古地形の指示者としての水底流走火山灰 : 魚沼層群のSK110およびSK100火山灰の例  [in Japanese] Subaqueous ash flow deposit as an indicator of paleotopography : A case study of the SK110 and SK100 ashes in the Uonuma Group, Niigata region, Central Japan  [in Japanese]

水底流走火山灰の堆積様式から古地形がどの程度復元できるかを, 新潟地域の魚沼層群中に近接して挟在するSK110およびSK100火山灰を例にして検討した。両火山灰はそれぞれ類似した4ユニットからなり, 基底のユニットIはash flow turbiditeのユニットである。それぞれのユニットの層厚・層相および粒度(Mdφ)の側方変化を堆積環境も考慮しつつ, その運搬・堆積機構と関連づけて解釈した。両火山灰の側方変化の特徴には強い共通性が認められ, 古地形の影響を示唆している。検討の結果, 調査地域の西半部(高柳町・小国町・柏崎市・西山町地域)ではトラフ状の凹地やバリャーが存在する複雑な地形に対し, 東半部(十日町市・小千谷市地域)では概して平坦な地形であったと想定された。復元された古地形は八石背斜南部以南では現在の褶曲構造とおおむね調和的であるが, それ以北では必ずしも調和的ではない。

An attempt to reconstruct the paleotopography was made using two adjacent subaqueous ash flow deposits (the SK110 and SK100 ashes) in the Niigata region, central Japan. These earliest Pleistocene ashes extend over 40×30 km^2 and are embedded in fluvial to marine sediments of the Uonuma Group in the southern area and of the Haizume and Nishiyama formations in the northern area.. The SK110 (6-380 cm thick) and SK100 (16-300 cm) ashes are silicic, and consist of four sedimentary units (Unit I〜Unit IV). The basal unit (Unit I) of the both ashes and Unit III of SK100 ash can be referred to the ash flow turbidite units. The other units are mostly ash cloud deposits. The paleotopography was reconstructed by the lateral changes of thickness, grain size (Mdφ) and lithofacies of these units. Both in two ashes, a distinct axis revealed from thickness and more clearly from grain size was extending from south to north in the western half of the study area, while a few faint axes southwest to northeast in the eastern half. The destinct axis can be interpreted to imply the trough-like depressions in paleotopography. Thus, trough-like depression from south to north in the western half and general flatness in the eastern half were reconstructed. Besides, in the western half, the up-slope topography (barrier) which damps down the flows was ascertained in the northern Hachikokuto southern Chuo Oil Field and Kawanishi area.