Title Reconstruction of Fujikawa Trough in Mio-Pliocene Age and Its

Reconstruction of Fujikawa Trough in Mio-Pliocene Age and Title its Geotectonic Implication

Author(s) Soh, Wonn

Memoirs of the Faculty of Science, Kyoto University. Series of Citation geology and mineralogy (1986), 52(1-2): 1-68

Issue Date 1986-03-31

URL http://hdl.handle.net/2433/186658

Right

Type Departmental Bulletin Paper

Textversion publisher

Kyoto University MEMolRs oF THE FAcuLTy oF SclENcE, KyoTo UNIvERsrTy, SERIEs oF GEoL.& MINERAL., Vol. LII, No. 1 & 2, pp. 1-68, pls. 1-14, 1986

Reconstruction of Fujikawa Trough in Mio-Pliocene Age and its Geotectonic Implication

By WONN SOH

(Received December 28, 1985)

Contents

Abstract ...... ,...... ,...... ¥¥.¥¥¥¥¥¥¥ny¥¥¥¥¥¥¥¥¥¥-¥¥¥¥¥¥¥¥ 1 1. Introduction ...... 2 2. Geological background ofthewestern halfofthe Southern Fossa Magna Region ... 4 2-A. GeologicsettingoftheSouthern FossaMagna Region ...... ,...... 4 2-B. Outline of geology and stratigraphy of the western halfof the South- ern Fossa Magna Region ...... ¥¥¥¥ 5 2-C. GeologicstructureoftheSouthernFossaMagnaRcgion ...... ,...... 8 3. Stratigraphy ofthe Minobu Formation and its equivalent strata ...... 8 4. Sedimentary facies of the Minobu Formation and its equivalent strata...... 15 5. Facies association ...... ,...... ,...... 19 6. Dispersal pattern of sediments ...... ,...... ¥¥.¥¥ 38 7. Petrography of the Minobu formation...,...... ,...... 45 8. Interpretation offacies association ...... ,...... ,...... 50 9. Reconstruction ofsedimentary environment ...... ,...... ¥¥¥¥ 55 1O. Comparison with modern sedimentary environments ...... ¥,¥¥¥¥¥¥¥¥¥¥¥¥¥¥¥ 59 11. Reconstruction of tectonic setting of the Southern Fossa Magna Region ...... 60 Acknowledgement...... ,...... ¥.¥¥¥¥¥¥¥¥¥¥¥¥¥¥¥¥¥¥¥¥-¥ 63 Reference ...... ¥.¥¥¥¥¥¥¥¥¥¥¥¥¥¥¥¥¥-¥¥-¥¥¥¥¥¥¥¥¥¥¥¥.¥¥¥¥¥¥¥¥¥¥¥.¥¥¥-¥¥ M

Abstract

The late Miocene to Pliocene Minobu Formation and its equivalents in the Southern Fossa Magna Region, centralJapan, consist of turbidites and associated coarse clastic deposits. The thickness of the Minobu Formation and its equivalents attains up to 3 km. In those sediments, sixteen sedimentary facies are recognized, and they are grouped further into eight facies associations; Conglomeratic, Conglomeratic-sandy, Conglornerate-Sandstone, Sandy, Silty, Sandy-silty, Siltyand Pebbly siltstone associations. Among them, the first four associations are interpreted as channel fiII sediments. While the second three associations are thought to be interchannel sediments. Interpretation ofsuch facies associations as mentioned above is based on the following points; l) comparative studies of sedimentary facies and facies association in the study area with those reported from modern and ancient turbidite basins, 2) morphologic analysis of sedimentary body of facies association and 3) dispersal pattern of sediments. The last of those associations is assumed from its characteristic sedimentary features to be slump deposits 2 SoH, N4J. which flowed down along such steep slope as fault scarp. Result of such interpretations leads further to the conclusion that sedimentary environments of the Minobu Formation and its equivalents is fi11ings ofa trough which extends in N-S direction throughout the area. Furthermore, fromsedimentological analysis of the trough fi11ings, feeder channel extending ENE-WSW direction, western boundary fault scarps and an axial channel of a few kilometres in width are revealed. Then it is named herein Fujikawa Trough. Petrography and dispersal pattern of clastic sediments indicate that the main source area was situated around the Kwanto mountainland, more than 40 kilometres away from Fujikawa Trough. Comparison of Fujikawa Trough with modern trough and trench is made. Consequently, Suruga Trough seems to show the closest similarity by the following reasons of 1) similarity of internal topographic features such as western boundary fault scarps and an axial channel of a few kilometres wide, 2) similarity of the axial direction of trough and 3) similarity of the sedimentary ratio of fi11 sediments. Such geomorphologic similarity of both trough is important, and suggests that Fujikawa Trough was formed as a consequence of collision of Izu-Bonin Arc into Southwest Japan Arc in the same mannar as Suruga Trough is developing now.

1. INTRODUCTION Fossa Magna Region is a large graben-like region located atjunction of Sou- thwest Japan and Izu-Bonin arcs, and oflrers a key towards clarification of geotectonic relationship between these two arcs (Fig. 1). Many surveys have been made since last century to establish the geologic development of the region (NAuMANN, 1887; HARADA, 1890; EHARA, 1953; MATsuDA, 1978; etc). In constract with geophysical interpretation, however, geological one of the region has not been developed further since the 1960's, owing to complex lithofacies changes and developments of folding and faulting. It has been known that turbidites and associated coarse clastic deposits, more than 4000m thick, are extensively developed in the southern part of the Fossa Magna Region (the Southern Fossa Magna Region). Sedimentological analysis of those sedimentary sequences, especially from a "tectono-sedimentologic viewpoint", is considered to be very important in clarifying the geologic development of this region. However, no such study has been carried out yet. Among the sedimetary sequences, the latest Miocene to early Pliocene Minobu Formation of the Fujikawa Group was selected for this study. Because this formation is well-exposed and most widely distributed. And many literally equivalent strata ofthe Minobu Formation, ranging from deep-sea to shallow marine environments, are recognized in the region. In this paper, the author aims first to describe the complex lithofacies of the Minobu Formation and its equivalent strata from viewpoints of sedimentary facies and facies association ofMuTTi & Ricci-LuccHi (1972; 1975). The second aim is to reconstruct the sedimentary environments, especially focusing on paleotopography. The following points are taken into account; 1) the comparative studies of sedimentary facies and their preferred transition in each facies association of the Minobu Formation with those reported from modern and ancient turbidite basins, 2) a mor- 1<.econstruction of Fujikawa. 'I'rougl] in ] lio-Plioccne Age and its C;eotcetonic Irnplication 3 phologic analysis ofthe sedimentary body as constituted by facies association, 3) the dispersal pattern of facies association and 4) the petrographic properties of the sediments. The third aim is to estimate the sedimentary and tectonic settings of the Minobu Formation and its equivalent strata by comparison with those of modern sedimentary and tectonic settings. Finally, a new tectono-sedimentological inter-

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2. GEOLOGICAL BACKGROUND OFWESTERN THE HALF OF THE SOUTHERN FOSSA MAGNA REGION

2-A. Geologic setting of the Southern Fossa Magna Region As the most important geologic features in the Southern Fossa Magna Region and its surrounding region, it should be taken up that E-W zonal arrangement of main geologic belts of Southwest Japan Arc bends acutely to north here (Fig. 2). In those belts, the Ryoke, the Sanbagawa, the Chichibu and the Shimanto belts are

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-¥,r ') '"'zl i¥ `' t 1 . {ll3""ll,lg`'V--rii'JI;"i , blJ ,,z;' t K'ct -t .l. y r-)C)'V 1t t---""tt' Ii r J;/.1>lk < L, A tt ' l'li" Tli' ,s L--I` ;xN)' ; 1 K)Jt t lt r: kx ?' ). rj- tf.} S`i Ii ]Xb L¥ts / 'i" J tt ftltt J.l - ' x"t '-'-t t Jft ; J-r 'f Fig. 2. Tectonic sketch and iocation map aroundin and the Fossa Southern rvlagna Region, showing bending morp}iology oi- geologicbelts Southwest in the Japan arc, and of the modern troughs. A. Koma mountainland, B. Misaka mountainland, C. Tanzawa mountainland, D. Iwadono area, E. Nishikatsura area, F. FuJ-ikawa area, G. Ashigara area. Reconstruction of Fujikawa Trough in M{o-Pliecene Age and its Geotectonic Implication 5 included. In addition, it is also noticeable that the axes of modern Suruga and Sagami troughs bend along this bending trend. Since NAuMANN (1887) and HARADA (1890), many geologists and geophysists have discussed the genesis ofthis interesting feature. With the introduction ofplate tectonic models to this region, it has been speculated that the bending structure of the pre-Miocene sequence is caused by non-uniform compression from the southern side (HARA et al., 1973; 1977; IcHiKAwA, 1980). And, it is believed that the curved line connecting Suruga and Sagami troughs through the Southern Fossa Magna Region is a modern plate boundary interpreted to have been constructed by the collision of Izu Peninsula to central Honshu (KAizuKA, 1972; MATsuDA, 1978; and others). Now, it is believed that this region has been located on a collision boundary between Izu-Bonin Arc on Philippine-sea Plate and Southwest Japan Arc on Eurasian Plate (e.g. NiiTsuMA, 1982, 1985). Accordingly, the late Miocene to Pliocene system in the Southern Fossa Magna Region should have recorded an unique geologic history of sedimentation and tectonism as the collision-accretion region between both arcs.

2-B. Outtine of geology and stratigraphy in the western half of the Southern Fossa Magna Region In the Southern Fossa Magna Region, the Neogene system is extensively developed (Fig. 3). Pre-Neogene basement rocks, however, are not exposed (Matsuda, 1984). The Neogene systemin the western part ofthis region is classified into two groups; the middle Miocene Nishiyatsushiro Group and the late Miocene to Pliocene Fujikawa Group in ascending order (MATsuDA, l961) (Table 1). The Nishiyatsushiro Group is widely distributed in Tanzawa Mountains, Misaka Mountains and Aimata area in the central and northern part of the Southern Fossa Magna Region. The group is mainly composed of tholeiitic altered lava and hyaloclastic sediments. Some of them distributed in the western part of the region consist of alkaline volcanic rocks. Black mudstone is found interbedded with sandstone. It attains a thickness of 2800 m and is subdivided in ascending order into the Furusekigawa Formation and the Tokiwa Formation. The Nishi- yatsushiro Group is comformably overlain by the Fujikawa Group (AKiyAMA, 1957; MATsuDA, 1958; FuJiKAwA CoL. REs. GR. 1976; 'I"oKuyAMA et al., 1981). The Fujikawa Group is mainly distributed in the western part of the Southern Fossa Magna Region. In contrast to the Nishiyatsushiro Group, it consists mainly ofclastic sediments such as conglomerate, thick-bedded sandstone and siltstone. The calkalkaline andestic volcanic deposits are intercalated. Those volcanic deposits, however, are only weakly altered different from those of the pervasively altered Nishiyatsushiro Group. The Fujikawa Group, more than 6000m thick, is sub- 6 SoH, W.

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o 10km 20km zgl[]z 1234 Fig. 3. Generalizcd geological and structurai map of the Southern Fossa .Pvlagna Region, showing half-dome structure facing north around Tanzawa rnountain, of the Miocene System. 1. Fault, 2. Thrust, 3, Anticline, 4. Synclinc. divided into three formations namely; the Shimobe, the Minobu and the Akebono formations in ascending order. The Fujikawa Group is unconformably overlain by the Kanbara Gravel of middle to late Pleistocene age (SuRuGAwAN CoL. REs. GR., 1981). Based on the first or last occurrences of designated planktic foraminifer taxa, it appears that the Nishiyatsushiro Group ranges at least from N.10 to N.18 ofBLow's zone, corresponding to middle to late Miocene age (SHiMAzu et al., 1971; CHiJi & KoNDA, 1978). Moreover, KoNDA (1980) suggested by the study of benthic foraminifer taxa that the paleobathymetry of the Nishiyatsushiro Group is nearly in upper bathyal zone. In the Fujikawa Group, UJiiE & MuRAKi (1976), CHiJi & KoNDA (!978) and KANo et al. (1985) stated that the Fujikawa Group with an except of the Akebono Reconstruction of Fujikawa Trough in Mio-Pliocene Age and its Geotectonic Implication 7

Table1 . Summary of stratigraphic succession and litho)ogy in the western halfof the Southern Fossa Magna Region.

Stratigraphy Blow's ,Geologic Group FormatÅ}on Lithology Thickness number 'T!Irte

thick-beddedconglomerate Akebono N2I volcaniclasticdeposits 1200m+ 'Formatlon siltstonewithsandstone

siltstoneinterbeddedwithsandstone Pliocene FuJlKAWA thick-beddedsandstone GRoup Minobu thick-¥beddedconglomerate

,Format¥ion pebblymudstone 3100m volcaniclasticdeposits N19

siltstoneinterbeddedwithsandstone ,Shimobe thick-beddedtuffaceoussandstone 2ate 'Forrnation volcaniclasticdeposits 1900m 'Nlocene N18 -----N17 mudstoneSnterbeddedwithtuff& NIsHI- Tokiwa tuffaceoussandstone N15 'Forrnatlon 2000m YATSUSHIR N14

middJe'Mlocene GRoup Nl3

Nl2 Furusekigaw basaltlavasandpyroclastics

.Forrnatlon tuffaceousmudststonesandsands¥tenes 800ru+ (NIO}

Formation, can be correlated with N.18 up to N.21 ofBLow's zones, ranging in age from late Miocene to Iate Pliocene. Additional}y, the most sequences of the Fujikawa Formation is deposited in upper bathyal environment or more deeper one (KANo et al., 1985). Based on KENNETT & SRiNivAsANs'timetable (1983), the period between N.18 and N.21 that is corresponding to the geologic period of the Fujikawa Group except for the Akebono Formation is estimated as about 3 million years. This estimation gives a sedimentation rate for the Minobu and the Shimobe formations ef the Fujikawa Group as approximazely 150 cmllOOO year. This value of sedimentation rate is nearly equal to the higher one (cÅí STow et al., 1984) for turbidites and associated coarse clastic deposits in the world. 8 SoH, W.

2-C. Geoiogic structure of the Southern Fossa Magna Region Neogene strata in the Southern Fossa Magna Region as a whole, excepting the Koma mountains, show a half-dome structure facing north (see Fig. 6 in SuGiyAMA, 1976) (Fig. 3). Thrust faults run not only along the boundary between the Neogene and pre-Neogene systems but also within the Necgene system, in which are of concordance with the half-dome structure. The diorite-gabbro complex intruding into the Neogene rocks (KuNo, l957; YAziMA, 1968; TAKiTA, 1974), are aJso distributed in parallel with the half-dome structure. Study area belonging to the western half of the Southern Fossa Magna Region, is mostly situated on the west wing ofthe half-dome structure. This area corresponds to the conversional region of two structural trends of NE-SW and N-S directions (OTuKA, I931). Folding structures of the ENE-WSW trend are developed in the eastern part of the study area, and gradually converge east- and southward to monoclinal structures of N-S strike, dipping west, which are parallel to the extension of "Fujikawa Tal'' of MATsuDA (1958) (AKiyAMA, 1957; MATsuDA, 1961). Such a strong deformation as repesented by bedding slip or slaty cleavage is not recognized in the study area. The plunge of folding is not steep and is less than 45 degrees. Accordingly, for the sedimentological investigation and reconstruction, the effect of structual deformation with an exception of probablely minor space shortening by folding and faulting are negligible, ifit is present.

