VOLCANOLOGY OF UBEHEBE CRATERS, DEATH VALLEY CALIFORNIA: GEOCHEMISTRY AND SIZE-FREQUENCY OF BASALT FRAGMENTS IN EJECTA

Ubehebe Craters, in Death Valley National Park, consist of a group of maar craters formed during Holocene time. The biggest and youngest of these is Ubebebe Crater (800 m wide, 235 m deep). Little Hebe Crater, which is about 100 m wide, lies just south of Ubehebe Crater. Earlier studies suggested that the ejecta blankets (covering 15 square km) were mainly composed of pyroclastic surge deposits. Although these earlier studies established style and sequence of eruption, little is known about the geochemistry of basalt clasts in ejecta, details of an eruption, or the origin of distal ejecta deposits. For this study, 32 samples were collected from four sites: three are 0.2 to 0.4 km west of the main crater and the fourth is Little Hebe Crater, 0.2 km south of the main crater. All of the samples are older than those associated with the eruption of Ubehebe Crater. After collection, each sample was washed, photographed, and the size of clasts digitally measured. Basalt clasts are less than 10 mm in size and are identical in chemistry to a lava flow exposed in the wall of Little Hebe Crater. These clasts, therefore, represent ripped up bedrock and not new magma. Chemically, Ubehebe basalt is similar to basalts in the Greenwater Range and volcanoes near Yucca Mountain. Yogodzinski and Smith (1995) proposed the Amargosa Valley Isotope Province (AVIP) outlining basalt fields with enriched Nd and Sr isotopes and distinctive trace-element signature. Originally the AVIP included volcanoes near Yucca Mountain and those in the Greenwater Range east of Death Valley. Basalt from Ubehebe Craters is identical in chemistry to AVIP, suggesting that the AVIP should be enlarged to include Ubehebe Craters. The sizes of over 2000 clasts were measured and median and Inman sorting coefficients were calculated for each sample. Compared to size-sorting data from other volcanic fields and work by Crowe and Fisher (1973), the sampled deposits are more typical of pyroclastic air-fall deposits than surge. Variations in median size and sorting values represent deposition of ejecta from a series of eruptions each having a different energy. In summary, eruptions at Ubehebe were dynamic, ripping up basalt with AVIP chemical signatures into fragments usually less than 10 mm in size and depositing them during numerous pyroclastic air-fall events.