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Akutan reported activity





Event Name : Akutan Cascade Bight Valley Lahars

Start: 600 Years BP Tephrochronology
Stop: 470 Years BP Tephrochronology

Tephrafall: BibCard BibCard
Lahar, debris-flow, or mudflow: BibCard BibCard
Eruption Type:Explosive

Description: From Waythomas and others (1998): "At least one lahar deposit and a sequence of thin, black, sandy ash layers in Cascade Bight (fig. 3 [in original text]) document minor eruptive activity directed down this valley 470 to 600 years ago. One or more of the lahars found in Flat Bight may be correlative with this period of eruptive activity."

From Waythomas (1999): "Cascade Bight valley is south of Akutan caldera, on the Pacific Ocean side of the island (Fig. 1 [in original text]). The valley heads on the volcano flank, and lahar and tephra deposits are exposed in several locations throughout the valley (Fig. 10 [in original text]). A compact, gray, massive, matrix-supported, noncohesive lahar deposit with angular to subrounded cobble and finer lithic clasts of andesite is present at sections 56, 57, 41, 4la, and 82 (Fig. 10 [in original text]). This basal gray lahar deposit underlies much of Cascade Bight valley and likely extends to the coast."

"The basal gray lahar is overlain by a sequence of interbedded fine, light-colored (distal?) ash beds, black sandy tephras, and sand-and-cobble alluvium that accumulated on a former flood plain. Organic detritus from the base of the ash and alluvium sequence at section 82 (Fig. 10 [in original text]) yielded a radiocarbon age of 630+/-110 years B.P. (Table 1 [in original text]). Soil-organic matter from a buried soil at the top of a correlative sequence of ash and alluvium at section 41a (Fig. 10 [in original text]) yielded a radiocarbon age of 25+/-80 years B.P. and is considered modern (Table 1 [in original text]). A third radiocarbon age on soil organic matter from a buried A/Cox soil developed on tephra that overlies the basal gray lahar is 395+/-125 years B.P. (section 56, Fig. 10 [in original text]; Table 1 [in original text]). These dates indicate that Cascade Bight valley was inundated by at least one noncohesive lahar more than 500-600 years ago. The Akutan tephra was not found in Cascade Bight valley, and it is not known if the basal gray lahar deposit is part of the eruption sequence associated with the tephra. This lahar deposit is similar to others that overlie the Akutan tephra and are associated with the caldera-forming eruption."

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