Introduction

The Fukuji district, Takayama City, Gifu Prefecture, central Japan, is well known as a locality of Paleozoic sedimentary rocks that form a component of the Hida Gaien belt (Figures 1.1, 1.2). The Devonian Fukuji, Carboniferous Ichinotani, and Permian Mizuyagadani formations comprise mainly fossiliferous limestone beds that have been investigated for over 60 years for their paleontological and lithological signature (e.g. Igo, 1956; Niikawa, 1978, 1980; Harayama, 1990). Both macro- and microfossils, including fusulinids, corals, brachiopods, cephalopods, trilobites, ostracods and conodonts have been reported from the three formations (Igo, 1956; Niikawa, 1978, 1980; Kuwano, 1987; Niko and Hamada, 1987; Tazawa et al., 2010; Tanaka et al., 2013; Stocker et al., 2018). In the Mizuboradani Valley (Figure 1.3), located on the northwest side of Okuhidaonsen Village, Carboniferous macro- and microfossils are reported from gray, greenish gray and dark gray limestone boulders (Goto and Okura, 2004; Tazawa et al., 2010; Isaji and Okura, 2014; Stocker et al., 2018). Goto and Okura (2004) reported 14 isolated teeth of Carboniferous and Permian cartilaginous fish. Isaji and Okura (2014) described well preserved molluscan larval shells, including gastropods and bivalves, from the Ichinotani Formation. Stocker et al. (2018) reported an ostracod assemblage from the Carboniferous Ichinotani Formation consisting of silicified carapaces of Amphissites, Kirkbya, Bairdia, Aechmina and Healdia species. According to Stocker et al. (2018), the ostracods indicate a shallow-marine ‘Eifelian type’ ecological assemblage of Becker (1971), which is characteristic of mid-Palaeozoic fore-reef ecosystems.

In this paper, we report newly discovered Carboniferous conodonts from float limestone boulders in the Mizuboradani Valley. Provenance of the boulders and the biostratigraphic distribution of the species/fauna are also discussed herein.

Geological setting

The three southern islands of Japan (Honshu, Shikoku and Kyushu) are comprised predominantly of Paleozoic, Mesozoic and Cenozoic accretionary complexes (Isozaki and Maruyama, 1991; Isozaki et al., 2010). The Jurassic to Cenozoic accretionary complexes form elongate tectonic belts bounded by major structural lineaments. The Median Tectonic Line, one of the major tectonic lineaments of Japan, subdivides rocks of the ‘Outer Zone’, composed of the Sambagawa metamorphic belt, Chichibu belt, Kurosegawa belt, and Shimanto belt, from those of the ‘Inner Zone’ consisting mainly of the Ryoke metamorphic belt, and Mino, Akiyoshi, Hida and Hida Gaien tectonic belts on Honshu Island (Taira, 1990; Isozaki et al., 2010).

Figure 1.

1, map showing southwest and central Japan with distribution of limestone blocks in the Akiyoshi and Hida Gaien belts, modified from Ishida et al. (2013). Abbreviations: MTL, Median Tectonic Line; ISTL, Itoigawa-Shizuoka Tectonic Line. 2, distribution of Hida Gaien belt, modified from Tsukada et al. (2004). 3, map showing the study area with the locality of conodont-bearing limestone boulders (marked by the star), Gifu Prefecture, central Japan modified from 1:25,000 scale topographic map ‘Yakedake’ of the Geospatial Information Authority of Japan (GSI). 4, geological map of the Fukuji area modified from Harayama (1990).

The Fukuji district is situated in the eastern part of the Fukuji-Hongo-Furukawa area, which is the stratotype area of the Hida Gaien belt (Kamei and Igo, 1957; Tsukada et al., 2004). According to Tsukada et al. (2004), in the Fukuji-Hitoegane district, Ordovician to Permian sedimentary rocks are divided into six formations in ascending order: the Ordovician to Silurian Hitoegane Formation, the Silurian Yoshiki Formation, the Devonian Fukuji Formation, the Carboniferous Ichinotani Formation, the Permian Mizuyagadani Formation, and the Permian Sorayama Formation (Figure 1.4). The physical stratigraphical relationships between these formations are summarized by Niikawa (1978, 1980), Harayama (1990), Tsukada et al. (1999, 2004) and Williams et al. (2014). The Ichinotani Formation, mainly composed of fossiliferous marine carbonate rocks, is subdivided into six fusulinid zones, namely, Eostaffella, Profusulinella, Lower Fusulinella-Fusulina, Beedeina, Upper Fusulinella-Fusulina, and Triticites zones in ascending order. The Mizuyagadani Formation consists of carbonates and siliciclastic rocks, in which the Pseudoschwagerina and Pseudofusulina fusulinid zones are recognised (Figure 1.4; Niikawa, 1980; Harayama, 1990). The Sorayama Formation conformably overlies the Mizuyagadani Formation, and is dominated by massive pyroclastic rocks and intercalating conglomerates, sandstones and mudstones (Tsukada et al., 1999).

