Mino terrane

The Mino terrane is defined as a subduction-generated accretionary complex composed of Lower Permian to Upper Jurassic unmetamorphosed sedimentary rocks with minor volcanic rocks (Mizutani, 1990). The most extensive exposure areas are in central Japan.

The constituent rocks of the Mino terrane comprise a terrigenous clastic rock assemblage of mudstone, sandstone, and conglomerate, and an oceanic rock assemblage of basaltic rocks, carbonates, and chert and related siliceous rocks (Wakita, 1988; Sano and Kojima, 2000). These terrane rocks are incorporated into complexly stacked and imbricated thrust sheets.

The Mino terrane rocks are tectonostratigraphically divided into two coherent units and five mélange units (Wakita, 1988). The coherent units are characterized by a stratigraphically orderly succession of Lower Triassic to Middle Jurassic chert and related siliceous rocks of pelagic facies, and overlying Middle Jurassic to lowest Cretaceous terrigenous rocks of trench-fill deep-sea turbidites. This lithologic succession records the Early Triassic to earliest Cretaceous ocean plate stratigraphy. The mélange units are characterized by a chaotic mixing of slabs and isolated blocks of Permian to Jurassic oceanic rocks with the matrix of Jurassic to lowest Cretaceous scaly mudstone. The lithologic association of the oceanic rocks and the age of the matrix characterize each of the five mélange units. For an instance, the Funabuseyama unit, which includes the examined PTB siliceous rocks, is characterized by large slabs of dominantly Permian and subordinately Triassic oceanic rocks and a Middle Jurassic mudstone matrix (Wakita, 1988).

Permian and Triassic oceanic rocks of Funabuseyama unit

The Mt. Funabuseyama area (Figure 1) is one of the major areas of exposures of the Permian and associated Triassic oceanic rocks of the Funabuseyama unit (Wakita, 1988). The oceanic rocks in the Mt. Funabuseyama area are divided into three, nearly timeequivalent stratigraphic units (Sano, 1988). They are designated as the Funabuseyama, Amanokawara, and Hashikadani formations. The former two have shallowmarine carbonate facies and are underlain by basaltic rocks. The Hashikadani Formation is dominated by deep-marine siliceous rocks resting upon basaltic rocks. Sano (1988) dated the Funabuseyama and Amanokawara formations as late Early to late Middle Permian and late Early Permian, respectively. The Hashikadani Formation ranges in age from the middle Early Permian to late Early Triassic (Sano, 1988; Kuwahara et al., 2010). Sano (1988) interpreted the Funabuseyama, Amanokawara, and Hashikadani formations as having accumulated at the top, upper flank, and lower flank, respectively, of a seamount in the mid-oceanic realm of the Panthalassa Ocean. The geochemical attributes of the basaltic rocks (Jones et al., 1993) indicate a hotspot origin, consistent with the interpretation by Sano (1988).

Figure 1.

Maps showing the locality of the study area in central Japan (A) and the examined section (NF 195) in the Mt. Funabuseyama area (B). The two PTB sections (NF 1212R and 1036R sections: Sano et al., 2010) are located near the study section. Abbreviations F, NH, and U stand for Mt. Funabuseyama, Neohigashidanigawa River, and Mt. Uokaneyama, respectively.

Stratigraphy and age of Hashikadani Formation

The Hashikadani Formation (>400 m thick) records a deep-marine pelagic sedimentation of biogenic siliceous deposits. The preserved sedimentary record ranges from the late Sakmarian, through the PTB transition, to the late Induan in age (Sano, 1988; Kuwahara et al., 2010; Sano et al., 2010).

The Hashikadani succession begins with a basal unit of basaltic lavas and volcaniclastic rocks (>150 m thick), directly succeeded by hydrothermally precipitated Mnrich chert (<10 m: Sugitani et al., 1988). The basal unit is followed by a thick unit (ca. 150 m) of upper Lower to upper Upper Permian ribbon-bedded radiolarian chert that contains resedimented dolomite breccia and sandstone at several levels. The Permian chert exhibits a change of the rock color, from reddish purple to red in the lower part, through greenish, and ultimately gray in the upper part. The uppermost Permian chert is black and overlain by the basal Triassic black claystone (Sano et al., 2010) and the upper Lower Triassic dark gray to black chert and siliceous claystone (Kuwahara et al., 2010).

Lithostratigraphy and microscopic characteristics of PTB siliceous rocks

The study section crops out in the upper stream of Iwaidani gully, where the Hashikadani oceanic rocks are exposed along with the Lower Permian limestone of the Funabuseyama Formation and the Jurassic mudstone (loc. NF 195 in Figure 2). These units strike northwestsoutheast, dip steeply to the northeast, and are in fault contact with one another.

The Hashikadani oceanic rocks in the Iwaidani gully are incorporated into the two major thrust sheets, separated by a steeply inclined fault (F-F’ in Figure 2). Though structurally repeated complexly on a smaller scale, the Hashikadani rocks are younging to the northeast within each thrust sheet.

