Aitchison 1986 Shimanto terrane

EARTH SCIENCE; JOURNAL OF THE ASSOCIATION FOR THE GEOLOGICAL COLLABORATION IN JAPAN

Vo1., 40 No.5 .pp. 337-363 SEPTEMBER 1986

Stratigraphy, Sedimentology and Tectonic Evolution of the

Shimanto Terrane, Southwest Japan

by

Jonathan C. AITCHISON*

Abstract

Shimanto terrane is a regionally extensive belt of rocks outcropping along the southeast side of SW Japan. Correlatable strata are mapped from Okinawa to Boso Peninsula. The rocks typically consist of broad zones of complexely folded turbidite sediments bounded by high angle reverse faults or thrusts along which zones of melange are located. These melange zones include allochthonous blocks of chert, basalt, and varicoloured shale within a pervasively deformed shaly matrix. Smaller zones of less complexely deformed turbidite or shallow water sediments are found stratigraphically above complexely deformed zones. Occasional acid tuff beds are found throught the terrane and sandstone petrography reveals that most strata were derived from the erosion of a mature, active volcanic arc.

Shimanto rocks were accreted to, or deposited on, the SE side of the Japanese archipelago during (at least) Cretaceous to Miocene time as a result of the northward subduction of the Kula Plate.

Several large scale, regionally extensive, thrust faults represent decollements developed within the accretionary prism and the Shimanto terrane (s.s.) may be a more extensive terrane than the belt of rock to which the name Shimanto is currently applied.

I Introduction

In the past ten years many works describing Shimanto terrane strata ha ve been published. This has resulted from an increase in interest in these somewhat monotonous, yet structurally complicated, rocks after it was discovered that many beds yield radiolarian fauna. Advances in radiolarian biostratigraphy have enabled geologists to more fully. understand Shimanto rocks now that age relationships of various strata can be more clearly ascertained. Biostratigraphic work shows that Shimanto terrane consists of an overall sequence of generally SSE younging strata, however, strata within individual fault bounded blocks often young northwards. Other advances in the understanding of Shimanto rocks have resulted in the recognition of widespread zones of tectonically deformed melange. Paleomagnetic study has shown that many melange blocks originated from significantly lower paleolatitudes than encompassing strata.

Numerous works, often in Japanese, with vague or rather poorly translated English abstracts, have been published about the Shimanto terrane. As a result of various groups of researchers working independantly and publishing local stratigraphies, a complicated lithostratigraphic subdivision of the Shimanto has evolved and there is some difficulty in correlating like strata between individual areas. In addition individual groups of workers have tended to belong to distinct schools of geological thought and their conception of Shimanto terrane development is often closely reflected in the manner in which individual outcrops are described. Classically many Japanese workers have interpretted these rocks as strata which developed in a geosyncline and at least one author (YANAI, 1984a, b) still adheres to this pre-plate tectonic era model.

Since KANMERA and SAKAI (1975) interpreted the sedimentary setting of the Shimanto rocks as a trench-forearc region on the basis of the similarity of their tectonic style, many other workers have

:c-;:----c----c--:-:::---:-----"--""* Department of Geology and Mineralogy, Faculty of Science, Niigata University, Ikarashi-2, Niigata 950-

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present address: <1 Crossleigh Crescent, Balclutha, New Zealand

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CRETACEOUS STRATA

PALEOGENE STRATA

MIOCENE STRATA

INTRUSIVES

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i Table 1: Shimanto terrane stratigraphy and possible correlation between areas.

F.=Formation, G_=Group, B.=Belt, M.=Melange. on Kyushu; (a) from IMAI et al. (1979), (b) from SAKAI and KANMERA (1981), on Shikoku; (c) from ISHIHARA et al. (1985), (d) Hata Peninsula from TAIRA et al. (1980b), (e) Muroto Peninsula from TAIRA et al. (1980b), (f) East Shikoku from KUMON (1983), on Kii Peninsula (w) = west. (e)=east, (N)= northern Muro Group, (S)=southern Muro Group, on Akaishi Mt_s; (h) after KANO and MATSUSHIMA (in press), CD after IUlMA (1981), on Boso; (k) after TAIRA et al. (1982), on Kanto; (1) after OGAWA and HORIUCHI (1978).

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-340 Jonathan C. AITCHISON

gradually come to accept the likelyhood that Shimanto terrane represents strata accumulated in an accretionary complex. In light of the somewhat complicated regional stratigraphies already pub~

lished for these strata, recent developments of the terrane concept (JONES et al., 1983), and refinements of plate tectonic models, I will attempt to synthesis existing data on the Shimanto rocks along with my own observations and intepret these data with regard to the evolution of the Shimanto terrane. In many cases descriptions of strata in existing literature are highly interpretive and occasionally Japanese nomenclature is somewhat different from that of the English speaking geological world. However, it is possible to describe regional stratigraphy and the similarity that exists between areas and finally make some suggestions as to the evolution of the terrane.

II Stratigraphy

Distribution and stratigraphy of the Shimanto terrane rocks are shown in Fig. 1 and Table 1.

A. N ansei Islands

Southernmost outcrops of Shimanto terrane rocks occur on the Nansei Islands. This area has not been studied in as much detail as areas such as Shikoku, nevertheless descriptions of Shimanto strata on Okinawa (FUKUDA and HA YASAKA, 1978), Amami Oshima (SAIJI, 1969; SAKAI et aI., 1977; OSOZAWA et al..1983) and Tanegashima (HAYASAKA et al., 1980; HAYASAKA, in press; TAIRA et al., 1982) suggest similarity with Shimanto strata to the north.

Oldest Shimanto rocks, on Okinawa Island, are represented by the early Cretaceous Nago Formation which includes melange rock consisting of phyllites with intercalated mafic volcanic blocks. Nago Formation is in fault contact with Eocene Kayo Formation to the east along a NNW dipping thrust. Kayo Formation includes alternations of quartz-rich, fine to medium grained turbidite sandstones and shales along with subordinate intraformational conglomerates (FUKUDA and HAYASAKA, 1978). Overall Kayo Formation is a composites fining upwards megasequence dominated by smaller fining upwards sequences. Trace fossils of the Nereites ichnofacies (SEILACHER, 1967) are reported from Kayo Formation and indicate lower bathyal to abyssal (3500-5000 ill water depth) deposition.

Further north a thicker sequence of Shimanto rocks crops out on Amami Oshima (SAur, 1969; OSOZAWA et al., 1983). Shimanto rocks are in fault contact, to the west, with Yuwan Formation which consists of Chichibu terrane melange. Oldest

( 34)

Shimanto strata are rocks of the Cenomanian to Turonian Naze Formation. Naze Formation consists of a melange which includes slate, phyllite, sandstone and basalt, and is in turn, overlain by the Turonian Odani Formation. Odani Formation consists of mostly sandstone and slate, however it also contains some allochthonous masses of Aptian slate and basalt. Ogachi Formation, is also of Turonian age and shows a coarsening and thickening upwards sequence of sandstones and shales. The youngest Shimanto strata are represented by the Eocene Wano Formation which consists of turhidite sandstones and shales with some conglomerates in a gradually fining upwards sequence (ISHIDA, 1969). Wano Formation is fossiliferous and contains numerous ichnofossils, gastropods, pelecypods, echinoids and plant fossils. Strata on Amami Oshima are complex and often repeated by folding and thrust faulting.

In the northern Nansei Islands Shimanto rocks on Tanegashima have been studied in detail and some of the results are published by HAYASAKA et al. (1980), HAYASAKA (in press) and TA[I{A et al. (1982). Shimanto strata on Tanegashima are of Paleogene age and are represented by the Kumage Group. Kumage Group is subdivided by HAYASAKA et al. (1980) in ascending order into Nijuban. I-Iamatsuwaki, Fukago and Sumiyoshi formations. Nijuban Formation is a thickening, coarsening upward sequence of turbiditic sandstones and shale with some interbedded tuff horizons. Hamatsuwaki Formation is described as having three members (HAYASAKA et al., 1980). These members show an overall thickening, coarsening upwards sequence of turbidite sandstones and shales which are themselves grouped into smaller thickening and coarsening upwards cycles. These members are developed on top of a melange of chaotically disturbed tuff, sandstone, basalt (including pillow lava), micritic limestone and umberlike blocks within a scaly cleaved mudstone matrix. Rocks of this and other correlative melange on Yakushima are discussed by SAKAI (1980) who suggested that they are of olistostromal origin. Fukago Formation is another series of thickening, coarsening upward sequences and coarsens upwards overall from thinly bedded alternations of sandstone and shale to coarse grained thick turbidite sandstones and shales. Sumiyoshi Formation represents yet another thickening, coarsening upwards sequence from thin bedded sandstone and shale alternations to medium to coarse sandstone beds with subordinate shale.

