Lithostratigraphy of the late Quaternary under the Syuso and Saijyo ...
Lithostratigraphy of the late Quaternary under the Syuso and Saijyo ...
Lithostratigraphy of the late Quaternary under the Syuso and Saijyo ...
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Journal <strong>of</strong> Geosciences, Osaka City University<br />
Vol. 51, Art. 5, p. 35-46, March, 2008<br />
<strong>Lithostratigraphy</strong> <strong>of</strong> <strong>the</strong> <strong>late</strong> <strong>Quaternary</strong> <strong>under</strong> <strong>the</strong> <strong>Syuso</strong> <strong>and</strong><br />
<strong>Saijyo</strong> Plains, Ehime Prefecture, southwest Japan<br />
Norihito KAWAMURA 1 <strong>and</strong> Kiminori MINAKUCHI 2<br />
I Marugame High School, Roku-Bancyo-l, Marugame, 763-8512, Japan<br />
E-mail: nori-ka@jt2.so-net.ne.jp<br />
2 Construction Research Institute <strong>of</strong> Ehime Prefecture, Higashi-Ishii 5-1-31, Matsuyama, 790-0932,<br />
Japan<br />
Abstract<br />
Subsurface geology in <strong>the</strong> coastal area <strong>of</strong> <strong>the</strong> <strong>Syuso</strong> <strong>and</strong> <strong>Saijyo</strong> Plains, Ehime Prefecture,<br />
southwest Japan, is divided stratigraphically into <strong>the</strong> Pleistocene <strong>Syuso</strong> <strong>and</strong> Holocene yugawa<br />
Formations. Additionally, <strong>the</strong> Nyugawa Formation is subdivided into <strong>the</strong> lower, middle <strong>and</strong> upper parts<br />
in a cending order. Two volcanic ashes, <strong>Saijyo</strong> Minato 1 <strong>and</strong> <strong>Saijyo</strong> Minato 2 Volcanic Ashes are<br />
interca<strong>late</strong>d in <strong>the</strong> upper part <strong>of</strong> <strong>the</strong> <strong>Syuso</strong> Formation <strong>and</strong> in <strong>the</strong> middle part <strong>of</strong> <strong>the</strong> Nyugawa Formation<br />
<strong>and</strong> corre<strong>late</strong>d with Aira-Tn <strong>and</strong> Kikai-Akahoya Volcanic A hes, re pectively.<br />
From <strong>the</strong> tephrostratigraphy <strong>and</strong> lithological characteristics <strong>of</strong> <strong>the</strong> two formations, <strong>the</strong> upper part<br />
<strong>of</strong> <strong>the</strong> <strong>Syuso</strong> Formation would have been deposited continuously until around 26ka ~ 29ka at river<br />
channels or on flood plains. The Nyugawa Formation has formed in <strong>the</strong> last 7300 years mostly as<br />
deltaic sediments.<br />
Key-words: Pleistocene <strong>Syuso</strong> Formation, Holocene Nyugawa Formation, <strong>Syuso</strong> <strong>and</strong> <strong>Saijyo</strong> Plains,<br />
volcanic ash, drilling-core<br />
I. Introduction<br />
The <strong>Syuso</strong> <strong>and</strong> <strong>Saijyo</strong> Plains occupy <strong>the</strong> central part <strong>of</strong><br />
<strong>the</strong> nor<strong>the</strong>rn coastal area <strong>of</strong> Shikoku Isl<strong>and</strong> facing <strong>the</strong> Seto<br />
Inl<strong>and</strong> Sea (Fig. 1). They are suitable areas for studying <strong>the</strong><br />
st<strong>and</strong>ard stratigraphy <strong>of</strong> <strong>the</strong> <strong>Quaternary</strong> in nor<strong>the</strong>rn<br />
Shikoku Isl<strong>and</strong> because <strong>of</strong> storage <strong>of</strong> many drilling logs<br />
<strong>and</strong> samples. But detailed research <strong>of</strong> sub urface geology<br />
based on drilling-core observation has not been done since<br />
1972.<br />
The subsurface geology in <strong>the</strong>se plains were first<br />
reported by Murashita <strong>and</strong> Noma (1964), Kitazaki <strong>and</strong><br />
Hiyoshi (1965), <strong>and</strong> Kitazaki <strong>and</strong> Shindo (1965).<br />
Murashita <strong>and</strong> Noma (1964) reported <strong>the</strong> general<br />
subsurface geology in terms <strong>of</strong> hydrogeology. Kitazaki <strong>and</strong><br />
Hiyoshi (1965) <strong>and</strong> Kitazaki <strong>and</strong> Shindo (1965) studied <strong>the</strong><br />
<strong>late</strong> <strong>Quaternary</strong> deposits as ground investigation <strong>of</strong> <strong>the</strong><br />
plains. After <strong>the</strong>se researchers, Saito et a1. (1972) studied<br />
<strong>the</strong> stratigraphy <strong>of</strong> a drilling-core at <strong>the</strong> nor<strong>the</strong>rn fringe <strong>of</strong><br />
<strong>the</strong> plains. Kurihara (1972) revised <strong>the</strong> lithological<br />
division <strong>of</strong> <strong>the</strong> <strong>Quaternary</strong> sediments in <strong>the</strong> <strong>Saijyo</strong> Plain.<br />
However, <strong>the</strong> lithostratigraphical division <strong>of</strong> Kurihara<br />
(1972) does not coincide with that <strong>of</strong> Kitazaki <strong>and</strong> Hiyoshi<br />
(1965) so detailed observation <strong>of</strong> <strong>the</strong> drilling-cores was<br />
required.<br />
Kawamura <strong>and</strong> Shiota (2005) described <strong>the</strong> lithology,<br />
molluscan fossils, <strong>and</strong> volcanic ashes <strong>of</strong> four drilling-cores<br />
from Minato area, <strong>Saijyo</strong> City. They insisted that <strong>Saijyo</strong><br />
Minato 1 <strong>and</strong> <strong>Saijyo</strong> Minato 2 Volcanic Ashes can be<br />
corre<strong>late</strong>d with Aira-Tn <strong>and</strong> Kikai-Akahoya Volcanic Ashes<br />
(Machida <strong>and</strong> Arai, 1992, 2003) ,respectively. Distribution<br />
<strong>of</strong> <strong>the</strong>se volcanic ashes beneath <strong>the</strong> <strong>Saijyo</strong> Plain, however,<br />
has not been clarified precisely.
