redoubt reported activity

Event Name : Redoubt Crescent River Lahars

Start: 3600

Years BP Tephrochronology

Lahar, debris-flow, or mudflow:

Description: From Beget and Nye (1994): "At least two clay-rich mudflows, known as the Crescent River lahars, travelled 30 km down the Crescent River valley to the coast of Cook Inlet, buried an area of almost 90 km2 and had an original volume of more than 400 × 106 m3 (Riehle and others, 1981). Both lahars contain significant amounts of hydrothermally altered rock debris, have compact clayey matrices, and are generally olive-gray to yellow to red in outcrop. The upper lahar locally contains a component of fresh-appearing gray andesitic silt, sand, and granule size material. Wood fragments incorporated in the lahars are not burned, although some appear slightly charred on their outer edges. Prismatically jointed blocks are present in some exposures."

"The age of the Crescent River lahars can be closely determined. Small, isolated wood fragments in the lower lahar have previously been dated at 3605+/-145 and 3450+/-140 yr B.P. (Riehle and others, 1981). A large tree still in growth position but inundated and broken off 2m above its base by the lower lahar was exposed by wave erosion in 1990. Wood from the outer rings of this tree dates to 3620+/-70 yr B.P. (Table 1 [in original text]). If taken together and averaged, the three dates on wood from the lahars suggest an age for this deposit of 3589+/-58 yr B.P., while an average of the two apparently concordant dates yields 3617 +/-63 yr B.P. Thus, the lower Crescent River lahar appears to date to about 3600 yr B.P., corresponding to a calibrated age between 3963 and 3875 calendar years ago (Table 1 [in original text]). There is no soil development visible between the upper and lower lahar, and the contact is sharp and conformable at several excellent exposures in sea cliffs, which suggests that the upper lahar was deposited very soon after the lower lahar."