September 7-10, 2014

FIELD EXCURSION GUIDE

Cretaceous forearc basin siliciclastic successions
along the Pacific coast, central Japan:
Choshi, Nakaminato and Futaba groups
Hisao Ando, Kenji Kashiwagi, Ren Hirayama
and Seiichi Toshimitsu

The Second International Symposium ofInternational Geoscience Programme (IGCP) Project 608“Cretaceous Ecosystems and Their Responses to
Paleoenvironmental Changes in Asia and the Western Pacific”

The Second International Symposium ofInternational Geoscience Programme (IGCP) Project 608“Cretaceous Ecosystems and Their Responses to
Paleoenvironmental Changes in Asia and the Western Pacific”

Field Excursion Guide
Cretaceous forearc basin siliciclastic successions
along the Pacific coast, central Japan:
Choshi, Nakaminato and Futaba groups
Hisao Ando, Kenji Kashiwagi, Ren Hirayama
and Seiichi Toshimitsu

September 7-10, 2014
Tokyo - Choshi - Tsukuba - Nakaminato - Kitaibaraki - Iwaki - Tokyo
JAPAN
Edited by Organizing and Scientific Committee of the IGCP 608	
 Itinerary
DAY 1: Sunday, September 7, 2014
8:00
Leave Takadanobaba, Tokyo at 8 a.m. for Inubosaki (small peninsula projected over the Pacific),
Choshi City, Chiba Prefecture by chartered bus
Stop 1-1 --- Kurohae Fishery Port: Cross-stratified conglomerate and conglomeratic sandstone, Ashikajima
Formation, bedded chert block of Atagoyama Unit as basement rock
Stop 1-2 --- Kimigahama: HCS sandstone and bioturbated sandy siltstone, Kimigahama Formation.
Picnic lunch
Stop 1-3 --- Cape Inubousaki, Geosite of Choshi Geopark, Choshi City: HCS sandstone and wave-formed
sedimentary structure, Inubouzaki Formation
Stop 1-4 --- Cape Nagasakihana, Choshi City: Massive fine sandstone, Nagasakihana Formation, Upper
Miocene to Pliocene Naarai Formation, Inubo Gr. and Lower Miocene Senninzuka Fm.
18:00 Arrival at Hotel Sunrise Choshi
19:00 Dinner and welcome party at Café Marina, Chiba Institute of Science near a sunset beach
20:30-21:00 Preliminary meeting and overview lecture
DAY 2: Monday, September 8, 2014
07:30 Breakfast
08:30 Leave Choshi for Kita-Ibaraki through Tsukuba and Nakaminato
Stop 2-1 --- Geological Museum, Geological Survey of Japan, AIST in Tsukuba City
Lunch at AIST restaurant
Stop 2-2 --- Wave cut bench along Hiraiso coast, Nakaminato City: Mudstone of Hiraiso Formation
Stop 2-3 --- “Seijo-seki”, north of Hiraiso coast: Turbidite facies of the lower Isoai Formation
Stop 2-4 --- north of Hiraiso coast: debris flow deposits of the Isoai Formation (pterosaur locality)
Stop 2-5 --- Isozaki coast, Nakaminato City: Thick sandstone succession of the upper Isoai Formation
18:00 Arrival and stay at Izura Kanko Hotel Taikanso along sea cliff of Izura Coast, Kitaibaraki City
19:00 Dinner (with fresh seafood and Japanese cuisine)
20:30 Explanatory lecture
DAY 3: Tuesday, September 9, 2014
07:30 Breakfast
08:30 Leave hotel for field along Izura Coast, Kitaibaraki City by footStop 3-1 --- North of “Rokkakudo”, Oh-izura: Calcareous concretions in Kokozura Fm., late E. Mio.
Stop 3-2 --- South of “Rokkakudo”, Ko-izura: Stratigraphic relations of late-E. to Mid. Miocene
Stop 3-3 --- Garden rocks: Calcareous concretion blocks bearing chemosynthetic bivalves
Stop 3-4 --- “Rokkakudo”, cultural monument, Izura Institute of Art and Culture, Ibaraki University
After North Ibaraki Geopark leave for Iwaki
Lunch at Iwaki Coal and Fossil Museum
Stop 3-5 --- Iwaki Coal and Fossil Museum (“Iwaki Sekitan Kasekikan”): Plesiosaur, dinosaur, coal, etc.
Stop 3-6 --- Iwaki City Ammonite Center: Ammonite shell bed of Ashizawa Formation, Futaba Group
17:30 Arrival at Hotel Iwaki Shin-Maiko Heights
19:00 Dinner
20:00 Explanatory lecturse
DAY 4: Wednesday, September 10, 2014
After visit to the Futaba Group, leave Iwaki for Tokyo
07:30 Breakfast
08:30 Leave hotel for field
Stop 4-1 --- Kotakidaira near Asami River, Hirono Town: Unconf. L. Cret. Granite & Futaba Gr.
Stop 4-2 --- Large cliff at Kitazawa, Hirono: Kasamatsu & Tamayama F., Futaba Gr., Eoc. Iwaki Fm.
Stop 4-3 --- “Kairyu-no-sato” at Irimazawa, Iwaki City: near plesiosaurid locality, Tamayama Fm.
Stop 4-4 --- Itakizawa, Iwaki C.: Boundary between fluvial and shallow-marine facies of Tamayama Fm.,
Futaba Gr. and unconformity between Up.Cretaceous Futaba Gr. and Paleogene Shiramizu Gr.
Arrival at Tokyo until 6 p.m.

Cretaceous forearc basin siliciclastic successions: Choshi, Nakaminato and Futaba groups

Cretaceous forearc basin siliciclastic successions along the Pacificcoast, central Japan: Choshi, Nakaminato and Futaba groups
Hisao Ando 1)，Kenji Kashiwagi 2), Ren Hirayama 3) and Seiichi Toshimitsu4)1) Department of Environmental Sciences, Faculty of Science, Ibaraki University, Bunkyo 2-1-1, Mito 310-8512, Japan(ando@mx.ibaraki.ac.jp)
2) Graduate School of Science and Engineering for Research, University of Toyama, 3190 Gofuku, Toyama 930-8555, Japan(kasiwagi@sci.u-toyama.ac.jp)
3) School of International Liberal Studies, Waseda University, Nishi-Waseda 1-6-1, Shinjuku-ku, Tokyo 169-8050, Japan(renhirayama@waseda.jp)
4) Geological Museum, Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, Central 7,
1-1-1 Higashi, Tsukuba 305-8567, Japan (s.toshimitsu@aist.go.jp)

Keywords: shallow-marine to offshore siliciclastic sedimentation, sequence stratigraphy, taphonomy, cool-seepcarbonate, Cretaceous forearc basin, Northeast Japan

forearc basin along the eastern margin of thepaleo-Asian continent, called the Yezo forearc basin.
On the other hand, Southwest Japan had been
uplifted by the tectonic movements after the Japan
Sea opening, e.g. subduction and transformmovements of Pacific and Philippine Sea Plates. As
many of Cretaceous forearc sediments had beeneroded away and tectonically deformed, their surface
distribution is very limited. Therefore, it is not easy to
study the sedimentary and faunal successions as wellas due to recent vegetation of inland areas. However,since the Nakaminato and Choshi groups are well
exposed along the Pacific coastline, their stratigraphic
successions can be appropriately traced andestablished.
Recently the boundary problems between
Southwest Japan and Northeast Japan, especially on
the differentiation during the pre-Neogene andpost-Neogene stages, are well noticed recently(Takahashi, 2006).

Introduction
As the Japanese Islands are situated in an active
convergent margin, their geologic structure is very
complicated in general. The pre-Neogene strata are
distributed reflecting a zonal arrangement of several
geotectonic provinces on the surface, some of whichwere intensely modified by post-Paleocene tectonism
partly related to the opening of the Japan Sea caused
by backarc spreading during the middle Miocene (Fig.
1). On a large scale, the pre-Neogene structure of theJapanese Islands is divided into Southwest Japan andNortheast Japan by the Tanakura Tectonic Line
(TTL).
Therefore, the Cretaceous strata had been affectedby tectonic movements several times on alarge/mid/micro
scale.
On
the
basis
of
lithostratigraphy, facies and sequence stratigraphy
and biostratigraphy, over 22 surface and subsurface
stratigraphic successions of the Cretaceous and lower
Paleogene were correlated from north Honshu toKanto region. Their distribution and geologic featuresof the Cretaceous strata are considerably different
between the two divisions (Fig. 2).
In Northeast Japan, Upper Cretaceous to Lower
Paleocene shallow-marine to non-marine sediments
as well as offshore-marine sediments are distributed
along the Pacific coast of north Honshu and theoffshore Pacific area (Fig. 1). They continue
northward toward Sakhalin through central Hokkaido,
1,400 km long in a north-south direction (Ando, 2003,
2005). They have been deposited in an ancient

Cretaceous Forearc Siliciclastics: Choshi,
Nakaminato and Futaba groups
In this excursion we observe forearc siliciclastic
successions exposed along the Pacific coast andnearby, central Japan, 100-250 km east to northeast
from Tokyo, central Honshu Island (Fig. 3). They
include fluvial to offshore turbidite facies throughstorm-dominated shallow-marine facies occasionallyassociated with several sedimentary structures andsuch fossil sites as vertebrate bone beds and bivalves/1

Hisao Ando, Kenji Kashiwagi, Ren Hirayama and Seiichi Toshimitsu

Fig. 1. Pre-Neogene geological map
and tectonic subdivision of Tohoku to
Kanto
regions(Ando,
2006).
Numbers of the locations refer tothose of columnar sections shown in
Fig. 2.

2

130

125

120

115

110

105

100

95

90

85

80

75

70

65

60

UPPER

55

TERTIARYCRETACEOUS

M

Lower

Upper

65.0

Danian60.9

Selandian

57.9

Thanetian

54.8

Hauterivian

127.0

Barremian

121.0

Aptian112.2

Albian98.9

Cenomanian

93.5

Turonian

89.0

Coniacian85.8

Santonian

83.5

Campanian

71.3

Maastrichtian

Lower

?

JNOC
(2000)

2

off
Sanrikuplant leaves
pollendinoflagellata
calcareous nannofossils
radiolaria
foraminifera
ammonites
inoceramids
shallow marine bivalves
brackish bivalvesoyster reef?

?

Sasaki &Iwasaki(1992)?

?

off Kuji

3

4

TaneichiAndo(1997)

?

vvvv

vvvvv

8

Miyako?

6

7

Sawamawari
Fm.

Kase et al.
(1984)

Iwaizumi

vvvvv

?

9

Facies

JNOC(1992)

Futaba Gr.

Ando etal. (1995)

vvv

vv

vv vvvvvvvv

Volcanics & volcaniclastics

16

vvvv

vvvv

Tanaka (1970)

18

Nakaminato

Oarai Fm.

?

?

Oarai

17

Nakaminato Gr.

thickness (m)fining-upward

19

?

22

Shimonita?

Arai & Takagi (1998)

22

Shimonita

Ito & Matsukawa (1997)

21

Sanchu

130

125

120

115

110

105

100

95

90

85

80

75

70

65

60

?
Kanohara Fm.

50

55

?

?
?
Atokura Fm. Atokura Fm.

?

?
Yorii Fm.

Obata et al. (1982)
Ito & Matsukawa (1997)

Choshicoarsening-upward

sharp facies boundaryravinement surface

sequence boundary

20

Makimoto & Takagi & Fujimori
Takeuchi (1992)(1989)Yorii

S o u t h w e s t J a p a n ( K a n t o ) S Ma

Boundary, succession& thickness

Kato et al.
(1996)

offKashima

Slump

Tuff

vvv
vvv

Coal

OffsTurbidite
hore mar&inbasine (ms)plain(ss<ms)basin plain
Turbidite&(ms<ss)?

Offshore marine (ss<<ms)Marine conglomerate

Fluvial to shallow marine(ss>ms)
Shallow marine (ss & ms)

Fluvial (ss>ms)

Fluvial- lacustrine (ss<ms)

Fluvial (conglomerate)

JNOC(1991)

15
Iwaki

10
Nara et al.
(1994)

?

vvvvv

12
off Soma

14 off Joban

?

vvvv

?

OshimaMatsumotoet al. (1982)

OfunatoJNOC(1985)

off
Kesennuma

11

FT71.2±4.4

Yokomichi
Fm.

vvvvv

Tanai et al.
(1978)

Kogawaunconformity 1

TaneichiFm.

