Event Name : Redoubt Cone Building Stage
|Start: 340000 (± 10000 Years) || Years BP K-Ar || |
|Stop: 184000 (± 12000 Years) || Years BP K-Ar || |
|Lava flow: ||
|Eruption Product: || other ||
From Till and others (1994): "Basalt and basaltic andesite lava flows and flow breccia form the lower part of the present cone at Redoubt Volcano. The flows occur as high as 2560 m on the northeast flank of the mountain, so the cone stood at least that high at the culmination of the early cone-building stage (1000-1200 m above the base). Because the lava flows crop out on all sides of the volcano and are distinctive in appearance, they are useful as a stratigraphic marker horizon. Three K-Ar ages on samples from lava flows generated during this stage have been obtained (Table 3 [in original text]). Because the relative stratigraphic position of the lava flows is not known, the full time span of the early cone-building stage has not been determined. It apparently spanned a period from at least 0.340+/-0.01 Ma to 0.184+/-0.012 Ma."
"One andesite flow on Redoubt Creek crops out among deposits of the early cone-building stage. The base of the flow is not exposed, but basaltic pyroclastic deposits of the early cone-building stage sit on either side of it. The andesite flow is petrographically and chemically different from most of the flows and clasts in the block-and-ash flows formed during this stage, however. It contains less than 5% phenocrysts and 2% more SiO2 than the next most silicic flow erupted during the early cone-building stage. A whole-rock K-Ar date gives an age of 0.193+/-0.008 Ma (locality 38, Table 3 [in original text]), close to that obtained from a flow more typical of the early cone-building stage, so it has been included in that stage. Because its chemical composition is atypical of the stage, it is separately designated on plots of chemical data (see Figs. 10, 11 [in original text])."
"The early cone-building stage produced voluminous flows, flow breccia, and pyroclastic deposits that built the cone up to a minimum of 1000 m above its base. The thickest exposures of early cone-building units are on the north side of the volcano and may indicate that activity was concentrated on that side."