Abstract
Two types of turbidites, i.e. shallow-marine or shelfal one in the east and deep-marine or submarine-fan one in the west, were concurrently formed and accumulated to form the Pliocene Kawaguchi Formation at the eastern margin of the Tertiary Niigata sedimentary basin, central Japan. Based on the paleocurrent and heavy-mineral analyses, these two types of turbidites were supplied westward from the different sources in the eastern mainland (Tokuhashi, 1992 a, 1996). As time-series fluctuation of turbidite deposition is controlled by allocyclic and autocyclic systems, the characteristics of the different depositional systems can be detected by numerical analysis. In the present paper, first, bed thickness frequency distributions and time-series fluctuations of turbidite successions from western deep-marine facies (4 section) and eastern shallow-marine facies (3 section) were analyzed. Then, sedimentary process and system of turbidites of the Kawaguchi Formation are discussed based on the results of these numerical analyses. The main results are as follows : (1) thickness frequency distributions of non-turbidite mudstone beds in western deep-marine facies have larger variance ; (2) time-series of both turbidite thickness and number of turbidite beds fluctuate discontinuously due to the existence of turbidite packets which are formed by dozens of turbidite bed sets during several tens of years ; (3) time-series fluctuation of the advent of turbidite packets in each sections of deep-marine facies varies in sync by the Milankovitch order cycles ; (4) time-series fluctuations in shallow-marine turbidites are consecutive and independent from deep-marine ones. These results suggest that the depositional system of the western deep-marine Kawaguchi Formation, comprised of discontinuous narrow depositional tongues, has a main feeder channel, which were detached and connected to the mouth of onshore main supply river by the influence of sea-level fluctuation. However, the eastern shallow-marine turbidites were not influenced by the same environmental change and deposited independently under the different sedimentary system.