3. STRATIGRAPHY OF THE MINOBU FORMATION AND ITS EQ)UIVA- LENT STRATA The Minobu Formation, the midd!e formation of the Fu.jikawa Group was first described in Minobu area by AKiyAMA (1957), and subsequently redefined by many workers (MATsuDA, 1958, 1961; FuJiKAwA CoL. REs. GR., 1976; ToKuyAMA et al., 1981; SoH, 1985). In this paper, the author follows his previous work (SoH, 1985) which is modified from Matsuda's works. Equivalent strata of the Minobu Formation are distributed not only to the south of the type locality (Manzawa area), but also to the north (Akebono area), to the northeast (Nishikartsura area) and to the west (Aimata area). Based on the similarity of predominated lithofacies, paleocurrent pattern and geologic structures, these areas can be grouped into three domains, that is, Nishi- katsura domain, Fujikawa domain and Aimata domain (Fig. 4). Nishikatsura domain, comprising Minobu and Nishikatsura areas, stretches east-northeastward from Minobu area to Nishikatsura area 35 km away. Nishikatsura area is separated from Minobu area by Q;uarternary Fuji lava area of Aokigahara between. Resedimented conglomerate and thick-bedded turbidite are well-developed in this domain. Reconstruction of Fujikawa FI"rough in .rV'lio-Pliocene Age and its Geotectonic Irriplic.ation 9

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Fujikawa domain extends along Fuji River including Akebono, Minobu and Manzawa areas. It corresponds to the northeastern part of "Fujikawa Tal" of rvl.yrsuDA (1961). The sequence of this domain consists mainly of conglomerate, thick-bedded sandstone, siltstone interbedded with sandstone, with volcaniclastic intercalations and slump deposits. The boundaries ofAkebono and Minobu, and of Minobu and Manzawa areas are drawn along axes of Tokiwa and Utsubuna anticlines, respectively (FuJiKAwA CoL. REs. GR,, 1976) (Fig. 5). It should be noted that Fujikawa and Nishikatsura domains overlap each other in Minobu area. Additionally, the westernmost Aimata domain stretches N-S and is characterized by hyaloclastic sediments and siltstone interbedded with tuffaceous sandstone. In this paper, the Aimata domain is only briefiy mentioned.

Nishikatsura domain Minobu area The Minobu Formation in the type area consists of a complex sequence of resedimented conglomerate, thick-bedded sandstone, siltstone interbedded with sandstone, volcaniclastic sediments, and slump deposits. Three mernbers have been 10 SOH, X¥V.

Table 2. Summary of stratigraphy of the Minobu Formation and its equivalent strata.

FujtkawaTal Ntshikatsuraarea Manzawaarea Akebonoarea Minobuarea Ntshikatsuraareo (Machiya-Nanbu (stratatype) {Nishikatsura- area) Iwadonoarea) Akebono Y.orihatapyrocl. Forrnation (FukushÅ}Tuff) Karasumoriyamapyroclastics

Machiyaalt¥ 'Minobu Hakiialt. Manzawa Marutaki ForTnation alt. Haramudstone congt. Toshima 'Yaglsawa alt, alt, Nishikatsura ? congl. Shimobe ? ,Uenota;raalt. Furuya 'Formation sandstone Takenoshimaalt., i'.',e//.{i,t/,/¥://i.t¥1/l,{it.i-l/¥////i/i//f,11'//////i.,.,,/il/{'";.E..,.. ¥1111///11'l¥s.:,.,1".'i//'.ll;iii.ili¥,1',lil..//.'1¥111i¥4,i,¥.lliil,,,I¥li..,.'1i.i'l..,iS- ¥¥- Nl.shÅ}y,f.t/.e..,tt..f..S.l,,,1.,,p.g.,,,,,,,,,,,.•,,,,,,S•/,tt,,,.1•,-,l,,,P•,.1/l••;,i•,i,

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Nishikatsura area In Nishikatsura area, the Nishikatsura Group, equivalent to the lower part of the Fujikawa Group, unconformably overlies the Nishiyatsushiro Group. The Nishikatsura Group is divided in ascending order into the Furuya Sandstone and the Katsuragawa Conglomerate (MiKAMi, 1962; WATANABE, 1954; FuKuDA & SHiNoKi, 1952; MANo et al., 1977; KoMATsu, 1984) (Table 2). Among them, the Katsuragawa Conglomerate corresponds to the Minobu Formation (MATsuDA, 1961j 1972). The Katsuragawa Conglomerate is exposed continuously for more than 10 km along the Nishikatsura fault of FuKuDA & SHiNoKi (1952) from Iwadono to Lake Kawaguchi (Fig. 6).

Fnjileawa domain Manzawa area The Manzawa Formation of MATsuDA (1961) in Manzawa area comprises four th

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.o 21?Oo. ={ ,o ,, "I:',''"'i'l"'" "go.. O.Os

g Ogg./.SSSb"ggliee23efll?2kowi,N ' O oooO.o . $,il:.::,¥ b.X 02 o-1fiUz5im2

oooooooooo oooooo o Fig. 5. Geological map of Fujikawa domain. The boundaries of Akebono and Minobu, and Minobu and Manzawa areas are drawn on the axes of the Tokiwa and Utsubuna Anti clines. 1. Intrusive rocks, 2. dyke rocks, 3. fault, 4. dip and strike. Reconstruction of FujikaxN'a Trough in ptIio-Pliocene Age and its Geotectonic Implication 13

Shimanto

Regend M' l Katsuroccra C, figjS :;;:i HitsuktrÅ} l:/s:" :'t"" k:[suro L Furuva S, .mt Grco E {s.s, t m,s alt,) Furuve S, 7t z ttur'feceeus s,s,J "At YJ' NÅ}sniva.tw Oamo F' .ov g Kawaoucni F, tsys

Nishivatsushiro,;.};, i.,... ' ,deei GrOUP Nxss ss

sN"IEIiililli{ilSISIi:iissilill}S`gtyLtOetr'asasTe>otwirpsisawa Group '

X ier

}ssg

--++------+------3't 5hi:-oyeshidi.so e Fig. 6. Geological map ofNishikatsura area. members; the Toshima Alternation, the Manzawa Alternation, the Machiya Alter- nation and the Fukushi Tuff in ascending order. The Manzawa Formation, however, is tentatively used here for the portion below the Fukushi Tuff (Fig. 5). This formation is considered to be equivalent to the Minobu Formation, because 1) some characteristic sequenceb of the underlying Shimobe Formation can be traced from Minobu area to Manzawa area bordering on Utsubuna Anticline, and 2) lithologically, the Fukushi Tuff (including the Yorihata Pyroclastics) are thought to correspond to the Karasumoriyama Pyroclastics, that is, the lowest member ofthe Akebono Formation in the Minobu and Akebono areas (FuJiKAwA CoL. REs. GR., 1976). The thickness of the Manzawa Formation is up to 2500m and is nearly equal to that ofthe Minobu Formation. The formation, however, is slightly different from the Minobu Formation in having more tuffaceous materials. 14 SoH, W.

The Toshima A]ternation is mainly distributed in the central part of Manzawa area (Fig. 5). As dacitic tuff and chaotic pebbly siltstone can be traced as key beds laterally from the Toshima Alternation to the Manzawa Alternation, most of the Toshima AIternation can be shown to be stratigraphically equivalent to the Manzawa Alternation. The Machiya Alternation overlies conformably both the Toshima Alternation and the Manzawa Alternation. Intrusive rocks of gabbro-diolite complex and porphyrite, named Sanogawa Intrusive Rocks, are very common in the eastern part of this area, with a local contact metamorphism (YAziMA, 1970).

Akebono area In Akebono area, the Fujikawa Group is divided into the Hara Mudstone and the Akebono Formation in ascending order (AKiyAMA, 1957; MATsuDA, 1958; 1961). The Hara Mudstone is safely correlated with the Shimobe Formation and the Minobu Formation of Minobu area. Both sequences of the underlying Nishi- yatsushiro Group and the overlying Akebono Formation, are traceable from Minobu area to Akebono area (AKiyAMA, 1957; MATsuDA, 1958; FuJiKAwA CoL. REs. GR., 1976). In addition, this correlation is supported by the study of foraminiferal biostratigraphy by UJiiE & MuRAKi (1976) in the Akebono area, and by CHiJi & KoNDA (1978) in Minobu area. The Hara Mudstone consists mostly of massive siltstone with thin-bedded sandstone that decreases abruptly in thickness to the north.

Aimata domain Aimata area Aimata area is located on the western side of Fuji River and bounded to the east by Minobu area at the Minobu fault. According to MATsuDA (1961) and SHiMAzu et al. (1984), three formations are recognized in the Aimata area; the Gotenyama Formation ofthe Nishiyatsushiro Group, the Kuon.ii Mudstone and the Aimata Formation (the Takatoriyama Pyroclastics) of the Fu.jikawa Group in ascending order. MATsuDA (1961) and SHiMAzu et al. (1984) correlated the Aimata Formation with the Minobu Formation. This correlation is accepted in this paper, although a further study is required to confirm it. The Aimata Formation, up to 2000 m thick, is composed mainly of hyaloclastic sediments such as andesitic andlor basaltic lava and tuff-breccia, intercalated with siltstone. The Aimata Formation is conspicuously different lithologically from the sequences of Fujikawa and Nishikatsura domains. I believe that the Aimata Formation is considered to have been deposited in a seamount environment basing on their characteristic sedimentary facies. Details on this interpretation, however, is out of duty of this paper. Reconstrtiction of Fujikawa Trough in Mio-Pliocene Age and its Geotectonic Implication 15

4. SEDIMENTARY FACIES OF THE MINOBU FORMATION AND ITS EQ)UIVALENT STRATA

Based on lithologic and sedimentologic characteristics such as clast or grain size, sedimentary structures and bedding style, turbidites and associated coarse clastic deposits can be classified into several "Facies" consisting of a layer or group of layers. In the present study, descriptive classification of "Facies" is preferred rather than a genetic one deduced from the inferred deposirional mechanism. In the Minobu Formation and its equivalent strata, sixteen facies are recognized, which can be grouped into five megafacies (Table 3).

1) Conglomerate and Pebbly Sandstone Megafacies

I7Tacies Al: thick-bedded disorganized conglomerate Facies Al comprises poor-sorted, clast-supported conglomerate, containing clasts of cobble to granule sizes. Bed thickness ranges from 50 cm to 8 m (2 m average). It shows rapid lateral thickness change. Most beds have erosional, uneven- bases. Beds of this facies are massive (ungraded) or massive to normal-graded (in the sense of DAvis & WALKER, 1975), occasionally inverse-graded to massive or more

Table 3. Classification of Sedimentary F acles.

1) Conglomerate and Pebbly Sandstone Megafacies Facies Al : Thick-bedded disorganized cong!omerate Facies A2 : ThÅ}n-bedded poor-sorted conglomerate FacÅ}es A3 : Well-sorted conglomerate Facies A4 : Ungraded parallel--stratified Åëonglomerate and sandstone Facies A5 : Ungraded cross-stratifÅ}ed conglomerate and sandstone Facies A6 : Massive pebbly sandstone 2) Sandstone Megafacies Facies Bl : Thick- to rnedium-bedded massive sandstone Facies B2 : Coarse-grained parallel-stratified sandstone Facies B3 : Cross-stratified sandstone Facies B4 : Fine-grained para!lel and thin-bedded sandstone 3) AIternations of Siltstone and Sandstone Megafacies Facies Cl : Sandstone interbedded with siltstone Facies C2 : Siltstone interbedded with ripple cross-laminated sandstone Facies C3 : Siltstone interbedded with thin-bedded coarse-grained sandstone 4) Chaotic deposits Megafacies Facies Dl : Pebbly siltstone Facies D2 : Chaotic deposit with coherent folded/ contorted strata 5} Volcaniclastic Megafacies Facies E : Volcaniclastic deposit 16 SoH, NNr.

rarely inverse-graded to normal-graded. Clast organization such as preferred clast orientation and imbrication has not been observed.

Racies A2: thin-beddedPoor-sortedconglomerate Facies A2 comprises very poor-sorted, matrix- or clast-supported conglomerate. Although the conglomerate contains boulders to coarse pebbles, it is thin-bedded ranging from 5 cm to 40 cm thick. Most of the conglomerates show lenticular bedding with erosional base and well-stratified upper surface. Beds are massive andl or crude inverse- or normal-graded. Clast organization is not so developed. This facies is considered to represent channel lag sediments.

Racies A3: well-sortedconglomerate Facies A3 is the second commonest facies in Conglomerate and Pebbly Sandstone Megafacies. It comprises matrix- or clast-supported and well-sorted conglomerate. Clasts are mostly coarse pebble to granule, The matrix consists of coarse- to medium-grained sand. Bed-thickness is 20 cm up to 6 m with average 80 cm. Most of the beds have sharp and erosional bases, and are either massive to normal-graded, or nomal-graded, or more rarely inverse- to normal-graded. Many of them show distinct clast imbrication. Inclination of clast is relatively high (20-35 degrees) in the lower part ofthe bed, but gradually decreases upward. Longest axes (A-axis) of clasts of this facies are aligned parallel to the paleocurrent direction. Flute marks are common on the sole of this facies.

Facies A4: ungradedParaiiet-strattfied conglomerate and sandstone

Facies A4 comprises parallel-stratified alternations of ungraded conglomerate, ranging from 10 cm to 60 cm in bed thickness, and ungraded coarse-grained sandstone or pebbly sandstone, ranging from 5 cm to 1 m thick. Thickness ofthe alternation is 20 cm to 6 m, and 60 cm on an average. Bed boundaries are poorly defined, this facies often grades upward or downward into other facies. The conglomerate is matrix-supported and weH-sorted. Clasts are mostly less than 6 cm across. A- axes of clasts are aligned parallel to bedding surface without any conspicuous clast imbrication.

Racies A5: Ungraded cross-strattfied conglomerate and sarzdstone

Facies A5 comprises ungraded conglomerate and pebbly sandstone with trough- cross stratification. This facies is uncommon. Internal stratification surfaces are smooth, but the base of cross-stratified units scour underlaying beds. Most units are 10 cm to 60 cm thick. Inclination of cross stratification is up to 30 degrees. Cong!omerate consists of well-sorted and matrix-supported. Most clasts are less than 3 cm across, and are aligned parallel to cross stratification. Clast imbrication seems to be absent. Reconstruction of Fujikawa Trou.gh in `Mio-Pliocene Age and its Geotectonic Impljcation 17

Facies A6: massivePebblpt sandstone Facies A6 comprises poorly sorted, coarse- to very coarse-grained sandstone with clasts of granules andlor pebbles totalling less than 50/,. Most clasts are concentrated on the bottom surface, but some ofthem are scattered throughout the sandstone bed. Abrupt normal grading is developed. Current ripples are often recognized in the uppermost part of the bed which are interpreted to have deposited from the entrained layer of MuTTi & Riaai-LuccHi (1972). Inverse grading is rarely seen. Many rip-up clasts are contained in the upper part of the bed. At bed bases, various sole marks such as fiute mark are recognjzed.

2) SandstoneMegafacies 17'acies Bl : thick- to medium-bedded massive sandstone Facies Bl is the commonest facies of Sandstone Megafacies. It comprises coarse- to very coarse-grained sandstone. Some of sandstones have dish structures andlor crude coarse-tail gradings, but most of them are structureless. Bed-thicknesses ofsandstones are 30 cm to 3 m (mostly 60 cm to 1 m). Base ofsandstone bed shows gentle scouring. Amalgamation of beds is common. Most sandstone beds contain many rip-up clasts which tend to be arranged on a certain plane in the upper part ofsandstone bed. Flute marks are frequently recognized on the sole surface.

Facies B2: coarse grainedParallel-stratzfied sandstone Facies B2 comprises fine- to medium-grained sandstone characterized by the development of horizontal thick-stratification. Beds are 10cm to 120cm thick, 40 cm on an average. The base ofthe sandstone bed is not sharp and does not show scouring. Most sandstones are crudely normal-graded, or ungraded. This facies often grades into other facies below and above. Facies B3: cross-stratiped sandstone Facies B3 consists of fine- to very coarse-grained sandstone characterized by development oftrough-cross thick-laminations. Co-set thickness ofcross laminations is 10 cm to 1 m (mostly 30 cm to 50 cm) occasionally up to 3m. The bases of the co-set are erosional scouring into subjacent layers. In each cross-stratification, crudely normal grading is recognized. Inclinations of cross lamination are less than 20 degrees. Facies B4: fine-grainedParallel and thin-laminated sandstone Facies B4 comprises parallel laminated alternations of fine- to medium-grained sand, and silt to very fine-grained sand. Some alternations show wavy laminations. Thickness of the sandstone is 2 cm to 1 m (mostly IO cm to 15 cm). Bases of sandstone beds are fiat and do not show any remarkable scourings. Basal boundary is generally poorly defined. Facies B4 often grades from Facies Bl. No grain 18 SoH, W.

organization except for parallel lamination can be observed.

