タービダイト砂岩分布予測のための堆積学的・岩石学的・数理学的研究 新潟および房総における解析例

Abstract

The controlling factors and principles which govern the spatial and chronological distribution of turbidite sandstones have been investigated based on the results of sedimentological, petrographical, and mathematical analyses on the Neogene and Quaternary turbidite sandstones in the forearc and backarc basins, central Japan. Main conclusions are as follows:1) The sedimentological research revealed that the three-dimensional forms of individual turbidite sandstone beds, which are part of a depositional lobe of submarine fan system and regionally correlated bed by bed by using many useful tuff key beds, are similar to each other. The similarity continues vertically within the interval of a single depositional lobe or of a certain stratigraphic unit. Their thickness and distribution area reflect the magnitude of turbidity currents, while their form greatly depends on the grain size distribution of the sediments.2) The petrographical research proved that the heavy mineral composition can be used as a good indicator to recognize turbidite sandstone bodies which share a single source area, and are useful to check the original relationship of turbidite sandstone bodies isolated each other.3) The mathematical research led to the conclusion that the development of turbidite sandstones is controlled by the global changes such as the eustaic sea level changes, because the periodicity in an order same as the Milankovitch's cycles was recognized in the vertical change of number and total thickness of turbidite sandstone beds within an interval delineated by a certain accumulated thickness of hemipelagite in several turbidite successions.