ARTICLE

Memoir of the Fukui Prefectural Dinosaur Museum 11: 1―18（2012）Ⓒ by the Fukui Prefectural Dinosaur Museum

NEW DETRITAL ZIRCON AGE DATA FROM THE TETORI GROUP IN THE MANAAND ITOSHIRO AREAS OF FUKUI PREFECTURE, CENTRAL JAPAN
1

2

3

1

Yuta KAWAGOE , Shin-ichi SANO , Yuji ORIHASHI , Hokuto OBARA ,1
1
Yoshikazu KOUCHI and Shigeru OTOH
1

Graduate School of Science and Engineering, University of Toyama,3190 Gofuku, Toyama 930-8555, Japan
2
Fukui Prefectural Dinosaur Museum, 51-11 Terao, Muroko, Katsuyama, Fukui 911-8601, Japan
3
Earthquake Research Institute, University of Tokyo,
1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan

ABSTRACTThe age of non-marine formations of the Tetori Group was reexamined in the Mana and Itoshiro areas in Fukui
Prefecture, central Japan. The Tetori Group in the Itoshiro area has been divided into the Kuzuryu, Itoshiro, andAkaiwa subgroups, whereas that in the Mana area is divided into the Middle and Upper formations of the “Kuzuryu
Subgroup”. Among them, the age of the following marine formations has been determined from ammonoid fossils:
the Kaizara (latest Bathonian–Callovian) and Yambarazaka (Oxfordian) formations of the Kuzuryu Subgroupin the Itoshiro area and the uppermost part of the Itoshiro Subgroup (Late Hauterivian–Early Barremian) in the
Uchinamigawa area, 5 km to northwest of the Itoshiro area. Moreover, the Middle Formation of the “Kuzuryu
Subgroup” in the Mana area likely yields Tithonian ammonoids. We obtained LA-ICPMS U-Pb ages of detrital
zircons in (1) sandstone of the Itsuki Formation in the uppermost part of the Itoshiro Subgroup in the Itoshiro areaand (2) lapilli tuff of the Upper Formation of the “Kuzuryu Subgroup” in the Mana area. As a result, the youngest
zircon from the sandstone of the Itsuki Formation has the concordant age of 127.2 ± 2.5 Ma (2σ; Barremian),
whereas the lapilli tuff of the Upper Formation of the “Kuzuryu Subgroup” in the Mana area has the intercept ageof 124.6 ± 2.3 Ma (2σ; late Barremian–early Aptian). Hence the Itsuki Formation of the Itoshiro Subgroup in the
Itoshiro area is presumably correlated with the ammonoid-bearing bed in the Uchinamigawa area and is correlatedwith the Lower Barremian. On the other hand, the Middle and Upper formations of the “Kuzuryu Subgroup” in the
Mana area are likely correlated with the Itoshiro and Akaiwa subgroups in the Itoshiro area.
Key words: U-Pb age, detrital zircon, LA-ICPMS, Tetori Group, Fukui Prefecture
川越雄太・佐野晋一・折橋裕二・小原北士・高地吉一・大藤 茂（2012）福井県大野市真名地域および石徹白地域の手取層群から得られた新たな砕屑性ジルコン年代．福井県立恐竜博物館紀要 11： 1−18．
福井県大野市の石徹白及び真名地域に分布する手取層群非海成層の年代を再検討した．石徹白地域の手取層群は下位より九頭竜，石徹白，および赤岩亜層群に，真名地域の手取層群は下位より“九頭竜亜層
群”中部層および上部層にそれぞれ区分される．これらの内，以下の海成層は産出するアンモナイトにより堆積時代が推定されている：石徹白地域の九頭竜亜層群貝皿層（Bathonian最後期～Callovian）及び山原坂層（Oxfordian）と，石徹白地域の北西５キロ，打波川地域の石徹白亜層群最上部（Hauterivian後期〜Barremian前期）．さらに，真名地域の“九頭竜亜層群”中部層はTithonianに対比される可能性がある．
筆者らは，（1）石徹白地域の石徹白亜層群最上部を占める伊月層の砂岩と（2）真名地域の“九頭竜亜層群”上部層の火山礫凝灰岩に含まれる砕屑性ジルコンのウラン－鉛年代を LA-ICPMS で測定した．その結果，伊月層砂岩中の最も若いジルコンのコンコーディア年代は 127.2 ± 2.5 Ma（2σ；Barremian），真名地域の火山礫凝灰岩のインターセプト年代は 124.6 ± 2.3 Ma（2σ；Barremian後期〜Aptian前期）となった．従って，石徹白地域の石徹白亜層群伊月層は恐らく打波川地域のアンモナイト産出層とともに，下部Barremianに対比される可能性が大きい．一方，真名地域の“九頭竜亜層群”中部層から上部層は，石徹
白地域の石徹白～赤岩亜層群に対比可能である．
Received June 26, 2012. Accepted November 13, 2012．
Corresponding author—Shigeru OTOH
E-mail: shige*sci.u-toyama.ac.jp（*を半角＠に変えてご入力ください）
1

2

YUTA KAWAGOE, SHIN-ICHI SANO, YUJI ORIHASHI, HOKUTO OBARA, YOSHIKAZU KOUCHI AND SHIGERU OTOH

FIGURE 1. Index map showing the distribution of the Tetori Group andthe locations of regions and areas mentioned in the text.

exact in situ U-Pb age determination of zircons using sensitive
high-resolution ion-microprobe (SHRIMP) or inductively
coupled plasma-mass spectrometry with laser ablation sampling(LA-ICPMS) (e.g., Compston, 1996; Williams, 1998; Kosler
and Sylvester, 2003; Jackson et al., 2004). It has thereby become
easy to measure the age distribution of hundreds of detrital
zircons in a single clastic-rock sample and to reconstruct theage structure of the hinterland (e.g., Rino et al., 2004; Iizuka etal., 2005; Rino et al., 2008). Moreover, the analytical technique
has helped many geologists to constrain the upper age limit ofsedimentation of a clastic-rock bed, including a pyroclastic-rock
bed, using the youngest age of detrital zircons (e.g., Tsutsumi etal., 2000; Kusuhashi et al., 2006; Aoki et al., 2007; Shimojo etal., 2010; Otoh et al., 2010; Lee et al., 2010).
Following the studies mentioned above, this paper aims
to constrain the sedimentary age of some non-marine clastic
rocks of the Tetori Group in the Mana and Itoshiro areas in
the southern part of Ono City, Fukui Prefecture, through LAICPMS, U-Pb dating of detrital zircons. Moreover, the paper
presents an idea of correlation of the Tetori Group in the Mana,
Itoshiro, and Kamihambara areas, combining the new age dataand previous studies of biostratigraphy.

INTRODUCTIONOUTLINE OF GEOLOGYThe Tetori Group exposes in the Hida and Hida Gaien belts
of Southwest Japan and is composed mostly of Middle Jurassic
to Early Cretaceous shallow marine to terrestrial clastic rocks.
The exposure of the Tetori Group is subdivided into the Hakusan
and Jinzu regions, northern central Japan; the former includes
the upper reaches of the Kuzuryugawa, Tedorigawa, Shogawa,Miyagawa, and Takaharagawa rivers (Maeda, 1961b; Fig. 1).
This study focuses on the detrital zircon chronology of the Tetori
Group in the Mana and Itoshiro areas along the upper reaches of
the Kuzuryugawa River in the southwestern part of the Hakusan
Region (Figs. 1 and 2).
The Tetori Group consists of three subgroups: the Kuzuryu,
Itoshiro, and Akaiwa subgroups, in ascending order (Maeda,
1957, 1961a, b). Among them, the timing of sedimentation ofmarine beds in the Kuzuryu and Itoshiro subgroups has been
well constrained using ammonoids and other index fossils (e.g.,
Sato and Westermann, 1991; Sato and Yamada, 2005; Goto,
2007; Sato, 2008). The age of non-marine formations, however,is mostly unknown because of the following reasons: (1) the
non-marine formations yield virtually no index fossils; (2) lateralcontinuity of lithofacies of the Tetori Group is not very well; and(3) key beds that help to correlate the strata in different areas
are very few. Accordingly, although various ideas of correlation
among formations in different areas were presented (e.g.,
Maeda, 1961b; Yamada et al., 1989; Fujita, 2003; Matsukawa etal., 2003; Matsukawa et al., 2006; Sano et al., 2008; Matsukawa
and Asahara, 2010), they have not been widely accepted.
Recent progress in analytical technique has enabled rapid and

The outline of geology of the Tetori Group in the Mana andItoshiro areas, and of the Kamihambara area on the east of theformer two areas (Fig. 2) is described below.
Mana area
The geology of the Tetori Group in the Mana area was studied
by Maeda (1952, 1957, 1961a), Kawai et al. (1957), and Yamada
et al. (1989). Here we mainly follow the descriptions of Yamada
et al. (1989), who clarified the geologic structure of the area
using sedimentological way-up criteria.
The Tetori Group in the Mana area is divided into the“Kuzuryu” and “Itoshiro” subgroups. The “Kuzuryu Subgroup”
in this area consists of the Middle and Upper formations; theLowest and Lower formations, which occur in other areas(Fig. 2), are lacking. The Middle and Upper formations arecorrelated with the Kaizara and Yambarazaka formations in the
Itoshiro area, respectively (Yamada et al., 1989). The “Kuzuryu
Subgroup” in the Mana area generally strikes east and dips
moderately to north, forming a northward-facing sequence,
although the subgroup strikes north and dips and faces to easton the west of the Managawa River. The “Itoshiro Subgroup”
of the Mana area, on the other hand, is in fault contact with theapparently overlying “Kuzuryu Subgroup” (Fig. 2) and does notform a continuous succession with the “Kuzuryu Subgroup”.
Thus the “Itoshiro Subgroup” of Yamada et al. (1989) in the
Mana area is excluded from the object of this study. As will be
discussed later, the “Kuzuryu Subgroup” of Yamada et al. (1989)

FIGURE 2. Geologic map of the Mana, Itoshiro, and Kamihambara areas in Ono City, Fukui Prefecture, central Japan, compiled from the geologic maps of Yamada et al. (1989) and Fujita (2002). a,
Post-Tetori volcanic rocks; b, Post-Tetori plutonic rocks; c, Nochino Formation (Akaiwa Subgroup); d–e, Itoshiro Subgroup (d, Itsuki Formation; e, Obuchi, Kamihambara, Ashidani, and Yambara
formations); f–h, Kuzuryu Subgroup in the Mana area (f, Upper Formation; g, pyroclastic rocks; h, Middle Formation), i–l, Kuzuryu Subgroup in the Itoshiro and Kamihambara areas (i, Yambarazaka
Formation; j, Kaizara Formation; k, Tochimochiyama Formation; l, Lowest Formation); m, Constituent rocks of the Hida Belt; n, Constituent rocks of the Hida Gaien Belt; o, Fault. Italicized numbers
denote dated samples.

