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Wrangell reported activity





Event Name : Wrangell Debris Avalanche

This is a questionable event.

Start: 342000 (± 16000 Years) Years BP K-Ar
Stop: 270000 (± 40000 Years) Years BP K-Ar

Debris-avalanche, volcanic avalanche, or landslide: BibCard
Eruption Type:Not an eruption.

Description: From Waythomas and Wallace (2002): "An areally extensive volcanic mass-flow deposit of Pleistocene age, known as the Chetaslina volcanic mass-flow deposit, is a prominent and visually striking deposit in the southeastern Copper River lowland of south-central Alaska. The mass-flow deposit consists of a diverse mixture of colorful, variably altered volcanic rocks, lahar deposits, glaciolacustrine diamicton, and till that record a major flank collapse on the southwest flank of Mount Wrangell...Deposits of the Chetaslina volcanic mass flow in the Chetaslina River drainage are primary debris-avalanche deposits and consist of two principal facies types, a near-source block facies and a distal mixed facies. The block facies is composed entirely of block-supported, shattered and fractured blocks with individual blocks up to 40 m in diameter. The mixed facies consists of block-sized particles in a matrix of poorly sorted rock rubble, sand, and silt generated by the comminution of larger blocks. Deposits of the Chetaslina volcanic mass flow exposed along the Copper, Tonsina, and Chitina rivers are debris-flow deposits that evolved from the debris-avalanche component of the flow and from erosion and entrainment of local glacial and glaciolacustrine diamicton in the Copper River lowland. The debris-flow deposits were probably generated through mixing of the distal debris avalanche with the ancestral Copper River, or through breaching of a debris-avalanche dam across the ancestral river. The distribution of facies types and major-element chemistry of clasts in the deposit indicate that its source was an ancestral volcanic edifice, informally known as the Chetaslina vent, on the southwest side of Mount Wrangell. A major sector collapse of the Chetaslina vent initiated the Chetaslina volcanic mass flow forming a debris avalanche of about 4 km3 that subsequently transformed to a debris flow of unknown volume."

"Analysis of a single pumice pyroclast in the pyroclasticflow

deposit (Fig. 8 [in text]) gave a K-Ar age of 342 ± 16 ka (D.H.Richter, personal communication, 2000 and unpublished data), and we consider this a maximum-limiting age for the debris avalanche. In the upper Chetaslina River drainage, debrisavalanche deposits are overlain by a lava flow that yielded a K-Ar age of 270 ± 40 ka (Nye 1983). Thus, the debris avalanche probably formed between about 340 and 270 ka."

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