Title Three Dimensional Analysis of a Large Sandy-flysch ...repository.kulib.kyoto-u.ac.jp/dspace/bitstream/2433...Title Three Dimensional Analysis of a Large Sandy-flysch Body, Mio-Pliocene

Title Three Dimensional Analysis of a Large Sandy-flysch Body,Mio-Pliocene Kiyosumi Formation, Boso Peninsula, Japan

Author(s) Tokuhashi, Shuichi

Citation Memoirs of the Faculty of Science, Kyoto University. Series ofgeology and mineralogy (1979), 46(1): 1-60

Issue Date 1979-09-30

URL http://hdl.handle.net/2433/186633

Right

Type Departmental Bulletin Paper

Textversion publisher

Kyoto University

)vlEMolRs oF THE FAcvLTy oF ScmNcE, KyoTo UNIvERsrry, SERrEs oF GEoL. Vo]. XLVI, No. 1, pp. I-60, pls. 1-5, 1979

& MrNERAL.,

Three Dimensional Analysis of a Large Sandy-Flysch Body,

Mio-Pliocene Kiyosumi Formation, Boso Peninsula, Japan

By

Shuichi ToKuHAsHI*

(Received January 13, 1979)

Contents

Abstract ............,.........................................................................

I. Introduction.....................................................,....................

A. Preliminary Remarks to the Present Study ............................. 1. Start ofmodern flysch sedimentology ,....,.............................

2. Unique investigation in the Boso Peninsula ,.........................

B. Necessity of the Three Dimensional Analysis ofFlysch Sequence .. 1. Submarine fan model and facies association ..........................

2. Some problerns on current submarine fan models .................... 3. Purpose and method ofthis study ................•.......•............. II. Geologic Outline....................................................................

A. Geologic Setting of the Neogene Sediments in the Boso Peninsula .. B. Geology ofthe Middle Part ofthe Boso Peninsula .................... 1. Preliminary remarks .................,......................................

2. Stratigraphy ..........................,......................................

3. Igneous activity ..............................................................

C. KiyosumiFormation...........................................................

1. General remarks.......................,......................................

2. Chronologicdata ...........................................................

3. Depositionaldepth...........................................................

4. Typeofbasin .......................................,.........................

III. Basic Sedimentological Data.....................................................

A. Main Tuff Marker-Beds Used to Divide the Formation into Units B. Facies Distribution ...........................................................

C. LateralVariationofThickness ............................................

D. Trough-likeBasalErosion .,................................................

E. Size and Constitution of Gravels......................................,.....

F. Paleocurrent ....................................................................

IV. Inferred Depositional Process .......................................,..........

A. On thc Origin of Gravels.....................................................

B. Depositional Patterns ofthe Representative Units .................... I. Depositional pattern ofthe Tk-Kr unit .....................,.,........

2. Depositional pattern ofthe Hk-Km unit ............................. 3. Depositional pattern of the Sa-Nm unit ...............................

C. Depositional ProÅëess ofthe Kiyosumi Forrnation ......................,

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* Geological Survey ofJapan, Osaka OMce

2 Shuichi ToKuHAsHr

D. Depositional Model ofTurbidite Bed and Facies V. Discussion .........-,..H.....................,.............

A. Comparison of the Depositional Process of the that of the Submarine Fan Modei ....,......,...... B. Early Stage ofFan Sedimentation ,..........,,..,.. C. Further Problems .......................................

VI. ConclusiveRemarks ......,........,.......,...............

Acknowledgements ......,,.......,........,.................,.....

References ...................,..................................,.....

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Abstract

Three dimensional analysis of fiysch sequence is very important as it makes substantially possible

to compare the depositional process of it with that of the current submarine fan model and to clarify

the phased development ofsubmarine fan sedimentation. Such study has been almost lacking until

now. The Kiyosumi Formation is composed of a large lenticular sandy-flysch body, measuring 850 min maximum thickness and more than 20 km in E-W direction aiong the folding axis and more than6 km in N-S direction. The Kiyosumi Formation is divided into five units by means ofsix main tuffmarker-beds. Each unit is composed mainlY of both channel deposits and depositional tongue.Channel deposits are characterized by an upward thinning cycle beginning with thick pebblysandstones and ending with thin siltstone-dominated alternations and by accompanying a largetrough-like erosive morphology at the base attaining up to some 50 m in maximum depth and morethan 5km in maximum width. The depositional tongue is characterized by extensive and thicksandstone-dominated alternations downcurrent of the channel cleposits and shows negligible basalerosion. Furthcr downcurrent, the depositional tongues are replaced by siltstone-dominated alterna-tions or massive siltstones.

The Kiyosumi Formation has been deposited by the lateral supply from north into the E-Wtrending restricted slope basin by the same process as that of the recent submarine fans. Channeldeposits in each unit seem to have been deposited on the upper-fan segment. Depositional tonguein each unit must have formed a suprafan (NoRMARK, 1970) on the middle fan segment. Siltstone-dominated alternations or massive siltstones must have been cleposited on the lower-fan segment orbasin floor. However each depositional tongue in the Kiyosumi Formation, covering a wide areamore than 20 km, occupys a much more extensive segment on a fan than suprafans on recent submarinefans. Individual thick sandstone beds also continue persistently as wide as the tongue. The channeldeposits and the depositional tongue in the lowermost unit show a peculiar distribution as fan sedi-

mentation. They seem to be the deposits at "a preparatory stage" of fan sedimentation or at "apre-fan-sedimentation stage'' during which preexisting reliefs on the basin floor are smoothed and anequilibrium profile of the slope-fan-basin is attained.

The phased retreat of the terminus of channel deposits from the lowermo,gt unit to the uppermost

unit caused the first order upward thinning cycle detected throughout the Kiyosumi Formation. Shiftof the channel to the different site was caused by beginning of each second order upward thinningcycle commonly observed in the channel deposits or by the rejuvenation of turbidity currents. Thecauses of these multiple upward thinning cycles, characterizing the vertical variation of the flyschsequence in the Kiyosumi Formation, are also discussed.

Three Dimensional Analysis ofa Large Sandy-Flysch Body 3

I. Introduction

A. Preliminary Remarks to the Present Study

1. Start of modern flysch sedimentology Modern flysch sedimentology started when the turbidity current theory wasapplied to geology on land. First paradigmatic works were performed by KuENEN's

two papers with two micropaleontologists (KuENEN and ])vliGLioRiNi, 1950; NATLAND

and KuENEN, 1951). Hereupon has come into the world an entirely new-styledscientific province which is based on land geology, marine geology and hydraulics.

Turbidity current theory has not only coined the term "turbidite", but aiso produced

the revolutionary effects on a realm ofclastic sedimentation. According to WALKER

(1973), the turbidity current hypothesis constitutes a revolution perhaps equal in

magnitude and importance to the continental drift revolution.

Until now, a large number of papers have been published on turbidite andflysch (KuENEN and HuBERT, 1964; DzuLyNsKi and WALToN, 1965; WALKER,1973; etc.). , Modern flysch sedimentology landed in Japan in 1960's. Thereafter, manyresults of flysch sedimentology have been obtained in succession in various areas in

Japan (SHiKi, 1961; HARATA, 1965; TANAKA, 1965; SuyARi, 1966; SuyARi et al.,1968; HiRAyAMA and SuzuKi, 1965, 1968; SAsAKi and UsHljiMA, l966; HARATAet al., 1967; KisHu SHiMANTo REsEARcH GRoup, 1968, 1970; TsuDA and NAGATA,1969; TERAoKA, 1970; TANAKA, 1970). In 1970's, much more papers on fiyschsedimentology have been continuously produced by many researchers.

2. Unique investigation in the Boso Peninsula Studies on turbidites and flysch in the Boso Peninsula occupy an unique portion

in flysch sedimentology not only in Japan but also in the world.

Neogene flysch deposits are developed thick and widely in the Boso Peninsula,

and they intercalate numerous extensive and characteristic tuff-beds. Since 1950's,

these tuff-beds have been used as very useful marker-beds in order to correlateformations separated from each other, to analyze the lateral change of thickness and

Iithofacies of succession between two specific tuff-beds and to estimate the quantity

of eroded succession below an unconformity (MiTsuNAsHi, 1954; MiTsuNAsHi andYAzAKi, 1958s MiTsuNAsHi et al., 1961; Boso PENiNsuLA REsEARcH CoLLABoRATiNGGRoup, 1965; NAKAJiMA, 1973, 1978; REsEARcH GRoup FoR NEoGENE TEcToNicsoF THE KANTo DIsTRIcT, l977). On the other hand, HiRAyAMA and SuzuKi (1965, 1968) succeeded in tracingindividual beds of a monoclinic sequence over 30 km along the strike with the aid of

4 Shuichi ToKuHAsHl

many thin tuff marker-beds. They clarified the geometry and sedimentary struc-tures of individual constituent beds of the sequence. Subsequently, various studies

on turbidites based on bed-by-bed correlation have been made in the Boso Peninsula

(HiRAyAMA et al., 1969; YAMAMoTo, 1971; ToKuHAsHi and IwAwAKi, 1975; ToKu-HAsHi, l976a, b).

WALKER (1973) pointed out that the first thing of the main three aspects which

still need "mopping up" of the "turbidite mess" is to know about the extent of indi-

vidual turbidite beds and the way in which bundles of beds are related to basingeography, because little is known about them. Such important studies have beendone in the Boso Peninsula that fiII a large gap in the knowledge of flysch sedimen-

tology in the world. The introduction of these works to the world has been long

overdue (HiRAyAMA and NAKAJiMA, l977).

B. Necessity of the Three Dimensional Analysis of Flysch Sequence

1. Submarine fan model and facies association Since "fluxo-turbidites" (DzuLyNsKi et al., 1959) and "Bouma sequence"(BouMA, 1962) were recognized, many active discussions on a domain of turbiditefacies and the interrelationship of the various kinds of facies have been made among

the flysch sedimentologists (RADoMsKi, 1961; STANLEy and BouMA, 1964; DzuLyNsKi

and WALToN, 1965; WALKER, 1966, l967; STAuFFER, 1967; CoRBERT, 1971; W.ScHLAGER and M. ScHLAGER, 1973; MuTTi, 1975). These discussions were com-bined with a submarine fan model and resulted in proposal of an overali facies asso-

ciation of the turbidites (WALKER and MuTTi, 1973; Ricci-LuccHi, 1975). Asa result, the domain of the turbidite facies was remarkably expanded. The workon recent submarine fans played a great role to interpret the turbidite facies on land

(ME! ARD, 1955; GoRsLiNE and EMERy, 1959; HAND and EMERy, 1964; GoRsLiNEand NELsoN, 1969; SHEpARD et al., 1969; NoRMARK and PipER, l969; NoRMARK,1970; NELsoN et al., 1970; HANER, 1971; NoRMARK and PipER, 1972; NELsoN andKuLM, 1973, SAKuRAi et al,, 1974; NoRMARK, 1974; etc.).

2. Some problems on current submarine fan models An overall depositional process of flysch is not clarified in terms of the current

submarine fan model. The present submarine fan model has at least two basicshortcomings as fo}lows.

In the first place, minutely investigated submarine fans until now are restricted

to those at stable continental margins. But little or nothing is known about turbidites

in the trenches, in the interarc and marginal basins (WALKER and MuTTi, 1973).Therefore, other depositional environments must be also considered in addition to

such submarine fan models.


In the second place, the present submarine fan model is based mostly on thesurface morphology and very thin core samples (WALKER and Mu rri, 1973; MuT"ri,1974). Consequently a little is known about the detail internal structures of the fan

and nothing is known or discussed about an early stage of fan deposition.

On the other hand, in case of studying flysch deposits on land, it is more easy

to recognize and analyze the vertical facies sequence but it is very diMcult to analyze

the areal or lateral facies change (NoRMARK and PipER, 1969; }vluTTi, l974; Ricc!-

LuccHi, 1975). Therefore, it is inevitable to depend on the current submarine fan

model, when flysch sedimentologists estimate the interrelationship among the various

types of facies and their extents and depositional sites.

But it is still unknown whether the depositional process of the early stage ofsub-

marine fan sedimentation is the same as that of the current submarine fan model.There is no guarantee that the extent of each facies in ancient flysch deposits is the

same as that of the submarine fan model which is estimated based only on the recent

submarine fans. Thus, there is still a large gap to be filled between the submarine

fan model and the overall depositional process ofthe real flysch sedimentation.

3. Purpose and method of this study If three dimensional analysis of a flysch body containing various types of facies

is possible and their depositional relation is clarified independently based on the field

evidence, flysch sedimentologists need not interpret the genesis entirely depending

on the current submarine fan model, but can reexamine it with their results from

a geologic new viewpoint. Moreover they may recognize more substantial problems

on submarine fan sedimentation. But such studies have not been performed yetexcept for few preliminary papers (PipER et al., l978). The main reason is that the

fiysch body is composed of thick and somewhat monotonous sequences and generally

and very unfortunately lacks usefu1 marker-beds. If many extensive marker-bedsindicating the time surface such as tuff-beds are intercalated in the fiysch sequence,

three dimensional analysis is really possible. Such is the case in the Boso study area.

In this paper, the author first presents a detailed three dimensional analysis of

a large sandy flysch body named the Kiyosumi Formation in the Boso Peninsula.The Kiyosumi Formation is composed mainly of sandstone-dominated alternationsattaining 850 m in maximum thickness. Several sedimentary cycles indicating up-

ward thinning can be recognized in its sequence. This formation is gently folded

and distributed for about 25 km along the folding axis and about 6 km across it.

Numerous tuff-beds are intercalated in the Kiyosumi Formation and many of them

are tracable throughout the peninsula for 40km along E-W direction and 6km .across lt.

ToKuHAsHi (1976a, b) correlated the individual sandstone beds in sandstone-dominated alternation of about 50 m in maximum thickness just below the Hk tuff

6 Shuichi ToKuHAsHI

named by MiTsuNAsHi and YAzAKi (1958) for about 40 km in E-W direction andabout 5 km in N-S direction. He demonstrated the geometry ofthem with a panel-

diagram method and analyzed the depositional process of them. Then he firstproposed that these sandstones were supplied not from the south but from the north,

and the depositional environment might have been a submarine fan.

In the present paper the author aims to clarify the depositional process of the

overall Kiyosumi Formation. The formation is divided into five units mainly based

on the upward thinning cycles in it. These units are tracable throughout the sur-

veyed area by means of six selected tuff marker-beds lying near the boundary of the

cycles. Exact geometry and detailed facies distribution of each unit are firstly

demonstrated with other sedimentological data. The depositional patterns of thethree representative units are analyzed and then the depositional process of the

overall Kiyosumi Formation is inferred. At last the difference from the currentsubmarine fan model, an early stage of fan sedimentation and other related problems

are discussed.