The measured PTB siliceous rocks comprise the eastern part of the approximately 30 m-wide and nearly eastwest-elongated section composed of the bedded chert and claystone (loc. NF 195 in Figure 2). Sano (1988) first reported the upper Upper Permian radiolarians from this locality. Also, Kuwahara (1997) defined the four Albaillella abundance zones in the chert section. Igo and Sakano (1996) and Kitao (1996) examined the Permian conodont biostratigraphy of the chert section. The preliminary radiolarian biostratigraphic study shows that the entire section at loc. NF 195 ranges from the upper Middle Permian to the upper Upper Permian (Follicucullus charveti-Albaillella yamakitai Zone to Neoalbaillella optima Zone: Sano and Kuwahara, 2011).

We measured the PTB siliceous rocks along the two subsections, NF 195A and NF 195B (Figure 3). The subsections are precisely correlated with each other using the physical tracing of individual beds in the field.

Subsection NF 195A

Although thin, subsection NF 195A comprises a stratigraphically intact succession of the PTB siliceous rocks, consisting of gray, dark gray, and black cherts and black claystone (Figures 4, 5–1). The thickness of the succession is approximately 2.1 m. The top is thought to be in fault contact with the Jurassic scaly mudstone (Figure 3).

Subsection NF 195A is lithologically divided into lower, middle, and upper units (Figure 4). Each of the units is characterized by gray to dark gray chert, black chert, and black claystone, respectively.

The chert of the lower unit (ca. 0.5 m thick) is ribbonbedded, with gray to grayish green clayey partings. Individual chert beds range from 2 to 10 cm in thickness, typically 4 to 5 cm. Black chert beds of the middle unit (ca. 0.7 m) are separated by black claystone partings, and each of the chert beds is slightly thinner than that of the grayish chert of the lower unit. The black claystone partings slightly thicken up-section, attaining 2.5 cm thick in the upper part of the middle unit. A thin bed of gray to greenish gray chert occurs at the top of the middle unit.

The upper unit of subsection NF 195A (ca. 0.9 m) consists dominantly of black claystone. It is sooty black, fissile, and carbonaceous. Black chert beds, 2 to 10 cm thick, intermittently occur at several levels. Slightly siliceous, gray parallel laminae with poorly defined top and bottom surfaces are intercalated at a few levels. The lithologie change from the middle unit to the upper unit is sharp and abrupt (Figure 5–2).

Subsection NF 195B

Attaining approximately 6.9 m in thickness, subsection NF 195B is much thicker than subsection NF 195A (Figure 4). The top and bottom of subsection NF 195B is covered with talus deposits and cut by faults, respectively (Figure 3).

Figure 2.

Location of the study section, indicated as NF 195 on the base map illustrating the approximate distribution of Permian basaltic rocks, chert, and dolomite of the Hashikadani Formation (Ph), Permian limestone of the Funabuseyama Formation (Pf), and Jurassic mudstone (Jm) including isolated blocks of Permian basaltic rocks and dolomite and Upper Triassic siliceous limestone (Sano et al., 2010) in the upper reaches of the Iwaidani Gully of the Mt. Funabuseyama area. See Figure 1 for the mapped area. Base map after Sano et al. (1992).

Subsection NF 195B consists almost entirely of gray chert with minor greenish gray, dark gray, and black cherts and black claystone (Figure 4). The succession is lithologically divided into a main unit and an upper unit.

The main unit (ca. 6.2 m thick) consists almost wholly of usually gray and locally greenish bedded chert with grayish clayey partings and has a small amount of black to dark gray chert and black claystone near the top (Figure 5–3). The lithologic property of the chert is almost identical with that of subsection NF 195A. Although subtle, the individual chert beds of subsection NF 195B, reaching up to 20 cm thick in places, are thicker than those of subsection A. Black claystone near the top of the main unit is also lithologically indistinguishable from that of the upper unit of subsection NF 195A.

The upper unit (ca. 0.7 m thick) predominantly comprises black chert with a small amount of gray chert and black claystone (Figure 4). The chert and black claystone are lithologically comparable with those of the main unit of subsection NF 195B and upper unit of subsection A.

Figure 3.

Field sketch illustrating the stratigraphic relation between the Upper Permian bedded chert and the overlying black claystone including the lower Induan chert bed at loc. NF 195. Thick arrows with A and B indicate sites of subsections NF 195A and NF 195B, respectively. See also Figure 5–1. The locality is indicated in Figure 2.

The upper unit contains conspicuous pyrite nodules embedded in the gray chert bed near the top of the upper part (NF 195–31 in Figure 4). The nodules have slightly flattened subspherical shapes with gently undulating, smooth surfaces (Figure 5–4), and consist entirely of an aggregate of fine pyrite grains. The boundary between the nodules and surrounding chert is lithologically sharp.

