Bull. Natl. Mus. Nat. Sci., Ser. C, 44, pp. 13–18, November 26, 2018

U–Pb age of the Sphenoceramus schmidti Zone (middle Campanian,Cretaceous) in Hokkaido, northern Japan
Yasunari Shigeta1 and Yukiyasu Tsutsumi1
1

Department of Geology and Paleontology, National Museum of Nature and Science,
4–1–1 Amakubo, Tsukuba, Ibaraki 305–0005, Japan

Abstract Radiometric age dating of zircons (238U/206Pb ratios, using the LA-ICP-MS method)
obtained from a tuff sample just below the Sphenoceramus schmidti-bearing beds of the Yezo Group in
the Nakagawa area, Hokkaido, northern Japan, yielded an age of 80.2±0.8 Ma, 95% conf., which suggests an early middle Campanian (Late Cretaceous) age. This result indicates that the age of the S.
schmidti Zone is the same in both southwestern and northern Japan. Thus, Sphenoceramus schmidti
should be regarded as an ideal zonal-index fossil for the lower middle Campanian of the Northwest
Paciﬁc region.
Key words: Campanian, Cretaceous, Sphenoceramus schmidti, U–Pb age, zircon

Introduction
The inoceramid bivalve Sphenoceramus schmidti(Michael, 1899) occurs abundantly in the Upper
Cretaceous of southwestern Japan (Noda et al.,
1996; Misaki and Maeda, 2009). Because its characteristic shell sculpture, e.g. divergent ribs, is easy
to recognize, it has been regarded as an ideal zonalindex fossil (Toshimitsu et al., 1995). Kodama(1990) correlated the S. schmidti Zone in the Izumi
Group with the uppermost part of polarity chronC33r, and Tsutsumi et al. (2014) reported an age of
79.7±0.7 Ma (95% conf.) for a tuff in beds containing S. schmidti in the Himenoura Group. In consideration of this evidence, Shigeta et al. (2016)
correlated the S. schmidti Zone with the lower middle Campanian based on the three-subdivision
scheme for the Campanian.
Sphenoceramus schmidti is also common in theUpper Cretaceous of the Yezo Group in Hokkaido,
northern Japan (Nagao and Matsumoto, 1940) and
Sakhalin, Far Eastern Russia (Zonova et al., 1993).
Kodama et al. (2000) recognized 13 magnetozones
in the Upper Cretaceous in the Naiba area, southern
Sakhalin, and concluded that the S. schmidti-bearing
beds correspond to the bottom of polarity chron
C32r (＝ middle upper Campanian). In contrast,
Tamaki et al. (2008) correlated the S. schmidti-bearing beds in the Urakawa area, Hokkaido with theupper part of polarity chron C33r and lower part ofC33n (＝ lower middle Campanian). Thus, the magnetostratigraphic correlation of the S. schmidti Zone
in the Yezo Group is still a matter of debate and theradiometric age of the zone had never been determined until now.
In this paper, we calibrate the international correlation of the Sphenoceramus schmidti Zone with thezircon-based geochronology of the Yezo Group in
the Nakagawa area, Hokkaido, and establish a more
precise global correlation of the zone.
Notes on stratigraphy
Even though the Yezo Group in the Nakagawa
area is complexly folded and faulted, a continuous
Upper Cretaceous succession is well exposed along
the Abeshinai River and its tributaries (e.g. Matsumoto, 1942; Takahashi et al., 2003). Within thegroup, the Osoushinai Formation yields numerous
relatively well-preserved Campanian megafossils
from various horizons.
The 600-m thick Osoushinai Formation consists
mainly of dark gray to gray siltstone, sandy siltstone
and muddy sandstone, intercalated with 1–100 cm
thick white, vitric tuff beds (Takahashi et al., 2003;
Fig. 1). The lower and middle parts contain theinoceramid Sphenoceramus naumanni (Yokoyama,1890) and ammonoids Yokoyamaoceras ishikawai(Jimbo, 1894), Gaudryceras tenuiliratum Yabe,
1903 and Polyptychoceras pseudogaultinum

14

Y. Shigeta and Y. Tsutsumi

Fig. 1. Index map showing distribution of the Yezo Group (black areas) in Hokkaido (A), locality (B, C) and stratigraphic horizon (D) of the tuff bed that provided the U–Pb zircon age in the Nakagawa area, Hokkaido, northern
Japan.