IL GeologicOutline

A. Geologic Setting of the Neogene Sediments in the Boso Peninsula

It is well known that the characteristic geologic belts in the southwest Japan,

such as the Ryoke, the Sambagawa, the Chichibu and the Shimanto belts fromnorth to south, stretch to the Kanto region (KoiKE, 1957; IsHii, 1962; IcmKAwAet al., l970; YosHiDA, 1975; Fig. 1). They constitute the basement rocks of mostly

the Quarternary sediments under the Kanto Plain located between the northern and

southern Kanto regions. Neogene sediments in the southern Kanto region including

the Boso Peninsula are developed mainly on the Shimanto belt. The zonal structures

along the Sagami and Suruga Troughs are also observed in the Neogene sedimentsdeveloped concordantly with the northern and western older geologic belts men-tioned above (OMoRi, 1960; MATsuDA, 1962; KiMuRA, 1977). KiMuRA (1977) stretched the zonal Neogene depositional trend in the southern

Kanto region to the slope region between the shelf and the Nankai Trough off the

Pacifie coast of the southwest Japan. On the slope are developed many basinscontaining Neogene and Quaternary sediments. The Kumano Trough, MurotoTrough, Tosa Terrace and Hyuga Terrace which are located in the inner part ofthe upper slope are basins of the first order in magnitude but less than 100 km in

their maximum length (Fig. I). These isolated basins are generally restricted with

the outer ridges, spurs and so on (MoGi and SATo, 1975; OKuDA, 1977). Theauthor calls these basins on the slope between shelf and trough (or trench) "slope

basins" here collectively.

Three Dimensional Analysis of a Large Sandy-Flysch Body

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(}'eologic and geographic setting of the Boso Peninsula (modified fromrvloGi and SATo, 1975).

1. Ryoke belt, 2. Sambagawa belt, 3. ChichibLt belt, 4. Shimantobelt, 5. Margin of the continental shelf, 6. Submarine canyon,7. Slope basin, 8. Trench (Trough),

a. Hyuga Terrace, b. Tosa Terrace, c. Muroto Trough,d. Kumano T'rough, e. Nankai Trough, f. Suruga Trough,g. Sagami Trough, h. Japan Trench.

B. Geology of the Middle Part of the Boso Peninsula

1. Preliminaryremarks A thick sequence ofthe Oligocene to Pleistocene marine sediments are successive-

ly developed throughout the Boso Peninsula, and have been regarded as one of the

most important and representative type successions of the Late Cenozoic Era in

8 Shuichi ToKuHASHI

Japan (AsANo, 1970). Since 1880's, a lot of geologists have studied this area and

numerous papers have been published. Geologic outline of the middle part of thepeninsula is briefly explained here according to the papers published after the World

War II. After the World War II, stratigraphic works by many geologists have resulted

in many different stratigraphic divisions (IKEBE, 1948; OTuKA and KoiKE, 1949;KoiKE, 1949, 1951, 1957; IDA et al., 1956; KAwAi, 1957, 1961; MiTsuNAsHi andYAzAKi, 1958; MiTsuNAsHi, 1968; NAKAJiMA, 1973, 1978). In this paper the author

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Width of the vertical lenticular forms in the right column means the relative degreeofdevelopment ofeach facies in the eastern halfofthe peninsula.


follows the latest division by NAKAJiMA (1978) (Fig. 2). For a description of each

formation except for the Kiyosumi Formation, the reader is referred to such papers

as KoiKE (1949, 1957), KiKucHi (1964) and NAKAJiMA (l972MS). Only a fewaspects are briefly explained here.

2. Stratigraphy In this area are developed the Oiigocene Mineoka Group, Oligo-Miocene HotaGroup, Mio-Pliocene Awa Group and Plio-Pleistocene Kazusa Group (Fig. 3).The uppermost Pleistocene Shimosa Group is distributed in the northern part of the

peninsula. Only the Mineoka Group occurs as an E-W trending block with tectonic

contacts. The younger groups are distributed in the more northern side of theMineoka Group than the older ones. The Shimosa Group and most of the KazusaGroup are monoclinic. Other groups underlying them are more or less folded.Trend ofthese folds is about E-W. The degreeoffolding becomes more complicatedin the lower groups (KoiKE, 1957).

The Mineoka Group is composed largely of interbedded sandstones and mud-stones. Chert, limestone, si}iceous shale and green tuff occur only in this group

in the Boso Peninsula.

The Hota Group includes massive tuffaceous sandstones, massive shaly mud-stones and sandstone-dominated alternations, Detailed stratigraphy ofthe MineokaGroup and Hota Group is not still clarified.

The Awa and Kazusa Group are composed of neritic coarse-grained sediments,argi11aceous sediments and various flysch-type alternations and respectively divided

into many formations as shown in Fig. 2.

Two main groups of sediments are recognized in the neritic coarse-grainedsediments. One is characterized by the existence of basal conglomerates overlain

by well stratified coarse sediments, such as the Okuzure Conglomerate and Senhata

Conglomerate in the Awa Group and the basal part of the Kurotaki Formation inthe Kazusa Group. The other group is characterized by a large massive or wellcross-bedded sandstone body which interfingers with flysch-type alternations and/or

massive siltstones. The Higashihigasa and Ichijuku Formations are the examples.

Thick argillaceous sediments develop below, above and between flysch-typealternations. Most ofthem are hemipelagic and composed ofcoarse- to fine-grained

siltstones. The Kiyosumi Formation is a main constituent ofthe upper Awa Group.

3. Igneousactivity Only around the Mineoka Mountains are exposed the igneous associations such

as serpentinized peridotite, basalt, picrite basait, gabbro, gabbroic pegmatite and

diorite which intrude the Mineoka Group and the surrounding Hota Group (SAME-

.yiMA,1950; KAwAi,1957; KANEHiRA,1976). These rocks are included in the

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Mn: MineokaGroup, Ht: HotaGroup, Kz: KozukaFormation, Kn: KinoneFormation, Am: AmatsuFormation,Ky: Kiyosumi Fotmation, In: Inakozawa Formation, Hg: Hagyu Formation, An: Anno Formation, Kzs: KazusaGroup, 1. Ultra-basic rock, 2. Basaltic rock.

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Okuzure and Senhata Conglomerates in the Awa Group as gravels (KoiKE, 1949;TAKAHAsHi, 1954). Therefore, the period of intrusion or eruption of these igneousrocks is estimated to be after the deposition ofthe Hota Group and before the deposi-

tion of the Awa Group (OTuKA and KoiKE, 1949; KoiKE, 1952, 1957; KAwAi,1957; KANEHIRA, 1976). FolloNNTing these igneous activities, the Boso Peninsula was divided into three

different tectonic regions characterized by different movements, namely, a geanticlinal

elevation ofthe Mineoka zone, a rapid submergence on its southern side and a gentle,

basin-forming down-NNrarping on its northern side (KoiKE, 1952, 1957). KANEHiRA

<1976) supposed that a submarine volcanic chain more than 20km long in E-Wdirection was formed along the Mineoka zone when basaltic magma erupted. A number of acidic to basic tuff-beds are intercalated in the Awa and Kazusa

Groups. Thus the active volcanic explosions are supposed to have taken place inthe surrounding areas during their deposition.

C. KiyosumiFormation

1. Generalremarks Sandstone-dominated alternations distributed around Mt. Kiyosumi and onthe northern side of it was first named the Kiyosumi Formation by WAKiMizu (1901)

(see Plate 3, Figures 1, 2,3 and 5). The Kiyosumi Sandstone is synonymous withthe Kiyosumi Formation (KoiKE, 1949). Distribution of the Kiyosumi Formation is controlled by a pair of anticiine and

syncline with axes in NWW-SEE trend (Fig. 3), The Kiyosumi Formation stretchesfor about 25 km in E-W direction and about 6 km in N-S direction from the eastcoast toward the central part of the peninsula. That is, this formation is distributed

in the drainage area of the Isumi, Yoro, Obitsu and Koito Rivers from east to west.

The Inakozawa Formation or the Inakozawa Mudstone, which is distributed in thedrainage area of the Minato River in the western part of the peninsula, interfingers

with the Kiyosumi Formation between the Koito and Minato Rivers. The Kiyosumi Formation consists mostly of thick-bedded sandstones attaining

about 850m in maximum thickness. The formation conformably overlies theAmatsu Formation or the Amatsu Mudstone attaining about 1,OOOm. The faciestransition from the former to the latter is very abrupt and the boundary between

them is very sharp (PIate 3, Figure 5). Many rounded small pebbles are included

near the base of the Kiyosumi Formation (WAKiMizu, 1901; SAwADA, 1939; KoiKEand NisHiKAwA, l955; IiJiMA and IKEyA, 1976) (Plate 2, Figures 1-5). Scouring

of the underlying mudstone is observed at the base of the Kiyosumi Formation(SAwADA, 1939; IiJiMA and IKEyA, 1976). KoiKE and NisHiKAwA (1955) recognizeda large upward thinning cycle throughout the Kiyosumi Formation.

12 Shuichi ToKuHAsHI

The Kiyosumi Formation is conformably overlain by the Anno Formation orthe Anno Alternation attaining 400 m to 500 m in thickness. The lower and middle

parts of the Anno Formation is composed largely of alternated sandstones and silt-

stones. Siltstone-dominated alternations predominate over sandstone-dominatedalternations (ToKuHAsHi and IwAw'AKi, 1975). This seems to indicate that thetendency of upward thinning of the Kiyosumi Formation continues to the overiying

Anno Formation. The alternated sandstones and mudstones of the middle and lower parts of the

Anno Formation pass upward into the massive siltstones and then sandy siltstones

of the upper part of it. Therefore, the flysch-type alternations of sandstones and

siltstones in the Kiyosumi and Anno Formations together constitute a positive turbidite

suite (Ricci-LuccHi, 1975).

KoMATsu (1958> and IiJiMA and IKEyA (1976) considered the Mineoka upliftzone on the southern side as the provenance of the Kiyosumi Formation. But, asalready mentioned, ToKuHAsHi (1976a, b) first indicated the supply of them from

the northern area and the probability of a submarine fan as a depositional environ-

ment of the Kivosumi Formation. i

2. Chronologicdata Recent geologic dating of the Late Cenozoic sequence in the Boso Peninsula is

discussed based on the planktonic fbraminifera and the paleomagnetic evidence.

According to ODA(1975), such chronologically important planktonicforaminifera as Globigen'na nepenthes, Pulleniatina primalis, Globorotalia margantae, SPhaero-

idinelloPsis spp. occur in the Kiyosumi Formation. Based on the range chart of these

foraminifera and the paleomagnetic column by NiiTsuMA et al. (1972), the boundary

between the Gilbert Reversed Epoch and the Epoch 5 is estimated in the lower part

of the Kiyosumi Formation. The Mio-Pliocene boundary is positioned near thebase of the Gilbert Epoch (BERGGREN, 1973). Therefore, most of the KiyosumiFormation seems to have been deposited in the period from N. 18 to N. 20 of BLow's

zone. According to ODA (1975), the term ofdeposition of the Kiyosumi Formation

is about one million years, Consequently, maximum depositional rate of theKiyosumi Formation is estimated at about 85 cm per 1,OOO years.

NiiTsuMA (1976) pointed out based on the paleomagnetic stratigraphy that thedepositional rate become remarkably large since the start ofdeposition ofthe Kiyosumi

Formation.

3. Depositionaldepth Since NATLAND and KuENEN (1951), benthonic foraminifera in the hemipelagicsediments have been noticed to be valuable to estimate the depositional depth.However, a few data on the benthonic foraminifera in the Kiyosumi Formation are

Three Dimensional Analysis o{'a Large Sandy-Flysch Body 13

available now. AoKi (1968) suggested that the fauna of benthonic foraminifera in

the Kiyosumi Formation has closest aMnity with the Glroidina-Metonis fauna in the

Kazusa Group, the most constituents of which are representative of the middle tolower bathyal biofacies. According to HATTA (unpublished data), benthonic forami-

niferal assemblages in a siltstone bed just below the Hk tuff at two localities indicate

upper bathyal environment. These data seem to indicate the middle to upperbathyal environment for the Kiyosumi Formation.

4. Type of basin The basin of the Kiyosumi Formation is considered to have been a slope basin,

because the geologic setting around the basin is similar to that of slope basins off the

southwestJapan. The Mineoka uplift zone seems to have been an outer ridge which

dammed up the sediments from north. In the southern area of the uplift zone, the

lower part of the Chikura Formation partly including flysch sediments were being

deposited in a more outer slope basin during the deposition of the Kiyosumi For-mation (NARusE et al., l951; MAiyA, 1972).

In the N-S cross-section through the Boso Peninsula, Neogene sediments areinterpreted to be thickest where they are now exposed (KiKucHi, 1964; NARusE,1968). Therefore, the basin of the Kiyosumi Formation was probably located atthe innermost or at least inner position on the slope. Based on such position and

size, the basin of the Kiyosumi Formation seems to have been a siope basin of the

first order in magnitude comparable to the Muroto Trough and so on formed onthe upper slope off the coast of the southwestJapan now (Fig. 1).

The type of these basins is more er less different in the surrounding geologic

setting from those of borderland basins off California, oceanic basins at the conti-

nental margins and marginal sea basins such as Japan Sea and so on,

III. BasicSedimentologicalData

A. Main Tuff Marker-Beds Used to Divide the Formation into Units

At Ieast more than two hundred tuffs are intercalated in the Kiyosumi For-

mation. Most of them are thinner than 50 cm. Many tuff marker-beds can beidentified by means of the individual and characteristic tuff-beds or the combination

of several associating tuff-beds. 'rhese tuff marker-beds are traceable over thewide area because of the remarkable consistency of their thickness and other features.

Among them six main marker-beds are selected to divide the Kiyosumi Formationinto five units. These are here narned Kr, Tk, Km, Hk, Nm and Sa for short respec-

tively in ascending. order (Plate 1, Figures1-6). Their localities and columnarsections along the type route are shown in Fig. 4 and Fig. 5, respectively. A geologic

]4 Shuichi ToKuHAsHI

gs

tttt ttttttilL,iili/••• - t..t t,.t ... .:.tttt t tt. - tttt.t ..t .b

i'''i''' l'l" 1`:')' 1":'= 'L:'"ge. i,'i{f,li ;. ;-..:-.-•`r-

'Yi'$':f' Zi6:.l.l•ii',Siii.iii;.--ii/i-i.,/:•2L,/l,,,•ii

Karneyama Station

t- ---- t '..•, R.Obitsu '• . I: :, tt 1 .1 t+ tt: tt ---.+t

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R.5asa •, ' ' ' ' f ',me,, .........,.t"/..liSUjSuhriotaki

tt . .-

1': z'1.II:•..:.,.i.-]... ..

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. .-.r ' t-ts.;.t::{.:.

-.-?t-.-

Localities of the main sixTall Road and Sasa River).

L2. San3.