NEW AGE DATA FROM THE TETORI GROUP
3

4

YUTA KAWAGOE, SHIN-ICHI SANO, YUJI ORIHASHI, HOKUTO OBARA, YOSHIKAZU KOUCHI AND SHIGERU OTOH

FIGURE 3. An exposure of the lapilli tuff of the Upper Formation of the “Kuzuryu Subgroup” in the Mana area (sample 11110805). a, Whole outcrop view;b, A close-up view showing the texture of the lapilli tuff.

in the Mana area is mostly correlated with the Itoshiro Subgroupin the Itoshiro area; the distribution of the coeval strata with theKuzuryu Subgroup in the Itoshiro area is, if any, very narrow in
the Mana area.
The Middle Formation of the “Kuzuryu Subgroup” in the
Mana area, about 600 m thick, overlies the Carboniferous
Fujikuradani Formation of the Hida Gaien Belt and is composed
mainly of black shale covering basal calcareous coarse sandstone
with the thickness of 3 m or less. The black shale is partly
interbedded with fine to medium sandstone and intercalates acouple of thin layers of coarse sandstone and conglomerate. Theinterbedded sandstone and shale are dominant in the middle partof the Formation. The conglomerate is well-round and contains
pebbles of granite, black shale, and sandy limestone (Yamada etal.,1989). Yamada et al. (1989) correlated the Middle Formationof the “Kuzuryu Subgroup” in the Mana area with the uppermost
Bathonian–Callovian Kaizara Formation (or Kaizara Shale),because the two formations are both ammonoid-bearing, shalerich formations, although the ammonoids reported by Yamada
et al. (1989) have not been identified (Sato, 2008). On theother hand, a Tithonian ammonoid, Parapallasiceras sp. cf. P.
pseudocontiguum , was reported from the Mana area, suggestingthat the ammonoid-bearing beds of the Middle Formation of
the “Kuzuryu Subgroup” can be correlated with the Tithonian(Yamada and Uemura, 2008).
The Upper Formation of the “Kuzuryu Subgroup” consists
mainly of sandstone, with intercalations of conglomerate, shale,
and interbedded sandstone and shale. A layer of pyroclastic rock
(tuff breccia or lapilli tuff), about 10 m thick, is intercalated
in the upper part of the formation (Fig. 3). The sandstone ismassive or bedded feldspathic arenite with feldspar fragments
of more than 50%, whereas the conglomerate bears granule- tocobble-sized clasts of granite and gneisses. The thickness of theUpper Formation of the “Kuzuryu Subgroup” is about 1,500
m, and the thickness between the base of the formation and the
pyroclastic rock layer is 1,000 m or more (Yamada et al., 1989).
Itoshiro and Kamihambara areas
Geology of the Tetori Group in the Kamihambara and Itoshiro
areas was studied by Oishi (1933), Maeda (1952, 1957, 1961b),
Kawai et al. (1957), Yamada et al. (1989), Fujita (2002), andMatsukawa et al. (2003, 2006, 2008). Most of them haveregarded that the Tetori Group along the Itoshirogawa River in
the Itoshiro area as a type section. Here we mainly follow thedescriptions of Fujita (2002).
The Tetori Group in the Kamihambara and Itoshiro areas
forms a northward-dipping and northward-facing succession,
comprising the Kuzuryu, Itoshiro, and Akaiwa subgroups
in ascending order. The Kuzuryu Subgroup consists of the
Tochimochiyama, Kaizara, and Yambarazaka formations,whereas the Itoshiro Subgroup consists of Yambara, Ashidani,
Kamihambara, Obuchi, and Itsuki formations, in ascending order(Fujita, 2002). The Akaiwa Subgroup in the two areas consists
only of the Nochino Formation (Oishi, 1933; Fujita, 2002). The
Itoshiro Subgroup covers the Kuzuryu Subgroup unconformably(Umagatani Unconformity; Maeda, 1952), and the Akaiwa
Subgroup covers the Itoshiro Subgroup conformably (Maeda,
1961b). The distribution width of the Kuzuryu Subgroup
gradually thins out from the Itoshiro area to the Kamihambara

5

NEW AGE DATA FROM THE TETORI GROUP

area (Kawai et al., 1957; Yamada et al., 1989; Fujita, 2002),
whereas the Kamihambara Formation of the Itoshiro Subgroupis absent along the Itoshirogawa River of the Itoshiro area
(Fujita, 2002). The southern boundary of the Tetori Group in the
Kamihambara area is in fault contact with the constituent rocksof the Hida Gaien Belt such as Ise Metamorphic Rocks andthe Shibasudani, Konogidani, Otani, and Motodo formations,whereas that in the Itoshiro area is in fault contact with
Tandoguchi Gneiss of the Hida Belt (e.g., Kawai et al., 1957).
The Tochimochiyama Formation of the Kuzuryu Subgroup,
about 500 m thick, consists mainly of interbedded granular
sandstone and fine sandstone with intercalations of shale.
The Kaizara Formation, about 250 m thick, consists mainlyof mica-rich dark gray shale with intercalations of sandstone.
The following three ammonoid assemblage zones have been
discriminated in the Kaizara Formation (Sato and Westermann,
1991): Pseudoneuqueniceras yokoyamai assemblage zone
(uppermost Bathonian), Kepplerites japonicus assemblage
zone (lowermost Callovian), and Oxycerites assemblage zone
(upper Lower Callovian?). The Yambarazaka Formation is
the uppermost formation of the Kuzuryu Subgroup and iscomposed of interbedded sandstone and siltstone. The thicknessof the Yambarazaka Formation is about 150 m in average,
but significantly changes because of the erosion during theformation of the Umagatani Unconformity. An ammonite
assemblage zone, Kranaosphinctes matsushimai assemblage
zone (Middle Oxfordian) has been discriminated in the
Yambarazaka Formation (Sato and Westermann, 1991).
The Yambara Formation of the Itoshiro Subgroup, 150
m thick, consists of conglomerate, granular sandstone, andcoarse sandstone. The conglomerate contains pebble- toboulder-sized clasts of gneisses, schists (Tsujimori, 1995),
granite, quartz porphyry, limestone, sandstone, and shale, of
which the latter two were likely originated from the Kuzuryu
Subgroup. The largest clast reaches up to 1 m in diameter. The
granular sandstone of the Yambara Formation yields Vaugonia
yambarensis (Kobayashi, 1956). The Ashidani Formation,
200–600 m thick, consists mainly of interbedded sandstoneand shale and intercalates conglomerate and coal seams. The
formation yields plant and brackish molluscan fossils. The
Kamihambara Formation, about 200 m thick, consists chiefly
of shale interbedded with thin sandstone on top of thin basal
conglomerate. The Kamihambara Formation is a Tithonian
formation yielding marine molluscan fossils such as Inoceramus
sp. cf. I. maedae (Fujita et al., 1998) and Parapallasiceras sp.
cf. P. pseudocontiguum (Sato and Yamada, 2005; point A in
Fig. 2). The Obuchi Formation, about 250 m thick, consistsof conglomerate and massive sandstone. The conglomeratecontains pebbles of chert, siliceous mudstone, shale, and granite.
The Itsuki Formation, about 300 m thick, consists of interbedded
sandstone and shale with some coal seams. The formation yields
common brackish water bivalves with the Monobegawa Groupin the Northern Chichibu Belt, Southwest Japan, such as the

Tatsukawa-type fauna (Hauterivian; Kozai et al., 2002, 2005)
and Myopholas sp. cf. M. semicostata (Hauterivian–Barremian;
Fujita, 2002). Moreover, Goto (2007) reported Late Hauterivian–
Early Barremian ammonoid Pseudothurmannia sp. from theuppermost part of the Itoshiro Subgroup in the Uchinamigawa area,
5 km to northwest of the Itoshiro area.
In spite of these interpretations of correlation, Yamada et al.
(1989) had colored the Ashidani, Kamihambara, Obuchi, andItsuki formations in the Kamihambara area of Fujita (2002) as
the Upper Formation of the Itoshiro Subgroup, i.e., the Itsuki
Formation. We cannot deny the interpretation of Yamada etal. (1989), because the Itoshiro Subgroup in the Itoshiro andKamihambara areas has not yet been correlated with marine
index fossils.
The Nochino Formation of the Akaiwa Subgroup consists
mainly of sandstone and siltstone with some intercalationsof conglomerate. The sandstone is feldspathic arenite andpartly contains some pebbles (Maeda, 1961b; Fujita, 2002).
The conglomerate characteristically contains pebbles oforthoquartzite and granite (Fujita, 2002).
SAMPLE DESCRIPTIONS
We studied the following two samples and constrained theirage of deposition: (1) sandstone of the Itsuki Formation in the
Itoshiro area, and (2) lapilli tuff of the Upper Formation of the“Kuzuryu Subgroup” in the Mana area (Fig. 2). Here follows thedescription of studied samples.
Sandstone (11042906) of the Itsuki Formation, Itoshiro area
(N35°55’28.9”, E136°40’56.5”)
Sample 11042906 of the Itsuki Formation was collectedfrom the exposure above the rock shed at the type locality of
the formation along the Itoshirogawa River (N35°55’28.9”,
E136°40’56.5”). The sample was collected from interbedded
sandstone and thin shale about 100 m above the base of the
Itsuki Formation. The sandstone is light brown, well-sorted,and sub-angular feldspathic arenite (Fig. 4a, b) composed ofmonocrystalline quartz (52.3%), polycrystalline quartz (3.5%),
feldspar (37.6%), lithic fragments (3.2%), and matrix (2.8%).
The polycrystalline quartz fragments show wavy extinction andsubgrain boundary, and were likely derived from metamorphic
rock or protomylonite. The lithic fragments are mostly of
volcanic rocks and mudstone.
Lapilli tuff (11110805) of the Upper Formation of the “Kuzuryu
Subgroup”, Mana area (N35°54’28.9”, E136°31’28.5”)
Sample 11110805 of the Upper Formation of the “Kuzuryu
Subgroup” was collected from an outcrop along a logging road to
the west of Lake Manahimeko (N35°54’28.9”,E136°31’28.5”),
about 1,000 m above the base of the formation. The lapilli

6

YUTA KAWAGOE, SHIN-ICHI SANO, YUJI ORIHASHI, HOKUTO OBARA, YOSHIKAZU KOUCHI AND SHIGERU OTOH

FIGURE 4. Photomicrographs of analyzed samples. a–b, Sandstone of the Itsuki Formation (sample 11042906). Scale bars denote 0.5 mm. a,Open nicol; b, Crossed nicols. c–d, Volcanic rock fragments in a lapilli tuff of the Upper Formation of the “Kuzuryu Subgroup” (sample 11110805). Scalebars denote 0.5 mm. e–f, Garnet-bearing metamorphic rock fragment in sample 11110805. Scale bars denote 0.1 mm. e, Open nicol; f, Crossed nicols.
Abbreviations Grt: garnet, Hbl: hornblende, Kfs: K-feldspar, Pl: plagioclase, Qtz: quartz, Rf: rock fragment.