3) Alternations of Siltstone and Sandstone Megafacies

Facies Cl: sandstone interbedded with siltstone

Facies Cl comprises rhythmic altemations of sandstone and siltstone referable to turbidite ofclassical definition (e.g. BouMA, 1962; WALKER, 1967). Bed-thicknesses ofthe sandstone and siltstone are 1O.2 cm and 5.1 cm on an average respectively, and the thickness ratio of sandstone and siltstone is 1.8 to 2.3, with 2.0 on an average. Sandstone is fine- to coarse-grained, and is normally graded. The base of sandstone beds are flat and sharp. Bouma sequences (Bauma, 1962), especially Tac and Tbc, are developed in the sandstone bed, but d divisicn (upper parallel lamination) is not recognized. Various sole markings and trace fossils on the bottom surface are abundant. Most of sandstone change upward gradually into siltstone. Siltstone is massive and unicolor, and can not be subdivided into turbiditic and hemipelagic parts.

Facies C2: sittstone interbedded with ri Ple cross-taminated sandstone

Facies C2 is one ofthe commonest facies in the study area. The facies comprises monotonous siltstone interbedded with sandstone. Sandstone is mainly fine- to medium-grained, but sometimes coarse-grained. The average thicknesses of sandstone and siltstone beds are 2.8 cm and 13.3 cm, respectively, with a thickness ratio of approximately O.2. Sandstone bed shows flaser bedding, current ripple lamination (BouMA's c) and crudely normal-grading. Siltstone is poorly sorted, massive and unicolor.

Facies C3: siltstone interbedded with thin-bedded coarse-gra71ned sandstone Facies C3 comprises siltstone interbedded with coarse-grained, sometimes pebble-bearing sandstone. At a glance, the bedding of sandstone seems to be continuous laterally in a orderly way, but as carefully observed, most sandstone beds are irregular in thickness, and show lenticular beddings pinching out laterally over a short distance. Siltstone is massive and poorly sorted. This facies is accompanied with facies C2 in many cases. Based on the characteristics ofintercalated sandstone bed, two subfacies are distinguished. Small size flute marks are often observed. Subfacies C3-a: Subfacies C3-a contains medium- to very coarse-grained sandstone which is crude normal-graded or massive. It shows well-stratified surfaces both at the base and at the top, and lateral thinning of bed-thickness. Sabfacies C3-b: Subfacies C3-b comprises fine- to very coarse-grained sandstone with a few granules, which is normal-graded, rarely with current ripple lamination in the upper part ofthe bed. Base of the bed is to scour the underlying beds. The shape of sandstone beds is lenticular. The sandstone of this subfacies gradually chan- Reconstruction of Fujikawa Trough in Mio-Piiocene Age and its Geotectonic Implication l9 ges upward into siltstone. Subfacies C3-b may be fill-sediments of minor channels.

4) ChaoticDepositMegafacies

Facies Dl: Pebbl] siltstone Facies Dl consists of poorly sorted muddy siltstone containing angular clasts of andesite and basalt, 5 cm to 1.2 m in size. Many slump-folded blocks andlor rip-up clasts are frequently contained in it. Pebbly siltstone beds form lenticular sedimentary bodies with erosional base, a few tens to hundreds of metres across and 40 cm to 20 m thick. Normal grading is often observed.

Ii'acies D2: chaotic dePosit with coherentfoldedlcontorted strata Facies D2 is least developed in the study area. It comprises poorly sorted siltstone with transported blocks of centorted strata suffering coherent folding and deformation, and some pebbles. Beds ofthis facies are 40 cm to 5 m thick. Basal boundary is flat, and scouring of the subjacent layer is not recognized.

5) VolcaniclasticSedimentsMegafacies Pacies E: volcaniclastic dePosit

Facies E is composed of volcaniclastic materials, such as dacitic to andesitic tufl' and tuff-breccia. Bed thickness is 2 cm to 2 m. Bed bases are flat and sharp. Most ofthe beds have normal grading, parallel stratification, and rip-up clasts ofsiltstone. Rill marks are observed on the sole of several beds.

5. FACIES ASSOCIATION

Facies association (MuTTi & Ricci-LuccHi, 1972; l975) is composed of a combination of facies, that make a preferred facies transition and form a characteristic sedimentary body of various scales and degree of organization. Facies association is considered to reflect the spacial expression ofa depositional environment and process, and not directly corresponds to stratigraphic units such as a member and formation. Since MuTTi 8c Ricci-LucaHi (1972), several attempts have been made to compare turbidites and associated coarse clastic deposits with those of reported modern and ancient turbidite basins, and have developed a concept of facies association for interpretion of turbidite paleoenvironments (MuTTi & Ricci-LuccHi, 1975, MuTTi, 1977, WALKER, 1978, UNDERwooD & BAcHMAN, 1983, and others). In the Minobu Formation and its equiva}ent strata, eight facies associations can be recognized, and they are grouped into coarse clastic association group, fine clastic association group and Pebbly Siltstone association (PMA). Coarse clastic association group is composed ofSandy association (SDA), Conglomeratic association 20 S(/)H, N¥Avi.

Table 4. List of Facies iXssociation in the four different areas. [ZZZZZZ-EI[ Nishikatsura domain

Manzawa Jikebono Minobu ,Nish;katsura area area area area

.Conglomeratic- sandyass.SP.IA; Conglorneraticass. Conqlornerate-v sandyass.S.!ILII sandstoneass. Sandy-siltyass. sÅ}ltyass•SIAm wa

pebblyrnudstoneassociation!l!wo.A

(GSA), Conglomeratic-sandy association (SCA) and Conglomerate-sandstone association (CPA). Fine clastic association group consists of Silty association (SIA), Sandy-silty association (SSA) and Silt association (SMA). The Minobu Formation and its equivalents in the four different areas mentioned above, are each represented by the characteristic combination of these facies associations (Table 4).

Deseription of 17Tacies Association (1) Sandy association (SDA) SDA can be recognized in the Hakii Alternation of Minobu area and the Hara Mudstone of Akebono area. SDA is exposed in many outcrops on the bank of Fuji River, for example at Loc. a and d in Fig. 7 and 8.

1-a: Facies component and facies transition SDA consists of Facies Bl, B2, B3, B4, C2 and C3 accompanied by Facies Al, A3 and A6. Vertical succession of these facies was observed in the fields and analysed by the modified markov chain analysis (PowER & EAsTERLiNG, 1982). Examples of the measured data of facies transition are shown in Fig. 9. Results of the analysis on the preferred sequence is shown in Fig. 10. Characteristic features of SDA are summarized as follows. 1) The preferred facies transition is interpreted as a fining upward sequence (Fig. 9), that is, Facies Al, A3, A6 and B1 with erosional bases tending to change into Facies C2 and C3 through Facies B3, B2 and B4. 2) The thickness of fining upward sequence ranges from 2m to 8m. 3) Coarse-grained sedimentary facies such as A3, Bl, B2 and B3 are predominant in the sequence, and fine-grained sedimentary facies such as Facies C2 and Facies C3 occupies only a few per cent of the total volume. 1-b: Geometry and internal structure One ofthe important features ofSDA for the analysis ofthe sedimentary process Rcconstruction of Fujikawa Trough in Mio-Pliocene Agc and its Geotectonic Implication 21 is the morphology of the sedimentary body. In a N-S transverse section (Fig. 7), normal to paleocurrent direction, SDA forms a lenticular sedimentary body several tens ofmeters to a few hundred meters wide and several meters to a few tens of meters thick without exception.

Fig.13 adaktsntmt' ¥¥" Z. -- --. s .. . GSA $.¥ z 1 x. ¥' :: .: SIA llil }8 f' Otslaw 2¥kn : SDA iief/itl't"1,FX";,r.a,as,,a,, i4%.e r:i.TS"-"i{;'¥

Obikane ,i,;-:,:s',C.`.;' Sil7i {.. s,::;Yi.`,- PMA g g. ¥N 1/,/,s/,'N. di .g ------`t)g,l,t:,z,/Zs:'Nb"tYNi.,/,,,r' gF.k{.i3 . 'ql.". t,,/,iss//li"s¥i/i/i'si$'x""is - -- t- x' 'g,l¥.'.' N¥ ¥ -- . . xx ----- i. . x em -- ,c¥¥e' .' : ' 8Å~ . . --. -t x . . ---' -kv ¥':¥¥ . ¥g e, ""Stspt ,,,, ,¥eN ..:' , . . ¥.'--¥ ¥:¥'¥ :'¥ s. b 7,k{:¥.-- f.K..lllr'a' "'sx,v/2'XS/',," . e5 t- /:.Flils.ylipt{I.i,¥ ... - .. ':.- : ee - -- . --t .-- - . ,..111..>,xxl,XX,lll.i¥1,/>S/,111 -i -t -- :---- t¥.g'i¥.' -- ¥'. ¥ -t::'¥s'¥. -- .-- - ¥' g ¥¥ ¥lk'lli,lj .Mtnobu . -t------}------¥;:¥)¥ tt ;iliX¥,;i..tE.l.tti,i.sti,l.i..l -" . -- -i-- . ./t --- -. -- x•Å~ 'i'{ l------t i.gllX/k"sl,l.i -- t------,sx' e ---- 1:ti":":i`}';"'¥'l""1""i""l: 1¥/LtE'¥i¥F'tli¥'Lci: x s' -- -t--t t- . 8 eSc -- ....t . -- raYaefi'ikl ,-et -t. i- I¥¥:,il' L..":l¥.i¥i¥.:s'fi¥.'¥'¥.¥.'¥.¥.¥ -¥ . - '¥ ts7¥ 't -e 1 .I¥: . .v t;' ...... 'g "ut beiteu-sh;t '.1. . 1.rny"S. --. .4--1i: ....:.N'. ..S. ;. .i.. -. e - --tt ...` . -- ,,¥l'l,il.l,sf},., -----i i- ' .1¥¥1/,¥/,¥/;l¥ r. , .:.q.")c ¥.' ¥.l¥l:l¥X'¥i&¥71'¥i¥I¥I'¥l'l'll¥l' , -, ¥:¥.""x N -- N ('¥ 1''1''¥¥¥/'/'¥1,s/s :/.; . . :,ii,".,...t}:. ',",/',//,/. 1""Y 9-/':" -4.i i:¥:i:::i'I" ':,',i':.:.il:.'Lly':'i 5!J',i'za¥¥ .,.g{ 1'l'. 2.n ),: ll,/ti,l//i,/111111,1;,//,,,,, $.i,,.,..,Nx g 't f' 8pti'A.`. i- -, i,.¥¥. z r`"t m I, Oehima = !r,¥lv-t ss K v "¥ w¥' M m Y E N7iN.i YLi, Nsx m- t.}LFI--'ny ' .- c 'tt'"-Åí$.,$lk,l%,,gi k"Si to , sA i/3if..g' Ns.; m = i.-1-i. ¥ ,/¥ "ttsugesawa -?¥ sS - K' : I x.NkNW o o o o tu -o " vc bj ' / Dykerock "-t gq l.¥, k,ctpa-l ,i i' '4 sit' sy .+ N hk`.:vss .4 v¥.¥ eteUm k 'IS /dip & strike

Fig. 7. Facies association ma p of Minobu area. 22 SoH, "r.

o 1km 20

) :l:I:]' GsA = 17 n --- m O""' 15 m 12 -1.,k:-cr, FAciEs E 1km [=] .--.-- o -"-- r] s/¥L (KEYBEDS) Z ro -.--....---J'r- ] ;Il'i. SIA OD 24 g m [] '.'tt:-.--L FAc[Es C1 30 '=t:-:- {KEY BED) -pMA g fi Illll s l9 Z '.-¥- fi 5 (EIXglgT.es... ¥ ¥ . l ¥:¥. o tt ee ilO oo 3 1 2] z gm ee --- 2 [l l) 15' 15 ! t ,-.T rt c lliilli -- ,1 1:.::: -:- 1- :.: , .. ¥¥ :::: - :l:--¥ 1-- c 1'1 :::: 111i :- ¥ 1i. Ilil-- 1ii ------Ill -:- :.:. 28 -:-:" ¥1:: [!n -t--- l:I, :.: .. e:-: ;;--s'v{E--¥,ny'¥.'k'. ¥¥ T [== --t --¥-- :-:- - .- .¥ .¥ ------.. I.:i e:- ---Ei-- :.:. --¥ .-- :-: - '9 ' -{¥ .. --. , Eg -:- : :l:. Ill -:- ge 2¥k,,gcl't}i't'ev3.ithllZ-%'i Sl,IC,..,ttt l ' 2.a ' -- ¥¥ Es e= es --- li¥l H --¥:- ¥ -- i/" --- ;/2;f/../ ' g 7-///ti -, :-: -. i g f/ti-. ?,2:: :.: -¥-- .;-i-..al;/lllzlg 'i''1i autijg 8 .fuT....\/E//ua Y//2 - -:- Zt;y .i :.: // 7I ///}' -i' y : Sl:i, south Z-Z -t north NishiyatsushiroGroup

Fig. 8. Morphology of GSA and SDA, in the profile normal to paleocurrent direction. Solid and broken lines indicate the boundaries of facies association and 1

Furuyashiki Sedimentary Body, named for one of the SDA, is well-exposed at the cliff to the west of Furuyashiki (Loc. a in Figs. 7 and 8). In the Cliff, N-S trending profiIes ofthe southern part ofthe overlying Furuyashiki Sedimentary Body and the surrounding SIA are exhibited. Dip and strike ofbeddings within the body and of the surrounding SIA can be observed to be parallel each other. However it should be noticeable that the boundary surface shown as line a-a' in Plate 2-1, which is conspicuous and sharp, is gently oblique to the bedding plane, constituting a "concave-up arch". When tilting correction of the bedding plane is made, this boundary surface dips northwest at a little Iess than 20 degrees. Paleocurrent direction measured within the body is of concordance with strike of the original boundary surface. Beds within the body thin rapidly toward the boundary surface abutting and onlapping at this boundary. Thus, the basal surface is considered to represent a part of erosional wall. Upper boundary of SDA can be observed at several outcrops. In an outcrop, east of Furuyashiki (see, Plate 2-3), upper boundary of Furuyashiki Sedimentary Body is observed to be nearly flat and gradational to the overlying SIA in contrast to that of the basal boundary. Detailed mapping work between inter-outcrops indicates that the upper boundary Reconstruction of FuJ'ikawa Trough in ILI io-Pliocene Age and its Geotec onic Implication 23 is in parallel with the bedding plane of adjacent SIA and bedding within the sedimentary body. On the other hand, an E-W profile of sedimentary body of s SDA, that is parallel to paleocur- .,.?.,;¥ 4:"- N rent direction, can be Facies observed Bl FaciesB4 at the cliff north of Arayashiki. The boundary of Arayashiki the sedimentary body and the unde- FaciesBl :zl::,:i¥! :'9¥.¥ 1'1¥¥- ;/ rlying beds (a-a' in plate 2'2) FaciesB2 shows a conspicuous surface nearly Facies C2 concordant and parallel to bedding FF ,a glgg B2 planes. Beds within the sedime- FaciesB2 ntary body are continuous and ::21g;g22 uniform in uniform thickness in and sedimentary structures, ln ¥ Facies B3 contrast to Facies B2 those of the N-S profile described Facies A4 above. Basedonthesetwoprofiles, different in relation to paleocurrent Facies Al direction, they can give a three- dimensional morphology Facies of B2 Sedi' Faciesc2 mentary body of SDA- DetailS FaciesBl. of the morphology are discussed FaciesC2 later. Facies Facies B2 B4 Internal structure of Furuya- FaciesBl shiki Sedimentary Body is charac- Facies B4 terized by a series of superimposed Facies Bl Facies c3 thinning and fining upward se- c'esBl quences with scoured bases often :aaCcii:g:g incised afew meters deep. Indi- FaciesBl vidual beds within the sedimentary Facies c2 2m body are variable in thickness, and Facies B4 n{v¥:¥::¥/:.,i'=,?t:t..: are laterally discontinuous. '-:W.n:.h"st These features of its morphology and internal structure are Facie$Bl o commonly recognized in many " 1 other sedimentary bodies of SDA, for examples, Arayashiki and Ki- Fig¥ 9¥ An measured of Kitaharasection Sedirnen- tary Body, showing sedimentaryfacies and tahara Sedimentary bodies¥ cycles in the sandy association (SDA). Ar- rows indicate fining-upward sequences. 24 SoH, W.

Bl >2.0 l.O.2.0

Al O.5->1.0 Bt2AiS'

gse/o conf.:6s.so G2test:76.85

Fig. 10. Transitional pattern of sedimentary facies in the sandy association (SDA). after Powers & Easterling (1982).