(Yokoyama, 1890) (Fig. 1D). Characterizing theupper part are gigantic specimens of the inoceramidS. schmidti, which occur abundantly in calcareous
concretions as well as in the host rock (Matsuda andUbukata, 1999; Takahashi et al., 2003), together
with the ammonoids Urakawites Matsumoto, 1955,
Teshioites Matsumoto, 1955 and Canadoceras
Spath, 1922 (Matsumoto, 1954, 1955).
Material and methods
Material: Radiometric age analysis was conducted on zircons extracted from a tuff sample
taken from a 100-cm thick white, vitric tuff bed
positioned immediately below the Sphenoceramus
orientalis-bearing beds, which occur about 18 m
below the S. schmidti-bearing beds in an exposureof the Osoushinai Formation along the Abeshinai

River (44°40′23.90″N, 142°1′15.55″E; Fig. 1).
Methods: The zircon grains were extracted bystandard techniques: crushing, heavy liquid separation and handpicking. Then, the zircon grains were
mounted in an epoxy disc with the FC1 zircon standard (206Pb/238U ＝ 0.1859; Paces and Miller, 1993),
and the SRM610 glass standard and polished until
the center of each grain was exposed. The backscattered electron and cathodoluminescence images ofthe zircon grains were used for selection of the sites
for analysis. U–Pb dating of the sample was carriedout using the LA-ICP-MS method, which consistsof a NWR213, a 213 nm wave length Nd-YAG laser
ablation system (Electro Scientiﬁc Industries) andan Agilent 7700x quadrupole ICP-MS (Agilent
Technologies), installed at the National Museum of
Nature and Science at Tsukuba, Japan. The experimental conditions and procedures for the measure-U–Pb age of the Sphenoceramus schmidti Zone (Cretaceous)

15

Table 1. LA-ICP-MS analyzed data and calculated ages of the tuff sample from the upper part of the Osoushinai Formation. Errors are 1 sigma. Pbc and Pb* indicate the common and radiogenic portions, respectively.
LabelsAbe_01
Abe_02
Abe_03
Abe_04
Abe_05
Abe_06
Abe_07
Abe_08Abe_09Abe_10
Abe_11
Abe_12
Abe_13
Abe_14
Abe_15
Abe_16
Abe_17
Abe_18
Abe_19Abe_20
Abe_21
Abe_22
Abe_23
Abe_24
Abe_25
Abe_26
Abe_27
Abe_28Abe_29Abe_30
Abe_31
Abe_32

206

Pbc(1)
(%)

U
(ppm)

Th
(ppm)

Th/U

238

207

1.64
0.54
0.48
0.11
0.68
0.77
0.29
0.77
2.01
0.94
0.13
0.81
2.64
0.79
0.73
0.17
0.13
1.89
0.23
1.02
0.01
0.00
0.60
0.00
0.25
1.08
1.09
2.14
1.60
0.00
2.62
0.33

183
406
209
207
290
173
492
117
170
490
353
172
137
196
364
474
503
171
312
375
139
258
663
117
190
99
137
335
226
99
100
139

72
126
111
84
186
74
254
66
111
262
209
95
77
94
147
158
455
102
157
299
69
118
269
62
138
50
76
174
125
52
50
74

0.40
0.32
0.55
0.41
0.66
0.44
0.53
0.58
0.67
0.55
0.61
0.57
0.58
0.49
0.42
0.34
0.93
0.61
0.52
0.82
0.51
0.47
0.42
0.54
0.75
0.52
0.57
0.53
0.57
0.54
0.52
0.55