Microscopic characteristics

The chert of subsections NF 195A and NF 195B essentially comprises radiolarian remains with minor sponge spicules in a matrix of microcrystalline quartz containing a small amount of clayey minerals and carbonaceous matter (Figure 6–1 to 3). The radiolarian remains are poorly preserved with highly obliterated outlines. Tiny pyrite grains occur scattered in the chert. The black chert of the upper unit of subsection NF 195A is more carbonaceous than the others, and includes carbonaceous matter-enriched, short and laterally discrete patches (Figure 6–3).

Black claystone of the upper unit of subsection A is extremely fine-grained, consisting mostly of clay minerals with a great admixture of carbonaceous matter (Figure 6–4). Thin, light-colored and siliceous layers, usually several tens of microns thick, with poorly defined top and bottom surfaces occur locally in the black claystone.

Acid-processed residues of the black chert at three levels of subsections NF 195A and 195B contain foraminifer individuals along with radiolarian tests (Figures 7, 8). Although unrecognized in the thin sections, a significant amount of the foraminifers occur in the residues.

The foraminifer individuals in the residues range in size from several tens to several hundreds of microns. They vary in shape, having short spindlelike, elongated conical, and slightly twisted cylindrical shapes (Figure 6–5). Although internal structures have been lost in most of the individuals, a chamberlike structure is locally discernible. The interiors of the chamberlike spaces are filled with carbonaceous matter. The tests are completely replaced by quartz.

Biostratigraphy and age of PTB siliceous rocks

The chert and black claystone of subsections NF 195A and NF 195B were dated on the basis of radiolarian and conodont fossils. We collected the chert samples by beds for thorough radiolarian biostratigraphic examination and processed 157 chert samples for microscopic observation. The black chert and black claystone samples at the five stratigraphic levels of the upper unit of subsection NF 195A and the gray chert sample at the top of the middle unit of subsection NF 195A were examined for conodonts. As most of the conodont elements have been dissolved, their molds were observed for identification on surfaces of the thinly crushed chips.

Figure 4.

Measured lithologic columnar sections of the PTB siliceous rocks of the Hashikadani Formation along subsections NF 195A and 195B at loc. NF 195. See Figure 2 for the locality.

Figure 5.

Outcrop views of the PTB siliceous rocks of the Hashikadani Formation at loc. NF 195. See Figure 2 for locality. 1, entire view of the PTB siliceous rock succession composed of the Upper Permian chert (P) and overlying black claystone (C). Arrows indicate the approximate position of their sharp lithologic boundary. See also Figure 3. 2, close-up view of the sharp lithologie boundary in Figure 5–1, indicated as b-b’. Abbreviations P and C stand for the upper Upper Permian chert and the black claystone, respectively. Subsection NF 195A. 3, gray chert with extremely thin clayey partings. Middle part of the main unit of subsection NF 195B. An arrow indicates the face of beds. 4, subspherical-shaped, fist-sized pyrite nodule (py) embedded in the upper Upper Permian gray chert (ch) near the uppermost part of the main unit of subsection NF 195B. NF 195–31. See Figure 9 for the stratigraphic level.

We follow Ishiga (1986), Kuwahara et al. (1998), and Xia et al. (2004) for the Permian radiolarian biostratigraphy and their age assignments. The Lower Triassic radiolarian biostratigraphy and correlation are based upon Kamata (2007), Kamata et al. (2007), and O'Dogherty et al. (2010). Triassic conodont biostratigraphic zones and their correlation are adopted from Yin et al. (2001), Jiang et al. (2007), and Orchard (2010).

Subsection NF 195A

Permian radiolarian biostratigraphy—Although the preservation is poor, the radiolarian fossils occur in nearly all the stratigraphic levels of subsection NF 195A (Figure 7). We identified 27 species of 20 genera of Permian radiolarians in the chert of the lower and middle units (NF 195–28 to NF 195–6 in Figure 7).

Figure 6.

Thin-section photomicrographs of the PTB siliceous rocks of the Hashikadani Formation and microscopic view of the foraminifer individual in the acid-processed residue. Stratigraphic levels of the samples are shown in Figure 4. Plane light. 1, radiolarian chert including poorly preserved radiolarian remains. NF 195–6 at the top of the middle unit of subsection NF 195A. Scale bar = 1 mm. 2, black chert bearing sparse radiolarian remains and abundant carbonaceous matter. NF 195–1, intercalated in the black claystone near the top of the upper unit of subsection NF 195A. Scale bar = 1 mm. 3, dark gray chert including poorly preserved and slightly flattened radiolarian remains, indicated by an arrow, and having thin, carbonaceous patches. NF 195–5 near the bottom of the upper unit of subsection NF 195A. Scale bar = 1 mm. 4, black claystone associated with dark gray chert shown in Figure 6–3. NF 195–5. Scale bar = 1 mm. 5, completely quartz-replaced foraminifer individual contained in the acid-processed residue of the black chert of NF 195–5 shown in Figure 6–3. Elongated conical-shaped test with faint vestige of a multichambered uniserial structure. Chamberlike internal spaces are filled with carbonaceous matter. Scale bar = 100 µm.