36 <strong>Lithostratigraphy</strong> <strong>of</strong> <strong>the</strong> <strong>late</strong> <strong>Quaternary</strong> <strong>under</strong> <strong>the</strong> <strong>Syuso</strong> <strong>and</strong> <strong>Saijyo</strong> Plains, Ehime Prefecture, southwest Japan<br />
The objective <strong>of</strong> <strong>the</strong> present study was re-examination<br />
<strong>of</strong> <strong>the</strong> chronostratigraphical division <strong>of</strong> <strong>the</strong> subsurface<br />
geology in <strong>the</strong> <strong>Syuso</strong> <strong>and</strong> <strong>Saijyo</strong> plains by <strong>the</strong> detailed<br />
observation <strong>of</strong> 11 new drilling-cores from <strong>the</strong> coastal area<br />
in <strong>the</strong> plains, especially on <strong>the</strong> basis <strong>of</strong> recognition <strong>of</strong> <strong>the</strong><br />
two volcanic ash layers. Likewise, <strong>the</strong> authors will discuss<br />
<strong>the</strong> sedimentation age <strong>of</strong> <strong>the</strong> <strong>late</strong> <strong>Quaternary</strong> in <strong>the</strong> coastal<br />
area <strong>of</strong> <strong>the</strong>se two plains.<br />
(a)<br />
133"E<br />
II. General geography <strong>and</strong> geology<br />
The Nakayamagawa <strong>and</strong> Kamogawa Rivers, two<br />
major rivers in <strong>the</strong> <strong>Syuso</strong> <strong>and</strong> <strong>Saijyo</strong> Plains, are running<br />
from <strong>the</strong> Ishiduchi Mountains to <strong>the</strong> Seto Inl<strong>and</strong> Sea. The<br />
rivers form alluvial plains along <strong>the</strong>ir lower reaches <strong>and</strong> <strong>the</strong><br />
coast <strong>of</strong> Hiuchinada (Fig. 1). The authors call <strong>the</strong> area that<br />
extends above T.P.(Tokyo Peil) 3m "<strong>the</strong> inl<strong>and</strong> area" <strong>and</strong><br />
<strong>the</strong> plain below T.P. 3m "<strong>the</strong> coastal area" because <strong>the</strong><br />
inl<strong>and</strong> area slopes gently toward <strong>the</strong> Hiuchinada <strong>and</strong> is<br />
generally flat l<strong>and</strong>.<br />
As for geomorphological division, <strong>the</strong> <strong>Syuso</strong> Plain is<br />
classified as <strong>the</strong> terrace, fan, flood plain <strong>and</strong> delta that was<br />
formed by <strong>the</strong> Nakayamagawa River, <strong>and</strong> reclaimed l<strong>and</strong><br />
(Shiki, 1965). The <strong>Saijyo</strong> Plain is also divided into <strong>the</strong> fan,<br />
flood plain <strong>and</strong> delta formed by <strong>the</strong> Kamogawa River, <strong>and</strong><br />
reclaimed l<strong>and</strong> (Shiki, 1965).<br />
Shiki also classified <strong>the</strong><br />
sou<strong>the</strong>rn part <strong>of</strong> <strong>the</strong> plains along <strong>the</strong> nor<strong>the</strong>rn foot <strong>of</strong> <strong>the</strong><br />
Ishiduchi Mountains as terrace <strong>and</strong> alluvial cone areas.<br />
Immediately to <strong>the</strong> north <strong>and</strong> west <strong>of</strong> <strong>the</strong> <strong>Syuso</strong> Plain,<br />
<strong>the</strong> Takanawa Mountains lie, which is <strong>under</strong>lain by <strong>the</strong><br />
Ryoke Granites (Miyahisa <strong>and</strong> Hiraoka, 1970; Ochi, 1982<br />
etc.). The plains' eastern neighbor, <strong>the</strong> Nishiyama<br />
Mountains, is made up <strong>of</strong> <strong>the</strong> Cretaceous Izumi Group<br />
(Momoi et aI.,1991). The Shikoku Mountains are found<br />
immediately south <strong>of</strong> <strong>the</strong> plains <strong>and</strong> are <strong>under</strong>lain by <strong>the</strong><br />
crystalline schists <strong>of</strong> <strong>the</strong> Sanbagawa Belt (Hide et ai. 1956).<br />
The Pleistocene Okamura Formation is distributed along<br />
<strong>the</strong> fringe <strong>of</strong> <strong>the</strong> plain (Saito, 1962; Takahashi, 1969;<br />
Mizuno et al.,1993).<br />
According to Mizuno et ai. (1993) <strong>and</strong> Nakata et ai.<br />
(1995), <strong>the</strong> active fault system <strong>of</strong> <strong>the</strong> Median Tectonic Line<br />
runs along <strong>the</strong> sou<strong>the</strong>rn fringe <strong>of</strong> <strong>the</strong> <strong>Syuso</strong> <strong>and</strong> <strong>Saijyo</strong><br />
Plains (Fig. 1). The Kawakami Fault (Okada, 1972) <strong>and</strong><br />
Komatsu Fault (Mizuno et aI., 1993) are located along <strong>the</strong><br />
southwestern rim <strong>of</strong> <strong>the</strong> plains. Ano<strong>the</strong>r fault is found at<br />
<strong>the</strong> sou<strong>the</strong>rn rim <strong>of</strong> <strong>the</strong> <strong>Saijyo</strong> plain <strong>and</strong> was named <strong>the</strong><br />
Okamura Fault (Nagai,1995). At <strong>the</strong> western border<br />
between <strong>the</strong> mountains <strong>and</strong> <strong>the</strong> <strong>Syuso</strong> Plain, <strong>the</strong> Kawane<br />
Fault is found (Okada, 1972). In contrast, any distinctive<br />
dislocation by <strong>the</strong> active faults have not been found in <strong>the</strong><br />
central <strong>and</strong> coastal parts <strong>of</strong> <strong>the</strong> plains.<br />
(b)<br />
Z?-.. .<br />
N ~;)ffi'<br />
~~~\<br />
rb '<br />
CD Kawane Fault (g) Kawakami Fault @ Komatsu Fault<br />
@ Okamura Fault<br />
I I Sea <strong>and</strong> lake C=:J Alluvial plain <strong>and</strong> delta<br />
~ Mountain <strong>and</strong> terrace<br />
Fig. 1 Study area <strong>and</strong> <strong>the</strong> drilling sites<br />
Location <strong>of</strong> <strong>the</strong> fault system is after The<br />
Research Group for Active Faults <strong>of</strong> Japan<br />
(1992) <strong>and</strong> Nakata et aI. (1995). Abbreviations<br />
<strong>of</strong> drilling-cores <strong>of</strong> TYP-K1, TYP-D1, KTG-1,<br />
<strong>and</strong> KTG-2 are followed from Kawamura <strong>and</strong><br />
Shiota (2005), <strong>and</strong> numbers 1 to 5 indicate <strong>the</strong><br />
location <strong>of</strong> drilling-cores CSt.1 to S1.5) used in<br />
this study.<br />
III. Methods<br />
o<br />
2 4km:<br />
Eleven new drilling-cores at five sites (Fig. 1) were<br />
used for discernment <strong>of</strong> lith<strong>of</strong>acies <strong>and</strong> precise analyses <strong>of</strong><br />
faunal fossils <strong>and</strong> volcanic ash layers. The authors fur<strong>the</strong>r<br />
used <strong>the</strong> 160 previous drilling logs (The Ministry <strong>of</strong><br />
Construction <strong>and</strong> Ehime Prefecture, 1965; Shikoku Ground<br />
Information Utilizing Association, 2007) for revision <strong>of</strong> <strong>the</strong><br />
subsurface geology <strong>and</strong> identification <strong>of</strong> <strong>the</strong> continuity <strong>of</strong><br />
sediments.