4. Sandysiltstones, 5.

x v-d-.-

v' .'tt/ '," .:':' .r/,

,:{ll-t•//:1,/ij.• •1!;,••

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Kamogawa Tot[ Road

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x

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xx xNF t-• "X,, "X'T 'l'X'

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'izl

ÌI

i:' :tt ,sg r•,/•js

i

Kanayama... Dam:•.•;, :-t''i'4:,

along the

c.9

1ELoLE=en

o

2

marker-beds

Sandstone-dominated alternations (in dstone-dominated alternations (inSiltstone-dominated alternations and

Siity sandstones.

EzM=EYX-

z}

?i:

.Iz'

•),•

Ls Mo

it

t.

;:

'lg

in. v, .

.i. 1-

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i' r•Li: I

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mtype

"

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ov

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o

t.}I•

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, :1'•' 1`/ /1I,l :'i

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':"f';'t':-

t'.u:.V"

route

Anno Formation),Kiyosumi Formation),

massive siltstones,

EN

co,"v rd

E v o L

= m - nyEg

(Kamogawa

Threc Dimensional Analysis of a Large Sandy-Flysch Body

Fig

Sa

Sa

itj-ii

---t---iiT"

-t

.

t--4-

"t't'l•n+••l•-•

Nm

Hk

Hk

Km

m

.

Tku

Tk

tt.

ttt

-//,1//,;-.:,

--a---a-tttt

ij

"

v

Tk

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Kr

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ssss

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cm10080604020

o

fi-- 2 ee

30 4 ',t.•{•-;•

5 ewi/ 6-I 7 ,iti,1,ti

i8pa 9 MflM]

10 [iil

11 ::t

12kil

.5. Colun]nar sections rieai' the six tuff marker-becls along the type

1'oute.

1, rvlassive sandstone, 2. I.aminatcd sandstone, 3. Siltstone, 4. Turbiditic siltstone, 5. Scoria-colored tutllaccous siltstone,

6. Scovia tufll 7. "Goinashio" tuff (white glassy and telsic rnitieralssvithrandomlyscatteringniaficminerals), 8. ``Haigoma" tufl' (dark grey scoriaccous tufT), 9. Very fine white tuff, 10. Pumice grains, 11. Scoriagrains, l2. Siltstone clasts.

15


map of the upper Awa Group in the central part of the Boso Peninsula is shown inFig. 6.

The }owermost marker-bed Kr is positioned just below the boundary between

the Kiyosumi and Amatsu Formations (see Figures 9A, B). The uppermost maintuff marker-bed Sa is positioned within a few meters just above the boundary of the

Anno and Kiyosumi Formations. Other main marker-beds, that is, Tk, Km, Hkand Nm are respectively included in the thin siltstone-dominated alternations (less

than 20m) in the Kiyosumi Formation. According to Ricci-LuccHi (1975), eachunit may correspond to a thick turbidite megasequence or a turbidite subsuite.Where thick pebbly sandstones are developed near the base of the unit, each unitshows a typical upward thinning cycle.

Sedimentological data on the Kiyosumi Formation are shown by dividing theminto these five units.

B. FaciesDistribution

In the Kiyosumi Formation occur not only the sandstone-dominated alternationsof main facies but also other associated facies. Various facies, which are observed

in the Kiyosumi Formation and its western equivalent, the Inakozawa Formation,are indicated with symbols so as to easily understand the facies distribution of each

unit. Classification and symbolization ofthe facies are presented in Table l. This

classification approximately corresponds to the descriptive characteristics of facies.

WALKER and MuTTi (1973) tried to classify the various kinds of facies observed in

flysch deposits, Their classification is very comprehensive as they give consideration

to the various facies reported by many flysch sedimentologists in the world. In this

table, the author uses a little modified classification and symbols mainly to express

the sandstone-dominated alternations more quantitatively, but the correspondenceto the WALKER and MuTTi's division is also indicated in this table.

Facies distribution of five units are shown ln Fig. 7 by means of symbols in

Table 1. Facies "b" group, that is, sandstone-dominated alternations are obviously

dominant. Facies "a", that is, pebbly sandstone is observed in places surrounded by

facies "b" group. The distribution area of facies "c", that is, normal alternations

is very narrow. Therefore it is concluded that the transition from facies "b" (sand-

stone-dominated alternations) to facies "d" (siltstone-dominated alternations) occurs

very rapidly or abruptly in the Kiyosumi Formation. On the other hand, facies"d" and "e" are observed in the surrounding areas ofthe facies "b" and "c". Argil-

laceous sediments in these facies are composed mostly of medium- to coarse-grained

silt. But in the particular sites, they are composed of sandy siltstone ("el") or silty

sandstone ("eO").

NR.Mtnato

N

t

. ,• R.Kotto

't: t" 't

t

r'`

'

,.

Bl

NR. Obitsu

---t i

p

-tvlt

HLN

t.x.:--, ,e; +T-jTV t V-Av "+{L" tL '-".ts.r"

,,nr

k

.' NtalpL=v,U- L

' t i'-'y ts{- ,'p+r .., NN''- s,t-.v-ft ,t, . )" . -t t '.."t- M vS l" .-?t.

- -'.--A-ut. v./..' .t.N:tLH'.tt--=Vu,.7i.;.'t.:..`.,...'

'

,.,-Eg..H...' 7"X.tnt

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vh

. -' l

.i - r' j

M

.=-"-sJ.=-.t.it.e,-".VTpa7,.

"=lxilJr--VV7 .. . r{l S

erA'i'A-":n..E:Oom

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N

c/

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l. .,

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tN'

7;

A

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;

t

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T .". -. ThL-` .

--: - -.-L )tL -t ""A

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st --- --+ N.- .rh x. . 's, K'b.." ' . N rs. '". N.v g asL Vr .Ix, N '"`ts- sv.-N "N..N

'. y. It N-tL/. B'

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;: r.". k

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itl. -

L v

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SSN NN " h VS-

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, in,N "I KI

ts N

'N.

A' )F

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:L-

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Mt K.A4t

Ttt

:

9S'um/

-c-' -tr- :-+""I-

-N.e:-"s'-Efft"H.,X,".N')L r3oorn

",.rr.i`, .'"LL: - -`"st cEoR.Kamo +-

-J -El -; - +

..tu71atr7t..

"- it 1- -- -

;u sl ..ei 'V i.N -1

x

-N.+";N

o 1

D-'

2 3 4 5 kmh== ,i.

Egcom

D

R.Obitsu

R.Koito

RYo ro

RJsumt

R.Antn

R.Kam

uskm.

e

k

.

iD2a3D4[]

a---b -vv--

c---d---e-v-f --'-g•••

h-t-'-'

Fig 6 Geologic map ofthe upper Awa Group in the central part ofthe Boso Penmsula.

1 Sandstone-dominated alternatuons (in Anno Formation), 2. Sandstone-dominated alternations (in Kiyosumi Formation), 3. Siltstone-dommated alternations and massive siltstones,

siltstonesandsiltysandstones, a' Sa, b: Nm, c Hk, d' Km, e' Tk, f' Kr, g: Ok, h: Thrustfault, i: KurotakiUnconformity.

Unpubhshed data by HiRAyAbEA and NAKAJrMA are partly referred.

4. Sandy

Three Dimensional Analysis of a Large Sandy-Flysch Body 17

Table 1. Classification of facies.

Facies

a

b

bO

bl

b2

b3

c

d

e

dO, eOdl, el

d2, e2d3, e3

d4, e4

Definition of Facies

Pebbly sandstones Amalgamation ofpebble conglornerates and massive sandstones with no interbedded siltstones Frequent occurrence of outsize siltstone-clasts

Sandstone-dorninated alternationes (Sandy flysch)

sandstone siltstone ratio, sd/st>5

Massive or amalgamated sandstones with few interbedded siltstones

Modal thickness ofsandstone beds, Mo.>2 m

O.5<Mo.<2m

Mo.<O.5 m

Nomal alternations (Normal flysch)

1<sd/st<5Sikstone-dominated alternations (Silty fiysch)

sd/stg1Massive siltstones

Partly including thin-bedded sandstones

Walker & Mutti (1973)

A3, A4

B

c

D

GSilty sandstone

Sandy siltstoneCoarse-grained siltstone

Medium-grained siltstoneFine-grained siltstone

Main grain size

of silty beds

In this table the existence ofintercalated pyroclastic beds is not considered.

C. LateralVariationofThickness

Lateral thickness variation of the five units are shown in Fig. 8. As a whole,

geometric characteristics of these units resemble each other. A detailed examination

indicates that the lower units are more remarkable in the lateral thickness variation

than the upper ones. The lowermost Tk-Kr unit shows the greatest lateral variation

in thickness. This unit is composed largely of siltstones about 10 m thick along the

north row, that is, on the northern side of the anticline. But it consists of the sand-

stone-dominated alternations of about 300 m in thickness along the south row, that

is, on the southern side of the syncline. Columnar sections near the base of the

Kiyosumi Formation on the both sides of the anticline, where the Tk-Kr unit isextremely thin, are shown in Fig. 9A. In Fig. 9B are shown the columnar sectionsnear the base of the formation on the southern side of the syncline, where the Tk-Kr

unit is remarkably thick, and in the eastern area of the peninsula.

It is definite from these data that on the southern side of the syncline and in the

eastern area of the peninsula, the Tk-Kr unit occupies the lowermost part of the


Sa-Nm ,-' ?<ts ` 'h

:'J.. '

3

Nrn-Hk ."' ./""

:t./ i

.

i

H k-- Km

L,.

b

6 X()r

'11..'

•/1

,.. :,i

tt

tttt i

"r,'

;t"ivk

x ged

•,• b

-E

`

Km-Tk •• -,"'

••' 2 t"' "

,i

12

/1/ttt tvttttt

.

Tk-Kr

.E:• .

. KX

... ua....:.

//' r•...'

.2

if

?

.

•ts

I,, 2 'ts,.,

•/

.

3

`

5," ' bl ', '/4'i' b2•.,. .... , tt/ ttt / t

xX

N

1` aa''

•aa .,.. )

'

6ibe v-"

O2468 10Fig. 7. Areal variation offacies ofeach unit.

For description of symbols see Table 1.

Kiyosumi Formation, but the overlying Km-Tk unit occupies the lowermost partof it on both sides of the anticline. It is the most important to analyze the depo-

sitional process of the lowermost Tk-Kr unit in clarifying the overall depositional


Sa 'tL %.-., e 23, 260

.e65.5'

,•S lkJA

x

:/l,l:I/I/I•

' A• `lx"1

ll'lllll..

r

N

'II//,.

,il{11'

7'•' G•

fis'

lt

'

•i

t.'n45

Nm-Hk

.,.. i

Hk--Km

5

•)

-,1", 's

?• ' ' ' '

') ' xg

N

.

oo

.ritst

x

Km-Tk

/tL

Tk -Kr(Tk-Mlt-Kt}

--

Fig. 8.

1. Pebbly sandstones,alternations and massive sikstones.

.liiil >E

-••-' -' •-

ii,''''"' l.'.'.,.,"l,ll',ll'i,l,liii..• ,,•,iiillii'i•liilili•I•i•'•l/,l,.iiii.I•ii•k,,i,l,'il,,,.,''•••,",,"'''lil.,,,,.,,.,.

tttt ll:' i'/..,''''''"::':':'•i'i'"•..`''5?,, "'::':'"i":'"•-.,..g,o,•(';.

It : tt ttt tttt tt /t'' "" '

".''"'"i' ' ' ' '' '''••-ill.l':')i.--••-';::.•--....i•'''`•""" ',r:ii""i'"'

ttt ttt 4o ,, ". xx

t .t tt 1 t /t tt ./t ./ .../ ttt ./ : t tt ttt.t t ttttt tt ttttttt tttttttttttttt ttt tt tt it ttttttt t/ttttttt-

tt ttttt. / t . tt..I= :t":-t .t....-:t....tt.t,tt.... 'L..t't'' '[t' .t'' tt't't'

- ... fs ("s ,ioo.,liKki, .

f"aj' 3',,) ca K"ilN..

• . -''''"'•s'i••11''''''1.-""---"'"""''''']'.i...,••""'

,-.'•,..•' o Areal variation of thickness and rnain facies

2. Sandstone-dominated alternations, 3.

'

246 ua kmofeach unit.

Siltstone-dominated

1. •::::::::'

2. i[illlllllll•I

3. -

4oom.,:ili 3oo:i:I::. 200I:}Iii ioo

':'1': o

8 10

process of the Kiyosumi Formation

All units are thicker in facies

(ToKuHASHI,

"b" group1976a, b).

(sandstone-dominated alternations)

20

m10

8

6

4

2

o

Go

'

:

1' -t

a

rv

i l l i :

, : !

YKr

9m

E"•-.. --.

Kr

b c

d

Shuichi

vX

4

.

a

Kr Kr

e

K'

1•l•• • -l: •- :• •:

-" I

ToKuHASHI

f

Mat,rL.•

i

g

Ma

t-/tt

I

r "tt

;/

t-Jt

Ma

h

kxOk ì..

4- :---

i' '

i

Go

Kr

j

k

Mit

Go

Kr

t

'j j""' /t - tt ttt t t - -. -

Fig. 9B. Columnar sections near the base ofthe Kiyosumi Formation (Part II)

These sections are in the area where "1'k-Kr sandstone-dominated alternations or pebbly sandstones and is remark bly thjck. Locality of each coiumn is shown jn the 1 legend see Fig. 9A.

than in facies "a" (pebbly sandstones). Every unit becomes thicker in the order of

facies "e") "d") "a" and "b".

Geometry of the Kiyosumi Formation is shown in Fig. 1O which shows the lateral

t, : 400m.:

''• ,••1/ I•:': 2oo

'o O2468 10th km .unit is composed ot'

a- ower figure. 1?or

m201

18

16

14

12

10

8

6

4

2

o

Tk

Kr

b

:tt'r;

'ii

Dal `::I

San

Hk

Km

a

'.i'x

ii,lll

lt-lll

;' llll: ku

Sb-

Mit

bo

c

Tk'

th

ii

.:-:.

A

Oo

Mtt

bo

v

Kr

s.s

--"" 4'.

d e

Mit

Go*5m

Kr

ligrk

bo

Kr

f

t'"'

4--

.

"1'k

g

Mit

h

:,..';•-•'- ,. . 1',.

,tlit

l

. r

j

:•:.I

't

i4

;:g

-.

mat

bo

k

;il..t

nt)

f::"

';v:,

Kr

.

i'i':

•1,

't''

:. ---l

-- t ttl-

•IP

'i'i

:`t"

l•l:•l.

s-"

l mnIi:.l13

tt tt

:'i' A';:: tt tt

-

1i'i •1•':l:l';'

;':i':' •I:

';i':'t-

•J.:1

:. :.

.

:,::

tt;' .I.

