VOLCANOLOGY OF THE NORTHERN ELDORADO MOUNTAINS, NEVADA: NEW EVIDENCE FOR THE SOURCE OF THE TUFF OF BRIDGE SPRING?

The Eldorado Mountains are located in the Colorado River extensional corridor in Clark County, Nevada, approximately 30 miles southeast of Las Vegas. The Eldorado Mountains are composed primarily of Miocene volcanic rocks deposited on Precambrian basement, and broken into a series of east-tilted blocks by the Eldorado detachment fault. Previous workers had reported the possible existence of a new caldera in the northern Eldorado Mountains, and had hypothesized that it was the source of the Tuff of Bridge Spring, a regionally extensive ash-flow sheet. New mapping within the northern Eldorado Mountains has revealed a steeply tilted stratigraphic succession of lag breccias, clast-supported pyroclastic breccia, sedimentary breccia including megabreccia and channelized fluvial sandstone, capped by intermediate flows and domes. This succession is interpreted to represent a caldera fill sequence. A hypabyssal intrusive suite located to the southwest is interpreted to represent either the caldera feeder system, or the presence of an earlier volcanic center. Elsewhere, the presence of an unidentified tuff deposited on Precambrian basement, and additional lag breccia and megabreccia deposits suggest the presence of a second caldera. Together these deposits may indicate the existence of a nested caldera system in the northern Eldorado Mountains.