Norihito KAWAMURA <strong>and</strong> Kiminori MINAKUCHI 37<br />
Lithological description <strong>of</strong> drilling-core samples<br />
Lithology <strong>of</strong> drilling-core samples were described by<br />
inspection with <strong>the</strong> naked eye <strong>and</strong> stereoscopic microscope.<br />
Grain-size distribution, fossils, <strong>and</strong> sedimentary structure<br />
were described by means <strong>of</strong> lithology. For color <strong>and</strong><br />
N-values <strong>of</strong> <strong>the</strong> drilling-core samples <strong>the</strong> authors refened<br />
to <strong>the</strong> provisional reports <strong>of</strong> borehole drilling companies<br />
since <strong>the</strong> samples concerned have been dried <strong>and</strong> oxidized<br />
for up to ten years. Volcanic ashes, faunal fossils such as<br />
Mollusca, Foraminifera, Ostracoda <strong>and</strong> Echinodea, <strong>and</strong><br />
secondary formed crystals <strong>and</strong> gypsum are all sampled<br />
during <strong>the</strong> description.<br />
Sampling <strong>of</strong> faunal fossils<br />
Faunal fossils were collected from core samples by <strong>the</strong><br />
following operation. Large-size molluscan fossils were<br />
directly picked up from drilling-core samples. Ten to<br />
twenty grams <strong>of</strong> sample blocks were collected <strong>and</strong> washed<br />
with water on a 1/8 millimeter mesh sieve, <strong>and</strong> <strong>the</strong> fossils<br />
were separated with <strong>the</strong> aid <strong>of</strong> a stereoscopic microscope.<br />
Petrographical description <strong>of</strong> volcanic ash<br />
The interca<strong>late</strong>d volcanic ash was studied for its<br />
mineral composition <strong>and</strong> <strong>the</strong> shape <strong>and</strong> refractive index <strong>of</strong><br />
glass shards in terms <strong>of</strong> petrography. Classification <strong>of</strong><br />
glass shard's shape (H, C, T <strong>and</strong> o<strong>the</strong>r types) is after<br />
Yoshikawa (1976). Refractive index was measured by <strong>the</strong><br />
temperature change method using <strong>the</strong> refractometer<br />
"MAIOT" (Furusawa, 1995). Accuracy <strong>of</strong> measurement by<br />
this method is approximately ± 0.0001. Refractive index<br />
<strong>of</strong> glass shards from each volcanic ash sample was<br />
determined by <strong>the</strong> measurements <strong>of</strong> twenty glass shards.<br />
IV. Description <strong>of</strong> lithological units <strong>of</strong> drilling cores<br />
On <strong>the</strong> basis <strong>of</strong> lith<strong>of</strong>acies, Kawamura <strong>and</strong><br />
Shiota(2005) divided <strong>the</strong> KTG-1, 2 <strong>and</strong> TYP-Kl, D1<br />
drilling-cores into <strong>the</strong> Lithological Units <strong>of</strong> 1 to 9. In this<br />
study, <strong>the</strong>se Lithological Units are applied to <strong>the</strong> newly<br />
observed drilling-core samples. Additionally, <strong>the</strong><br />
Lithological Units 4a <strong>and</strong> 8m are newly defined. These<br />
Lithological Units are summarized in Table 1.<br />
Unit 1: The Unit consists <strong>of</strong> <strong>the</strong> grayish massive s<strong>and</strong>y<br />
gravel which contain subrounded to subangular pebbles<br />
with coarse s<strong>and</strong>.<br />
Unit 2: Massive silt beds, a few meters thick, are<br />
interbedded with gravel beds. The mud sometimes<br />
includes s<strong>and</strong> or rounded pebbles <strong>and</strong> is rarely peaty.<br />
Unit 3: This unit is <strong>the</strong> inverse grading bed with<br />
subrounded to subangular pebbles, in which silt beds grade<br />
into s<strong>and</strong>y gravels upward.<br />
Unit 4: This unit is dark gray in color <strong>and</strong> comprises fine<br />
s<strong>and</strong>y silt or clay.<br />
Unit 4a: This unit is an alternating bed <strong>of</strong> clay <strong>and</strong> silt 1<br />
millimeter to several centimeters thick each.<br />
Unit 5: This unit is comprised <strong>of</strong> dark gray clay <strong>and</strong><br />
contains ordinary humus. Gypsum crystals are partly<br />
formed in <strong>the</strong> core samples.<br />
Units 6m <strong>and</strong> 6s: The Unit 6m is dark gray, massive clay<br />
or silty beds, hence Unit 6s is s<strong>and</strong>y silt beds occasionally<br />
with unclear laminae. Both <strong>of</strong> <strong>the</strong> units bear mollusk,<br />
sponge, benthic foraminifer, ostracod, <strong>and</strong> fragments <strong>of</strong><br />
echinus. Some examples are Elphidium spp. as marine<br />
benthic foraminifer, Ammonia beccari as brackish benthic<br />
foraminifer,<strong>and</strong> Bicornucy<strong>the</strong>re bisanensis,Trachyleberis<br />
scabrocuneata <strong>and</strong> Spinileberis quadriaculeata as innerbay<br />
living ostracod.<br />
Unit 7: This unit is dark gray, massive silty s<strong>and</strong> that bears<br />
mollusk rarely.<br />
Units 8 <strong>and</strong> 8m: These units are comprised <strong>of</strong> dark gray to<br />
dark bluish gray, medium to coarse sized massive s<strong>and</strong><br />
beds containing gravel. Unit 8 yields no marine molluscan<br />
fossil but mollusk are found rarely in <strong>the</strong> <strong>under</strong>lying Unit<br />
8m like Batillaria zonalis <strong>and</strong> Batillaria multiformis as<br />
intertidal zone living species, <strong>and</strong> Eufenella rufocincta as<br />
intertidal zone to 20 meters depth living species.<br />
Unit 9: This Unit bears a gravely s<strong>and</strong> or silt bed<br />
frequently with blocks <strong>of</strong> concrete <strong>and</strong> wood.<br />
V. Petrographical characteristics <strong>of</strong> volcanic ash layers<br />
Table 2 shows petrographical characteristics <strong>of</strong><br />
volcanic ash layers in <strong>the</strong> plains.<br />
<strong>Saijyo</strong> Minato 1 Volcanic Ash (SjM-1)<br />
The volcanic ash, SjM-1, is light grayish, silt to fine<br />
s<strong>and</strong> sized vitritic volcanic ash.<br />
SjM-1 is mainly composed <strong>of</strong> clear glass shards with<br />
rare orthopyroxene <strong>and</strong> plagioclase crystals. The shapes <strong>of</strong><br />
<strong>the</strong> glass shards are mainly classified into H<strong>and</strong> C types.<br />
The refractive index(n) ranges from 1.498 to 1.501(mode:<br />
1.498 ~ 1.499).<br />
<strong>Saijyo</strong> Minato 2 Volcanic Ash (SjM-2)<br />
The volcanic ash, SjM-2, is a slightly brownish color,<br />
silt to fine s<strong>and</strong> sized vitritic volcanic ash that contains<br />
fine-grained pumice.<br />
SjM-2 is mainly composed <strong>of</strong> clear glass shards but<br />
includes brown-colored glass shards <strong>and</strong> rare<br />
orthopyroxene <strong>and</strong> plagioclase crystals. The shapes are H,<br />
C <strong>and</strong> T types (H>C>T). The refractive index ranges from<br />
1.508 to 1.514.
