Revision of the Stratigraphy of the Toyora and Toyonishi Groups in the Ouchi-Kikugawa Area, Yamaguchi Prefecture, West Japan

Jour. Geol. Soc. Japan, Vol. 111, No. 7, p.389－403, July 2005

Revision of the stratigraphy of the Toyora and Toyonishi Groups in the Ouchi-Kikugawa area, Yamaguchi Prefecture, west Japan

Abstract

Toshihiro Yamada＊ and The stratigraphy of the Toyora（Lower to Middle Jurassic）and Toyo- nishi Upper Jurassic to Lower Cretaceous Groups in the Ouchi-Kiku- Terufumi Ohno＊＊（） gawa area, south of the Tabe Fault, in the western part of Yama- guchi Prefecture is revised. The revised succession is, in ascending Recived August 26, 2004 order, the Higashinagano, Nishinakayama and Utano Formations in the Accepted May 11, 2005 ＊ Department of Geology and Paleontology, Toyora Group, the Kiyosue and Yoshimo Formation in the Toyonishi National Science Museum, 3-23-1 Hyakunincho, Group. The Kiyosue Formation is further subdivided into the Nakao Shinjuku, Tokyo 169-0073, Japan Siltstone and Nanami Sandstone Members. The Utano Formation has ＊＊ The Kyoto University Museum, Kyoto Universi- been thought to be marine in the type Toyora area, while non-marine in ty, Kyoto 606-8317, Japan the studied area. However, marine facies, characteristic in the Utano Formation, is ascertained in the formation hitherto classified as the Corresponding author; T.Yamada, ptilo@ “Nishinakayama Formation”in the studied Ouchi-Kikugawa area. The mb.infoweb.ne.jp deposits referred to the non-marine sequence of the“Utano Forma- tion”by previous authors are re-classified into the Nakao Siltstone Member of the Kiyosue Formation. Newly established stratigraphy suggests that the age of the so-called “Utano flora”, reported from the Kiyosue Formation as well as the “Kiyosue flora”, of the studied area is younger than Bathonian, con- trarily to the previous age assignment older than Bathonian. The rela- tionship of these two florae should be explored in the future.

Key words: Ouchi-Kikugawa area, Kiyosue flora, Toyonishi Group, Toyora Group, Upper Jurassic to Lower Cretaceous, Utano flora, Yamaguchi Pref.

bachian to Toarcian）and the Utano（Toarcian to Batho- Introduction nian）Formations in ascending order（Kobayashi, 1926; The Jurassic-Cretaceous sediments are distributed in Matsumoto and Ono, 1947; Takahasi et al., 1966; Hirano, the western part of the Yamaguchi Prefecture（formerly 1971, 1973a, b）. The geological age of these formations called the Nagato Province, as seen in old papers）. The are well established by the fossil ammonoids contained sediments are classified generally into three groups; Toyo- （Hirano, 1971, 1973a, b）. ra（Lower to Middle Jurassic）, Toyonishi（Upper Jurassic In the studied Ouchi-Kikugawa area, described herein, to Lower Cretaceous）and Kwanmon（Lower Cretaceous） the Toyora and overlying Toyonishi Groups are distri- Groups in ascending order（e.g., Yabe, 1920; Kobayashi, buted. The latter group consists of the Kiyosue（lower） 1926; Matsumoto, 1949; Hase, 1960; Takahasi et al., and the Yoshimo（upper）Formations. The type area of 1965,1966）. These groups are distributed in two separate the Kiyosue Formation was designated in this area（Oishi, areas, with NW-SE striking Tabe Fault between them 1933; Matsumoto, 1949; Hase, 1960）. （Kobayashi, 1936）; the northern area（type area of the However, the correlation of the succession in the type Toyora Group）and the southern area（the Tabe Basin area of the Toyora Group（northern part）and that in the including present studied Ouchi-Kikugawa area）（Fig. 1）. Ouchi-Kikugawa area（southern part）is controversial The Toyora Group has its stratotype in the northern according to previous authors（see Fig. 2）. The sedimen- area as mentioned above. The group in the type area has tary environments of the Toyora Group have been inter- been studied by many authors since Yokoyama's（1904） preted as changing gradually southwards from offshore to first report of ammonites（e.g., Yabe, 1920; Kobayashi, deltaic by many authors. In particular, its upper part, the 1926; Matsumoto and Ono, 1947; Hase, 1960; Takahasi et Utano Formation, was thought to be entirely non-marine al., 1966; Hirano, 1971, 1973a, b）. The Group is further in the Ouchi-Kikugawa area（Kobayashi, 1926, 1931; Mat- divided into three formations; the Higashinagano（Sine- sumoto, 1949; Hase, 1960; Takahasi et al., 1965）. Oishi murian to Pliensbachian）, the Nishinakayama（Pliens- （1933, 1940）, however, classified these non-marine beds CThe Geological Society of Japan 2005 389 390 T. Yamada and T. Ohno 2005―7

Fig. 1. Index map showing the study area and type area of the Toyora Group（based on Takahasi et al., 1965, 1966; Hirano, 1971）. These two areas are separated by the Tabe Fault with a NW-SE strike.

