秋田・山形地域における新第三系の層序と古地理・古環境  [in Japanese] Neogene paleogeography and paleoenvironment in Akita and Yamagata Prefectures, Japan Sea side of northeast Honshu, Japan  [in Japanese]

秋田・山形地域の新生界の層位学的研究成果にもとづいて, 新生代の古地理・古環境の変化を考察した。秋田・山形地域には, 前期中新世末に海水が広範に進入した。この時期の海底は非常に起伏に富んでいたが, 中期中新世前期の中頃に出羽丘陵が急速に沈降して, 全域が沈水した。引き続く中期中新世中期には日本海は主として北方のみに開いた海となり, 以後この状態が基本的に維持された。海岸線は, その後, 後期中新世に脊梁山脈が, そして鮮新世に出羽丘陵が隆起に転じたため, 順次西側に移動し, あわせて浅海堆積相が支配的になる。しかし, 男鹿半島地域で大陸棚の底生有孔虫群集が出現するのは中部更新統鮪川層以上のことであり, この地域における浅海化の遅れが明かになった。

Neogene paleogeography and paleoenvironment in the region of Akita and Yamagata Prefectures were discussed based on the recent development of litho-, bio- and chrono-stratigraphic studies. In this region, the first Neogene marine transgression occurred at the end of Early Miocene, and a subtropical shallow water fauna of early Middle Miocene age including larger foraminifera distributed in this region and far north. The sea-bottom relief might be high during this initial stage of the Sea of Japan as shown by the occurrence of benthic faunas of shallow waters and bathyal zone within a short distance. At the middle of early Middle Miocene, this region drowned widely with the subsidence of previous land areas. During late Middle Miocene this subsidence was culminated and characteristic siliceous shale deposited throughout this region. The sea was cold as indicated by planktonic fauna and flora suggesting that the Sea of Japan opened to the north in this time. The backbone range of northeast Honshu uplifted in Late Miocene, later the Dewa Hills in Pliocene. Thus the coast line shifted gradually to the west since Late Miocene. Then this region of northeast Honshu completely emerged until earliest Pleistocene time leaving Late Pliocene shallow water deposits. The Oga Peninsula area, however, maintained a deep water condition in Early to Middle Pleistocene, and this area became the shallow shelf condition later in late Middle Pleistocene time. This westward shift of the sedimentary basin was caused by the uplift of mainland Honshu with a series of thrust faults along the present coast line.