7

NEW AGE DATA FROM THE TETORI GROUP

tuff consists of lapilli (25%), other lithic fragments (5%), andmatrix (70%). The matrix is crystal tuff composed of euhedral tosubhedral plagioclase, hornblende, and augite of 0.3–1.3 mm indiameter.
The lapilli are composed mostly of andesite lava withsome felsic lava and tuff: (1) aphanitic andesite lava with
m i c r o c r y s t a l l i n e p l a g i o c l a s e , ( 2 ) a n d e s i t e l ava w i t hglomeroporphyritic plagioclase (Fig. 4c), (3) porphyritic andesite
lava with phenocrysts (ca. 15%) of plagioclase and hornblende(Fig. 4d), (4) porphyritic andesite lava with phenocrysts(ca.
10%)of plagioclase and augite, (5) aphyric andesite lava or
hypabyssal rock consisting of plagioclase and hornblende, (6)porphyritic dacite or rhyolite lava with phenocrysts of quartz andbiotite, (7) welded pumice, and (8) felsic crystal tuff consistingof fragments of plagioclase, quartz, and opaque minerals.
Besides the lapilli listed above, fragments of sandstone,
mudstone, and metamorphic rock are included. Among them,the metamorphic rock fragments consist of recrystallized quartz
and small amounts of garnet (Fig. 4e, f), and are most likely offelsic gneiss.
ANALYTICAL METHODThe U-Pb dating method makes use of the growth of leadisotopes caused by the decay of uranium isotopes. The method
relies on two parallel decay chains: the uranium series from 238U
to 206Pb and the actinium series from 235U to 207Pb (e.g., Kaneoka,
1998; Hirata and Nesbitt, 1994). The U-Pb dating is commonly
performed on zircon (ZrSiO4), a non-magnetic silicate mineralwith the density of 4.6–4.7. Zircon is one of the best minerals
to determine the timing of crystallization from magma or
metamorphic fluids, because of the following reasons. (1) Zircon
contains certain amount of uranium but virtually no common
lead; (2) it has high closure temperature (ca. 900°C; Cherniak
and Watson, 2001); and (3) it is highly resistant to weathering,alteration, and metamorphism. Hence the youngest age of
detrital zircons in a clastic rock sample is interpreted to show
the youngest age of igneous activity in a provenance before thedeposition of the sample, and in turn constrains the oldest age
limit of the deposition.
The zircon samples for analyses were prepared in thefollowing procedures. Zircon grains were separated from eachsample using standard crushing, sieving (< 250 μm), panningand heavy-liquid (diiodomethane; density up to 3.3 kg/m3)
techniques. We picked up 400 zircon grains from the sampleof the Itsuki Formation (sample 11042906), and 52 zircon
grains from the sample of lapilli tuff in the Mana area (sample
11110805). The grains were mounted in acrylic resin andpolished. The internal textures, inclusions, and microcracks ofthe zircons were observed using transmitted and reflected optical
microscopy and cathodoluminescence (CL) imaging. CL images
were captured on a scanning electron microscope (JEOL JSM5910LV) equipped with a JEOL MP-Z01118T CL detector in

the Fukui Prefectural Dinosaur Museum. Grains showing clear
oscillatory zoning in CL images (Fig. 5), a common feature of
igneous zircons (Corfu et al., 2003), and having few inclusions
or microcracks were chosen for analyses to avoid a possible
effect of common lead on the age dating.
The measurement was carried out on a laser ablationinductively coupled plasma mass spectrometer (LA-ICPMS)
equipped in the Earthquake Research Institute of the Universityof Tokyo. The ICPMS instrument used was a Thermo Elemental
Plasma Quad 3 quadropole-based ICPMS connected with a New
Wave UP-213 LA system, which used the frequency quintupled
Nd-YAG 213-nm wavelength (Orihashi et al., 2008). Orihashi
et al. (2008) measured 7 standard zircons on this system witha NIST SRM610 glass reference material, and found out
the best conditions for precise measurement and determinedthe correction factor, 206Pb/238U = 0.2302; the conditions andcorrection factor have enabled the system to measure the ageof the 7 standard zircons within ±2% of their published ages
obtained with both isotope dilution-thermal ionization mass
spectrometry (ID-TIMS) and SHRIMP. The measurement
conditions were as follows: the ablation pit size of 30 μm, energy
density of 11–13 J/cm2, pulse width of 4 ns nominal and pulse
repetition rate of 10 Hz. All analyses were carried out in a peakjumping mode and the peaks of 202Hg, 204(Hg+Pb), 206Pb, 207Pb,
232
Th, and 238U were monitored. Data were acquired in sequencesof 28 analyses, consisting of 5 analyses of gas blank, 4 SRM610
NIST (National Institute of Standards and Technology, U.S.A.)
glass standard, 1 standard zircon (91500 zircon with the206
Pb/238U age of 1062.4 ± 0.4 Ma; Wiedenbeck et al., 2004), 9
unknown, 4 SRM610 standard, and 5 gas blank.
After the analyses, transmitted and reflected opticalphotomicrographs of zircons were reexamined. If an ablation
spot contains inclusions or microcracks, which may containcommon lead, the analytical data from the spot were rejected.
The obtained data were plotted on concordia diagrams and therelative probability diagram using the Isoplot 3.70 software(Ludwig, 2008).
RESULTS
Sandstone (11042906) of the Itsuki Formation, Itoshiro area
The detrital zircons from sample 11042906 are commonly
abraded and have anhedral to subhedral shapes. They havecolumnar shape with the aspect ratio ranging from 1.1 to 2.9(1.5–2.0 in average). More than half of the zircons are coloredbrown or purple and tend to be Precambrian in age, and theothers are colorless. Most of the zircons show oscillatory zoningin CL images, a common feature of igneous zircons (Corfu etal., 2003; Fig. 5).
From the 400 zircon grains we picked up, we chose 148 grains
with remarkable oscillatory zoning in CL images, sampled anouter part (rim or mantle) of each grain and an inner part (core)

8

YUTA KAWAGOE, SHIN-ICHI SANO, YUJI ORIHASHI, HOKUTO OBARA, YOSHIKAZU KOUCHI AND SHIGERU OTOH

FIGURE 5. Cathodoluminescence images of some zircons analyzed in this study. a, Zircons from sandstone of the Itsuki Formation in the Itoshiro area (sample
11042906); b, Zircons from lapilli tuff of the Upper Formation of the “Kuzuryu Subgroup” in the Mana area (sample 11110805).

of 5 grains with the laser ablation technique, and analyzed thesample with the ICPMS. After the analyses, we rejected theanalytical data of 27 spots from the criteria described in theprevious section.
The analytical data from 126 spots in 121 detrital zircon
grains were thus adopted (Table 1). The Th/U ratio of each spot
was 0.11–2.8 and falls in the range of igneous zircons, Th/U >0.1 (Rubatto and Hermann, 2003). Table 1 and the concordia
diagram drawn with Isoplot 3.70 (Fig. 6a, b) show that theconcordance of 206Pb/238U and 207Pb/235U ages is quite well; 104
spots out of 126 have the % conc value (100·(207Pb/235U age)/(206Pb/238U age)) between 90 and 110. Fig. 6c is the probability
distribution of 206Pb/238U ages of 102 concordant outer spots on
102 zircons from sample 11042906. Fig. 6a and 6c show someage clusters: 3500–3300 Ma (2 grains, 2%), 2500–1500 Ma(86 grains, 84%), 260–210 Ma (6 grains, 6%), 180–165 Ma (5grains, 5%), and around 130 Ma (2 grains, 2%). The concordant
age of the youngest zircon was 127.2 ± 2.5 Ma (Fig. 6b).

From the 52 zircon grains we picked up, we chose 33
grains, sampled an outer part (rim or mantle) of each grain withthe laser ablation technique, and analyzed the sample with the
ICPMS. After the analyses, we rejected the analytical data of 11
spots from the criteria described in the previous section.
The analytical data from 22 spots in 22 detrital zircon grains
were thus adopted (Table 2). The Th/U ratio of each spot was
0.12–1.03 and falls in the range of igneous zircons, Th/U > 0.1(Rubatto and Hermann, 2003). The concordia diagram drawn
from all the analytical data, including some discordant ones,
with Isoplot 3.70 (Fig. 7a, b) shows two distinct age clusters:
2500–1450 Ma (13 grains, 60%) and 130–120 Ma (6 grains,27%). The zircons forming the younger age cluster are allcolorless and euhedral. The intercept age of the younger age
cluster, which shows some influence of common lead, was 124.6± 2.3 Ma (Fig. 7b).

Lapilli tuff (11110805) of the upper part of the “Kuzuryu
Subgroup”, Mana area

Age of deposition of the investigated samples

Most detrital zircons in sample 11110805 exhibit euhedral
shapes with some abraded subhedral crystal faces. They havecolumnar shape with the aspect ratio ranging from 1.0 to
3.3 (about 1.5 in average). More than half of the zircons arecolorless and euhedral, and the others are brown or purple. Mostof the zircons show oscillatory zoning in CL images, a common
feature of igneous zircons (Corfu et al., 2003; Fig. 5).

DISCUSSION

Sandstone of the Itsuki Formation.—The youngest 206Pb/238Uage cluster of detrital zircons from the sandstone (sample
11042906) of the lower part of the Itsuki Formation in the
Itoshiro area consists only of two concordant grains (IT-1.2
and IT-1.40; Table 1) and one discordant grain (IT-1.65; Table1). We interpret that the small number of Early Cretaceous
zircon is due to the magmatic hiatus (Sagong et al., 2005) inthe provenance as discussed in a later section. The concordant

NEW AGE DATA FROM THE TETORI GROUP

9

FIGURE 6. Analytical data of detrital zircons from sandstone of the Itsuki Formation in the Itoshiro area (sample 11042906). a, Concordia diagram for all data;b, Concordia diagram for 500–100 Ma data set; c, Relative probability diagram.

Formation of the “Kuzuryu Subgroup” in the Mana area contains
zircons of various ages, but the intercept age of the younger age
cluster was 124.6 ± 2.3 Ma (Fig. 7b). A youngest age cluster ofzircons extracted from a pyroclastic-rock sample most likely
show the age of formation of essential volcanic-rock fragment,
and hence show the age of sedimentation of the pyroclastic
rock. Accordingly we regard that the age of sedimentation of thelapilli tuff of the Upper Formation of the “Kuzuryu Subgroup”
is 124.6 ± 2.3 Ma, corresponding to late Barremian–early Aptianof Early Cretaceous (ICS，2010).
Age of deposition of the Tetori Group in the Itoshiro andMana areas

FIGURE 7. Analytical data of detrital zircons from lapilli tuff of theUpper Formation of the “Kuzuryu Subgroup” in the Mana area (sample
11110805). a, Concordia diagram for all data; b, Concordia diagram for
140–120 Ma data set.

age of the youngest detrital zircon (grain IT-1.40; Table 1) insample 11042906 was 127.2 ± 2.5 Ma, which corresponds to theBarremian age of Early Cretaceous (International Commissions
on Stratigraphy (ICS), 2010). Hence the Itsuki Formation waspresumably deposited in Barremian or later, except for its
lowermost part.
Lapilli tuff of the Upper Formation of the “Kuzuryu
Subgroup”.—The lapilli tuff (sample 11110805) of the Upper

Here we discuss the age of the Tetori Group in the Itoshiro andMana areas from previous stratigraphical and paleontologicalstudies and the U-Pb ages presented in this study.
Itoshiro area.—The Tetori Group along the Itoshirogawa
River in the Itoshiro area is a type section of the group (e.g.,
Maeda, 1961b). The Tetori Group in the Itoshiro area consistsof Kuzuryu, Itoshiro, and Akaiwa subgroups in ascendingorder. The Kuzuryu Subgroup, in turn, is composed of the
Tochimochiyama, Kaizara, and Yambarazaka formationsin ascending order. Latest Bathonian–Early Callovian andOxfordian ammonoid fossil assemblages have been reported
from the Kaizara and Yambarazaka formations, respectively(Sato and Westermann, 1991), although the age of the
Tochimochiyama Formation is still unknown. According toFujita (2002), the continuous succession of the Itoshiro andAkaiwa subgroups in the Kamihambara area (Fig. 2), except for
the Kamihambara Formation, extends to the Itoshiro area. The

10

YUTA KAWAGOE, SHIN-ICHI SANO, YUJI ORIHASHI, HOKUTO OBARA, YOSHIKAZU KOUCHI AND SHIGERU OTOH

FIGURE 8. Age and correlation of the Tetori Group in the Mana, Itoshiro, and Kamihambara areas. Numerical ages on geologic-age boundaries are from ICS(2010). Parenthesized numbers denote dated samples. Abbreviations E: Epoch, Fm.: Formation, P: Period, Sg.: Subgroup.