into one single entity, under the name of the Kiyosue 1986; Kimura and Ohana, 1987a, b）, while late Jurassic to Plant Beds, although he neither presented any detailed Early Cretaceous if the bed is judged to belong to the lithological description nor its geographical distribution. Kiyosue Formation（Oishi, 1933, 1940）. Thus, this controversy seems to have resulted from the In this condition, the re-examination of the plant-bear- different interpretation of the sedimentary environments ing stratigraphic horizons in the Ouchi-Kikugawa area is of the Utano Formation. strongly needed, as the basin plays an important role for The above-mentioned non-marine beds contain abun- deciphering the floristic and phytogeographic problems dant plant fossils（Oishi, 1933, 1940; Takahasi, 1959; in East Asia during the early Jurassic to earliest Creta- Takahasi et al., 1965; Kimura et al., 1986; Kimura and ceous time period（Kimura, 1987, 1988）. Ohana, 1987a, b）. The age of these plant fossils is decid- Recently, new exposures became available for the geo- ed on the basis of the geologic age of the contained beds; logical survey, as the result of the construction of a new Middle Jurassic if the beds belong to the Utano Forma- road from Shimonoseki to Kikugawa. We have had the tion（Takahasi, 1959; Takahasi et al., 1965; Kimura et al., opportunity to re-examine the stratigraphy in the Ouchi- Jour. Geol. Soc. Japan 111（ 7 ） The Mesozoic in the Ouchi-Kikugawa area 391

Fig. 2. Comparison of the stratigraphic scheme proposed in this study to those of previous studies. *Kobayashi's study（1926）referred to this as the“Nanami Group”.

Kikugawa area, which covers the parts of Shimonoseki （1）Higashinagano Formation［Kobayashi, 1926］ City and Kikugawa Town（Figs. 1, 3）. Our study focused Type locality: Higashinagano, Toyota Town. on two points: to determine the lithological facies of the Lithology in Ouchi-Kikugawa area: In the studied Utano Formation（presence or absence of the marine Ouchi-Kikugawa area, the Higashinagano Formation is in facies）and to position the horizons of the plant-bearing fault contact with the basement rocks（Loc. 209 in Fig. 3; beds in the newly established successions. Kamiozuki Fault: Takahasi et al., 1965）. This formation consists mainly of alternating mudstone and fine-grained Stratigraphy sandstone beds. The granule- to pebble-graveled con- The Mesozoic strata exposed in the Ouchi-Kinugawa glomerate beds are intercalated in the upper part. The area（Figs. 1, 3）can be classified into the Jurassic Toyora thickness is 350 to 900 m. The lithofacies show gradual Group and the Jurassic to Cretaceous Toyonishi Group lateral variation, i.e., the sandstone beds become thicker （Figs. 4－6）. The Lower Cretaceous Kwanmon Group lies and more frequently intercalated within the mudstone unconformably on the Toyonishi Group in the northwest- beds towards north. Mudstone beds become dominant ern corner of this area（Hase, 1960; Figs. 4－6）. The Toy- towards the top of the formation. Mudstone beds are less ora Group is in fault contact with the Permian Toyohi- frequent and conglomerate beds are not observed in the gashi Group at Loc. 209 in Kamiozuki（Fig. 3; Takahasi et type area（Hirano, 1971）. Although the basal conglomer- al., 1965）. This group is the basement of the Mesozoic ate（Nbc in the type area（Hirano, 1971）; see Fig. 2）is strata and is composed of hornfelsized sandstone and lacking in the studied Ouchi-Kikugawa area, the uncon- mudstone. The Mesozoic strata strike mostly in NE-SW formable relation between the Higashinagano Formation direction and dip toward the NW direction, except in the and the basement rock is observable at Shimoohno, east area of Takaji Pass and Imou, where the Mesozoic strata of the present area（Takahasi et al., 1965; Fig. 1）. are folded and cut by some faults and the strike of the This formation is composed, in the Kamiozuki and beds is predominantly in NW-SE direction（Fig. 4）. These Ipponmatsu areas, of gray to black, massive or hummocky Paleozoic and Mesozoic strata are intruded by middle to cross-stratified fine-grained sandstone and weakly bio- late Cretaceous granitic rocks（Iizumi et al., 1985）at turbated sandy siltstone. It is subdivided into two parts Imou and Yamane. according to the quantity of sandstone beds: the lower 1．Toyora Group［Kobayashi, 1926］ part（ca. 350 m thick）composed of sandstone beds（2 to The type section of the Toyora Group is in Toyota and 20 m thick）intercalating the sandy siltstone beds（30 to Kikugawa Towns, in the northeast of the Tabe Fault（Fig. 50 cm thick）and the upper part（ca. 250 m thick）of 1）. It is characterized by mudstone irregularly interbed- alternating mudstone and sandstone beds of almost the ded with sandstone beds of marine origin. This group is equal thickness（20 to 30 cm）. A very thick（ca. 40 m） divided into the Higashinagano, Nishinakayama and Utano conglomerate bed, comprising subrounded pebbles of Formations in ascending order. chert, phyllite and siliceous rock, is locally intercalated 392 T. Yamada and T. Ohno 2005―7

Fig. 3. Locality map of the study area. Plant fossil localities of previous studies（Takahasi et al., 1965; Kimura et al., 1986; Kimura and Ohara, 1987a, b）are boxed. The previous locality numbers are summarized in Table 3. Jour. Geol. Soc. Japan 111（ 7 ） The Mesozoic in the Ouchi-Kikugawa area 393

Fig. 4. Geological map of the study area and profiles along A-B and X-Y. Quaternary deposits are omitted. Upper direction of strata is indicated by arrows in profiles. 394 T. Yamada and T. Ohno 2005―7 are indicated by arrowheads. Dashed ） he locality numbers shown in Fig. 3. The horizons of the plant fossil ltstone bed. Takahasi et al., 1965; Kimura 1986; and Ohara, 1987a, b （ Lithological correlation of columnar sections along five routes in the studied area. Numbers to right are t . 5 localities in this study are indicated by trilobed leaflet marks and those of previous studies line: conformable boundary, bold fault waved unconformable indent traceable tuffaceous si Fig. Jour. Geol. Soc. Japan 111（ 7 ） The Mesozoic in the Ouchi-Kikugawa area 395