In the Mizuboradani Valley, carbonate and siliciclastic rocks of the Lower Fusulinella-Fusulina Zone to Triticites Zone of the Ichinotani Formation, carbonate rocks of the Pseudoschwagerina Zone of the Mizuyagadani Formation, and tuffaceous rocks of the Sorayama Formation crop out (Figure 1.4; Harayama, 1990). The Ichinotani and Mizuyagadani formations dominate the southern part of the valley, with the Sorayama Formation in the northern part. In the downstream of the valley, Carboniferous and Permian float limestone boulders that contain abundant macro- and microfossils have been reported (e.g. Goto and Okura, 2004). In this study, we collected float limestone boulders from the upstream of the valley to extract conodont microfossils.

Figure 2.

1, polished slab of conodont-bearing greenish gray float limestone boulder (T-01). Abbreviations: C, crinoid stems; B, brachiopod shell. 2–5, photomicrographs of thin section of float limestone boulders. 2, bioclastic limestone (right side) covered by tuffaceous mudstone (left side) including fusulinid foraminifera (white arrow) (T-01). Dashed line shows the boundary between limestone and mudstone. 3, bioclastic limestone (T-01). 4, bioclastic limestone covered by tuffaceous mudstone (I-02). Dashed line shows the boundary between limestone and mudstone. 5, bioclastic limestone (K-01).

Material and methods

We analyzed four greenish-gray limestone boulders (numbered I-01, I-02, K-01, T-01) collected as float in the downstream of the Mizuboradani Valley (Figures 1–3). These limestone boulders, 10–40 cm in diameter, associated with greenish-gray tuff and tuffaceous mudstone (Figures 2.2, 2.4). Goto and Okura (2004), and Isaji and Okura (2014) yielded well preserved microfossils from I-01, and Stocker et al. (2018) reported well preserved ostracods from three boulders (I-01, I-02, K-01: Figure 3). In the laboratory, these limestones were crushed into pellets 2–3 cm in diameter and immersed in 5–6% acetic acid to extract the phosphatic microfossils. Conodont elements were obtained from I-01, K-01 and T-01 (Figure 3). This process also recovered abundant silicified fusulinid foraminifers, gastropods, bivalves, brachiopods and ostracods from the limestones.

We used the Olympus SZX7 light microscope (Olympus Co., Ltd., Tokyo) for picking microfossils. Conodonts were coated with platinum using the magnetron sputter MSP-1S (Vacuum Device Co., Ltd., Ibaraki) for capturing images with an Ace-700-S Scanning Electron Microscope (Sanyu Electron Co., Ltd., Tokyo).

Systematic paleontology

Species were identified based on the P₁ element. The specimens studied in this paper are stored in the Micropaleontology Collection (MPC) of the National Museum of Nature and Science, Tsukuba, Japan. Orientation, terminology and measurement of P₁ elements are shown in Figure 4, following Clark and Mosher (1966), Sweet (1988), Purnell et al. (2000), and Rosscoe and Barrick (2009). All elements are well preserved and of a dark-gray color.

Figure 3.

Conodonts and other fossil from the four float limestone boulders (I-01, I-02, K-01 and T-01) in the downstream of the Mizuboradani Valley (this and previous studies), and from a dark gray limestone boulder reported by Tazawa et al. (2010). Open circles show our original data. Solid circles show the fossils figured in previous studies. Double circles show the fossils observed by Stocker et al. (2018).

Figure 4.

Orientation, terminology and measurement for P₁ elements of Gondolella and Idiognathodus. Abbreviations: L, length; H, height; W, width; b, blade; bc, basal cavity; bg, basal groove; c, cusp; ca, carina; cr, crest; fb, free blade; iar, inner adcarinal ridge; il, inner lobe; k, keel; me, medial carina; oar, outer adcarinal ridge; ol, outer lobe; pf, platform; pt, posterior tip; tr, transverse ridge.

Figure 5.

SEM images of P₁ elements of Gondolella sublanceolata Gunnell, 1933. 1, MPC32956; 2, MPC32957; 3, MPC32958; 4, MPC32959. 1–3 from I-01; 4 from K-01. For 1–4; a, d, lateral views; b, upper view; c, lower view.

Figure 6.

SEM images of P₁ elements of Idiognathodus sulciferus Gunnell, 1933. 1, MPC32960; 2, MPC32961; 3, MPC32962; 4, MPC32963; 5, MPC32964; 6, MPC32965; 7, MPC32966; 8, MPC62967; 9, MPC32968; 10, MPC32969; 11, MPC32970; 12, MPC32971. 1, 2, 4–6, 8 from I-01; 3, 7, 9–12 from K-01. For 1–12; a, lateral view; b, upper view; c, lower view; d, postero-upper view.

