砂礫底化石群集とシーケンス層序 : 二戸・三戸地域の中部中新統産貝類化石群の例

抄録

The pectinid assemblage is found in open shallow-marine environments, especially in the gravel-sand bottom. Some problems concerning both paleoecologic and sequence stratigraphic significances of this assemblage in the Ninohe-Sannohe district are discussed as an example. The Pectinid Assemblage occurs in the base of transgressive coarse-grained deposits and in the shell-sandstone near the maximum flooding surface from the sequence stratigraphic point of view. It also exists in the marginal facies around submarine volcanoes. Modes of occurrence for these three types suggest that the first two suffered post-mortem transportation, while that from volcaniclastic deposits is para-autochthonous. Reworked fossils from older strata in the transgressive lag deposit can be discriminated. Three gravel-sand bottom assemblages were recognized based on co-occurrence of species in the middle Miocene strata of the Ninohe-Sannohe district : Mizuhopecten kimurai-Masudapecten, Gloripallium-Kotorapecten, and Nanaochlamys-Nipponopecten-Kotorapecten Assemblages. The recurrence of these assemblages are recognized in other lower middle Miocene strata of Honshu. Thus they can be treated as indigenous. The Mizuhopecten kimurai-Masudapecten Assemblage is restricted to lower part of the transgressive systems tracts in addition to volcaniclastic deposits, while other two occurred in the shell-sandstone around the maximum flooding surface. The paleogeographical position and sedimentary facies indicated that the Gloripallium-Kotorapecten community inhabited more offshore environments than the Nanaochlamys-Nipponopecten-Kotorapecten community did.