DEPOSITION, DIAGENESIS, AND CEMENTATION HISTORY OF PLEISTOCENE VOLCANICLASTIC SEDIMENTS IN LONG VALLEY CALDERA, CA

Understanding how volcaniclastic sediments are deposited, cemented, and altered in an active caldera is critical for interpreting the evolution of hydrothermal systems. An extensive package of such sediments was first described from Long Valley Caldera by Bailey et al. (1976), who documented a >350 m thick package of lacustrine diatomite, shale, sandstone and conglomerate that filled Long Valley Lake following the ca. 0.76 Ma eruption of the Bishop tuff, through later periods of resurgent dome uplift, until the lake drained completely by 100 Ka. Two major pulses of hot-springs discharge, at approximately 300 Ka and 40 Ka, took place along the caldera’s resurgent dome (Sorey et al., 1991). In this study, we examine the petrography and sedimentology of the lacustrine deposits, with particular focus on the role of hydrothermal activity in genesis and alteration of the abundant, opaline silica cements.

Preliminary oxygen isotopic analyses of siliceous cements in the sandstones suggest cementation at temperatures of <100°C, assuming equilibrium with water that was isotopically similar to modern geothermal waters. Petrographic analyses confirm that the primary sediment source is the resurgent dome, from which clasts of volcanic glass and pumice were derived. Plutonically derived microcline K-feldspar is absent proximally, and increases in abundance distally. Sediment sorting increases away from the dome, toward the east moat of the caldera. Porosity, ranging from <1% to 25%, is highly variable among and within localities, even at cm scale.

Despite the heterogeneous porosity, sandstones consistently show isopachous cement, 10-20 µm thick, rimming the grains. Meniscus and void-filling cements occur along with the isopachous cements and, in some samples, there is evidence of complete porosity occlusion after the initial cementation event. X-ray diffraction reveals the presence of opal-A, opal-A/CT, and chalcedony, suggesting variable digenetic maturation of opaline cements. These observations support a scenario of initial, pervasive cementation followed by pulses of localized cementation infilling relict porosity as the epithermal fluid flow system in Long Valley Caldera evolved.