Three Dimensional Analysis of a Large Sandy-flysch Body, Mio-Pliocene Kiyosumi Formation, Boso Peninsula, Japan

Abstract

Three dimensional analysis of flysch sequence is very important as it makes substantially possible to compare the depositional process of it with that of the current submarine fan model and to clarify the phased development of submarine fan sedimentation. Such study has been almost lacking until now. The Kiyosumi Formation is composed of a large lenticular sandy-flysch body, measuring 850 m in maximum thickness and more than 20 km in E-W direction along the folding axis and more than 6 km in N-S direction. The Kiyosumi Formation is divided into five units by means of six main tuff marker-beds. Each unit is composed mainly of both channel deposits and depositional tongue. Channel deposits are characterized by an upward thinning cycle beginning with thick pebbly sandstones and ending with thin siltstone-dominated alternations and by accompanying a large trough-like erosive morphology at the base attaining up to some 50 m in maximum depth and more than 5 km in maximum width. The depositional tongue is characterized by extensive and thick sandstone-dominated alternations downcurrent of the channel deposits and shows negligible basal erosion. Further downcurrent, the depositional tongues are replaced by siltstone-dominated alternations or massive siltstones. The Kiyosumi Formation has been deposited by the lateral supply from north into the E-W trending restricted slope basin by the same process as that of the recent submarine fans. Channel deposits in each unit seem to have been deposited on the upper-fan segment. Depositional tongue in each unit must have formed a suprafan (NORMARK, 1970) on the middle fan segment. Siltstone-dominated alternations or massive siltstones must have been deposited on the lower-fan segment or basin floor. However each depositional tongue in the Kiyosumi Formation, covering a wide area more than 20 km, occupys a much more extensive segment on a fan than suprafans on recent submarine fans. Individual thick sandstone beds also continue persistently as wide as the tongue. The channel deposits and the depositional tongue in the lowermost unit show a peculiar distribution as fan sedimentation. They seem to be the deposits at "a preparatory stage" of fan sedimentation or at "a pre-fan-sedimentation stage" during which preexisting reliefs on the basin floor are smoothed and an equilibrium profile of the slope-fan-basin is attained. The phased retreat of the terminus of channel deposits from the lowermost unit to the uppermost unit caused the first order upward thinning cycle detected throughout the Kiyosumi Formation. Shift of the channel to the different site was caused by beginning of each second order upward thinning cycle commonly observed in the channel deposits or by the rejuvenation of turbidity currents. The causes of these multiple upward thinning cycles, characterizing the vertical variation of the flysch sequence in the Kiyosumi Formation, are also discussed.