CHIRP SEISMIC-REFLECTION PROFILES FROM MONO LAKE: THE EFFECT OF VOLCANIC ERUPTIONS ON LACUSTRINE STRATIGRAPHY

High-resolution, CHIRP seismic-reflection profiles from Mono Lake, California, reveal the effects of recent volcanic processes on the stratigraphy of the lake deposits. Late Quaternary lake sediments, documented from previous core studies, were seen in the western embayment in water depths shallower than about 20 meters. These sediments appear to be conformably overlain by volcanic deposits of variable thickness, probably deposits related to the Mono Craters eruption of about 600 cal BP. The upper part of this volcanic unit is acoustically stratified and highly reflective, suggesting that the material is largely tephra, possibly including pumice. Below about 20 m water depth, the tephra unit is unconformably overlain by distinctive slump deposits, which have been cored in several places. The slump deposits have a distinctive radial pattern around Paoha Island, first thickening away from the island to more than 20 m thick, and then thinning to a feather edge and onlapping pre-existing tephra deposits. The slump deposit is acoustically massive in most places, but strong internal reflections in a few places suggest multiple local surges. This pattern indicates that the slump deposits are related to the formation of Paoha Island, an abrupt event that resulted from the shallow intrusion of magma shortly after CE 1650. The entire sequence is conformably overlain by 1-2 m of lake sediments, thinning toward the lake margins. This upper lacustrine sequence has been extensively cored by several groups and is comprised of very finely laminated sediments of various color that apparently can be correlated across the lake. Piston coring the tephra deposits in relatively shallow water, where they are close to the lake floor and not covered by slump deposits, has proven remarkably difficult.

The seismic-reflection data allow mapping of the parts of Mono Lake that are undisturbed by the uplift of Paoha Island and the distribution and thickness of the 600 BP tephra where it is not disturbed by the Paoha Island event. Near-continuous lacustrine deposition was seen only near the basin margins in water shallower than about 20 m. The seismic data do not indicate any major low stands of the lake deeper than a few meters below the level of the lake in 2009.