Three Dimensional Analysis of a Large Sandy-flysch Body, Title Mio-Pliocene Kiyosumi Formation, Boso Peninsula, Japan
Author(s) Tokuhashi, Shuichi
Memoirs of the Faculty of Science, Kyoto University. Series of Citation geology 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.& MrNERAL., Vo]. XLVI, No. 1, pp. I-60, pls. 1-5, 1979
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 ...... ,...... m...."H 2 I. Introduction...... ,...... ".".".""HH" 3 A. Preliminary Remarks to the Present Study ...... ""..".H-."". 3 1. Start ofmodern flysch sedimentology ,....,...... ".".."H"H"" 3 2. Unique investigation in the Boso Peninsula ,...... """H-...... 3 B. Necessity of the Three Dimensional Analysis ofFlysch Sequence .. ".H.H..H.H.." 4 1. Submarine fan model and facies association ...... ""..."H...Hm 4 2. Some problerns on current submarine fan models ...... ".""..".m"" 4 3. Purpose and method ofthis study ...... ¥...... ¥...... "...".".".." 5 II. Geologic Outline...... -"..".""""" 6 A. Geologic Setting of the Neogene Sediments in the Boso Peninsula .. .H"...""""." 6 B. Geology ofthe Middle Part ofthe Boso Peninsula ...... "-.H"" 7 1. Preliminary remarks ...... ,...... --.H"."H" 7 2. Stratigraphy ...... ,...... H"...H-..."" 9 3. Igneous activity ...... "H.H"H"."" 9 C. KiyosumiFormation...... ------i------i 11 1. General remarks...... ,...... ------i---- 11 2. Chronologicdata ...... -""""."."" 12 3. Depositionaldepth...... H"..,-""."" 12 4. Typeofbasin ...... ,...... "."...... "." 13 III. Basic Sedimentological Data...... "H"...-"" 13 A. Main Tuff Marker-Beds Used to Divide the Formation into Units ..""H"."."" 13 B. Facies Distribution ...... ¥¥¥-----..."" 16 C. LateralVariationofThickness ...... "."H.....HH". 17 D. Trough-likeBasalErosion .,...... -¥---¥¥¥""". 21 E. Size and Constitution of Gravels...... ,..... ¥¥¥-¥-¥¥--..".. 21 F. Paleocurrent ...... ¥-¥¥¥¥--.....H 23 IV. Inferred Depositional Process ...... ,...... "."-.".""", 30 A. On thc Origin of Gravels...... ".".m.H."" 30 B. Depositional Patterns ofthe Representative Units ...... "".H"...m. 31 I. Depositional pattern ofthe Tk-Kr unit ...... ,.,...... "."".-.""." 31 2. Depositional pattern ofthe Hk-Km unit ...... "...."-" 34 3. Depositional pattern of the Sa-Nm unit ...... H".m.H.""". 34 C. Depositional ProÅëess ofthe Kiyosumi Forrnation ...... , "".-.-".."" 37
* Geological Survey ofJapan, Osaka OMce 2 Shuichi ToKuHAsHr
D. Depositional Model ofTurbidite Bed and Facies . . ` 40 ------. . V. Discussion ...... -,..H...... ,...... 45 ------4 . ------li --- . ------. . . . ` A. Comparison of the Depositional Process of the Kiyosumi Formation with that of the Submarine Fan Modei ....,...... ,...... 4Jr -+------d-- . +------. ------. , B. Early Stage ofFan Sedimentation ,...... ,,..,.. 47 ------. - . . . C. Further Problems ...... 48 ------. . VI. ConclusiveRemarks ...... ,...... ,...... ,...... 50 ' ----+------+--+-l . Acknowledgements ...... ,,...... ,...... ,...... ,..... 53 ------+-t . ------. . . References ...... ,...... ,..... 53 ------i------l----- . . . .
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 m in maximum thickness and more than 20 km in E-W direction aiong the folding axis and more than 6 km in N-S direction. The Kiyosumi Formation is divided into five units by means ofsix main tuff marker-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 pebbly sandstones and ending with thin siltstone-dominated alternations and by accompanying a large trough-like erosive morphology at the base attaining up to some 50 m in maximum depth and more than 5km in maximum width. The depositional tongue is characterized by extensive and thick sandstone-dominated alternations downcurrent of the channel cleposits and shows negligible basal erosion. 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-W trending restricted slope basin by the same process as that of the recent submarine fans. Channel deposits in each unit seem to have been deposited on the upper-fan segment. Depositional tongue in 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 or basin floor. However each depositional tongue in the Kiyosumi Formation, covering a wide area more than 20 km, occupys a much more extensive segment on a fan than suprafans on recent submarine fans. Individual thick sandstone beds also continue persistently as wide as the tongue. The channel deposits 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 "a pre-fan-sedimentation stage'' during which preexisting reliefs on the basin floor are smoothed and an equilibrium 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. Shift of the channel to the different site was caused by beginning of each second order upward thinning cycle commonly observed in the channel deposits or by the rejuvenation of turbidity currents. The causes of these multiple upward thinning cycles, characterizing the vertical variation of the flysch sequence 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 was applied 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-styled scientific 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 and flysch (KuENEN and HuBERT, 1964; DzuLyNsKi and WALToN, 1965; WALKER, 1973; etc.). , Modern flysch sedimentology landed in Japan in 1960's. Thereafter, many results 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; HARATA et al., 1967; KisHu SHiMANTo REsEARcH GRoup, 1968, 1970; TsuDA and NAGATA, 1969; TERAoKA, 1970; TANAKA, 1970). In 1970's, much more papers on fiysch sedimentology 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 correlate formations 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 and YAzAKi, 1958s MiTsuNAsHi et al., 1961; Boso PENiNsuLA REsEARcH CoLLABoRATiNG GRoup, 1965; NAKAJiMA, 1973, 1978; REsEARcH GRoup FoR NEoGENE TEcToNics oF THE KANTo DIsTRIcT, l977). On the other hand, HiRAyAMA and SuzuKi (1965, 1968) succeeded in tracing individual 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 basin geography, because little is known about them. Such important studies have been done 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 turbidite facies 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). As a result, the domain of the turbidite facies was remarkably expanded. The work on 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; GoRsLiNE and NELsoN, 1969; SHEpARD et al., 1969; NoRMARK and PipER, l969; NoRMARK, 1970; NELsoN et al., 1970; HANER, 1971; NoRMARK and PipER, 1972; NELsoN and KuLM, 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 basic shortcomings 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. Three Dimensional Analysis ofa Large Sandy-Flysch Body 5
In the second place, the present submarine fan model is based mostly on the surface 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 yet except 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-beds indicating 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 alternations attaining 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 and about 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 first proposed 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 the three representative units are analyzed and then the depositional process of the overall Kiyosumi Formation is inferred. At last the difference from the current submarine 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 from north to south, stretch to the Kanto region (KoiKE, 1957; IsHii, 1962; IcmKAwA et 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 sediments developed 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 basins containing Neogene and Quaternary sediments. The Kumano Trough, Muroto Trough, Tosa Terrace and Hyuga Terrace which are located in the inner part of the 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). The author 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 7
135' 14e' V 145' -4st
13O' oldidiPp
it' ! 4e' i]s}
E2el e 2e h
o + +N {p ++z.,.,[.,. . 'A + --- - 35' y .+c +-. f g ¥:¥; of 1+..
:- -- -- e BOSO . -; PENINSULA
30'
1Eg] 2ww 3[RMM 4:i¥::. sE!IE] 6[M 7EIII] sEiii
Fig. !. (}'eologic and geographic setting of the Boso Peninsula (modified from rvloGi and SATo, 1975). 1. Ryoke belt, 2. Sambagawa belt, 3. ChichibLt belt, 4. Shimanto belt, 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 the peninsula 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 and YAzAKi, 1958; MiTsuNAsHi, 1968; NAKAJiMA, 1973, 1978). In this paper the author
Shimosa :t Group AA UMA UNC. : . Sunami, Sanuki Fms Kasamori Fm e Naga ama, an ano ms e N onan m e e uz Kakinokidai Fm v a IchiJ'uku Fm e o = Kokumoto Fm coF o g e L ------e - mu Higashihigasa Fm Limegase Fm g at" ÅÄ- rd ca Otadai Fm = g :: m N p= as o ca Y Kiwada Fm g E e 'r. p= Kurotaki Fm ard m zo o ami ana Fm . vÅë e E e m 'r- u KUROTAKI UNC. atsuura m vÅë bjz e , ` , ca v enU o Hagyu Fm Anno Fm en H 5 = a- LF o =n Fm Kiyos umi o Inakozawa Fm rdU osc o ' Senhata Con lomerate Mb e --r- e uL SENHATAVNC. Amatsu Fm e < e3 tu < -z Kinone Fm ov - Kozuka Fm e = Okuzure Con lomerate e OKUZUREUNC. , Hota Group e
, ? ezo tu Mineoka Group e e oHU"v Fig. 2. Stratigraphic division of sequence in the middle and northern parts of the Boso Pe- ninsula (NAKAJiMA, 1978; partly modified). Width of the vertical lenticular forms in the right column means the relative degree ofdevelopment ofeach facies in the eastern halfofthe peninsula. Three Dimensional Analysis ofa Large Sandy-Flysch Body 9 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 few aspects are briefly explained here.