(2) Conglomeratic association (GSA)

GSA is exposed in Minobu area only, along the highland area on eastside of Fuji River from Hadakazima to Nanbu, which correspond to most ofthe Marutaiki Conglomerate as a stratigraphic unit. Good outcrops of GSA can be observed along Kuwagara Stream. 2-a: Facies components and facies transition GSA consists mainly of Facies Al, A3, A6, Bl, B2, B3, B4, C3 and C2 interca- Iated with A5 and El. Facies Dl can not be observed. Facies described above are complexly interbedded one another, so that, the modified Markov chain analysis (PowER & EAsTERLiNG, 1982) is made for determining preferred facies transition. According to this analysis, the following relationships of facies transition can be suggested (Figs. 11 & I2). 1) GSA has a preferred facies sequence changing upward from Facies Al, A3, A6 and Bl (with erosional bases) into Facies C2 and C3 through Facies A4, B3, B2 and B4 (Fig. 12). 2) Preferred sequenceb of GSA are also interpreted as fining upward sequences, although they are more complex and irregular than those ofSDA. 2-b: Geometry and internal structure GSA forms a conspicuously sedimentary body like SDA, but larger than the Reconstruction of Fujikawa Trough in Mio-Pliocene Age and its Geotectonic Implication 25

l¥E:i¥l:'1/i,i¥. I'

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fi'/lft'rLi':t.r¥l7¥,.Li't¥;',/r']/ Facies A4 Facies A6 .:.,1¥:.,1."¥o.¥.¥,-.',1: Facies A5 Facies C2 ¥1¥.¥:-'¥'.";'.,.' /' tt¥'i.¥'ii¥i¥:¥ , x Facies A6 "' e:":'e. :r ¥¥' ".' P ' ' x Fig. 11. An measured sectionshowingsedimentary facies and cycles in GSA. Arrows indicate fining-upward sequences. latter being 3 km to l3 km across in the N-S section and 20 m to 2000m thick. Marutaki Sedimentary Body, shown as c in Fig. 7 and 8, is the largest and most widely distributed in the east of Marutaki. It offers a good example for investi- gating detailed morphology and internal structures of the sedimentary body of GSA. Outline of the N-S profile, normal to paleocurrent direction, of Marutaki Sedimentary Body looks like a section of "baseball-type cap" placed upside down (Fig. 8). Unfortunately, lowest base of the sedimentary body is not exposed, but basal boundary of the northern marginal part, that is the wing of "cap", is well- exposed (Loc. b. and c. in Fig. 13). In those outcrops, the base of the sedimentary body, usually sharp and conspicuous one, seems to be in parallel with the underlying bedding surface. However, the base of the sedimentary body should be interpreted 26 SoH, W.

Bl . >2.0 A1 ------...till(4 - 1.o-2.e O.5.I.O x A3 Nx , B3 A6 t xx B4.Q'>,xxx(llliB2 Å~ C2 950/o conf.:82.7l

G2test:I05.64

Fig. 12. Transitional pattern of sedimentary facies in GSA. after Powers & Easterling (1982). to scour the underlying SIA. This is because it is slightly oblique to the bedding plane of the underlying SIA, when tracing the several outcrops laterally (Fig. 8). Additionally, according to the detailed facies map (Fig. 13-A), the disruption ofthe key bed in the underlying SIA is recognized to the north of Onta, indicating the erosion of the underlying SIA. At the northern wing ofthe sedimentary body, the boundary surface between the sedimentary body and the laterallyjuxtaposed SIA, clearly discord with the bedding plane of both GSA and SIA. Judging from the detailed facies map at the north- western tip of "cap" in Hadakashima area, the dip and strike of the boundary surface (as shown A-B line in Fig. 13-A) is estimated to be approximately N80W 65S. In contrast, these bedding planes display approximately N40W30SW. In addition, the same characteristic phenomenon is recognized at the northern part of the "cap'' in Okuzure (Fig. 13-B). Judging from the topographic map in which the locations of outcrops are plotted (Fig. 13-B), the basal boundary of Marutaki Sedimentary,Body at Okuzure is conspicuously oblique to the bedding planes of GSA and SIA, and scours to the underlying SIA. The boundary contacts around Okuzure are also observed to be sedimentary, and not to be faulted (Loc. e in Fig. 13-B). Such erosional structure of the basal boundary to the underlying SIA, and the obliqueness of the boundary surface to the bedding planes are strongly infiuenced by the original topography reconstructed as a bottom of large-scale "groove structure" on the sea-fioor, and a wall of "groove structue" with a ralative high angle, relatively. Reconstruction of Fujikawa Trough in rv(io-Pliocene Age and its Geotectonic Implication 27

Basal boundary of the southern half of the sedimentary body, however, is not so clear because of dense vegetation and Iimited outcrops. The upper boundary of Marutaki Sedimentary Body can be traced for 15 km in N-S direction. It is of structural concordance with the overlying beds of SIA (Fig. 7), and a gradational transition to the latter can be observed in many outcrops such as Loc. c in Fig. 13-A. As a whole, the distribution of Marutaki Sedimentary Body extends along NNE-SSW parallel with the direction of paleocurrent measurements of the sedimentary body (MATsuDA, 1958).

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A: Hadakashima area, on tip of the "cap" of Marutaki Body. Heavy line is the boundary of the conglomeratic (dotted part) and silty associations. Thin lines indicate a key bed.

Fig. 13. 28 SOH, "T.

N g ¥i }K< ,, 71ig,IL X ii xÅ~jsifiijlj[eti ,, !c}eek Geem ft B 7 e 500m ¥R,Eg'N l,]}}l,e le pm e,' kX .' t",,.S,x'H' 80 v- G$lki,tkk ,liS,) iLV' o 4 v v d ,i ,1S5. ftgR¥' E ' ,g es N( x> "Å~ / M 69 li E$l :"i"'.D )}lls.xl>'" ea )' t t Tsubakizori e6 ,, ". ' IA !¥ t. a,,l,i,liSSii'R,ta)!,Ites.a NN .(i,S> twa ff ' e,x pu,i, K , ,}e ), ;- S e /¥i .l,t 3o iii?2ty/;i,ilrtpt,o.8.;)::;;:..csg,, e"nyc`

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r--- KEY BED " E¥i;il:lili D - iliill¥Il ilS¥,,¥lii'1)¥¥g¥. M

Mega- Mega- Mega- FaciesE FaciesCl FaciesD2 faciesA faciesB faciesC B: Okuzure area, on the northern part of Marutaki body. Boundary of conglomeratic and silty associations is oblique to the dip and strike of beddings and key bed. 1. dyke rocks, 2. fioating block, 3. dip and strike. a, b, c, d, e, means localities of the outcrops showing the contact of two associations.

Fig. 13. Topographic maps with location of outcrops in the northern part of Marutaki Sedimentary Body.

Internal structure of Marutaki Sedimentary Body is seems to be represented by a series of superimposed fining upward sequences with basal scouring up to a few meters deep. Most beds within the sedimentary body are Iaterally discontinuous. Basically identical characteristic features are recognized in Obikane Sedimentary Body of GSA which has similar facies components to those of Marutaki Sedimentary Body of GSA. These characteristics are thought To be common features of the sedimentary bodies of GSA. Reconstruction of Fujikawa Trough in Mio-Pliocene Age and its Geotectonic Implication ee

(3) Silty association (SIA)

SIA is widely exposed along Fuji River from Minobu to Akebono areas, and composes the most part of the Yagisawa Mudstone, the Hakii Aternation and the Hara Mudstone. SIA is partially heterotopic but contemporaneous with GSA and SDA. 3-a: Facies component and facies transition SIA is composed mainly ofFacies C2 intercalated with Facies C3, and sometimes contains Facies Cl, El and D2 (Plate 3-1). Positive preferred facies transition in this association can not be recognized. Distinct sedimentary cycles such as a thinning or thickening upward sequences are also not observed. 3-b: Distributionalpattern SIA does not form any sedimentary body of a definite shape, but is extensively developed surrounding GSA and SDA (Figs. 7 and 8). Broadly speaking, SIA is monotonous throughout the study area, However, local variations ofthe dominated sedjmentary facies, and of thickness ratio of sandstone and siltstone of the same sedimentary facies are recognized. For examples, in Tsubakizori near the boundary of Marutaki Sedimentary Body, Facies Cl is better developed than facies C2 and C3. Additionally, the thickness of sandstones in Facies C3 tends to decrease around Teuchizawa, and to increase around Mitsugozawa.

(4) Conglomerate-sandstone association (CPA) CPA is distributed in Nishikatsura area. Details of the sedimentary features of CPA can be observed in Tonoirisawa Stream (Loc. a in Fig. 6). The Katsuragawa Conglomerate corresponds to this facies association.

4-a: Facies component and facies transition CPA consists of Facies Al and A3 with intercalation of facies A2, A4, A5, A6, B2 and B3 (Fig. 14 & Plate 1-3, 1-4, 1-5, 1-6). Positive preferred facies transition can not be obtained from the statistical analysis. It appears that the association is monotonous laterally as well as vertically. However, in some vertical sequence of CPA, crude finning upward sequence, that is Facies Al, A2 and A3 with erosional base tending to shift gradually Facies B2 and B3 through Facies A4, are recognized. 4-b: Geometryandinternalstructure CPA is continuously distributed as a narrow belt along Nishikatsura Fault, extending in an ENE-WSW direction for 20 km, from Iwadono to Lake Kawaguchi. Longitudinal profile of the distribution is parallel to the paleocurrent direction measured from CPA. 30 SoH, W.

Two different kinds ofthe boundary between CPA and the underlying strata are observed. Around Tonoirisawa FaciesA3n.edi"su FaciesB2 't' ,"- (Loc. a in Fig. 6), the underlying Furuya Sandstone changes gradually into CPA 8.eNfo with an increasing the intercalation of FaciesA3 N;-¥:¥¥t:.kiF8g/E conglomeratebeds. However,thebou- FaciesB Facies i/¥i,:/i,:'{-:'tV;','Fl¥:':'-:l,'if¥Y:-':¥¥ ndary is sharp in outcrops forming a ¥-tSS6iES?.,edpcs.i -c>u¥9"CS.'e" base of channel structure (Plate 1-1). '' t¥e::¥c'b6'bee"'e.ve .t@,BE'-"e'ttdiV'.S;-'e:'oepeng:.6'es",ee'C)Feo.o-.e-.e.9po'-otso'e.'.;.tt•-oÅéBEEtg2:i?.lsl::-t'eeR.".g,g96g The boundary surface is mapped approximately parallel to the bedding planes of both CPA and the underlying Furuya

Sandstone. On the other hand, in the FaciesA4 area from Kamiohata to Asari (Fig. 6), g"b"D the boundary is oblique to bedding plane FaciesAl oftheunderlyingstrata. Manyangular gts"g.g"R.:o.... clasts of the underlying Nishiyatsushiro Group, are observed on the uneven FaciesA3 boundary surface (e.g. outcrops south FaciesB2 to Asari) indicating the erosion to the ..:.h.I:-.=th". X"gpssetwh.siil"ts-. underlyings. Judging from a geologic map (Fig. 15), such erosional surface can D be traced from Asari to Kami-Ohata. Original strike of the erosional surface, 3m when making bedding correction, is ""rslj parallel with the paleocurrent direction FaciesA3 esoO measured within CAP, that is, in parallel -it-'tt"')t"',t:S:'.:-.:-.J"":.:.:::.-v-:.tterv FaciesB2 .Y..,"{:-"--::;.::"'-:.:. with the elongation of the distribution "d".cSees1' o of CPA. Fig. 14. An example of the measured section Upper boundary of CPA can not ofCPA. AtHishakuruzawa. be observed in Nishikatsura area due to cutting by Nishikatsura Fault. As to the internal structure, CPA seems to be structureless with the exception of minor channel structures (Plate 1-1). Beds of CPA show to be rather continuous in thickness than that ofGSA mentioned above.

(5) Conglomeratic-sandy association (SCA) SCA is widely distributed in Manzawa area (Fig. 16), which corresponds to the Toshima Alternation and the most part of coarse-grained sediments within the Utubuna Alternation and the Manzawa Alternation. Good exposure is found on the right bank of Fuji River west of Hashinoue. ReÅëonstruction of Fujikawa Trough in Mio-Pliocene Age and its Geotectonic Implication 1 31

- [llllill O 'i;/;/g///¥g¥//¥/g';: IEIEI [Z) N 1 2345 6 7 Fig. I5. Topographic map with location of outcrops in Maruta, see Fig. 6 for map location. Note that CPA directly about to the underlying Nishiyatsushiro Group. 1. black shale of "Shimanto Group", 2. alternations of sandstone and mudstone (Kawaguchi Formation, Nishiyatsushiro group), 3. altered pyroclastics (Onuma Formation, Nishiyatsushiro group), 4. Conglomerate-sandstone association (CPA), 5. intrusive rocks (porphyrite), 6. dip and strike, 7. floating block.

5-a: Facies component and facies transition SCA is composed of Facies A3, A6, Bl, B2, C2 and C3 interbedded with Facies Al, A2, A4, B3, B4, Cl and El. Facies D2 is also frequently contained within them (Fig. 17). Result of the modified Markov chain analysis indicates that the association has positive preferred facies transitions, which are principally regarded as fining-upward sequences (Fig. 18). An example of measured data of facies transition, is shown in Fig. 17. SCA is more complicated and variable laterally than GSA due to frequent intercalations of both chaotic deposits and fine clastic sediments classified as Facies Dl, D2 C2 and CI. Noteworthy is that the distribution of Facies Al, A2 and A6 tends to be confined to the central part northwest of 32 SoH, "r.

NK"er ; bg "" N g!li!-l2e x,.))l.ll],. t si Shlmobe F,l-.

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Facies A3

Facies C3 Facies C2 Facies Al Facies C3 fining Facies Bl upward cycle Facies Cl Facies C2 Facies B2 FaciesC1 Facies A6 Facies C2 Facies C3 Facies B4 Facies C2 FaciesCl

Facies D2

Facies Dl

Facies D2 fining Facies A6 upward Facies C3 cycle Facies C2 Facies C3

Facies A4 fining upward cycle Facies A4 3m

Facies A4 Facies B4 Facies A4

FaciesA3 o Fig. 17. An example of the measured section of SCA, showing fining-upward sedimentary cycles.

Hashinoue. Many thinning-upward sedimentary cycles of 20m up to 100m thick are observed in SCA (Fig. 19). Alternations of Siltstone and Sandstone Megafacies, a few metres to 30m thick, are frequently contained in the upper part of the sedimentary cycles. Most sequences of this cycle, however, comprise Conglomerate and Pebbly Sandstone Megafacies and Sandstone Megafacies only. Facies Dl, D2 and E are minor compornent, occasionally intercalated in the upper part ofthe cycles.

5-b: Geometry and internal structure SCA forms a lenticular sedimentary body in the profile normal to the paleocurrent direction measured within SCA. In morphology, two different types are recognized in the sedimentary body of SCA. The first one is single-storeied 34 SoH, wr.

Bl - >2,O 1,O-2.0

------O,5-> 1.0 Al l A4 B3 A3l I B2NNN A6 /,.,c B4 e C3 gsa/o conf.ls2,7o G2test:U3.36

Fig. 18. Transitional pattern of sedimentary facies of SCA, showing fining-upward sequence. sedimentary body, which is developed western part of Manzawa area (S in Fig. 16). Sedimentary bodies of the single storeyed type are less than several tens of meters wide and ten to twenty of meters thick. Geometric characteristics of this sedimentary body is similar to that of SDA. The second type is the multi-storeyed sedimentary body consisting of a pile of many smaller scale sedimentary bodies. The sedimentary body of this type, distributed around Hashinoue, is 5-7 km wide and more than 2000 m thick only (M in Fig. 16). It is characterized in having lateral branches, that is, SCA oftenly interfingers with SSA laterally, and is completely replaced by the latter over a few kilometers (Figs. 16 & 20). As to internal structure, many thinning-upward sedimentary cycles of 20 m up to 100 m thick are observed. The basal boundary of the branched body can be traced laterally to the basal surface of the large-scale thinning- upward sedimentary cycle. Channel structures of various sizes are developed in SCA. Emphasis is that the mutual relationship of sedimetary body of SCA and thejuxtaposed SSA is quite similar to thar ofFuruyashiki Sedimentary Body ofSDA and the surrounding SIA mentioned above. For example at Loc. A in Fig. 20, the basal boundary surface of the branched body of SCA is conspicuously sharp and gently curved. It is clear that the surface discords with the bedding planes of the sedimentary body and underlying SSA, indicaring the erosion ofthe underlying SSA. Beds within the sedimentary body thin out towards boundary surfaces onlapping the boundary plane. Strike of the origina] surfaces of the boundary is NNW-SSE parallel to predominant paleocurrent direction of this area. Upper boundary of the sedimentary body is of structual concordance with the overlying SSA not only Reconstruction of Fujikawa Trough in Mio-Piiocene Age and its Geotectonic Implication 35

B TThinning- upward

A 10 ThThinning- upwardu

Measured K,B. tu.s ,= "" t bed number

Thinning- upward g

loo n J B taf.as"eg. .. o t 10 100 1000CM Bed-thickness Fig. 19. Generalized section of SCA, showing developments of thinning-upward cycles through the section. A: measured section at Hashinoue. K.B; keybeds of Facies E. B: Bed-thicness of B portion in A, arrows shows thinning-upward cycles. in outcrops, but also in mappable order (Figs. 16 & 20). Beds constituting the sedimentary body are laterally discontinuous, and are variable in thickness in contrast with continuous and uniform beds of SSA.