Nagahama &Terui (1992)

5

Kuji

?

?

13 off Iwaki

Sasaki & Iwasaki (1992)

Joban subbasi n

K/T gap: unconformity 2

unconformity 3

Kitakami subbasin

In d ex fo s si l

?

vvvv

?

Sasaki &Iwasaki(1992)

vvvv

?

vvvv

?
vvvv

?

vvvv

?

off
Hachinohe

1
560

34

Ypresian

Upper

L

Upper

?

671
1000+

49.0

1123
334
348

150
700

Lutetian

Kuji Gr.

PALEOGENE
PALEOCENE EOCENE

LOWER

120
200

100
417
316+ 133

Northeast Japan (Yezo f orearc basin: Tohoku)

350

1000
1100
800

50

N

135

MiddleM U L U Low

M U

M U L

Low

Up

Middle

Lower

Upper

Lower

Low Upper

Up

L

70
180 170
250
>3500
400

1335
400

Ofunato Gr.

Miyako Gr.
Omoto F. Harachiyama Fm.

>2500

1050
Oshima Gr.

Hiraiso Isoai Fm.
Fm.

900

1825

Tanakura Tectonic Line
800

420
900

Stage

HiraisoFm.

460
460 270 110

1000

System &seriesvvvvvvvvvvv

MavvvvvvFig. 2. Stratigraphic correlation of Cretaceous to Paleogene sediments from Tohoku to Kanto regions (Ando, 2006).
vvvvvvvvvv

Eustatic curves (Haq etal. ,1988) calibrated byGradstein et al. (1995)(m) 200 150 100 50 0

Choshi Gr.

500

Sanyama F.
Sebayashi F.
Ishido F.
Shiroi F.

563

vvv
vvv

vvvvvv

vv

v
vvv
v

3
vvvvvvvvvvv
vvvvvv
vvv vv
vvvvvvvvvvv
vvvvvvvvvvv
vvvvvvvvvvv
vvvvvvvvvvv
vvvvvvvvvvv

Chronostratigraphy

Cretaceous forearc basin siliciclastic successions: Choshi, Nakaminato and Futaba groupsHisao Ando, Kenji Kashiwagi, Ren Hirayama and Seiichi Toshimitsu

Fig. 3. Distribution of isolated Cretaceous to Paleogene strata around the Kanto Region.

/ammonite shell beds.
The Choshi Group is situated at the eastern marginof Southwest Japan, and also the eastern extremity of
Chiba Prefecture facing the Pacific with rocky shoreand steep sea cliffs (Figs. 1 and 2, no. 19). The
Choshi Group is characterized by moderately
consolidated
rocksof
storm-dominated,shallow-marine sandstone and mudstone less than 950
m thick. Its age is regarded as Barremian-Aptian
based on ammonites and foraminifers.
The Nakaminato Group is also located in theeastern margin of Southwest Japan, forming a fault
block on the southern extension of the Tanakura
Tectonic Line (Figs. 1 and 2, no. 18). This groupconsists of the sandy turbidite and offshore mudstonefacies of late Campanian to early Maastrichtian age.
The characteristic heteromorph ammonite occurrence
suggests that the Nakaminato Group is the easternextension of the thick sequence of the Izumi Group in
Shikoku and west Honshu 600 to 800 km WSW of
the Nakaminato area.
The last Futaba Group lies on the southern marginof Northeast Japan (Figs. 1 and 2, no. 15). It is fluvial
to shallow-marine sandstone and mudstone sequencesof the Coniacian to lower Santonian. Due to simple
geologic structure and low-grade diagenesis, thesediments are not well consolidated. There is even asand quarry still working. Invertebrate fauna
represented by such as ammonites, etc., vertebrate
fauna and recently discovered angiosperm flower
fossils make possible to reveal the late Cretaceous
ecosystem of shallow-sea and land areas nearby.
In the Choshi and Nakaminato areas, there aresome geosites of Japanese Geoparks, Choshi Geoparkand North Ibaraki Geopark, respectively. Even in the
Iwaki area local people are now under investigation
for the nomination to the Japanese Geopark Network.
We can also know the outline of geopark activities at
each geosite.
Furthermore, we visit the Izura Coast Geosite ofNorth Kitaibaraki Geopark, Kitaibaraki City as extra
stops. There we can observe the late Early Miocene
strata bearing a large amount of calcareous
concretions and a chemosynthetic bivalve communityof methane seep origin. The striking natural sceneryon sea cliff and rocky coast is derived from thewave-resistant rocks of naturally cemented under sea
bottom during late Early Miocene.
We can see a few excellent museums and theirexhibition facilities highlighting the Cretaceous
fossils and geology: Geological Museum of
Geological Survey of Japan, Iwaki Coal and Fossil
Museum and Iwaki Ammonite Center.

4Cretaceous forearc basin siliciclastic successions: Choshi, Nakaminato and Futaba groups

Furthermore, as this block is subdivided into a few
blocks by other smaller-scale faults crossing the
Kurohae-Togawa Fault with high-angles, thestratigraphy of the Choshi Group is difficult to beestablished (Obata et al., 1975, 1982, etc.). The
Choshi Group may be in fault contact with the
pre-Cretaceous basement rocks, Atagagoyama Unit.

Choshi Group: Shallow-marine
storm-dominated sandstone facies
The Choshi Group, extending ca. 4 km in anorth-south direction along the eastern coastline of
Choshi Peninsula, is Barremian-Aptian in age,approximately 935 m thick, and is made up of
terrigenous siliciclastic rocks (Fig. 4). The Choshi
Group is bounded by inferred faults with the
pre-Cretaceous basement rocks, which are possibly
the equivalent strata of the Jurassic accretionary
complexes. The Choshi Group is unconformably
overlain by the upper Miocene to Pliocene Naarai
Formation at Inubousaki, west of Toriakeura,
Nagasakihana and Tokawa (Obata et al., 1975;
Takahashi et al., 2003).

“Byobugaura”,
steep
coastalcliff
of
Plio-Pleistocene Inubo GroupIn the Iioka Terrace, the Plio-Middle Pleistocene
Inubo Group is distributed NE strike and dip gently
northwestward at less than 10 degree. Especially, thelower to middle parts (Naarai, Kasuga and Obama
formations) are exposed along the steep Pacificcoastal cliff 40 to 50 meter high called “Byobugaura”(means folding screen in Japanese) 9 km long fromwest Cape Gyobu to east Naarai (Fig. 6). It issomewhat similar to White Cliff of Dover. This isalso a Geosite highlighting Choshi Geopark.

Geologic Setting of the Choshi area
Quaternary Terrace Geomorphology
The Choshi area situated in the eastern extremityof the Kanto Plain geomorphologically forms thewestern Iioka (about 50m high) and eastern Choshi(about 30 m) terraces (Figs. 4 and 5). The twoterraces had been formed during post-Middle
Quaternary glacioeustatic sea-level changes, MIS(marine isotopic stage) 5e (ca. 130-115 ka) and 5a (ca.
85-80ka), respectively (Sugihara, 2000).

Pre-Cretaceous basement rocks

Pre-Neogene tectonic block
Below the Pleistocene unconsolidated sediments
the older rocks such as the Cretaceous Choshi Groupand pre-Cretaceous basement rocks are distributed
only in the Choshi Peninsula projected over the
Pacific. The Choshi area represents a tectonic high ofthe eastern margin of Kanto basin formed by thelarge-scale tectonic movement related to platemotion.
According to Takahashi et al. (2003), thepre-Neogene basement rocks of the Choshi Peninsula
are distributed only on the east side of the NE-SW
trending Metogahana-Kayakarijima fault. This
basement area is subdivided into two areas by the
NNE-SSW trending Kurohae-Togawa fault. Here, wename the two as the western Atagoyama block andthe eastern Inubousaki block.
The Atagoyama block is characterized by thesporadically and isolatedly distributed the Takagami
Conglomerate and the Atagoyama Unit (Takahashi,
2008; Figs. 4 and 6). This block forms a horst-likehighland.
In the Inubousaki block the Cretaceous Choshi
Group crops out along the N-S trending Pacific coast.

Pre–Cretaceous basement rocks have been
described as the Atagoyama Formation (Ozaki, 1959)
or the Atagoyama Group (Shikama and Suzuki, 1972;
Takahashi, 1990). Although small, discontinuous
outcrops and lacking of fossils indicative of
sedimentary age make it difficult to establish theprecise correlation and age determination. In the lastten years, Pre–Cretaceous basements have been
tectonically and stratigraphically subdivided into 2
systems: the Kurohae Formation and the Atagoyama
Formation (Tazawa and Hasegawa, 2007); theAtagoyama Unit and the Takagami Conglomerate(Takahashi, 2008). Here we follow Takahashi’s(2008) criteria (Figs.6, 7).
The Takagami Conglomerate crops out northwest
and west of Atagoyama hill and northwest ofNagasakihana, and is mainly composed of sandstone
and conglomerate. Intraformational conglomerate
beds within thick sandstone show polymictic gravelcomposition: limestone, plutonic and volcanic rocks,
sandstone, slate and so on (Kano, 1958; Ozaki, 1959).
A fusulinoidean fauna including Lepidolina
shiraiwensis and Yabeina columbiana obtained fromthe limestone clasts indicates that the age of the

Pleistocene marine sand cover
The Late Pleistocene Katori Formation is mainlycomposed of unconsolidated marine sand andunconformably covers the older strata distributed bothin the Iioka and Choshi terraces (Fig. 4, S2)

5

Hisao Ando, Kenji Kashiwagi, Ren Hirayama and Seiichi Toshimitsu

Fig. 4. Geological map of the Choshi area. After Geological Map, 1:200,000, Chiba, Unozawa et al. (1983).
A: Atagoyama Unit (Jurassic); C: Choshi Group (Cretaceous); Me: Metogahana Fm. (Mio.); K1: Naarai Fm.
(Plio.); K2: Iioka Fm. (Plio.); S1: lower Shimosa Gr. (Pleist.); S2: Katori Fm. of upper Shimosa Gr. (Pleist.)
Fig. 5. Satellite image of
the Choshi Peninsula.
After Google Earth.

6Cretaceous forearc basin siliciclastic successions: Choshi, Nakaminato and Futaba groups

Fig. 6. Detailed geological map of the Choshi area. Modified after Takahashi et al. (2003). Basic map provided by
Dr. Takahashi’s courtesy.

Takagami Conglomerate is late Permian (Tazawa andHasegawa, 2007).
The Atagoyama Unit crops out isolatedly in thenorth and south of the Atagoyama block. Bedded
chert at Kurohae in the northeastern part of the blockyields Middle to Late Triassic conodont genera such

7

Hisao Ando, Kenji Kashiwagi, Ren Hirayama and Seiichi Toshimitsu

Fig. 7. Stratigraphy of the
Choshi area. After Takahashi
et al. (2003).
The
AtagoyamaGrouprepresents the Atagoyama
Unit and the Takagami
Conglomerate in Fig. 6.

as Neogondolella, Epigondolella and Parvigondolella(Kunihiro et al., 1984). The Atagoyama Unit at
northwest of Nagasakihana in the southern partconsistsofsandstone,sandstone-mudstonealternations and pebbly mudstone including lots ofsandstone and small amounts of greenish tuff andchert (Takahashi, 1990). Due to the broken mixture ofvarious lithologies, the Atagoyama Unit is possiblyinterpreted as a part of Jurassic accretionary
complexes (Tazawa and Hasegawa, 2007; Takahashi,2008).

Stratigraphy of the Choshi Group
Owing to the block fault structure, restricted
outcrops to the sea cliff and lacking of continuous
strata overall, the stratigraphy of the Choshi Group
has not yet reached a united opinion, although many
researchers have accepted Obata’s et al. (1982)
stratigraphic scheme.
The Choshi Group consists of five formations
based on their lithofacies and ammonoid ages: the
Ashikajima, Kimigahama, Inubouzaki, Toriakeura,and Nagasakihana formations in ascending order(Figs. 7-10) (Obata et al., 1982; Obata andMatsukawa, 2009a). Explanations for its eachformation below are referred to their works.
The Ashikajima Formation forms the base of the
Choshi Group, and is composed of cross-stratified
very coarse to fine sandstone and conglomerate.
Clasts within the conglomerate are dominated bychert with minor sandstone, shale, limestone andporphyry (Obata et al., 1975).
The main part of the formation shows
upward-fining facies changes from the lower
conglomerate into coarse to medium sandstone
gradually, in general. Macrofossils are rare, butbelemnite and petrified wood fragments werereported by Obata et al. (1975).
Fig. 8. Geological map of the Lower Cretaceous Choshi
Group. Modified from Obata et al. (1982) and Obata andMatsukawa (2009a).