calated by conglomerate beds（20 cm to 1 m thick）. The intercalating conglomerate is composed of clast-supported subangular granules and pebbles of chert, phyllite and siliceous rock. The upper part of the formation is composed of black parallel-laminated mudstone intercalated by gray fine- grained sandstone beds of less than 50 cm in thickness. The siltstone is sometimes bioturbated and contains sporadically plant fragments. The fossils discovered from this formation are listed in Table 1. （2）Nishinakayama Formation［Kobayashi, 1926］ Type locality: Nishinakayama, Kikugawa Town. Lithology in Ouchi-Kikugawa area: The Nishinakaya- ma Formation lies conformably on the Higashinagano For- mation. Sandstone beds decreases in frequency and thickness towards the upper part of the Higashinagano Forma- tion which thus changes gradually into the Nishinakayama Formation. This fact is typically observable in the succession between Locs. 11 and 12 near Oka, in Fig. 3. The Nishinakayama Formation is composed mainly of well- laminated black claystone and siltstone. The total thickness varies locally and ranges from 450 to 900 m. The lithofacies correlative with Na in the type area（Hirano, 1971, 1973b; alternating beds of mudstone and sandstone）is lacking in this area（see Fig. 2）. Fig. 6. Schematic standard columnar section in the study area The well-laminated black mudstone is composed of very showing fossil-bearing horizons. Legends are the same as in Figs. 3, thin alternating layers of sand and clay. They are trace- 5. able laterally, in the order of several meters Grayish yellow fine-grained sandstone beds（approximately 50 cm in the alternating sandstone and siltstone beds of the thick）are intercalated in the lower part（Locs. 18, near upper part（between Locs. 230 and 231 in Fig. 3）. This Oka and 246 in Ipponmatsu; Fig. 3）. bed thins out towards both ends. Fossil ammonites are discovered from one horizon In the Yamane area southwest to Kamiozuki, the litho- developed at the Loc. 31 and near Oka（Fig. 7）; they facies of the formation is different from those observed include Protogrammoceras nipponicum（Matsumoto）, in the Kamiozuki and Ipponmatsu areas. The formation is Harpoceras chrysanthemum（Matsumoto）, H. okadai subdivided in ascending order into the lower（alternating （Yokoyama）and Fuciniceras nakayamense（Matsumo- sandstone and siltstone beds: ca. 130 m）, middle（fine- to）, which indicate Pliensbachian to Toarcian Age. The grained sandstone: more than 120 m）, and upper parts fossils are listed in Table 1. （mudstone: more than 150 m）. （3）Utano Formation［Kobayashi, 1926］ The lower part of the formation is composed of alter- Type locality: Utano-dani, Utano, Kikugawa Town. nating beds of parallel laminated black sandy siltstone Lithology in Ouchi-Kikugawa area: The Utano Forma- （1－3 m thick）and gray parallel-laminated or hummocky tion conformably lies on the Nishinakayama Formation. cross-stratified fine-grained sandstone（20 cm to 1 m The total measurable thickness varies from 100 to 850 m, thick）. Convolute lamination or flame structure is often owing to the southward increasing erosion, covered observable. Ripple lamination is frequent at the top of the obliquely by the overlying Kiyosue Formation. The For- parallel-laminated or hummocky cross-stratified fine- mation is subdivided into basal（ca. 50 m）and main parts grained sandstone beds. The siltstone is sometimes bio- （ca. 50 to 800 m）. turbated and contains sporadically plant fragments. The basal part is composed of weakly bioturbated gray The middle part of the formation is composed of gray to black mudstone with rare very thin interbeds of fine- hummocky cross-stratified fine-grained sandstone inter- grained sandstone（less than 5 cm thick）. The lower 396 T. Yamada and T. Ohno 2005―7

