Yoshioka et al.
Progress in Earth and Planetary Science(2024) 11:46
https://doi.org/10.1186/s40645-024-00651-w

RESEARCH ARTICLE

Progress in Earth andPlanetary Science

Open AccessIntegration of new zircon U–Pb
ages with biostratigraphy to establish
a high‑precision age model of the Miocene
Nakayama Formation on Sado Island in Central
Japan
Jumpei Yoshioka1,2* , Kenji M. Matsuzaki1, Sota Niki3,4, Junichiro Kuroda1 and Takafumi Hirata3

Abstract
The most common age constraint for the diatomaceous sediments is biostratigraphy of siliceous microfossils.
Although biostratigraphy is a powerful tool to establish stratigraphy and correlate with sedimentary sequencesin other sites, biostratigraphy generally includes uncertainties difficult to evaluate. In this study, we measured zircon
U–Pb ages of eight tuff beds intercalated with diatomaceous mudstone of the Nakayama Formation on Sado Island
in Central Japan and integrated the U–Pb ages with diatom and radiolarian biostratigraphy, whose ages and errors
were re-evaluated by this study, to establish an age model precisely representing the sedimentary age. Two tuff
beds in the upper and middle part of the formation offered zircon U–Pb ages of 6.7 ± 0.2 Ma and 10.87 ± 0.07 Ma,
which are consistent with biostratigraphy, and provided a good example of effective integration of zircon U–Pb
ages with the biostratigraphy. On the other hand, zircon U–Pb ages of the other six tuff beds in the lower part arearound 12 Ma and not distinguishable from each other. In addition, older zircon grains in the 6 tuff beds are assembled in the interval from 30 to 20 Ma, which is consistent with the age of the volcanic basement rocks forming mostpart of Sado Island. Similarities in chemical compositions of glass shards and age distributions of zircon grains indicatethat the volcaniclastic components in the tuff beds should originate from single or associated magmatic activities.
Keywords Zircon U–Pb age, Diatom biostratigraphy, Radiolarian biostratigraphy, Middle to late Miocene, The Japan
Sea, The Nakayama Formation*Correspondence:
Jumpei Yoshioka
j-yoshioka@aist.go.jp
1
Atmosphere and Ocean Research Institute, The University of Tokyo,
5‑1‑5 Kashiwanoha, Kashiwa, Chiba 277‑8564, Japan
2
Geological Survey of Japan, National Institute of Advanced Industrial
Science and Technology (AIST), 1‑1‑1 Higashi, Tsukuba, Ibaraki 305‑8567,
Japan
3
Geochemical Research Center, Graduate School of Science, The
University of Tokyo, 7‑3‑1 Hongo, Bunkyo‑ku, Tokyo 113‑0033, Japan
4
Institute for Space‑Earth Environmental Research, Nagoya University, 3‑3
Furo‑cho, Chikusa‑ku, Nagoya 464‑8601, Japan

1 Introduction
Diatoms, siliceous marine algae, contribute ca. 40% of
marine primary production and particulate organic carbon export as part of the biological pump (Tréguer et al.
2018). Diatoms and organic matter transported to theseafloor form into diatomaceous sediments. Therefore,
diatomaceous sediments have attracted scientific attention in their role in organic carbon burial, which is oneof the important factors controlling the atmospheric­CO2 concentration (e.g., Ragueneau et al. 2000; Renaudie
2016). In fact, the prosperity of diatomaceous sedimentsin the North Pacific marginal basins including the Japan© The Author(s) 2024. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which
permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to theoriginal author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images orother third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line
to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory
regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of thislicence, visit http://​creat​iveco​mmons.​org/​licen​ses/​by/4.​0/.

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Sea since the middle Miocene has been associated with
global climate change (e.g., Vincent and Berger 1985;
Flower and Kennett 1994). However, the vulnerability ofthe old diatomaceous sediments to silica diagenesis andthe scarcity of carbonate microfossils are preventing the
paleoceanographic and paleoclimatologic use of diatomaceous sediments.
One of the greatest concerns in the use of diatomaceous sediments for paleoceanographic studies is theprecision of their sedimentary age. The scarce content
of carbonate makes it difficult to use chemostratigraphysuch as the oxygen stable isotope. The most common age
constraint for the diatomaceous sediments is diatom andradiolarian biostratigraphy. Biostratigraphy is a powerful tool to establish chronostratigraphy and correlate asedimentary sequence with those in other sites. International Ocean Discovery Program/Integrated Ocean Drilling Program (IODP), Ocean Drilling Program (ODP),
and Deep Sea Drilling Project (DSDP) have significantly
improved the Neogene diatom and radiolarian biostratigraphy in the North Pacific region (e.g., Barron et al. 1995;
Yanagisawa and Akiba 1998; Kamikuri et al. 2017). However, biostratigraphy generally includes uncertainties
difficult to evaluate, for example, geographic or regional
differences in biostratigraphic events and uncertainties
deriving from age models of the reference sequences, like
deep-sea drilling cores, which are used to construct thebiostratigraphy.
In case there are tuff beds intercalated with diatomaceous sediments and the tuff beds contain zircon grains,zircon U–Pb dating is available. The recent improvementof in situ U–Pb age measurement and the establishmentof new standard zircon preferable to the measurementof young zircon have enabled precise U–Pb dating for
Miocene samples (Iwano et al. 2013; Hattori et al. 2017;
Obayashi et al. 2017). Whereas a zircon U–Pb age is an
excellent indicator of the crystallization age of the zircon,
the zircon U–Pb age does not always represent the ageof the deposition of the tuff bed, for example, early crystallization of zircon a long time before the eruption andre-sedimentation of old pyroclastic materials containing
zircon (e.g., Danhara et al. 2005).
The Miocene Nakayama Formation distributed on Sado
Island has a great potential to be used for paleoceanographic studies during the middle to late Miocene. It
has orbital-scale oscillations in sedimentary structures
such as laminations and bioturbations representing thebottom water oxygenation level (Sakamoto 1992). Wellpreserved siliceous microfossils and good exposures ofoutcrops of the Nakayama Formation have contributed tothe establishment of high-resolution diatom biostratigraphy (Yanagisawa and Watanabe 2017a). The Nakayama
Formation also has a lot of felsic tuff beds intercalatedwith diatomaceous sediments (Sakamoto 1992; Yanagisawa et al. 2009; Yanagisawa and Watanabe 2017a), whichenables the integration of biostratigraphy and radiometric dating and establishment of more plausible age
constraints.
In this study, we measured zircon U–Pb ages of eighttuff beds intercalated with diatomaceous mudstone of
the Nakayama Formation and compared their U–Pb ages
with diatom biostratigraphy already reported by Yanagisawa and Watanabe (2017a) and radiolarian biostratigraphy newly established by this study in order to confirm
that these age constraints indeed represent the sedimentary age. The ages and error ranges of diatom and radiolarian biostratigraphy were re-evaluated by constructing
new age models of reference sedimentary sequences forbiostratigraphy. In addition to age determination, we
described the detailed stratigraphy of the lowest part of
the Nakayama Formation (SW Section), where a lot of
thick tuff beds have been identified.
1.1 Geological setting

Sado Island (854.8 k­ m2) is located in the Japan Sea northwest of Niigata Prefecture in Central Japan. Sado Island
has two highland areas, Osado Mountains and Kosado
Hills, with the Kuninaka Plain area between them (Fig. 1).
Most parts of the highland areas are composed of volcanic rocks, which are mainly formed during the early
Miocene volcanism (e.g., Metal Mining Agency of Japan
1994). Sedimentary rocks on Sado Island started to beformed during the middle Miocene, preceding the marinetransgression widely reported from the Japanese Islands(Kobayashi 2000; Yanagisawa and Watanabe 2017b), andare mainly exposed along the southeastern foot of Osado
Mountains and in the southern part of Kosado Hills(Fig. 1; Shimazu et al. 1977). The Neogene and Quaternary sedimentary rocks are classified as, from bottom totop, the Orito, Hanyugawa, Nakayama, Nozaka, Kawachi,
Kaidate, and Shichiba Formations (Fig. 2; the Compilation Committee of the Geological Map of Niigata Prefecture 2000; Yanagisawa and Watanabe 2017b).
The first phase of the sedimentary rocks is called the
Orito Formation. The Orito Formation is composed ofconglomerate, sandstone, and mudstone, which are considered fluvial to inner bay deposits (Yanagisawa and
Watanabe 2017b). It is thought that the depositionalage of the Orito Formation is within the Neogene North
Pacific diatom zone (NPD) 3­ A1 (ca. 16.9–16.6 Ma) due tothe occurrence of Arcid-Potamid Fauna from the tropical
Kadonosawa Fauna (Yanagisawa and Watanabe 2017a).
The Hanyugawa Formation, which conformably covers the Orito Formation, is composed of conglomerate,calcareous sandstone, glauconite sandstone, and graymudstone (Yanagisawa and Watanabe 2017b). A basal

Yoshioka et al. Progress in Earth and Planetary Science(2024) 11:46

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38°20′N

The Japan Sea

Sado

38°10′N

500 kmPliocene-Pleistocene
sedimentary rocks
Miocene-Pliocene
sedimentary rocks
Middle Miocene
Ogi Basalt

Studied areain Fig. 3

Early Miocene
volcanic rocks

38°0′N138°30′E

Rhyolitic
intrusive rocks

138°15′E

Oligocene
volcanic rocks

37°50′N
Fig. 1 Location and a simplified geological map of Sado Island. The target formation in this study, the Nakayama Formation, is included
within the dark brown shade category, Miocene-Pliocene sedimentary rocks. A black rectangle shows the studied area, whose detailed map
is described in Fig. 3. Geological distribution is based on Sumi (1989) and the Compilation Committee of the Geological Map of Niigata Prefecture(2000), and stratigraphy is based on Yanagisawa and Watanabe (2017b)

conglomerate of the Hanyugawa Formation is recognized because of the marine transgression (Yanagisawa
and Watanabe 2017b). The age of the top of the Hanyugawa Formation was estimated as ca. 12.3 Ma based on
extrapolation of a sedimentation rate curve, which was
obtained from diatom biostratigraphy established in theupper Nakayama Formation (Yanagisawa and Watanabe
2017a). While the thickness of the Hanyugawa Formation

is no more than 15 m, the existence of glauconite beds
supports the long duration of the Hanyugawa Formation equivalent to ca. 4 Myr (Yanagisawa and Watanabe
2017b).
The Hanyugawa Formation is conformably covered by
the Nakayama Formation, which is composed of diatomaceous mudstone and a few glauconite beds. Diatom
biostratigraphy shows that the Nakayama Formation,

Yoshioka et al. Progress in Earth and Planetary Science

Age

Pleistocene
Pliocene
Late
MioceneFormations

Lithology

Shichiba

sandy silt, sand, & gravel

Kaidatesand & gravel

Kawachisandy mudstone

Nozakamassive mudstone

Nakayama

diatomaceous mudstone
hard mudstone

MiddleMiocene Hanyugawa

glauconitic sandstone
calcareous sandstone
conglomerate

Orito

sandstone, mudstone
conglomerate

Kimpokusan

dacite lava & pyroclastic rocks
andecite lave

Early
Miocene Masaragawa
Oligocene(2024) 11:46

altered dacite
pyroclastic rocks

Aikawaaltered andecite lava
& pyroclastic rocks

Nyukawa

dacite pumice tuff

Fig. 2 Cenozoic stratigraphy and simplified lithologies of Sado Island
in Niigata Prefecture, Central Japan. Stratigraphy and lithologies are
based on Yanagisawa and Watanabe (2017b). Color bars (dark gray,light gray, dark brown, and light brown) represent the classification
categories used in Fig. 1

Page 4 of 26

Formation is unconformably covered by the Kaidate
Formation, which is composed of Pleistocene sand andgravel with seashell fossil layers (Yanagisawa and Watanabe 2017b). The Kaidate Formation is conformably
overlain by the Shichiba Formation, which is composedof alternation of gravel, sand, and sandy silt with a lot ofseashell fossils and foraminifera (Yanagisawa and Watanabe 2017b).
The studied area is in the southern part of the Osado
Mountains foot area (Fig. 1). In this area, the Orito, Hanyugawa, Nakayama, and Nozaka Formations are exposedwith east dips and N–S to NE–SW strike directions
without major unconformities, and the Kawachi Formation unconformably covers the Nozaka Formation in thenorthern part and the Nakayama Formation in the southern part (Fig. 3; Sakamoto 1992; Yanagisawa and Watanabe 2017a). There are inferred faults in the middle part,and the degree of silica diagenesis is lower in the southof the faults than in the north (Yanagisawa and Watanabe 2017a). Diatom biostratigraphy was established inmost parts of the Nakayama and Nozaka Formations(Yanagisawa and Watanabe 2017a). On the other hand,
the lowest part of the Nakayama Formation, exposed in
SW (Hanyugawa) Section, is poorly constrained, but it
has been assigned within the diatom zone ­NPD5B3–4 (ca.
12.3–11.5 Ma; Yanagisawa and Watanabe 2017a).

