The East Range, northwestern Nevada: A neglected key to the tectonic history of the region

The East Range occupies a strategic position at the western edge of the exposed western facies terrane near the Paleozoic continental margin. There a Cambrian and Ordovician deep-water sequence including the Preble and Valmy Formations is overlain by middle to upper Paleozoic strata that are assigned to three separate stratigraphic sequences on published geologic maps of the range. The three sequences, which include the Havallah sequence, the Inskip Formation, and the Harmony Formation, originally were perceived to be of different ages and lithic composition. The strata assigned to the Havallah are now known to contain conodonts of Late Devonian to Permian ages, and fusulinids of Permian age; those assigned to the Inskip contain conodonts of Early Mississippian and Permian ages and corals of Mississippian age. Strata assigned to the Harmony are undated paleontologically. All three are composed mainly of siliceous deep-water sedimentary deposits. Lower beds of the Havallah and Inskip totalling hundreds of meters in thickness are composed of arkosic sandstone and conglomerate identical to that composing most of the Harmony. The Havallah and Inskip lie disconformably on the quartzitic, middle part of the Valmy Formation. The Harmony lies concordantly on distinctive chert beds at the top of the Valmy Formation. Assuming the latter contact is disconformable rather than a bedding-parallel fault, the East Range displays the stratigraphic bases of all three arkosic map units. The disconformable relation of the Havallah and Inskip to the Valmy Formation indicates elevation of deep-water deposits, subaerial erosion down to Middle Ordovician, and in Late Devonian a return to deep immersion. There is no evidence here of Late Devonian to Early Mississippian folding. Major folds in the East Range must be of Jurassic or younger age because Upper Triassic strata are involved. Evidence of large-scale thrust-faulting is notably scarce. The existence of the Willow Creek thrust shown on published maps of the East Range was based on the mistaken belief that strata below the Valmy Formation are of Triassic age whereas they are actually of Cambrian and earliest Ordovician age.