Abstract
Detailed mapping of the Ryokami area in the Kanto Mountains, Saitama Prefecture, revealed that the Ryokami-yama chert unit is divided into three subunits (Subunit 1, 2 and 3 from bottom to top). Each subunit is composed mainly of chert beds and overlying clastic rocks. This unit is characterized by a pile-nappe structure of chert-elastics sequences. Middle Jurassic radiolarian fossils were found from the uppermost part of chert beds of Subunit 1 and Subunit 2 as well as from the siliceous mudstone of Subunit 1 and Subunit 2. The transitional horizon from chert to clastics in these subunits is of Bajocian to early Bathonian age. Judging from lithological features and geological age, the Kashiwagi Formation and the Hashidate Group in the Northern Chichibu terrane should not be the origin of the Ryokami-yama chert unit. This disagrees with the previous interpretation. The Ryokami-yama chert unit is better compared to the Unazawa Formation in the Southern Chichibu terrane or to the Kamiyoshida Formation in the Northern Chichibu terrane.
