西南日本秋吉带石炭系高山(Ko-yama)灰岩群的石炭纪(Visean – Moscovian)牙形石序列

西南日本秋吉带石炭系高山(Ko-yama)灰岩群的石炭纪(Visean – Moscovian)牙形石序列

E-mail: ishidak@ias.tokushima-u.ac.jp.

1 Introduction

The Carboniferous conodont biostratigraphy in Japan has been studied mainly in the 20th Century(Igo and Koike, 1964; Koike, 1967; Igo, 1973; Igo and Kobayashi, 1974; Watanabe, 1975; Igo and Igo, 1979;Haikawa, 1988). The Middle Carboniferous conodont zonation has been studied in relation with the Mississippian–Pennsylvanian boundary in the Hina Limestone of the Akiyoshi Belt (Mizuno, 1997). The Akiyoshi Belt is regarded as one of the Permian accretionary complexes in the Inner Zone of SW Japan(Isozaki, 1997; Sano et al., 2004). The complex encompasses large limestone masses, as they are well known as the Akiyoshi, Taishaku, Atetsu, Hina, and the Ko-yama limestones with associations of basaltic pyroclastics on their bases. On the sedimentology of the Akiyoshi Limestone Group, Nakashima and Sano(2007) gave the implication that the limestone group is of a resedimented origin shed from Mississippian –Permian mid-oceanic atoll-type buildup into slope-to-basin facies.

We studied the conodont faunas of the lower part of Ko-yama Limestone Group in NW of Okayama with respect to the biostratigraphic resolution in the clastic carbonate sedimentary field. The obtained faunal succession that appeared in concordance with the lithostratigraphic order was correlative with those of global Visean, Serpukhovian, Bashkirian and Moscovian stages, respectively. The Visean/ Serpukhovian boundary of the section was considered to be represented by the occurrence ofLochrieaspecies (e.g.Blanco-Ferrera, et al., 2005; Groves et al., 2003; Higgins, 1975; Kullmann et al., 2008; Nemyrovska, 1999,2005; Nikolaeva, et al., 2009; Somerville, 2008).

2 Geological setting of the Ko-yama Limestone Group

The Ko-yama Limestone (Yokoyama et al., 1979)in the Ko-yama Limestone Group (after Yoshimura,1961: Koyama Group) is one of the large masses in the Akiyoshi Belt. The biostratigraphic research revealed that the Ko-yama Limestone ranges from Carboniferous to Middle Permian. The six foraminifer-zones ofEndothyra,Millerella,Pseudostafella,Profusulinella,Fusulinella–Fusulina, andPseudoschwagerina–Parafusulinawere recognized (Yokoyama et al.,1979).

The lower part of the Ko-yama Limestone Group is mainly composed of clastic carbonates, which are dominated by the crinoid–bryozoan bioclastic packstone. These carbonates were considered to be formed by the transportation of carbonate clasts into the chert depositional deeper open sea bottom (Yokoyama et al.,1979). According to the shed-model (Nakashima and Sano, 2007), the limestone–basaltic pyroclastic sequence with association of the spiculite cherts in the Akiyoshi Limestone Group is suggestive as a marginal deeper and slope facies of the mid-oceanic atoll-type buildup.

3 Faunal succession of the lower part of the Ko-yama Limestone Group

Among the extracted conodont materials, P1elements were used for the chronological analysis. The faunas appear in concordance with the lithostrati-

graphic order (Inada et al., this issue), and are correlative with the faunas from the studied conodont zonation in the Akiyoshi Belt (Fig. 1). The faunal succession of the lower part of the Ko-yama Limestone Group is as follows in ascending order.

Gnathodus semiglaber Fauna (upper Visean): is characterized by the occurrence ofGnathodus semiglaber.The fauna was obtained from a wackestone block in the basaltic pyroclastics at the basal part of the group.Gnathodus praebilineatus – Lochriea multinodosa Fauna (upper Visean): is composed ofGnathodus praebilineatus,Gnathodusex. gr.bilineatus,Pseudognathodus homopunctatus,Lochriea multinodosa,Lochriea commutata, andCavusgnathus unicornis.

