ARTICLE

Memoir of the Fukui Prefectural Dinosaur Museum 15: 33–84 (2016)© by the Fukui Prefectural Dinosaur Museum

PROVENANCE ANALYSIS BASED ON DETRITAL-ZIRCON-AGE SPECTRAOF THE LOWER CRETACEOUS FORMATIONS IN THE RYOSEKI–MONOBE AREA,
OUTER ZONE OF SOUTHWEST JAPANTakuji IKEDA1, Takuya HARADA1, Yoshikazu KOUCHI1, Sachiko MORITA1,
Miwa YOKOGAWA1, Koshi YAMAMOTO2 and Shigeru OTOH1
Graduate School of Science and Engineering, University of Toyama, 3190 Gofuku, Toyama 930-8555, Japan
Graduate School of Environmental Studies, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8601, Japan1

2

ABSTRACT
Detrital-zircon-age spectra of the Lower Cretaceous Monobegawa and Nankai groups in the Ryoseki–Monobe area,
Southwest Japan, are studied to make a provenance analysis and evaluate previous tectonic models. We also studied,for reference, the U-Pb zircon age of igneous rock cobbles, the detrital-zircon-age spectra of sandstone cobbles, andthe detrital-zircon-age spectra of a sandstone samples of the Permian basement. The results are as follows. (1) The
Ryoseki Formation of the Monobegawa Group contains many Permian zircons that were presumably reworked from thePermian basement sandstone. (2) The two groups contain Early Cretaceous detrital zircons and igneous rock cobbles
of some 125 Ma, suggesting active acidic volcanism in the hinterland. (3) The Yunoki Formation of the Monobegawa
Group contains 18% of 460 – 400 Ma zircons. It is concluded that the hinterland of the two groups were the Zhejiang –Fujian provinces of South China, where 145 – 100 Ma and 460 – 400 Ma felsic igneous rocks are widely exposed.
Key words : detrital zircon, Monobegawa Group, Nankai Group, provenance analysis, sinistral strike-slip motion,
Southwest Japan
池田拓司・原田拓也・高地吉一・森田祥子・横川実和・山本鋼志・大藤 茂（2016）砕屑性ジルコン年代分布を用いた西南日本外帯領石―物部地域下部白亜系の後背地解析．福井県立恐竜博物館紀要 15：33–84．
西南日本外帯高知県領石 ‒ 物部地域に分布する，秩父累帯下部白亜系物部川層群及び南海層群砂岩の砕屑性ジルコンのウラン－鉛年代を検討した．また，礫岩中の砂岩礫・火成岩礫及び物部川層群基盤のペルム系砂岩のジルコン年代も検討した．結果と考察は以下の通りである．①物部川層群領石層砂岩はペルム紀ジルコンを多量に含み，これらは基盤のペルム系砂岩からリワークされたと見られる．②両層群は前期白亜紀の砕屑性ジルコンを含み，火成岩礫のジルコン年代も約 125 Maであることから，後背地では堆積と同時に活発な酸性火成活動が生じていた．③物部川層群柚ノ木層中部層砂岩は 460 ‒ 400 Maのジルコンを豊富に含む．
以上より，両層群は，145 ‒ 100 Ma及び 460 ‒ 400 Maの酸性火成岩が露出する南中国浙江省～福建省を後背地にしたと解釈した．

INTRODUCTION

bivalve fauna of the Nankai Group (Tethyan Fauna) indicatesa warmer environment than that of the Monobegawa Group(Northern-Tethyan Fauna) and that the Nankai Group formed tothe south of the Monobegawa Group. Since the Nankai Group
lies on the Pacific side of the Monobegawa Group, Tashiro(1985, 1986, 1994) presumed that an arc-subparallel sinistral
strike-slip fault lay between the two groups and had carried the
Nankai Group relatively northward by the Albian. Matsukawa
and Eto (1987), on the other hand, attributed the lithofacies andfaunal differences between the two groups in the Katsuura areaof Tokushima Prefecture, 80 km to the east of the study area, tothe differences in sedimentary environments and ocean currents

Lower Cretaceous ﬂuvial to shallow marine formations occur
in the Chichibu Composite Belt in the Outer Zone of Southwest
Japan. Tashiro (1985) and Tashiro and Ikeda (1987) divided themfrom lithofacies and faunal assemblages into the Monobegawa,
Nankai, and Pre-Sotoizuimi groups (Tashiro, 1985; Tashiro and
Ikeda, 1987). Tashiro (1985, 1986, 1994) suggested that theReceived August 11, 2016. Accepted November 24, 2016.
Corresponding author — Takuji IKEDA
E-mail: takujiikeda2000＊yahoo.co.jp
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T. IKEDA, T. HARADA, Y. KOUCHI, S. MORITA, M. YOKOGAWA, K. YAMAMOTO AND S. OTOH

FIGURE 1. Index map showing the distribution of the Lower Cretaceous strata of the Chichibu Composite Belt and the locations of the Monobe and Ryoseki–
Birafu areas drawn in Figs. 2 and 3. Modiﬁed after Yamakita (1998).

in a single sedimentary basin. They, however, also suggested a
possibility of the initial separation of the basin because of thedifference in clast composition of Hauterivian conglomerate in thetwo groups. Recent studies, reporting the presence of Tethyan–
North-Tethyan mixed fauna, tend to argue that the sinistral strikeslip model only cannot explain the faunal distribution in the
Monobegawa and Nankai groups (Kozai and Ishida, 2003; Kozai
et al., 2007; Terabe and Matsuoka, 2009).
The Cretaceous sinistral strike-slip faulting has likely affected
the distribution of Mesozoic plant fossils and major pre-Aptian
geologic units of Japan. Berriasian (?)–Barremian plant fossils
in the continental side of Japan (Tetori-type ﬂora) contain many
common taxa with those of Northeast China and southeast Siberia.
On the other hand, Oxfordian–Barremian plant fossils in the
Paciﬁc side of Japan (Ryoseki-type ﬂora) contain many common
taxa with those of South China and the Indochina and Malay
peninsulas (Kimura, 1987); the Ryoseki-type flora invaded into
the continental side of Japan in the Aptian (Yabe et al., 2003). The
ﬂoral contrast between the continental and Paciﬁc sides of Japan
can also be explained by an arc-parallel sinistral strike-slip fault in
between that carried the geologic units with the Ryoseki-type ﬂora
relatively northward (e.g., Otoh and Yanai, 1996). The sinistral
strike-slip faults can be responsible for the duplication of the pairsof pre-Aptian accretionary and non-accretionary geologic units inthe continental and Paciﬁc sides of Japan (e.g., Taira and Tashiro,
1987; Yamakita and Otoh, 2000a, b). Yamakita and Otoh (2000a,b) attributed the duplication of Permian accretionary complex(AC) in the Inner and Outer zones of Southwest Japan to the Late
Cretaceous sinistral strike-slip motion along the Median Tectonic
Line (MTL).
We have started provenance analysis of the Jurassic andCretaceous formations in Japan to validate the hypothesis of theCretaceous strike-slip faulting. In particular, detrital zircon age
spectra, which reﬂect the changes of provenance, paleogeography,
and tectonic setting, can be a powerful tool for evaluating largescale displacements caused by plate motion (e.g., Okawa etal., 2013). In this paper, we will present the detrital zircon age
spectra of sandstone samples from all the formations of the Lower
Cretaceous Monobegawa and Nankai groups in the Ryoseki–
Monobe area, Outer Zone of Southwest Japan (Fig. 1). We will
also provide the U-Pb zircon age of igneous rock cobbles andthe detrital zircon age spectra of sandstone cobbles from theconglomerate of the two groups. Moreover, we will present
the detrital zircon age spectra of two pre-Cretaceous sandstone
samples of the Chichibu Composite Belt for comparison. Finally,
we will compare the provenance transitions of the two groups andverify the sinistral strike-slip and other related models.
GEOLOGIC SETTINGThe Chichibu Composite Belt comprises three belts: theNorthern Chichibu, Kurosegawa, and Southern Chichibu belts
from the continental side (from present-day north to south;
Yamakita, 1998). The Northern Chichibu Belt consists ofa Jurassic AC, tectonically overlying Permian AC, and theunconformably covering Lower Cretaceous Monobegawa Group.
The Kurosegawa Belt consists of the following geologic bodies
that have been cut and fragmented by belt-subparallel faults:
pre-Jurassic metamorphic rocks, serpentinite, pre-Carboniferous
basement rocks, Permian (?) chaotic rocks, Permian–Jurassic
shallow marine beds, Jurassic chaotic rocks, and the Lower
Cretaceous Nankai Group partly covering the Upper Jurassic–
lowest Cretaceous Torinosu Group (Yamakita, 1998). The
Southern Chichibu Belt consists mainly of late Early Jurassic toFIGURE 2. Geologic map of the Monobe area showing the sampling locations. Modiﬁed after Tashiro and Kozai (1984). Gp.: Group, Fm.: Formation, Mb.: Member.

LOWER CRETACEOUS FORMATIONS IN THE RYOSEKI–MONOBE AREA
35

FIGURE 3. Geologic map of the Ryoseki–Birafu area showing the sampling locations. Modiﬁed after Kozai (2008). Gp.: Group, Fm.: Formation, Mb.: Member.

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T. IKEDA, T. HARADA, Y. KOUCHI, S. MORITA, M. YOKOGAWA, K. YAMAMOTO AND S. OTOH

LOWER CRETACEOUS FORMATIONS IN THE RYOSEKI–MONOBE AREA

earliest Cretaceous AC, partly covered with the Middle Jurassic
to earliest Cretaceous Naradani and Torinosu groups (Matsuoka etal., 1998).
The Monobegawa Group in the Ryoseki–Monobe area isin fault contact with or unconformably covers the Permian
Shirakidani Group (AC) of the Northern Chichibu Belt on thenorth (Ikuma, 1980). The group is in fault contact with thePermian Suita Group, Lower Cretaceous Nankai Group, andUpper Cretaceous Sotoizumi Group on the south (Kozai et al.,2006; Hada et al., 1982; Figs. 2 and 3). The Monobegawa Groupconsists of the Ryoseki (Hauterivian–lowest Barremian), Monobe(Barremian), Yunoki (Aptian–lower Albian), and Hibihara(Albian) formations in ascending order and is composed mainlyof siliciclastic rocks such as conglomerate, sandstone, andmudstone. Among the four formations, the Monobe Formation
was originally a member of the Ryoseki Formation; Tanaka et al.
(1984) separated the upper marine part of the Ryoseki Formationof the day as the Monobe Formation. The lower three formations
form a conformable sequence whereas the Hibihara Formation
likely covers the Yunoki Formation by disconformity (Tanaka etal., 1984; Tashiro, 1985; Fig. 4). Moreover, the Yunoki Formation
does not occur in the Birafu–Ryoseki area on the west of the
Kubokawa River (Kozai, 2008; Fig. 2). The Ryoseki Formation37

is likely of ﬂuvial to brackish deposits because of the abundanceof red beds in the lower part and the occurrence of the Ryosekitype ﬂora in the upper part (Tanaka et al., 1984). The Monobe to
Hibihara formations, on the other hand, are of brackish to marine
deposits substantiated by abundant fossil fauna (Tashiro, 1985;
Tanaka et al., 1984).
The Nankai Group in the study area consists of the Birafu(Oxfordian–Valanginian or Hauterivian), Funadani (upper
Hauterivian–Barremian), and Hagino (Aptian) formations,in ascending order, and is in fault contact with the Suita andMonobegawa groups and the Permian Yukigamine Formation(Kozai et al., 2006; Fig. 3). The Nankai Group is mostly ofbrackish to marine deposits (Kozai and Ishida, 2000; Morino,
1993; Tashiro, 1985). There are some conflicting views on theposition of the Birafu Formation. Morino et al. (1989) ﬁrst deﬁned
the formation as a siliciclastic-rock formation with some Torinosutype limestone bodies. He assigned the Birafu Formation to amember of the Nankai Group, for two reasons. First, the mudstone
in the formation contained late Valanginian to Barremianradiolarians and was significantly younger than the Torinosu
Group, despite the occurrence of the Torinosu-type limestone.
Secondly, the formation contained many bivalve species common
with those of the Nankai Group. Kozai et al. (2004), on the other

FIGURE 4. Stratigraphic correlation of the Upper Jurassic to Lower Cretaceous strata in the Ryoseki–Monobe area.