38 <strong>Lithostratigraphy</strong> <strong>of</strong> <strong>the</strong> <strong>late</strong> <strong>Quaternary</strong> <strong>under</strong> <strong>the</strong> <strong>Syuso</strong> <strong>and</strong> <strong>Saijyo</strong> Plains, Ehime Prefecture, southwest Japan<br />
Table I<br />
Lithological Units <strong>and</strong> interpretation <strong>of</strong> <strong>the</strong>ir sedimentary environments (modified from Kawamaura <strong>and</strong><br />
Shiota, 2005)<br />
Lithology<br />
Sedimentary env-<br />
ironment<br />
Fossil <strong>and</strong> secondary formed<br />
mineral<br />
1 grayish massive s<strong>and</strong>y gravels~river channel<br />
consisting <strong>of</strong> subrounded to<br />
subangular pebbles with<br />
coarse s<strong>and</strong> as matrix<br />
----------<br />
2 massive mud beds interbedded back marsh<br />
in <strong>the</strong> gravelly beds<br />
3 inverse grading from silt to natural levee or<br />
s<strong>and</strong> or silt to s<strong>and</strong>y gravel<br />
back marsh<br />
upward ~<br />
----------<br />
4 dark gray fine s<strong>and</strong>y silt or<br />
back marsh<br />
clay<br />
----------<br />
4a alternating beds <strong>of</strong> clay <strong>and</strong> back marsh<br />
silt layers<br />
5 dark gray clay containing gypsum crystals brackish or sea<br />
humus<br />
water marsh<br />
6 mainly dark gray massive silt intertidal zone, <strong>and</strong> intertidal bay-head<br />
zone to upper shallow sea<br />
mollusks including inner-bay<br />
species;e.g. Paphia (Neotapes)<br />
undu/ata <strong>and</strong> Veremo/pa micra<br />
foraminifer: Ammonia beccari ,<br />
E/phidium spp.<br />
inner-bay ostracods; Bicornucy<strong>the</strong>re<br />
bisanensis •<br />
Trachy/eberis scabrocuneata<br />
<strong>and</strong> Spini/eberis quadriacu/eata<br />
othrs; sponge <strong>and</strong> Echinodermata<br />
7 dark gray massive silty s<strong>and</strong> rare occurence <strong>of</strong> intertidal intertidal zone<br />
zone to upper shallow sea <strong>and</strong> subtidal zone<br />
mollusk; e.g. Batil/aria zona/is<br />
8 dark gray to dark bluish gray<br />
massive s<strong>and</strong> or gravelly s<strong>and</strong><br />
<strong>of</strong> medium to coarse size<br />
~(beach ridge)<br />
8m same as <strong>the</strong> Unit 8 intertidal <strong>and</strong> subtidal zone intertidal to submollusk;e.g.<br />
----------<br />
Crassostrea gigas tidal zone<br />
9 gravelly s<strong>and</strong> including blocks reclaimed l<strong>and</strong><br />
<strong>of</strong> concrete <strong>and</strong> wood<br />
Table 2 Petrographical characters <strong>of</strong> <strong>the</strong> volcanic ash layers<br />
For abbreviations <strong>of</strong> <strong>the</strong> drilling sites, see Fig.l.<br />
Name<br />
<strong>Saijyo</strong> Minato 2<br />
Volcanic Ash<br />
Kikai-Akahoya<br />
<strong>Saijyo</strong> Minato 1<br />
Volcanic Ash<br />
Aira-Tn<br />
Drilling site <strong>and</strong><br />
elevation (T.P.)<br />
Glass shards<br />
f--..."s-ch-a-p-e---;C(%::-'0"")--=r='=R:::"e'-'fr""'a"'c:':::ti-ve--:-in-d-c ēx --:-(n-:)--I Mineral<br />
composition<br />
H C T Oth ranae mode<br />
opX, Oth<br />
Reference<br />
Kawamura <strong>and</strong> Shiota(2005)<br />
- (present study)<br />
- (present study)<br />
------------------------------------------.<br />
H > T 1.508-1.516: - opx>Cpx Machida <strong>and</strong> Arai (2003)<br />
KTG-1 -11.2m 55.6 26.1 16.5 1.7 1.508-1.514: -<br />
81.4 a.85m 43.8 37.2 17.8 1.1 1.508-1.514: -<br />
.~!'.~.:?: !~ _!"!.~..S;.. ~_.~ ]:~.~~:~ :?~~.l ~ .<br />
KTG-1 -18.3m<br />
St.4 -11.65m<br />
_____________________________<br />
69.3 27.4 3.2 0<br />
51.8 34.9 11.5 1.8<br />
• ._._.<br />
1.498-1.501: 1.498-1.499<br />
1.498-1.500: 1.498-1.499<br />
._. L _<br />
H > T 1.498-1.500 : 1.498-1.499<br />
Shape <strong>of</strong> glass shards are after Yoshikawa(1976)<br />
Minerals; Ho: hornblend, Opx: orthopyroxene, Cpx: clinopyroxene, Oth: o<strong>the</strong>rs<br />
opX, Oth Kawamura <strong>and</strong> Shiota(2005)<br />
- (present study)<br />
-------------------------------------------<br />
Opx>Cpx>Ho Machida <strong>and</strong> Arai (2003)
Norihito KAWAMURA <strong>and</strong> Kiminori MINAKUCI-U 39<br />
VI. Revised lithostratigraphy<br />
Takahashi (1958) called <strong>the</strong> Tertiary beds found at <strong>the</strong><br />
sou<strong>the</strong>rn fringe <strong>of</strong> <strong>the</strong> plains <strong>the</strong> Okamura Formation. Saito<br />
(1962) regarded <strong>the</strong> Okamura Formation as <strong>the</strong> Plio<br />
Pleistocene Mitoyo Group(Saito <strong>and</strong> Nakayama, 1954).<br />
Saito et al. (1972) <strong>and</strong> Kurihara (1972) ascribed <strong>the</strong><br />
Tertiary beds in <strong>the</strong> <strong>Saijyo</strong> Plain just to <strong>the</strong> Mitoyo Group.<br />
In this study, <strong>the</strong> authors call <strong>the</strong>se Tertiary systems <strong>the</strong><br />
Okamura Formation following Takahashi (1958). The<br />
formation must <strong>under</strong>lie <strong>the</strong> Pleistocene sediments in <strong>the</strong><br />
plains, but <strong>the</strong> authors have not found it in <strong>the</strong> drilling-core<br />
samples observed.<br />
The <strong>Quaternary</strong> system beneath <strong>the</strong> plains was divided<br />
into "Delluvium" <strong>and</strong> "Alluvium" by Kitazaki <strong>and</strong> Shindo<br />
(1965) <strong>and</strong> called "Alluvium" by Kurihara (1972).<br />
Stratigraphy <strong>of</strong> subsurface geology <strong>of</strong> <strong>the</strong> plains studied by<br />
Kitazaki <strong>and</strong> Hiyoshi (1965), Saito et al. (1972), <strong>and</strong><br />
Kitazaki <strong>and</strong> Shindo (1965) is summarized as follows (Fig.<br />
2); lower, middle, <strong>and</strong> upper Delluvium (lowermost Beds),<br />
uppermost Delluvium (lower Beds), <strong>and</strong> Alluvium (upper<br />
<strong>and</strong> uppermost Beds). Hence, Kurihara (1972) pointed out<br />
<strong>the</strong> difference <strong>of</strong> lithostratigraphy between <strong>the</strong> inl<strong>and</strong> <strong>and</strong><br />