The general trend of Kumage Group is NNE-SSW and strata typically dip steeply westwards. An inbricate structure is often developed due to numerous thrust faults and tight folds. TAIRA et

Stratigraphy, sedimentology and tectonic evolution of the Shimanto Terrane, 341

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CRETACEOUS SHIMANTO S'.rRATA

(Cretaceous melange bearing zone) MOROTSUKA BELT

PALEOGENE MELANGE ZONE MIKADO BELT

EOCENE-LOWER MIOCENE SHIMANTO STRATA

(olistostrome bearing strata)

lIYUGA & NICHINAN BELTS

lo\IOCENE ~IIYAZAKI GIlOUP

ACID INTRUSIVES

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Fig. 2 Distribution of Shimanto terrane rocks in southern Kyushu after IMAI et al. (1982) and others.

al. (1982) reported Kumage Group strata somewhat differently. They described a latest early Eocene to earliest middle Eocene radiolarian assemblage and suggested that the group consists of three

35

tectonostratigraphic units. These units are: flysch with variegated claystones, chaotic rocks, and thick cross bedded sandstones which include occasional thin coaly beds above seat earths. They also

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342 Jonathan C. AITCHISON

reported some clastic dikes and sand volcanoes. Although stratigraphic descriptions are somewhat different between HAYASAKA et al. (1980), HAYASAKA (in press) and TAIRA et al. (1982) there appears to be broad agreement about the lithologies present.

B. Kyushu

Shimanto terrane crops out throughout much of southern Kyushu (Fig. 2). In the north Shimanto rocks are in fault contact with Chichibu terrane along the Butsuzo Tectonic Line. Overall the rocks young SE although within individual fault bounded formations rocks often young northwards. Various stratigraphic subdivisions have been published CIMAI et at., 1971, 1979, 1982; SAKAI and KANMERA, 1981; SAKAI, 1978, 1985; TAKESHITA, 1982; NISHI, 1985; etc.) and there is general agreement that these rocks can be subdivided into at least four belts. These belts are Morotsuka, Mikado, Hyuga and Nichinan belts.

Morotsuka Belt is of Cretaceous age and is subdivided differently by various authors (eg. SAKAI and KANMERA, 1981; IMAI et al., 1982). It includes some zones of melange along with other more coherent rocks. Melange includes chert, metabasalt, dolerite, gabbro, hyaloc1astite, acidic tuff and sandstone blocks within an intensely deformed, cleaved, sheared, shale matrix. Melange is succeeded by a coarsening upward sequence, beginning with shale dominated alternations of turbidite sandstone and shale with interbedded acidic tuff and coarsens upsection to become sandstone dominated. Moro· tsuka--Belt geology is complicated by numerous low angle, NE-SW trending, NW dipping, thrust faults which shorten and repeat many sedimentary sequences. Actinolite zone of the greenschist facies regional metamorphism is widespread and most sandstones and shales are psammite and pelite respectively.

Mikado Belt is dominated by a large zone of Paleocene melange and is separated from Morotsuku Belt to the north by the NE trending, NW dipping, Nobeoka Thrust. Mikado Belt includes blocks of prehnite'pumpellyite metamorphosed basalt, including pillow basalts, along with hyaloclastite, minor gabbro, varicoloured shale and sandstone in a highly deformed, intensely sheared, cleaved, shaly matrix.

Above the melange is a coarsening, thickening upwards sequence of sandstone and shale alternations.

SE of Mikado Belt and separated from it by Oyabu, Hikageyama or Mikado thrusts is the Eocene to lower Miocene Hyuga Belt. Hyuga Belt is subdivided by SAKAI and KANMERA (1981) into four formations; Aradani, Tashiro, Unama and Ohchibaru formations in ascending order. Aradani

36

Formation is a melange unit which includes metabasaltic blocks in a highly sheared shale matrix. Lower Tashiro Formation is dominated by shale with subordinate sandstone and upper Tashiro Formation consists of slumped blocks of sandstone in an olistostrome which has a shale matrix. Successively higher Hyuga Belt formations complete a thickening, coarsening, upwards sequence which culminates in coarse, thick sandstone.

Another NE trending, NW dipping, thrust separates Hyuga Belt from the Oligocene-Miocene Nichinan Belt which lies further to the south. Nichinan Belt rocks are described by NISllI (1985) who reported an overall composite, fining upwards, megasequence which includes both smaller, fining and coarsening, upward sequences. Lowermost beds include some conglomerates. Forams found throughout the belt suggest deposition above the CCD. Nichinan Belt is notable for an extensive olistostromal belt comprised of large scale, gravity slides with beds and blocks of sandstone and alternations of sandstone and shale chaotically dispersed in a sheared and intensely contorted muddy matrix. NISHI (1985) reports megablocks up to 35km in size completely surrounded by chaotic rocks. Late Miocene and younger beds of the Miyazaki Group overlie Nichinan Group with angular unconformity.

C. Shilwlm

Outcrops on Shikoku probably represent some of the best exposed, least metamorphosed and de· formed, Shimanto terrane strata and are consequently some of the most intensively studied of these rocks. Shimanto strata take their name from the Shimanto River of Hata Peninsula. Many works describing aspects of Shikoku Shimanto geology in various ways, have been published and include those by SUZUKI and HADA (1979, 1983), TAIRA et al. (1980. 1982b). SAKAI (1981). YANAI

(1983, 1984a), KODAMA et al. (1983), ISIlIIIARA et al. (1985). TERAOKA et al. (1986). KUMON (1981. 1983). Much excellent interpretive work has recently emanated from Kochi University and for this reason the stratigraphy proposed by Kochi workers is preferred in this paper. TAIRA et at. (1980b) summarized Shimanto stratigraphy in Shikoku on the eastern and central parts of Rata Peninsula, around Kochi city, and on SW and southern parts of Muroto Peninsula (Fig. 3). They subdivided into a northern belt of Cretaceous strata (possibly a correlative of Morotsuka Belt in Kyushu) which is in fault contact with a southern belt of Tertiary strata. These two belts are further subdivided into four subbelts; northern Shinjogawa and Taisho, and southern Murotohanto, and Nabae subbelts. Correlation with Kyushu is difficult, although

Stratigraphy, sedimentology and tectonic evolution of the Shimanto Terrane,

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Murotohanto subbelt probably correlates with both Mikado and Hyuga belts of Kyushu. Uppermost Murotohanto subbelt and Nabae subbelt correlate with Nichinan Belt in Kyushu.

Shinjogawa Group (subbelt) strata are of Neocarnian to Turonian age. Shindoi Formation (I-Iata Peninsula) is comprised of zeolite facies metamorphosed. sandstone dominated beds of alternating turbidite sandstones and shales. Doganaro Formation (near Koehi city) consists of sandstone and shale which yield late Neocomian to Aptian brackish to shallow marine molluscan fossils and contains occasional limestone olistoliilis, possibly derived from southern Chichibu terrane strata to the north. Hayama Formation (both Rata and Muroto penin" sulas) contains sandy turbidites of late Aptian to Albian age and Susaki Formation (cropping out south of Hayama Formation) consists of Ceno" manian mud dominated turbidites with intercalated red hemipelagic shale. Uwagumi Formation (in the north of Muroto Peninsula) is dominated by sandstone and mudstone which contains a shallow marine, Turonian"Coniacian, fauna.

Taisho Group (subbelt) rocks are of Coniacian to Maastrichtian age and is mapped on Hata Peninsula whereas similar correlative rocks, although known on Muroto Peninsula, are not subdivided by TAIRA et at. (1980b). Shimotsui Formation is Coniacian to Santonian in age and consists of turbidites with frequent intercalations of hemipelagic red shale. Nonokawa Formation is also of Coniacian to Santonian age and is dominated by sandy turbidites. Nakamura Formation is also of Coniacian to Santonian age and is dominated by synsedimentarily deformed muddy turbidites which contain some basaltic blocks. Arioka Formation yields Campanian to Maastrichtian fossils from its slump folded sandstone and shale beds. Formations of Taisho Group are generally inclined at a high angle and individual formations young northwards. They are separated by high angle, north dipping, reverse faults and zones of melange rock. These melange zones (Ohyu, Yokonami, Tel, Kure, Tsukimiyama melanges of TAIRA et at., 1980a, which are discussed later) contain allochthonous blocks of basaltic pillow lava, chert, red pelagic shale, hemipelagic varicoloured shale, acidic tuff, limestone, hyaloclastite, etc. of Jurassic to Cenomanian age in black to grey sheared, highly deformed. shale matrix which yields Coniacian to Santonian radiolarian fossils.