8Cretaceous forearc basin siliciclastic successions: Choshi, Nakaminato and Futaba groups
Fig. 9. Stratigraphic column
showing ammonoid occurrences
modified from Obata et al.,(1982)
and
Obata
and
Matsukawa (2005, 2007, 2009a,b).

The Kimigahama Formation consists of medium
to fine sandstone and bioturbated mudstonealternations. On the basis of ammonoids andforaminiferal biostratigraphy, the Kimigahama
Formation is safely assigned to the Barremian.
The Inubouzaki Formation is characterized byamalgamatedhummockycross-stratified
finesandstone, and alternated HCS fine sandstone andmudstone.	
  Ripple marks, trace fossils and plantfragments are abundant. The facies changes from theKimigahama to Inubouzaki formations are abrupt.
From the occurrence of five Aptian ammonite genera,

9Hisao Ando, Kenji Kashiwagi, Ren Hirayama and Seiichi Toshimitsu

the Inubouzaki Formation is surely deposited in the
Early Aptian age, although foraminifers are notknown from the formation.
The Toriakeura Formation shows a general
upward-fining trend from alternating sandstone andmudstone in the lower half to more muddy alternating
beds facies in the upper. From the ammonites andforaminifers occurrences, the formation is referred tobe the Upper Aptian.
The Nagasakihana Formation is comprised ofmostly amalgamated, thick-bedded to massive,
medium to fine sandstone. Obata and Matsukawa(2009a) assumed the formation as the lower Albian.

Sedimentary facies and environments of the
Choshi Group
Katsura et al. (1984) conducted detailed
sedimentological analyses of sedimentary successions
in the Choshi Group, and revealed its depositional
environmental changes. The Choshi Group was
deposited in storm-dominated shallow marine
environments as below (Figs. 10, 11):
Nagasakihana Fm.
Offshore (turbidite)
Toriakeura Fm.
Inner to outer shelf
Inubouzaki Fm.
Lower shelf
Kimigahama Fm.
Lower shelf -inner shelf
Ashikajima Fm. Up. Upper – lower shoreface
Ashikajima Fm. Low. Upper shoreface

Macrofauna of the Choshi GroupThe Choshi Group has also been well known forabundant occurrences of macrofossils: ammonoids,
bivalves, gastropods, brachiopods, echinoids, shark
teeth, plant fossils (petrified woods and ambers) andso on (e.g., Shikama and Suzuki, 1972; Hayami andOji, 1980; Kase and Maeda, 1980; Obata et al., 1982;
Saiki et al., 1991; Obata and Matsukawa, 2007, 2009a,b). In particular, ammonoid fossils have been studied
precisely as important age diagnostic taxa in a seriesof Obata and Matsukawa’s researches (Obata et al.,1975, 1982; Obata and Matsukawa, 2005, 2007,2009a, b), and have contributed the precise age
controls of the Choshi Group as Barremian to early
Albian.

Microfossils of the Choshi GroupRadiolarians have been firstly extracted from 3
horizons of the Choshi Group: chert gravels in the
Ashikajima Formation; sandy shell bed intercalated in

10

Fig. 10. A stratigraphic column of the Choshi Group,
based on Katsura et al. (1984).

the Kimigahama Formation; calcareous nodule within
the Toriakeura Formation. Here we introduce someradiolarian occurrences as below (Fig. 12).
Several chert pebbles and cobbles withinconglomerate beds of the Ashikajima Formation
sometimes contain abundant radiolarian fossils.
Sample12072301-03
contains
poorlyto
moderately-preserved Bajocian to early Callovian(Middle Jurassic) radiolarians: Eucyrtidiellum sp.,
Hsuum sp., Striatojaponocapsa sp. (identified as
Striatojaponocapsa conexa or Striatojaponocapsa
plicarum), Syringocapsa sp. and Williriedellum sp.
Also sample 12072301-06 indicates a Permian age forthe presence of Pseudoalbaillella spp. Chert gravels
in the Ashikajima Formation have been probably
derived from the Jurassic accretionary complex.

Cretaceous forearc basin siliciclastic successions: Choshi, Nakaminato and Futaba groups

Fig. 11. Diagrammatic reconstruction of the sedimentary environments and their inferred facies successions (Katsura
et al., 1984).

The Kimigahama Formation (middle Barremian toearliest Aptian) contains a scarce and poorly
preserved radiolarian fauna in the sandy shell bed
intercalation
withinthesandstone–mudstonealternations. Most of radiolarians show spherical tosubspherical outline of test, and 180–380 µm in
length/diameter of test due to sorting by size andshape during deposition. Only two forms were
Fig. 12 Paleozoic and Mesozoic radiolarians
from the Choshi Group.
1-2. Striatojaponocapsa sp.
3. Williriedellum sp.
4. Stichocapsa sp.
5. Eucyrtidiellum sp.
6-7. Pseudoalbaillella spp.
8-9. Praeconosphaera spp.
10-11. Sethocapsa ? orca
12. Dictyomitra pseudoscalaris
13. Archaeodictyomitra mitra
14. Holocryptocanium sp.
15. Triactoma sp.
1-7. chert pebbles of the Ashikajima
Formation (1-5. sample 1207230103; 6-7.
sample 1207230106); 8-11. sandy shell bedof the Kimigahama Formation; 12-15.
calcareous nodule of the Toriakeura
Formation.

11

Hisao Ando, Kenji Kashiwagi, Ren Hirayama and Seiichi Toshimitsu

identified: Praeconosphaera sp. and Sethocapsa?
orca Foreman. The radiolaria-bearing shell bed
indicates early Hauterivian to early Aptian or later
based on the presence of Sethocapsa ? orca.
The Toriakeura Formation (early late Aptian)contains radiolaria-bearing calcareous nodules in the
muddy facies. The radiolarian assemblage shows ahigh diversification, and is listed as follows:Archaeodictyomitra mitra Dumitrica, Dictyomitra
pseudoscalaris
(Tan)
sensu
Schaaf,
Holocryptocanium sp. as nassellarians, andAngulobracchia sp., Cenosphaera sp., Orbiculiformasp., Praeconocaryomma sp. and Triactoma sp. as
spumellarians. A radiolaria-bearing calcareous nodule
indicates an age interval of late Valanginian to early
Aptian or later due to the coexistence ofArchaeodictyomitra
mitra
and
Dictyomitra
pseudoscalaris.

The basal conglomerate less than several meters
thick is composed of various gravel consisting ofPaleozoic rocks, Cretaceous sandstone and calcareous
rocks, lower Miocene volcanic rocks, etc. withintuffaceous silty matrix (Ozaki, 1958; Matoba, 1967).
It overlies the probably Jurassic Atagoyama Unit, theLower Cretaceous Choshi Group and the Lower
Miocene Metogahama Formation with a distinct
angular unconformity (Ozaki, 1954, 1958). At present,
these unconformity outcrops had been artificiallycovered by reclaimed land for building a fishery port.
The overlying tuffaceous sandstone beds crop out
continuously along the Byobugaura Sea-Cliff, andcontain pumice and glauconite with many
intercalations of pumice and silty tuff (Ozaki, 1958;
Matoba, 1967; Sakai, 1990). The stratigraphic
relationship between the tuffaceous sandstone and theunderlying basal conglomerate has been unknown due
to lacking of outcrops showing their contact horizon.

Naarai Formation: Upper Miocene-Pliocene
bearing Cretaceous reworked ammonites

Macro- and micro-fossils and depositional age
In the basal conglomerate, various fossils areincluded: 62 species of gastropods, 51 species ofpelecypods, 26 species of brachiopods, 70 species ofbenthonic foraminifers, 8 species of planktonic
foraminifers and other taxa (barnacles, corals,echinoids, ophiuroids, bryozoans, teeth of shark,otolith of whales, mammalian bones and plant leaves

The Cenozoic Naarai Formation, the base of the
Inubo Group, consists of the basal conglomerate(probably Late Miocene to Early Pliocene) and theoverlying tuffaceous sandstone beds (probably
Pliocene to Pleistocene). It is estimated to 80–100 m
in total thickness (Ozaki, 1958; Matoba, 1967).

Fig. 13. Some microfossils from the calcareous rocks in the basal conglomerate of the Naarai Formation. 1-2.
Holocryptocanium barbui; 3ab. Cryptamphorella sp.; 4. spumellarian radiolaria; 5. multisegmented tower-like nassellaria;
6-8. fragmental pieces of ichthyoliths.

12Cretaceous forearc basin siliciclastic successions: Choshi, Nakaminato and Futaba groups(Ozaki, 1954a, 1958). Also many microfossils occur
in the tuffaceous sandstone: foraminifers (Matoba,
1967; Ishikawa and Hatta, 1984), nannoplankton(Nishida, 1980), and radiolarians (Sakai, 1990). (Fig.
13)
The basal conglomerate was firstly assigned to theupper Pliocene based on the molluscan similarity with
the Zushi Formation, Miura Peninsula, Kanagawa
Prefecture (Ozaki, 1954a, b). And then Matoba(1967) regarded the basal conglomerate as the upper
Miocene owing to the presence of some planktonic
foraminifera (Globigerina nepenthes, Globorotalia
menardii miocenica, Sphaeroidinellopsis seminulina).
On the other hand the tuffaceous sandstoneintercalates with some Pleistocene tuff beds (2.2-2.3
Ma, ca. 2.5 Ma and 2.65 Ma tuff beds) (Tamura et al.,2010). Thus the stratigraphic relationship between thebasal conglomerate ant the overlying tuffaceous
sandstone have been a matter of debate.
An interesting fact is that several ammonoids havebeen discovered sporadically in the basal
conglomerate of the Naarai Formation: Albian
Mortoniceras sp. and Puzosia subcorbarica (Obata etal., 1975, 1982), and Tithonian Virgatosphinctes sp.
and Usseliceras (Usseliceras) sp. (Obata andMatsukawa, 2005). These ammonoids are interpreted
as derived fossils from unexposed Cretaceous andJurassic formations.
Some calcareous rocks in the basal conglomerate
yield abundant radiolarians and foraminifers with rareichthyoliths. Kashiwagi et al. (2013) reported latest
Late Jurassic to early Late Cretaceous radiolarianassemblage(e.g.,
Holocryptocanium
barbui
Dumitrica, Cryptamphorella spp., etc.) fromvertebrate bone fragment-bearing calcareous rock(Fig. 13).

Choshi Geopark
The Choshi Geopark approved by Japan Geopark
Network in November 2012, covers the Choshi
Peninsula and nearby in Choshi City, Chiba
Prefecture. There are major four geosites, 1) Kurohae,
Metogahana and Houman, 2) Inubousaki, 3)Atagoyama, Sengaiwa and Inuiwa and 4) Byobugaura.
In this excursion, we visit and observe 1) and 2).

13

Hisao Ando, Kenji Kashiwagi, Ren Hirayama and Seiichi Toshimitsu

DESCRIPTION OF DAY 1

chert is redefined as the Atagoyama Unit by
Takahashi (2008). The stratigraphic relation with the
Choshi Group is thought to be in fault contact.
(Kurohae-Tokawa Fault; Fig. 6).

Stop 1-1	
  Kurohae Fishery Port, Choshi City:
Cross-stratified conglomerate and conglomeratic
sandstone, Ashikajima Formation
Conglomerate and interbedded coarse to medium
sandstone of the lower part of the Ashikajima
Formation can be observed in the Kurohae Fishery
Port (Fig. 14). Conglomerate consists of round andsubround, pebble to cobble of chert, sandstone, slate,granite, etc. first three of which seem to have been
derived from the pre-Cretaceous Atagoyama
Formation. Trough cross-stratification common insandstone and conglomerate suggests the deposition
on upper shoreface of wave-dominated environments.
In the fishery port, a few rocky reefs of andesite
massive lava and its volcanic breccia of the Miocene
Senninzuka Formation are sporadically distributed,
though the contact with the Choshi Group cannot be
detected on outcrops.
A bedded chert block is isolatedly exposed inreclaimed land near the Kurohae Port (Fig. 15). This

Stop 1-2 	
  Kimigahama, Choshi City: HCS
sandstone and bioturbated sandy siltstone, lower
part of Kimigahama Formation
Along a rocky to sandy beach, north ofKimigahama, we can observe the facies changes fromthe Ashikajima to Kimigahama formations.
The upper part of the Ashikajima Formation iscomposed of amalgamated hummocky cross-stratified(HCS) sandstone. The lower part of the Kimigahama
Formation changes into alternated HCS sandstone andbioturbated sandy siltstone facies (Fig. 16).
In the sandstone part, there are wave-formed
structure such as HCS and wave-ripples. Several
kinds of vertical and horizontal burrows filled withdark gray muddy matrix are also common. Molluscan
shells mostly of bivalves such as trigoniids, etc. are
included as basal lags of beds, intervening lamina or
scattering fragments.
Mudstone is much mottled and homogenized byheavy bioturbation of benthic organisms. Initial
sedimentary structures were considerably destroyed.