Biostratigraphy of the conodonts

The conodont species Gondolella sublanceolata and Idiognathodus sulciferus from limestone boulders I-01, K-01 and T-01, derived from the upstream of the Mizuboradani Valley (Figure 1.3), indicate an early Missourian (Kasimovian, Late Pennsylvanian) age. Previously, Kuwano (1987) reported Devonian and a few Carboniferous conodonts from the Fukuji and Ichinotani formations, in the Ichinotani Valley located about 500 m south of the Mizuboradani Valley (Figure 1.4): the Carboniferous conodonts he reported are Idiognathodus sinuosus Ellison and Graves, 1941 and Streptognathodus lateralis Higgins and Bouckaert, 1968, from the Ichinotani Formation. These conodonts indicate an Early Pennsylvanian, Bashkirian age (Barrick et al., 2013). Niikawa (1978, 1980) established fusulinid range zones in the Ichinotani and Mizuyagadani formations in the Fukuji district, and estimated the age to be from Visean (Middle Mississippian) to Asserian (earliest Permian). However, Kasimovian strata have not been reported, because the Moscovian (Middle Pennsylvanian) and Gzhelian (uppermost Pennsylvanian) parts of the lithostratigraphy are structurally juxtaposed (Niikawa, 1980). Thus, this is the first report of Kasimovian conodonts in the Hida Gaien belt.

In Japan, Carboniferous conodonts have mainly been reported from Carboniferous to Permian massive limestone olistoliths in the Akiyoshi belt by several authors: Mississippian to Middle Pennsylvanian conodont assemblages were reported from the Omi Limestone, Niigata (Igo and Koike, 1964), the Akiyoshi Limestone, Yamaguchi (Igo, 1973), the Atetsu Limestone, Okayama (Koike, 1967), the Hina Limestone, Okayama (Mizuno, 1997), and the Ko-yama Limestone, Okayama (Ishida et al., 2012, 2013). An Early Pennsylvanian conodont assemblage was also reported from limestone blocks in the Chichibu belt (Ishida et al., 2005), and Gzhelian (latest Pennsylvanian) conodonts were reported from the Tamba belt (Kusunoki et al., 2004). However, prior to this study, Kasimovian conodonts have not been reported, and their discovery is important for biostratigraphic correlation with other global areas.

Possible source of the conodont-bearing limestone boulders

In the Mizuboradani Valley, Carboniferous marine fossils have been documented from limestone boulders (Goto and Okura, 2004; Tazawa et al., 2010; Isaji and Okura, 2014; Stocker et al., 2018). The dark gray limestone blocks reported by Tazawa et al. (2010) contain fusulinids (Eostaffella? sp., Millerella aff. marblensis Thompson, 1942, Ozawainella sp.) that indicate the upper Bashkirian (Lower Pennsylvanian), probably within the Profusulinella Zone that exists in the Ichinotani Formation. Isaji and Okura (2014) reported larval shells of gastropods and bivalves from greenish-gray limestone boulders (I-01) that contain fusulinids, brachiopods, ostracods, trilobites, crinoids, conodonts and isolated teeth of cartilaginous fish described by Goto and Okura (2004). Isaji and Okura (2014) suggested that the limestone was possibly derived from the interval of the Fusulinella-Fusulina Zone within the Ichinotani Formation. Sample I-02, reported by Stocker et al., (2018) contains the fusulinid Ozawainella mosquensis Rauser-Chemoussova in Rauser-Chemoussova et al. (1951), which was only reported from the Beedeina Zone, just below the Upper Fusulinella-Fusulina Zone of Niikawa (1980) in the upstream of the Mizuboradani Valley.

We have been unable to find exposures of greenish-gray tuff and tuffaceous mudstone in the Beedeina Zone of the Ichinotani Formation. Thus, the Missourian conodont-bearing limestone boulders might be derived from uninvestigated exposures and sections in the valley.

