title sedimentology and basin analysis of the paleogene
TRANSCRIPT
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"
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svte6u -HieaMgOWa
xtV's
.--
Belt"
Fig.
mf'essge
=
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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. '/ '' .'. ,...
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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' :'
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---T ' -l;Ss}Sio'o'e'o
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9oe
o
eo
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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.
Sedimentology and Basin Analysis of the Paleogene Muro Group 191
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$tone bearing
Bs
M!rozu- Ameshima B4
S4t5
B3
B2
Bl
-E!i]EIIill
EIIIII
[Iii[IEI!!] angular
----"-fi---------"ttm-t----------
ttt
fragment
N N sfi"'22 s hs
16
Generalized Cotumnar Sectlon
H
GN N Å~ s e-Ns F ssxNs Es s. -.sN ss D s '-- xC N.
t-
l
B
!-A
km
Fig. 3. Columnar sections of the Muro group. Each section was compiled from the followings;generalized section: KIsHu SHIMANTo REsEARcH GROUP (1975), Hiki-Mirozu: TATEIsHl(1976), Mirozu-Ameshima: KIsHu SHIMANTo REsEARcH GROuP (1973),Ameshima-Tano-saki: KisHu SHIMANTo RESEARcH GRoup (!969).The numbers at the right of sections show the horizons of localities examined sedimentologi-cally.
192 Masaaki TATEISHI
3,500 m in thickness, and obtained columns can be correlated to the generalized strati-
graphy of the Muro group as shown in Fig. 3. The middle formation of the presentarea is subdivided into five members by intercalations of mudstone and muddy flysch isNE-$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.
Sedimentology and Basin Analysis of the Paleogene Muro Group 193
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
194 Masaaki TATEISHI
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.
Sedimentology and Basin Analysis of the Paleogene Muro Group 195
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
196 Masaaki TATEISHI
.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
Sedimentology and Basin Analysis of the Paleogene Muro Group 197
-!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.
Sedimentology and Basin Analysis of the Paleogene Muro Group 199
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
200 Masaaki TATEISHI
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
Sedimentology and Basin Analysis of the Paleogene Muro Group 201
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
202 Masaaki TATEISHI
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
Sedimentology and Basin Analysis of the Paleogene Muro Group 203
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
206 Masaaki TATEISHI
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
Sedimentology and Basin Analysis of the Paleogene Muro Group 207
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
Sedimentology and Basin Analysis of the Paleogene Muro Group 213
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.
214 Masaaki TATEISHI
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.
Sedimentology and Basin Analysis of the Paleogene Muro Group 215
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.
216 Masaaki TATEISHI
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).
Sedimentology and Basin Analysis of the Paleogene Muro Group 217
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
220 Masaaki TATEISHI
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.
Sedimentology and Basin Analysis of the Paleogene Muro Group 221
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 direc- tions 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
Sedimentology and Basin Analysis of the Paleogene Muro Group 229
(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.
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' (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,
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-- (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
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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.