I:1:

ttt

tt. t

u

tt

•1•.•

tt :tt' 1ii

•l•11.,T :"t'ee"tt

t"/, Tk /it:..lj•••

't; --.t

t-tt t

r, :..

tt ':i

'

ttt

' tttt

ttttt

Bm.tt. i::: :

:-tt

' •liOo t::

oo':'t .rfrl:

:,:,

;[.;.., ••- ;;•

=?;:'t;-tt' -t--

. "i:

l/•Ls•.::

iil'I::tt

-t :t

I:.:

t- ::

::.:,::

t-

4-4 v,:; u

Mit' tt:

l-4 Mittt:t

-:

Tktt

i Bm-t-

Oo

Go

Mit

Go

Ht{r•:. :"t::

Ht Kr Kr

L... Le•i•i

..J

•.. 400m'"'"" IE•i•l' 2oo

;::' o

O2468 10

v1. EIS]

zo3. Ege

4.N5.-

f

:s:';i

-4-

hr

o

6.ggg7. Eiffg

8. Etw9. I[ilM

10. Rww

p

11. rw12. [!E]

13. {Zi]

14. as15. Nl

r

- ''t':' '' '':"' ' ' km Fig. 9A. Columnar sections near the base ofthe Kiyosumi Formation (Part I).

These sections are in the area where Tk-Kr unit is composed mainly of siltstone beds and remarkably thin.Locality of each column is shown in the lower figure. 1. Sandstone, 2. Siltstone, 3. Turbiditic siltstone,4. Scoria-colored tuffaceous sikstone, 5. Scoria tuff, 6. "Pumisco" tuff (random mixture of pumice andscoria grains), 7. "Haigoma" tuff (intermediate between scoria and pumisco tuff), 8. "Gomashio" tuff(white glassy and felsic minerals with randomly scattering mafic minerals), 9. Very fine white tuff, IO. Veryfine grey tuff, 11. Very fine pink and pinkish tuff, 12. Gravels of hard basement rocks, 13. Scoria grains,l4. Pumice grains, 15. Shell fragments.


i

l•

r..

J...

:t --tJt t't -t-'tt' ttvtt

'.220 20

" ' > ' i t; , l-.,

tJ/ :,/' Ll-

t t---: --}----

M.-"---------------

t -e--

'

l:

t' '"' :" . I:s - ei f.. ... ".-,......:JS' .n

--x -:-t-:-:-:-:

::-i-i

-- --

.'t

. ---: •.:i li

(

40.

Fig. 10.

1 :,

r'

:.:: :.:

--qo

'i

.l.,,l .ill

-jt-

20-;-- -

-;--::

- t+'-tt-t-i'

tttl

k•l K

O.

s

$

.. -.,•- ...... i-"" t" ...r' 8eOm L• !llll' 4oo "" o O2468 10 w kmAreal variation of total thickness of five units between Sa and Kr marker-beds.

Dotted part means facies composed mainly of sandstone-dominated alternationsand partly of pebbly sandstones, Black part means facies composed of siltstone-dominated alternations and massive siltstones.

variation of the thickness and main facies between Sa and Kr main tuff marker-beds.

It is obvious as also shown in Fig. 8 that the thickness of the Kiyosumi Formation

decreases both westward and eastward. Therefore, the Kiyosumi Formation con-sists of such a large lenticular sandy-fiysch body attaining 850 m in maximum thick-

ness, more than 20 km long in E-W direction and more than 6 km in N-S direction.

D. Trough-likeBasalErosion

Where facies "a" (pebbly sandstones) is developed, the underlying beds aremore or less widely eroded and the trough-like erosive morphology is formed at the

base of the units (Fig. 11). Especially at the base of the lowermost Tk-Kr unit is

developed the largest trough-like morphology measuring about 50 m deep and more

than 5 km wide. Even the Ok tuff (MiTsuNAsHi and YAzAKi, 1958) in the uppermostpart of the Amatsu Formation, is eroded in the area of the maximum erosion (see

locality i in Fig. 9B). Profile of this large trough-like morphology at the base and

the Tk-Kr unit fi11ing it is illustrated in Fig. 12.

ttt t .tt. t tt ttt tt ttt tE. Size and Constitution of Gravels

Distribution of the maximum diameters of the hard gravels included in the'turbidite sandstones of the Kiyosumi Formation is shown in Fig. 13. In this figure

are shown the maximum diameter and the mean value of the larger ten diameters

22

Sa-Nm ,,=• ..

" :i i' 'i :t:t- tt - ttt /-Jlt--ttt't'ttttttth.t ttttttttttt-t.t..ittt.' .....

. Nm-Hk ,/' /'

tt t- li, . 2K i'' ' tt ' tt / t tt t...••-•' -• ., --••.... .•-•......J;-•.,.

tt .ttt..t x i Hk-Km "-" •"''' "' t/tltttt.tttt tttttt ttttt.t

t'H't' ''t"

tt tt - 't !:,- i

Km-Tk ,,••' .."-•"'h" ':.

tttt. ttt 4 : , ,],/t, /1. i tttt /tt tt ttt t ttt.ttt tt..,. .,.....,....,................,.....tt...,...,..

f`

Tk-Kr ,••' .• "'

('i'r• '

' ' "'i ' tt ttt t

. (

Fig. 11.

Each number means a

of the gravels observed at each

centimeters in diameter. Butin places,

' ' •' 4

i

''jl''"' '

•-tt"

Shuichi ToKul-IAsHI

f/l,/lil"'''1''"''t''''''"''t,t/••ll,l• 'x '1

-/. ttttt tt/ttttttt tt / ttt'ht .'i:'t'"H'' "/H .....t:'t''' ',

,,iiii"""''"""'"""'L'•iiiiii g i4'" ':••iii ''v

ttt.t ttt .t/t 11t t.

tt tt ttt ttttt.ttt tt ttt.t

••-.I/•..

,s 'it

:I"'"'""'

'

.::i

l

x

•//.•• 1•

KX

xR

ti:"

K

""

tt]

?fi•

t tttttttt 'ttttttt ltlttt'tttttt t

-/ 4

value of the

outcrop.in the

'tl'..

xtt

•i

i

t/

N

:ttit

tt tttt:ttttttt

KX

'''"'i '"''il'iil'11iis•••-•• i IO,.

.-"••••••./-.•.• -...:•••. ,.- ••.••,•

' '' 0246S IO L==tisi==f kmTrough-like ero:ive morphology at the base ofeach unit.

estimated maximum thickness of eroded succession.

Most gravels observed are iess than a few

uppermost Sa-Nm unit, larger gravels occur


.

E' ,. q'"

n•- ;,

.( Ol 2km.:t: -.. .t . ... .... .t

•••,..,..!$,,,b,,, "-•iiV,,.

61.E•

' G.,

H .'

.

's :.."S'

:ttt

'l:• '

ii

.:,.

,=

:s.."--.'

-l,.

•. ..l, gi. .,

C.'tt'.V. =/':i.:. '4,

L

m

T";i:

-t:tttltt

;t., .xt':

i'

50

o

50

Fig.

Constltutlon

sandstone

commonlycrops at locality a,

the larger

served.

Theseof the

or well

than 3.2 cm)

(1941).

J' '"•••1',.•• o 2 4 e s 10km b== i=in==U==ti=Å}==ti

-tirin if!zv-TVt '- '-t-"V-V-"'=i' iES=;'S -.--.N.sKtts-

...2•f. I, ,(. i• f•-- ,,'i i• i. // l. i/ i• li ;l 11 i•l//• l,ll /i• l• I• :• ;• i• 'i. 'i, il i:• l•ii/ li 11iI• l/ l• I- i- i• i• :- ,;li,ifll,ili,l}sli. ,},,, s.-,.....

.'

l•'•i•i//•,,,•,',,.l,,//,/•,••,'iili':i'ig'il/l/ii'l/:/g.//l//•l/i/i•:•}i•:c'i/iii;i'i:i//i•i'//•i•i'ii//'ll///{:•i'ii•;l•iii/ii•i/i/;/;-i/i•i,Lg/•;/i/i•i/{i,ilii''ll/-ii,li],E/,}•i/.:,:•i•:•l-•.'k

' ',•,,i,rl.i,;i[i.ii.i•ii:•i.iil\l,lil,iiiliill•i•i'i•i•l,IiiiiPi?P,91!i9,a,Pe,el?Pieil;1:•i'iiIlIIliiiIili•ilii•]l:'ii,,,1,,..,L-;•"i',,'i•i ,,,,

Kr -- "' '.;.I .: •; •: E/ i:/i l• i•::• lli [, iiil i- i•ii• ii i• il:/ i• l' i./i• i• liil//iili i. l' l'i'l i• ii/i:• li: •i:"""" '-'-'---'-Kr .:,.' R" stnr"'

l li;lt I- ;Fill'.I'Iil'.ii' i' i' I'i[i' l' ['i.II i' l':i'ilI• llii• ll ll i• [i 11' 11liil l' ii ll'i l'll' l'lll[l' [i l"I .' .I.... """"""'ì-A'-'"Mma'A'"AA"',

sbr'''"''"''"'-"J'"M''"''''"" -`' -': l- iiil l/ I :, : 11 iilli l' lillvl i• ll•-lllilEil 'I ' " """"''"'" O-kin

Ok ----.---•---•-•--••-- -- --•--.- •:::::::::••• •. ""•"". ..-.,. ."."Hv"," Obdyvvvvvvvvvvvtvvvyyv"vtv yvvvvvvtiv vv vvvv vvvvvtv-Tvv tyvvvvvvvvv vvvv vvvtvvvvvv

A BC DEF G HI12. A profile of the largest trough-like erosive morphology filled with the largest pebbly

sandstone body of the lowermost Tk-Kr unit.

' ' of these gravels is shown in Fig. 14 and Table2. Chert and are predominant. Rhyolite, andesite, hornfels and quartz rock are observcd. Crystalline schist is also commonly observed except for out-

b and c which belong to the uppermost Sa-Nm unit and include gravels. In addition, shale, granitic rock and meta-quartzite are ob-

gravels is generally rounded to well rounded. However, only gravelscrystalline schists show edged and discal forms although some are rounded

rounded. Data on sphericity, shape and roundness of the gravels (larger at the locality a in Fig. 14 are shown in Fig. 15 according to KRuMBEiN

F. Paleocurrent

In the case of turbidite

currents can be measured by

sandstones, the orientation

means of the external and

and sense of transporting

internal sedimentary struc-

24 Shuichi ToKuHASHI

Sa-Nm

•/•1.•• -

Nm-Hk -"'

•1' 1 '

t t:ttt

tttttt

'.tt tt

Ee

tiF-iSlii

rt.tt

" 't'

'c {-tt

H

llililiii>ti,rsEiiill•lg

t9M,x'

lptfbb.

tt

1/

c5.YY

L 3i"/x

y

il

k,r ,'

}/

x.,.•i••-

'

kXxx

i

Hk-Km .•i' <clj /tt

pt.SIIIII'Ig )s-,? ?x

im'Tk ,"bs , ,k KX

'k-" X,,r. 3e).)y"siX ,i,,;,9,Mi

in km Fig. 13. Size ofgravcls included in each unit.

1. Diameter of the largest gravel among gravels occuring in each outcrop, 2. Meanva!ue ofdiameters of the larger ten gravels among them.

x `x,tllxt,

tures. Among theseare summarized in

data, those which at

Fig. 16A. They areIeast indicate definite current orientation

obtained from sole markings, preferred

Three Dimensional Analysis ofa Large Sandy-Flysch Body

Table 2. Basic data on constitution ofgravels in the Kiyosumi Formation

25

1 2 3 4 5 Ch ss Sh Rh GR An Hf QR MQ CSa

a

b

c

c

d

e

f

ghi

Sa-NmSa-NmSa-NmSa-NmSa-NmSa-NmHK-KmKm-TKTk-KrTk-KrTk-Kr

32-64

t832-64

32-64

l8-8

2-4

4-8

4-8 4-8

4-8

l35 31 55260 115 47 84 41 43 82 34 50345 l30 604tlO 354 55301 301 47329 260 49 79 54 49378 187 52461 209 52

24

9

21

15

25

11

15

15

l8

26

25

4

3

o

o

2

1

3

2

4

2

3

6

7

17

28

7

7

7

7

4

4

3

o

o

o

o

o

3

1

1

1

o

o

3

29

13

2

2

6

7

2

5

2

2

6

2

6

5

2

3

3

1

3

2

1

2

3

o

o

2

8

9

ll

5

9

IO

o

o

o

o

o

2

2

3

1

o

o

o

o

o

o

o

45

9

IO

3

4

l. Locality (Fig. I4>, 2. Unit jncluding grave]s, 3. Size (mm), 4. Total number of gravels, 5. Total number ofgravels examined under thin section. Ch; Chert, Ss; Sandstone, Sh; Shale, Rh; Rhyolite, Gr; Granitic rock, An; An- desite, Hf; Hornfels, QR; Quartz Rock (Meta-Chert, Vein Quartz), MQ; Meta- Quartzite, CS; Cry$tallineSchist.

orientation of wood fragments and strike of walls of channel-like scores filled with

small pebbles. Sole markings imprinted on the top of siltstone beds are revealedby carefu11y removing the overlying semi-consolidated sandstones with a set of tools

(Plate 4, Figures3 and 4). It is confirmed based on both the laboratory workand field observation that the preferred orientation of wood fragments generally

included in the upper part of the sandstone beds approximately coincide with thedirection of currents and the orientation ofsole markings (NAKAJiMA, 1977).

Data which indicate the definite sense but broad orientation of currents aresummarized in Fig. 16B. These data are based on the current ripple cross lamina-tion observed in the cross sections of sandstone beds (Plate 4, Figures 1 and 2) and

imbrication of outsize siltstone clasts developed mainly in facies "a", that is, pebbiy

sandstone facies (Plate 5, Figures 1-6). Most of these data indicate the currents

from north or northwest, but in the Nm-Hk unit, such current ripple cross lami-

nations which indicate the currents from south are observed only at one locality in

the eastern part of the peninsula.

These various sedimentological data on the Kiyosumi Formation are examinedand used to clarify the depositional process ofit in the next chapter.


Sa-Nm /i' l'i•

tf"" "''

f"' i

.,i 1 Lt----t.--tt.t-ttt..nyttt-tt.-

Nrn-Hk ,s" L••--•.- •--•

I•: .

" .l,.,.

i•1'•

i

`

Hk-Km -t.ttt.t

i'

':''h S

Km-lk

-.t:.-tt t

:ttt

[•1. .

L/"

`

Tk-Kr

II,,., -

i

:''' ' r-.

-tt.

. =th..tt tl t'"'t

"k /,, i ,g;,

,s... 'i..

.sE..

I]

i

`

ll'

tt tJtttttt

z.. :'

,L

X'L"'

t '

r

crW

't'

!it t""t'tt'tt'"'t'"'"'

.

-t."L-' "'""'v

/t trtt ttt tt

e ...

.1

/Lt t.tttt t-tt-t ttt'-t-- :"

. -'' '-.k.

