Lithostratigraphy of the late Quaternary under the Syuso and Saijyo ...

Journal of Geosciences, Osaka City University 

Vol. 51, Art. 5, p. 35-46, March, 2008 

Lithostratigraphy of the late Quaternary under the Syuso and 

Saijyo Plains, Ehime Prefecture, southwest Japan 

Norihito KAWAMURA 1 and Kiminori MINAKUCHI 2 

I Marugame High School, Roku-Bancyo-l, Marugame, 763-8512, Japan 

E-mail: nori-ka@jt2.so-net.ne.jp 

2 Construction Research Institute of Ehime Prefecture, Higashi-Ishii 5-1-31, Matsuyama, 790-0932, 

Japan 

Abstract 

Subsurface geology in the coastal area of the Syuso and Saijyo Plains, Ehime Prefecture, 

southwest Japan, is divided stratigraphically into the Pleistocene Syuso and Holocene yugawa 

Formations. Additionally, the Nyugawa Formation is subdivided into the lower, middle and upper parts 

in a cending order. Two volcanic ashes, Saijyo Minato 1 and Saijyo Minato 2 Volcanic Ashes are 

intercalated in the upper part of the Syuso Formation and in the middle part of the Nyugawa Formation 

and correlated with Aira-Tn and Kikai-Akahoya Volcanic A hes, re pectively. 

From the tephrostratigraphy and lithological characteristics of the two formations, the upper part 

of the Syuso Formation would have been deposited continuously until around 26ka ~ 29ka at river 

channels or on flood plains. The Nyugawa Formation has formed in the last 7300 years mostly as 

deltaic sediments. 

Key-words: Pleistocene Syuso Formation, Holocene Nyugawa Formation, Syuso and Saijyo Plains, 

volcanic ash, drilling-core 

I. Introduction 

The Syuso and Saijyo Plains occupy the central part of 

the northern coastal area of Shikoku Island facing the Seto 

Inland Sea (Fig. 1). They are suitable areas for studying the 

standard stratigraphy of the Quaternary in northern 

Shikoku Island because of storage of many drilling logs 

and samples. But detailed research of sub urface geology 

based on drilling-core observation has not been done since 

1972. 

The subsurface geology in these plains were first 

reported by Murashita and Noma (1964), Kitazaki and 

Hiyoshi (1965), and Kitazaki and Shindo (1965). 

Murashita and Noma (1964) reported the general 

subsurface geology in terms of hydrogeology. Kitazaki and 

Hiyoshi (1965) and Kitazaki and Shindo (1965) studied the 

late Quaternary deposits as ground investigation of the 

plains. After these researchers, Saito et a1. (1972) studied 

the stratigraphy of a drilling-core at the northern fringe of 

the plains. Kurihara (1972) revised the lithological 

division of the Quaternary sediments in the Saijyo Plain. 

However, the lithostratigraphical division of Kurihara 

(1972) does not coincide with that of Kitazaki and Hiyoshi 

(1965) so detailed observation of the drilling-cores was 

required. 

Kawamura and Shiota (2005) described the lithology, 

molluscan fossils, and volcanic ashes of four drilling-cores 

from Minato area, Saijyo City. They insisted that Saijyo 

Minato 1 and Saijyo Minato 2 Volcanic Ashes can be 

correlated with Aira-Tn and Kikai-Akahoya Volcanic Ashes 

(Machida and Arai, 1992, 2003) ,respectively. Distribution 

of these volcanic ashes beneath the Saijyo Plain, however, 

has not been clarified precisely.

36 Lithostratigraphy of the late Quaternary under the Syuso and Saijyo Plains, Ehime Prefecture, southwest Japan 

The objective of the present study was re-examination 

of the chronostratigraphical division of the subsurface 

geology in the Syuso and Saijyo plains by the detailed 

observation of 11 new drilling-cores from the coastal area 

in the plains, especially on the basis of recognition of the 

two volcanic ash layers. Likewise, the authors will discuss 

the sedimentation age of the late Quaternary in the coastal 

area of these two plains. 