The two units are characterized by the occurrence of diverse Permian radiolarian fossils, comprising Albaillellaria, Latentifistularia, Entactinaria, and Spumellaria. However, the faunal diversity of the Permian radiolarians exhibits a marked decrease immediately above the level of NF 195–6 (Figure 7).

We recognized the occurrence of Albaillella triangularis and Neoalbaillella optima at several levels of the lower and middle units (Figure 7). Both of these albaillellarian species characterize the Neoalbaillella optima Assemblage Zone of Kuwahara et al. (1998) indicative of the upper Upper Permian (Changhsingian). Also, Albaillella spp. occur at most levels of the lower and middle units.

Together with the albaillellarian fossils, many species of Latentifistularia, Entactinaria, and Spumellaria occur (Figure 7). Identified are Hegleria mammilla, Hegleria spp., Ishigaum klaengensis (Figure 9–10), Ishigaum spp., Latentifistula spp., Raciditor gracilis, Triplanospongos spp., Copicyntra spp., Megaporus spp., Paracopicyntra akikawaensis, P. ziyunensis (Figure 9–17), Paracopi- cyntra spp., Stigmosphaerostylus spp., Archaeospongoprunum spp. (Figure 9–21), Copicyntroides asteriformis, Copicyntroides spp., Copiellintra spp. (Figure 9–23), Orbiculiforma? sp., Pseudospongoprunum? spp., Srakaeosphaera minuta (Figure 9–27), Tetrapaurinella? spp., Tetraspongodiscus spp. (Figure 9–28), and Yujingella spp. (Figure 9–29). Latentifistula spp., Copicyntra spp. with concentric shells, and Paracopicyntra spp., including P. akikawaensis and P. ziyunensis, occur at many levels (Figure 7).

Figure 7.

Stratigraphic distribution of Permian and Triassic radiolarians and conodonts of subsection NF 195A. The two alternative stratigraphic positions of the PTB are indicated. Abbreviations A, N, and C stand for Albaillellaria, Nassellaria, and conodonts, respectively. See Figure 4 for lithologie symbols.

Figure 8.

Stratigraphic distribution of Permian radiolarians of subsection NF 195B. See Figure 4 for lithologic symbols.

Triassic radiolarian biostratigraphy—In contrast with the lower and middle units, the upper unit yields radiolarian fossils of low diversity (Figure 7). Several types of sphaerioid-shaped spumellarians and minor nassellarians characterize the upper unit. Its radiolarian assemblage markedly changes within a few-centimenter-thick interval between the stratigraphic levels of NF 195–6 and NF 195–5. The preservation of the radiolarian fossils in the upper unit is very poor.

Sphaeroid spumellarians in the upper unit include five types differing in the size of tests and the number of spines. We morphologically discriminated sphaeroids A, B, D, E, and F, characterized by a large test with no spines (Figure 10–1 to 4), a smaller test with no spines (Figure 10–5 to 7), a test with three spines (Figure 10–8), a test with four spines (Figure 10–9), and a test with over five spines (Figure 10–10), respectively.

These sphaeroid spumellarians as well as Sphaeroid C were reported from the upper Lower Triassic (Olenekian) siliceous rocks (Kuwahara et al., 2010) and recently from the lower Lower Triassic (Induan) chert of the PTB siliceous rocks succession (Sano et al., 2010). Also, Yao and Kuwahara (1997) reported many sphaeroid radiolarians from the Lower Triassic chert (Dienerian? to lower Spathian), and tentatively set up the Sphaeroids zone below the middle to upper Spathian Parentactinia nakatsugawaensis Zone of Sugiyama (1992).

We detected a nassellarian radiolaria of Triassospongocyrtis? sp. (Figure 10–11) from the chert bed in the middle part of the upper unit (NF 195–3: Figure 7). It is noted that Triassospongocyrtis? sp. occurs together with Hindeodus parvus indicative of the early Induan (Figure 7).

The genus Triassospongocyrtis was described as a new nassellarian genus from the Ladinian sections of Hungary and Southern Alps by Kozur and Mostler (1994). Kamata (2007) reported three species of the genus Triassospongocyrtis from the upper Dienerian chert (Neospathodus cristagalli Zone: Kamata et al., 2007) at Arrow Rocks, New Zealand. Our biostratigraphic examination shows that this Mesozoic primitive nassellarian genus appeared as early as the early Induan (Griesbachian), much earlier than previously thought (O'Dogherty et al., 2010).

The upper unit also yields several species of Permiantype radiolarians (Figure 7). They include Albaillella? spp. (Figure 10–12, 13), Hegleria? sp. (Figure 10–14, 15), Stigmosphaerostylus sp. (Figure 10–18), Pseudospongoprunum? sp. (Figure 10–19), Grandetortura? sp. (Figure 10–20), and Latentifistularia gen. et sp. indet. A and B (Figure 10–16, 17). Most of these Permian-type radiolarias are poorly preserved.