"\

::-•i

/L

,a

,• ;,-

t't '

k

.L'I:, ...,.

t--

K

lttttttt

:tt-t."p---4

x

l•

. s:

ttttttlt

:':'t

.".....s

'- t'

[/

r.t

.-

"t .ktttt -t.'tt t t E .K, - ..t.-ttFt..

:, x. ., "t

//

.

'II:"•.

,,.' t n, .'

•(

:'t:'t'

't"'

/

t-ttttt

'i]" "';

...i .. -1.tJ 11.. 1- " t-tt'ttt--jt

LXX

,I• i,. t ttt. ,. :: .. .1/ .- tt t/ ttt]tttttt tt

g.,::::

tt :/ tt

kX

l•' "'

xX

, "l .-r --t ---" . t-- : :JL"" !.'

'i

: '-"

:-..:L'-"

s,.

:/

-t-ttt

-. I

-Jl

i ttt s

Fig. 14. Constitution ofgravels included in each unit

1. Chert, 2. Sandstone, 3. Shale, 4. Rhyolite, 5. Granitic7. Hornfels, 8. Quartz rock (meta-chert and vein quartz),IO. Crystalline schist.

The constitution is analyzed for gravels of2-4, 4-8 and 32-64 mm in diameter

((E})

({E})

({ilii)

. L-"'

0246 . rock, 6. Andesite, 9. Meta-quartzite,

i.-2. .:.. n.

3. Etw4. ewgs, E##gg

6. EeefiY

zN8. ewg. N

n- s ro km

.

Three Dimensional Analysis ofa L arge Sandy-Flysch Body 27

ROUNDNESS SPHERICITY SHAPE

35

CHERT 58

6S

11

63

25

24 37

124

22 2014

2

SANDSTONE 269

12

31-- li

B9M [kL- slO 7[D==b

RHYOLITE 13

6.s2-ni.Eun32 11 11333 [lti[2L[s4

ANDESITE 2

2 2 2

HORNFELS 6

42 11112--..ll.-rrri=== 33

54

TOTAL

Fig

1055 23

10 9 81

O.? O,3 O.4 O.5 Q6 O.7 O,6

37

6 8

2021

83 2

,50 .55 ,60 65 .70 .75 BO .B5

3335 34

3

r ll III N

.15. Roundness, sphericity and shape of gravels (>3.2 cm in rnaximum diameter) occuring at locality a in Fig. 14.

Each value is measured based on the method by KRuMBEiN (1941). These gravels are rounded to well rounded, and sphericity is relatively

high. Disc-shaped gravels (type III) are very rare.

28 Shuichi ToKuHAslll

Sa-Nm / .. bY. 1-.. ..

r' i

Nrn-H k

Ekl:,)

/,

i

)

Hk- Km

1;i

Km-"Tk

ttt..

ii. .

Tk - Kr

11.. .

Fig. 16A.

Y

/ttt.tt

i' "

' vi-..,1

;,tt/

t.ttt t t

'L•,

tt

ttt .t tttt t

e 1 ttttt

ttttt t.tt

tttttt

tt tt

i

ttt t -tt.

4

5tsJ

..vs

'f

i

ttttt

i..A VI

.

g //i,rwvRl•K,

l

(/,

1./

L

tt t

.

h., .

tw•llif

.+ 'z,

A5ktt tti / /

1. th /•1

ttttt

r"'

'•5

ttti /1

ttt t /.TJ..'t"''' '' '/='"''"

t' '

'twfttut9sR.

., B

'sS'i., -.,,.. Keq(

y-Slx

.t

xY

x ttt e

Xl

K

/ '',N '/ 'b,

•:• @ •--•-;-

.//

,s

.E(gi•I?lfi' i•v.

' 's-f"DNe i rf.

xX

?g .

't' "''

;t ...t.:t"....tt tt

•te

N

'

th2, q

3. Q

4. N

1 '-r ;:r' 1 /tt ttt tt tt tttt tt ttttt ` :., oth246810km 'Paleocurrent directions measured by some sedimentary structures indicating distinct ttorientation.

1. Sole marking (Sense is unknown.), 2. Soie marking (Sense is known.),3. Preferred orientation of wood fragments, 4. Mean orientation of strikes ofwalls of both sides of a distinct channel-like scour in the pebbly sandstone facies.

The valuc in each circie is number of sandstone beds whose sole markings weremeasured.

Three Dimensional Analy sis' ofa Large Sandy-Flysch Body

Sa-Nm ,,"" .. •..' '''"' '"""• 'g,i

1

i

l':'

. '

v'-'

Nm-Hk

z + e -i '

•.•

ttttt ttttt ttttttt ttttttttt

t'

Hk--Km

Km-lk i"

,r'

,tr .

tt rt

"`

.

i•-

....LL.' ... ,r'

tt/ t[ttt

• 1 ,,.g tt ' lliiicit•""•••/,i/l.

/•...r'•i,......•x..,. :.i

LJA".S, 2x

't"'t' 'ttttii

,, ll .tt.."'.. '''' .:.'z''..... '

ttttttttlt-tt

gscg

. .,....... L- 'l:....t. :-' ,...t

ML,,.)

ttt i

t- tt tttvtt trtltt tt-tttttlt tttttt. tttt

' t Ttt., ttt..tltt:ttt tt tttttt tttt.ttttlttl t.. t/ttt

/,. '

`

t tt-

K'

k

gf

"

&

Tk-•Kr .1""'"

'}' i

Fig. l6B.

.

i

'ii

tit ttttttttt ttt tt't' t'

....y

x 1:

t -ttt

1

t-JJ- II tt.t

k

ji

.,-

`

.' s..

. i•X t, : . ,1,l .,• 2.8 ' v '"'•-- -••..-•',. . ' ttt ' / tt tt ttt ' " 02468 10 " kmPaleocurrent directions measured by some sedimentary structures indicatingdistinct sense but broad orientation.

I. Current ripple cross lamination. The value in each circle means numberofsandstone beds whose cross laminations were rneasured.2. Imbrication oflarge siltstone clasts mainly in the pebbly sandstone facies.

One datum per one locality.

29

30 Shuichi ToKuHAsHi

IV. Inferred Depesitional Process

A. On the Origin of Gravels

Before discussing the depositional process of the Kiyosumi Formation, the origin

ofgravels is considered here. Gravels offer the very important geologic information

on the provenance of the gravels themselves and the enclosing sandstones as well,with the paleocurrents responsible for the turbidite sandstones.

Gravels ofigneous rocks exposed around the Mineoka Mountains on the southernside, such as ultra-basic rocks, basalt, gabbro, diorite and so on, are not included

in the Kiyosumi Formation (Fig. I4; Table 2). IiJiMA and IKEyA (1976) also no-ticed the same fact though they inferred the supply from south. In addition, rhyolite,

andesite and hornfels, which are commonly included in the gravels of the Kiyosumi

Formation, are not exposed around the present Mineoka Mountains. Some of the rhyolite gravels closely resemble the Cretaceous Okunikko Rhyolites,

the Paleogene Katashinagawa Rhyolites and Neogene Kinugawa Rhyolites whichare now distributed around the Ashio Massifin the northern Kanto regions (SuDo,

1976). The Cretaceous Rhyolites may have stretched widely in the northern Kanto

region in the Late Cretaceous Period (TANAKA and KAwADA, 1971). Also theNeogene Rhyolites may have been distributed more widely than now in the north-western Kanto region.*i)

Gravels of crystalline schist consist mostly of such low-grade metamorphic rocks

as pelitic schist (quartz-albite-muscovite-graphite), basic schist (quartz-albite-

chlorite-pumpellyite-actinolite-sphene) and psammitic schist, as well as such high-

grade metamorphic rocks as siliceous schist (quartz-muscovite-garnet-biotite).These rocks are different from those metamorphic rocks that were reported from two

very small islands near the Kamogawa port Iocated in the Mineoka uplift zone(KANEHiRA et al., 1968; KANEHiRA, 1976). These rocks are characteristic to theSambagawa metamorphic belt, Mikabu green rocks and Ryoke metamorphic belt.*2)As already mentioned, these belts are well known to be distributed below the Kanto

Plain (IsHii, 1962).

These facts mentioned above strongly support the supply from north, that is,

from the middle and northern Kanto regions. Moreover, the imbrication of theaggregates of the outsize siltstone clasts which occur together with these hard gravels,

also indicates the supply from north at the several outcrops (see Fig. I6Bi Plate 5,

Figures 1-6).

*i) Personal suggestion by Dr. NT. YAMADA, Geological Survey of.Japan.*2} Personal advice by Profi K. KAyt EmRA, Chiba University.


B. Depositional Patterns of the Representative Units

1. Depositional pattern of the Tk-Kr unit The sedimentological data on the lowermost Tk-Kr unit of the Kiyosumi For-mation are summarized in Fig. 17A to E. Main characteristics of the Tk-Kr unitare as follows.

First, the unit abruptly thickens southward in the central part of the peninsula.

Then the thickest pebbly sandstones (facies "a") are developed, filling the largest

trough-jike morphology by erosion attaining about 50m in maximum depth andmore than 5 km in width in the eastern part of the peninsula (Fig. I2).

Paleocurrent directions measured in the southern area, where sandstone-domi-nated alternations are thick, indicate the currents from east to west. On the other

hand, paleocurrent directions measured in the eastern area indicate the currents

from north to south.

Fig. 18 shows the detailed columnar sections between the Mit and Go tuffmarker-beds in the middle part of the Tk-Kr unit. Lateral change of thickness andfacies between the Mit and Go tuff marker-beds closely resembles that of the overall

Tk-Kr unit. It may well be said that the former is a miniature of the latter. Be-

tween the Mit and Go tuff marker beds, bed-by-bed correlation of individual sand-

stone beds is possible. Geometry of each correlated sandstone bed is shown inFig. 19. This figure shows that several sandstone beds observed along the northrow (localities a-k) on the northern side of the anticline, such sandstone beds 1 to 4,

continue to the sandstone beds developed along the south row (localities q-u) on the

southern side of the syncline. Along the north row, including localities v, w and x

near the east coast, these sandstone beds increase their thickness and their capacity

to scour the underlying beds both from west and from east to the middle part where

pebbly sandstones are thick developed. Along the south row, they increase theirthickness much more but decrease their scouring capacity (see sandstone bed 1).

These data strongly indicate that the turbidity currents were funneled southward

through the area in the eastern part of the peninsula where a large trough was formed

and thick pebbly sandstones were deposited. Then they changed their directiontoward west and deposited thick-bedded sandstones of the lowermost Tk-Kr uniton the southern side of the syncline. Therefore, it is most reasonable to conclude

that the thick pebbly sandstones fi11ing the trough are channel deposits. Thisconclusion coincides well with WALKER and MuTTi (1973) in interpretation of theirfacies A3 and A4.

The depositional pattern of the Tk-Kr unit are shown in Fig. 17F. A topo-graphic high as a tectonic uplift zone (KoiKE, 1952, 1957; IsHii, 1962) or a submarine

volcanic chain (KANEHiRA, 1976) has been considered to have existed around the


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Mineoka Mountains. Therefore the distribution ofthe turbidite sandstones supplied

from north seems to have been restricted to the northern side of the Mineoka uplift

zone. The width ofthe topographic high at that time is not considered to have been

much larger than that of the present distribution area of the Mineoka Group. Be-cause the Mineoka uplift zone is interpreted to have been lifted along the faults by

the tectonic movements. In Fig. 17F, the distribution boundary is drawn along the

Kamo River located just on the northern side of the Mineoka Mountains.

2. DepositionalpatternoftheHk-Kmunit Sedimentological data on the Hk-Km unit are summarized in Fig. 20A to E.ToKuHAsHi (1976a, b) already reported the geometry and depositional process ofthe individual sandstone beds in the upper half of the unit. It is characteristic that

the Hk-Km unit thickens toward south very gradually but not as abruptly as in thecase of the lowermost Tk-Kr unit, and that the facies of the sandstone-dominated

alternations in the central area of the peninsula changes into siltstone-dominated

alternation facies both in the eastern and western areas. Paleocurrent data on this

unit indicate such a pattern fanning out toward south in the central area of the

peninsula much like the results obtained from the upper halfofthe unit by ToKuHAsHi

(op. cit.). Pebbly sandstones (facies "a"), though being much thinner and narrower

than those in the Tk-Kr unit, is distributed with a small-scale trough•-like erosive

morphology at the base just near the area where the paleocurrent begins to diverge.

Therefore, the area where the pebbly sandstones are observed seems to be the nearest

portion to the terminus of the channel. Depositional pattern of the Hk-Km unit is

shown in Fig. 20F.

3. Depositional pattern of the Sa-Nm imit Sedimentoligical data on the Sa-Nm unit are summarized in Fig. 21A to E. Itis the characteristics of this unit that the unit thickens toward north, and that the

sandstone-dominated alternations (facies "b") continue to the east coast. In ad-dition, two a little different groups ofconglomeratic gravels occurs in this unit. One

is characterized by containing a iot of larger gravels and including no crystalline

schists not only in gravels but also in sand-size grains. It occurs only in this unit

(localities a, b, c and so on). The other one consists ofsmaller gravels and commonly

includes gravels of the crystalline schist. It also constitutes the pebbly sandstone

facies ofother underlying units. It is clear that the former group was also supplied

from north, because the imbrication of the outsize siltstone-clasts coexisting with

hard gravels indicates their supply from north (Plate 5, Figures 1-4).

Based on these facts, two different channels are presumed on the northern side

in the case ofthe Sa-Nm unit (Fig. 21F). The terminus ofthe channel which haveplayed the most important role in supplying the turbidite sediments ofthe underlying


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-J -t O2468 10 ua kmSummary of some basic sedimentological data on the Hk-Kman inferred depositional pattern ofthe unit (F).


d2

unit (A-E) and

36 Shuichi ToKuHAsHr

A

B

c

D

E .

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'Summary ofsome basic sedimentological data on the Sa-Nm unit (A-E) and aninferred depositional pattern ofthe unit (F).



units further regressed toward north.

the western side of the older one and

areas than the older one.

But the other new channelseems to have stretched to the

was formed atmore southern

C. Depositional Process of the Kiyosiuni Formation

Depositional process of the overall Kiyosumi Formation can be drawn by thesuccessive figures of the depositional patterns of individual units... .,

The following two assumptions play the very important roles to infer the process.

Firstly, the pebbly sandstone facies (facies "a") is channel depQsits. The main

reasons of this assumption are as follows. .• ''• .' (1) A large trough-like erosive morphology is observed at the base of the pebbly sandstone facies of each unit.

(2) Pebbly sandstone facies disappears rapidly in a few kilometers in the lateral

directions.