2. Stratigraphy In this area are developed the Oiigocene Mineoka Group, Oligo-Miocene Hota Group, 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 the Mineoka Group than the older ones. The Shimosa Group and most of the Kazusa Group are monoclinic. Other groups underlying them are more or less folded. Trend ofthese folds is about E-W. The degreeoffolding becomes more complicated in 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 Mineoka Group 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-grained sediments. 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 in the Kazusa Group. The other group is characterized by a large massive or well cross-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-type alternations. 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 OH R.Isumi
¥- Ky -¥¥ Am xKy.-,
trm ¥¥¥ z:: E, a. v. :fliK llll H :":i: tt'' R s Kamogawa s R.Kamo 'u $ e H
1.- 2.EIS O 5 10 PACIFC OCEAN Q O-pakm - Bww f-octs inttw VCorVto f-eglon Fig. 3. Geologic map in the middle part ofthe Boso Peninsula (GEoL. SuRv.JApAN, 1978; partly modified). Mn: MineokaGroup, Ht: HotaGroup, Kz: KozukaFormation, Kn: KinoneFormation, Am: AmatsuFormation, Ky: Kiyosumi Fotmation, In: Inakozawa Formation, Hg: Hagyu Formation, An: Anno Formation, Kzs: Kazusa Group, 1. Ultra-basic rock, 2. Basaltic rock. Three Dimensional Analysis ofa Large Sandy-Flysch Body 11
Okuzure and Senhata Conglomerates in the Awa Group as gravels (KoiKE, 1949; TAKAHAsHi, 1954). Therefore, the period of intrusion or eruption of these igneous rocks 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-W direction 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 in the surrounding areas during their deposition.
C. KiyosumiFormation 1. Generalremarks Sandstone-dominated alternations distributed around Mt. Kiyosumi and on the 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 with the 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 stretches for about 25 km in E-W direction and about 6 km in N-S direction from the east coast 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 the drainage 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 the Amatsu Formation or the Amatsu Mudstone attaining about 1,OOOm. The facies transition 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; KoiKE and 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) recognized a large upward thinning cycle throughout the Kiyosumi Formation. 12 Shuichi ToKuHAsHI
The Kiyosumi Formation is conformably overlain by the Anno Formation or the 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-dominated alternations (ToKuHAsHi and IwAw'AKi, 1975). This seems to indicate that the tendency 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 uplift zone on the southern side as the provenance of the Kiyosumi Formation. But, as already 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 planktonic foraminifera 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 the base of the Gilbert Epoch (BERGGREN, 1973). Therefore, most of the Kiyosumi Formation 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 the Kiyosumi Formation is estimated at about 85 cm per 1,OOO years. NiiTsuMA (1976) pointed out based on the paleomagnetic stratigraphy that the depositional rate become remarkably large since the start ofdeposition ofthe Kiyosumi Formation.
3. Depositionaldepth Since NATLAND and KuENEN (1951), benthonic foraminifera in the hemipelagic sediments 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 to lower 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 upper bathyal 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 are interpreted to be thickest where they are now exposed (KiKucHi, 1964; NARusE, 1968). Therefore, the basin of the Kiyosumi Formation was probably located at the 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 on the 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 be identified by means of the individual and characteristic tuff-beds or the combination of several associating tuff-beds. 'rhese tuff marker-beds are traceable over the wide area because of the remarkable consistency of their thickness and other features. Among them six main marker-beds are selected to divide the Kiyosumi Formation into 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 columnar sections along the type route are shown in Fig. 4 and Fig. 5, respectively. A geologic ]4 Shuichi ToKuHAsHI
Karneyama Station
t- '..¥, ----R.Obitsu t '¥ I: :,. tt .1 1 tt: t+tt ]a ---.+t o E L N o " -tt o m E J o gs R.5asa ' ¥, N '' rd goLLLt f' tttt ,me,, ...... ,.t"/..liSUjSuhriotaki ' x ttttttilL,iili/¥¥¥ - oo t..t t,.t ... l'l" 1`:')' 1":'= 'L:'"ge. i,'i{f,li ;. ;-..:-.-¥`r- .b Unconformity r:T i'''i''' tttt .:. t tt. - tttt.t tt . .-..t 'Yi'$':f' Zi6:.l.l¥ii',Siii.iii;.--ii/i-i.,/:¥2L,/l,,,¥ii !" 't'i' i¥9"! J.tt-ii/,hies" 1': .,' ov z'1.II:¥..:.,.i.-]... .. "t(i.,,.,:.l li.)`il.!.t'i f ll1:::.'e=::'''"rt'', Nt ttttt-- t ttt '"-t-. - ¥l;¥ Y 119''i.l'f,,l!lt-,.,:iii/-2i'dii/[2Z t- .'l Ll, 111.IV!titli, -t t-t tt t! "L ,i f21- l,s•il,,lll/11/l/l.'l;i•i,ÅÄ,iiiri.ti-ll'/g,, io ':'¥:i.¥,, -- i.' s i,'Ssii 'Q',/S'I'.'¥i/tt- ¥¥,-¥:¥ X 'N/iN,'. --- -st t-' .26 . ,"-';st;i Ok :-t"st--r. -- 4 'x XN,N. ",. .' ¥: ,,,':tr¥t Kr.Tk == 'I >}¥.: =2.0 . :¥x:..: ' tdttSt-T t- tt .} t xx xNF t-¥ '''' '''""N¥' '''' rX;' --.-=NL-"'''s' .:..,,...-= Km '1I¥ '''I:;:1"t'S '¥':1: '' ;-r",.t'rr.vt .,:,.,;.ts' ,:i Hk "X,, "X'T 'l'X' 'lg xi¥ ¥. -.: ¥¥ 'l'l 'i'[yi{¥ti.`}t , '.:fr':"T. i
v; :t 'V l-' Kamogawa 4. . .-.r Tot[ Road m' 5, s.;.t::{.:. t-t -.-?t-.- m Fig . 4. Localities of the main six tuff marker-beds along the type route (Kamogawa Tall Road and Sasa River). L Sandstone-dominated alternations (in Anno Formation), 2. San dstone-dominated alternations (in Kiyosumi Formation), 3. Siltstone-dominated alternations and massive siltstones, 4. Sandysiltstones, Siity sandstones.5. Threc Dimensional Analysis of a Large Sandy-Flysch Body 15
Sa Hk Tk Kr
tt. cm itj- ii 100 -- ttt -t-- 80 iT"-i
-//,1//,;-.:, ' -t 60 . 40 20 t--4- o --a- --a- "t't' tttt l¥n+¥ Sa ¥l¥-¥
ij "
ssss
tt"..l QzaC? Hk Tku
v Nm Km fi-- U 2 ee 30 4 ',t.¥{¥-;¥ 5 ew m i/ 6- I 7 ,iti,1,ti . i8pa 9 MflM] 10 [iil 11 ::t 12kil
Tk
Kr
Fig .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. 16 Shuichi ToKuHAsHI map of the upper Awa Group in the central part of the Boso Peninsula is shown in Fig. 6. The }owermost marker-bed Kr is positioned just below the boundary between the Kiyosumi and Amatsu Formations (see Figures 9A, B). The uppermost main tuff 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, Hk and Nm are respectively included in the thin siltstone-dominated alternations (less than 20m) in the Kiyosumi Formation. According to Ricci-LuccHi (1975), each unit may correspond to a thick turbidite megasequence or a turbidite subsuite. Where thick pebbly sandstones are developed near the base of the unit, each unit shows a typical upward thinning cycle. Sedimentological data on the Kiyosumi Formation are shown by dividing them into these five units.
B. FaciesDistribution
In the Kiyosumi Formation occur not only the sandstone-dominated alternations of 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 correspondence to 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"). ,. N N . ,¥ R.Kotto R. Obitsu R.Yoro
't: ---t i t t " 't r' . -' l t ` .i - B - r' j l. ., ' tN' R.Mtnato l
t
t / c A 7 p ; D N -tv t.x.:--, , .' NtalpL=v,U- L lt Mt Mlts eshl ' HLN e; +T-jTV t V- ' t i'-'y ts ' Av "+{L" tL M -fi tt '-".ts.r" {- ,'p+r .., NN' - '- s,t-.v-ft ,t, . )" . t-V" x -t t '.."t- T .". -. ThL-` . M vS l" .-?t. 1 --: - -.-L )tL -t tN!{itlryr rVt Si . X.tnt x ""A ,.,-Eg..H...' 7" ft,fi.ijR}ij"-".--tt T>J .l ¥t ki$t.y Jl ' sc XX V7--N-t- 7¥A N i -i L N- .=-"-sJ.=-.t.it.e,-".VTpa7,. SN x, >"N"Y" KKY "Ar.ts --- t vh fll'Y-: 'K ,-s-. .s'1 ys - -'.--A-ut. v./ "=lxilJr--VV7 .. N- t tLt ..' .t.N:tLH'.tt--=Vu,.7i.;.'t.:..`.,...' k N: . s. ; x ---vi f h- Vt.s SN. " M . r{l S esx 'N .N. AF te-- "- --+ K,- Y. N.- . l. - S-A x. st --- . Y-h.- `:t 's rh . 's, K'b.." ' . v fi" ,,nr N '".N rs.N.v "-. , in,N "I KI . -." Jt: k Ix, gN '"`ts-asL sv.-N Vr "N..N. it x¥ x,. ¥ ts N thk" / ti- N - v t ;- L :s' Mt K 9S'um/ A' : erA'i'A-":n.. N SSN L- '. y. J It 4- N-t 'N. .A4t 7L.. E:Oom k -) ti '" INA ;tAti tt 'd? Vpt'f S.L L =, - iS. ;N .Ss 'c ' NN " .e A' )F ;=Ft "eJ h VS- Tt kecA " t IJ "J 1 -- {f L/ ;: r.". N. i . B' k .tu 7" :Af--s" 11i ,,, "-l w- r +.s ,N . D' t aTt.-c s ys -et'i'"' }l¥i!;.¥kt t. Eg)o- it "tg"f"' - : ti,- - t,tr7.' ¥ ge Tf tt -e7i bF !!-.T...`Å} )s /¥ -i s- -r4-1 B c'
-N.e:-"s'-Efft"H.,X,".N')L r3oorn R.Kamo +- c-' - -tr- :- ",.rr.i`, .'"LL: - -`"st cEo x
-J -El -; - + +""I- ..tu71atr7t.. R.Obitsu
"- it 1- -- - -N .+ ";N RYo ro ba--- -vv-- ;u sl ..ei 'V i.N Egcom R.Koito RJsumt D-' -1 c--- D iD d--- R.Antn e-v- o 1 2 3 4 5 km 2a f --'- h== ,i. g¥¥¥ . k 3D R.Kam 4[] h-t-'-' uskm e . 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, 4. Sandy 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. Three Dimensional Analysis of a Large Sandy-Flysch Body 17
Table 1. Classification of facies.
