有機相からみた古環境 : 津軽半島新第三系の場合  [in Japanese] Paleoenvironmental determination using visual kerogen analysis : A study of the Neogene sediments around the Tsugaru Peninsula, northern Honshu, Japn  [in Japanese]

ビジュアル・ケロジェン分析法により得られる有機組成は, 堆積環境に影響され, 続成作用や埋没変成作用などの堆積後の過程にほとんど影響されない。したがって, ビジュアル・ケロジェン法は, 古環境推定に役立つ。津軽半島付近に分布する新第三系の古環境変遷史を, 堆積岩中のビジュアル・ケロジェン組成から推定した。新第三紀の海は, 磯松層の堆積時(18 Ma頃)に津軽半島の北西部, 小泊半島付近にまず侵入した。この海侵は, 小泊層の堆積完了時(8 Ma頃)まで引き続き進行し, そして海退へと移り変わった。しかし, 津軽半島の西方から南西方にかけては, これらと同様の地質現象が数百万年程遅れて起こっている。以上のように推定した地史は, 津軽堆積盆の新第三系に関して, 今までに報告されている古生物学的及び堆積学的データにより支持される。

Visual kerogen analysis is a useful method for determining paleoenvironment, because kerogen is present in almost all sedimentary rocks and is indicative of sedimentary conditions. Kerogen is classified into three types by visual kerogen analysis. Coaly-woody kerogen consists of charcoal and woody fragments derived from terrestrial plants. Herbaceous kerogen is composed of pollen, spores and terrestrial plant leaves. Amorphous kerogen without any clear outline, distinct shape and identified structure, is considered to be derived from marine microorganisms (Tissot and Welte, 1984). Therefore, terrigenous sediments contain much more coaly-woody and herbaceous kerogen than amorphous kerogen. Terrigenous sediments contain less than 35% amorphous kerogen in our samples. The content of amorphous kerogen in sedimentary rocks tends to be higher with an increase of depositional depth in the sea. The paleoenvironmental history during the Neogene around the Tsugaru Peninsula was determined by visual kerogen composition. The sea first invaded around the Kodomari Peninsula in the northwest part of the Tsugaru Peninsula at the time of deposition of the Isomatsu Formation (about 18 Ma). This transgression continued until the end of deposition of the Kodomari Formation (about 8 Ma). Then the transgression was replaced by regression. But in the west to southwest area of the Tsugaru Peninsula similar geological events took place a few million years later. This history is consistent with all data on paleontology and sedimentology of the Neogene in the Tsugaru basin reported by other authors.