(a) 

133"E 

II. General geography and geology 

The Nakayamagawa and Kamogawa Rivers, two 

major rivers in the Syuso and Saijyo Plains, are running 

from the Ishiduchi Mountains to the Seto Inland Sea. The 

rivers form alluvial plains along their lower reaches and the 

coast of Hiuchinada (Fig. 1). The authors call the area that 

extends above T.P.(Tokyo Peil) 3m "the inland area" and 

the plain below T.P. 3m "the coastal area" because the 

inland area slopes gently toward the Hiuchinada and is 

generally flat land. 

As for geomorphological division, the Syuso Plain is 

classified as the terrace, fan, flood plain and delta that was 

formed by the Nakayamagawa River, and reclaimed land 

(Shiki, 1965). The Saijyo Plain is also divided into the fan, 

flood plain and delta formed by the Kamogawa River, and 

reclaimed land (Shiki, 1965). 

Shiki also classified the 

southern part of the plains along the northern foot of the 

Ishiduchi Mountains as terrace and alluvial cone areas. 

Immediately to the north and west of the Syuso Plain, 

the Takanawa Mountains lie, which is underlain by the 

Ryoke Granites (Miyahisa and Hiraoka, 1970; Ochi, 1982 

etc.). The plains' eastern neighbor, the Nishiyama 

Mountains, is made up of the Cretaceous Izumi Group 

(Momoi et aI.,1991). The Shikoku Mountains are found 

immediately south of the plains and are underlain by the 

crystalline schists of the Sanbagawa Belt (Hide et ai. 1956). 

The Pleistocene Okamura Formation is distributed along 

the fringe of the plain (Saito, 1962; Takahashi, 1969; 

Mizuno et al.,1993). 

According to Mizuno et ai. (1993) and Nakata et ai. 

(1995), the active fault system of the Median Tectonic Line 

runs along the southern fringe of the Syuso and Saijyo 

Plains (Fig. 1). The Kawakami Fault (Okada, 1972) and 

Komatsu Fault (Mizuno et aI., 1993) are located along the 

southwestern rim of the plains. Another fault is found at 

the southern rim of the Saijyo plain and was named the 

Okamura Fault (Nagai,1995). At the western border 

between the mountains and the Syuso Plain, the Kawane 

Fault is found (Okada, 1972). In contrast, any distinctive 

dislocation by the active faults have not been found in the 

central and coastal parts of the plains. 

(b) 

Z?-.. . 