Facies Definition of Facies Walker & Mutti Pebbly sandstones (1973) Amalgamation ofpebble conglornerates and massive a sandstones with no interbedded siltstones A3, A4 Frequent occurrence of outsize siltstone-clasts Sandstone-dorninated alternationes (Sandy flysch) b sandstone siltstone ratio, sd/st>5 Massive or amalgamated sandstones bO with few interbedded siltstones
bl Modal thickness ofsandstone beds, Mo.>2 m B
b2 O.5 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 is extremely thin, are shown in Fig. 9A. In Fig. 9B are shown the columnar sections near 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 18 Shuichi ToKuHAsHI Sa-Nm ,-' '11..' ;t" 3 ¥/1 ? i :'J.. ' ,.. :,i Nrn-Hk ."' ./"" . KX 6 tt tttt i :t./ i X()r -E ... ua....:. //' r¥...' . H k-- Km x ge d ¥,¥ b L,. ` if "r,' Km-Tk ¥¥ -,"' /1/ttt tvttttt 5," ,i ¥¥' 2 12 . N .2 bl '', /4'i' b2¥.,. ' .... , t"' " ttt tt// t . Tk-Kr xX ¥ts 3 I,, 2 'ts,., ` .E:¥ . ? ` aa'' 1 ¥/ ¥aa 6ibe v-" .,.. ) ' . O2468 10 Fig. 7. Areal variation offacies ofeach unit. For description of symbols see Table 1. Kiyosumi Formation, but the overlying Km-Tk unit occupies the lowermost part of 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 Three Dimensional Analysis of a Large Sandy-Flysch Body 19 Sa 23, 260 ') ' lt t. ' ll'lllll.. 'II//,. e :/l,l:I/I/I¥ -,1 g x tL %.-., ,il{11' ", 's 'n45 .ritst .e65 .5' N 7'¥' G¥ ' r ' A¥ fi s' ,¥S ?¥ x lkJA lx` N ' ¥i ' .liiil >E "1 ' x ii,''''"' l.'.'.,.,"l,ll',ll'i,l,liii..¥ ,,¥,iiillii'i¥liilili¥I¥i¥'¥l/,l,.iiii.I¥ii¥k,,i,l,'il,,,.,''¥¥¥,",,"'''lil.,,,,.,,.,. Nm-Hk -' -¥¥-' ¥- 5 ll:' i'/..,''''''"::':':'¥i'i'"¥..`''5?,, "'::':'"i":'"¥-.,..g,o,¥(';. tttt .,.. i '' It ".''"'"i' : ' ' ' tt'' '''¥¥-ill.l':')i.--¥¥-';::.¥--....i¥'''`¥""" ttt tttt tt ',r:ii""i'"' /t "" ' tttttt Hk--Km 4o ,, ". xx ¥) oo . tttt tt ttttttt /t ttttttttttttttt tt.t ./t ttt tttt tt ./it ttttttt 1.../ t/ttttttt- t . tt..I= ttt ttt.t :t":-ttt .t....-:t....tt.t,tt.... ttttt. ./ / 'L..t't'':tt '[t't t.t'' tt't't'tt - ... Km-Tk fs ("s ,ioo.,liKki, . /tL 1. ¥::::::::' Tk -Kr f"aj' 3',,) ca K"ilN.. 2. i[illlllllll¥I (Tk-Mlt-Kt} 3. - ' 4oom .,:ili 3oo -- :i:I::. 200 I:}Iii ioo ':'1': o ¥ . -''''"'¥s'i¥¥11''''''1.-""---"'"""''''']'.i...,¥¥""' ,-.'¥,..¥' o ua246 km 8 10 Fig. 8. Areal variation of thickness and rnain facies ofeach unit. 1. Pebbly sandstones, 2. Sandstone-dominated alternations, 3. Siltstone-dominated alternations and massive sikstones. process of the Kiyosumi Formation (ToKuHASHI, 1976a, b). All units are thicker in facies "b" group (sandstone-dominated alternations) 20 Shuichi ToKuHASHI m a b c f j 10 Go rv i k 8 g t 6 d Go 4 i t-Jt h l e l Mit :i Go 9m 2 :, ! YKr Kr Kr o Kr Kr Kr Ma Ma Ma kxOk `i.. t,rL.¥ E"¥-.. --. t-/tt K' ' . i I : a 1¥l¥¥ ¥ -l: ¥- :¥ ¥: t vX 4- .: :--- , : 400m 1' r " i' '¥ ,¥¥1/ I¥:':' 2oo 'j tt j""' ;/ -t -" I ' 'o - tt ttt /t t t - -. - 4 O2468 10 Fig. 9B. Columnar sections near the base ofthe Kiyosumi Formation (Partth II) km . These sections are in the area where "1'k-Kr unit is composed ot' sandstone-dominated alternations or pebbly sandstones and is remark a- bly thjck. Locality of each coiumn is shown jn the 1 ower figure. 1?or 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 m 20 1 b c f h l j k l Ii:.l13 mn:'i' p r tt tt A ';:: tt tt - l:l';' 1i'i ¥1¥': :tt'r; d ;':i':' ¥I: e ;il..t ';i':'t- A :,:: nt) 18 f::" 'ii ¥J.:1 ;' .I. ';v:, tt :. :. I:1: g . ttt Dal `::I tt f u ¥1¥.¥ tt. t tt :tt 16 San Oo ' 1ii ¥l¥11.,T :"t'ee"tt t"/, /it:..lj¥¥¥ '.i'x Tk 't; --. t ii,lll t-tt t r, :.. t'"' tt ':i Hk lt-lll 14 ;' llll: ku ' ttt :¥:.I ttttt ' ttt t .t i::: 't Bmt. : ligrk "1'k 12 :-tt 'i'i :`t" Mtt ;:g t:: l¥l:¥l. ' ¥liOo s.s bo -. oo ':'t rfrl: :,:, . ;[.;.., ¥¥- ;;¥ =?;:'t;-tt' -t- 10 - . "i: Km iil'I::tt Tk' l/ ¥Ls¥.:: bo Mit :s:';i -t :t 4-- Mit I:.: Go t- :: :,:: ::. Sb- *5m t- 8 mat 4-4 v i4 s-" ,:; u . Mit a ' tt :tt:t -: l-4 Mit bo Tk tt i ii Bm -t- 6 Kr Kr Oo Go Tk th .:-:. 4 Mit -4- o Go Ht 2 Mit {r¥:. :"t:: bo v Kr Kr Kr Kr hr o Ht Kr --"" 4'. :,..';¥-¥'- ,. . 1',. lit v ,t 1. EIS] 6.ggg 11. rw . r 7. Eiffg i'i': 12. [!E] :. - -- t ttl- 3.zo Ege 8. Etw 13. {Zi] . L... L --l ¥.. 400m e¥i¥i ¥1, '"'"" IE¥i¥l' 2oo 4.N 9. I[ilM 14. as 't'' ¥IP ;::' o 10. Rww 15. Nl - ''t':' '' '':"' ' ' ..J 5.- km O2468 10 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 and scoria 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. Very fine grey tuff, 11. Very fine pink and pinkish tuff, 12. Gravels of hard basement rocks, 13. Scoria grains, l4. Pumice grains, 15. Shell fragments. Three Dimensional Analysis ofa Large Sandy-Flysch Body 21 1 :, $ " ' > ' i l-., tJ/ Ll- t; :,/' , tttl ( r' k¥l K l: -jt- :t --tJt t't -t-'tt' ttvtt ' . 40 :.:: :.: 20 .l.,,l .ill -;-- - J... t -- '.220 ' 20 . -;--:: -- t---: O. t qo t' '"' :" --}---- -:-t- 'i i M.-"- :-:-:-: . I:s .------x t -e-- ---:::-i-i -- l¥ ¥.:i li-- r.. f ei .. ... ".-,...... :JS' .n s - t+'-tt-t-i' .. -.,¥- ...... i-"" t" ...r' 8eOm L¥ !llll' 4oo "" o O2468 10 w km Fig. 10. Areal variation of total thickness of five units between Sa and Kr marker-beds. Dotted part means facies composed mainly of sandstone-dominated alternations and 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 are more 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 uppermost part 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. t .tt. t ttt E. Size andttt Constitution t tt ofttt Gravels tt 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 Shuichi ToKul-IAsHI Sa-Nm ,,=¥ .. f/l,/lil"'''1''"''t''''''"''t,t/¥¥ll,l¥ 'x '1 x ' ¥'' 4 i " i' 'i" :i " :t:t- tt ttttt tt/ttttt- -/. tt /-Jlt--ttt't'ttttttth.t ttttttttttt-t.t..ittt.'- ttt ..... tt / ttt'ht .'i:'t'"H'' "/H .....t:'t''' ', ¥//.¥¥ 1¥ . Nm-Hk ,/' /' ,,iiii"""''"""'"""'L'¥iiiiii g KX tt t- li, . 2K i'' i4'" ':¥¥iii ''v tt' t...¥¥-¥' ' -¥ ., --¥¥.... .¥-¥...... J;-¥.,. tt / ''jl''"' ' t tt .ttt..t tt t.t ttt .t/t 11t t. tt x tt tt ttt ttttt.ttt tt ttt.t i ¥¥-.I/¥.. Hk-Km "-" ¥"''' "' t/tltttt.tttt tttttt ttttt.t xR t'H't' ''t" tt 'ttt - ,s : ttit !:,- i 'it .::i tt tttt:t tttttt l ti:" 'tl'.. :I"'"'""' tt] Km-Tk ,,¥¥' .."-¥"'h" ':. ¥- tt" K tttt. ttt 4 x : , ,],/t, ?fi¥ tt /1. i ¥i tttt /tt i tt..,. .,.....,....,...... ,.....tt...,...,..t ttt.ttt tt ttt t tttttttt 'ttttttt ltlttt'tttttt t t/ f` Tk-Kr ,¥¥' .¥ "' KX N ('i'r¥ ' "'i ' '' -/ '''"'i 4 '"''il'iil'11iis¥¥¥-¥¥' i IO,. tt ttt t .-"¥¥¥¥¥¥./-.¥.¥ -...:¥¥¥. ,.- ¥¥.¥¥,¥ . ' '' 0246S IO ( L==tisi==f km Fig. 11. Trough-like ero:ive morphology at the base ofeach unit. Each number means a estimated value maximum of the thickness of eroded succession. of the gravels observed at each outcrop. Most gravels observed are iess than a few centimeters in diameter. But inuppermost the Sa-Nm unit, larger gravels occur in places, Three Dimensional Analysis ofa Large Sandy-Flysch Body 23 ' ,. q'" ii E n¥- .:,. ;, ,= .( 2km 's T :.."S' .:t: -.. .t ...... t Ol -l,. ";i: ¥¥¥,..,..!$,,,b,,, "-¥iiV,,. -t:tttltt ;t., .xt': ¥. ..l, gi. ., . :ttt 61.E¥ C.'tt'.V. =/':i.:. '4, . 'l:¥ ' ' i' G., :s.."--.' L HJ' .' '"¥¥¥1',.¥¥ o 2 4 e s 10km b== i=in==U==ti=Å}==ti m -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.-,..... 50 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 ,,,, o 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 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.... """"""'`i-A'-'"Mma'A'"AA"', ' R" stnr"' .:,. sbr'''"''"''"'-"J'"M''"''''"" -`' -': l- iiil l/ I :, : 11 iilli l' lillvl i¥ ll¥-lllilEil 'I ' " """"''"'" O-kin Ok ----.---¥---¥-¥--¥¥------¥--.- ¥:::::::::¥¥¥ ¥. ""¥"". ..-.,. ."