boundary of this formation is demarcated by this weakly bioturbated mudstone, which can be seen at the Locs. 271 and 272 in Kamitabe（Figs. 3, 5）. The main part is composed of also bioturbated siltstone and fine-grained sandstone. The siltstone is gray to black in color when fresh, but becoming whitish yellow when weathered. The siltstone beds are 50 to 200 cm in thickness, bioturbated, and contain trace fossils like Planolites and Chondrites. The fine-grained sandstone beds are 15 to 50 cm in thickness, dark gray and massive or graded. Inoceramus utanoensis Kobayashi occurs from the lower（Loc. 272 in Kamitabe）and upper parts（Loc. 279 Fig. 7. Selected molluscan fossils collected from the Toyora Group. in Kamitabe and Loc. 360 in Toriyma）（Figs. 3, 7）. Accord- A: Protogrammoceras nipponicum（Matsumoto）, at Loc. 31 near ing to Hirano（1973b）, the bed containing this inoceramid Oka（Fig. 3）（#NSM-PM-17121）. B: Inoceramus utanoensis Kobayashi, at Loc. 272 in Kamitabe Fig. 3 #NSM-PM-17136 . is correlative with the Bathonian. The fossils and their （（）） Scale bars ＝ 1 cm. localities are listed in Table 1. 2．Toyonishi Group［Matsumoto, 1949］ The Toyonishi Group was originally defined by Mat- mation by Toriyama（1938）, and to the Utano Formation sumoto（1949）as a unit composed of both Kiyosue and by Matsumoto（1949）and Hase（1960）. The lower part of Yoshimo Formations. Its type area is in the western part the Utano Formation by Takahasi et al.（1965）is also of the Toyora-gun（including Toyonishi Village in former equivalent of the present member. The Kiyosue Plant administrative regime, which belongs to Shimonoseki City Beds of Oishi（1933, 1940）includes a part corresponding at present）. We have adopted essentially his definition in to this member（Fig. 2）. this study, but the Kiyosue Formation should be partly Thickness: Variable. More than 250 m in the Takaji Pass, redefined as described later. The Toyonishi Group con- 500 m in the Nakano Forestry Road, 450 m in Ono, and sists of alternating sandstone and mudstone beds, with 260 m in Toriyama. conglomerate beds intercalated sometimes in sandstone Lithology: This member is composed predominantly of beds. very fine-grained sandstone, siltstone and carbonaceous （1）Kiyosue Formation［Oishi, 1933］ claystone. The conglomerate of granule- to pebble-sized Type area: Around the former Kiyosue Village（present- gravels forms the basal part, which uncomformably over- ly included in Shimonoseki City）and Toyohigashi Village lies the Utano Formation. In the upper part of the member （presently Kikugawa Town）. The stratotype is not defined （50 to 130 m below the upper boundary of the member）, in the original definition. Here we designate the succes- a fine- to coarse-grained massive sandstone beds are sion along the Nakano Forestry Road（Locs. 99 to 119; intercalated in the predominant lithofacies（Locs. 73 to Figs. 3, 5）as the stratotype. 77 in Ono and 106 to 107 in Nakao（Figs. 3, 5）. This mem- Lithology: The Kiyosue Formation consists of alternating ber is comformably overlain by the Nanami Sandstone sandstone and mudstone beds, with conglomerate beds Member. intercalated in the sandstone beds. It is subdivided into The basal conglomerate bed（5 m thick）lies on the two new members, the Nakao Siltstone Member for the Utano Formation with almost parallel unconformity, as lower part and the Nanami Sandstone Member for the seen at the Locs. 53 in Ono, 99 and 139 in Nakao, 296 in upper part. Toriyama（Figs. 3, 5, 8）. It is composed of angular to sub- a. Nakao Siltstone Member［new name］ angular gravels of sandstone, mudstone and siliceous Stratotype: Succession exposed along the Nakao rock. The matrix is dark gray to black in color, coarse- to Forestry Road（Locs. 99 to 107）in Nakao（Fig. 3）. very coarse-grained sandstone. History of classification: The stratigraphic position of The very fine-grained sandstone in the main part is gray this member has been subject to argument since long to dark gray in color and contains mud-draped asymmet- time. The corresponding beds were first described by ric ripple laminae（climbing ripples in some cases）, 0.5 to Kobayashi（1926）as the Nishi-Nakayama“Group”and 2 cm high, with the wavelength of about 4 to 6 cm. The Nanami“Group”, but soon modified to Beds by Kobayashi laminae are often weakly deformed by bioturbation. The （1931）. The Nakao Siltstone Member introduced herein ripples suggest the paleocurrent direction roughly from corresponds to the upper part of the Nishinakayama For- SE to NW. The sandstone beds are usually 50 to 200 cm in Jour. Geol. Soc. Japan 111（ 7 ） The Mesozoic in the Ouchi-Kikugawa area 397

Table 1. Fossils from the Toyora Group. The detailed stratigraphic level of the locality 127 is uncertain.

thickness, but occasionally attaining 400 cm. The siltstone first described by Kobayashi（1926）as the Nishi-Nakaya- beds are dark gray to black in color and are parallel-lami- ma“Group”and the Nanami“Group”, but soon modified nated. The lenticular beds of very fine-grained sandstone to“Beds”by Kobayashi（1931）. The Utano Formation of with asymmetric ripple laminae are developed at the base Toriyama（1938）, the upper part of the Utano Formation of the siltstone beds. The siltstone beds are 50 to 400 cm of Takahasi et al.（1965）and the Kiyosue Formation of in thickness in the main part. In many cases, very fine- Matsumoto（1949）and Hase（1960）are equivalents of grained sandstone grades upwards into the siltstone. The this member. The upper part of the Kiyosue Plant Beds of carbonaceous claystone beds are 30 to 100 cm in thick- Oishi（1933, 1940）is also corresponding to this member ness. The root fossils in upright position are often （Fig. 2）. observed in the claystone and siltstone beds. The acidic Thickness: Variable. More than 120 m in the Takaji Pass, tuffaceous siltstone bed, 8 to 10 cm in thickness, is inter- 450 m in Nakao, 900 m in Kaminanami, and more than 300 calated in the main part as seen at the Locs. 56 in Ono, m in Toriyama. 141 and 144 in the Takaji Pass（Fig. 3）. The tuffaceous Lithology: This member is predominantly composed of siltstone is sandwiched between tuffs. medium- to coarse-grained sandstone intercalating mud- The fine- to coarse-grained massive sandstone beds stone layers with the intervals of 10 to 15 m. The granule- with the erosional base in the upper part are 3 m in thick- to pebble-graveled conglomerate beds are sometimes ness at maximum and sometimes intercalate thin beds of intercalated in the sandstone beds. Although the Nakao gravel-supported conglomerate of angular to subrounded Siltstone Member changes gradually into the Nanami pebbles. Sandstone Member with increasing the frequency and the Plant fossils occur in almost all the lithofacies of all the thickness of the fine- to coarse-grained sandstone beds, horizons, except for conglomerate（Table 2, Figs. 5, 6）. the lower boundary of the Nanami Sandstone Member is Sedimentary environments: The facies characterized recognizable as the predominance of medium- to coarse- by fine-grained sandstone with asymmetric ripple laminae grained sandstone beds in contrast to the predominance and mudstone containing abundant upright root fossils of more fine-grained deposits in the Nakao Siltstone Mem- suggests the fluvial environments. ber（Locs. 73 to 77 in Ono and 106 to 110 in Nakao: Figs. b．Nanami Sandstone Member［redefinition］ 3, 5）. The Nanami Sandstone Member is conformably Stratotype: Succession along the Nakao Forestry Road overlain by the Yoshimo Formation. （Locs. 107 to 119; Figs. 3, 5）. The medium－to coarse-grained sandstone is arkosic Original author: Kobayashi（1926）, who designated the and whitish gray to white in color on the fresh surface, member as a“group”. but becomes reddish yellow to red when weathered. History of classification: The corresponding beds were Trough cross-laminae, ca. 6 cm in height and 20 cm in 398 T. Yamada and T. Ohno 2005―7

