The Quaternary Research (Daiyonki-Kenkyu)

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Drastic changes in the sedimentary environment occurred during the Heian Period (AD 794 to 1192) in the central Tsugaru Plain. Observation of sedimentary facies in trenches of the excavated Jusanmori archaeological site, one of the largest village of the middle Heian Period (around AD1000) in this area, as well as data from subsurface sediments at 21 sites obtained by hand auger, have revealed an extensive distribution of volcaniclastic sands (lahar deposits) abruptly overlying organic-matter rich (peaty) silt and peat in the area. Radiocarbon ages from the peaty silt, mineral compositions, and shape and refractive index of volcanic glass shards show a sudden sedimentary facies change from peaty silt to volcaniclastic sand in the early Heian Period. The volcaniclastic sand bed contains a high proportion of glass shards derived from the Towada-a tephra (AD 915 erup-tion of Towada volcano). This indicates that pre-eruptive low-energy wetland to floodplain in the central Tsugaru Plain was inundated by lahar after the AD915 eruption. The Baegdusan-Tomakomai tephra fall (B-Tm tephra : ∼AD 930 to 940) overlying the lahar deposits implies the termination of the lahar event within decades.
In the central Tsugaru Plain, many archaeological sites appear above the horizon of the volcaniclastic sand bed (i.e., lahar sediments after the 915 eruption). This is probably because the extensive sandy plains formed by lahar events were preferable places to settle for Heian people and eventually caused an increase in habitants in the area.

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