."Hv"," 50 Obdyvvvvvvvvvvvtvvvyyv"vtv yvvvvvvtiv vv vvvv vvvvvtv-Tvv tyvvvvvvvvv vvvv vvvtvvvvvv Fig. A12. A profile BC of the largest trough-likeDEF erosive morphology G filled HI with the largest pebbly sandstone body of the lowermost Tk-Kr unit. Constltutlon ' ' of these gravels is shown in Fig. 14 and Table2. Chert and sandstone are predominant. Rhyolite, andesite, hornfels and quartz rock are commonly observcd. Crystalline schist is also commonly observed except for out- crops at locality ba, and c which belong to the uppermost Sa-Nm unit and include the larger gravels. In addition, shale, granitic rock and meta-quartzite are ob- served. These gravels is generally rounded to well rounded. However, only gravels of the crystalline schists show edged and discal forms although some are rounded or well rounded. Data on sphericity, shape and roundness of the gravels (larger than 3.2 cm) at the locality a in Fig. 14 are shown in Fig. 15 according to KRuMBEiN (1941). F. Paleocurrent In the case of turbidite sandstones, the orientation and sense of transporting currents can be measured by means of the external and internal sedimentary struc- 24 Shuichi ToKuHASHI llililiii>ti,rsEiiill¥lg Sa-Nm tttttt L 3i"/x t tt.t' t t:ttt Ee tiF-iSlii t9M,x' c5 ¥/¥1.¥¥ - x.,.¥i¥¥- rt.tt y " 't' .YY ' 'c Nm-Hk -"' {-tt il kXx lptfb H x b. tt 1/ ¥1' 1 ' k,r ,' }/ i `x,tllxt, Hk-Km .¥i' 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. Mean va!ue ofdiameters of the larger ten gravels among them. tures. Among these data, those which at Ieast indicate definite current orientation are summarized in Fig. 16A. They are obtained from sole markings, preferred Three Dimensional Analysis ofa Large Sandy-Flysch Body 25 Table 2. Basic data on constitution ofgravels in the Kiyosumi Formation 1 2 3 4 5 Ch ss Sh Rh GR An Hf QR MQ CS a Sa-Nm 32-64 l35 31 55 24 4 6 o 3 6 2 o o a Sa-Nm t8 260 115 47 9 3 7 o 29 2 3 o o b Sa-Nm 32-64 84 41 43 21 o 17 o 13 6 o o o c Sa-Nm 32-64 82 34 50 15 o 28 o 2 5 o o o c Sa-Nm l8 345 l30 60 25 2 7 o 2 2 2 o o d Sa-Nm -8 4tlO 354 55 11 1 7 3 6 3 8 2 4 e HK-Km 2-4 301 301 47 15 3 7 1 7 3 9 2 5 f Km-TK 4-8 329 260 49 15 2 7 1 2 1 ll 3 9 g Tk-Kr 4-8 79 54 49 l8 4 4 1 5 3 5 1 IO h Tk-Kr 4-8 378 187 52 26 2 4 o 2 2 9 o 3 i Tk-Kr 4-8 461 209 52 25 3 3 o 2 1 IO o 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 revealed by 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 work and field observation that the preferred orientation of wood fragments generally included in the upper part of the sandstone beds approximately coincide with the direction of currents and the orientation ofsole markings (NAKAJiMA, 1977). Data which indicate the definite sense but broad orientation of currents are summarized 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 examined and used to clarify the depositional process ofit in the next chapter. 26 Shuichi ToKuHAsHI Sa-Nm /i' l'i¥ tf"" "'' f"' r l¥ i crW . s: .,i 1 "\ .L'I:, ...,. Lt----t.--tt.t-ttt..nyttt-tt.- t-- ttttttlt Nrn-Hk ,s" !it t""t'tt'tt'"'t'"'"' L¥¥--¥.- ¥--¥ k " .l,.,. x K ` i¥1'¥ ll' . I¥: . ::-¥i /L :':'t .".....s lttttttt r.t -t."L-' "'""'v :tt-t."p---4 i Hk-Km /t trtt ttt tt LXX -t.ttt.t e ((E}) ` ... i' ,a i . t-ttttt ,I¥ i,. ':''h S 1 ,¥ ;,- 'i]" "'; ,. :: t ..ttt. .1/ ({E}) : r-. ''' ' ttt]tttttt t/tt .- ...i .. -1.tJ tt tt tJtttttt /Lt 11 ttt t.t t-tt-t ttt'-t-- .. :" t-tt'ttt--jt 1- " . '-.k.. -'' ({ilii) Km-lk =th.. z.. :' t't ' kX tt tl t'"'t 'i -.t:.-tt t "k : '-" s,. :ttt /,, i ,g;, ,L :/ -t-ttt [¥1. . '- t' -. I i.- [/ -Jl ,s... 'i.. :-..:L'-" 2. .:.. n. L/" -tt. .- 3. Etw "t .ktttt -t.' 'II:"¥. l¥' "' tt i t t E ..t.-ttFt...K, - 4. ewg :, x. ., "t ,, / Tk-Kr .' t n, .' s, E##gg X'L"' // xX .s ¥( E.. 6. EeefiY t ` ' I] 't' . 8.zN ew II,,., - g :'t:'t' , .,:::: 't"' "l .-r g. N ---" . . L-"' --tt-- :J: tt :/ tt L"" !.' n- i ttt s ro s 0246 km Fig. 14. Constitution ofgravels included in each unit . 1. Chert, 2. Sandstone, 3. Shale, 4. Rhyolite, 5. Granitic rock, 6. Andesite, 7. Hornfels, 8. Quartz rock (meta-chert and vein quartz), 9. Meta-quartzite, IO. Crystalline schist. The constitution is analyzed for gravels of2-4, 4-8 and 32-64 mm in diameter . Three Dimensional Analysis ofa L arge Sandy-Flysch Body 27 ROUNDNESS SPHERICITY SHAPE 35 1 25 1 22 20 CHERT 58 14 12 7 4 6S 63 24 3 2 SANDSTONE 26 12 9 B9 slO 7 31 -- li M [kL- [D==b RHYOLITE 13 [lti[2L[s4 .s2-ni.Eun32 6 11333 11 ANDESITE 2 2 2 2 HORNFELS 6 42 33 --..ll.-rrri=== 11112 54 37 3335 34 TOTAL1055 23 2021 10 9 8 6 8 8 1 3 2 3 O.? O,3 O.4 O.5 Q6 O.7 O,6 ,50 .55 ,60 65 .70 .75 BO .B5 r ll III N Fig .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 .+ 'z, / .. bY. 1-.. .. Ekl:,) y A5k -Slx . L /, tt t tt r' i tti / / 1. th /¥1 ttttt i Nrn-H k 4 r"' xY .t ) ' / '',N Y h., . ¥5 '/ 'b, llif tw¥ ttti t' ' /1 ¥:¥ @ ¥--¥-;- l x ttt e ttt t / .TJ..'t"''' '' '/='"''" 5tsJ Hk- Km 'twfttut9sR. Xl ..vs K ' vi-..,1 'f ,s 1;i .// tt/ ;, t.ttt t t ., B 'L¥, i tt 'sS'i., -.,,.. Km-"Tk /ttt.tt ttttt t.tt ttttt i' " ttt tttttt xX .t tttt t K tt tt ttt.. i.. eq( 't' "'' A VI . ii. . e i E(gi¥I?lfi' i¥v. 1 ' ttttt ttt t -tt. ( ;t ...t.:t".... /, tt tt . th Tk - Kr 2, q ¥te 3. Q ,rwv//i N Rl¥K, ' 's 4. N g 1. / -f"DNe i rf. ?g 11.. . . 1 '-r ;:r' 1 ` /tt ttt tt tt tttt tt ttttt :., oth246810km Fig. 16A. Paleocurrent directions ' measured by some sedimentary structures indicating distinct orientation. tt 1. Sole marking (Sense is unknown.), 2. Soie marking (Sense is known.), 3. Preferred orientation of wood fragments, 4. Mean orientation of strikes of walls 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 were measured. Three Dimensional Analy sis' ofa Large Sandy-Flysch Body 29 Sa-Nm ,,"" K' ¥.¥ .. ¥..' '''"' '"""¥ 'g,i tt/ t[ttt 1 i ¥ 1 ,,.g lliiicit¥""¥¥¥/,i/l. tt ' :' l' . ' gf ttt ttttt ttttttt ttttttttt v'-' tt Nm-Hk /¥...r'¥i,...... ¥x..,. :.i k t' tt rt LJA".S, 2x . z + 'e-i 't"'t' 'ttttii ji ,, ll ....y " " .tt.."'.. '''' .:.'z''..... ' ` ttttttttlt-tt Hk--Km x 1: i¥- gscg 1 & t -ttt ....LL.' ... ,r' .'l:....t. .,...... :-' ,...t L- t-JJ- II tt.t . Km-lk i" k ,r' .,- i ML,,.) 'ii ,tr . ttt i tttvtt trtltt t- tt-tttttlt tttttt.tt tttt ` .' s.. Tk-¥Kr ttt..tltt:ttt t Ttt.,' tt tttttt tttt.ttttlttl t.. t/ttt tit ttttttttt ttt tt't' t' .1""'" ' /,. ` . i¥X '}' i t t,tt- : ' . ,1,l .,¥ 2.8 v '"'¥-- -¥¥..-¥',. . ' ttt ' / tt tt ' ttt " 02468 " km 10 Fig. l6B. Paleocurrent directions measured by some sedimentary structures indicating distinct sense but broad orientation. I. Current ripple cross lamination. The value in each circle means number ofsandstone beds whose cross laminations were rneasured. 2. Imbrication oflarge siltstone clasts mainly in the pebbly sandstone facies. One datum per one locality. 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 southern side, 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 which are 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 the Neogene 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 the Sambagawa 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 the aggregates 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. Three Dimensional Analysis ofa Large Sandy-Flysch Body 31 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 unit are 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 and more 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 tuff marker-beds in the middle part of the Tk-Kr unit. Lateral change of thickness and facies 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 in Fig. 19. This figure shows that several sandstone beds observed along the north row (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 their thickness 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 direction toward west and deposited thick-bedded sandstones of the lowermost Tk-Kr unit on the southern side of the syncline. Therefore, it is most reasonable to conclude that the thick pebbly sandstones fi11ing the trough are channel deposits. This conclusion coincides well with WALKER and MuTTi (1973) in interpretation of their facies 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 32 Shuichi ToKuHAsHi xX 7 3 ;35 n ¥,s A Mit . --i:--t. ,¥, /.. ,fVN---S 5 w" =. 