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redoubt reported activity





Event Name : Redoubt Rust Slough Lahar and tephra

This is a questionable event.

Start: 400 Years BP Tephrochronology
Stop: 200 Years BP Tephrochronology

Lahar, debris-flow, or mudflow: BibCard
Fumarolic or hydrothermal activity: BibCard
Geomorphologic change: BibCard BibCard BibCard
Eruption Type:Not an eruption.

Description: From Beget and Nye (1994): "An assemblage of young flood deposits and a slightly older lahar are exposed in stream cuts along Rust Slough, only a few kilometers to the southwest of the Drift River Terminal (Fig. 11 [in original text]). The basal lahar in this assemblage was exposed just at the 1990 water level along Rust Slough, is commonly yellow to red, and is referred to here as the Rust Slough lahar. It consists primarily of highly oxidized and altered lithic clasts in a clayey matrix of hydrothermally altered material and is 2-3 m in thickness (Fig. 12 [in original text]). The abundance of hydrothermally altered material suggests that this deposit records a partial collapse of part of a prehistoric summit edifice of Redoubt Volcano. The reddish color of the waters in Rust Slough may in part reflect the leaching of iron oxides and the erosion of yellow and reddish clays from stream cuts through this deposit. This thick lahar is exposed for several kilometers along upper Rust Slough, but probably underlies a much larger area of the Drift River fan where it has been buried by younger deposits."

"A 2-4-cm-thick peat horizon buried by the Rust Slough lahar was dated at 1060+/-70 yr B.P. (Table 1 [in original text]). The peat overlies thinly bedded alluvium that is locally as much as 2 m thick, and extends below the current level of Rust Slough. The yellow lahar itself is overlain by as much as 2 m of alluvium that has buried a thin soil and peat horizon developed on the surface of the Rust Slough lahar. Tree stumps as much as 30 cm in diameter are preserved in growth position above the yellow lahar; one stump was dated at 260+/-60 yr B.P. and provides an upper limiting date on the underlying yellow lahar. A concordant upper limiting date was obtained from a similar site in the upper Drift River valley (see below)."

"There are isolated and rounded pumice lapilli in the lake- sediment section, but no deposits of airfall tephra layers or pyroclastic flows, which suggests they formed during a dormant interval at Redoubt Volcano. The apparent existence of a Drift Glacier dam of the upper Drift River valley also suggests a period without major eruptions. The 1966 eruptions largely destroyed the upper parts of the Drift Glacier, and produced a prolonged period of stagnation that stopped an advance that might have dammed the upper Drift River. The Drift Glacier had largely reformed by the late 1980s and was advancing again (Sturm and others, 1986), but the 1989-1991 eruptions again removed its upper parts. Similar destruction of the glacier by eruptions almost certainly did not occur during this prehistoric interval when it was large enough to dam the upper valley."

"The lake sediments are overlain by a ca. 1-m- thick, clay-rich yellow lahar, and a similar deposit of yellow, hydrothermally altered clayey debris is locally exposed in stream cuts for more than hundred meters along the Drift River downstream from the lacustrine section. This distinctive yellow lahar is thought to be correlative with the thick yellow lahar found at Rust Slough. Both overlie non-volcanic deposits, as the upper lahar rests upon the lacustrine sequence, while the clayey lahar at Rust Slough overlies thinly bedded alluvium. In addition, a thin soil above the yellow lahar at the lake sediment section was dated to 210+/-50 yr B.P., close to the date of 250+/-60 yr B.P. on wood from above the clayey yellow lahar at Rust Slough (Fig. 12 [in original text]). The very weak soil development on this deposit suggests it is not much older than the radiocarbon dated soil, so that this widespread clay-rich lahar appears to record a massive slope failure of hydrothermally altered debris from the north side of Redoubt at ca. 200-400 yr B.P."

"The presence of the Rust Slough lahar near the Drift River Terminal more than 30 km downvalley from Mount Redoubt indicates this deposit was originally quite extensive. We cannot directly reconstruct its areal extent and volume, as the Rust Slough lahar has been largely buried by younger lahars and flood deposits. However, if a distribution similar to the flood deposits of the 1989-1990 eruption is assumed, its original volume may have been on the order of 100-200 X 106 m 3. For comparison, the volume of the Crescent River lahars has been estimated at 435 X 106 m 3 (Riehle and others, 1981). The modern summit basin, partly exposed by melting of the upper Drift Glacier during the 1989-1990 eruptions, is about 200-250 m deep, open to the north, and has a volume of about 100 X 10 6 m 3. Perhaps this crater reflects a massive slope failure several hundred years ago that produced the Rust Slough lahar."

From Beget and others (1994): "Recent field studies at Redoubt Volcano have identified a number of prehistoric lahars and intercalated weak soils preserved in the Rust Slough near the Drift River which were deposited approximately 250 14C yr B.P., corresponding to a calibrated calendar age of ca. 350 years ago (Beget and Nye, 1994). The similarity in age between the pair of Redoubt tephras found in Skilak Lake and the Rust Slough lahars suggests that the newly discovered tephras in Skilak Lake and the prehistoric lahars in the Rust Slough are broadly coeval, and apparently record a series of eruptions of Redoubt Volcano which occurred sometime between 300 and 400 years ago."

From Beget and others (1994): "The next lower tephra occurs at a depth of 24-40 cm, depending on local core sedimentation rates (Fig. 2 [in original text]). At core sites with low sedimentation rates this tephra lies close to or is even mixed with the overlying Crater Peak ash discussed above. This tephra is geochemically quite similar to the tephra erupted in 1989-1990 and 1902 at Redoubt Volcano, and appears to record a previously unknown Redoubt eruption at ca. 350+/-50 years ago, based on sedimentation rate extrapolations."

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