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T. IKEDA, T. HARADA, Y. KOUCHI, S. MORITA, M. YOKOGAWA, K. YAMAMOTO AND S. OTOH

hand, separated the Birafu Formation from the Nankai Group, forthree reasons. First, radiolarian and bivalve fossils indicated thatthe formation was of the Oxfordian to the early Valanginian andwas signiﬁcantly older than the Hauterivian?–Barremian Funadani
Formation (Kozai, 2008), which was the oldest formation of the

Nankai Group. Secondly, the Birafu Formation is in fault contact
with the Nankai Group. Finally, the southward-dipping and facing
Birafu Formation is discordant in geologic structure with the
Nankai Group on the north, forming a syncline. In this paper, weassign the Birafu Formation as a member of the Nankai Group

FIGURE 5. Clast compositions of the conglomerate samples from the Monobegawa Group published by Miyamoto and Nakai (1974) and Miyamoto (1980).
Fm.: Formation.

LOWER CRETACEOUS FORMATIONS IN THE RYOSEKI–MONOBE AREA

39

from the following reasons: (1) a sandstone sample from the
middle part of the Birafu Formation contains Valanginian zircons,
as will be described later, and is presumably of the Valanginianor younger age; and (2) the Birafu Formation lies adjacent to the
Nankai Group although a fault runs in between.
A REVIEW OF PETROGRAPHIC STUDIES OF
THE LOWER CRETACEOUS CLASTIC ROCKS IN THE
STUDY AREA
In this section, we will review petrographic studies of theLower Cretaceous clastic rocks in the Ryoseki–Monobe area.
Since there are differences in the distribution of each formation
among the previous studies, all the sample locations of theprevious studies have been plotted on the geologic map of Tashiro
and Kozai (1984) and used their formation names for description.
Clast composition of conglomerates of the Monobegawa Group
Miyamoto and Nakai (1974) and Miyamoto (1980) madeprecise petrographic studies of the Monobegawa Group in the
study area. According to the studies, the Monobegawa Group
contained various kinds of clasts such as igneous-, sedimentary-,and metamorphic-rock clasts. There was a tendency that
sedimentary-rock clasts prevailed in the lower part, whereas
felsic-to intermediate-igneous-rock clasts increased upwards (Fig.
5). Sedimentary-rock clasts consisted mainly of chert, sandstone,
and shale, among which chert clasts were most abundant. In someconglomerate horizons of the Ryoseki Formation, sandstone clasts
were exceptionally more than chert clasts. The average proportionof the sedimentary-rock clasts in the Ryoseki (R), Monobe (M),
Yunoki (Y), and Hibihara (H) formations was 72.8%, 59.3%,
43.1%, and 39.2%, respectively. On the other hand, the average
proportion of the felsic to intermediate igneous rock clasts inthe four formations was 12.5% (R), 23.1% (M), 37.1% (Y), and54.7% (H), in ascending order. The conglomerates also contained
small amounts of“schalstein”and metamorphic-rock clasts.
Sandstone modal composition of the Monobegawa Group
According to the classiﬁcation of Okada (1971), the sandstoneof the four formations of the Monobegawa Group was mostly
assigned to lithic wacke (R; Ryoseki Formation), lithic wacke(M; Monobe Formation), lithic to feldspathic wacke (Y; Yunoki
Formation), and feldspathic wacke (H; Hibihara Formation),in ascending order (Miyamoto and Nakai, 1974; Miyamoto,1980; Fig. 6). Only one sample from the Yunoki Formation andthree samples from the Hibihara Formation were assigned toarenite out of the 103 sandstone samples they studied. Thus, thegreywacke type sandstone, relatively rich in the matrix, represents
the Monobegawa Group. The average proportions of quartz,
feldspar, rock fragments, and the matrix in the four formations
were as follows. Quartz grains were 13.3% (R), 22.2% (M),22.2% (Y), and 19.1% (H); feldspar grains were 13.8% (R),20.8% (M), 28.8% (Y), and 37.1% (H); rock fragments were

FIGURE 6. A Q-F-R diagram showing the sandstone compositions of theMonobegawa and Nankai groups provided by Miyamoto and Nakai (1974),
Miyamoto (1980), and this study. Gp.: Group, Fm.: Formation, Q: quartz, F:
feldspar, R: rock fragments.

36.8% (R), 32.4% (M), 22.1% (Y), and 20.0% (H); and thematrix was 36.1% (R), 24.6% (M), 26.9% (Y), and 23.8% (H).
There are overall trends that the proportions of rock fragments
and the matrix decrease and feldspar grains increase upwards.
Miyamoto (1980) described species of rock fragments and their
proportions, along the Kubokawa River route, suggesting that theratio of sedimentary rock fragments decreased and volcanic rockfragments increased upwards (Fig. 7). The trend is concordant
with the upward transition of clast proportion in conglomerates.
Lithology of the clastic rocks of the Nankai Group
Morino (1993) described that the Birafu Formation consisted
mainly of mudstone and interbedded arkose sandstone andmudstone, with minor intercalations of the Torinosu-typelimestone, sandstone, conglomerate, and tuff. The clasts ofconglomerates were mostly pebble to granule size and consisted
chiefly of greenish gray chert, with minor amounts of reddishbrown chert and granitic rocks (Morino et al., 1989). The Funadani
Formation contained thick conglomerate beds having the clasts ofchert and sandstone, with minor amounts of tuffaceous sandstone
and limestone. The Hagino Formation was characterized by arepetitive occurrence of medium to ﬁne sandstone, rich in arenite,
and gray silty mudstone (Tashiro, 1993). Moreover, the formationin the Sano area (former Igenoki Formation; Kozai, 2008; Fig.
3) intercalated conglomerate with felsic igneous rock clasts inthe bed of the Monobegawa River (Kozai and Ishida, 2000). The
Nankai Group as a whole was characterized by the absence of red
clastic rocks and abundance of arenite (Tashiro, 1985). Moreover,
the occurrence of felsic igneous rock clasts in the conglomerate
and arkose sandstone from each formation indicated the exposureof felsic igneous bodies in the provenance.

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T. IKEDA, T. HARADA, Y. KOUCHI, S. MORITA, M. YOKOGAWA, K. YAMAMOTO AND S. OTOH

FIGURE 7. Compositions of rock fragments in sandstone samples of the Monobegawa Group published by Miyamoto and Nakai (1974) and Miyamoto (1980).
Fm.: Formation.

Paleocurrent and sediment transport
Miyamoto (1980) concluded from the slump structures andsole marks developed in the Hibihara Formation in the study areathat there was a land to the north (in the present coordinate) of thesedimentary basin and that the clastic sediments were supplied
from north to south. Miyamoto (1980) further proposed that Early
Cretaceous igneous rock bodies that were exposed along thesouthern boundary of the Ryoke Belt but were completely eroded
away supplied the igneous-rock clasts of the Monobegawa GroupLOWER CRETACEOUS FORMATIONS IN THE RYOSEKI–MONOBE AREA

41

FIGURE 8. Cathodoluminescence images of some zircons analyzed in this study. For grain numbers, see Table 1.

in the study area. Kozai and Ishida (2000), on the other hand,found an Early Devonian radiolarian Devoniglanssus unicus froma tuffaceous-sandstone clast of the Funadani Formation of the
Nankai Group. Since the radiolarian species had commonly beenreported from the tuffaceous sandstone in the G4 horizon of the
Yokokurayama Group (Wakamatsu et al., 1990), they suggested
that the clast was derived from the Siluro–Devonian of theKurosegawa Belt.
In summary, the source of sedimentary-rock clasts in theconglomerate and sandstone of the Monobegawa Group in the
study area is likely the pre-Cretaceous rocks of the Northern
Chichibu Belt. The reasons are as follows: (1) the Monobegawa
Group unconformably covers the Permian AC of the Northern
Chichibu Belt; (2) the pre-Cretaceous AC of the Northern Chichibu
Belt mainly occur on the north of the Monobegawa Group andsedimentary structures indicate the sediment supply from the north;and (3) the sediment source of the Monobegawa Group in the
Katsuura area has been proposed to have been the pre-Cretaceous
rocks of the Northern Chichibu Belt (Ishida and Hashimoto, 1997;
Matsukawa and Eto, 1987; Ogawa, 1971). A sediment source of
the Nankai Group was likely a geologic entity that now constitutes

the Kurosegawa Belt (Kozai and Ishida, 2000).
SAMPLE DESCRIPTIONS
We studied the following seventeen samples: (1) ﬁve sandstone
samples from the Monobegawa Group, (2) ﬁve sandstone samples
from the Nankai Group, (3) three sandstone cobbles from the
Ryoseki, Yunoki, and Funadani formations, (4) two igneousrock cobbles from the Yunoki and Hibihara formations, and (5)two sandstone samples from the Shingai Unit and ShirakidaniGroup, members of the Permian AC of the Northern Chichibu
Belt. Here follows the description of the samples studied. The
modal composition of a sandstone sample was measured froma thin-section using a petrological microscope and an automatic
point counter. We counted ﬁve hundred points for each sandstone
sample (three hundred points for each sandstone cobble) andcalculated the percentages of (1) single quartz, (2) single feldspar,(3) the rock fragment, including chert fragments and mineral
grains other than quartz and feldspar, and (4) matrix.

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T. IKEDA, T. HARADA, Y. KOUCHI, S. MORITA, M. YOKOGAWA, K. YAMAMOTO AND S. OTOH