Other northern belt Cretaceous strata from western Shikoku are discussed by ISHIHARA et at. (1985). They subdivide into two belts; the Uwa and Nakasuji belts. Northern portions of their Uwa Belt are of late Neocomian to Santonian age and southern areas are Conician to Campanian.

38 )

Nakasuji Belt is of Coniacian to Maastrictian age. ISHIHARA et al. (1985) subdivide Eocene to early Oligocene strata further south into Hiromi and Shimizu belts which TAIRA et al. (1980b) map somewhat differently.

Geology of the Uwajima area (Uwa Belt) is complicated and complex relationships exist between shallow marine fossil bearing sandstone facies and sandstone and shale alternations'deposited as turbi" dites in a deeper marine environment (Y. Teraoka pers. comm.; TERAoKA et ai., 1986). A clear difference can be seen between the chemical and detrital sandstone compositions of these rocks which developed somewhat differently to typical Shimanto rocks (TERAOKA, 1979). These rocks represent autocannibalised Shimanto rocks the mineralogical maturity of which resulted from recycling of existing Shimanto strata by erosion and subsequent redeposition in a slope basin. TERAOKA et at. (1986) map rocks in the Uwajima district as a series of nine structurally divided fault blocks which include rocks ranging in age from Barremian to Santonian.

In papers on east and west Shikoku. YANAI (1983, 1984a) has mapped Shimanto rocks. Despite presenting an alternative interpretation of the3e rocks to that of most other workers (cf. TAIRA et al .• 1980b, 1982; SUZUKI and HADA, 1979; and myself), non genetic, lithological. descriptions of Yanai indicate that strata in these areas follow similar Shimanto patterns. Tightly folded and faulted Cretaceous turbidite deposits bounded by major faults and melange zones are reported from both areas. Melange is generally confined to southernmost parts of the northern belt of Cretaceous strata. Cretaceous shallow marine sandstones which are markedly less deformed than adjacent lithologies comprise the Uwajima Group of YANAI (1984a). Uwajima Group consists of three sequences which fine upwards from conglomerate and sandstone to finer grained lithologies. Some thick conglomerate horizons are known, along with. acidic tuff horizons. In eastern Shikoku Yanai (1984a) subdivides Cretaceous Shimanto strata into Northern, Middle and Southern Shimanto terranes and assigns Paleogene strata to another terrane, the Setogawa-Nakamura terrane. He then changes from use of the tectonostratigraphic term • terrane' to the lithostratigraphic 'Supergroup', the strata of which he proceeds to describe on the basis of biostratigraphy. This does not allow for easy correlation with stratigraphies of other workers. However. it is likely that the Northern Shimanto Terrane of YANAI (1984a) can be broadly correlated with Shinjogawa Group of TAIRA et at. (1980b) and his Middle and Southern

Stratigraphy, sedimentology and tectonic evolution of the Shimanto Terrane,

Shimanto terranes with their Taisho Group. Setogawa-Nakamura terrane corresponds with the southern Shimanto Belt of TAIRA et at. (1980b).

KUMON (1981. 1983) also maps and describes r eastern Shikoku Shimanto strata and subdivides

northern Shimanto Belt (similar usage to TAIRA et al., (1980b) into three fault bounded zones. The northernmost zone is represented by a coarsening, thickening upwards sequence which is further subdivided into two formations. Akamatsu Formation is dominated by muddy rocks which are conformably overlain by mainly thick bedded sandstones and minor alternations of turbidite sandstone and shale as well as conglomerate of the Hinotani Formation. Kumon also reports rare greenstone and chert from Hinotani Formation which may indicate the presence of melange. Strata of this northernmost zone range in age from Aptian to Cenomanian.

The middle zone of KUMON (1983) consists of the Taniyama Formation which is 2000m thick and includes a melange zone with sandstone, acidic tuff, and allochthonous chert, red shale and green" stone in a muddy matrix followed, to the south, by sandstone and coarse acidic tuff. The age of this formation is reported as Turonian to Santonian.

The southern zone of KUMON (1983) begins with a melange unit, the Mugi Formation, which includes greenstone and acid tuff blocks dispersed in a muddy matrix. This formation is succeeded by Hiwasa Formation which is mainly composed of sandstone and conglomerate. These rocks are late Santonian to Maastrichtian in age. Shimanto rocks in eastern Shikoku are frequently faulted and their geology is complex.

Paleogene Shimanto strata are separatl;!d from Cretaceous rocks by the Nakasuji Tectonic Line on Hata Peninsula and the Aki Tectonic Line on Muroto Peninsula. As most of these rocks lie within Kochi Prefecture their stratigraphy is described by TAIRA et al. (1980b). Murotohanto subbelt consists of two correlative Eocene to early Oligocene Groups; Hata and Murotohanto groups each restricted to the peninSUla from which their name is taken. These groups are further subdivided into a series of broadly correlative formations. Hirata Formation on Hata Peninsula consists of shallow marine sandstone and siltstone. This formation lies above the Cretaceous Arioka Formation and possibly represents a continuation of deposition in the same environment. Ohyamamisaki (Muroto Peninsula), Tanokuchi and Tatsugasako (Hata Peninsula) formations contain resedimented conglomerate and turbidite sandstones along with some slump deposits. Conglomerates within these formations contain a variety of lithol-

ogies including boulders of crystalline schist. KUMON (1983) calls similar beds, in eastern Shikoku, Kaifu Formation and mentions that muddy facies dominate the lower part of this formation. Hiromi Complex (:t-Iata Peninsula) rocks are similar although they are more shale rich and contain some basic igneous blocks which may be slump derived (TAIRA et al., 1980b). Kurusuno and Naharigawa formations (Rata and Muroto peninsulas respectively) contain sand dominated turbidite deposits with some slumps. The upward thickening and coarsening Naharigawa Formation possibly represents a basin fill sequence. Some minor orthoquartzite is found as conglomerate clasts (KUMON, 1983) in this formation. Muroto and Shimizu formations (Muroto and Rata peninsulas respectively) are possibly correlative and contain syndepositionally deformed, shale dominated, turbidite and debris flow deposits with a small amount of intercalated tuff. Both formations are extensively slumped and contain olistostromal beds. Muroto Formation is in tectonic contact with Sakihama Melange. Sakihama Melange inclUdes huge blocks of basalt along with some red pelagic shale. intercalated in a sheared, deformed, scaly, shale matrix of probable Eocene age. Miocene shallow marine beds of the Misaki Formation are developed on top of Shimizu Formation on Hata Peninsula.

Late Oligocene to lower Miocene Nabae Subbelt is known only from Muroto Peninsula and is subdivided into thl-ee formations. l-Iioid Complex consists of basaltic blocks in a deformed muddy matrix of late Oligocene age. Shijujiyama Formation consists of shallow marine. fossiliferous sand" stones. Tsuro Formation rocks are of early Mio" cene age and composed of turbidite sandstones, red shale and olistostromal rocks.

Shimanto terrane rocks on Shikoku have complicated structural relationships. Individual formations often young northwards with an overall younging polarity to the south. Numerous strike parallel. northwards dipping, high angle reverse or thrust faults occur and typically mark formation boundaries. These faults are often accompanied by zones of melange rock.

D. Kii Peninsula

Shimanto rocks on Kii Peninsula are quite similar to those exposed on eastern Shikoku. Much research has been carried out by members of the Kishu Shimanto Research Group (KSRG) in the last decade and stratigraphy, especially that of western KE Peninsula, is well established (KUMON, 1985). However. the interpretation of this work differs from many interpretations previously published by KSRG workers (eg. HARATA et at., 1978).

( 39 )

346

:i :J

'" z ffi Q.

Jonathan C. AITCHISON

II

Z -=:::j'

( 40 )

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" " w w z z w w u u B B 0 z z


Shimanto terrane rocks are generally separated from Chichibn terrane strata to the north by the major thrust of the Butstlzo Tectonic Line. However, in part of central Kii Peninsula, Chichibu terrane rocks are thrust over Shimanto rocks and form a klippe above Shimanto rocks. Shimanto rocks there are in direct contact with Sambagawa terrane schists along the Aridagawa Tectonic Line (KURIMOTO, 1982). Kii Peninsula Shimanto rocks are less tectonically disrupted and less metamorphosed than correlative strata further to the north or in Kyushu.

