Tephra layers of the Wilson Creek Formation, Mono Basin, California.

Tephra deposits within the Wilson Creek Formation were formed during the late Pleistocene, and are interbedded with lacustrine deposits formed in Lake Russell (the ancestor of present-day Mono Lake). Most of the tephra layers are rhyolitic in composition, and were produced from the Mono Craters. In this study, we present detailed stratigraphy and sedimentology of these tephra layers sampled at ten outcrops near the shoreline of Mono Lake and the Mono Craters, and implement grain size, componentry, and surface morphology analysis to characterize their physical properties. Sub-unit correlation is proposed for certain tephra units. Interpretation on B7 and tephras in Sequence A is given, which includes the number of eruption pulses, dispersal direction, occurrence of pyroclastic flow and surge, and other processes taking place during and after the eruptions. Noticeable features of the tephras, such as the occurrence light rounded or highly vesicular pumice within certain sub-units, are highlighted with interpretation presented. The abundant obsidian, lithics, and ostracods within many sub-units suggest that the associated eruption pulses were involved with water-magma interaction. We show that eruptions from the Mono Craters during the late Pleistocene were consistent in neither frequency nor volume. The Mono Craters were most active during the eruptions of Sequences C and A, and reached its peak during the eruption of C11. Tephras in Sequence B may have their vents located in the southern half of the Mono Craters, or are smaller in volume (except for B7), and vents for A4, A3, and A1 are located near the northern end of the Mono Craters. The stratigraphy of B7 and A1 suggests unstable depositional environment, which can be used to further constrain the water-level history of Lake Russell during the late Pleistocene.