title sedimentology and basin analysis of the paleogene

Title Sedimentology and Basin Analysis of the Paleogene MuroGroup in the Kii Peninsula, Southwest Japan

Author(s) Tateishi, Masaaki

Citation Memoirs of the Faculty of Science, Kyoto University. Series ofgeology and mineralogy (1978), 45(2): 187-232

Issue Date 1978-12-09

URL http://hdl.handle.net/2433/186629

Right

Type Departmental Bulletin Paper

Textversion publisher

Kyoto University

MEMOIRS OFTHE FACULTYOF SCIENCE,KYOTO UNIVERSITY,SERIESVol. XLV, No. 2, pp. 187-232, 1978

oF GEoL.&MINERAL.,

SedimentologyMuro Group in

and the

Basin Analysis

Kii Peninsula,

of the PaleogeneSouthwest Japan

By

Masaaki TATEISHI

(Received June 23, 1978)

Contents

Abstract

I. Introduction ll. Geologic Setting Stratigraphy and geologic structure Lithofacies M. Sedimentology of Conglomeratic Rocks, Thick-bedded Sandstones and Flysch-type Alternations A. Sedimentary structures and fabrics

1. Conglomerates 2. Thick-bedded sandstones 3. Conglomeratic rnudstones 4. Flysch-type alternations B. Transportation and deposition 1. Conglomerates 2. Thick-bedded sandstones 3. Conglomeratic mudstones 4. Flysch-type alternations rv. Submarine Fan Sedimentation A. Channel-scours and --fi11s B. Channel directions and paleocurrents in channel-fi11s V. Basin Analysis of the Muro Group A. Lithostratigraphic feature B. Properties of coarse sediments C. Sedimentation of coarse sediments D. Paleocurrent Analysis E. Submarine fan sedimentation F. Paleogeographic reconstruction VI. Summary and ConclusionAcknowledgementsReferences

Abstract

The Oligocene to lower Miocene Muro group in the Kii Peninsula represents the later stage ofthe Shimanto geosyncline which was widely extended along the Pacific coast of Southwest Japanfrom late Jurassic to earliest Miocene time. The Muro group is composed mainly of flysch-type

188 Masaaki TATEISHI

alternations of sandstone and mudstone, intercalating conglomerates, thick-bedded sandstones and con-

glomeratic mudstones. These sequences are well exposed in the southern coast area in the peninsula. Based on the analyses of the sedimentary features such as grading, stratification and clast fabric,and field o6currence as well, the conglomerates'are referred to have been transported and deposited

by grain flow and turbidity current cornbined. Thick-bedded sandstones are considered to have beentransported and deposited by turbidity current. It is safely concluded that conglomeratic mudstonessuch as pebbly mudstone and angular fragment-bearing mudstone were transported and deposited bydebris flow. All of these coarse sedirnents were deposited in proximal suit of deposition.

There are observed several channel-scours and -fi11s in the upper forrnation of the Muro group,which indicate the submarine fan deposition of the formation. Paleocurrent analysis based on sole marks and clast fabrics shows that the provenances existednot only to the north, but also to the south of the basin. General trends of the channels and paleo-

currents in channel fillings indicate the distributary channel system developed on the submarine fan

in the southern side of the basin. The existence of the Kuroshio Paleoland estimated to the southof the Shimanto terrain by the Kishu Shirnanto Research Group is strongly supported by the presentstudy.

I. Introduction

The Shimanto terrain situated in the southernmost part of outer zone o'f Southwest

Japan extends more than 1,OOO krn in length with a width of more than 200 km. Little

has been known about the geology of the Shimanto terrain for long years. However,energetic surveys have been done for the past ten and several years, and the geology of

the Shimanto terrain has been made fairly clear. In '1975, KisHu SHIMANTo REsEARcH

GRoup summarized the geology of the Shimanto terrain in the Kii Peninsula, anddiscussed the development of the Shimanto geosyncline. In this paper the group at-tempted the paleogeographic reconstruction of the Shimanto geosyncline from the late

Cretaceous to the early Miocene, and inferred that the source land must have existed

not only to the north as generally considered, but also to the south of the Shlmanto

geosyncline. This hypothesis based mainly on the paleocurrent analysis and properties of

coarser clastic sediments is important not only for the consideration of the geologic

development of the Shimanto terrain, but also for the study of basement rocks of theJapanese Islands and furthermore of the development of the Philippine Sea. So it must

be confirmed by the more detailed reconstruction of paleogeography. ' The Oligocene to lower Miocene Muro group in the Kii Peninsula representing thelater stage of the Shimanto geosyncline has been studied in detail stratigraphically and

sedimentologicall•y, and its paleo-environment has been made fairly clear (KisHuSHiMANTo REsEARcH GRoup, 1975). In order to clarify in more detail the sedimentary

environment of the Muro group, the present writer examined the sedimentation ofcoarse sediments such as conglomerate and thick-bedded sandstone, and of channel-scours

and -fills. Main purposes of the present paper are 1) to describe the sedimentary struc-

tures and fabrics of coarse sediments and to consider the sedimentary process of them,

2) to describe the channel structures and their sedimentation, and 3) to clarify the

Sedimentology and Basin Analysis of the Paleogene Muro Group 189

paleogeography and sedimentary environment of the Muro group. The field-work was done mainly in the southern coast area of the Kii Peninsulawhere wave-cut terraces and cliffs are well developed and the various excellent sedimen-

tary features can be observed.

II. Geologic Setting

Stratigraphy and geologic structure

The Shimanto terrain occupies the vast area south of the Butsuzo Tectonic Line.

The Shimanto tercain in the Kii Peninsula is divided into three belts by the Gobo-Hagi

tectonic line and the Hongu fault, namely, the Hidakagawa, the Otonashigawa and the

Muro belts from north to south (Fig. 1). The Hidakagawa group (mainly late Creta-

ceous), having eugeosynclinal facies, distributed in the Hidakagawa belt. The Otonashi-

gawa beit is occupied by the Eocene Otonashigawa group, and the Muro belt by the

f "e ch'ich'tb"

' .>tTecto i

svte6u -HieaMgOWa

xtV's

.--

Belt"

Fig.

mf'essge

=

t7- r/LGgb-e=Ua-gL.

llX/'-N"- ..--

Fa a. 1-.t-. .Y.E k. tpt S. .- . ---- ---

,eEgki

Linewrt Belt 1 '

.7-?tL.l-:i.t,iii/!,/L,L•i'i'Llll.Iij.:',:,:,

- - t' - 1''' -.-...JL - -- t-HL ltL --lrtttttt- . - - ..:r. '/ '' .'. ,...

--=t-,., ,...... r-r-.Ltt,Lt; -..-i 'i g• :, :.4•t'• 'I:,il'(.=

'- ' 't =-.-:r-;,Elll ': ': i• ii ill'.p -• J:

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:' []Ill]Middie E

tatLowerF. -

1. Geologic outline of

a: Median Tectonic 1: Kimidani Anticline 4: Samoto Fault,

'o'eGreuPg

O'-'

:1'illpt'9fi'i'.L:'t/l'il'.Ll•

11:. 1--L i31'3o'

tttttt tt-ttttttt::::Åë::::::::1•; 1

.:• x-s-'

i

." iFo

Ioal

t- oo'o' :'

t-t oottt o.o.tt

---T ' -l;Ss}Sio'o'e'o

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9oe

o

eo

eo

o

e

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'i nr' 11

the Shimanto

Line, b:, 2: 5:

L ' o' o' ' o7- -1•-oos, ttto.e.o.

"y

'-oo

i-l-33xo•

Fig.2 OL-"t---.= tU(m

terrain in the Kii Peninsula.

Butsuzo Tectonic Line, Uchikoshi Anticline, 3: Susarni Anticline, Wabuka Anticline.

:tJ3r3ot

190 Masaaki TATEISHIO!igocene to lower Miocene Muro group, composed entirely of terrigenous sediments

lacking in ophiolites and radiolarian cherts. The Otonashigawa and Muro groups areoverlain with clino-unconformity by the middle Miocene Tanabe and Kumano groups inthe west and east respectively.

Stratigraphic, structural and sedimentologic studies of the Muro group have been

much advanced by the collaborative research group for the Shimanto terrain in the KiiPeninsula (KisHu SHiMANTo REsEARcH GRoup), of which the present writer is a member,

for the past ten and several years. The results were summarized by KisHu SHiMANTo

REsEARcH GRoup (1975). According to this paper, the stratigraphy and lithofacies of

the Muro group are summarized as in the followings. The generalized geologic map of

the Muro belt is shown in Fig. 1, and the generalized columnar section of the Muro

group is shown in Fig. 3. The Muro group, of which total thickness attains to 7,500to 9,OOO m, is composed mainly of fiysch-type alternations of sandstone and mudstone,

often intercalating thick-bedded sandstones, conglomerates and conglomeratic mudstones.

The geologic age of the Muro group is assigned as the Oligocene to the lowerMiocene based on molluscan fossil evidence. The group is subdivided lithologically into

the lower (A), the middle (B) and the upper (C to H) formations. The lower forma-

tion is composed mainly of mudstone and muddy fiysch. The middle formation is com-posed predominantly of sandy flysch and thick-bedded sandstone. The upper formation

is composed of various kinds of sediments such as mudstone, muddy fiysch, conglomerate,

thick-bedded sandstone, pebbly mudstone and angular fragment-bearing mudstone. The Muro belt can be divided into northern and southern blocks by the Matsune-

Hirai fault. Main folding structures in the Muro belt are the Uchikoshi anticline and

the Kogawa synclinorium in the northern block and the Susami anticline and the Wabuka

,teees/k•igiivaz•/vaxk'kVlll'

Yokoshima Bt A ' Tenpseki 2. Geologic map of the studied area, compiled from KISHu SHIMANTO RESEARCH GROup (1969, 1973) and the present writer (TATEIsHr, 1976).

N •o.o o,o- T • o.

•: ' ge • '

mb[- -.. --- -J-t

. .g •:'•'

ar2 :' 'zii2'

ll,lpm•i

Fig.


The present study deals with the southern part of the Muro belt shown by aquadrangle in Fig. 1. The geology along the southern coast exceeding 30 km in length

was reported in detail by KisHu SHiMANTo REsEARcH GRoup (l969, 197S) and thepresent writer (TATEisHi, 1976). The geologic map and columnar sections of this areaare shown in Figs. 2 and 3 respectively. The Muro group in the present area attains to

Columnar Sections

Hlkl- Mlrozu

at the Coastal Area

Ameshima-', Tanosakl

lg"

NN

-1

2

t2tt13

3to

694s8

7

MtidstoneMuddy f!yschNorrnal flyschSandy flyschSandsteneConglomeratePebbly raudstoneMud$tone bearing

Bs

M!rozu- Ameshima B4

S4t5

B3

B2

Bl

-E!i]EIIill

EIIIII

[Iii[IEI!!] angular

----"-fi---------"ttm-t----------

ttt

fragment

N N sfi"'22 s hs

16

Generalized Cotumnar Sectlon

H

GN N Å~ s e-Ns F ssxNs Es s. -.sN ss D s '-- xC N.

t-

l

B

!-A

km

Fig. 3. Columnar sections of the Muro group. Each section was compiled from the followings;generalized section: KIsHu SHIMANTo REsEARcH GROUP (1975), Hiki-Mirozu: TATEIsHl(1976), Mirozu-Ameshima: KIsHu SHIMANTo REsEARcH GROuP (1973),Ameshima-Tano-saki: KisHu SHIMANTo RESEARcH GRoup (!969).The numbers at the right of sections show the horizons of localities examined sedimentologi-cally.


3,500 m in thickness, and obtained columns can be correlated to the generalized strati-

graphy of the Muro group as shown in Fig. 3. The middle formation of the presentarea is subdivided into five members by intercalations of mudstone and muddy flysch isNE-$W to ENE-WSW in the Kvestern and eastern parts of the present coast area. On

the other hand, that in the central coast area b'etween Mirozu and Ameshima is N-S, ttbeing discordant to the regional trend of the Muro belt. The lithofacies of the Muro

group is briefly descrlbed in the following section. Localities of conglomerates, thick-

bedded sahdstones and channel structures examined i-n •detail in' the present study are

shown in Fig. 4.

' Fig., 4. Localitttes of conglomerates, thick-bedded sandstones and channel structures observed

and exaniined. '' ' [ Lithofacies

••. . /. The lower formation: The lower formation is found in the axial part of the / ,./.,.tt,t..Wabuka anticline. It is about 600 m in thicknegs, ghowing a fining-upward sequence as

a whole. The lower part is composed mainly of sandy to normal flysch, in which solemarks and ripple marks are well developed, and frequently intercalates thick-bedded

sandstone and conglomerate. The upper part consists of muddy flysch and mudstone, ' 'rarely intercalating sandy flysch, thick-bedded sandstone and conglomerate. Thick-bedded sandstones and sandy fiysch, that will be described later, are found in the west

of Wabuka (loc. 16, Fig. 4). ' ttt The middle formation: The middle formation lies in the western half of thepresent.area. It qttains to 2,500 to 3, 10e m in total thickness. It is composed mainly of

sandy flysch and thick-bedded sandstone, intercalating mudstone, muddy flysch and con-

glomerate, and is subdivided into five members (Bl to B5 members) by mappable inter-

calations of mudstone and muddy flysch (B2 and B4 members). Molluscan fossils were

discovered in mudstone of the B2 member at Mirozu.


The Bl member distributing around Susami Bay consists mainly of sandstone 1 to 3

m thick in bed. Graded bedding and parallel lamination are well developed in thesethick-bedded sandstones. Several conglomerate beds are intercalated. These are mostly

granule- to pebble-sized, and are mostly bedded 1 to 3 m, sometimes up to 5 m. Thick-

bedded sandstone and conglomerate were well observed at five localities around Susami

Bay (locs. 4, 5, 7, 8 and 9) and at one locality north of Magari (loc. 6).

The B3 member is composed mainly of sandy fiysch and thick-bedded sandstone,frequently intercalating conglomerate. It shows a thickening- and coarsening-upward

sequence in •the lower part and a thinning- and fining-upward sequence in the upper

part. Pebble conglomerates of 5 to 10 m thick are frequently intercalated in the middle

part, which can be seen well at the east of Shirashima (locs. 10 to 13). Sandy flysch

and thick-bedded sandstone are well developed at Esuzaki (loc. 14).

The B5 member is found around Natachi. It consistsofthick-bedded sandstone and

sandy fiysch. Linguoid ripple marks and sole marks are well developed in these flysch

beds. Pebble conglomerates are intercalated in the upper part. Thick-bedded sandstones

were observed at locs.land 2. ' The upper formation: The upper formation is found in the east of the Azashifault. It is composed of many kinds of sediments such as mudstone, muddy flysch, sandy

flysch, thick-bedded sandstone, conglomerate, pebbly mudstone and angular fragment-

bearing mudstone. It is 900 m in total thickness, and is subdivided into C toGmembers

lithologically. The H member, the uppermost member of the Muro group, is not found

in the present area. Not a few molluscan fossils were found in the D, E and G mem-bers, especially in the last one.

The C and E members consist of muddy flysch and mudstone. The D member ischannel-fi11 sediments scouring out a part of the C member. It is made up ofthinning-

and fining-upward sequences from pebbly muds,tone or conglomerate to muddy flysch

through sandy flysch. The member is well observed at the east ofWabuka (loc. 17), the

west of Azashi (loc. 18), Yokoshima Island (loc. 19), Azashi (loc. 20), Soshima Island

(loc. 21) and Nagoshima Island (loc. 22) which will be descr•ibed later.