2 Methods/experimental
whose thickness is 230–320 m, corresponds to NPD5B
to NPD7A (ca. 12.3–6.5 Ma; Yanagisawa and Watanabe
2017a). In the lowest part of the Nakayama Formation,
diatomaceous mudstone has often altered into hard siliceous mudstone due to silica diagenesis, and the hard
mudstone used to be called the Tsurushi Formation(Sakamoto 1992). However, the use of “the Tsurushi Formation” as a formation name is not recommended since
the horizon of the silica diagenesis boundary is not consistent among sites (Yanagisawa and Watanabe 2017b).
Then, we avoid using the name of the Tsurushi Formationin this study.
The Nakayama Formation is conformably overlain by
the Nozaka Formation, which is composed of massive
slightly diatomaceous mudstone formed during the late
Miocene to Pliocene (Yanagisawa and Watanabe 2017b).
The boundary between the Nakayama and Nozaka Formations is a tuff bed named WKA-5, which is one of oursamples for zircon U–Pb dating. Glauconite is also developed around the boundary.
The Kawachi Formation, which is composed of the
Pliocene to Pleistocene sandy mudstone with a lot ofseashell fossils, unconformably covers the Nozaka Formation (Yanagisawa and Watanabe 2017b). The Kawachi

2.1 Sampling strategy

In the studied area, many felsic tuff beds and several
mafic tuff beds are intercalated with the muddy sedimentof the Nakayama and Nozaka Formations (Fig. 4). Since
most of the tuff beds are thinner than 5 cm and finer than
medium sand, we took samples for zircon U–Pb datingfrom coarse-grained felsic tuff beds selectively. Each sample was equally collected from the top to the bottom ofeach tuff bed.
The two tuff samples (WKA-5 and TBA-3) weretaken from the sections with better-constrained diatom
biostratigraphy, WK (Wakamiya) and TB-II (Shichibagawa-II) Sections, corresponding to the upper and middle
part of the Nakayama Formation (Figs. 3 and 4). WKA-5,
which is exposed in WK Section, is a 30-cm-thick coarse
felsic glassy tuff with laminated intervals (Fig. 4). WKA-5
is intercalated with glauconite-bearing mudstone, andthe top of WKA-5 is defined as the boundary between
the Nakayama and Nozaka Formations (Yanagisawa and
Watanabe 2017b). TBA-3, which is exposed in TB-II Section, is a 10-cm-thick coarse felsic glassy tuff with pumice fragments (Fig. 4). While its lower boundary with
diatomaceous mudstone is sharp, the upper boundary is
weakly bioturbated.

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Fig. 3 A detailed geological map of the studied area. The geological distribution and diatom biohorizons in each section are based on Yanagisawa
and Watanabe (2017b). The Nakayama Formation is divided into two colors (sky blue and purple) depending on the degree of the silica diagenesis:
The sky-blue area represents diatomaceous mudstone in the opal-A zone, and purple area represents hard mudstone in the opal-CT zone. The
measured tuff samples were taken from WK Section (WKA-5), TB-II Section (TBA-3), and SW Section (SWT-1aU, SWT-2U, SWT-3U, SWT-3L, SWT-5U,
and SWT-5L). Brown zigzag lines represent sampling locations of WKA-5 and TBA-3. Sampling locations in SW Section are shown in Supplementary
Fig. 1

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WK & SN
composite (Yanagisawa & Watanabe, 2017)

Close-upcolumnar section

NozakaFm.

D77 Diatom biohorizon(Yanagisawa & Watanabe, 2017)

D75
D73

Columnar section

D70

laminateddiatomaceous mudstone

D68

weakly laminateddiatomaceous mudstone

300
mmassivediatomaceous mudstone

D66

slightly diatomaceous
mudstone

WKA-5

mudstone

Nakayama Formation

250
m

D65

mudstone
containing glauconite

D60
D59
D58

hard mudstone

D57
D56

20 cm

sandstone
tuff bedlaminated diatomaceous
mudstone(This study)
TB-I

mudstone
containing glauconite

D55.8

tuff

TB-III

150m

TBA-1’

lamination
patches of glauconite

TB-II

100
m

TBA-3
D55.2

50
m

massive diatomaceous
mudstone

SW

20 cm(This study)

D55

North

South
assumed fault

Orito
Fm.

Hanyugawa Fm.

Fig. 4 Columnar sections in the studied sections and close-up columnar sections around WKA-5 and TBA-3. The sampling locations of WKA-5
and TBA-3 are shown in Fig. 3. A composite columnar section in the north of the assumed fault (WK and SN Sections composite) is basedon Yanagisawa and Watanabe (2017a). The sedimentary sequence covers six Neogene North Pacific diatom zones from NPD5C to NPD7Ba
and contains 15 diatom biohorizons from D55 to D77 (Yanagisawa and Watanabe 2017a). Abbreviation: Fm. Formation

Yoshioka et al. Progress in Earth and Planetary Science

A

Radiolarian

Page 7 of 26

Laminateddiatomaceous mudstone
Massive diatomaceous
mudstone
Diatomaceous mudstone
containing glauconite

FO

60
m

Outcrop

SW Section(2024) 11:46

Hard siliceous mudstone

Nakayama Formation

50m

Diatombiohorizon
D55

B

SWT-1aU

Sandstone

SWT-1aL

Tuff (tuff sample)

C
SWT-1bU

40
m

SWT-2UD

SWT-2L

LO

E
F

SWT-1bL

20 cm

30
m

G
LO

SWT-3U

20m

SWT-5U

Orito
Fm.

Hanyugawa
Fm.

Hexaconthium akitaensis
Eucyrtidium inflatum
Calocyclas motoyamai
Lithopera renzae

SWT-3LH

20 cm
SWT-5L

20 cm

Lamination

Frame structure

Cross lamination

Graded bedding

PamiceGomashio

Pseudoconglomerate

Fig. 5 A composite columnar section with age-diagnostic radiolarian occurrences in SW Section. Three close-up columnar sections around fivesets of thick tuff beds (SWT-1a, SWT-1b, SWT-2, SWT-3, and SWT-5) are also displayed. Tuff names with underlines express the measured tuff
samples in this study. Alphabets A to H beside the composite columnar section represent outcrops described in the route map of SW Section(Supplementary Fig. 1). Only one diatom biohorizon (D55) was recognized in SW Section (Yanagisawa and Watanabe 2017a). Abbreviations: FO firstoccurrence, LO last occurrence, Fm. formation

Yoshioka et al. Progress in Earth and Planetary Science(2024) 11:46

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From SW Section, six tuff samples (SWT-1aU, SWT2U, SWT-3U, SWT-3L, SWT-5U, and SWT-5L) weretaken for zircon U–Pb dating (Fig. 5; Supplementary
Fig. 1). They are coarse felsic glassy tuff, whose thickness ranges from 15 to 60 cm. Detailed descriptions are
given in Sect. 3.1. Additionally, 15 diatomaceous mudstone samples were taken from SW Section for radiolarian analysis to further constrain the age model (Fig. 5).
Almost all sampling intervals are shorter than 4.2 m (withone exception of 11.9 m).

of Pb isotopic ratios and 238U/206Pb ratios, respectively,
before and after 10–20 spot analyses of unknown samples. We also analyzed OD-3 zircon (Iwano et al. 2013)as a secondary standard to evaluate the reliability of themeasurements. OD-3 zircon shows a weighted mean
206
Pb/238U age of 32.90 ± 0.12 Ma (n = 29), which agrees
with previously reported ages in Iwano et al. (2013)
within the 2 σ error ranges. We used Isoplot 3.75 program (Ludwig 2012) to calculate weighted means of206
Pb/238U ages and 207Pb-corrected ages and to draw
Tera-Wasserburg concordia diagrams.

2.2 Major element analysis of volcanic glass shards

Volcanic glass shard samples for major element analysis
were separated as a light-mineral fraction by a panning
process. After sieved with a 63-µm mesh, residue grains
were mounted in epoxy resin discs and polished until
their surfaces were flattened. We measured 10 major element compositions ­(SiO2, ­TiO2, ­Al2O3, FeO, MnO, MgO,
CaO, ­Na2O, ­K2O, and ­P2O5) of volcanic glass shards byusing a JEOL JXA-8900R electron probe microanalyzer(EPMA) equipped with wavelength-dispersive X-ray
spectrometers (WDS) at the Atmosphere and Ocean
Research Institute, the University of Tokyo (Kashiwa,
Japan). EPMA operating conditions were 15 kV acceleration voltage, 7 nA beam current, beam diameter of
10 µm, and counting time of each element was 10–60 s at
the peak position and 10–30 s at the background (Matsumoto et al. 2015; Nakanishi et al. 2020).
2.3 Zircon U–Pb measurement

Heavy minerals including zircon grains were separated
from tuff samples by elutriation, panning, magnetic
separation, and heavy liquid techniques at the National
Museum of Nature and Sciences (Tsukuba, Japan). Zircon grains were mounted in epoxy resin discs by handpicking. The zircon samples were polished until themidsection of the grains was flatly exposed.
Zircon samples were analyzed by a Nu Plasma II (Nu
Instruments, UK), multi-collector inductively coupledplasma-mass spectrometry (MC-ICP-MS), connected toa Jupiter solid nebulizer system (ST Japan, Japan) at the
Geochemical Research Center, the University of Tokyo(Tokyo, Japan). The laser with a wavelength of 257 nm
from CARBIDE (Light Convection, Lithuania) ablated
a square-shaped area with 20-µm sides. The mass spectrometer simultaneously detected 202Hg, 204Pb (+204Hg),
206
Pb, 207Pb, 208Pb, 235U, and 232Th (Obayashi et al. 2017;
Hattori et al. 2017). The 238U signal intensity was calculated based on the 235U signal intensity by normalizing
the 238U/235U ratio at 137.818 (Hiess et al. 2012). We used
NIST SRM 612 and GJ-1 zircon (Jackson et al. 2004) as
primary standards. Three spots of both NIST SRM 612
and GJ-1 were analyzed to calibrate the fractionation

2.4 Construction of radiolarian biostratigraphy

Samples for radiolarian analysis were prepared following the procedures described by Kamikuri et al. (2017).
The dried sediment samples were placed in beakers with
15% hydrogen peroxide to remove organic materials forseveral hours. A 5% solution of hydrochloric acid is generally added to remove the calcareous fraction from thesediments; however, because of the very poor content incalcareous matters of the used sediments, we did not usehydrochloric acid this time. Subsequently, after the samples were washed and sieved through a 45-μm mesh, they
were dried again in an oven at 40℃. Then, each sample
was scattered randomly on a glass slide. Next, Norland
Optical Adhesive #60 (refractive index = 1.56) was used
as a mounting medium for a 24 × 36-mm cover glass.
Then, all species on a slide were checked to identify key
biostratigraphic species useful to constrain sedimentages. Approximately at least 500 specimens were checked
for each sample.
2.5 Estimation of age errors of diatom and radiolarianbiostratigraphy

There are a lot of error factors difficult to estimate, forexample, errors of the age model of the reference sedimentary sequence and inter-regional effects on the timing of the species extinctions and occurrences. Theinter-regional effects could be minimized by adopting the biostratigraphy constructed in the neighboringbasins. In the case of diatom and radiolarian biostratigraphy of the Japan Sea sediments, the seafloor sediments
taken by IODP, ODP, and DSDP in the Japan Sea and theNorth Pacific are usually used as reference sedimentary
sequences.
In this study, we calculated errors of some diatom andradiolarian biohorizons by estimating 2 σ error ranges of
the age models of the reference sedimentary sequencesby using an open-source age-depth modeling routine, theUndatable software in MATLAB produced by Lougheed
and Obrochta (2019). Based on a sediment deposition
simulation, the Undatable can estimate the uncertainty
resulting from inconstant sedimentation rates. New age