Itsuki Formation of the Itoshiro Subgroup in the Itoshiro area
was correlated with the Hauterivian from the occurrence of the
Tatsukawa-type fauna (Kozai et al., 2002, 2005). On the other
hand, Goto (2007) reported a Late Hauterivian–Early Barremianammonoid, Pseudothurmannia sp., from the uppermost partof the Itoshiro Subgroup in the Uchinamigawa area, 5 km tonorthwest of the Itoshiro area. The ammonoid-bearing horizon
is most likely correlated with the uppermost part of the Itsuki
Formation in the Itoshiro area. In this study, we reported thatthe youngest age of the detrital zircons from a sandstone sample(11042906) of the lower part of the Itsuki Formation was 127.2± 2.5 Ma (Barremian; ICS, 2010). Our new data, combined withthe previous paleontological data, strongly suggest that the ageof the Itsuki Formation, except for the lowermost part, is Early

Barremian (Fig. 8).
Mana area.—The Tetori Group in the Mana area, reexaminedin this study, was assigned to the Middle and Upper formationsof the “Kuzuryu Subgroup”; the two formations were correlatedwith the Kaizara and Yambarazaka formations in the Itoshiro
area, respectively (Yamada et al., 1989). However, theammonoid fossils reported from the Middle Formation of
the “Kuzuryu Subgroup” by Yamada et al. (1989) have notbeen identified (Sato, 2008). On the other hand, the occurrenceof another ammonoid species, Parapallasiceras sp. cf. P.
pseudocontiguum, from a float of the Middle Formation of
the “Kuzuryu Subgroup” has been informally reported,suggesting that the Middle Formation of the “Kuzuryu
Subgroup” in the Mana area can be correlated, at least partly,11

NEW AGE DATA FROM THE TETORI GROUPTABLE 1. U-Pb isotopic data for zircons from sandstone of the Itsuki Formation in the Itoshiro area (sample 11042906). % conc = 100· ( 207Pb/235U age)/(206Pb/238U age) is a measure of concordance between 206Pb/238U and 207Pb/235U ages. Analyses shown in italics are discordant and are not included in theprobability plot.

Grain

Spot

IT-1.1IT-1.2IT-1.3IT-1.4

Outer
Outer
Outer
Outer
Outer
Inner
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Inner
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Inner
Outer
Outer
Outer
Inner
Outer
Outer
Outer
Outer
Outer
Outer
Inner

IT-1.5IT-1.6IT-1.7IT-1.8IT-1.9IT-1.10IT-1.11IT-1.12IT-1.13IT-1.14IT-1.15IT-1.16IT-1.17IT-1.18IT-1.19IT-1.20IT-1.21IT-1.22IT-1.23IT-1.24IT-1.25IT-1.26IT-1.27IT-1.28IT-1.29IT-1.30IT-1.31IT-1.32
IT-1.33
IT-1.34
IT-1.35
IT-1.36IT-1.37
IT-1.38
IT-1.39IT-1.40IT-1.41IT-1.42IT-1.43
IT-1.44IT-1.45206

Pb /238U (2σ)

0.29010.020220.30910.35750.03827
0.03935
0.46320.33130.30130.30070.02665
0.32420.02763
0.028610.40070.32800.30300.52920.68830.32940.42630.33070.32710.35520.32090.30950.46050.33030.34230.29130.40890.31080.32210.43100.30460.24600.42690.03394
0.03306
0.32530.44480.026830.24180.01994
0.33870.25720.32340.40510.03761
0.03808± 0.0039± 0.00033± 0.0042± 0.0049± 0.00055± 0.00045± 0.0062± 0.0044± 0.0041± 0.0040± 0.00040± 0.0045± 0.00039± 0.00036± 0.0056± 0.0044± 0.0042± 0.0075± 0.0093± 0.0046± 0.0060± 0.0047± 0.0046± 0.0050± 0.0045± 0.0044± 0.0066± 0.0046± 0.0066± 0.0056± 0.0078± 0.0060± 0.0062± 0.0083± 0.0058± 0.0047± 0.0068± 0.00070± 0.00050± 0.0052± 0.0073± 0.00044± 0.0025± 0.00039± 0.0054± 0.0043± 0.0044± 0.0054± 0.00068± 0.00069

207

Pb /235U (2σ)

4.620.1450
5.05
7.39
0.290
0.2826
10.25
5.26
4.846
4.747
0.188
5.30
0.20540.2260
8.65
5.20
4.75
17.54
24.69
5.52
9.02
4.83
5.18
5.69
5.47
5.20
9.95
5.28
5.22
5.36
7.80
4.96
5.19
9.85
4.76
3.747
9.33
0.243
0.262
5.17
10.02
0.17823.75470.1308
5.39
4.00
5.23
7.63
0.282
0.290

± 0.10± 0.0076± 0.11± 0.16± 0.010± 0.0087± 0.21± 0.11± 0.081± 0.098± 0.007± 0.12± 0.0064± 0.0090± 0.20± 0.11± 0.11± 0.40± 0.51± 0.12± 0.19± 0.11± 0.11± 0.12± 0.12± 0.12± 0.23± 0.11± 0.13± 0.13± 0.18± 0.12± 0.13± 0.24± 0.11± 0.093± 0.24± 0.019± 0.015± 0.14± 0.28± 0.0061± 0.0663± 0.0090± 0.14± 0.12± 0.13± 0.17± 0.019± 0.019206

Pb /238U age(2σ) in Ma
1642 ± 22129.1 ± 2.1
1736 ± 231970 ± 27
242.1 ± 3.5
248.8 ± 2.82453 ± 33
1845 ± 24
1698 ± 231695 ± 23169.5 ± 2.5
1810 ± 25
175.7 ± 2.5
181.9 ± 2.3
2172 ± 30
1829 ± 25
1706 ± 242738 ± 39
3376 ± 46
1836 ± 26
2289 ± 32
1842 ± 26
1824 ± 25
1959 ± 28
1794 ± 251738 ± 252442 ± 35
1840 ± 26
1898 ± 37
1648 ± 32
2210 ± 421744 ± 34
1800 ± 34
2310 ± 441714 ± 33
1418 ± 27
2292 ± 36
215.1 ± 4.5
209.7 ± 3.2
1816 ± 29
2372 ± 39
170.7 ± 2.81396 ± 14127.3 ± 2.5
1880 ± 301476 ± 25
1806 ± 242193 ± 29
238.0 ± 4.3
240.9 ± 4.4

207

Pb /235U age% concTh/U(2σ) in Ma
1752 ± 39
106.7
0.28
137.5 ± 7.2
106.6
0.71
1828 ± 39
105.3
0.31
2159 ± 46
109.6
0.19
258.6 ± 8.5
106.8
0.88
252.7 ± 7.8
101.6
1.58
2458 ± 50
100.2
0.61
1863 ± 38
101.0
0.70
1793 ± 30
105.6
0.54
1776 ± 37
104.8
0.27
174.7 ± 6.7
103.0
0.25
1868 ± 42
103.2
0.30
189.7 ± 5.9
108.0
1.0
206.9 ± 8.2
113.8
0.84
2302 ± 52
106.0
1.3
1852 ± 41
101.3
0.33
1776 ± 42
104.1
0.72
2965 ± 67
108.3
0.95
3296 ± 68
97.6
0.67
1904 ± 40
103.7
0.18
2340 ± 50
102.2
1.61790 ± 3997.2
0.171849 ± 39
101.3
0.56
1930 ± 4298.50.221897 ± 42
105.7
0.19
1852 ± 41
106.5
1.12430 ± 55
99.5
1.11865 ± 39
101.4
0.31
1855 ± 47
97.80.261879 ± 46
114.0
0.13
2208 ± 52
99.90.121813 ± 46
103.9
0.27
1851 ± 45
102.8
0.36
2421 ± 59
104.8
2.5
1778 ± 42
103.7
0.37
1581 ± 39
111.5
0.14
2371 ± 61
103.4
0.45
221
± 17
102.5
0.25
236
± 14
112.6
0.85
1847 ± 50
101.7
1.12436 ± 68
102.7
0.39
166.5 ± 5.7
97.6
0.60
1583 ± 28
113.4
0.94
124.8 ± 8.6
98.0
0.32
1883 ± 50
100.1
0.25
1634 ± 48110.7
1.41857 ± 46
102.8
0.77
2189 ± 50
99.8
0.58
252
± 17
106.0
0.55
258.5 ± 17.1
107.3
0.56
(continued on next page)12

YUTA KAWAGOE, SHIN-ICHI SANO, YUJI ORIHASHI, HOKUTO OBARA, YOSHIKAZU KOUCHI AND SHIGERU OTOHTABLE 1.（Continued）

Grain
Grain

Spot
Spot

IT-1.1IT-1.1IT-1.2IT-1.2IT-1.3IT-1.3IT-1.4IT-1.4

Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Inner
Inner
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Inner
Inner
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Inner
Inner
Outer
Outer
Outer
Outer
Outer
Outer
Inner
Inner
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Outer
Inner
Inner

IT-1.5IT-1.5IT-1.6IT-1.6IT-1.7IT-1.7IT-1.8IT-1.8IT-1.9IT-1.9IT-1.10IT-1.10IT-1.11IT-1.11IT-1.12IT-1.12IT-1.13IT-1.13IT-1.14IT-1.14IT-1.15IT-1.15IT-1.16IT-1.16IT-1.17IT-1.17IT-1.18IT-1.18IT-1.19IT-1.19IT-1.20IT-1.20IT-1.21IT-1.21IT-1.22IT-1.22IT-1.23IT-1.23IT-1.24IT-1.24IT-1.25IT-1.25IT-1.26IT-1.26IT-1.27IT-1.27IT-1.28IT-1.28IT-1.29IT-1.29IT-1.30IT-1.30IT-1.31IT-1.31IT-1.32
IT-1.32
IT-1.33
IT-1.33
IT-1.34
IT-1.34
IT-1.35
IT-1.35
IT-1.36IT-1.36IT-1.37
IT-1.37
IT-1.38
IT-1.38
IT-1.39IT-1.39IT-1.40IT-1.40IT-1.41IT-1.41IT-1.42IT-1.42IT-1.43
IT-1.43
IT-1.44IT-1.44IT-1.45IT-1.45

206
206Pb /238
238U (2σ)

Pb / U (2σ)
0.29010.2901 ±± 0.0039
0.00390.020220.02022 ±± 0.00033
0.00033
0.30910.3091 ±± 0.0042
0.00420.35750.3575 ±± 0.0049
0.00490.03827 ±± 0.00055
0.00055
0.03827
0.03935 ±± 0.00045
0.000450.03935
0.4632
0.4632 ±± 0.0062
0.00620.33130.3313 ±± 0.0044
0.00440.30130.3013 ±± 0.0041
0.00410.3007 ±± 0.0040
0.00400.30070.02665 ±± 0.00040
0.000400.02665
0.32420.3242 ±± 0.0045
0.00450.02763
0.02763 ±± 0.00039
0.000390.02861
0.02861 ±± 0.00036
0.000360.40070.4007 ±± 0.0056
0.0056
0.3280 ±± 0.0044
0.00440.32800.30300.3030 ±± 0.0042
0.00420.52920.5292 ±± 0.0075
0.00750.68830.6883 ±± 0.0093
0.00930.32940.3294 ±± 0.0046
0.0046
0.4263 ±± 0.0060
0.00600.42630.3307 ±± 0.0047
0.00470.33070.32710.3271 ±± 0.0046
0.0046
0.35520.3552 ±± 0.0050
0.00500.32090.3209 ±± 0.0045
0.00450.3095 ±± 0.0044
0.00440.30950.4605 ±± 0.0066
0.00660.46050.33030.3303 ±± 0.0046
0.0046
0.34230.3423 ±± 0.0066
0.0066
0.29130.2913 ±± 0.0056
0.0056
0.40890.4089 ±± 0.0078
0.00780.3108 ±± 0.0060
0.00600.31080.3221
0.3221 ±± 0.0062
0.00620.43100.4310 ±± 0.0083
0.00830.30460.3046 ±± 0.0058
0.00580.24600.2460 ±± 0.0047
0.00470.4269 ±± 0.0068
0.00680.42690.03394 ±± 0.00070
0.000700.03394
0.03306
0.03306 ±± 0.00050
0.00050
0.32530.3253 ±± 0.0052
0.00520.44480.4448 ±± 0.0073
0.00730.02683 ±± 0.00044
0.000440.026830.2418 ±± 0.0025
0.00250.24180.01994
0.01994 ±± 0.00039
0.000390.3387
0.3387 ±± 0.0054
0.00540.25720.2572 ±± 0.0043
0.00430.32340.3234 ±± 0.0044
0.00440.4051 ±± 0.0054
0.00540.40510.03761
0.03761 ±± 0.00068
0.00068
0.03808
0.03808 ±± 0.00069
0.00069