TheF member is composed of angular fragment-bearing mudstone in the lowerpart and of alternation of poorly sorted conglomerate and coarse sandstone in the upper.

The former contains various-sized angular clasts quite randomly. The clasts are pebbie-

sized gravels to large blocks up to 8 m in diameter of conglomerate, sandstone andmudstone. It was called "Sarashi-kubi (gibbeted head) bed" as a field name by KisHu

SHiMANTo REsEARcH GRoup (1969). The upper part of the formation contains non-turbidite sandstones which show a wedge-shaped large cross-lamination. Some kinds of

trace fossiis considered to show Cruziana facies are observed in the upper sequence.

The G member is composed of angular fragment-bearing mudstone, crudely bedded

1.5 to 2 m thick in the lower part, and of poorly sorted siltstone and mudstone in the

upper. The former contains clasts smaller than those in the F member, and most of


them 'are pebble- or cdbble-sized.

III. Sedimentology of Conglomeratic rocks, Thick-bedded Sandstones and Flysch-type Alternations

The Muro group is composed mainly of flysch-type alternations, intercalating fre-

quently coarse sediments such as thlck-bedded sandstone, conglomerate and congiomer-

atic mudstones. Textural properties and mineral or gravel composition of sandstone and

conglomerate in geosynclinal sediments have been examined especially to get information

on source tocks and provenance. To examine the depositional process of coarse sedi-

ments is important for reconstructing the paleogeography and estimating the sedimentary

environment of the geosyncline. The study of sedimentary process of coarse sediments is

not as advanced as the study of flysch-type alternation, which has been remarkably

developed for the last two decades since the turbidity current theory was presumed by

KuENEN and MiGLIoRiNi (1950).

There have been papers on conglomerate such as FisHER and MATTisoN (1968),WALKER and PETTiJoHN (1971), HENDRy (1973, 1976), RocHELEAu and LAJoiE (1974),MRAKovicH and CooGAN (1974) and WALKER (1975a). They have mainly describedsedimentary structures such as grading and stratification, textural properties and clast

fabrics. Recently DAviEs and WALKER (1974) and WALKER (1975b) proposed themethods how to describe the sedirnentary features of conglomerate in geosyncline. As for

massive and/or thick-bedded sandstone, there have been papers such as STAuFFER(1968), CHipplNG (1972), CoRBETT (1972), TyLER (1972) and LiNK (1975). They ex-amined and discussed mainly the internal sedimentary structures and lateral changes of

lithofacies. The process of transportation and deposition of coarse grains such as gravels

and sand grains was discussed in detail by MiDDLEToN and HAMpToN (1976).

In order to clarify their sedimentary process, it is important that sedimentary struc-

tures and clast or grain fabric are examined in detail. The present writer examined

sedimentary structures and fabrics of conglomerate and thick-bedded sandstone at 22

localities along the coast area shown in Fig.4. Conglomeratic mudstones and fiysch-type

alternations were also treated and are described briefly.

A. Sedimentary structures and fabrics

1. Conglomerates Layers of conglomerate with sandy matrix are frequently intercalated in the Muro

group, especially in the middle and upper formations (Fig. 3). The observed localities

and their stratigraphical horizons are shown in Figs. 3 and 4. Internal sedimentary struc-

tures, especially grading and stratification, were observed on 70 beds. Clast fabrics were

examined in 24 beds, and several beds were examined at plural points. Examined pointson clast fabrics attained to 29 in total.


a. General occurrence ' ' Conglomerate layers occur usually as a part of thick proximal sequ'ence together

with thick-bedded sandstones and sandy flysches. Typical successions composed mainly of

conglomerate are shown in columnar sections (Fig. 5). Each layer of cQnglomerat'e is

'

5M

o

KeLdtecm,;rl

•. 1.

f.P,

g- g•

iltp,

9,ss.

c.,p,

g•

M:g:.rk• N

'f,p•

f.p•

T!3 lck'b.b,

g-+p•)

fi:bb.j

[n :et:,Tliima

t•P•

rn,ss,

c,ss. tg•I

g•

/F-i.4

[p•]

g•

ShTtashima,t ." .T .' ;',eb-vv

f[P.&p:

fi/.t

'tt't'i..B

'i.".',,

ShirsshimaLX Z':`-E

ll,2,.:cob

c,p"i[COb.I

f.p, c.p 11{Lc.ob,i

t.p,

Esuia Lkei

c.

g,Ss,'

m.ss,

to•

t.sSt

t,p.

"1".S.lt',,W

fi•:':'.•: •: t ap

:.: :.t.;s:, .'. S f-P'

:t:"" '"}' -

o peb. g tnds,:•' U;'t'..:..." •:, J e.,p.• .y .

e"

.,-

llts'/{'i'f/E,i"'

=.l stretlfbcetiog t.-'.',.'. crude stnltltcetie" T grad/"g l crude grsding

Fig. 5. Columnar sections of conglomerates showing internal sedimentary features and average gravel size. g: granule, c.p.: coarse pebble, f.p.: fine pebble, cob.: cobble, boul.: boulder, g.ss.: granular sandstone, c.ss;: coarse sandstone, m.ss.: medium sandstbne, (): associated gravel size.

Nurnerals at the right are'the ones of clast fabric data (see Table 2).

generally 40 to 200 cm in thickness, and consistituting gravels are mostly fine pebbles,

partially coarse pebbles and rarely cobbles. Granule conglomerates are also found fre-

quently. These granule conglomerates are intercalated in thick-bedded sandstones as lens

or lamina, and sometimes change gradually to sandstones without sharp boundary. The

bottom of conglomerate generally shows erosional or irregular surface, but sometimes

non-erosional planar surface.

b. Internal sedimentary structures

As shown in Fig. 6, DAviEs and WALKER (1974) recognized seven sedimentary struc-

ture types in resedimented conglomerates of the Cambro-Ordovician Cap Enrage forma-

tion at Gaspe, Quebec. They examined the relationship between sedimentary structuretype and gravel size, and made clear that sedimentary structures of conglomerate are

related intimately to an average gravel size. That is, an inverse grading is found at the

lower part of cobble conglomerate with or without boulder gravels. On the other hand,pebble and granule conglomerates are characterized by the normal grading and stratifi-

cation. Adding much more observation on other conglomerate beds, WALKER (1975b)proposed three descriptive models on the sedimentary structures of conglomerates,

namely, i) inverse-to-normal grading model, ii) graded-stratified model, and iii) disorgan- 'ized model (Fig. 7). . .' i


.o "/l/l,i/"i/i&•/L'/lo2eb

e.04Q"??"?ee•.5.or.o

?29.ebe2..O"2.3M'D'

"ie-%e-- -e".% 2'..,. ge.

.o.8:.g"O.e"6tt'.

gaO.oDbC,'6.eg

--- -i --•r.e .oLe. ., 4. . .: ..

'e' :e.d'"'"'e

- -a - t-e.' aeO'.eo"-tt-tt-tttod a.

o"e- e"-e'o"-"d

-a--Odb-eeoeo.eo.e.ot"tii':"t':::":"t:/

Oee6odoeo"60enJ' :---.;- e..-: e

:t. -e:-' :e•1=` .: -. /" .b::

'r' -:' 'e .:" . o. .-.t O'lf.:. -...- -f..;-T. orTv- tii-o,iLg".,,"/S/;.?i

'e.eOb.ij.leWas'e

-?=esgpttL.SS;JldyStd

-e-"..es "'ae

normal.Inverse

massive

.Inverse

pt1

N > ft

Jil

norrnal ts {w. mass.X

crude stratifiedit

x

t

Jl

l

g

Davies & Walker-=- T!EZ!)

'L/l.ge" //Åédi/t/..2,.

OelO.e.e.e.o

" ea-oOo- 2

a".Ob"ts""'

oeoAaa"DeaOSeVo"as'a"V.:..eeOe..P.e".".

: .e..e. o. e, e.

e ' e o"ba o O:.9.0.aaOd.`ea"a

".e g".e".8tfl'.O..a.eO

..ots.eo•" .obesa.":.

,Pet,OaeZ•'rdt"O.}..:.

gtratifi'ed 21b

'massLve

rnverge

norrnal

.Inverse

norrnal

inverse & norrnal ,

norrnal

cross stratified

.rnasslve

le"o'.sv8o."e.'e,

e•go;r•g&.o,g

eg. b:e. o.

Fig.

stratified

partially stratified

.masslve

6. Types of internal sedimentary features of conglomerates in the Muro group (present study) and in the Cap Enrage formation at Gaspe, Quebec (DAvlES and WALKER, 1974).

t- Descrzptzve Sedimentary Model

i. inverse•-to-normal .grading model:g.Q:.g".g,i,iii-'."e:S

norrnal oreblZt.""ga"g.o"o,"""ta.e.,, massive

#t.t".i/O..tSgasa.O.aCoaa inverse

ii. graded-stratifiedeeOaOcoeooa"non-ooeebeoeo---"." e".ea"." ." .e

eooae4e.lg60,.D.gt•a.e,q,o

g.-g .̀'g.

.[Oisg,rg2.

Dae.9ISeoo

iii'aoDo[>i"DOO =S OQssd".k9e2.E'beiltl."otba.g,.Q..:",Qa

t..•?sD9ge,9•trOg

Fig. 7.

rnode1

stratified

norrnal

disorganized' .rnodel

.masslve

Descriptive sedimentarymodels of conglomerates

proposed by WALKER(1975b).

Conglomerates of the Muro group were examined on the basis of the same methodsas their ones. Flrst, conglomerates of the Muro group were classified into seven sedi-

mentary structure types, which are different in a few points from their types (Fig. 6) ;

that is, their 3rd type (lower: inverse and normal, upper: normal) is omitted and a

cross-stratified type is newly added. Secondly, the relationship between each type and

average gravel size was examined, and summarized in Table 1, which differs from that

obtained by them in the following respects; i) the gravel size of conglomerates charac-

terized by inverse grading in the Muro group is smaller than that of conglomerates in

the Cap Enrage formation, ii) although the sedimentary structure types of conglomerates

in the Cap Enrage formation are classified clearly by their mean gravel size, there are

observed both types of inverse-to-normal grading and graded-stratified in the fine pebble

conglomerates of the Muro group. It may be said, however, that the three descriptive

sedimentary models, which were proposed by WALKER (1975b), are also recognized in


-!Tyeg..ofi//tttttitiits,

2Elll•J:'S"S'n:i

/-"--b- le.e.'.e.s 3(`S..e:.tTt.":':"N

4l/11/Lllli,iii

1::::.,::. 5 iii//'g•'ii';,

t -ttttt ' tt" tlt t -:

6 •'1'/',.-.:•

tt''ti'!:l/e:

'7IE'.2",7•gi.,e

wa"s..io.a..

sedi.st.rysg.u.r"e....L--co..b. ! g, .p.gb.If, peb• I grarr-7

normul , 3.S 4 lnverse • ' '--i'" ''"" 1 ' "' massive i 4 7• lnv.elie. - .-----'-' '-+-"-''--- 7-

•1 r?//";fS.assi.v.e2..+L.--. L, 5 /.fl ml .-3.--

Cg:Se.tifi.d 3 , 1 l 1 / 11'-r'-"'-'- +"'"' I ' l' '''' i-" ' stratified / '3 11 i cross "'' ' 'i I V' -"-r ..st.g:.t.ifieg. / ... . I s i 1 •-•i--- -] --

1• ]il

Table 1. Frequency of internal feature types

of 70 conglomerate beds classifiedaccording to clast size.

the conglomerates of the Muro group as a whole. Features of internal sedimentarystructures, especially of grading and stratification, are described in the followings.

i. Inverse-to-normal grading model

Cobble conglomerates and most of coarse pebble conglomerates are 6haracterized by

distinctive inverse grading at the base. Inverse grading part is generally 10 to 20 cm in

thickness. Usually a zone composed of larger clasts comes above the inverse grading part,

of which thickness is generally 10 to 20 cm. Normal grading or massivedivision is found

generally in the upper part. A typical example is observed at the west of Azashi (loc.

18 in Fig. 5, Plate I-1). A few conglomerate layers in which inverse grading develops

throughout a bed are observed.

Stratification is generally indistinctive in this type of conglomerates. Sometimes there

develops a crude stratification formed by arrangement of calcareous mudstone clasts of

several to ten cm in diameter.

ii. Graded-stratified model

Fine pebble conglomerate is characterized by development of normal gradingthroughout a bed or by development of inverse grading at the base. Both features occur

at the roughly equal rate in fine pebble conglomerates. Typical example of fine pebble

conglomerate with normal grading is observed at the north of Magari (loc. 6 in Fig. 5).

Granule conglomerate is characterized by development of normal grading throughout a

bed, and there develops no inverse grading part. Most of granule conglomerates aretransitional upward to sandstone as shown in Plate I-3.

Stratification is remarkably developed in this type of conglomerates. A typical ex-

ample of stratification is observed at the west of Shirashima tunnel (ioc. 10 in Fig. 5,

Plate I-4).

iii. Disorganized model

There are found several conglomerates which do not show sedimentary features such

as grading and stratification. These conglornerates are massive throughout a bed, and are

usually 40 to 200 cm in thickness. Sometimes these are transitional laterally to conglomer-

198 Masaaki TATEIsHIate showing some sedimentary structures of sedimentary models i and ii.

c, Clast fabrics

Fabrics of conglomerate are represented by clast orientation and clast imbrication.

Generally speaking, clast orientation means the preferred direction of long axis on

bedding plane, while clast imbrication means the inclination of clasts observed at cross

section vertical to the bedding plane. It is worth to say that secondary fabrics caused

by later tectonic deformation may sometimes be observed in conglomerates, however,there is no possibility of such fabrics in the conglomerates of the Muro group. There

can be found no rotation around each clast and no deformational structures at anyobserved localities. So all the clast fabrics treated here can safely assigned as primary

ones.

Although the orientation of conglomerate cannot be found easily at a glance at the

exposure because of bearing a large amount of spherical gravels, it can be detected by

paying attention to elongated clasts that have a ratio of long axis to short axis greater

than 1.5 and by treating them statistically. Clast imbrication can be observed rather

easily, because discoidal clasts also indicate the imbrication in the same way as rod-

shaped ones. Clast fabrics were observed and measured in 24 beds, and at 29 points in

total. The several points are shown in columnar sections in Figs. 5 and 10. The results

of measurements is shown in Table 2.

i. Method and results of measurements Clast orientation; After marking the strike of the bed, each exposure surfaceparallel or nearly parallel to the bedding plane was photographed. Only elongated clasts,

having a ratio of long axis to short axis greater than 1.5, were choosed and measured

(Fig. 8). All elongated clasts in each photograph were measured. The measured num-

bers of clasts range between 35 and 65. The clast orientations are shown collectively in

52

Fig. 8. Clast orientation atTNagoshima (loc. 22). A measure

indicates the strike (N 200 E) of bedding plane, and a rose diagram shows distribution of elongated 52 clasts.


a rose diagram, after rotating bedding plane around the strike horizontally. Several ex-

amples of rose diagrams are shown in Fig. 10.

It is worth to mention here that two types of clast orientation pattern are recognized.

One is the type that only one preferred orientation is obtained, and the other is the

type that two preferred directions are distinguished. In the latter type, more preferen-

tial direction can be detected by comparison of numbers of clasts arranged on eachdirection.

Clast imbrication; After marking the line paraHel to the bedding plane, the expo-

sure surface nearly vertical to the bedding plane was photographed. The angle between

the marked line and the long axis of each clast was measured (Fig. 9). The measured

numbers of clasts range from 23 to 56. Examples of rose diagrams representing clast im-

brication are shown in Fig. 10 by small solid patterns. In order to obtain maximum angle

of inclination, the measurements were corrected mathematically in considering the angle

between the obtained orientation and the direction of the photographed exposure.Corrected values are shown in Table 2. There may be an intimate relatlon betweenmean angle of inclination and mean clast size. As shown clearly in Table 3, the larger

the mean clast size is, the higher the mean angle is.

