Title Ryukyu Limestone of Okinawa-Jima, South …...KoNisHi et al,, 1970; KoNisHr et al., 1974)...

Title Ryukyu Limestone" of Okinawa-Jima, South Japan : AStratigraphical and Sedimentological Study

Author(s) Takayasu, Katsumi

Citation Memoirs of the Faculty of Science, Kyoto University. Series ofgeology and mineralogy (1978), 45(1): 133-175

Issue Date 1978-07-31

URL http://hdl.handle.net/2433/186623

Right

Type Departmental Bulletin Paper

Textversion publisher

Kyoto University

MEMolRs oF THE FAcuLTy oF SclENcE, KyoTo UNIvERsny, SERIEs oF GEoL. & MINERAL., Vol. XLV, No. 1, pp, 133-175, Pls. 14--16, 1978

"Ryukyu Limestone" of Okinawa-jima, South Japan

A Stratigraphical and Sedimentological Study

By

Katsumi TAKAyAsu

(Text and Plates)

Abstract

The "Ryukyu Limestone", the host of the Quaternary geology of the Ryukyu Islands, is dividedfundamenta!ly into the rnain limestones and the accessory limestones. This division is based on thedifferences in lithological characters, fossil assemblages, distribution and thickness ofbeds, age ofsedimen.

tation, etc. The rnain limestones composing the major part of the "Ryukyu Limestone" constitute theRyukyu Group together with the contemporaneous noncalcareous deposits, and they are the basement of

the accessory limestones.

The Ryukyu Group is divided stratigraphically into two formations; the Lower and the Upper•The Lower Formation is the sediments which accumu!ated in the basins of the tectonic origin or in the

valleys of the eroded basement and is represented by various lithofaÅëies. On the other hand, the Upper

Formtion is more widely distributed than the Lower Formation, and its stratigraphical sequence of the

lithofacies is rather constant everywhere. The Upper Formation is subdivided into the A, B and CMembers based on characteristicfossil assemblages, Within these Members, theA Memberis represented

by some foraminiferal and molluscan fossils showing deeper water than a coral reef environment. Fur.thermore, the Cptcloe(ypm'-OPerculiua bed of this Member, being fairly continuous, is a good horizon marker

for the correlation of the Ryukyu Group not only in Okinawa-iima but also other islands. The biolithite

facies, though poor in the Ryukyu Group as a whole, is recognized in the C Member of the Upper Forma-

tion and a part of the Lower Formation. In order to analyse the sedimentary environments of the Ryukyu Group, the petrography and thefossil analysis under the optical microscope were made in connection to the field observation. As a result,

the sedimentary environments of the Ryukyu Group can be generally inferred to have been shifted fromthe land prevailing condition disqualified for coral reef formation to the favourable condition for reef

building, though the process of reef forming of the latter was interrupted for a short time by the intervcn-

tion of deep sea environment as represented by the A Member in the Upper Formation. As for the age of the Ryukyu Group, it is regarded to be Early and early Middle Pleistocene from the

paleontological and stratigraphical aspects. The geologic stmcture of the Ryukyu Group is composed

of many tilting blocks brought in by faulting after the completion of sedimentation. These tectonic

movements of the maximum stage brought about the original configuration of the present islands. Since

then, to the fringing areas of these emerged blocks the accessory limestones represented by the Minatogawa

Limestone Formation were attached. However, it is also evident that mainly the tectonic movementsaffected the mode of the Ryukyu Group sedimentation such as !ocal unconformities between the Lower

and the Upper Formation and accelerated the change to shallow water of the sedimentary environment. Therefore, the Ryukyu Group is the product of transgression due to tectonism, especially shown in

the later stage. Contrasting with- the main limestone deposition, the accessory limestones wcre thedeposits formed under the influence of the eustatic sea level changes.

134 Katsumi TAKAyAsu

Lastly, the Quaternary geohistory of the Ryukyu Islands is reviewed from the viewpoint of the"Ryukyn limestone" stratigraphy.

I. Introduction

One of the most peculiar features in the geology of the Ryukyu Islands is the wide

distribution of the Quaternary limestone, collectively called the "Ryukyu Limestone".

The name "Ryukyu Limestone", originally spelled Riukiu Limestone, was first proposed

by YABE and HANzAwA (1930) with regard to distinctions among raised coral reefs.Thereafter, by a series ofworks, especially by the paper of 1935, HANzAwA was highly

esteemed for carrying out a detailed description of the geology of the Ryukyu Islands.

Owing to HANzAwA's works, many researchers first recognized the significance of the

"Ryukyu Limestone" in considering the geology of the Islands.

The studies on the "Ryukyu Limestone", however, together with the studies on

the other geological formations, were forcibly interrupted for a long time by un-fortunate circumstances in the Ryukyus such as the years of the Second World Warand the succeeding occupation. After the War, with the remarkable progress of the

geological sciences, several new interpretations and methods were rapidly introduced

into the studies of the "Ryukyu Limestone" with in suthcient stratigraphical andsedimentological data.

During the time under occupation, the "Ryukyu Limestone" of Okinawa-jimawas subdivided into three formations, that is, the Naha formation (the Naha limestone),

the Yontan and the Machinato limestones ascendingly, and they were collectivelycalled the Ryukyu Group in the reports on military geology by U.S. Army Forces and

associated studies (FuNT et al., 1959; MAcNEiL, 1960). NAKAGAwA (1967; 1969a, b),who surveyed in Tokuno-shima and its neighbouring islands, Kagoshima Prefecture,

applied the concept of sea level changes obtained from the studies at the Northeast

Japan coast to the study of the Ryukyu Islands, and he regarded each formation of

the Ryukyu Group as the terrace deposits which are correlated with each other by the

altitude of their distribution. In this sense, the studies of HiRATA (1956, 1958) and

YAMAzATo (1959, 1960) are similar to NAKAGAwA's work. They also divided the"Ryukyu Limestone" on the basis of the altitude of the distribution, and tried to

compare the topographical and the ecological characters of the living coral reefs with

those of the "fossil reefs". Further, KoNisHi and his colleagues (KoNisHi et al., 1967;

KoNisHi et al,, 1970; KoNisHr et al., 1974) applied for the first time the radiometry of

Th230 and Pa23i growth methods for the dating of the "Ryukyu Limestone", and theyinsisted in the explanation that neotectonism is a more effective gauge than eustatic

sea level changes. They stated little about the stratigraphy and sedimentology of

the limestones, but it seems that they subscribe to ToKuNAGA's opinion (1901), thelimestones of the Ryukyu Islands are composite of raised coral reefs.

It is also true that several researchers have the opinion that the sedimentary

"Ryukyu Limestone" of Okinawa-jima, South Japan 135

environment of the "Ryukyu Limestone" was not always so shallow as the coral reefs

environment; HANzAwA (1948) already pointed out from the viewpoint of the studyon larger foraminifers that sedimentation underwent at the depth of not shallow but

of moderate.

Since the retrocession of"Okinawa" in 1972, manyJapanese geologists have muchinterest in the study of the "Ryukyu Limestone", and a lot of information has been

accumulated. YAzAKr (1976) and OyAMA (1976) reported the presence of deep seamo!luscan fossils from the "Ryukyu Limestone" of Miyako-jima, Furthermore, theOKINAwA QuATERNARy REsEARcH GRoup (1976) clarified that most of the "RyukyuLimestone" has nothing to do with the terrace deposits, though only a few limestones

younger than the Middle terrace can be the object of the topographical correlation.

Putting a heavy emphasis on this point, the writer stated in his paper dealing with

the Quaternary limestones ofthe Motobu Peninsula, Okinawa-jima (TAKAyAsu, 1976a),that the name of the Ryukyu Group should be used for the main part of the "Ryukyu

Limestone" and the sediments of their contemporaneous facies other than youngerterrace limestone. OKiMuRA (1976) summarized his work on the Quaternary geologyof Yoron-jima, Kagoshima Prefecture, in the same manner as the writer's proposal.

On the age of the main part of the Ryukyu Group or the "Ryukyu Limestone",there are divergences of opinion; some geologists regard it as the Middle and LatePleistocene terrace deposits or the raised coral reefs, while others regard it as the

Pliocene or the Early to Middle Pleistocene basement rocks.

In such circumstances, it was requested at first to establish an accurate stratigraphy

of the "Ryukyu Limestone" as far as possible and to criticize those various opinions

concerning the "Ryukyu Limestone". The writer started his work as a member ofthe Okinawa Quaternary Research Group in 1973, and surveyed the "RyukyuLimestone" of about twenty islands in Okinawa Prefecture. An outline of the results

obtained by the Research Group and the writer has been reported in several articles

(OKiNAwA 9uATERNARy REsEARcH GRoup, 1976; KizAKi and TAKAyAsu, 1976;OKrMuRA and TAKAyAsu, 1976; TAKAyAsu, 1976a, b). In this paper, the writer gives

more detailed stratigraphical and sedimentological considerations on the "RyukyuLimestone", with special reference to the Ryukyu Group of Okinawa-jima. TheQ;uaternary geohistory of the Ryukyu Islands is briefly discussed from the viewpoint

of the "Ryukyu Limestone".

II. GeologicOutlineofOkinawa-jima

A. Topography Okinawa-jima, the largest islands of the Ryukyu Islands, is located at the middle

ofthe Islands which extend from 310N lat. to 240N lat. (Fig. 1). From the viewpoint

of geography, this island is divided into several units as shown in Fig. 2.

136 Katsumi TAKAyAsU

sc..11e ----iifiS,iiSi

32' K3"bNlt-

)F 7 ;,`(r....

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12r 124."-- - 1---"e 1' i' 1 ji F -.Li -l. I•'rns"Xt'j 1

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KYUSI ' icEIL,,,, L.==..),- 1

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1-f s, Daito•stvtoi vPrefecture

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Fig. 1. Map showing Iocation of the Ryukyu Islands. The Islands are divided geographically into

three parts; the Northern Ryukyus (Osumi-shoto), the Central Ryukyus (Amami- andOkinawa-shoto) and the Southern Ryukyus (Sakishima-shoto consisting of Miyako- andYaeyama-shoto). The numbers in the map are index for Table 4 in Chapter III; 1. 0kinawa-jima, 2. Yoron-jima, 3. Kume-jima, 4. Aguni-jima, 5. Miyako-jima, 6. Tarama-jima, 7.Ishigaki-jima, 8. Taketomi-jima, 9. Kohama-jima, 10. Iriomote-jima, 11. Hateruma-jima,12. Yonaguni-jima. (shoto; islands,jima; island)

The topographical divisions of Okinawa-jima are summarized as shown in Table1. Among these topographical divisions, the terraces are classified into the Higher,

the Middle and the Lower in accordance with their surface altitudes. The Higher

terraces are distributed in Kunigami and Motobu areas, Northern Okinawa, andusually covered by so called "Kunigami gravels". HANzAwA (1935) considered thatgravels as the deposits overlying the "Ryukyu Limestone", but his "Kunigami gravels"

is known as the collective name for gravel deposits of various stages (NAKAGAwA,1969b; TAKAyAsu, 1976b). It is conspicuous that the height of the surfaces lower than of the Middle terrace

of the Minatogawa Plane are rather constant without any local variation. This ischaracteristic of the terrace topography throughout Okinawa-jima as well as in otherislands.

In addition, it is unique in Okinawa-jima that the land form of the "Ryukyu


H -m;saki

le-shima

c>se.s.jSAO.

Yarna -on n

Northern partof Motoby Peninsula

MOTOBU AREA

1sin-y

NA t.ltrgldi,Åë.."Noj

CENTRA OKlNAW {Nakagami courity) kimi

NSOUTHERN OKI NAWA CShlncjiri County)

Fig. 2.

Kin-wan shlkawa

.- t/rr."ine.?&.Xps•st

elkoi-jima

Miyagi-jirna

im"jimoOKINAWA VHcLrnahiga-jima(KOZA) Gushikawa-Kats'

urn Nakagusuku eTsuken-jirna , .wan

iprKuctc ka-jima

10Krn SOUtheef paoktt•inanNa-jima -

Index map of Okinawa-jima showing location of study areas.

Limestone" is represented by mesa and cuesta topographies in southern part (Pl, 14,

Fig. 1), and butte in central part. It may due to the effect of lithologic or tectonic

control otherwise. Furthermore, the limestone walls observed in everywhere along

cliffs are considered to be case hardening ofexposed limestone beds (FuNT et al., 1953).

B. Geology The basement rocks of the "Ryukyu Limestone" are composed of various sedi-mentary rocks and small intrusive rocks ranging from Permian to Neogene (Fig. 3).They are arranged zonally and divided structurally from inner (continental side) to

outer (oceanic side), the Motobu belt (Permian and Triassic), the Kunigami belt(Jurassic?-Eocene) and the Shimajiri belt (Neogene) respectively (KoNisHi, 1965).

Among them, the Shimajiri Group of the Shimajiri belt is the most important de-

posit in relation to the "Ryukyu Limestone". It is more than 2,OOO rn in total thick-

ness, and is composed of thick siltstone with intercalations of sandstone and tuff(FuKuTA et al., 1971). According to the studies ofplanktonic foraminifers and calcare-


Table 1. Classification oflandforms ofOkinawa-jima

Topographic division

Mountain land

Hilll and

Limestone landforms(mesa, cuesta, butte)

Terrace

Higher

Middle

Lower

Coastal landforrns(bench, dune, reef,mangrove swamp)

Type area

Northern Okinawa(Kunigami, Motobu)

Central and Southern Okinawa

Central and Southern Okinawa

Northern Okinawa(Kunigami, Motobu)

aoastal area

Altitude

496 m (Yonaha-dake)453 m (Yae-dake)

Less than 100m

The highest point is198 m

80-200 m

'" 4'O'-6o ifi'' "20-30m (MinatogawaPlane)

10-15m5-7 m

Less than 5 m

Geology

Pre-Neogene basement rocks

Shimajiri Group

Ryukyu Group (the main partof the "Ryukyu Limestone")

"Kunigami gravels"

Gravels and the accessory partof the "Ryukyu Limestone"

Postglacial raised 6oral reeflimestone, calcareous beachdeposits, alluvial clay, etc.

a 10 km

-ILL,-.o:

` 11 1,• e-I-lil /.