207
207Pb /235
235U (2σ)

Pb / U (2σ)
4.62
±
4.62
± 0.100.10
0.14500.1450 ±± 0.0076
0.00765.05±
5.05± 0.11
0.11
7.39
±
7.39
± 0.160.16
0.290 ±
± 0.0100.010
0.290
0.2826 ±± 0.0087
0.00870.2826
10.25
10.25 ±
± 0.21
0.21
5.26
±
5.26
± 0.11
0.11
4.846
4.846 ±
± 0.081
0.081
4.747 ±
± 0.098
0.098
4.747
0.188 ±
± 0.007
0.007
0.188
5.30
±
5.30
± 0.120.12
0.20540.2054 ±± 0.0064
0.00640.22600.2260 ±± 0.0090
0.00908.65±
8.65± 0.200.20
5.20± 0.11
0.11
5.20
±
4.75
±
4.75
± 0.11
0.11
17.54
17.54 ±
± 0.400.40
24.69
24.69 ±
± 0.51
0.51
5.52
±
5.52
± 0.120.12
9.02± 0.19
0.19
9.02
±
4.83
± 0.11
0.11
4.83
±
5.18
±
5.18
± 0.11
0.11
5.69
±
5.69
± 0.12
0.12
5.47
±
5.47
± 0.120.12
5.20± 0.12
0.12
5.20
±
9.95
± 0.23
0.23
9.95
±
5.28
±
5.28
± 0.11
0.11
5.22
±
5.22
± 0.13
0.13
5.36
±
5.36
± 0.13
0.13
7.80
±
7.80
± 0.180.18
4.96± 0.12
0.12
4.96
±
5.19
±
5.19
± 0.13
0.13
9.85
±
9.85
± 0.24
0.24
4.76
±
4.76
± 0.11
0.11
3.747
3.747 ±
± 0.0930.093
9.33± 0.240.24
9.33±
0.243 ±
± 0.019
0.019
0.243
0.262
0.262 ±
± 0.015
0.015
5.17
±
5.17
± 0.140.14
10.02
10.02 ±
± 0.280.28
0.1782 ±± 0.0061
0.00610.17823.7547 ±± 0.0663
0.06633.75470.13080.1308 ±± 0.0090
0.00905.39±
5.39± 0.14
0.14
4.00
±
4.00
± 0.12
0.12
5.23
±
5.23
± 0.130.13
7.63± 0.17
0.17
7.63
±
0.282
0.282 ±
± 0.0190.019
0.290
0.290 ±
± 0.019
0.019

206
206Pb /238
238U age

Pb / U age(2σ)(2σ) in
in Ma
Ma
1642
1642 ±
± 22
22
129.1
129.1 ±
± 2.1
2.1
1736
±
1736 ± 232319701970 ±± 27
27
242.1 ±
± 3.5
3.5
242.1
248.8 ±
± 2.8
2.8
248.8
2453
2453 ±± 33
33
1845
1845 ±± 24
24
1698
1698 ±± 23
231695 ±± 23
23
1695
169.5 ±
± 2.5
2.5
169.5
1810
1810 ±
± 25
25
175.7
175.7 ±
± 2.5
2.5
181.9
±
181.9 ± 2.3
2.3
2172
2172 ±± 30
30
1829 ±
± 25
25
1829
1706
1706 ±± 24
24
2738
2738 ±± 39
39
3376
3376 ±± 46
46
1836
1836 ±± 26
262289 ±± 32
32
2289
1842 ±
± 26
26
1842
1824
1824 ±± 25
25
1959
1959 ±± 28
28
1794
1794 ±± 25
251738 ±± 25
25
17382442 ±
± 35
35
2442
1840
1840 ±± 26
26
1898
1898 ±± 37
37
1648
1648 ±± 32
32
2210
2210 ±± 42
421744 ±
± 34
34
1744
1800
1800 ±± 34
34
2310
2310 ±± 4444
1714
1714 ±± 33
33
1418
1418 ±± 27
272292 ±± 36
362292
215.1 ±
± 4.5
4.5
215.1
209.7
209.7 ±
± 3.2
3.2
1816
1816 ±± 29
29
2372
2372 ±± 39
39
170.7 ±
± 2.8
2.8
170.7
1396 ±
± 14
14
1396
127.3
127.3 ±
± 2.5
2.5
1880
1880 ±± 30
30
1476
1476 ±± 25
25
1806
1806 ±± 24
242193 ±
± 29
29
2193
238.0
238.0 ±
± 4.3
4.3
240.9
240.9 ±
± 4.4
4.4

207
207Pb /235
235U age

Pb / U age% conc
conc
Th/U
%
Th/U
(2σ)
(2σ) in
in Ma
Ma
1752
106.7
0.28
1752 ±
± 39
39
106.7
0.28
137.5
±
7.2
106.6
0.71
137.5 ± 7.2
106.6
0.71
1828
±
39
105.3
0.31
1828 ± 39
105.3
0.31
2159
109.6
0.19
2159 ±
± 46
46
109.6
0.19
258.6 ±
± 8.5
8.5
106.8
0.88
258.6
106.8
0.88
252.7 ±
± 7.8
7.8
101.6
1.58
252.7
101.6
1.58
2458
100.2
0.61
2458 ±
± 50
50
100.2
0.61
1863
101.0
0.70
1863 ±
± 38
38
101.0
0.70
1793
±
30
105.6
0.54
1793 ± 30
105.6
0.54
1776 ±
± 37
37
104.8
0.27
1776
104.8
0.27
174.7 ±
± 6.7
6.7
103.0
0.25
174.7
103.0
0.25
1868
103.2
0.30
1868 ±
± 42
42
103.2
0.30
189.7
±
5.9
108.0
1.0
189.7 ± 5.9
108.0
1.0
206.9
±
8.2
113.8
0.84
206.9 ± 8.2
113.8
0.84
2302
106.0
1.3
2302 ±
± 52
52
106.0
1.3
1852 ±
± 41
41
101.3
0.33
1852
101.3
0.33
1776
104.1
0.72
1776 ±
± 42
42
104.1
0.72
2965
±
67
108.3
0.95
2965 ± 67
108.3
0.95
3296
±
68
97.6
0.67
3296 ± 68
97.6
0.67
1904
±
40
103.7
0.18
1904 ± 40
103.7
0.18
2340 ±
± 50
50
102.2
1.6
2340
102.2
1.6
1790 ±
± 39
39
97.2
0.17
1790
97.2
0.17
1849
101.3
0.56
1849 ±
± 39
39
101.3
0.56
1930
±
42
98.5
0.22
1930 ± 42
98.5
0.22
1897
±
42
105.7
0.19
1897 ± 42
105.7
0.19
1852 ±
± 41
41
106.5
1.1
1852
106.5
1.1
2430 ±
± 55
55
99.5
1.1
2430
99.5
1.1
1865
101.4
0.31
1865 ±
± 39
39
101.4
0.31
1855
97.8
0.26
1855 ±
± 47
47
97.8
0.26
1879
±
46
114.0
0.13
1879 ± 46
114.0
0.13
2208
99.9
0.12
2208 ±
± 52
52
99.9
0.12
1813 ±
± 46
46
103.9
0.27
1813
103.9
0.27
1851
102.8
0.36
1851 ±
± 45
45
102.8
0.36
2421
±
59
104.8
2.5
2421 ± 59
104.8
2.5
1778
±
42
103.7
0.37
1778 ± 42
103.7
0.37
1581
111.5
0.14
1581 ±
± 39
39
111.5
0.14
2371 ±
± 61
61
103.4
0.45
2371
103.4
0.45
221
± 17
17
102.5
0.25
221
±
102.5
0.25
236
±
112.6
0.85
236
± 14
14
112.6
0.85
1847
101.7
1.1
1847 ±
± 50
50
101.7
1.1
2436
±
68
102.7
0.39
2436 ± 68
102.7
0.39
166.5 ±
± 5.7
5.7
97.6
0.60
166.5
97.6
0.60
1583 ±
± 28
28
113.4
0.94
1583
113.4
0.94
124.8
98.0
0.32
124.8 ±
± 8.6
8.6
98.0
0.32
1883
±
50
100.1
0.25
1883 ± 50
100.1
0.25
1634
±
48
110.7
1.4
1634 ± 48
110.7
1.4
1857
±
46
102.8
0.77
1857 ± 46
102.8
0.77
2189 ±
± 50
50
99.8
0.58
2189
99.8
0.58
252
±
106.0
0.55
252
± 17
17
106.0
0.55
258.5
107.3
0.56
258.5 ±
± 17.1
17.1
107.3
0.56
(continued
on
next
page)
(continued on next page)

NEW AGE DATA FROM THE TETORI GROUP

13

TABLE 1.（Continued）

Grain
IT-1.1IT-1.2IT-1.3IT-1.4

Spot

206

Pb /238U (2σ)

207

Pb /235U (2σ)