Fig. 9. Clast imbrication at Nagoshima (loc. 22).

parallel to bedding plane, and a rose distribution of elongated 56 clasts.

A measure isdiagram shows


Yekoshim2I .t.. , . . ,.,.1 ,,

ibill,r, ". 'e

!.,I Set/ :• ;e .; .. e

!e.

i•o

s•'

o-b ••a. ' ' o' ob

l

tec 19

f•p- (c.p.)

--iS}>L

oie o e T7.

2t

N

I:• g.".

.-IZ'o,

1

1-ttt/t tttI'

/tt

ee

oVe e e'ob o? da o". e:

O Oo ee o.O ea eeeeO

g eee .e oO; o. o

w"-7t--i 'Lgtfi"'•i .• •. .T o- --pr o

t/ f.p• /

...t. "t

12

)x.LLu>

. o--"

2•r11!

ol

':o;' 3'

i Pe Oeb"! Og : eeOO

ee OeOx d"

t.--"o n oOO

led-e+.. o- O-1'o•"ao"ao

eJoo-ooee-Od -eo'-ot o-

"e"d" eenaoaooa -f•P•/-- ogL-- od

-edO e-e

di-

oO

;e- t t pR"}.'.'es' /v:s't.t.'.b`s'oS• i

1."o'."aOe,?.e.c.:.o.e.:`eo .p

,.Oe.2o:":ett"/e.OoLi,e."f, -l'L f p

e/li' IMIi./,,l.'//rb,//'o.,tlltli9$'g;-11

24

N

.7.; ----J-..

,'Z--< hx231

- '

t

! agoshima

"v---.

-""L/,

`i--i

'o

.

o

o cl

"

eo

peb, mds,

s 25

,

-;l}- .

o

Å~ '` i) "

st'' 'iiii]

C•••

'j '

1--Jts >/

i c'

-f-

m21

'

-

o]

Loc. 22

oe o sDe'o"- n o -

sh.tlvshs

ls

Fig.

NVabuka-E Loe 17

ln.ss.

A/ g•

peb. rnds.

Lfe• .m

Y c,ss.,

Ieben.

l Legend

i' i'l

t/:1l

1il"il 'l• ii

I[;

1

L '

x

J

N

/

-

•s-l--

k,,

7<iw3"

s

'

fk' (.

2 .)t 4t ''

)

7

,r

"i

10.

t5

N N 114 i

'

+

c.ss.

1

' "

Columnar sections and clast fabrics of conglomerates.

average gravel size are the same as in Fig. 5.

Legend 1: paleocurrent deduced from sole marks,paleocurrent deduced from clast fabric, 4: trend ofwas observed, 5: diagram of imbrication drawn in aof clast fabric data (see Table 2).

1

1'

s

Symbols for internal

" g-

t,p,

features and

2: rose diagram of orientation, 3:section along which clast inclination

first or fourth quadrant, x: number


No Locaiity Hori-zon

trype .Slze Orientation' Imbri-cation

Paleo-current

t' Kodimari-W Bl A f.p• t*kN780E{82.5)(39)' **20e(42) 78eE,

2 Kodomari-W Bl A f.p• N260E(91.3)(44)N74.50W93.413 27e(48) 26.0E

3 Magari•-N Bl' A f.p• 'N72eEC79.7)(31) 22e(24) 720E

4 rnazurnijima Bl B' g•' N450W(90.3)(45)N480E97.8 21e(29) 1350W

5 susami-r Bl A E.P• N85.50E{95.0){36)NS96.35 21e{44) 85.seE

6 Susarrti-IZ 'BZ A f.p• N32.5eE{86.4)C44) 45eC45) 32.5eE

7 Susami-:r' Bl B g. N57eW(89.0}(26)N460E96.1)(8) 330(30) 330E

8 Shirashima-1 B3 .A c.p. NIOeE(78.7)(65> 480(32) 100E

9 Shirashima-: B3 A f.p• N14eE(89.4)(54) 4oec3o) 14eE

10 Shirashima-rr B3 B c.P' N600E,(91.5){38) 32e(29) 1500E

11 Shirashima--II B.3 A c.p. NlieE(88.6)(27)N800E(96.9)(10} 490(33) 169OW

'12

Esuzaki B3 A. g• N130E(86.9)C54) 200(34) 167eW

13 Esuzaki B3 A f.p• N370E(88.4){24)N400W(94.8)Cll) 31e(38} 143ew

14' Wabuka-E D B f,p• N290W(95.1)(24)N61eE(90.2)(22)

37e(35)' 610E

15 Wabuka-E D B f.p• N250W(89.2)(45) 350(41) 65eE

16 Azashi-W D A c.p. N60.5eWC78.1)'(51) 30e(32) IZ9..5eE

17 Azashi-W D A c.p. N23eW(84.2)(50) 1570E

18 Azashi•-W D A c.p. N20eW(73.0)(47) 460(23) 1600E

19 Yokoshima-N D ,A f.p• N4eW(96.4)(25)N850E{88.9)(17) 37e(44} 176eE

20 Yokoshima-N D A f.p• N350W(79.1){43) 23e(38)' 1450E

21 Yokoshima-S D A c.p. N85.5eW(89.4)•(45) 270(35) 94.seE

22 Yokoshima-S D A .c.p. N620W'{92.1)(45)N350E{95.7){10) 21e(27} 'Zl80E

23 Yokoshima-S D ,A c.p. N47eW{85.1){47) 133eE

24 Yokoshima-S D A c.p. NleW{74.4)(40) 30e(25) 1790E

25 Yokoshirna-S D A c.p. N520E{86.9)(35)N41eW96.1(14} 34O(39) 52eE

26 Soshima D A c.p. N830W:(82.6)(40) 46e(35) 970E

27 Soshima D A c,p. N170W(87.l)(40) 280(43) 163eE

28 Soshima D A c.p. N36eE(77.2)(37) 13e(41) 1440w

29 Nagoshirna' D A c.p. NlleE(86.7)(31) 550(48) lleE

Table 2. CIast fabric and paleocurrent.

*consistency ratio ** the Shirashima-I: Shirashima tunnel-W

number of clasts measured Shirashima-II: Shirashima tunnel-E


clastsize number -lincll-•.natlon

c.peb. 13 37o

f.peb. li 31e

gra. 3 2so

Table 3. Inclination

larger the higher the

of imbricated clasts. Theaverage gravel size is, theinclination is.

ii. RelationshiP between clast orientation and imbrication

Two types of clast fabrics, which are shown diagramatically in Fig. 11, can be

recognized in the conglomerates of the Muro group. The one is a type that the long

axes arrange parallel to imbrication (type A),the other is a type that the long axes

arrange transverse to imbrication (type B). Typical examples of both types are shown in

Fig. 10, in which two examples at the east of Wabuka (loc. 17) are of type B, and five

examples at the south of Yokoshima (loc. 19) and an example at Nagoshima (loc. 22)are of type A. The type of all clast fabrics measured is collectively shown in Table 2,

in which only five exarnples belong to type B, and the remaining to type A. The rela-

tionship between the type of clast fabric and clast size is shown collectively in Table 4.

It can be inferred that there is a tendency that type B occurs more in finer conglomer-

ates.

e"ssetS>{

G"tte"t

xti/f.il'0..0'tai'S,ozo

e

B &

" sits"

X3. ss" ss

"ssN" ss > "ssts ss "ts

ss ss s>

:. 2o".;"e."O.

e" 0•

Fig.

Table

type c. 'peb. f' peb.' gra.

: i4./15

l/159/112/li

1/3•

2/3

4. Relationship between clast fabric type and average gravel size. The denominaters are the total numbers of conglomerates having each clast size. The smaller the average gravel size is, the higher the ratio of B-type

fabric is.

11. Diagrams showing clast fabric types A and B. 1: bedding plane or plane parallel to bedding plane (orientation) 2: plane vertical to bedding plane and parallel

to current direction (imbrication) 3: plane vertical to bedding plane and current direction•

iii. RelationshiP between clast fabrics and sole

It is well known that the actual dip of clasts

ment indicates the direction of transport of clasts,

(JoHANssoN, 1965; PoTTER and PETTiJoHN, 1963).

marks

deposited in fiuvial or beach ehviron-

namely the direction of paleocurrent

That relationship is considered to be


realized in conglomerates deposited in geosynclinal environment (DAviEs and WALKER,

1974; MRAKovicH and CooGAN, 1974). The paleocurrent direction inferred from clastfabrics is shown in Fig. 10, and that deduced by sole marks are also shown in the same

figure. As shown in the examples (Yokoshima-S and Nagoshima in Fig. 10), there isgood relationship between the paleocurrents from clast fabrics and those by sole marks.

On the other hand, there are a few cases that these directions are oblique each other,

the examples of which can be observed at Wabuka-E in Fig. 10. Such cases occur in

type-B conglomerates. .

2. Thick-bedded sandstones Thick-bedded sandstones are frequently intercalated in the Muro group, especially

in the middle formation. Sandstone beds exceeding 80 cm in thickness are treated here

as "thick-bedded". Mineral composition and size distribution of these sandstones in the

present area were reported by the present writer (TA'rEisHi, 1976). These are poor in

matrix (mostly less than 15o/o), and are mostly medium- to fine-grained, although some-

times there are granular to coarse sandstones. These have two modes in general, and

show moderate to poor sorting. Most of them belong to feldspathic arenite, and a part

of them to feldspathic wacke. Internal sedimentary structures and grain fabrics ofthick-

bedded sandstones are described in the followings.

a. General occurrence There are two types of occurrence of thick-bedded sandstones. One is the sand-stones that make thick sequences in which several thin beds of mudstone, fiysch bed and

conglomerate are intercalated. These sequences vary in thickness from 40 to 200 m, and

sometimes contain a thick sandstone bed up to 10 m. Such sequences are the mostwave resistant rocks, making most of promontries. The other is the sandstones intercalate

solely within muddy flysch beds or mudstones. These sandstones vary in thickness along

strike, and are lenticular-shaped in general. Typical occurrence of sequences mainly

composed of thick-bedded sandstone are shown in Fig. 12. Their horizons and localities

are shown in Figs. 3 and 4 respectively. - • ' b. Classification based on internal sedimentary structures

In total, 100 beds of thick-bedded sandstones were examined. Three types, namely,i. composite bed (type 1), ii. structureless bed' with parallel lamination at the top (type

2) and iii. Iaminated bed (type 3), are distinguished on the basis of internal sedimentary

structures (Fig. 13). In 100 beds of thick-bedded sandstones, 17 beds belong to type 1,

52 beds to type 2 and 28 beds to type 3. Most of sandstones belonging to type 1 are

considered to be composed of thick-bedded sandstones belonging to type 2 or 3. There

exist 22 beds having the intermediate feature between types 2 and 3.

i. ComPosite bed ' ' This type is composed of plural sandstone layers, although these look like a single

bed apparently. This type of sandstone was reported as to have been formed by amalga-

"

,

m

ICodomeri W Loc.3

Fig. 12.

loedsh.

sh.

sole

sele

toTe

ShirTst)Vne T"nnel W Loc.to

peb. cgt.

f'

t

m.

f'

r;

m,

dish

Kuchivvsbuke Lec• t2

•tg•)

f'

"L

m

v.f,

m.

v.t.

t.

f'

t

t

Esuiskt Loc,t4

-Lmm t+uti CIS

t'

t'

f'

m.

pt..ma-l2}

r;P2Vtoss

cala sh.

c{a)

t.fteme

C3)

Columnar sections of sequences composed mainly of thick-bedded sandstone. These showthe grain size, internal sedimentary feature (grading and lamination) and external sediment-

ary structure. Symbols of grain size are same as in Fig. 5, though the symbol of sandstone(ss.) is omitted. Numerals at the right of section (Esuzaki, loc. 14) show the sample num-bers of sandstones collected in order to examine grain fabrics (see Fig. 16).

No"

Kst

Btr-

H>Htaza

=-

::

:gs

F:.

:

Type 1

Type 2

Type 3

Sedimentology and Basin Analysis of the

Cotvtposite bed

' lamima changed laterally to bedding plane

appar::.ti:::.b,eig.Ugit:g.2ti.g

Stnscturetess bed

T parallel lamination grading Fig. c=ude para'ilel lamination dish structure composite grading " , shale clast l crude grading ,

Leminated bedt crude trOUgh c:oss lamination grading rarely plane cross lamina- parallel larnination tion

crude parallel lamination

Paleogene Muro Group

13. Sedimentary types of sandstone based on sedimentary features.

ax

thick-beddedinternal

Fig. I4. Lateral change of amalgamated bed into two thin beds observed at Wabuka-W (loc. 16).Scale bar is 1 m in length (see PIate II-1).

mation sometimes accompanied with loading by WALKER (1967) and CoRBETT (1972).In the typical example shown in Fig. 14 and Plate II-1, two beds (beds a and b) canbe distinguished clearly with interca}ation of thin mudstone in the left of the figure,

while they change suddenly to an amalgamated bed (bed c) in the right. There theboundary between the two original beds is indicated only as lamination. Bed-a is 30 to

32 cm thick, and its base is very irregular. Clasts of calcareous mudstone are contained

in the basal part. Grading can be observed throughout the bed from very coarse in the

base to medium sandstone in the upper. Bed-b is thinning leftward from 58 to 18 cm

thick. It is composed of medium sandstone, and is associated with thin muddy lamina-

tions at the top. Grading is indistinctive excepting a transitional zone from sandstone to

mudstone. Two zones of mudstone patches are observed in the bed. Bed-c is 89 cmthick, and its base is irregular. The fiame structures are found there. It is composed


mostly of medium sandstone having some laminations of calcareous mudstone patch. The

basal part consists of very coarse sandstone having clasts of calcareous mudstone. Thin

muddy laminations are found at the top. Grading is indistinctive excepting the basal

part and the top of the bed.

Another example shown in Fig. 15 shows clearly that a contemporaneous loadingmust have played an important role together with amalgamation for uniting plural layers.

.

t

-- --- cS . .essX.

tm , .;2:1

:- - -- --

-. ".- ---i . -- .

.tA,k-'

'

.' :".l-:.-::1-:':-:-:;..

:

--t

•,

e'il,.,,.

-- -

; t.- .e..t .i

-- - ---..- -- - "- -- .-a -- -- -- --- '' -O ---- --- -- . It /j-t -tJ-." :x:'le ;,- ; - - yt"-t tit- t.:il.

.ffi.-`:.

t1- --i . -- i- -. ----- ,ve---' • --. - ....t --.

-J

-:-- :-.;

.

-•s ;'i:•,

4:- ".s ..- {'

:•;(• l`•:, :

I.•"l,X.:

••:1:-:

•.•l

--

---

-

L-' -

.

--

.

:

•r,. -..-t ..•C3 -e.:,::.::r.t.TT..,lf•!:;...,.-I•L.1'ii

::.:':-1'i'1"

---. i- t

.-.; .-:

-- - - t- -- -- -- -------- - --

l- t .- ' 'i'

.

-- -- - t ILnZ

l`ditr,;f

.-•:srrs•---t;.`e•t.ftv.

--- - --Ti e'. -e-e-tS- ---i- -- -`

Fig: 15.

. ----

Lower part of an amalgamated sandstone

about 180 cm thick (scale bar is 20 cm

length) observed at Kodomari (loc. 5).

