Stratigraphical study on the Middle Pleistocene pyroclastic flow deposits, northern Tochigi and southern Fukushima Prefectures, Northeast Japan

Abstract

This study describes the revised stratigraphy and correlations of the middle Pleistocene tephra units around northern Tochigi and southern Fukushima prefectures. We have identified three pyroclastic successions consistent with the volcanic deposits So-OT, So-TN, and So-KT (from youngest to oldest). So-OT consists of a fall pumice deposit, a surge deposit, and three pyroclastic flow units. We observed the fall deposits of So-OT at six localities, with the northernmost occurrence in southern Fukushima city. So-TN is divided into four pyroclastic flow units on the basis of observed differences in the refractive index values of the orthopyroxene and the glasses. Importantly, the refractive index values of the So-TN orthopyroxene are higher than those of the So-OT and So-KT tephra units. From the distribution of pyroclastic flow deposits, and trends in grain size and thickness of the fall deposits, the So-OT tephra is confidently attributed to the Shiobara caldera. The So-TN and So-KT tephra units become thicker and coarser toward the Shiobara caldera; furthermore, similarities in the glasses to those of So-OT indicate a shared volcanic provenance. The stratigraphy of these pyroclastic flow deposits and other important tephra units is reconstructed as follows (from youngest to oldest): the Tobiyama tephra (Az-MiP), So-OT, So-TN, A₄Pm, So-KT, and A₁Pm or A₂Pm. The So-TN and the stratigraphically younger So-OT tephra units are constrained to between the dated A₄Pm (337 ka) and Az-MiP (300 ka). The stratigraphically older So-KT tephra is constrained to below the A₄Pm (337 ka) and above the A1Pm or A2Pm (337-410 ka).