SOFT ROCKS IN A RESTLESS CALDERA: THE LONG VALLEY CALDERA-FILL SEDIMENTS

Following the caldera-forming eruption of the Bishop Tuff (0.76 Ma), a lake quickly filled the Long Valley Caldera of east-central California. An intracaldera sedimentary package records the sediment infill of this caldera lake and the post-eruption history of Long Valley; this package also provides an opportunity to study sedimentation processes within an active caldera. Spatially diverse measured sections, extensive field sampling, and detailed field observations provide the means by which to interpret the upper exposed section of these rocks. Proximal, texturally immature to submature volcarenite sandstones and pebble conglomerates that were sourced from the resurgent dome preserve evidence of a fluvial-dominated Gilbert-type delta system. These deposits exhibit well-developed topset and foreset beds and coarsen up-section. Medium- to fine-grained sandstone beds also contain localized soft sediment deformation that is suggestive of disturbance by volcanic unrest and that also may represent hydrothermal degassing through the sediments (Bailey et al. 1976, JGR, v. 81, no. 5). As several authors have noted (Bailey et al. 1976; Lipshie, SCGS 2001 Guidebook), in one location on the southeastern flank of the resurgent dome, fossilized plant remains within silt and mud suggest the existence of a marsh environment behind the delta front. The distal facies is comprised of marl and diatomite interbedded with sparse, very thin, angular, fine to very fine obsidian sands. Consolidated and unconsolidated beach gravels mark lake stands, and tufa deposited both along shorelines and on an island created by moat rhyolite documents chemical sedimentation within the lake. These records are used to develop a detailed geologic history of Long Valley, specifically with respect to the caldera lake, following the eruption of the Bishop Tuff and more broadly to test and refine models of intracaldera sedimentation.