Event Name : Akutan Lava Bight Lahar
|Stop: 7860 (± 50 Years) || Years BP C-14 (raw) || |
|Lahar, debris-flow, or mudflow: ||
|Pyroclastic flow, surge, or nuee ardente: ||
From Waythomas (1999): "The Lava Bight-Reef Bight area is on the west side of Akutan Island, directly west of the caldera (Figs. 1, 8 [in original text]). Limited time was spent mapping and studying this side of the island; as a result, the interpretation of volcaniclastic deposits given here is preliminary. One or more noncohesive lahar deposits are present in the Lava Bight area (section 62, Fig. 8 [in original text]). These deposits contain no clay but in outcrop are hard, compact, matrix supported, and contain mostly angular to subangular lithic clasts up to small boulder size. A matrix supported pyroclastic-flow deposit at least 15-m thick is exposed along an unnamed creek near the divide between Lava Bight and Reef Bight (section 105, Fig. 8 [in original text]). Vertical gas-escape structures and juvenile clasts of basaltic andesite are common in this deposit. Steep wave-cut cliffs at the head of Reef Bight expose a pyroclastic-flow deposit and several lahar and tephra deposits (sections 61 and 76, Fig. 8 [in original text]). Organic matter from a thin A/Cox soil on the basal lahar yielded a radiocarbon age of 7860+/-50 years B.P. (Fig. 8; Table 1 [in original text]). The basal lahar is overlain by a 10- to 20-cm-thick, dark gray (2.5Y 4/1 ), scoriaceous lapilli tephra that grades upward into a scoria-bearing pyroclastic- flow deposit containing juvenile clasts and cauliflower-shaped bombs of vesicular andesite up to cobble size. The calibrated age of the radiocarbon-dated soil provides a maximum-limiting date for the tephra and associated pyroclastic flow, and it indicates that a relatively large(?) eruption occurred approximately 8500 years B.P. Two undated noncohesive lahar deposits overlie the scoria-bearing pyroclastic-flow deposit (sections 61 and 76, Fig. 8 [in original text]). The lower of the two lahars is part of the eruption sequence recorded by the underlying pyroclastic-flow and tephra deposits. Both lahars are composed of cobble and finer, subangular to subrounded lithic clasts; matrix samples contain < 1 % clay. The lahar deposits are moderately stratified and inversely graded."