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)

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'

Fig. 3. Geologic outline ofOkinawa-jima. I; Motobu belt Nakijin Formation, Trias), II; Kunigami belt (the Nago Eocene), III; Shimajiri belt (the Shimajiri Group, Neogene).

on KoNisHi (1965).

OAIIuvium

'/''/' Ryukyu G. t Terrace deposits

EllilZ sh;mdjiri G.

Iii.Il Nage F. g )cayo F.

Eiliilll Nakij,n E

OIIrelm Motobe G,

(the Motobu Group, Permian, and the and the Kayo Formations, Jurassic?- The name of each belt is based


Table 2. Stratigraphic sequence of the QLuaternary System of Okinawa-jima (Takayasu, 1976b)

FLINT et at,"gsg) TAKAyAsu "g76b) eKww/NAWA 8tU l]M/gW6)x8g

:g2

wzur

U

o

"

a

Beach deposits

Machinato Ls.

Yontan Ls.

Kunigami Grcrvet

''

'

Nbha E

/Chinen Sand

Guga Grave(

Shimoiiri F,

:. ---. .NxNs

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RecentCoralReet,etc.

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'LowerTerracet6J

III

gEgE,

(VeneerType)

(VeneerType) t-51

.x--xit

MinatogawaLs.E(AccumutationType)t40)

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t60.1

'li'

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Raised Caul Reel

'M'

s

Lower Terraoe Ls,orgfqvet

"Awaishi'Ls. ?'5ango'Ls.

Yemiten Ls.

Upper TerTuoe Gtvvel

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Atternated"Ls.

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ttokazu Ls.

Shimajiri F.

xi2

ul

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ous nannoplanktons, the Shimajiri Group is regarded as the sediments of the LateMiocene to Early Pleistocene (NAToRi et al., 1972; NisHmA, 1973; IBARAKi and TsuaHi,

1976; NAToRi, 1976). As the result of geological re-examination (TAKAyAsu, 1976b), the QuaternarySystem of Okinawa-jima was summarized as in Table 2.

III. Stratigraphical Study of the "Ryukyu Limestone"

A. GeneralStratigraphy1. Fundamental division ofthe "Ryukyu Limestone" The writer proposed tentatively to divide the "Ryukyu Limestone" into theRyukyu Group and the "terrace limestones" (TAKAyAsu, 1976a, b). Though thisconcept is still valid, the terminology of "terrace" seems to cause confusion in the

definition. Therefore, in this paper the writer adopts the new term ofaccessor2 limestones,

i.e. the accessory part of the "Ryukyu Limestone", instead of the "terraec limestones".

The Minatogawa Limestone distributed in Southern Okinawa is a representative ofthe accessory limestones in Okinawa-jima. The limestones of the Ryukyu Group arecalled the main limestones, i.e. the main part of the "Ryukyu Limestone" excluding

the accessory limestones. The reason why the "Ryukyu Limestone" should be divided

into two parts would be understandable in Table 3. ,


Table 3. Comparison between the main limestones and the accessory limestones

Distribution

Thickness

Lithology

Fossils

Identity ofstratigraphy

Preservation of depositional surface

Structure

Main limestones

widely (overlapping whole area of theShimajiri Group and also moderate areaof the pre-Neogene basements).

about 100 m or more.

bioclastic limestone for the most partand biolithite in part. micrite matrixrich.

including some fossils showing not suchshallow water as the coral reef can beformed.

fairly continuous laterally in accordancewith the more upper horizon.

extremely poor

faulted and tilted. more than 20e in dipnear fault.

Accessory limestones

limited (accessory distribution at thecircumference of each island).

less than 40 m.

biolithite rather predominant. sparrycalcite cement rich.

representatives at shallow Envir6iifiiEn-t-"similar to the littoral zone aroundpresent islands.

not clear as its limited distribution.

clear as the terrace topography

scarcely disturbed by tectonic move-ments.

Besides, following the definition of YABE and HANzAwA (1930), the name of"Ryukyu Limestone" is adopted for collective use to the Pleistocene limestones*excluding Postglacial raised coral reef limestone.

2. The Ryukyu Group-the main limestones and their contemporaneous sediments In consideration of the lithofacies and the fossil assemblages, the Ryukyu Group

is divided stratigraphically into two formation units, the Lower and the Upper.Furthermore, the Upper Formation is subdivided by the characteristic fossil assemblages

into three members ofthe A, B and C ascendingly (TAKAyAsu, 1976a, b). The LowerFormation is the sediment which accumulated in the tectonic basins or in the valleys

of the eroded basements. Therefore, the thickness of the Formation varies from place

to place. The lithofacies of the Lower Formation also vary from noncalcareous tocalcareous facies according to the basement topography and the quantity of theterrigeneous fragments. But, in general, it becomes more calcareous towards the upper

part and at the top the coralline and/or algal biolithite facies are observed distinctly.

The Upper Formation overlies the Lower Formation conformably, and is distri-

buted more widely than the Lower Formation. The unconformity between thebasement and the Upper Formation is rather smooth as compared with the relationbetween the Lower Formation and the basement. From the evidence ofoverlapping,it is clear that the depositional area of the Upper Formation spreaded out successively.

Therefore, the lateral facies change is not conspicuous in comparison with the Lower

* Though HANzAwA (1935) considered the age of the "Ryukyu Limestone" to be the Pliocene to Early

Pleistocene, the present researchers regard it as the Pleistocene based on the relation to the Shimajiri

Group.


Formation. In the lowest part of the Upper Formation, the CLycloclL7Peus-OPerculina beds in the

limestone facies are fairly continuous laterally, and form a good horizon marker.The Operculina bed occupies a position slightly lower than the C2clocl)Peus bed generally,

but sometimes either one is missing.

Cl!clocllpeas assemblage is dominated by C. carPenteri. According to HANzAwA(1948), this larger foraminifera is usually known at the depth of about 100 m in the

present sea bottom, and some other larger foraminifers accompanied with it are also in-

habitants of the moderate depth. Therefore, he considered that the "Ryukyu Lime-stone" was deposited at deeper environment than the depth of coral reef formation.In addition to this, a bivalve, Plicatula mulicata, is sometimes found at a horizon immedi-

ately above the Clclocl!Petts bed. This bivalve is also present in the "Ryukyu Lime-

stone" of Miyako-jima. The molluscan assemblages of the Plicanta horjzon arerepresented by shells which are considered to be indicative of the depth between 60

and IOO m (OyAMA, 1976). From the facts mentioned above, the C)clocl7Peus bed and its associated horizons are

significant to consider the sedimentary environment ofthe Ryukyu Group.

The writer called the beds of OPerculina and Clclocl"Peusas the A Member. In the

lower half of the A Member are often some terrigeneous fragments, and in a part of the

Operculina swarm, generally about two meters in thickness, are sometimes intercalatedone or two thin layers of the tenigeneous reddish brown clay.

The A Member is overlain by the B Member which is cheracterized by the presenceof algal limestone. The algae appears to be ball-like in many cases, but sometimes is

broken into fine fragments in situ. In the B Member, the number of fossil species in-

creases upward, while the content of terrigeneous fragments decreases more and more.

The C Member covers the B Member, and is represented by coralline bioclastic limes-

tones. In some areas it changes laterally into coralline biolithites.

3. The Minatogawa Limestone Formation-as a representative of the accessory lime-

stones The Minatogawa Limestone corresponds roughly to the Machinato limestone ofFuNT et al. (1959). But now, the topographical situation of type locaiity ofthe latter

has been strongly changed artificially and moreover is hard to access owing to the mili-

tary establishments. Therefore, the writer made the re-definition of that limestone

formation in Minatogawa, Southern Okinawa, and requests to shift the type locality

(TAKAyAsu, 1976b).

The Minatogawa Limestone Formation fi11ed Iow relief on the erosion surface of

both the Ryukyu Group and Shimajiri Group with remarkable unconformities. TheFormation is divided lithologically into the Lower and the Upper Members. TheLower Member represented by coralline biolithites usually contains the huge limestone

blocks of the Ryukyu Group. The Upper Member conformably overlying the Lower


Member is represented by foraminiferal calcarenite or calcirudite with many roundedintraclasts.

The distribution of the Minatogawa Limestone is followed up to about 30 m above

sea level continuously, and forms the Minatogawa Plane of20-30 m in height. Besides,

it must be mentioned that even at some places of 55-60 m in height, other limestones

which look like the Upper Member of the Minatogawa Formation distributed in asmall amount near the distribution area of the Minatogawa Limestone Formation.The relation between those two limestones is not yet clarified.

As for other accessory limestones, the small distributions ofcoralline biolithite andl

or algal ball limestones are observed on the Lower surfaces of 10-15 m and 5-7 m in

height respectively. Their thickness is generally less than 5 m.

B. LocalStratigraphyoftheRyukyuGroup!. NorthernpartofMotobuPeninsula

The writer already reported on the Quanterary limestones ofthis area (TAKAyAsu,

1976a), and the summary will be mentioned briefly in this article.

The lithofacies ofthe Lower Formation differs with each other between the eastern

area (Nakijin Village area) and the western area (Motobu Town area) (Fig. 12a, inChapter IV). In the eastern area, it is represented by calcaroeus sand beds called the

Untensandbeds. The Unten sand beds become more calcareous westwardly. Thelower Formation of the western area is stratified bioclastic limestones of the Urasaki

limestone beds. It should be noted that the Urasaki limestone beds reveal sometimes

remarkable cross bedding or lamination (Pl. 15, Fig. 5), with many terrigeneousfragments int he lower part. The difference between the lithofacies of the eastern and

the western areas may be explained as that of contemporaneous heterotopism due to

the difference in sedimentary environments, but the calcareous sand of the Untensand beds is recognized below the bioclastic facies of the Urasaki limestone beds atseveral localities. There fore, the sedimentation of the Unten sand beds might have

preceeded that of the Urasaki limestone beds during the first stage of the deposition.

The Lower Formation abuts on the basement rocks, having fragments or boulders

ofbasement rocks abundantly in the basal part. On the other hand, the LowerFormation of Kouri- jima, off Unten, Nakijin Village, is represented by the stratified

bioclastic limestones with poor cross bedding, different from the Urasaki limestone beds.

As for the Upper Formation of this area, it is easy to distinguish among the three

Members, A, B and C, especially in the eastern area. The Operculina bed ofthe A Mem-ber (Pl. 14, Fig. 4) has a good lateral continuity all over the Peninsula and also in Kouri-

jima. In the western area ofthe Peninsula, the C2clocl7Peus limestone Iies directly above

the Operculina swarm part. The B Member is slightly thicker in the eastern area than

in the western area. Furthermore, in the western area it is often difficult to distinguish

lithologically the B Member from the C Member. The C Member is represented byweakly layered bioclastic limestones (Pl. 15, Fig. 4), and the coralline biolithitic lime-

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LEGEND

At{uvium & beach sand

Lower terrace deposits

Middle terrace deposits

5 Ftyukyu Group 1:terrigeneous graveLsand,and clay,3 and catcareous sand(Lower-•UpperFormation) 2:b'mstic sandy or brecciated timestone<Lower F,) gi.[lgS:S9g',,it.Y.Pg"Sb..t!,l9h9t. 4:b'DCtaStic tS'}cupper E)

Chinen sand stone (Upperrnost formation ct Shimajiri Gtvup)-colc, silty sand.

Shlmajiri Group silt stone. sand stone, tuff.

Nago Formation Me sozo ic? phyU ite

. t- Fault t slte

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WHITE BEACHIU,S. NAVAt BASE)

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:. ttt:ttt/1km tt tIt-t:N

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c-o1:.-.-==L::-.m-."500 m

Fig. 4. Geologic map of Gushikawa-Katsuren area.

D

"Ryukyu Limestone" of Okinawa-jima, South JapOn 143

stones are also distributed sporadically around the salient part of the basement (Pl. 14,

Fig. 3). Hitherto, these biolithitic limestones had been once regarded as a part of the

"terrace limestones" of the upper surface of the Middle terraces (TAKAyAsu, 1976a)•

However, after detailed surveys, the writer has come to conclusion that these biolithitic

limestones are correlated with the coralline biolithitic limestones of the C Member, and

both are contemporaneous. At several localities in the eastern area of the Peninsula,

the biolithite ofthe C Member unconformably overlies the Unten sand beds.

2. Gushikawa-Katsuren area This area is divided lithologically into three areas as foilows; the western area (the

western area of Gushikawa City) where the Lower and Upper Formations are bothpredominated by noncalcareous facies, the central area (the eastern area of Gushikawa

City and the neck ofKatsuren Peninsula) where the Lower Formation is dominant in the

noncalcareous facies but the Upper Formation is characterized by the calcareous facies,

and the eastern area (the largest part of Katsuren Peninsula) where both Formations are

represented by calcareous facies (Fig. 4). The correlation between the three areas is

not yet suMciently confirmed because a good horizon marker is absent in the noncal-

careous facies. In calcareous facies, however, the C!clocllpetas bed can be fairly well

traced laterally.

The Lower Formation of the western area consists mainly of noncalcareous gravels

or sands, and contains many molluscan shells and sometimes coralline fragments. The

Upper Formation intercalatcs with the limestone pebble beds in the lower part.Though this information was obtained from a drilling core (GuB-3, in Fig. 5), it is

possible to say that these limestone pebbles were derived from the limestones of the

Lower Formation somewhere in the more eastern area. The calcareous parts increase

upward in the Upper Formation, but most of the Upper Formation is occupiedby noncalcareous sand and gravel facies. In the noncalcareous parts of the UpperFormation, the fossils are not yet reported. The calcareous parts are intercalatedwithin the noncalcareous parts in large lenticular limestone bodies. These limestones

remain as buttes which escaped erosion.

