Ignimbrite Flareup volcanic and sedimentary rocks
Ignimbrite Flareup volcanic and sedimentary rocks
early Miocene to early Oligocene
Rhyolite ignimbrites and interbedded fluvial deposits occurring in east-west paleochannels crossing the central Sierra (where mapped as Valley Springs Formation) and northern Sierra (where mapped as Delleker Formation).
Pyroclastic flows erupted from calderas of the western Nevada volcanic field in central Nevada (Henry and John, 2013). These eruptive products are dominantly rhyolitic with lesser trachydacite and minor dacite; typically high-K to shoshonitic (Henry and John, 2013). Pyroclastic material is present as nonwelded to densely welded and thick (up to >100 m) primary pyroclastic flow deposits and reworked tuffaceous sediments. At least 15 discrete ignimbrites have been identified within the Sierra Nevada, and range in age from 31.7 Ma (Holland and others, 2015; O’Neal, 2023) to 23.0 Ma (Roche and others, 2022; Holland and others, 2015). These flows exhibit a general pattern of southward younging, which reflects southward younging of source calderas (e.g., Henry and others, 2012; Henry and John, 2013; Best and others, 2015). 
Sedimentary strata consist predominantly of heterolithologic conglomerates and tuffaceous sandstones with lesser tuffaceous siltstone and mudstone. Conglomerate clasts typically consist of variable proportions of metamorphic, plutonic, and volcanic (ignimbrite) clasts; however, proportions vary substantially and locally may be predominantly rhyolitic tuff or other rock types. Sedimentary strata are dominantly fluvial, but some marsh and lacustrine deposits are present locally (Bartow, 1994).
In more proximal parts of the paleochannels at higher elevations, the unit is thicker (up to >300 m) and dominated by ash-flow tuff (or ignimbrite). In more distal parts of the paleochannels, in the foothills, the unit is thinner, with a higher proportion of tuffaceous sandstones and conglomerate commonly deposited in alluvial fans (Cherven, 2020) and locally passing distally into ephemeral lake and marsh deposits (Bartow, 1994). Unit conformably (to locally unconformably) overlies the Tertiary pre-volcanic gravels (map unit pvg) and Ione Formation (map unit io) and is overlain by Miocene andesitic rocks of the Ancestral Cascade arc (map unit aca) in erosional unconformity. Contact of fluvial strata with underlying pre-volcanic gravels may be indistinct or gradational.