The boundary is shown clearly by thedifference of -grain size, and the Ioading

structute is developed.

ii. Structureless bed with Parallel lamination at the toP

There are many sandstones which consist of structureless division in the lower part

and parallel-laminated division in the upper. These sandstones are treated here as type

2. The lower division is predominant in general, and the upper one may be developed

only along the top of the bed as shown in Plate II-2 and -3. Grading is usually indis-tinct, although it develops well at a transitional zone between the two divisions. This type

attaining to 8 to 10 m thick, and is thicker in general than type-3 sandstone.

iii. Laminated bed

Parallel laminations develop dominantly throughout this type of sandstone. It is usu-

ally 80 to 150 cm thick. A typical example is shown in Plate II-4. Although laminations

are mostly parallel, cross laminations and convolute laminations are sometimes observed.

In cross laminations, there are three types, such as trough (Plate III-1), plane andripple-drift types (Plate III-2).

c. Relationship between grain fabrics and sole marks

The relationship between grain fabrics and sole marks in flysch-type alternations was

examined by STAuFFER (1967) and CoLBuRN (1968). The relationship in thick-beddedsandstone, however, has not been reported yet. The relationship gives an eMcient clue

to discuss sedimentary process of thick-bedded sandstones.

Grain fabrics of thick-bedded sandstones were tentatively examined at fourpoints. After marking the strike of the bed, hand-specimens were sampled from four


thick-bedded sandstone bed which have sole marks on the bottom, Their hOrizons areshown in Fig. 12. Grain orientation was measured mlcroscopically on thin sectlons cut

parallel to bedding plane and, then, grain imbrication was determined on newly made

thin sections cut vertical to bedding plane and parallel to the grain orientation. '

The results are shown in Fig. !6. It is apparent that long axes of sand-sized grains

are arranged well in a preferred direction, and that grains show imbricated structures

although these are indistinct. Paleocurrents deduced by sole marks are also shown in the

same figure. It is clear that there exists a good relationship between paleocurrents

obtained by means of grain fabrics and that by sole marks.

t O'et ,s r.

3

ttt-

.;I,Is

2

e,s'

stn r. '

`

tO,3'.-' ea.7$

Fig. 16. Relationship between paleocurrentsfrgm grain fabric and sole rnarksin the thick-bedded sandstonecollected at Esuzaki (loc. 14) (seeFigL 12), Both of paleocurrents are'

well coincided.

Legend

Ietio

otOm

ttom sele msrking

5 s,e'•

< 11

lr

as'

208 Masaaki TATEIsHI 3. Conglomeratic mudstones There exist two kinds of conglomeratic mudstones, that is, pebbly mudstone and

angular fragrnent-bearing mudstone, in the Muro group. Pebbly mudstones are fre-quently intercalated within the upper formation. Thick layers of pebbly mudstone (usu-

ally 6 to 10m thick) occur in the basal part of the channel fi11ing sediments which will

be described later (Figs. 18, 19 and 20). Thinner layers (1 to 3 m thick) are intercal-

ated within flysch sequences and sequences composed of thick-bedded sandstone and

conglomerate. The pebbly mudstones consist of predominant dark-grey mudstone andscattered clasts in the matrix. The mudstone matrix is massive and structureless in

general. Clasts are pebb!e- te granule-sized, rounded to subangular gravels of acidic

volcanic rocks, orthoquartzite, greywacke, mudstone and chert. In general, these gravels

are randomly scattered through the matrix, though they are sometimes aligned making

laminae (Fig. 18b). The large blocks of conglomerate and sandstone, sometimes reaching

3 m in diameter, are contained in pebbly mudstone which forms the basal part of chan-

nel-fi11 (Figs, !8 and 19). These rocks are identical in lithology to those ofstratigraphical-

ly lower horizon. These blocks show sometimes contorted shape similar to slumping.

The angular fragment-bearing mudstones are an important constituent ofthe F and

G members. These in the lower part (about 200 m thick) of the F member are called"Sarashi-kubi (gibbeted head) bed" as a field name by KisHu SHiMANTo REsEARcH GRoup

(1969). Dark-grey mudstone matrix occupies more than 90 per cent and clasts are scattered

throughout the matrix. Although the muddy matrix is apparently massive and structureless,

if observed in detail, it is formed by assemblage of small fragments of mudstone, which

suggests the secondary fragmentation during transportation. The clasts are composed of

various sized, subangular to angular sedimentary rocks, such as mudstone, sandstone,

flysch-type alternation and conglomerate, all of which are identical in lithology to those

of stratigraphically lower horizon. Some clasts are up to 5 to 8 m in diarneter. These

clasts are randomly scattered in the matrix in general, but in the lower part of the F

member, are aligned with long axis parallel to general trend ofthe neighbourhood.

Angular fragment-bearing mudstones forming the lower part (about 100 m thick)of the G member are different from those of the F member : smaller clasts than cobble-

size ; crude bedding ; and are called "Ko-sarashi bed (ko- means small) " as a field name

by KisHu SHIMANTo REsEARcH GRoup (1969). These angular fragment-bearing mudstonein the F and G members are considered to be endolistostrome (ELTER andRAGGr, 1965).

4. Flysch-type alternation

The Muro group is composed predominantly of flysch-type alternation of sandstone

and mudstone. The flysch-type alternation of the Muro group has been examined sedi-

mentologically (HARATA, 1965; SHiKi et aL, 1968; KuRoKAwA, 1972; KisHu SHiMANToREsEARcri GRoup, 1970, 1972, 1976). The flysch beds are classified in this paper as sandy

flysch (sandstone>mudstone), normal flysch (sandstone=mudstone), and muddy fiysch

Sedimenotology and Basin Analysis of the Paleogene Muro Group 209

(sandstone<mudstone) on the basis of approximate ratio of sandstone to mudstone.Typical examples of these flysch beds are shown in Plate IV. In general, the bottom ef

sandy part is sharp and distinctive, and alternatively the boundary between sandy part

and overlying muddy part is transitlonal and indistinctive. Most of the fiysch beds have

internal sedimentary structures such as parallel lamination and ripple cross lamination.

Grading structure is sometimes observed in the lower part of the bed. These internal

sedimentary features have a good correspondance with those in Bouma's flysch sequences

(BouMA, 1962). Most of sandy fiysch beds are Tb-e or Tc-e sequences of Bouma's

sequence. A part of sandy fiysch beds begins from grading division, and consists of

complete sequence (Ta-e). On the other hand, normal flysch and muddy flysch showpredominantly Tc-e or Td-e sequence, and there are few beds with grading and lowerparallel lamination divisions.

There are observed many kinds of external sedimentary structures such as directional

sale marks, load casts and ripple marks in the fiysch beds. There are also observed many

kinds of trace fossils such as HelminthoPsis, CesmerhaPhe, Nereites, 'Paleedict2on. These struc-

tures play important roles to reconstruct sedimentary environment of the Muro group,

and several studies were already reported as mentioned above.

B. Transportation and deposition On the basis of the above-mentioned data, the mechanism of transportation anddeposition of conglomerate, thick-bedded sandstone, conglomeratic mudstone and fiysch.

type alternation will be discussed in the following.

1. Cong!omerates It is well known that large gravels which are rolled on the bottom are finally align-

ed with their long axes transverse to flow direction and with their intermediate axes

dipping upstream (JoHANssoN, 1965; REEs, 1968; RusT, 1972). Most of congiomerates

of the Muro group show the preferred fabric. Two types (types A and B) can berecognized in their clast fabrics (Fig. 11). Type A is that Iong axes of elongated clas{s

are arranged parallel to direction of clast imbrication, in other words, parallel to direc-

tion of current. Type B is that long axes are aligned transverse to current. B--typefabric, which occurs not so many, implies that constituting clasts were rolled for appreci-

able distance. A-type fabric, which is most common in the conglomerates of the Muro

group, clearly indicates that clasts were not rolled for a long distance, but were trans-

ported in state of suspension and deposited in accordance with the decrease of energy.

Concerning to the current that is able to transport coarse detritus in state ofsuspension,

there have been proposed turbidity current (WALKER, 1967), grain flow (STAuFFER, 1967)

and submarine debris flow. The sediments deposited by submarine debris flow arenot sorted well and are scarcely organized, The conglomerates in the Muro group showmostly imbrication, inverse and/or normal gradings and sometimesstratification. Sothe

210 Masaaki TATEISHIsubmarine debris flow can be eliminated safely, and the other two cases remain logically

as a transportation mechanism of the conglomerates of the Muro group. Large clast-bearing conglomerates are characterized by development of inverse grading in the lower

part (Model i). It is known that the inverse grading structure is formed by the effect of

the dispersive pressure arised from clast collision under shear, namely, by the so-called

Bagnold's effect (BAGNoLD, 1954). It is most probable that transportation and deposition

of conglomerate, especially composed of large clasts, are attributed to grain flow that

supports the clasts above the bed by dispersive pressure caused by clast collision. Small

clast-bearing conglomerates are characterized by development of normal grading andstratification (Model ii). Their sedimentary features indicate that these conglomerates

were transported and deposited mainly by turbidity current. The mechanism ofturbidi-

ty current is distinguished theoretically from that of grain fiow. However, it is likely that

the relationship between the two mechanisms is transitional in real flows (MiDDLEToN

and HAMpToN, 1976; WALKER, 1975b), and most of the gravels are transported and be

deposited by the currents combining two types of mechanisms. The dispersive pressureby clast collision plays the important role to transport a lot of larger gravels, and forms

the inverse grading. The turbulence plays an important role to transport small gravels,

and forms the normal grading and stratification.

It is most likely that the disorganized model of conglomerates, which is not sorted

and stratified so well, deposited in the most proximal suite where the current is not

developed to segregate clasts (WALKER, 1975b).

As for the B-type fabric in the conglomerate of the Muro group, it is possible that

the rolling of clasts took place at the last stage of the turbidity current. It may be likely

that the normal bottom current acts to retransport clasts at that stage.

2. Thick-bedded sandstones Three types (1. composite bed, 2. structureless bed with parallel lamination at the

top, and 3. Iaminated bed) are distinguished in the thick-bedded sandstones of the Muro

group (Fig. 13). As the first type is assigned to have been formed secondarily by amal-

gamation, the present writer would like to treat here the second and third types. There

Type3 Type2 Laminated bed Struct,ureTess bed

::.I--.:;;:•z':'.i' Fig. 17. Inferred relationship between sedim-

Lr:,I:itf•'i'Tr: ': r•T•i'• entary types 2and3of thick-bedded t- ' Lt.. ' 1•ttr::• il:.:•:i::- sandstones. It is probable that type 2 --:-t-- -:t tttt-tttt. - - :.:::.:'i•:•',:•'• beds are located m more proximal Å~'x itti•1•'/i'S• .;i"/ 'i. suits than type 3 beds. Xx /'':'i;'il•;',•f•'-1>ii;i:iii:i'

Å~ 1{.`:,:::;'i.r',:,r::,'il.::•I•:

---"------.----.---•---nt'i" proximalitV

Sedimentology and Basin Ana!ysis of the Paleogene Muro Group 211

are many thick-bedded sandstones of intermediate type, and the relationship between the

two types is gradational (Fig. 17). Sedimentary features such as grading and various

kinds of laminations are similar to those in typical "turbidite" sequences reported by

BouMA (1962), KuENEN (1967) and WALKER (1967). These facts suggests that the origin

of thick-bedded sandstones of the Muro group must be same as in typical turbiditesequences. The present writer examined the grain-size distribution ofthick-bedded sand-

stones of the Muro group, and drew the C-M diagram (PAssEGA and ByRAMJEE, 1969) ofwhich pattern suggests that these sandstones might have been transported and deposited

by turbidity current (TATEisHi, 1976). The grain fabrics of thick-bedded sandstone support

that sand grains were transported by the currents which made the directional sole marks.

There exist some different features between thick-bedded sandstones and flysch-type

alternations. Compared with flysch-type sandstone, thick-bedded sandstone is generally

coarser, and poorly sorted. Furthermore, structureless division is thicker, and grading is

not so remarkable in thick-bedded sandstone. These features in the thick-bedded sand-

stones of the Muro group coincide well with the characteristic features in proximal

turbidites reported by WALKER (1967).

The sedimentary process of thick-bedded sandstones in a geosyncline, has beenargued extensively for the last decade. Most of sedimentologists considered that these

were transported and deposited by a kind of sediment gravity flows (MiDDLEToN andHAMpToN, 1976), although there has been a controversy whether sand grains could bedeposited by turbidity current or by grain flow. Theoretically turbidity current and

grain fiow can be distinguished from each other, however, the sediments yielded by them

cannot be distinguished practically (MiDDLEToN and HAMpToN, 1976). The character-istlc features of grain flow assigned by STAuFFER (1967) cannot be found in the thick--

bedded sandstones in the Muro group except for dish structures. Dish structure itself

cannot be assigned as one of the features of grain flow, but as a secondary water-

escaped structure (LowE and LopiccoLo, 1974; RAuTMAN and DoTT, Jr., 1977).. It isprobable that the grain flow did not play main role in emplacement ofsandstones ofthe

Muro group, and that most of thick-bedded sandstones of the Muro group were depos-

ited in proximal site by turbidity currents.

3. Conglomeratic mudstone

There exist pebbly mudstone and angul'ar fragment-bearing mudstone in the Murogroup. CRowELL (1957) examined the pebbly mudstone in California ranging from Juras-sic to Pliocene in age, and assigned the origin of pebbly mudstone to subaqueous sliding

or slumping. Thereafter, sedimentation of tilloids or pebbly mudstones in sedimentary

sequences was examined by several workers and was assigned to be of the submarine

mass movement such as sliding, slumping, or mudfiow (ScHERMERHoRN and STANToN,1963; DoTT, 1963). MiDDLEToN and HAMpToN(1976) summarized the mechanisms anddeposits of various kinds of sediment gravity fiows, and called the flow including the

212 Masaaki TATEISHIfiows of above-mentioned mechanisms the debris flow. The summary ofthe type ofsedi-

mentary features that might be found in beds deposited by debris flow is presented by

HAMpToN (1972) and MiDDLEToN and HAMpToN (1976). The pebbly mudstones in the Muro group have the following features; i. clasts are

scattered at random throughout the matrix, ii. grading is scarcely observed, iiL contorted

blocks of sandstone or conglomerate are frequently included. Judging from these features,

the origin of pebbly mudstones in the Muro group is ascribed safely to the debris flow.

Angular fragment-bearing rnudstones in the Muro group have quite similar features

to the above-mentioned pebbly mudstones and are also considered to have been trans-

ported and deposited by the debris flow. However, they differ from the pebbly mud-stoncs in having angular clasts of various sizes and various degrees of consolidation which

are derived from the lower beds of the Muro group, and are certainly referred to as

endolistostrome. Olistostrome has been reported from geosynclinal sediments in various

places (ABBATE, et al. 1970; KAy, 1976; De JoNG, 1974), and is generally considered

to be of submarine slide or submarine mudflow origin (ABBATE, et aL, 1970). Theorganized arrangement of clasts are observed in some parts of the angular fragment-

bearing mudstones of the F member. This indicates that the mass movement was ad-vanced, and the segregation of clasts took place in the flow. The crude bedding in the

G member suggests that the small-scale mass movement occurred frequently at shortintervals. Source materials of the angular fragment-bearing mudstones differ from those

of pebbly mudstones as mentioned already. They must have been caused by the break-down of the geanticlinal upheaval within the basin. On the other hand, pebbly mud-

stones are derived from the exotic provenance same as the other turbidite sequences.