(3) In the area downcurrent of the pebbly sandstone facies are developed the thick

sandy-flysch facies (facies "b"). '

Secondary, the channel deposits and their downcurrent deposits debauchedfrom the almost same outlet located at the northern portion, except for those of the

uppermost Sa-Nm unit which contain the larger gravels and include no crystallineschist gravels. Therefore, the discontinuous migration of the pebbly sandstone

facies from the Tk-Kr unit to the Hk-Km unit through the Km-Tk unit was causedby the shifting of the channel (Fig. 8). The main reasons of the second assumption

are as follows. ', ' ' •(1) Constitution of gravels in the pebbly sandstones of these units closely resemble

each other (Fig. I4, Table 2).

(2) The thickness of the channel deposits is much larger than the depth of the trough-like erosive morphology formed at the base of the units (Figures 8 and

Il).

(3) The channel deposits indicate an upward thinning cycle from the thick pebbly

sandstone facies at the base to the thin siltstone-dominated alternations at the

top through the sandstone-dominated alternations. . tt(4) The migration of the pebbly sandstone facies from one unit to the overlying

unit is discontinuous and complete. ' On the basis of these assumptions the depositional process of the KiyosumiFormation is shown in Fig. 22 and interpreted as follows.

Tk-Kr imit (see Fig. 17) The channel of this unit was extended toward southeast and passed through the

eastern part of the investigated area, and then turned toward west. Downcurrent

38 Shuichi ToKvHAsHI

lii

Sa-Nm

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v..,.•... =":e- ':i"•l:i•'::''I{/:z .i6 •"i''•i•.:::;:r•i

. 1/: ..:. .1./..../.:., ,.:.:.:.,,...,.:.:.. . -L:.,/,i,E,il•,'I :E:•:,i. ,iii ii[,ilIil,':l'1:.i:i'i,;. . . ,d2. E•r:

' -.•:•.:i::•,/•ii:-:,l-il•[:gllliiiiEIII':,l•lll•iil•l,II,l•I:'l•I'il-i•{tY

`;v;t•;•:•l•:L:tt•jtt•J;L•:.:';'

:,I{II.::•

d;i::i,-'-t'S

';tt:•. r 'v• ',. g t. :z '•t.-rt. -', - 'r=. - -.:- ....,, - -it. ../11.:tttst- -. - .tttt- -,,1. - ./,1.., ,.1,- e :1/' 1',,''/-1:1:::/:li..i-..I',";':•/..Ts•/T/<.''iii'ira:::'Ii•i•i'iiizl'l''i/;•,

.' .'.:..,:a:'::' :al' ,:l:,ili":::'::,lll'

iil:•ii.• :'':'::•t:::,•'..,lii•/•::'..i,•- '•"

-;.'v . - - .'... ::,'=..:.'.'.e v:::11i/./.:./././.:./,.,•/,/•z•:-/::::::::/:::::::V

'v;•}';';'/:/L:',i:::'//I:•:.l::,'•i:::Li::'`"

400m Egoo

O246810km

Fig. 22. Inferred depositional process ofthe Kiyosumi Formation.

Detailed explations are given in the text. Outlines of areal variations of facies

and thickness ofeach unit are also shown in this figure.


of the pebbly sandstones fi11ing a large channel are extensively developed thick

sandstone-dominated alternations.

Km-Tk unit In this unit braided channels were created at the northern part ofthe investigated

area, The main reasons are as follows.

(1) Trough-like erosive morphology is observed at the two areas apart from each other at the base of this unit (Fig. 11).

(2) Deposits fi11ing these two erosive morphologies were formed concurrently in

the saine period.

(3) The thickness of the unit in the area between these two morphologies is very

small (Fig. 8).

Two depositional tongues composed of the sandstone-dominated alternationswere formed and incorporated with each other in the area downcurrent of eachbraided channel.

Hk-Km unit (see Fig. 20) The braided channels of the Km-Tk unit were replaced by a single new channel

extended from north to the northernmost portion of the investigated area. A single

depositional tongue composed of the sandstone-dominated alternations was formed

downcurrent of the channel. Further downcurrent in the eastern and western partsof the peninsula were deposited the siltstone-dominated alternations.

Nm-Hlc unit The channel of the Hk-Km unit gave place to a new one formed in the neigh-bourhood. Its terminus retreated further northward, because pebbly sandstonefacies (facies "a") are not exposed anywhere in this unit within the investigated

area. The depositional tongue composed of the sandstone-dominated alternationsstretched beyond the east coast of the peninsula.

As already mentioned, current ripple cross laminations indicating the currents

from south are observed in the Nm-Hk unit at only one locality in the eastern part

of the peninsula (Fig. 16B). But these sandstone beds are all thin and composedonly of laminated sandstones. On the other hand, the orientation of currents at

the same locality obtained from sole markings and preferred wood fragments of thick

sandstone beds containing massive sandstone are concordant with the NWW-SEEcurrent directions in the central part of the peninsula (Fig. 16A). Moreover, at

the east coast near that locality, current cross laminations of both thick and thin

sandstone beds in the same unit are all concordant with those, too (Fig. 16B). There-

fore, the author attributes such northward current cross laminations to shifting of

current directions controlled by microreliefs on the basin floor and not to direct


supply from the southern area,

Sa-Nm urrit (see Fig. 21)

The terminus of the surviving channel deposits also did not reach to the investi-

gated area and the depositional tongue composed of the sandstone-dominated alter-

nations successively stretched beyond the east coast. The new channel comingfrom a different outlet intruded to the more southern area than the older one on the

western side of it. The new channel deposits contain a little different gravels from

those of the older channel deposits. In the area downcurrent of the new channel

deposits was also formed the depositional tongue of the sandstone-dominated alter-

nations. Therefore, in the central part of the peninsula, two tongues were formed

overlapping with each other.

From this depositional process, it is clear that each unit is composed of channel

deposits, depositional tongue of sandstone-dominated alternations, and surrounding

siltstone-dominated alternations or massive siltstones. The Kiyosumi Formationis formed by superposition of several pairs of the channel deposits and depositional

tongue. Therefore, the migration of the channel and depositiona! tongue hasplayed the most important role in vertical facies variation ofthe Kiyosumi Formation.

D. Depositional Model of Turbidite Bed and Facies

ToKuHAsHi (1976) traced the sandstone-dominated alternations constitutingthe upper half of the Hk-Km unit widely for 40 km in E-W direction throughoutthe peninsula along the folding axis and for 5 km in N-S direction across it. He

correlated many individual sandstone beds in the alternations by means of manythin tuffmarker-beds in the siltstone beds (Fig. 23). He reconstructed schematically

a three-dimensional geometry of each sandstone bed (Fig. 24A) and illustrated a

common lateral change of sedimentary structures and textures in each turbidite bed

along the downcurrent direction (Fig. 24B). But these models lack the most up-current part, because the sediments in the exactly upcurrent area such as pebbly

sandstones are not included in the upper half of the Hk-Km unit.

However in the overall Kiyosumi Formation are also observed such thick pebblysandstones as channel deposits filling a large-scale erosive morphology, that is, a

large channel. Therefore, a more complete diagrarnatic model of the lateral change

of sedimentary structures and textures in each turbidite bed can be illustrated now

(Fig. 25A). Characteristics of sedimentary structures and textures of each division

in the model are summarized in Table 3. Depositional models of the turbiditesequence corresponding to individual units in the Kiyosumi Formation are illus-trated in Figures 25B and C with facies symbols in Table 1.

The depositional model of each turbidite bed illustrated here relatively well

Three Dimensional Analysis of a Large Sandy-Flysch Body

o

p

.o

q pq

2

85 160 as tt tt t11 "''''"'z::, 't

Q Q.z.--.o.-.e..

o- 2 62

. 145

2

•• 37

i:R,.E9,Tso.!92 iss

17 40•::• :::••:': •::••• :.143 ••'•;:::::•1 150 ":':li:•:1,1::•,•:•.167

:,:•.:. 20 '' ,-;.'• :::, IS ..•.:'

2 O-l2 54"i

6

7

o

p

9K

.

o

o

Q

'

3

.

11 1

ve130

1

'82

9

,::::1 :O:,7

'• 4o

T5

11

N . O-•1 --s-

16 sl

Q g..g L, ..q Q

.

75

1

Q,vsu-vs'Sl

6

3

'

130 115. 't'.'•r.. 95

•.i•. 100 t-t tt

" ÅÄ

1

IS

:8. Q

.

IZ

oo

IM

174 ::.

4s2o

12

12

'- - O.5

t3..o..2 ".r --:-

o q.-";t}.1.9-i.. -5

4

3

9

2

ÅÄ2

A

Al

33

5

5s

9

4

1

6•26

B

1

.P o ,

16

5

10

9

Q

2

Q.-sveugSF

9.s.,:S

5 3 ' ff200cm llO -O km ' 'Fig. 23. Geometry ofindividual sandstone beds in the upper halfofthe Hk-Km unit (ToKuHAsHi, 1976). Each number means thickness in cm at each locality. The lower point ofeach section corresponds to each locality. Triangles mean the absence ofbeds due to erosion by the overlying thick sandstone beds.

41


A , ••;I4.•.' L..t•z" ' t ft.'

.'

-. .1,1-/. .1.. -l tt t t ,

::M

, "IILN, ,.,,..es"i

,. •:•i. f;•: .::•; q t-ttt ttt 4 t ;ttt -- -tJ t- t ttt V•" .:: '.:{;x. t•.t . ,L•-; ,.••.•1,l.i.. s';.:/li•{';:i .g?'i'i. md ..i,i

•' " tt'. :I I/"': .il:V k ::.. n .,,::::: .:.

ttttt .tt. t - - ttt.t..1.:I•:' il.pt.[':1.::i,.,, •t ...:.,•,,,/,-i•.g,.••,iiii,1'i'ili,•1'111•

tt tf' • li 'i l": ir: ': ';'7Ili;li' l.:'ii "ii'i'il':i'i:.' )ii',li 'i ':;' '.

' -" ' ' ;' :':l ,e•,,'. ,n.il el 'iil t. lhl.S .T .vl x: :I- L i .

t ...t ttt.t-tt.ttt

lit ng MlfieekS

pt

N'c'''•,

:•S•'i•bZ ••'.•' "•••.x' . •.'.• .• :--' N. ,

tt tttt t t t- --t-tt t+t t t t

E"ioocmzone

O 10 L = ==z===l km

m-c. sitt t.sitt

vf• t'vi'

t'

sand

m. m-cm,.c, stlt

1.

Ehgt.--

p"p:c-

A3

----

A2

---Al

Eh

?.

-- -- ..'

Centrel thlck

-- -

pert+Marginel

. . .s

h

Al

.

.

h

sN

ss

s.

s

s-

s.

.

.N

xx

kNsx S-s s x xS xA2 's

NS

x s- ssA3 .

I-

,

ti

c'..

.. s . . ---- . -- -

thln

t.s,.S') J

p 111se 4.[E:l s.:t•.,

of individual partly modified)

----

part-

i !l M.'CJS tt Eh-- --

Fig. 24.

••::,,• 2.ESffS 3.as 6.EE] 7.:;: s.E{l] g.U

Depositional modet turbidite beds in the Kiyosumi Formation(ToKuHAsHi, 1976; . Vertical scale is remarkably exaggerated.A. Geornetric model ofa moderately thick sandstone bed.B. Schematic model of the lateral variation orsedimentary structures and texturesin a considerably thick sandstone bed along the current direction.

1. Massive sandstone, 2. Laminated sandstone, 3. XNTood fragments, 4. Silt-stone clasts, 5. Pumice grains, 6. Scoria grains, 7. Small pebbles of basementrocks, 8. Shell fragments, 9. Siltstone. For description of symbols see Table 3.

Fig . 25. Depositional model ofindividual trubidite beds and facies in the Kiyosumi Formation along the current direction. Vertical scale is remarkably exaggerated.

A. Schematic model of the lateral variation of sedimentary structures and textures in a considerably thick sandstone bed. For description ofsymbols see Table 3. B. Schematic profile model of the lateral facies change of flysch sequence corresponding to an upward thinning cycle unit. C. Schematic plan model of the same unit as B. For description of symbols in B and C sec Table 1.


Chlnne4 part --- F-- Central thick part --Merginat thin part

A

iliii.iisiS•ee</tlill,i]/,.A,.l.,t.vltillici,,as

..,] .ny

Ai

lilllll•::::;ct:sR;i13•41,, .. .

L}ESIt4.it/.Siwo..DHEt

Eh

- Channel depesits--+Depesitienel tengue- Mefginei B .- ;=1- -i!z/ .tk' ... ...ttt-.-.7-.-tth.if,t-gtff-m}op-iLaE'e"1'l-fll-NtNNr'L,tfi"lliSI.'ix.-,i',s.s-.g-`.---wt

Ygtg/sks.tt,..,...,...

s}.ii. '. ,,,.t. d ,''"

depesits

-- =,=••.-fi.

c

e

---------- --• i.- 11 1 bOI i

'

bl-3 c d .

Fig. 25.

Table 3. Characteristios of each division of turbidite beds and h emipelagic siltstone beds in the Kiyosumi Formation

Symbo1

AO

Al

A2

A3

BC

D

Et

Eh

Characteristics

Pebbly sandstone division Small hard pebbles occur crowdedly, contacting with eachother. They occur fi11ing small channel-like scours, enclosing verylarge angular siltstone-clasts or forming elongated lenses in the mas-sive sandstones.Lower massive sandstone division (Bouma's a division) Small hard pebbles and hard scoria grains and shell fragmentsoccur scattering in the massive sandstones. Grading and imbri-cation of these pebbles and grains are often observed. Dish struc-tures are sometimes developed.Middle massive sandstone division (Bouma's a d Well sorted sandstone with few conspicuousstructures are sometimes developed.

ivision)grains in it. Dish

Upper massive sandstone division (Bouma's a division) Massive sandstone with scattering rounded sikstone-clasts,wood fragments ofvarious sizes but smaller than siltstone clasts, andpumice grains. Pumice grains are sometimes parallel aranged in theuppermost part ofthe division. Preferred orientation ofwood frag-ments are sometimes observed.Laminated sandstone division (Bouma's b and c division) Various sedimentary structures with carbonaceous wood flakes,such as parallel, wavy, convoluted, current-ripple-cross laminationsoccur. Division into lower-parallel-lamination part (Bouma's bdivision) and overlying current-ripple-cross-lamination part(Bouma's c division) is very diMcult so they are treated as a single B-C division here.Finely laminated silty sandstone division (Bouma's d division) Very fine sedimentary structures such as thinly-alternatedparallel laminations and other minute laminations with very smallwood flakes and micas are observed. Weathered surface is pro-truded as in the case of overlying siltstone bed.Turbiditic siltstone division (lower part of Bouma's e division) Turbiditic siltstone is easily distinguished from overlying hemi-pelagic siltstone (Eh) by finer-grained and better sorting and sharpcontact with the latter. Distinct grading is observed in the case ofthick turbiditic siltstone.

