2005 Volume 62 Issue 62 Pages 3-16
The lower Pliocene Kiyosumi Formation in the middle part of the Boso Peninsula, central Japan, is mainly formed of sandstone-dominated flysch-type alternation attaining more than 850m in maximum thickness, and outcrops under the control of a pair of the east-west trending northern anticline and southern syncline. A depositional model was proposed previously by the present author that the turbidite sandstones in the formation were deposited by the turbidity currents which flowed from the central northern outlet of the forearc basin, named the Kiyosumi Basin, restricted by an outer ridge called the Mineoka Uplift Zone along the southern margin. As a different idea that the turbidite sandstones in the formation were formed by the turbidity currents from the western outlet, flowing eastward, was proposed recently, additional measurement of the paleocurrents of the turbidite sandstones was conducted in the lowermost unit among five units of the formation in the Seiwa Prefectural Forest where the unit is exposed widely. The thick turbidite sandstones attaining 300m in maximum thickness in the lowermost unit were exclusively distributed in the southern part of the basin along the syncline axis.
The results of the measurement of the paleocurrent based on the current ripple cross lamination in the turbidite sandstones indicate westward flows, not eastward, which support the previous depositional model by the present author that the turbidite sandstones in the unit were deposited by the turbidity currents which were introduced into the basin from the central northern outlet and flowed along the feeder channel developed in the eastern part of the peninsula and then flowed downward to the west in depressions developed along the southern syncline axis and deposited coarse-grained thick turbidite sandstones there.
