2021 Trip Report - Inbound Geology Tour Near Shoshone
Inbound Geology Tour Near Shoshone
by trip leader Bill Neill
Photos by Bill Neill and Janet Austin
After meeting in Shoshone on Friday afternoon, Oct 22, our inbound group examined two nearby outcrops of volcanic rock, then caravanned to Pahrump for the Rendezvous. Both volcanic rock units are described by a 1997 book, Geology Underfoot in Death Valley and Owens Valley, by Robert Sharp and Allen Glazner.
From the center of downtown Shoshone, first we drove about 1200 feet to the Southwest, to Dublin Gulch where homeless men during the 1930's excavated living quarters in a layer of lithified volcanic ash. The book chapter describing this unit is titled "A Collector of Volcanic Ashes - Ancient Lake Tecopa".
During the Ice Ages, until about 500,000 years ago, ancient Lake Tecopa covered about 85 square milesand was over 400 feet deep. Over time, mud and volcanic ash were deposited on the lake bottom and eventually filled the lake basin. These sediments now form rounded hills of tan-colored mudstone along Highway 127 south of Shoshone. The lake sediments also include a dozen volcanic ash layers, derived from explosive eruptions elsewhere in the western states. The thick volcanic layer at Dublic Gulch has been identified geochemically as derived from the enormous eruption that formed Yellowstone caldera, 650 miles tothe northeast. Most of the powdery ash near Shoshone settled on surrounding slopes and was washed into the lake by rainfall.
Our second and final stop on the way to Pahrump was a roadcut 3 miles east of Shoshone. Here we examined another volcanic ash layer, of Miocene age, about 9.5 million years old. This layer also was formed of volcanic ash, but erupted from a nearby vent and deposited while ash particles were still hot, not cooled by the atmosphere like the Lake Tecopa ash layers. The black layer is volcanic glass like obsidian; but unlike obsidian at Mono Craters that was slowly extruded as viscous molten rock, this layer is a “welded tuff”, formed of compressed “shards” of hot volcanic ash derived from an explosive eruption. Above and below the black layer is pink to orange volcanic ash with pumice blocks that cooled faster after eruption and emplacement, so did not nearly remelt with all porosity squeezed out.