Shimanto strata are subdivided, by KSRG, into three groups; Hidakagawa, Otonashigawa and Muro groups which are separated by high angle, northwards dipping, reverse faults (Fig. 4).

Hidakagawa Group strata are of Cretaceous age and are further subdivided into six typically reverse fault bounded formations which have an overall southwards younging polarity. Hanazono Formation was formerly mapped as part of Chichibu terrane, but KURIMOTO (1982) demonstrated that it should be included within the Shimanto terrane. It consists of mainly massive sandstone and boudinaged lenses or blocks of sandstone in shale. Some blocks of red bedded chert, red shale and metabasalt are also known. Radiolarian fossils indicate a Coniacian to Campanian age.

Yukawa Formation consists of thickly bedded, massive, shallow marine bivalve bearing, synclinally folded, sandstones in the north. To the south the formation consists of isoclinally folded, structurally imbricated, turbidite sediments. Some allochthonous light grey to black coloured chert blocks, possibly derived from the Chichibu terrane, are also known. Radiolarian and molluscan ages from the formation indicate a mainly Albian to Cenomanian age range. KUMON (1983) correlates Yukawa Formation with rocks on eastern Shikoku which he mapped as Akamatsu and Hinotani formations of his northern zone.

Miyama Formation consists of Albian to Cenomanian turbidites with intercalated fault bounded zones of melange rock and can be correlated with similar zones on Shikoku (Taniyama Formation of the middle zone_ of KUMON, 1983). Slaty cleavage is developed throughout the formation especially in melange units. More coherent turbidite units are typically composed of alternations of sandstone and shale which often include acidic tuff beds up to several -metres thick. Melange units, some of which were described by NAKAZAWA et al. (1983), include blocks of Tithonian to Albian red bedded chert, basaltic lava, including pillow lava, hyaloclastite, Albian to Cenomanian siliceous sandstone and acidic tuff, varicoloured shale, sandstone and

black shale all in an intensely sheared black shale matrix. Melange matrix yields radiolaria of Turonian to Santonian age similar to those from shale interbeds in more coherent turbidites which surround the melange zones.

Ryujin Formation is of Campanian to Maastrichtian age and correlated by KUMON (1983) with his middle zone Mugi and Hiwasa Formations on eastern Shikoku. Ryujin Formation is dominated by shaly turbidite sandstone and shale alternations with sandstone dominant zones occurring in some areas. Paleocurrent analyses indicate longitudinal, E-W, sediment supply. Acidic tuff is present as vitric tuff, vitric crystal tuff, pumice and fine 1apilli ash in beds up to a few metres thick, all of which have typically been redeposited by gravity flows. Some basalts (including pillow lava) as well as hyaloclastites are seen as allochthonous blocks along with umberlike manganiferous rocks and siliceous red shale. Overall, rocks of this formation represent a thickening, coarsening, upward sequence with some cobble conglomerates present in uppermost beds.

Nyunokawa Formation is another coarsening upwards sequence from shale through turbidite sandstone and shale alternations to conglomerate at the top of the formation. Once again, some melange is present with basaltic blocks seen in a shaly matrix. An ultrabasic block is reported from melange in this formation by SUZUK[ and HADA (1983) at Karuigawa. Strata are folded and repeated by faulting making stratigraphy complicated, however, radiolarian fossils indicate a probable Maastri~ chtian age.

Terasoma Formation, of Turonian to Santonian age, contrasts with other Cretaceous Shimanto formations on Kii Peninsula in that it contains numerous megafossils. Few bedding disruptions are seen and strata form a simple synclinorium. Basal strata are composed of mainly shale and shaly alternations which coarsen upwards to a middle member which is characterised by a dominance of sandy alternations. Strata subsequently fine upwards to black shale with thin intercalated sandstone at the top of the formation. Nyunokawa and Terasoma formations are considered to be lithologically and structurally similar to, though they are different chlonologically from, Hiwasa Formation of KUMON (1983) in eastern Shikoku. Terasoma Formation is in fault contact, with Miyama Formation beds of correlative age to the south.

Otonashigawa Group is bounded by northward dipping, reverse faults. To the north the GoboHagi Tectonic Line separates Otonashigawa Group from Hidakagawa Group and to the south Hongu

( 41 )

348 Jonathan C. AITCI-lTSON

t it from Muro Group. Strata are I separa es Fan t 1 folded and imbricated by several narth i~te~se Yreverse faults. Their precise age is not d1PP1D-g

1 known, however, the position of the

sent Y pre between Cretaceous and late Eocene strata grOUP a probable Paleocene to early Eocene age.

ggests . . d b H h' gU P is subdivide y atenas t Research The g~~~73, 1980) into three formations; Uridani, Group nel Fudono formations in ascending order. II raku a '.

a oN" (1983) Suggeste? correlatlO.ll Of. Otonasht-KUMa Graup with Kaifu FormatIOn m eastern gaW Greenstone and chert are not known Shikoku,

Otonashigaw~ Group ~ocks. frorn. l' FarmatlOn consists of black, bedded

Undall , h d d '1' with minor greems mu stone an Sll" udstone

nl udstone. Some calcareous nodules are ceous rn In the uppermost part of the formation

esent. pr istic marker beds of green and/or red character

e are present. I1lUdsto2u Formation is a coarsening and thicken-Ilaro~ [ b'd' 1 '

ards sequence 0 tUT lite a ternatlOns of . tlPW lDg and mudstone. Sandstone gradually sandstone dominant upsection with crystalline schist becO~es boulder to cobble conglomerates, similar bearlIlg found in Ohyamamisaki Formation on

those to . present in uppermost beds. Paleocurrents ShlkakU'dominantly longitudinal E-vV transporta-. dicate . ' 1 I~ of sediment wIth mmor ateral transport from tWD orth and south. both n Formation is a restricted thin sequence

Fudono . ddy sandstone and shale alternatlOns.

.of mU

Group outcrops extensively over a wide :rvl~ro outhern Kii Peninsula. Detailed study of

area JJl ~roup (KSRG, 1970; SUZUKI et al., 1979; Muro I 1978' TATEISHI et al., 1979; etc.) has 'fATEISH, ' .. ' .

d in its subdlvlslOn mto two blocks each resu~te'ng of three, correlative, formations. The conslstl e separated by the Matsune-Hirai Fault. blocks. ar . . the individual blocks formations are con-Wlthl~le. Structure is somewhat sjmpler than forrn~f older Shimanto strata on Kii Peninsula. that . the northern block are characterised by S ata JU tr . whereas those in the southern block are foldlngt risticallf block faulted with s.ome slump charac e I . 'n ·the south (KUMON et a ., in pl"ess). An fold101~ ;hickening and coarsening upwards mega~ avera is present in Muro Group strata and seCju~cerepresents basin infilling (TATEISHI, 1978). pOSs! Y rent data indicate longitUdinal and lateral PaleoCtlr

t Lateral transport was not only from transpor .

th but also from the south (TATEISHI, 1978). the nor rtzite clasts which occur as a minor Ol'thoqua 1 onent of polymictic cong omerates of the Muro camp decrease in size and abundance northwards. Group, Group strata are' presumed to be of-· Eocene Mur

o 1 Miocene age and are overlain, with angular

to ear Y

( 42 )

unconformity, by middle Miocene Tanabe and Kumano Group rocks.

Yasukawa Formation, in the northern block, outcrops in the centre of a major structure, the Uchikoshi Anticline. Strata represent a thickening, coarsening, upwards sequence of sandstone and mudstone. Sandstone dikes are frequently found within sandstone and shale alternations. This formation correlates with Wabuka Formation in the southern block which outcrops in the core of the Wabuka Anticline and consists of bedded black mudstone and muddy alternations which contain a marine molluscan fauna.

Uchikoshi Formation in the north is a thick monotonous sequence of massive sandstone and sand dominant flysch alternations. Some intercalated thick, polymict conglomerates are also known. Marine molluscan fossils are found in rare bedded mudstones. Uchikoshi Formation thins rapidly eastwards and correlates with Mitogawa Formation in the south. Mitogawa Formation is also dominated by massive sandstones with a thick sequence of muddy turbidites intercalated mid-section.

