A previously unreported sedimentary complex has recently been discovered in the northern part of Choˇnsu Bay. It overlies both unconformably the Precambrian low-grade metamorphosed sedimentary sequence(Taean Formation) and the Jurassic biotite granite. The sedimentary complex is proposed to be designated as Choˇnsuman Formation. The Formation is composed of conglomerate, tuff, basalt, tuffbreccia, and shale. It is distributed in the northwest and northeast coasts of Choˇnsu Bay and in the Chukdo. Age of the Formation seems to be of late Mesozoic. The Choˇnsu sedimatary basin is considered to be originated by block faulting presumably during the Cretaceous, as suggested by the development of N-S trending structural lineation in the study area and its vicinity. The Chonsuman Formation seems to be formed by sedimantation in close association with block faulting and volcanic activities nearby. Sedimentation is thought to be developed from west to east in the basin.