5:15 PM - 6:30 PM
[BPT07-P01] Conodonts and other microfossils from the Omi Limestone of the Akiyoahi Belt, Niigata Prefecture
Keywords:microfossil, Paleozoic Era, Akiyoshi Belt, Omi Limestone, conodont
The Omi Limestone is located around the Omi Town, Niigata Prefecture, which is the most northeastern extension of the Akiyoshi belt. It is distributed along the Omi River with a length of approximately 8 km and a width of approximately 2 km (Nagamori et al., 2010). Many previous paleontological investigations have been conducted on the microfossils of the Omni Limestone, including foraminifers (Igo, 1960; Watanabe, 1973; Kobayashi, 1988; Ueno and Nakazawa, 1993), conodonts (Igo and Koike, 1964; Watanabe, 1975), corals (Rowett and Minato, 1968; Yoshida et al., 1987; Yoshida and Okimura, 1992; Niko and Hasegawa, 2000; Niikawa, 2001), brachiopods (Hayasaka, 1918; Tazawa et al., 1983; Tazawa et al., 2004), bryozoans (Sakagami, 1962, 1963), conuralia (Makiguchi, 1993), and others. This aim of this study was to re-examine the conodont biostratigraphy and investigate the Late Paleozoic microfossil fauna recovered from the limestone using the acetic acid method. Limestone samples were systematically collected from the Tomi (Myojyou Cement Co., Ltd.) and Omni mines (Denkikagaku Kogyo Co., Ltd.).
We recovered conodonts, fish teeth, ostracods, smaller foraminifers, bryozoans, sea urchin spines, holothurian sclerites, calcareous algae, and other unidentified fossils. The following conodont genera, indicative of the late Carboniferous Bashkirian–Moscovian time scale, were identified: Declinognathodus, Idiognathoids, Idiognathodus, Streptognathodus, Neognathodus, Mesogondolella, and others. With respect to the limestone lithology, limemudstone and grainstone predominated based on the thin section analysis. Lime mudstone consists of very fine calcareous mud and often contains fossil fragments. Grain stone is characterized by a concentration of fragments and ooids within a sparitic matrix. As these lithological microfacies are continuously observed laterally and stratigraphically, these limestones seem to have been deposited in a back reef environment during the late Carboniferous period. This indicates that the microbiota were diversified in pelagic back reef environments during this time.