WALKER and MuTTi (1973) attempted to classify several turbidite facies. According to

them, the pebbly mudstone and olistostrome are grouped in the slope-channel facies

association. The conglomeratic mudstones in the Muro group are considered to bedeposited in the lower part of submarine slope, and in the major channels or canyons

incised into the slope.

4. Flysch-type alternations

There have been reported numerous papers as for the sedimentology of the fiysch

sequences, characterized by rhythmic alternations of sandstone and mudstone. SinceKuENEN and MiGuoRiNi (1950) put forward the suggestion that turbidity flow causes a

graded bedding, the concept of turbidite has been greatly elaborated and popularized.

Diagnostic characteristics of turbidites were summarized by BouMA and BRowER (1964),

PoTTER and PETTiJoHN (1963), KuENEN (1967), SELLEy (1970), etc. Turbidites can beidentified by many kinds of sedimentary structures, such as external and internal struc-

tures. The former are current scours, obstacle scours and tool marks, and the latter are

graded bedding, parallel lamination and cross lamination. The typical sequence of in-

ternal structures in turbidites was established for the first time by BouMA (1962). It has


been called as Bouma's sequence, being considered as one of the most important fea-

tures of turbidites. The fiysch sequences of the Muro group show the excellent rhythmic

bedding as shown in Plate IV. External sedimentary structures such as flute mark,

groove mark and current crescent mark can be abundantly observed. Although the beds

possessing the idealized Bouma's sequence (complete sequence) are not so many, most of

the flysch beds show the Tb-e or Tc-e sequences of Bouma's sequence. These factsindicate apparently that the fiysch-type alternations of the Muro group were mostlytransported and deposited by turbidity currents.

IV. Submarine Fan Sedimentation

Facies analysis of sediments deposited on ancient submarine fan has been reported

by many sedimentologists (NoRMARK, 1970; SMiTH, 1971; NrLsEN and SiMoN!, Jr., 1973;

WALKER and MuTTi, 1973;MuTTi, 1974, 1977; NELsoN and NiLsEN, 1974; CARTER andLiNDQvisT, 1975; WALKER, 1975c). Channel-scours and -fi11s of various scales areobserved in the Muro group, suggesting submarine fan deposition. In order to make

clear the sedimentary environment of the Muro group these channel-scours and -fi11swere examined in detail.

A. Channel-scours and -fills

A number of channel-scours and -fi11s of various scales are observed in the coast

area. Most of these channel structures belong to the middle and upper formations.Typical channel structures which belong to the D member are well observed at the east

of Wabuka (loc. 17), Yokoshima Island (loc. 19) and Soshima Island (loc. 21).

1. East of Wabuka (loc. 17)

The channel structure is well exposed at the sea cliff of 25 m in width and 10 m in

height (Plate V). The geologic map and a brief sketch of the channel are shown inFig. 18. The sequence underlying the channel structure consists of thin-bedded muddy

fiysch in the lower part and pebble conglomerate in the upper. The former, in which

bioturbations are well developed, ls alternations of fine sandstone or siltstone (less than 1

cm thick) and mudstone (2 to 3 cm thick). The latter overlying conformably the for-

mer is 2 m in thickness. Mudstone clasts up to 30 cm in diameter are scattered in the

upper part. The pebble conglomerate has N 300E strike, dipping 400 southward at point

B. The channel cut into the underlying conglomerate and even muddy flysch as well

about 4 m in depth at point A. There, the bottom surface of the channel has N 100Estrike, dipping 200 eastward. The channel-fi11 consists of pebbly mudstone about 10 mthick. It contains pebble gravels and large blocks ofsandstone, granule congtomerate and

muddy flysch in the basal part, and intercalates two medium sandstone beds in lenticular

shape. Pebble conglomerate overlies the pebbly mudstone. Flute marks are found on itsbottom surface, indicating the paleocurrent from NNW to SSE.


7t -.. .- r -t------" ,.,...x.kk%.ZtU2 .. .Y -Z•. : 1• .9e t":. - 'Y ".rt?- va A .S' /7Ak[zs:z -,zl,lil.i 'l.. .)hs.aa2az.2t.i l'6 x'

..

.te,;y 1'55'L-.x,>..

tt `:.itt'iii`)l-'-'A-...,;,g,2.sAi,.'i./[/./i,tE,tE.,,t,/.E/;l`"xltliiZ/Ziz/llzti

'.;1 a 4 .•7fl':,')fi,i',fti'

./ a4 ge '" 'l' e somta

S Mudstone E!i] Mudy fTysch

EIE!i] cong}omerate [Iii] Pebbly mudstone

Å~ s x t,,.,'kJ•li-1•::7';lilll't. aj

.4;',:•lfi/u7ee ttttt.Za-'g.i.}XKt

:ez-.-ft j .=j;=.:- v:-nt.i- ;Hl.T

". .. ... P]. - xr- L.. ,m gpt;'•-..:']•..1:li':rt•'tilr'i

. " --. -- N /'-''. ':' " ' v ''';''''''' --..... ,,.•.. . :-.tpa.....--Lav==-7-:!-Z/'.frsrs-tN-Nyttt

Fig. 18. Channel structure at Wabuka--E (loc. 17). See Plate V.

A: Geologic map (after KISHu SHIMANTO REsEARcH GROuP, 1969). The arrow shows the point of channel structure. B: A brief sketch of the channel structure. The arrow shows the channel base.


2. Yokoshima Island (loc. 19)

Two large channels (lower and upper ones)

brief sketch of the upper channel are shown in

are observed. TheFig. 19.

geologic map and a

N

t tr" tt. ="="J; -=- ttr.;-IJI.-..;iil.:.uiii.21#.--;-.ttLi.•2'•1. g

:- =-- -:=. .,w:. 1•t' -- ... ...:,1:. 1,1 ,4,5•iS: . ...... ..ot ''' . -- . ,..tc: .- .. ti ' ''' "-:t'''' ' ' ' [• •:i ,l'jil.1 .:f, Ilg•,;; '••i •ll.'Å}• ';•'• •

e

•---7--.- ---r;." -r, 'IE.;l.x.I.3S .-'--..::.:]-'--=-L=-X-.7.---

tV/;L--=.tt'-iL{ii'("iti-'ttt"'i'i:tlllllliilSll.llillliiil-..-gl-l"Å}'-i'3'-"

'i-'x.g...F..--.-i'Fi'-i-'l;',i;'lii///'iii/lr!/i,,;.I'X'i"it-pt-'-'F'-X"`";J=tF,'.-=."'3".'ttillltlSl'/'-"/','ll-,i S/'ln;:lilliiii.SiO:1::h.,

.-{.- -.•--.t-i.•.Z.-.214-=.',-.i.i.-iE.-F.-:---'`C.7i '`r-:='.J'. !i;,.{'.'- ..----"--g';-{l}:----;'.'-J-'.t.e--'-'.ii -r E•l!lli,,.,i Sandv flvsoh

- - ftt - "ti.'s ..... .- . .. .. ({;> [;ijl.--.i.i.ii'.-'iXJ '' •1•i:I: Sandstone '.'.; Congtomerete ' .;i'- ft:::i-•f4g ii:.-.i<7i--'-- mudstgne

=.t.;.r•;. r. =7-i=j.'`j

5eM

t-.:a ',-,."'gif.•f.:t-':'-:-LS.L`-:t..'l"li';-tl"ili"

--ix '•'•.'•'•=.'•''.'::•',',•:L•Sr 'i t.tt.tt t-- t- --t"111'/4•ii.i•J,•,,,'i•••;i.'k-•g/L'i.li.i,:' ii-:i•i:1.{':1/l:•iiJl•i:;/:ll,lll,.;:/'tii•;••;"'.t••

:;t;FJrt, e.. EZI pbthly' .:-tl'.tr.ti-t/'lli-';]'lj-'J;"-.3;l(...."oo

- tt t.tt ....;t..xT"' ;'r,r:-:-J..tt]-, "''"':"")':'i"'`''''

:tt -t

-•

.J. TL,.. . ..,",. :. •v t;-r :.::,:;:.;! :;:'r •I L'•T7 s Ti •. -. •.: •, •:.: :. :: :r. :.:.;:f;!. •:';• :'l' `#' 'il'1==-: -ef. •

2;-ew=:.91'.=•==.i:rr=.=h';tLl.;..-P...f,.t/:.;.:.:,l•::

. .. •L:'-'.'••f/•.•. -' Si'/ t' i't. i7/H :':.1:i :' I• .:.: :.: :.: :i•:'r z': i zl•'z l• /'•.: ': '1:S- l' :lili'::',7:'R•:'1 :: :' :•:.: : :::;

"' "::.:"•::;:••t'.•' . ":•:.:::.•ii. •, ..::::':•::::•}• i'•2'il:. ;-- ;=.T=

ESI.}IEttE]L.-==-i.-L-ss:-'::'t"l--.;ii:=/...-'-==-.-

.h figL. s-.SL " -- -. -e ."+. ... t-.-

-Cn.=lli;ili--t•-)..Fi-illlli..,L,-,s-.."lllg{.lg.=i-.1.,l't-'t.ll'iJr.i.''i'il•-'i'"'.-'-•{-;'.ili•fiM

.L='-=-•=r.=-.--":!.'.'=='--==-==C-:--:-'--=--Hf-.'-- -::S i-""l='i;J=:z's"•.=in'."l-•=--t' - ----. -.m -m.-- i=-.J--h--; --.- -- - -.- - .-E- -=.=-=-=--e---. e-=-pTL---"--EIiv 35m -,w

Fig. 19. Channel structuresA: Geologic map large channelB: A brief sketch

at Yokoshima Island (loc. 19) (see Plate VI).

(after KISHu SHIMANTo REsEARcH GROuP, 1970). White arrows show thestructures, and black arrows show the small channel structures.

of the upper large channel. The black arrow shows the channel base.


a. Lower channel The sequence underlying the channel consists of bedded mudstone, muddy flysch

and normal fiysch in ascending order. These have N 50eW to EW strike, dipping 450 to

650 southward, and are deeply scoured out by the channeL At the east of the island,

normal and muddy fiysch are entirely scoured out. Unfortunately the eastern part of the

channel wall is not exposed. The western half of the channel is 20 m deep and 200 mwide. Bottom surface of the channel has N 600 to 800E strike, dipping 550 southward.

The channel is fi11ed with pebbly mudstone, pebble conglomerate, thick-bedded sandstone,

sandy to normal flysch and muddy flysch in ascending order. The fi11ing sediments as a

whole comprise typical thinning- and fining-upward sequence as shown in Fig. 20. The

lowest pebbly mudstone, of which thickness varies from 150 to 450 cm, contains blocks

of sandstone and mudstone. Several small channels can be seen in the fi11ing sediments.

The thick-bedded sandstones are scoured at least 8 m in depth by overlying sandy fiysch,

and the sandy to normal flysch beds are also scoured, forming small channels (Plate VI).

;. l•: ': 1•::: ':- I1•l:

v."-- .=.-o- ts-h"- tar- es.sm. w "=tame.--t==a=.ntr twhem[pt) :a "' wh

-.ql- Upper

channet

. Muddy flysch .

m20

o

v-tr-e=.e;-E;=m=n=-- [-.EIEUS!$= ;t)ajilee:-

tuh lge-tuma=!==[!.srtr-pa=rt-tc- ===t .t-

in]]=mE[ E ==qu-!=:!7.ttr-m;•be'EliF!=ur-Mz=r nj-ptn-=!=:t[SE=-'•`ptR "=-tr.-:gr4.-[=L =-FI=...

"=CXi-t zS•i =:gel=tl!-gnE;, . ..

-- Normal flysch

• Sandy to normal ftysch

s"vai "-

.o.us;"v-- . I =J-tSi

nt ------L:4. .=.

+Composed thick-bedded sandstone

- --t -/- )-IE•--.kS-9.-e :.-

,.l,

:'r:: .-egtr;-.

-:'r--'-

r- {EeL•.tL:--0"

h=.

'- Peboly mudstone Lovter channel

Fig. 20. Columnar section of fi11ing sediments of the

lower channel at Yokoshima Island (loc. 19).


b. Upper channel The sequence underlying the channel consists of muddy flysch and granule sand-stone. It has generally N 800E strike, dipping 40e southward. The granule sandstone is

about 40 cm in thickness, and contains mudstone clasts up to 20 cm in diameter in theupper. The channel scours out the underlying sequence in a scale of 1 m in depth and

10 m in width. The bottom surface has EW strike, dipping 300 southward. The channel

is fi11ed with pebbly mudstone, pebble conglomerate and thick-bedded sandstone. The

lowest pebbly mudstone is 6 to 7 m in thickness, and contains blocks of pebble to gran-

ule conglomerate and sandstone in the middle horizon of the bed. Sole marks are ob-

served on the bottom of conglomerates, indicating paleocurrents from ESE.

3. Soshima Island (loc. 21)

The geologic map of Soshima Island is shown in Fig. 21. The sequence underlyingthe channel consists of muddy flysch of sandstone or siltstone (O.5 to 5 cm thick) and

mudstone (3 to 15 cm thick). It has N 75eE strike, dipping reversely 50e to 600 north-

ward. The channel scours out these beds in 25 m in depth and 150 m in width. Bottomsurface of the channel has N 50 OW strike, dipping 500 northward, though it ls over-turned. The filling sediments, attaining to 80 m in total thickness, are subdivided into the

lower and the upper parts. The lower part is composed of pebble conglomerate of 30m thick, intercalating thick-bedded sandstone which thin out towards the channel wall.

The upper part is composed of pebbly mudstone, pebble conglomerate and thick-beddedsandstone. Current marks are observed on the bottom surfaces ofthese fi11ingsequences,

N

'ftr':'::':'

'-l -J! ..,:yt;::..':.

.it:.:•f.::i:"=L--•-xis •.:,.

tttttttt;:t- -le::rt'iYt"':- .v;t .tlttl'- t--

t. ttt t tttttt tt.t t tt t}t l

..t ...:•,//./1.'1.1.•...L...

11'•Ilili'ii'i':'1''[:'.:r","iti"';;;'`':-'tz

'' .=' '::'.• ':':::':g:::. tt t t. ,'...tr,,. ..t t ti'i{,i,,i-'111iill,lii'li•l',,11,2•,,.,,illli:•-1•'1•i"ill"ill'

.,r:','•:'' rt,':,'.f',,L"•'/'g:l,i",/i'','tL•.,.'t•/•

.:• ,•:

,-.'iS•'::::'•':•I•z:.:.{'.

.'.:.-'::'IL:":. /. .-..`,V;tt...ev.i"{{..,..}iiJ.,/4,:i,:-I•tl.,},:,i:iL://i,;,l•J.-Fi,/l,I'l.,tA.-.--;-•••

.-; .,/.{.;.-i..': :'/' {-tt,'lfrt'i`illfs,f,t-.ittt'/'=."--r'ri=i:.i"..=-t.i.t,t..'..-.'.r.-r'i.r-T-.ti.t-I-..=J.;-

•-"li'/'l,i•ll2-9'"' r'--'r" --'r"'L`" ' - ."..•.- tt...."' t't '" '''r' ' '

....y.

,.,•-.".-tt.-LteL-'

g,!t5.-ttL.-:t.ttttt-,ti'.-'t.fi--ll.l-.-L•.tti-:':"k•-"L=SI.ii---.::l;.?-ff•

-. .. -• S5:'/t-f:'"'ti "'-t"'t'"

l'il.//l'Si,I;i.lk•-li.t.:l-ttt•il/i.T•,"E•tl,.;'•.,l.Åíli.•1,l.-i;,I.,i.;'-.

.

,r• •: .,

.. ,itJfrt. -..•:v.•."• '--t, t/ L -.

