MARINE EVIDENCE OF THE LATEST PLEISTOCENE MT. EDGECUMBE TEPHRA IN SOUTHEAST ALASKA

The Mt. Edgecumbe Volcanic Field is located on Kruzof Island near Sitka Sound in southeast Alaska. Previous evidence has indicated a complex eruptive history that was initiated 611 ± 74 kyr, and has had intermittent activity through the mid-Holocene (Riehle et al., 1992). An important terrestrial stratigraphic marker exists as a variable composition tephra horizon that has been correlated with a large multiple-stage regional eruption at 11.25 ± 50 ¹⁴C yr BP (Beget and Motyka, 1998). New marine sediment cores recovered from adjacent basins to Kruzof Island in 2004 contain volcaniclastic deposits that consist primarily of rhyolitic tephra, with minor dacitic, andesitic, and basaltic members as based on total alkali-silica compositions. Ten individual tephras have been identified within a core recovered from Sitka Sound (EW0408-40JC). Eight of these are rhyolitic and geochemically indistinguishable based on similarity analyses; one dacitic and one andesitic tephra were also identified. Preliminary geochemical electron microprobe analyses of these tephra deposits correlate with the Latest Pleistocene eruption. A bivalve shell recovered from the Sitka Sound core was dated at 10922 ± 58 ¹⁴C yrs, assuming a marine reservoir correction of 600 years; this shell was located stratigraphically above the tephras, thus indicating a minimum age of marine deposition and volcanic activity. However, there is a need to assess the appropriateness of applying this reservoir correction because a transition from marine to freshwater deposition occurs between this shell and the uppermost tephra based on changes in sedimentology and gamma-ray bulk density measurements. Earlier studies have shown that the Latest Pleistocene eruption of the Mt. Edgecumbe Volcanic Field produced useful regional chronostratigraphic markers in terrestrial southeast Alaska, but the present study proposes that marine cores in the adjacent offshore region may also be correlated using tephras from the Mt. Edgecumbe eruptions.