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From volcanic sink to source: Unravelling eruption records from volcanic wet mass flow deposits in fluvial and lacustrine successions

Kyoko S. Kataoka 1, Atsushi Urabe1, Yoshitaka Nagahashi2

  • Affiliations: 1 Research Institute for Natural Hazards and Disaster Recovery, Niigata University, Japan; 2 Faculty of Symbiotic Systems Science, Fukushima University, Japan

  • Presentation type: Talk [Invited]

  • Presentation time: Tuesday 14:15 - 14:30, Room R280

  • Programme No: 3.5.5

  • Theme 3 > Session 5

Abstract

Geological records of primary tephra fall and pyroclastic density current deposits are often incomplete due to post-depositional degradation of primary deposits and low preservation potential at high relief volcanic slopes proximal to the eruption source. These effects are especially pronounced with small-scale eruptions.  This study evaluates the past volcanic activity at Adatara and Banda volcanoes (Japan) by describing and analysing volcanic wet mass flow deposits in fluvial and lacustrine successions (Kataoka and Nagahashi, 2019; Kataoka, 2023). In detail, we study eruption-associated lahar deposits and runout deposits from upstream to downstream by: 1) surveying outcrops along rivers, 2) drilling cores from fluvial terraces, alluvial and delta plains, and 3) collecting piston cores from Lake Inawashiro---the terminus of the river basins.               We identify terrestrial lahar deposits associated with small-scale phreatic eruptions at Adatara and correlate these with sub-lacustrine density current deposits. In addition, we find sub-lacustrine density current deposits in the lake core sediments associated with large-scale edifice collapses at Bandai. We characterise these event deposits in terms of sedimentary facies, mineral composition, whole-rock chemistry, clay content, alteration (clay) mineral composition, and volcanic glass-shard composition. The 14C ages from lahar deposits and the depositional ages of the lacustrine deposits constrain timing of these events and provide a more detailed chronology of volcanic eruptions at the two volcanoes. The observation that some event deposits do not correspond to previously reported tephra and lahar deposits is indicative of multiple unknown eruptions and implies a higher frequency of eruption-associated lahar events than previously assumed.