Event Name : Trident 58000 yBP
|Start: 58000 (± 15000 Years) || Years BP Ar/Ar || |
|Pyroclastic flow, surge, or nuee ardente: ||
From Hildreth and others (2003): "...the southwest slope (fig. 7 [in original text]) is made up of steeply dipping (25 to 35 degrees) sheets, 0.1 to 3 m thick, of agglutinated mafic fallout rich in accidental lithic fragments, subordinate thin pyroclastic-flow deposits, and associated avalanche rubble. Derived from the central vent of Trident I, these deposits (unit tca, fig. 3 [in original text]) are generally gray brown to brick red, but parts were altered ocherous yellow or orange brown, probably while still hot. Fresh juvenile scoriae and spatter in these deposits are olivine-rich andesite, the most mafic material (53-55 weight percent SiO2) known to have been erupted from the Trident group. A dense spatter blob from agglutinate on the south slope gave a 40Ar/39Ar plateau age of 58+/-15 ka (Hildreth and others, 2003). The strata also contain abundant accidental clasts of the pyroxene andesite that makes up most of the edifice."
"An andesitic lava flow that directly underlies the summit dome of Trident I, about 100 m southwest of peak 6115, yields an age of 73+/-12 ka. Most of the edifice appears to lie stratigraphically beneath this lava, except for the mafic pyroclastic assemblage (unit tca, fig. 3 [in original text]), which apparently drapes many of the andesites and yields a 40Ar/39Ar age of 58+/-15 ka (Hildreth and others, 2003). Because much of the Trident I pile is banked against its eastern neighbor (dated at 143+/-8 ka), the active lifetime of Trident I appears to be reasonably well bracketed."