Paleoenvironmental Studies of the Nishiyatsushiro and Shizukawa Groups, South Fossa-Magna Region
スポンサーリンク
概要
- 論文の詳細を見る
The forearc basin-trench system is generally characterized by a paleoenvironmental complex which is reflected in various lines of geological and paleontological evidence. The Fujikawa area in the South Fossa-Magna region belongs to a part of such a complex system along the tectonically active margin of southwest Japan. Analysis of foraminiferal biofacies and lithofacies of the Neogene Nishiyatsushiro and Shizukawa Groups, distributed in the area, reveals complicated patterns of their interaction, which are attributed to the Neogene depositional trends innate to a tectonically complex portion of the Pacific rim. An emphasis is placed on the estimation of depth ranges of fossil benthic foraminiferal species by assessing both the distribution of modern counterparts and sedimentological properties in a similar active continental margin setting. Such data are used to reconstruct depositional environments of the Neogene sequences. A quantitative analysis of benthic foraminiferal species in the Neogene sequences reveals four important paleoenvironmental factors influencing the species composition. These factors include such environmental parameters as water depth, dissolved oxygen, and effects of both bottom water currents and coastal water mass. Furthermore, nine types of paleoenvironments are deduced by integrating results of analyses of principal components and sedimentary properties as well as ecological data on modern benthic foraminifera. Each of the nine types of paleoenvironments is represented by a particular biofacies which is named after dominant species as follows : Type a (Rhabdammina abyssorum biofacies), Type b (Melonis sphaeroides-Nodosaria longiscata biofacies), Type c (Globobulimia auriculata-Melonis sphaeroides biofacies), Type d (Nodosaria longiscata biofacies), Type e (Melonis sphaeroides-Stilostomella lepidula biofacies), Type f (Stilostomella lepidula biofacies), Type g (Globobulimina pupoides-Stilostomella lepidula biofacies), Type h (Globobulimina auriculata biofacies) and Type i (Ammonia ketienziensis-Ammonia takanabensis biofacies). Foraminiferal distributions in the NW Pacific off southwest Japan and around Hachijojima Island show that three ancient biofacies (Types a, h, and i) were also distributed under the comparable environments. The present author developed a dynamic model of sedimentation history which is closely related to the developmental processes of several sedimentary basins in the Fujikawa area. Such sedimentological data as the direction of grain transport and sedimentary structures in fine-grained sedimentary rocks are obtained together with geological and micropaleontological data. Selected biogenous grains in foraminifer-bearing rocks are examined in the laboratory. The distributional pattern of these biogenous grains in the Nishiyatsushiro and Shizukawa Groups resemble that in sediments of the present-day forearc basin. These paleontological and sedimentological data together provide the basis to conclude that the Nishiyatsushiro and Shizukawa Groups were deposited in a forearc basin setting. The Misaka Group, which accumulated contemporaneously with the basal part of the Nishiyatsushiro Group, consists mostly of oceanic materials, and exhibits geological features characteristic of the accretional zone. The sedimentary basin active during the Neogene is here named the Nishiyatsushiro paleo-forearc basin and the accretional prism which is located beneath the forearc basin the Misaka paleo-trench.South Fossa Magna regionNeogeneForaminiferal biostratigraphyEnvironment of sedimentationPaleo-plate margin
- 東北大学の論文
- 1991-02-05