Yoshioka et al. Progress in Earth and Planetary Science(2024) 11:46

Page 9 of 26

models containing the 2 σ error ranges were establishedfrom five sedimentary sequences (Supplementary Fig. 2),
ODP Sites 887C and 884B (the North Pacific), DSDP Site
438B and ODP Site 1151A (the Northwest Pacific), and
IODP Site U1430 (the Japan Sea), in order to obtain theage errors of diatom biohorizons reported in the Nakayama Formation by Yanagisawa and Watanabe (2017a)
and radiolarian biohorizons newly identified in SW Section in this study (Table 1). The input age constraints for
the Undatable calculations are listed in Supplementary
Table 1. The age models of Sites 887C and 884B wereconstructed based on magnetostratigraphy (Barron et al.
1995). The age model of Site 438 was based on selective
diatom biohorizons (Yanagisawa and Akiba 1998). The
age model of Site 1151A was based on magnetostratigraphy and diatom biohorizons (Motoyama et al. 2004).
The age model of Site U1430 was based on cyclostratigraphy (Kurokawa et al. 2019), which was established on theperfectly continuous spliced sequence (Irino et al. 2018).
The geomagnetic polarity timescale used in this study
was revised after Geologic Time Scale 2020 (Ogg 2020).
The ages and 2 σ error ranges of diatom biohorizons
used for Sites 438 and 1151A were derived from the diatom biostratigraphy in Sites 887C and 884B (Barron and
Gladenkov 1995; Watanabe and Yanagisawa 2005) andrevised after the new age models. The input 2 σ errors of
the age constraints were set at 5 kyr for magnetostratigraphy, individual widths of the calculated 2 σ error ranges
for diatom biostratigraphy, and 100 kyr for cyclostratigraphy (there are listed in Supplementary Table 1). The
calculation on the Undatable was executed under thefollowing conditions: nsim = ­105, bootpc = 0, xfactor = 0.1,
and combine = No (details of the parameters are given in
Lougheed and Obrochta 2019).

the lower (Outcrop G) is composed of massive diatomaceous mudstone, thick coarse tuff beds, and hard mudstone (Fig. 5).
Following the Neogene radiolarian biostratigraphic
scheme proposed in the Japan Sea by Kamikuri et al.
(2017), we identified four key radiolarian biohorizonsfrom the sedimentary sequence of SW Section: (1) theFirst Occurrence (FO) of Hexaconthium akitaensis in
Outcrop A, (2) The Last Occurrences (LO) of Eucyrtidium inflatum between Outcrops D and F, (3) the LO of
Calocyclas motoyamai between Outcrops D and F, (4) the
LO of Lithopera reanzae in Outcrop G (Fig. 5). The mean
ages of the FO of H. akitaensis and the LOs of E. inflatum,
C. motoyamai, and L. renzae were estimated as 11.35 Ma,
11.75 Ma, 11.75 Ma, and 12.07 Ma, respectively, in thisstudy (Table 1). This radiolarian biostratigraphy in SW
Section is consistent with the diatom biohorizon D55(11.55–11.33 Ma) previously reported by Yanagisawa and
Watanabe (2017a).
SW Section has several thick and coarse felsic tuff beds.
These tuff beds are useful for correlations among outcrops because of their unique sedimentary structures andtheir appearances as a set with another nearby tuff bed(Fig. 5). In the following paragraphs, we described below
the detailed sedimentary structures of five sets of thickfelsic tuff beds in SW Section: SWT-1a, SWT-1b, SWT2, SWT-3, and SWT-5.
SWT-1a consists of the upper 15-cm-thick tuff bed(SWT-1aU) and the lower 10-cm-thick tuff bed (SWT1aL) with intercalation of 10-cm-thick massive diatomaceous mudstone between the tuff beds. The top boundaryof SWT-1aU is gradual, and the bottom boundary is
sharp. SWT-1aU shows laminations in the middle part.
The top boundary of SWT-1aL is gradual, and the bottom boundary is erosional. The grain size of both tuff
beds is coarse to medium. SWT-1a is exposed in Outcrops B and C.
SWT-1b consists of the upper 35-cm-thick coarse tuff
bed (SWT-1bU) and the lower 40-cm-thick coarse-tofine tuff bed (SWT-1bL) with intercalation of 10-cmthick massive diatomaceous mudstone between the tuff
beds. The top boundary of SWT-1bU is gradual, andthe bottom boundary is erosional. The uppermost partof SWT-1bU looks like gomashio (dark-colored grains
in white matrix grains). The top boundary of SWT-1bL
is gradual, and the bottom boundary is sharp. SWT1bL shows laminations in the upper and lower parts andgraded structures in the middle part. SWT-1b is partially
exposed in Outcrop B and fully exposed in Outcrop C.
SWT-2 consists of the upper 35-cm-thick coarse tuff
bed (SWT-2U) and the lower 5-cm-thick coarse tuff
bed (SWT-2L) with intercalation of 15-cm-thick massive diatomaceous mudstone between the tuff beds. The

3 Results
3.1 Stratigraphy of SW section

Outcrops of SW Section are dispersedly located along
the Hanyugawa River (Supplementary Fig. 1). In thisstudy, the Nakayama Formation represented by diatomaceous mudstone with tuff and hard siliceous mudstone(Outcrops A to G), and the Orito Formation represented
by sandstone and conglomerate (Outcrop H) were recognized, but the lack of continuous exposure prevented us
from finding the outcrop containing the Hanyugawa Formation represented by thin glauconite sandstone, which
was described in Yanagisawa and Watanabe (2017a). The
thickness of the sedimentary sequence of the Nakayama
Formation we described in SW Section measures up to
ca. 48 m. The upper (Outcrops A–C) and middle parts(Outcrops D–F) of the sequence are composed of laminated and massive diatomaceous mudstone, several thickcoarse tuff beds, and thin hard mudstone layers, while

Yoshioka et al. Progress in Earth and Planetary Science(2024) 11:46

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Table 1 Newly calculated ages of diatom and radiolarian biohorizons(D ­codea)

Biohorizon

Calculated age (Ma)

Site

Reference

Mean

Upper limit 2σ

Lower limit 2σ

5.53

5.34

5.80

5.551

5.374

5.785

884B

1

5.487

5.269

5.827

887C

1

Diatom(D77)

FO Shionodiscus oestrupii(D75)

LO Rouxia californica

6.47

6.37

6.58

438

2

(D73)

LO Nitzschia pliocena

6.72

6.49

7.08

438

2

(D70)

LCO Thalassionema schraderi

7.68

7.62

7.75

7.678

7.623

7.739

884B

1

7.910

7.464

8.341

887C

1

(D68)

FO Nitzschia pliocena

7.92

7.79

8.05

884B

1

(D66)

FCO Thalassionema schraderi

8.49

8.36

8.61

438

2

(D65)

LO Denticulopsis katayamae

8.63

8.48

8.73

(D60)

(D59)

LO Denticulopsis dimorpha

FO Denticulopsis katayamae

8.653

8.511

8.725

884B

1

8.356

7.957

8.792

887C

1

9.24

9.14

9.30

9.246

9.160

9.301

884B

1

8.755

8.281

9.328

887C

1

9.40

9.35

9.43

9.400

9.352

9.425

884B

1

9.253

8.793

9.690

887C

1

(D58)

LO Denticulopsis dimorpha var. areolata

9.45

9.38

9.56

438

2

(D57)

FO Denticulopsis praekatayamae

9.59

9.43

9.87

438

2

(D56)

LO Denticulopsis dimorpha var. dimorpha

10.01

9.96

10.10

10.021

9.979

10.097

884B

1

9.641

9.170

10.044887C

1(D55.8)AC + LCO Denticulopsis hustedtii

10.19

10.07

10.35

438

2

(D55.2)FO Denticulopsis hustedtii

11.23

10.78

11.50

438

2

(D55)

LCO Denticulopsis praedimorpha

11.47

11.33

11.55

11.49411.19111.806884B

1

11.46311.344

11.531

887C

3

11.35

11.16

11.50

11.53011.18911.848884B

611.428

11.203

11.566887C

611.59611.06012.4821151A

6

11.27011.12911.387

U1430

7

11.75

11.62

11.86

11.874

11.60611.995884B

511.676

11.497

11.914887C

511.734

11.64811.823

U1430

7

11.75

11.62

11.86

11.874

11.60611.995884B

511.676

11.497

11.914887C

511.734

11.64811.823

U1430

7

12.07

11.93

12.23

12.19812.07812.318884B

511.676

11.497

11.914887C

512.561

11.63713.7761151A

4Radiolarian
FO Hexaconthium Akitaensis

LO Eucyrtidium inflatum

LO Calocyclas motoyamai

LO Lithopera renzae

Reference: 1 = Barron and Gladenkov (1995), 2 = Yanagisawa and Akiba (1998), 3 = Watanabe and Yanagisawa (2005), 4 = Kamikuri et al. (2004), 5 = Kamikuri et al.
(2007), 6 = Kamikuri (2010), 7 = Kamikuri et al. (2017)a

D code is a code number of a useful diatom biohorizon used in Yanagisawa and Akiba (1998)

Yoshioka et al. Progress in Earth and Planetary Science(2024) 11:46

Page 11 of 26

top boundary of SWT-2U is bioturbated, and the bottom boundary is sharp. SWT-2U shows laminations andpumice fragments. The top boundary of SWT-2L is bioturbated, and the bottom boundary is erosional. SWT-2L
also shows laminations. SWT-2 is exposed in Outcrops
D, E, and F.
SWT-3 consists of the upper 60-cm-thick coarse-tomedium tuff bed (SWT-3U) and the lower 40-cm-thickcoarse-to-fine tuff bed (SWT-3L) with intercalation of
10-cm-thick glauconite-bearing massive diatomaceous
mudstone between the tuff beds. The top boundary of
SWT-3U is bioturbated, and the bottom boundary is
erosional. SWT-3U shows laminations and pumice fragments in the upper part and graded bedding in the lower.
The top boundary of SWT-3L is gradual, and the bottom boundary is erosional. SWT-3L shows laminations,cross-laminations, frame structure, and pumice fragments. SWT-3 is exposed in Outcrop F.
SWT-5 consists of the upper 60-cm-thick coarse tuff
bed (SWT-5U) and the lower 40-cm-thick coarse tuff bed(SWT-5L) with intercalation of 35-cm-thick glauconitebearing massive diatomaceous mudstone between the
tuff beds. The top boundary of SWT-5U is gradual, andthe bottom boundary is erosional. SWT-5U is almost
massive except for a thin silt band. The top boundaryof SWT-5L is gradual, and the bottom boundary is erosional. SWT-5L shows pseudoconglomerate in the upper
part, cross-laminations in the middle part, and laminations in the lower part. SWT-5 is exposed in Outcrop G.

0.087–0.093 wt%). Interestingly, the U-group consistsof the upper tuff beds of SWT-1a, SWT-2, SWT-3, andSWT-5 (SWT-1aU, SWT-2U, SWT-3U, and SWT-5U),
and the L-group consists of the lower ones of SWT-3 andSWT-5 (SWT-3L and SWT-5L).
In this study, the classification of the shape of volcanic glass shards is mainly based on Yoshikawa (1976):
the bubble-wall type (Bw), the pumiceous type (Pm),the intermediate type (Im), and the irregularly shapedtype (Irr). Bw-type glass shards show planar shapes likefragments of large bubbles. Pm-type shards have a lot ofsmall bubbles or elongated ones in shards and are usually porous. Im-type shards show intermediate characteristics between Bw- and Pm-type shards. Irr-type shards
show different shapes from other types, for example,
glass shards by quench fragmentation. The observational
characteristic of each tuff sample is described below.
WKA-5 contains abundant high-temperature quartz
and glauconite other than volcanic glass shards. The volcanic glass of WKA-5 is composed of abundant Bw-type
glass shards with minor Pm-type ones. TBA-3 shows anunusual combination of heavy minerals: zircon, monazite, and barite. The volcanic glass of TBA-3 is composedof abundant Im-type glass shards with minor Pm- andBw-type ones. SWT-1aU contains a small portion oflithic fragments and sponge spicules. The volcanic glassof SWT-1aU is composed of abundant Pm-type glass
shards with minor Im- and Bw-type ones. SWT-2U contains a small amount of glauconite and sponge spicules.
The volcanic glass of SWT-2U is composed of abundant
Im-type glass shards with minor Pm- and Bw-type ones.
SWT-3U contains a small amount of glauconite. The volcanic glass of SWT-3U is composed of abundant Im-type
glass shards with minor Pm- and Bw-type ones. SWT-3L
contains a small amount of glauconite and sponge spicules. The volcanic glass of SWT-3L is composed of abundant Im-type glass shards with minor Pm- and Bw-type
ones. SWT-5U contains a small amount of glauconite anda lot of weathered lithic fragments and sponge spicules.
The volcanic glass of SWT-5U is composed of abundant
Im-type glass shards with minor Pm- and Bw-type ones.
SWT-5L contains weathered lithic fragments and sponge
spicules with a small amount of glauconite. The volcanic
glass of SWT-5L is composed of abundant Im-type glass
shards with minor Pm- and Bw-type ones.

