Abstract

Studies of the sedimentary and igneous rocks of the Pilot Range and of parts of the adjacent Silver Island Mountains, Pigeon Mountains, Leach Mountains, Goshute Range, and Toano Range reveal newly recognized stratigraphic and structural complexities as well as a wide variety of granitoid rocks. Pre-Cenozoic sedimentary rocks range in age from late Proterozoic to Triassic, whereas Cenozoic strata are Oligocene and Miocene. Mesozoic deposits younger than Early Triassic are absent. A major low-angle fault, the Pilot Peak decollement, emplaced Cambrian to Triassic miogeoclinal strata over Proterozoic and Cambrian metamorphosed miogeoclinal strata. A probable tear fault, confined to the allochthon above the decollement, has juxtaposed platform-facies Cambrian through Permian strata in the southern part of the Pilot Range area against platform to ramp facies Ordovician through Triassic strata in the northern part.

Oligocene deposits are predominantly pyroclastic materials and lava flows. Miocene strata are composed largely of lacustrine rocks whose primary source was reworked volcanic detritus derived from the north. The Oligocene and Miocene basins were probably more extensive than the Basin and Range valleys within which they presently lie.

Plutonic rocks are of Jurassic and Oligocene age, and show wide compositional variations. Jurassic plutons include coarse-grained granite, granodiorite, quartz monzonite, and diorite; field studies suggest that their compositions span the peraluminous, metaluminous and alkalic fields. These plutons typically form contact metamorphic aureoles in Paleozoic wallrocks. Oligocene plutons and dikes are generally hornblende-biotite granodiorite, and hypabyssal textures typify the smaller bodies.