N ~;)ffi' 

~~~\ 

rb ' 

CD Kawane Fault (g) Kawakami Fault @ Komatsu Fault 

@ Okamura Fault 

I I Sea and lake C=:J Alluvial plain and delta 

~ Mountain and terrace 

Fig. 1 Study area and the drilling sites 

Location of the fault system is after The 

Research Group for Active Faults of Japan 

(1992) and Nakata et aI. (1995). Abbreviations 

of drilling-cores of TYP-K1, TYP-D1, KTG-1, 

and KTG-2 are followed from Kawamura and 

Shiota (2005), and numbers 1 to 5 indicate the 

location of drilling-cores CSt.1 to S1.5) used in 

this study. 

III. Methods 

o 

2 4km: 

Eleven new drilling-cores at five sites (Fig. 1) were 

used for discernment of lithofacies and precise analyses of 

faunal fossils and volcanic ash layers. The authors further 

used the 160 previous drilling logs (The Ministry of 

Construction and Ehime Prefecture, 1965; Shikoku Ground 

Information Utilizing Association, 2007) for revision of the 

subsurface geology and identification of the continuity of 

sediments.

Norihito KAWAMURA and Kiminori MINAKUCHI 37 

Lithological description of drilling-core samples 

Lithology of drilling-core samples were described by 

inspection with the naked eye and stereoscopic microscope. 

Grain-size distribution, fossils, and sedimentary structure 

were described by means of lithology. For color and 

N-values of the drilling-core samples the authors refened 

to the provisional reports of borehole drilling companies 

since the samples concerned have been dried and oxidized 

for up to ten years. Volcanic ashes, faunal fossils such as 

Mollusca, Foraminifera, Ostracoda and Echinodea, and 

secondary formed crystals and gypsum are all sampled 

during the description. 

Sampling of faunal fossils 

Faunal fossils were collected from core samples by the 

following operation. Large-size molluscan fossils were 

directly picked up from drilling-core samples. Ten to 

twenty grams of sample blocks were collected and washed 

with water on a 1/8 millimeter mesh sieve, and the fossils 

were separated with the aid of a stereoscopic microscope. 

Petrographical description of volcanic ash 

The intercalated volcanic ash was studied for its 

mineral composition and the shape and refractive index of 

glass shards in terms of petrography. Classification of 

glass shard's shape (H, C, T and other types) is after 

Yoshikawa (1976). Refractive index was measured by the 

temperature change method using the refractometer 

"MAIOT" (Furusawa, 1995). Accuracy of measurement by 

this method is approximately ± 0.0001. Refractive index 

of glass shards from each volcanic ash sample was 

determined by the measurements of twenty glass shards. 

IV. Description of lithological units of drilling cores 

On the basis of lithofacies, Kawamura and 

Shiota(2005) divided the KTG-1, 2 and TYP-Kl, D1 

drilling-cores into the Lithological Units of 1 to 9. In this 

study, these Lithological Units are applied to the newly 

observed drilling-core samples. Additionally, the 

Lithological Units 4a and 8m are newly defined. These 

Lithological Units are summarized in Table 1. 

Unit 1: The Unit consists of the grayish massive sandy 

gravel which contain subrounded to subangular pebbles 

with coarse sand. 

Unit 2: Massive silt beds, a few meters thick, are 

interbedded with gravel beds. The mud sometimes 

includes sand or rounded pebbles and is rarely peaty. 

Unit 3: This unit is the inverse grading bed with 

subrounded to subangular pebbles, in which silt beds grade 

into sandy gravels upward. 

Unit 4: This unit is dark gray in color and comprises fine 

sandy silt or clay. 

Unit 4a: This unit is an alternating bed of clay and silt 1 

millimeter to several centimeters thick each. 

Unit 5: This unit is comprised of dark gray clay and 

contains ordinary humus. Gypsum crystals are partly 

formed in the core samples. 

Units 6m and 6s: The Unit 6m is dark gray, massive clay 

or silty beds, hence Unit 6s is sandy silt beds occasionally 

with unclear laminae. Both of the units bear mollusk, 

sponge, benthic foraminifer, ostracod, and fragments of 

echinus. Some examples are Elphidium spp. as marine 