3.2 Petrology of volcanic glass shards

The results of the major element analysis of volcanic glass
shards are listed in Table 2. The total of 10 major element
concentrations ranges from 88.93 to 92.95 wt%. They are
relatively lower than those of the Quaternary tephra beds
measured by the same machine (Nakanishi et al. 2020).
It possibly resulted from the hydration of volcanic glass
shards due to the longer duration after its sedimentation.
After the normalization of the major element concentrations as the total got to 100 wt%, the measured
volcanic glass shard samples similarly show high ­SiO2
concentrations ranging from 77.09 to 78.60 wt% and areplotted in the rhyolite zone of the total alkali-silica diagram by Le Bas et al. (1986) (Fig. 6a). On the other hand,the ­K2O–TiO2 diagram provides some variances, 3.45–
4.93 wt% of ­K2O concentrations and 0.093–0.205 wt% of­TiO2 concentrations (Fig. 6b). WKA-5 displays the lowest­K2O concentration and the highest T­ iO2 concentration
among the measured samples. In the K­ 2O–TiO2 diagram,the samples from SW Section were clearly divided into
two groups: U-group (the higher K­ 2O: 4.66–4.93 wt%
and the lower T­ iO2: 0.036–0.059 wt%) and L-group (themoderate ­K2O: 4.27–4.30 wt% and the moderate T­ iO2:

3.3 Zircon U–Pb ages

Obtained analytical data of the zircon grains are listed
in Table 3. We measured 18–39 euhedral zircon grains
for each sample. The non-radiogenic Pb ratios andthe 207Pb-corrected ages were calculated for all grains
by estimating the 207Pb/206Pb ratios of non-radiogenic Pb based on the two-stage model of Pb isotopic

11

11

SWT-5uSWT-5 l

0.09

0.09
30.70

77.95

0.77

50.73

0.05

18.41

1.19

77.45

1.09

77.91

1.43

11.91

1.42

12.37

1.39

11.91

0.84

12.45

11.81

1.36

14.62

1.07

7.86

1.34

3.69

1.00

0.07

40.37

0.04

25.22

0.07

48.49

0.04

20.50

0.07

16.44

The major element composition was normalized as the total got to 100 wt%

Classification of the shape of volcanic glass shards is based on Yoshikawa (1976)ab

CV coefficient of variation, Bw the bubble-wall type, Pm the pumiceous type, Im the intermediate type

CV (%)CV (%)CV (%)

0.04
65.96

3.08

1.00

0.06

27.49

0.03

27.04

0.05

26.07

0.03

18.39

0.05

15.67

0.07

22.94

8.44

0.93

21.41

0.71

6.46

0.94

4.87

0.66

4.01

0.76

4.41

0.70

4.95

3.35

3.80

3.55

5.22

3.45

4.58

3.34

3.21

3.49

4.96

3.52

4.25

3.62

4.73

4.30

5.75

4.66

3.38

4.27

1.86

4.86

2.76

4.90

2.34

4.93

5.46

4.71

0.03

95.70

0.03

112.390.02

136.030.02

144.90

0.01

85.44

0.02

140.110.02

104.83

90.28

90.90

90.25

90.42

90.92

91.11

90.86

91.31

11

0.52

77.53

1.00

12.56

0.06
35.85

7.91

0.48

97.89

0.02

SWT-3 lCV (%)

0.05
32.36

0.99
11.31

0.05
12.70

14.92

3.45

11

0.69

77.09

0.90

12.57

0.05
32.37

3.37

3.70

SWT-3uCV (%)

0.06
26.40

3.11

0.99

24.33

0.96

11

1.34

77.10

0.71

12.69

46.52

0.15

SWT-2uCV (%)

0.05
37.35

23.06

0.05

10

0.74

77.34

13.90

1.20

SWT-1au

CV (%)

3.75

11.68

11

41.41

0.21

TBA-3

1.53

78.59

12

WKA-5

CV (%)

SiO2 (wt%) TiO2 (wt%) Al2O3 (wt%) FeO (wt%) MnO (wt%) MgO (wt%) CaO (wt%) Na2O (wt%) K2O (wt%) P2O5 (wt%) Raw ­totala(wt%)

Tuff sample N

Table 2 Major element composition and shape of the volcanic glass shard samples

Im > Pm, Bw

Im > Pm, Bw

Im > Pm, Bw

Im > Pm, Bw

Im > Pm, Bw

Pm > Im, Bw

Im > Pm, Bw

Bw > Pm

Shape of glass­shardsb

Yoshioka et al. Progress in Earth and Planetary Science(2024) 11:46
Page 12 of 26

Yoshioka et al. Progress in Earth and Planetary Science(2024) 11:46

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a 16

Error ellipses are 1 σ

14

Na2O + K2O (wt%)

WKA-5
TBA-3
SWT-1aU
SWT-2U
SWT-3U
SWT-3L
SWT-5U
SWT-5L

Phonolite

12
10

Phonotephrite

8

Tephrite

6

Basanite

Trachybasalt

4
BasaltPicrobasalt

2040

Trachyte

Tephriphonolite

Foidite45

50

Basaltic
trachyandesite

Trachyandesite

Basaltic
andesite

55

Trachydacite

Andesite

60

DaciteRhyolite

65

70

SiO2 (wt%)

b 6

80

85

Error ellipses are 1 σ

U-group5

K2O (wt%)

75

L-group

4
3
2

U-group1

L-group
0

0

0.05

0.10

0.15

TiO2 (wt%)

0.20

WKA-5
TBA-3
SWT-1aU
SWT-2U
SWT-3U
SWT-3L
SWT-5U
SWT-5L

0.25

0.30

Fig. 6 a The total alkali-silica diagram (Le Bas et al. 1986) and b the ­K2O-TiO2 diagram. a All of the volcanic glass shard samples are plottedin the rhyolite zone. Error ellipses are 1 σ. b The volcanic glass shard samples from SW Section were clearly divided into two groups: U-groupand L-group in the ­K2O-TiO2 diagram. Error ellipses are 1 σ

evolution constructed by Stacey and Kramers (1975).
Zircon grains with higher non-radiogenic Pb than 5%
were excluded from the following U–Pb age determination protocol. To obtain information about the

sedimentary age, we distinguished the youngest groupof zircon grains from older grains. The youngest group
was defined as grains whose 207Pb-corrected ages overlap with any of the 207Pb-corrected ages overlapping

Yoshioka et al. Progress in Earth and Planetary Science(2024) 11:46

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Table 3 U–Pb measurement data of zircon grains from the sampled tuff beds in the Nakayama Formation
SampleWKA-5Grain noNonradiogenica
Pb (%)

Th/U

238

U/206Pb

207

Pb/206Pb

238

U/206Pb*

238

U-206Pb* age (Ma)555 ± 12

11.61 ± 0.25

g1

D

1.12

756

0.73

549 ± 120.0551 ± 0.0028

g2

D

34.88

606

0.57

655 ± 150.3217 ± 0.0099

g3

Y

0.67

323

0.71

1005 ± 32

0.0515 ± 0.0056

1012 ± 32

6.37 ± 0.21

0.20

669

0.40

556 ± 120.0478 ± 0.0028

557 ± 1211.55 ± 0.25

0.00

115

0.68

928 ± 440.0418 ± 0.0080

928 ± 44

6.94 ± 0.33

0.04

343

0.82

860 ± 25

0.0465 ± 0.0047

860 ± 25

7.49 ± 0.23

0.24

1415

0.88

968 ± 200.0480 ± 0.0026

971 ± 20

6.64 ± 0.14

1.07

130

0.70

895 ± 400.0547 ± 0.0085

905 ± 40

7.12 ± 0.32

g4
g6

Y

g8
g9

Y

g10
g13

D

0.97

342

0.97

530 ± 140.0539 ± 0.0040

536 ± 1412.03 ± 0.32

g15

D

1.03

263

0.53

561 ± 160.0544 ± 0.0048

567 ± 16

11.36 ± 0.33

0.32

380

0.77

555 ± 140.0488 ± 0.0037

557 ± 14

11.56 ± 0.30

g16
g17

D

7.35

252

0.57

58.9 ± 1.2

0.1064 ± 0.0025

g19

D

10.49

202

0.79

851 ± 310.129 ± 0.011

g20

D

13.73

257

0.85

490 ± 150.1548 ± 0.0093

0.14

5129

0.70

550.9 ± 9.5

0.0474 ± 0.0012

551.7 ± 9.5

11.67 ± 0.20

Y

0.38

1146

0.72

958 ± 220.0492 ± 0.0033

962 ± 226.70 ± 0.16

0.53

193

0.71

889 ± 370.0504 ± 0.0077

894 ± 38

7.21 ± 0.31

g26

D

11.16

3742

0.66

854 ± 160.1343 ± 0.0031

g27

D

15.47

561

0.89

799 ± 220.1684 ± 0.0084

g28

0.13

630

0.83

553 ± 130.0473 ± 0.0033

553 ± 13

11.64 ± 0.28

g29

0.22

1053

1.19

555 ± 120.0480 ± 0.0026

556 ± 12

11.58 ± 0.25

0.00

538

0.69

956 ± 280.0433 ± 0.0044

956 ± 28

6.74 ± 0.19

0.41

613

0.73

557 ± 130.0495 ± 0.0034

559 ± 13

11.52 ± 0.28

g21
g23
g25

g31

Y

g32
g33
g34
TBA-3

U content (ppm)

Y

0.00

651

0.45

561 ± 130.0422 ± 0.0031

561 ± 13

11.49 ± 0.27

0.00

385

0.84

934 ± 30

0.0457 ± 0.0053

934 ± 30

6.90 ± 0.22

g1

Y

0.23

516

0.63

594 ± 130.0481 ± 0.0037

595 ± 1310.82 ± 0.24

g2

D

2.45

805

0.43

90.5 ± 1.1

0.0668 ± 0.0016

92.8 ± 1.2

69.10 ± 0.88

g4

D

17.46

1985

2.53

478.1 ± 6.3

0.1843 ± 0.0041

g8

Y

0.46

581

1.70

585 ± 120.0499 ± 0.0036

588 ± 1210.96 ± 0.23

g9

D

26.52

661

2.62

437.5 ± 7.8

0.2559 ± 0.0077

0.00

161

0.61

96.9 ± 2.0

0.0474 ± 0.0027

96.9 ± 2.0

66.2 ± 1.4

g10
g12

Y

0.60

381

0.78

578 ± 140.0510 ± 0.0044

582 ± 1411.07 ± 0.28

g13

YD

1.04

617

1.11

592 ± 120.0545 ± 0.0037

598 ± 1210.77 ± 0.22

g15

D

26.63

915

1.94

434.3 ± 7.5

0.2567 ± 0.0067

g16

Y

0.03

879

0.69

595 ± 110.0465 ± 0.0029

595 ± 1110.83 ± 0.21

g17

Y

0.13

1358

1.73

591.1 ± 9.8

0.0473 ± 0.0024

591.9 ± 9.8

10.88 ± 0.18

g18

0.20

1464

0.32

112.4 ± 1.5

0.0487 ± 0.0011

112.6 ± 1.5

57.00 ± 0.74

g19

0.14

302

0.83

94.9 ± 1.7

0.0485 ± 0.0020

95.0 ± 1.7

67.5 ± 1.2

g21

Y

0.18

452

1.73

591 ± 140.0477 ± 0.0041

592 ± 14

10.89 ± 0.27

g23

Y

0.36

809

1.00

600 ± 120.0491 ± 0.0031

603 ± 12

10.69 ± 0.21

g24

YD

0.78

2752

0.35

581.1 ± 8.2

0.0524 ± 0.0018

585.6 ± 8.2

11.00 ± 0.16

Yoshioka et al. Progress in Earth and Planetary Science(2024) 11:46

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Table 3 (continued)
SampleGrain nog26
SWT-1aU