Monobegawa Group
Sandstone of the Ryoseki Formation (Sample Ry-1; 33˚ 37'
11.67" N, 133˚ 36' 39.87" E)
Sample Ry-1 of the Ryoseki Formation was collected from an
exposure of the lowermost part of the formation on the northernside of Kochi Prefectural Road 269 along the Kasanokawa River(Fig. 3). The sample was of non-marine, red, ill-sorted, angular to
sub-rounded, medium- to coarse-grained lithic wacke in the senseof Okada (1971), consisting of quartz (12.2%), feldspar (7.8%),
rock fragments (37.2%), and matrix (42.8%). The rock fragments
were mostly of sedimentary and metamorphic rocks with volcanic
rocks and chert. The metamorphic rock fragments were mostlyof an aggregate of elongated quartz grains with sutured grainboundaries and the longest dimensions arranging subparallel toeach other (sheared quartz or stretched metamorphic quartz).
Among the zircon grains we collected, 43% were euhedral, and
57% were abraded. They were mostly colorless, and only 3%
were purple. The euhedral zircons had the aspect ratio of 1.2–3.0(2.0 in average). Most of the collected zircons showed oscillatory
zoning in CL images (Fig. 8).
Sandstone of the Monobe Formation (Sample Mo-1; 33˚ 42'
51.15" N, 133˚ 50' 05.49" E)
Sample Mo-1 of the Monobe Formation was collectedfrom an exposure of the lower part of the formation along the
Hibiharagawa River (Fig. 2). The sample was of light-gray,moderately- to well-sorted, sub-angular to rounded feldspathic
arenite, consisting of quartz (48.1%), feldspar (20.3%), rockfragments (18.6%), and matrix (13.0%). Among the zircon
grains we collected, 62% were euhedral, and 38% were abraded.
They were mostly colorless, and 6% were purple or brown. The
euhedral zircons had the aspect ratio of 1.0–3.3 (2.0 in average).
Most of the collected zircons showed oscillatory zoning in CL
images (Fig. 8).
Sandstone of the lower part of the Yunoki Formation(Sample Yu-1; 33˚ 42' 31.17" N, 133˚ 50' 01.71" E)
Sample Yu-1 of the Yunoki Formation was collected from an
exposure of the Lower Marine Member (Tanaka et al., 1984)
along the Hibiharagawa River (Fig. 2). The sample was of lightgray, moderately-sorted, sub-angular to sub-rounded mediumgrained feldspathic arenite, consisting of quartz (38.0%), feldspar(38.8%), rock fragments (15.2%), and matrix (8.0%). Among thezircon grains we collected, 65% were euhedral, and 35% wereabraded. They were mostly colorless, and 5% were purple. The
euhedral zircons had the aspect ratio of 1.5–2.8 (2.1 in average).
Most of the collected zircons showed oscillatory zoning in CL
images (Fig. 8).
Sandstone of the middle part of the Yunoki Formation(Sample Yu-2; 33˚ 42' 23.83" N, 133˚ 50' 13.85" E)
Sample Yu-2 of the Yunoki Formation was collected froman exposure of the Middle Non-marine Member (Tanaka et al.,1984) along the Hibiharagawa River (Fig. 2). The sample was ofdark-gray, ill-sorted, sub-angular to sub-rounded, fine- to veryfine-grained feldspathic wacke with abundant carbonized plantfragments. The sample consisted of quartz (23.2%), feldspar(18.2%), rock fragments (11.7%), and matrix (47.0%). Amongthe zircon grains we collected, 68% were euhedral, and 32% wereabraded. They were mostly colorless, and 4% were purple. The
euhedral zircons had the aspect ratio of 1.4–3.0 (2.1 in average).
Most of the collected zircons showed oscillatory zoning in CL
images (Fig. 8).
Sandstone of the Hibihara Formation (Sample Hi-1; 33˚ 42'
15.36" N, 133˚ 50' 34.28" E)
Sample Hi-1 of the Hibihara Formation was collected froman exposure of the Middle Marine Member (Tanaka et al., 1984)
along the Hibiharagawa River (Fig. 2). The sample was of darkgray, ill- to moderately-sorted, sub-angular to sub-rounded, ﬁneto very fine-grained feldspathic wacke, consisting of quartz(17.6%), feldspar (20.6%), rock fragments (13.0%), and matrix(48.8%). Among the zircon grains we collected, 39% were
euhedral, and 61% were abraded. They were mostly colorless,and 3% were purple. The euhedral zircons had the aspect ratio of
1.3–2.8 (1.9 in average). Most of the collected zircons showedoscillatory zoning in CL images (Fig. 8).
Nankai Group
Sandstone of the lower part of the Birafu Formation(Sample Bi-1; 33˚ 38' 55.27" N, 133˚ 47' 52.38" E)
Sample Bi-1 of the Birafu Formation was collected from an
exposure of the A2 Member (Kozai et al., 2004, 2006) along the
Nishinokawa River (Fig. 3). The sample was of gray, moderatelyto well-sorted, sub-angular to rounded, very fine-grained
feldspathic arenite, consisting of quartz (43.0%), feldspar (42.6%),
rock fragments (4.8%), and matrix (9.6%). Among the zircon
grains we collected, 67% were euhedral, and 33% were abraded.
They were mostly colorless, and only 1% were purple. The
euhedral zircons had the aspect ratio of 1.5–2.7 (2.1 in average).
Most of the collected zircons showed oscillatory zoning in CL
images (Fig. 8).
Sandstone of the middle part of the Birafu Formation(Sample Bi-2; 33˚ 38' 55.66" N, 133˚ 48' 07.93" E)
Sample Bi-2 of the Birafu Formation was collected from an
exposure of the B1 or B2 Member on the eastern side of Kochi
Prefectural Road 30 along the Nishinokawa River (Fig. 3). The
sample was of white, ill- to moderately-sorted, angular to rounded,coarse-grained feldspathic arenite, consisting of quartz (70.9%),
feldspar (20.2%), rock fragments (8.7%), and matrix (0.2%).
Among the zircon grains we collected, 62% were euhedral, and
38% were abraded. They were mostly colorless, and 7% were
purple or brown. The euhedral zircons had the aspect ratio of
1.6–4.0 (2.1 in average). Most of the collected zircons showedoscillatory zoning in CL images (Fig. 8).
Sandstone of the Funadani Formation (Sample Fu-1; 33˚
37' 22.79" N, 133˚ 41' 59.69" E)
Sample Fu-1 of the Funadani Formation was collected froman exposure near the Meoto Pond (Fig. 3). The sample was ofweathered, brown, moderately-sorted, sub-angular to rounded,ﬁne- to medium-grained lithic wacke, consisting of quartz (22.4%),
feldspar (24.4%), rock fragments (32.3%), and matrix (20.9%).

LOWER CRETACEOUS FORMATIONS IN THE RYOSEKI–MONOBE AREA

Among the zircon grains we collected, 65% were euhedral, and
35% were abraded. They were mostly colorless, and 8% were
purple or brown. The euhedral zircons had the aspect ratio of
1.6–3.3 (2.1 in average). Most of the collected zircons showedoscillatory zoning in CL images (Fig. 8).
Sandstone of the Hagino Formation from Sano (Sample
Ha-1; 33˚ 37' 16.89" N, 133˚ 42' 42.87" E)
Sample Ha-1 of the Hagino Formation was collected from an
exposure in Sano (Fig. 3). The sample was of weathered, brown,
well-sorted, sub-angular to rounded, fine- to very fine-grained
feldspathic wacke, consisting of quartz (22.0%), feldspar (26.2%),
rock fragments (25.4%), and matrix (26.4%). Among the zircon
grains we collected, 61% were euhedral, and 39% were abraded.
They were mostly colorless, and 3% were brown. The euhedralzircons had the aspect ratio of 1.4–5.0 (2.4 in average). Most of thecollected zircons showed oscillatory zoning in CL images (Fig. 8).
Sandstone of the Hagino Formation from Hagino (Sample
Ha-2; 33˚ 38' 03.12" N, 133˚ 45' 25.74" E)
Sample Ha-2 of the Hagino Formation was collected from an
exposure along the Haginogawa River in Hagino (Fig. 3). The
sample was of light-gray, moderately- to well-sorted, angular torounded, medium- to coarse-grained lithic arenite, consisting ofquartz (60.8%), feldspar (14.8%), rock fragments (24.2%), andmatrix (0.2%). Among the zircon grains we collected, 60% were
euhedral, and 40% were abraded. They were mostly colorless, and
5% were purple or brown. The euhedral zircons had the aspectratio of 1.4–3.4 (2.0 in average). Most of the collected zircons
showed oscillatory zoning in CL images (Fig. 8).
Cobbles from conglomerate
A sandstone cobble from the Ryoseki Formation, Monobegawa
Group (Sample RySs-1; 33˚ 38' 17.23" N, 133˚ 45' 23.01" E)
S a m p l e Ry S s - 1 w a s c o l l e c t e d f r o m a n e x p o s u r e o fconglomerate of the Ryoseki Formation, Monobegawa Group,
along the Haginogawa River (Fig. 3). The sample was of asubangular cobble of dark-gray, ill-sorted, angular to sub-rounded,coarse-grained lithic arenite, consisting of quartz (38%), feldspar(13.7%), rock fragments (37%), and matrix (11.3%). Among thezircon grains we collected, 53% were euhedral, and 47% wereabraded. They were mostly colorless. The euhedral zircons had
the aspect ratio of 1.3–2.7 (2.0 in average). Most of the collectedzircons showed oscillatory zoning in CL images.
A sandstone cobble from the Yunoki Formation, Monobegawa
Group (Sample YuSs-1; 33˚ 42' 43.20" N, 133˚ 50' 06.09" E)
S a m p l e Yu S s - 1 w a s c o l l e c t e d f r o m a n e x p o s u r e o fconglomerate of the middle part of the Yunoki Formation,
Monobegawa Group, along the Hibiharagawa River (Fig. 2).
The sample was of a round cobble of dark-gray, moderatelysorted, sub-angular to sub-rounded, medium- to coarse-grained
feldspathic arenite, consisting of quartz (37.3%), feldspar (31.3%),
rock fragments (25.3%), and matrix (6%). Among the zircon
grains we collected, 56% were euhedral, and 44% were abraded.
They were mostly colorless, and 8% were purple. The euhedralzircons had the aspect ratio of 1.1–2.6 (1.9 in average). Most of43

the collected zircons showed oscillatory zoning in CL images.
A sandstone cobble from the Funadani Formation, NankaiGroup (Sample FuSs-1; 33˚ 37' 21.28" N, 133˚ 42' 02.22" E)
Sample FuSs-1 was collected from an exposure ofconglomerate of the Funadani Formation, Nankai Group, near
the Meoto Pond (Fig. 3). The sample was of a subround cobble
of dark-gray, ill-sorted, angular to sub-rounded, medium-grained
feldspathic arenite, consisting of quartz (36.8%), feldspar (43.4%),
rock fragments (10.6%), and matrix (9.3%). Among the zircon
grains we collected, 68% were euhedral, and 32% were abraded.
They were mostly colorless, and only 1% were purple. The
euhedral zircons had the aspect ratio of 1.4–2.8 (1.7 in average).
Most of the collected zircons showed oscillatory zoning in CL
images.
A rhyolite cobble from the Yunoki Formation, Monobegawa
Group (Sample YuIg-1; 33˚ 42' 43.20" N, 133˚ 50' 06.09" E)
Sample YuIg-1 was collected from the same exposure withsample YuSs-1 (Fig. 2). The sample was of a round cobble ofrhyolite with quartz and plagioclase phenocrysts floated in agroundmass (Fig. 9a-c). Among the zircon grains we collected,
78% were euhedral, and 22% were abraded. They were mostly
colorless, and 5% were purple or brown. The euhedral zircons had
the aspect ratio of 1.5–3.2 (2.2 in average). Most of the collectedzircons showed oscillatory zoning in CL images (Fig. 8).
A granodiorite cobble from the Hibihara Formation, Monobegawa
Group (Sample HiIg-1; 33˚ 39' 34.69" N, 133˚ 46' 01.87" E)
Sample HiIg-1 was collected from an exposure of conglomerateof the Hibihara Formation, Monobegawa Group, along a northernbranch of the Monobegawa River (Fig. 3). The sample was of around cobble of granodiorite with quartz, plagioclase, and minor
chlorite (Fig. 9d-f). Among the zircon grains we collected, 92%
were euhedral, and 8% were abraded. They were mostly colorless,and 3% were brown. The euhedral zircons had the aspect ratio of
1.3–3.6 (2.1 in average). Most of the collected zircons showedoscillatory zoning in CL images (Fig. 8).
Pre-Cretaceous sandstone of the Northern Chichibu Belt
Sandstone of the Shingai Unit (Sample 13072103; 33˚ 36'
16.44" N, 133˚ 28' 22.83" E)
The Shingai Unit is a geologic unit of the Permian AC of theNorthern Chichibu Belt, correlative to the Shirakidani Group in
the Ryoseki area (Fig. 3). The Shingai Unit consists mainly ofsandstone, mudstone, chert, basalt, and limestone (Wakita et al.,2007). Sample 13072103 was collected from an exposure along
the Kochi Prefectural Road 33 at Kagamiimai, Kochi City. The
sample was of grayish-green, moderately sorted, ﬁne- to mediumgrained lithic arenite, consisting of quartz (32.9%), feldspar(14.2%), rock fragments (52.9%), and matrix (12.8%). The rockfragments were mostly of volcanic rocks. Among the zircon grains
we collected, 56% were euhedral, and 44% were abraded. They
were mostly colorless, and 2% were purple or brown. Most of thecollected zircons showed oscillatory zoning in CL images.