Kogawa Formation is of variable lithology and outcrops discontinuously in the northern block. Strata include both massive and bedded mudstone with occasional pebbly mudstone, sandy turbidites, massive medium sandstone associated with conglomerate beds, turbidite mudstone and sandstone, conglomerate and pebbly mudstone and muddy turbidites. Marine molluscan fossils including Glycimeris are known. Shimotsuyu Formation, in the southern block, is correlative and of similar variable lithology. Shimotsuyu Formation is notable as it includes the Sarashikubi (gibbeted head) beds which are an unusually eroded series of allolistostromal beds which outcrop on a wave-cut platform in SE Kii Peninsula. Abundant OligoMiocene fossils are found in these two formations: Out~rop in NE Kii Peninsula is not as good as

in other areas and Shimanto rocks are often covered by younger rocks. However; MIZUTANI et at. (1982) report on the rocks in the Nanto-Nansei area of Mie Prefecture. Here zones of melange rock:which includes blocks of basalt, red bedded chert, siliceous shale, and varicoloured shale in a sheared, muddy matrix are intercalated between zones o~ coherent, turbidite sandstones. Radiolarian ages range from mid-Triassic to late Cretaceous. Middle Triassic radiolaria from chert blocks along with Coniacian to Santonian chert interbedded with basaltic blocks are found in melange with a matrix of late Creta~ ceous age. Paleomagnetic study of allochthonous blocks in this melange indicates that they formed at equatarial latitudes (HIROOKA et al., 1983).


E. Akaishi Mountains

Rocks characteristic of the Shimanto terrane including turbidite sandstones and shales with intercalated melange zones outcrop throughout the Japanese Minami (Southern) Alps. These alps are also known as the Akaishi Mountains. Strata in the south trend NE-SW and those in the north have been tectonically bent and now trend N-S. Rocks are strongly deformed in a manner similar to that of other Shimanto rocks elsewhere, although, further post Paleogene disruption has increased the complexity of these strata. Large scale, left lateral faults, regional metamorphism (to actinolite zone of the greenschist facies in the north) (YAMADA et al., 1983; AITCHISON, in prep), development of slaty cleavage, as well as contact metamorphism in the far north, in these rocks which outcrop in rugged, mountainous, heavily forested, terrain with few roads have precluded detailed study of these strata. YAMADA et at. (1983) have published details on northern parts of the range. However, the stratigraphy of KANO and MATSUSHIMA (in press) is preferred in this work as their paper deals with the whole of the Akaishi Mountains and their stratigraphy is easy to follow (pers. obs.) and generally accepted by those working in the district (Fig. 5). However, they only describe strata older than those of mid-Eocene to lower Miocene age and complex younger strata are described by IUIMA et at. (1981) and IBARAKI (1984) .

Shimanto terrane is in fault contact with Chichibu terrane strata to the NW along the Butsuzo Tectonic Line. As in other areas Shimanto rocks form an overall SE younging sequence of tectonically separated units bounded by high angle NW dipping, reverse faults.

Strata in the northwestern part of the region include the Akaishi Group from which Albian to Cenomanian radiolaria have been found (KA WA ~

BATA, 1984). Akaishi Group is in fault contact with Chichibu terrane to the NW. The group is dominated by sandy turbidites and alternations of sandstone and shale with subordinate conglomerate and mudstone. Acidic tuff and tuffaceous mudstone are also known. Rare, thin melange zones with blocks, often stretched, of dolerite, basalt. hyaloclastite and varicoloured shale as well as acidic tuff are known north of Mt. Akaishi (AITCHISON, in prep.). Slaty cleavage and incipient schistosity in sandstones is seen in the north. Tight isoclinal folds and tectonic disruption make accurate detailed mapping difficult.

Shirane Group consists of both melange and coherent zones of turbidites which, in some instances, may possibly be large megablocks within

the-;:melange (KAWAI3ATA, 1986). Melange includes dolerite, metabasalt, hyaloc1astite, limestone, acidic tuff, siliceous shale, red bedded chert, grey chert, sandstone and shale blocks in a pervasively sheared deformed muddy matrix (AITCHISON, in prep). Jurassic radiolaria are known from grey chert blocks in the Toyama River area (ITo, 1981) and Albian to Cenomanian radiolaria are known from red bedded chert south of Mt. Akaishi (KA WABATA, 1984). Muddy tuffaceous mudstone and melange matrix shale yields Conician to Campanian radiolaria (KAWABATA, 1984). A major melange zone occurs immediately east of the fault contact of Shirane Group with Akaishi Group to the west. Sandstone dominated turbidites with interbedded acidic tuff outcrop east of the faulted eastern boundary of this major melange zone. Strata become finer grained eastwards and although further melange zones are present to the east they do not appear to be as extensively developed as the westernmost zone. The original nature of Shirane Group beds is extensively disrupted by tectonic effects and finer detail is often destroyed by shearing and development of slaty cleavage. North of Mt. Akaishi, Shirane Group rocks are regionally metamorphosed. Schistosity developed in sandstones is a little higher than that which is developed in Akaishi Group strata to the west.

Sumatagawa Group is characterised by rhythmically bedded alternation beds of sandstone and mudstone. Beds become more sandstone rich in a SW direction. Strata are relatively nncomplicated in structure and include occasional slumped beds, frequent pale green, acidic tuff beds and tuffaceous mudstones up to several metres thick, along with, occasional, polymictic, cobble conglomerates. Radiolaria extracted from these strata (lTo, 1981. MURAMATSU, 1984) indicate a late Campanian to early Maastrichtian age.

Inui Group outcrops SE of Sumatagawa Group. This group includes chaotic mixtures of sandstone and mudstone in the north. Elsewhere there are thick, coherent zones of turbidite sandstone and shale with interbedded, white to pale coloured. acidic tuff up to a few metres thick. Melange zones incorporated into the group include allochthonous blocks of metabasalt (including pillow lavas, pillow breccias and hyaloclastites), red shale and red bedded chert in a sheared shale matrix. Some sandstone zones may represent megablocks in melange. Veins of calcite and quartz, possibly associated with dewatering, are seen sub-parallel to major disrupted melange zones (pers obs). Late Maastrichtian radiolaria extracted from mudstone indicate a late Cretaceous to early Paleogene age for the group.

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350

AKAISHI MOUNTAINS

LEGEND.

I CRETACEOUS SHIMANTO STRATA

(melange bearing zone)

PALEOGENE SHIMANTO STRATA

EO-MIOCENE SHIMANTO STRATA

,.(olistostorne bearing

ACID INTRUSIVES

AKAISHI GROUP

SHIRANE GROUP

SUMATAGAWA GROUP

INUI GROUP

5 MIKURA GROUP

SETOGAWA GROUP

7 KOMYO GROUP

8 ACID INTRUSIVES

9 COVERING STRATA

Jonathan C. AITCI-lISON

Q

1IJ,1f/ // /'---l:l\. :

'p

.... '.' ' ..

///l;~ ...

'.

Fig. 5 Distribution of Shimanto terrane rocks in the Akaishi Mountains after KANO and MATSUSHIMA (in press)

(44 )


Mikura Group is of Oligocene age and includes three dominant lithofacies. One is a chaotic mixture of sandstone and mudstone with no intercalated greenstone or chert. Soft sediment deformation slump folds can be clearly observed in some beds and their shape suggests derivation from the north. Massive black mudstone with occasional nodular «IOcm) marls and alternations of sandstone and mudstone constitute the other dominant lithofacies. Some rare, acidic tuff is also known. Oligocene fossils include radiolaria, foraminifera, molluscs and pollens (MATSUMOTO, 1971; IUIrvIA et at., 1981). This group is similar in part to Muro Group of Kii Peninsula.

In the SE of the Minami Alps strata are divided into two group by KANO and MATSUSIIlMA (in press). Komyo Group which is probably a correlative of Shirane Group to the north outcrops south of the Butsuzo Tectonic Line and Chichibu strata. Lithology is similar to that of the Shirane Group and includes large melange zones along with more coherent flysch zones.

Amahatagawa Group outcrops along the Haya River and may be a correative of the lower Setogawa Group (ONO, 1973). This group includes probable Paleogene melange in which metabasaltic blocks occur in a sheared mudstone matrix.

Setogawa Group (Setogawa terrain of IUIMA et al., 1981) is of middle Eocene to middle Miocene age. It includes both coherent sequences of alternating sandstone and shale along with melange zones. Melange includes chert, basalt, gabbro, sandstone, shale and limestone blocks in a sheared shale matrix. Oigawa Group is also considered to be part of the Setogawa terrane (IMAI, 1977) and is dominated by reef limestone which unconformably overlies Setogawa rocks.

