Reddish brown clastic beds associated with chert beds and greenstones were newly found in the Sumaizuku Unit of the Northern Chichibu Belt in the Kanto Mountains. These beds consist mainly of thick sandy mudstone beds containing thin conglomerate layers. Sandy mudstone beds are massive and structureless, while conglomerate layers are normally sorted. Mudstone and matrix of conglomerates are composed of microcrystalline quartz, finer grain of hematite, opaque minerals and radiolarian tests. Conglomerates consist mainly of subrounded granule of chert, siliceous mudstones, limestone and volcanic rocks accompanied with clasts of volcanic glass and sponge spicules. Therefore, sandy mudstone and conglomerate are lacking in terrigenous clastic particles. The modal composition of the clastic beds is quite similar to that of interbedded clastic beds in bedded cherts of the Mino Terrane that were redeposited in the open oceanic basin. However, reddish brown clastic beds are much thicker than clastic beds of the Mino Terrane, which suggests that they were deposited in hemipelagic area around a seamount situating near trench in the Early Jurassic.