.-

..•--s, ": ": 'i't/.'t/ttlil;/3 ',:':S''''

Tffht '"t" pt =' l;,1,vEl/illlii,I•i•/'

-1 - 1 .l; rt' '" "- tii=iL:,;F.'../"../-r'.' ..... --.t.- ..i.;.lf:;.,.I.}•.sJ-

='":.-"-. rt L- .J rt r rr r. ttL L.t -$=iLt=-".tLt' Ji rt'tt

. ' iE.. "',1.=--i -=F,'=-=un: :-: ;l ;r. 'tTT. -1 // '''

.-i .t.i.-g /".,giser""

:•,ii,lvii';it.t•r•tl;',?:t:'t'l'•'`'..,,.•:.,1•,-.l..,..

tb-m

Fig. 21. Geologic map of Soshima IslandLegend of lithofacies is as same

(Ioc. 21)

as in Fig.

•;1-li,,t"•i,;.:.'.:-.•,'ff.:1'/.;,•i.it•g:•ti•i/{:.:•i'•i/ir,:i/-'l'li,i/

-,•i.;.,,c•;-i';i-,t,',:.;c,7.ill/1i'11;li.i'tJ,•'/i41i!li•'k-/l//i',/t,iii'.i,SI.,.i;ii•[li,L':'

`' ••i••s,.:..i.i'.,i/;"i'l`l`ili/l'lli-,l//:-,{-l•/-.,//i.•i'f/i'tt•1.i"'g'.'f":'

-:'i-dis!sp :i...

(after KISHU SHIMANTO RESEARaH GROup,19. The arrow shows the channel base.

1969)

218 Masaaki TATEISHIindicating paleocurrents from ESE or E.

B. Channel directions and paleocurrents in channel-fills

The directions of elongated channels and paleocurrents found in channel-fi11s are

examined in the following.

1. The direction of elongated channels

In order to reconstruct a dispersal pattern of a submarine fan, it is important to

obtain the direction of elongated channels. AIthough the channel-wall and -base have

different geometries from place to place, the general trend of an elongated channel can

be obtained by averaging some measurements of strikes of channel-wall and -base (Fig.

22). In order to obtain original strikes and dips of channel-wall and -base, it is neces-

sary to correct data for tilt. The compensations were done by means of the samemethods as the tilt correction of cross-bedding (PoTTER and PETTrJoHN, 1963;p. 259-

260). Using the strike-line as a hinge line, the bed under channel structure is rotated

back to horizontal position. By means of the rotation, original strike and dip of the

bottom surface of channel can be obtained. Example of compensation is shown in Fig.

23. It shows the compensation about the lower channel at Yokoshima Island. The data

N

--t ---. - xr lt;yx g. -- - -.- t t s-- v.4/.it(-SSI;-,tz.--itlii-t'Vh

-x"s s- - -tS. X. ??7f.'3':'., tttt'5W E .tr2./,t"."•2.g.L z,,.s- --/- ."xX- rv --.-- .kl[/llll -` -ort..rri.t--u=,•K''' L' X' !f!.?.s:i-T.E.--i;'-"' -'"

- F;:' -tL.=' =tt'NF'f- ' !..ti y.v v;=. v- ., r. -'

Z,"V 1 - K. H. sS g' N'S .-1-L ' t./ s --..x '" xE,l -- - -" '• L.. xS)?rr=: - - .. -. -- •- x H. Mfx='yh --- -.-. v ..

Fig. 22.

Fig. 23.

-. .r-=...,\ sNH/ f•- .r sv

B'--• N: -- ejV.=-'i, -'N)g.l.-;Lg#.y.,;.vtt?rk;•

Fig. 22.

Illustration of channel

by averaging sorne strike

dip of them are differedTilt correction to obtain

the same as correction forby numerals are the strike

shown by dashed numeralschannel at Yokoshima

3'

3 r

2 l

'

'

Fig. 23.

structure. General trend of channel (white arrow) can be obtained measurements of channel-base and -wall, though the strike and from place to place. original strike and dip of channel-base and -wall. The method ls

cross-bedding by PoTTER and PETTIJOHN (1963). Points shown and dip of the beds underlying channel structures, and points

are the ones of the channel-base and -walL Data are of lowerIsland (loc. 19) (see Table 5).

Sedimentology and

sequenae underlied channel structure East of Wabuka N700.E, 15eS lower at Yokoshima 1 N55eW, 650S 2 N720W, 500S 3 N520W, 45es 4 EW', 45eS upper at Yokoshima• N850E, 400S Soshima l N650E, 850N(o) 2 N540E, 740N(O) 3 N750E, 60eN(O) 4 N740E, 700N(O) 5 N800E, 550N(O) 6 Nsoow, soeN(o) 7 NsoeE, 7oeN(o) 8 EW, 65eN(O) 9 N650E, 800N(O) 10 N750E, 700N<O) 11 N550E, 700N(O) 12 N600E, 550N(.O) 13 N550E, 800N(o)

Table 5. General trends of channels. These can or can be shown by the compensated value.

'and compensated. values are shown in Table

pensated values for strike of the bottom surface

the channel. General trends of four channels

east of Wabuka: lower one at Yokoshima Is.: upper one at Yokoshima Is. :

Soshima Is.:

2. Paleocurrents in channel-fills

Although the general trend of elongated

tioned method, its sense cannot be known.

indirectly by paleocurrent analysis of fi11ing

The general trends of channels and thein rose diagrams in Fig. 24. These show that

flowed down from southeast, suggesting that

the south.

Basin Analvsis of the Paleogene

bottom surface of channel

NIOOE, 300E

N650E, 380s N800E, 380S N480E, 360S N600E, 550Svector mean

N7sew, 6oos

N800E, 200N(O) N550W, 80eN (O) N800W, 500s N600w, 700s N680W, 350N(O) N75eW, 650N(O) N750E, 60•ON(O) N55eW, 80eN(O) N700W, 700N(O) N850W, 800N(O) N350W, 550E(O) NsooE, 7ooN(o) N860W, 740N(O)vector mean

be obtained

5 about 4 channels.

of channel in are shown in the

N 200E N 170W N 500W N 40W

channel is obtained

The sense of channelsediments.

paleocurrents of fi11in

the fi11in

the mouth

Muro Group

compensated value

N200E, 250E

N520W, 38eNN250W, 24eEN 3eW, 50eENIOeE, 250EN170w 'N50eW, '25eS

N16eE, 20eWNI7eW, 780WN55eE, 740NN55eW, 600NN40eE, 300EN650W, 150NN760W, !OOsNl60W, 350EN 80W, 450EN4oew, 2ooEN 20W, 78eEN680W, 250NN22eW, 380EN 40w

219

by averaging the compensated values,

Vector mean of com- d:,cates a general trend of

followings.

by the above-men- trend can be deduced

g sediments are showng sediments excepting one example

of submarine canyon existed to


N

W

N

Lo.w,e v.c,h.a",,"•e,:l.XN,'

'

-yschbecL

UpperchannelatYokoshima

N...fij'ppLe-m-e.r-kJ

'

-=::

be6

eere

efeieette6tieof

triarkingSSendstoneS

erde,b`od""PSote

eftateGenerettrend

ofchannel

elestfabric

Mark;ng5flysch

conglepte

s of

s

loptesvSe

con9x'Generattrend

of channet

W-

WabukaN

Generel trer)d

of channel

etb

vee

96 Ce o e di E W

SoshimaN

-en e •-. = "e - -. o" G .tiseNeeS-

ozeet

tebri tate C ofcengLe/Irle

s

-s.[:

eOknwn e se" eS Nese godMstaer il/ngSce"g

teeteSt febric ef ce"gNoCC'eSS

xv "bte

sGeneret trend of channet

E

Fig. 24. General trends of channels and paleocurrents in each channel-fi11ing sediments.


V. Basin Analysis of the Muro Group

The sedimentary environment of the Muro group was reconstructed mainly on thebasis of stratigraphical study, paleocurrent analysis and petrographical examination of

conglomerates (KisHu SHiMANTo REsEARcH GRoup, 1975). In order to make clear theenvironment in more detail, the present writer examined sedimentary structures of conglo-

merates and thick-bedded sandstones, and sedimentary features of channel structures.

Adding these new data, the paleogeographical reconstruction is attempted in the follow-

ing.

A. Lithostratigraphic features

Stratigraphy and lithofacies of the Muro group were described in some papers

(KisHu SHiMANTo REsEARcH GRoup, 1969, 1972, 1973; TATEIsHi, 1976), and were sum-marized by KisHu SHiMANTo REsEARcH GRoup (1975). Based on these papers, the litho-

stratigraphic features of the Muro group is summarized as the followings. .

The Muro group is composed predominantly of marine sequences of terrigenousclastic rocks, but lacking in greenstones and radiolarian chert of oceanic origin. Such

lithofacies indicates the miogeosynclinal environment. The Muro group is divided into

three formations, namely the lower, middle and upper formations. The lower formation

is composed mainly of muddy flysch and mudstone, intercalating sandy flysch and thick-

bedded sandstone. The middle formation is composed mainly of sandy fiysch and thick-

bedded sandstone, frequently intercalating conglomerate. The upper formation consists

predominantly of alternations of coarse sediments such as conglomerate, conglomeratic

mudstone and thick-bedded sandstone. The Muro group'shows an upward coarseningsequence as a whole (Fig. 3). Such sequence suggests that fi11ing up ofthe sedimentary

basin was more rapid than its subsidence, and that the basin gradually became shallow

as a result.

B. Properties of coarse sediments

Sedimentary petrol•ogic examination of conglomerate and sandstone gives us valuable

informations for the source rocks. Such studies on the Muro group have been done by

ToKuoKA (1967, 1970),KisHu SHiMANTo REsEARcH GRoup (1970, 1976) and the presentwriter (TATEisHi, 1976). Based on these papers, the compositional properties of their

coarse sediments are summarized as the followings.

Conglomerates of the Muro group are polymictic, and consists of gravels of various

kinds of rocks predominated by acidic volcanic rocks, greywacke-type sandstone, ortho-

quartzitic sandstone, chert, shale, granitic rocks and muddy limestone. Orthoquartzite

gravels are commonly contained in conglomerates, especially more in the upper formation.

Exotic gravels in pebbly mudstone and angular fragment-bearing mudstone are the samekinds of rocks as those of conglomerate.

222 Masaaki TATEISHI Sandstones of the Muro group are mostly poor in matrix, and mostly medium to fine

grain-sized. These belong mostly to feldspathic arenite, and partially to feldspathic wacke.

Main constituents are quartz, K-feldspar, plagioclase and rock fragments.

C. Sedimentation of coarse sediments

Transportation and deposition of the coarse sediments such as conglomerate, con-

glomeratic mudstone, thick-bedded sandstone and flysch-type alternation were examined

in Chap. III, based on sedimentary features such as internal sedimentary structure and

fabric. The sedimentation of these sediments is summarized as the followings.

It is apparent that the flysch-type a!ternation in the Muro group are considered to

be mostly typical turbidite. Thick-bedded sandstone can also be assigned as turbidite in

origin. Although there are some differences of sedimentary features between fiysch-type

alternation and thick-bedded sandstone, these are mainly due to the difference of deposi-

tional environment, that is, flysch-type alternations are deposited in the more distal site,

and on the contrary, thick-bedded sandstones in the more proximal one. • Most of gravels of conglomerates are considered to be transported in suspension and

deposited by the flows combining two mechanisms of turbidity current and grain flow.Grain flow is resulted from dispersive pressure by clast collision under shear. That pres-

sure plays a more important role than turbulence when a lot of large clasts exist in the

flow, and forms the inversive grading of gravels in the lower part of conglomerate bed.

The turbulence being an agency of turbldity current plays an important role to transport

fine gravels, and forms the normal grading and stratification. Conglomerates are consid-

ered to be deposited in the more proximal environment. Conglomeratic mudstones such

as pebbly mudstone and angular fragment-bearing mudstone were deposited by themechanism of debris flow in the lower part of submarine slope, that is, in the most

proximal slte. Angular fragment--bearing mudstone is endolistostrome, and is considered

to have been deposited from the breakdown of the geanticlinal upheaval within the

basin.

D. Paleocurrent analysis Paleocurrent and channel direction play an important role to estimate the situation

of provenance. In this section, the paleocurrents decided by directional sole marks and

clast fabrics of conglomerate are examined.

Many kinds of directional sole marks can be found in the Muro group. Paleocur-

rent analysis of the Muro group was made by HARATA (1965) for the first time, and

thereafter many data were obtained by KisHu SHiMANTo REsEARcH GRoup (1968, 1970,1973, 1976) and the present writer (TATEisHi, 1976). These are summarized in Fig. 25,

and the following conc!usive results can be shown.

i. Lower formation: Eastern longitudinal currents predominant. ii. Middle formation: Current systems vary in horizons. Eastern longitudinal and

Sedimentology and Basin Analysis of the Paleogene Muro Group 2ns

SouthernCoastalAreaStandardStrati-graphy

WestTATErSHI(l976)

Middle'

K.S.R.G.(1970)East

K.S.R.G.(1976)

GF Upper

Noaap

ED Middle

C CBs• UpperB4

oe.VA:

B B3 MiddletB2Blxt

oA Lower Lower

K.S.R.G.: Kishu Shimanto Research Group

Fig. 25. Paleocurrent system of the Muro group at the coast area. The arrows represent the directions of currents. The upside in figure shows the north. The scale of arrow is proportionate

to the measured numbers.

' southern lateral currents are predominant in the lower and middle horizons, while

in the upper, northern lateral currents become common, though several eastern longitudinal currents are found in the top.

iiL Upper formatlon: Southern lateral currents are predominant, and northeastern

!ongitudinal ones are observed partially.

There are two types of clast fabrics in conglomerates of the Muro group, in whichA-type indicates the direction of currents transported in suspension gravels. Paleocurrents

deduced from clast fabrics of conglomerates are shown collectively in Fig. 26. The

results are summarized as the followings.

i. Longitudinal currents from the northeast are observed in the B member, while

in the B member, lateral currents frorri the north-northeast and south-southwest

exist together. . iL Lateral currents from the southeast are predominant in the D member.

2an Masaaki TATEISHI

N

o

byN

"Tetal Bl

Fig. 26. Paleocurrent system deduced fromclast fabrics of conglomerates. The

arrows in outer circle show thedirection of currents from A-type

fabric, and the ones in inner circle

the ones from B-type fabric.

s

;`-i8•t - '

-

s

It can be safely concluded from the above-mentioned paleocurrent analysis that the

sediments were transported mainly by northeastern longitudinal and southern lateral

currents. The southern lateral currents occurred for the first time in the middle forma-

tion, and became predominant in the upper formation.

E. Submarine fan sedimentation

Four large channels found in the D member a!ong the coast area were examined in

detail. Their Iocalities, general trends and paleocurrent in fi11ing sediments are collectively

shown in Fig. 27. The channel at the east of Wabuka is correlated stratigraphically to

the lower one at Yokoshima Island, both of which belong to the lower part of the D

member. The other two channels belong to the upper part of the D member. Consider-ing their stratigraphical relation, the distributary systems of channels as shown in the

upper right of Fig. 27 are obtained.

The fi11ing sediments in each channel show the thinning- and fining-upward se-quence, of which typical example is shown in Fig. 19. Thinning- and fining-upward se-

quences are known from many channel fi11ing sediments, and have been considered toresult from vertical accretion (aggradation) upon gradual channel abandonment (MuTTi,

Sedimentology and Basin Analysis of the Paleogene Muro Group am

N

tX'"nt;.,, .:, ;

i't' t..1 .. - l-- H"• S"" i :'" t"' t .." n- --s -.t- t :." " " t tl' ;t:' i :

Wabuka.' "tt'7'

4t t. tttt-

V.hc.

