Tuttle Lake Formation and upper Lake Tahoe sequence
Tuttle Lake Formation and upper Lake Tahoe sequence
Cretaceous and Late to Early Jurassic
Harwood (1988, 1992) mapped the Tuttle Lake Formation northwest of Lake Tahoe. It is chiefly andesite tuff breccia, tuff, and minor volcaniclastic sandstone, unconformably overlying the Sailor Canyon Formation (part of unit NSTS). John and others (1994) reported a whole-rock Rb-Sr isochron age of ~165 Ma from hypabyssal plagioclase-porphyry breccia in this formation. Harwood (1992) reports a late Early through Middle Jurassic pelecypod in a volcaniclastic siltstone interbed at English Mountain, the northernmost body of this unit. Fisher (1989) mapped the Tuttle Lake Formation at the Mount Tallac roof pendant, southwest of Lake Tahoe. There it is composed of diamictite and conglomerate overlain by intermediate to mafic hypabyssal and extrusive breccia, lava flow rock, and tuffaceous sandstone.
West of Lake Tahoe, Harwood (1988, 1992) mapped the Lake Tahoe sequence, the upper part of which is roughly coeval with the Tuttle Lake Formation. The upper Lake Tahoe sequence is mostly sedimentary, made up of slate, limestone, and volcaniclastic sandstone debris-flow deposits overlain by quartz arenite and calcareous pelite with local volcaniclastic sandstone. Ammonite fossils from the debris-flow deposits are suggestive of Early Jurassic deposition (Harwood, 1992). Christe and others (2018) report a detrital zircon MDA of ~177 Ma for a quartz sandstone in this unit.
East of Lake Tahoe (Genoa Peak pendant), this unit includes the mafic metavolcanic and minor metaclastic rocks of the Peavine sequence, which has a zircon U-Pb age of ~163 Ma, the metaconglomerate, meta-arenite, and slate of the South Camp sequence with a detrital zircon MDA of ~142 Ma, and the overlying tuff breccia of Genoa Peak (Christe and Schweickert, 2020). This unit also includes the type Peavine Peak sequence mapped northwest of Reno, composed predominantly of mafic to intermediate, locally felsic, flow rock, ignimbrite, breccia, and tuff with subordinate volcaniclastic sedimentary rocks. Godwin (1958) reported Jurassic(?) plant fossils from this area. Garside (1998) reported whole-rock Rb-Sr ages of ~167 Ma and ~117 Ma. Garside noted the younger age is roughly equivalent to the intruding plutonic units (e.g. unit Glkt), though somewhat older than the closest dated intruding pluton at the head of Long Valley Creek (~97 Ma; Langenheim and others, 2025). 
Fisher (1989) notes that mafic to intermediate plutonic rocks in the partly coeval Tuttle intrusive complex (unit Gtt) are the same lithology as dikes and hypabyssal rocks within the Tuttle Lake Formation in the Mount Tallac pendant. Similarly, Howell (1998) noted that the older part of the Emigrant Gap intrusive suite (Black Butte quartz monzonite) was coeval with and equally deformed as this unit, and suggested it might be part of the magmatic source for the volcanic rocks. These observations suggest that the lower part of this unit forms the volcanic part of the Tuttle arc that includes the older part of the Tuttle Intrusive Sequence (unit Gtt) and Emigrant Gap intrusive suite granitic unit (unit Gegg). Christe (2010) suggests a correlation of the Middle Jurassic Peavine sequence and Tuttle Lake Formation of this unit with coeval volcanic rocks in the Kettle Rock terrane (unit KR) and marine volcanic rocks of Walker Lake terrane (unit WLV), and suggests they form a Middle Jurassic overlap assemblage linking the three terranes at that time.