F. Kanto Mountains

Some Shimanto rocks are exposed in the Kanto Mountains. Oguichi Group described by OGAWA and HORIUCHI (1978) is in fault contact with Chichibn terrane rocks along the Butsuzo Tectonic Line. Melange rock incorporating blocks of basalt, acidic tuff, chert, limestone and conglomerate in a sheared shale matrix is overlain by quartzofeldspathic, Inoceramus bearing, sandstones. Strata are tightly folded and a schistosity is developed. Further south, late Cretaceous strata of Kobotoke Group are dominated by coherent alternations of sandstone and shale (OGAWA and HORIUCHI, 1978). Kobotoke Group includes some minor 7,ones of melange which incorporates blocks of metabasalt, bedded chert, acidic tuff and limestone in a sheared pelitic matrix. Kobotoke Group rocks are also folded and sheared and there is widespread develop-

ment of slaty cleavage.

Boso Peninsula

Mineoka Group in the southern part of Boso Peninsula is considered to represent the easternmost extent of Shimanto terrane (TAIRA et al., 1982). These rocks are of unknown precise age and include coherent alternations of sandstone and shale as well as zones of melange similar to those in other areas.

III Shimanto Lithofacies

A. Turbidites and associated coarse clastics

These are the dominant Shimanto terrane lithofacies. Such beds are generally composed of sandstone of varying thicknesses interbedded with the ratio between the two components variable. Beds of this type, are typically described, in Japan, as alternations of sandstone and shale, with the dominant component indicated. Thickening and coarsening upwards megacycles as well as thinning and fining upwards megacycles are found throughout the Shimanto with the former being the most common. The volumetrically most common turbidite facies is dominated by thick, coarse grained, often amalgamated, sandstones. This corresponds with facies B of the turbidite facies classification of MOTTl and RICCI-Luccl-II (1972). Many turbidite zones are highly deformed and beds are tightly folded with their original stratigraphy disrupted. Individual turbidite zones are often separated from others or from other lithofacies by low angle thrusts or higher angle reverse faults which typically dip northwards and are subparallel to regional strike. Other turbidite zones are relatively undisrupted and such zones are often folded sync1inally. Turbidites typically have a poor macrofauna, although many tuff and mudstone interbeds have recently yielded radiolaria allowing significant advances to be made in unraveling Shimanto stratigraphy.

B. Shallow marine facies

In some areas, of limited regional extent, sequences of typically thick, often cross bedded, sandstone beds have rich in situ shallow marine macrofauna. Beds of this lithofacies are typically more simply folded compared with other Shimanto rocks and rest, with angular unconformity, on underlying deformed turbidite facies beds. Notable examples of this lithofacies are found in the Uwajima Group (TERAOKA and OBATA, 1975; TERAOKA et at., 1986) and Doganaro and Uwagumi formations on Shikoku, as well as, Yukawa and Terasoma formations on Kit Peninsula (NAKAZAWA et at.,

( 45 )


352

Formation on Amani Oshima. 1979) and Wano . this lithofacies tend to present

t with1t1 1 ° Paleocurren S lieated pattern t Ian those III

a much more CO~!ajima Group compositions are zones of flysch.. logicaly mature than canteronotably more mtn

era beds suggesting possible re-

t rbidite poraneous ~ older Shimanto rocks. cycling of slightly

C. Acid Tuff h' h have generally been 'd tuff, w le

Beds of aC1 bidity currents, are found redeposited by t~r anto terrane in strata of all 1 h "he ShlI11 ° ° t HOUg au beds are qUIte thick. ~nd coarse

ages. Some tuff.. is typically daClilc. These . C poslttOn

gramed. om b penecontemporaneous vo:can-beds indicate near ~ 1 of Shimanto accumulation. ism during the perloe

. mal facies D. Ohstostro .' Shimanto lithologies have been

In some localttleSd ositional slumping as chaotic redeposited by syn- e~any of these beds are doolistostromal beds'

d matrix which contains large

minated by a :mud y blocks. None of the olistocontorted sandsto:

e scalY matrix, however, some

stromal beds has uch as those in the Shimotsuyu olistostromal beds.' s exotic lithologies, including Formation conta~nd terminate origin. Olistostro-

't of In e orthoquartz i e lent in Paleogene strata indud-mal ,beds are preva Muroto, Shimizu and Shimo-° N° hOnan Belt, G ° h 109; IC 1 d Mikura roup, In t e

t'ons an tsuyu forma 1 the Shimanto terrane. southern part of

E. Melange f :melange outcrop within the

E . zones 0 '11 ° xtenslve d a large l'eglOna y extensIve Shimanto terrane an

hich formed in the late Creta

zone of melange, W ver the entire strike length of able 0 ,

ceous, is trace h' work the term melange IS In t IS

the terrane., refer to an internally frag-used descriptively to j{ body containing a variety

d 'xed roC mented an ml commonly contained in a

h· hare of blocks w lC d matrix (after SILVER and

° j deforme . pervasIve y l' is consistent wIth a further BEUTNER,1980). ! l~S finition by RAYMOND (1984a) refinement of thIS e as "a body of rock, mappable who defined melan;;o or smaller, and characterised at a scale of 1 : 24£ ' ternal continuity of contacts both by the lack 0 ~: inclusion of fragments and or, strata and ~y t both exotic and native in a blocks of all SIZ~S, of finergrained material". No fragmented mat~l:X:: are intended with this usage

. ' licatIons genetic Imp .' of Shimanto melange zones although the Orlg111the end of this section. Highly will be discussed at d turbidite zones (refercd to

° d d deforme . dlsrupte an . a paper on North Amencan as type! melange 1ll(1985) are not termed melange, examples by CoWAN

in this work, although in some cases they may be blocks within a large melange zone. If such zones occur within otherwise coherent flysch sequences then I prefer the term broken formation.

Shimanto melange typically contains a wide range of lithologies including; metamorphosed basaltic rocks, often with pillow lavas of various types, hyaloclastites, calcareous nannofossil lime~ stones, red coloured shale, ribbon cherts, acidic tuff, dismembered sandstone and shale, and occa~

sional ultramafic rocks (Kii and Muroto Peninsulas) as blocks or lenticular bodies in a commonly, pervasively sheared scaly argillaceous matrix. These blocks have a large size range from micro· scopic to km scale. As Jarge blocks are often mOl-e resistant than their enveloping strata to weathering, relationships within melange zones and especially at the contacts between melange zones and surrounding turbidites are often equivocal. The ratio of blocks to matrix is also hard to determine accurately.

Melange zones do not always parallel the regional strike and dip of strata with which they are in contact. Rather the melange zones are often subparallel to strike along which they often appear to pinch and swell and are, possibly, occasionaly anastomosed. In some areas turbidite rocks become more progressively disrupted across strike with zones of broken formation seen before melange zones are reached. In other areas melange appears to lie in fault contact with the, top of zones of coarse turbidite sandstone and is apparently stratigraphically succeeded by coarsening upwards turbidite sequences.

Most Shimanto melange zones have an intensely foliated matrix which is clearly not the result of some primary sedimentological process and may ha ve developed in response to tectonic stress although the formational process of scaly cleavage in melange rocks is at present equivocal (JACOBI, 1984; BYRNE, 1984). I have observed web structure in sandstone blocks, similar to that described from Alaska by BYRNE (1984), in melange rocks of. Shikoku and Akaishi mountains. BYRNE (1984) considers it unlikely that sedimentary processes are responsible for the formation of these struc-. tures. As we now see them these melange zones are tectonised and their primary genesis is difficult to determine.

A notably large melange zone exists amongst late Cretaceous Shimanto strata. Other smaller melange zones are found both in older and younger strata in some areas. Paleocene melange is found in Mikado Belt in Kyushu, and Inui Group in the Akaishi Mountains with slightly younger melange found in Shikoku in the Sakihama Melange and

( 46 )


in the Setogawa Group in Shizuoka Prefecture. Recently, the large Cretaceous melange zone has

been studied intensively in Shikoku and other areas (SANO et al., 1979; SUZUKI and HADA, 1979, 1983; SUZUKI et al., 1978; KODAMA et al., 1983; HADA, in press). Developments in radiolarian biostratigraphy have enabled workers to establish that blocks and matrix are not penecontemporaneous although the matrix is of similar age to enveloping strata. At Tei and Ryoji on Shikoku blocks of metabasalt, with associated pillow lava, aTe found in a well exposed melange which includes blocks of Valanginian red bedded chert intimately associated with basalt pillows. Other blocks of bedded chert have ages as young as Cenomanian. Acidic tuff yields Coniacian to San" tonian radiolaria and melange matrix yields radio~ Jaria as young as Campanian. Surrounding nonmelange strata are of similar age. Similar age ralationships aTe reported from similar melange zones on Kii Peninsula (NAKAZAWA et al., 1983) and in the Akaishi Mountains (KAWABATA, in prep).