Fig. 8. Boundary（unconformity: indicated by medium dashed line）between the Utano Formation and the Nakao Siltstone Member of the Kiyosue Formation at loc. 99 in Nakao. The boundary is slightly shifted on account of the small fault（along bold dashed line）. The bedding planes a are indicated by narrow dashed line.

wavelength, are often developed in this sandstone. The and orthoquartzite. In some cases, pebbles are imbricated, cross- laminae suggest the paleocurrent direction from SE indicating the paleocurrent direction roughly from SE to to NW. Intercalated mudstone beds are less than 1 m in an NW. The medium- to very coarse-grained sandstone beds average thickness with the eroded upper surface. Plant are 30 cm to 1 m thick, white in color, quartzose in lithol- fragments and root fossils in upright position are common ogy, and intercalating very thin siltstone beds. Ripple lam- in this mudstone（Table 2）. The intercalated conglomer- inae（sometimes mud-draped）, 2 cm in height and 10 cm ate beds are composed of rounded pebbles of chert and in wavelength, are often observable in these sandstone orthoquartzite, and are about 2m thick. These beds are beds. well observable in the section along the runnel between Brackish bivalves, such as Tetoria yokoyamai the Locs. 113 and 114 in Nakao（Figs. 3, 5）. （Kobayashi and Suzuki）and Eomiodon nipponicus Ohta Sedimentary environments: The unidirectional trough occur in the medium-grained sandstone at the Loc. 401 in cross-laminae in sandstone and the upright root fossils in the north of Nakao（Fig. 3）. mudstone indicate the fluvial environments. The boundary between the Yoshimo Formation and （2）Yoshimo Formation［Oishi, 1933］ overlying Kwanmon Group is not observed in the studied Type locality: Yoshimo beach, Yoshimo, Shimonoseki area. However, they are readily distinguishable by tuffa- City. ceous lithology of the latter group. The Kwanmon Group Lithology in Ouchi-Kikugawa area: The formation con- unconformably overlies the Toyonishi Group according to sists mainly of pebble-graveled conglomerate and medi- Hase（1960）. um- to very coarse-grained sandstone. Discussion The base of the formation is demarcated by the abrupt increase of grain size, compared to the Kiyosue Forma- 1．Stratigraphic interpretation tion, as seen at Locs. 177 in the north of the Takaji Pass As shown in Fig. 2, Toriyama（1938）designated the and 408 in the north of Nakao（Figs. 3, 5）. The thickness base of the Nishinakayama Formation in this area at the is 190 to 450 m（Figs. 5, 6）. bottom of the conglomerate bed around the Tabe Pass The conglomerate beds mentioned above are 30 cm to 1 （e.g., Loc. 230 in Ipponmatsu, Fig. 3）. This conglomerate m thick and contain rounded pebbles of chert, metapelite is interpreted therefore as the basal conglomerate of the Jour. Geol. Soc. Japan 111（ 7 ） The Mesozoic in the Ouchi-Kikugawa area 399

Table 2. Plant fossils from the Kiyosue Formation. The asterisk indicates that the detailed stratigraphic level of the locality is uncertain.

Toyora Group. Later this was interpreted as the basal con- （black, compact and massive silty shale）and Ut（black glomerate of the Nishinakayama Formation by Takahasi sandy shale and alternating beds of sandstone and shale）. et al.（1965）. These authors included the Ipponmatsu Among them, Up is comparable to the basal part and Ub is Formation of Toriyama（1938）in the Higashinagano For- comparable to the main part of Ouchi-Kikugawa area, mation（Fig. 2）. However, the conglomerate should be while the lithofacies comparable to Uh and Ut is not dis- channel fill deposit, judged from its localized distribution cerned. These lithofacies assigned to the Utano Forma- and intraformational nature. tion in this study were overlooked by some previous We could distinguish two different lithofacies in the researchers; the horizons were assigned to the upper part Utano Formation of the studied Ouchi-Kikugawa area: the （Kobayashi, 1926, 1931; Takahasi et al., 1965）or to the basal part（weakly bioturbated sandy mudstone with rare middle part（Toriyama, 1938）of the“Nishinakayama For- thin sandstone interbeds,as seen at Locs. 271 to 272 in mation”. However, the occurrence of Inoceramus uta- Kamitabe）and the main part（bioturbated mudstone with noensis from Locs. 272, 279, 360（Figs. 3, 5-7）indicates sandstone interbeds, e.g., Locs. 272 to 277 in Kamitabe） that the beds bearing this fossil should be correlated to （Figs. 3, 5）. In the type area of the Toyora Group, the the Utano Formation, because this species is known only Utano Formation is divided into the following four mem- from the type Utano Formation（Hirano, 1973b）. bers（Hirano, 1971, 1973a, b; Fig. 2）: in ascending order, The Kiyosue Formation overlies unconformably the Up（black massive sandy mudstone rarely intercalated by Utano Formation. We are of opinion that the Kiyosue For- thin sandstone layers）, Ub（black massive or laminated mation could be divided into two members: the Nakano mudstone frequently intercalated by sandstone beds）, Uh Siltstone Member and the Nanami Sandstone Member. 400 T. Yamada and T. Ohno 2005―7

