魚沼層群にみられる海成層の堆積サイクルと相対的海水準変動  [in Japanese] Depositional cycle of marine beds and relative sea-level changes of the Plio-Pleistocene Uonuma Group, Niigata, central Japan  [in Japanese]

新潟県の東頸城丘陵には, 最大層厚3000 mにもおよぶ更新統の魚沼層群が分布している。魚沼層群は, 海成層と非海成層の繰返しからなり, 多くの火山灰鍵層を挟む。これらは, 岩相の特徴から下位より塚山層, 小国層に区分される。これらの海成層と非海成層の繰返しは, 氷河性海水準変動と構造運動による海進・海退により形成されたことが示唆されている。魚沼層群の海成層の堆積相解析から, 海成層は, 内側陸棚, 外浜, 前浜や内湾の堆積物からなる。これらの堆積サイクルは, 塚山層と小国層では異なり, 塚山層では前浜-外浜タイプ, 小国層では内湾-沿岸タイプからなる。魚沼層群の各海成層と酸素同位体比曲線, 火山灰層の年代, ナンノ化石層序との対比から, 魚沼層群の海成層と非海成層の繰り返しの周期は, 4万〜10万年周期の氷河性海水準変動に起因している。

The Plio-Pleistocene Uonuma Group, well exposed in the Higashikubiki Hills of Niigata Prefecture, central Japan, attains more than 3,000 m in the total thickness. Clastic sediments of the Uonuma Group mainly consist of cyclic changes in marine and non-marine beds intercalated with many volcanic ash markers. They are divided into the Tsukayama and Oguni formations based on lithofacies features of the sediments. The marine beds interbedded with the non-marine beds suggest that the transgressive and regressive processes were controlled by glacio-eustatic sea-level changes. According to analysis of sedimentary facies, marine beds consist of inner-shelf, shore face, beach and bay-lagoon sediments. Depositional cycles of marine beds differed in the Tsukayama and Oguni formations. That is, marine beds of the Tsukayama Formation were formed by the the "Beach-Shoreface type". In contrast, those of the Oguni Formation were formed by the "Bay-Nearshore type". By correlating each marine bed with the stable oxygen isotope sea level index, based on an age framework for the Uonuma Group, integrated with the volcanic ash age and nanno-fossil analysis, the periodicity of cyclic changes in marine and non-marine sediments of the Uonuma Group can be estimated at about 40,000 to 100,000 years.