Event Name : Augustine Yellow Cliffs Debris Avalanche
|Start: 2160 (± 60 Years) || Years BP C-14 (raw) || |
|Stop: 1830 (± 80 Years) || Years BP C-14 (raw) || |
|Debris-avalanche, volcanic avalanche, or landslide: ||
|Eruption Type:||Not an eruption.|
From Waitt and Beget (1996): "Between about 2100 and 1800 yr B .P. (between tepbra layers G and D) two large debris avalanches swept east and southeast to the sea."
"Overlying the G tephra (sparsely exposed) that caps the East Point debris-avalanche deposit is a highly oxidized massive diamict containing angular altered andesite boulders commonly 1-2 m in diameter, some as large as 3.5 m. This distinctively yellowish unit 5 to 9 m thick forms the middle to upper part of the sea cliff for at least 25 km along the east coast. The matrix and large clasts alike are strongly altered to soft zeolite (?) and clay, but the unit also contains sporadic pods meters in diameter of gray to reddish coarse diamict of scarcely altered andesite. Midway between East Point and Northeast Point its surface shows sharp local relief of 3-4 m where directly buried by the Northeast Point debris-avalanche deposit. At the south of Southeast Point the Yellow Cliffs diamict is overlain by the gray Southeast Point debris-avalanche deposit, which is capped by the I tephra."
"The hummocky surface topography and the massive and coarse texture clearly identify the Yellow Cliffs diamict as a debris-avalanche deposit. The enclosed pods of unoxidized diamict and the fact that the unit is both overlain and underlain by unoxidized to weakly oxidized similar diamicts shows that the heavy alteration had occurred before the avalanche."
"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."