Nonradiogenica
Pb (%)
Y

Th/U

238

U/206Pb

207

Pb/206Pb

238

U/206Pb*

238

U-206Pb* age (Ma)

0.34

1219

1.07

593 ± 100.0490 ± 0.0025

595 ± 1010.82 ± 0.19

g27

0.00

689

1.68

95.0 ± 1.4

0.0472 ± 0.0014

95.0 ± 1.4

67.53 ± 0.97

g4

0.02

409

0.95

280.3 ± 4.2

0.0467 ± 0.0025

280.4 ± 4.2

22.95 ± 0.35

g5

0.08

539

0.56

281.6 ± 3.7

0.0471 ± 0.0022

281.8 ± 3.7

22.84 ± 0.31

g6

Y

0.00

380

0.64

543 ± 100.0456 ± 0.0035

543 ± 1111.87 ± 0.23

g7

D

0.43

458

1.19

283.0 ± 4.0

0.0499 ± 0.0024

284.3 ± 4.1

22.64 ± 0.33

534.1 ± 7.4

12.06 ± 0.17

285.9 ± 5.0

22.51 ± 0.40

g8

Y

0.20

797

0.43

533.0 ± 7.4

0.0478 ± 0.0025

g11

D

5.70

153

0.75

271.4 ± 6.4

0.0916 ± 0.0057

0.33

300

0.84

285.0 ± 4.9

0.0491 ± 0.0030

g12
g15

D

23.16

155

0.95

220.6 ± 4.8

0.2297 ± 0.0087

g16

D

26.34

278

0.54

395.4 ± 8.0

0.2545 ± 0.0093

0.07

565

0.91

279.8 ± 3.6

0.0471 ± 0.0021

280.0 ± 3.6

22.98 ± 0.30

g17
g18

Y

0.05

348

0.45

538 ± 110.0467 ± 0.0037

538 ± 1111.97 ± 0.25

g20

D

0.28

3878

1.11

511.0 ± 3.8

0.0485 ± 0.0011

512.4 ± 3.8

12.568 ± 0.094

g22

Y

g23
g24

Y

g25
g26

YD

g28
g29

D

g30

SWT-2UU content (ppm)

0.00

384

0.56

539 ± 100.0451 ± 0.0034

539 ± 1011.95 ± 0.23

0.00

1100

1.47

282.9 ± 5.5

0.0462 ± 0.0017

282.9 ± 5.5

22.75 ± 0.44

0.25

472

0.52

535 ± 130.0482 ± 0.0033

536 ± 1312.01 ± 0.30

0.37

296

0.69

254.2 ± 6.1

0.0494 ± 0.0030

255.2 ± 6.1

25.21 ± 0.61

1.16

578

0.71

530 ± 120.0554 ± 0.0033

537 ± 1312.00 ± 0.29

0.17

1916

1.38

294.1 ± 5.5

0.0478 ± 0.0014

294.6 ± 5.5

21.84 ± 0.41

1.50

230

0.75

280.9 ± 7.3

0.0583 ± 0.0038

285.1 ± 7.4

22.57 ± 0.60

0.01

102

0.79

276.6 ± 9.4

0.0466 ± 0.0050

276.6 ± 9.4

23.26 ± 0.80

g33

Y

0.15

1148

1.18

537 ± 110.0475 ± 0.0022

538 ± 1111.98 ± 0.25

g34

Y

0.55

242

0.57

540 ± 160.0506 ± 0.0048

543 ± 16

11.85 ± 0.36

g35

0.16

183

0.96

280.6 ± 7.8

0.0477 ± 0.0038

281.1 ± 7.8

22.90 ± 0.65

g37

0.28

214

0.68

269.6 ± 7.1

0.0487 ± 0.0035

270.3 ± 7.1

23.80 ± 0.63

540 ± 1111.93 ± 0.24

281.5 ± 5.9

22.86 ± 0.49

g38

D

12.18

262

0.66

461 ± 130.1548 ± 0.0080

g40

Y

0.19

1257

0.87

539 ± 110.0478 ± 0.0021

g43

D

15.26

219

0.59

441 ± 130.1669 ± 0.0090

0.02

413

0.96

281.4 ± 5.9

0.0467 ± 0.0029

g1

0.95

240

0.78

272.6 ± 6.8

0.0540 ± 0.0040

275.2 ± 6.9

23.38 ± 0.60

g3

g2

D

0.00

618

0.50

295.1 ± 5.5

0.0454 ± 0.0024

295.1 ± 5.5

21.81 ± 0.41

g4

0.00

235

0.56

251.2 ± 6.2

0.0464 ± 0.0036

251.2 ± 6.2

25.61 ± 0.63

g5

0.15

740

0.93

234.0 ± 4.0

0.0478 ± 0.0021

234.3 ± 4.1

27.45 ± 0.48

g6

D

2.38

1646

0.39

267.9 ± 4.1

0.0653 ± 0.0019

274.4 ± 4.2

23.45 ± 0.36

g7

Y

0.37

406

0.56

543 ± 140.0492 ± 0.0041

545 ± 14

11.81 ± 0.30

g8

0.17

552

1.13

276.5 ± 5.3

0.0478 ± 0.0026

277.0 ± 5.3

23.23 ± 0.45

g9

0.00

436

0.62

230.8 ± 4.6

0.0461 ± 0.0026

230.8 ± 4.6

27.87 ± 0.55

Yoshioka et al. Progress in Earth and Planetary Science(2024) 11:46

Page 16 of 26

Table 3 (continued)
SampleGrain noNonradiogenica
Pb (%)

U content (ppm)

Th/U

238

207

238

238

g13

0.00

1295

0.89

279.3 ± 4.4

0.0454 ± 0.0017

279.3 ± 4.4

23.04 ± 0.36

g14

0.00

496

1.44

277.9 ± 5.5

0.0464 ± 0.0027

277.9 ± 5.5

23.15 ± 0.46

1.45

490

0.99

279.3 ± 5.6

0.0580 ± 0.0030

283.5 ± 5.6

22.70 ± 0.46

0.02

1598

0.64

36.09 ± 0.50

0.04974 ± 0.00095

36.10 ± 0.50

176.1 ± 2.4

D

0.81

731

0.55

281.1 ± 5.0

0.0529 ± 0.0024

283.4 ± 5.1

22.71 ± 0.41

0.28

330

0.77

248.3 ± 5.4

0.0487 ± 0.0031

249.0 ± 5.5

25.83 ± 0.57

Y

0.27

1535

1.11

549.6 ± 9.4

0.0484 ± 0.0022

551.1 ± 9.4

11.69 ± 0.20

0.12

909

0.72

280.2 ± 5.1

0.0474 ± 0.0019

280.5 ± 5.1

22.94 ± 0.42

3.07

76

0.34

560 ± 280.070 ± 0.011

578 ± 29

11.14 ± 0.57

g24

0.00

494

0.55

278.5 ± 5.7

0.0458 ± 0.0024

278.5 ± 5.7

23.11 ± 0.47

g25

0.10

466

0.73

255.8 ± 5.2

0.0473 ± 0.0024

256.1 ± 5.2

25.12 ± 0.52

10.35

309

0.40

476 ± 130.1281 ± 0.0070

0.10

388

0.54

299.7 ± 6.6

0.0472 ± 0.0029

300.0 ± 6.6

21.46 ± 0.48

g15

D

g17
g18
g19
g20
g22
g23

g26

YD

D

g27
g28

Pb/206Pb

U/206Pb*

U-206Pb* age (Ma)

0.08

2076

0.78

277.4 ± 4.5

0.0471 ± 0.0013

277.7 ± 4.5

23.18 ± 0.38

4.02

306

1.03

238.4 ± 5.4

0.0784 ± 0.0038

248.4 ± 5.6

25.90 ± 0.60

0.00

347

0.80

245.2 ± 5.4

0.0461 ± 0.0027

245.2 ± 5.4

26.24 ± 0.57

0.87

1524

0.84

275.7 ± 4.7

0.0534 ± 0.0016

278.1 ± 4.7

23.14 ± 0.39

g33

0.08

243

0.66

255.5 ± 6.3

0.0472 ± 0.0033

255.8 ± 6.3

25.16 ± 0.63

g36

0.02

315

0.59

276.7 ± 6.4

0.0467 ± 0.0030

276.7 ± 6.4

23.25 ± 0.54

0.44

603

1.12

283.6 ± 5.6

0.0500 ± 0.0023

284.9 ± 5.6

22.59 ± 0.45

0.25

566

1.70

234.4 ± 4.5

0.0485 ± 0.0022

235.0 ± 4.5

27.37 ± 0.53

g29

D

g31
g32

g38

D

D

g39
g40

D

1.84

156

0.70

282.6 ± 8.3

0.0610 ± 0.0050

287.9 ± 8.4

22.35 ± 0.67

0.08

337

1.32

240.7 ± 5.3

0.0472 ± 0.0028

240.9 ± 5.3

26.71 ± 0.59

0.00

694

0.77

276.7 ± 5.3

0.0425 ± 0.0020

276.7 ± 5.3

23.26 ± 0.44

g43

0.14

335

0.81

283.0 ± 6.4

0.0476 ± 0.0030

283.3 ± 6.4

22.71 ± 0.52

g44

0.08

551

0.41

277.5 ± 5.5

0.0471 ± 0.0023

277.7 ± 5.5

23.17 ± 0.47

g45

0.14

393

0.80

263.0 ± 5.6

0.0476 ± 0.0027

263.4 ± 5.7

24.43 ± 0.53

g1

0.18

2801

1.31

297.1 ± 3.2

0.0478 ± 0.0012

297.7 ± 3.2

21.62 ± 0.23

g2

0.15

282

0.79

284.5 ± 5.9

0.0476 ± 0.0033

284.9 ± 5.9

22.59 ± 0.48

g41
g42

SWT-3U

U/206Pb

D

g3

0.22

856

0.63

279.4 ± 3.9

0.0482 ± 0.0019

280.0 ± 3.9

22.98 ± 0.32

0.27

856

0.46

539.6 ± 8.8

0.0484 ± 0.0026

541.1 ± 8.9

11.90 ± 0.20

g5

0.19

158

0.43

228.8 ± 5.7

0.0481 ± 0.0039

229.2 ± 5.7

28.06 ± 0.71

g6

0.07

446

0.72

280.1 ± 4.8

0.0470 ± 0.0026

280.3 ± 4.8

22.96 ± 0.40

g4

Y

g7

Y

0.12

280

0.50

560 ± 150.0472 ± 0.0046

561 ± 15

11.48 ± 0.31

g8

YD

2.23

200

0.59

522 ± 15

0.0639 ± 0.0061

534 ± 1612.06 ± 0.37

g9

Y

0.01

1572

1.07

540.5 ± 7.3

0.0463 ± 0.0019

540.6 ± 7.3

11.91 ± 0.16

g10

D

0.32

1124

0.63

277.7 ± 3.6

0.0490 ± 0.0017

278.6 ± 3.6

23.10 ± 0.30

g11

Y

0.00

326

0.48

540 ± 130.0435 ± 0.0040

540 ± 13

11.93 ± 0.29

Yoshioka et al. Progress in Earth and Planetary Science(2024) 11:46

Page 17 of 26

Table 3 (continued)
SampleGrain noNonradiogenica
Pb (%)

U content (ppm)