The Lower Formation of the central area reveals particularly complicate facieschanges. From the drilling (GuB-1, in Fig. 5) near the estuary of the Tengan-gawa,dark greyish silt beds containing organic matter were found. In the silt beds, brackish

water foraminifcrs represented by Ammonia were yielded. Similar silt beds were also

found in the dri11ing cores ofthe Ishikawa City area (IsB-1 and IsB-2, in Fig. 5). Well

sorted and weakly cross laminated noncalcareous sand beds which look like dune sandbeds are distributed in the neck of Katsuren Peninsula (CGS-Ol, in Fig. 5), where the

altitude of the basement Shimajiri Group is generally higher than in other areas. To-

wards the west, the sand beds change gradually into startified sand beds intercalating

thin silt layers (CGS-09, in Fig. 5), and towards the east they change to sand beds con-

taining sporadic calcareous parts (CYN-Ol, in Fig. 5). Another variety of the Lower

1" Katsumi TAKAyAsu

lshkowa

:se-? <l)itv:a

AISH;KAWA C,

)i)))it...":•n-...

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1 KATsuREN V.

y6oNAousuxu v. ,t'"ket.,deA

r-• h- wiLmo r-

Fig.

(a)

5. Columnar sections (right) and theirlocalities in Gushikawa-Katsuren area (on legend see Fig. 6).

Formation in this area is represented by well stratified bioclastic limestones. The

limestones are distributed in the northern area of Tairagawa and Gushikawa, Gushi-

kawa City. They overlie the noncalcareous sand beds, a part of which shows crossbedding (Pl. 15, Fig. 6).

The Upper Formation of the central area is represented by stratified bioclastic

limestones. Though the coralline biolithites correlative with the C Member are also

distributed on the top of the ridge north ofTengan, in most of this area, it seems that

only the B Member is distributed. In the neck ofKatsuren Peninsula, all ofthe Upper

Formation is absent. Additionally, near the neck ofKatsuren Peninsula (CGS-09, inFig. 5), the Upper Formation overlies the Lower Formation unconformably (Pl. 15,Fig. 3). Giving attention to these facts of distribution and local uncornformity, the

writer inferred that the Ryukyu Group of this area suffered the tectonic movementsduring the deposition, and the sedimentary basin was separated into the east and the


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west by uplift of the neck ofKatsuren Peninsula (TAKAyAsu, 1976b).

In the eastern area, it is known by the results of drillings (KaB-2 and KaB-4, in

Fig. 5) that the thick Lower Formation is distributed under the axial part of the Penin-

sula. The lithofacies changes from the calcareous sand of the west to the clastic lime-

stone ofthe east, At several outcrops, the clastic limestones show remarkable stratifi-

cation and cross bedding. The Upper Formation of this area is distributed mainlyalong the southeastern edge of the Peninsula, and it overlies directly the Shimajiri

Group in general. The sequence from the OPercutina-C!ctocl2Petts limestone of the A

Member to the algal ball limestone ofthe B Member can be observed at several outcrops

(CKT-02, in Fig. 5), while the C Member is absent.

3. SouthernpartofOkinawa-jima In this part, the Lower and Upper Formations are both represented by calcareousfacies. As shown in Fig. 7, this part is divided on the bsais ofthe difference in tectonism

into the western area, west to the Minatogawa Limestone, and the eastern area of the

opposite side. In the western area (mainly Itoman City area), tectonic movementswere vigorous after the deposition ofthe Ryukyu Group, while in the eastern area (main-

ly the area ofChinen Peninsula) the erosion was more predominated than the tectonic

s s

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Fig. 6. Columnarsections (right) ancl theirlocalities in thesouthern partofOkinawa-jima.

Om

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LEGENDNtitude{abeve sea levcL)

wa

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control of topography.

The Lower Formation of the western area is limited in the eroded depressions onthe Shimajiri Group, and in some basal horizons the presence ofcalcareous sand beds is

recognized (SIT-03 and SGS-04, in Fig. 6). At the top ofthe Lower Formation, coral-

line or algal biolithites are sometimes recognized (SIT-03, in Fig. 6). The Upper

Formation of the western area is divided into three Members lithologically. Amongthem, the algal ball limestone of the B Member (Pl. !4, Fig. 5) and coralline bioclastic

limestone or coralline biolithite ofthe C Member are distinctive.

In the eastern area, the unconformity between the Shimajiri Group and the Ryu-

kyu Group has a position up to more than 100 m above sea level. The bioclastic lime-

stones of thc Lower Formation are distributed in the neighborhood of Oyakebaru(STM-02, in Fig. 6), and the sequence from the Clclocl7peus limestone of the A Member

(Pl. 14, Fig. 6, Pl. 16, Fig. 2) to the algal ball limestone of the B Member of the Upper

Formation is observed at several outcrops around the plateau of Itokazu (SIT-Ol, in

Fig. 6). The limestones correlative with the C Member are absent. The corallinelimestones are observed in only a part of the top of the Lower Formation.

Furthermore, the Itokazu limestone and the "Alternated limestone" of the OKi-

NAwA QuATERNARy REsEARcH GRoup (1976) are correlated respectively with the Up-per Formation and with a part of the Lower Formation and the Upper Formation ofthe Ryukyu Group in the writer's stratigraphical definition.

4. Hedo-misaki The limestones of the Ryikyu Group are distributed in a small area at Hedo-misaki

cape. In this area the limestones cover the basement of the Triassic limestone of the

basement and those include numerous granules of the latter. As for the fossi!s, algae

which coat the granules in ball shape ofless than 1 cm in diameter are abundant, and

ClcloclyPeus is common.

Hitherto, the limestones were correlated with the Machinato limestone, since the

topography ofthis cape shows the flat plane of20-30 m in height (FuNT et al., 1959).

From the lithological characteristics, however, those limestones may belong to the upper

part of the A Member. Besides, the limestone similar to the algal ball-Cyctoct!peus limestone ofHedo-misaki

is also recognized in Yoron-jima, about 20 km offthe cape. OK:MuRA (1976) correlated

this limestone with the lowest horizon ofthe Middle Formation ofthe Ryukyu Group ofhis stratigraphical division.

5. CircumferenceofHaneji-naikai In the neck of Motobu Peninsula, loose sand beds with gravels of about 30 m in

thickness, are laid horizontally on the Goga gravel beds and Nakoshi sand beds whichincline towards the southeast. Sand beds with gravels intertongue with the calcareous

sand beds near Nakoshi. Calcareous sand beds with numerous granules and quartzgrains are also distributed in O-jima and Yagachi-jima, northern partitions ofHaneji-

CHZNA•SAKI

a A"uvium.beach sand & dune

[IllllllillgS !wtny/.a....tg.!;S!y!gT JtEgupgS.bt Q!o/

calcarenite- -rudite(Vpper Member)biolithite (Lower M.)

wntraltmectasticlimestone•-bio[ithite

}(Upper FQrmattan) bioclasttc ts.-• alpat ball ls.gyL!!bgs!clyps!,Is bed

bioclastic sandy ts..calcareous sand (Lower F.)

hima'iri rouca careous sand silt (Chinen sandstone)

Sitt StOne tLrff

' thlt Z

e.te Recent corqtreef

. -TR. ll. .t. L..,i -L• ,+;LS

r 1̀"2..

-e9."! Fqult

th7 -7

,.t7

zt12i3fi

/ZT' i•iilt•,,.,tf'•s.

T//za"

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1is;lllllLrfir

) eATEN

HARBOR 9

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o

}iilil

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'

i-.i.,l. ,..Zt//. . .D

fl

7z•Zz:- zi/

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u- i,2 •••••.• .• .7s>-'tt''tl]..

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ilr:T

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-

'T ' T -- .-'Hr'r15"'''

;.,ri• ;: =' .r. iL L

----

i.tLrX.ck.st3,Fgt3i8i;#?-T.trxr' r/ t'

'

;lÅ}i!ili7' '-•,/

T -= -TL XI ,..: t••' -- .l] [: •,

. . .. =.S---J.'

T

Arqgus,u .U'rll

.:

ptIPstzfiica'Or""'I'i-,•-- rep-

lriT' •':`", yoZA' KE .-t

.• JLYipt' ]

-

9/e

f !

163-e-e

.• 12•1--•:::

t-tt"it'rrU,f;.

•-

,g

k"•-.J'.:..t,

- !..

-:yAE.'i l-DSI'i:fitl.iig;g2/

-•- ."D :Å}r' -

- ee.tntttt

1.2. 4,k,

L.lnt:

'T lti i -

. I,•

Fl

T:P

.eù

tt..'.t.T.S'--:iX...Yvm#t,t]f,S'`" i.,l!

YTt ./t,,t -" -

Tt'r It irf.tt.itlt.igglgi.llll-- T..;1 .L.

/ /"1'tt r {..l

-•: ; i' .!H yptune

' •!!' 5

illlll.Z./t•-y

iikk

.: -.

-..

Gus tchgn

tMinatg.awa.

lt.

i' '4.-=-"rl"----.c/

-vJ" 1

? N"J- J

.• /t

teece

Ecct

c

;EEc

' O-JIM.A

-v "hi s -J v-.t '-`-.-"-

ge

M'ait)ltSt-tu

y•Z.' T I

..Zl//2' )s ' N

T :"=-Iy 9t)

'-F /'. :./rJtt .T-[

:l;.Li-ii ,i

--Tv't l]ltTtT-

,•l;ll.l-IEi •k' .r' i /J

2v

tfll --e'I-'r-'t -

T .v. )rj.'tt!,•

:ILtlT•.,' ttl) t:'t" i ' :

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Eee

t

e t- t ! i•trR ...LT 'f '', t..h' ",'.s'.t'

Z/yK,/Ki.• .Z. .,..

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Makabe

o/UII!Si..i.II- Itl•ttLIZiJ• IIi

;---•' tlll'l,iillilllt'mi

•/l:"'':ì-i:}iiN';./i/lii2/;7iJ/ -' 10!Lt=ETi-.-";n"

ttL!•!5Fl:i=t-O;-lts.V

r:r.rtrl)l3x-•--- "7 1,i.i'rU.nt'4:L::

tt': I-' iiC. Iil.LllirLitlii-t.

t .l !I'ti ,liILE,ILt,Ii/ii,,.,•E-'--=•fi

I/Ltii-L- 1.L " tL..,Lfi...r"rL-"'t .

" .:=N.lj , .

-L ;t/l!EISill.I L;`/r. . JII

f

2 '•:k

iif{[Lili.-:,u.....'

' t' ';' ;pt'

;.;t!5IlÌEi-;il'lit:•lll'l"Trll-IIIIi."=---T .L

tL','#'-'{'1'-'tt"•;"';!'lllZTTolk?ri-ilit:liJitt7'7;;'ti,ri;7"

' n.;Korne5U' .-F. '::[;.-Ti

-M' -thTiT'i =- [J]I[;th=- ' •,' .' . L' t'=T• ]- . • Ma tiunLi. =

ii5.9"a, " [} Å}, s`:il;li[ FF'T'i `- 'X.H]

uS iEEEIIIII!lltt• "',xr, r'"'rL ,"-••i••,•.- iili[• Ji"t".'.

'1'1'' JLj ""J-

`'' u--''.'

L'' pt"-'Jt'-' '

ttfwJ Yv

ILliElli!.JSg";.i'."1x't-.: .....uJ' v

o

v" aa-4t-'

1 2km

Cg

KYAN-MtSAKt/

s})--

l.L ' Eiile..- T it,l-]

Eli•Ii fARA-SAKI

m200-

100-

KYAN

YOZA•DAKE

A 1

MiNATOGAWAN

-

B c D

1kmFig. 7. Geologic map of the southern part of Okinawa-jima.


naikai. The calcareous sand beds ofthese islets are considered to be contemporaneous

with the loose sand beds in the neck ofMotobu Peninsula. OPerculina swarm parts are

found in these calcareous sand beds, while their lateral continuity has not been clarified.

The calcareous sand beds of the western area of Yagachi-jima contain abundant re-mains of Pecten and they are lithologically similar te the Unten sand beds of the eastern

area in Motobu Peninsula, which is the opposite coast with a narrow strait.

Most of the sediments in this area are probably correlated with the Lower Forma-

tion of the Ryukyu Group.

6. Yamada-onsen The lowest part is represented by noncalcareous sand beds with a large amount of

phyllite fragments of the Nago Formation. Fossils have not yet been found in these

sand beds. The noncalcareous sand facies change promptly to calcareous sand faciesand upwardly to bioclastic limestone (Fig. 8). In accordance with such Iithological

change, the amount offossils and also the species number seem to be increased.

The amount of terrigeneous fragments increase again nearby in the middle horizon

of this outcrop, and in this horizon thin noncalcareous sand beds are sometimes partly

intercalated.

Between this horizon and the upper horizon 7-8 m above, OPerculina and C)clo-

cllpeus are found sporadically. Therefore, it seems that this part is correlated with the

A Member. On the other hand, this part also shows different characteristics; theamount of foraminifers is less than that of the A Member in other areas, and Operculina

is observed abundantly in the lower horizons rather than in this part. Moreover,algal fragments are found commonly and coralline debris is sometimes contained in this

part. These characteristics seem to be intermediate between the A and B Members.It is necessary to establish more suMcient stratigraphical divisions and the correlation

ofthis outcrop with those ofother areas should be studied more precisely in the future.

7. Tako-yama By means of the geological survey nearby, it is confirmed that noncalcareous sand

beds are buried below this outcrop, the calcareous facies occupies most of this outcrop

(Fig. 8).

