山陰地方の中新統層序と古地理  [in Japanese] Miocene stratigraphy and paleogeography of the San'in district, Southwest Japan  [in Japanese]

山陰地方の中新統について, 各堆積区ごとに従来の資料を整理して層序対比を行った。山口県油谷湾以西の西海区は, 漸新世から引き継がれた北九州の堆積盆の延長であり, 厚い外洋性堆積物からなる中部中新統が漸新統を不整合で覆う。中国山地に模式的に分布する瀬戸内区中新統は, 中期中新世の海侵に伴う熱帯浅海堆積物が顕著であるが, 上部は深海化と閉鎖的海域の出現で特徴づけられる。本地方の大部分を占める山陰北陸区では, 激しい火成活動を伴う非海成の下部中新統と海成中部中新統からなり, 山陰中部, 山陰東部, 島根半島, および隠岐諸島で, 堆積時の構造運動を反映して層序と岩相に違いがみられる。層序対比をもとにして本地方の中新世を波多期(22-16.5 Ma), 川合期(16.5-16 Ma), 久利期(16-14.5 Ma), 大森期(14.5-14 Ma), 布志名期(14-12 Ma), 松江期(12-10 Ma)および都万期(8-6 Ma)に分けて, それぞれ古地理図を示した。

Stratigraphic correlation of the Miocene formations in the San'in district along the Sea of Japan side of southwest Japan, was made on the basis of recent biochronological and radiometric age data (Fig. 2). The western area of the district, belonging to the Saikai sedimentary province, consists of Middle Miocene marine sediments uncomformably overlying thick Oligocene sediments. The Middle Miocene is characterized by thick turbidites with rare igneous activity. The inland area of the district, with comparatively thin marine sediments of the Middle Miocene, belongs to the Setouchi sedimentary province. The sediments indicate a marine invasion under a tropical environmentat about 16.5 Ma which rapidly changed into a deep and stagnant environmentat about 14.5 Ma. The Setouchi Miocene formations are also sporadically distributed along the western coast of the district. Another areas of the district belong to the San'in-Hokuriku sedimentary province. It is characteristic of this province that the marine formations of the Middle Miocene overlie the non-marine Lower Miocene formations which are accompanied by vigorous acidic and intermediate igneous activity. Several differences in the Miocene sequence are recognized among the formations of the central San'in, eastern San'in, Shimane Peninsula and Oki Islands. In the central San'in area the marine formations, unconformably underlain by the Lower Miocene, have a discontinuance of sedimentation between 14.5 and 14 Ma caused by andesitic volcanics of land origin. After14 Ma a cool shallow marine environment prevailed in the area and regressed gradually until the volcanism of alkali basalt at 10 Ma. The Miocene formations in the eastern San'in area show the same sequenceas in the central San'in area until 14 Ma, but marine sediments are absent there after. On the Shimane Peninsula the relation between the Lower Miocene and the overlying marine formations is generally conformable, and just after the beginning of marine invasion, an abrupt deepening of the basin proceeded with vigorous submarine volcanism. On the Oki Islands the formations conformably overlying the Lower Miocene are characterized by rare igneous activity and deep marine and diatomaceous sediment diposited until about 10 Ma.