(6) Sandy-silty association (SSA) SSA composed of most of the Utubuna Alternation and the Manzawa Alter- nation in Manzawa area. It borders with SCA in the highlands in the west of Kobayama (southwestern part) and in the mountains east ofUtsubuna (northeastern 36 SoH, W, part) in distribution. However, outcrops in the latter area are limited, because of wide distribution of the Sanogawa intrusive rocks. Several key beds can be traced throughout SCA to SSA (Fig. 20).

, e9,¥

N 't" " R52 ' 1 0 t m/ fi:' s Htrayama ' :.; 1. X7o ./i'7s o o,)r km /''-:'IT' f/''''' 1. ,, t. s /- t $.gorn }sEi:llili ttt9¥¥i.,i,, i8o 6.g Ni.i..i,iii... ¥"' ¥¥¥¥ t tttt ttt t' It.'....:.'.::.:l t t.tttttttt. xX/XR{3o.. ¥¥El6I'L'i,,,¥,.. 70aj t t.ttt r .. ' 1' tt.., .. ...t... t //tt ;¥¥¥ //.1.,.., .1...,.t. ,sllj - /// - ttttt - gt E. ,, 2 t. t "iss t lt ttttt ' tt Åít...... t. T3g' ';'11 tLtt :t.tttt/t ts .' -"..tY-S'.h ]o $,-V' tt C1,.] ,. IN' '"' ' ,/

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'` 'l: J '"t' ";'' ""I" x,xs}Sliix¥:¥$ll¥g "l.i.tt.-;Ji,l!.i']i'('{ ....' . /.s,JSoth "' t /tttttttt/t /. g•/$,t"/"tk,,Å~ tttt t {i' i,i,tt-'1,e' 2 t tttt tt ttt . tt t oJ 1,lttt 1 RSe6 '/''

'g : /' "' ''""'t"''' , ,...;li" eq25 1 ,,7., ,.',. ''' ttt < tt.t 6g¥l ./,,, >¥.1,, .t-t.. -¥ i¥¥i.7 '"' .l;i-deÅ~- t.t rg-pi '¥ ¥;¥,<'X¥ -::'. l¥-'A

'' T''1'' 1 kobayamo x/otV'5atw}tO .R52 . istX.¥) ,x. I 'be 1-¥.1¥.;i'I' . ' 40 - 5'iioXxX.xgx$ ¥-kY- ¥ ¥ ¥ ' ' ' x 1 '. 0, ..SCA ' 'l' l-," t-',t')tslN. . S" 9&)?'Z'N e' ..' -XD Xs .2S .X r,¥¥/' 1 lll,..125.Dpt'''.'..'Li N it ig .., . .,.20. .. ..t.va: geF< 45J"7¥ ," 'ao,::' ii¥`c.:.:'i::i.,Jl, ' ' "x Å~ ge• Elllli ' I,1 z.EOo EII] 8 ,-;,s).¥ Eil] 12 34 5 67 Fig. 20. Topographic map with locations of outcrop around Yashil

6-a: Facies components and facies transition SSA consists mainly of Facies C3, Cl intercalated with Facies C2 and-E, and occasionally Facies D2. Subfacies C3-b is most predominant. Facies C2 is not well-developed comparing with SIA and SMA. Conspicuous preferred facies transition of SSA is not recognized by field observations and by statistical analysis. 6-b: Distributional pattern and internal structure SSA is widely distributed surrounding the sedimentary body of SCA, and partly interfingers with the latter as already noticed (Fig. 16). Noteworthy is that, in SSA, lateral changes of sedimentary facies are recognized. The predominant sedimentary facies of SSA tends to change from Facies Cl to Facies C2 and C3-b lateraJIy far away from the margin of the sedimentary body of SCA. Additionally, thickness and grain-size of sandstone beds in the same facies also decreases to the same direction.

(7) Silt association (SMA) SMA is developed in lowland in and around Machiya. It consists mainly of two facies, Facies C2 and E with intercalation of Facies C3. The association, which corresponds to the Machiya Alternation as a stratigraphic unit, seems to be monotonous. No special facies transition are recognized either laterally or vertically, but thickness of siltstone beds in SMA tends to increase upward. Its distribution, however, is confined (Fig. 16). SMA reaches a few hundred meters in thickness.

(8) Pebbly siltstone association (PMA) PMA comprises Facies Dl (Fig. 21), and is scattered in Fujikawa domain stretching for 30 km from Akebono area to Manzawa area (Fig. 22). Comparing with the other facies associations, it has many particular characteristics in its distributional pattern, internal structure, geometry and dispersal pattarn. PMA is distributed fairly continuous in N-S direction, in contrast with its limited distribution in an E-W profile. For example, the distribution of PMA in Manzawa area is restricted in the western part and is thinning out eastward. Pebbly siltstone bed is regarded as the product of infi11ing of channel structure. Pebbly siltstone beds, well-observed at many locations (Fig. 22), are mostly not persistent more than 50 meters, and form lenticular bodies (Plate 4-4). Most bases of the beds are sharp, irregular and erosional (PIate 4-5). In contrast, top of the bed gradually merges into the overlying SIA or SSA. At the west of Minobu, some of PMA show the normal grading (Fig. 21). In the beds, the heavy clasts, mostly of massive andesite and basalt, tend to gradually decrease upward in size (maximum size of the clast 1.2 m in diameter, Plate 4-2). 38 SoH, W.

. -l.slii-t ..- --.J.A - 'i":{/v.tv t-nX e i:-1-.,I; e- -:r-i'- .- (-;t- .- r sJ - :.h -. `,);:¥;r..':'.¥r.:,"v.'': --.-.----.-=-.-¥ '- -,a poor--sorted ii,li./5.!.7Si"ias¥-:"oolO6g.egoo'qbNl$ir4{Es mud

"es.!q...... ii{I;2iai..iig@tooo.'

t-R'--i- `} ,lii'i¥'.];.; i :

ipo.l.i!il';.;-' m = .l i','k i, ;eq:¥(Oi ., N e. ., h'J¥ ) laminated IVE,.',-Y.tU.II.trs o te.Sonfoeo..lx ':r: ftltt/.:. ,.6 :ut tuff ':i.:' ii'1',l t' o- - i'ii' ,fossq',lt¥-=' Z ¥t t::: 6koh¥o.,ttr3C] ,--T.' m /kiQ 0¥ ; ." ..q: ':. ss tr'/''i's')¥se,z; 1`s)s O.s¥.g -. - ;t"L tÅébl .N, Qg(2./i` i' andesite { i3.'e4i' a : -Q:6..." 2m t ' o g -"i .- .O-- --ti9S c:Oo' Oe ::¥ b sl 0 l- 6 t- -" - ." o v¥-tQ. '' " S.J .-e '- v, '. ' ' ' frag. s eriP up" Fig. 2l. An example of PMA showing internal sedimentary structure.

In the middle to upper part of the pebbly siltstone bed, the clasts composed of porous and laminated tuffpredominate. The uppermost consists ofpoor-sorted siltstone with some andesite fragments. Any modification structure such as ripple- laminations do not observed in the bed. Thus, such different organization of clasts with in a bedis caused by the difference of clast density between the heavy massive andesite and the lighit porous tuff, and not by different flows.

6. DISPERSAL PATTERN OF SEDIMENTS In the Minobu Formation and its equivalent strata of the western half of the Southern Fossa Magna Region, three main paleocurrent directions are recognized. The first paleocurrent direction is west-southwestward transport. SDA, GSA, SIA (Minobu and Akebono areas) and PCA (Nishikatsura area) are mostly transported by sediment gravity fiow in this direction. The second is south-southeastward transport, developed in SCA and SSA (Manzawa area). The last one is observed in PMA (Akebono to Manzawa areas) and a part of SMA, which shows Reconstruction o fFujikawa Trough in Mio- Pliocene Age and its Geotectonic Implication 39

hr..Kr Teuchizawn Z'-z2//

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Hadakashime

;-¥e7T-'¥N,, Pl.4-5 Minobu 'J.' tt.e . :L

fi i"i'/ tl ' /i/ ¥' ¥'. NN .:1 ',¥ .-ir.' NN .1 iiliillll' tt tt oshi'"a' '.,..:;e,,.

,.i,,f' YCk"V. Illilli$..ilseC¥.tS' RNs,

Machiya Toshirnn

e Manzawa{g,

I =ma =SW i, = O 3km Pl,4 -3

Fig. 22. Locality map of PMA. Outcrops of PMA are shown as closed circles. Arrows indicate the direction of transporting of PMA. 40 SoH, W. eastward transport that is quite different from the first paleocurrent direction.

West-southwestward Paleocurrent Direction SDA, GSA and SIA (Minobu and Akebono areas) In the Fujikawa Tal, the west-southwestward paleocurrent direction is Vi Hadakashirna S e..."'"""" ---. ':. 2 .q------flute rnark ¥¥:¥ 3 : groove ¥..' 4 mark . clast o im -ff'; .2s:;:' - fabric Fu ruya?: hJil]1: ,k, '/i: . :¥¥ ; 7 ,¥L¥ 6 ¥k Onta ------parting 't ,¥U 1ineation .8 .. 1 x -i 1,, z,zlll. , ti ":'1'i 9NS . ,f2vaiit?,vaf ' ---- .:i NN 11 gxX il 'i¥N

:. ,-- iN -:. :15 N :' ji( :. -/t if' : Tsubakizori l: t,t, :. : ii :- .------h- " 17 : - :: .:. . t- t-- ¥,lxz ' ''ti' . .... ---. t ,] 1,.t, t. : -- 'f'i't 't/.tJJti.J t :. li ¥lt¥ siyi,,' ;: t::- - ;tllt l/t,, s:.': Z -i/lfttl/'' .ti.tt 1. -:-: I:J - -: l: : 18 :"l2 ! . . t;-t lttt tt 9¥': -- =- 16 1:" ;'"'' l' .;:-l----" :: 1¥g Arayashikl¥ , {- -i 3,1. izt,[ { dd.P ---s ,::- :,l . rltttt,t 1 .s".'¥' ii 1" {¥v'Ii,1¥lt; 't :- 'f! -l--:" Nt r,:,r ','f,4", Nl . 'i 22 3 NS 111', N, l,. --tti- ,N'¥ .. ::¥ : :- 1"1;//,;. :. ;-- / ------: -:-t .

. i !- t - t. :"' l 'i'' t-. 24 il ¥¥'i¥ --l -- -. t-t. .-- -j oshima . ,¥tt - --:- --:---- Jlt Z/,,,,. - -- --.------'i/; -- - tt-ti---- 23 i . :-- ' -:- "- '6 --: J. - ."" ---- >(li)ljil(i < N •Å~ C 29 N 27 N ----i 1 1: 2Stu"l t

Fig. 23. Paleocurrent directions of SDA, GSA and SIA in Minobu area, showing fairly constant west-southwest transpoting, and concordant with that of the three facies associations. Reconstruction of Fujikawa Trough in Mio-Pliocene Age and its Geotectonic Implication 41 measured mainly in SDA, GSA and SIA, by fiute marks, groove marks, clast fabrics and parting lineations in Facies A3, A6, Bl, Cl and C3. SIump folding showing the sliding to this direction has scarcely been observed. Results ofpaleocurrent direction in Minobu area, for example, are shown in Fig. 23. Noteworthy is that the significant difference of paleocurrent directions among three facies associations is not recognized.

CPA (Nishikatsura area) Paleocurrent direction Sole markings are infrequent in CPA of Nishikatsura area, but paleocurrent direction can be obtained by clast fabrics such as preferred clast orientation and imbrication (Plate 1-2). The paleocurrent direction is consistent with that measured from grain fabric and preferred orientation of carbonaceous fragments. All of those paleocurrent directions of sediments indicate west-southwestward transport,

M' ez

N Hatsukari

.ll :, F'uein'

"tl/t ': ' .: ¥,,t-¥ ¥¥¥ -t ....g.,,. Lt+' NVi,';¥, .v"pt, -/."-,lj.eq/ag: " A 'i '' tt p Mt.Mitsuishi ,i

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' c/' "

'' :VL ,.,,,s¥ ,cT- ,,,.1/f¥lgys'ss¥ '-, i'" :L

:; "' -i "' O 1 2km Nishikatsura ti stNN¥ Kawaguchi, lt1 ke. 12 x Vv34

Shimoyoshida Fig. 24. Palcocurrent ofdirection CPA, showing that the paleocurrent direction is concordant to the distribution of CPA. 1. clast orientation, 2. orientation of carbonaceous fragments, 3. scour mark, 4. sandgrain fabric. 42 SoH, W.

parallel to the long axis of the distribution of CPA (Fig. 24). Clast fabric pattern It is recognized that the pattern of the clast fabric is of difference between that in Iwadono (the northernmost part of Nishikatsura area) and that in Nishikatsura (the main part of Nishikatsura area) (Fig. 25). Clast fabric of Facies A3 in Nishi- katsura is of typical A-type proposed by DAvis & WALKER (1974), that is, long (a-) axis ofimbricated clasts is parallel to flow and dipping upstream (Fig. 25). A-type clast fabric is interpreted to have been made by clast interaction such as collision within a high-density and high-concentration sediment gravity fiow (WALKER, 1978). On the contrary, in Iwadono, fabric pattern is B-type of DAvis & WALKER (l974), that is, long axis of clasts is usually transverse to flow direction with inter- mediate (b-) axis dipping upstream. This type is thought to be formed by drag of

NlSHlKATSURA area a

c.p. x... b f.p- normat lnverse B-type f. p¥ to normal

normal B f. p¥ B f. p¥ normal o Otsuki inverse to cp. "t lm normai i62t ' <6FeS lnverse to cob. normal .- .- tl - votcanic congL o f, p- o A Tsuru mainly Mt.Mitsutoge crude bedded " f. p¥ congl. Im b m

.,..¥, -.¥¥¥¥,'-"/' a Fujiyoshida o A-type IwADoNo area Fig. 25. Transition of clast fabric pattern from Iwadono of B type to Nishikatsura of A type, and bedding style, showing change of flow mechanism from fluid gravity flows to sediment gravity flow. Reconstruction ofFujikawa Trough in Mio-Pliocene Age and its Geotectonic Implication 43 fluid gravity flow ofMiDDLEToN & HAMpToN (1973; 1976) (DAvis & WALKER, 1974). Iwadono is situated at more upstream side judged from the relationships ofpaleocurrent direction and its distribution. The transition from A-type to B-type of clast fabric is found around Asari where the Katsuragawa Conglomerate corresponding to CPA becomes to unconformably overliy the Nishiyatsushiro Group. Such change of clast fabric pattern from B-type to A-type is interpreted to be resulted from the change of transportational mechanism from fluid gravity fiows to sediment gravity flows, due to rapid deepening of the sedimentary environment.

N)f);,ima K ."o .itXI(- tony/ bg""71 Rl) /1 x<)]) sljY .// Utsubuna X /2 J. % .L-l )t 3

./.. / "- 4 /-t;' --z- 05 --/ '7-///

-.--./- ./ .' x''"f t't"' '-J--f-r-. /-// - '" - lde tJ'JJ .f Machiya -Jtf" ft .tt... '/""ttJ; Toshimac'. Jl ' 'pt N x -IJ-t. . ¥..¥;../. i .J-;. Å~ -'J'/-.'- f" '/ ) 't-. t¥' c/f- ;l /-i.-/IJ;' -Z;' -tJt f- t- ttttttttttt 't.. t- tttttt ' f.ltJIJ ..f-.i ' --f';' Manzawa "'t'/ -.;1. ./ 0 f ;' ¥ -.1!--- ..t ` 5Rrii"'f X ill> o t======' -'?' //)/' = "' '1,

ltl.lli.il.....,7..1..J,/.tt¥/I' f/' t' /. -. - / ' 'i-Yfi-!,i.rg.yany .t a -{

Fig. 26. Dispersal pattern of SCA, SSA, SMA and PMA in Manzawa area, showing a centripetal directional pattern of paleo-current, 44 SoH, N"

This po.ssibly suggests the transition from a subaerial fan-delta to a submarine channel on a slope setting.