Fig. 14. Conglomerate of the Ashikajima Formation at
Stop 1-1, Kurohae Fishery Port.

Fig. 16. Sandstone-dominated, alternating HCS finesandstone and bioturbated sandy mudstone of the
Ashikajima Formation at Stop 1-2, Kimigahama
Formation, Kimigahama coast.

Stop 1-3	
  Cape Inubousaki, Geosite of Choshi
Geopark, Choshi City: HCS sandstone andwave-formed sedimentary structure, Inubouzaki
Formation
This stop is the Inubousaki Geosite, a highlight of
the Choshi Geopark. It was also selected as one ofJapan's Top 100 Geological Features in 2007.
It is also a sightseeing spot where a small capeFig. 15. A bedded chert block at Stop 1-1, Kurohae
Fishery Port.

14

Cretaceous forearc basin siliciclastic successions: Choshi, Nakaminato and Futaba groups

projects over the Pacific from the Choshi Peninsula.
Shallow-marine sandy deposits of the Inubouzaki
Formation, the middle of the Lower Cretaceous
Choshi Group, crop out on large outcrops along a sea
cliff, forming a gentle anticline with an eastward
plunging E-W axis. In comparison with both of thenorthern and southern coasts, the harder lithology ofapparently massive sandstone of the Inubouzaki
Formation makes resistant geomorphology of thesteep coastal cliff against wave erosion.
The Inubouzaki Formation is characterized by
HCS fine sandstone and occasionally associated withsandy siltstone (Fig. 17). On the coastal cliff androcky shore, several kinds of sedimentary structure
are observable as follows: the most frequent HCS andamalgamated and thickly stacked HCS, flat/parallel
lamination, wave ripples as top marks or in crosssection, planar cross-stratification (PCS), troughcross-stratification (TCS) and several kinds of trace
fossils. Ammonites, trigoniid bivalves, amber, plantremains (fern, conifer and cycad, etc.) rarely occurs
from certain horizons (Obata et al., 1982; Hayami andOji, 1980; Kase and Maeda, 1981 etc.).
Hummocky cross-stratification is well known tobeindicativeofstorm-dominatedsandy
shallow-marine environments (e.g., Duke, 1985;
Walker, 1992). Ishigaki and Ito (2000) suggested thatHCS might be formed by combined flows withdominant oscillatory flows on the basis of the log
normal distribution of HCS wave length with a mode
at 300 cm detected in this formation. They also
pointed out that HCS size decreases in offshore finer
facies reflecting the relation among substrate grainsize, wave energy and water depth.

The Inubouzaki Formation seems to have been
deposited on the storm-dominated lower shoreface
sandy bottom of open-sea shallow shelf.
The strata has been designated a naturalmonument in Japan as “Cretaceous shallow-marine
sediments at Inubousaki” in 2000 (Fig. 17). We cansee some familiar figures and photo for geologists andsedimentologists on the explanatory board.
Stop 1-4 	
  Cape Nagasaki-hana, Choshi City:
Massive fine sandstone, Nagasakihana Formationand overlying Upper Miocene to Pliocene Naarai
Formation, Inubo Group and Lower Miocene
Senninzuka Formation
The uppermost Nagasakihana Formation of the
Choshi Group can be observable only here (Figs. 6, 8).
This formation is composed of thick, mostly massive
and slightly laminated, medium to fine sandstone
considerably different from other sandy formations.
While convolute structure is common (Fig. 8),
bioturbated traces are rare, suggesting highsedimentation rate. The sediments seem to be gravity
flow deposits, though not typical turbidite. Outer
shelf to upper slope is a reasonable candidate for the
sedimentary environments.
No fossils are found except plant fragments.
Owing to no good index fossils, the age determination
is difficult. In terms of the geologic structure, theformation has been regarded as the uppermost
stratigraphic position in the Choshi Group (Obata etal., 1982).

Fig. 18. Convolute structure observed in massive medium
sandstone of the Nagasakihana Formation at Stop 1-4,
Nagasakihana coast.
Fig. 17. Amalgamated HCS fine sandstone of the
Inubouzaki Formation at Stop 1-3, Cape Inubousaki. The
right-side stone monument marks “Cretaceousshallow-marine sediments at Inubousaki” in Japanese as anatural monument in Japan.

15

Hisao Ando, Kenji Kashiwagi, Ren Hirayama and Seiichi Toshimitsu

Senninzuka Formation: Miocene andesite
On rocky reefs northeast of the Nagasakihana
lighthouse, the Miocene Senninsuka Formation iscomposed of cross-stratified tuffaceous sandstone andthe overlying bronzite andesite lava according to
Takahashi et al. (2003).
In contrast with the other northern rocky andsandy coasts along Choshi Peninsula, theNagasakihana coast is dark-colored gravel beach due
to abundant black-colored andesite beach gravel
derived from the Miocene Senninzuka Formation.
The gravel is derived also from the basal
conglomerate of the Upper Miocene to Pliocene
Naarai Formation described below and the Cretaceous
Nagasakihana Formation.

Various macrofossils occur from the basal
conglomerate and its reworked gravel on the recent
gravel beach. So Nagasakihana is one of thewell-known fossil localities in the Choshi area where
many fossil hunters have visited to collect fossils.
You can also gather microfossil-bearing calcareous
gravel with round to subround shape and much-boredsurfaces by recent boring shells.
Outcrop observations of the basal conglomerateand collecting fossils depend on the ebb tide andwave condition on 7th September (ebb tide at 8:43;
flood tide at 15:36 by tidal forecasts of the Japan
Meteorological Agency).
At Café Marina, Chiba Institute of Science, we
can observe the excellent fossil collections by a local
fossil collector.

Naari Formation, Inubo Group: Pliocene bearing
Cretaceous ammonites and radiolarian-bearing
pebbles
Owing to the recent gravel beach, the basal
conglomerate of the Naarai Formation crops out
intermittently (Fig.19). It shows poorly sorted,
matrix- or clast-supported, angular to subround gravel
facies with tuffaceous matrices.

Fig. 19. Field view at Nagasakihana, Stop 1-4. 1) Panoramic view of Nagasakihana. 2) The basal conglomerate of the Naarai
Formation. Black arrow indicates an angular cobble of microfossil-bearing calcareous rock. 3-4) Boulders of
microfossil-bearing calcareous rocks washed out from the basal conglomerate of the Naarai Formation.

16

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17

Hisao Ando, Kenji Kashiwagi, Ren Hirayama and Seiichi Toshimitsu

3. Sedimentary facies changes
The facies changes from the Hiraiso to Isoai
formations were inferred as the basin-plain to lower
submarine-fan mudstone and turbidite facies to themid to upper submarine-fan coarser turbidite facies
by Masuda and Katsura (1978) and Katsura andMasuda (1978).

basis of those inoceramid and ammonite occurrences.
This age is nearly equivalent to that of the Izumi
Group (Morozumi, 1985) in Southwest Japan and the
Hakobuchi Group in the Sorachi-Yezo Belt of
Hokkaido (Toshimitsu et al., 1995; Ando, 2003).

Microfossils from the Hiraiso Formation

Macrofauna and depositional age

A calcareous nodule collected from massive
mudstone facies of the Hiraiso Formation on 7th
March 2014 yields abundant diatoms with rareradiolarians and foraminifera (Fig. 21). Radiolarians
show various preservation states, although they don’t
contribute to the age assignment due to lacking in
age-determinablespecies.
Extracted
diatomspecimens have circular disc-like tests, and are almost
filled by framboidal pyrite in their inner side.
As late Cretaceous diatom fossils occurred from asfollows: Santonian Omagari Formation (Shimada etal., 2013) and lower Campanian Osoushinai
Formation (Iwata et al., 1998) of the Yezo Group in
Hokkaido, and Upper Campanian to Lower

The Hiraiso Formation occasionally contains
helicoid heteromorph ammonite Didymoceras
awajiense, U-shaped heteromorph Polyptychocerassp., planispiral ammonite (Pachydiscus sp.),
Inoceramus cf. balticus, some small bivalves andechinoids (Saito, 1958, 1959, 1961, 1962; Sakamoto
et al., 1972). The Isoai Formation very rarely contains
some inoceramids as Inoceramus cf. shikotanensis
and Inoceramus kusiroensis in a few horizons(Sakamoto et al., 1972; Ando, 2006). The lateCampanian to early Maastrichtian age is inferred for
the Hiraiso and Isoai formations respectively, on theFig. 21. Radiolarians, diatoms andforaminifera from the Hiraiso
Formation of the Nakaminato
Group.
1-4. Spumellarian radiolarians
5-10b. Diatoms
11. Foraminifera

18Cretaceous forearc basin siliciclastic successions: Choshi, Nakaminato and Futaba groups

Maastrichtian Shoya Formation in the Kanto Region(Takahashi et al., 1999). Therefore, the diatom
occurrence from the Hiraiso Formation is the fourthcase in the pre-Cenozoic strata in Japan.

On the other hand, Ono (2000) reported 64.2 Ma
K-Ar age of granitic rock gravel from conglomerate
in addition to the older different gravel ages. Judging
from these circumstances and the geologic setting
nearby, the Oarai Formation is possibly regarded as
the late Paleocene in age.
The Oarai Formation may represent marine
channel facies deposited at high sedimentation rateand derived from gravelly river, taking the extremely
large thickness (over 1,000m), the relatively
monotonous conglomeratic lithofacies and no clear
evidence of fluvial origin (Ando, 2006) into account.
The Nakaminato Group and Oarai Formation arerecognized as geologic elements of Southwest Japanon the basis of the geographic distribution of theequivalent contemporaneous strata.

Tectonic Setting
In terms of the lithofacies similarity andmacrofossil biostratigraphy, the Nakaminato Group isthought to be a part of the eastern extension of the
Izumi Group that is widely distributed along theMedian Tectonic Line in Shikoku and western
Honshu of Southwest Japan, 600 to 800 km WSW of
the Nakaminato area. In Northeast Japan, there are noequivalent contemporaneous strata similar inlithofacies and biofacies (Fig. 2). During the latest
Cretaceous, a wide forearc (or strike-slip) basin seems
to have been distributed in Southwest Japan facing
the Paleo-Asian continent.
The stratigraphic relations of the Nakaminato
Group with the underlying and overlying strata areunknown due to an inferred fault block distribution
and the Quaternary sediment cover. Taking thelarge geologic structure and the geographic
distribution into account, the Nakaminato Groupseem to be a large tectonic block within the southern
extension of the Tanakura Tectonic Line (Fig. 20).

North Ibaraki Geopark

Paleogene Oarai Formation
To the south of the Nakaminto Group, the possibly
Paleogene Oarai Formation is distributed from rivermouth side of the Naka River to the Oarai coast
southward (Fig. 20). The Oarai Formation (Oyama,
1960; Saito, 1961) dominated by very thick,gravel-supported, pebble to cobble conglomerate,
apparently underlies the Nakaminato Group on thebasis of its dip and strike, though there is no exposureof the contact. Its thickness reaches 1,000 m.
Since plant macrofossils and pollen fossils occur
from intervening sandstone layers, the Oarai
Formation was uncertainly interpreted to be late
Cretaceous in age ("Oarai flora": Oyama, 1960, 1961),
and fluvial sediments (Saito, 1961). Because of bad
preservation of the fossils (specimens' repository:
Ibaraki Univ.) and lacking of index taxa, a reliable
age assignment was difficult (Sakamoto et al., 1972).
According to Saiki and Miyahashi (2005), most of
the specimens of the Oarai flora do not show any
characteristics of Cretaceous plant elements such as
Bennettitales. They concluded the Oarai Formation as
the Cenozoic strata, but they did not find any goodevidence to discriminate the Paleogene or Neogene.

