1993 Volume 38 Issue 38 Pages 73-84
The Middle Pleistocene (0.5-0.7Ma) Saijo Formation consists of braided river- and meandering river siliciclastic deposits, 30-50m thick, developed in three basins, Hokubu-Saijo Basin (NSB), Nambu-Saijo Basin (SSB) and Kurose Basin (KB) from the northeast to the southwest, in the central-south Chugoku region. The braided river deposits represented by thick-bedded sand-sandy gravel facies mainly occur in NSB, whereas the meandering river deposits are developed throughout both SSB and KB, which are characterized by marked fluvial cyclic depositional facies. The cyclic facies comprises four depositional unit types, types I-IV, with varying thickness, showing different environments from of high energy to low energy. Analysis of the distribution pattern of the each facies, combined with paleo-current data, suggest that the Saijo Formation was formed in a serial fluvial system comprising upper stream area of NSB to lower one of KB via intermediately situated SSB. Development of the cyclic depositional facies in SSB and KB suggests that a frequent migration of channels and floodplains occurred in the meandering river. The Saijo fluvial depositional basins are suspected to have formed affected by left-lateral faulting under marked E-W regional stress-field, and E-W flexural uplift of SSB relative to KB after the deposition may have been related to regional upheaval of the Chugoku region, although relevant tectonic aspect is still open together with detailed depositional processes of the Saijo Formation.