.,,

l,) lll!)[ll ""

t k•-

o lkM early - st

x

tate sta e

,r;i tt'itti'ti'i"'t::.ss

-"-s ---' it"""":: :'t: r't

lower o tkm ptFig. 27. General trends of channels, and paleocurrents deduced

ripple marks (dotted bution of the D member. tem inferred from considering of these

1974; Ricci LucaHi, 1975; WALKER,

son with recent submarine fans.

area are now abandoned duevertical accretion in the interchannel

shifting of the distributary channel

sedimentary basin of the Muro

fining-upward sequences in

F. Paleogeographic reconstruction

KISHu SHIMANTO RESEARcHto the northern provenance,Shimanto'geosyncline and is

the paleocurrent analysis by

the basis of petrographicpresumed to be consisted of

rocks such as sandstone, shale,

(KIsHu SHIMANTo REsEARcH GRovp,can be confirmed strongly by

t't---v : A. 't' 1' Tako r.,3 ..t•K`/1:-2tJtt'"'ii-si! '"iL '-' nL...--.-..... s.ev/iiii nx ttz" .:....t. s/it

ma :..h.,:•::•s '`-' //,:.ti.; k" .i`J iili'

--- shime Ta"h6saki'": /{-i•;<l..s..!.lg::

--- t /•..tL.1. :'..,i 1.f

from sole marks (black arrows),arrow) and clast fabric (white arrows). Dotted areas show the distri-

Illustration in the upper right show the distributary channel sys-

channel's stratigraphy.

1975b). This interpretation is based on a compari-

In recent deep-sea fans, many channels in middle fan

to shifting of the distributary systems along with rapid

area (NoRMARK, 1970). As shown in Fig. 27, thc systems are considered to have taken place in the group. This shifting seems to cause the thinning- andchannel-fi11ings of the Muro group.

.

GRoup (1975) proposed the hypothesis that in addition the source land rnust have located to the south of the named Kuroshio Paleoland. This hypothesis was based on sole marks and the occurence of orthoquartzite gravels. On

studies on coarse sediments, the southern source land was

Precambrian sedimentary quartzite, various kinds of covering

chert, acid volcanic rocks, etc., and granitic plutonic rocks

1975). The existence of the southern former--land paleocurrents deduced from clast fabrics ofconglomerates

2as

-<>Nx /N- x L.! -s Lz i " -x-• . - t-.st ---t . t -L t tt -- -- s '"'' 'Ss. -NX XLNNw-.L-.,--. V .N =./L)ix--. 1' -c/?g,gft7•li".ill•////'"'t2'tl,I-s/t,,11----L

Masaaki TATEISHI

fr So-7o Km --- .1

l -'N K. K/N, !V- ! -s .-L

'" KN--(::i

- S-x X-- Le v-. N=-

--. X. ".- x - -al ptx L. -.L.

PF-...s----

xs !'L ,Å~.,,,.islE /pzittK tt?i .

,K.scg`

1-,'l/L//91iiiifL

, -- va. -"•

t+ of the

deposited

system 1976)

f6rmation

.reconstructlon

and also to the

sequences ;a part

D member

Geanticlinal

of the

found

of the basin

into the shelf.

by a

x ,gl:-zt/ "',fas:s,y}" ..>;II?..il),i..,L:---

),./l,/2s,,,ll//)/,//;-il,,,//,/1,lek'/i,\l#/,{.E'g/•,k'

-t rs. r- xt >

S -s )'

-- Ne <:It'}

t' ..1 Å~ ?Y•

K--v ? t .

group' fiow.

(TATEISHI,

the middle of histurbidity currents

north, but

the proximal

stage of the

this

formation.

difl'erentiation

cannot be complicated

part partially

exposed partially

The upheaval

1Nt

-/Å~ •..-/x-- ./X-- f--.V(t'y'7nxE.

! lt:/Y. i'-V 7'--e

f-f.k-,"gxz/iwixE.}-.\./,v.t/i'E-

.i!lgeSiJo= /- l + 74 r.i / Shimanto geosyncline the stage when the middle forma- (TATEISHI, 1976). arrows represent the turbidity

reconstructed in present paper. reconstructed illustrated the sedimentary of the group deposited (Fig. 28). are: the were transported and partially by fiow; the provenances exi- south of the ;the lateral currents from the bottom currents might haveof sediments.

of the the

upheaval seems to have taken basin. This inferred by the facts that

northward Uchikoshi anticline, andpaleocurrents in (KIsHu SHIMANTo REsEARaH at the of the middle formation was At the of the F and G members certain and supplied angular movement grew , and the Precambrian

f f N- L ----) t"x-- Å~ nrtH ,N /- sy;:- N t-- "/-'ST t! .ust --!' Ns.- f X-" y. x .-.-- SLL'

x L1

x p-M X<Ls[:' /

'-----v--' '-Y- ttFig. 28. Reconstructed illustration at tion of the Muro The current or gram

and the submarine fan-channel the The present writer andbasin at the stage when MuroThe important points sedimentsdeposited mainly by grainsted not only to the basinthe south deposited normalretransported and redeposited

The basin at the upper formation is reconstructed asshown in Fig. 29. At stage, the basin became shallower and narrower than that at

stage of the middle movementplace, and yielded the isorthoquartzite gravels beyond thethat there are observed the basinGRoup, 1975). The marginal stageupheaved and changed stagethe Muro group was fault movement,fragments to the basin. intense

Sedimentology and

bJN[v'N,L.,L--....

-= ...",f"<ilxttl'isx.

'"fX LSSL

Basin Analysis of the Paleogene Muro Group

"'-..>ll

.-Å~-x>>.t--- :f$IIrl 50-6o krn'•

v-. , x.ys .xNx. Irs)trX.xNSt-"x -`" "LNY'

.,I. tt -4 t: Nt: • ••: • --:/•

::• xiri' i' '"K

. rt... 1i, ! s' f/Vr' ' ..,7` >x'

.. x"--NN<ll'k i-su.i•-!:A.Bl":.l'xl

,, `1

rrr I

lx<' u'.

x { s, KV,;:VV.

t. 1,' 'f..sLt' .,= xu-•,- ' -tE, ,f"1, ':,

i-t-t-

' 77N--Å~-Å~h/=-/xK-,

2.:'N x -sx/; S. N... N A .-X ]'- Kf XN'

xN4 N..

Ek,

N

2or

.1.J-j ii :- ..a:.N

.:

ly.i,.,..,/kl'."Illl/(i-li••

f t,•,N

'i:',S'sX K'xi, ,1•1-1•f,-••, 'k , )k'SJi' //CI.t;IICt.L

'•' '' .••i';'.'lr').,,f.i fxh'x : g•-.'J'.'tL ,.'L.,L'('f

i YXY. IJ/Ar= t' 'V'L '

t' ts. -

'.. / A .:r(,..., .,t

" lttk'lfVTtb).• ' . .ei.iiili(llii/tli:2NZ7"

.t. tL,. ,f. t,'• y•-, ,r' 2,:;T -:T ')S li a

.t i-e Ll/'.XJ,:'

J,:,,.tr.1•i•;i(iL,•

' 't x' ''

"r" !-rx •--•

Å~ .J7"h" .

. ts - - --fx ... -S 'X L-- ls

"t

KvNtL. L. LL

e Jll .LKXX

/'E )lx" /-t.S.N.xN.-

K- ' Cr Nx "" N" P-M c?) N xx Å~ +x +

eef, Slurnp .

Fig. 29.

the Muro

basement containing

provenance. This is

formation (KisHucoarse sands were

Several submarine

channel systems

thick-beddedcurrents, which are

These sedimentsor debris flows.

mudstone weredeposited mainly by

dvL."

:. ,t.• 'K

E

P-Me)

sL? +• +deposits

group. The

e. ' )e o. e." L- l.. 't::' ' ••,r '

-T Xl-ll"))xj',,i•Ii.

'x • <ilt) .,.rf,•

/-i-" "ts /x' ,• '; t-.

isif.'([1,1 ...:.

-"M:, .3,iilll.i,'l.."'kNtt,-'•ii,,l-'

l"r"itT

x tttl •v t, '-"'trLtk'"gi.Ei'i,'l'5•,,.,,,.,

' ...'i'"'ISI..' ;•, sgei;•li.tVt .-" ,

•V,"'i'eE"l-•'•`;'tt•c;'• • ,

L N'.thr,v.r,g

•• rl

,,

//i?;Ti<•i. /r-',rtt•r,•

tx., .tgl:s;I ..,};,:::t

tt,t,Z7iR3::r.nf'.L'k•`)e

-" -" ... •1..J..rrt':X(••tttr,S•f.•,'2,i;.2•,,t//,/ll•L'

,! .r st Ll,r. }- :'

]S;"` vl'J Ufi,r-'I,;.lie

..t. i>l.1"

7

E L- S'-•'I

P .s. 7v +•+Reconstructed illustration of the

arrows

orthoquartzite

suggested from the '

SHIMANTO RESEARCH supplied abundantly

fans were deve!oped were formed on sandstones were deposited

inferred apparently

were deposited mainly The distal sediments

deposited on outer fan or turbidity currents

.N--

-- ~"XN: -1

u-V .Z-

,.P ',i) , '. 1' r o7 t-+ + + Basement

Shimanto geosyncline at the stage of the D member of represent the turbidity current or grain flow.

sandstones was exposed extensively in the southern

mcrease of orthoquartzite gravels in the upper

GRoup, 1976). Coarse clastics such as gravels and

and were deposited at marginal part of the basin.

at the margin of the basin, and distributarythese fans. Conglomerates, conglomeratic mudstones and

there as proximal sequences by southernlatera!

by sole marks and clast fabrics of conglomerates.

by turbidity currents, and partially by grain flows

such as a part of sandy flysch, muddy flysch and

on basin plain. These were transported and from the east and from the southeast.

- Lh= Lv: L..NN/X

L.--`l.K

-,.•

==LL..L. L- "7/:- t:- -'-"

v/7- -.l" rN

' Po-' -.

+

)vayY" x.v" ./.

'L7-'

hyvx.

tr r-xt- x k

LxK.- N.

t+

The

VI. Summary and Conclusion

sedimentary structures of conglomeratic rocks, thick-bedded sandstone and

ms Masaaki TATEISHI

fiysch-type alternation were described, and the sedimentary processes of these sediments

were discussed. Furthermore, channel-scours and -fi11s observed in the upper formation

were examined. On the basis of sedimentological data, the sedimentary environment of

the Muro group was reconstructed. The results are summarized as the followings.

1.- Three descriptive sedimentary models of the conglomerates, which were proposed

by WALK•pR 0975b) are also recognized in the Muro group; namely, i. inverse-to-nomal

grading model, iL graded-stratified model, iii. disorganized modeL Two types of clast

fabrics are• distiguished; Type A - long axis of clasts arranged parallel to current direc-

tion, Type B - long axis arranged transverse to current direction. Most ofconglomerates

of the Muro group show the Type-A fabric. Pa!•eocurrent directions deduced from c!ast

fabrics showing' Type-A coincide well with paleocurrent directions indicated by sole

' ' 2. Conglomeratic mudstones such as pebbly mudstone and angular fragment-bearing

mudstone consist of predominant dark-grey mudstone and scattered clasts in the matrix.

These rocks ' are massive and structureless, and clasts are scattered randomly"'Within

matrix mudstone. Angular fragment-bearing mudstone is an endolistostrome.

3. There are three•sedimentary types of thick-bedded sandstones; i. composite bed

ii. structureless bed with parallel lamination at the top, iii. Iaminated bed. There are

abundant sandstones showing intermediate type between the latter two. Paleocurrent

directions deduced from grain fabrics show an excellent coincidence with sole markstrends.

Most of•flysch-type beds show the succession of internal sedimentary structuressimilar to the Bouma's sequence (BouMA, 1962).

4. Most of the sediments excepting congtomeratic mudstones are concluded to betransported and deposited mainly by turbidity current and partially by grain flow. In the

real fiow, the relation between turbidity current and grain fiow is trJnsitional. The grain

fiow may have played an lmportant role for emplacement of conglomerate composed oflarge clasts. Conglomeratic mudstones are concluded to be deposited by debris flow.

5. Large channels are observed in the upper formation. These channels show near-ly N-S trend in general, and the fi11ing sediments were transported from the south.

These channels are referred to constitute a distributary system. The fi11ing sediments

show the thinning- and fining-upward sequences, suggesting progressive abandonment of

channels due to shifting of the distributary system.

6, The Muro group is composed of miogeosynclinal sediments of 3, 500 m in thick-

ness, and contains abundantly proximal sediments such as conglomeratic rocks and thick-

bedded sandstone. The Muro group make the coarsening- and thickening-upward sequ-ence as a whole. The sedimentary basin of the Muro group was fi11ed up progressively,

and became shallower.

7. The hypothesis that the provenance existed to the south of the Shimanto geosyn-

cline in addition to the north, which proposed by the KisHu SHIMANTo REsEARcm GRoup


(1975), was supported by paleocurrents deduced from clast fabricsofconglomerates and

the occurrence of channel structures. Especially, the supplies from the southern source

land became remarkable at the stage when the upper formation deposited. ,,, 8, Reconstruction of sedimentary environment at the stage when the upper forma-tion deposited was attempted and illustrated (Fig, 29). At the latest stage ofthe Shima-

nto geosyncline, the sedimentary basin was fi11ed up by sediments that were supplied by

intensive and intermittent upheaval of provenance, and dissapeared in the Early Miocene.

Acknowledgments ' I wish to thank the members of the research group on sedimentation in geosynclines

of Dept. of Geology and Mineralogy of Kyoto University. Especially 1 thank Prof. Dr.

Keiji Nakazawa and Dr. Takao Tokuoka of Kyoto University for thelr guidances andencouragements. Also I thank Dr. Tsunemasa Shiki of Kyoto University for his suggestion

on sedimentology and improvement of the manuscript.

I obtained many suggestions from dlscussions with the Kishu Shimanto ResearchGroup, of which I am one of the members. Special thanks are due to Prof. Dr. Tetsuro

Harata of Wakayama University and Dr. Hiroyuki Suzuki of Doshisha University fortheir suggestions and encouragements.

The thin sections of sandstones to examine grain fabric were made by Messers.Hisao Tsutsumi and Kinzo Yoshida of Kyoto University. Helps for typing the manuscript

were received frorn Messers. Fujio Kumon and Kunfhiko Hisatomi. I wish to thankthem.

References

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pp. 521-557.BAGNOLD, R. A• (1954), Experiments on a gravity-free dispersion of large solid spheres in a Newto- nian fluid under shear. Proc. Ray. Sec. A, 225, pp. 49-63.BOuMA, A. H. (1962), Sedimentelogy of.seme flisch deposits. Elsevier Publ., Amsterdarn. 168 p•

- and BROuwER, A. (1964), Turbidites. Elsevier, Amsterdam. pp. 264.CARTER, R. M. and LINDQvlsT, J. K. (1975), Sealers bay submarine fan complex, Oligocene, southern New Zealand. Sedimentolog", 22, pp. 465-483.CHIPPING, D. H. (1972), Sedimentary structure and environment of some thick sandstone beds of turbidite type. Jour. Sedi. Petrol. 42 (3), pp. 587-595.COLBuRN, I• P. (l968), Grain fabrics in turbidite sandstone beds and their relationship to sole mark

trends on the same beds. Jour. Sedi. Petrel. 38 (1), pp. 146-158.CoRBETT, K. D. (1972), Features of thick-bedded sandstones in a proximal flysch sequence, upper Cambrian, southwest Tasmania. Sedimentelogy, 19, pp. 99-114.CROwELL, J. C. (1957), Origin of pebbly mudstones. Bull. Geol. See. Amer. 68 (8), pp. 993-1010.DAvlES, I. C. and WALKER, R. G. (1974), Transport and deposition of resedimented conglomerates; the Cap Enrage formation, Cambro-Ordovician, Gaspe, Quebec. Jour. Sedi. Petrol., 44 (4), pp•

l200-l216. 'De JONG, A. K. (1974), Melange (olistostrome) near Lago Titicaca. Bult. Amer. Assoc. Petrol. Geot.,

230 Masaaki TATEISHI . .58 (4), pp. 729-741.

DOTT, R. H. Jr. (1963), Dynamics of subaqueous gravity depositinal processes. Bult. Amer. Assoc. Petrol. Geol. 47 (1), pp. 104--128.