The stratigraphic level of the Nakao Siltstone Member was Table 3. Comparison of the locality numbers of the present study thought to correspond to various horizons of the Toyora to those of previous studies. The stratigraphic assignment of the localities is indicated by“K”（Nakao Siltstone Member）and“N” Group, being classified as the Utano Formation by some “ ” （Nanami Sandstone Member）in parentheses. previous authors（Kobayashi, 1926, 1931; Matsumoto, 1949; Hase, 1960; Takahasi et al., 1965; refer to Fig. 2）or “Nishinakayama Formation”by others（Kobayashi, 1926, 1931;Toriyama, 1938）. The Utano Formation in the type area of the Toyora Group is lithologically different from “the Utano Formation”in the Ouchi-Kikugawa area, and the difference was interpreted as gradual facies change from marine to terrestrial（Kobayashi, 1926, 1931; Toriya- ma, 1938; Matsumoto, 1949; Hase, 1960; Takahasi et al., 1965）. This interpretation can not be retained any longer for the following reasons:（1）the lithofacies which has been assigned to the type Utano Formation is in fact seen only in the horizons lower than that of the“Utano Forma- tion”（sensu Kobayashi, 1926, 1931; Toriyama, 1938; Taka- hasi et al., 1965）in the studied Ouchi-Kikugawa area, and （2）considerable erosion occurred after the Utano For- mation and the sedimentary environments were abruptly changed from offshore to terrestrial（as discussed below） between the Utano Formation and the Nakao Siltstone Member. This change suggests an unconformable relation between the Utano Formation and the Nakao Siltstone Member. Thus the redefined Nanami Sandstone Member Ohta, 1965; Tamura, 1990）. corresponds to the“Kiyosue Formation”of some previ- 2．Sedimentary environment of the Toyora and ous authors（Matsumoto, 1949; Hase, 1960）（see Fig. 2）. Toyonishi Groups In their papers, the boundary between their“Utano”and In a large scale, the water depth of the basin increased “Kiyosue”Formations was not precisely indicated. We gradually during the depositional time of the Toyora could not ascertain the distinct unconformity between the Group. The Higashinagano Formation, the lowest forma- so-called“Utano Formation”and the so-called“Kiyosue tion of the group, is of shallow marine facies as indicated Formation”（ sensu Matsumoto, 1949; Hase, 1960） by the hummocky cross-stratification（HCS）. In more because the relation between the two seems gradual. In detail, the HCS is discrete in the southern part of the conclusion, our stratigraphic classification is quite similar study area, while the HCS are amalgamated into several to that of Oishi（1933）. tens of meters in thickness in the northern part. This indi- The exact geologic age of the Kiyosue Formation is not cates that the basin became shallower towards north, certain. It is a remarkable fact that the Yoshimo Forma- judged from the fact that the amalgamation of HCS is tion overlying the Kiyosue Formation is intercalated by greater in extent towards the coast（Cheel and Lekie, the Berriasian reef limestone in the Murotsu area, to the 1993）. It is also supported by the fact that the shallow- west of this study area（Yoshidomi and Inoue, 2001; Shi- marine molluscan fossils reported from the Higashinagano raishi and Yoshidomi, 2005）. Recently Crybelosprites- Formation of the type area（Hayami, 1959, 1960）are not type spore was discovered from the Loc. 102（Yamada and discovered from the formation in the studied Ouchi-Kiku- Kato, 2002; Figs. 3, 5, 6）, indicating the latest Jurassic to gawa area. However, most of previous researchers under- earliest Cretaceous Period and thereafter（Dettmann, stood that the depth of the basin had become shallower 1963; Lupia et al., 2000）. These suggest that the Kiyosue towards south（Kobayashi, 1931; Toriyama, 1938; Mat- Formation is correlative with the uppermost Jurassic to sumoto, 1949; Hase, 1960; Takahasi et al., 1965）. Berriasian. This comparison is concordant with previous The exact water depth at the time of the Nishinakayama provisional correlation that the Toyonishi Group is con- Formation is not known, but the well-laminated mudstone sidered to be the uppermost Jurassic to the lower Neoco- （so-called black shale）suggests an anaerobic bottom con- mian and is comparable to the lower part of the Itoshiro dition. In taking the scarcity of the supply of coarse sedi- Subgroup of the Tetori Group（Oishi, 1933; Hase, 1960; ment into account, this formation was presumably Jour. Geol. Soc. Japan 111（ 7 ） The Mesozoic in the Ouchi-Kikugawa area 401