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T. IKEDA, T. HARADA, Y. KOUCHI, S. MORITA, M. YOKOGAWA, K. YAMAMOTO AND S. OTOH

FIGURE 9. Photographs of studied igneous-rock clasts. a–c, A rhyolite clast from the Yunoki Formation (sample YuIg-1). a, A hand specimen (Scale bar = 1.0cm); b, A photomicrograph (Open nicol; Scale bar = 0.2 mm); c, A photomicrograph (Crossed nicols; Scale bar = 0.2 mm.). d–f, A granodiorite clast from the
Hibihara Formation (HiIg-1). d, A hand specimen (Scale bar = 1.0 cm); e, A photomicrograph (Open nicol; Scale bar = 0.2 mm); f, A photomicrograph (Crossed
nicols; Scale bar = 0.2 mm). Qtz: quartz, Pl: plagioclase.

LOWER CRETACEOUS FORMATIONS IN THE RYOSEKI–MONOBE AREA

45

FIGURE 10. Analytical data of detrital zircons from sandstone of the Ryoseki Formation (sample Ry-1). a, Concordia diagram for all data; b, Concordia
diagram for a younger data set; c, Probability density plot with a histogram for all data of concordant grains; d, Probability density plot with a histogram for the
0–500 Ma data set. Open (light blue) circles in the concordia diagrams from Fig. 10 to Fig. 26 show the analytical data for discordant grains. N: total number,%Pc: percentage of Precambrian zircons.

FIGURE 11. Analytical data of detrital zircons from sandstone of the Monobe Formation (sample Mo-1). a, Concordia diagram for all data; b, Concordia
diagram for a younger data set; c, Probability density plot with a histogram for all data of concordant grains; d, Probability density plot with a histogram for the
0–500 Ma data set. N: total number, %Pc: percentage of Precambrian zircons.

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T. IKEDA, T. HARADA, Y. KOUCHI, S. MORITA, M. YOKOGAWA, K. YAMAMOTO AND S. OTOH

Sandstone of the Shirakidani Group (Sample 16091701; 33˚
37' 16.18" N, 133˚ 35' 47.18" E)
The Shirakidani Group is a geologic unit of the Permian ACof the Northern Chichibu Belt in the Ryoseki area (Fig. 3). The
Shirakidani Group consists mainly of sandy mudstone, sandstone,
and mudstone, with blocks (olistoliths; Suyari et al., 1983) ofchert, limestone, and basaltic tuff and lava. Sample 16091701
of the Shirakidani Group was collected from an exposure on
the northern side of Kochi Prefectural Road 269 along theKasanokawa River (Fig. 3). The sample was of gray, moderatelyto ill-sorted, sub-angular to rounded, medium- to coarse-grained
lithic arenite, consisting of quartz (18%), feldspar (24.4%), rockfragments (51.2%), and matrix (6.4%). The rock fragments were
mostly of volcanic rocks. Among the zircon grains we collected,
50% were euhedral, and 50% were abraded. They were mostly
colorless, 2% were purple, and 5% were brown. The euhedralzircons had the aspect ratio of 1.4–4.3 (2.2 in average). Most of
the collected zircons showed oscillatory zoning in CL images.
ANALYTICAL METHOD
The zircon samples for analyses were prepared following theprocedures described in Kawagoe et al. (2012). The measurement
was carried out on laser ablation inductively coupled plasma mass
spectrometers (LA-ICPMS) equipped in the Graduate Schoolof Environmental Studies, Nagoya University. The ICPMS
instrument was an Agilent 7700x quadrupole-based ICPMS
connected with a New Wave Research NWR-213-type LA
system, which used the frequency quintupled Nd-YAG 213-nm
wavelength. The measurement conditions, optimized to reduce
matrix effects, were as follows: energy density of 11.7 J/cm -2,
pulse repetition rate of 10 Hz, pre-ablation time of 8 s, ablation
time of 10 s, and the ablation pit size of 25 µm (Kouchi et al.,2015). The analyses were carried out in a peak-jumping mode,and the peaks of 202Hg, 204(Hg+Pb), 206Pb, 207Pb, 232Th, and 238U
were monitored. Data were acquired in sequences of 28 analyses,
consisting of 5 analyses of gas blank, 4 NIST (National Instituteof Standards and Technology, U.S.A.) SRM 610 glass standard,
one standard zircon (91500 zircon with the 206Pb/ 238U age of1062.4 ± 0.4 Ma; Wiedenbeck et al., 2004), nine unknown, 4
SRM 610 standard, and ﬁve gas blank.
RESULTSWe sampled an outer part (rim or mantle) of collected zircon
grains with the laser ablation technique and analyzed with the
ICPMS. After the analyses, we plotted all the data on a concordia
diagram. Then we chose concordant grains with the %conc value(100・(206Pb/238U age)/(207Pb/235U age)) between 90 and 110 anddrew a probability density plot and a histogram with the datainterval of 50 Myr ( 206Pb/ 238U age). Among the peaks in theprobability density plot that consist of two or more zircon data, theyoungest one will be designated as the youngest peak (YP) in thispaper. The data processing was carried out using the Isoplot 4.15
software (Ludwig, 2012). Here follow the results of our analyses.

Monobegawa Group
Sandstone of the Ryoseki Formation (Sample Ry-1)
We obtained 128 analyses from 128 zircon grains collectedfrom sandstone sample Ry-1 of the Ryoseki Formation. 118 grains
out of 128 gave concordant results (Fig. 10). We detected three
age groups and five single plots of concordant detrital zircons
on the concordia diagram: 205–298 Ma (105 grains; 89% of 118
concordant grains), 342–359 Ma (3%), 1635–1846 Ma (3%), 180
Ma (1%), 437 Ma (1%), 929 Ma (1%), 2318 Ma (1%), and 2552
Ma (1%) (Each age denotes the 206Pb/238U age at the centers of
the youngest and oldest age plots of each cluster or at the centerof each single plot.) (Fig. 10a, b). The percentage of Precambrianzircons (%Pc) was 6, and the 206Pb/238U age of the youngest
zircon (YZ) was 179.8 ± 4.8 Ma. The probability density plot for
the 0–500 Ma data set showed a single peak at 252 Ma (Fig. 10d).
Sandstone of the Monobe Formation (Sample Mo-1)
We obtained 117 analyses from 117 zircon grains collectedfrom sandstone sample Mo-1 of the Monobe Formation. 87 grains
out of 117 gave concordant results (Fig. 11). We detected six age
groups of concordant zircons: 161–306 Ma (60%), 127–147 Ma(14%), 1787–1973 Ma (13%), 2199–2566 Ma (7%), 425–466
Ma (3%), and 1471–1629 Ma (3%), with %Pc of 23 and the YZ
of 126.8 ± 4.2 Ma (Fig. 11a–c). The probability density plot for
the 0–500 Ma data set showed the largest peak at 174 Ma, second
largest peaks at 146 Ma and 190 Ma, and small peaks at 127 Ma,
132 Ma, 161 Ma, 211 Ma, 230 Ma, 244 Ma, 252 Ma, 267 Ma, and
290 Ma (Fig. 11d). The youngest peak (YP) was at 127 Ma, andthe concordant age of the three grains that constitute the 127 Ma
peak was 127.4 ± 2.9 Ma (MSWD = 0.14, Probability = 0.71).
Sandstone of the lower part of the Yunoki Formation(Sample Yu-1)
We obtained 131 analyses from 131 zircon grains collectedfrom sandstone sample Yu-1 of the lower part of the Yunoki
Formation. 92 grains out of 131 gave concordant results (Fig. 12).
We detected three age groups and two single plots of concordant
detrital zircons: 116–237 Ma (74%), 1580–2036 Ma (16%),
254–301 Ma (8%), around 2209 Ma (1%), and around 2961 Ma(1%), with %Pc of 19 and the YZ of 115.5 ± 2.7 Ma (Fig. 12a–c).
The probability density plot for the 0–500 Ma data set showed thelargest peaks at 173 Ma, 181 Ma, and 189 Ma, a second largest
peak at 131 Ma, and small peaks at 124 Ma, 221 Ma, 235 Ma, and
259 Ma (Fig. 12d). The YP was at 124 Ma, and the concordant
age of the two grains that constitute the 124 Ma peak was 124.3± 2.4 Ma (MSWD = 0.67, Probability = 0.41).
Sandstone of the middle part of the Yunoki Formation(Sample Yu-2)
We obtained 125 analyses from 125 zircon grains collectedfrom sandstone sample Yu-2 of the middle part of the Yunoki
Formation. 91 grains out of 125 gave concordant results (Fig. 13).
We detected four age groups and one single plot of concordant
detrital zircons: 107–318 Ma (65%), 379–466 Ma (20%), 1638–
1852 Ma (12%), 2067–2198 Ma (2%), and around 2407 Ma(1%), with %Pc of 15 and the YZ of 107.3 ± 3.1 Ma (Fig. 13a–c). The probability density plot for the 0–500 Ma data set showedLOWER CRETACEOUS FORMATIONS IN THE RYOSEKI–MONOBE AREA

47

FIGURE 12. Analytical data of detrital zircons from sandstone of the lower part of the Yunoki Formation (sample Yu-1). a, Concordia diagram for all data;b, Concordia diagram for a younger data set; c, Probability density plot with histogram for all data of concordant grains; d, Probability density plot with ahistogram for the 0–500 Ma data set. N: total number, %Pc: percentage of Precambrian zircons.

FIGURE 13. Analytical data of detrital zircons from sandstone of the middle part of the Yunoki Formation (sample Yu-2). a, Concordia diagram for all data;b, Concordia diagram for a younger data set; c, Probability density plot with histogram for all data of concordant grains; d, Probability density plot with ahistogram for the 0–500 Ma data set. N: total number, %Pc: percentage of Precambrian zircons.

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T. IKEDA, T. HARADA, Y. KOUCHI, S. MORITA, M. YOKOGAWA, K. YAMAMOTO AND S. OTOH

FIGURE 14. Analytical data of detrital zircons from sandstone of the Hibihara Formation (sample Hi-1). a, Concordia diagram for all data; b, Concordia
diagram for a younger data set; c, Probability density plot with a histogram for all data of concordant grains; d, Probability density plot with a histogram for the
0–500 Ma data set. N: total number, %Pc: percentage of Precambrian zircons.

the largest peaks at 117 Ma, 173 Ma, and 220 Ma, second largest
peaks at 274 Ma and 185 Ma, and small peaks at 108 Ma, 135
Ma, 147 Ma, 191 Ma, 243 Ma, 274 Ma, 424 Ma, and 447 Ma(Fig. 13d). The YP was at 108 Ma, and the concordant age of thetwo grains that constitute the 108 Ma peak was 108.2 ± 2.4 Ma(MSWD = 3.7, Probability = 0.053).
Sandstone of the Hibihara Formation (Sample Hi-1)
We obtained 154 analyses from 154 zircon grains collectedfrom sandstone sample Hi-1 of the Hibihara Formation. 91 grains
out of 154 gave concordant results (Fig. 14). We detected four age
groups and four single plots of concordant detrital zircons: 102–
204 Ma (78%), 1517–2009 Ma (9%), 264–285 Ma (7%), 226–233
Ma (2%), 391 Ma (1%), 454 Ma (1%), 572 Ma (1%), and 2251
Ma (1%), with %Pc of 11 and the YZ of 102.0 ± 4.8 Ma (Fig.
14a–c). The probability density plot for the 0–500 Ma data set
showed the largest and youngest peak at 115 Ma, a second largest
peak at 187 Ma, and small peaks at 145 Ma, 165 Ma, 179 Ma, and
269 Ma (Fig. 14d). The YP was at 115 Ma, and the concordant
age of the thirty grains that constitute the 115 Ma peak was 114.9± 0.8 Ma (MSWD = 5.9, Probability = 0.015).
Nankai Group
Sandstone of the lower part of the Birafu Formation(Sample Bi-1)
We obtained 128 analyses from 128 zircon grains collectedfrom sandstone sample Bi-1 of the lower part of the Birafu

