The Sloan Sag: A mid-Miocene volcanotectonic depression, north-central McCullough Mountains, southern Nevada

Hayden Lloyd Bridwell, University of Nevada, Las Vegas


In the Hidden Valley area of the north-central McCullough Mountains, southern Nevada, mid-Miocene andesite and dacite domes, flows and pyroclastic units (the Sloan volcanics) partially fill a sag in the underlying Hidden Valley volcanics. Sagging was accommodated during and/or after the eruption of the Sloan volcanics by a combination of movement on the McCullough Wash normal fault system, and subsidence into evacuated chambers; Major, trace and rare-earth element geochemistry suggests that the rocks of the Sloan volcanics belong to four groups, each of which were produced by partial melting of chemically distinct sources; The Mount Hanna andesite member of the Sloan volcanics erupted as a hot, dry aphyric lava by a mechanism of lava-fountaining. The only other documented felsic-to-intermediate composition lava-fountaining event in the southern Basin-and-Range is the Taylor Creek Rhyolite of southwest New Mexico.