Outer 0.2901 ± 0.0039
4.62± 0.10Outer 0.02022 ± 0.00033
0.1450 ± 0.0076
Outer 0.3091 ± 0.0042
5.05± 0.11
Outer 0.3575 ± 0.0049
7.39± 0.16
Outer 0.03827 ± 0.00055
0.290 ± 0.010
IT-1.5
Inner 0.03935 ± 0.00045
0.2826 ± 0.0087
IT-1.6
Outer 0.4632 ± 0.0062
10.25 ± 0.21
IT-1.7
Outer 0.3313 ± 0.0044
5.26± 0.11IT-1.8Outer 0.3013 ± 0.0041
4.846 ± 0.081
IT-1.9Outer 0.3007 ± 0.0040
4.747 ± 0.098
IT-1.10Outer 0.02665 ± 0.00040
0.188 ± 0.007
IT-1.11Outer 0.3242 ± 0.0045
5.30± 0.12Outer 0.02763 ± 0.00039
0.2054 ± 0.0064
IT-1.12Inner
0.02861 ± 0.00036
0.2260 ± 0.0090
IT-1.13
Outer 0.4007 ± 0.0056
8.65± 0.205.20± 0.11IT-1.14
Outer 0.3280 ± 0.0044
IT-1.15
Outer 0.3030 ± 0.0042
4.75± 0.11IT-1.16
Outer 0.5292 ± 0.0075
17.54 ± 0.40
IT-1.17
Outer 0.6883 ± 0.0093
24.69 ± 0.51
IT-1.18
Outer 0.3294 ± 0.0046
5.52± 0.12IT-1.19
Outer 0.4263 ± 0.0060
9.02± 0.19IT-1.20
Outer 0.3307 ± 0.0047
4.83± 0.11IT-1.21
Outer 0.3271 ± 0.0046
5.18± 0.11IT-1.22
Outer 0.3552 ± 0.0050
5.69± 0.12IT-1.23
Outer 0.3209 ± 0.0045
5.47± 0.12IT-1.24
Outer 0.3095 ± 0.0044
5.20± 0.12IT-1.25
Outer 0.4605 ± 0.0066
9.95± 0.23IT-1.26
Outer 0.3303 ± 0.0046
5.28± 0.11IT-1.27
Outer 0.3423 ± 0.0066
5.22± 0.13withthe Tithonianand Uemura,In this study,
IT-1.28
Outer(Yamada
0.2913± 0.00562008).5.36± 0.13Outer
0.4089± 0.0078
7.80Formation± 0.18
weIT-1.29
reported thatthe lapilli
tuff layer
in the Upper
Outer
0.31080.00604.96± 0.12of IT-1.30
the “Kuzuryu
Subgroup”
in the±Mana
area waspresumably
IT-1.31at 124.6
Outer
0.3221± 0.0062
5.19 Fig.
± 0.13
deposited± 2.3 Ma(late Barremian–early
Aptian;8).
IT-1.32
Outer 0.4310 ± 0.0083
9.85± 0.24IT-1.33of previous
Outer ideas
0.3046± 0.0058
4.76± 0.11
Revisionof chronology
and correlation
IT-1.34
Outer 0.2460 ± 0.0047
3.747 ± 0.093
IT-1.35
Outer 0.4269 ± 0.0068
9.33± 0.24
Correlation of the Tetori Group in the Mana and Itoshiro
Outer 0.03394 ± 0.00070
0.243 ± 0.019
IT-1.36
the foregoing discussion, previous ideas ofareas.—FromInner
0.03306 ± 0.00050
0.262 ± 0.015
chronology and correlation of the Tetori Group in the Mana
IT-1.37
Outer 0.3253 ± 0.0052
5.17± 0.14area(e.g., Yamada
et al.,0.4448
1989) need
to be revised.
The Middle
IT-1.38
Outer± 0.0073
10.02± 0.28
Formation of the“Kuzuryu
Subgroup”
in the Mana
area yields
Outer0.02683± 0.00044
0.1782± 0.0061
IT-1.39 ammonoid (Yamada and Uemura, 2008), though
TithonianInner0.2418 ± 0.0025
3.7547 ± 0.0663
froma float, andthe lapilli
tuff layer
in the Upper
Formation
IT-1.40Outer
0.01994± 0.00039
0.1308± 0.0090
of IT-1.41
the “Kuzuryu
Subgroup”
dated to5.39
124.6 ±± 2.3
Outer0.3387has ±been
0.00540.14
MaIT-1.42(late Barremian–early
Aptian).
On the other± 0.0043
4.00hand,± the
0.12
Outer 0.2572
IT-1.43Subgroup
Outerof the0.32340.0044
5.23Oxfordian± 0.13
Itoshiro
Itoshiro± area
covers theIT-1.44
Outer 0.4051± 0.0054 Unconformity,
7.63
± and
0.17
Yambarazaka
Formationby the Umagatani
Outer the0.03761± 0.00068± 0.019
theIT-1.45
Itsuki Formation,
youngest
formation of0.282
the Itoshiro
Inner likely
0.03808± 0.00069± 0.019Subgroup, is mostan Early
Barremian 0.290
formationas
discussed above. Although the age of the Itoshiro Subgroup in
the Itoshiro area has not precisely been constrained except for

206

207
Pb /238U age
Pb /235U age% concTh/U(2σ) in Ma(2σ) in Ma
1642 ± 22
1752 ± 39
106.7
0.28
129.1 ± 2.1
137.5 ± 7.2
106.6
0.71
1736 ± 23
1828 ± 39
105.3
0.31
1970 ± 27
2159 ± 46
109.6
0.19
242.1 ± 3.5
258.6 ± 8.5
106.8
0.88
248.8 ± 2.8
252.7 ± 7.8
101.6
1.58
2453 ± 33
2458 ± 50
100.2
0.61
1845 ± 24
1863 ± 38
101.0
0.70
1698 ± 23
1793 ± 30
105.6
0.54
1695 ± 23
1776 ± 37
104.8
0.27
169.5 ± 2.5
174.7 ± 6.7
103.0
0.25
1810 ± 25
1868 ± 42
103.2
0.30
175.7 ± 2.5
189.7 ± 5.9
108.0
1.0
181.9 ± 2.3
206.9 ± 8.2
113.8
0.84
2172 ± 30
2302 ± 52
106.0
1.3
1829 ± 25
1852 ± 41
101.3
0.33
1706 ± 24
1776 ± 42
104.1
0.72
2738 ± 392965 ± 67
108.3
0.95
3376 ± 46
3296 ± 68
97.6
0.67
1836 ± 26
1904 ± 40
103.7
0.18
2289 ± 32
2340 ± 50
102.2
1.61842 ± 26
1790 ± 39
97.2
0.17
1824 ± 25
1849 ± 39
101.3
0.56
1959 ± 28
1930 ± 4298.5
0.221794 ± 25
1897 ± 42
105.7
0.19
1738 ± 25
1852 ± 41
106.5
1.12442 ± 352430 ± 55
99.5
1.11840 ± 26
1865 ± 39
101.4
0.31
1898 ± 37
1855 ± 47
97.8
0.26
the1648
Itsuki Formation,
the1879“Kuzuryu
Subgroup”
in the Mana
± 32
± 46
114.0
0.13area
2210 correlated
± 42
2208
± 52 Subgroup99.9in the Itoshiro
0.12
is mostly
withthe Itoshiro
1744
34
1813
46correlated
103.9
0.27
area,
and its± uppermost
part may± bewith the Akaiwa
1800 ±
34 are other
1851ideas± that45 assume102.8
Subgroup.
There(1) that the0.36
Itsuki
2310 ±
2421 ± 59 marine 104.8
2.5thatFormation
is a44Tithonian–Berriasian
formation, (2)1714± 33 time interval
1778 between± 42 the Itsuki
103.7 and Nochino
0.37
there
is a large
1418
±
27
1581
±
39
111.5
0.14
formations (e.g., Matsukawa et al., 2003, 2006), and (3) that2292 ± 36
2371 ± 61
103.4
0.45
the ammonoid-bearing Late Hauterivian–Early Barremian bed
215.1 ± 4.5
221
± 17
102.5
0.25
in the Uchinamigawa area (Goto, 2007) is correlated with the209.7 ± 3.2
236
± 14
112.6
0.85
Amagodani Formation (equivalent to the Nochino Formation)
1816 ± 29
1847 ± 50
101.7
1.1
in 2372
the Shokawa
is neverwith the0.39
Itsuki± 39 area but
2436± 68correlated
102.7
Formation(Matsukawa
andAsahara,2010).
However,none
170.7 ± 2.8
166.5 ± 5.7
97.6
0.60 of
these
ideas
are
consistent
with
our
data
and
interpretation.
1396 ± 14
1583 ± 28
113.4
0.94TheItsuki
Formation
Itoshiro ±area
127.3± 2.5 in the 124.8
8.6and the ammonoid-bearing
98.0
0.32
bed1880in the ±Uchinamigawa
area ±
can50both be correlated
with
30
1883
100.1
0.25theLower
1476Barremian.
± 25 Although
1634 the± age
48 of the Nochino
110.7 Formation
1.4± 24been constrained,
1857 ± the46 conformable
102.8 relationship0.77
has1806
not nicely
2193 the± Itsuki
29
± 50
0.58
betweenand2189
Nochinoformations99.8(Maeda, 1961b),
238.0 with± 4.3
252data presented
± 17
106.0
0.55
together
the new age
in thisstudy, indicates± interval
4.4
258.5 the± Itsuki
17.1 and Nochino
107.3 formations
0.56
that240.9
the time
between(continued
on
next
page)
must be much shorter than Matsukawa et al. (2003, 2006) assumed.

14

YUTA KAWAGOE, SHIN-ICHI SANO, YUJI ORIHASHI, HOKUTO OBARA, YOSHIKAZU KOUCHI AND SHIGERU OTOHTABLE 2. U-Pb isotopic data for zircons from lapilli tuff of the Upper Formation of the “Kuzuryu Subgroup” in the Mana area (sample 11110805).

Correlation of the Tetori Group in the Mana and Kamihambara
areas.—The Kamihambara Formation in the Kamihambara
area yields Tithonian ammonoid Parapallasiceras sp. cf. P.
pseudocontiguum, the same ammonoid with that reported froma float of the Middle Formation of the “Kuzuryu Subgroup” in
the Mana area (Sato and Yamada, 2005; Yamada and Uemura,2008). If the Itsuki and Nochino formations of the Itoshiro area
are correlated with the Itsuki and Nochino formations in the
Kamihambara area, respectively, as Fujita (2002) proposed, thecontinuous succession of the “Kuzuryu Subgroup” in the Mana
area can directly be correlated with the continuous succession
from the Kamihambara Formation to the Itsuki (or Nochino)
Formation in the Kamihambara area. However, the correlationof the Itoshiro Subgroup between the Itoshiro and Kamihambara
areas needs further examination from the following reasons:(1) a marine bed correlative with the Kamihambara Formation
has not been found in the Itoshiro area; (2) the sandstone
and fossil samples that Fujita (2002) used as the basis for thecorrelation were mostly collected in the western part(Itoshiroside)of the Kamihambara area; and (3) a different interpretationof correlation was addressed by Yamada et al. (1989) who
had colored the Ashidani, Kamihambara, Obuchi, and Itsuki

formations in the Kamihambara area of Fujita (2002) as theUpper Formation of the Itoshiro Subgroup, i.e., the Itsuki
Formation.
Geologic structure of the Tetori Group in the three areas
studied.—From the discussion in the previous paragraphs, it
is presumable that the Tetori Group in the Mana, Itoshiro, andKamihambara areas all exhibit a northward-facing, Tithonian–
Barremian (or earliest Aptian) succession. Although the Tetori
Group in the Mana area is apparently overlain by the Tetori
Group in the Itoshiro area, they contain coeval successions andmust have been juxtaposed by faulting. Moreover, although
the Tetori Group in the Kamihambara area has been assumed
to extend to the Tetori Group in the Itoshiro area (e.g., Maeda,
1957; Fujita, 2002), there arises another possibility that the
Tetori Group in the Kamihambara area laterally extends to theTetori Group in the Mana area. In any case, further geologicalstudy of the blank area in the eastern part of Fig. 2 is necessary
to clarify the geologic structure of the three areas.
Transgressive phases recorded in the Tetori Group andthe Umagatani Unconformity.—Matsukawa et al. (2007)
discriminated three phases of marine transgression from theirstudy of the Tetori Group in the Shokawa and Hida-Furukawa

15

NEW AGE DATA FROM THE TETORI GROUP

areas (Fig. 1): (1) Bathonian–Oxfordian, (2) Tithonian–
Berriasian, and (3) Hauterivian–Barremian. They further discussthat the Kamihambara and Itsuki formations are correlative
formations deposited during the second (Tithonian–Berriasian)transgressive phase. The three transgressive phases can also
be discriminated in four neighboring areas in Ono City, i.e.,the Mana, Kamihambara, Itoshiro, and Uchinamigawa areas.
Namely, the first transgressive phase corresponds to the age of
sedimentation of the Kaizara and Yambarazaka formations of theKuzuryu Subgroup in the Itoshiro area (Bathonian–Oxfordian),
the second phase corresponds to the timing of sedimentationof the Kamihambara Formation in the Kamihambara area andthe Middle Formation of the “Kuzuryu Subgroup” in the Mana
area (Tithonian), and the third phase corresponds to the age of
sedimentation of the uppermost part of the Itoshiro Subgroup(Itsuki Formation) in the Uchinamigawa and Itoshiro areas (Early
Barremian). The Umagatani Unconformity at the base of the
Itoshiro Subgroup in the Itoshiro area has been interpreted as a
product of regression between the first and second transgressive
phases as shown in Fig. 8 (e.g., Fujita, 2002). So far, however,
the age of formation of the Umagatani Unconformity has notwell been constrained. Considering the facts that the marine
bed correlative with the Kamihambara Formation has not beenfound in the Itoshiro area, and that there is a significant timeinterval (> 15 m.y.) between the Kamihambara and Itsuki
formations (Fig. 8), there arises another possibility that the
Umagatani Unconformity was formed between the second andthird transgressive phases and the Kamihambara Formation waseroded out in the Itoshiro area. With further geochronologicalstudies, more precise age constraints (particularly Yambara
Formation) and correlation of transgressive phases have potential
to yield a greater understanding of the Tetori Group.

