抄録
The Tsuike volcanic ash bed (TsA)(newly defined) is found in the marine silt-sand beds of the Uonuma Group over 24 km in the Uonuma Hill. The Tsuike volcanic ash bed is 6 to 35 cm thick, fine sand to silt sized, mainly composed of vesiculated thin glass shards with a few crystals of plagioclase, hornblende and biotite. It is intercalated about 25 m above the Tsujimatagawa volcanic ash bed (1.75 Ma), and about 70 m below the SK110 volcanic ash bed (1.61 Ma) at Tagawa, Tokamachi city and the age of the Tsuike volcanic ash bed was estimated to be about 1.7 Ma. The ash bed is also identified in the Niitsu Hill at Jojo, Kamo City (4 cm thick) and Sugenosawa, Gosen City (5 cm thick). At Sugenosawa in central Niitsu Hill, it is intercalated about 5 m above the basal part of the marine mud of the Taira Formation. The Tsuike volcanic ash bed is thus useful marker tephra for correlation of the 1.7 Ma marine beds widely interbedded in the Uonuma Group and Taira Formation in Niigata Prefecture.
