BASE-SURGE MECHANICS AND STRUCTURES, AN EXAMPLE FROM SINKER BUTTE, ID, WESTERN SNAKE RIVER PLAIN

A base surge is defined as a type of pyroclastic surge that forms at the base of eruption columns and travels outward during some hydroclastic eruptions (Fisher and Schmincke, 1984). They move radially, gradually thinning, as they get further away from the vent. Base-surge deposits are wedge shaped, thickest near the vent, and thinnest towards the distal ends of the tuff cone/ring. Dune-form bedding set thickness decreases approximately logarithmically away from the vent (Wohletz and Sheridan, 1979) and average wavelength also decreases away from the vent (Waters and Fisher, 1971). The process of deposition within base surges is very complex, as the mechanics of the base surges have many constantly changing parameters. As base surges move laterally away from the eruptive vent, they lose heat, sediment load, and velocity, which directly affects the resulting deposits. The types of sedimentary structures observed within surge deposits are also directly related to the amount of liquid water involved.

At Sinker Butte, the surge deposits proximal to the vent display large, low-angle regressive cross-strata that can be easily broken out from the more planar, laterally continuous airfall deposits. The wavelengths and bedding thicknesses of these cross-strata decrease nearly logarithmically away from the vent, which is consistent with observations at other phreatomagmatic volcanoes. Grain sizes within the surge deposits vary from fine ash to fine lapilli near the vent and from fine ash to medium ash in distal areas. This is due to the loss of sediment carrying capacity as the surge loses velocity. The presence of abundant armored lapilli, vesiculated tuffs, and the regressive or up-stream migrations of the cross-strata in deposits near the vent all indicate that the base surges at Sinker Butte were “wet” three-phase flows. However, as the surge deposits are traced further from the vent, the cross-strata become progressive, or migrate away from the vent. This is likely due to the decrease in particle concentration and flow velocity, as regressive cross-strata also indicate upper flow-regimes.