benthic foraminifer, Ammonia beccari as brackish benthic 

foraminifer,and Bicornucythere bisanensis,Trachyleberis 

scabrocuneata and Spinileberis quadriaculeata as innerbay 

living ostracod. 

Unit 7: This unit is dark gray, massive silty sand that bears 

mollusk rarely. 

Units 8 and 8m: These units are comprised of dark gray to 

dark bluish gray, medium to coarse sized massive sand 

beds containing gravel. Unit 8 yields no marine molluscan 

fossil but mollusk are found rarely in the underlying Unit 

8m like Batillaria zonalis and Batillaria multiformis as 

intertidal zone living species, and Eufenella rufocincta as 

intertidal zone to 20 meters depth living species. 

Unit 9: This Unit bears a gravely sand or silt bed 

frequently with blocks of concrete and wood. 

V. Petrographical characteristics of volcanic ash layers 

Table 2 shows petrographical characteristics of 

volcanic ash layers in the plains. 

Saijyo Minato 1 Volcanic Ash (SjM-1) 

The volcanic ash, SjM-1, is light grayish, silt to fine 

sand sized vitritic volcanic ash. 

SjM-1 is mainly composed of clear glass shards with 

rare orthopyroxene and plagioclase crystals. The shapes of 

the glass shards are mainly classified into Hand C types. 

The refractive index(n) ranges from 1.498 to 1.501(mode: 

1.498 ~ 1.499). 

Saijyo Minato 2 Volcanic Ash (SjM-2) 

The volcanic ash, SjM-2, is a slightly brownish color, 

silt to fine sand sized vitritic volcanic ash that contains 

fine-grained pumice. 

SjM-2 is mainly composed of clear glass shards but 

includes brown-colored glass shards and rare 

orthopyroxene and plagioclase crystals. The shapes are H, 

C and T types (H>C>T). The refractive index ranges from 

1.508 to 1.514.


Table I 

Lithological Units and interpretation of their sedimentary environments (modified from Kawamaura and 

Shiota, 2005) 

Lithology 

Sedimentary env- 

ironment 

Fossil and secondary formed 

mineral 

1 grayish massive sandy gravels~river channel 

consisting of subrounded to 

subangular pebbles with 

coarse sand as matrix 

---------- 

2 massive mud beds interbedded back marsh 

in the gravelly beds 

3 inverse grading from silt to natural levee or 

sand or silt to sandy gravel 

back marsh 

upward ~ 

---------- 

4 dark gray fine sandy silt or 

back marsh 

clay 

---------- 

4a alternating beds of clay and back marsh 

silt layers 

5 dark gray clay containing gypsum crystals brackish or sea 

humus 

water marsh 

6 mainly dark gray massive silt intertidal zone, and intertidal bay-head 

zone to upper shallow sea 

mollusks including inner-bay 

species;e.g. Paphia (Neotapes) 

undu/ata and Veremo/pa micra 

foraminifer: Ammonia beccari , 

E/phidium spp. 

inner-bay ostracods; Bicornucythere 

bisanensis • 

Trachy/eberis scabrocuneata 

and Spini/eberis quadriacu/eata 

othrs; sponge and Echinodermata 

7 dark gray massive silty sand rare occurence of intertidal intertidal zone 

zone to upper shallow sea and subtidal zone 

mollusk; e.g. Batil/aria zona/is 

8 dark gray to dark bluish gray 

massive sand or gravelly sand 

of medium to coarse size 

~(beach ridge) 

8m same as the Unit 8 intertidal and subtidal zone intertidal to submollusk;e.g. 

---------- 

Crassostrea gigas tidal zone 

9 gravelly sand including blocks reclaimed land 

of concrete and wood 

Table 2 Petrographical characters of the volcanic ash layers 

For abbreviations of the drilling sites, see Fig.l. 

Name 

Saijyo Minato 2 

Volcanic Ash 

Kikai-Akahoya 

Saijyo Minato 1 

Volcanic Ash 

Aira-Tn 

Drilling site and 

elevation (T.P.) 

Glass shards 

f--..."s-ch-a-p-e---;C(%::-'0"")--=r='=R:::"e'-'fr""'a"'c:':::ti-ve--:-in-d-c ēx --:-(n-:)--I Mineral 

composition 

H C T Oth ranae mode 

opX, Oth 

Reference 

Kawamura and Shiota(2005) 

- (present study) 


------------------------------------------. 

H > T 1.508-1.516: - opx>Cpx Machida and Arai (2003) 

KTG-1 -11.2m 55.6 26.1 16.5 1.7 1.508-1.514: - 

81.4 a.85m 43.8 37.2 17.8 1.1 1.508-1.514: - 

.~!'.~.:?: !~ _!"!.~..S;.. ~_.~ ]:~.~~:~ :?~~.l ~ . 

KTG-1 -18.3m 

St.4 -11.65m 

_____________________________ 

69.3 27.4 3.2 0 