A sand bed ofO.5-1 m in thickness containing many of the basement lithoclasts is

intercalated in the middle horizon of the outcrop, and between the base ofthis sand bed

and the upper horizon about 6 m above, OPerculina and C!cloct"Peus are abundantly yield-

ed. Therefore, the writer correlated this part with the A Member of the Upper For-mation. Both the A and B Members decrease in thickness northward (toward right inFig. 8), while the biolithites of the C Member increase in thickness. Biolithitic lime-

stones intertongue with the bioclastic limestones in the Lower Formation. The sedi-

ments of the Ryukyu Group are dislocated frequently by faults in this outcrop, while a

plane of about 75 m above sea level is developed which uniformly cuts these geological

structures.

som-

10-

TAK-1 TNC-2 TAK-3 TAK-4clv, 75m 74.Sm 7asm 71Dm

c eht."c

eÅëh- -9 .r- e

- ---- i4 e"tlLS-ee e

--- ---- B..4 ;si-•.L}i

B

"t?1q.,'itotA"B--

Bse-::'=-----S . --

A

14.}•!"-s••

:..LtFt t-

----r . -- J--ototo-o--t-

tTt9g.Ll

A #!•-l

A ./

w•"•.vh••,:- 1•:••

•y.'•i.ls,

,"-L•

A

tt e..t

p.s/'/

"Li-`e-sv

.e.,e

''' t"'6S.

i"'h"

l'

toAfii-eL

k`1"A-iirt-i

ei..--..

""--eL

':'i'tLt"

n:Dljtttie

L1•H6.s

.o•t)ttt

g"=tt.Yl,1

'lt

-ttt ctoL m"ts-1"x,rs'

klC)

dt.t"

i`iii

"tlo

gs

EAST CHINA SEA

YAM-1

v-..-`"f"'X.

Llillmada

,/ f2

TakovameHabe Center

V"Mll .

ygme.Zll'el9?" pi ra

foaj/ooom

Nn

TAK-1

tsO tO 20 30m

Fig.

r•'"e•.tl,ti.tt.e,f./•,

ls •Sv -. v Sp'1';'tt•Z:•t"-i,.[i•./i:s/"• .e"

-e- "i.

3oM-

..xgÅë.

"i •' 20-

lo-

YAM-2

8. Sketches and columnar sections of the outcrops

YAM-3 YAM-4 YAM-5m 26Sm 32.0m ss.sm 24.Y! 25.0mo -

ett

Bt-

eE)-

bo

A

"t

tiLht"t

A ."sf/e/./)

A

eL)L ;t't

}'j

L

'T'.L).e'

ttt

L

t!sh

.tl'.?L"

t)v'l.fi'l}i••.1.

;.11.:1

=."

20

Llt

L

rc-21-i.,l

L

"nAAIeArt"A

g)"a-i,)oe

,'

.lc...

{•i-1•1•,;•

IZ;1-•

':'SiZ,.

t.t., ...

ri-./

.t-. -l.

[l.L `JIA

'{"

ii•,7i

30

-2-spt-gs-.t,.o-

o le 2o 3omat Takoyama (TAK) and

1e.s:.......'e

YAM-bot.tes

-.'i'l.,r-.."-.

YAM-3

"Lthe")-----' t-.AMYAM-5"tpt' fii-sst :tb"ais}s";It-isir-i-=r'-s:LlL.rit,..

.er.Ln

s.:.;"i-.l''s/'.•//•.,..:..L..r.:;..-,irxgTk

iii.ili-:11i,,llS•3•l;,f,:it•ii'ii';'i",•'ii/Fi,11i'i/ili.Ii"igl,/11•.i/l••.I,,l-ll/'iill.,llli-1111'll-l'il.i,.,g,,tth;'..':':J:'.{'i.1't"1,';t/.i].t'/V:.:l:.'::li'vti"txt.'.-J

Yamada-onsen(YAM).

-ao

g:

sE.

?ge

"8

x

't•

'

t.

OL,,

2 100m

1n.

x 'c.

4ox,

ltt

1,

o. "Oe

) or.Yr e.. I el] 'e" J

itt

nile,,,

x-it t

i

, 'K h t/.i/ Va ' x, " ..,.m X'7o. .t/ . t t.. ti YOMITAN '•.. ' Vi-N,, t"t/, 'ttx '"' 'k L-",,,, x, .

..t x. .. tt ttt ttttt ' x

eoOe

' -" iesi

L

-;

l,

t'

'I

,sl-i 1 't;..

-Lflt:.

-- S s"LL ;-

LEGENDee Ceraltine bioLithite and itsclastics

E eorutthe bbiithite Lertse

an] eedded

l.'ttF/-:- eedded

wa Feraminiferous scLncly

':t' :tt.i•:': Catcareous sand

--'

0Ok;nowa

ouar

ctastic tienestone

muddy tirnestonewith terrigeneeus gruvel

t.s.TOYA

a-.•

ttt .tYoM8.".",ryi'>-x.

-- - " > ) L

.'i "" N'; N-- N

.- k

i., 'x.,t

E.

Pe

a' ca .sb

•1'1

.:tt t .., .. ,•f, 1

t' / .e' 'r tttN 't'." ' 1;l"'i's'.itii

o ",' r'•. •t h. tL-' "/ "t<tt' ;X .1

t ttt ttt ttt

.. N{. •t 50 t., Y.-..... ',• ''"S>s"

. .,, b /,,

-, L..,.... •,s",Å~`6

tN .'.N b" Y..k

)

x

(v

ts

'j

N

t.

E)} x.. 'x'x..

illLSIrpf.,2seFk .

/

. r.-. St/t""-•;,,,

. tt x. < tNL 1,, ) .l,stt "k Y/.. ;.. x,>X 'N., Xy" ' i" x

C.

'A.'

CoraUinebiotithiteanditsc{astics

BeddedclasticLs.withbiotithitetense

Uc

----Ub----

Ua

. Atgatbatltimestone

-j--------------Beddedmuddyt.s.withterrigeneousgravet-t------.t.!+;.-9,'',•nm•.ttttt

Foraminiferoussandyt.s.

L

I,N•,•XX"c,i/ec.kX.t.-.t-.

Catcareoussand

Fig. 9. Sketch map and columnar section of Yomitan quarry.

pS

g1

rr.

HPA

s:n:

O..,

orr.

npt

$rp

ll:s

"pt

mo=e-Etr

kpo

vpt

p

-inN


8. Yomitan The limestone quarry ofYomitan was designated as the type locality ofthe Yontan

limestone by FLiNT et al. (1959). The writer, however, regards most ofthis limestone in

this quarry to the C Member of the Ryukyu Group, notwithstanding its large thickness

(Fig. 9). Therefore, it may be concluded that the Yontan limestone of FLINT et al. is

synonymous with the writer's C Member in this paper.

At this quarry, the writer observed the conformable relation ofthe C Member with

the lower beds. Therefore, it should be discarded to postulate the presence of the

unconformity between the Yontan limestone and the Naha formation as pointed outby FLiNT et al. (1959).

Near Zakimi Castie, the northern part ofYomitan Village, the coralline biolithite

overlies unconformably the noncalcareous sand and gravel beds which are correlated

with the noncalcareous sand beds ofthe outcrop ofYamada-onsen. On the other hand,the coralline biolithite seems to be corelated with the C Member. The stratigraphical

relation ofthe local unconformity between the Lower Formation and the C Member ofthe Upper Formation is similar to that in the eastern part of Motobu Peninsula.

9. Makiminato Well stratified calcareous sand beds and bioclastic limestone with numerous quartz

grains are distributed in Makiminato, Urasoe City. They are rich in mollusks, bryo-

zoans, echinoderms and brachiopods, and similar lithologically to the Lower Formation

of Katsuren Peninsula' and the southern part of Okinawa-jima.

Besides, though it is hard to approach the good outcrops ofthe Machinato limestone

ofFuNT et al. (1959) in this area, as already mentioned, a few small outcrops ofthe cross

laminated calcarenite can be seen in the former foreigner's residental district nearby the

shore. The limestone is similar to the Minatogawa Limestone having abundantforaminifers and intraclasts, but the grain size of the former is finer than that of the

latter.

C. Discussion As mentioned above, for the stratigarphy of the Ryukyu Group, the main part of

the "Ryukyu Limestone", the writer's division is shown to be available everwhere in

Okinawa-jima, ifsome local differences in lithology are disregarded. Thus, the writer's

Ryukyu Group is considered to be approximately eqivalent to the Naha formation plus

the Yontan limestone formation ofFLiNT et al. (1959). In this sense, the algal limestone

beds of the horizon marker by FLiNT et al. (1959) and SHoJi (1968) seem probably to be

equal to the B Member in the writer's stratigraphical division; even though, the algal

limestones are also recognized in other horizons ofthe Ryukyu Group. In addition, the

local differences of the mode of occurrence of fossil algae and the lithology of the B

Member make it diMcult to discriminate definite horizons ofthe algal beds stratigraphi-

cally. Neverthless, the C!clocl2Peus-OPerculina swarm bed is usually recognized as one

definite horizon everywhere, with remarkable petrographic characteristics.

Table 4. Tentative correlation of the late Pliocene and the QLuaternary formations of the southern half of the Ryukyu Islands

a=e-e

OKINAWA KUME ISHIGAKI TAKETOMI IRIOMOTEHAT YONAGUNI0to,cC

Kunigami Motobu Nakagarni Shirnajiri

YORONcr<IMURA,t9 CorMWAaUAT

AGUNIRES.G.1976)

MlYAKO(YAIAKI.

TARAMA1976)

KOHAMA(SHtNOHAHA&KezAKp

Dunesorld,beachroek,Recentreef,etc• NIslsaDunebend bmidaiLs. ShirDLmeSand Dunesorid,beachrock,Recentreef.etc,HOLOC.

Raisedcerelreef.beecÅ}ideposits{1•3} RaisedreetCIO-) lribisNts.t2} RaLsedreetts Shiratori•selciLs.aedocia[bLs 2'3nihictibeach Reisedreef(3] Raiscdbeachreck 3mhighbench Faisedreel(5-) Reisedteel(2)(5-7)(n-15)

Numb-rstepit-nthtsesend4tttethehe-ghtolttrttcts"tJeÅ}-s.tnm-ret "6)'K7p;.t? l5-71 (5-7}t15•1

{5-10)

{15-20)t20•30)Kyodaterrece

(ro-30)'imeLsh (20-30}--Ybsiiiwata4s [30)btomiL.s,"

(20)'imlit.

CIO-15)

(20-30)

(40-oo)

ooMSnetogawe

L.s.F.

(201"Awaislii'LIike

Ls.

'OharaL.s.

ShirTx)ji-jima

(5-10)ao-1s)

"5-20}

(50-60}

llO}

(20-30)

f40-so)

Ls.

(20-30)

C40-501

(oo}

PanariL.s.

(40-60)

75

(40-60) (30-50) t30-50)

t55-oo)C45) C40}{60)

{BO)

"-

c corattinel.s.Åëinnerabiolithite)feeie$

coratlineLs.

.

coral"nei,s. HiraraL.s.

.U[1,

' co

mZmoobm-mAm

L'

Loaa=

algelI,s.U;'innerobielitNte-facies

aigalhaIIl.s.

meliuscanend

eorallineUpper

1corellinebieclasticLs.

UruroL.s.

ut1inet,s.

algelbetl1.s.

L'

Loaa=

coralSineLs.

brecclatedLs.

bioelasticLs.cla

BA

-.misS(i

CrctoCtvpeuqyokutypevs

ewrcultna

.8Eouter

clasticLs.f.percuhne Crctec+rpeus

j=Lenre,Exo-

CycteCtypaul

dij•:oEFO

Middle

AmagawaL.s. CycbeJvpt"s Opercutina sand,gravel browny

----

------

sand.biodestic

Ls.

corellinebieelasticI.s

:T

elgelbellemdfo(ambioclestic

lsLevver rt

ul

Le)oJ

bioclastic

gravetI.s.eaic.sen

terrigeneousLL'pebt"e-

granule

LCORglolectasticts.f.

---

coralline

bieclasti'c

Ls.

? ealc.sand

ul

LÅëloJ

sandysilt BoraL.s.Tpt

G-veiBunera

clay Ng

UoctasticLs.

sendysilt

t'iconglo,

Nskeshi5and ChinenSend tL rv

tio:N kecon9la GegaGravel Sh;nzetoTuff

?

?rv

IM lm ImSonaiCongle.

q

e:Higas:=

YonaberuF.ShimajiriGroup

Elr-utpt

pS

gE:r•

B96g

:gotr.

:pEpt

ei:

B"po

mog-trgpo

vpo

v

paee


Further, the continuity of the ClcloclJpetts-OPerculina bed is verified by the writer not

only in Okinawa-jima but also in other islands of the Ryukyu Islands. Therefore, it is

probable that the stratigarphy ofthe "Ryukyu Limestone" established in Okinawa-jima is applicable to so-called "Ryukyu Limestone" in other islands throughout the

Ryukyu Islands. In this way, the writer compiled the stratigarphy of "Rhukyu Lime-stone" in a correlation chart ofTable 4 on the basis of his data together with others.

IV. Sedimentological Study of the "Ryukyu Limestone"

In order to consider the sedimentary environment of the "Ryukyu Limestone",the writer made petrographical observations and fossil analysis on the samples of main

outcrops and drilling cores under the optical microscope. The results will be described

below briefly.

A. TexturalComponents The fundamental textural components ofthe "Ryukyu Limestone", are classified as

follows: matrices, framework grains (allochemical grains or allochems, by FoLK, 1959)

and pores. The matrices in the writer's usage imply not only the microcrystalline calcite ooze

but also sparry calcite cement. They are easily distinguished from each other under the

microscope. Although diagenetic fabrics are observed in some cases and seem to ex-plain only partially the hysteresis of the limestone, the writer could not find out the

pertinent facts on this point.

The constituents of the framework grains are occupied by skeletal grains of fossil for

the most part. Besides, the presence oflithoclasts and quartz grains ofthe terrigeneous

fragments are confined to some specific horizon ofthe Ryukyu Group. On the contra-ry, it is characteristic that the Minatogawa Limestone contains well rounded intraclasts

abundant and sometimes lithoclasts of the Ryukyu Group. Pellets are rarely found,

and oolites have not been found yet in the "Ryukyu Limestone". As for fossils, though it is necessary to distinguish detrital skeletal grains from ske-

letons in situ, it is almost impossible in practice to discriminate them in thin sections.