Th/U

238

207

238

238

g12

0.12

662

0.83

278.4 ± 4.2

0.0474 ± 0.0021

278.7 ± 4.2

23.09 ± 0.35

0.19

1040

0.78

539.9 ± 8.3

0.0478 ± 0.0024

540.9 ± 8.3

11.91 ± 0.19

g14

0.00

397

0.63

276.7 ± 4.9

0.0447 ± 0.0026

276.7 ± 4.9

23.26 ± 0.42

g15

0.14

744

0.65

287.1 ± 4.2

0.0476 ± 0.0021

287.5 ± 4.2

22.39 ± 0.33

g16

0.01

523

0.70

277.5 ± 4.5

0.0466 ± 0.0024

277.5 ± 4.5

23.19 ± 0.38

g13

Y

Pb/206Pb

U/206Pb*

U-206Pb* age (Ma)

g17

Y

0.00

497

0.35

533 ± 110.0452 ± 0.0033

533 ± 1112.08 ± 0.24

g18

YD

0.62

1611

0.43

527.9 ± 7.0

0.0512 ± 0.0020

531.2 ± 7.1

12.12 ± 0.16

0.26

208

0.94

284.2 ± 6.7

0.0485 ± 0.0038

285.0 ± 6.7

22.58 ± 0.54

g19
g20

YD

0.76

319

0.77

544 ± 130.0522 ± 0.0044

548 ± 1311.75 ± 0.29

g21

D

1.07

202

0.65

284.8 ± 6.6

0.0550 ± 0.0040

287.9 ± 6.7

22.36 ± 0.53

g22

0.14

507

0.57

289.9 ± 4.8

0.0475 ± 0.0024

290.3 ± 4.8

22.16 ± 0.37

g23

0.33

340

0.72

282.0 ± 5.3

0.0491 ± 0.0029

283.0 ± 5.3

22.74 ± 0.44

Y

0.00

596

0.55

537.3 ± 9.9

0.0449 ± 0.0029

537.3 ± 9.9

11.99 ± 0.22

0.00

397

0.68

280.3 ± 5.0

0.0462 ± 0.0026

280.3 ± 5.0

22.96 ± 0.41

g26

D

1.53

109

0.93

287.2 ± 8.8

0.0585 ± 0.0056

291.6 ± 8.9

22.07 ± 0.69

g28

YD

1.55

416

0.74

539 ± 11

0.0585 ± 0.0040

547 ± 1211.76 ± 0.26

0.02

655

0.58

233.8 ± 3.4

0.0468 ± 0.0019

233.8 ± 3.4

27.51 ± 0.40

1.77

362

0.70

279.3 ± 5.1

0.0605 ± 0.0032

284.3 ± 5.2

22.63 ± 0.42

g31

0.00

699

1.04

278.7 ± 4.1

0.0449 ± 0.0020

278.7 ± 4.1

23.09 ± 0.34

g33

0.06

1215

0.79

524.7 ± 7.6

0.0467 ± 0.0021

525.0 ± 7.6

12.27 ± 0.18

D

3.49

212

0.82

237.9 ± 5.1

0.0741 ± 0.0042

246.5 ± 5.3

26.10 ± 0.58

0.27

153

0.74

266.3 ± 6.8

0.0486 ± 0.0042

267.0 ± 6.8

24.10 ± 0.63

g37

D

0.74

205

0.52

264.5 ± 6.0

0.0524 ± 0.0037

266.5 ± 6.0

24.14 ± 0.56

g38

D

0.92

497

0.68

281.2 ± 4.6

0.0537 ± 0.0026

283.8 ± 4.7

22.67 ± 0.38

g39

YD

0.65

412

0.34

534 ± 110.0514 ± 0.0037

537 ± 1111.99 ± 0.26

g40

D

0.44

1138

0.62

301.0 ± 3.9

0.0499 ± 0.0017

302.3 ± 3.9

21.29 ± 0.28

0.00

245

0.72

257.5 ± 5.4

0.0443 ± 0.0031

257.5 ± 5.4

24.98 ± 0.52

g24
g25

g29
g30

g34

D

g35

g43
SWT-3L

U/206Pb

g1

Y

0.00

280

0.37

536 ± 130.0458 ± 0.0042

536 ± 1312.01 ± 0.29

g2

Y

0.00

801

0.54

530.3 ± 8.0

0.0454 ± 0.0025

530.3 ± 8.0

12.14 ± 0.18

g3

D

6.83

334

0.76

526 ± 120.1002 ± 0.0059

g5

Y

0.26

432

0.45

524 ± 100.0483 ± 0.0035

525 ± 10

12.27 ± 0.25

g6

YD

0.59

586

1.19

539.4 ± 9.3

0.0509 ± 0.0031

542.6 ± 9.4

11.87 ± 0.21

g7

D

59.52

461

1.28

219.5 ± 3.1

0.5172 ± 0.0091

g9

0.00

1639

0.36

65.52 ± 0.51

0.04776 ± 0.00065

65.52 ± 0.51

97.64 ± 0.75

g10

0.07

1968

0.74

280.3 ± 2.5

0.0470 ± 0.0012

280.5 ± 2.5

22.94 ± 0.21

g11

0.00

857

0.67

280.8 ± 3.4

0.0455 ± 0.0018

280.8 ± 3.4

22.91 ± 0.27

0.00

261

0.67

541 ± 140.0460 ± 0.0044

541 ± 14

11.89 ± 0.30

g12

Y

Yoshioka et al. Progress in Earth and Planetary Science(2024) 11:46

Page 18 of 26

Table 3 (continued)
SampleGrain noU content (ppm)

Th/U

238

U/206Pb

207

Pb/206Pb

238

U/206Pb*

238

U-206Pb* age (Ma)

g13

YD

0.64

475

1.16

547 ± 110.0514 ± 0.0035

551 ± 1111.70 ± 0.23

g15

D

1.80

840

1.17

275.2 ± 3.3

0.0607 ± 0.0020

280.2 ± 3.4

22.96 ± 0.28

0.02

1474

0.52

280.1 ± 2.8

0.0467 ± 0.0014

280.1 ± 2.8

22.97 ± 0.23

g16
g18

Y

0.02

614

0.85

540 ± 13

0.0464 ± 0.0030

540 ± 13

11.93 ± 0.29

g19

Y

0.06

321

0.77

538 ± 15

0.0468 ± 0.0041

538 ± 16

11.97 ± 0.35

g20

D

63.60

239

0.62

206.4 ± 5.3

0.550 ± 0.014

g21

Y

0.57

244

0.55

544 ± 170.0508 ± 0.0049

547 ± 17

11.78 ± 0.38

g22

Y

0.03

981

0.78

542 ± 120.0465 ± 0.0024

542 ± 1211.88 ± 0.27

g23

D

1.07

463

0.87

279.8 ± 6.5

0.0549 ± 0.0027

282.8 ± 6.5

22.76 ± 0.53

g24

Y

0.00

358

0.94

532 ± 15

0.0453 ± 0.0038

532 ± 15

12.10 ± 0.34

g25

0.00

1249

0.35

233.7 ± 4.6

0.0463 ± 0.0014

233.7 ± 4.6

27.52 ± 0.55

g26

0.05

675

0.70

280.2 ± 6.1

0.0469 ± 0.0021

280.3 ± 6.1

22.95 ± 0.50

g27

0.03

978

0.57

277.7 ± 5.7

0.0467 ± 0.0017

277.7 ± 5.7

23.17 ± 0.48

g28

Y

g29

SWT-5U

Nonradiogenica
Pb (%)

0.38

407

0.92

531 ± 140.0493 ± 0.0037

533 ± 1412.09 ± 0.33

0.53

168

1.00

248.8 ± 7.3

0.0508 ± 0.0040

250.1 ± 7.3

25.72 ± 0.76

g30

YD

0.00

220

1.02

536 ± 170.0385 ± 0.0045

536 ± 1712.01 ± 0.39

g31

Y

0.00

267

0.70

535 ± 160.0456 ± 0.0044

535 ± 1612.04 ± 0.37

g32

Y

0.00

135

0.64

523 ± 200.0440 ± 0.0060

523 ± 20

12.32 ± 0.48

g33

Y

0.00

301

0.32

537 ± 160.0445 ± 0.0041

537 ± 16

11.99 ± 0.35

g34

Y

0.45

245

0.75

535 ± 170.0498 ± 0.0048

538 ± 17

11.97 ± 0.38

0.32

708

0.64

280.5 ± 4.1

0.0490 ± 0.0022

281.4 ± 4.1

22.87 ± 0.34

g1

0.38

427

0.64

544 ± 120.0493 ± 0.0040

547 ± 1211.78 ± 0.27

g3

g2

Y

0.20

784

0.82

283.4 ± 4.0

0.0481 ± 0.0021

283.9 ± 4.0

22.66 ± 0.33

g4

0.45

391

0.72

263.9 ± 4.8

0.0501 ± 0.0029

265.1 ± 4.8

24.27 ± 0.45

g5

0.04

944

0.94

267.4 ± 3.5

0.0469 ± 0.0018

267.5 ± 3.5

24.06 ± 0.32

g6

0.21

964

0.57

233.0 ± 2.9

0.0482 ± 0.0017

233.5 ± 2.9

27.55 ± 0.35

g7

0.00

664

0.49

233.2 ± 3.3

0.0456 ± 0.0020

233.2 ± 3.3

27.59 ± 0.40

g8

0.17

272

0.40

233.6 ± 4.8

0.0479 ± 0.0032

234.0 ± 4.8

27.49 ± 0.58

g9

0.00

702

0.66

274.5 ± 4.0

0.0449 ± 0.0021

274.5 ± 4.0

23.45 ± 0.34

0.30

753

0.82

279.7 ± 4.0

0.0489 ± 0.0021

280.5 ± 4.0

22.94 ± 0.34

g12

g10
Y

0.10

460

0.93

536 ± 120.0471 ± 0.0037

536 ± 1212.00 ± 0.26

g13

Y

0.49

313

0.83

543 ± 140.0501 ± 0.0047

545 ± 14

11.81 ± 0.31

g14

D

0.83

995

0.97

276.4 ± 3.6

0.0530 ± 0.0019

278.7 ± 3.6

23.09 ± 0.30

g15

Y

0.11

1772

0.87

541.7 ± 6.9

0.0471 ± 0.0019

542.3 ± 6.9

11.88 ± 0.15

g16

Y

0.30

290

0.74

535 ± 140.0486 ± 0.0047

537 ± 14

11.99 ± 0.33

g17

D

3.74

971

1.09

248.4 ± 3.2

0.0761 ± 0.0023

258.1 ± 3.3

24.93 ± 0.33

g18

Y

0.28

674

0.87

535.8 ± 9.9

0.0485 ± 0.0031

537.3 ± 9.9

11.99 ± 0.23

0.00

550

0.60

278.0 ± 4.5

0.0452 ± 0.0024

278.0 ± 4.5

23.15 ± 0.37

g19

Yoshioka et al. Progress in Earth and Planetary Science(2024) 11:46

Page 19 of 26

Table 3 (continued)
SampleNonradiogenica
Pb (%)

U content (ppm)

Th/U

238

207

238

238

0.04

477

0.50

533 ± 110.0466 ± 0.0036

534 ± 1112.07 ± 0.26

g21

0.08

374

0.92

279.0 ± 5.3

0.0471 ± 0.0030

279.2 ± 5.3

23.05 ± 0.44

g22

0.11

497

0.71

282.2 ± 4.2

0.0473 ± 0.0024

282.5 ± 4.3

22.78 ± 0.35

g23

0.02

507

0.40

231.5 ± 3.3

0.0467 ± 0.0021

231.5 ± 3.3

27.79 ± 0.40

g24

0.44

143

0.85

244.9 ± 6.2

0.0500 ± 0.0042

245.9 ± 6.2

26.16 ± 0.67

Grain nog20

SWT-5LY

U/206Pb

Pb/206Pb

U/206Pb*

U-206Pb* age (Ma)