19

North Ibaraki Geopark approved by Japanese
Geopark Network in 2011, covers the northern half ofIbaraki Prefecture 100 to 180 km north of Tokyo. It
includes 1) Abukuma mountains composed of
Paleozoic sediments (Cambrian and Carboniferous toPermian), their metamorphic rocks during early
Cretaceous and granitic rocks intruded during early to
mid-Cretaceous, 2) Yamizo Mountains of Jurassic toearly Cretaceous accretional complexes, 3) Pacificside hilly area of Abukuma Mountains composed of
Eocene to Pliocene sediments (Joban forearc basin),4) Kuji Mountains and the southern hilly area of
Miocene to Pliocene sediments (Tanakura basin
formed by strike-slip movements of Tanakura
Tectonic Line, and 5) northern part of Hitachi Terraceof Pleistocene shallow-marine sand deposits.
Among 15 selected major geosites, the Hiraiso
Coast Geosite is characterized by the Upper
Cretaceous Nakaminato Group and its exposed rocky
shore and wave cut benches. Offshore marine
sediments of 76 to 70 Ma attract the public
imagination on the Late Cretaceous world withheteromorph ammonite swimming ocean andpterosaur-flying sky. At Stop 2-3 an explanatoryboard was recently set up for the Geopark tourism.

Hisao Ando, Kenji Kashiwagi, Ren Hirayama and Seiichi Toshimitsu

DESCRIPTION OF DAY 2
Stop 2-1 Geological Museum of Geological
Survey of Japan, AIST: Excellent geologicalcollections
The Geological Museum of Geological Survey ofJapan, AIST (National Institute of Advanced
Industrial Science and Technology) was establishedin 1980 in Tsukuba Science City (Fig. 22). Its main
purpose is to exhibit and keep large amounts ofgeological specimens collected by many GSJ
geologists over a number of years, and extend
research processes and fruits of GSJ to the generalpublic.

Fig. 23. Plastic replica of a folded Jurassic strata.

Fig. 22. Front view of the Geological Museum, GSJ.
Fig. 24. Reconstructed skeleton model of Desmostylus.

The exhibition areas of the Geological Museum
are divided into four major display rooms themed on
1) History of earth and life, and geology of theJapanese Islands, 2) Human life and mineral
resources, 3) Geological hazards, such as earthquakes
and volcanic eruptions and 4) Systematic exhibitionsof minerals, rocks and fossils. A total of exhibitionareas including the entrance hall reach 1500m2 in themuseum building. The remarkable highlights ofmajor exhibits are as follows:
Entrance hall:
1. Distribution patterns of earthquakes beneath theJapanese Islands on the ceiling of the entrance hall.
2. Peeling cast of the Jogan tsunami deposits (A.D.
869) collected from Sendai City, Miyagi Prefecture,
Northeast Japan.
3. Replica of folded Jurassic strata from the Ojika
Peninsula, Miyagi Prefecture, NE Japan (Fig. 23).
1st display room:
4. Schematic 3-D geological map of the JapaneseIslands.
5. Reconstructed whole skeleton model of a Miocene
extinct mammal Desmostylus from Hokkaido,
Northern Japan (Fig. 24).
2nd display room:

Fig. 25. Display room of the geological specimens.

6. Elemental periodic table with related naturalminerals.
7. 3-D topographic map of the Pacific Ocean floor.
3rd display room:
8. 3-D geological map of Fuji and Hakone volcanoes,
central Japan.
4th display room:
9. Over 1000 geological specimens, such as minerals,
rocks and fossils (Fig. 25).

20

Cretaceous forearc basin siliciclastic successions: Choshi, Nakaminato and Futaba groups

Fig. 26. Dark gray mudstone facies of the Hiraiso Formation, Nakaminato Group exposed on a wide wave cut bench at the
low tide in the spring tide during summer. A left rocky reef represents thick massive medium sandstone projected on the flatbench.

Stop 2-2: Wave cut bench along Hiraiso Coast,
Nakaminato City: Basin plain mudstone faciesof the Hiraiso Formation
Only during low spring tides from March toAugust, the Hiraiso Formation is well exposed on awide wave cut bench 100 to 150 meter long in aseaward direction (Fig. 26). This rocky shore consistsof dominant dark gray mudstone and often
intercalated thin layers of fine sandstone (distal
turbidite). They seem to have deposited in an offshore
deep-sea environment. A few very thick (> 5 m)layers of medium sandstone (possibly high density
turbidite) can be observed as rock reefs projected on aflat bench reflecting high resistance against wave
erosion. The strata were often cut and translated byminor faults in several ways and offset lengths due totectonic movements after deposition.
There sporadically contains calcareous concretionsof various size and occasionally forms nodular layers.
Spiralheteromorphammonite,
Didymoceras
awajiense rarely occurs from the concretions andmother rocks.

ranges a few tens centimeter to 1.5 meters. Reflecting
differential resistance of each bed against wave
erosion, alternated resistant sandstone and depressed
mudstone form small-scale, serrated high and low
relief like a washboard.
This turbidite succession shows various
sedimentary structures as grading, parallel/crosslamination, dish, convolute and load structures,
burrowing traces by benthos as well as flute andgroove casts as sole marks.

Stop 2-3: “Seijo-seki”, north of Hiraiso Coast,
Nakaminato City: Turbidite facies of alternatedsandstone and mudstone in the lower part of the
Isoai Formation
Rocks at this stop were named as “Seijo-seki” thatmeans pure stone in Japanese, by Mitsukuni
Tokugawa, the second lord of the domain of Mito(now nearly Ibaraki Prefecture) in 17th century. Hererhythmically alternated sandstone and mudstone of
the lower part of the Isoai Formation is exposed on awave cut bench narrower than Stop 2-2 (Fig. 27). The
thickness of the sandstone and mudstone coupletsFig. 27. Alternated sandstone and mudstone in the lower
part of the Isoai Formation, Nakaminato Group, north of
Hiraiso coast.

21

Hisao Ando, Kenji Kashiwagi, Ren Hirayama and Seiichi Toshimitsu

Stop 2-4: North of Hiraiso Coast, Nakaminato
City: Debris flow deposits as pebbly sandstone
in the Isoai Formation (pterosaur locality)
In the rocky shore 300 m north of Stop 2-3, pebbly
sandstone is intercalated in a thick succession ofsandstone-dominated, interbedded sandstone andmudstone (Fig. 28). Poorly sorted gravel scattered
within a large amount of massive sand matrix
indicates of subaqueous debris-flow origin. Beds ofgranule to pebble conglomerate and conglomeratic
sandstone are also occasionally intercalated. A
pterosaur bone and a few kinds of bivalve shells(articulated Opis and inoceramid fragments) werediscovered from the pebbly sandstone. Only a pieceof mosasaurid caudal vertebra was recently foundfrom a conglomerate layer.

Stop 2-5: Isozaki Coast, Nakaminato City:
Thick sandstone succession of the upper part of
the Isoai Formation
Along cobble to boulder gravel beach, thicksandstone-dominated succession typical in the upper
part of the Isoai Formation is exposed with NNE dip(40º) and WNW strike (Fig. 30).
Beach gravel is mostly eroded and reworked
sandstone of the Isoai Formation. Rarely some
Miocene calcareous mudstone gravel bearing bivalvesfragments can be collected. This may be derived fromthe some Miocene strata nearby already eroded away.
The white-walled building here on the late middle
Pleistocene terrace about 20 meters high from sea
level is a beautiful hotel called “Hakuaki-so”.
“Hakuaki” and “so” mean Cretaceous System andinn/hotel/apartment house in Japanese, respectively.

Fig. 28. Pebbly sandstone of debris flow deposit in thelower part of the Isoai Formation, Nakaminato Group,
north of Hiraiso coast. A pterosaur bone was recently
discovered from the central portion.

Fig. 30. Thick sandstone facies in the upper part of the
Isoai Formation, Nakaminato Group, Isozaki coast.

To 150 m northward, there are southward dipping
strata forming rocky reefs within the NNE dipping
Isoai Formation, locally called “Chikusho-iwa”(dump animal rock in Japanese) (Fig. 29). This seems
to be a large-scale slump block, possibly tilted bysubmarine sliding. 	
 Fig. 29. Reversely inclined strata regarded as slump block
due to submarine sliding in the lower part of the Isoai
Formation, Nakaminato Group, north of Hiraiso coast.

22

Cretaceous forearc basin siliciclastic successions: Choshi, Nakaminato and Futaba groups

Fig. 31. Stratigraphy of the Upper Cretaceous to Neogene sequence in the Joban area (Ando et al., 2011).

23

Hisao Ando, Kenji Kashiwagi, Ren Hirayama and Seiichi Toshimitsu

DESCRIPTION OF DAY 3
Stop 3-1 North of “Rokkakudo”, Ohizura, Izura
coast, Kitaibaraki City: Calcareous concretions
in Kokozura Formation, Takaku Group (late
Early Miocene)
The Izura coast is one of highlights of North
Ibaraki Geopark in culturally and geologicallystriking features.
We can see a large amount of calcareous
concretions of various shapes are included in the late
Early Miocene Kokozura Formation, Takaku Group
along the Izura (five inlet in Japanese) coast forming
five small inlets of rocky shore (Figs. 32, 33). The
Kokozura Formation is mainly composed oftuffaceous, intensely bioturbated, very fine silty
sandstone to sandy siltstone, which may have been
deposited on inner shelf of the Miocene forearc basin.
Bivalve shells occur scatteringly or somewhat

gregariously often in articulated life position.
Ueda et al. (2005) described the mode ofoccurrence of calcareous concretions and fossil shells
on the cliff and rocky reef section less than ten meter
thick. They detected six morphotypes of calcareous
concretions on the basis of their shapes, sizes andmodes of occurrence, and defined three stratigraphic
units. Judging from carbon and oxygen isotopic ratiosin carbonate concretions as well as mode of fossil(Lucinoma
acutilineatum)occurrences,these
concretions seem to have been formed in a methane
seepage zone associated with chemosynthetic
community on a shallow shelf environment.
Kohda et al. (2007) discovered the shark teeth
aggregation of Carcharodon megalodon, the largest
shark species during the Earth history.
.

Fig. 32. Geological map of the Hirakata-Otsu area, Kitaibaraki City showing Paleogene to Neogene strata. The Kokozura
Formation is generally equivalent to the Numanouchi Formation, Takaku Group in Fig. 31. The overlying Taga Group
indicates the submarine channel-fill deposits composed of several sedimentary units of different ages (Ando et al., 2011).

24

Cretaceous forearc basin siliciclastic successions: Choshi, Nakaminato and Futaba groups

Fig. 33. Outcrop photo and sketch of Ohizura, Izura coast, Kitaibaraki City (Stop 3-1). Photo was taken during a low springtide on 16 June 2006. The excavation research on a giant shark teeth assemblage was operated at the Carcharodon locality inthe central part of wave cut bench. The lower unit of the Kokozura Formation (Ueda et al., 2005) is dominated by type I ofcarbonate concretions. The dark-colored upper unit forms a gently inclined platform seaward of types V and VI concretions(Ando et al., 2011).

Stop 3-2 South of “Rokkakudo”, Koizura, Izura
coast, Kitaibaraki City: Stratigraphic relations
between Kokozura Formation, Takaku Group(late Early Miocene) and Taga Group (latest
Early-Middle Miocene)
In the small inlet called “Koizura” (=small inlet in

Japanese), we can observe the complicatedunconformable relations between the Kokozura
Formation, Takaku Group and the Taga Group. On
the basis of the detailed diatom biostratigraphy,
outcrop observations and wide-areal geologicalmapping, three stratigraphic units were described as

Fig. 34. Route map and photograph showing the wall of the Otsu Submarine Channel of the Miocene Taga Group at
Koizura, Izura Beach, Kitaibarki City (Stop 3-2). Units Ot1 and Ot2 overlie the Kokozura Formation of the Takaku Group.
The Izura Tuff is intercalated in unit Ot3 (Ando et al., 2011).