Therefore, the writer discrirninated the clastic limestones from the biolithites on the

occasion ofsampling, and made an anlysis in the laboratory only for the former in order

to know the constituents ofthe skeletal grains.

B. MethodofFossilAnaylsis Even though the skeletal grains which constitute most of the "Ryukyu Limestone"

are supposed to be detrital, they play an important role in the inference ofthe sedimen-

tary environments and also in the accurate correlation of the stratigraphical sequences.

From the viewpoint stated above, the writer tried with his colleagues to establish

the method for the recognition of the skeletal constituents of the "Ryukyu Limestone"

more objectively and quantitatively, as far as possible. Consequently, two methods

"Ryukyn Limestone" of Okinawa jima, South Japan 155

used here are condidered. Though these methods connote many problems to be solved

and improvements to be made**, it seems that these methods contribute to objectiveand quantitative studies on the skeletal consituents of those materials.

Method-A: Counting each skeletal grain as one unit, and after summing them up

the composition of each taxon is shown by percentage. In order to keep the stability in the analytical value, the total nulpber of skeletal

grains should be counted more than 500. Though the amount of indeterminable ske-letal grains often reachs to the value more than thirty percent, the outline of the quanti-

tative distribution of main taxa can be known from the determinable skeletal grains.

Method-B : Using a mechanical stage, move the thin section sideways and forwardsand backwards in equal distances ofO.2 mm each, and identify constituents juSt under

the cross hair. Then summing up the number of the points, the composition of each

constituent is shown in percentage.

In this method, all of the constituents including pores are calculated. But at this

time, as the problem on the origin of the pores is still unsolved, the proportion to the

value subtracted of the point numbers of pores from the total count is substituted as the

approximate proportion of each constituent,

By means of this method, the ratios ofthe microcrystalline calcite matrix, the sparry

calcite cement and allochemical grains which are fundamental components for petro-graphicai classification oflimestone, can be represented by numerical values . There-

fore, this method can be applied to the fossil analysis as well as the petrographical des-

cription. For keeping the stability in the analytical value, the total count is recom-

mended to be more than 1,OOO points in the case of the calculation of those three com-

ponent ratios and more than 2,OOO or 2,500 points in the case of the examination of the

skeletal proportion.

C. Discussion The results of the analysis are summarized in Fig. 10-15.

Only SHoJi (l968) has carried out a study on the microscopic petrography and

sedimentology ofthe "Ryukyu Limestone". Though it was conventional SHoJi statedthat some sedimentary subcycles are present in each limestone ofgravel, sand, sparite,

sparmicrite and micrite succession, and there are three cycles in the Naha limestone,

two cycles in the Yomitan limestone, and one in the Machinato limestone.

** In Method-A, it is impossible to denote quantitative comparison among different sel•mples in the strict

sense, because there is a certain difference in the amount ofskeletal grains in each sample. In the case

of the "Ryukyu Limestone", however, this anxiety can be negated, for the s'keletal grains show high

occupancy in most samples. In Method-B, there is suspicion that the large fragments show a large

value of proportion, because the value of the constituent proponion obtained by this method is cal-

culated as the ratio of area. Furthermore, as for both methods, all skeleta! grains are regarded as

objects having equal valuation whether the grain is broken or not, because the observation is made

only on one section of each skeleton. Therefore, the analitical value may also be affected by the

differences in the structural intensity of the individual skeleton. - •


ALLOCHEM GRAINS

(a)

N=179.

.

:e-- , -- --et e- :e .: t"-. --"t"'tt"t""':"

::e re t-:

.ee

.e..:

----.

e-. e-- "1

.

..

-e.-- -- .

.

.

MICROCRYSTALLINE CALCITE MATRIX

SPARRY CALCtTE CEMENT

Okinawa-jima

-o

ee 5eo

ALLOCHEM GRAiNS

A

A

A

AA"A Oo A

N=35

o MhatoDcpNct Ls. kyukvu Group o eMe"sber"''''--'"--""'"' VPPerE - AMembtr A Lewtr Formation

(b)

.A

A.

MtCROC V5TALLINE CALCITE MATRIX

5n4RRY CALCITE CEMENT

'

ALLOCHEM GRAINS

(c)

Kouri-j ima

A-

Bnes . n

atp noAAo

AeO. n -

A

oop

Aaa

A

A

a

D

eVo

oA

a

A

Ns,6e

mCMemberT eBM"mbtrtUFPePr:ation

eAMember1 A Lewer Formation

Fig. 10.Triangle diagramg showing the pro-

portion of three fundamenta1 com-ponents. (a) All analyzed samples(b) Sarnples of Okinawa-jima (c)Samples of Kouri-jima.

MICROCRYSTALLINE CALCITE MATRtX

SPARRY CALCITE CEMENT

o

SStseigigLQLpms 50 foo o

Fo rami nifers 50 1OO

M 26

16m

Ub

Ua

ma

rEl

mlL

25m

Ua

Ua

L

E9

-.:i ;---: t- .-. tr: .'i)" ':I);

t- :.- .- ----•T.: r.•

- --tt --t'..r:f' l: T:'•.

- :.L 'tt tt-t

t-. -•s ---.' res=:..:;'";!il•,

-

----vt

Å~----e----

tt-

.tttMB-3-

13.8m - --------------.L -

,. ----------I',;,f•'ii••,•,:,tl•'l•

Ua

'N., -.

:---:---

t-t '

-ttt t. r

i,L/t•r,l{-

----'

t, ttl "' t'I'.:. :- :tl:. ----

ttt-;-

tt -

-...Nts

-tt

s,."'•/z:.,:.t:;.."

.ttt

-:•:.:,r.s, 1.'

MB;gL

Eopt

EEI Massive

IIil] Loose

Fig. Il.

S Colcareous algae(Corallinaceoe) Eii!!i Plonktonic

g Foraminifers .E opercult'nag Bryozoons E c)tcloclLvpeus ge Amphistegina :':.;.:•'i:i,r.:•:/i.:.:',;s. Echir)oderrns

' MoHusks g CalcarinaaOthers eAcervutt'na

EEEEEEII Homotrematidae

Iirnestone "'i•i.L:;':.: Catcareoussand l•l'::'i:tr;,':ee't:'IS-{';"• Soritidoe

Arenoceoustimestone :t.:.,,;.i: Terrigeneous sand,gravel ,. .- -J a others Diagrams ofskeletal components ofsome columnar sections in Motobu Peninsula (analyzed by Method-A).

N Bise

. .B3. ...

B4

'// t

..:.. . :l::.::: :'.... !:1.B.2.:il:: •

- - :: - -:li:--i.

:--:::

M leXM26z"s./.,/,,..,,,

ttt ' ttt. ..t..{ ..•,• Åé ••••

Nakason.e

--'

,

KOURt-JIMA.

t.t/

t/tt

... il."'f''

}

XM5

Akk#:;gbeE

''fi'

.•

Terrace deposltsIZZ!]gravel EIIIIIIItimestone

Ryukyu Group

Upper E es Lower F. E::] timestone

Biofacies

E[EXIcaicareous sand

E9]

li'ten

1km

1

i

(1 ) Sand and gruvel (2) Calcareeus sand (3)Terrigeneous fragrnents bearing ctastic timestone (4)Fine qainedmassiye lirTvestone (5}Loosely hrecciated timestene

Fig. 12. Geologic outline of the northern part of Mobuto Peninsula

correlation among the sections shown in Fig

---: :-i --: -: -- --: t-

CF-A)

CF-B-E)y////x

F, Forwrinifers B

A/ Calcareous aLgae

.

CF-A) t

Il'::•1:::,1•f,','.': ',:,l•1:1•1'.:,:' :'.:'1::i-r•

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CCoraLEnaceae)

(above), 11, (below).

Foraminiferal assemblage

--" -- ------.--pa= --- -tt---

. -s- t:t:::-t-

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(Ac•H)

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----

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-

--.....- .- N-tu-- tt ----

Arn'A-tphisteg,!nlg Ca CgLgpm C) Op• OpRcg-LIDg Ac,Acervut[na H P: Plaakton[c foraminifers

and lithological and paleontological

CycÅ}n,ririypegs.

Hcnr,o:refixitidae

:N"t

EE:.B2s:o:

Oe

oa:p}po

kel :

e-po

m2e

cis'

g:

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- --!SS7-J-t-J --SZ--- --•Ti -

llX. It lntTi' T- ---J t2-!w,'twli41il

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f

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jvIt/ 'i' Il i 1/"1 '' I t'-tt

11

l'

' il

r•

hLYfi

•1i l,,i ti r, :Ll

[ gs.L-''if' to soom

MATRIX!Gl!gL!-Ebg2g-&-m!Lsgrainsh a

..,••IJil:g

,:ii'i"' V

:," -''/' f"ii' /,/i: ''ILtis I"'

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i ::•)'i'

t ':'j

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e oopa`er' m,̀Otr•tc `O' kOt/iV'O

Limestone X Siotithite

(a)

./ii''' ,Xt

t/. t t// t// t /tt tttlttl gttttt

/tt t/ t //. / ., ttt 1/ t/ ,/ t. ,/t tt tttt/ t "i,, x,

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' ' ttt i'7

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Reunded I•SubrO-nded }Subanpttar -

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y]-.{V) ,"

'" /.,i/,E,iLl,g>,k

K .),?-IliXi;rL'>f'. . III'

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04eeeeate)i i'

5A '<V' .

't

'l";Ei;I-.[Z'(' //.tLJ"/"..,,,fx

t.tt /Z'

/t% ' i,x,Cgelssr63"2:,Ol) O.pa,,rc,VS',,",,., YY"tl//1-11/it,oOill A.m,p,7J.sfg,'"a5*,O,'cHeg:;"g;7smatLdae

t CO-i25-O,25) eenthonic farorninifer oCg-O,125) 1'' cornposit[on ' ' tt 'Fig. 13. Distributive patern of each component of the Ryukyu Group of Kouri-jima. shape and size ofgrains. (b) Skeletal components (analyzed by Method-B),

(a) Matrix and

tlt .

Calcareous algae (Corallinaceae)

-''

"" t't/"k,,

t"t/

t/!•

.1.

,; .,.: N..

I;t.)

Halimeda

o

•tt,

•itt

'le,,.,

7,.,l.iSl,.:ll.lliilk,,.L.itLg.k),,,,.N

x.e''' P 1./

>1

Corals

scIXk>r%

'N X.

Å~•

.t

X

e

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Planktonic foraminifers

,1i,1•

l,i,'

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<

g.,,,--.le,••e• •,,-,,•• •

t- .........-..t. t".t..

Benthonic foraminifers

'' be. .

K

i SX.?

,C tt•/tit tt

'1" '1'i '' t"

. -/•t. ',.)x ' 't.

XI•<

,'e }1 xl[' 41'' '::"'

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Bryozoans

sx8 o ' eX, e

't/tX ts 1

k

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Mollusks

' . o iaG7 -- .e-'' 9• ,,. .. .tttttt'VS --' ',-!t ttt.tt.

x.

tt// ttt

i ft

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-t ix

x.

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-Xx.

Echinodertns . •; M)pa e

f(,'

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x,J'

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tt lttt

Fig. 13. (b)

N

-9

e

x

c.,'.,

x -s

s

e

/ !) :' ,'

pa:

gE:•s2o-

g:

o.

o5:po

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-pt

ge

gtr

gpt

vpo

p

Go

160 Katsumi TAKAyASU

To.t.a,,t

3, fS.'5.:

2

le

10

H,g i l'li

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o

ro

U;nB '

H,-,A

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t-'.l

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j

ForBlt aFerPltcdotted}

t. {:

t/'i

t..

•1tt

t:1:E.t.t't' :'I'•i,.iI•'l•S'./{rl.t'.'trI"IriS.l

t..t

ttt

.. -

tttt l

ttt

tt/

•1tt

.,• T•li'tt':'

Corlt

I

Brzlt

Å~

Mo[tt

i li

l•l{ ...ttr..'i'

t

'

''

tt

Fig. 14.

/

tt

I

t

Echlt

x

e b11 tN lv v 1twu IX x

N

O1" IU rvVVI .UX O1lt Å}111yvv/ ) lrX e1 li t" )VVV" V"V lrX Ob" ldb lVvvt ml vl lxX

Stratigraphical distribution of the skeletal components of the Ryukyu Group in Kouri-jima.Lateral axis shows content: O; less than O.1250/., I; O.125-O.250/., II; O.25-O.50/., III; O.5-1O/o,

IV; 1-20/., V;2-4%, VI; 4-8%, VIII; 8-160/,, VIII; 16-320/., IX; 32-640/., X; more than640/o to total counts without pores (t). Longitudinal axis shows frequency. AlgC; Corallina-

ce2e, AlgH; Halimeda, ForB; Benthonic foraminifers, For P; Planktonic foraminifers, Cor;Corals, Brz; Bryozoans, Mol; Mo]lusks, and Ech; Echinoderms.

SHoJi considered the formation ofthose cycles as the reflection ofthe developrnental

process of the coral reef controlled by sea level change or tectonic movement, and re-

garded the microcrystalline limestone as product of the last stage of each cycle, that is, a

stagnant shallow lagoon environment.

As mentioned formerly, MAcNEiL's stratigraphical division on which SHoJi relied

is different from the writer's. Therefore, it is diMcult to compare directly SHoJi's work

with the results of the writer's analysis. But, on the constituent of matrices, to which

SHoJi paid much attention, the following facts must be pointed out from the results of

the writer's analysis.

(a) In the matrices of the "Ryukyu Limestone", microcrystalline calcite is thedominant constituent in general (Fig. 10a), and the typical spartie is observed to be

limited in the Minatogawa Limestone and a part of the Ryukyu Group (Fig. 1Ob, c).

