KAGUYAK VOLCANO AND ITS HOLOCENE CALDERA, ALASKA PENINSULA

Kaguyak Caldera lies in a remote wilderness corner of Katmai National Park, 135 km NW of Kodiak and 90 km SSW of Augustine volcano. The 2.5-by-3-km caldera collapsed ~6 ± 0.5 ka during emplacement of a radial apron of poorly pumiceous crystal-rich dacitic pyroclastic flows (61-67% SiO₂). Accompanying proximal pumice-fall deposits are thin and sparsely preserved, but an oxidized coignimbrite ash is found as far as the Valley of Ten Thousand Smokes, 80 km southwest. Postcaldera events include filling of a 150-m-deep lake, emplacement of two intracaldera silicic domes (62-63% SiO₂), and phreatic ejection of lakefloor sediments onto the caldera rim. CO₂ and H₂S bubble up through the lake, weakly but widely. Camped on an island (dome) in the lake with a small inflatable boat, we sampled, photographed, and sketched panoramas of all intracaldera lavas. By helicopter, we later sampled the pyroclastic apron and all extracaldera lavas. About 120 analyzed samples (54-74% SiO₂) define one of the lowest-K arc suites on the Alaska Peninsula.

The precaldera edifice was not a stratocone but was, instead, six contiguous but discrete clusters of lava domes, each a stack of a few stubby exogenous lobes, some as old as 60 ka. Three clusters are extracaldera, but the other three were truncated by the collapse and now make up the steep north, east, and south walls inside the caldera. The climactic ignimbrite was shortly preceded in the early Holocene by radial emplacement of a 100-m-thick sheet of monolithologic glassy dacite breccia (64±1.5% SiO₂), rich in angular 1-10 m blocks in a crystal-rich, sandy-gritty matrix. Filling the notches between the three truncated dome clusters, the breccia now makes up three segments of the steep caldera wall, which beheads gullies incised into the breccia deposit prior to caldera formation. On the inside wall, the breccia sheet is unstratified, but at distal exposures as far as 3 km beyond the rim, it consists of a few crudely (normally) graded but ultracoarse flow units. They were probably shed by a big dome extruding centrally, where the caldera lake is today, in the hollow between the three now-truncated dome clusters.