25

Hisao Ando, Kenji Kashiwagi, Ren Hirayama and Seiichi Toshimitsu

follows (Ando et al., 2011; Fig. 34):
1) Ot3: Ohtsu submarine channel-fill unit 3(Late Miocene: ca. 10-10.5Ma)
2) Ot2: Ohtsu submarine channel-fill unit 2(Late Early Miocene: ca. 16.3-16.5Ma)
3) Kokozura Formation (late Early Miocene)(Late Early Miocene: ca. 16.7Ma)
Though all of three strata are apparently similar eachother in lithology, erosional surfaces (unconformity)
as submarine channel bases can be detected on theoutcrop surfaces.
	  In the Ot3, Izura Tuff (Yanagisawa, 2000) isintercalated within thick and massive diatomaceous
siltstone. This marker bed is characterized by coarse
mica, quartz and pumice grains. The F-T age dating
for the correlated Ayukawa Tuff in the Hitachi area
40 km south resulted in 11.1±0.5Ma a little older thanthe estimated position of the uppermost NPD 5c(North Pacific Diatom Zone 5c) in Ot3 (Figs. 31, 34).

30 cm long of Acharax yokosukensis, the lagest as thegenus.

Stop 3-4 Izura Institute of Art and Culture,
Ibaraki University: “Rokkakudo”, a small
hexagonal cottage on calcareous concretions“Rokkakudo” a red-walled small hexagonalcottage stands on a narrow cliff step in the Izura
Institute of Art and Culture, Ibaraki University (Fig.
36).
The institute was established in 1955, after thehouses, Rokkakudo and an entrance gate built duringlate Meiji Period by Mr. Tenshin Okakura, apioneering Japanese artist, were donated to Ibaraki
University, a regional national university. Aroundhere the Japanese-style modern painting art
movement occurred during late Meiji to early Taisho
Period, early 1900s to 1910s by Mr. Okakura his
following painters. Here is one of highlights of North
Ibaraki Geopark and sightseeing of Kitaibaraki City.

Stop 3-3 Garden rocks on a marine terrace
above Stop 3-2: Calcareous concretions bearing
chemosynthetic bivalves
There are many blocks of calcareous concretions
in a garden on a marine terrace above Stop 3-2 (Fig.
35). These occurred from the ground of Izura Kanko
Hotel Taikanso (north of Stop 3-1) during
construction works. The stratigraphic horizon may beequivalent to the lower unit of Ueda et al (2005) at
Stop 3-1.

Fig. 36. “Rokkakudo”, a red-walled small cottage for sea
viewing built by Mr. Tenshin Okakura, a pioneering
Japanese Modern Artist during early 1900s, now kept in
the Izura Institute of Art and Culture, Ibaraki University.
After completely swept away by the tsunami attack on 11
March, it was reconstructed in April 2012. Stop 3-4.

Fig. 35. Calcareous concretion block bearing a large
amount of chemosynthetic bivalves at Stop 3-3.

Of dark/light gray-colored concretions, light gay
ones contains a large amount of chemosynthetic
bivalves as Adulomya 	  sp., Conchocele bisecta,
Lucinoma acutilineatum, Acharax yokosukensis,
Nipponothracia? sp. The predatory gastropod species
such as Cryptonatica clausa and Megasurcula
yokoyamai are also associated to a small extent.
Amano and Ando (2011) described a specimen about

26

Cretaceous forearc basin siliciclastic successions: Choshi, Nakaminato and Futaba groups

Futaba Group: Western marginal facies of
the Yezo Basin

Lower Cretaceous Abukuma granitic rock mass
(Ohisagawa Granodiorite) with nonconformity in thewestern margin, and the Permian Takakurayama
Group with angular unconformity in the southwestern
margin (Fig. 37). In the eastern margin it underlies
the upper Eocene-lower Oligocene Shiramizu Group
with gentle angular unconformity. In the southern
margin, the group is truncated by the Futatsuya Fault
and is not distributed southward. However, theequivalent strata were detected undergroundsouthward by the coal exploration drilling (Eguchi et

The Upper Cretaceous Futaba Group is narrowly
distributed 2 to 3 km from east to west and about 15
km from north to south along the foot area of the
Abukuma Mountains parallel to the Pacific coast,
covering the southernmost Naraha and Hirono towns
and the northern part of Iwaki City (Fig. 37).
This area is tectonically regarded as the Abukuma
Belt, Northeast Japan. The Futaba Group overlies theFig. 37. Geologicalmap of the Futaba
Group (Ando et al.,1995).

27

Hisao Ando, Kenji Kashiwagi, Ren Hirayama and Seiichi Toshimitsual., 1959) and the sea-bottom dredge survey (Sugai etal., 1957). Its correlatives are also distributed under
the sea bottom off Soma to Joban in the Pacific (Ando,
2003).
The Futaba Group represents sandy and subordinately
silty and gravelly, fluvial to shallow-marine
siliciclastics deposited on the western margin of amatured forearc basin during the Coniacian toSantonian. It is characterized by simple geologic
structure with NNE-SSW strike and isoclinally
eastward dip of 10 to 20 degrees. The group ranges
350m in thickness at maximum, but pinches out
northward by the angular unconformity with theoverlying late Eocene Shiramizu Group containing
coal measures (Figs. 31, 38).

Fig. 38. Synthesized stratigraphic column, and depositional model and history of the Upper Cretaceous Futaba Group.
As: Asamigawa Member, Ashizawa Formation, Ob: Obisagawa Member, Ashizawa Formation, Km: Kasamatsu
Formation, Tm: Kohisagawa Member, Tamayama Formation, Tu: Irimazawa Member, Tamayama Formation.
Modified from Ando et al. (1995).

28

Cretaceous forearc basin siliciclastic successions: Choshi, Nakaminato and Futaba groups

Stratigraphy and Sedimentary facies

Early Coniacian ammonites and inoceramids, suchas Forresteria alluaudi and Inoceramus uwajimensis,
occurred from the middle part of the Ashizawa
Formation, and lower Santonian Inoceramus(Platyceramus) amakusensis from the Irimazawa
Member of the Tamayama Formation (Obata, 1967;
Obata and Suzuki, 1969; Matsumoto et al., 1990).
Therefore, the age of the group is approximately
regarded as early Coniacian to early Santonian (Fig.
38).

The Futaba Group is divided into three formations,
the Ashizawa (the Asamigawa and Obisagawa
members), Kasamatsu and Tamayama (theKohisagawa and Irimazawa members) formations in
upward sequence (Figs. 31, 38; Saito, 1961, 1962;
Ando et al., 1995; Kubo et al., 2002, etc.).
Sedimentary facies and their spatial distribution wereanalyzed in detail by Ando et al. (1995) and Kubo etal. (2002). According to Ando et al. (1995), fifteen
depositional facies were discriminated based on theirlithology, sedimentary structure, etc. (Fig. 39). The
group includes talus, alluvial fan, meandering river,braided river, transgressive lag, upper shoreface toinner shelf sediments.

1. Asamigawa Member, Ashizawa Formation
This member is represented by basal conglomerate
and interbedded often cross-stratified coarse to
medium arkosic sandstone and sandy mudstone
indicative of upper to lower alluvial fan environment.
Fig.
39.
Faciesdivision and theircharacteristics of the
Futaba Group (Ando etal., 1995).

29

Hisao Ando, Kenji Kashiwagi, Ren Hirayama and Seiichi Toshimitsu

The basal conglomerate contains pebbles to boulders
mainly of granitic rocks and subordinately of shaleand sandstone, amphibolite, schist, chert derived fromthe basement. In the southern periphery of the
distributed area, it includes talus or upper fan
conglomerate composed of angular gravel ofbasement rocks as black shale, chert, granite, etc. In
the northern part the sandstone unconformably covers
the granitic basement without basal conglomerate.
The thickness of this member varies from less than 30
to a few meters depending upon the undulated basal
unconformity. A variety of mesofossils of plants
including angiosperm flowers, pollen and spores havebeen found from the lower part of the member at
Kamikitaba, Hirono Town, and reported in several
papers (e. g., Takahashi et al., 2014) since Takahashi
et al. (1999a, b).

composedofcross-stratified
coarseto
medium-grained, channel sandstone and flood-plain
grayish-black bituminous siltstone containing coaly
seams and plant remains. The uppermost part
indicates the lagoonal environments locally, judging
from common bioturbated and burrowed sedimentary
structures. The repetitive sandstone and mudstone
couplets
form
five
to
six,
higher-order
upward-fining/thinning units 5 to 20 meter thick. The
ratio and thickness of sandstone/mudstone show thenorthward fining and thickening of mudstonesuggesting the northward distal/down-stream trend.
4. Kohisagawa Member, Tamayama Fm.
The Kohisagawa Member, the lower to middle
parts about 200 meter thick of the Tamayama
Formation is braided river sediments characterized bygenerally monotonous, medium to large-scale troughcross-stratified, arkosic, coarse to medium sandstone.
Coaly plant fragments are rarely contained insandstone. The uppermost part is locally intercalatedwith coaly and carbonaceous mudstone layers andoccasionally bioturbated and vertically burrowed,
suggesting the lagoonal environments.
Amber, pollen and spore and angiosperm flower
occur from intercalated coaly mudstone in the upper
part (Miki, 1977; Takahashi, 1988; Takahashi et al.,1999). Dinosaur bones and teeth are also found fromthe same coaly mudstone facies.

2. Ohisagawa Member, Ashizawa Formation
The Obisagawa Member 60 to 150 meters thick is
dominated by the upper to lower shoreface finesandstone and inner shelf sandy siltstone above thebasal transgressive conglomerate of subround to
round pebble to cobble traceable over the area. The
lowest part is generally coarser and contains medium
to coarse, partly cross-stratified sandstone intercalatedwith pebble to granule conglomeratic layers andconglomerate. A few layers of intercalated calcareous
pebbly sandstone form coquina beds of trigoniid,
glycimerid and other bivalves.
The main part of the Obisagawa Member ismainly composed of much bioturbated very fine tofine sandstone, but rarely hummocky cross-stratified
in fine sandstone. Calcareous concretions arecommon, though their size and shape are variable.
There are several fossiliferous horizons and layers
mainly of molluscs such as bivalves including
inoceramids, gastropods, scaphopods and ammonites,
though their stratigraphic positions are different by
area. Furthermore, shark teeth, turtle bone, anddinosaur bone and teeth rarely occur from a few
different localities. The ammonite shell bed observedat the Iwaki Ammonite Center is stratigraphicallyequivalent to the lower one third of this member. One
of common fossils, Inoceramus uwajimensis indicatedthe Early to Middle Coniacian age.

5. Irimazawa Member, Tamayama Fm.
The Irimazawa Member less than 60 meter thick
exposed, of the Tamayama Formation forms lower
shoreface to inner shelf fine sandstone facies above apebble conglomerate layer overlying lagoonal
deposits of the uppermost part of the Kohisagawa
Member. The basal conglomerate can be regarded
as transgressive lag deposits diachronously formed byshoreface erosion during transgression. Finesandstone of the lowest part shows low-angle
cross-stratification or hummocky cross-stratification,
but the middle to upper parts are generally
bioturbated and massive, and occasionally containlenticular calcareous-concretion layers. Marine
molluscs, fish and marine reptile (plesiosaurid:
Futabasaurus suzukii Sato et al., 2006) occurred froma few horizons. An inoceramid fossil as Inoceramus(Platyceramus) amakusensis indicates this member as
the Lower Santonian.

3. Kasamatsu Formation
The Kasamatsu Formation about 120 meter thickis characterized by fluvial upward-fining units

30Cretaceous forearc basin siliciclastic successions: Choshi, Nakaminato and Futaba groups

Fig. 40. Correlated columnar sections of the Futaba Group showing facies distribution and successions (Ando et al., 1995)

31

Hisao Ando, Kenji Kashiwagi, Ren Hirayama and Seiichi Toshimitsu

Sequence Stratigraphy and Depositional History

successions composed of channel sandstone andflood-plain siltstone couplets in the Kasamatsu
Formation, shows large-scale river channel shifts byperiodic changes of the fluvial system or relative
sea-level changes.
The sudden lithofacies changes from theKasamatsu into the Tamayama Formations imply
some large-scale changes in fluvial systems by
sea-level fall, possibly meandering to braided river,though an erosional base is not observable. Becausea sequence boundary (SB3) is expected at the base of
the Tamayama Formation or nearby, the Kasamatsu
Formation seems to form a depositional sequence(DS2) as TST due to thick aggradational successionsof fluvial plain deposits and the uppermost
subordinate lagoonal deposits.
The third sequence (DS3) is characterized by LSTof thick braided river sediments of the Kohisagawa
Member of the Tamayama Formation.
The basal conglomerate of the Irimazawa Memberof the Tamayama Formation is interpreted astransgressive lags just above a ravinement surface(RS2). The Irimazawa Member exposed about 50 m
thick only in the central area, shows the slight
upward-fining/deepening trend from upper shoreface
fine to inner shelf very fine sandstone. The member
may represent thick TST of DS3. The upper sequenceboundary (SB4) of DS3 shows the conspicuous
angular unconformity with the Paleogene Shiramizu
Group.
In summary, each formation of the Futaba Group
forms a depositional sequence. They may haveformed in the small-scale fan delta, coastal plain toshallow shelf on the foot area of the Early Cretaceous
granitic basements by three relative sea-level changes.