35 s xX 9 3 e3 2 B . . 2sbtw iwn a "nL---S aa bl it > fiA. x)V N x c . . . ` xX . D fr-pt jfr,sifipm'xi(/S . ,fLv'xv---z wx . E j IYIiYwo-cli, . g'Vx---, -:,:,. .: t ,;:iJl.., kill:: ". :' -tttt-."t - - -..-. -tt ;'e' -.s":"rt'-- 2 ':.:'.":::::', -- - ... x. . -tt- -ttt-tt lt - 't'- --s '- F .t,,, ,'.'.¥"'1¥'"r. e .'",:::¥ .'.'".:.:::::::¥1:'i: ',',s ,t"'" , "' ¥-¥ -.... .ti,gi:. '-..., 'iU;¥;¥ ¥-¥¥LV¥'i;:;:;:,:. I,¥i,'¥:¥I[ilil ,¥,.I¥I¥ , 'ia:'.:¥'..q'. ¥¥:I¥::¥:¥'':':i:li" 'v ¥:;:,¥: :I¥:':.I .¥ t::' :", ".:;'.,:ii'::::::,i: ' th -;tttt tt i t ttttt -----t--t-t------t Owt46810k. ""`i'i:i:¥,,t/i.:':¥1,i,:'ii,::1{ii:f;,I¥)lillll:¥L/L'¥' t...... -,. ttt-tt----t-tt--t-----t- tttt t Fig. I7. Summary of some basic sedimentological data on the Tk-Kr unit (A-E) and an inferred depositional pattern ofthe unit (F). For explanation see the text. a b c d e f g h i ] k iiiiil Mit = :::¥ -..L.- Hs-- -.- vi}- t"' - l m - 444 Å~ :i':' 1 - Go - N 20 -N s --- s --4t x------i"at sxxlZt"4 :=:: -' 2 iti - N y N i- s N 4. s n N {m N N '""t ym-ta -Å~ s N N 18 N t N o p q r s u x v W x .E-¥.I Mit it Åí./•.. OEt.t.. - rtt: -- : ;':'i' TiS"- N tt tt I:'r--- tt './[/-Etl'.i'i ::: - :- --- u"il i:" r.:.t .{¥¥l,tl 'r', 1 rl::' 'i'=" 1..).. 1 Go t';"'t 1'r:¥'- . :t ;:. tt tt tt t ' :1 16 ' tt:' .: '' tt va ttt - -li ' ' . --:. ' IL -.i.s : -:t- ttt ' .' ' tt ' N ' ::: ', ,:. Go tt.:.' t:- ;:'- 2 N . 1¥¥: ':t '¥' :,:'Jl "tt EIZ' "t .: 3 tt '' t:t X, 3 t ttt N2 x .: : :.-.r t:tt tt J tt .. J:I tt .:' . NN .: : ' ,, ' ttt :t " ttt tt :. ' tt ' . ,:(¥ '' :tt : t. 4 NN ' '' ,.. :, '' ' ,:1'¥ tt 4-l:' l. t : I:: tt ' 14 tt :.?. ttt ' . .1:.' ::tt -i: ':::- :,¥: ,: 1 ..: . ;tttt :: 'il't- 1¥' .: I¥',ttttt tt tt- :- ::. ¥1¥: ¥,¥ ttt I" :.. t ::1 5 ttt tt .:.: ¥" -- ::1: :¥, J f '' i'i'i' ttt ¥Jil1¥' :"tt- tt //i'ii/! tt /ti' 12 I¥¥I i/"'/ - /x,'i'/,/ ttt t-t t: ::¥.¥ '::. /¥,// :.:,: ;-::, 1' l,::' .tZ-: ¥.1:¥ :::.1 :¥ :¥ --- -t ::¥:¥::'1::t.-.:::. ;,: --: .I..+ 8 10 --- ''..: .:.: :-:- i:--- -.t --Ji -. .:4 t--;: ;¥.1 :::,- i:` ----- ;:. ' . :;tl::'J¥;.!;':tl::r.' .. "t --., : ' --:-- :,;.:1 -- 12 i-jl ' t::t:t :1: . . ';-521g- -:. ttt tt t--- -lt, .- '' ---i--a--tit-- ::;ttt-t- t/ .¥ p,T13 -.-.- :.:: z:14/ .-:;.:J.-ri--tt-tttt-'t'--t--i---.--t -- :?'tttt /10 --- ,. l¥//¥ 8 1¥.: ¥;. ' e¥:¥ttt:ttt 1¥.:.: i:-- ;.-:-, II)hs"--s. x . ' --t ' 1:'. tt :tt tt .:I ,i.f''''"'ttv-.-..s 4ii-i I¥jI' .:..: . ll. : t'--- t i , 1 -:- L tt- xN -- tt : 11 1,t.i t { ttt -:tt -' tt i¥1,1' 6 ' ,1:: :-tt 1 1 a, g -;- -tt ' . ' 'f 'p ".x .# '. -t . ¥'t i t-j --/ '' :<¥:¥. t- n , J:: li , - q,/r JJ f " U':.-A 1 ,l.-.; . -. 12 .1¥ t'.''N .''tj :¥ L: s'¥ r A:. ,' 1] -. t--l1 : '' - .. tt '-" VtW. r¥,.-= .t.-...st '. td :--- ;, ,z¥ -tt "'-'.. ¥..--I L x tl:-t 13 't- -.Ji . ., x ' t.- SE tlr . 'esti /7 {- ,i`" :¥ 14 :Ll I,¥.1 :I ::II 'J.:tJL--t.------.-:- ,,,, >,,K' g.g,g" -+ . 4 i;t- 3,, XX :-tt- t :l.. -:-- ' :-l . ' 2j -:: , ': :.:,',I.: ' :. O,6 < .5 :.. - t , .7 ' j -t ttlL,-''L 14¥ lt.lsi5':'1:¥::li::'I: Iif¥1¥¥l: ¥"it .i'sj( 17. 9. ':.' ',i.l ".. :, ,.' r¥ IL8') :-s"'i ", -t .M'¥lt-L"G6>'<(NG -t -.,. - !t'-- i- ./ 3,4 o.s 2a m -t- --d- -- -;J Jt L-l--l :':: lao 2 : i: o .--- tt 4futt' /i"' i7i 'X4 , .- -- --Il t. rs8..9 " --i -:it':i .77 -.- xX ;.-tt+.tt- tt--' 8. ..,...,2.,7..5 3 1 s., ' 1,: . : -J tth :"i" ' 1 ', ,:,.¥:¥ 1,¥,Mi{:i ,, ,. 3 ' : . t+ :30 '' 135:. ':::.:,,l,/.::i,',f.:.¥ .,,1so¥, J: --- ' ';'t"':;""--t .- ' . . 't' i:u ... - ttht-t ,r :.: :, : ss. Ji--J :-.- 1' 320 -t tti -. li-J u' '--tTt+-i-e-t ti:itc : j; 215 t --II t: . '(' o (Tk-Kr) -ny L ------tj .i' ..i ------s-- :t-t -it t .,` 400m 6 024 8 10 ::.: 200 ua km l¥:¥1 i-s o Fig. 18. Columnar sections between the Mit and Go marker-beds in the middle part of the Tk-Kr unit. Numbers beside the columns designate areally correlated individual sandstone beds. For legend see Fig. 9A. 1 xx ,X"il}iXX . 8- o o¥ ,/ -:il¥:.i!.. ¥O o . l' O ,.N t¥ " . 1' ' . X-.'V`' . s ¥, (ww .r'?IIRg¥- "¥' i9(rw s'i. 'trt-- -"vi- paNde-. i¥ 2 9 ask' Ol ,xyxx .2 o. o ¥i,o . i'o i¥i >o io -,9., 1 , c:¥ , ,/. i , H s' .2 .""' ':. '. ;' el"\ -. <'x..rhv;LL tt g . ¥'¥.- W"\ s;' :-'-""'' -(rigk"¥tx-. ¥. l itt.t g 3. xk to ..... / (1ts o, o g. o, ,o iS g : ,,- oo WX 8. .- . .¥ . . ' g vetw 11" w"\ e ¥-¥ (nc po- "'" ''K j' l- (""s ;¥ 4 11 . o gslg) oi o o. o iY 1 ,,xvx s o ' 'f' o o E. . > ./,. I e g . 4y"r? ¥,. j ¥. (#\ 'o s -pt ev" j s 12 .'¥- gskh ,- es 5 ' --. o¥iOo o,XYI .sp, xX o 'IO . oo 8 -tet)l s ` B (1'(tiN] I,1 .qtw i .? ,li ¥¥ tv- s ekvy. j'- eM .p 6 13 ".. --- , - o oo 5Yl x o o,,XYX tr o . co i' ¥' l¥ o .. o o ]ti`ilg)ziii 4 6`' O.. 2 pt xv(2¥riggl)ll .- wv)?.¥¥, iV K ' 'ttj 7 poo ,X YX .' D 14 i .¥.. ,, &YX o¥ o o -. ,i'o t:, ¥l'o o ;'o ' . . r "- t-t (bl. .... ,l- W"N] ,,,, ¥' ¥"" tr' j: e'k-x.ti' S-, .,1 ''; ':- i E Ouat46sro Fig. 19. Geometry of individual sandstone beds between the Mit and Go marker-beds. ifx, km 8 34 Shuichi ToKuHAsHI 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 of the 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 the case 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. It is 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 have played the most important role in supplying the turbidite sediments ofthe underlying Three Dimensional Analysis of a Large Sandy-Flysch Body 35 Xo o "v, 'i2iO 60 40 .i4()// ep- .5 t.. A .: oe . / .- ` :e ,se-, .. WS>""li e-xhS 5 xX ,,tli il iilllttIIO bl iilikll" 2.,,]ct,}2,2yt.(}"" B X7 Kb2'" . t e {- dl pt 1 xX ` 5S c . j twA(!"v (;nx-.-"V.-. jge,Jiil)agTitl!]X.,ii t xX . 7 D "fg, . ('VLx.-- gi,c.>XBI ) yx ` E . & CV'"--x '`i .R " ii .tr¥. ,:.,. -=---= ..-.,.- d ..,fl, ::ii,'11¥1:.i.:i¥¥ .:..l.'i tttt ''::Il t :i: ..t- :':'til:, :t... F t '' '' '='""'"'' ''''t :''':ÅÄ:e- ,t.,:¥ll¥I¥ :i:¥:i::i¥,il¥i¥¥l¥:,til.' - .. =,.. l¥''il:':i/,lil¥il¥I¥:. =:. ,.t ¥':ii.I.ii:':l¥tttt,.. .. .'.,....ttt ¥i:..,..=..=p t tt"'{i'I'l'l'lil,'i'i' i'I"l'' I '.l,51' ..,''L"'' "Xi'il'' ¥¥c d2 -J -t O2468 10 ua km Fig. 20. Summary of some basic sedimentological data on the Hk-Km unit (A-E) and an inferred depositional pattern ofthe unit (F). For explanation see the text. 36 Shuichi ToKuHAsHr , ss&.i2gsscr(,ged,`igl("t .i. A oo . f itt>//ag'/Plj1tige¥"., d3 1 B yi?i' ` tttl¥pa?'til"Cee3scts,Xllllk c . { ' (tr,S> xsggx- t t/tttt '" t.ll'll"'L';" ,iiiiiibl 's"'¥ . t ttttt ih /tsL-L-x R .-.,i¥l'llii ' D .A ililfx[(¥u"> k sv7t OV-X-E s< . X 'hv , ts .lll,N " > " . Ar" N k .-. I x E . t -;""-L- f b s ` 'it'A ,'' '; V -'t "' t'-- t' 1''- F ot246patc iok. bi'`'i'i:izik:iii:',tii¥,.:.i//f;.fi.:,;'i':J.ii,if.ii:;,:',/'//i'i::i':¥::.:';;"' ' Fig, 21. Summary ofsome basic sedimentological data on the Sa-Nm unit (A-E) and an inferred depositional pattern ofthe unit (F). For explanation see the text. Three Dimensional Analysis ofa Large Sandy-Flysch Body 37 units further regressed toward north. But the other new channel was formed at the western side of the older one and seems to have stretched to the more southern areas than the older one. C. Depositional Process of the Kiyosiuni Formation Depositional process of the overall Kiyosumi Formation can be drawn by the successive 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 debauched from 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 crystalline schist 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 caused by the shifting of the channel (Fig. 8). The main reasons of the second assumption are(1) Constitution as of gravelsfollows. 