redoubt reported activity

Event Name : Redoubt Harriet Point Debris Avalanche

Start: 13000	Years BP Tephrochronology
Stop: 10460 (± 80 Years)	Years BP C-14 (raw)

Debris-avalanche, volcanic avalanche, or landslide:

Description: From Beget and Nye (1994): "A previously unrecognized debris avalanche deposit from Redoubt Volcano covers almost 40 km 2 in the lower Redoubt Creek area (Beget and Nye, 1990). We name this deposit the Harriet Point debris avalanche for excellent exposures in sea-cliffs near Harriet Point (Figs. 2, 3 [in original text]). The deposit forms a light-yellow to gray bouldery, hummocky layer 5-15 m thick that can be traced in outcrops along almost 7 km of sea cliffs north of Redoubt Creek. The yellowish avalanche deposit typically overlies gray clays and silt of late Pleistocene age, which appear in part to consist of the Bootlegger Cove Clay (Riehle and Emmel, 1980). The avalanche itself is overlain in most places by 1-2 m of peat containing several thin tephra layers. In a few areas, particularly on the northeast border of the deposit, it is overlain by coarse alluvium."

"The Harriet Point debris avalanche is the largest and most extensive such deposit known associated with any Cook Inlet or Alaska Peninsula volcano. Several debris avalanches have been mapped at Augustine Volcano, but these travelled no more than 8-12 km (Siebert and others, 1987; Beget and Kienle, 1992). A large prehistoric debris avalanche preserved south of Spurr Volcano travelled about 20 km (Nye and Turner, 1990). It is not possible to accurately estimate the original volume of the Harriet Point deposit as it has been completely eroded away near Redoubt Volcano where it was probably originally the thickest. However, a comparison of its areal extent to that of the less extensive 1980 Mount St. Helens avalanche suggests it may originally have been of similar or somewhat greater volume (Fig. 7 [in original text]). The Mount St. Helens avalanche deposited about 2.8 km 3 of debris (Glicken, 1991)."

"The Harriet Point debris avalanche must be no older than latest Pleistocene in age, as it clearly postdates the deglaciation of much of Cook Inlet, including the lower Redoubt Creek drainage ca. 14-15 kyr B.P. (Hamilton and Thorson, 1983; Schmoll and Yehle, 1986). The avalanche deposit overlies clays and silt thought to correlate with the Bootlegger Cove Formation (Riehle and Emmel, 1980). The Bootlegger Cove Clay has been dated at other localities to ca. 13-15 kyr B.P. (Schmoll and others, 1972)."

"Several radiocarbon dates were obtained from peat sections that immediately overlie the avalanche near Harriet Point (Table 1 [in original text]). The oldest date was 10,460+/-80 yr B.P., obtained from a horizon separated from the avalanche by 10-20 cm of silt. There was no soil development visible below the peat, within the silt, or on the avalanche surface, which suggests that this date constitutes a close upper limiting age. The available data therefore suggest the Harriet Point debris avalanche was deposited sometime between 10.5 and 13 kyr B.P."