Stratigraphic and sedimentologic framework for tephras in the Wilson Creek Formation, Mono Basin, California, USA

TL;DR

This study provides a detailed stratigraphic and sedimentologic framework for late Pleistocene tephras in the Wilson Creek Formation, revealing eruption patterns, physical properties, and environmental implications of volcanic activity near Mono Lake.

Contribution

It offers new detailed stratigraphy, physical characterization, and interpretation of tephra layers, including sub-unit correlation and eruption source insights, for the Mono Craters during the late Pleistocene.

Findings

Tephras are mainly rhyolitic and produced from Mono Craters.

Eruption activity was variable in frequency and volume.

Tephra layers B7 and A1 indicate unstable depositional environments.

Abstract

Numerous tephra layers occur within the late Pleistocene Wilson Creek Formation, where they are interbedded with lacustrine deposits of 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. We present detailed stratigraphy and sedimentology of the tephra layers, sampled at twelve 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. Noticeable features of the tephras, such as the occurrence of low-density rounded or highly vesicular pumice within certain sub-units, are highlighted. The abundant obsidian, lithics, and ostracods within many sub-units suggest that the associated eruption pulses involved water-magma interaction. Eruptions from the Mono Craters during the late Pleistocene were consistent neither in frequency nor volume. The Mono Craters were most active during the eruption of tephras in Sequences C ($\sim$42.5-39.4 ka) and A ($\sim$14.1-12.9 ka), and Pleistocene volcanic activity reached its peak during the eruption of tephra layer C11. Detailed interpretation of tephra layer B7 and tephras in Sequence A is given. Tephras in Sequence B ($\sim$26.2-23.1 ka) may have had their vents located in the southern half of the Mono Craters, or are smaller in volume (except for B7), compared to the other tephras in the Wilson Creek Formation. Vents for tephra layers A4, A3, and A1 are located near the northern end of the Mono Craters. The stratigraphy of tephra layers B7 and A1 suggests an unstable depositional environment, which can be used to help constrain the water-level history of Lake Russell during the late Pleistocene.