Early Middle Jurassic (late Aalenian) radiolarian assemblage in a manganese nodule from the Northern Chichibu Belt in the Ino area, Kochi Prefecture, Southwest Japan

Abstract

Well-preserved radiolarians were obtained from a manganese nodule collected from the Northern Chichibu Belt in the Ino area, Kochi Prefecture, Southwestern Japan. This is the first discovery of manganese nodules containing radiolarians in the Chichibu Belt. The radiolarian assemblage in the manganese nodule consists of 111 nassellarians and 110 spumellarians. The assemblage contains 38 described species and 43 unknown morphotypes as shown by Yao [Yao, A., 1997. Faunal change of Early–Middle Jurassic radiolarians. News of Osaka Micropaleontologists, Special Volume, no. 10, 155–182 (in Japanese with English abstract)]. The assemblage suggests the late Aalenian age based on the biostratigraphic data in Japan and the Western Pacific. It shows a low diversity of ring and disc shaped spumellarians like the genera Hexasaturnalis, Parasaturnalis and Orbiculiforma. The genera Paronaella, Parvicingula and Parahsuum are highly diversified. Faunal composition of the examined assemblage in the Chichibu Belt is different from the contemporaneous assemblages in the Mino and Kuzumaki–Kamaishi Belts. This difference seems to indicate that the depositional regions of manganese nodules are different among the Chichibu, Mino and Kuzumaki–Kamaishi Belts. The examined manganese nodule may have been deposited during late Aalenian time in a higher latitudinal area than its present location in the Kuzumaki–Kamaishi Belt judging by the number of species belonging to Parvicingula.