新潟県中央・西山油帯における椎谷層(下部鮮新統)のテフラ層序と火山活動の変遷

概要

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Early Pliocene tephra marker beds in the Shiiya Formation in the Chuo and Nishiyama Oil Fields of the Niigata sedimentary basin, central Japan, were described in their horizon, field occurrence and petrograhy. Chemical composition of glass shards of these tephras was also shown. The Shiiya Formation is 5.3-4.3 Ma in age and intercalates more than 20 tephra beds. Among these, the Gtp, Km-1, Km-2, Ckz, Ztg, Obg, Nig and Cbg Tephra Beds can be traced through the surveyed area more than 30km. In the early half of the Shiiya stage (5.3-4.7Ma), tephra beds are found only 1 bed per 150,000 years and gravity flow deposits dominate, while in the later half (4.7-4.3Ma), they are frequently intercalated as 1 bed per 20,000 years and fallout deposits dominate. Most of these tephras are rhyolitic in composition, among which the Km-2 Tephra (about 4.5Ma) is per-aluminous and the Obg Tephra (about 4.4Ma) is low in CaO content (about 0.4%). The Cbg Tephra (about 4.3Ma) is alkali-rhyolitic and bluish-gray in color. It shows high alkali contents (K_2O=5.4% and Na_2O=4.4%), which is an unique tephra in the Plio-Pleistocene in the Niigata region.
2003-11-28