51.8 34.9 11.5 1.8 

• ._._. 

1.498-1.501: 1.498-1.499 

1.498-1.500: 1.498-1.499 

._. L _ 

H > T 1.498-1.500 : 1.498-1.499 

Shape of glass shards are after Yoshikawa(1976) 

Minerals; Ho: hornblend, Opx: orthopyroxene, Cpx: clinopyroxene, Oth: others 

opX, Oth Kawamura and Shiota(2005) 


------------------------------------------- 

Opx>Cpx>Ho Machida and Arai (2003)

Norihito KAWAMURA and Kiminori MINAKUCI-U 39 

VI. Revised lithostratigraphy 

Takahashi (1958) called the Tertiary beds found at the 

southern fringe of the plains the Okamura Formation. Saito 

(1962) regarded the Okamura Formation as the Plio 

Pleistocene Mitoyo Group(Saito and Nakayama, 1954). 

Saito et al. (1972) and Kurihara (1972) ascribed the 

Tertiary beds in the Saijyo Plain just to the Mitoyo Group. 

In this study, the authors call these Tertiary systems the 

Okamura Formation following Takahashi (1958). The 

formation must underlie the Pleistocene sediments in the 

plains, but the authors have not found it in the drilling-core 

samples observed. 

The Quaternary system beneath the plains was divided 

into "Delluvium" and "Alluvium" by Kitazaki and Shindo 

(1965) and called "Alluvium" by Kurihara (1972). 

Stratigraphy of subsurface geology of the plains studied by 

Kitazaki and Hiyoshi (1965), Saito et al. (1972), and 

Kitazaki and Shindo (1965) is summarized as follows (Fig. 

2); lower, middle, and upper Delluvium (lowermost Beds), 

uppermost Delluvium (lower Beds), and Alluvium (upper 

and uppermost Beds). Hence, Kurihara (1972) pointed out 

the difference of lithostratigraphy between the inland and 

coastal areas of the plains. 

Considering the inconsistency of previous 

stratigraphic divisions, the authors propose a revised 

lithostratigraphy of the late Quaternary beneath the Syuso 

and Saijyo Plains (Fig. 2) on the basis of 17 drilling-cores 

used in this study and in Kawamura and Shiota (2005). 

Syuso Formation (new usage) 

(definition) The Syuso Formation is defined as sandy 

gravel in the Syuso Plain that overlies the Okamura 

Formation and underlies the Nyugawa Formation. The 

formation corresponds to a part of the sediments underlying 

the Alluvium which Kurihara (1972) correlated with the 

Plio-Pleistocene Mitoyo Group (Saito and Nakayama, 

1954). Beneath the southwestern part of the Syuso Plain, 

the formation unconformably overlies basement rocks of 

the Izumi Group. 

(type locality) Typical beds are distributed between 

TP. -25.5 and -11.5 meters of the St.l Drilling-core (33 0 

55'44"N, 133 0 

06'40"E, see Fig. 1), Hiroe area, Saijyo City. 

(distribution) Distributions of the formation are 

different between the inland and coastal areas (see Fig. 3 

(c)). Fig. 3 shows three typical cross-sections across the 

plains. In the inland area, the Syuso Formation 

immediately lies beneath the earth surface in the Syuso 

Plain, which lie between TP. 3 and approximately 50 

meters (Fig. 3 (c)). Hence, it is distributed in the coastal 

area at least below TP. -10 meters. The lower boundary of 

the formation is unidentified from any of the observed 

drilli ng-cores that are not deep enough to reach to the 

basement rocks. According to the referenced drilling logs 

between St.3 and TYP-K1 (Fig. 3 (a)), the formation 

distributes below TP. -30 meters at least. 

(thickness) More than 17 meters. 

(lithology) In the inland area of the plains, especially 

in the southern neighboring area of the Nakayamagawa 

River, the formation has simple facies and mainly consists 

of sandy gravels (lithological unit 1). To the contrary, in 

the coastal area the upper part of the formation comprises 

of mud, sand and gravel (units 1,2,4 and 5). 

(correlation) This formation is correlated with the 

middle and upper Delluvium (Kitazaki and Hiyoshi, 1965), 

Delluvium (Saito et a!., 1972), and the lower sandy gravels 

of the Alluvium (Kurihara, 1972). 

Kitazaki and Correlation Age 

Reference Hiyoshi (1965). Kurihara (1972) Saito et al. This study of wide- [ka] 

Kitazaki and Shindo (1972) spread (after Machida 

(1965) tephra and Aral. 2003) 