Hemipelagic siltstone division (upper part of Bouma's e division) This siltstone is usually ill sorted and often contains scoria andpumice grains randornly. Many continuous tuff beds are includedin the division. Grading is not observed. Trace-fossils are some-times observed.

Grain size of matrix

very coarse- to me-dium-grained sands

coares- to medium-grained sands

medium- to fine-grained sands

fine-grained sands

fine- to very fine-grained sands

very fine-grained sandsto coarse-grained silts

medium- to fine-grained silts

mostly of medium- tocoarse-grained silts

Occurrence

channel de POSItS

lower to middle parts of thbedded sandstones

ick-

middle to upper parts of thick-bedded sandstones and lowerpart ofsandstone beds thinnerthan about 1 m.

upper orrelatively

stones

uppermost part ofthick-bedded sand-

upper or uppermost part ofthick-bedded sandstones andlower part of thin-beddedsandstones

uppermost part of thick-bedded sandstones and upper partof thin-bedded sandstones

usuallyrelatively

stones

accompanied wtihthick-bedded sand-

lying en the sandstone beds orturbiditic siltstone beds

AA

mtr:5't6oxcm>ua:H


coincides with that hypothetically proposed by MEiscHNER (l964). But the lower-most pre-face underlying the main face in the MEiscHNER's model is not observedhere. In addition, not only hemipelagic siltstone but also turbiditic siltstone are

both observed on the relatively thick sandstone beds and they are easily distinguished

from each other. Generally the turbiditic siltstone is finer-grained and better-sorted

than the overlying hemipelagic siltstone. Detailed differences between the turbiditic

and hemipelagic siltstones are discussed in O'BRiEN et al. (in press).

The depositional model of the turbidite sequence illustrated here (Figures 25B

and C) fundamentally coincides with the hypothetical lithologic variation proposed

by DzuLyNsKi and WALToN (1965) with symboJs such as "a" to "e" used to expressthe characteristic facies commonly occurring in flysch. But in the present model,the concept of upward thinning cyc!e are also incorporated (Fig. 25B) and pebbly

sandstone facies (facies "a") is distinguished and stressed as channel deposits by

their narrow distribution and large basal erosion (Figures 25B and C).

V. Discussion

A. Comparison of the Depositional Process of the Kiyosurni Formation with that of the Submarine Fan Model

The prevailing submarine fan model is as follows. A submarine fan whichstretches from a mouth of a submarine canyon is generally divided into three seg-

ments, that is, upper or inner fan, middle fan and lower or outer fan. In the upper

fan, is developed a submarine valley or channel with natural levees which debouches

from the mouth of the submarine canyon. When tubbidity currents flow downthrough the valley or channel and arrive at an intersection zone (HANER, 1971),

they release and deposit a large quantity of suspended loads on the surroundedarea. Consequently a depositional lobe or suprafan (NoRMARK, !970) is formeddowncurrent of the zone, that is, on the middle fan segment. Therefore the middlefan is characterized by the active deposition and by a convex-upward segment on a

radial profile. The lower fan has a very smooth and nearly flat surface with nochannels. Uniform parallel bedding is observed on reflection profiles. The shifting

of channel in the upper fan is followed by the migration of the suprafan in the middle

fan. Submarine fan model shows that the repetition of these processes makes a fan

to grow up (NELsoN et at., l970; NoRMARK, 1970; HANER, 1971; NoRMARK andPipER, 1972; NELsoN and KuLM, 1973; WALKER and MuTTi, 1973; SAKuRtu et al.,1974; NoRMARK, 1974; NELsoN and NiLsEN, 1974; CLEARy and CoNoLLy, 1974;Ricci-LuccHi, 1975; PipER et al., 1978; NoRMARK, 1978).

On the other hand, the inferred depositional process of the Kiyosumi Formation

is fundamentally the same as that of the submarine fan model. Because, as already

46 Shuichi'ToKuHAsHImentioned, the Kiyosumi Formation was formed by the superposition of depositional

tongues or lobes which were deposited in the area downcurrent of the termini of

channel deposits. The terminus of the channel deposits seems to correspond ac-curately to the intersection zone of the submarine fan model.

Submarine channel deposits in the Kiyosumi Formation are characterized byaccompanying a trough-like erosive morphology at the base and by an upwardthinning cycle beginning with thick pebbly sandstones and ending with thin siltstone-

dominated alternations or massive siltstones. Pebbly sandstone facies are verynarrow laterlaly and rapidly change into siltstone-dominated alternations. Threestages are recognized in the formation of the channel deposits.

First stage The period of growth of an channel morphology by the passage of a large quan-tity of sediments and concurrent erosion of the underlying layers.

Second stage The period of deposition of pebbly sandstones in the channel and consequentdisappearence of the channel morphology on account of the regression of the inter-

section zone, which follows the reduction of the power and magnitude of turbidity

currents.

Third stage The period of burial of the pebbly standstones in the channel by deposition of

sandstone- and siltstone-dorninated alternations on them, due to the further regression

of the intersection zone.

When the magnitude and power of turbidity currents were recovered again,a new channel of the first stage was formed at a different site. That is, therejuvenation of turbidity currents scems to have been the most important factor to

cause the shift of the channel. The characteristics of the depositional site of these

channel deposits coincide well with those of the upper fan especially of the lower

half of the upper fan of the submarine fan model.

On the other hand, the depositional tongue, consisting of thick sandstone-dominated alternations devcloped in the area downcurrent from the terminus ofthe channel deposits, must have formed a convex-upward depositional bulge onthe surface with the negligible erosion at thc base of the tongue. Therefore, the

depositional tongue seems to correspond exactly to the suprafan of the submarine

fan model.

The suprafans in the Kiyosumi Formation extend more than 20 km in E-Wdirection and they overlap most parts of their extents with each other (Fig. 22).

Moreover each suprafan of the unit occupies the most part of the main depositional

area. Because of these features, the depositional process or pattern of the Kiyosumi

Formation does not completely coincide with that of the current submarine fan

model. In the case of the submarine fan model, each suprafan occupies only a


portion ofthe overall surface ofa fan (NoRMARK, 1970; Ricci-LuccHi, 1975; MuTTi,

1977). As a result, the lateral extent of individual turbidite beds on the suprafan

are remarkably underestimated such as 1 to 2 km wide (WALKER and MuTTi, l973).But in the case of the Kiyosumi Formation, the individual thick sandstone bedsextend consistently in thickness at least over 10 to 20 times as wide as the estimated

value (ToKuHAsHi, 1976a; HiRAyAMA and NAKAJiMA, 1977). Accordingly, in thecase of the Kiyosumi Formation, the remarkably large-scale turbidity currents seem

to have occurred forming remarkably wider suprafans than those in the currentsubmarine fan model. Siltstone-dominated alternatjons and successive siJtstones which are distributed

in the areas downcurrent ofthe sandstone-dominated alternations (suprafan) seem to

have formed the lower fan deposits and!or basin fioor deposits.

B. Early Stage of Fan Sedimentation

One of the most serious shortcomings of the prevailing submarine fan model is

that it makes no reference to the early stage of the submarine fan sedimentation.

That is, it refers only to the depositional process on the surface of the adequately

matured submarine fan. Here the author will discuss the early stage of the deposi-

tional process of the Kiyosumi Formation which corresponds to the early stage of

fan sedimentation.

When the inferred depositional process of the Kiyosumi Formation is examined,the peculiar behaviour ofthe lowermost Tk-Kr unit should be first noticed (Fig. 22).

Why does such a peculiar behaviour occur only in the lowermost Tk-Kr unit?What does it mean in forming the submarine fan? Ofcourse, the configuration ofa basin and the site of a feeder canyon obviously have an overall influence over the

growth of a fan. As already mentioned, the Kiyosumi Formation was formed bylateral suppiy from north into a restricted basin elongated in the E-W direction on

the continental slope. The Mineoka uplift zone formed an outer ridge along thesouthern side of the basin and dammed up the sediments supplied from north.

On the other hand, the bottom topography such as general slope and relief of

the basin before the incoming of turbidity currents seems to have had an important

influence on the depositional process in the early stage of fan sedimentation.

Bottom topography of the basin in the depositional period of the uppermost part

ofthe Amatsu Formation, which underlies the Kiyosumi Formation and is composedmainly of hemipelagic siltstones, seems to have been formed mainly under the infiu-

ence of tectonic movements for a long time. Especially in the case of the slope basins

in the geologically active region, the bottom topography mainly reflects the tectonic

movements, and the tectonic topography is dominant unless differential deposition

occurs to bury the relieÅí Therefore, on the bottom of the basin just before the


beginning of the deposition of the Kiyosumi Formation, some tectonic topographyseems to have been developed. The first group of turbidity currents which flowedinto the basin from north must have necessarily been controlled by the configulation

of the basin and the bottom tectonic topography.

The Mineoka uplift zone on the southern side must have changed the direction

of turbidity currents of the lowermost Tk-Kr unit from southward to westward.The southern area in the basin fioor must have been deeper than the northern area,

because the thick sandstone-dominated alternations of the Tk-Kr unit are distributed

only in the southern area in the basin.

The turbidite sandstones of the Tk-Kr unit seem to have smoothed thepreexisting relief to attain an equilibrium profile of slope-fan-basin. Therefore, it

may be said that the lowermost Tk-Kr unit showing the peculiar depositional process

exhibits the deposits of "a preparatory stage" of fan sedimentation, or the deposits of

"a pre-fan-sedimentation stage". Such stage must always exist, although thesedimentary features may not be same due to the characteristics of the various basins

in the various geologic frameworks.

C. FurtherProblems

The Kiyosumi Formation occupies the lower half of a turbidite suite, andtherefore, reveals the process of fan sedimentation of the first-half stage including

the early stage. The Kiyosumi Formation consisting of a large sandy-flysch body

formed the fundamental framework ofa submarine fan on the basin floor madeof the Amatsu Formation or Amatsu Mudstone. The Inakozawa Formation orInakozawa Mudstone, the western equivalent of the Kiyosumi Formation, wasformed on the lower fan or the basin floor around the fan. On the submarine fan

composed of the Kiyosumi Formation was deposited the Anno Formation, which iscomposed of several small-scale sandstone-dominated alternations surrounded respec-

tively with siltstone-dominated alternations both laterally and vertically. Tounderstand the process of fan sedimentation of the later-half stage including the

last stage, the depositional process ofthe Anno Formation must be analyzed. Thenthe depositional process of the overall fan sedimentation or of a complete turbidite

suite can be clarified.

On the other hand, the origin of upward thinning cycles observed in theKiyosumi Formation seems to be the most important problem to be further discussed,

because they not only play such a substancial role as channel shifting in fan sedimen-

tation but also may refiect the chronologic and geologic events in those days.

At least the two different orders of upward thinning cycles are recognized in

the Kiyosumi Formation. An upward thinning cycle of the first order is detected

throughout both the Kiyosumi Formation and the overlying Anno Formation. As


already mentioned, these two formations together form a positive or transgressive or

recessional turbidite suite (Ricoi-LuccHi, 1975>. In the case of the Kiyosumi For-

mation, a phased retreat of the terminus of the channel deposits is clearly recognized

unit by unit from the bottom to the top as shown in Fig. 22.

Upward thinning cycles of the second order are cleariy recognized at the ancient

submarine channel sites of the individual units. Each of them may correspond toa positive or transgressive turbidite megasequence or subsuite (Ricci-LuccHi, 1975).

These cycles were repeated several times in the Kiyosumi Formation. The rejuve-nation of turbidity currents, which means a new start of the next unit or cycle, played

the most important role to form a new channel at a different site.

What caused such upward thinning cycles of the first and second orders in the

Kiyosumi Formation? These phenomena might refiect the eustatic movements,that is, a general tendency of gentle rise of sea Ievel with small-scale oscillations.

Obviously the level of the sea surface effects the possibility for the coastal sediments

to reach the canyon heads or the shelf break, and therefore, the frequency and magni-

tude of turbidity currents. Generally the lower level of the sea surface is considered

to permit more frequent and larger turbidity currents (DALy, 1936; KuENEN, 1950;

HAND and EMERy, 1964; CALsoN and NELsoN, 1969; NoRMARK and PipER, 1972;NELsoN and KuLM, 1973). Therefore, the gentle ascention of the sea level reduces the frequency andmagnitude of turbidity currents and may produce the upward thinning cycle in the

depositional area. The following relatively rapid descent of the sea level mayproduce, on the contrary, the rejuvenation of turbidity currents. Therefore, the

tendency of a general rise of the sea level with such small-scale fluctuation might

have produced such upward thinning cycles of the first and second orders as observed

in the Kiyosumi Formation. If the idea is correct, we can read the ancient eustatic

movements or glacial activities by analyzing the upward thinning cycles in flysch

sequences. However, a different way of thinking may be possible. The depositional pro-

cesses in the depositional basin are no more than the reflection of the destructive

processes in the source area. That is, the turbidity current action is the process to

attain a new stable equilibrium occuring on a large scale in the submarine realm.

An abrupt destructive fall of the unconsolidated metastable sediments at the first

stage may occur on the largest scale and form a new canyon head on the uppermost

part of the slope. As the canyon head grows forward, each quantity of sediments

produced by a single destructive fall may decrease gradually and then the magnitude

of turbidity currents may become smaller than that at the earlier stage. In response

to this, the upward thinning megasequence or cycle may be formed in the deposi-tional area.

The formation of a new canyon head at a different site due to a next new


large destructive fall of the unconsolidated sediments may be responsible for therejuvenation of turbidity currents or the shift of channel site in the depositional area.

FEux and GoRsuNE (1971) pointed out the lateral shifts of the Newport SubmarineCanyon off California in response to the changing point of input of sand to thecanyon system. The constructive process of a transgressive or recessional turbidite

suite may correspond to a general destructive process of a large body of the uncon-

solidated sediments such a delta through the formation of a canyon system of the

dendritic tributaries on the upper slope.

Moreover, the tectonic movements including a change of lifting rate at theprovenance may have sensibly controlled the rate of supply of sediments to thecanyon heads. In this case, the tectonic movement may be a main factor to haveproduced the upward thinning cycle. Of course, the combination of these factors may have produced the first- and

second-order upward-thinning cycles in the Kiyosumi Formation. Accumulation ofthe data on the eustatic rnovements in those days and on the initiation of turbidity

currents at the canyon heads will help to analyze the generation of such cycles.

Anyway, the clarification of origin of the upward thinning cycles demands thebroader knowledge and background as in the case of the other cyclic sedimentation.

It may be very important to recognize that any submarine fan sedimentationand its sediments reflect and are characterized by not only the geologic setting such

as size, configuration and depth of basin, and rate of sediment supply, but also the

chronologic events such as eustatic movements. Geologic time is an important factor ,in changing growth patterns and differing sedimentary regimes during the deposi-

tional history of fans (NELsoN and KuLM, 1973).

