Analytical study of turbidites, Otadai Formation, Boso Peninsula, Japan

On the Boso Peninsula, southeast of Tokyo, individual turbidite layers have been traced over about 38 km with the aid of tuff marker-beds. Sandstones are generally much more variable in thickness than the interbedded mudstones, showing lenticular shapes with great lateral continuities up to 30 km or more. But the latter have greater extents than the former, forming extensive sheets with a slight upslope thickening. Most sandstones show vertical and lateral grading, while mudstones gradually coarsen upslope. Sandstones have a variety of sedimentary structures typical of turbidites, whereas mudstones are virtually devoid of them throughout. Sandstones contain intensely abraded shallow-water fauna in contrast with mudstones with well-preserved deep-water fauna. Thus, mudstones are mostly of hemipelagic origin, and turbiditic mudstones, if present, are of negligible amounts. In general, individual sandstone beds are much more variable in thickness on the upstream side than on the downstream side. This is probably because of the existence of some minor channels and the adjacent levee-like embankments on the upslope side. At the base of thicker sandstone beds local penecontemporaneous erosion has usually taken place, giving rise to a few minor channels bordered by small levee-like embankments on both sides. The downslope side of the largest channel is commonly flanked by the greatest embankment that is coincident with the thickest part of each sandstone layer. Sandstones in the minor channels and levee-like embankments contain a lot of mud lumps torn out of the substratum, which decrease in size and amount and improve in roundness with increasing distance from the channels. In general, thicker sandstones show Bouma abcde sequences around the thickest part. Bouma d-division and convolution are developed preferentially on the downstream side of the thickest part. With decreasing bed thickness laterally, the complete depositional sequences arc successively replaced by Bouma bcde, then cde, and de sequences downslope, while by Bouma bc/c and then c/e sequences upslope. The facies transition of the horizon studied probably represents a section extending from the downslope end of the channelized portion of a suprafan to the outer portion with a smooth surface.