GEOLOGY AND PETROGRAPHY OF GROUNDHOG CONE, GOLDEN TROUT WILDERNESS, SOUTHERN SIERRA NEVADA

Groundhog Cone (2738 m) is an unglaciated basaltic cinder cone located in the south central Sierra Nevada of California. Geologic mapping reveals that Groundhog Cone is comprised of a 140-m-high pile of loose scoria interbedded with thin, poorly vesicular, pahoehoe lava flows and agglutinate spatter flows, indicating that: (1) magma was alternately erupted in both explosive and effusive styles from the summit; and (2) the eruption style alternated from Hawaiian fire-fountaining to powerful Strombolian explosions. Ballistic bombs ranging up to 30 m in diameter are found up to a km away from vent and bombs 0.5 m in diameter are found up to 3 km away from vent. Isopach mapping of the scoria fall deposit indicates eruption column heights of up to 2,000 m and a scoria fall deposit covering at least 30 km2. Preliminary geochronological results suggest an eruption age of 5,500 yrs BP based on 14C dating of charcoal found within and immediately underlying the scoria fall deposit. Following the formation of the cinder cone, ascending magma within the cone breached the northeast flank, leaving behind an 80-m-wide and 200-m-deep amphitheater. Large (up to 10 m) blocks of the breached cone rafted atop a 0.03 km3 lava flow that effused 5 km west towards Kern River Canyon, where it spilled over the rim down the steep canyon walls. The total erupted volume of the Groundhog eruption is estimated at 0.05 km3. Petrographical examinations of the erupted material indicate that the scoria and lava flows vary systematically in mineralogy. Whereas scoria fragments are composed of almost entirely of plagioclase (up to 20 microns) with lesser orthopyroxene (up to 15 microns) and trace amounts of 5-micron diameter clinopyroxene and olivine, lava flow samples are composed of subequal proportions of plagioclase, orthopyroxene, and clinopyroxene, with only slightly lesser concentrations of olivine and resorbed quartz and orthoclase feldspar. It is uncertain whether any regular variation in the mineralogy of the lava flow exists with increasing distance from vent owing to the compound and complex nature of the Groundhog flow field.