Permian and Triassic conodont biostratigraphy— Reliable conodont fossils were found from the chert beds at the two stratigraphic levels of subunit NF 195A (Figure 7). One is the gray chert at the top of the middle unit (NF 195–6) and the other is the black chert in the middle part of the upper unit (NF 195–3).

The gray chert of NF 195–6 yields Clarkina sp. The specimens are segminiplanate elements with a broad and relatively smooth platform that narrows strongly toward the anterior end, a downwardly deflected anterior margin, and an anterior free blade.

Since Kozur (1989) discriminated the genus Clarkina from the Permian and Triassic gondolellid conodonts, this genus is reported in many regions (west Texas: Lambert et al., 2010; Iran: Sheng and Mei, 2010; South China: Ji et al., 2007; Southwest Japan: Nishikane et al., 2011). Though the genus Clarkina ranges down to the upper Middle Permian (e.g. Wardlaw and Nestell, 2010) and up to the lower Lower Triassic (e.g. Metcalfe and Nicoll, 2007; Chen et al., 2009), its dominant occurrence is recorded in the Upper Permian.

Sample NF 195–3 yields Hindeodus parvus (Kozur and Pjatakova), H. sp. A, and H. sp. B (Figure 11). Specimens of H. parvus are carminiscaphate elements with a large cusp that is approximately twice the height of the posterior denticles. Their posterior bars bear 5 to 9 subequal denticles (Figure 11–1 to 6).

The first appearance datum of H. parvus defines the PTB (Yin et al., 2001), and the occurrence of Triassic Hindeodus species is limited to the lower Induan (e.g. Kozur, 1998; Orchard and Krystyn, 1998). Thus, our conodont biostratigraphic examination reveals that the chert of NF 195–3 is correlated with the lower Induan (Figure 7).

H. parvus characterizes the basal Triassic H. parvus Zone with several species of the genera Hindeodus and Isarcicella (Orchard, 2010). According to Orchard (2010), however, the occurrence of H. parvus spans the four Griesbachian conodont zones, comprising the H. parvus, I. lobata, I. staeschei, and I. isarcica zones in ascending order. The stratigraphic distribution of H. parvus ranges up into the upper Griesbachian. Thus, the conodont zones of the upper unit of subsection NF 195 remain uncertain, because it only yielded H. parvus and a few species of the genus Hindeodus (Figure 7).

Subsection NF 195-B

The biostratigraphic examination revealed that subsection NF 195B consists wholly of the Upper Permian (Figure 8). We identified more than 55 species of 31 genera of Permian radiolarian fossils in subsection NF 195B. Although moderately to poorly preserved, the Permian radiolarians are as diverse as in the lower and middle units of subsection NF 195A.

The Permian radiolarians in subsection NF 195B are characterized by the prolific occurrence of diverse albaillellarian fossils (Figure 8). The extracted albaillellarians are Albaillella flexa (Figure 9–3), A. angusta (Figure 9–1), A. excelsa (Figure 9–2), Neoalbaillella ornithoformis (Figure 9–8), A. triangularis (Figure 9–5), A. kuwaharai (Figure 9–4), and N. optima (Figure 9–7). The former three albaillellarian species and the latter three are characteristic of the upper part of the Neoalbaillella ornithoformis and N. optima assemblage zones of Kuwahara et al. (1998), respectively.

Kuwahara (1997) recognized the four Albaillella abundance zones in the chert section that correlates to subsection NF 195B. These are the A. levis, A. flexa, A. excelsa, and A. triangularis abundance zones in ascending order.

Following the definition by Kuwahara (1997), we also recognized three Albaillella abundance zones in subsection NF 195B. They are the A. flexa (from NF 195–120–3 to NF 195–118 of Figure 8), A. excelsa (from NF 195–112 to NF 195–88), and A. triangularis (from NF 195–86 to 195–32) abundance zones. The A. levis Abundance Zone is missing due to coverage by talus deposits. According to Kuwahara (1997), the A. flexa and A. excelsa abundance zones in subsection NF 195B are correlated nearly with the upper part of the Neoalbaillella ornithoformis Assemblage Zone (Figure 8). The A. triangularis Abundance Zone is included in the N. optima Assemblage Zone.

