Event Name : Edgecumbe Second Mid-Holocene Rhyolitic Ash
|Start: 4310 (± 140 Years) || Years BP C-14 (raw) || |
|Stop: 4030 (± 90 Years) || Years BP C-14 (raw) || |
|Pyroclastic flow, surge, or nuee ardente: ||
From Riehle and others (1992): "One or, locally, two thin layers of rhyolitic ash (3R287; Table 2 [in original text]) occur in soil above the latest Pleistocene fallout deposits on Kruzof Island and at Sitka. A 1-2 mm bed of very fine ash in sediment cored in a lake (not shown here) 45 km north of the MEF confirms a fall- out origin of the deposits. Deposits are coarsest (lapilli to 2 cm) and thickest (maximum of 5 cm) at Crater Ridge and southwest of Mount Edgecumbe (site 37, Fig. 1 [in original text]), implying two sources. Coarse deposits contain clasts of altered greywacke and subround pumiceous lapilli, suggesting eruption of a subsurface rhyolitic dome at a contact with greywacke."
"Riehle and Brew (1984) report overlying and underlying peat ages for these Holocene deposits of 4,030+/-90 and 4,310+/-140 yr B.P. (Beta 6004, 6005) at one site on Kruzof Island, and a single age of 5,760+/-70 yr B.P. (Beta 6003 ) from another site. The ages imply one eruption about 4.2 ka and another about 5.8 ka. The presence of only one deposit at some sites may be due to mixing of two layers by bioturbation and to limited overlap of fall- out from each ash plume. The ash bed in the sediment core is midway between the main fallout deposits and the sediment-water inter- face, consistent with a middle Holocene age."