Formation. 82 grains out of 128 gave concordant results (Fig.
15). We detected six age groups of concordant detrital zircons:
1489–2181 Ma (44%), 150–221 Ma (35%), 241–270 Ma (11%),
292–320 Ma (4%), 2413–2625 Ma (4%), and 397–420 Ma (3%),
with %Pc of 48 and the YZ of 149.9 ± 3.6 Ma (Fig. 15a–c). The
probability density plot for the 0–500 Ma data set showed thelargest peak at 179 Ma, second largest peaks at 188 Ma and 197
Ma, and small peaks at 169 Ma (YP), 214 Ma, 221 Ma, 241 Ma,
and 267 Ma (Fig. 15d).
Sandstone of the middle part of the Birafu Formation(Sample Bi-2)
We obtained 126 analyses from 126 zircon grains collectedfrom sandstone sample Bi-2 of the middle part of the Birafu
Formation. 104 grains out of 126 gave concordant results (Fig.
16). We detected six age groups of concordant detrital zircons:
128–255 Ma (66%), 1509–1980 Ma (23%), 290–317 Ma (3%),
2110–2205 Ma (3%), 2535–2605 Ma (3%), and 2390–2409 Ma(2%), with %Pc of 31 and the YZ of 128.1 ± 4.7 Ma (Fig. 16a–c). The probability density plot for the 0–500 Ma data set showed
the largest peak at 172 Ma, the second largest and youngest peakat 136 Ma, and small peaks at 151 Ma, 221 Ma, and 242 Ma (Fig.
16d). The eight grains that constitute the YP gave the concordant
age of 136.5 ± 2.5 Ma (MSWD = 0.65, Probability = 0.42).
Sandstone of the Funadani Formation (Sample Fu-1)
We obtained 123 analyses from 123 zircon grains collectedfrom sandstone sample Fu-1 of the Funadani Formation. 98 grains
out of 123 gave concordant results (Fig. 17). We detected four

LOWER CRETACEOUS FORMATIONS IN THE RYOSEKI–MONOBE AREA

49

FIGURE 15. Analytical data of detrital zircons from sandstone of the lower part of the Birafu Formation (sample Bi-1). a, Concordia diagram for all data;b, Concordia diagram for a younger data set; c, Probability density plot with a histogram for all data of concordant grains; d, Probability density plot with ahistogram for the 0–500 Ma data set. N: total number, %Pc: percentage of Precambrian zircons.

FIGURE 16. Analytical data of detrital zircons from sandstone of the middle part of the Birafu Formation (sample Bi-2). a, Concordia diagram for all data;b, Concordia diagram for a younger data set; c, Probability density plot with a histogram for all data of concordant grains; d, Probability density plot with ahistogram for the 0–500 Ma data set. N: total number, %Pc: percentage of Precambrian zircons.

50

T. IKEDA, T. HARADA, Y. KOUCHI, S. MORITA, M. YOKOGAWA, K. YAMAMOTO AND S. OTOH

FIGURE 17. Analytical data of detrital zircons from sandstone of the Funadani Formation (sample Fu-1). a, Concordia diagram for all data; b, Concordia
diagram for a younger data set; c, Probability density plot with a histogram for all data of concordant grains; d, Probability density plot with a histogram for the
0–500 Ma data set. N: total number, %Pc: percentage of Precambrian zircons.

FIGURE 18. Analytical data of detrital zircons from sandstone of the Hagino Formation from Sano (sample Ha-1). a, Concordia diagram for all data; b, Concordia
diagram for a younger data set; c, Probability density plot with a histogram for all data of concordant grains; d, Probability density plot with a histogram for the
0–500 Ma data set. N: total number, %Pc: percentage of Precambrian zircons.

LOWER CRETACEOUS FORMATIONS IN THE RYOSEKI–MONOBE AREA

51

FIGURE 19. Analytical data of detrital zircons from sandstone of the Hagino Formation from Hagino (sample Ha-2). a, Concordia diagram for all data; b,
Concordia diagram for a younger data set; c, Probability density plot with a histogram for all data of concordant grains; d, Probability density plot with ahistogram for the 0–500 Ma data set. N: total number, %Pc: percentage of Precambrian zircons.

FIGURE 20. Analytical data of detrital zircons from a sandstone cobble from the Ryoseki Formation (sample RySs-1). a, Concordia diagram for all data;b, Concordia diagram for a younger data set; c, Probability density plot with a histogram for all data of concordant grains; d, Probability density plot with ahistogram for the 0–500 Ma data set. N: total number, %Pc: percentage of Precambrian zircons.

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T. IKEDA, T. HARADA, Y. KOUCHI, S. MORITA, M. YOKOGAWA, K. YAMAMOTO AND S. OTOH

FIGURE 21. Analytical data of detrital zircons from a sandstone cobble of the Yunoki Formation (sample YuSs-1). a, Concordia diagram for all data; b,
Concordia diagram for a younger data set; c, Probability density plot with a histogram for all data of concordant grains; d, Probability density plot with ahistogram for the 0–500 Ma data set. N: total number, %Pc: percentage of Precambrian zircons.

FIGURE 22. Analytical data of detrital zircons from a sandstone cobble of the Funadani Formation (sample FuSs-1). a, Concordia diagram for all data; b,
Concordia diagram for a younger data set; c, Probability density plot with a histogram for all data of concordant grains; d, Probability density plot with ahistogram for the 0–500 Ma data set. N: total number, %Pc: percentage of Precambrian zircons.

LOWER CRETACEOUS FORMATIONS IN THE RYOSEKI–MONOBE AREA

53

FIGURE 23. Analytical data of zircons from a rhyolite cobble of the Yunoki Formation (sample YuIg-1). a, Concordia diagram for a younger data set; b,
Concordia diagram for the data set forming the youngest cluster (the light blue ﬁlled ellipse denotes the concordant age). N: total number.

FIGURE 24. Analytical data of zircons from a granodiorite cobble of the Hibihara Formation (sample HiIg-1). a, Concordia diagram for all data (except for
no.15 zircon of Table 1); b, 206Pb/238U ages for the 14 concordant zircons with the calculated weighted mean age. N: total number.

age groups and two single plots of concordant detrital zircons:
143–393 Ma (73%), 1711–1981 Ma (15%), 2076–2215 Ma (5%),
422–481 Ma (4%), around 1570 Ma (1%), and around 2424 Ma(1%), with %Pc of 22 and the YZ of 143.4 ± 3.2 Ma (Fig. 17a–c).
The probability density plot for the 0–500 Ma data set showed thelargest peak at 269 Ma, second largest peaks at 254 Ma and 292
Ma, and small peaks at 144 Ma (YP), 166 Ma, 187 Ma, 216 Ma,
and 237 Ma (Fig. 17d).
Sandstone of the Hagino Formation from Sano (Sample
Ha-1)
We obtained 113 analyses from 113 zircon grains collectedfrom sandstone sample Ha-1 of the Hagino Formation. 52 grains
out of 113 gave concordant results (Fig. 18). We detected four
age groups and two single plots of concordant detrital zircons:
170–262 Ma (60%), 1521–1867 Ma (26%), 148–159 Ma (6%),
2193–2227 Ma (4%), 297 Ma (2%), and 413 Ma (2%), with %Pc
of 31 and the YZ of 148.1 ± 3.5 Ma (Fig. 18a–c). The probability
density plot for the 0–500 Ma data set showed the largest peaks at
177 Ma and 191 Ma, a second largest peak at 170 Ma, and small
peaks at 149 Ma (YP), 159 Ma, 202 Ma, 210 Ma, 239 Ma, and
250 Ma (Fig. 18d).

Sandstone of the Hagino Formation from Hagino (Sample
Ha-2)
We obtained 137 analyses from 137 zircon grains collectedfrom sandstone sample Hs-2 of the Hibihara Formation. 110
grains out of 137 gave concordant results (Fig. 19). We detected
three age groups and one single plot of concordant detrital zircons:
113–292 Ma (86%), 1677–2024 Ma (11%), 2222–2233 Ma (2%),
and around 2525 Ma (1%), with %Pc of 14 and the YZ of 112.9± 3.9 Ma (Fig. 19a–c). The probability density plot for the 0–500
Ma data set showed the largest peak at 175 Ma, the second largest
and youngest peak at 120 Ma, and small peaks at 135 Ma, 150
Ma, 202 Ma, 220 Ma, 243 Ma, 255 Ma, and 267 Ma (Fig. 19d).
The ten grains that constitute the YP gave the concordant age of
119.3 ± 1.8 Ma (MSWD = 0.051, Probability = 0.82).
Cobbles from conglomerate
A sandstone cobble from the Ryoseki Formation, Monobegawa
Group (Sample RySs-1)
We obtained 110 analyses from 110 zircon grains collectedfrom sandstone cobble sample RySs-1 of the Ryoseki Formation.
90 grains out of 110 gave concordant results (Fig. 20). We
detected three age groups and three single plots of concordant
detrital zircons: 240–336 Ma (89%), 450–506 Ma (6%), 380–383

54

T. IKEDA, T. HARADA, Y. KOUCHI, S. MORITA, M. YOKOGAWA, K. YAMAMOTO AND S. OTOH

FIGURE 25. Analytical data of detrital zircons from sandstone of the Shingai Unit (sample 13072103). a, Concordia diagram for all data; b, Concordia diagram
for a younger data set; c, Probability density plot with a histogram for all data of concordant grains; d, Probability density plot with a histogram for the 0–500
Ma data set. N: total number, %Pc: percentage of Precambrian zircons.

FIGURE 26. Analytical data of detrital zircons from sandstone of the Shirakidani Group (sample 16091701). a, Concordia diagram for all data; b, Concordia
diagram for a younger data set; c, Probability density plot with a histogram for all data of concordant grains; d, Probability density plot with a histogram for the
0–500 Ma data set. N: total number, %Pc: percentage of Precambrian zircons.

LOWER CRETACEOUS FORMATIONS IN THE RYOSEKI–MONOBE AREA

Ma (2%), 423 Ma(1%), 626 Ma (1%), and 1779 Ma (1%), with
%Pc of 2 and the YZ of 240.2 ± 7.7 Ma (Fig. 20a–c). The
probability density plot for the 0–500 Ma data set showed thelargest peak at 263 Ma and small peaks at 330 Ma, 380 Ma, and465 Ma (Fig. 20d).
A sandstone cobble from the Yunoki Formation, Monobegawa
Group (Sample YuSs-1)
We obtained 79 analyses from 79 zircon grains collected fromsandstone cobble sample YuSs-1 of the middle part of the Yunoki
Formation. 60 grains out of 79 gave concordant results (Fig. 21).
We detected three age groups and one single plot of concordant
detrital zircons: 184–314 Ma (70%), 1721–2070 Ma (23%), 358–
380 Ma (5%), and 2633 Ma (2%), with %Pc of 25 and the YZ of
183.6 ± 2.6 Ma (Fig. 21a–c). The probability density plot for the
0–500 Ma data set showed the largest peak at 263 Ma, a second
largest peak at 295 Ma, and small peaks at 183 Ma (YP), 199 Ma,
214 Ma, 230 Ma, 281 Ma, and 378 Ma (Fig. 21d).
A sandstone cobble from the Funadani Formation, NankaiGroup (Sample FuSs-1)
We obtained 87 analyses from 87 zircon grains collected fromsandstone cobble sample FuSs-1 of the Funadani Formation. 59
grains out of 87 gave concordant results (Fig. 22). We detected
one age group and two single plots of concordant detrital zircons:
260–363 Ma (97%), 1940 Ma (2%), and 2143 Ma (2%) with %Pc
of 3 and the YZ of 259.7 ± 7.2 Ma (Fig. 22a–c). The probability
density plot for the 0–500 Ma data set showed the largest peak at
295 Ma and second largest peaks at 275 Ma (YP) and 329 Ma (Fig.
22d).
A rhyolite cobble from the Yunoki Formation, Monobegawa
Group (Sample YuIg-1)
We obtained 34 analyses from 34 zircon grains collected fromrhyolite cobble sample YuIg-1 of the middle part of the Yunoki
Formation. 12 grains out of 34 gave concordant results (Fig. 23).
We detected two age groups and two single plots of concordantzircons: 122–137 Ma (8 grains), 174–181 Ma (2 grains), 269 Ma (1grain), and 2334 Ma (1 grain) (Fig. 23a). The concordia age of theyoungest 6 grains was 125.4 ± 2.3 Ma (MSWD = 7.0, Probability= 0.008), which we interpreted as the formation age of the rhyolite(Fig. 23b).
A granodiorite cobble from the Hibihara Formation,
Monobegawa Group (Sample HiIg-1)
We obtained 24 analyses from 24 zircon grains collected from
granodiorite cobble sample HiIg-1 of the Hibihara Formation.
14 grains out of 24 gave concordant results, which form a single
age group at 120–133 Ma (Fig. 24a). The weighted mean of the
206Pb/238U ages of 14 grains was 126.4 ± 2.5 Ma (MSWD =6.7, Probability = 0.0), which we interpreted as the formation ageof the granodiorite (Fig. 24b).
Pre-Cretaceous sandstone of the Northern Chichibu Belt
Sandstone of the Shingai Unit (Sample 13072103)
We obtained 110 analyses from 110 zircon grains collectedfrom sandstone sample 13072103 of the Shingai Unit. 69 grains
out of 110 gave concordant results (Fig. 25). We detected six age