Reliable paleomagnetic data obtained from Shikoku Shimanto melange zones (KODAMA et al., 1983) and similar strata in Kii Peninsula (HmooKA et al., 1983) show that Cretaceous basaltic rocks formed at equatorial latitudes whereas mudstones in flysch which surround the melange yields bedding corrected paleomagnetitc results which indicate a position similar to that of the Japanese archipelago today. Assuming that these Valanginian basalt blocks which formed at low latitude were incorporated into melange which formed near its present latitude in the Campanian, KODAMA et at. (1983) postulated that plate movement of equivalent to 50Ma of northward travel at 10cm/yr must have occurred. This is not inconsistant with present day rates of plate motion. Paleocene basaltic blocks incorporated in Paleocene melange basalt blocks show similar paleolatitudes to those of their en~

veloping strata which are not significantly different from present day latitude suggesting minimal transport before inclusion into zones of melange. There also appears to be little significant age difference between block and matrix ages in Shimanto Paleocene melange. Extensive ribbon chert is not known in Paleocene Shimanto melange examples from Shikoku.

Geochemical studies of basaltic lithologies within Shimanto melange (eg. SUGISAKI et at., 1979; SHiIDA et at., 1971 ; TSUCHlYA et at., 1979; SUZUKI and HADA, 1983; TANAKA, 1975, 1977; SUZUKI and YAMAGUCHI, 1981; etc.) show that basaltic blocks are typically metamorphosed to gl-eenschist facies. Compositions are alkali or tholeiitic which reflects the origin of blocks at the site of a mid ocean ridge 01' sea-

mount. The origin of bedded (em scale) chert incorpo

rated into the melange is problematic. Bedded chert is intimately associated with basaltic rocks of spreading ridge origin. However DSDP results (HEIN and KARL, 1983) do not support the classical model of open·ocean origin for this type of rock. Geochemical work by SUGISAKI (1984), particularly on Mn ratios in ocean floor sediments, strongly suggests that many Shimanto chert rocks are not pelagic and may have formed relatively close to land. Both basalt and chert may have been deposited in a marginal sea environment, an analogue of which would be the contemporary Japan Sea. Chert beds show evidence of current deposition with occasional grading and cross bedding. Partings between chert beds are typically thin layers of basic tuff. The origin of other megablock litho· logies such as acid tuff, sandstone and shale is less equivocal as these lithologies can be found elsewhere in the Shimanto.

Mapping of melange zones is restricted by a lack of continuous exposure and many zones appear to either anastomose or are repeated by numerous thrust faults_ Many Japanese geologists have previously labelled these rocks olistostromal deposits. However,' a priori' use of this genetic term is inappropriate as the origin of the melange is at present quite equivocal. Recent work on the mechanical properties of Shikoku melange (HADA, in press) indicates that melange mudstones were once significantly denser than mudstones in s\lr" rounding coherent units and he suggests a tectonic origin on the basis of this and other works. Various models for melange generation have been proposed and strongly argued by numerous authors (RAY~ MOND, 1984a; COWAN, 1985). These models include development as a result of the deformation of pre"existing olistostromes, layer parallel extension, offscraping and accretion from a subducting plate, flow within fault conduits in an accretionary prism and diapirism. Some or all of these processes could be valid and there may be various ways in which melange strata can form. However, it is generally accepted that melange is commonly (although not exclusively) developed in subduction zones.

I would interpret many Shimanto melange zones, interpreted as olistostromal in origin by other workers (such as those on Kii Peninsula described by NAKAZAWA et at., 1983), as tectonic in origin. Many Japanese melange zones have been interpreted in an 'a priori' manner and no clear genetic indicators are present at individual outcrops. Tn Japan a model involving an extensive zone of slumping from the subducting plate into the trough in front of a subduction complex is widely favoured.

( 47)

354

Fig. 6

I I

I , I

I

, , , ,

8

( DisseCTED)


• o

N

Q

RECYCLED

OROGENIC

p

B2C G 3 1

'-

MAGMATIC ARC

(TRANSITIONAL)

2

3

4 5

• 7 8 9 A

• C 0 E F G

" , J K L

M

" 0 p

(UNOISSECTED)

L

d trital (QFL) sandstone compositions of Shimanto rocks. Suggested

A rage e ve fields after DICKINSON et at. (1983). enance ' prov . K:::::Cretaceons, P=Paleogene, .KeY, . h' M f T d 'K (1979)

b Is' Akals 1 ts. rom OKUOKA an UMON ;

sym 0 '. . !:::::Inui G. 2=M,kura G. 3=Setogawa G. 4=Olgawa G.

C Peninsula from KUMON (1983); 111 5:::::Kogawa F. 6=Uchikoshi F. 7=Yasllkawa F. 8=Haroku F.

9:::::Nyunokawa F. A=Ryujin F. B=Terasoma F. C=Miyama F.

D:::::Yukawa F. E tern Shikoku from KUMON (1983);

as E:::::Naharigawa F. F=Kaifu F. G=Hiwasa F. H=Mugi F. I:::::Taniyama F. J=Hinotani F. K=Akamatsu F.

"(T t Shikoku and eastern Kyushu from TERAOKA (1979); "es Me' ' St' (U .. ) L :::::oLower Cretaceous = omaClan- an oman waJlma

N ::::oCenomanian-Santonian a =Campanian-Maastrichtian

p :::::oPaleocene-lower Miocene

( 48 )

Kip A A .. .. .. '" .. ..

" ,. II

" .. .. .. II

" • ,. ., II .. II

" ...


However this model fails to account for occasional massive sandstone blocks within melange zones and some of the basaltic blocks presumed to have slumped are of enormoUS size. I favour a model of simple accretion and offscraping with some recycling of existing subduction complex lithologies due to slumping at the toe of the accretionary prism. After offscraping some melange rocks may have flowed along major fault zones within the Shimanto accretionary prism incorporating or enveloping zones of sandstone and shale around which the melange now appears to anastomose . Near the surface of the accretionary prism some melange may have risen diapirical1y. However, the exact origin of Shimanto melange is not yet clear and a model for the development of melange at one locality may not necessarily be applicable to other locality. Much work remains to be done on Shimanto melange particuarly with regard to such aspects as rock density, Youngs modulus, vitrinite reflectances, illite crystallinities etc. which may give some clearer indication of the history of these chaotic rocks compared to the history of their encompasing strata. Even if the primary origin of some of thcse roc~s was olistostromal they are now often extensively tectonised and as such are best described non genetically as melange.

IV Detrital Compositions of Shimanto Terrane Clastic Rocks

Remarkably similar detrital compositions from penecontemporaneous strata have been observed throughout the Shimanto terrane by numerous workers (TERAoKA, 1979; KUMON, 1983; TOKUOKA and KUMON, 1979; OKADA, 1977; ISUIDA, 1969; IMAI et at., 1982; ISI-IIHAI{A et aI., 1985; AITCI!ISON, in prep; KUMON and KA\,,IAllATA, 1986; etc.) (Fig. 6). Comparison of these sandstone detrital compositions with other data from modern and ancient sedimentary environment gives us some idea of the nature of the source terrane for Shimanto clastic strata. DICKINSON and SUCZEK (1979), DICKINSON (1982), and DICKINSON et al. (1983) have demonstrated that, in general, strong correlation can be shown between sandstone detrital compositions and geotectonic setting. Demonstrable change in Shimanto compositions with time allows us to see how the provenance evolved during the development of the terranc. There is little or no significant difference betwecn compositions of these penecontemporaneous strata whIch have been deposited in similar depositional environments along the entire strike length of the Shimanto. Cretaceous sandstone compositions are dominantly those of volcaniJithicfeldsarenites (terminology after FOLK et al., 1970)

whereas Paleogene sandstone compositions become progressively more quartzose upsection, although they are still classified as volcanilithic-feldsarenites. The ratio of K feldspar to total feldspar also increases markedly upsection. Lithic clasts in both sandstones and conglomerates show a comparable wide range of compositions. They are dominated by acidic to intermediate volcanic rock fragments. Subordinate lithologies include; gabbro, granitoid, chert, sandstone, shale, schist, basalt, limestone and occasional orthoquartzite. Acidic tuff beds are also found intercalated with Shimanto sand

stones. Consideration of detrital composition and com

parison with those of other clastic rocks from a wide range of tectonic settings (DICKINSON and SUCZEK, 1979; DICKINSON, 1982; DICKINSON et at., 1983; ENKEBOLL, 1981; STEWART, 1977,1978; YERlNO and MEYNARD, 1984) suggests that Shimanto terrane rocks were derived from dissection of a volcanic arc. Schist, granitoid and orthoquartzite clasts as well as K feldspar to total feldspar ratios suggest that it was a continental arc. Increasing mineralogical maturity of Paleogene strata along with the presence of clasts of chert, basalt, sandstone and shale derived from other pre-existing Shimanto lithologies suggests increasing reworking of strata, or auto-cannibalism. More mature compositions of some Cretaceous strata, particularly shallow marine facies (TERAoKA, 1979) suggests that recycling of existing lithologies was not

restricted to the Paleogene.

