山陰地域を中心とした第三紀軟体動物化石群と古環境の変遷  [in Japanese] Molluscan faunal and paleoenvironmental changes of the Tertiary formations in the San'in district, Southwest Japan  [in Japanese]

山陰を中心とする地域に分布する第三系に含まれる軟体動物化石の産出層準はI期(漸新世), II期(前期中新世末〜中期中新世初頭), III期(中期中新世)に分けられる。I期の日置層群からはCrassostrea, Glycymeris-Dosinia, Venericardia, Angulus-Cultellus, Corbicula群集が, II期の備北層群からはCrassostrea, Tateiwaia tateiwai-Crassostrea, Vicarya-Crassostrea, T. yamanarii-Crassostrea, Vasticardium, Felaniella, Macoma-Mizuhopecten群集が, III期の出雲層群からはOstrea, Dosinia-Cardium, Macoma-Cultellus, Pecten, Lucinoma-Saccella, Glycymeris-Saxidomus群集がそれぞれ識別される。山陰地域の第三系に含まれる軟体動物化石群集を浅海砂質底相, 浅海泥質底相の群集ごとに整理して軟体動物化石群の変遷について検討した結果, 第三紀軟体動物化石群は汎世界的な海水準変動と日本海の拡大にともなう古地理の変化の影響を強く受けて変遷したものと考えられる。

Tertiary formations, containing many marine molluscan fossils, are widely distributed in the San'in district, Southwest Japan. On the basis of biochronologic framework, faunal changes with this area are divided into three stage as follow: Stage I (Late Oligocene), Stage II (late Early Miocene-early Middle Miocene) and Stage III (Middle Miocene). Marine sediments of Stage I are exposed in the western most part of Honshu. Molluscan fauna of this stage is composed of the Crassostrea, Glycymeris-Dosinia, Venericardia, Angulus-Cultellus and Corbicula assemblages of the Hioki Group. Marine fossiliferous strata of Stage II are sporadically distributed in the coastal area along the Sea of Japan and the Setouchi Sedimentary Province. This molluscan fauna consists of the Crassosryea, Tateiwaia tateiwai-Crassostrea, Vicarya-Crassostrea, Tateiwaia yamanarii-Crassostrea, Vasticardium, Felaniella and Macoma-Mizuhopecten assemblages of the Korematsu Formation in the Setouchi Sedimentary Province. Marine formations of Stage III are mainly distributed in the vicinity of Lake Shinji, central San'in district. The molluscan fauna is composed of the Ostrea, Dosinia-Cardium, Macoma-Cultellus, Pecten, Lucinoma-Saccella and Glycymeris-Saxidomus assemblages of the Izumo Group. Molluscan faunal change of the Tertiary formations in the San'in district were discussed based on comparative paleoecological characteristics of the molluscan assemblages of each stage. The changes of them were controlled by the immigration and emigration of species, along with change of marine climatic conditions. It seems to be due to paleogeographic changes, related to eustatic sea level changes and the opening of the Japan Sea.