HYDROVOLCANISM IN THE HIGH ROCK CALDERA, NORTHWESTERN NEVADA

Hydrovolcanic tuff cones are scattered throughout the interior of the High Rock Caldera, in northwestern Nevada. One set of tuff cones, at latitude 41.487° N, longitude 119.433 ° W., informally named "the Bear Paws" was chosen for study. The cone assemblage is about 1.4 km x 0.6 km, elongated north-south. Aerially, the Bear Paws appear to be two cones, North and South Paw. The tuff cones erupted through caldera-filling lacustrine deposits and are likely to be the youngest magmatic event of the approximately 16 Ma High Rock Caldera, one of the oldest calderas along the NE-trending Yellowstone hotspot path.

Stratigraphic and textural evidence reveals that the Bear Paws tuff cone eruptions were emergent to subaerial. Only the innermost vent facies is preserved with the uppermost and exterior portions of the cones eroded away. There is one emergent and two subaerial stratigraphic units surrounded by older, interbedded caldera fill and lacustrine deposits. The matrix of the cones has been moderately to highly palagonized and is weathered, with many volcanic clasts also showing secondary alteration. Hyaloclastite beds dip 45-68° towards the vent, vastly greater than typical tuff cones, which dip 20-25°. Soft sediment deformation and repeated slip failure back into the vents accounts for the steep dips with subsequent cementation by palagonitization. Dike swarms along the periphery of both cones appear to have slumped outward from the interior of the cone.

The Bear Paws are basalt composition and erupted after siliceous and intermediate volcanism ended at the High Rock Caldera. The Bear Paws vented along a migrating path from south to north, producing the double cone morphology. The migration may have followed the course of a dike. After the hydrovolcanic eruptions, lake levels rose and fluctuated creating two wave-cut platforms and eroding much of the upper and outer parts of the cones.

Geochemical and petrographic analyses show that the Bear Paws are the most mafic magmas erupted at the High Rock Caldera, with juvenile clast silica content as low as 43%. On major and trace element variation diagrams the basalt lies along linear and curvilinear differentiation trends of High Rock Caldera magma compositions. The basalt appears to represent a parental composition that may have evolved to form the entire High Rock Caldera magmatic suite.