CO-EVOLUTION OF VOLCANIC AND LACUSTRINE SYSTEMS IN PLEISTOCENE LONG VALLEY CALDERA, CALIFORNIA, USA

摘要

Sedimentary records from intracaldera lakes document unique interactions between volcanic and sedimentary processes and hold valuable information about the geologic evolution of the associated volcanic systems. Here, we present a comprehensive analysis of the lacustrine sedimentary package within Long Valley Caldera, California, which provides new constraints on the history of the Long Valley system as well as new insights into the development of intracaldera sedimentary systems in general. After the eruption of the Bishop Tuff (760 ka), the resulting caldera was partially filled by a large lake, within which volcaniclastic, chemical, and biogenic sediments were deposited. Contrary to previous interpretations, Long Valley Lake was initially evaporative and did not become relatively fresh and deep until later in its history. This observation, coupled with paleoshorelines documented on the resurgent dome, suggests an extended timeframe for resurgent dome uplift. Near-shore volcaniclastic sedimentation in Long Valley Lake was predominantly episodic and was concentrated in an extensive Gilbert-type delta system that built outwards into the lake from intracaldera rhyolite highlands. Deposition in distal environments was dominated by pelagic sedimentation that included accumulation of diatomite and marl with ostracod shell beds. The sedimentary package is herein assigned to two coeval formations that exhibit fundamentally different characteristics and modes of deposition. The Sagehen Formation comprises the proximal volcaniclastic succession, which was influenced by ongoing deformation and volcanism during the evolution of the Long Valley system. The Benton Crossing Formation comprises the distal pelagic sedimentary package, which appears to offer a unique paleoclimatic record within a well-constrained chronostratigraphic framework. Together these formations support models of intracaldera sedimentation in which event-driven deposition and diatomite accumulation are dominant sedimentary processes, although they also illustrate the importance of delta development and catchment-area evolution in focusing sedimentation and controlling the architecture of the deposits. The Long Valley intracaldera sedimentary package highlights the utility of lacustrine sediments in understanding caldera evolution.