South-southeastward Transport Direction SCA, SSA, SIA and SMA (Minobuand Manzawa areas) South-southeastward transport is ob- W tained in SCA, SSA, SIA and SMA by v fiute marks, and less commonly by groove marks, c]ast fabrics and ripple marks (Fig. ve U 26). Significant difference of paleocur- -es T rent directions between SCA and SSA is s not recognized, However, SMA is not so clear because of" lack of indicative R structure of the paleocurrent direction. Q When observing it in detail, the pal- P ec current directions of S(IA, SSA do not sho", ca critically consistent south-south- o e,astward transport. Because those in the N western part of the distril[)ution of SCk4y, M SIA a. nd SS.A, to the west or south of L .NIanzawa, sometimcs show eastward transport. On the contrary, paieocurrents K in the northeastern part of SC.LN. and SSA J associate with southwestward transport l (Fi.cr. 26). As summing up, south-south- H eastxvard transport show a centripetal and conver,crtlng pattern. G F Eastward Transport Direction E PMA (Akebono to Manzawa areas) D Sedimc.ntarv structure indicative oi' ' o c paleocurrent direction has not been ob- rm B served in Facies Dl. Dispersal pattern ([iii) of PMA, however, is able to presume from A analyses of the strike of channel walls Fig. 27. (Plate 4-5), and of slump folding associ-- Bed-by-bcd paleocurrent (lirections of SCA. ated xvith Facies Dl (Plate 4-3). Dis- Note that paleoeurrent directions oi' Sand- sione. ptIegaf'acies (S) i.s quite concordant persa] pattern of PMA displays eastward with tliose ot' Alternation of Siltstone and Sancl- transport (Fig. 22). Noteworthy is that stone .N¥legafacies. At west of Hashin()kami. Reconstruction of Fujikawa Trough in Mio-Pliocene Age and its Geotectonic Implication 45

prodominant eastward transport of PMA are quite different from that of the other clastic sediments.

7. PETROGRAPHY OF THE MINOBU FORMATION

Conglomerate clast composition Conglomerate clast composition is a good indicator in consideration for the source and dispersal pattern of sediments. Conglomerate and Pebbly Sandstone Megafacies of the Minobu Formation and its equivalents are distributed in such three areas as Minobu, Nishikatsura and Manzawa areas. Clast compositions in the first twoare as were briefly reported by MATsuDA (1958) and YAMANAsHi PREFEcTuRE (1969), but detailed study has not been carried out. In the present study, more than 40 pebbles of over 1cm in diametre, were sampled from 60 cm square space in both conglomerate and pebbly sandstone beds, and were examined by the naked eye. To confirm the results, eighteen pebbles were reexamined under microscope. 1) SDA and GSA (Minobu area) In SDA and GSA, amount of Conglomerates and Pebbly Sandstone Megafacies attain approximately 30/. and 620/. of the measured sections composed of 128 meters in SDA and 214 meters in GSA. Clast composition was examined in 18 points ofboth SDA and GSA. Conglo- merate clast compositions of both facies associations are quite similar to each other. Matrix is medium to coarse-grained lithic wacke similar to sandstones of study area

Mtncou area

---tvvl- t'1 -'', : ,i¥'.¥.¥L/,".''Y¥k-i' ;::= ¥¥¥¥--¥ .,.','.'¥/-u¥ -il'J,, ¥-¥ ¥7.. i' v/Nishikatsura '}¥; ,area Manzowa area :i../ /.. /.. 'T'-VV-Vi--iT tl--t---Itltt '/ . ill}!f . hitJ :.Ns '";:s T ,''.¥'¥ ., ::.::;;:¥ ., , :: .. ,,,h ',t. t.t; :: agJtutA ViiiTiiiT-i---T' "' I;:, ''' N"t-1i;}:"! :: 'tl'tl'tl' ::t--- /''/ :'t"'' "'''' '/1' -IJ-tl

111 tl't.t-?//,//'' ;:;;;::::::: :: ''' V+ t.,L ---- ::: /.,li¥s.1,!,1' K :'W.'`'¥'S3t,,li¥ L.Tt". ;:,;":,L:.

t/ t lt/tt//t/ ::Illll'l;l.I':' i -`v/ M :# llllll " ¥:'}i'1'. D :::' .; .¥s. 1234 56 Fig. 28. Clast composition of conglomerates, showing similar compositions to one another. 1. chert and quartz rocks, 2. diolite and granite, 3. volcanic rocks, 4. black shale, 5. angular clast of "andesitic rocks", 6. sandstone. 46 SoH, W.

% 5

30 20 10

o O1234 5cm Ol234 Sem O1 234 5om O12 34 5cm O12 34 5cm sandstone mudstone aeidic volcanics basic volcanics diolite--qranite Fig. 29. Size distribution of the measured pebbles. in composition and texture (see, sandstone petrography in next section). 983 clasts examined consist of 600/, sandstone (including meta-sandstone), 250/. black shale (including meta-shale), 80/, volcanics (dacite-basalt), with 30/, plutonics (diolite- granodiolite) and 40/. other kinds of rocks (reddish chert, vein quartz and limestone, etc,) (Fig. 28). Size distribution ofclasts are shown in Fig. 29. Clasts are generally subangular to subrounded with exception of diorite clasts that are subrounded to well-rounded (Fig. 30), and are 120/, blade, 200/. prolate, 290/. spheroid and 390/, oblate in shape (Fig. 31).

2) PCA (Nishikatsura area) PCA comprises Conglomerate and Pebbly Sandstone Megafacies in an amount of up to 800/,, and Sandstone Megafacies. Conglomerate clast composition of PCA

Minobu Nishikatsura Manzawa Area Area Area o/o a 40

o A o/o SA SR R WR A SA SR R WR A SA SR R WR

40 b

to o A SA SR R WR A SA SR R WR A SA SR R WR Fig. 30. Roundness of sandstone and shale clasts. a: sandstone, b: black shale, A: angular, SA: subangular, SR: subrounded, R: rounded, WR: well- rounded. Reconstruction of Fujikawa Trough in Mio-Pliocene Age and its Geotectonic Implication 47

: [$) : 9 -.1- [}] l % 1 i 1 i ' %,.,-t---.L5s.''':' '-tt

'¥21--tt,'.'tt''t---:tL,..t4---- 43x .,N' /.:S' ¥ 25 "':t'-ttt :'..1¥:1'-¥

.-- , :v- , ttt tt "¥?r:'f¥;-t-t-12-'.. ---tt t-- -41--t- '-'" t' t-:--t- :.l, -- 14.i ,, 17 .. ¥t5 , ', ' .-

Nlshtkatsura Mtnobu Manzowa Fig. area 31. Clast shape of thearea measurec! conglomerates. area was examined at eight localities. Matrix of the conglomerate is medium- to coarse- grained lithic wacke similar in texture and composition to sandstone. Clasts, on average, consist of 500/, sandstone, 250/, black shale, 200/, volcanics (dacite & basalt), with 30/. plutonics (diolite & granite) and 50/. others (reddish chert, etc.) (Fig. 28). Pebble-size clasts are generally subangular to subrounded (Fig. 30) and 140/. blade, 21O/, spheroid, 200/, prolate and 450/, oblate in shape (Fig. 31). 3) SCA(Manzawaarea) Conglomerate and Pebbly Sandstone Megafacies occupies less than 40/. of SCA (the measured section of 600 m) in Manzawa area. On an average, clasts consist of240/. angular andesite and basalt, 390/, sandstone, 270/o black shale, 70/o volcanics and 30/. other rocks (diorite and quartz rocks) (Fig. 28). Pebble-size clasts are generally subrounded (Fig. 30) with exception of andesite clasts which are mostly subangular (Plate 3-6). Clasts consist of 180/, blade, 250/. spheroid, 140/, prolate and 430/. oblate in shape (Fig. 31). It is noteworthy that conglomerate clast composition of SCA differs from those of SDA and GSA in having many subangular andesitic clasts like that in Facies Dl (Plate 3-6). This andesitic clasts are commonly larger in size than those of other rocks. However, clast composition excluding the andesitic clast is similar to those of GSA and PCA. 4) PMA(AkebonotoManzawaareas) Pebbly siltstone differs greatly from the conglomerate in clast composition. In pebbly siltstone, andesitic clasts attains to more than 800/. of total clast composition. Andesitic rocks which include andesite and basalt clasts are mostly 48 SOH, X¥V.

angular to subangular (Plate 4-1, 4-2). As the others clasts, black shale and sandstone are included. Moreover, rip-up clasts and transported ciasts of thin- bedded turbidite are contained in PMA. Difference in local clast composition is recognized. In Manzawa area, pyroxene andesite and greenish altered andesite predoiiiinate as main volcanic clasts, but in Akebono area, most clasts are basalt.

Sandstone Petrography Sandstone is predominant element of the most sequence of the Minobu Formation and its equivalents, but a minor component of CPA and SIA, In the present study, samples for petrographic examination were taken mostly from massive part ofmedium to coarse-grained sandstones ofSandstone Megafacie and Alternations of Sandstone and Siltstone Megafacies. Modal composition was obtained by counting more than 500 points in one thin-section for each specimen. Numbers of analyzed samples are as foIJows. SDA, GSA and SIA (Minobu area).,,...... 18 PCA (Nishikatsura area) ...... ,...... ,...... ,...... 8 SCA and SSA (Manzawa area) ...... ¥¥¥¥¥¥...... ¥¥¥¥¥+ 9 Main framework grains are monocrystalline quartz, plagioclase, rock fragments ofvolcanic and argillite. The result ofstudy on composition is shown in Fig. 32. 1) SDA, GSA and SIA (Minobu and Akebono areas) Sandstones of SDA, GSA and SIA are quite similar in petrographic character-

A 50 se Q so Minobu Nishikatsura Maniawa

o8o o SB o oS o o O ope o R F o% oO F RF oo R 50 se 50

50 NishikatsuVa se B Minobu

o o oOoo oo OoO o aci 50 arg 50 arg 50 arg Fig. 32. Sandstone composition of the study area, showing simi]ar compositien to one another. A: QN-F-R diagram. B: compositional diagram of lithic ftagments. bas: basic volcanics, acid: acidic volcanics, arg: argillite, Reeonstruction of Fujikawa Trough in Mio-Pliocene Age and its Geotectonic Implication 49 istics with one another. They belong mostly to lithic wacke with modal compositions of quartz: l2.30/,, feldspar: 11.50/., lithic fragments: 76.20/,. Mineralogically, they are volcanic lithic and argillite lithic-rich wacke. Average matrix content is 14.80/,. They are generally moderaely to poorly sorted with subangular to subround grains. Lithic fragments contain argillite, basalt and dacite, and some plutonic fragments. It is important that argillite and dacite rock fragments constitute the majority in the sandstone composition (Fig. 32-B). Matrix is composed of true matrix of fine grains smaller than O.03 mm in diameter, and subordinate pseudomatrix of DiaKiNsoN (1970), so for it is mixed with calcite cement. Accessory minerals are pyroxene and opaque minerals (iron oxides) comprising up to 1O/. of the terrigneous material fraction. Small amount of biotite, epidote and chlorite are also contained. Accessory minerals can be regarded to be derived mostly from basaltic rocks.

2) PCA (Nishikatsura area) Sandstones of PCA are mostly classified as lithic wacke with modal composition of quartz: 21.50/,, feldspar: 14.20/, and lithic fragrnents: 64.30/.. Mineralogically, they are volcanic lithic and argillite lithic-rich wacke which are 13.80/. of matrix in an average. Sandstone is moderately to poorly sorted, with subangular to subrounded grains. Sandstone composition of PCA is nearly the same as that of sandstones of SDA, GSA and SIA in Minobu area. Matrix of many sandstones are mixed with calcite and silica cements, and pseudomatrix. Accesory minerals comprise pyroxene, opaque minerals, biotite, epidote and chlorite. Pyroxenes are mostly altered into pseudomorph of calcite. 3) SCA and SSA (Manzawa area) Sandstones of SCA and SSA in Manzawa area have the same petrographic characters. They are mostly classified into lithic wacke with rnodal compositions of quartz: 15.70/., feldspar: 16.00/., lithic fragments: 68.30/o. Mineralogically, they are lithic wacke with 17.50/o matrix on an average. Most sandstones are moderately to poorly sorted, with subangular to subrounded grains. Q;uartz and feldspar are minor compositions in comparison with lithic fragments, which are mainly composed of two different origins; basaltic and dacitic rocks, and argillite. Matrix consist of true matrix of fine grains smaller than O.03 mm, and pseudomatrix. Interstitial calcite cement is also seen instead of matrix. Accessory minerals include pyroxene and opaque minerals, reaching O.60/o of total amount, along with biotite, epidote and chlorite. In sandstone petrographic composition, the Minobu Formation and its equivalent strata has very good similarity. 50 SoH, W.

8. INTERPRETATION OF FACIES ASSOCIATION In order to interprete the facies associations, the following points must be taken into account; 1) morphologic analysis of facies association, 2) dispersal pattern and 3) comparative studies of sedimentary facies and facies association in the study area with modern and reported typical ancient examples. As a result, it is inferred that coarse clastic association group represents channel fi11ings, and fine clastic association group are interchannel sediments. PMA, showing the opposite dispersal pattern to clastic sediments, is regarded as a product ofdebris flow running down on a steep slope. a) SDA and GSA as channel sediments As stated already, SDA and GSA are developed in Minobu and Akebono areas. At first, morphology of SDA and GSA are analysed. In N-S profile, normal to the paleocurrent direction, the sedimentary body constituted by two associations is always lenticular in shape (Figs. 6 and 7). The bottom surface of SIA and GSA is demarcated by distinctive boundary. It is concave-up and erosional to underlying SIA. Additionally, strike of the boundary surface is originally parallel to paleocurrent direction of the sedimentary body. Each individual bed within the sedimentary body pinches out toward the boundary surface, and it abuts directly on the erosional surface. In contrast to those in N-S profile, the sedimentary body in E-W profile parallel to paleocurrent direction, does not show the lenticular geometry. It seems to be continuous and uniform in thickness as well as the feature ofsedimentary structures. The sedimentary body of SDA and GSA changes gradually into overlying SIA. Upper boundary of the sedimentary body is parallel to bedding plane not only in outcrop but also in mappable orders. Furthermore, the distribution of the sedimentary body extends parallel to the direction of paleocurrent measurements. Thus, such morphologic features of two different profiles indicates that the sedimentary bodies of SDA and GSA form concaved "shoestring bodies" elongated parallel to paleocurrent direction in three dimentions having the erosional base. In respect to sedimentary facies ofthe two associations, they are thought to be transported by of high density and high concentrat sediment gravity flows, and sedimentary facies consisting of the two associations, show fining-upward sequences starting from an erosional surface. These sedimentary facies and fining-upward regularity observed by the two associations are similar to those of channel fi11ings of fluvial sediments (e.g. Scott type and!or Donjek type in MiALL, 1977; l978) in spite of the fact that these sediments of SDA and GSA are deposited in a deep-sea environment. In addition, these characteristics are also described and interpreted as those of the paleo-submarine channel-fi11s by MuTTi & Ricci-LiccHi (1972, 1975), MuTTi (1977), WiNN & DoTT (1979), ERiKssoN (1982), HEiN & WALKER (1982), PicKERING (1982) and so on. Reconstruction of Fujikawa Trough in Mio-Pliocene Age and its Geotectonic Implication 51

Table 5. Summarv of channel fi11 sediments in Minobu area.

sedimentary facies paleocurren strikeof dipof widthof .maxlmum sedimentary bodysname dssociation direction channel channel channel thickness. sequence locationt M FURUYASHIKI frorn aboutse SDA finingand (Loc.a) N25eE N35eE to15e 650m+ 73m+ thinningup- N400E warduence OBIKANE from about (Loc.d) GSA N5oeE N600E ? ? N600E 3.2km 300m+ fran N40eE 'finingupward MARUTAKI to 1800m+ GSA N300E about3oe 4km+ (Loc.b) N70eE N800E to40o sequence