floristic province in Late Jurassic to Early Cretaceous times (e.g.,
Kimura, 1987).
Another distinctive feature of the two investigated samples isthat they include 240–170 Ma zircons. If we consider geologicalrelationships between the Archean to Paleoproterozoic rocks
mentioned above and the Tetori Group, we can find the 240–
170 Ma granitoids in the Hida and Hida Gaien belts in Japan(Kunugiza and Kaneko, 2001) and 180–170 Ma granitoids in the
Yeongnam Massif of South Korea (Sagong et al., 2005). These
areas must also have constituted the provenance of the Tetori
Group. This idea is consistent with that of Kim et al. (2007)
who addressed that the quartz arenite clasts in the Yambara andNochino formations were supplied from the Cambro–Ordovician
formations in the east–central part of the present-day Korean
Peninsula.
In the Korean Peninsula, 158–110 Ma magmatic hiatus(Sagong et al., 2005) has been known as a distinctive geotectonic
event. Although we could not find 165–135 Ma zircons from thetwo samples, the 130–125 Ma zircons in the sandstone sampleof the Itsuki Formation and the 125 Ma lapilli tuff of the Mana
area suggest an igneous activity in the nearby provenance of
the Tetori Group. Minor igneous activities have already beenreported in East Asia: e.g., in the metamorphic core complexeson the north of Beijing and in Inner Mongolia, China (e.g., Daviset al., 1996, 2001), volcano-plutonic complex in Northeast Japan(e.g., Kanisawa, 1974), and the Early Cretaceous detrital zircons
in the Early Cretaceous Shindong Group, South Korea (Lee etal., 2010). Early Cretaceous zircons and pyroclastic rocks in the
Tetori Group may have been related to local contemporaneous
igneous activities in East Asia.

Provenance of clastic rocks of the Tetori Group in the studyarea

We obtained LA-ICPMS U-Pb ages of detrital zircons from (1)a lapilli tuff sample of the Upper Formation of the “Kuzuryu
Subgroup” of the Mana area and (2) a sandstone sample of the
Itsuki Formation of the Itoshiro Subgroup in the Itoshiro area,
Ono City, Fukui Prefecture, central Japan. The results of thestudy and their implications can be summarized as follows.
1．The youngest zircon from the sandstone of the Itsuki
Formation was 127.2 ± 2.5 Ma (Barremian). Combining
the results of previous paleontological studies, the Itsuki
Formation must be correlated with the Lower Barremian.
The lapilli tuff of the Upper Formation of the “Kuzuryu
Subgroup” in the Mana area, on the other hand, was dated to124.6 ± 2.3 Ma (late Barremian–early Aptian).
2．A Tithonian ammonoid reported from the Middle Formationof the “Kuzuryu Subgroup” in the Mana area (Sato and
Yamada, 2005) indicates that the formation is correlatedwith the Kamihambara Formation of the Kamihambara
area. The Barremian–earliest Aptian lapilli tuff of the Upper
Formation of the “Kuzuryu Subgroup” dated in this study
can be correlated with the Itsuki or Nochino Formation in

The analyzed samples in this study, the sandstone (sample
11042906) of the Itsuki Formation in the Itoshiro area andthe lapilli tuff (sample 11110805) of the Upper Formationof the “Kuzuryu Subgroup”, were both deposited in Barremianand/or earliest Aptian and contained many Archean to
Paleoproterozoic zircons (84% in sample 11042906 and 60%
in sample 11110805). The fact indicates that Archean to
Paleoproterozoic rocks were widely exposed in the provenanceof the Tetori Group in Barremian–earliest Aptian time. The
exposure of Archean to Paleoproterozoic rocks around presentday Japanese Islands has been reported from northeast China
and North Korea, in the northeastern part of the Eastern Blockof the North China Craton, and from the Jiao–Liao–Ji Belt, thefailed rift along the eastern margin of the Eastern Block (e.g.,
Zhao et al., 2005). The Archean to Paleoproterozoic rocks inthese areas were most likely exposed in the provenance of thetwo samples. These areas were included in the ‘Tetori-Type’

CONCLUSIONS16

YUTA KAWAGOE, SHIN-ICHI SANO, YUJI ORIHASHI, HOKUTO OBARA, YOSHIKAZU KOUCHI AND SHIGERU OTOH

the Itoshiro area. The continuous succession of the Itoshiro
Subgroup and Nochino Formation in the Itoshiro area wasassumed to be correlated with the Itoshiro Subgroup andNochino Formation in the Kamihambara area, although the
Kamihambara Formation of the Itoshiro Subgroup is missingin the former area (Fujita, 2002). Further study is howeverneeded to better constrain the age of the Itoshiro Subgroup
and Nochino Formation.
3．Three phases of marine transgression can be discriminatedfrom the Tetori Group in nearby four areas in Ono City,
the Mana, Kamihambara, Itoshiro, and Uchinamigawa
areas. They are (1) age of sedimentation of the Kaizara
and Yambarazaka formations of the Kuzuryu Subgroupin the Itoshiro area (Bathonian–Oxfordian), (2) the ageof sedimentation of the Kamihambara Formation in the
Kamihambara area and the Middle Formation of the“Kuzuryu Subgroup” in the Mana area (Tithonian), and(3) the age of sedimentation of the uppermost part of the
Itoshiro Subgroup (Itsuki Formation) in the Uchinamigawa
and Itoshiro areas (Early Barremian).
4．Judging from the age distribution of detrital zircons in
the two samples, the provenance of the Tetori Group in
Barremian–Early Aptian time was likely (1) the northeasternpart of the Eastern Block of the North China Craton where
Archean–Paleoproterozoic rocks were widely exposed, and(2) Early Mesozoic volcano-plutonic province of the Hida
and Hida Gaien Belt in Japan and the Yeongnam massif inSouth Korea.
ACKNOWLEDGMENTS
We would like to express our sincere gratitude to Emeritus
Professor Kenji Konishi of Kanazawa University for his
encouragement to submit our work to this issue; to Associate
Professor Toshihiro Yamada of Kanazawa University for his
information on the occurrence of the Tithonian ammonoid in
the Mana area; and to Professor Masaaki Shimizu, Associate
Professor Kenji Kashiwagi, and Associate Professor Yasuo
Ishizaki of the University of Toyama for various discussions
and instructions during the course of this study. Critical reviewsof the submitted manuscript by Associate Professor Tatsuki
Tujimori of the Institute for Study of Earth’s Interior, Okayama
University, Dr. Nao Kusuhashi of Ehime University, Emeritus
Professor Kenji Konishi, and Dr. Hiroto Ichishima of the Fukui
Prefectural Dinosaur Museum are greatly appreciated. This
study was supported by the Earthquake Research Institutecooperative research program, University of Tokyo.
REFERENCES
Aoki, K., T. Iizuka, T. Hirata, S. Maruyama and M. Terabayashi.
2007. Tectonic boundary between the Sanbagawa belt andShimanto belt in central Shikoku, Japan. Journal of theGeological Society of Japan 113: 171–183.
Cherniak, D. J., and E. B. Watson. 2001. Pb diffusion in zircon.
Chemical Geology 172: 5–24.
Compston, W. 1996. SHRIMP: Origins, impact and continuing
evolution. Journal of the Royal Society of Western Australia
79: 109–117.
Corfu, F., J. M. Hanchar, P. W. O. Hosikin and P. Kinny. 2003.
Atlas of zircon textures; pp. 469–500 in J. M. Hanchar and
P. W. O. Hoskin (eds.), Zircon, Reviews in Mineralogy
and Geochemistry 53. Mineralogical Society of America,
Washington, DC.
Davis, G. A., X. Qian, Y. Zheng, H.-M. Tong, H. Yu, C. Wang,
G. E. Gehrels, M. Shafiquallah and J. E. Fryxell. 1996.
Mesozoic deformation and plutonism in the Yunmeng Shan: Ametamorphic core complex north of Beijing, China; pp.
253–280 in A. Yin and T. M. Harrison (eds.), The Tectonic
Evolution of Asia. Cambridge University Press, Cambridge.
Davis, G. A., Y. Zheng, C. Wang, B. J. Darby, C. Zhang and G.
E. Gehrels. 2001. Mesozoic tectonic evolution of the Yanshan
fold and thrust belt, with emphasis on Hebei and Liaoning
provinces, northern China; pp. 171–197 in M. S. Hendrix
and G. A. Davis (eds.), Paleozoic and Mesozoic Tectonic
Evolution of Central Asia: From Continental Assembly to
Intracontinental Deformation, Geological Society of America
Memoir 194. Geological Society of America, Boulder.
Fujita, M. 2002. A new contribution of the stratigraphy of theTetori Group, adjacent to Lake Kuzuryu, Fukui Prefecture,
Central Japan. Memoir of the Fukui Prefectural Dinosaur
Museum 1: 41‒53.
Fujita, M. 2003. Geological age and correlation of thevertebrate-bearing horizons in the Tetori Group. Memoir of
the Fukui Prefectural Dinosaur Museum 2: 3‒14.
Fujita, M., T. Miyamoto and H. Tanaka. 1998. Inoceramus fromthe Itoshiro Subgroup of the Tetori Group in the east of Izumi,
Fukui Prefecture, central Japan, and its significance. Journalof the Geological Society of Japan 104: 52‒55.**
Goto, M. 2007. An Early Cretaceous ammonoid from the
Itoshiro Subgroup of the Tetori Group in the Uchinami River
area of Ohno City, Fukui Prefecture, Central Japan.
Memoirs of Fukui Prefectural Dinosaur Museum 6: 27‒34.*
Hirata, T., and R. W. Nesbitt. 1994. Uranium-lead geochronologyof zircon by laser probe-inductively coupled plasma mass
spectrometry. Chishitsu News 482: 53–65.**
Iizuka, T., T. Hirata, T. Komiya, S. Rino, I. Katayama, A. Motoki
and S. Maruyama. 2005. U-Pb and Lu-Hf isotope systematics
of zircons from the Mississippi River sand: Implications forreworking and growth of continental crust. Geology 33: 485–
488.
International Commission on Stratigraphy (ICS). 2010.
International stratigraphic chart.
Jackson, S. E., N. J. Pearson, W. I. Griffin and E. A. Belousova.
2004. The application of laser ablation-inductively
coupled plasma-mass spectrometry to in situ U-Pb zircon

