Time-series analysis of turbidite sequence in the upper part of the Awa Group in the Boso Peninsula, central Japan

Abstract

The Mio-Pliocene Awa group exposed in the Boso Peninsula, central Japan, consists mostly of a series of turbidite and hemipelagic sediments. As many key tuff beds, traceable through the whole area, are intercalated in these sediments, the three-dimensional structure of the sequence have been investigated on the basis of these key beds by previous workers. In this study, the bed thickness of the Kiyosumi and Anno formations in the Awa Group are numerically examined by statistical investigations of thickness frequency distribution and by time-series analysis of turbidite frequency and magnitude along several study routes. The fact that the frequency distributions of hemipelagic approximate and exponential distribution implies that the accumulations are ruled by a Poisson process. The frequency distributions of turbidite thickness approximate a log-normal distribution of the Kiyosumi Formation. However, those in the Anno Formation show a bimodal distribution which suggests the effect of cross-sectional geometry of the turbidite beds. The time-series variations in both turbidite number and thickness in a unit hemipelagite interval reveal a marked periodicity. The dominant periods are estimated to 6 to 20 ky, which are comparable to the Milankovitch band. Each sedimentary cycle in the Anno Formation can be traced to the other sections, from proximal to distal position, using key tuff beds. The fluctuation of turbidite number sometimes does not coincide with that of turbidite thickness. This may be caused by a particular geometry of each turbidite layer, as implied by the bimodal thickness distribution.