Holocene and late Pleistocene tephra stratigraphy of the Mono Craters, eastern California
Meyn, Charles Robert
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Previous obsidian hydration rind dating of dome material of the Mono Craters in eastern California and radiocarbon dates from associated deposits suggests that after scattered eruptive activity in the Pleistocene the Mono Craters ceased activity ∼13,000 y B.P and resumed activity at a much higher rate ∼6,000 y B.P. A stratigraphic study of the tephras of the Mono Craters was conducted to test whether the gap in eruptions occurred or was merely a product of sparse data. No radiocarbon dates or stratigraphic relationships between tephra units logged in this study contradict the inferred gap in activity. The petrologic lithologies described by Kelleher (1986) are preserved in the tephra and pyroclastic deposits from the Mono Craters. Though these lithologies are useful for narrowing source areas, the presence of several domes with each lithology prevents the establishment of one to one correlations between domes and tephra on the basis of petrology alone without further data. Geochemical trends between the units before and after the inferred gap are vague. Small variations in trace element chemistry point to a slight fractionation trend from the Pleistocene biotite-bearing units to the Holocene volcanic products. While a sample from a pyroclastic flow deposit originating from Wilson Butte is very geochemically similar to the Mono Craters volcanics it is not directly descended by fractionation from any of the erupted Mono Craters products. Contamination of the chemical samples from lithics and other non-juvenile products from Mono and elsewhere adds significantly to uncertainty to the observed geochemical trends. A large eruptive sequence dated ∼1680 C-14 y B.P. consists of multiple petrologic lithologies in a single eruption; a behavior not previously recorded in the Mono Craters. The eruption likely consisted of several eruptions of pyroclastic material from multiple vent sources sampling different regions of a weakly zoned magma chamber. At least one component, the orthopyroxene-bearing unit 06802-I, is not likely to have had an associated dome extruded following the explosive phase, deviating from the typical Mono behavior. The obsidian hydration rind dating methods hydration rate for the Mono Craters was recalibrated producing slightly more accurate dates for Holocene eruptions older than the ∼600 y B.P. North Mono Eruption, but poorer dates for the youngest and oldest domes. This slight refinement in the OHR dating helps constrain the eruption ages of the porphyritic orthopyroxene and fayalite bearing units which remain poorly constrained by radiocarbon dates.