The Mount Edgecumbe Tephra Deposits, a Marker Horizon in Southeastern Alaska Near the Pleistocene-Holocene Boundary

Abstract

Late Pleistocene tephra deposits found from Sitka to Juneau and Lituya Bay are assigned to a source at the Mount Edgecumbe volcanic field, based on similarity of glass compositions to nearvent deposits and on thinning away from Kruzof Island. The sequence of near-vent layers is basaltic andesite and andesite at the base, rhyolite, and mixed dacite and rhyolite on top. The only breaks in the tephra sequence are two 1-mm-thick silt partings in a lake-sediment core, indicating a depositional interval from basaltic andesite to dacite of no more than about a millennium. Tephra deposits at sites >30 km from the vent are solely dacite and rhyolite and are 10,600 to 11,400 14C yr old based on interpretation of 18 radiocarbon ages, including 5 by accelerator mass spectrometry (AMS). Basaltic andesite and andesite deposits nearer the vent are as much as 12,000 yr old.Discrepancy among radiocarbon ages of upland tephra deposits provisionally correlated as the same grainfall is resolvable within ±2 σ of analytical uncertainty. Comparison of bulk and AMS ages in one sediment core indicates a systematic bias of +600 to +1100 yr for the bulk ages; correlation of tephra deposits among upland and lacustrine sites implies an additional discrepancy of 200–400 yr between upland (relatively too young) and lacustrine ages. In any case, the Mount Edgecumbe tephra deposits are a widespread, latest Pleistocene stratigraphic marker that serves to emphasize the uncertainty in dating biogenic material from southeastern Alaska.

Late Quaternary Vegetation Development Following Deglaciation of Northwestern Alexander Archipelago, Alaska

“…In the Sitka area east of Kruzof Island, volcanic ash deposits 1-3 m thick overlie glacial till and bedrock (Yehle, 1974). The volcanic deposits recovered in 40JC do not represent the entire postglacial eruptive sequence, as a longer series of tephras have been described from an undated lake sediment core from Chichagof Island (Riehle et al, 1992b;Addison et al, 2010). The older tephras in the lower part of that Chichagof lake core included a basaltic andesite tephra that does not occur in 40JC, and was produced during an early stage of the eruption sequence (Addison et al, 2010).…”

Using cryptotephras to extend regional tephrochronologies: An example from southeast Alaska and implications for hazard assessment

“…An SC less than 0.90 indicates no correlation (Borchardt et al, 1972;Riehle, 1985;, although these general rules need to be treated with caution. SC matrices were constructed to compare tephras within our data set and with a large data set compiled from previously published tephra analyses in southern Alaska (Riehle, 1985;Riehle et al, 1987;Riehle and Bowers, 1990;Riehle et al, 1992Riehle et al, , 1999 (selected data); Downes, 1985;Begét et al, 1991;Richter et al, 1995;Begét and Motyka, 1998;Child et al, 1998). The SC technique has the advantages of being 3.14 0.08E E E M e a n M e a n norm.…”

“…Tephra from a Younger Dryas-age Edgecumbe eruption (ca. 11,250 14 C yr BP) is found in many sites in the region (McKenzie, 1970;Riehle et al, 1992;Begét and Motyka, 1998). In the Holocene, it is believed that the volcano had two more minor eruptions around 4000-6000 14 C yr BP, but tephra from these events is only found closely adjacent to the volcano (Riehle and Brew, 1984;Riehle, 1996).…”

Tephrochronology of the Brooks River Archaeological District, Katmai National Park and Preserve, Alaska: what can and cannot be done with tephra deposits

“…Replicate analyses of sample splits show that values of 0.96 0.98 are typical of pairs of samples of the same deposit. In rare cases of unusually heterogeneous deposits, values as low as 0.93 have been reported from the same deposit (Riehle et al 1992).…”