coastal areas <strong>of</strong> <strong>the</strong> plains.<br />
Considering <strong>the</strong> inconsistency <strong>of</strong> previous<br />
stratigraphic divisions, <strong>the</strong> authors propose a revised<br />
lithostratigraphy <strong>of</strong> <strong>the</strong> <strong>late</strong> <strong>Quaternary</strong> beneath <strong>the</strong> <strong>Syuso</strong><br />
<strong>and</strong> <strong>Saijyo</strong> Plains (Fig. 2) on <strong>the</strong> basis <strong>of</strong> 17 drilling-cores<br />
used in this study <strong>and</strong> in Kawamura <strong>and</strong> Shiota (2005).<br />
<strong>Syuso</strong> Formation (new usage)<br />
(definition) The <strong>Syuso</strong> Formation is defined as s<strong>and</strong>y<br />
gravel in <strong>the</strong> <strong>Syuso</strong> Plain that overlies <strong>the</strong> Okamura<br />
Formation <strong>and</strong> <strong>under</strong>lies <strong>the</strong> Nyugawa Formation. The<br />
formation corresponds to a part <strong>of</strong> <strong>the</strong> sediments <strong>under</strong>lying<br />
<strong>the</strong> Alluvium which Kurihara (1972) corre<strong>late</strong>d with <strong>the</strong><br />
Plio-Pleistocene Mitoyo Group (Saito <strong>and</strong> Nakayama,<br />
1954). Beneath <strong>the</strong> southwestern part <strong>of</strong> <strong>the</strong> <strong>Syuso</strong> Plain,<br />
<strong>the</strong> formation unconformably overlies basement rocks <strong>of</strong><br />
<strong>the</strong> Izumi Group.<br />
(type locality) Typical beds are distributed between<br />
TP. -25.5 <strong>and</strong> -11.5 meters <strong>of</strong> <strong>the</strong> St.l Drilling-core (33 0<br />
55'44"N, 133 0<br />
06'40"E, see Fig. 1), Hiroe area, <strong>Saijyo</strong> City.<br />
(distribution) Distributions <strong>of</strong> <strong>the</strong> formation are<br />
different between <strong>the</strong> inl<strong>and</strong> <strong>and</strong> coastal areas (see Fig. 3<br />
(c)). Fig. 3 shows three typical cross-sections across <strong>the</strong><br />
plains. In <strong>the</strong> inl<strong>and</strong> area, <strong>the</strong> <strong>Syuso</strong> Formation<br />
immediately lies beneath <strong>the</strong> earth surface in <strong>the</strong> <strong>Syuso</strong><br />
Plain, which lie between TP. 3 <strong>and</strong> approximately 50<br />
meters (Fig. 3 (c)). Hence, it is distributed in <strong>the</strong> coastal<br />
area at least below TP. -10 meters. The lower boundary <strong>of</strong><br />
<strong>the</strong> formation is unidentified from any <strong>of</strong> <strong>the</strong> observed<br />
drilli ng-cores that are not deep enough to reach to <strong>the</strong><br />
basement rocks. According to <strong>the</strong> referenced drilling logs<br />
between St.3 <strong>and</strong> TYP-K1 (Fig. 3 (a)), <strong>the</strong> formation<br />
distributes below TP. -30 meters at least.<br />
(thickness) More than 17 meters.<br />
(lithology) In <strong>the</strong> inl<strong>and</strong> area <strong>of</strong> <strong>the</strong> plains, especially<br />
in <strong>the</strong> sou<strong>the</strong>rn neighboring area <strong>of</strong> <strong>the</strong> Nakayamagawa<br />
River, <strong>the</strong> formation has simple facies <strong>and</strong> mainly consists<br />
<strong>of</strong> s<strong>and</strong>y gravels (lithological unit 1). To <strong>the</strong> contrary, in<br />
<strong>the</strong> coastal area <strong>the</strong> upper part <strong>of</strong> <strong>the</strong> formation comprises<br />
<strong>of</strong> mud, s<strong>and</strong> <strong>and</strong> gravel (units 1,2,4 <strong>and</strong> 5).<br />
(correlation) This formation is corre<strong>late</strong>d with <strong>the</strong><br />
middle <strong>and</strong> upper Delluvium (Kitazaki <strong>and</strong> Hiyoshi, 1965),<br />
Delluvium (Saito et a!., 1972), <strong>and</strong> <strong>the</strong> lower s<strong>and</strong>y gravels<br />
<strong>of</strong> <strong>the</strong> Alluvium (Kurihara, 1972).<br />
Kitazaki <strong>and</strong> Correlation Age<br />
Reference Hiyoshi (1965). Kurihara (1972) Saito et al. This study <strong>of</strong> wide- [ka]<br />
Kitazaki <strong>and</strong> Shindo (1972) spread (after Machida<br />
(1965) tephra <strong>and</strong> Aral. 2003)<br />
Alluvium<br />
Upper s<strong>and</strong>y gravels.<br />
s<strong>and</strong>s <strong>and</strong> silts<br />
Upper Alluvium<br />
u.:<br />
upper part<br />
til<br />
~<br />
Middle s<strong>and</strong>s,<br />
middle part<br />
E<br />
til<br />
:::J<br />
.;;;<br />
Lower Alluvium OJ - -- - -- :::J<br />
SjM-2<br />
>,<br />
- - K-Ah - -- --- 7.3<br />
.2 silts <strong>and</strong> muds z<br />
Stratigraphy Uppermost
40 <strong>Lithostratigraphy</strong> <strong>of</strong> <strong>the</strong> <strong>late</strong> <strong>Quaternary</strong> <strong>under</strong> <strong>the</strong> <strong>Syuso</strong> <strong>and</strong> <strong>Saijyo</strong> Plains, Ehime Prefecture, southwest Japan<br />
(a)<br />
[T.P.m]<br />
10<br />
0<br />
A St1 St.2 St3 TYP-K1 TYP-Dl<br />
1 J 1 I 1<br />
B<br />
CJ<br />
materials<br />
artificial<br />
~<br />
muddy beds<br />
NU:::: ~ ....<br />
s<strong>and</strong>y beds<br />
-10 NM<br />
00000<br />
0000<br />
SjM-2<br />
gravelly<br />
beds<br />
-20 III<br />
s<strong>and</strong> stone<br />
0 2km<br />
-30 I I<br />
v --<br />
volcanic<br />
ash layer<br />
(b)<br />
[T.P.mJ<br />
30 o<br />
10<br />
o<br />
-10<br />
-20<br />
-30<br />
-40<br />
o<br />
I<br />
2<br />
I<br />
4km<br />
I<br />
Fig. 3 Geological sections in <strong>the</strong> <strong>Syuso</strong> <strong>and</strong> <strong>Saijyo</strong> Plains, Ehime Prefecture<br />
(a) Line A-B, (b) Line C-D<br />
Iz: Izumi Group, Syu: <strong>Syuso</strong> Formation, NL: <strong>the</strong> lower part <strong>of</strong> <strong>the</strong> Nyugawa Formation, NM: <strong>the</strong> middle part<br />
<strong>of</strong> <strong>the</strong> Nyugawa Formation, NU: <strong>the</strong> upper part <strong>of</strong> <strong>the</strong> Nyugawa Formation.
