Tertiary pre-volcanic gravels
Tertiary pre-volcanic gravels
early Oligocene to Eocene
Consists predominantly of fluvial sandstones and conglomerates with lesser siltstone and minor clay interbeds deposited in paleochannels at moderate to lower elevations of the central and northern Sierra. Referred to as Tertiary gravels, auriferous gravels, and pre-volcanic gravels (Lindgren, 1911). Clast composition typically exhibits moderate to high compositional maturity (though maturity decreases up-section); sandstones are quartzose to arkosic, and conglomerates are commonly dominated by vein quartz and quartzite clasts. Attains maximum recorded thickness of ~160 m on the San Juan Ridge in the lower foothills (Yeend, 1974). Cassel and Graham (2011) interpret a variably high-energy braided stream environment wherein coarser sand and gravel packages represent high-energy channel deposits and finer siltstone and clay intervals, locally with thin paleosols, represent floodplain deposits.
Generally interpreted to be time-equivalent and interfingering with the Ione Formation (map unit io), 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.
Sparse Cenozoic detrital zircon ages of ~43 to ~33 Ma (Cassel and others, 2012; Cecil, 2009) constrain maximum depositional ages for parts of those strata and support correlation with Ione Formation (map unit io) and broadly conformable contact with overlying Valley Springs and Delleker Formations (ignimbrite flareup volcanic and sedimentary rocks; map unit ivs). Sparse and possibly reworked pyroclastic deposits are also interbedded with fluvial strata in at least two localities in the northern Diamond Mountains near Susanville, California and contain ~35 Ma zircon grains which constrain maximum depositional age to <35 Ma for those strata (Lovelock, 2010). Maximum age of unit is poorly constrained, though generally regarded as Eocene. Separated from underlying basement (minimum age ~90 Ma) by major erosional unconformity. The minimum age of the unit is well constrained by overlying Oligocene tuffs (~31.7 Ma or younger; varies spatially; see unit ivs).
Excepting the sparse pyroclastic material near Susanville, the unit contains no primary volcanic deposits and pre-dates input of Oligocene ignimbrites from the Nevada caldera belt (see map unit ivs). Note, however, that the contact between units pvg and ivs is not always well located in source mapping, thus, as mapped, some syn-volcanic (ivs) deposits are erroneously included in unit pvg and vice versa. The Tertiary gravels host substantial placer gold and were the primary target of historic hydraulic and drift mining within the Sierra (e.g., Lindgren, 1911; Yeend, 1974).