CORRELATION OF MIDDLE JURASSIC VOLCANIC ROCKS IN THE KETTLE ROCK SEQUENCE OF NORTHERN CALIFORNIA AND PINE NUT TERRANE OF WESTERN NEVADA, AND IMPLICATIONS FOR STRIKE-SLIP FAULTING IN THE TAHOE BASIN AREA

Correlation of Jurassic units in the Sierra Nevada and western Nevada figures prominently in any analyses of post-Middle Jurassic strike-slip faulting in this part of the Cordillera. In particular, correlation of the Early Jurassic Sailor Canyon Fm (SCF) of the Northern Sierra Terrane (NST) and Gardnerville Fm (GF) and related units of the Pine Nut Terrane (PNT) has been used to argue against structural breaks across this region. New analysis of Mesozoic strata in the area challenges this interpretation.

The early Mesozoic stratigraphy of the NST is marked by a thin (or absent) Triassic sequence, distal tuffaceous turbidites of the SCF, and Middle Jurassic, submarine, mafic volcanics (Tuttle Lake Fm) with an oceanic-arc geochemistry. To the east, the PNT contains a thick Triassic section of shallow marine carbonate and intermediate-felsic volcanics, overlain by slope-facies siltstones of the GF, nearshore-facies quartz arenite and local gypsum. Middle Jurassic volcanic strata of the PNT are mostly felsic and were deposited subaerially. While the GF and SCF are both Early Jurassic marine clastic units, they occur within very different stratigraphic successions that reflect very different paleogeographic settings. The lack of a transitional facies between the PNT and NST (only 20km separation in the Tahoe Basin area), strongly suggests the terrane boundary represents a fault and not solely a change in depositional environment.

In the northernmost NST, Middle Jurassic strata of the Kettle Rock sequence (KRS), which are separated from the rest of the NST by the Taylorsville Thrust, are felsic, dominantly subaerial, and have a geochemistry suggestive of eruption through continental crust. The volcanic section is overlain by Oxfordian(?) ash flow tuffs, and fluvial to nearshore deposits locally containing abundant plant fossils, dinosaur bone fragments, and crocodile teeth. The KRS contrasts with age-equivalent volcanics of the NST, but shares lithologic and geochemical similarities to those of the PNT. Two distinctive KRS lithologies - a pale-green, heterolithic tuff and a red dacite porphyry - can be traced SE to the Nevada border, and have correlatives in the PNT. We conclude that the KRS is a fragment of the PNT displaced at least 100km northward along the aforementioned fault sometime after the Middle Jurassic.