55groups and one single plot of concordant detrital zircons: 245–311
Ma (65%), 746–997 Ma (15%), , 1156–1604 Ma (7%), 482–538
Ma (6%), 341–357 Ma (3%), 408–444 Ma (3%), and 2690 Ma(1%), with %Pc of 23 and the YZ of 245.2 ± 7.0 Ma(Fig. 25a–c).
The probability density plot for the 0–500 Ma data set showed asingle peak at 258 Ma (Fig. 25d).
Sandstone of the Shirakidani Group (Sample 16091701)
We obtained 96 analyses from 96 zircon grains collected fromsandstone sample 16091701 of the Shirakidani Group. 66 grains
out of 96 gave concordant results (Fig. 26). We detected two age
groups and two single plots of concordant detrital zircons: 232–
316 Ma (94%), 1800–1815 Ma (3%), 377 Ma (2%), and 2007 Ma(2%), with %Pc of 5 and the YZ of 232.2 ± 7.5 Ma (Fig. 26a–c).
The probability density plot for the 0–500 Ma data set showed thelargest peak at 259 Ma, second largest peak at 279 Ma, and small
peaks 233 Ma (YP) and 301 Ma (Fig. 26d).
DISCUSSION
Age of deposition of the Lower Cretaceous formations in the
study area
The mineral zircon is crystallized from acidic to intermediatemagma (e.g., Hoskin and Schaltegger, 2003) and memorizes theage of formation of the felsic to intermediate-igneous-rock bodyfrom the magma. Due to the erosion of the igneous-rock body, thezircon grains in the rock body are transported to a sedimentarybasin, together with reworked older zircons from sedimentary
and/or metamorphic rock bodies. Okawa et al. (2013) concluded,from their study of detrital zircon ages from 14 sandstone samplesof the South Kitakami Belt, that the YP of a sandstone sample
on the probability density plot is a good measure for the upper
age limit of the deposition of the sample. Following Okawa et al.
(2013), we assume that the YP age or the concordia age of the YPforming zircons (if calculated) of a sample is the upper limit of theage of deposition. In this study, we found that the upper age limitof the Yunoki and Birafu formations inferred from detrital zircon
ages is signiﬁcantly younger than the age of deposition inferredfrom fossils. In the following discussion, we will revise the age ofdeposition of the two formations.
Yunoki Formation
The Yunoki Formation has been correlated with the upper
Barremian from a Neocomian type ammonite, Paracrioceras (?)
sp. from the upper part of the formation (Tanaka et al., 1984; Fig.
4). The overlying Hibihara Formation, on the other hand, wascorrelated with the Aptian–Albian from ammonites. The lower
Middle Member of the Hibihara Formation yields Cheloniceras(C .) sp. and is correlated with the lower Aptian; the upper Middle
Member yields Eodouvilleiceras sp. and Nolaniceras (?) sp. andis correlated with the upper Aptian; and the Uppermost Member
yields Hysteroceras aff. H. carinatum, Engonoceras aff. E.
stolleyi, Tetragonites cf. T. timotheanus, Idiohamites sp., andPseudhelioceras sp. and is correlated with the Albian (Tanaka
et al., 1984; Fig. 4). In this study, we found that the concordia
age of the two zircon grains forming the YP of sample Yu-1

56

T. IKEDA, T. HARADA, Y. KOUCHI, S. MORITA, M. YOKOGAWA, K. YAMAMOTO AND S. OTOH

FIGURE 27. A diagram comparing the age constraints from fossils and zircons of the Monobegawa Group. Keys I: The age-range of index fossils, II: The agerange of zircons (with 2σ errors) forming the YP of each sample of the Monobegawa Group, III: The age-range of zircons (with 2σ errors) forming the second
YP in sample Yu-2, Fm.: Formation, YP: youngest peak in the probability density plot.

from the lower part of the Yunoki Formation was 124.3 ± 2.4
Ma. For sample Yu-2 from the middle part, the concordia age ofthe two zircon grains forming the YP was 108.2 ± 2.4 Ma, andthe concordia age of the five zircon grains forming the second
youngest peak was 116.4 ± 1.5 Ma (MSWD = 0.7, Probabilility= 0.4). Moreover, the age of the rhyolite cobble from the middle
part of the Yunoki Formation (sample YuIg-1) was 125.4 ± 2.3
Ma. According to International Commission for Stratigraphy (ICS)(2016), these age data strongly suggest that the Yunoki Formationis correlated with the Aptian (–Albian) (Fig. 27). The reason
for the discrepancy between the fossil and zircon ages is still
unknown. We have to wait for the deﬁnition of the base Aptian
GSSP and its numerical age assignment, and the reexaminationof the first appearance and last appearance biohorizons of theammonite fossils from the Yunoki and Hibihara formations.
Birafu Formation
There were two interpretations on the correlation of the Birafu
Formation, which was subdivided into A1–3, B1–2, and C
members, in ascending order (Kozai et al., 2004, 2006). Morino etal. (1989) found radiolarian fossils from two locations including
the B members along the Nishinokawa River and correlated
the formation with the upper Valanginian–Barremian. Kozai etal. (2004, 2006), on the other hand, studied the radiolarian andbivalve fossils from the formation and correlated the it with
the Oxfordian–lower Valanginian, suggesting that the Jurassic–
Cretaceous boundary lies between the A3 and B1 members. We
found that sample Bi-1 from the A2 Member had the YZ and YPof 149.9 ± 3.6 Ma and 169 Ma, respectively, and did not contain
Early Cretaceous zircons. Since a tuff layer just above Bi-1
yields Oxfordian radiolarians such as Kilinora spiralis (Kozai etal., 2006), we propose that sample Bi-1 is Oxfordian or youngerin age, and our data do not contradict with the interpretation of

Kozai et al. (2004, 2006) that the A members are correlated with
the Upper Jurassic. We also found that sample Bi-2 from the B
members had the YZ and YP of 128.1 ± 4.7 Ma and 136 Ma,
respectively, and 9 zircon grains out of 104 were of the Early
Cretaceous. Since the concordia age of the 8 grains forming
the YP was 136.5 ± 2.5 Ma, the age of deposition of sample
Bi-2 must have been 139 Ma (Early Valanginian; ICS, 2016) or
younger. Considering the fact that the C Member of the Birafu
Formation, overlying the B members, consists of more than 200
m thick fine-grained rocks such as mudstone and interbedded
fine sandstone and mudstone, we suppose that the age of the
Birafu Formation ranges from the Oxfordian (Fig. 20) to the Late
Valanginian or even the Hauterivian.
Provenance change of the Monobegawa and Nankai groups
Monobegawa GroupThe detrital-zircon-age spectra of the Monobegawa Groupare summarized as follows. The detrital zircons of the Ryoseki
Formation (sample Ry-1) consisted mostly of Permian (51%)
and Triassic (38%) zircons (Table. 1; Figs. 28 and 29), and theprobability density plot showed a prominent peak at 252 Ma. On
the other hand, the Monobe and Yunoki formations (samples Mo-1
and Yu-1) contained abundant Mesozoic zircons: i.e., Triassic,
Jurassic, and Early Cretaceous zircons, whereas the uppermost
Hibihara Formation (sample Hi-1) contained 60% of Early
Cretaceous zircons (Figs. 28 and 29). Previous petrographicalstudy of clastic rocks of the Monobegawa Group indicated thatthe clastic grains of the Ryoseki Formation were mainly derived
from older sedimentary rocks, whereas the contribution of felsic
to intermediate (the former in particular) igneous-rock fragments
gradually increased upwards from the Monobe to Hibihara

LOWER CRETACEOUS FORMATIONS IN THE RYOSEKI–MONOBE AREA

FIGURE 28. Bar graphs showing the age composition of detrital zircons
from sandstone samples including the samples of sandstone cobbles.

formations (Miyamoto and Nakai, 1974; Miyamoto, 1980). We
will discuss the provenance of detrital zircons in the Monobegawa
Group from these data and previous studies.
The predominant Permian and Triassic zircons in the Ryoseki
Formation strongly indicate the rework of detrital zircons fromthe pre-Cretaceous AC of the Northern Chichibu Belt, for four
reasons. (1) Previous studies suggested that the clastic grainsof the Ryoseki Formation were mainly supplied from the preCretaceous sedimentary rocks of the Northern Chichibu Belt
(Miyamoto and Nakai, 1974; Miyamoto, 1980). (2) The sandstone
cobbles in the Ryoseki and Yunoki Formations (samples RySs1 and YuSs-1) contain many Permian and Triassic zircons. (3) In
particular, the detrital-zircon-age spectra of the sandstone cobble(sample RySs-1) and sandstone (sample Ry-1) of the Ryoseki
Formation are very similar. (4) The detrital-zircon-age spectraof the sandstone of the Shingai Unit (sample 13072103), theShirakidani Group (sample 16091701) and the Ryoseki Formation(sample Ry-1) are also very similar (Figs. 28 and 29).
On the other hand, our data and previous studies imply that
igneous activity gradually became active in the hinterland during
the deposition of the Monobe, Yunoki, and Hibihara formations.
The reasons are as follows. (1) The proportion of Early Cretaceous
zircons, which were absent in the Ryoseki Formation, gradually
increases upwards (Fig. 28). (2) The proportion of feldspar and57

