Abstract
The Oi Formation of the Ultra-Tamba Terrane and sedimentary complexes of the Tamba Terrane are widely distributed in the Wakasa area, Fukui Prefecture, Southwest Japan. The sedimentary complex in the western part of the Obama area (central Wakasa) is poorly understood because the Oi Formation of the Ultra-Tamba Terrane has similar lithofacies to those of the sedimentary complex of the Tamba Terrane. The present study proves for the first time that gray felsic tuffaceous mudstone of the sedimentary complex in the western part of the Obama area contains chronostratigraphically significant Early Jurassic radiolarians. The identified radiolarian fauna includes Canoptum spp., Natoba spp., and Pantanellium browni Pessagno and Blome, among others. The fauna is consistent with the Canoptum assemblage and indicates a late Hettangian to early Sinemurian age. Because the Ultra-Tamba Terrane does not contain Jurassic rocks, whereas the Tamba Terrane includes Lower Jurassic clastic rocks, the discovery of a Lower Jurassic radiolaria-bearing felsic tuffaceous mudstone suggests that the sedimentary complex in the study area belongs to the Tamba Terrane and can be correlated with an Early Jurassic sedimentary complex (Shuzan Complex) within this terrane.