deposited under the hemipelagic and extensive anoxic Museum（transferred from the University of Tokyo）, and condition. the exact sites of some specimens became clarified. Concerning the depositional environments of the Utano According to the labels attached to the specimens, Formation, the overall development of turbidite deposits, “Rokumambo”is located at the shore of the pond in Kam- which are characterized by mudstone with sandstone inanami, near the Locs. 302－307（Fig. 3）. This locality is interbeds of graded bedding, suggests the slope environ- therefore assigned to the Nanami Sandstone Member. ments. Most of Oishi's specimens collected from“Takazi”are The Toyonishi Group exhibits the upward coarsening accompanied with the labels indicating the localities as sequence from the Nakao Siltstone Member（lower）to the “southern foot of the Takaji Pass”, where only Toyonishi Yoshimo Formation（upper）, via the Nanami Sandstone Group is exposed. His specimens are contained in silt- Member（middle）. As the Kiyosue Formation is fluvial and stone with lenticular beds of sandstone or in very fine- the Yoshimo Formation yields brackish water mollusks, grained sandstone, which are characteristic lithology of this sequence could be related profoundly with the pro- the Nakao Siltstone Member. His specimens from“Outi” gression of deltaic environments. show similar lithology to the Nakao Siltstone Member. 3．Stratigraphic levels of plant-bearing horizons It seems probable that the“Utano flora”is synonymous Many paleobotanical studies were conducted in the with the“Kiyosue flora”and we are of opinion that as the Ouchi- Kikugawa area（Oishi, 1933, 1940; Takahasi, 1959; Kiyosue flora including the“Utano”and“Kiyosue”florae Takahasi et al., 1965; Kimura et al., 1986; Kimura and should be redefined after further studying the exact rela- Ohana, 1987a, b）. Oishi（1933）assigned the plant-bear- tion between the two based on their floristic composi- ing horizons to the Kiyosue Plant Beds of the Kiyosue tions. Nevertheless, it is obvious that both florae flour- Group. Later, Oishi's（1933, 1940）Kiyosue Plant Beds ished in the period between Late Jurassic to earliest Cre- was divided into the“Utano Formation”of the Toyora taceous（Berriasian）and that the“Utano flora”is much Group and the“Kiyosue Formation”of the Toyonishi younger in age than previously thought（as Toarcian to Group（Takahasi, 1959; Matsumoto, 1949; Hase, 1960）. Aalenian）by Kimura et al.（1986）and Kimura and Ohana Further, Takahasi（1959）called the fossil flora of the （1987a）. “Utano Formation”as Toyora flora and that of the Acknowledgement “Kiyosue Formation”as Toyonishi flora, while he pointed out that there are not distinct differences between the We thank Mr. Hideki Nakano, Toyota Branch of Agri- two florae in their floristic compositions. According to culture and Forestry Office of Yamaguchi Prefecture for Takahasi（1959）, Oishi's“Rokumambo”（Kaminanami） the permission of conducting our field works in the con- assemblage belongs to the Toyonishi flora,“Outi”（Ouchi） struction sites of farm roads. We also thank the Taniseki's assemblage to the Toyora flora, and“Takazi”（Takaji Pass） of Jinjoji Temple in Toyota, for the kind assistance pro- assemblage includes both. Later, the fossil plant assem- vided us during our fieldwork. Dr. Tadashi Sato, chairman blage of the“Kiyosue Formation”（sensu Matsumoto, of Fukada Geological Institute, Professor Takeshi 1949; Hase, 1960）, which includes Takahasi's（1959） Setoguchi, Professor Fujio Masuda, Dr. Haruyoshi Maeda Toyonishi flora in part, was referred as the“Kiyosue and Dr. Toru Tamura of the Department of Geology and Flora”（Hase, 1960）and that of the“Utano Formation” Mineralogy, Kyoto University, and Dr. Kazuhiko Uemura （sensu Takahasi et al., 1965）as“Utano Flora”（Kimura et of the National Science Museum for their helpful advice al., 1986; Kimura and Ohana, 1987a, b）. on our geological survey and preparation of this paper. All the localities of mega-plant fossils of the“Utano For- We are grateful to Professor Masahiro Kato of the Depart- mation”, shown in Table 3, by previous authors（Takahasi ment of Biological Sciences, University of Tokyo, for his et al., 1965; Kimura et al., 1986; Kimura and Ohana, financial support. The paleobotanical specimens cited in 1987a, b）are attributed to the Kiyosue Formation in this this paper are deposited in the Kyoto University Museum paper（Figs. 3, 5）. Among these localities, those of Ono 1 and the molluscan specimens are in the National Science to 3, Toriyama 2 to 6, 061, 063 and 065 are included in the Museum. Nakao Siltstone Member and the remaining localities in References the Nanami Sandstone Member（Figs. 3, 5; Table 3）. Cheel, R.J. and Lekie, D.A., 1993, Hummocky cross-stratification. In Up to now, the exact localities of Oishi's 1940 speci- （） Wright, V.P., ed., Sedimentology Review 1, Blackwell Sci. Publ., mens are not known. He only cited obscure place names Oxford, 103-122. such as Outi, Rokumambo or Takazi. We have re-exam- Dettmann, M.E., 1963, Upper Mesozoic microfloras from south-eastern Australia. Proc. R. Soc. Vic., 77, 1-148. ined some of his specimens stored in the National Science Hase, A., 1960, The Late Mesozoic formations and their molluscan fossils 402 T. Yamada and T. Ohno 2005―7