(b) Even in the same horizon (for example, in the B Member or in the C Member

"Ryukyu Limestone" of Okjnawa-jima, South Japan 161

ofthe Upper Formation ofthe Ryukyu Group, shown in Fig. 1Ob, c), the constituents of

matrices are variable in accordance with the difference ofsampling point.

(c) The A Member ofthe Upper Formation ofthe Ryukyu Group is representedgenerally by microcrystalline limestones (Fig. 10b, c).

From these facts, the writer does not agree with SHoJi's opinion that the "Ryukyu

Limestone" consists of many sedimentary subcycles. According to the writer's con-sideration, the Ryukyu Group itself, the main part of the "Ryukyu Limestone", is a

product ofa Iarge sedimentary cycle as a whole. But in general, in this large cycle,

there are small two cycles which are regarded as corresponding to the Lower and the

Upper Formations respectively.

From this viewpoint, the writer gives his consideration to explain the sedimentary

environment ofthe Ryukyu Group. 1) With good correlation between lithofacies and biofacies, the sedimentary facies

of the Lower Formation varies from place to place. For example, in Motobu Penin-sula, as shown in Fig. 12, the biofacies change from [foraminifers-bryozoans-echino-

derms] to [foraminifers-bryozoans], and [foraminifers-calcareous algae] in parallel with

the change oflithofacies from abundant terrigeneous fragment situation to poor ones.

2) The correlation between lithofacies and biofacies mentioned above can be

XBD A

Fig. 15.

buts ti

ts=GVts

l

b Rg"8-g..

Ry.g.b

b"sC"

g

oE.9 o

O y-F

abundantGeneral tendency of the stratigraphical distribution of skeletal components and terrigeneous

fragments, on the basis of field observation and indoor anal ysis,


generally ascertained in vertical change (Fig. I2) as well. It suggests that the sedi-

mentary environment of the Lower Formation changed from the sea unfavorable forcoral reef forming to those of favorable. Actually, this is attested by the presence of

coralline biolithite at the top horizon of the Lower Formation.

Uc

UB

•xt

UA

U-L

Lu

LL

Fig. 16. Schema of the sedimentary process of the Ryukyu Group. LL and Lu are the stages of thelower and the upper parts of the Lower Formation respectively, and UA, UB and Uc are thestages of the A, B and C Members of the Upper Formation respectively. U-L is the interval

stage between the Lower and the Upper Formations. Showing the imaginative section inCentral Okinawa, E-W trend.

"Ryukyu Limestone" of Okinawa-jima, SouthJapan 163

3) During the deposition of the A Member of the Upper Formation, the sedi-mentary environment turned to those of unfavorable for coral reef forming. Thisinference is based on the fact that confined species such as C"clocl2peus and allies of

deeper environment are chief components of the biofacies of the A Member and arerich in number as shown in Fig. 1 1. As for the A Member, some other characteristics

were already mentioned, and they are as follows; terrigeneous fragrnents are often

contained in the lower part of the A Member, and the thin layers of terrigeneousreddish brown clay are sometimes intercalated in the OPerculina swarm part, and

C2cloclJpeus are often yielded in swarm immediately above the OPerculina swarm part.

These facts suggest that the coral reef environment deteriorated rapidly due to the

inflow process oflithoclasts and residual clay from the land area, and the increase of

the depth ofthe sea.

4) In the stage of the Upper Formation, the area ofsedimentation expanded ra-

pidly more than the Lower Formation as mentioned in the former chapter. Thefact that the uniform stratigraphical sequence of the Upper Formation is tracable in

wider areas than those of the Lower Formation, suggests the advent ofa uniform sedi-

mentary environment by transgression. Succeedingly the sea became gradually shal-lower after the A Member stage, and then in the C Member stage, the typical coral reef

environment was initiated. This explanation is supported by the increase of Coral-

linaceae and the decrease of bryozoans upwardly in the Upper Formation, and therapid increase of Halimeda and corals in the C Member (Fig. 13, 14). The decrease in

the number ofplanktonic formainifers towards the upper in the Upper Formation (Fig.

13, I4) also suggests the environmental change from open sea to reef.

The stratigraphical distribution of the fossils in the Ryukyu Group is shown sche-

matically in Fig. 15, and the environmental changes during the sedimentation of the

Ryukyu Group are shown in Fig. 16.

Incidentally, the Minatogawa Limestone Formation is considered to be the sedi-ments of an extreme shallow environment affected by constant wave action or a sub-aerial sand beach environment in part, because these limestones are represented bysparite and all grains are well rounded without exeption (Pl. 16, Fig. 1).

V. General Discussion on the Quaternary Geohistory of the Ryukyu Islands

A. Age of the Ryukyu Group There are very few reports of fossils available for determining the age of "Ryukyu

Limestone". But the following discussion will be possible.

YABE and HATAi (1941) reported the occurrence of AmussioPecten praesignis and

Pecten naganumantts in the "Ryukyu Limestone" in Naha City. Though they described


these fossil horizons with uncertainty, it is probable that they were yielded from the

Lower Formation of the Ryukyu Group in the writer's division. In Honshu, A. Pra-esignds is known from the Pliocene and the Lower Pleistocene, and P. naganumanus is

abundant especially in the Lower Pleistocene. Therefore, these mollusks supportHANzAwA's opinion (1935) that the age ofthe "Ryukyu Limestone" is Late Pliocene or

from Late Pliocene to Early Pleistocene.

On the other hand, the uppermost part of the Shimajiri Group, below the RyukyuGroup, is considered Lower Pleistocene according to the latest micropaleontological

works. As for the planktonic foraminifers, NAToRi et al. (1972) reported the first ap-

pearance of Ctoborotaria (Gtoborotaria) truncatutinoides from the top of the Shinzato For-

mation (upper Shimajiri), and IBARAKi and TsucHr (1975) reported abundant occur-rences of that species from the Chinen Sandstone (upper-most Shimajiri). On the cal-careous nannoplankton, NisH:DA (1973) recognized that Clclococotithtts macint2rei and

Discoaster brouweri range up to the overlying Chinen Sandstone. He described the ap-

pearence of GePh2rocaPsa oceanica from the Nakoshi Sand Beds.

These reports suggest that the age ofthe Chinen Sandstone is the Early Pleistocene•Therefore, it is certain that the lowest horizon of the Ryukyu Group does not go back to

the Pliocene.

Several vertebrate fossils have been obtained from cave or fissure deposits in the

"Ryukyu Limestone" (Table 5). Among them, Metacervulus astylodon is most abundant,and is regarded as an important element of the paleovertebrate fauna of the Ryukyu

Islands.

M. ast21odon was also reported from the Lower Formation of the Ryukyu Group of

Ishigaki-jima (FosTER 1965; OTsuKA and HAsEGAwA, 1973). The "Muntiacus" thatoccurred in the basal part of the Ryukyu Group (SHiKAMA and OTsuKA, 1971) is con-sidered to be identified as Metacervultts astylodon, too (OTsuKA and HAsEGAwA. 1973)•

According to ToKuNAGA and TAKAi (1939), M. ast21odon ofthe Ryukyu Islands and

M. kendengensis, a member ofthe Trinil marnmalian fauna ofJava, are both the nearest

desendants of M. caPreolinus which occurs in the Yushe Series (Zone II and Zone III,

Villafranchian equivalent), South-Eastern Shansi, China. And they considered that

M. kendengensis is a relative of living muntjacs of Southeast Asia and Taiwan, while ex-

tinct M. astllodon was the insular form in Ryukyu.

Based on their opinions, the lowest horizon ofthe Ryukyu Group deduced from M.astltodon may go back to Early Pleistocene.

The elephantoid molar teeth discovered from the cave deposits ofMiyako-jima and

from the "Ryukyu Limestone" at Kyan-misaki, Southern Okinawa, are also worthy ofnotice among the vertebrate fossils in the Ryukyu Islands. Hitherto, two specimens

have been discovered in Miyako-jima; one was reported by ToKuNAGA (1940) as amolar of Pataeoloxodon namadicus or ElePhas trogonterii and the other was reported by

OTsuKA (1941) as a molar of Palaeotoxodon?. After the re-examination of the site and


Table 5. List of fossil vertebrates of the Ryukyu Islands

Mammalia Rodentia DiPlethrix te,gata (THoMAs) Miyake-jima (Amagawa and Tanabaru Caves)

Okinawa-jima (Minatogawa fissure)

Artiodactyla

CaPreelus mi akoenst's OTsuKA Miyakorjima (Amagawa Cave)

CaPreottts sp. Miyakorjima (Tanabaru and Nakabaru Caves)

Metacerr,ultts astylodon (MA rsuMoTo) Miyako-jima (Amagawa Cave)Ie-jime (caves)

Okinawa-jima (fissures and Ryukyu Group)

Ishigaki-jima (Ryukyu Group)

Proboscidea

TriloPhodon sp. Miyakorjima (Shimajiri Group)

Palaeoloxodon or "Archidiskodon" Miyako-jima (Tanabaru aave)

Okinawa-jima (Ryukyu Group)

Reptilia

Chelonia Tetudo cf. emvs ScHLEGER et MuLLER Miyakojima (Amagawa Cave>

Okinawa-jirna (caves)

Clemm)s cft rrtutica (CArgToN) Miyako-jima (Arnagawa Cave)

by the articles ; ToKuNAaA and TAKAi (1939),OmsuKA and HAsEaAwA (l973),(1973)

ToKuNAGA (19. 2K)), OlsuKA (1941), HAsEGAwA et al. (1973), NoHARA

FosTER (1965),and HAsEGAWA

the photographs of those fossil molars, KAMEi (1970) suggested that these were "Archi-

diskodon" rather than Pataeoloxodon.

The fossil horizon of Kyan-misaki is the B Member of the Upper Formation in the

writer's dividion. NoHARA and HAsEGAwA (1973) identified it as a fragmentary molarof Palaeoloxodon? sp., but they stated that the possibility of "Archidiskodon" still remains.

If these elephants are "Archidiskodon", it is possible to correlate the Ryukyu Group

with the upper part of the Osaka Group and the upper part of the Kazusa Group.The highest horizon of the Ryukyu Group, therefore, is considered to be Early and early

Middle Pleistocene.

Additionally, it was reported by SHiKAMA et al. (1975) that the Cho-chen fauna ofTaiwan, Early and Middle Pleistocene in age, is represented by Mummuthus armeniaczes

taiwanicus and ElePhas hlsudricus. The relation between the fossil elephants of Taiwan

and the Ryukyu Islands is a very interesting problem.

Based on the paleontological aspects mentioned above and the geological relation


between the terrace formations and the Ryukyu Group, the writer considers the age of

the Ryukyu Group as Early to early Middle Pleistocene. It may be possible that theRyukyu Group is correlated with the upper part of the Osaka Group.

After the general report of Th230 -Pa2Si ages of the "Ryukyu Limestone" by Ko-NisHi et al. (1974), all measured values are included in Late Pleistocene. But it seems

that these values are inconsistent with paleontological evidence and the results of the

geohistorical considerations of the Ryukyu Islands mentioned below.

B. "UrumaCrustalMovement" Many faults caused network dislocation in the Ryukyu Group, and consequently,many tilting blocks are formed (Fig. 7). In so far as Southern Okinawa area in con-

cerned, the C!clocllPeus bed is distributed in various heights from near sea level to about

155 m above sea level due to such dislocation. Covering the area from Central to Sou-

thern Okinawa, a dome structure with a long axis ofNE to SW trend is confirmed (Fig.

17), which is called the Nakagusuku Dome (FLiNT et al., 1959). The fault system ofthese districts is supposed to be urdergone through the destruction process of the Naka-

gusuku Dome (KizAKi and TAKAyAsu, 1976; TAKAyAsu, 1976b). The fault system which dislocate the Ryukyu Group are also developed in Miyako-

jima and Kikai-jima, and those structures are clearly recognized from topographical

features. In these areas without exception, the thick Shimajiri Group exists below the

Ryukyu Group. This fact suggests that the tectonic movement such as dome formingand the fault system development is closely related with the extermination of the sedi-

K2,.e,,,,,i//,,//,I/i'i'•i/1/l'l-liii/illil"i,'i,''kl'i'''''

1,

' ./PENINsuL ,.1./.,,i;.,,,/Si,/1:,.l./ .1

tt /it/ttttttttttt t"ttt 1

/

eAv

llllllif1((S diXs,

'Xgek @

e

lliiX.ii

CHINENf5km

3.[E]

4.Y 5./ 6./

4

g

7. ><E'

18.

"

1lt

t

Fig. 17. Structural outline of Central andSouthern Okinawa (TAKAyAsu, 1976b). 1; "Rynkyu Limestone", 2;Shimajiri Group, 3; Basements, 4;General structure of the RyukyuGroup, 5; General structure of theChinen Sandstone, 6; General struc-ture of the Shimajiri Group, 7; Axis

of the Nakagusuku Dome, 8; Fault.


mentary basin being continued from Neogene time.

It is accepted by all researchers that the relation between the Shimajiri Group and

the Ryukyu Group is generally represented by an angular unconformity. This relation

is clearly observed in the west coast of Okinawa-jima, where the Shimajiri Group in-

clines to the east or southeast, while the Ryukyu Group has gentle structure or slightly

inclines to the west in direction. In the east coast of Central and Southern Okinawa 'however, both ofthe Groups incline to east or southeast gently. So, it is possible to say

that the relation between the two Groups is represented as a whole by an indistinct un-

conformity or disconformity. In several places, the Ryukyu Group is successively with-

out any marked break with the Chinen Sandstone, as in the cases of Chinen Peninsula

and Henza-jima, off the coast ofKatsuren Peninsula,

The writer, with reference to these significant relations, tried to explain the conver-

sion from the "Shimajiri Sea" to the "Ryukyu Coral Sea". It was explained by means

of the shifting of the upheaval center from west to east (Fig. 18; KizAKi andTAKAyAsu, 1976; TAKAyAsu, 1976b). That is, at the close of the "Shimaijri Sea"stage, the upheaval center of the area where was to the west of Okinawa-jima became

to be covered widely by shallow facies, which was represented by the uppermost part

ofthe Shimajiri Group (LERoy, l965; NoHARA, 1976). Succeedingly, thesedimentarycenter of the basins was shifted toward the east. As a result, the Shimajiri Group was

characterised in having the structure with uniform inclination to the east or southeast,

and thus the emerged areas came to be under erosion. Succeedingly, the "RyukyuCoral Sea" was initiated by the next transgression.