The nearly shore-parallel facies distribution
schematized from correlated columnar sections (Figs.
39, 40 and 41) enables us to interpret sequencestratigraphy. A thick acidic tuff layer recognized in
the upper part of the Kasamatsu Formation provides
only a good time marker horizon. Each formation of
the Futaba Group consists of a third-orderdepositional sequence (DS) during 4 to 4.5 m.y.
The lowest sequence boundary (SB1) is equal to
the nonconformity or angular unconformity withbasement rocks. The first depositional sequence(DS1) has an LST as the main part of the Asamigawa
Member distributed in the northern half of the area. It
is lithologically characterized by upper alluvial fan
conglomerate and mid to lower fan sandstone. In thesouthern periphery of the distributed area the member
contains talus or upper fan angular conglomerate.
The basal conglomerate of the Obisagawa
Member distributed over the area is interpreted to betransgressive conglomerate with a sharp base called aravinement surface (RS1) that was formed bytransgressive shoreface erosion during transgression.
The overlying thick sandstone of the Obisagawa
Member shows a gently upward-fining trend frommedium, fine to very fine sandstone, suggesting thewide and prolonged development of sandy shallow
shelf and the environmental changes from upper
shoreface to inner shelf. Therefore, this member canbe regarded as transgressive systems tract (TST).
A sharp erosional surface between the Obisagawa
Member and the Kasamatsu Formation appears an
unconformity (SB2) showing sea-level fall andsubaerial erosion.
Five to six upward-fining

Fig. 41. Diagrammatic facies distribution and sequence stratigraphy of the Futaba Group. Ia: Inoceramus
uwajimensis, Yo: Yabeiceras orientale, Fa: Forresteria (Forresteria) alluaudi (Lower Coniacian). Modified
from Ando (1997).

32

Cretaceous forearc basin siliciclastic successions: Choshi, Nakaminato and Futaba groups

Judging from the time duration of Early Coniacian to
Early Santonian (89.8-85.5: Gradstein et al., 2012),
these sequences are regarded as third-order ones.

and subsequently excavated during a few years(Obata et al., 1970), the specimens called “Futaba
Suzuki Ryu” and the Futaba Group as the vertebrate
fossil-bearing Cretaceous strata have been wellknown to the Japanese public. According to Goto etal. (1996), Saegusa and Tomida (2011), Sato et al.
(2006, 2012) and Shimada et al. (2010), the Futaba
Group includes the following vertebrate fossils: 1)
partial shells of a terrestrial turtle, family
Nanhsiungchelyidae (Order Testudines), 2) tooth ofMosasauridae (Order Squamata), 3) vertebra ofElasmosauridae and 4) isolated teeth of
Polycotylidae? (Order Sauropterygia) from the
Ashizawa Formation of Hirono Town and Iwaki City
(Tokunaga and Shimizu, 1926; Sato et al., 2012); 5)
isolated shark teeth such as Scapanorhynchus
mophiadon Agassiz, ?Paranamotadon angustidens,
and Cretolamna appendiculata, 6) a rostal spine of aosteichthyan fish, Ischyrhiza iwakiensis, 7) vertebraof Mosasauridae (Order Squamata), 8) a wellpreserved skeleton of famous marine reptile,
Futabasaurus suzukii Sato, Hasegawa and Manabe,
2006 (Order Sauropterygia), and 8) teeth oftitanosaurian sauropod (Order Saurischia) from theTamayama Formation of Iwaki City. Several taxa ofdinosaurs and turtles from the Futaba Group areremained unpublished.

Coniacian to Early Santonian Fauna andFlora detected from the Futaba GroupFutaba Fauna
1.Molluscan fauna
A variety of molluscs as ammonites andshallow-marine, fine sandy-bottom dwelling bivalves,
gastropods and scaphopods occur from the
Obisagawa Member of the Ashizawa Formation
gregariously or sporadically. The abundance of thebenthos and the common burrowing trace fossils inthe bioturbated massive sandstone facies suggest thata shallow-marine benthic ecosystem during Coniacian
were prolific in biomass and active in substrate
mixing, though not so many in deep burrowing
bivalves.
The bivalves fauna is characterized by Yaadia,
kimurai (thick-shelled large trigoniid), Loxo and
Glycymeris in occasionally pebbly, fine to medium
calcareous sandstone, and by Apiotrigonia (small
trigoniid), Inoceramus uwajimensis, Eryphyla,
Nanonavis, etc. in fine to very sandstone, reflecting
the substrate characteristics.
Though ammonites generally occur from somecertain horizons and localities sporadically, thepeculiar ammonite shell bed is observable at the
Iwaki Ammonite Center. A few tens huge ammonites
mainly of Mesopuzosia yubarensis, the middle-sizedAnagaudriceras limatum and other species including
heteromorphs are swarmed and associated with
common bivalves and gastropods on a bedding plane.
Shark, rays and telestei teeth, plesiosaurid bones andamber are also rarely found. In comparison with the
Yezo Group of Hokkaido well known in wellpreserved diverse ammonite fossils, the ammonite
preservation of the Futaba Group is not so good.
However, their species composition is similar,
suggesting the same ammonite fauna within the
Northwest Pacific region.
Furthermore, a small amount of molluscs rarely
occur also from the Irimazawa Member, Tamayama
Formation, but their detail is only briefly reported by
Obata and Suzuki (1969). The member was regarded
as the lower Santonian due to the occurrence of
Inoceramus amakusensis.

Futaba Flora (Kamikitaba Mesofossil Flora)
Takahashi et al. (1999a, b) found new
assemblages of well-preserved plant fossils frompoorlyconsolidated,carbonaceous,
black,
poorly-sorted, sandy siltstone of the lower
Asamigawa Member, Ashizawa Formation and theupper part of the Kohisagawa Member, Tamayama
Formation. The former prolific assemblage wasnamed as Kamikitaba mesofossil flora. The fossils
basically extracted by water washing and sieving, aregenerally small, three-dimensional and charcoalified
or lignitized (mesofossils) and yield elaborate
structural details when observed with SEM. This flora
importantly extends current knowledge of Coniacianplant evolution and biogeography in Asia. The
Kamikitaba assemblage contains well-preserved
angiosperm flowers, fruits, seeds, leaf fragments andwood, as well as shoots, leaves, pollen cones, conescales and seeds of confer. They include a varietyof angiosperm taxa such as Lauraceae (クスノキ科),
Hamamelidaceae (マンサク科), Fagaceae (ブナ科),
Cornales ( ミズキ科 ), Combretaceae ( シクンシ科 ),and probably Ericales (ツツジ目), Magnoliaceae (モク レ ン 目 ), Nymphaeales ( ス イ レ ン 目 ) and2.Vertebrate fauna
Since plesiosaurid fossils were discovered in 196833

Hisao Ando, Kenji Kashiwagi, Ren Hirayama and Seiichi Toshimitsu

Taxodiaceae ( ス ギ 科 ). Besides, leaf and rachis
fragments of ferns, sporangia with in situ spores, andmegaspores also document the presence of
Selaginellaceae (イワヒバ科) and Schizaeaceae (フサシダ科). Several new taxa (mostly new genera andspecies) were described as follows: Esgueiria
futabensis Takahashi et al., 1999b (Combretaceae
flower); Lauranthus futabensis Takahashi et al., 2001
(Lauraceous flower); Hironoia fusiformis Takahashi
et al., 2002 (Cornalean fruit); Symphanale futabensis
Takahashi et al., 2007 (Annonaceae seed);
Archaefagacea futabensis Takahashi et al., 2008a(Fagales flower); Futabanthusa samigawaensis
Takahashi et al., 2008b (Annonaceae flower);
Microlaurus perigynus Takahashi et al., 2014
(Lauraceous flower).
In comparison with the palynofloral analyses of
the Ashizawa Formation by Miki (1977), Takahashi
et al. (1999a) concluded that the early Coniacian
vegetation represented by the Futaba Group was
probably dominated, in terms of abundance, by bothconifers and angiosperms, with angiosperms probably
more diverse in terms of species.	
 The charcoalified
preservation of many plant fossils from the Futaba
Group suggests that the vegetation was periodicallysubjected to fire.

34

Cretaceous forearc basin siliciclastic successions: Choshi, Nakaminato and Futaba groups

DESCRIPTION OF DAYS 3 AND 4
DAY 3
Stop 3-5: Iwaki Coal and Fossil Museum(“Iwaki Sekitan Kasekikan”)
Exhibitions on Upper Cretaceous plesiosaur,
dinosaur and several molluscan fossils occurredfrom the Futaba Group, including Paleogene
coal mine facilities
The Iwaki Coal Fossil Museum was opened in
October 1984 a few years after the closing of all coalmines, near the hot spring town Iwaki-Yumoto (Fig.
42). The purpose of this museum is to explain the 125
years history of the Joban Coal Field in Iwaki Cityand exhibit the various and valuable fossils specimens
mainly occurred around the city. The exhibition of themuseum now consist of four sections: 1) Paleozoic to
Cenozoic major fossils occurred from Iwaki City andthe surrounding, 2) natural history of Iwaki City
including coal, rocks and geology, 3) the history of
the Joban Coal Field, by using a undergroundsimulated mine equipped with archived coal-mining
facilities and machineries and 4) life in coal mines
and city as well as coal mining customs, apartment
houses and local community.
The remarkable highlight is a reconstructed
plesiosaur described as Futabasaurus suzukii Satoet al., 2006, displayed in the entrance hall (Fig. 43).
Its specimens was firstly found from the Cretaceous
Futaba Group distributed in the northern part of Iwaki
City, by a local high school student Mr. Suzuki in
1968, and subsequently excavated during a few years(Obata et al., 1970). This finding and excavation
has long attracted not only visitors of the museum but
also young generation in Japan to paleontology andvertebrate fossils including dinosaurs.

Fig. 43. Recontructed Futabasaurus suzukii Sato,
Hasegawa and Manabe, 2006 in the entrance hall of the
Iwaki Coal and Fossil Museum.

The Iwaki Coal and Fossil Museum was recently
renovated in 2010 and nicknamed “Horuru” whichmeans an abbreviated Japanese word “horu” (=excavate) two times: excavate coal and fossils. Manykinds of large attractive fossils mainly of the
Mesozoic and Cenozoic have been sophisticatedly
exhibited in a renewed large hall (Fig. 44): some
Cretaceous huge ammonite such as Mesopuzosia,
several Cretaceous dinosaur, pterosaurid, turtles,
pliosaurid and shark teeth, as well as Cenozoic
mammals such as elephant, whales, and fin-footed
mammals, and Oligocene protopterid bird similar to
penguin. Other common and paleontologicallyimportant fossils from Iwaki City are also displayed
sophistically.

Fig. 44. Main exhibition hall of the Iwaki Coal and Fossil
Museum.

Fig. 42. Large statue of plesiosaurid on a frontal yard of
the Iwaki Coal and Fossil Museum. Red-colored irontower is a facility of the underground simulated mine.

