• Harvey Thorleifson, Chair
    Minnesota Geological Survey
  • Carrie Jennings, Vice Chair
    Minnesota Geological Survey
  • David Bush, Technical Program Chair
    University of West Georgia
  • Jim Miller, Field Trip Chair
    University of Minnesota Duluth
  • Curtis M. Hudak, Sponsorship Chair
    Foth Infrastructure & Environment, LLC


Paper No. 3
Presentation Time: 2:05 PM


BURSIK, Marcus I., Geology, University at Buffalo, 411 Cooke Hall, Buffalo, NY 14260 and SIEH, Kerry, Earth Observatory of Singapore, Nanyang Technological University, Singapore, 639646, Singapore,

The youngest eruptive material in the Mono--Inyo Craters volcanic chain is the tephra and associated volcanics of Paoha Island. Eruption of these materials accompanied island uplift, which was dated by Stine (S. Stine, 2005, pers. com.) using radiocarbon and tree rings to 1650--1750 CE.

Most of the well-dated older eruptions were much more explosive and generated numerous pyroclastic flows, surges and falls as well as domes or lava flows. The most thoroughly studied of the older deposits are those from the North Mono--Inyo--Mammoth eruptive sequence. The work of Millar et al (2006, QR), coupled with the best radiocarbon data and stratigraphic relationships suggest that these eruptions are nearly contemporaneous at 1325--1350 CE. The most explosive phases of this eruption sequence were subplinian to small plinian, and comparison of pyroclastic material in numerous outcrops suggests that all older events were likewise of low intensity.

The next oldest sequence was the South Mono eruption, which is dated in proximal sections at 594--648 CE (this and all dates reported below are 2-sigma. Where pooled, carbon dates were the same at the 95% confidence limit). At this time, the vent under South Coulee was active, as well as numerous pyroclast producing vents to North and South. The South Mono eruption was preceded by eruptions from the vent underlying Northwest Coulee and related vents in 322--430 CE.

Logs within the Wilson Butte pyroclastic flow deposit along Highway 395 produced diachronous results for Miller (1985, Geology) (c. 1300 14C y BP) and others (ourselves and Wood: c. 1700 14C y BP). Three of our dates are consistent with an age of 240--339 CE, which overlaps slightly with the Northwest Coulee deposits.

Going back further, there are Mono--Inyo eruptions at 262--37 BCE, 1682--1445 BCE, 3347--3089 BCE and < 7685--7382 BCE. These last tephras, which include that from Red Cones, directly overly Pleistocene deposits.

One particular problem is worth noting. Wood's "Tephra2" is widespread and has numerous dates of c. 1200 +/- 100 14C y BP associated with it. But this is clearly not the same layer everywhere as delineated by Wood. At Deadman Summit, "Tephra 2" is the uppermost beds of the North Mono eruption of 1325--1350 CE. In distal sites, where Wood's dates were obtained, it must be an older tephra, or there is a problem with the dating.

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