Lochriea ziegleri – Gnathodus girtyi girtyi s.l. Fauna(lower Serpukhovian): in addition toLochriea ziegleri,Gnathodusex. gr.bollandensisandVogelgnathus campbelli, transition forms ofGnathodus girtyi girtyis.l. toG.girtyi simplex, andLochriea nodosas.l. toL.crusiformiswere recognized from the lower horizons.Neoganthodus symmetricus – Idiognathodus primulus Fauna (middle–upper Bashkirian): is composed ofNeoganthodus symmetricus,Idiognathodus primulus,Idiognathodussp. 1,Idiognathodussp. 3, andStreptognathodusexpansus.The fauna from a particular horizon contains reworked upper Visean to Serpkhovian elements ofGnathodus semiglaber,Gnathodussp.1,Pseudognathodus homopunctatus, andVogelgnathus postcampbelli.

Fig. 1 Correlation of the conodont zones in the Akiyoshi Belt, Japan

Idiognathoides convexus – Gondolella clarki Fauna(lower Moscovian): is composed ofGondolella clarki,Gondolellasp. 1,Idiognathoides sulcatus sulcatus,Idiognathoides macer,Idiognathoides attenuates,Idiognathoidesconvexus,Idiognathodus delicatuss.l.,Streptognathodus suberectus,Neognathodus bothrops,Neoganthodus symmetricus, andIdiognathodussp. 1.

4 Results and Discussions

Conodont faunal succession: The conodont faunas from the lower part of the Ko-yama Limestone Group were studied with respect to the biostratigraphic reliance in the “clastic carbonate” sedimentary field. The existence of conodont faunal succession as well as the faunal stability was basically confirmed.The conodont faunal succession is:Gnathodus semiglaberFauna (upper Visean),Gnathodus praebilineatus–Lochriea multinodosaFauna (upper Visean),Lochriea ziegleri–Gnathodus girtyi girtyis.l. Fauna(lower Serpukhovian),Neoganthodus symmetricus–Idiognathodus primulusFauna (middle – upper Bashkirian), andIdiognathoidesconvexus–Gondolella clarkiFauna (lower Moscovian). The faunas are correlative with those from the conodont zones of the Hina, Atetsu, Akiyoshi, and Omi limestone groups in the Akiyoshi Belt, respectively.

Visean/Serpukhovian boundary in the section:The FAD ofLochriea ziegleriin the study section marked the Visean/Serpukhovian boundary.

Mixed fauna in relation with the early Bashkirian global low-standing eustatic condition: A reworked Serpukhovian and older elements were recognized in someNeoganthodus symmetricus–Idiognathodus primulusFauna (middle – upper Bashkirian)from the particular clastic carbonate horizon. The early Bashkirian global low-standing eustatic condition (ex. Hallam, 1992; Nakazawa and Ueno, 2009)might be considered for the erosional event in the provenancial shallow-carbonate buildup.

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Carboniferous (Visean–Moscovian) Conodont Faunal Succession in the Ko-yama Limestone Group, Akiyoshi Belt, SW Japan:the Biostratigraphic Resolution in the Clastic Carbonate Sedimentary Field

Keisuke ISHIDA1), Sigeyuki SUZUKI2), Noriyuki INADA2), Shinji YAMASHITA2)
1)Laboratory of Geology, Institute of SAS, Tokushima University,770-8502,Tokushima, Japan;
2)Dept. Earth Sci., Okayama University,700-8530,Okayama, Japan

The Carboniferous conodont faunal succession of the lower part of Ko-yama Limestone Group was studied. The confirmed upper Visean–lower Moscovian lithostratigraphy of the group is characterized by the clastic carbonates with common association of the basaltic pyroclastics and some intercalation of spicular chert beds.The faunal succession ofGnathodus semiglaber(upper Visean),Gnathodus praebilineatus–Lochriea multinodosa(upper Visean),Lochriea ziegleri–Gnathodus girtyi girtyis.l. (lower Serpukhovian),Neoganthodus symmetricus–Idiognathodus primulus(middle – upper Bashkirian), andIdiognathoidesconvexus–Gondolella clarki(lower Moscovian) faunas, appears in concordance with the lithostratigraphic order. The faunas are correlative with those from the conodont zones of the Hina, Atetsu, Akiyoshi and Omi limestone groups in the Akiyoshi Belt. The Visean/Serpukhovian boundary of the section was recognized by the FAD ofLochriea ziegleri.

conodont; Ko-yama Limestone; Carboniferous; Akiyoshi Belt; SW Japan

10.3975/cagsb.2012.s1.14