76.30±2.50
81.01±1.8180.38±2.41
84.89±2.47
81.98±2.5580.42±2.4779.85±1.77
78.96±3.48
86.84±3.05
78.26±1.6877.93±1.8780.24±2.6479.78±3.3682.22±2.77
79.86±1.92
73.89±1.55
81.53±1.73
80.39±2.8579.30±2.4279.10±2.2078.72±3.4378.71±2.09
80.39±1.5877.36±3.24
87.08±3.2176.07±3.76
79.24±3.51
78.39±1.95
82.76±2.4373.16±3.02
83.43±4.85
79.67±3.32

0.0505±0.0138
0.0418±0.0064
0.0474±0.0116
0.0497±0.0115
0.0455±0.0102
0.0430±0.0135
0.0479±0.0070
0.0469±0.0216
0.0477±0.0172
0.0425±0.0084
0.0535±0.0090
0.0474±0.0125
0.0378±0.0215
0.0437±0.0132
0.0442±0.0077
0.0410±0.0060
0.0520±0.0095
0.0525±0.0176
0.0488±0.0098
0.0381±0.0115
0.0431±0.0218
0.0507±0.0061
0.0499±0.0060
0.0540±0.0109
0.0505±0.0177
0.0623±0.0325
0.0573±0.0213
0.0255±0.0093
0.0378±0.0117
0.0500±0.0126
0.0441±0.0358
0.0504±0.0188

U/206Pb* (1)Pb*/206Pb* (1)U/206Pb* age (1)
(Ma)

238

83.9±2.779.1±1.879.7±2.475.5±2.2
78.2±2.479.7±2.480.2±1.881.1±3.673.8±2.681.8±1.882.2±2.079.8±2.680.3±3.477.9±2.680.2±1.986.7±1.878.6±1.779.7±2.880.8±2.581.0±2.281.4±3.581.4±2.1
79.7±1.682.8±3.573.6±2.784.2±4.180.8±3.681.7±2.077.4±2.387.5±3.676.8±4.4
80.4±3.3

238

U/206Pb* age (2)(Ma)
83.7±2.779.6±1.779.8±2.375.3±2.2
78.4±2.480.1±2.580.2±1.781.2±3.373.8±2.482.4±1.781.6±1.979.9±2.681.3±3.2
78.3±2.680.6±1.986.7±1.778.1±1.579.1±2.680.6±2.481.7±2.0
81.3±3.180.9±2.2
79.3±1.582.0±3.673.2±2.482.5±4.1
79.8±3.4
83.4±1.978.3±2.287.2±3.877.1±3.780.0±3.1(1) Common Pb corrected by assuming 206Pb/238U–208Pb/232Th age-concordance(2) Common Pb corrected by assuming 206Pb/238U–207Pb/235U age-concordance

ments were based on the methods described in Tsutsumi et al. (2012). The spot size of the laser was 25µm. Corrections for common Pb was made on thebasis of the measured 207Pb/206Pb ratio (207Pb correction) or 208Pb/206Pb and Th/U ratios (208Pb correction) (e.g. Williams, 1998) as well as the model forcommon Pb compositions proposed by Stacey andKramers (1975). In this paper, we adopt the 207Pb
correction for age discussion because it is more
effective in calculating the Phanerozoic 238U–206Pb*
age than the 208Pb correction (e.g. Williams, 1998).
The pooled ages presented in this study were calculated using Isoplot/Ex software (Ludwig, 2003).
The uncertainties in the mean 238U–206Pb* ages represent 95% conﬁdence intervals (95% conf.). 206Pb*
indicates radiometric 206Pb.

