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Augustine reported activity





Event Name : Augustine North Bench Debris Avalanche

Start: 1470 (± 160 Years) Years BP C-14 (raw)
Stop: 1110 (± 70 Years) Years BP C-14 (raw)

Debris-avalanche, volcanic avalanche, or landslide: BibCard BibCard
Eruption Type:Not an eruption.

Description: From Waitt and Beget (1996): "Between about 1400 and 1100 yr B.P. (between tephra layers H and C) one debris avalanche swept to the sea on the south, another on the southwest, and perhaps a third on the north-northwest."

"North Bench diamict comprises angular porphyritic andesite fragments as large as 3.5 m, bearing a mildly hummocky topography with local relief of 5 m. Seaward the deposit is truncated by a gently arcuate wave-cut scarp as high as 23 m, the highest near-coastal topographic feature along the west and north coasts. This sea cliff is now isolated 340 to 450 m from the sea by younger deposits that have flowed coastwise in the manner shown in figure 6c [in original text]. Beget and Kienle (1992) ignored the eastern part of this feature and lumped the western part with their Grouse Point debris-avalanche deposit. The North Bench bouldery diamict is probably a debris-avalanche deposit, but an origin by lahar or lithic pyroclastic flow cannot be disqualified."

'North Bench diamict is geomorphically distinct from three neighboring debris-avalanche deposits: West Island, Grouse Point, Rocky Point (described below). The two segments of the arrested sea cliff truncating North Bench are nearly surrounded by theses younger diamicts. A straight scarp behind West Lagoon partly burned by West Island debris-avalanche deposit (see below) may be a west extension of this sea cliff."

"The North Bench diamict is overlain by a pyroclastic-flow deposit (undistinguished on map [in original text]) and by the Katmai 1912 ash and two older sand ashes. This tephra stratigraphy is about the same as that overlying the nearby clearly younger debris avalanche deposits such as at West Island and Grouse Point. But eh geomorphic expression including a nearly continuous straight sea cliff of moderate height well back from present coast indicates a much older age of the North Bench diamict, perhaps in the Hc or IH interval. Probably the overlying tephra stratigraphy is highly incomplete, perhaps because the veneering pyroclastic-flow deposit is itself much younger than the North Bench diamict."

"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."

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