NEW AGE DATA FROM THE TETORI GROUP

geochronology. Chemical Geology 211: 47–69.
Kaneoka, I. 1998. Numerical Dating: From a Decade to 46
Hundred Million Years Ago. University of Tokyo Press,
Tokyo, 315 pp.**
Kanisawa, S. 1974. Granitic rocks closely associated with theLower Cretaceous volcanic rocks in the Kitakami Mountains,Northeast Japan. Journal of the Geological Society of Japan
80: 355–367.
Kawai, M., K. Hirayama and N. Yamada. 1957. Geological Sheet
Map “Arashimadake”, Scale 1: 50,000, and its Explanatory
Text. Geological Survey of Japan, 110 pp.*
Kim, Y.，Y. I. Lee and K. Hisada. 2007. Provenance ofquartzarenite clasts in the Tetori Group (Middle Jurassic to
Early Cretaceous), Japan: Paleogeographic implications.
Journal of Asian Earth Sciences 29: 116–126.
Kimura, T. 1987. Chapter 6: Geographical distribution of
Palaeozoic and Mesozoic plants in East and Southeast Asia;
pp. 135–200 in A. Taira and M. Tashiro (eds.), Historical
Biogeography and Plate Tectonic Evolution of Japan andEastern Asia. TERRAPUB, Tokyo.
Kobayashi, T. 1956. Some Jurassic trigonians from central andwest Japan. Studies on Jurassic trigonians in Japan. Part V.
Japanese Journal of Geology and Geography 27: 1–8.
Kosler, J., and P. J. Sylvester. 2003. Present trends and thefuture of zircon in geochronology: Laser ablation ICPMS;
pp. 243–275 in J. M. Hanchar and P. W. O. Hoskin (eds.),
Zircon, Reviews in Mineralogy and Geochemistry 53.
Mineralogical Society of America, Washington, DC.
Kozai, T., K. Ishida, S.-O. Park and K.-H. Chang. 2002. Early
Cretaceous non-marine bivalves from Korea and Japan.
Abstracts with Programs, The 2002 Annual Meeting, The
Palaeontological Society of Japan: 16–17.
Kozai, T., K. Ishida, F. Hirsch, S.-O. Park and K.-H. Chang.
2005. Early Cretaceous non-marine mollusc faunas of Japanand Korea. Cretaceous Research 26: 97–112.
Kunugiza, K., and K. Kaneko. 2001. Metamorphic rocks andgranites of the eastern Hida Belt, Toyama Prefecture; pp.
137–156 in A. Ishiwatari (ed.), Excursion Guidebook of the108th Annual Meeting of the Geological Society of Japan.
Geological Society of Japan.**Kusuhashi, N., A. Matsumoto, M. Murakami, T. Tagami, T.
Hirata, T. Iizuka, T. Handa and H. Matsuoka. 2006. Zircon
U-Pb ages from tuff beds of the upper Mesozoic Tetori Groupin the Shokawa district, Gifu Prefecture, central Japan. Island
Arc 15: 378–390.
Lee, Y. I., T. Choi, H. S. Lim and Y. Orihashi. 2010. Detrital
zircon geochronology of the Cretaceous Sindong Group,
Southeast Korea: Implications for depositional age and Early
Cretaceous igneous activity. Island Arc 19: 647–658.
Ludwig, K.R. 2008. Isoplot 3.70:Geochronological Toolkit
forMicrosoft Excel. Berkeley Geochronology Center Special
Publication 4, 77 pp.
Maeda, S. 1952. A stratigraphical study on the Tetori series of17

the upper Kuzuryu district in Fukui Prefecture. Journal of theGeological Society of Japan 58: 401‒410.*
Maeda, S. 1957. Stratigraphy and geological structure of theTetori Group along the Uchinami and Itoshiro rivers, Fukui
Prefecture. Journal of the Geological Society of Japan
63: 357‒365.*
Maeda, S. 1961a. Stratigraphy of the Tetori Group in thesouthern part of the Kuzuryu River, Fukui Prefecture. Journalof the Geological Society of Japan 67: 23‒31.*
Maeda, S. 1961b. On the geological history of the Mesozoic
Tetori Group in Japan. Journal of the College of Arts andScience, Chiba University 3: 369‒426.*
Matsukawa, M., and T. Asahara. 2010. Stratigraphy of the Tetori
Group in the Uchinamigawa District, border area of Ishikawa
and Fukui prefectures, central Japan, and correlation between
its Hakusan Area and Kuzuryugawa Region. Bulletin of Tokyo
Gakugei University, Division Natural Science 62: 119‒130.*
Matsukawa, M., M. Fukui, K. Koarai, T. Asakura and H. Aono.
2007. Discovery of a third marine transgression in the Tetori
Group based on the restudy of stratigraphy of the group in
Hida-Furukawa region, Gifu Prefecture, Japan. Journal of theGeological Society of Japan 113: 417‒437.*
Matsukawa, M., M. Ito, N. Nishida, K. Koarai and M. G.
Lockley. 2008. Reply to the discussion of Sano et al.
Cretaceous Research 29: 174‒181.
Matsukawa, M., M. Ito, N. Nishida, K. Koarai, M. G. Lockley
and D. J. Nichols. 2006. The Cretaceous Tetori biota in Japanand its evolutionary significance for terrestrial ecosystems in
Asia. Cretaceous Research 27: 199‒225.
Matsukawa, M., N. Nishida, K. Koarai, K. Hayashi, H. Aono
and M. Ito. 2003. Stratigraphy of the Tetori Group in theeastern part of the Kuzuryugawa area, Fukui Prefecture,
Japan and correlation with its main part in the vicinity of
Mount Hakusan, central Japan. Bulletin of Tokyo Gakugei
University, Section 4, 55: 191‒200.*
Oishi, S. 1933. On the Tetori Series, with special references to
its fossil zones. Part 1. Journal of the Geological Society ofJapan 40: 617‒644.**
Orihashi, Y., S. Nakai and T. Hirata. 2008. U-Pb age
determination for seven standard zircons using inductively
coupled plasma-mass spectrometry coupled with frequency
quintupled Nd-YAG (λ = 213 nm) laser ablation system:
Comparison with LA-ICP-MS zircon analyses with a NIST
glass reference material. Resource Geology 58: 101–123 (doi:
10.1111/j.1751-3928.2008.00052.x)
Otoh, S., M. Shimojo, K. Aoki, T. Nakama, S. Maruyama andS. Yanai. 2010. Age distribution of detrital zircons in the
psammitic schist of the Sanbagawa Belt, Southwest Japan.
Journal of Geography 119: 333‒346.*
Rino, S., T. Komiya, B. F. Windley, I. Katayama, A. Motoki,
and T. Hirata. 2004. Major episodic increases of continental
crustal growth determined from zircon ages of river sands:
Implications for mantle overturns in the Early Precambrian.

18

YUTA KAWAGOE, SHIN-ICHI SANO, YUJI ORIHASHI, HOKUTO OBARA, YOSHIKAZU KOUCHI AND SHIGERU OTOH

Physics of Earth and Planetary Interiors 146: 369–394.
Rino, S., Y. Kon, W. Sato, S. Maruyama, M. Santosh and D.
Zhao. 2008. The Grenvillian and Pan-African orogens:
World’s largest orogenies through geologic time, and theirimplications on the origin of superplume. Gondwana Research
14: 51‒72.
Rubatto, D., and J. Hermann. 2003. Zircon formation during fluid
circulation in eclogites (Monviso, Western Alps): Implications
for Zr and Hf budget in subduction zones. Geochimica et
Cosmochimica Acta 67: 2173–2187.
Sagong, H., S.-T. Kwon and J.-H. Ree. 2005. Mesozoic episodic
magmatism in South Korea and its tectonic implication.
Tectonics 24: TC5002 (doi: 10.1029/2004TC001720)
Sano, S., K. Kubota and A. Yabe. 2008. “The Cretaceous Tetori
biota in Japan and its evolutionary significance for terrestrial
ecosystems in Asia”［Cretaceous Research 27 (2006) 199–
225］— Discussion. Cretaceous Research 29: 168–173.
Sato, T. 2008. Synoptic list of ammonites from the Tetori Group.
Fukada Geological Institute, Annual Report 9: 79‒107.
Sato, T., and G. E. G. Westermann. 1991. Japan and SouthEast Asia; pp. 81–108 in G. E. G. Westermann and A. C.
Riccardi (eds.), Jurassic Taxa Ranges and Correlation Charts
for the Circum Pacific 4, Newsletters on Stratigraphy 24.
Schweizerbart Science Publishers, Stuttgart.
Sato, T., and T. Yamada. 2005. Early Tithonian (Late Jurassic)
ammonite Parapallasiceras newly discovered from the
Itoshiro Subgroup (Tetori Group) in the Hida Belt, northern
Central Japan. Proceedings of the Japan Academy 81 (SeriesB): 267–272.
Shimojo, M., S. Otoh, S. Yanai, T. Hirata and S. Maruyama.
2010. LA-ICP-MS U-Pb age of some older rocks of theSouth Kitakami Belt, Northeast Japan. Journal of Geography
119: 257‒269.*
Tsujimori, T. 1995. Staurolite-bearing sillimanite schist cobble
from the Upper Jurassic Tetori Group in the Kuzuryu area,

Hida Mountains, central Japan. Journal of the GeologicalSociety of Japan 101: 971–977.
Tsutsumi, Y., K. Yokoyama, K. Terada and Y. Sano. 2000.
SHRIMP U-Pb dating of zircons in the sedimentary rocks
from the Akiyoshi and Suo zones, Southwest Japan. Journalof Mineralogical and Petrological Sciences 95: 216–227.
Wiedenbeck, M., J. M. Hanchar, W. H. Peck, P. Sylvester, J.
Valley, M. Whitehouse, A. Kronz, Y. Morishita, L. Nasdala, J.
Fiebig, I. Franchi, J.-P. Girard, R. C. Greenwood, R. Hinton,
N. Kita, P. R. D. Mason, M. Norman, M. Ogasawara, P. M.
Piccoli, D. Rhede, H. Satoh, B. Schulz-Dobrick, O. Skar, M.
J. Spicuzza, K. Terada, A. Tindle, S. Togashi, T. Vennemann,
Q. Xie and T.-F. Zheng. 2004. Further characterisation of
the 91500 zircon crystal. Geostandards and Geoanalytical
Research 28: 9–39.
Williams, I. S . 1998. U-Th-Pb geochronology by ion
microprobe; pp. 1–35 in M. A. McKibben, W. C. Shanks
and W. I. Ridley (eds.), Applications of Microanalytical
Techniques to Understanding Mineralizing Processes,
Reviews in Economic Geology 7. Society of Economic
Geologists, Littleton.
Yamada, K., S. Niwa and M. Kamata. 1989. Lithostratigraphyof the Mesozoic Tetori Group in the upper reaches of theKuzuryu River, central Japan. Journal of the GeologicalSociety of Japan 95: 391‒403.*
Yamada, T., and K. Uemura. 2008. The plant fossils fromthe Kaizara Formation (Callovian, Jurassic) of the Tetori
Group in the Izumi district, Fukui Prefecture, Central Japan.
Paleontological Research 12: 1–17.
Zhao, G., M. Sun, S. A. Wilde and S. Li. 2005. Late Archean to
Paleoproterozoic evolution of the North China Craton: Key
issues revisited. Precambrian Research 136: 177–202.
* : in Japanese with English abstract**: in Japanese＜ 地名・地層名 ＞
Akaiwa Subgroup
Amagodani Formation
Ashidani Formation
Fujikuradani Formation
Fukui Prefecture
Hakusan Region
Hida Belt
Hida Gaien Belt
Ise Metamorphic Rocks
Itoshiro
Itoshiro Subgroup
Itoshirogawa River
Itsuki Formation
Jinzu Region
Kaizara Formation
Kaizara Shale

赤岩亜層群
アマゴ谷層
葦谷層
藤倉谷層
福井県
白山区
飛騨帯
飛騨外縁帯
伊勢変成岩類
石徹白
石徹白亜層群
石徹白川
伊月層
神通区
貝皿層
貝皿頁岩層

Kamihambara
Kamihambara Formation
Konogidani Formation
Kuzuryu Subgroup
Kuzuryugawa River
Lake Manahimeko
ManaManagawa River
Miyagawa River
Monobegawa Group
Motodo Formation
Nochino Formation
Northern Chichibu Belt
Obuchi Formation
Ono City
Otani Formation

上半原
上半原層
此木谷層
九頭竜亜層群
九頭竜川
麻那姫湖
真名
真名川
宮川
物部川層群
本戸層
後野層
北部秩父帯
大淵層
大野市
大谷層

Ryoseki Formation
Shibasudani Formation
Shikoku Island
Shogawa River
Shokawa
Takaharagawa River
Tandoguchi Gneiss
Tatsukawa
Tedorigawa River
Tetori Group
Tochimochiyama Formation
Uchinamigawa
Umagatani
Yambara Formation
Yambarazaka Formation

領石層
子馬巣谷層
四国
庄川
荘川
高原川
谷戸口片麻岩
立川
手取川
手取層群
栃餅山層
打波川
馬ヶ谷
山原層
山原坂層