Event Name : Augustine Grouse Point Debris Avalanche
|Start: 350 || Years BP Tephrochronology || |
|Stop:|| 1812 ||Tephrochronology|
|Debris-avalanche, volcanic avalanche, or landslide: ||
|Eruption Type:||Not an eruption.|
From Waitt and Beget (1996): "Between about 350 yr B.P. (after tephra layer B) and historic time, three separate debris avalanches swept to the sea on the west-northwest, north-northwest, and northflanks. One of them (West Island} was large and fast, most of it having rode to sea beyond a sea cliff cut back into older deposits."
"Grouse Point diamict is hummocky with local relief of 5 m and is composed of reddish porphyritic andesite including anfular boulders at least as large as 2.5 m. Overlying stratigraphy in upward stratigraphic succession is lithic pyroclastic-flow deposit, possibly waterlaid (tsunami?) deposit, two thin ash layers, and the distinctive white Katmai 1912 ash."
"At lowest tides a nearly continuous field of boulders extends nearly a kilometer seaward of the present beach. The deposit has been eroded back into a nearly continuous, sharply curving sea cliff 4-7 m high. Thus geomorphically this deposit seems older than the similarly coarse and hummocky West Island and Rocky Point deposits but much younger than the South Point diamict and others on the south and east coasts. It may be an eastern arm of Lagoon debris-avalanche deposits."
"Grouse Point debris-avalanche deospit covers part of the sea cliff cut into North Bench deposit. By this and its far more hummocky surface texture, Grouse Point diamict is geomorphically distinct from North Bench diamict. Also contrasting the arrested sea cliff of Borth Bench, Grouse Point deposit juts into the sea. Grouse Point deposit must be considerably younger than North Bench diamict."
"Grouse Point diamict is not proven to predate the B tephra, exposed in gullies upslope from North Bench diamict 8unit IMan). Beget and Kienle (1992) having not distinguished North Bench diamict from Grouse Point diamict, they inferred the Grouse Point as well as North Bench diamict to underlie the B tephra."
"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."