Norihito KAWAMURA <strong>and</strong> Kiminori MINAKUCHl<br />
41<br />
(c)<br />
[T.P.mJ<br />
50 E<br />
40<br />
30<br />
20<br />
10<br />
o<br />
F<br />
-10<br />
-20<br />
-30<br />
o<br />
I<br />
2km<br />
I<br />
'-NM<br />
Fig. 3 (continued)<br />
(c) Line E-F in Fig.<br />
<strong>Saijyo</strong> Minato 1 Volcanic Ash (SjM-l; Kawamura <strong>and</strong><br />
Shiota, 2005)<br />
The volcanic ash layer was found by Kawamura <strong>and</strong><br />
Shiota (2005) at TP. -18.3 meters <strong>of</strong> KTG-l drilling-core in<br />
Minato area, <strong>Saijyo</strong> City. The upper part <strong>of</strong> <strong>the</strong> <strong>Syuso</strong><br />
Formation interbeds this volcanic ash layer occasionally.<br />
The layer is 1.1 meters in maximum thickness <strong>and</strong> found<br />
between TP. -22 <strong>and</strong> -17 meters.<br />
On <strong>the</strong> basis <strong>of</strong> similar lithological characteristics <strong>and</strong><br />
stratigraphy, SjM-1 beneath <strong>the</strong> plains can be corre<strong>late</strong>d<br />
with Aira-Tn Volcanic Ash (AT). The shape <strong>and</strong> refractive<br />
index <strong>of</strong> glass shards in AT (H>T <strong>and</strong> n=1.498 ~ 1.500;<br />
Machida <strong>and</strong> Arai, 1992,2003) are in good agreement with<br />
those <strong>of</strong> SjM-l (Table 2).<br />
Nyugawa Formation (new usage)<br />
(definition) The Nyugawa Formation is defined as a<br />
series <strong>of</strong> finer sediments beneath <strong>the</strong> coastal area <strong>of</strong> <strong>the</strong><br />
<strong>Saijyo</strong> Plain, which unconformably overlies <strong>the</strong> <strong>Syuso</strong><br />
Formation.<br />
(type locality) The stratotype section is between TP.<br />
-11.5 <strong>and</strong> -1.5 meters <strong>of</strong> <strong>the</strong> St.l Drilling-core (33 0<br />
55'44"N, 133°06'40"E, see Fig. 1), <strong>and</strong> <strong>the</strong> parastratotype<br />
section is between TP. -10 <strong>and</strong> -0.5 meters <strong>of</strong> <strong>the</strong> St.2<br />
Drilling-core (33°55'39"N, 133°06'28"E), Hiroe area,<br />
<strong>Saijyo</strong> City.<br />
(distribution) The formation is distributed just beneath<br />
<strong>the</strong> coastal area <strong>of</strong> <strong>the</strong> <strong>Saijyo</strong> Plain, at depth between TP.<br />
-20 <strong>and</strong> 3 meters (Fig. 4).
42 <strong>Lithostratigraphy</strong> <strong>of</strong> <strong>the</strong> <strong>late</strong> <strong>Quaternary</strong> <strong>under</strong> <strong>the</strong> <strong>Syuso</strong> <strong>and</strong> <strong>Saijyo</strong> Plains, Ehime Prefecture, southwest Japan<br />
N<br />
~ o<br />
2<br />
4km<br />
/~-- f{juchlnada ---------1<br />
Limit <strong>of</strong> distribution <strong>of</strong> <strong>the</strong> Nyugawa Formation<br />
Fig. 4 Distribution <strong>of</strong> <strong>the</strong> Nyugawa Formation<br />
Numbers beside contour lines indicate elevation <strong>of</strong> <strong>the</strong> basement <strong>of</strong> <strong>the</strong> Nyugawa Formation in T.P. meters.<br />
Numbers beside drilling site indicate station numbers in Fig.I. KI, DI, Gl, <strong>and</strong> G2 mean TYP-Kl, TYP-DI,<br />
KTG-l <strong>and</strong> KTG-2 in Fig.l respectively.<br />
(thickness) The formation is 20 meters in maximum<br />
thickness.<br />
(stratigraphy) This formation is lithologically<br />
subdivided into <strong>the</strong> lower, middle <strong>and</strong> upper parts, in<br />
ascending order (Fig. 5). The lower or middle part <strong>of</strong> <strong>the</strong><br />
formation abuts with <strong>under</strong>lying <strong>Syuso</strong> Formation.<br />
(llthology)<br />
lower part: The part comprises <strong>of</strong> s<strong>and</strong> or s<strong>and</strong>y gravel<br />
(Unit 1) less than 6 meters in maximum thickness.<br />
middle part: Thickness <strong>of</strong> <strong>the</strong> part ranges from 3 to 7<br />
meters. Accumulation <strong>of</strong> clay <strong>and</strong>/or silt (Units 4, 5, 6 <strong>and</strong><br />
7) characterizes most <strong>of</strong> this part. Beneath <strong>the</strong> nor<strong>the</strong>rn<br />
area <strong>of</strong> <strong>the</strong> plains, <strong>the</strong> part turns into s<strong>and</strong>y beds in general.<br />
Marine molluscan shells are contained in whole in <strong>the</strong><br />
mud, such as Mactra (M.) veneriformis, Paphia (P.)<br />
euglypta, <strong>and</strong> Babylonia japonica. Spines <strong>of</strong> Echinodea are<br />
also found (Kawamura <strong>and</strong> Shiota, 2005). The volcanic<br />
ash, <strong>Saijyo</strong> Minato 2 Volcanic Ash, is interca<strong>late</strong>d in <strong>the</strong><br />
part.<br />
upper part: This part mainly consists <strong>of</strong> non-mollusk<br />
bearing s<strong>and</strong> or gravel(Units 1, 8 <strong>and</strong> 8m) in <strong>the</strong> coastal<br />
area <strong>of</strong> <strong>the</strong> <strong>Saijyo</strong> Plain. The upper part, 5 meters thick at<br />
maximum, becomes thicker northward.<br />
(correlation) The lower <strong>and</strong> middle parts, <strong>and</strong> <strong>the</strong><br />
upper part can be corre<strong>late</strong>d with <strong>the</strong> middle <strong>and</strong> upper<br />
Alluvium <strong>of</strong> Kurihara (1972), respectively.<br />
<strong>Saijyo</strong> Minato 2 Volcanic Ash (SjM-2; Kawamura <strong>and</strong><br />
Shiota, 2005)<br />
The most distinctive lithological characteristic <strong>of</strong> <strong>the</strong><br />
lower part <strong>of</strong> <strong>the</strong> Nyugawa Formation is <strong>the</strong> interbedding <strong>of</strong><br />
a volcanic ash layer, <strong>Saijyo</strong> Minato 2 Volcanic Ash (SjM-2;
Norihito KAWAMURA <strong>and</strong> Kiminori MINAKUCHI 43<br />
3-<br />
St. 1 St. 2 St. 3 St. 4 St. 5<br />
1-<br />
0-<br />
-1-<br />
--..,<br />
--....::..: line<br />
correlation<br />
~ artificial<br />
materials<br />
C<br />
B<br />
clay<br />
silt<br />
-5-<br />
[IT] fine ........<br />
s<strong>and</strong><br />
-6 -<br />
-7 -<br />
-8 -<br />
-10-<br />
-9-<br />
medium<br />
0... ,-<br />
s<strong>and</strong><br />
CJ<br />
.... coarse<br />
s<strong>and</strong><br />
CJ gravel<br />
-11-<br />
-13-<br />
-12-<br />
-14-<br />
-15 -<br />
c o<br />