35

Hisao Ando, Kenji Kashiwagi, Ren Hirayama and Seiichi Toshimitsu

Stop 3-6: Iwaki City Ammonite Center(Ammonite shell bed: shallow-marine finesandstone facies of Ashizawa Formation, Futaba
Group)
Around Stop 3-6, bioturbated, gray massive silty
fine to very fine sandstone about 10 m thick of the
Ohisagawa Member, Ashizawa Formation of the
Futaba Group, are exposed and inclined eastward at
15 to 20 degrees. The main building of the Iwaki
Ammonite Center was founded on an inclined
bedding surface to observe an ammonite shell bed andassociated fossils (Fig. 45). A few tens of ammonites(Mesopuzosia yubarensis) larger than a few tens cm
in diameter, the smaller planospiral and heteromorphammonites and other molluscan fossils such as
inoceramids (Inoceramus uwajimenis), trigonid
bivalves (Yaadia kimurai, Apiotrigonia minor), otherbivalves, gastropods, shark teeth, plant remains, etc.
are sporadically and irregularly swarmed within lessthan a meter thick horizon. Many of large ammonites
are preserved within carbonate concretions ofsandstone, though the central inner whorls of shells
were mostly dissolved away. Some external molds ofMesopusosia less than 20 cm in diameter also occur
even in less calcified mother rocks. Small patches a
few cm thick and in diameter composed of swarmed
and poorly preserved fragmentary shells are often
scattered. Anyway, taphonomically interesting modeof fossil occurrence can be observed on the bedding
surface of the ammonite bed.
At an exposure outside for experimental fossil
excavations, the irregularly scattered patchy mode ofshell occurrence can be also well observed in massive
silty fine sandstone facies. Though shell preservation
state is not so good, articulated bivalves are not rare,
and thin-shelled bivalves as inoceramid andDidymotis in butterfly position rarely occurred. Thereis no evidence for secondary distant transportation ofthese shell fossils after initial deposition. Judging
from these taphonomical characteristics as well as thestratigraphic succession of depositional facies, theammonite shell bed may have formed on Early
Coniacian fine sandy inner shelf bottom by storm
waves or storm-induced strong currents (or perhaps
tsunami?). After deposition, intensive bioturbation by
prolific benthic organisms homogenized the shellconcentrations and physical sedimentary structures.

DAY 4
Stop 4-1: Kotakidaira near Asami River, Hirono
Town:
Unconformity
between
Lower
Cretaceous Abukuma granitic rock basement
and Upper Cretaceous Futaba Group
Stop 4-1 is a small outcrop where the Upper
Cretaceous Futaba Group unconformably overlies a
granitic rock mass of Lower Cretaceous Abukuma
Granitic Rocks (Ohisagawa Granodiorite in this case:
Kubo et al., 2002) with an undulated erosional surface.
The basal part of the group, the Asamigawa Member,
Ashizawa Formation is inclined eastward at 15 to 20
degree, and composed of gravel-supported round to
subround, pebble to cobble conglomerate with arkosiccoarse sand matrix. Some lenticular layers of coarse
sandstone with cross-stratification are intercalatedwith sharp facies boundary. Gravel contains mainly
granitic rocks derived from the basement, butsiliceous older rocks are also common. This
conglomerate shows the oldest strata exposed in the
Joban subbasin and the beginning of thesedimentation in the western margin of the Joban
subbasin, Yezo forearc basin.
Stop 4-2: Large cliff next to a solar power plantat Kitazawa, Hirono Town: Kasamatsu andTamayama formations, Futaba Group andunconformity with Eocene Iwaki Formation,
Shiramizu Group
This large outcrop about 50 m high next to a
recently established solar power plant shows thefluvial facies of the (1) Kasamatsu (the upper two
third 50 m thick) and (2) Tamayama (the lowest a few
m thick) formations, Futaba Group (upper Coniacian)and the alluvial-fan conglomerate facies of the upper
Eocene (3) Iwaki Formation, Shiramizu Group (Fig.
46).
The lowest Kasamatsu Formation forms
alternating beds of dominant, cross-stratified, poorly
to medium-sorted, arkosic medium to fine sandstoneand subordinate dark gray to grayish black
carbonaceous mudstone. The thickness of sandstoneFig. 45. Ammonite shell bed in the Iwaki Ammonite
Center.

36

Cretaceous forearc basin siliciclastic successions: Choshi, Nakaminato and Futaba groups

Fig. 46. Large outcrop of the Upper Cretaceous Futaba Group (Kasamatsu (Km) and Tamayama (Tm) formations), thePaleogene Shiramizu Group (Iwaki Formation: Iw) and their unconformity (un) in between in north of Kitazawa, Hirono
Town (Stop 4-2).

and mudstone couplets is less than a few meters. The
basal surfaces of sandstone layers sometimes show
shallow scour-and-fill structure associated with someload casts. Furthermore, a few to tensandstone-mudstone couplets form the higher-order,
upward fining and thinning units five to ten meters
thick. Four to five units at least can be detected over
the area. The basal parts of the higher-order units
consist of gravelly very coarse sandstone with
undulated erosional bases. These higher-order facies
successions seem to reflect river channel shifting of ameandering river system.
As there are common in three dimensional
network burrows filled with sand within sandy
siltstone of the uppermost part of the Kasamatsu
Formation, the fluvial system seems to have changed
into tide-influenced estuary due to a transgression or
marine incursion.
The overlying massive and medium-sorted,arkosic coarse to medium sandstone a few meters
thick represents the lowest part of the Tamayama
Formation. The facies changes between the twoformations are relatively sharp. Grayish black
mudstone clasts derived from the underlying
Kasamatsu Formation are included at the base of thesandstone. Due to the unconformity with the Iwaki
Formation, Shiramizu Group, the most of the
Tamayama Formation at Stop 4-2 was eroded away,
though nearly 150 m thick of the formation is
exposed at the fossil locality of plesiosaur,
Futabasaurus, Irimazawa (Stop 4-5) about 5 km
southward. The sandstone facies shows the braided
river environments.
The lowest part of the late Eocene Iwaki
Formation, Shiramizu Group is composed mainly of
welltomoderate-sorted
gravel-supported

conglomerate dominant in subround chert pebble tocobble. The conglomerate is partly cross-stratified
and higher in sorting and maturity than conglomerate
in the Futaba Group. Large-scale forest cross bedding
is well observed, suggesting gravel bar progradation
on mid alluvial fan. Lenticular gravelly coarse to
medium sandstone is also intercalated.
Stop 4-3: 	 “Kairyu-no-sato” at Irimazawa,
Ohisa-cho, Iwaki City: Near the fossil locality ofplesiosaurid from the Irimazawa Member of the
Tamayama Formation, Futaba Group	  A small exhibition hall and rest lounge was built in1991 to provide a tourist spot for the fossil locality ofplesiosaurid from the shallow-marine fine sandstonefacies of the Irimazawa Member of the Tamayama
Formation, Futaba Group.

Stop 4-4:	
  Itakizawa, Ohisa-cho, Iwaki City:
Boundary between fluvial and shallow-marine
facies of Tamayama Formation, Futaba Groupand unconformity between Upper Cretaceous
Futaba Group and Paleogene Shiramizu GroupThe facies transition of the 1) Kohisagawa to 2)
Irimazawa members of the Tamayama Formation,
Upper Cretaceous Futaba Group and theunconformably-overlying conglomerate of the Eocene3) Iwaki Formation, Shiramizu Group can beobserved in this outcrop near a small village (Fig. 47).
All strata dip eastward at 15 to 25 degree in general.
1) The uppermost part of Kohisagawa Member,
Tamayama Formation, Futaba GroupThe lowest part of the outcrop consists ofapparently massive, but somewhat cross-stratified
coarse to medium sandstone about 7 to 8 m thick andintercalated grayish black sandy siltstone 1 m thick,37

Hisao Ando, Kenji Kashiwagi, Ren Hirayama and Seiichi Toshimitsu

Fig. 47. Depositional facies and stratigraphic relations between the Kohisasgawa and Irimazawa members, Tamayama
Formation of the Futaba Group (Upper Cretaceous) and the Iwaki Formation of the Shiramizu Group (Eocene-Oligocene) at
Itakizawa, Obisa-cho, Iwaki City (Stop 4-4). Center: panoramic view of outcrop; A and B: unconformity (SB: sequenceboundary) between the Iwaki Formation and the Tamayama Formation; C: imbricate structure of gravel indicating leftward(westward) flow in the basal part of the Iwaki Formation; D: grayish black coaly sandy siltstone (salt marsh facies)
intervening in trough cross-stratified (TCS) sandstone (braided river facies) of the Kohisagawa Member; E: hummocky
cross-stratified (HCS) sandstone of the Irimazawa Member leftward (westward) onlapping TCS sandstone of the Kohisagawa
Member with a sharp-based ravinement surface (RS) and transgressive lags. Low-angle cross stratification within the lags
laterally (rightward) grades into HCS of fine sandstone; F: close-up of HCS sandstone.	
  From Ando et al., (2011)

showing sandy braided river environment (Ando et al.,1995). Some vertical burrows observed in thesiltstone suggest marine water influence under estuary
environments.
2) Irimazawa Member, Tamayama Formation,
Futaba GroupThe Irimazawa Member thins westward (leftward)
from 10 to 4 meters and seems to pinch out further
westward due to an unconformity with the Eocene
Iwaki Formation. The boundary with the underlying
Kohisagawa Member forms a flat wesward onlapping
surface where the basal transgressive conglomerate
and overlying hummocky cross-stratified (HCS) finesandstone obliquely cover the Kohisagawa Member
with low angle less than 10 degree. Cross-lamina
planes developed in conglomerate also cross the
onlap surface at less than 10 degree and laterally
continued into HCS on the other side (eastward).
Therefore, the basal conglomerate layer showsinterfingered lateral facies changes within the outcrop.

38

These facies distributions suggest transgressive lags
and the following storm fine sand layers
diachronously deposited above a ravinement surface
gently inclined seaward formed by shoreface erosion
during transgression.
The fossil horizons of plesiosaurid Futabasaurus
suzukii Sato, Hasegawa and Manabe, 2006 discovered
along two river-floor exposures at Irimazawa about
200 and 400 m southeast seem to be stratigraphicallysituated a few tens meter upper than this stop.
3) Iwaki Formation, Shiramizu Group
Gravel-supported, subround pebble to cobble
conglomerate of the lowest part of the Eocene Iwaki
Formation covers the Irimazawa Member of the
Futaba Group with a relatively gentle angular
unconformity. As the unconformity eroded the lower
horizon westward, the Irimazawa Member pinches
out westward and the Iwaki Formation covers thelower Kohisagawa Member. At Stop 4-2, Kitazawa
about 5 km northward, only a few meters thick of theCretaceous forearc basin siliciclastic successions: Choshi, Nakaminato and Futaba groups

lowest part of the Kohisagawa Member is exposed
below the unconformity. The conglomerate of the
Iwaki Formation here is partly cross-stratified andshows some imbricate structure (Fig. 47C), though it
is generally massive in lithofacies.

Eguchi, M., Shoji, R., Arakawa, T. and Suzuki, S., 1959,
Boundary between the Cretaceous and Tertiary in the
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Friedrich Pfeil, Muenchen, Germany.
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Hayami, I. and Oji, T., 1980, Early Cretaceous bivalvia
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Ishigaki, A. and Ito, M., 2000, Size population of
hummocky bedforms: an example from the Lower
Cretaceous Choshi Group, northeastern Boso
Peninsula, Japan. Jour. Geol. Soc. Japan, 106, 472–
481. *
Ishikawa, H. and Hatta, A., 1984, The foraminiferal
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Fac. Educ. Chiba Univ., 33, 21-42. *Iwata, K., Sugai, Y. and Hikida, Y., 1998, Radiolarians
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Kano, H., 1958, On a granite-bearing conglomerate fromthe Choshi Peninsula, Chiba Prefecture, Japan Studies on the granite-bearing conglomerates in Japan,
No. 5 -. Jour. Geol. Soc. Japan, 64, 551-556. *Kase, T. and Maeda, H., 1980, Early Cretaceous
gastropoda from the Choshi District, Chiba Prefecture,
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originated from the basal conglomerate of the
Pliocene Naarai Formation, Choshi area of Kanto
Region, central Japan. Jour. Geol. Soc. Japan, 119,647-652. *Katsura, Y. and Masuda, F., 1978, Sequences ofstructures in submarine-fan depostis in the Cretaceous
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Acknowledgements
We wish to thank the Choshi Educational Board,
Promotion Council of Choshi Geopark, Iwaki City
Educational Board, Iwaki Coal and Fossil Museum,
Iwaki Ammonite Center, Dr. Takao Ando and Dr.
Takeyuki Ueki of Chiba Institute of Science, Izura
Institute Art and Culture, Ibaraki University for their
help and support in many ways. We must express our
gratitude to Mr. Akiyoshi Yamashita, Mr. Akira
Murata, Mrs. Wakako Murata of Izura Kanko Hotelfor their excellent hospitality. We would also like tothank Café Marina, Hotel Sunrise Choshi and Iwaki
Shin-Maiko Heights.
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2014 September

Organizing Committee of the 2nd
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