Abstract
Early Jurassic radiolarians were recovered from the Sannokou Complex, which is a part of the accretionary complex of the Chichibu Composite Belt in the central Kii Peninsula, Southwest Japan. The Sannokou Complex, which tectonically overlies the Middle Jurassic to middle Early Cretaceous complexes, predominantly consists of thickly bedded sandstone with conglomerate and mudstone intercalations; and minor amounts of siliceous mudstone, bedded chert, and sheared mudstone including various kinds of blocks. Radiolarian fossils were extracted from three and four samples of siliceous mudstone and chert, respectively. The siliceous mudstone samples bear well-preserved radiolarians, which are assigned to be early and middle Toarcian of late Early Jurassic in age. Age of cherts, ranging from Sinemurian to Pliensbachian, is slightly older than that of siliceous mudstone.
It has been argued that the Chichibu Composite Belt in the central Kii Peninsula has been divided into seven complexes. Furthermore the six complexes except the Sannokou Complex are tectonically stacked with a downward-younging polarity from early Middle Jurassic to middle Early Cretaceous due to radiolarian biochronology. On the basis of occurrences of Early Jurassic radiolarians, the Sannokou Complex, which occupies the uppermost tectonic horizon in the Chichibu Composite Belt, shows the older age than these complexes. Thus the tectonic stacking with a downward-younging polarity can be recognized overall in the Chichibu Composite Belt of the central Kii Peninsula.