In this study, we focused on the Sorayama Formation as one of the possible sources of the conodont-bearing float limestone boulders. According to Igo (1956), Tsukada and Takahashi (1998), and Tsukada et al. (1999), the Sorayama Formation is widely distributed in the Fukuji district and consists mainly of mafic to intermediate pyroclastic rocks with intercalated conglomerates, sandstones and mudstones. The conglomerates are clast-supported and contain pebble- to boulder-sized limestone clasts. The limestones contain Devonian corals Favosites and Heliolites, and Carboniferous fusulinids such as Ozawainella sp., Beedeina ichinotaniensis (Igo, 1957), Quasifusulina longissima (Möller, 1878), and Triticites sp. (Igo, 1956; Tsukada et al., 1999). In contrast, the matrix (tuffaceous mudstone and/or calcareous sandstone) of the conglomerates contains middle Permian fusulinids such as Nankinella sp., Parafusulina aff. gigantea (Deprat, 1913) and Russiella pulchra Miklukho-Maclay, 1958. Tsukada et al. (1999) suggested that the limestone clasts of the Sorayama Formation were derived from both the Devonian Fukuji and Carboniferous Ichinotani formations. The limestone boulders documented for our paper are lithologically similar to the above-mentioned limestone clasts of Carboniferous age. According to Tsukada et al. (1999), tuffaceous clastic materials which are components of the matrix of the conglomerate of the Sorayama Formation fill the chambers of some Permian fusulinids. This feature is observed in our sample T-01 (Figure 2.2). These observations enable us to consider the conglomerates of the Sorayama Formation as one of the likely sources of our Carboniferous-age limestone boulders (Figure 7).

Figure 7.

1, stylised image of a conglomerate bed of the Permian Sorayama Formation. These beds are clast-supported and contain abundant pebble- to boulder-sized subangular and subrounded clasts of sandstone, mudstone, various igneous rocks, and Devonian and Carboniferous fossiliferous limestones in a tuffaceous mudstone and/or calcareous sandstone matrix. The limestones were probably derived from the Devonian Fukuji and Carboniferous Ichinotani formations. 2, float limestone boulder as it would appear if reworked from the conglomerates of the Sorayama Formation.

Biogeography

The Missourian (Kasimovian, Late Pennsylvanian) conodont species Gondolella sublanceolata and Idiognathodus sulciferus, are commonly reported in the Pennsylvanian of the USA (Gunnell, 1933; Clark and Mosher, 1966; Barrick et al., 2004; Rosscoe and Barrick, 2009). Idiognathodus sulciferus is also known from the Moscow Basin of Russia (Goreva et al., 2009), and from Guizhou, South China (Qi et al., 2013). The similarity of the Kasimovian conodonts of Japan with those of southern China and the USA continues a trend that is also apparent from Early and Middle Pennsylvanian faunas. Ishida et al. (2012, 2013) reported Bashkirian to early Moscovian (Early to Middle Pennsylvanian) conodonts from the Ko-yama Limestone Group, Akiyoshi Belt, Okayama, Japan. The Moscovian conodont assemblage consists of Mesogondolella clarki (Koike, 1967), Idiognathoides attenuatus (Harris and Hollingsworth, 1933). Id. convexus (Ellison and Graves, 1941), Id. macer (Wirth, 1967), Idiognathodus delicatus Gunnell, 1931, Streptognathodus suberectus Dunn, 1966, Neognathodus bothrops Merrill, 1972 and N. symmetricus (Lane, 1967). Seven species (M. clarki, Id. attenuatus, Id. convexus, I. delicatus, S. suberectus, N. bothrops and N. symmetricus) from the Ko-yama Limestone are reported from the Midcontinent area of the western USA (Ellison and Graves, 1941; Dunn, 1966, 1970; Merrill, 1972; von Bitter and Merrill, 1977; Barrick et al., 2013). Mesogondolella clarki, Id. convexus, I. delicatus, S. suberectus, N. bothrops and N. symmetricus are also known from Guizhou, South China (Qi et al., 2013). Mesogondolella clarki, N. bothrops and N. symmetricus appear to be cosmopolitan species in the Moscovian (Middle Pennsylvanian).

Concluding remarks

In this study, we report the Missourian (Carboniferous) conodonts Gondolella sublanceolata Gunnell, 1933 and Idiognathodus sulciferus Gunnell, 1933 from float limestone boulders collected in the downstream of the Mizuboradani Valley, Fukuji district, central Japan. According to the conodont age, the float limestone boulders might be from the Triticites Zone of the Ichinotani Formation, or derived from boulder-sized limestone clasts in the conglomerates of the Permian Sorayama Formation.

Gondolella sublanceolata has previously been reported only from the Carboniferous of North America, and this is the first report of the species from a western Panthalassic setting. This suggests that Gondolella was a widespread pan-Panthalassic Ocean species during the Middle to Late Pennsylvanian, and thus of biostratigraphical value for international correlation.

Author contributions

T. Maekawa described conodonts collected from limestone boulders and discussed their biostratigraphic age. T. Komatsu, G. Tanaka, M. Okura, and A. Umayahara investigated the Ichinotani Formation in the Mizuboradani Valley and collected limestone boulders. T. Maekawa, C. P. Stocker and M. Okura collected microfossils from prepared residues of the boulders. T. Maekawa, T. Komatsu, M. Williams and C. P. Stocker identified the biogeographical significance of the Pennsylvanian conodonts. All authors contributed to writing the paper.