Results
Analyzed data from zircons; common 206Pb, U,
and Th concentrations, Th/U, 238U/206Pb*, and207
Pb*/206Pb* ratios, and radiometric 238U/206Pb*
ages are listed in Table 1. All errors state 1 sigma
level. All zircons in the samples show rhythmic
oscillatory and/or sector zoning on cathodoluminescence images, which is commonly observed in igneous zircons (e.g. Corfu et al., 2003), and theirhigher Th/U ratios (＞0.1) also support their igneous origin (Williams and Claesson, 1987; Schiøtte
et al., 1988; Kinny et al., 1990; Hoskin and Black,2000). Figure 2 shows a Tera-Wasserberg concordia
diagram and an age distribution plot for all analyzed
spots of the sample. All zircon U–Pb ages cluster inthe range 73–87 Ma. The weighted mean age, after
the rejection of three data points, yield 80.2±0.8
Ma (MSWD ＝ 0.8; 95% conf.), which is thought to16

Y. Shigeta and Y. Tsutsumi

Fig. 2. U–Pb zircon ages of tuff sample collected from the upper part of the Osoushinai Formation. A, cathodoluminescence image (CL) of zircon grains from the sample. Circles on the grains represent spots analyzed by LA-ICPMS. Spots are 25 μm across. B, Tera-Wasserburg U–Pb concordia diagram of zircons. C, 238U–206Pb age distribution
plot of tuff sample. 207Pb* and 206Pb* indicate radiometric 207Pb and 206Pb, respectively.

represent the magmatism/deposition age of the tuff
sample.
Discussion
Zircon geochronology in this study reveals that
the U–Pb age of the tuff just below the Sphenoceramus schmidti-bearing beds is 80.2±0.8 Ma (95%conf.), which infers an early middle Campanian age.
This result conﬁrms the validity of the work of
Tamaki et al. (2008), who correlated the beds in the
Urakawa area, Hokkaido with the upper part ofpolarity chron C33r and lower part of C33n (＝lower middle Campanian). In contrast, our result
differs from the work of Kodama et al. (2000), whocorrelated the S. schmidti-bearing beds in the Naiba
area, Sakhalin with the bottom of polarity chron
C32r (＝ middle upper Campanian). We question
the accuracy of this magnetostratigraphic correlation primarily because strata just below the S.

schmidti-bearing beds are not well exposed in the
Naiba area (Shigeta et al., 2015, 2016).
The Sphenoceramus schmidti Zone in southwestern Japan was shown to be lower middle Campanian by both magnetostratigraphy and zircon geochronology (Kodama, 1990; Tsutsumi et al., 2014),
thus suggesting that the age of the zone in southwestern Japan is the same as in Hokkaido. Therefore, Sphenoceramus schmidti should be regarded
as an ideal zonal-index fossil for the lower middle
Campanian of the Northwest Paciﬁc region
Sphenoceramus schmidti also occurs in the Upper
Cretaceous of the Paciﬁc coastal region of North
America (e.g. Usher, 1952; Matsumoto, 1960;
Jones, 1963). Ward et al. (1983, 2012) indicatedthat the S. schmidti Zone of the Great Valley
Sequence in northern California should be placednear the top of polarity chron C34n (＝ Santonian)and lowest part of polarity chron C33r (＝ lowest
Campanian), which differs greatly from that of theU–Pb age of the Sphenoceramus schmidti Zone (Cretaceous)

Northwest Paciﬁc region. We question the accuracyof this conclusion because the magnetostratigraphyof northern California has only been studied in very
short stratigraphic sequences. Further magnetostratigraphic studies of a continuous sequence spanning the Santonian to Maastrichtian as well asradiometric age determinations are needed settle
this problem.
Acknowledgments
We thank Yasuyuki Tsujino (Tikushima Prefectural Museum, Tokushima) for valuable commentson the ﬁrst draft. We are very grateful to Toshihiro
Sakai (Asahikawa, Hokkaido) for his help duringﬁeld work. Thanks are extended to Jim Jenks (West
Jordan, Utah) for his helpful suggestions andimprovement of the English text. This research was
ﬁnancially supported by the National Museum of
Nature and Science project, Chemical Stratigraphy
and Dating as a Clue for Understanding the Historyof the Earth and Life.
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