During the "Ryukyu Coral Sea" stage, the upheaval center was shifted further to

the east, and the embryo of the Nakagusuku Dome became to exist. The local uncon-

formity present between the Lower and Upper Formations in the Ryukyu Group, asmentioned in Chapter III, might be the result of that dome forming. Further, thedevelopment of the coral reef in the C Member of the Upper Formation may also beaccompanied with the appearance of the shallow environment due to the accentuation

of the Nakagusuku Dome. Later, after the collapse of that dome by faulting, the pre-

sent configuration of Okinawa-jima was brought about. A series of these tectonic

movements is called the "Uruma*** Crustal Movement" (OKiNAwA QuATERNARyREsEARcH GRoup, 1976; KizAKi and TAKAyAsu, 1976; TAKAyAsu, 1976b).

Taking a general view of the Ryukyu Group, it is possible to say that the Plio-

Pleistocene sea of this area went to extinction step by step through the course of the

"Uruma Crustal Movement", and the estate of the "Ryukyu Coral Sea" of the laststage is the Ryukyu Group itself.

Thought the details of the "Uruma Crustal Movement" will be discussed in ano-

ther paper, it is possible to infer that similar tectonic movement had an influence throu-

*** "Uruma" means coral is!and in the dialect of Ryukyu.


(T}

Kume

vY: ,saAt

Tonaki Kerama Okinawa

--+

"+

x ',tt.q l--)t-lst.';

eea•. ':.. .

//'ltt' /'..'1/l'/. ,."/t.1

{R}vJvNv ,-,A -+ --

--t

Cs)

clN[Tlu-.

g•/'/g"'i''

i`/L/////i/i.,/,/i//il/•i/.,,-,.///i'//11/l//l///"

L

S'-1•r

s,

z(>

-.-S'

' "i'I"" ff

vv vv A, AA '

.

E

-;+: b l•k{•i-

'1111;/'t/li/i l'1}11: '

..tll,/.l.i.i.. ,.ul••leS.tse;m.tt.-J

.•' -.: Å}. ,x'v w...' IX. .'.'.{ .•` .'

••• • ",f i,-/w•"•/,}'/'/l/r/:\/,•'//l•,f'••/'ii'l•

Fig. 18. Schema showing the process ofthe "Uruma Crustal Movement" (KizAKi and TAKAyAsv, 1976). (S) The last stage of the "Shimajiri Sea". (R) The stage of the "Rytikyu Coral Sea". (T) Post-"Ryukyu Coral Sea" stage.

ghout Ryukyu Islands, , Therefore, as the Quaternary tectonic movement, the"Uruma Crustal Movement" has a significant position in the establishment of theRyukyu arc.

C. OutlineoftheQuaternaryGeohistoryoftheRyukyuIslands In the foregoing chapters, the writer has come to conclude that the Ryukyu Group,

main part of the "Ryukyu Limestone", was the product of the tectonism during Earlyto early Middle Pleistocene, and not ofeustatic movement alone. From this viewpoint,

he tries to make an outline of the Q;uaternary geohistory of the Ryukyu Islands here-

inafter. For converience, it will be mentioned in following stages successively.

(I) The stage prior to the Ryukyu Group sedimentation, or the last stage of the

"Shimajiri Sea" (Earliest Pleistocene).

(II) The "Ryukyu Coral Sea" stage (Early-early Middle PIeistocene)

(II/III) The maximum stage ofthe "Uruma Crustal Movement" (middle MiddlePleistocene)

(III) The Minatogawa stage (late Middle Pleistocene-Late Pleistocene)

(IV) The Ryukyu Islands stage (Late Pleistocene-Holocene)

(I) The last stage of the "Shimaj'in' Sea"

As a result of depth decrease of the Shimajiri Sea, calcareous sands deposited and

"Ryukyu Limestone" of Okinawa-jima, South Japan r69

the Chinen Sandstone was formed. Accompanied with it, vast land areas emerged and

extended near and a!ong the present position of the Ryukyu Islands as the hinterland

for successive sedimentation. Probably deer of Metacervulus migrated into the Ryukyu

Islands area from the continental area through land bridge of East China Sea. It is

probable that Honshu and its adjacent islands had been already separated from theRyukyu Is!ands, and yet the latter might be without connection to Taiwan, because

those fossil deer have not been reported from Honshu and Taiwan.

(U) The "R2ukyu Coral Sea" stage It is the stgae in which transgression initiated the deposition of the Ryukyu Group

and completed its accumulation. This stage is subdivided further into the early and the

late substages.

The early substage is represented by the sedimentation of the Lower Formation of

the Ryukyu Group. At first, a large quantity of terrigeneous materials flowed into the

marginal sea from the land area of hinterland which still remained widely nearby.Consequently, the resultant decreasing oftemperature, salinity, transparency, etc. ofsea

water might spoil the activity of reef-building organisms. Nevertheless, following the

depression ofbasin floor oceanic conditions were introduced to the Ryukyu Islands area.

Thus, some parts of the sea area might promote favourable conditions for the coral reef

building. But some temporary pause ofbasin depression brought about suspension orrelative drop of the sea level, and thus, the sedimentation of the Lower Formation of the

Ryukyu Group ceased. The late substage is the sedimentation of the Upper Formation of the RyukyuGroup. As the sea level was rising again rapidly, the sea covered the Iand area morewidely and the depth deepened for a while. After that, the depth of the sea decreased

gradually, and at last the coral reef building became more active than the previous time.

Alongside ofthis, the tectonic movements such as the Nakagusuku Dome forming mightcontrol or influence the sedimentary facies variations. Early in this substage, archaic

elephant or primitive "Mammuthus", "Archidiscodon", existed in the Ryukyu Islandsarea, and these elephants seem to have close relationships to the fossil elephant of the

Cho-chen fauna ofTaiwan of that time. Therefore, it is probabale that the transient

land connection between Taiwan and the Ryukyu permitted elephant migration.

(II/III) The maximum stage ofthe "Urama Crustal Movement"

It was the stage ofdestruction ofthe Nakagusuku Dome and dislocation ofthe Ryu-

kyu Group. Thus, many tilting blocks were formed. Isolation of the Ryukyu Islands

were completed, because continental land mammals of the Middle Pleistocene Chou-k'outien fauna and the Late Pleistocene Huangt'u fauna, which were present in Taiwan

in one hand and Honshu area on the other hand, are not known from the RyukyuIslands.

(III) The Minatogawa stage Since this stage, it semes that the influence of the eustatic sea level changes surpass

Table 6. Summarized chart of the geohistory during the late Neogene and the QLuaternary periods of the Ryukyu Islands

AgeHOLCENE

t.1.1

zwUo'ut

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Ja

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E

A-Lew

wzt.L.1

UoJa

Stratigraphy & Lithotogy

IV

111

H

Recent Reefeeachde osits

limestone. sand. gravet

Rt ised CotvtReef

LoAtTerracedep. &

vefitti s

coraUine biotithite

mdiUmooLnLtutuvF

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a=oLeL-

l=UE

=ut

i"

corattine biotithite.

gravet.ctay

aLirnestoneForTnafon

co

•+-.

oE6LLeaaD

c

BMem.

AMem

sparry calcarenite

corattine bio{ithite

ccralline biolithite

coralline andlor atgat bioclastic Ls.

foraminiferou$ rnicritic L.s.

co•.-"oE6Lts

)oJ

Chinen Sand

c o •;oU6E2 ts

•- L=ut

i'i" r, iti. -•, //.g

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bioc[astic Ls.,

catcareous sand,

noncaLc. sand, graveL. silt

cdcareous sand

"t -..'tlf.

::Il•/ •,•1,/.l./•i.

;t

;tLtlt-1'l-l-t-'{t-i-t-l-1-tsll-

tuff,

sandstone sittstone

c=oL' gUoeE;.di.

sitts{one.

sandstone

Pateontot ogy

Mammat

Capreetus "A rchidiskoden"

Metacervutus

(Corat-Hatimeda abundant)

(Corattinaceae abundant)

X Pticatuta t XCyctectypeus ' X oparcblina

O "A rchidiskodon "

11il

il

I

l

ists

tsxE•!

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t'

'

,

,11'

1

N"

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Rs.RR"ljSE6

t)

Catcareous nannofossiLr)

Geophrrocapsaoceanica Zone

Pseudoemilia taeunesa. Z.

.Discoaster

btounvi Z.

Disccasterpentaradlatus Z,

Retievtotenestrv

tettculate Z.

lMetaceryuius

Potten flora s)

leh.N" cRgeNljNgg

ts

ih e

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Geohi story

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Ee2

an.E•==oL

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:,

,

1) Natori

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ti Postglaciat transgression(+2rT"

Mi,natogcrwa tranagression

{+55m?,+20-30m)

opotuut

oLoUJhxJ

lnnerubllttoral

uterSutiittor

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or lniet-Estuari

c.9ut

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Outerubtittoral

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Separated fromContinent

Utuut

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•=ut Upper

ath at

Mstecervukes

immlgrcte from China continent

et al. (1972), Ibaraki and ttTsuchi (G975) ; 2) Nishida (1975) ; 5) Nishida and Itokazu (1976)

-Nlo

g:

gs.

?ge

K%a


that oftectonic movements in the sedimentation. The terraces and the accessory lime-

stones were formed around each isolated land. The latter is represented by the Minato-

gawa Limestone Formation, and the highest sea level of that time is inferred to be 20-

30 m (or maximum about 55 m) above present sea level. Most of the deposits in caves

and fissurs of the "Ryukyu Limestone" might belong to this stage.

(IV) TheR2uk2ulslandsstage Formation of the raised coral reefs at the height of about 2 m above present sea

level. Deposition of the dune sand and other beach sediments. Formation of the pre-

sent coral reefs.

The sequence of the geological events metioned above is shown in Table 6.

Acknowledgements

In carrying out fieldwork and in completing this study, I have been aided by many

persons and several organizations to whom I wish to express my sincere thanks. Es-pecially, I am grateful to Prof. T. KAMEi, Prof. K. NAKAzAwA, Associate Prof. S. IsHiDA,

Associate Prof. T. SHiKi, Dr. D. SHiMrzu and Dr. T. ToKuoKA of Kyoto University for

their helpfu1 suggestions and critical readings ofthe manuscript, and to Prof. lkzAKi and

Associate Prof. H. FuRuKAwA of the University of the Ryukyus, Associate ProÅí Y. OKi-

MuRA of Hiroshima University and Dr. T. SHiBAsAKi a consulting geologist for their

continuing guidances and encouragements. I am also much indebted to the members ofthe Geologist Association of Okinawaand the Okinawa Quaternary Research Group and fellow students of the Department ofGeology and Mineralogy, Kyoto University for their criticisms, insights, arguments and

general commentaries and dialogues, which have added up to a large contribution to

this work.

Acknowledgements are likweise due to Mr. M. KuRoKAwA of Okinawa GeneralBureau, Dr. K. ONo and Mr. M. TANAKA ofKokusai Aerial Survey Co. Ltd. and Dr.K. TsuJi of Nippon Koei Co. Ltd. for their ready offering of the unpublished data and

helpfu1 suggestions, and Mr. H. TsuTsuMr of Kyoto University for preparation ofmany

rock thin sections.

References

Fur(T, D. E., CoRwiN, G., DiNGs, M. G., FuLLER, W. T., MAeNEiL, F. S. and SApLrs, R. A. (1953), Limest one Walls ofOkinawa. Bll. Geol. Soc. Amer., 64, pp. 1247-1260. , SApus, R. A. and CoRwiN, G. (1959), Military Geology of Okinawa-Jirna, Ryukyu-Retto. 5. Ceol. Surv. Branch, lntell. Div. Ofice Eng. Hg. U. S. Arm] Ferces, Far East, Personnel of U. S. Ceol. Surv,

88 p.FoLK, R. L. (1959), Practical Petrographic Clasbification of Limestone. Butl. Amer. Petrl. Geet., 43,

pp. 1-38.FosTER, H. L. (1965), Geology of Ishigaki-shima, Ryulcyu-retto. Prof. Paper, U. S. Geol. Surv., 339-A,


119 p.FuKuTA, O. et al. (1971), Neogene of the Ryukyu Islands. S"mposium on Geological Problems in the AdJ'acent

Sea Province ofKJushu, pp. 91-101. (inJapanese)HAr"zAwA, S. (1935), Topography and Geology of the Riukyu Islands. Sci. ReP. Tohoku ImP. Univ., 2nd Ser., 17, 59 p.

(1948), A Comparative Study of the Fossil Foraminiferal Fauna of the Ryukyu Limestone and the Recent Fauna ofthe Ryukyu Islands. Jaztr. Geol. Soc. JaPan, 54, pp. 123-124. (inJapanese)

HiRATA, K. (1956), Ecological Studies on the Recent and Raised Coral Reefs in Yoron Island. Sci. ReP. Kagoshima Univ., (5), pp. 97-118.

(1958), Ecological Studies on the Recent and Raised Coral Reefs in Yoron Island II. ibid., (7), pp. 69-84.

IBARAKi, M. and Tsucm, R. (1975), Planktonic Foraminifera from the Upper Part of the Neogene Shimajiri Group and Chinen Sand, the Okinawa Island. ReP. Fac. Sci. Shizuoka Univ., 10, pp. I29-

143.KAMEi, T. (1970), Horizon of the Elephant Fossil in Miyako Island. Tsttkttmo-Chigaku, (5), pp. 1-8.