rock fragments in sandstone and of felsic-igneous-rock clasts inconglomerate gradually increases upwards (Figs. 5–7). (3) The
age of igneous rock cobbles in the Monobegawa Group (samples
YuIg-1 and HiIg-1) is approximately 125 Ma.
Thus the provenance of the Monobegawa Group in the
Ryoseki–Monobe area changed from the pre-Cretaceous rocksof the Northern Chichibu Belt (Ryoseki period) to an Early
Cretaceous igneous province and pre-Cretaceous rocks of theNorthern Chichibu Belt (Monobe–Hibihara period). A similar
provenance change has been proposed in the Lower Cretaceous
Sanchu Group in the Kanto Mountains from the petrography ofclastic rocks (Takei, 1980) and the temporal change of the detritalzircon-age spectra (Nakahata et al., 2016).
Nankai GroupThe detrital-zircon-age spectra of the Nankai Group aresummarized as follows. The Funadani Formation (sample Fu-1)
was characterized by the abundance of Permian (37%) zircons,
whereas the Birafu and Hagino formations (samples Bi-1, Bi2, Ha-1, and Ha-2) were characterized by the abundance ofJurassic (28–48%) zircons (Figs. 28 and 29). Among them, thePermian zircons in the Funadani Formation were likely derived
from older sedimentary rocks from the following reasons. (1)The conglomerate of the Funadani Formation mostly contains
sedimentary-rock clasts, and (2) the sandstone cobble of the
Funadani Formation (sample FuSs-1) was rich in Permian zircons(Figs. 28 and 29).
Origin of Triassic and Jurassic zircons
It is a little hard to judge if the Triassic and Jurassic zircons in
the two groups have been derived from older sedimentary rocks orolder igneous rocks. However, we propose that Jurassic igneousrocks were widespread in the hinterland of the Birafu Formation,
because the sandstone of the Birafu Formation (samples Bi-1 andBi-2), containing many Jurassic zircons, is feldspathic arenite
derived from felsic plutonic rocks. Further, the Aptian Kurohara
Formation (Kozai and Ishida, 2006) in the Sakawa area, some 50
km to the west of the study area, contains a 227 Ma granite cobble(Ikeda et al., 2016), suggesting that Triassic igneous rocks werealso exposed in the hinterland of the Lower Cretaceous beds ofthe study area.
Paleogeography of the Lower Cretaceous formations of the
Ryoseki–Monobe area
Sediment supply from South China
We inferred that Early Cretaceous igneous rocks were exposedin the hinterland of most of the Lower Cretaceous formationsin the study area from the detrital-zircon-age spectra. Along thepresent continental margin of East Asia, Early Cretaceous rockbodies that can supply enough detrital zircons occur most widelyalong the eastern margin of South China (145–90 Ma: e.g., Li etal., 2014; Wang et al., 2013). Smaller Early Cretaceous igneousrock bodies also occur in the Kitakami (130–110 Ma: Tsuchiya
et al., 2015) and Abukuma (120–85 Ma: e.g., Kawano and Ueda,
1965; Shibata and Uchiumi, 1983; Ishihara and Orihashi, 2015;
Kon et al., 2015) belts of Northeast Japan, in the Higo Belt of

58

T. IKEDA, T. HARADA, Y. KOUCHI, S. MORITA, M. YOKOGAWA, K. YAMAMOTO AND S. OTOH

FIGURE 29. Histograms showing the detrital-zircon-age distribution of sandstone samples including the samples of sandstone cobbles. Fm: Formation.

LOWER CRETACEOUS FORMATIONS IN THE RYOSEKI–MONOBE AREA

Kyushu, Southwest Japan (120–110 Ma: e.g., Sakashima et al.,2003), and around the Bohai and West Korea bays, Northeast
China (135–110 Ma: e.g., Kiminami and Imaoka, 2013; Wang etal., 2012; Wu et al., 2007). Among these areas, we propose that
the Early Cretaceous zircons in the Lower Cretaceous formationsof the study area were supplied from the Zhejiang–Fujian coast ofSouth China, for three reasons.
(1) The detrital-zircon-age spectra of the Monobegawa andNankai groups do not show the inﬂuence of the magmatic hiatus
in Korea, which is supposed to have been 158–110 Ma (Sagong etal., 2005). The distribution of 145–100 Ma felsic-igneous rocks islimited along the eastern margin of South China among the above
areas.
(2) Possible provenances of the 125 Ma felsic-igneous-rockclast (cobble) are along the east coast of South China, Kitakami
Belt, and around the Bohai and West Korea bays, where felsicigneous-rocks of approximately 125 Ma are widely exposed (Fig.
30). Considering the size and abundance of igneous rock clasts,
it is hard to imagine that they were transported from rock bodies
far from the coast. In this sense we exclude the 125 Ma felsicigneous-rock-bodies around the Bohai and West Korea bays fromthe candidates of the provenance.
(3) The Ryoseki Formation of the Monobegawa Group yields
many elements of the Ryoseki-type ﬂora, which occurs in South
China (Zhejiang Province and to the south), Indochina, andthe Malay Peninsula in continental Asia (Kimura, 1987). The
distribution of the flora is concordant with our idea that theMonobegawa Group was deposited along the eastern coast ofSouth China.
The Monobegawa and Nankai groups also contain many
Triassic and Jurassic zircons, and at least parts of them are
igneous origin as discussed above. However, the Triassic–Jurassic
zircons were not enough useful to detect the provenance, because
Triassic–Jurassic (> 158 Ma) igneous rocks widely occur in East
Asia. Nevertheless, the presence of Triassic–Jurassic zircons does
not contradict with our idea; Triassic–Jurassic igneous rocks,
including those formed during the magmatic hiatus of Korea,
are widely distributed in the eastern part of the Zhejiang–Fujian
provinces of South China.
Sandstone from the middle part of the Yunoki Formation(sample Yu-2) of the Monobegawa Group (Northern ChichibuBelt) contains a certain amount of 460–400 Ma zircons (18%). In
Japan, 460–400 Ma felsic-igneous-rock bodies occur in (1) theSouth Kitakami Belt, Northeast Japan (e.g., Shimojo et al., 2010),(2) the Yakuno Complex, Inner Zone of Southwest Japan, and (3)the Kurosegawa Belt, Outer Zone of Southwest Japan (Mitaki
igneous rocks; e.g., Hada et al., 2000), but the present areas of
their distribution are very narrow. Although we cannot completely
deny the possibility that the 460–400 Ma zircons in sample Yu-2
were supplied from the 460–400 Ma rock bodies in present-day
Japan, the age composition of igneous rocks in South China (Fig.
30) and the presence of the Ryoseki-type ﬂora in the Monobegawa
Group strongly indicates that the 460–400 Ma Kwangsian Granite
in South China (e.g., Wang et al., 2013) was most likely theprovenance of the 460–400 Ma zircons.

59

Evaluation of sinistral strike-slip models
In the previous section, we concluded that the provenance of
Early Cretaceous zircons in the Monobegawa and Nankai groups
and 460–400 Ma zircons in the Yunoki Formation was Early
Cretaceous and 460–400 Ma igneous rock bodies in the Zhejiang–
Fujian province of South China. The distance from the Ryoseki–
Monobe area and the Zhejiang–Fujian province is roughly 1,500
km. It is also possible that the basement of the Yellow Sea Basinconsists partly of Early Cretaceous igneous rocks that cropped
out on the land surface in the Early Cretaceous and supplied Early
Cretaceous zircons to the Monobegawa Group and/or NankaiGroup. Even in this case, the distance between the Ryoseki–
Monobe area and the mouth of the Yellow Sea is about 500 km.
Hence we interpret that the Monobegawa and Nankai groups
have moved at least 500–1,500 km northeastward (Fig. 30). We
further interpret that the sinistral strike-slip motion along theMedian Tectonic Line (Miyata and Iwamoto, 1994; Yamakita andOtoh, 2000a, b) was responsible for at least a part of the above
displacement.
It is still hard to verify, only with our zircon data, the strikeslip model of Tashiro (1985), which described that the arcsubparallel sinistral strike-slip motion between the Monobegawa
and Nankai groups had carried the Nankai Group relatively
northward by the Albian. There are signiﬁcant differences in the
detrital-zircon-age spectra between the coeval samples of theMonobegawa and Nankai groups, but we cannot so precisely
specify the site of deposition of each sample as to evaluate thestrike-slip model of Tashiro (1985). For example, samples Ry-1
(Ryoseki Formation, Monobegawa Group) and Bi-2 (middle partof the Birafu Formation, Nankai Group) can be contemporaneous,
but the differences in the positions of the largest peak (Ry-1 at
252 Ma and Bi-2 at 172 Ma; Figs. 10d and 16d) and in %Pc
value (6 for Ry-1 and 31 for Bi-2; Figs. 10c and 16c) suggest
different provenances. Sample Fu-1 (Funadani Formation, NankaiGroup) can also be coeval with sample Ry-1, but the positionof the largest peak (269 Ma; Fig. 17d) and %Pc value (22; Fig.
17c) differ from those of sample Ry-1. Sample Mo-1 (Monobe
Formation, Monobegawa Group) in turn can be coeval withsample Fu-1, but the position of the largest peak (174 Ma; Fig.
11d) differs from that of sample Fu-1. Moreovre, the following
differences in sandstone and conglomerate petrography also
indicate the different provenances between the Monobegawa andNankai groups. (1) The sandstone of the Monobegawa Group is
primarily ill-sorted wacke, whereas that of the Nankai Group ismainly well-sorted arenite (Tashiro, 1985). (2) The conglomerateof the Shobu Formation of the Nankai Group in the Katsuura area
characteristically contains felsic-igneous-rock clasts, whereas
the coeval conglomerate of the Tatsukawa Formation of theMonobegawa Group is characterized by sedimentary-rock clasts(Matsukawa and Eto, 1987). In contrast, the detrital-zircon-age
spectra of the following two Aptian samples are similar with eachother: sample Yu-1 from the lower part of the Yunoki Formationand sample Ha-2 from the Hagino Formation from Hagino (Fig.
31). The similarity indicates the common provenance of theMonobegawa and Nankai groups in the Aptian. Even if the strike-

60

T. IKEDA, T. HARADA, Y. KOUCHI, S. MORITA, M. YOKOGAWA, K. YAMAMOTO AND S. OTOH

FIGURE 30. A paleogeographic map showing the possible reconstruction model for the sedimentary basins of the Lower Cretaceous formations in the Ryoseki–
Monobe area, Outer Zone of Southwest Japan.

LOWER CRETACEOUS FORMATIONS IN THE RYOSEKI–MONOBE AREA

FIGURE 31. Probability density plots showing the similarity of detritalzircon-age distribution between sample Yu-1 (lower part of the Yunoki
Formation) and Ha-2 (Hagino Formation from Hagino).

slip model of Tashiro (1985) is close to the reality, the movement
may have ceased by the end of Aptian.
Evaluation of the Paleo-Ryoke model
Many previous studies implied that a geologic entity called the
Paleo-Ryoke Belt, consisting of Permian and Early Cretaceous
granite and metamorphic rocks, was once exposed between the
Ryoke and Sambagawa belts but have mostly been eroded away
(Ichikawa, 1964; Takagi and Shibata, 2000; Miyamoto et al.,2000). Miyamoto (1980) attributed the origin of igneous-rockclasts in the Monobegawa Group of the study area to the missing
geologic entity along the southern margin of the Ryoke Belt. As
we discussed above, however, we concluded that the igneous-rockclasts in the Monobegawa Group were supplied from igneous-rockbodies in the Zhejiang–Fujian provinces of South China from thebotanical paleogeography (Kimura, 1987) and the sinistral strikeslip motion along the Median Tectonic Line separating the Ryokeand Sanbagawa belts (e.g., Miyata and Iwamoto, 1994; Yamakita
and Otoh, 2000a, b). Moreover, (1) the presence of Permian61