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. . (4) The migration of the pebbly sandstone tt facies from one unit to the overlying unit is discontinuous and complete. On the basis of these assumptions ' the depositional process of the Kiyosumi Formation 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 't .,ti Sa-Nm --- tt ::-L ¥l ¥E¥ :-¥s- .r' ,i ' --t i' i>'di':i t':: r.' . 'i"' :: .s-1 ¥rt. tt tt' :! 4tt" ttt t:' ' Ita. 't:' :¥:,: li/r:rF, c;:. tltt t ¥¥[i':1¥i'i:l'i¥//,.. ttt"tt tt v-'.'. '...... ¥:11¥::1:::::l v-'.'....., i' ,;¥ i: i' t.tttttttttl v., :l. tttttttttt /ttttttttt) ;.d. /ttt;t:t:t:I ` tt tttttttt- tt t. tt .L};.:.:,:.:ÅÄ:,:.:.:.. : tttt- t .b..-;"t;-:;:.:,:.:,:,:.tt:::;::.:"tt;,Ui't'. tt-.t- .-vvi Nm-Hk s- :: :: ¥. ¥ :1. 't':i "":i" -'f':' ' .. .'¥rt,: .rtt:1-' 1/Il:- ,¥:. t'::=' ttt. ''. .. ::7.-} dt :¥,i' '"' ::-':¥r-:".. .t.t:¥T¥i:¥;:T::f.. :: :l:'::1:::I.1,¥.¥¥'''¥:1:::.:'::l:::.S.'L¥t/t/:;:;/,il¥.lz:.]:,::¥i.I:izi:1'. ,: L¥}¥::' il¥jliii¥,':.11i.i-l iiil[iiii.i:ili,:,'...[iE . ::,1[:. ...II, - E.' . i"" ='=r=',' --- vt'' v.vt ' --- ./ :I. ili{.I:i:¥i'i.:{:,l':l[¥I¥I¥:.i¥i'liill¥1¥lll,,i.,,.g.,e,.}li¥i{¥Il:L;' '-kk::::::,.¥:::::¥:.:I::1:::¥i:¥/:S¥I]//:::]::::/::;-) '"';']L':s::i':'li¥111'lil,i.'/fi:¥:',/'i/i',i';";" Hk-Km tr:, ':¥ "ei. ,¥:,. tt- lii 't:" tt 'f:i" ' ''': :¥ ::r¥. d .tl•: :: :Il::i::= ÅÄ.•.. j:'E'/1/l¥r,:, {'tli'ii'l',lili'lii'lllli'Il";i:i.;I'::}xl'ii"ii''lii'l ::,i.:I::.:il"- t ,.,.:.'.'n,,.:'.'.',v=.'=.'.'. t;¥¥'t¥;,t,t¥:;L,ir-I:.:¥//.{ii'li//,ii'¥i'//,:l:g:.lE d2 ;':'i" I:¥::¥ -;` Km-Tk ':f k ¥¥ r¥, c,;. ¥r,. - ..,- "i: tt, ¥R.}¥ -i¥¥r, , e -'.¥.'¥ ..¥, v/ ------1--!- .I{,l 3 ..:..1.:¥.i -'¥ ', .'//' l:.r,r; ,:., t.t'¥.,i.i.' ill":i:-' iti .. - .../.tttttttt .:.v.' './ÅÄ'.1.=. ':i"¥l:i¥'::''I{/:z v..,.¥... .i6 ¥"i''¥i¥.:::;:r¥i=":e- ;;/F¥;i¥t,::¥¥1,:l.:':¥iiitiL,llll ttttttttt . 1/: ..:. .1./..../.:., ,.:.:.:.,,...,.:.:.. . - i;' L:.,/,i,E,il¥,'I :E:¥:,i. ,iii ii[,ilIil,':l'1:.i:i'i,;. . . ,d2. E¥r: ;t ' -.¥:¥.: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¥:.:';' Tk---Kr :,I{II.::¥ d;i::i,-'-t'S ';tt:¥. r 'v¥ ',. :zg '¥t.-rt. t. -', - 'r=. - 2 -it.-.:- ../11.:tttst- ....,, - -,,1. -. - -./,1.., .tttt- ,.1,- .i{iii' ,,i',"¥l,¥ e :1/' 1',,''/-1:1:::/:li.. Egoo 400m 'f':'"li'' i-..I',";':¥/..Ts¥/T/<.''iii'ira:::'Ii¥i¥i'iiizl'l''i/;¥, ttt t tttt ttt tt,li[iill¥lli ..... :¥:¥:-J¥:S¥ .¥ /:'::'/i:i[l/1'l/[l:l:': .:..,:a:'::' :al' ,:l:,ili":::'::,lll' .' .' --- :.:::¥ iil:¥ii.¥ :'':'::¥t:::,¥'..,lii¥/¥::'..i,¥- '¥" O246810 -. ;.;t km -;.'v v:::11i/./.:./././.:./,.,¥/,/¥z¥:-/::::::::/:::::::V . - - .'... ::,'=..:.'.'.e 'v;¥}';';'/:/L:',i:::'//I:¥:.l::,'¥i:::Li::'`" 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. Three Dimensional Analysis of a Large Sandy-Flysch Body 39 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 alternations were formed and incorporated with each other in the area downcurrent of each braided 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 parts of 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 sandstone facies (facies "a") are not exposed anywhere in this unit within the investigated area. The depositional tongue composed of the sandstone-dominated alternations stretched 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 composed only 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-SEE current 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 40 Shuichi ToKuHAsHi 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 coming from 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 Formation is formed by superposition of several pairs of the channel deposits and depositional tongue. Therefore, the migration of the channel and depositiona! tongue has played 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 constituting the upper half of the Hk-Km unit widely for 40 km in E-W direction throughout the 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 many thin 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 pebbly sandstones 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 turbidite sequence 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 41 i:R,.E9,Tso.!92 iss 85 160 17 40¥::¥ :::¥¥:': ¥::¥¥¥ as tt tt t :.143 ¥¥'¥;:::::¥1 11 "''''"'z::, . 145 150 ":':li:¥:1,1::¥,¥:¥.167 ,::::1 :O:,7 .o 't :,:¥.:. 20 '' ,-;. '¥ :::, IS ..¥.:' o '¥ 4o 11 2 O-l2 Q Q.z.--.o.-.e.. "i 54 q pq 2 6 N . O-¥1 --s- ' . p 3 Q,vsu-vs'Sl ve130 9 o- 2 62 7 2 1 ¥¥ 37 o T5 '8 2 1 6 3 IS IZ IM . 75 130 115 1 16 sl . 11 1 't'.'¥r.. 95 174 ::. 12 ¥.i¥. 100 p t-t oo tt :8. Q 9 . 12 K 9 Q g..g L, ..q Q " 4s2o ÅÄ ÅÄ2 '- 1 o - O.5 9 2 o A . 9 Q.-sveugSF 4 .P o t3..o..2 ". Al , 4 5 Q ' r --:- 33 1 9.s.,:S 6¥26 10 o q.-";t}.1.9-i.. -5 5 3 16 Q 5s 2 B 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. 42 Shuichi ToKuHAsHI ::M pt , "IILN, ,.,,..es"i N ,. ¥:¥i. f;¥: .::¥; q t-ttt ttt 4 t ;ttt -- V¥" .::-tJ '.:{;x. t- t¥.t t .ttt ,L¥-; ,.¥¥.¥1,l.i.. s';.:/li¥{';:i .g?'i'i. md ..i,i 'c'''¥, ¥' " tt'. :I I/"': .il:V k ::.. n .,,::::: .:. :¥S¥'i¥bZ ¥¥ '.¥' "¥¥¥.x' . ¥. , ¥¥;I4.¥. '.¥ .¥ :--' N. , tt- tttil.pt.[':1.::i,.,, .tt. ¥t ...:.,¥,,,/,-i¥.g,.¥¥,iiii,1'i'ili,¥1'111¥t - - ttt.t.. t ft.' 1.:I¥:' tt tttt ' L..t¥z" ' .' t t t- -- A t-tt t+t t t t f' ¥ li 'i l": ir: ': ';'7Ili;li' l.:'ii "ii'i'il':i'i:.' )ii',li 'i ':;' '. .1,1-/. tt t -. ' -" ' ' ;' :':l ,e¥,,'. ,n.il el 'iil t. lhl.S .T .vl x: :I- L i . .1.. E"ioocm -l,tt t t t ...t ttt.t-tt.ttt zone MlfieekS lit ng L =O ==z===l 10 km m-c. sitt Eh t.sitt gt.-- vf¥ Centrel thlck thln t'vi' p:c-p" pert+Marginel part- t' A3 -- -- .. -- ' - . h ss s- . ---- N kN . sx S-s . . s NS h s sand . s x x A2 ? N xS s- s . ss . s x s A3 . . A2 's c'. Al . I- . x . --- , . x ti s . . Al ---- . ------t.s,.S') J i !l M.'CJS tt Eh m-c m. m,.c, stlt Eh p 111se 1. ¥¥::,,¥ 2.ESffS 3.as 4.[E:l s.:t¥.,6.EE] 7.:;: s.E{l] g.U Fig. 24. Depositional modet of individual turbidite beds in the Kiyosumi Formation (ToKuHAsHi, 1976; partly . modified)Vertical scale is remarkably exaggerated. A. Geornetric model ofa moderately thick sandstone bed. B. Schematic model of the lateral variation orsedimentary structures and textures in 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 basement rocks, 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. Three Dimensional Analysis of a Large Sandy-Flysch Body 43 Chlnne4 part --- F-- Central thick part --Merginat thin part lilllll¥::::;ct:s A R;i13¥41,, .. . L}ESIt4.it/.Siwo..DHEt Ai iliii.iisiS¥ee .ny ..,] - Channel depesits--+Depesitienel tengue- Mefginei depesits 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 ,''" . -- =,=¥¥.- fi c i 11 e .