(in Japanese)

KizAKi, K. and TAKAyAsu, K. (1976), Outline of Geohistory of the Ryukyu Arc. "Kai o Kagaku" (Marine Sct'ences), 8, pp. 50-56. (in Japanese with English abstract)

KoNtsHi, K. (1965), Geotectonic Framework of the Ryukyu Islands (Nansei-shoto). Jottr. Geol. Soc. Japan, 71, pp. 437-457. (in Japanese with English abstract)

, OMtJRA, A. and KiMuRA, T. (1967), Dating and Rates of Vertical Displacement of Reefy Limestones in the Marginal Facies of the Pacific Ocean. JaPan. Quart. Res., 6, pp. 207-223. (in

Japanese with English abstract) ,ScHr.ANGER, S. O. and OMuRA, A. (1970), Neotectonic Rates in the Central Ryukyu Islands derived from Th2SO Coral Ages. Mar. GeeL, 9, pp. 225-240. , OMuRA, A. and NAK2iMicHt, O. (l974), Radiometric Coral Ages and Sea Level Records from the Late Quaternary Reef Complexes of the Ryukyu Islands. Proc. 2nd lnt. Coral Reef SJm. 2. (Great Barrier Reef Committee, Brisbane) pp. 595-613.

LERoy, L. W. (1965), Smaller Foraminifera frorn the Late Tertiary of Southern Okinawa. Proc. PaPer, U. S. Geol. Suri,., 454-F, 58 p.

MAcNEit, F.S. (1960), The Tertiary and Quaternary Gastropoda of Okinawa. Prof. Pa]ber, U. S. Geol. Surv., 339, 148 p.

NAKAGAwA, H. (1967), Geology of Tokuno-shima, Okierabujima, Yronto and Kikaijima, Amami Gunto (I). Contrib. Tohoku Uni.v. Inst. Geol. Paleent., 63, pp. 1-3. (inJapanese with English abstract)

(1969a), Geology of Tokuno-shima, Okierabujima, Yoron o and Kikaijirna, Amami Gunto (II). ihid., 68, pp. 1-17. (inJapanese with English abstract)

(1969b), Quaternary Sea-level Changes in the Ryukyu Tslands. QLuaternar7 Svstem of JaPan (ed.

Research Group on the Quaternary ofJapan), Monograph 15, The Assosiation for the Geological Col- laboration inJapan (Chidanken), pp. 429-435. (inJapanese with English abstract)

NAToRi, H., FuKuTA, O. and IsHmA, M. (1972), Younger Cenozoic Foraminiferal Biostratigraphy in Okinawa and Miyazaki Prefecture,Japan (Preliminary Rep.). Jour. JaP. Asoc. Petrol. Thech., 37,

pp. 416-421. (in Japanese)

(1976), Planktonic Foraminiferal Biostratigraphy and Datum Planes in the Late Cenozoic sedimentary Sequence in Okinawa-jima, Japan. Progress in MicroPaleontologl (ed. TAKAyANAGr, Y.

and SATTo, T.), pp. 214-243.NisHrDA, S. (1973), Preliminary Study of the Upper Cenozoic Calcareous Nannoplankton Assemblages from the Nansei Islands. Mem. Geol. Soc. JaPan. (8), pp. 65-75. (inJapanese with English abstract)

and IToKAzv, Y. (1976), A Palynological Study of the Shimajiri Group, Nansei Islands. "Kai o-Kagaku" (Marine Sct'ences), 8, pp. 201-206. (inJapanese with English abstract)


NoHARA, T. and HAsEGAwA, Y. (1973), An Elephantoid Tooth from Kyan, Okinawa-jima. (Studies of the Palaeovertebrate Fauna of Ryukyn Islands,Japan. Part III). Mem. Nat. Sci. Mus., (6), pp.

59-63. (inJapanese with English abstract) (1976), The Shimajiri Group on the Basis ofOstra-coda. Geologicat Stt"lies ofthe Rpt]u Islanals (ed. Kizaki, K.), 1, pp. 71-74. (in Japanese)

OKiMuRA, Y. and TAKAyAsu, K. (1976), Quaternary Geology in Limestone Area. (The Latest Studies on Quaternary Geology. 8). " Tsuchi to Ktso" (Soil andFoundations), 24, pp. 65-72. (inJapanese)

(1976), The QLuaternary System of Yoron lsland, Ryulcyu Islands: as regards the Stratigraphy and Sedimentology in the "Ryukyu Limestone" Area. Geological Studies of the RLpmkLJ,tt Istands (ed.

KizAKi, K.), 1, pp. 97-109. (inJapanese with Eng!ish abstract)

OkiNAwA QuAiERNARy REsEARcH GRoup (1976), Quaternary System ofOkinawa and Miyako Gunto, Ryukyu Islands -Especially on the Stratigraphy of "Ryukyu Limestone". "ChikLyu Kagaku" (llarth

Science), 30, pp. 145-162.

OTsuKA, Y. (1941), On the Stratigraphic Horizon ofElephas from Miyako Is., Ryukyu Islands,Japan. Proc. ImP. Acad. Tok2o, 17, pp. 43-47.

O:suKA, H. and HAsEGAwA, Y. (1973), Fossil Deer from Ishigaki Island. (Studies ofthe Palaeovertebrate

Fauna ofRyulcyu Islands,Japan. Part II). Mem. Nat. Sci. Mus., (6), pp. 53-57. (inJapanese with

English abstract)

OyAMA, K. (1976), Biofacies of the Miyako-jima Limestone and the Discovery of Pleurotomariidae Fossil. GeologicalStudiesoftheR]rkluIslands (ed. KizAKi, K,), 1, pp. 125-126. (inJapanese)

SHrKAMA, T. and OTsuKA, H. (l971), Land Bridge of the East China Sea. SvmPosium on Geological Pro- blems in the Adj'acent Sea Province ofjK77 ushu, pp. 131-139. (inJapanese)

, and ToMiDA, Y. (1975), Fossil Probosidea from Taiwan. Sci. ReP. Yokehama IVat. Um'v., 2nd Sec., 22. pp. 13-62.

SHrNoHARA, Y. and KrzAKi, K. (1976), Geology of Kohama-jima, Yaeyama-gunto. Geologieal Studies ofthe R2ttkptu Islands (ed. KizAKi, K.), 1, pp. 37"ll. (inJapanese)

SHoJi, R. (1968), Sedimentological Studies on the Ryukyu Limestone of Okinawa, Ryukyu Islands. Sci. ReP. Tohoku Univ., 2nd Ser., 40, pp. 67-77.

TAKAy.4su, K. (1976a), The QLuaternary Limestones in the Northern Part of the Motobu Peninsula, Okinawa Islands. Jour. Geet. Soc. Japan, 82, pp. 153-162. (inJapanese with English abstract)

(1976b), Re-examination on the QLuaternary System of Okinawa-jima, Ryukyu Islands. Geologt'cal Studies of the Rlttk.yu Istands (ed. KTzAKi, K.), 1, pp. 79-96. (in Japanese)

ToKuNAGA (formerly YosHiwARA), S. (1901a), Note on the Raised Coral Reefs in the Islands of the Riukiu Curve. Jour. Cell. Sci. Imp. Tok),o, 16, (1.1), pp. 1-14.

(1901b), Geologic Structure of the Riukiu (Loo Choo) Curve, and its Relation to the Northern Part of Formosa. ibid., 16, (1.2), pp. 1-67.

and TAKAr, F. (1939), A Study of Metacervulus astylodon (Matsumoto) from the Ryukyu Islands, Japan. Trans. Biogeo.or. Soc. JaPan, 3, pp. 221-248.

(19iro), A Fossil Elephant Tooth Discovered in Miyakozima, an Island of the Ryukyu Archi- pelago,Japan. Proc. Imp. Acad. Toltvo, 16, pp. 122-124.

YABE, H. and HANzAwA, S. (1930), Tertiary Foraminiferous Rocks ofTaiwan (Formosa). SctL ReP. Tohoka lmp. Univ., 2nd Ser., 14, (1), pp. 1-tK}.

. and HATAi, K. M. (1941), A Sapplementary Note on the Fossil Marine Fauna from Okinawa- jima, Ryukyu Islands. Jour. Geol. Soc. JaPan, 48, pp. 78-83.

YAMAzATo, K. (1959), Ecological Studies on the Coral Reefs of Kume Island. I. Disuibution of the Coral Reefs. Bull. Arts. and Sci Div. Univ. R),tcklus (Math. and IVat. Sci.), (3), pp. 53-64.

. (1960), On thc Limestone ofMiyako Island. ibid., (4), pp. 88-92.

YAzAKi, K. (1976), Stratigraphy and Sedimentary Process of the Miyako-jima Limestone. Geologicat Studies ofthe R"uklu Zslartds (ed. lkzAKi, K.), 1, pp. 111-121. (inJapanese)


Chinen joft..

Haneji-naikai ntept-uaIshikawa EIjll

Katsuren maifKyan-misaki Sesrtthrp

Motobu AzzNakijin A-ratOyakebaru meeetnTako-yama ;iLtJUrasaki 'mathYagachi-jima msRteB

Place Narnes

Goga,Guga geaHedo-misaki mu)!fMqu

Itokazu "SxXKouri-jima ili"Iii*UU

Machinato,Makiminato t5cueNaha aseeNakoshi tvE71lShimajiri EntTengan )EivaUrasoe thutYomhan,Yontan tiiiii' 2:i

Gushikawa R,=Li:.JII

Henza-jima SIZImbesItoman "SN'ra

Kunlgami MeeMinatogawa 'rs)i]Nakagusuku rptwo-jima eqrkBTairagawa SIZNJII

Unten if:EYamada-onsen LIJNim'7Rzakimi ptggik

Explanation of PIate 14

Fig. 1. Cuestatopographyformedbytiltingblocks,westernpartofSouthernOkinawa•Fig• 2. Well stratified clastic limestone facies of the Lower Formation of the Ryukyu Group bent by

faulting, near Minatogawa, central part of Southern Okinawa.Fig. 3. Coralline biolithite facies of the C Member of the Upper Formation of the Ryukyu Group, near

Yamagawa, Motobu Town, western part of Motobu Peninsula.Fig. 4. 0Perculina limestones of the A Member of the Upper Formation of the Ryukyu Group, at Nakasone, Nakijin Village, eastern part of Motobu Peninsula.Fig• 5. Algal ball limestone of the B Member of the Upper Formation of the Ryukyu Group, near Itoman, western part of Southern Okinawa.Fig. 6. C)clecl)Peus in the A Member of the Upper Formation of the Ryukyu Group, near Itokazu,

eastern part of Southern Okinawa.

Explamation of Plate 15

Fig. 1. 0utcrop ofNo. M26, near Nakasone, Motobu Peninsula. Lcs; calcareous sand facies of the Lower Formation, Lcl; clastic limestone facies of the Lower Formation, UA; A Member of the Upper Formation represented by Opercttlina limestone with a dark band of terrigeneous clay layer. All this section belongs to the Ryukyu Group.

Fig. 2. 0utcrop showing the relation between the Lower and the Upper Members of the Minatogawa Limestone Formation, coast of Minatogawa, central part of Southern Okinawa. C; coralline biolithite of the Lower Member, F; foraminiferal sandy limestone of the Upper Member.Fig. 3. 0utcrop showing the regional unconformity between the Lower and the Upper Formation of the Ryukyu Group, near Gushikawa, the neck part ofKatsuren Peninsula. L; noncalcareous sand and silt of the Lower Formation, U; bioclastic limestone of the Upper Formation.Fig. 4. Weakly layered coralline bioclastic limestone facies ofthe C Member of the Upper Formation

of the Ryukyu Group, in Kouri-jima, off the north coast of Motobu Peninsula.Fig. 5. Cross lamination in the clastic limestone facies of the Lower Formation of the Ryukyu Group,

near Urasaki, Motobu Town, Motobu Peninsula.Fig. 6. Cross bedding in theclastic limestone facies ofthe Lower Formation ofthc Ryukyu Group, near

Tairagawa, Gushikawa City, Central Okinawa.


Fig

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Explanation of Plate 16

. 1. Foraminiferal and algal biosparudite, the Upper Member of the Minatogawa Limestone Formation. Co; coral, Al; coralline algae (Corallinaceae), Cl; Calcarina, Am; AmPhiste.gina, ic; intraclast, RG; grain of the Ryukyu Group, probably A Member as Operculina micrite. Loc.,

Minatogawa, central part of the Southern Okinawa.. 2. C7clocl7peus biomicrudite, the A Member of the Upper Formation of the Ryukyu Group. Cy; C]clocl),PetLs, Bz; Bryozoa. Loc., STM-Ol, near Itokazu, eastern part of Southern Okinawa.

. 3. Forarniniferal and algal biosparmicrudite, the B Member of the Upper Formation ofthe Ryu- kyu Group. Al; coralline algae (Corallinaceae), Ha; Halimeda. Loc., MB-1, Koechi, Nakljin

Village, Motobu Peninsula.. 4. 0Perculina and C7clocllPeus biomicrudite, the A Member of the Upper Formation of the Ryukyu

Group. Op; OPerculina, Cy; CJclocCyPeus. Loc., MB-2, Sakiyama, Nakijin Village, Motobu Peninsula.. 5. Foraminiferal and algal biosparudite, the top part of the Lower Formation of the Ryukyu Group. Ec; Echinoid spine, TG; terrigeneous grain, ic; intraclast, Al; coralline algae (Coral-

limaceae), Op; OPerculina. Loc., same to Fig. 3.

. 6, Well sorted foraminiferal and algal biosparite, the upper middle part of the Lower Formation

of the Ryukyu Group. Loc., M-50, Urasaki, Motobu Town, Motobu Peninsula.

Mem. Fac. Sci., Kyoto Univ., Ser. Geol. & Min., Vo]. XLV, No. 1. Pl. 14

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Title Ryukyu Limestone of Okinawa-Jima, South …...KoNisHi et al,, 1970; KoNisHr et al., 1974)...

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