Geology of the Virginia City quadrangle, Nevada

Abstract

Most of the Virginia City quadrangle, Nevada, is covered by a thick series of Tertiary and Quaternary volcanic deposits, but pre-Tertiary rocks are exposed in small patches. The pre-Tertiary rocks consist of metamorphosed sedimentary nud volcanic rocks probably of Triassic age. They have been folded along axes Avhose average trend is a little east of north, and are intruded by granitic rocks of Cretaceous age. The oldest of the Tertiary rocks are rhyolite pyroclastic rocks, which are tentatively considered to be of Oligocene age. Overlying these is a thick and complex sequence of andesitic flows and pyroclastic rocks, which have been divided into an older unit, the Alta formation, and a younger unit, the Kate Peak formation. The Davidson granodiorite is intruded into the Alta formation. Hypabyssal intrusions of andesite porphyry cut both the Alta and Kate Peak formations. The Alta formation and older rocks, and to a lesser extent the Kate Peak formation, were extensively altered. Much later, surficial weathering of the altered rocks that contain pyrite produced large areas of bleached rocks. Stream and lake deposits of the Pliocene Truckee formation overlie and partly interfinger with the Kate Peak formation. The lake deposits were laid down in structural basins formed by warping and block faulting. Contemporaneous rhyolitic eruptions contributed pumice to the Truckee formation and also formed a large rhyolite dome at Washington Hill. The sediments of the Truckee formation were warped and faulted, and a surface of low relief was eroded on the soft sediments. Later, flows of basalt and basaltic andesite were erupted from several vents and covered large areas. Still younger volcanic rocks, Quaternary in age, include andesite and olivine basalt. These are scattered and relatively small in volume. Quarternary stream and lake deposits were laid down in the intermontane basins and are still accumulating in parts of the basins. Extensive Quaternary hot-spring deposits have accumulated at Steamboat Springs. The Quarternary deposits have been warped and faulted like the older rocks, though to a lesser extent. The pre-Tertiary structures, insofar as they are known, trend about north, and the Cenozoic folds and fault blocks, although divergent in detail, also trend generally north. The similarity between the older and younger structures is limited to their trend, however; for the older structures are compressional folds, like those of the Appalachians and other folded mountains. On the other hand, the Cenozoic (basin and range) structures are more diverse; they include many normal faults (exemplified by the Comstock fault), warped and tilted blocks