g25

0.34

288

0.73

263.1 ± 4.9

0.0492 ± 0.0031

264.0 ± 4.9

24.37 ± 0.46

g26

0.19

331

0.77

293.7 ± 5.3

0.0480 ± 0.0030

294.3 ± 5.4

21.87 ± 0.41

g27

0.20

864

0.85

285.7 ± 3.5

0.0481 ± 0.0019

286.3 ± 3.5

22.48 ± 0.28

g28

0.00

1177

0.43

231.8 ± 2.4

0.0464 ± 0.0014

231.8 ± 2.4

27.75 ± 0.29

g29

0.13

1244

0.53

233.0 ± 2.4

0.0476 ± 0.0014

233.3 ± 2.4

27.57 ± 0.28

g30

0.27

181

0.74

258.2 ± 5.9

0.0487 ± 0.0038

258.9 ± 5.9

24.85 ± 0.58

g31

D

0.55

626

0.54

276.2 ± 3.7

0.0509 ± 0.0022

277.7 ± 3.8

23.17 ± 0.32

g32

D

1.12

275

0.64

292.3 ± 5.8

0.0553 ± 0.0035

295.6 ± 5.8

21.77 ± 0.44

g33

0.14

887

1.05

281.1 ± 3.4

0.0476 ± 0.0018

281.5 ± 3.4

22.86 ± 0.28

g34

0.01

259

1.39

254.9 ± 4.9

0.0466 ± 0.0031

254.9 ± 4.9

25.24 ± 0.50

g35

0.00

552

0.43

281.8 ± 4.1

0.0464 ± 0.0022

281.8 ± 4.1

22.83 ± 0.33

g36

0.07

606

0.93

282.0 ± 3.9

0.0471 ± 0.0022

282.2 ± 3.9

22.80 ± 0.32

g37

0.00

203

0.55

231.6 ± 4.9

0.0463 ± 0.0033

231.6 ± 4.9

27.78 ± 0.58

g38

0.06

1129

0.36

232.4 ± 2.4

0.0471 ± 0.0015

232.6 ± 2.4

27.66 ± 0.29

g39

0.00

175

0.76

247.1 ± 5.7

0.0462 ± 0.0037

247.1 ± 5.7

26.04 ± 0.60

g40

0.17

926

0.37

231.5 ± 2.6

0.0480 ± 0.0016

231.9 ± 2.6

27.74 ± 0.31

g1

D

1.60

445

0.71

278.5 ± 4.2

0.0591 ± 0.0029

283.1 ± 4.3

22.73 ± 0.35

g2

Y

0.00

239

0.88

534 ± 140.0451 ± 0.0047

534 ± 1412.06 ± 0.32

g3

YD

0.91

588

0.98

530.6 ± 9.0

0.0535 ± 0.0033

535.5 ± 9.1

12.03 ± 0.21

g4

YD

3.71

71

0.69

499 ± 24

0.076 ± 0.011

518 ± 24

12.42 ± 0.61

g5

Y

0.33

398

0.91

543 ± 110.0489 ± 0.0038

545 ± 1111.82 ± 0.25

g6

Y

0.20

364

0.74

540 ± 120.0479 ± 0.0039

542 ± 12

11.89 ± 0.26

0.01

568

0.56

102.8 ± 1.0

0.0473 ± 0.0014

102.8 ± 1.0

62.41 ± 0.63

g7
g8

YD

1.39

285

0.91

544 ± 130.0572 ± 0.0049

552 ± 14

11.67 ± 0.30

g9

Y

0.02

546

1.31

541.0 ± 9.6

0.0464 ± 0.0032

541.1 ± 9.6

11.90 ± 0.22

0.12

323

0.83

290.5 ± 5.2

0.0474 ± 0.0031

290.9 ± 5.2

22.12 ± 0.41

g11
g12

Y

0.00

469

1.09

545 ± 100.0446 ± 0.0034

545 ± 1011.82 ± 0.23

g14

Y

0.08

1321

0.86

539.3 ± 6.2

0.0469 ± 0.0021

539.7 ± 6.2

11.93 ± 0.14

g15

Y

0.00

709

1.02

546.7 ± 8.5

0.0456 ± 0.0028

546.7 ± 8.5

11.78 ± 0.18

g16

D

0.65

3293

1.13

99.74 ± 0.48

0.05242 ± 0.00068

100.39 ± 0.48

63.89 ± 0.31

0.40

245

0.43

2.761 ± 0.030 0.1256 ± 0.0010

g17

2.772 ± 0.030 1986 ± 22

Yoshioka et al. Progress in Earth and Planetary Science(2024) 11:46

Page 20 of 26

Table 3 (continued)
SampleaGrain noNonradiogenica
Pb (%)

U content (ppm)

Th/U

238

U/206Pb

207

Pb/206Pb

238

U/206Pb*

238

U-206Pb* age (Ma)

g19

D

0.74

209

0.86

256.9 ± 5.5

0.0524 ± 0.0038

258.8 ± 5.5

24.86 ± 0.54

g20

YD

0.97

459

0.75

527 ± 11

0.0539 ± 0.0037

532 ± 1112.10 ± 0.26

g22

Y

0.31

308

0.87

531 ± 130.0487 ± 0.0043

533 ± 1312.08 ± 0.31

g23

Y

0.02

462

0.82

532 ± 110.0464 ± 0.0034

532 ± 1112.10 ± 0.26

Non-radiogenic Pb means Pb initially trapped in the zircon grain and/or contaminated during the experimental procedure206

Pb* represents 206Pb after the 207Pb correlation

Alphabets, Y and D, represent zircon grains in the youngest group and discordant zircon grains, respectively

with that of the youngest grain within the 2 σ error
range. We reported a weighted mean age of the 207Pbcorrected ages in the youngest group as a zircon U–Pb
age of each tuff bed (Fig. 7). The weighted mean 207Pbcorrected age of each tuff bed is consistent with its
concordia age (except for SWT-2U, which has only 2concordant grains and no concordia age).
The zircon U–Pb ages of the upper two tuff beds
revealed that they were formed by the late Miocene volcanic eruptions. The youngest group of WKA-5 yieldeda weighted mean 207Pb-corrected age of 6.68 ± 0.18 Ma(2 σ, MSWD = 3.3, n = 6) (Fig. 7a). The older grains of
WKA-5 are divided into two groups, 7.49–7.12 Ma (n = 3)
and 12.03–11.36 Ma (n = 10). The youngest group of
TBA-3 yielded a weighted mean 207Pb-corrected age of
10.874 ± 0.065 Ma (2 σ, MSWD = 1.09, n = 10) (Fig. 7b). The
older grains of TBA-3 range from 69.10 to 57.00 Ma (n = 5).
As for the zircon U–Pb ages of the youngest group, allof the measured tuff beds in SW Section show the middle
Miocene ages, which are undistinguishable within the 2
σ error ranges. In addition, most of the older grains display similar 207Pb-corrected ages, typically ranging from28 to 21 Ma. The youngest group of SWT-1aU yielded aweighted mean 207Pb-corrected age of 11.972 ± 0.080 Ma(2 σ, MSWD = 0.31, n = 9) (Fig. 7c). The 207Pb-correctedages of older grains range from 25.21 to 21.84 Ma (n = 10)
except for one grain representing 12.568 Ma. The youngest group of SWT-2U yielded a weighted mean 207Pbcorrected age of 11.68 ± 0.51 Ma (2 σ, MSWD = 2.2, n = 3)
(Fig. 7d). The 207Pb-corrected ages of older grains range
from 27.87 to 21.46 Ma (n = 31) except for one grain representing 176.1 Ma. The youngest group of SWT-3U yieldeda weighted mean 207Pb-corrected age of 11.957 ± 0.065 Ma(2 σ, MSWD = 1.07, n = 11) (Fig. 7e). The 207Pb-correctedages of older grains range from 28.06 to 21.29 Ma (n = 25)
except for one grain representing 12.27 Ma. The youngest group of SWT-3L yielded a weighted mean 207Pb-corrected age of 11.943 ± 0.073 Ma (2 σ, MSWD = 0.73, n = 17)(Fig. 7f). The 207Pb-corrected ages of older grains range

from 27.52 to 22.76 Ma (n = 9) except for one grain representing 97.64 Ma. The youngest group of SWT-5U yieldeda weighted mean 207Pb-corrected age of 11.925 ± 0.088 Ma(2 σ, MSWD = 0.68, n = 7) (Fig. 7g). The 207Pb-correctedages of older grains range from 27.79 to 21.77 Ma (n = 32).
The youngest group of SWT-5L yielded a weighted
mean 207Pb-corrected age of 11.927 ± 0.082 Ma (2 σ,MSWD = 1.3, n = 13) (Fig. 7h). The 207Pb-corrected ages ofolder grains are divided into three groups, 24.86–22.12 Ma(n = 3), around 63 Ma (n = 2), and 1986 Ma (n = 1).

4 Discussion
4.1 Comparison of U–Pb ages with biostratigraphy
4.1.1 The upper and middle part (WKA‑5 and TBA‑3)

One of the outstanding results of this study is enabling
us to treat biostratigraphic age constraints as quantitative ages. Based on our newly calculated ages and errorsof diatom biostratigraphy (Table 1) originally reported
by Yanagisawa and Watanabe (2017a), zircon U–Pb
ages of WKA-5 and TBA-3 are concordant with diatom
biostratigraphy within the 2 σ errors (Fig. 8a). Especially,
the zircon U–Pb age of TBA-3 completely falls within therange of the diatom biostratigraphic age model.
TBA-3 occupies an intermediate horizon between
the diatom biohorizons D55.8 (10.35–10.07 Ma) andD55.2 (11.50–10.78 Ma). Therefore, the zircon U–Pb
age of TBA-3 (10.87 ± 0.07 Ma) is a significant age constraint compensating the ca. 1.05-Myr interval with noage restriction by diatom biostratigraphy. As a result,the combination of the dual age constraints by our highprecision zircon U–Pb ages and newly evaluated diatom
biostratigraphic ages was able to provide a more accurate
age model with a smaller error range.
WKA-5 is recognized as the boundary between the
Nakayama and Nozaka Formations near the diatom biohorizon D75 (6.58–6.36 Ma). The proximity of WKA-5
to the diatom biohorizon has an advantage for comparison of their ages. Our results revealed that the zircon U–Pb age of WKA-5 (6.7 ± 0.2 Ma) seems slightly

Yoshioka et al. Progress in Earth and Planetary Science(2024) 11:46

Page 21 of 26

Fig. 7 Weighted mean age plots and Tera-Wasserburg concordia diagrams of the youngest zircon group. Results of a WKA-5, b TBA-3, c SWT-1aU,
d SWT-2U, e SWT-3U, f SWT-3L, g SWT-5U, and h SWT-5L are shown. In the weighted mean age plots (left sides), green and purple bars represent
accepted and rejected 207Pb-corrected ages of individual grains with 2 σ errors, respectively, and black bars represent weighted mean ages. In
the Tera-Wasserburg concordia diagrams (right sides), black solid and dash ellipses represent concordant and discordant data with 2 σ errors,
respectively, and red ellipses represent weighted means with 2 σ errors

older (but not significantly) than the age of the diatom
biohorizon D75. The zircon U–Pb age generally represents the age of crystallization of zircon in magma,
but not strictly the sedimentation age of volcanic tuff.
Thus, some zircon U–Pb ages could be older than thesedimentation ages depending on the magmatic evolution and the eruption–deposition system.
Another explanation comes from the occurrence ofglauconite above and beneath WKA-5 (Fig. 4). Glauconite is an authigenic clay mineral, and one of the key

Yoshioka et al. Progress in Earth and Planetary Sciencea350(2024) 11:46

Page 22 of 26

The upper and middle part
NozakaFm.

laminated diatomaceous
mudstonemassive diatomaceous
mudstone
diatomaceous mudstone
containing glauconite

D77

WKA-5

D75
D73

300

b

D70
D68

The lower part (SW Section)

50

FO H. akitaensis

D65

40

D60
D59
D58
D57

Nakayama Formation

D55

D56

SWT-1aU30
D55.8

Thickness (m)Thickness (m)

tuff

D66

250

upper 2σ limit
mean
lower 2σ limit
diatom age

100

2σ

zircon U-Pb age

TBA-3

biostratigraphic
age model

50

D55.2

5

6

7

8

9

Age (Ma)

10

SWT-2Uupper 2σ limit
mean
lower 2σ limit
diatom age
radiolarian age

11

2σ

12

0

SWT-3U
SWT-3LLO E. inflatum
LO C. motoyamai

10

D55

0

hard siliceous mudstone

zircon U-Pb age

SWT-5U
SWT-5L

biostratigraphic
age model

LO L. renzae

11.0 11.2 11.4 11.6 11.8 12.0 12.2 12.4

Age (Ma)

Fig. 8 Summary of age constraints by zircon U–Pb ages and biostratigraphy in the Nakayama Formation. a The upper and lower partof the Nakayama Formation contains two zircon U–Pb ages (black bars; this study) with diatom biostratigraphy (red rectangles; Yanagisawa
and Watanabe 2017a), while b the lower part (SW Section) contains six zircon U–Pb ages (black bars; this study) with diatom (red rectangles;
Yanagisawa and Watanabe 2017a) and radiolarian (green rectangles; this study) biostratigraphy. Newly calculated diatom and radiolarian ages areused. All age constraints are described with 2 σ errors

factors for its formation is long-time residence near thewater–sediment interface (e.g., Fernández-Landero andFernández-Caliani 2021). Actually, diatom biostratigraphy proved that some Miocene glauconite-rich beds
observed in the Japan Sea coastal area of Japan, such as
the Noto Peninsula in Central Japan and Sasamori Hills
in northern Japan, represented a hiatus or an extremely
low sedimentation rate (Kato and Yanagisawa 2021;
Watanabe 1990; Yanagisawa 1999). The slow sedimentation inferred from glauconite around WKA-5 could

explain the minor misalignment between the diatom andzircon ages.
4.1.2 The lower part (SW section)