-16- .~<br />
-17 -<br />
-18-<br />
-19 -<br />
E o<br />
lJ..<br />
o<br />
:J '" >-<br />
UJ<br />
I·:·/l 1/-::<br />
IIIIIIiI ~;u~I<br />
c<br />
.., o<br />
'"E o<br />
lJ..<br />
o<br />
:J '" ><br />
UJ<br />
.... L- '--<br />
[ZJ clayey<br />
[Z] silty<br />
[Z] s<strong>and</strong>y<br />
C2J<br />
• gravely<br />
volcanic<br />
ash<br />
gypsum<br />
• crystal<br />
-20-<br />
-21-<br />
SjM-!<br />
M<br />
marine<br />
mollusca<br />
-22 -<br />
SjM-l<br />
SjM-2<br />
<strong>Saijyo</strong> Minato<br />
1,2 Volcanic<br />
Ashes<br />
Fig. 5 Geological columnar maps in <strong>the</strong> <strong>Syuso</strong> <strong>and</strong> <strong>Saijyo</strong> Plains<br />
Abbreviations <strong>of</strong> <strong>the</strong> drilling sites are same as Fig. 1<br />
Kawamura <strong>and</strong> Shiota, 2005). The layer widely occurs<br />
beneath <strong>the</strong> coastal area <strong>of</strong> <strong>the</strong> plains between TP.<br />
-16 <strong>and</strong> -5 meters with a maximum thickness <strong>of</strong> 1 meter<br />
<strong>and</strong> becomes a useful key bed.<br />
SjM-2 is a light brownish-gray, fine-grained s<strong>and</strong><br />
sized vitritic volcanic ash. It becomes light brownish-gray<br />
in color after purification.<br />
Possible correlatives <strong>of</strong> SjM-2 in <strong>the</strong> plains are two<br />
widespread volcanic ashes, Kikai-Akahoya Volcanic Ash<br />
(K-Ah, n=1.508 ~ 1.516 ; Machida <strong>and</strong> Arai, 1992,2003)<br />
or Ata Volcanic Ash (Ata, n=1.508 ~ 1.512; Machida <strong>and</strong><br />
Arai, 1992, 2003). Considering <strong>the</strong> stratigraphy <strong>of</strong> SjM-2<br />
above SjM-l that is corre<strong>late</strong>d with AT, SjM-2 can be<br />
corre<strong>late</strong>d with K-Ah.<br />
On <strong>the</strong> basis <strong>of</strong> <strong>the</strong> elevations <strong>and</strong> stratigraphy <strong>of</strong> this<br />
volcanic ash layer, SjM-2 can be corre<strong>late</strong>d with <strong>the</strong><br />
volcanic ash in <strong>the</strong> upper Delluvium <strong>of</strong> Kitazaki <strong>and</strong><br />
Hiyoshi (1965) <strong>and</strong> also with <strong>the</strong> volcanic ash layers that<br />
Kurihara (1972) recognized at T.P. -15 <strong>and</strong> -20 meters.<br />
VII. Interpretation <strong>of</strong> sedimentary environments <strong>of</strong><br />
lithological units<br />
Kawamura <strong>and</strong> Shiota (2005) interpreted <strong>the</strong><br />
sedimentary environments <strong>of</strong> <strong>the</strong> lithological units <strong>of</strong><br />
drilling-core samples in <strong>the</strong> <strong>Saijyo</strong>-Minato area.<br />
According to Kawamura <strong>and</strong> Shiota (2005), we can also<br />
interpret <strong>the</strong> depositional environments <strong>of</strong> <strong>the</strong> revised<br />
lithological units beneath <strong>the</strong> <strong>Syuso</strong> <strong>and</strong> <strong>Saijyo</strong> Plains<br />
(Table 2).
44 <strong>Lithostratigraphy</strong> <strong>of</strong> <strong>the</strong> <strong>late</strong> <strong>Quaternary</strong> <strong>under</strong> <strong>the</strong> <strong>Syuso</strong> <strong>and</strong> <strong>Saijyo</strong> Plains, Ehime Prefecture, southwest Japan<br />
For <strong>the</strong> interpretation <strong>of</strong> sedimentary environments,<br />
<strong>the</strong> authors follow <strong>the</strong> ecological data <strong>of</strong> mollusk-like<br />
vertical distribution after Higo <strong>and</strong> Goto (1993) <strong>and</strong> Higo<br />
et al. (1999).<br />
Units 1 to 3: The Lithological Unit 1 was infelTed as river<br />
channel sediments because <strong>of</strong> resembling <strong>the</strong> Facies Code<br />
Gm <strong>of</strong> Miall (1978). Likewise, Unit 2 was inferred to be<br />
back marsh sediments, which can be corre<strong>late</strong>d with Facies<br />
Code Fsc <strong>of</strong> Miall (1978). Inverse grading <strong>of</strong> mud to very<br />
fine s<strong>and</strong> <strong>and</strong> mud to medium s<strong>and</strong> is formed at back marsh<br />
<strong>and</strong> natural levee when a river floods (Masuda <strong>and</strong> Iseya,<br />
1985). So Unit 3, which includes inverse grading, was also<br />
inferred as natural levee or back marsh sediments.<br />
Units 4 <strong>and</strong> 4a: Units 4 <strong>and</strong> 4a are back marsh sediments<br />
(Kawamura <strong>and</strong> Shiota, 2005). The alternating beds <strong>of</strong> clay<br />
<strong>and</strong> silt <strong>of</strong> Unit 4a may indicate <strong>the</strong> repetition <strong>of</strong> flash<br />
floods <strong>and</strong> consequent swamp formation.<br />
Unit 5: The lower part <strong>of</strong> Unit 5 was inferred as brackish<br />
or sea water marsh sediments because <strong>the</strong> drilling-core<br />
samples corresponding to this part freq uently yielded<br />
gypsum crystals that indicate marine sediments<br />
(Itihara,1960).<br />
Unit 6: This unit bears abundant species <strong>of</strong> a molluscan<br />
assemblage that consist mainly <strong>of</strong> intertidal zone <strong>and</strong> upper<br />
shallow sea mollusks. Some <strong>of</strong> mollusks are inner-bay<br />
species; e.g. Paphia (Neotapes) undulata, <strong>and</strong> Veremolpa<br />
micra. As for micr<strong>of</strong>ossils, brackish foraminifer <strong>and</strong> some<br />
inner-bay ostracods are found from this unit. It is<br />
concluded from <strong>the</strong> fossil evidence that Unit 6 is bay-head<br />
sediments as infelTed by Kawamura <strong>and</strong> Shiota (2005).<br />
Unit 7: Inteliidal zone to upper shallow sea mollusks are<br />
also found from this unit. Therefore, Unit 7 is inferred to<br />
be <strong>the</strong> sediments formed <strong>under</strong> <strong>the</strong> conditions between<br />
intertidal <strong>and</strong> subtidal zones.<br />
Unit 8: No evidence was found useful for <strong>the</strong><br />
reconstruction <strong>of</strong> sedimentary environment. As it overlies<br />
Unit 7, Unit 8 may be considered as sediments formed at<br />
beach ridge.<br />
Unit 8m: This unit bears intertidal <strong>and</strong> subtidal zone<br />
mollusks like Crassostrea gigas. We can assume <strong>the</strong> unit<br />
was formed in <strong>the</strong> intertidal to subtidal zone.<br />
