ERUPTION STYLES AND LATE PLEISTOCENE AGES FROM 36CL DATING OF GROUNDHOG CONE, THE MOST RECENT AND UNGLACIATED BASALTIC VOLCANO IN THE GOLDEN TROUT VOLCANIC FIELD, KERN PLATEAU, SIERRA NEVADA

Approximately 40 x 10⁶ m³ of basalt, trachy-basalt, basaltic trachy-andesite, and basaltic andesite was erupted from four Quaternary volcanoes in the Golden Trout Volcanic Field (GTVF), located within the Toowa Valley (~2,780 m) atop the Kern Plateau in the southern Sierra Nevada. Previous geochronological studies indicate a ~743 ka volcanic history. The three oldest vents (Little Whitney Cone, South Fork Cone, and Tunnel Cone) show evidence of glacial modification in the form of partial burial by glacially-derived deposits, removal of delicate lava flow surfaces, and lava flows littered with subrounded granitic boulders interpreted to be glacial erratics. In contrast, deposits erupted from Groundhog Cone, the youngest mafic vent, do not show evidence of glaciation. As a result, previous studies speculated that the undated 70 m-tall scoria and spatter cone was Holocene. To better understand the eruption styles and timing of the youngest basaltic eruption from the GTVF, field mapping and cosmogenic ³⁶Cl dating of deposits were performed. Field mapping indicates that the eruption occurred in three phases. Phase 1 began as a Strombolian style eruption that produced the scoria cone, which is composed of poorly developed and nonwelded lapilli-sized pyroclasts and spindle bombs. Phase 2 was characterized by a Hawaiian-style eruption where lava fountaining produced densely welded spatter agglutinate and fusiform pyroclasts that partially cap the scoria cone rim. The eruption transitioned to Phase 3 as rising lava destabilized and breached the north flank. Rafts of the northeast wall were carried 7.5 km west to Kern River Canyon atop the 13 x 10⁶ m³ blocky lava flow. Rafts of nonwelded scoria were spread across the lava surface as the flow widened and narrowed downstream from the vent, whereas rafted welded agglutinate remained intact as 15 m-high hummocks. Cosmogenic ³⁶Cl exposure ages for quenched and streamlined crusts from three fusiform bombs scattered atop the Groundhog Cone crater rim indicate an eruption between 52 and 59 ka. The late Pleistocene ³⁶Cl ages reveal that the eruption of Groundhog Cone occurred after the Tahoe glaciation maximum and that subsequent Tioga glaciers did not extend down to the Toowa Valley floor so as to modify the Groundhog Cone deposits.