In conclusion, basing on such characteristic features of its morphology and facies and its transitional pattern, SDA and GSA are regard as the fi11 sediments of submarine channel. Width and direction ofsubmarine channels reconstructed from their morphologic analyses are given in Table 5. When reconstructing GSA in detail, GSA are interpreted as fillings of a single submarine channel system. This is because the reconstructed channel fi11ed by GSA has a width of more than a few kilometres, which is larger than that fi11ed by SDA, and they are not associated with any other sedimentary body in the same horizon. In the stratigraphy, the sedimentary bodies of GSA are confined to the lower part ofthe Minobu Formation. On the contrary, sedimentary bodies ofSDA are confined to the upper part ofthe Minobu Formation. In case of SDA, two or more sedimentary bodies are distributed on the same stratigraphic horizon (Figs. 7 and 8). The reconstructed channels fi11ed by SDA are up to a few hundred metres wide. Thus, they are suggested to form multi-branching, net-work channel systems. In summary, the two different submarine channel systems, single submarine channel system and net-work multichannel systems should have developed in the Minobu Formation in Minobu area. Accordingly, channel systems in the Minobu Formation had shifted from the former to the latter. b) SIAasinterchannelsediments SIA differs from SDA and GSA in its distributional pattern and facies transition. SIA is considered as interchannel sediments (sense ofNoRMARK, 1980) by the following evidence. 1) SIA is distributed surrounding SDA and GSA of channel fi11s, that is, vertical and lateral juxtaposition with both associations. 2) As a whole, characteristic sedimentary structures and bedding styles are similar to those of interchannel sediments that have been described by many authors (e.g. MuTTi, 1977; WiNN & DoTT, 1979; CARTER, 1979; MooRE et al., 1980; ERiKssoN, 1982; PIcKERING, 1982). It is noteworthy that the interchannel sediments in the area differ from "typical" 52 SoH, W.

ones which show a divergent paleocurrent system (e.g. Mutti, 1977). Predominant paleocurrent direction of SIA as interchannel sediments in the study area is parallel to that of GSA and SDA as main channel fiII sediments as mentioned already. The direction of supposed crevasse channels of the area, does not cross at a high angle, approximately 40 degreeatits maximum, to the main paleocurrent direction of the submarine channel. Such difference, however, may not reject the interpretation that SIA is interchannel sediments. Because the paleocurrent system is controlled not only by the sedimentary process but also by topographic condition. I believe that a relatively steep slope of this area must have compelled current direction of the spilled over flows from main channel parallel to paleoslope direction. A similar phenomenon of interchannel sediments is reported by CARTER (1979) in the Jurassic Otekura Formation of New Zealand, which is interpreted as the interchannel sediments on trench slope.

c) CPA as channel fill sediments The sedimentary body constituted by CPA is distributed along longitudinal profile parallel to paleocurrent direction only. So that, three dimensional reconstruction of the sedimentary body was hardly taken. To interpretate the sedimentary environment of CPA, the following facts are made much accounts. 1) The sedimentary body is continuously distributed along paleocurrent direction, and the original strike of erosional base of the sedimentary body is parallel to elongation of the sedimentary body. 2) The sedimentary facies show that CPA was deposited from high-density and high-concentration flow similar t'o the case of GSA. Such sediments are generally explained to be transported and deposited under "confined condition", because high-density and high-concentration flows must be maintained to the ]ast depositional stage. In comparison with GSA, sedimentary facies of CPA indicate a deposition directly from the higher energy level of sediment gravity flows, because of development of Facies Al and A2 in CPA. Thus, the prolonged shoestring shape and erosional base of the sedimentary body ofCPA are interpreted as a channel origin, and the association is also regarded as channel fill sediments. d) SCA as channel fi11 sediments Complete three dimensional morphology of sedimentary body of SCA can not be reconstructed due to lack of transverse profile in many cases. The sedimentary body of SCA, however, has the same characteristics as those of SDA and GSA in Minobu area not only in sedimentary facies, pattern of facies transition and bed features, but also the contact relationship to underlying strata. Thus, SCA is also interpreted as fi11 sediments of the paleo-submarine channels. Reconstruction of Fujikawa Trough in Mio-Pliocene Age and its Geotectonic Implication 53

Multi-storeyed sedimentary body is beleived to be a geometric record of the multi-s[oreyed channel fiIl sediments. Thick multi-storeied sedimentary body in this area is considered to be constructed by the frequent shifting of channel course as a sedimentary process, and by continuous subsidence to keep the channel in the same pasition. Similar morphologic characteristics of channel fi11ings in the geologic record is known in the fluvial sediments under conditions prevailing in an active synsedimentary subsidence, such as the Trassic Lower Kane Springs Member, Chile Formation (BLAKEy & GuBiTosA, 1984). The depth ofpaleosubmarine channels fi11ed by SCA is not known. The widths are roughly estimated from lateral extension of the sedimentary body as several hundred meters to a couple of kilometers. b) SSA as interchannel sediments SSA is distributed surrounding SCA, and is comtemporaneous but heterotopic with SCA channel fills. This association shows a decrease in coarse-grained sediments away from the margin of SCA, channel-fi11 sediments as mentioned above. Characteristic features of the sedimentary facies are also similar to those reported as interchannel sediments (e.g, MuTTi, 1977; WiNN & DoTT, 1979). Sediment dispersal of SSA, however, is of the same pattern as that of SIA, and does not directly correspond to that of the "typical" interchannel sediments.

3) Pebbly siltstone association (PMA) As mentioned previously, many pebbly siltstone (facies Dl) are intercalated in turbidites and associated coarse clastic deposits along Fujikawa domain. PMA forms a lenticular sedimentary body, up to few meters across, with an erosional base and crude normal grading (Fig. 29). It is generally believed that pebbly mudstone, with the exception of drop stone origin, is formed by debris flow. SinceJoHNsoN (1970) and HAMpToN (1972), Bingham plastic model has been regarded as rheological behavior of debris flows. Because debris flow has the matrix strength, that is, yield strength due to cohesion of fine-grained particle in the matrix. It is also generally considered that behavior of debris fiows is laminar rather than turbulent (JoHNsoN, 1970; HAMpToN, 1972; 1975; 1978; CARTER, 1975; MiDDLEToN & HAMpToN, 1973; 1976; HiscoTT & MiDDLEToN, 1979 ; and others). Laminar debris flow produces characteristic internal and external sedimentary structures such as inverse grading andlor massive structures (FisHER, 1971; NAyLoR, 1980; SHuLTz, 1984) and a smooth bed base without erosional structures (JoHNsoN, 1970; SwARBRicK & NAyLoR, 1980; MiDDLEToN & HAMpToN, 1973; 1976, and others). Sedimentary structures ofFacies D1 ofPMA in the study area, however, are quite different from sedimentary structure probably making laminar debris flow described 54 SoH, W. above. I interprete that PMA could be formed by the freezing from the flow in an intensely turbulence or at least due to the failure of turbulence rather than in a laminar fashion, which could make the characteristic sedimentary structure such as the erosional base and the grading. Based on his laboratory work, HAMpToN (1975) pointed out that the transitional condition of Bingham materials from laminar to turbulent, depends on the relationships of two dimensionless numbers; critical Reynolds Number (Re) and Bingham Number (B) (Fig. 33). RefB>1ooo pVc2/k>1OOO Add{tionally, according to ENos's (1979) empirical equation, the transitional condition is given as; log k== -2,88+2.03 log Vc Where k, Vc and p are matrix strength, velocity and density of fiow respectively. Ifk is defined, the transitional condition depends directly on the flow velocity of Bingham materials. Considering that the largest clast diameter within the association, with approximately 2.6 glcm in density, is 120 cm (Plate 4-2), and that

Kpti 1000 o

CriticalReforNewtonianfluids ' inopenchannels

1OO Ol B Bingham 1 Number 10 100 Fig. 33. Transitional condition of Bingham material from Iarninar fashion to turbulent one (after Hisccot & Middleton, 1979). Reconstruction of Fujikawa Trough in Mio-Pliocene Age and its Geotectonic Implication 55 matrix density of flow is supposed to be 1.5 glcm, and approximately 500/. volume concentration of non-cohesive particles are included, matrix strength k of 7 Pa is obtained from HAMpToN (1975; 1979)'s equations. Thus, the critical maximum velocity of approximately 21 m!sec is calculated from these equations as transitional condition. Therefore, turbulent condition can be expected even in Bingham materials containing Iarge clasts, if flow velocity is sudiciently high. Moreover, according to HAMpToN (1975), flow of Bingham materials is sheared once through the fiow, it is changed into turbulent flow, and value of matrix strength of the flow is abruptly decreased. As a result, the flow comes under more favorable condition for turbulent flow. In this case, driving force of high speed flows is considered to have a fairly positive relationship to slope gradient. On the other hand, TAKAHAsHi (l981) proposed that flow behavior of debris flow having a relative high-concentration of non-cohesive particles, is well-explained by his dilatancy model rather than Bingham plastic model. If this model is acceptcd, however, relative steeper slope is also required to be initiated and developed as the large-scale debris flow under the problem. In conclusion, to produce PMA with such sedimentary structures, a steep slope setting is required.

9. RECONSTRUCTION OF SEDIMENTARY ENVIRONMENT Kwanto Mountainland as provenance With regard to regional geologic distribution, two different interpretations on the material provenance of the Minobu Formation and its equivalent strata have been proposed. One provenance stated by MATsuDA (1958, 1961) is located in and around the Kwanto mountainland, and the other was inferred by TsuNoDA (1979) to be in and around the Akaishi mountainland. Basing on the results of sediment petrography as stated above, especially those represented by clast composition with the exception of the angular andesitic clasts of PMA, can be divided into two different rock types, sedimentary and volcanic rock origin. The former consists mostly of sandstone and black shale with a small amount of reddish chert. It is similar to the composition of sedimentary rocks of the Shimanto Supergroup in Kwanto and Akaishi mountainlands in lithologic characteristics. However, the latter clast type, comprising dacitic and basal[ic volcanic sediments, can be correlated with the sequence ofthe Nishiyatsushiro Group itself of the northern part of the Southern Fossa Magna Region, because of lack of clasts of alkaline volcanic rocks and of lithologic similarity. In addition, an important fact on the problem is the dispersal pattern of clastic sediments ofthe Minobu Formation and its equivalent strata. The main paleocurrent direction of clastic sediments show westward to west-southwestward transport. Therefore, the compositional feature and dispersal pattern of clastic sediments 56 SoH, W. indicate that provenance of the clastic sediments is located on north-eastward ofthe study area. The author believes that the main provenance of the Minobu Formation and its equivalents is situated around Kwanto mountainland rather than around Akaishimountainland. Additionally,largeamountofclasticsedimentsalsoprovided from Kwanto mountainland, which formed the equivalent turbidite sequence in Boso Peninsula to the east off the Southern Fossa Magna Region (ToKuHAsHi, !977).

Nishileatsura Channel Although Nishikatsura area is separated 35 kilometres from Minobu area by the presence ofthe Quarternory Fuji Lava at Fuji Lava of Aokigahara, CPA interpreted as channel fi11 sediments is considered to continue to GSA and SDA ofchannel fi11 sediments in Minobu area. This is deduced from the following evidence. 1) As stressed formerly, compositional features of clastic sediments are similar to each other, and predominant paleocurrent directions in these areas are concordant with the distribution of these facies associations in two areas (Fig. 46). 2) As shown in Fig. 34, the talweg of the low Bouguer anomaly area is traceable along Minobu-

N o ., twadonO >

-r s/ 1 M"tl!re,ji 40mgat Q E/' E," geL /v Q ....------c / qu! AsHIGARA AREA - -20mgat --10mgat 1! Omgat

tt/t o : 10 20 10m t 2ng.i rg:03L .g.i atOg,t

Fig. 34. Distribution and paleocurrent direction of Nishikatsura and Minobu areas, and Bouguer anomaly (from Hagiwara personal communication). Note that southern one of talwegs of low Bouguer anomaly is considered to be modern plate boundary by Nakamura et al. (1984). 1. Fuji lava, 2. distribution of channel fi11 sediments, a. paleocurrent direction, b. talweg of low Bouguer anomaly. Reconstruction of Fujikawa Trough in Mio-Pliocene Age and its Geotectonic rmplication 57

Aokigahara-Nishikatsura line (HAGiwARA, Y., personal communication, 1984), which suggests the presence of thick sediments of lower density than those of the underlying Nishiyatsushiro Group. Thus, it is reasonable to consider that thick piles of equivalent coarse clastic sediments are buried beneath Fuji Lava at Aokigahara. Sequences of channel fi11 sediments in Nishikatsura and Minobu areas, thus, are thought to constitute a continuous channel system, that is, two areas are interpreted as upper and lower reaches of the same submarine channel respectively. Judging from the enormous volume ofchannel fi11ings, this channel running through Nishikatsura to Minobu areas played an important role as a feeding system. It appears that the submarine feeder channel provided c]astic sediments from shallow marine environments such as fan-delta setting around Kwanto mountain paleoland to the deep Fujikawa basin, and that had been located on a relatively steep slope as discussed above. It is named herein Nishikatsura Channel (Fig. 35). It suggests that Nishikatsura Channel was the tectonic channel which had been originally controlled by tectonic movement such as folding and faulting. This is deduced from the following evidence. 1) Nishikatsura Channel has a nearly straight-course and shows a very low-sinuosity (Fig. 34). Additionally, there is no sedimentological evidence to suggest the presence of tributary submarine channel.

, KwantoMts. t"v4.YE.YZI.Tl!."V-N.f.r-¥- ... . ¥.il,,.v.2¥llllllll2i¥t¥9irli;d/ttrl'l.i/(li/:gl.,//tttttl;il

non-Lchannel trough ', ,/ - 't '...gtw-.i"'"N. .- r t' UL;tt .'-t -v. ¥-1::>¥-.. Jll,.E. N;fi;:r .-,' :¥,:: :..:::.. ... 't L' ;':;'illt.;;i¥;.i.l.,/'/t`.-:,,,es"'`'-Y' .t .-.t ;;.. ' .trttilli'i'i. '¥¥Ji'.-- ..' .. iitfi.g¥:.i :¥.l..c.rr','":'J- "' tli,IZ"e' ilSi.il?>)l¥]'''''J]k¥i,N',li'Ailig',ind" "" fault<¥f.""/llr k'i! -' ---""''--;-'¥ scarp ":"¥}.It t: "" 'F.pt,,i,;e.,.(se{'t"/E"'l't,': (X"`"i'li' 1 fi 'x t' .. .. -ti 'lt'y .z¥r tti¥.:'..¥i li .7'.'t' ', Ak:Akebono area ...k L..-tt ¥¥:- ::,'¥ 4tt. :.. i-Is{¥$',111-/`}il'llv-,-},'X"'S`ig,..t s" t trough with axial channel -' .-' 1 ,Mi:Minobu area ts?;ij', K. 1¥ 'lt, t-:¥i i,t [f,i:¥ If,/i J l: zilit}L¥J'I¥//'iil//,tf,.tkl'f{-.'1.:l.'J.iM.11M..e".zawaarea tJs:

depOSItSr¥:: ¥¥:i,,¥t-/L2,J`iAk¥¥.ll,¥¥- - - --s:" '' ttr'- .. ...:7:.... ;;¥li)1,I{..v"n.K,w. -.t-t.t t;'l¥i': " .-tt.tt=n¥. --t- vv t ' "'-':tt'- s Fig. 35. Reconstructional modelg of sedimentary setting of the western half of the Southern Fossa Magna Region. 58 SoH, W.

2) The course of Nishikatsura Channel, extending ENE-WSW, is concordant with the tectonic trend in the region (Figs. 3, 6 & 34).

