkins, 1946; Creely and Force, 2007) but these units may have different maximum/minimum ages. Some upper(?) Ione Formation strata contain middle Eocene mollusks (Creely and Force, 2007). 
Sparse Cenozoic detrital zircon ages of 44.1 and 43.6 Ma (Sharman and others, 2015) constrain maximum depositional ages for those strata and support correlation with Tertiary pre-volcanic gravels (map unit pvg). Conformably overlies older Eocene strata (mainly in the subsurface) but is separated from underlying basement by major erosional unconformity.