VIII. Age <strong>and</strong> sedimentary process <strong>of</strong> <strong>the</strong> <strong>Syuso</strong><br />
<strong>and</strong> Nyugawa Formations<br />
<strong>Syuso</strong> Formation<br />
Most <strong>of</strong> <strong>the</strong> formation formed at river channels <strong>and</strong> in<br />
flood plains because this formation is comprised <strong>of</strong> <strong>the</strong><br />
Lithological Units 1,2,3,4, 4a <strong>and</strong> 5 (Fig. 4). These<br />
sediments would have deposited as river systems <strong>of</strong> <strong>the</strong><br />
paleo-Nakayamagawa <strong>and</strong> paleo-Kamogawa Rivers.<br />
The upper part <strong>of</strong> <strong>the</strong> <strong>Syuso</strong> Formation bears SjM-l<br />
Volcanic Ash. It is corre<strong>late</strong>d with AT which erupted<br />
between 26 ka <strong>and</strong> 29 ka (Machida <strong>and</strong> Arai, 2003).<br />
Accordingly, <strong>the</strong> formation had been fonned during <strong>the</strong> <strong>late</strong><br />
Pleistocene until about 26 ka.<br />
Nyugawa Formation<br />
The lower part, composed <strong>of</strong> <strong>the</strong> Lithological Unit 1,<br />
developed in <strong>the</strong> nor<strong>the</strong>rn area <strong>of</strong> <strong>the</strong> plains <strong>and</strong> was<br />
formed <strong>under</strong> <strong>the</strong> river system by <strong>the</strong> paleo-Nakayamagawa<br />
River. The lower part is made <strong>of</strong> <strong>the</strong> Lithological Units 6<br />
<strong>and</strong> 7, <strong>and</strong> <strong>the</strong> latter two units are regarded as <strong>the</strong> bottomset<br />
<strong>and</strong> foreset <strong>of</strong> <strong>the</strong> present delta system <strong>of</strong> <strong>the</strong><br />
Nakayamagawa River. During deposition <strong>of</strong> <strong>the</strong> lower part,<br />
<strong>the</strong> river mouth would have been located around <strong>the</strong><br />
northwestern area in <strong>the</strong> <strong>Syuso</strong> Plain, although <strong>the</strong><br />
Nakayamagawa River flows into <strong>the</strong> Hiuchinada at <strong>the</strong><br />
eastern edge <strong>of</strong> this plain (Fig. 1). This is suggested by<br />
coarser grain-size distribution <strong>of</strong> <strong>the</strong> lower part in <strong>the</strong> area<br />
that may result from <strong>the</strong> sediment influx from <strong>the</strong> river.<br />
The upper part, made up <strong>of</strong> Lithological Units 1 <strong>and</strong>/or 8<br />
<strong>and</strong> overlies <strong>the</strong> forset beds <strong>of</strong> <strong>the</strong> delta, is considered as<br />
<strong>the</strong> topset <strong>of</strong> <strong>the</strong> delta system <strong>of</strong> <strong>the</strong> Nakayamagawa or <strong>the</strong><br />
Kamogawa River.<br />
The lower part <strong>of</strong> <strong>the</strong> Nyugawa Formation interca<strong>late</strong>s<br />
SjM-2 Volcanic Ash that is corre<strong>late</strong>d with K-Ah, which<br />
erupted at approximately 7.3ka (Machida <strong>and</strong> Arai, 2003).<br />
Judging from <strong>the</strong> eruption age <strong>of</strong> K-Ah, <strong>the</strong> lower part had<br />
been formed during <strong>the</strong> early to middle Holocene, <strong>and</strong> <strong>the</strong><br />
upper part have deposited since <strong>the</strong> middle Holocene.<br />
IX. Conclusions<br />
Lithostratigraphical division <strong>of</strong> subsurface geology in<br />
<strong>the</strong> <strong>Syuso</strong> <strong>and</strong> <strong>Saijyo</strong> Plains is as follows: <strong>the</strong> Pleistocene<br />
<strong>Syuso</strong> Formation <strong>and</strong> <strong>the</strong> Holocene Nyugawa<br />
Formation(<strong>the</strong> lower, middle <strong>and</strong> upper parts, in ascending<br />
order).<br />
<strong>Saijyo</strong> Minato 1 Volcanic Ash (SjM-l) is interca<strong>late</strong>d<br />
in <strong>the</strong> upper part <strong>of</strong> <strong>the</strong> <strong>Syuso</strong> Formation <strong>and</strong> is con-e<strong>late</strong>d<br />
with <strong>the</strong> Aira-Tn Volcanic Ash. <strong>Saijyo</strong> Minato 2 Volcanic<br />
Ash (SjM-2) in <strong>the</strong> middle part <strong>of</strong> <strong>the</strong> Nyugawa Formation<br />
is a good key bed <strong>and</strong> is corre<strong>late</strong>d with <strong>the</strong> Kikai-Akahoya<br />
Volcanic Ash.<br />
The upper part <strong>of</strong> <strong>the</strong> <strong>Syuso</strong> Formation would have<br />
been deposited until around 26ka at river channels or in<br />
flood plains. The Nyugawa Formation occurred at around<br />
7.3ka <strong>and</strong> had been formed through <strong>the</strong> Holocene mainly as<br />
deltaic sediments by <strong>the</strong> Nakayamagawa <strong>and</strong> <strong>the</strong><br />
Kamogawa Rivers.
Norihito KAWAMURA <strong>and</strong> Kiminori MINAKUCHl<br />
45<br />
Acknowledgements<br />
The authors would like to thank Dr. Muneki Mitarnura<br />
<strong>of</strong> Osaka City University for permitting <strong>the</strong> use <strong>of</strong> facilities<br />
<strong>of</strong> Natural History <strong>of</strong> Anthropogene Laboratory for help in<br />
<strong>the</strong> measurement <strong>of</strong> refractive index <strong>of</strong> volcanic glass<br />
shards. Gratitude is also due to <strong>the</strong> following public <strong>of</strong>fices<br />
<strong>and</strong> drilling companies: <strong>the</strong> <strong>Saijyo</strong> Office <strong>of</strong> Water for<br />
Industrial Use, Ehime prefecture, <strong>the</strong> Shikoku Branch <strong>of</strong><br />
Japan Society <strong>of</strong> Civil Engineers, Naiba Co. Ltd., Canaan<br />
Georesearch Inc., <strong>and</strong> Gijyutsu Kaihatsu Inc., for providing<br />
borehole samples <strong>and</strong> logs. The authors would like to<br />
express <strong>the</strong>ir deepest gratitude to Drs. Shigehiro Kato<br />
(Museum <strong>of</strong> Nature <strong>and</strong> Human Activities, Hyogo) <strong>and</strong><br />
Muneki Mitamura for reviewing <strong>and</strong> improving <strong>the</strong><br />
manuscript.<br />
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Revised m.anuscript accepted Decem.ber 25, 2007.