A Topic Session entitled “The Jurassic +” has been held during the annual meeting of the Geological Society of Japan since 2003. This paper summarizes the achievements of the topic session over the past ten years, listing the presentation titles and authors. This session has provided a regular opportunity for Jurassic researchers in Japan to exchange data and ideas. It has also contributed to providing the international Jurassic research community with information on the topic sessions through the Newsletters of the International Subcommission on Jurassic Stratigraphy (ISJS). This paper provides an introduction to the recent activities of the subcommission.
This study reports on a newly discovered radiolarian assemblage from the Sakamoto Formation in the Shiburi belt, which is an element of the Kurosegawa belt, in the Sakamoto area, Kumamoto Prefecture, western Kyushu. The assemblage, which contains Bagotum kimbroughi Whalen and Carter, B. modestum Pessagno and Whalen, B. maudense Pessagno and Whalen, Broctus ruesti Yeh sensu Goric̆an et al. (2006), B. selwynensis Pessagno and Whalen, Parahsuum simplum Yao, P. ovale Hori and Yao, P. edenshawi (Carter), P. izeense (Pessagno and Whalen), Droltus sanignacioensis Whalen and Carter, Zhamoidellum yehae Dumitrica, and Stichocapsa biconica Matsuoka, indicates an Early Jurassic (Pliensbachian) age and provides the first age dating of the Sakamoto Formation in the Shiburi belt. Over the last 10 years, Lower Jurassic formations have been discovered in four areas of the Kurosegawa belt in western Kyushu. The radiolarian assemblage plays an important role in undertaking correlations among these formations. For example, the Sakamoto Formation in the Shiburi belt correlates exactly with the Hirasawatsudani Formation in the Sakamoto belt, lithostratigraphically and chronologically. In addition, according to chronologic relationships in western Kyushu, it is suggested that deposition of the Lower Jurassic formations in the Kurosegawa belt was synchronous with the Early Jurassic development of accretionary complexes in the Outer Zone of Southwest Japan.
The Lower Jurassic Higuchi Group, exposed in the western part of Shimane Prefecture, Japan, includes, in ascending order, the Orojidani and Higuchidani formations. The Orojidani Formation is composed of conglomerate, sandstone, and mudstone, the latter containing Oxytoma sp. and “Pleuromya” sp. The Higuchidani Formation consists mainly of dark gray mudstone, from which six genera and six species of bivalves were collected: Kolymonectes staeschei, Palmoxytoma cygnipes, Ryderia texturata, Pseudomytiloides matsumotoi, Oxytoma sp., and Pleuromya sp. This bivalve assemblage is characterized by Boreal elements such as K. staeschei and P. cygnipes, which are known exclusively from the Lower Jurassic of Russia and northern Canada; it contains no typical Tethyan species.
We report on the first known occurrences of earliest Cretaceous belemnites in Japan, including: Cylindroteuthis aff. knoxvillensis Anderson (Cylindroteuthididae) from the Mitarai Formation (Berriasian) of the Tetori Group in the Shokawa area, Gifu Prefecture, central Japan; and Hibolithes spp. (Mesohibolitidae) from the Isokusa Formation (Berriasian-Valanginian) in the Kesennuma area, Miyagi Prefecture, and the Koyamada Formation (Berriasian-Valanginian) of the Somanakamura Group in the Minamisoma area, Fukushima Prefecture, both in the South Kitakami region, northeastern Japan. According to belemnite biogeography, Cylindroteuthis is considered a typical Boreal genus and Hibolithes a typical Tethyan genus. The recognition of Tethyan faunal elements in the South Kitakami region, which is presently located farther north than the Tetori region, and Boreal faunal elements in the Tetori region, contributes to the reconstruction of earliest Cretaceous paleogeography and oceanic current systems in the northwestern Pacific during this time. As the ammonoid assemblages of the Mitarai, Isokusa, and Koyamada formations are characterized by dominant Tethyan and Pacific genera and the absence of Boreal elements, it appears that Boreal and Tethyan faunas co-occurred in the paleo-Tetori region in the earliest Cretaceous.