Ful'ikatva Trough Because the paleocurrent direction is strongly controlled by topographic condition of sedimentary basin, the pattern of the paleocurrent direction is usefu1 for reconstruction ofpaleotopography ofthe sedimentary basin. As mentioned above, there are three main directionsj SSE, WSW and E, in the study area. The first of them corresponds to the elongation axis of Fujikawa domain. It is the main paleocurrent direction of Fujikawa domain from Akebono area to Manzawa area through Minobu area. Among them, it is dominated in Manzawa area and the southern part of Minobu area. The second direction is of concordance with the direction of Nishikatsura Channel as the main feeder channel, and is developed in Minobu and Akebono areas. Eastward transport is observed as the minor paleocurrent element in these three area. In summary, the paleocurrent pattern ofFujikawa domain, on the whole, shows a convergent and centripetal pattern to southwest- southward. In addition, the paleoslope direction estimated by slump folding is of concordance with the paleocurrent pattern. Therefore, the paleotopography of Fujikawa domain can be interpreted to be a trough basin extending NNW-SSE in direction with a southsouth-westward tilted axiai fioor. When giving a circumstancial account of the internal topography of this trough basin, the existence of western boundary paleoscarp in parallel to elongation of this trough basin is postulated. Because the results of facies analysis on PMA shows 1) to need a steep slope gradient as discussed above, 2) to need abundant supply of angular and large-size andesitic blocks and ofslump-folded breccias, and 3) to need scattered sources ofsediments extending along N-S direction west ofthe trough basin, causing the difference in local clast composition. On the other hand, SCA of Manzawa area is interpreted as fi11 sediments of the axial channel of the trough basin by the following reasons. 1) The channel fi11 sediments are located in the central part of Manzawa area, and are enclosed in interchannel sediments. Considering the paleotopography estimated from the paleocurrent pattern and slumping, the channel fi11 sediments always kept their position in the lowest part of the basin. 2) Paleocurrent directions of the channel fi11 sediments are parallel to the extension of this trough basin (Fig. 35). Recently it has been shown that axial channels are prominant features in modern troughs and trenchs where active sedimentation is proceeding (e.g. the Aleutian Trench: voN HuENE, I972; the Southern Chile Trench: ScHwELLER & KuLM, 1978; the Middle America Trench: MooRE et al., 1982; the Nankai Trough: MoGi, 1977; TAiRA & NnTsuMA, in press). In addition, benthic foraminifer analysis by KANo et al. (1985), suggest that Reconstruction of Fujikawa Trough in Mio-Pliocene Age and its Geotectonic Implication 59 this basin is located on upper bathyal or more deeper zone. Judging from these geomorphologic features, it is possible to state that Fujikawa domain was fi11 sediments of a trough extending N-S with an western boundary fault and an axial channel. It is named herein Fujikawa Trough.

10. COMPARISON WITH MODERN SEDIMENTARYENVIRONMENTS

Sedimentary setting of the Minobu Formation and its equivalent strata are probably not unique, when compared with modern and ancient sedimentary environments reported. In comparison with features of the Minobu Formation and its equivalent strata with modern analogues of the trough environment, the modern Suruga trough has the closest similarity. There are several facts in favor of this; 1) similarity of intra-basinal topography between Fujikawa trough and the modern Suruga trough, 2) similarity of the axial direction of both troughs, 3) similarity of sedimentary ratio of fi11 sediments within both troughs. Concerning the first point, it is confirmable that those two troughs are morpho- logically similar to each other, although complete reconstruction ofFujikawa Trough is out ofhands due to a lack ofmarginal deposits. Results ofanalysis ofSCA indicate the existence of an axial channel with a width probably more than a few kilometres. In Suruga Trough, it is recently reported from acoustic records, that there is a large- scale deep-sea axial channel which is a few kilometers in width (KATo et al., 1982; H.D.M.S.A. & G.S.J., 1981). It is also considered that this channel is fi11ed with turbidites and associated coarse clastic deposits. According to the reconstructed model in Fig. 47, there were fault scarps extending in N-S direction which demarcate the western boundary of Fujikawa Trough, and at the eastern side of the trough basin, there existed a steep slope as mentioned already. Similarly, in the modern Suruga Trough, fault scarps extending NNE to SSW form the western boundary of the trough axis (SAKuRAi & MoGi, 1980). On the eastern margin ofSuruga Trough, a steep slope is also developed toward Izu Penninsula and Zenisu Spur. In respect to the second point, the reconstructed sedimentary setting is composed of three main sedimentary environments; Kwanto mountainland as main source region, Nishikatsura Channel of ENE-SSW trend and the N-S trending Fujikawa Trough. Distributional trend of Nishikatsura Channcl sequence and Fujikawa Trough fi11 sediments are concordant with its dispersal pattern mentioned above. Furthermore, the deepening direction of the sea-bottom is also concordent with the azimuth of the paleocurrent from ENE to WSW. The three main environments, thus, are considered to have kept their original position since late Miocene age. This leads to the conclusion that the reconstructed Fujikawa Trough and the modern Suruga Trough have the same axial trend oblique to the general trend of the continental margin of the southwest Japan. This is a strikingly characteristic 60 SoH, W. feature common to these two troughs, because many modern and ancient trough axes are parallel to the general trend of the continental margin (McBRiDE, 1962). Considering the third point, the sedimentary ratio of the Minobu Formation is nearly equal to that of modern Suruga Trough, which has very high values for turbidites and associated coarse clastic sediments. The sedimentary ratio of coarse clastic sediments of piston core samples obtained from the axial part of the southern- most portion of Suruga Trough in the cruise KT-78-19 of Tansei-maru, attains up to approximately 150cmllOOOyear (SHiKi T., personal communication, 1985). Thus, sedimentary ratio in both troughs are similar to each other.

11. RECONSTRUCTION OF TECTONIC SETTING OF THE SOUTHERN FOSSA MAGNA REGION

Geomorphology in deep-sea environments differs basically from that ofsubaerial one where it is always destroyed by various kinds of external erosional processes such as storm and glacier. The geomorphology in the deep-sea must be strongly eontroll- ed by tectonic movements (MoGi, 1977). The similarity ofgeomorphologic features between the modern Suruga and Fujikawa Troughs is the most important. Because it suggests that similar tectonic movements to those now active in the modern Suruga Trough took place in Fujikawa Trough in Mio-Pliocene age.

Modern Suruga Trough as aplate boundary

In relation to the genesis of the bending geomorphology in and around the present region, EHARA (1953) proposed first that it was formed by the collision of his "Izu-shichito batholith" into central Honshu. Since 1970's, it has been argued that Suruga and Sagami troughs represent the plate boundary berween Philippine-sea and Eurasian plates (e.g. SuGiMuRA, 1972), and that Izu Peninsula on Philippine-sea Plate has been continuously driven to collide into Southwest Japan Arc on Eurasian Plate (KAizuKA, l975; MATsuDA l978; NAKAMuRA & SmMAzAKi 1981 ; NAKAMuRA et at., 1984; MoGi et al., 1982). On the geomorphologic setting of Philippine-sea and Eurasian plates around Izu Peninsula, geophysical and geological studies have been carried out. NAKAMuRA et al. (1984) pointed out that the material boundary of Philippine-sea and Eurasian plates is situated on the north of Hakone volcano running along talweg of the low Bouguer anomaly (A-A' line in Fig. 36). IsHiBAsHi (1976) proposed that the modern plate boundary has already jumped to the Izu-toho Line, off the eastern and southern coast of Izu Peninsula (B-B' line in Fig. 36), basing on analysis of local- uplifting phenomenon in the Kwanto earthquake of 1923. Furthermore, on the basis ofchange ofthe stress field pattern, MATsuDA (1978) reconstructed the tectonic Reconstruction of Fujikawa Trough in Mio-Pliocene Age and its Geotectonic Implication 61

:-: - -- .-- --: --

:: :-- :. :. -- ;- --- - .: ------. .

A '

Fig. 36. Proposed plate boundaries in the southern Fossa Magna Region between Eurasian and Philippine-Sea plates. A-A': boundary of Nakamura et al. (1984), B-B': boundary of Ishibashi (1976), history ofthis region as a subduction zone ofthe Miocene, which in turn changed to a collision zone in the Quaternary, and also proposed that a new subduction zone may have begun to shift to south.

Collision of Tanzawa block Assuming that modern geomorphology of Suruga and Sagami troughs has been constructed by collision of Izu Peninsula into Southwest Japan Arc and that the plate boundary is shifting to south, it follows that Fujikawa Trough might be also constructed by ancient collision event. A remaining question is what did collide at that time into Southwest Japan Arc, instead of Izu Peninsula. It is important facts for the question that the main frame work of geologic structure in the Southern Fossa Magna Region shows a half-dome structure around Tanzawa Mountains (Fig. 3), and that the thick-bedded coarse clastic sediments of the Minobu Formation and its equivalent strata are developed around Tanzawa Mountains only (Fig. 2). Additionally, there is a resemblance in rock appearance and in chemical composition between the sequences, mostly of volcanic origin, in Tanzawa Mountains and Izu Peninsula (MATsuDA, 1978). The sequence of Tanzawa Mountain seems to have belonged originally to the northern part of Izu-Bonin Arc, and not to the Neogene system ofthe southwest Japan Arc. I believe that the sequence of Tanzawa Mountain, here called Tanzawa Block, played 62 SoH, W. the same role as a collision mass just as Izu Peninsula does at present, and that t ' , collision of Tanzawa Block began prior t to deposition of the latest Miocene to b early Pliocene Minobu Formation. N s s t B Geologic history of the Southern A Ci ,

' r Fossa Magna Region t ' ' ' - The similarity of tectonic setting of '

modern and ancient troughs described b , before, brought a new interpretation to -. - s N the geologic history of the Southern A B Ciss Fossa Magna Region. It will be briefly t l ' mentioned below (Fig. 37). ' t' ' ' It has been stated that Southwest ' Japan Arc suffered the following tectonic events during early to middle Miocene a

, ' age; rotation of Southwest Japan Arc , , due to opening ofJapan Sea (OToFuJi A B c¥ - & MATsuDA, 1983; 1984) and initiation of subduction of Philippine-sea Plate after opening of Shikoku Basin (eg. KARiG & MooRE, 1975). After early to middle Miocene @ events, the first collision took place in A B c the Southern Fossa Magna Region in middle Miocene age (ToNoucHi & KoBAyAsHi, 1983). As a result of the collision, the structural belts of the eastern part of the southwest Japan have suffered bending action to form an arch (stage 2 in Fig. 37). Until now, how- A B c ever, details on the first collision have Fig. 37. not been made c]ear. Schematic reconstruction of development of With the beginning of the second the Southern Fossa Magna Regjon. Heavy line is trench or trough as plate boundary. a: collision by Tanzawa Block, the geomo- probably "Kushigata block", b: Tanzawa block, rphologic frame work of the western c: Izu block, A: Sanbagawa belts, B: Chichibu belts, C: Shimanto belts. Arrows mean the part of the Southern Fossa Magna probably moving direction of Philippine-sea Region was constructed. This includes Plate. Reconstruction of Fujikawa Trough in Mio-Pliocene Age and its Geotectonic Implication 63 features such as Fujikawa Trough, Nishikatsura feeder valley and uplift of Kwanto mountainland (stage 3 in Fig. 37). In the following stage 3, Kwanto mountainland uplifted enough to provide abundant coarse clastic sediments during latest Miocene to early Pliocene in age. Nishikatsura Channel and the axial channel of Fujikawa Trough were constructed and filled by clastic sediments. Fill-sediments deposited thickly but had not suffer- red remarkably synsedimentary deformation, same as that observed in the present of Suruga Trough ab a collision zone. This contrasts to the features of accretionary prism in Nankai Trough as a subduction zone. Afterjumping ofthe plate boundary to the north of Hakone, the third collision by Izu Block took place during 1 Ma to O.5 Ma (SuGiMuRA, 1972; KAizuKA, 1975; MATsuDA, 1978). As a result ofthis collision, thesedimentary basin doposited coarse clastic sediments was migrated to zhe Ashigara area north of Izu Peninsula and the Minobu Formation suffered tectonic deformation such as faulting and folding causing abrupt uplift of the northern half of the southern Fossa Magna Region. At this stage, the main provenance in and around the Southern Fossa Magna Region shifted abruptly from Kwanto mountainland to Akaishi mountainland. Thus, it is thought that the direction of plate motion of Philippine-sea Plate changed from northward to northwestward (MATsuDA, 1978; NAKAMuRA, et al, 1984) (Stage 5 in Fig. 37). In conclusion, it should be stressed to say that multiple collisions have played an important role for the geologic development of the Southern Fossa Magna Region.

ACKNOWLEDGMENTS This study has been done under the direction of Emeritus Professor Keiji Nakazawa of Kyoto University. To him I am particularly indebted for guidance and encouragements throughout the doctor course of Kyoto University. I am indebted to Associate Professor Tsunemasa Shiki of Kyoto University who supervised me and offered helpful suggestions, and to Associate Professor Shiro Ishida, Dr. Daikichiro Shimizu, Kyoto University, for their usefu1 discussions and kindly considerations. Information on Bouguer anomaly was recieved from Professor Yukio Hagiwara of Tokyo University. Suggestions on local stratigraphy were given from Professors Tokihiko Matsuda of Tokyo University and Fumio Tsunoda of Saitama University. Thanks are extended to Professor Franco Ricci-Lucchi of Bologna University and Dr. Kelvin T. Pickering of University of London for their helpfu1 advice in the field. I am also gratefu1 to Messers. Kinzo Yoshida and Hisato Tsutsumi for preparing many thin sections, and Kenjilrino for advice on many figures. 64 SoH, W.

References

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* in Japanese with English abstract ** inJapanese Plates 14 Explanation of Plate 1

1 Relationships between CPA and the underlaying Furuya Sandstone at Hishakurusawa showing erosional base of channel structure (A-A'). Clinometer for scale with scouring. 6 An example of facies transition, from Facies A4 to Facieg A3 through Facies B2. Fac. Sci., Kyoto Univ., E er. Geol. & Min., Vol. LII Nos. pl. 1 Mem. ' 1&2

SOH: Reconstruction ofF ujikawa Trough. Explanation of Plate 2

1 An outcrop of profile normal to paleocurrent direction, Loc. a in Fig. 7 & 8, showing relationships between SDA (Furuyashiki Sedimentary Body) and the underlying SIA. Note the curved and slightly eroded boundary surface (a-a'). Dip and strike; 1, N5E 60W; 2, NS 4SW,: (boundary surface); 3, N20E 40NN'; 4, N15E 40W,: (bedding plane). 2 An outcrop of profile paralell to paleocurrent direction, Loc. d in Fig. 7 & 8, showing the boundary between SDA (Arayashiki Sedimentary Body) and the underlying SIA, and the boundary surface of two associations (a-a') is concordant with the bedding planes of both associations. 3 An outcrop at east Furuyashiki, showing the upper boundary (a-a') of SDA (Furuyashiki Sedimentary Body), and the overlying SIA. Note that SDA changes gradually into SIA. Mem. Fac. Sci., Kyoto Univ., Ser. Geol. & Min., Vol. LII, Nos. 1 &2 Pl. 2

SOH: Reconstruction ofF ujikawa Trough. Explanation of Plate 3

1 Sedimentary facies ofinterchannel sediments; a view ofSIA. Beds younging to right. Clinometer 12 cm long. At Oshima in Fig. 7. 2 Closed-up of typical facies of SIA.

.g Three types of sandstone beds; upper and lower beds are subfacies C3-b and C3-a respectively, and middle one is Facies C2. Coin 2.5 cm in diameter. 4 Lenticular sandstone bed of subfacies C3-b, probably fi11 sediments of h minor crevasse channel. Measure 8 cm. Jr An outcrop of Facies Al contained with angular ande3itic clast in SCA. 6 Closed-up angular andesitic clasts. Hammer 40 cm long. Mem.Fac. Sci. , Kyoto Univ., Ser. Geol. & Min., Vol. LII, Nos. l&2 Pl. 3 2

e.."tt.It?,lii

.ttt ge.

4 -¥y,.. . F ve 1'

SOH: Reconstruction ofF ujik awa Trough. Explanation of Plate 4

1 Andesitic clasts in PMA, at Hadakashima. Scale board 14 cm long. 2 One of the maximum-size clast of andesitic rock in PMA. Boundary of PMA and SIA is shown as a-a'. 3 A slump-folding of PMA in SSA, showing a westward transporting at west Kobayama (See, Fig. 22). 4 An outcrop showing lenticular sedimentary body of PMA in profile normal to transport direction. Note the erosional basal boundary of PMA. 5 An example of outcrop showing erosional basal boundary ofPMA. Boundary "a"; N85W 80VN', and bedding plane of the underlying SIA "b';; N20E 60W (Sef, Fig. 22). Mem. Fac. Sci., Kyoto Univ., Ser. Geol. & Min., Vol. LII, Nos. 1 &2 Pl. 4

SOH: Reconstruction ofF ujik awa Trough.