Alluvium 

Upper sandy gravels. 

sands and silts 

Upper Alluvium 

u.: 

upper part 

til 

~ 

Middle sands, 

middle part 

E 

til 

:::J 

.;;; 

Lower Alluvium OJ - -- - -- :::J 

SjM-2 

>, 

- - K-Ah - -- --- 7.3 

.2 silts and muds z 

Stratigraphy Uppermost


(a) 

[T.P.m] 

10 

0 

A St1 St.2 St3 TYP-K1 TYP-Dl 

1 J 1 I 1 

B 

CJ 

materials 

artificial 

~ 

muddy beds 

NU:::: ~ .... 

sandy beds 

-10 NM 

00000 

0000 

SjM-2 

gravelly 

beds 

-20 III 

sand stone 

0 2km 

-30 I I 

v -- 

volcanic 

ash layer 

(b) 

[T.P.mJ 

30 o 

10 

o 

-10 

-20 

-30 

-40 

o 

I 

2 

I 

4km 

I 

Fig. 3 Geological sections in the Syuso and Saijyo Plains, Ehime Prefecture 

(a) Line A-B, (b) Line C-D 

Iz: Izumi Group, Syu: Syuso Formation, NL: the lower part of the Nyugawa Formation, NM: the middle part 

of the Nyugawa Formation, NU: the upper part of the Nyugawa Formation.

Norihito KAWAMURA and Kiminori MINAKUCHl 

41 

(c) 

[T.P.mJ 

50 E 

40 

30 

20 

10 

o 

F 

-10 

-20 

-30 

o 

I 

2km 

I 

'-NM 

Fig. 3 (continued) 

(c) Line E-F in Fig. 

Saijyo Minato 1 Volcanic Ash (SjM-l; Kawamura and 

Shiota, 2005) 

The volcanic ash layer was found by Kawamura and 

Shiota (2005) at TP. -18.3 meters of KTG-l drilling-core in 

Minato area, Saijyo City. The upper part of the Syuso 

Formation interbeds this volcanic ash layer occasionally. 

The layer is 1.1 meters in maximum thickness and found 

between TP. -22 and -17 meters. 

On the basis of similar lithological characteristics and 

stratigraphy, SjM-1 beneath the plains can be correlated 

with Aira-Tn Volcanic Ash (AT). The shape and refractive 

index of glass shards in AT (H>T and n=1.498 ~ 1.500; 

Machida and Arai, 1992,2003) are in good agreement with 

those of SjM-l (Table 2). 

Nyugawa Formation (new usage) 

(definition) The Nyugawa Formation is defined as a 

series of finer sediments beneath the coastal area of the 

Saijyo Plain, which unconformably overlies the Syuso 

Formation. 

(type locality) The stratotype section is between TP. 

-11.5 and -1.5 meters of the St.l Drilling-core (33 0 

55'44"N, 133°06'40"E, see Fig. 1), and the parastratotype 

section is between TP. -10 and -0.5 meters of the St.2 

Drilling-core (33°55'39"N, 133°06'28"E), Hiroe area, 

Saijyo City. 

(distribution) The formation is distributed just beneath 

the coastal area of the Saijyo Plain, at depth between TP. 

-20 and 3 meters (Fig. 4).


N 

~ o 

2 

4km 

/~-- f{juchlnada ---------1 

Limit of distribution of the Nyugawa Formation 

Fig. 4 Distribution of the Nyugawa Formation 

Numbers beside contour lines indicate elevation of the basement of the Nyugawa Formation in T.P. meters. 

Numbers beside drilling site indicate station numbers in Fig.I. KI, DI, Gl, and G2 mean TYP-Kl, TYP-DI, 

KTG-l and KTG-2 in Fig.l respectively. 

(thickness) The formation is 20 meters in maximum 

thickness. 

(stratigraphy) This formation is lithologically 

subdivided into the lower, middle and upper parts, in 

ascending order (Fig. 5). The lower or middle part of the 

formation abuts with underlying Syuso Formation. 

(llthology) 

lower part: The part comprises of sand or sandy gravel 

(Unit 1) less than 6 meters in maximum thickness. 

middle part: Thickness of the part ranges from 3 to 7 

meters. Accumulation of clay and/or silt (Units 4, 5, 6 and 

7) characterizes most of this part. Beneath the northern 

area of the plains, the part turns into sandy beds in general. 

Marine molluscan shells are contained in whole in the 

mud, such as Mactra (M.) veneriformis, Paphia (P.) 

euglypta, and Babylonia japonica. Spines of Echinodea are 

also found (Kawamura and Shiota, 2005). The volcanic 

ash, Saijyo Minato 2 Volcanic Ash, is intercalated in the 

part. 

upper part: This part mainly consists of non-mollusk 

bearing sand or gravel(Units 1, 8 and 8m) in the coastal 

area of the Saijyo Plain. The upper part, 5 meters thick at 

maximum, becomes thicker northward. 

(correlation) The lower and middle parts, and the 

upper part can be correlated with the middle and upper 

Alluvium of Kurihara (1972), respectively. 