in west Chugoku and north Kyushu, Japan. Jour. Sci. Hiroshima Matsumoto, T. and Ono, A., 1947, A biostratigraphic study of the Jurassic Univ. Ser. C, 3, 281-342. Toyora Group, with special reference to ammonites. Sci. Rep. Fac. Hayami, I., 1959, Lower Liassic lamellibranch fauna of the Higashinagano Sci., Kyushu Univ., Geol., 2, 20-31. Formation in West Japan. Jour. Fac. Sci., Univ. Tokyo, sec. 2, 12, Ohta, Y., 1965, On the Corbiculidae from the lower Neocomian of Japan. 115-130. Geol. Rep. Hiroshima Univ., 14, 165-171. Hayami, I., 1960, Lower Liassic gastropods from the Higashinagano For- Oishi, S., 1933, On the Tetori Series, with special references to its fossil mation in West Japan. Japan. Jour. Geol. Geogr., 31, 99-106. zones. Part 2. Jour. Geol. Soc. Tokyo, 40, 669-699. Hirano, H., 1971, Biostratigraphic study of the Jurassic Toyora Group. Pt. Oishi, S., 1940, The Mesozoic floras of Japan. Jour. Fac. Sci. Hokkaido 1. Mem. Fac. Sci., Kyushu Univ., Ser. D, 21, 93-128. Imp. Univ., Sec. 4, 5, 123-480. ＊＊ Hirano, H., 1973a, Biostratigraphic study of the Jurassic Toyora Group. Pt. Shiraishi, F. and Yoshidomi K., 2005, Depositional environment of the 2. Trans. Proc. Palaeont. Soc. Japan, N. S., no. 89, 1-14. upper Jurassic-lower Cretaceous reef limestone in western Yam- Hirano, H., 1973b, Biostratigraphic study of the Jurassic Toyora Group. Pt. aguchi Prefecture, Southwest Japan. Jour. Geol. Soc. Japan, 111, 3. Trans. Proc. Palaeont. Soc. Japan, N. S., no. 90, 45-71. 21-28. ＊＊ Iizumi, S., Sawada, Y., Sakiyama, T. and Imaoka, T., 1985, Cretaceous to Takahasi, E., 1959, Floral changes since the Mesozoic age of western Hon- Paleogene Magmatism in the Chugoku and Shikoku Districts, Japan. shu, Japan. Sci. Rep. Yamaguchi Univ., 10, 181-237. ＊＊ Earth Sci., 39, 372-384. ＊＊ Takahasi, E., Mikami, T., Kitaoka K., Yoshida, T. and Chikura, H., 1966, Kimura, T., 1987, Recent knowledge of Jurassic and Early Cretaceous flo- Palaeozoic and Mesozoic formations of the eastern and the northern ras in Japan and phytogeography of this time in East Asia. Bull. parts of the Tabe Basin, Yamaguchi Prefecture. Sci. Rep. Yam- Tokyo Gakugei Univ., Sect. 4, 39, 87-115. aguchi Univ., 16, 53-65. ＊＊ Kimura, T., 1988, Jurassic macrofloras in Japan and palaeophytogeogra- Takahasi, E., Uto,S., Mihara, A. and Takahasi, H., 1965, Palaeozoic and phy in East Asia. Bull. Tokyo Gakugei Univ., Sect. 4, 40, 147-164. Mesozoic formations of the eastern and the southern parts of the Kimura, T. and Ohana, T., 1987a, Middle Jurassic and some Late Liassic Tabe Basin, Yamaguchi Prefecture. Sci. Rep. Yamaguchi Univ., 15, plants from the Toyora Group, southwest Japan（I）. Bull. Natn. Sci. 33-49. ＊＊ Mus. Ser. C, 13, 41-76. Tamura, M., 1990, Stratigraphic and palaeontologic studies on non-marine Kimura, T. and Ohana, T., 1987b, Middle Jurassic and some Late Liassic Cretaceous bivalve faunas in Southwest Japan. Mem. Fac. Educ. plants from the Toyora Group, southwest Japan（II）. Bull. Natn. Sci. Kumamoto Univ., no. 39（Suppl.）, 1-47. ＊＊ Mus. Ser. C, 13, 115-148. Toriyama, R., 1938, Toyora Series in Toyora-gori, Yamaguchi Prefecture. Kimura, T., Ohana, T., Kurihara, Y. and Komori, K., 1986, Middle Jurassic Jour. Geol. Soc. Japan, 45, 247-258. ＊＊ Utano flora and its significance for biostratigraphy and palaeophyto- Yabe, H., 1920, On the so-called Mesozoic formations in the Kiku Penin- geography in East Asia（abstract）. Proc. Japan Acad., 62B, 341- sula, Buzen. Jour. Gr., 32, 513-520. ＊ 344. Yamada, T. and Kato, M., 2002, Regnellites nagashimae gen. et sp. nov., Kobayashi, T., 1926, Notes on the Mesozoic formations in Prov. Nagato, the oldest macrofossil of Marsileaceae, from the Upper Jurassic to Chugoku, Japan. Jour. Geol. Soc. Tokyo, 33, 1-9. Lower Cretaceous of western Japan. Intern. Jour. Plant Sci., 163, Kobayashi, T., 1931, On the Mesozoic crustal movements in Southwest 715-723. Japan. Jour. Geol. Soc. Tokyo, 38, 563-583. ＊ Yokoyama, M., 1904, Jurassic ammonites from Echizen and Nagato. Jour. Kobayashi, T., 1936, The geological structure of Southwest Japan and its Coll. Sci. Imp. Univ. Tokyo, 19, art. 20, 1-17. Mesozoic palaeogeography, pt. 4. Jour. Geol. Soc. Tokyo, 43, 531- Yoshidomi, K. and Inoue, Y., 2001, Late Mesozoic coral-bearing limestone 541. ＊ in the Murotsu area, Toyoura Town, western Yamaguchi Prefecture, Lupia, R., Schneider, G.M., Moeser, G.M., Pryer, K.M. and Crane, P.R., Southwest Japan. Jour. Geol. Soc. Japan, 107, 794-797. ＊＊ 2000, Marsileaceae sporocarps and spores from the Late Cretaceous of Georgia, U.S.A. Intern. Jour. Plant Sci., 161, 975-988. ＊ in Japanese Matsumoto, T., 1949, The late Mesozoic geological history in the Nagato ＊＊ in Japanese with English abstract Province, southwest Japan. Japan. Jour. Geol. Geogr., 21, 235-243.

toponym Imou 伊毛 Nakao 中尾 Shimoohno 下大野 Ipponmatsu 一本松 Nanami 七見 Tabe 田部 Kaminanami 上七見 Oka 岡 Takaji（Takazi）高地 Kamiozuki 上小月 Ono 小野 Toriyama 通山 Kamitabe 上田部 Ouchi（Outi）阿内 Rokumambo 六萬坊 Kanda River 神田川 Shimonanami 下七見 Yamane 山根 Kikugawa 菊川 Jour. Geol. Soc. Japan 111（ 7 ） The Mesozoic in the Ouchi-Kikugawa area 403

（要旨）

Yamada, T. and Ohno, T., 2005, Revision of the stratigraphy of the Toyora and Toyonishi Groups in the Ouchi-Kikugawa area, Yamaguchi Prefecture, west Japan. Jour. Geol. Soc. Japan, 111, 389－403.（山田敏弘・大野照文，2005，山口県阿内・菊川地域の豊浦・豊西層群層序の再検討．地質雑，111, 389－403）山口県下関市東部および菊川町（阿内・菊川地域）に分布する豊浦層群（下部～中部ジュラ系）および豊西層群（上部ジュラ～下部白亜系）の再検討を行った．本地域の豊浦層群は下位より，東長野層，西中山層，歌野層から，豊西層群は清末層，吉母層からなり，新たに清末層を中尾シルト岩部層および七見砂岩部層に区分した．これまで非海成層であるとされた本地域の“歌野層”は，清末層の中尾シルト岩部層であり，模式的な海成の歌野層は従来の“西中山層”の上部にあたることが確認された．この新しい層序解釈に基づくと，“清末植物群”とともに本地域の清末層から報告され，先バトン期とされた“歌野植物群”の時代は，バトン期以降と考えられ，今後両植物群の関係を明らかにしていく必要がある．