ELTER, P. and RAGGI, G. (1965), Contributo alla conoscennza dell Appennino ligure: Osservazioni preliminari sulla posizione delle ofioliti nella zona di Zignago (La Spezia) ; 2. Considerazione sul

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FISHER, R, V. and MATTIsON, J. M. (1968), Wheeler gorge turbidite-conglomerate series, California:

inverse gradlng. Jeur. Sedi. Petrol. 38 (4), pp. 1013-1023. •HAMpTON, M• A. (1972), The role of subaqueous debris flow in generating turbidity currents. Jour. Sedi. Petrol., 42 (4), pp. 775-793.

HARATA, T. (1965), Some directional structures in the flysch-like beds of the Shimanto terrain in the

Kii Peninsula, southwest Japan. Memoirs Coll. Sci. Univ. K2oto, Series B, 32 (2), pp. 103-176.

HENDRY, H. E. (1973), Sedimentation of deep water conglomerates in lower Ordovician rocks of Quebec -composite bedding produced by progressive liquefaction of sediment.Jour. Sedi. Petrol. 43 (1), pp. 125-136. '

' (1976), The orientation of discoidal clastsin resedimented conglomerates,Cambro-Ordovician, Gaspe, eastern QLuebec. Jeur. Sedi. Petrol. 46 (1), pp. 48-55.JOHANSSON, q. E. (1965), Structural studies of sedimentary deposits. Geol. I7eren. Stockh. Forhand. 87,

pp. 3-6!.KAY, M. (1976), Dunnage melange and subduction of Protacadic ocean, Northeast Newfoundland. Geol. Soc. Amer. SPec. Paper, 175, pp. 1-49.

KrSHu SHIMANTO RESEARcH GRoup (1968), The study of the Shimanto terrain in the Kii Peninsula, southwest Japan (part 2)-the present states of the research and southern former land in the Pacific Ocean -. Earth Science, 22, pp. 224-231. (in Japanese).

-- (1969), The stratigraphy and geologic structure of the southern coastal region of the Kii

Peninsula- the study of the Shimanto terrain in the Kii Peninsula, southwest Japan (part 3) -. Bull. Fac. Education, Wakalama Univ. Natural Sci., No. 19, pp. 19-29. (in Japanese). (1970), Sedimentological and paleontological studies of the Muro group at the southern coastal region of the Kii Peninsula - the study of the Shimanto terrain in the Kii Peninsula, southwest Japan (part 4) -. Bull. Fac. Education, Wakayama Uniu., Natttrat Sci., No. 20, pp. 75-102. <in Jap

- (1972), The Muro group in the upperreaches of Koza River in Wakayama Prefecture - the study gf the Shimanto terrain in the Kii Peninsula, southwest Japan (part 5)-. Earth Science, 26,

pp. 195-204. (in Japanese). •- (1973), The stratigraphy and geologic structure of the southern coastal region (Satono-Mirozu) of the Kii Peninsula- the study of the Shimanto terrain in the Kii Peninsula, southwest Japan (part 6) -. Bull. Fac. Education, Mlakayama Univ., IVatural Sci., No. 23, pp. 93-101. (in Japanese).

--•- (1975), The development of the Shimanto geosyncline. Monograph Assoc. Geel. Collab. Japan, No, 19, pp. 143-156. (in Japanese). (1976), Sedimentological and paleontological studies of the Muro group at the southern coastal

region (Satono-Mirozu) of the Kii Peninsula - the study of the Shimanto terrain in the Kii Peninsula, southwest Japan (part 7) -. Bull. Fac. Education, Waka7ama Univ., Natural Sci., No. 25,

pp. 39-51. (in Japanese).

KUENEN, P. H• (1967), Emplacement of flysch-type sand beds. Sedimentolo.gv, 9, pp. 203-243.

and MIGLIORINI, C. I. (1950), Turbidity currents as a cause of graded bedding. Jour. Ceol. , 58, pp. 91-127. 'KuROKAwA, K. (1972), Internal sedimentary features in flysch beds of the Muro group at the southern coastal district of the Kii Peninsula. Earth Science, 26, pp. 235-242. (in Japanese).LINK M. H. (1975), Matilija sandstone: a transition from deep-water turbidite to shallow-marine


deposition in the Eocene of California. Ioar. Sedi. Petrol., 45, pp, 63-78.

LOwE, D. R. and LoplccoLO, R. D. (1974), The characteristics and otigins of dish and pillar struc-

tures. Jonr. Sedi. Petrol., 44 (2), pp. 484-501. 'MIDDLETON, G. V. and HAMPToN, M. A. (1976), Subaqueous sediment transport and deposition by sedirnent gravity flows. In STANLEy, D. J. and SwlFT, D.J. P. (ed.), Marine sediment transport and

environmental management. John Wiley & Sons. Inc. pp. 197-218. 'MRAKOvlCH, J. V. and CooGAN, A. H. (1974), Depositinal environment of the Sharon conglornerate member of the Pottsville formation in northeastern Ohio, Jour. Sedi. Petrol., 44(4), pp. !186-1199•MuTTI, E. (1974), Examples of ancient deep-sea fan deposits from circum-Mediteranean geosynclines. In DOTT, R. H. and SHAvER, R, H. (ed.), Modern and ancient geesynelinat sedimentation. SP"ec. Pubt.

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Eocene Hecho group (south-central Pyrenees, Spain). Sedimentology, 24 (1), pp. 107-131•NELSON, C. H. and NILsEN, T. H. (1974), Depesitional trends of medern and ancientdeep-sea fans. In DOTT, R. H. and SHAvER, R. H. (ed.), Modern and ancient geos7nclinal sedimentatien. SPec. Publ. Sac. Ecen. Paleont. Mineral., 19, pp. 69-91.

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439-452.NORMARK, W. R. (1970), Growth patterns of deep-sea fans. Bull. Amer. Assoc. Petrel. Geol. 54, pp.

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ROCHELEAu, M. and LAJolE, J. (1974), Sedimentary structures in resedimented conglomerate of the Cambrian flysch, Llislet, Quebec Appalachians. Jeur. Sedi. Petrol. 44 (3), pp. 826-836.

RuST, B. R. (1972), Pebble orientation in fluvial sedirnents. Ieur. Sedi. Petrol., 42 (2), pp. S84-388.

SCHERMERHORN, L. J. G. and STANToN, W. I. (1963), Tilloids in the West Congo geosyncline. Quart. Jear. Geel. Sec. Londen, 119, pp. 201-241.SELLEY, R. C. (1970), Ancient sedimentary envirenments. Chapman and Hall Ltd., London. pp. 237.SHIKT, T. KIMuRA, H. and HARATA, T. (1968), On "flysch" in geosynclines. Memoirs Geol. Soc.JaPan,

No. 1, pp. 211-232•SMITH, T. M. (1971), A proximal-distal turbidite sequence and a probable submarine canyon in the Siegas forrnation (early Llandovery) of northeastern New Brunswick. Jrour. Sedi. Petrol., 41(3), pp,

752-762.STAUFFER, P. H. (1967), Grain-flow deposits and their implications, Santa Ynez mountains, California.

.Iour. Sedi. Petrol. 37 (2), pp. 487-508.

TATErSHI, M. (l976), The Muro group in the southwestern part of the Muro belt of the Shimanto terrain. Jour. Geol. Sec. Japan, 82 (6), pp. 395-407. (in Japanese).

TOKUOKA, T. (1967), The Shimanto terrain in the Kii Peninsula, southwest Japan - with special reference to its geologic developrnent viewed from coarser clastic sediments -. Memeirs Fac. Sci.

KJoto Univ. Series of Ceelog2 and Mineralog7, 34 (1), pp. 35-74.

232 Masaaki TATEISHI-- (1970), Orthoquartzite gravels in the Paleogene Muro group, southwest Japan. Memoirs Fac. Sci. K]oto Univ:, Ser. Ceot. Mlinerat. 37 (1), pp. 113-132.

TYLER, J. H. (1972), Pigeon Point formation: An upper Cretaceous shoreline succession, central California coast. Jour. Sedi. Petrol., 42 (3), pp. 587-595.

WALKER, R. G. (1967), Turbidite sedimentary structures and their relationship to proximal and distal

depositional environments. Jour. Sedi. Petrol. 37 (1), pp. 25-43. ' '' (1975a), Upper Cretaceous resedimented conglomerates at wheeler gorge, California: descrip- tion and field guide. Jour. Sedi. Petrol. 45 (1), pp. 105-112.

(1975b), Generalized facies models for resedimented conglomerates of turbidite association. Butl. Ceol. Soc. Amer. 86 (6), pp. 737-748.

(1975c), Nested submarine fan channels in the Capistrano formation, San Clemente, California.

BuU. Geet. Soc. Amer. 86 (7), pp. 915-924. and MuTTr, E. (1973), Turbidite facies and facies associations: turbidites and deep water sedimentation. Soc. Econ. Paleent. Miner. Shert course note, pp. 119--158.

-- and PETTIJOHN, E J. (1971), Archaean sedimentation: analysis of the Minnitaki basin, north- western Ontario, Canada. Geol. Soe. Amer. Bull,, 82 (8), pp. 2099--2130.

Plate I sedimentary features of conglomerates1: Inverse-graded conglomerate, of wh'ich basal part is composed of coarse pebbie-sized gravels, and upper part is

cobble-sized. This is stratigraphically the lowest bed at Azashi-W (loc. 18) (see Fig. 5). •2: Fine pebble conglomerate with cobble-sized calcareous mudstone clasts. It shows a crude stratification formed by arrangement of mudstone clast (Azashi, the east of loc. 18).3 Conglomerate graded from coarse pebble-sized to medium sandstone (Shirashima tunnel-E, ioc. 11).4 Normal-graded conglomerate (lower left) and stratified conglornerate (rniddle). The latter is 5 m thick, and is composed of laminations of fine pebble-sized gravels and granule-sized gravels (shirashima tunnel-W, loc. 10) (see Fig. 5).

Plate II

1

2

3

4

lwi ' $

.4gEwwva •'- \

'nest

ee.eefi.,r,tw......

•/,t.ag

ssit

"ll

",la.t

ajza

ex

tweggtm :lge , .

•mte l:'

/..W .

essR.•

,fiv-.,.ig

i'

1,r'4tt"'Eij'

Sedimentary features of thick-bedded sandstones: Composite bed formed by amalgamation, a sketch of which is shown Fig. 14 (Wabuka-W,: Structureless bed partially with parallel lamination. It is about 8 m thick (Inazumijima, loc.

: Thick-bedded sandstone in which structureless division of parallel lamination is observed about 4 m thick (Kuchiwabuka, loc. 12).: Laminated bed of 90 cm thick. Its lowest part is massive (Esuzaki, Ioc. 14).

ttsny

-ectvsi.gei

'i'l"$':i'"'ee.ieet..,ee/eef'

di"xas'tima'tk'lsg-.,vtf/•x

'Ree•.:lj'X.•F.,

tt}mfithi/

'Y' ri'"ttJ/i.is.

b 'tu"e"/} .wt

loc. 16).

7).

at the upper part. It is

sva".g.ft'.'-'-

an . ipm1Åí pt

k-

dese J•X•'

,,' )?x

t" ''

'•'

su//"$'s.:1-•}i'ttr--as.'s.

ajuteq$. •-

---c:s''

vtM'

ee

eeaj.,.•,.W'zt .-aj.tt.Sl,fiw. .'

ee•..`...,sek.mu,tw.,....l.:,Y

XY',ige,"thdiim-,k' "/tsS't"

v, g-iwtk' -..

"l•S-l/,"1'if{-i, I.,wh':'e.':' .l.

. .'"A./fff,.'ts/va,

.lk.de

as

/' [im, "V

s'.'xme.I. -, .,

-l.tts2

me

• ?eess•

Plate III

1: 2: 3: 4:

ew

"if,tS$.geeetsitee.s.•eeeYts '.

tw: •eq-•

.g

$

,,tla,• if,•sc

Vt''x"

tw.ig

y. slJr"

:;ft;

$pmab-sk •

paa\.

.pt

s n-bt-, .

ss

Internal sedirnentary structures of thick-bedded sandstonesTrough-type cross lamination developed at the top of laminated thick-bedded sandstone (Natachi, loc. 2).Ripple-drift c,ross lamination developed at the middle part of thick-bedded sandstone (Esuzaki, loc. 14).

Dish structure developed with in massive part of thick-bedded sandstone (Shirashima tunnel-W. Ioc. 10) (seePillar structure developed within laminated part of amalgamated bed (Kodomari-W, loc. 3) (see Fig. 12).

Fig. 12)•

Plate IV1:

2:

3:

4:

tt.rpa. ,Irtw .

wuae

igreW.E,,,p

.=egege

sstw•. ."mdizz.1 ,eq Tpm IS. S..b

srvbu

Flysch-type alternationSandy flysch; alternation of sandstones (7 to 50 cm thick) and rnudstones (2 to 5 cm thick) (Wabuka-W, loc. 16).Normal flysch; alternation of sandstones (usually 3 to 5 crn thick, often 20 cm thick) and mudstones (3 to 5 cmthick) (shirashima tunnel-W, loc. 10).Ta-e sequence of sandstone bed 45 cm thick (Wabuka-W, loc. 16).Muddy flysch; alternation of sandstones or siltstones (2 to 3 cm thick) and mudstones (3 to 5 cm thick) (Kuchiwabuka-S,the south of loc. 12).

gg' ,i ' 'z,,

ajfEg./.

in}t/t-;,tt- •

s-wh".",,.rt-lk,,-ppm,,,i

gtwt.•...

Plate V 1:

2:

3:

gel., t:.t'i,

..nge•kts

Channel structures and fi11ing sediments at Wabuka-EChannel-base and fiIIing sediments. F,11ing sediments

which two sandstone layers are involved in lens shape The quadranglar part is magnified into Plate V-2. Basal part of channel structure magnified the quadranglar basal part of fi11ing sediments. Conglomerate bed underlying the channel structure Fig. 18). A lot of pseudo-clasts of calcareous mudstone which bedding plane is irregular and indistinct.

1

(loc. 17) :are 'composed of pebbly mudstones of about 10 m thick, in and large blocks of sandstone and conglomerate are contained.

part of PIate V-1. Gravels are concentrated into the

(right) and the pebbly mudstone (left) near Point 160 (see is contained in the upper part of conglomerate bed, of

Plate VI1:

2:

3:

Channel structure and channel fi11ing sediments at Yokoshima Island (Ioc. 19)

Sandy to muddy flysch sequences fi11ed the lower channel (Iower) and the upper channel structure (upper right).Minor scour and fiIl observed within the fi11ing sediments of the lower channeL

Channel base of the lower channel. The sequences underlying the base (lower) is consist of muddy flysch, andfi11ing sediments (upper) is pebbly mudstone.

title sedimentology and basin analysis of the paleogene

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