While only one diatom biohorizon (D55) was recognized in SW Section (Yanagisawa and Watanabe 2017a),
we established radiolarian biostratigraphy and identified
four key radiolarian biohorizons from three stratigraphic
horizons (Fig. 5). Age constraints by diatom and radiolarian biostratigraphy are stratigraphically consistent,

Yoshioka et al. Progress in Earth and Planetary Science(2024) 11:46

Fig. 9 a An age histogram of zircon grains and b age distributionsof the basement rocks. A yellow shade represents the age range
from 20 to 30 Ma. Colors in the histogram represent individual tuff
samples taken from SW Section. Reported ages of the basement
rocks on Sado Island, the Nyukawa, Aikawa, Masaragawa,
and Kimpokusan Formations, are shown. FT ages are basedon Ganzawa (1982) and Metal Mining Agency of Japan (1987). K–Ar
ages are based on Shibata et al. (1979), Konda and Ueda (1980),
Metal Mining Agency of Japan (1987), (1988), (1989), and Shinmura
et al. (1995). These ages are listed in Supplementary Table 2. (1) The
Aikawa Formation includes the Oonogawa and Sanze Formations. (2)The Masaragawa Formation includes the Toyooka Formation. (3) The
Kimpokusan Formation includes the Kyozukasan Formation

whereas zircon U–Pb ages are not completely consistent(Fig. 8b).
Except for SWT-2U, which has only three youngestgroup zircon grains and a long 2 σ error bar, highly accurate U–Pb ages around 12 Ma were obtained from thetuff beds in SW Section, and not distinguishable fromeach other within the 2 σ errors although the maximum
stratigraphic distance is over 20 m (equivalent to ca. 0.2–
0.4 myr assuming the sedimentation rate of 6–10 cm/kyr). The similarity of the zircon U–Pb ages of the tuff
beds possibly indicates the same origin of the youngest
zircon grains in each tuff bed.
Compared with the biostratigraphic age model, the zircon U–Pb age of SWT-1aU is relatively older, and thoseof SWT-3U and SWT-3L are probably older. On theother hand, zircon U–Pb ages of SWT-5U and SWT-5L
are in conformity with biostratigraphy and can be considered as depositional ages of the tuff beds.
The major element compositions of the volcanic glass
shards of the measured tuff beds in SW Section also show

Page 23 of 26

similar properties. The ­K2O-TiO2 diagram, which is often
used to classify Quaternary tephra beds (e.g., Aoki and
Machida 2006), reveals that the volcanic glass shards ofthe tuff beds in SW Section are divided into two groups:
U-group and L-group (Fig. 6b). The U-group is characterized by higher ­K2O and lower ­TiO2 concentrations
than the L-group and consists of all upper counterparts(SWT-1aU, SWT-2U, SWT-3U, and SWT-5U). L-groupcontains the lower counterparts (SWT-3L and SWT5L). The thickness of diatomaceous mudstone intercalated between the upper and lower tuff beds of each tuff
set ranges from 10 to 35 cm, which corresponds to thetime interval of 1–6 kyr assuming a sedimentation rate of
6–10 cm/kyr. If each tuff set in SW Section was formedby each series of eruptions, the similarity of the compositional difference or transition between the upper andlower tuff beds of each tuff set may reflect some indigenous features in the magmatic evolution, for example,
assimilation and crystallization differentiation.
The thickness and the sedimentary structures of thetuff beds in SW Section indicate that the tuff beds areformed by proximal eruptions or resedimentation of
nearby pyroclastic materials (Fig. 5). Considering thesimilarity in the chemical compositions of the volcanic
glass shards and the U–Pb ages of the youngest zircon
grains, volcaniclastic components in the thick tuff beds
in SW Section could originate from single or associatedmagmatic activities around Sado Island.
4.2 Possible source of older zircon grains

Although considerable zircon grains have younger U–Pb
ages of ca. 12 Ma, many more grains show U–Pb ages
from 30 to 20 Ma (Fig. 9a). Especially, SWT-1aU, SWT2U, SWT-3U, and SWT-5U have large proportions of thezircon grains representing 20–30 Ma: 55%, 89%, 66%, and82%, respectively. These older zircon grains seem to be
inherited zircon grains incorporated during the eruption
or the assimilation in the magma chamber.
The basement rocks forming most part of Sado Island
are volcanic rocks formed during the Oligocene to theearly Miocene (Figs. 1 and 2). Figure 9b shows the compiled data of fission-track (FT) ages and K–Ar ages ofthe volcanic basement rocks on Sado Island, the Nyukawa, Aikawa, Masaragawa, and Kimpokusan Formations (Supplementary Table 2; Shibata et al. 1979; Konda
and Ueda 1980; Ganzawa 1982; Metal Mining Agencyof Japan 1987, 1988, 1989; Shinmura et al. 1995). Someconsideration would be needed because all FT ages werereported before the standardization of FT dating calibration by International Union of Geological Sciences subcommission on geochronology (Hurford 1990) and somesamples possibly experienced thermal alterations (Metal
Mining Agency of Japan 1988). However, the ages of theYoshioka et al. Progress in Earth and Planetary Science(2024) 11:46

Page 24 of 26

volcanic basement rocks are possible to explain the age
distributions of the older zircon group in the tuff samples
from SW Section (Fig. 9). The proximity of the volcanic
eruption site inferred by the thickness and the sedimentary structures of the tuff beds in SW Section also supports that the older zircon grains representing 30–20 Ma
derive from the volcanic basement rocks around Sado
Island.
Thick tuff beds in the lower part of the Nakayama Formation were also reported in the Kosado region. In the
Ogi-Donokama area, several tuff beds with dozens of centimeters to several meters in thickness are intercalatedwith hard siliceous mudstone equivalent to the Nakayama
Formation. Ages ranging from 13 to 11.5 Ma are reported
as FT ages of the tuff beds though the detailed sampling
points are unclear (Kanzo 1989). In the Ogi-Shiroyama
area, several thick felsic tuff beds, one of which is almost
10 m thick, were recognized in diatomaceous mudstonecorresponding to the diatom zone NPD 5­ B3 (ca. 12.3–
11.7 Ma) in the Nakayama Formation (Yanagisawa 2012).
Comprehensive studies including those tuff beds in the
Kosado region are needed to reveal the magmatic systemand the volcanic activities that formed thick tuff beds in
SW Section during the middle Miocene.

4. The U–Pb ages of the older zircon grains make anassemblage from 30 to 20 Ma. These older ages canbe explained by the contamination of zircon grainsfrom the volcanic basement rocks around Sado
Island.

5 Conclusions
In this study, we obtained new zircon U–Pb ages of 8 tuff
beds intercalated with the middle to late Miocene diatomaceous mudstone in the Nakayama Formation on Sado
Island in Central Japan. In addition, we newly evaluatedthe ages and error ranges of the diatom and radiolarianbiostratigraphy used in the Japan Sea region to compare its ages with our zircon U–Pb ages. As a result, we
obtained the following findings,
1. The zircon U–Pb ages of WKA-5 (6.7 ± 0.2 Ma) andTBA-3 (10.87 ± 0.07 Ma) are consistent with the diatom biostratigraphy within the 2 σ. The combinationof the zircon U–Pb ages with the biostratigraphic age
constraints resulted in a more accurate age modelwith a smaller error range.
2. The zircon U–Pb ages of the measured tuff beds in
SW Section are around 12 Ma and not distinguishable within the 2 σ. The only 2 zircon U–Pb ages fromthe lowest set of the tuff beds, SWT-5U and SWT5L, are consistent with the diatom and radiolarianbiostratigraphy and can be used as sedimentary ages.
3. The similarities in the U–Pb age distributions of zircon grains and the chemical compositions of volcanic
glass shards obtained from the measured tuff beds in
SW Section indicate that they should originate fromsingle or associated magmatic activities.

As the final remark, we emphasize that integrationof zircon U–Pb dating with biostratigraphy realizes
stronger age determination by making up for the individual weak points.
Abbreviations
IODP	International Ocean Discovery Program/Integrated Ocean Drilling Program
ODP	Ocean Drilling Program
DSDP	Deep Sea Drilling Project
NPD	Neogene North Pacific diatom zone
EPMA	Electron probe microanalyzer
WDS	Wavelength-dispersive X-ray spectrometer
MC-ICP-MS	Multi-collector inductively coupled plasma-mass spectrometryFO	First occurrenceLO	Last occurrence
FT	Fission-track

Supplementary Information
The online version contains supplementary material available at https://​doi.​
org/​10.​1186/​s40645-​024-​00651-w.
Supplementary Table 1. Input age constraint data for Undatable age
model constructions.
Supplementary Table 2. Reported ages of volcanic rocks from the Nyukawa, Aikawa, Masaragawa, and Kimpokusan Formations.
Supplementary Fig. 1. Columnar sections and a route map of SW Section.
A composite columnar section of SW Section was composed of 8 columnar sections of outcrops (Outcrops A to H). Some outcrops have key tuff
beds described in Fig. 5. Sampling locations of the measured tuff beds areshown in the route map of SW Section.
Supplementary Fig. 2. New age models containing 2 σ error ranges of
IODP, ODP, and DSDP cores. ODP Sites 887C and 884B are in the North
Pacific, DSDP Site 438 and ODP Site 1151A are in the Northwest Pacific,
and IODP Site U1430 is in the Japan Sea. The age constraints used toconstruct the age models are magnetostratigraphy for Site 887C and
884B (Barron et al. 1995), diatom biostratigraphy for Site 438 (Yanagisawa
and Akiba 1998), magnetostratigraphy and diatom biostratigraphy for
Site 1151A (Motoyama et al. 2004), and cyclostratigraphy for Site U1430(Kurokawa et al. 2019).
Acknowledgements
We thank Dr. Kenichiro Tani and Mrs. Yoko Kusaba for their assistance in the zircon extraction, Dr. Norikatsu Akizawa and Dr. Ryo Nakanishi for their assistancein the EPMA measurement, Dr. Hanaya Okuda for the use of his high-spec
PC for the Undatable software, and Dr. Asuka Yamaguchi for his laboratory
management. We are also grateful to Dr. Takeru Kochi, Dr. Hironao Matsumoto, Dr. Yuichi Okuma, Dr. Masayuki Ikeda, and Mr. Yusuke Umemiya for theirassistance in the fieldwork and the sample collection. We give a special thanks
to Dr. Yukio Yanagisawa and Dr. Mahito Watanabe for their establishment ofhigh-resolution diatom biostratigraphy in the studied area. Finally, we appreciate the helpful advice and comments from two anonymous reviewers and Dr.
Yasufumi Iryu (editor of PEPS) on how to improve our manuscript.
Authors’ information
JY was a student of JK. KMM is a colleague of JK. SN was a student of TH.

Yoshioka et al. Progress in Earth and Planetary Science(2024) 11:46

Author contributions
JY proposed the topic, conceived and designed the study, collected thesamples, calculated the biostratigraphic ages, carried out the EPMA and U–Pb
measurements, and analyzed the data. KMM helped in the sample collection,
carried out the investigation of radiolarian biostratigraphy, and collaborated
with the corresponding author in the construction of manuscript. SN carriedout the U–Pb measurement, analyzed the data, and helped in their interpretation. JK helped in the design of the study, the sample collection, and theconstruction of manuscript. TH provided the resources for the U–Pb measurement and helped in the data interpretation. All authors read and approved thefinal manuscript.
Funding
This work was supported by JSPS KAKENHI Grant Numbers 22J12168,
22KJ0845 (JY), 22K18426, 22H01342 (JK), and 23K03559 (KMM).
Availability of data and material
All data generated and analyzed during this study are included in this published article and its supplementary information files.

Declarations
Competing interests
The authors declare that they have no competing interest.
Received: 26 May 2024 Accepted: 19 August 2024

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