Saijyo Minato 2 Volcanic Ash (SjM-2; Kawamura and 

Shiota, 2005) 

The most distinctive lithological characteristic of the 

lower part of the Nyugawa Formation is the interbedding of 

a volcanic ash layer, Saijyo Minato 2 Volcanic Ash (SjM-2;

Norihito KAWAMURA and Kiminori MINAKUCHI 43 

3- 

St. 1 St. 2 St. 3 St. 4 St. 5 

1- 

0- 

-1- 

--.., 

--....::..: line 

correlation 

~ artificial 

materials 

C 

B 

clay 

silt 

-5- 

[IT] fine ........ 

sand 

-6 - 

-7 - 

-8 - 

-10- 

-9- 

medium 

0... ,- 


CJ 

.... coarse 


CJ gravel 

-11- 

-13- 

-12- 

-14- 

-15 - 

c o 

-16- .~ 

-17 - 

-18- 

-19 - 

E o 

lJ.. 

o 

:J '" >- 

UJ 

I·:·/l 1/-:: 

IIIIIIiI ~;u~I 

c 

.., o 

'"E o 

lJ.. 

o 

:J '" > 

UJ 

.... L- '-- 

[ZJ clayey 

[Z] silty 

[Z] sandy 

C2J 

• gravely 

volcanic 

ash 

gypsum 

• crystal 

-20- 

-21- 

SjM-! 

M 

marine 

mollusca 

-22 - 

SjM-l 

SjM-2 

Saijyo Minato 

1,2 Volcanic 

Ashes 

Fig. 5 Geological columnar maps in the Syuso and Saijyo Plains 

Abbreviations of the drilling sites are same as Fig. 1 

Kawamura and Shiota, 2005). The layer widely occurs 

beneath the coastal area of the plains between TP. 

-16 and -5 meters with a maximum thickness of 1 meter 

and becomes a useful key bed. 

SjM-2 is a light brownish-gray, fine-grained sand 

sized vitritic volcanic ash. It becomes light brownish-gray 

in color after purification. 

Possible correlatives of SjM-2 in the plains are two 

widespread volcanic ashes, Kikai-Akahoya Volcanic Ash 

(K-Ah, n=1.508 ~ 1.516 ; Machida and Arai, 1992,2003) 

or Ata Volcanic Ash (Ata, n=1.508 ~ 1.512; Machida and 

Arai, 1992, 2003). Considering the stratigraphy of SjM-2 

above SjM-l that is correlated with AT, SjM-2 can be 

correlated with K-Ah. 

On the basis of the elevations and stratigraphy of this 

volcanic ash layer, SjM-2 can be correlated with the 

volcanic ash in the upper Delluvium of Kitazaki and 

Hiyoshi (1965) and also with the volcanic ash layers that 

Kurihara (1972) recognized at T.P. -15 and -20 meters. 

VII. Interpretation of sedimentary environments of 

lithological units 

Kawamura and Shiota (2005) interpreted the 

sedimentary environments of the lithological units of 

drilling-core samples in the Saijyo-Minato area. 

According to Kawamura and Shiota (2005), we can also 

interpret the depositional environments of the revised 

lithological units beneath the Syuso and Saijyo Plains 

(Table 2).


For the interpretation of sedimentary environments, 

the authors follow the ecological data of mollusk-like 

vertical distribution after Higo and Goto (1993) and Higo 

et al. (1999). 

Units 1 to 3: The Lithological Unit 1 was infelTed as river 

channel sediments because of resembling the Facies Code 

Gm of Miall (1978). Likewise, Unit 2 was inferred to be 

back marsh sediments, which can be correlated with Facies 

Code Fsc of Miall (1978). Inverse grading of mud to very 

fine sand and mud to medium sand is formed at back marsh 

and natural levee when a river floods (Masuda and Iseya, 

1985). So Unit 3, which includes inverse grading, was also 

inferred as natural levee or back marsh sediments. 

Units 4 and 4a: Units 4 and 4a are back marsh sediments 

(Kawamura and Shiota, 2005). The alternating beds of clay 

and silt of Unit 4a may indicate the repetition of flash 

floods and consequent swamp formation. 

Unit 5: The lower part of Unit 5 was inferred as brackish 

or sea water marsh sediments because the drilling-core 

samples corresponding to this part freq uently yielded 

gypsum crystals that indicate marine sediments 

(Itihara,1960). 

Unit 6: This unit bears abundant species of a molluscan 

assemblage that consist mainly of intertidal zone and upper 

shallow sea mollusks. Some of mollusks are inner-bay 

species; e.g. Paphia (Neotapes) undulata, and Veremolpa 

micra. As for microfossils, brackish foraminifer and some 

inner-bay ostracods are found from this unit. It is 

concluded from the fossil evidence that Unit 6 is bay-head 

sediments as infelTed by Kawamura and Shiota (2005). 

Unit 7: Inteliidal zone to upper shallow sea mollusks are 