At last, we can't forget the facts that the detailed and fundamentai work on the

recent submarine topographies and sediments has made a great advance in inter-preting the genesis of various facies in flysch sequences on land. But unfortunately,

as correctly pointed out by WALKER and MuTTi (1973), only a little is known about

the detailed topographies, sediments and their interrelationships of the trenches,

troughs, slope basins, borderland basins and marginal-sea basins around the island

arcs includingJapan Island Arcs. Urgent and vigorous work on them is demanded.Such work will bring further usefu1 informations in interpreting the depositionalprocesses of flysch and other sediments which were deposited in ancient geosynclines

and other various basins and are now exposed on land.

VI. ConclusiveRemarks

Three dimensional analysis of the Mio-Pliocene Kiyosumi Formation with theaid of many tuff marker-beds disclosed the depositional process of the formation and

enabled it to be compared with the current submarine fan model. Main results are


as follows.

(1) The Kiyosumi Formation or the Kiyosumi Sandstone is composed of alenticular sandy-flysch body attaining 850 m in maximum thickness and distributed

more than 20 km in E-W direction along the folding ax;s and more than 6 km inN-S direction in the central and eastern parts of,the Boso Peninsula and interfingers

with the Inakozawa )vludstone in the western part of the peninsula.

(2) The turbiditic sediments were supplied laterally from north into a re-stricted slope basin elongated in E-W direction. Supply from north is supported not

only by paleocurrent evidences of turbidites but also by composition of gravels in the

formatjon. The Mineoka uplift zone on the southern side of the basin is considered

not to have been a source area of the formation, but a submarine outer ridge of the

basin.

(3) The depositional depth of the Kiyosumi Formation is inferred from ben-thonic foraminifera in the interbedded hemipelagic sediments as middle to upper

bathyal environments (AoKi, 1968; HATTA, unpublished data). Hemipelagic sedi-ments are composed mostly of medium- to coarse-grained siltstone.

(4) Submarine channels played the most important role in forming theKiyosumi Formation as passage ways for the large quantity of turbiditic sediments.

Channel deposits are characterized by an upward thinning cycle beginning withthick pebbly sandstones and ending with thin siltstone-dominated alternations or

massive siltstones and by accompanying a basal trough-llke erosive morphology up to

50 m in maximum depth and more than 5 km in maximum width. (5) In the area downcurrent of the channel deposits were deposited a largedepositional tongue composed of thick sandstone-dominated alternations withnegligible erosion at the base. Siltstone-dominated alternations are distributed in

the area further downcurrent of it.

(6) The Kiyosumi Formation was formed by shifting the submarine channelseveral times. Such depositional process of the Kiyosumi Formation is essentially

the same as that ofthe current submarine fan modei (NoRMARK, 1970; HANER, 1971;

WALKER and MuTTi, 1973; Ricci-LuccHi, 1975; etc.) The shift of the submarinechannel is related most intimately to the beginning of a new upward thinning cycle,

that is, to the rejuvenation of turbidity currents.

(7) Depositional environment of the channel deposits seems to have been anupper-fan segment. The terminus of the channel deposits probably corresponds toan intersection zone (HANER, l971) between the upper- and middle-fan segments.

(8) The depositional tongue of thick sansdtone-dominated alternations seemsto have been an ancient suprafan (NoRMARK, I970) on the middie fan segment.Siltstone-dominated alternations deposited in the area downcurrent of the deposi-

tional tongue seems to have formed the outer-fan deposits and/or the surroundingbasin floor deposits.

52 Shuichi ToKuHAsHI (9) The extent of the individual depositional tongues in the Kiyosumi Forma-

tion is much larger than that of the suprafan estimated based on the currentsubmarine fan model. The depositional tongues in the Kiyosumi Formation covermost of the middle-fan segment, so they largely overlap each other. The extent of

the constituent individual sandstone beds is also much larger than that estimated

based on the current submarine fan model (ToKuHAsHi, 1976a, b). This seems torefiect the much larger-scale turbidity currents in the depositional period of the

formation.

(10) The channel deposits and depositional tongue in the lowermost unit ofthe Kiyosumi Formation indicate a peculiar distribution as fan sedimentation. This

unit seems to have been allotted to smooth a preexisting relief on the basin floor

largely caused by tectonic movements for a long time and to attain an equilibrium

profile of slope-fan-basin. Such "a preparatory stage" of fan sedimentation or "a

pre-fan-sedimentation stage" must have an especially important meaning when afan is formed in such a geologically active basin as a slope basin.

(11) The Kiyosumi Formation occupies the lower half of a positive or trans-gressive or recessional turbidite suite (Ricci-LuccHi, l973) and so discloses thedepositional framework ofa fan. To clarify the depositional process of the later-half

stage including the last stage, the overlying Anno Formation must be analyzed.This formation contains several small-scale sandstone-dominated alternations sur-rounded with siltstone-dominated alternations both laterally and vertically.

(l2) The first-order and second-order upward thinning cycles are recognizedin the Kiyosumi Formation. The first-order upward thinning cycle is detectedthroughout the whole Kiyosumi Formation. It was formed by a phased retreat ofthe terminus of channel deposits or the intersection zone on the fan from theIowermost unit to the uppermost unit. The second-order upward thinning cycle isclearly observed in the channel deposits of each unit. It is repeated severa! times

in the Kiyosumi Formation. This cycle caused not only the formation and disap-pearence of the channel morphology but also the shift of the channel site.

(13) As causes of the multiple upward thinning cycles are considered aneustatic rise of sea level with small-scale fluctuations, a general destructive process of

a large unconsolidated sediments such as a delta through the development of acanyon system on the uppermost slope, a change of rate of sediment supply due to

the tectonic movements at the provenance area andlor the combination of thesefactors. It may be very important to recognize that fan sedimentation and itsdeposits may refiect not only the geologic setting such as size and configulation of

the basin and the rate of sediment supply in the provenance and so on but also the

worldwide chronologic events such as eustatic movements in the Miocene-Pliocenetime.

(14) Recent submarine geology has played the most important role to establish


the overall turbidite facies association based on the submarine fan sedimentation.

Many detailed and fundamental works on submarine topography and sediments inthe basins around the island arcs including the Japan Island Arc, such as marginal-

sea basins, troughs, trenches and so on are needed not only to fi11 the gap ofinforma-

tions on them (WALKER and MuTTi, 1973), but also to promote the further markeddevelopment of studies on flysch sequences which were deposited in the ancientgeosynclines and other basins.

Acknowledgements

The author wishes to express his sincere thanks to ProÅí Keiji NAKAzAwA andAssociate Prof. Tsunemasa SHiKi, Kyoto University, for their appreciated supervision

and permanent encouragement. He must extend his hearty thanks to Drs.JiroHiRAyAMA and Terumasa NAKAJiMA, Geo}ogical Survey ofJapan, for their initialand continuing encouragement and support to undertake the sedimentological studyin the Boso Peninsula.

He is much indebted to ProÅí Keiichiro KANEHiRA, Chiba University, Drs.Naotoshi YAMADA, Sadahisa SuDo and Hiroshi MAKiMoTo, Geological Survey ofJapan, for their examination of many thin sections of gravels in the Kiyosumi For-

mation. He also thanks Mr. Akio HATTA, Kisarazu-higashi High School, for hisinformation on the benthonic foraminiferal fauna in the siltstone bed just below the

Hk tuff.

He expresses his deep gratitude to Profs. Tadao KAMEi and Sadao SAsAJiMA,Associate ProÅí Shiro IsHmA, Drs. Daikichiro SHiMizu and Takao ToKuoKA, KyotoUniversity, for their critical and repeated reading of the rough drafts and their

helpfui advice to them. He also expresses it to Profi Neal O'BRiEN, State Univer-

sity of New York, Potsdam, for refining the manuscript.

He is also grateful to Messrs. KinzoYosHiDA and HisaoTsuTsuMi, KyotoUniversity, for preparing many thin sections of gravels, and to the colleagues of

Department of Geology and Mineralogy, Kyoto University, for their useful discus-sion and kindly consideration.

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SuDo, S. (1976): Geology of the Katashina area, Gunma Prefecture, centralJapan. A4em. GeoL Soc. Japan (13): Studies on the boundarJ area between east and west JaPan. 229-240. (in Japanese with English abstr.)SuyARi, K. (1966): Studies on the Izumi Group in the eastern Asan Mountain range, Shikoku (1). Jeur. Sci., Tokushima Univ., 1, 9-14. (inJapanese with English abstr.) et ae. (1968): ditte (2). ibid., 2, 7-16. (inJapanese with English abstr.>

TAKAHAsHi, S. (1954) : Notes on a minor structure and a bone-bed on the coast near Kanaya Village, Boso Peninsula. Sci. Rep. Yokohama Nat. Univ., Sec. ll, Biot. Geol. Sci., 3, 109-120. (inJapanese


with English abstr.)TANAKA, K. (1965): Izumi Group in the central part of the Izumi Mountain range, southwest Japan, with special reference to its sedimentary facies and cyclic sedimentation. ReP. Geot. Surv.

JaPan (212), 1-33. (inJapanese with English abstr.) -- (1970) : Sedimentation of the Cretaceous fiysch sequence in the Ikushumbetsu area, Hok- kaido,Japan. ibid.(236),1-102.TANAKA, K. and KAwADA, K. (1971): On the volcanic pebbles in the Upper Cretaceous conglomerates otb the Nakaminato-Oarai area, Ibaraki Prefecture,,Japan. Bult. Geot. Surv. JIaPan, 22, 655-

660. (inJapanese with English abstr.)TERAoKA, Y. (1970): Cretaceous formations in the Onogawa Basin and its vicinity, Kyushu, south- westJapan. ReP. Geol. Surv. JaPan (237), 1-84. (inJapanese with English abstr.)ToKuHAsHi, S. (1976a): Sedimentological study of the flysch-type alternation of Hk horizon in the Kiyosumi Formation (part I) - Constitution of the alternation and form of individua! sandstone bed -. Jour. Geot. Soc. JaPan, 82, 729-738. (inJapanese with English abstr.) (1976b): Ditto, (part II) - Dipositional processes and circumstances of sandstone beds . ibid. 82, 757-764. (inJapanese with English abstr.)

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WAKitsfizu, T. (1901): Preliminary reports on geology of the Agricultural College (Nohka-daigaku) Forest in Chiba Prefecture. Jour. Ceet. Soc. JaPan, 8, 411-424, 465-476. (inJapanese)WALKER, R. G. (1966): Shale grit and Grindslow shales; transition from turbidite to shallow water sediments in the Upper Carboniferous on northern England. Jour. Sediment. Petrol., 36, 90-1 14.

-- (1967): Turbidite sedimentary structures and their relationship to proximai and distal depositional environments. ibid. 37, 25-43.

-- -(1973): Mopping up the turbidite mess. In: Evolving concepts in sedimentotog] (Ed. by R.N. GiNsBuRG), 1-37,Johns Hopkins Univ. Press, Baltimore and London, 191 p. and rvluTTi, E. (1973): Turbidite facies and facies associations. In: Turbidites and deep- water sedimentation, 1l9-157. Pacific Section, Soc. Econ. Paleontol. Mineral., Los Angels.

YAMAMoTo, H. (1971): On the convolute lamination occuring in the flysch-type sandstones. Jour. Geot. Soc. IaPan, 77, 23-36. (inJapanese with English abstr.)YosHmA, T. (Ed.) (1975): An outtine ofthe geotogy ofJaPan. Th{rd Edition, Geol. Surv.Japan, 61 p.

Three Dimensional A nalysis ofa Large Sandy-Flysch Body 59

Explanation of Plate 1

Six main tuff marker-beds1. Sa : Reverse grading is prominant in this bed.2. Nm: This bed is composed oftwo-layered "gomashio'' tuffand is often overlain by the slump- structured, very fine white tuff.

3. Hk : This bed is composed ofthick "gomashio'' tuffranging about 130 to 150 cm in thickness. Various sedimentary structures are observed in it. The current ripple cross lamination in this figure indicates the southward current.

4. Km: This bed is very thin measuring about 5 cm in thickness and composed ofsmall lapilli scoria and associated with thin white "gomashio" tuff-bed below it. A thick sandstonÅë bed is usually developed just above the bed.5. Tk : This bed is characterized by the three-layered structure; the lower finer-grained scoria tuff, the middle coarser-grained scoria tuff and the upper fine to very fine sandstone.6. Kr : This bed is also characterized generally by the three-layered structure; the lower finer- grained black scoria tuff band, the middle coarser-grained dark brown scoria tuff zone and the upper coarser-grained brown scoria tuff zone.

Explanatien of Plate 2

Small channel-like scours in pebbly sandstone facies (facies "a").

1. A cutting outcrop beside a forest-iand path named Higashi- University Forest around Mt. Kiyosumi. Tk-Kr unit.2. An enlarged view of the same outcrop.3-5. Wall outcrops along a small tributary near the locality D in

Fukurokura Line in the Tokyo

Fig.12. Tk-Krunit.


Other sedimentary facies in the Kiyosumi Formation.1-2. Wall outcrops along the Sasa River cornposed ofthin-bedded sandstoncs (facies "b3") in the Sa-Nm unit. Siltstone-dominated alternations (facies "d") is partly included in the latter

outcrop.3. A cutting outcrop beside the Kamogawa Toll Road composed of relatively thick-bedded sandstones (facies "b2") in the Hk-Km unit.4. A cliff outcrop near the east coast composed of siltstone-dominated alternation (facies "d") with many tuff-beds in the Hk-Km unit. This succession is the eastern equivalent of the succession in Fig. 3.

5. A cutting outcrop near the Mt. Kiyosumi composed ofthe lower massive siltstones of the Amatsu Formation and the upper thick-bedded sandstones (facies "bl") of the Tk-Kr unit. The con- tact between the Amatsu Formation ancl the Kiyosumi Formation is very sharp.


Sedimentary structures ofsandstone beds indicating paleocurrent.

1. Current ripple cross lamination (localityiin Fig. 24A).2. Current ripple cross lamination (locality k in Fig. 24A).

6o Shuichi ToKuHAsHi

3.

4.

Groove cast imprinted on the underlying siltstone bed (locality n in Fig. 9A).Flute cast imprinted on the underlying siltstone beds (locality f in Fig. 24A). Current di-rections ofadjacent sandstone beds coincide well with each other.

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brication of siltstone clasts in pebbly sandstone facies.

A wall outcrop of the Sasa River (locality a in Fig. 14). Transport direction is from right to left (southward).

An enlarged view of the same outcrop. A wall outcrop of a small tributary of the Koito River (locality b in Fig. 14). Transport direction is from right to ieft (southward).

An enlarged view of the same outcrop. A cutting outcrop beside a small path (locality i in Fig. 14). Transport direction is from right

to left (southward).

An enlarged view of the same outcrop.

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Title Three Dimensional Analysis of a Large Sandy-flysch ...repository.kulib.kyoto-u.ac.jp/dspace/bitstream/2433...Title Three Dimensional Analysis of a Large Sandy-flysch Body, Mio-Pliocene