Together with the albaillellarians, subsection NF 195B yields diverse latentifistularians, entactinarians, and spumellarians (Figure 8). Latentifistularians including Cauletella paradoxa (Figure 9–9), Hegleria spp., Ishigaum trifustis (Figure 9–11), Ishigaum. spp., Latentifistula spp., and Ormistonella cf. adherens (Figure 9–12), entactinarians including Copicyntra spp., Paracopicyntra akikawaensis (Figure 9–16), P. ziyunensis, P. spp., and Stigmosphaerostylus spp. (Figure 9–19), and spumellarians including Copicyntroides spp., Copiellintra spp. (Figure 9–23), Pseudospongoprunum? spp. (Figure 9–26), and Tetraspongodiscus spp. are characteristic (Figure 8). Although less abundant than these characteristic radiolarian fossils, Follicucullus? spp. (Figure 9–6), Cauletella porosa, Foremanhelena triangula, Gustefena sp., Hegleria mammilla, Ishigaum klaengensis, Latentibifistula spp., Ormistonella robusta, Pseudotormentus spp., Quadricaulis inflata (Figure 9–13), Raciditor gracilis (Figure 9–14), R. scalae, Tetragregnon spp., Triplanospongos musashiensis (Figure 9–15), Megaporus spp., Stigmosphaerostylus itsukaichiensis (Figure 9–18), Triaenosphaera sp. (Figure 9–20), Trilonche pseudocimeria, Archaeospongoprunum spp., Kashiwara aff. magna (Figure 9–24), Orbiculiforma? sp., Pseudospongoprunum? fontainei (Figure 9–25), Tetrapaurinella? spp., Yujingella spp., and a disc-spiral form Spumellaria A are recognized (Figure 9–30).

Figure 9.

Late Permian radiolarians extracted from the chert of the Hashikadani Formation along subsections NF 195A and NF 195B. See Figures 7 and 8 for the stratigraphic levels of the samples. All scale bars represent 100 µm. 1, Albaillella angusta Kuwahara, NF 195–90; 2, Albaillella excelsa Ishiga, Kito and Imoto, NF 195–90; 3, Albaillella flexa Kuwahara, NF 195–120–1; 4, Albaillella kuwaharai Xia, Zhang, Wang and Kakuwa, NF 195–15; 5, Albaillella triangularis Ishiga, Kito and Imoto, NF 195–90; 6, Follicucullus? sp., NF195–121; 7, Neoalbaillella optima Ishiga, Kito and Imoto, NF195–104–1; 8, Neoalbaillella ornithoformis Takemura and Nakaseko, NF195–69; 9, Cauletella paradoxa Shang, Caridroit and Wang, NF195–48; 10, Ishigaum klaengensis Sashida, NF195–9; 11, Ishigaum trifustis DeWever and Caridroit, NF195–61–2; 12, Ormistonella cf. adherens Feng, NF195–48; 13, Quadricaulis inflata (Sashida and Tonishi), NF195–90; 14, Raciditor gracilis (DeWever and Caridroit), NF195–29; 15, Triplanospongos musashiensis Sashida and Tonishi, NF195–121; 16, Paracopicyntra akikawaensis (Sashida and Tonishi), NF195–121; 17, Paracopicyntra ziyunensis (Feng and Gu), NF195–17; 18, Stigmosphaerostylus itsukaichiensis (Sashida and Tonishi), NF195–48; 19, Stigmosphaerostylus sp., NF195–29; 20, Triaenosphaera sp., NF195–69; 21, Archaeospongoprunum sp., NF195–7; 22, Copicyntroides asterifbrmis Nazarov and Ormiston, NF195–27; 23, Copiellintra sp., NF195–53; 24, Kashiwara aff. magna Sashida and Tonishi. NF195–121; 25, Pseudospongoprunum? fontainei Sashida, NF195–29; 26, Pseudospongoprunuml sp., NF195–29; 27, Srakaeosphaera minuta Sashida. NF195–27; 28, Tetraspongodiscus sp., NF195–27; 29, Yujingella sp., NF195–17; 30, Spumellaria form A (disc-spiral), NF195–93–3.

The detailed stratigraphic distribution of the latentifistularians, entactinarians, and spumellarians in subsection NF 195B remains unclear. However, Copicyntra sp., Paracopicyntra akikawaensis, P. ziyunensis, and Pseudospongoprunum? spp. are relatively rich in the upper part of the subsection (Figure 8).

On the basis of the occurrence of the albaillellarians diagnostic of the upper part of the Neoalbaillella ornithoformis Assemblage Zone, we correlate the lower part of the main unit (Figure 8) with the uppermost part of the lower Upper Permian (upper Wuchiapingian) to lower part of the upper Upper Permian (Changhsingian). Following the occurrence of the albaillellarians characteristic of the N. optima Assemblage Zone, we correlate the remaining part of this subsection with the upper Upper Permian (Changhsingian).

Subsection NF 195B contains the Wuchiapingian Changhsingian boundary. According to Xia et al. (2004), we position the base of the Changhsingian at the first appearance datum of Albaillella flexa, NF 195–120–3, approximately 0.2 m above the base of subsection NF 195B (Figure 8).

Figure 10.

