Event Name : Kaguyak 6400 yBP
|Start: 6400 (± 500 Years) || Years BP Ar/Ar || |
|Stop: 6010 (± 160 Years) || Years BP C-14 (raw) || |
|Lahar, debris-flow, or mudflow: ||
|Lava dome: ||
From Fierstein and Hildreth (2008): "A generally unstratified, coarse, grey fragmental deposit forms a Holocene apron distributed west and northeast around Kaguyak caldera (Fig. 3 [in original text]). As shown in photo/sketch Figs. 4, 8, and 10 [in original text], the deposit thinly laps onto margins of the Eastern, Southern, and Northern Edifices. Both the northeast and west caldera walls are constructed of this diamicton; there would be no caldera lake if the deposit were not there. It fills what were low saddles between existing edifices, filling what were apparent drainages (WNW and NE) between the precaldera dome clusters. There is no evidence that this breccia ever covered the peaks or outer flanks of either the Northern or Eastern Edifices, and no evidence that it traveled far from source. Only when funneled into the Big River did it go farther than ~3 km. Three radiocarbon dates (locations along Big River marked with red Xs on Fig. 16 [in original text])-two on organic soil from two locations on top of the breccia and one on charred twigs found in the deposit-yield essentially identical results: respectively, 6010+/-100; 6010+/-110, and 6010+/-160 14C yrs B.P. (Fierstein 2007). In agreement, a 40Ar/39Ar date for a lava block within the breccia yields 6.4+/-5.0 ka (Table 1 [in original text])."
"Similar in all exposures around the caldera, the grey-brown, gritty matrix includes almost no clay and is seriate with the rest of the deposit, which includes angular to subangular lava blocks, commonly up to 4 m and as big as 8 m. All blocks are largely glassy, fresh to oxidized (none hydrothermally altered), dense to micropumiceous lava; there is no pumice in this deposit. Young and unglaciated, the breccia apron is cut radially by V-shaped gullies that project skyward at the caldera rim (Fig. 13 [in original text]), indicating that the gullies were beheaded by caldera collapse and thus predate it."
"[Fierstein and Hildreth (2008)] infer that the Big Breccia deposit resulted from catastrophic disruption and collapse of what was probably a new dome extruding close to the position of present-day Dome 2015."