igneous and metamorphic rocks along the southern margin of theJurassic AC of the Ryoke Belt and (2) the short distance between
the Lower Cretaceous AC (i.e., Early Cretaceous subductionzone) of the Shimanto Belt and the Ryoke Belt (ca. 60 km) havenot been explained in the Paleo-Ryoke model in a plate-tectonic
framework.
Aoki et al. (2014) proposed that there was a topographic barrierthat obstructed the southward transport of continent-derived
sediments in the Early Cretaceous, because the Ryoseki Formationof the Hauterivian forearc basin contained very few continentderived Proterozoic zircons that dominated in the brackish to
shallow-marine Toyonishi Group (Inner Zone of Southwest Japan)of the latest Jurassic to earliest Cretaceous back-arc basin. Aoki
et al. (2014) concluded that the barrier was the Early Cretaceous
igneous arc developed in the older orogen and was thrusted
southward for more than 200 km along the Median Tectonic
Line during the opening of the Sea of Japan in the Middle
Miocene. However, we detected more than 20% Proterozoic
zircons from the Monobe (Barremian; 22%), lower Birafu (latest
Jurassic; 46%), middle Birafu (early Early Cretaceous; 28%), andFunadani (Hauterivian–Barremian; 22%) formations. Moreover,
the Monobegawa and Nankai groups contained many Triassic
to Jurassic zircons, in spite of the fact that there are virtually no
Triassic to Jurassic igneous rocks to the south of the distributionof the Toyonishi Group. Thus, our data are not concordant withthe barrier model of Aoki et al. (2014). We tentatively think thatthe amount of Proterozoic detrital zircons depends on the area of
exposure of Proterozoic igneous rocks in the hinterland and the
distance between the Proterozoic igneous rock bodies and thesedimentary basin. In addition, we doubt the Miocene southwardthrust model, because the distance between the Paleogene
volcanic front (the southern boundary of the San-in granite belt)
and trench (Southern Shimanto Belt) was 300 km, close to thearc-trench gap in the present-day Northeast Japan, and we expect
no arc-orthogonal shortening in the Miocene.
Importance of reworked zircons in the study of detritalzircon-age spectra
Nakahata et al. (2016) conducted a similar study with oursof the Sanchu Cretaceous in the Kanto Mountains, north ofTokyo. They concluded that Permo–Triassic igneous rock bodies
were widely exposed in the hinterland of the Hauterivian Shiroi
Formation, because Permo–Triassic zircons occupied 76% of
the detrital zircons in the formation. They further suggested thatclastic materials from Permo–Triassic igneous rock bodies were
widely supplied to the Hauterivian forearc of Kanto and Shikoku,
because the detrital-zircon-age spectra of the Shiroi and Ryoseki
formations are very similar, and the Ryoseki Formation contains
many 300–200 Ma zircons (Aoki et al., 2012). However, thedistribution of Permian igneous rocks near Japan is limited to
Northeast China and Indochina, although there are few Permian
igneous rock bodies in the Maizuru Belt in Southwest Japan (e.g.,
Herzig et al., 1997). This study implied that the detrital zirconsof the Ryoseki Formation were mainly derived from the Permianclastic rocks of the Northern Chichibu Belt. The sandstone of theSanchu Cretaceous is lithic with many sedimentary-rock fragments

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T. IKEDA, T. HARADA, Y. KOUCHI, S. MORITA, M. YOKOGAWA, K. YAMAMOTO AND S. OTOH

in the lower part and felspathic in the upper part, suggesting thechange from a sedimentary-rock-rich hinterland to a graniterich hinterland (Takei, 1980). Moreover the conglomerate of theHauterivian Tatsukawa Formation of the Monobegawa Groupin the Katsuura area consists mostly of older-sedimentary-rockclasts (Ishida and Hashimoto, 1997; Matsukawa and Eto, 1987;
Ogawa, 1971). Hence we propose that the Hauterivian formationsof the Outer Zone of Southwest Japan from Kanto to Shikoku had
a hinterland widely occupied by older sedimentary rock bodies
and received detrital zircons from them. We further propose that
it is misleading to think that the detrital zircons were all derived
from igneous-rock bodies in the hinterland. To make a proper
provenance analysis using detrital zircons, we have to make acomprehensive study including (1) the modal composition of thesandstone for detrital zircon study and (2) the detrital-zircon-age
spectra of the basement sedimentary rocks and sandstone clasts
in the conglomerate. Although the proportion of abraded zircons
might be a criterion for the recognition of reworked zircons, thecriterion cannot be applied to our present study. The proportion ofabraded zircons in the sandstone of the Ryoseki Formation (Ry1), the sandstone clast in the Ryoseki Formation (RySs-1), and the
300–230 Ma zircons in the sandstone of the Shirakidani Group(16091701) were 57, 54, and 50%, respectively, and did not show
a signiﬁcant difference. The result may reﬂect a short distance oftransportation of the reworked zircons.
CONCLUSIONS
We studied detrital-U-Pb-zircon-age spectra of the sandstone
sample from every formation of the Lower Cretaceous
Monobegawa and Nankai groups (part of the Nankai Group isUpper Jurassic) of the Chichibu Composite Belt in the Ryoseki–
Monobe area, Southwest Japan. In addition, we measured the
U-Pb zircon age of (1) igneous rock cobbles in these formations
and detrital-zircon-age spectra of (2) the sandstone cobbles fromthese formations and (3) the basement Permian sandstone of theNorthern Chichibu Belt. The major results are summarized asfollows.
1. The detrital-zircon-age spectra of the sandstone andsandstone cobble of the Ryoseki Formation and the Permiansandstone of the Northern Chichibu Belt are very similar.
Combining the data with previous studies, we conclude thatthe clastic materials of the Ryoseki Formation were supplied
from the pre-Cretaceous sedimentary rocks of the Northern
Chichibu Belt.
2. Early Cretaceous zircons were absent in the Ryoseki
Formation, but suddenly increased in the Monobe–Hibihara
formations. Moreover, the U-Pb zircon age of the igneousrock cobbles from the Yunoki and Hibihara formations
was approximately 125 Ma. Combining with the previous
petrographical studies of sandstone and conglomerate, we
conclude that an acidic to intermediate igneous activity took
place in the hinterland concurrently with the deposition of
the Monobegawa Group.
3. The petrography of clastic rocks implies that felsic-igneous-rock-bodies were exposed in the hinterland of the NankaiGroup. The detrital-zircon-age spectra of the group indicatethat the igneous rock bodies contained Early Cretaceousones.
4. The hinterland of the two groups must have been the
Zhejiang–Fujian provinces of South China, because (1)the study of botanical paleogeography of Kimura (1987)
indicated that the Ryoseki Formation and South China
commonly yield the Ryoseki-type ﬂora, (2) detrital zircons
formed during the magmatic hiatus in Korea were included
in both the Monobegawa and Nankai groups, and (3) 460–
400 Ma detrital zircons from the middle part of the Yunoki
Formation were most likely from the Kwangsian Granite ofSouth China.
5. The Monobegawa and Nankai groups likely had different
hinterland by the Aptian. The previous petrographical studies
and the similarity of detrital-zircon-age spectra of the lower
part of the Yunoki Formation (Monobegawa Group) and the
Hagino Formation (Nankai Formation) indicate that the twogroups had common hinterland in the Aptian.
6. The two groups were deposited with the Zhejiang–Fujian
provinces as the hinterland. The two groups must haveshifted relatively northward by 500–1,500 km along theMedian Tectonic Line sinistral fault system.
7. Detrital zircons were not all supplied from igneous rockbodies in the hinterland, but can be provided from older
sedimentary and metamorphic rocks as reworked zircons. To
make a proper provenance analysis using detrital zircons, wehave to make a comprehensive study including (1) the modal
composition of the sandstone for detrital zircon study and (2)the detrital-zircon-age spectra of the basement sedimentaryrocks and sandstone clasts in the conglomerate.
ACKNOWLEDGMENTS
We would like to express our sincere gratitude to Dr. Shinichi Sano of the Fukui Prefectural Dinosaur Museum for his
encouragement to submit our work to this volume; and toProfessor Masaaki Shimizu and Associate Professor Yasuo
Ishizaki of the University of Toyama for various discussions andinstructions during the course of this study. Critical reviews of thesubmitted manuscript by Professor Haruyoshi Maeda of Kyushu
University, Professor Makoto Takeuchi of Nagoya University, and
Dr. Soichiro Kawabe of the Fukui Prefectural Dinosaur Museum
are greatly appreciated. This study was supported by JSPS
KAKENHI Grant Number 25400484.
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Abukuma Belt ……………… 阿武隈帯
Birafu ………………………… 美良布
Birafu Formation…………… 美良布層
Chichibu Composite Belt … 秩父累帯
Funadani Formation ………… 船谷層
Hagino …………………………… 萩野
Hagino Formation …………… 萩野層
Haginogawa River …………… 萩野川
Hibihara Formation ………… 日比原層
Hibiharagawa River ……… 日比原川
Higo Belt ……………………… 肥後帯
Igenoki Formation ……… 神母ノ木層
Kagamiimai …………………… 鏡今井
Kajisakogawa River ……… 楮佐古川
Kasanokawagawa River …… 笠ノ川川
Katsuura area ……………… 勝浦地域
Kitakami Belt ………………… 北上帯
Kochi City …………………… 高知市
Kochi Prefecture ……………… 高知県
Kubogawa River ……………… 久保川
Kurohara Formation ………… 黒原層

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地名・地層名＞Kurosegawa Belt …………… 黒瀬川帯
Kyushu ………………………… 九州
Kanto Mountains…………… 関東山地
Meoto Pond …………………… 女夫池Mitaki igneous rocks … 三滝火成岩類
Monobe Formation …………… 物部層
Monobegawa Group …… 物部川層群
Monobegawa River …………… 物部川
Yokokurayama Group …… 横倉山層群
Nankai Group ……………… 南海層群
Nishinokawa River …………… 西の川
Northeast Japan …………… 東北日本
Northern Chichibu Belt … 北部秩父帯
Odochi ………………………… 大栃
Pre-Sotoizumi Group … 先外和泉層群
Ryoke Belt …………………… 領家帯
Ryoseki Formation …………… 領石層
Ryoseki–Monobe area……………………… 領石–物部地域
Sakawa area ……………… 佐川地域
Sanbagawa Belt …………… 三波川帯

Sano …………………………… 佐野
Sasagawa River ………………… 笹川
Shikoku ………………………… 四国
Shimanto Belt ……………… 四万十帯
Shingai Unit ………… 新改ユニット
Shirakidani Group ……… 白木谷層群
Shobu Formation …………… 菖蒲層
Sotoizumi Group ……… 外和泉層群
Southern Chichibu Belt … 南部秩父帯
Southwest Japan …………… 西南日本
Suita Group ……………… 杉田層群
Tachibana ……………………… 立花
Tatsukawa Formation ………… 立川層
Tokushima Prefecture ………… 徳島県
Torinosu Group …………… 鳥巣層群
Torinosu-type Limestone……………………… 鳥巣式石灰岩
Tosayamada………………… 土佐山田
Yakuno Complex … 夜久野複合岩類
Yukigamine Formation …… 雪ヶ峰層
Yunoki Formation ………… 柚ノ木層

TABLE 1. U-Pb isotopic data for zircons analyzed in this study. All errors are 2σ. % 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 the probability density plots and histograms.

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73

74

TABLE 1. (Continued )T. IKEDA, T. HARADA, Y. KOUCHI, S. MORITA, M. YOKOGAWA, K. YAMAMOTO AND S. OTOHTABLE 1. (Continued )LOWER CRETACEOUS FORMATIONS IN THE RYOSEKI–MONOBE AREA

75

76

TABLE 1. (Continued )T. IKEDA, T. HARADA, Y. KOUCHI, S. MORITA, M. YOKOGAWA, K. YAMAMOTO AND S. OTOHTABLE 1. (Continued )LOWER CRETACEOUS FORMATIONS IN THE RYOSEKI–MONOBE AREA

77

78

TABLE 1. (Continued )T. IKEDA, T. HARADA, Y. KOUCHI, S. MORITA, M. YOKOGAWA, K. YAMAMOTO AND S. OTOHTABLE 1. (Continued )LOWER CRETACEOUS FORMATIONS IN THE RYOSEKI–MONOBE AREA

79

80

TABLE 1. (Continued )T. IKEDA, T. HARADA, Y. KOUCHI, S. MORITA, M. YOKOGAWA, K. YAMAMOTO AND S. OTOHTABLE 1. (Continued )LOWER CRETACEOUS FORMATIONS IN THE RYOSEKI–MONOBE AREA

81

82

TABLE 1. (Continued )T. IKEDA, T. HARADA, Y. KOUCHI, S. MORITA, M. YOKOGAWA, K. YAMAMOTO AND S. OTOHTABLE 1. (Continued )LOWER CRETACEOUS FORMATIONS IN THE RYOSEKI–MONOBE AREA

83

84

TABLE 1. (Continued )T. IKEDA, T. HARADA, Y. KOUCHI, S. MORITA, M. YOKOGAWA, K. YAMAMOTO AND S. OTOH