Early Triassic and Permian radiolarians extracted from the chert intercalated in the upper section NF 195A. See Figure 7 for the stratigraphic levels of the samples. 1–4, Sphaeroid A; 1, NF195–1; 2, NF195–2; 3, NF195–3; 4, NF195–5; 5–7, Sphaeroid B; 5, NF195-1; 6, NF195-2; 7, NF195–3; 8, Sphaeroid D, NF195–2; 9, Sphaeroid E, NF195–3; 10, Sphaeroid F, NF195–3; 11, Triassospongocyrtis? sp., NF195–3; 12,13, Albaillella? sp.; 12, NF195–3; 13, NF195–5; 14,15, Hegleria? sp.; 14, NF195–3; 15, NF195–5; 16, Latentifistularia gen. et sp. indet. A, NF195–3; 17, Latentifistularia gen. et sp. indet. B, NF195–3; 18, Stigmosphaerostylus sp., NF195–2; 19, Pseudospongoprunum? sp., NF195–1; 20, Grandetortura? sp., NF195–5.

Figure 11.

Early Triassic conodont fossils from the black chert bed intercalated in the upper unit of subsection NF 195A. All specimens are from sample NF 195–3. See Figure 7 for the stratigraphic level. All scale bars = 100 µm. 1–6, Hindeodus parvus (Kozur and Pjatakova); 1, 2, Pl element, a pair of stereo microscopic views; 3, 4, Pl element, a pair of stereo microscopic views; 5, 6, counterpart of the specimen in Figures 11–3 and 4, a pair of stereo microscopic views; 7, Hindeodus sp. A, Pl element; 8, 9, Hindeodus sp. B; 8, Pl element, cast and mold of one specimen; 9, Hindeodus sp. B, counterpart of the specimen in Figure 11–8.

Main extinction horizon of Permian radiolarians

The Changhsingian radiolarian fauna in both two subsections are highly diverse (Figures 7, 8). However, as recorded in subsection NF 195A, the Changhsingian radiolarians exhibit a sudden, marked diversity loss across the marked lithologic boundary between the chert of the middle unit and the black claystone of the upper unit. The level defined as the top of the middle unit marks the main extinction horizon of the Permian radiolarians. Also, the radiolarian faunal assemblage of the upper unit largely differs from that of the middle and lower units. The pronounced biotic turnover of radiolarians took place across the PTB, which occurs in the stratigraphic interval between the bottom of the upper unit and the level of NF 195–3. The comparable extinction event and biotic turnover across the marked lithologic boundary are also recorded at the correlative lithologic boundary of the PTB siliceous rock section of the Hashikadani Formation in the Mt. Funabuseyama area (NF 1212R: Sano et al., 2010).

Foraminifers in the radiolarian chert

The occurrence of the foraminifers in the chert is noteworthy (Figure 6–5). The foraminifer-yielding residues also contain a few Permian-type radiolarian fossils and sphaeroid spumellarians (Figures 7, 8).

Using the quantitative analysis of the stratigraphic variation of the size of well preserved Changhsingian to Olenekian foraminiferal tests, Song et al. (2010) illustrated the sudden and marked size reduction that coincides with the first or main stage of the end-Permian foraminiferal extinction shown by Song et al. (2009). The mean test sizes of Changhsingian and Induan species are 6.9 log µm³ (i.e., ca. 500 to 700 µm shell length for conical and cylindrical forms) and 5.6 log µm³(i.e., < 100 µm shell length), respectively. The size of the foraminifers exhibits a slow increase in the late Induan to Olenekian, reaching up to 6.3 log µm³. Song et al. (2010) considered the marked decrease in the test size across the PTB transition as the Lilliput effect (Urbanek, 1993) on foraminifers in the aftermath of the end-Permian crisis.

Although volumetric measurement of the test size was not undertaken, the foraminifers in the residue of NF 195–5 are most commonly a few to several hundreds microns in size. In terms of the size variation of the PTB foraminifers (Song et al., 2010), the foraminifers in the residue are comparable to the Changhsingian species rather than the Induan species. We prefer a Changhsingian age for the foraminifers in the residues.

The cooccurrence of the foraminifers and radiolarians in the acid-processed residues is enigmatic (Figures 7, 8). The coocurrence of foraminifers and radiolarians is reported from Permian successions at several localities (e.g. Dalong Formation, South China: Shang et al., 2003; Ruteh Formation, northern Iran: Vaziri et al., 2005). However, these successions comprise shallow-marine carbonate and siliciclastic facies, largely different from the Hashikadani chert. With an emphasis upon the deepmarine and pelagic sedimentation of the Hashikadani chert, following the most common occurrence of Permian foraminifers in a photic zone (Flügel, 2004), we infer that the Changhsingian foraminifers in the residues are allochthonous relative to their host sediment of radiolarian chert. The foraminiferal individuals were shed from a shallow-marine environment into a deep-marine environment where radiolarian chert accumulated.

Stratigraphic position of the PTB in subsection NF 195 A

According to the biostratigraphic constrains, the PTB is present within the lower part of the upper unit (Figure 7). However, no fossils available for the determination of the PTB occur in this stratigraphic interval. Therefore, we propose the two possibilities for the stratigraphic position of the PTB.

