Event Name : Augustine West Lagoon Pyroclastic Flow/Lahar
|Start: 760 (± 60 Years) || Years BP C-14 (raw) || |
|Stop: 380 (± 20 Years) || Years BP C-14 (raw) || |
|Pyroclastic flow, surge, or nuee ardente: ||
From Waitt and Beget (1996): "Between about 700 and 400 yr B.P. (between tephra layers M and B) a debris avalanche swept to the sea on the west and a small one on the south-southeast. Large lithic pyroclastic flows shed to the southeast; smaller ones descended exiting swales on the southwest and south."
"Atop a sea cliff now isolated along the inner margin of West lagoon is a 4-m massive cobbly sand, apparently lithic pyroclastic-flow (or lahar?) deposit (unit MBplo). It is capped by B (?) tephra, which at one site is underlain by M (?) tephra. Upslope at altitudes 40-90 m the deposit is far more boulder and diversified by intricately lobate termini (unit MBpli). The outboad portion formerly extended at least 300 m seaward into West lagoon, its legacy a lag of boulders as large as 2.5 m. This wave erosion must have occurred during the M-B period, after which a growing sand spit enclosed the lagoon and isolated the sea cliff from ocean waves."
"During this prehistoric period numerous domes must have been emplaced at the summit, repeatedly renewing the source for catastrophic debris avalanches. Remnants of these older domes form the east and south sides of the present summit-dome complex. Below the summit area at least three domes were emplaced on the upper flanks, one on the south (Karnishak dome), two on the northwest (domes "I" and "H"). Another undated and nearly buried dome or lava flow diversifies the upper south flank."