Event Name : Augustine Northeast Point Debris Avalanche
|Start: 1725 (± 25 Years) || Years BP C-14 (raw) || |
|Stop: 1470 (± 160 Years) || Years BP C-14 (raw) || |
|Debris-avalanche, volcanic avalanche, or landslide: ||
|Eruption Type:||Not an eruption.|
From Waitt and Beget (1996): "Between 1700 and 1400 yr B.P. (between tephras I and H) a large debris avalanche shed east and east-northeast to the sea."
"Underlying the H tephra, a coarse and massive diamict extends coastwise for more than 4 km from north of Northeast Point to south of East Point. The deposit contains angular boulders of gray andesite as large as 7 m in intermediate diameter. The surface is broadly hummocky, having a relief of 3 m over distances of 50 m. At East Point is at least 20 m thick (base nor exposed); it tapers south 1.5 km south of East Point to 7 m thick, from where it pinches out within a few hundred meters. Its south part overlies three older debris-avalanche deposits (see above). The north side of the main body of deposit is delineated by a straight levee 3-8 m high. But similarly hummocky, bouldery deposit also lies north of (outside) this levee. This northern bouldery debris may also be part of the Northeast Point debris avalanche, but of a phase that immediately preceded the levee-forming phase. This coarse rubbly diamict is exposed in the walls of incised gullies upslope, apparently tracable up to the base of the summit dome complex. Mantling tephra (lowest coarse tephra = H) and humus three meters and more thick have considerably smoothed this topography. But beneath this mantle in coastal exposure it has a sharp local relief of at least 6 m over distances of 20m. A few boulders larger than 2.5 m in the top of the deposit protrude through the thick tephra-humus blanket."
"About 0.8 km south of Northeast Point the upper part of the deposit is strikingly monolithologic, every fragment angular, only a low proportion finer than pebble size. Here the deposit appears to be a single dome block at least 30 m long; it is more disaggregated than so-called "jig-saw" blocks in the 1980 Mount St. Helens avalanche described by Glicken (1986), yet it did not dilate and disaggregate enough to form or admit matrix. This huge block must have piggybacked more or less intact atop the moving avalanche, for such a gigantic block cannot have been moved by pyroclastic flow or lahar. Detterman (1973) mistook this bouldery fragmental deposit, like the others along this coast, for in-situ lava flow."
"At Northeast Point the deposit crosses a straight east-facing scarp 650 m long and 30 m high that lies about 450 m back from the sea at Northeast Point. This sea cliff had been cm into tile Yellow Cliffs (and older?) debris-avalanche deposits. The capping section including a lowermost coast pumice (layer H) is the same before the scarp as it is behind. Thus the scarp was overridden and buried by the Northeast Point debris avalanche albeit not thickly enough to obliterate it utterly (fig. 6B [in original text]). The debris avalanche deposit is at least 20 m thick (base not exposed) seaward of the buried sea cliff whose original height therefore must have been at least 40 m."
"Nearly continuous boulders, some as large as 5 m, extend at least as far as 600 m offshore of Northeast Point, showing that the original deposit has been eroded back at least that far into its present sea cliff."
"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."