- 1 bl-3 c d . ------bOI i --¥ ' Fig. 25. Table 3. Characteristios of each division of turbidite beds and h emipelagic siltstone beds in the Kiyosumi Formation A Symbo1 Characteristics Grain size of matrix Occurrence Pebbly sandstone division very coarse- to me- channel de POSItS Small hard pebbles occur crowdedly, contacting with each dium-grained sands AO other. They occur fi11ing small scours, channel-like enclosing very large angular siltstone-clasts or forming elongated lenses in the mas- sive sandstones. Lower massive sandstone division (Bouma's a division) coares- to medium- lower to middle parts of th ick- Small hard pebbles and hard scoria grains and shell fragments grained sands bedded sandstones Al occur 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 ivision) medium- to fine- middle to upper parts of thick- Well sorted sandstone with few conspicuous grains in it. Dish grained sands bedded sandstones and lower A2 structures are sometimes developed. part ofsandstone beds thinner than about 1 m. Upper massive sandstone division (Bouma's a division) fine-grained sands upper or uppermost part of Massive sandstone with scattering rounded sikstone-clasts, relatively thick-bedded sand- trm wood fragments ofvarious sizes but smaller than siltstone clasts, and : stones 5' A3 pumice grains. Pumice grains are sometimes parallel aranged in the uppermost part ofthe division. Preferred orientation ofwood frag- t ments are sometimes observed. 6o x Laminated sandstone division (Bouma's b and c division) fine- to very fine- upper or uppermost part of c Various sedimentary structures with carbonaceous wood flakes, grained sands thick-bedded sandstones and m > such as parallel, wavy, convoluted, current-ripple-cross laminations ua lower part of thin-bedded : occur. Division into lower-parallel-lamination part (Bouma's b sandstones H BC division) 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-grained sands uppermost part of thick-bed Very fine sedimentary structures such as thinly-alternated to coarse-grained silts ded sandstones and upper part D parallel laminations and other minute laminations with very small of thin-bedded sandstones wood 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) medium- to fine- usually accompanied wtih Turbiditic siltstone is easily distinguished from overlying hemi- grained silts relatively thick-bedded sand- Et pelagic siltstone (Eh) by finer-grained and better sorting and sharp stones contact with the latter. Distinct grading is observed in the case of thick turbiditic siltstone. Hemipelagic siltstone division (upper part of Bouma's e division) mostly of medium- to lying en the sandstone beds or This siltstone is usually ill sorted and often contains scoria and coarse-grained silts turbiditic siltstone beds Eh pumice grains randornly. Many continuous tuff beds are included in the division. Grading is not observed. Trace-fossils are some- times observed. Three Dimensional Analysis ofa Large Sandy-Flysch Body 45 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 observed here. 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 express the 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 which stretches 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 down through the valley or channel and arrive at an intersection zone (HANER, 1971), they release and deposit a large quantity of suspended loads on the surrounded area. Consequently a depositional lobe or suprafan (NoRMARK, !970) is formed downcurrent of the zone, that is, on the middle fan segment. Therefore the middle fan 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 no channels. 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 and PipER, 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'ToKuHAsHI mentioned, 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 by accompanying a trough-like erosive morphology at the base and by an upward thinning cycle beginning with thick pebbly sandstones and ending with thin siltstone- dominated alternations or massive siltstones. Pebbly sandstone facies are very narrow laterlaly and rapidly change into siltstone-dominated alternations. Three stages 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 consequent disappearence 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, the rejuvenation 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 of the channel deposits, must have formed a convex-upward depositional bulge on the 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-W direction 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 Three Dimensional Analysis ofa Large Sandy-Flysch Body 47 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 beds extend consistently in thickness at least over 10 to 20 times as wide as the estimated value (ToKuHAsHi, 1976a; HiRAyAMA and NAKAJiMA, 1977). Accordingly, in the case of the Kiyosumi Formation, the remarkably large-scale turbidity currents seem to have occurred forming remarkably wider suprafans than those in the current submarine 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 of a 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 by lateral 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 the southern 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 composed mainly 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 48 Shuichi ToKuHAsHi beginning of the deposition of the Kiyosumi Formation, some tectonic topography seems to have been developed. The first group of turbidity currents which flowed into 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 the preexisting 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 the sedimentary 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, and therefore, 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 made of the Amatsu Formation or Amatsu Mudstone. The Inakozawa Formation or Inakozawa Mudstone, the western equivalent of the Kiyosumi Formation, was formed 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 is composed of several small-scale sandstone-dominated alternations surrounded respec- tively with siltstone-dominated alternations both laterally and vertically. To understand the process of fan sedimentation of the later-half stage including the last stage, the depositional process ofthe Anno Formation must be analyzed. Then the 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 the Kiyosumi 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 Three Dimensional Analysis ofa Large Sandy-Flysch Body 49 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 to a 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 and magnitude of turbidity currents and may produce the upward thinning cycle in the depositional area. The following relatively rapid descent of the sea level may produce, 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 50 Shuichi ToKuHAsHi large destructive fall of the unconsolidated sediments may be responsible for the rejuvenation 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 Submarine Canyon off California in response to the changing point of input of sand to the canyon 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 the provenance may have sensibly controlled the rate of supply of sediments to the canyon heads. In this case, the tectonic movement may be a main factor to have produced 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 of the 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 the broader knowledge and background as in the case of the other cyclic sedimentation. It may be very important to recognize that any submarine fan sedimentation and 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 depositional processes 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 the aid 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 Three Dimensional Analysis of a Large Sandy-Flysch Body 51 as follows. (1) The Kiyosumi Formation or the Kiyosumi Sandstone is composed of a lenticular 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 