also found from this unit. Therefore, Unit 7 is inferred to 

be the sediments formed under the conditions between 

intertidal and subtidal zones. 

Unit 8: No evidence was found useful for the 

reconstruction of sedimentary environment. As it overlies 

Unit 7, Unit 8 may be considered as sediments formed at 

beach ridge. 

Unit 8m: This unit bears intertidal and subtidal zone 

mollusks like Crassostrea gigas. We can assume the unit 

was formed in the intertidal to subtidal zone. 

VIII. Age and sedimentary process of the Syuso 

and Nyugawa Formations 

Syuso Formation 

Most of the formation formed at river channels and in 

flood plains because this formation is comprised of the 

Lithological Units 1,2,3,4, 4a and 5 (Fig. 4). These 

sediments would have deposited as river systems of the 

paleo-Nakayamagawa and paleo-Kamogawa Rivers. 

The upper part of the Syuso Formation bears SjM-l 

Volcanic Ash. It is correlated with AT which erupted 

between 26 ka and 29 ka (Machida and Arai, 2003). 

Accordingly, the formation had been fonned during the late 

Pleistocene until about 26 ka. 

Nyugawa Formation 

The lower part, composed of the Lithological Unit 1, 

developed in the northern area of the plains and was 

formed under the river system by the paleo-Nakayamagawa 

River. The lower part is made of the Lithological Units 6 

and 7, and the latter two units are regarded as the bottomset 

and foreset of the present delta system of the 

Nakayamagawa River. During deposition of the lower part, 

the river mouth would have been located around the 

northwestern area in the Syuso Plain, although the 

Nakayamagawa River flows into the Hiuchinada at the 

eastern edge of this plain (Fig. 1). This is suggested by 

coarser grain-size distribution of the lower part in the area 

that may result from the sediment influx from the river. 

The upper part, made up of Lithological Units 1 and/or 8 

and overlies the forset beds of the delta, is considered as 

the topset of the delta system of the Nakayamagawa or the 

Kamogawa River. 

The lower part of the Nyugawa Formation intercalates 

SjM-2 Volcanic Ash that is correlated with K-Ah, which 

erupted at approximately 7.3ka (Machida and Arai, 2003). 

Judging from the eruption age of K-Ah, the lower part had 

been formed during the early to middle Holocene, and the 

upper part have deposited since the middle Holocene. 

IX. Conclusions 

Lithostratigraphical division of subsurface geology in 

the Syuso and Saijyo Plains is as follows: the Pleistocene 

Syuso Formation and the Holocene Nyugawa 

Formation(the lower, middle and upper parts, in ascending 

order). 

Saijyo Minato 1 Volcanic Ash (SjM-l) is intercalated 

in the upper part of the Syuso Formation and is con-elated 

with the Aira-Tn Volcanic Ash. Saijyo Minato 2 Volcanic 

Ash (SjM-2) in the middle part of the Nyugawa Formation 

is a good key bed and is correlated with the Kikai-Akahoya 

Volcanic Ash. 

The upper part of the Syuso Formation would have 

been deposited until around 26ka at river channels or in 

flood plains. The Nyugawa Formation occurred at around 

7.3ka and had been formed through the Holocene mainly as 

deltaic sediments by the Nakayamagawa and the 

Kamogawa Rivers.

Norihito KAWAMURA and Kiminori MINAKUCHl 

45 

Acknowledgements 

The authors would like to thank Dr. Muneki Mitarnura 

of Osaka City University for permitting the use of facilities 

of Natural History of Anthropogene Laboratory for help in 

the measurement of refractive index of volcanic glass 

shards. Gratitude is also due to the following public offices 

and drilling companies: the Saijyo Office of Water for 

Industrial Use, Ehime prefecture, the Shikoku Branch of 

Japan Society of Civil Engineers, Naiba Co. Ltd., Canaan 

Georesearch Inc., and Gijyutsu Kaihatsu Inc., for providing 

borehole samples and logs. The authors would like to 

express their deepest gratitude to Drs. Shigehiro Kato 

(Museum of Nature and Human Activities, Hyogo) and 

Muneki Mitamura for reviewing and improving the 

manuscript. 

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