One option is that the PTB occurs at the bottom of NF 195–3, on the basis of the occurrence of H. parvus (Figure 7). It is reasonable to place the PTB at this level in terms of the world-accepted designation of the PTB defined by the first appearance datum of H. parvus (Yin et al., 2001). Given this option, the rapid environmental change recorded by the abrupt facies change from the chert of the middle unit to the black claystone of the upper unit likely commenced in the Changhsingian. Also the main extinction of the Permian radiolarians is thought to have occurred prior to the PTB, and thus some Permian-type radiolarians yielded from NF 195–5 may represent survivors of the main extinction event (Figure 7). The presence of the stratigraphic interval between the main extinction horizon of Permian taxa and the PTB is reported from PTB sections at several localities (e.g. Yin et al., 2001; Ji et al., 2007; Takahashi et al., 2009).

The alternative view is that the PTB occurs at the base of the upper unit, directly overlying the Changhsingian chert of NF 195–6 (Figure 7). This option is based upon the correlation of subsection NF 195A with the recently reported PTB section of the Hashikadani Formation in the Mt. Funabuseyama area (NF 1212R: Sano et al., 2010). Lithologically, section NF 1212R is very similar to subsection NF 195A. The PTB of section NF 1212R is precisely located by the occurrence of H. parvus from the basal part of the black claystone directly overlying the Changhsingian chert. Given this option, the main extinction horizon of the Permian radiolarians corresponds to the abrupt facies transition from the middle unit to the upper unit. Thus, the main extinction event is posited to have occurred concurrently with the drastic environmental change. However, this latter option does not fit the GSSP designation of the PTB by Yin et al. (2001).

The two options differ significantly from each other in the inferred position of the PTB. The first option seems more likely than the second in terms of the GSSP designation of the PTB. However, there remains the possibility that the stratigraphic interval between NF 195–6 and NF 195–3 yields H. parvus. Furthermore, it seems unlikely that the two comparable sections belonging to the same stratigraphic unit differ from each other in the stratigraphic relation of the PTB to the main extinction horizon and marked lithologic boundary. With an emphasis upon the comparison with section NF 1212R, we prefer the second option that corresponds to the lithologic boundary between the middle and upper units and the major extinction horizon of the Permian radiolarians (Figure 7). Nevertheless we cannot rule out the first option and need further examination to position the PTB definitely.

Permian-type radiolarians in the upper unit of subsection NF 195A

Some Permian-type radiolarian fossils occur from the black chert beds at the two levels within the upper unit of subsection NF 195A (NF 195–5 and NF 195–3 in Figure 7). Except for Grandetortura? sp., the Permian-type radiolarian fossils occur also in the upper Upper Permian chert beds of the lower and middle units. The cooccurrence of the Permian-type radiolarians and the lower Induan-indicative Hindeodus parvus in NF 195–3 is enigmatic (Figure 7).

Sano et al. (2010) reported the occurrence of some Permian-type radiolarians from the lower Induan chert beds of the PTB siliceous rock succession of the Hashikadani Formation. The stratigraphic levels of the Permian-type radiolarians-yielding chert beds are approximately 70 to 100 cm higher than the precisely age-constrained PTB. However, due to the lack of conclusive evidence, Sano et al. (2010) suggested two alternative explanations for the enigmatic occurrence of Permiantype radiolarians in the lower Induan chert beds, i.e., the survival and reappearance as Lazarus taxa in the lower Induan, or reworking of these fauna up into the lower Induan. As reliable data remain insufficient to date, we reserve critical discussions on this enigma.

Compilation of two subsections

The physical tracing of individual beds shows that the top of the middle unit of subsection NF 195A (NF 195–6 in Figure 7) corresponds to the top of the upper unit of subsection NF 195B (NF 195-29 in Figure 8). In addition, the middle unit of subsection NF 195 A and the upper unit of subsection NF 195B are litho- and biostratigraphically comparable with each other (Figures 7, 8). Thus, we combine the two subsections into one composite section, NF 195PTB (Figure 12).

Reaching ca. 7.8 m thick, the lithologic succession of the composite section begins with the gray chert unit (ca. 6.2 m thick) with small amounts of black chert and black claystone in the upper part, which grades up to the black chert unit (ca. 0.7 m) with a small amount of black claystone and gray chert, including pyrite nodules. This unit exhibits an abrupt lithologic changes up-section into the black claystone unit (ca. 0.9 m) with thin, intermittent beds of black chert. Composite section NF 195PTB ranges from the lower Upper Permian (upper Wuchiapingian) to the lower Lower Triassic (lower Induan). Following the definition of the Neoalbaillella optima Zone by Kuwahara et al. (1998), we position its bottom and top at the levels of NF 194–94–3 of subsection NF 195B and NF 195–6 of subsection NF 195A, respectively (Figures 7,8).