in N-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 the Kiyosumi Formation as passage ways for the large quantity of turbiditic sediments. Channel deposits are characterized by an upward thinning cycle beginning with thick 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 large depositional tongue composed of thick sandstone-dominated alternations with negligible 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 channel several 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 submarine channel 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 an upper-fan segment. The terminus of the channel deposits probably corresponds to an intersection zone (HANER, l971) between the upper- and middle-fan segments. (8) The depositional tongue of thick sansdtone-dominated alternations seems to 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 surrounding basin 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 current submarine fan model. The depositional tongues in the Kiyosumi Formation cover most 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 to refiect the much larger-scale turbidity currents in the depositional period of the formation. (10) The channel deposits and depositional tongue in the lowermost unit of the 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 a fan 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 the depositional 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 recognized in the Kiyosumi Formation. The first-order upward thinning cycle is detected throughout the whole Kiyosumi Formation. It was formed by a phased retreat of the terminus of channel deposits or the intersection zone on the fan from the Iowermost unit to the uppermost unit. The second-order upward thinning cycle is clearly 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 an eustatic 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 a canyon 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 these factors. It may be very important to recognize that fan sedimentation and its deposits 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-Pliocene time. (14) Recent submarine geology has played the most important role to establish Three Dimensional Analysis ofa Large Sandy-Flysch Body 53 the overall turbidite facies association based on the submarine fan sedimentation. Many detailed and fundamental works on submarine topography and sediments in the 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 marked development of studies on flysch sequences which were deposited in the ancient geosynclines and other basins. Acknowledgements The author wishes to express his sincere thanks to ProÅí Keiji NAKAzAwA and Associate Prof. Tsunemasa SHiKi, Kyoto University, for their appreciated supervision and permanent encouragement. He must extend his hearty thanks to Drs.Jiro HiRAyAMA and Terumasa NAKAJiMA, Geo}ogical Survey ofJapan, for their initial and continuing encouragement and support to undertake the sedimentological study in the Boso Peninsula. He is much indebted to ProÅí Keiichiro KANEHiRA, Chiba University, Drs. Naotoshi YAMADA, Sadahisa SuDo and Hiroshi MAKiMoTo, Geological Survey of Japan, 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 his information 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, Kyoto University, 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, Kyoto University, 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. References AoKr, N. (1968): Benthonic foraminiferal zonation of the Kazusa Group, Boso Peninsula. Trans. Prec. Pataeont. Soc. Japan, N. S., 70, 238-266. AsANo, K. (Ed.) (1970) : Micropateontotogy (Bikoseibtttsugaktt) Part L 278 p., Asakura-shoten, Tokyo. (in Japanese) BERGGREN, V"'.A. (1973): The Pliocene time scale; Calibration of planktonic foraminiferal and calcareous nannoplankton zones. Nature, 213, 391-397. 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(in Japanese with English abstr.) IcHiKAwA, K., FuJiTA,Y. and SHiMAzu, M. (Eds.) (1970): The geotogic develoPment of the Japanese Istands. 232 p., Tsuklji-shokan, Tokyo. (inJapanese) IDA, K., Mr'rsuNAsHi, T. and KAGEyAMA, K. (1956): On the new stratigraphic division of Upper Cenozoic in Tokyo district. Bull. Geol. Surv. JaPan, 7, 435-436. (in Japanese with English abstr.) IrJiMA, A. and IKEyA, N. (1976>: Geology of the Tokyo University Forest in Chiba. Misc. Inf., Toklo Univ. Forest (Enshurin), 20, 1-38. (inJapanese) IKEBE, N. (1948): On the Amatsu Group in the Boso Peninsula. Jour. JaPan. Assoc. Petrol. Technol., 13, l32-133. (inJapanese) Is}m, M. (1962): Basement ofthe Kwanto Plain, ibid., 27, iro5-430. (inJapanese) IsH", T. (1973): Wavelength ofturbidite ripples, Mar. Sci. (KaioKagaku), 5, 402-407. (inJapanese with English abstr.) KANEHiRA, K. (1976): Modes of occurrence of serpentinite and basalt in the Mineoka district, southern Boso Peninsula. Mem. Ceol. Soc. 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(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 conglomer- ates 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.) and IwAwAKi, T. 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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-beds 1. 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- Fukurokura Line in the Tokyo 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 Fig.12. Tk-Krunit. Explanation of Plate 3 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 suc- cession 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. Explanation of Plate 4 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. Groove cast imprinted on the underlying siltstone bed (locality n in Fig. 9A). 4. Flute cast imprinted on the underlying siltstone beds (locality f in Fig. 24A). Current di- rections ofadjacent sandstone beds coincide well with each other. Explanation of Plate 5 Imbrication of siltstone clasts in pebbly sandstone facies. L A wall outcrop of the Sasa River (locality a in Fig. 14). Transport direction is from right to left (southward). 2. An enlarged view of the same outcrop. 3. A wall outcrop of a small tributary of the Koito River (locality b in Fig. 14). Transport direction is from right to ieft (southward). 4. An enlarged view of the same outcrop. 5. A cutting outcrop beside a small path (locality i in Fig. 14). Transport direction is from right to left (southward). 6. An enlarged view of the same outcrop. Mem. Fac. Sci., Kyoto Univ., Ser. Geol. & Min., Vol. XLVI, No. 1. Pl. 1 . -,ft Ii." l'ee,.I¥ l,tt,¥.--¥.i¥S:-i,,.g¥lkss'¥ ,.tJ,'X,t"'as¥tt' .. .. li. '¥,/' i`mp.me -' P;st-L l2st"fftll-sskiesft.,"s'se.ttr" S' kli¥ltL/ffc X, itu ,wp .ts g ' ., -$ 'fiigittt'llliiitlk'r.t"."i?.fgf-,th¥/:;'i',i- ' ma,,imxx{)wl"s ,ifre.yeettt ToKuHASHI: Large Sandy-Flysch Body Mem. Fac. Sci., Kyoto Univ., Ser. Geol. & Min., Vol. XLVI, No. 1. Pl, 2 `L Sig.'..t' X."" -ge"ts'et. ToKuHASHI : Large Sandy-Flysch Body Mem. Fac. Sci., Kyoto Univ., Ser. Geol. & Min., Vol. XLVI, No. 1. Pl. 3 vx,'.- ¥ Xf': 3tw, Sil,}:ixvataR ¥lrixX 's' ajthwh ,-' . . -¥ ''xs:t. ..;.:3¥, ,¥- ,;.ix" ijes" eJSopil,.tdi.. "L. , . . k¥iLtt- 'l¥: ,'" Nx !' ToKuHASHI: Large Sandy-Flysch Body Mem. Fac. Sci., Kyoto Univ., Ser. Geol. & Min., Vol. XLVI, No. 1. Pl. 4 Il evst 3 ,".-tw.te...Z-E- "tt '.:,.;, -Is'-t""tll ' ¥pa. ¥: R¥ 1/ i}tsth: ma}- .L r" ry" wa a3, ¥.es,. ee s$.ll{i/Iilxge- 'pt tt,"i/l),iiiilll ' 'i; ' Ifaeg¥ik ,. "e. .lj4 as- ToKuHASHI: Large Sandy-Flysch Body Mem. Fac. Sci., Kyoto Univ., Ser. Geo]. & Min., Vol. XLVI, No. 1. Pl. 5 4 '?t wf. -{ ToKuHASHI : Large Sandy-Flysch Body