V Relationship of Shimanto terrane to other rock units

Generally accepted defmitions of the Shimanto Belt (Supergroup) include only strata of Cretaceous to earliest Miocene age. Shimanto Belt can typically be seen in fault contact with older sonthern Chichibu Belt (Sambosan Belt of some workers) strata which outcrop on the north or west side of the regionally extensive Butsuzo Tectonic Line. In some instances (eg. central Kii Peninsula) Shimanto rocks are in direct contact with Sambagawa schists.

To the south Shimanto Belt rocks are overlain with angular unconformity by middle Miocene sedimentary formations. These rocks range in lithology from turbidites through shallow marine facies to fluvial deposits. On Kii Peninsula these rocks are extensively developed and are named Tanabe Gronp in the west and Kumano Group in the east. Similar strata are found on Shikoku in the Misaki Group and on Kyushu in the Miyazaki

Group. In late Miocene time granitoid rocks intruded

( 49 )


356

u1' areas of the Shimanto. These intrusive '!lto rna d· 1 .. 1 although pre ommant y aCidic, are in places rocks, variable lithology. Their most extensive of JP-orerD-ent is found in the Kumano Acid Igneous develoP J{ii Peninsula where, along with granitoid R c1{S on . o cidic pyroclastIcs, quartz porphry, hypers-

cks,· I· 1 yO hyolite, basa be ava and basic plutonic thetW ;rop Qut on Shionomisaki (MIYAKE 1985; rOc1{S and HrsATOMI, 1986). JV1r'l M<E ther seaward, as far as the Nankai Trough,

Fur 'eS similar to those of older Shimanto rocks lithOIoi;l I·· M· . been deve opmg smce lOcene time and their have '(on and accretion is continuing. dep~:~~t1gh traditional lithostratigraphic subdivi-. .A of Japanese strata restricts Shimanto Belt to

S10n ous to Paleogene strata the Shimanto tercreta(ces) is a tectonostratigraphic unit and may

e s' . ra.," what larger in extent. b sorne

e . n to terrane rocks represent strata deve-Sh1ll1a . . ( in an accretlOnary pnsm see below). As

loped are fault bounded entities of regional t rranes . e teach charactensed by a geological history exteD: ' different from the history of contiguous h t 15 t a ,(JONES et ai., 1982, 1983), 1 consider that

t rranes e ld be useful to expand the use of the term 't WOll . I 1. to terrane to lUC ude all similar strata Shlrnan .' h· . .

1 d wlthIll t IS accretlOnary pnsm. There-

d ve ope e e Shimanto terrane (s.s.) should probably include for which have developed in the same accre-strata Th· . .

Y Prism. IS accrebonary pnsm appears

. nar tlO have developed seaward of the Kurosegawa to e Indeed, HADA and SUZUKI (1983) have t rran . e dy proposed a southern terrane south of the alrea gaWa terrane in which they included both !Curose . h·b d Sh· b

hern ChiC 1 U an Imanto elts. Kurose-

~~ . t·· I h· terrane IS a serpen mite me ange w lch gawa. s many blocks with continental affinities contain . . d· ··1 Sh· such, IS qUite ISSlml ar to Imanto ter-and, a~{owever, southern Chichibu Belt (SCE) rocks rane'

l ped in a similar manner to those of the

cleve auto Belt and SCE also includes large zones Shilua

lange, coherent turbidite sequences and shal-of 11l-e h arine rocks, T ese rocks include blocks of loW In . h· hi· . an to JurassIc age w IC ,a ong With coherent perltl i . accumulated largely III the late Jurassic strata, .

hugh some shallow manne strata developed on

alt 0 h· h·b k· h of older C IC 1 U roc s In t e Cretaceous. I top 'der that the Butsm:o Tectonic Line (BTL), conSi . d . . . ieally recoglllse as a major tectoUlc bound-lustor by Japanese workers, represents a major decol-ary t which developed as a result of duplexing Ie11l-eU . . . h' this accretIOnary pnsm allowing Chichi-WIt III ckS to be thrust over younger Shimanto bu ro .

groUP strata, ThIS does 'mean that SCB Super

Id be regarded as a seperate terrane, rather

shoU h h· n

ition of t e nature of t IS fault provides recog

further evidence for the accretionary prism hypothesis. However, as extension of the Shimato terrane to include SCB rocks is not widely accepted here in Japan further discussion in this paper is restricted to strata developed seaward of the BTL.

Shimanto related accretionary prism development did not end in the early Miocene, rather it has continued until now in areas further offshore, Strata such as those in the Kumano and Tanabe groups represent slope basin deposits and developed in a manner similar to other rocks such as the Cretaceous Uwajima Group.

Shimanto terrane development is still continuing with present day subduction along the Nankai Trough. In the Akaishi and Kanto mountains as well as the Miura-Boso area Shimanto terrane has been highly complicated by the collision of the Izu block and the accretion of other smaller allochthonous terranes,

VI Geological evolution of the Shimanto terrane and related penecontem

poraneous strata

Similar deposits to those of the Shimanto terrane are found in both modern and ancient environments which are interpreted as subduction complexes or accretionary prisms. Numerous examples of subduction complexes have been described from Nias Island (MOORE, 1979; MOORE and KARIG, 1980), California; Fransiscan Terrane (various authors), Alaskan Pacific margin (CONNELLY, 1978; COWAN and Boss, 1978; BYRNE, 1984), Australia; New England (FERGURSSON, 1984), New Zealand; Torlesse terrane (MACKINNON, 1983), Caples terrane (NB:LSON, 1982), Barbados (PUDSEY and READING, 1982), South Scotia Ridge (DALZIEL, 1985) etc. Although there is much variability between individual subduction complexes, related to variations in subduction and sedimentation rates, as well as, the nature of the subducting material etc, a broad overall similarity does exist, Accretionary complexes typically include zones of melange which outcrop between other zones of more coherent turbidite strata. The turbidites are often complexly folded and repeated by numerous high angle reverse or thrust faults. On top of these turbidites and melange zones younger, structurally simpler, turbidites, shallow marine, and/or fluvial sediments are often deposited in slope or forearc basins, Seismic profiles of sediment wedges developed on contemporary subduction margins (KARIG and SHARMAN, 1975; MOORE and KARIG, 1976; ARTHUR et ai., 1980; LEGGETT et aI" 1985) show a similar internally complicated structure developed within accumulated sediments which are typically folded and cut by numerous

( 50 )


high angle reverse faults. Occasional major, low angle, thrust fault, decollements occur where duplexing has caused older sediment to be thrust over younger strata (SILVER et at" 1985; MCCARTHY and SCHOLL, 1985). Shimanto terrane strata include typical accretionary complex lithologies and were deposited on, or accreted to, the south west coast

of Japan since late Jurassic time. Following accretion of the Kurosegawa micro

continent to southwest paleo-Japan, subduction of oceanic plate material was initiated along the southern flank of this south accreted terrane. Initial development of an accretionary prism resulted in the development of SCB rocks.

Continuing subduction during the Cretaceous resulted in the deposition of lowermost Shimanto Supergroup rocks. Duplexing within the accretionary prism resulted in SCE rocks thrusting over some of the Cretaceous Shimanto strata and this

decollement is noW represented by BTL. Cretaceous Shimanto deposition and accretion followed a similar pattern to that which occurred previously, Thick turbidite sequences were extensively developed in submarine fans on the trench floor at the foot of the accretionary prism. These beds later became structurally complicated during subsequent deformation as they were progressively rotated to structurally higher positions within the prism due to the continuing accretion and sedimentation at the toe of the prism. Other turbidite beds were deposited in slope basins where subsequent folding was less severe. Finer grained hemi-pelagic material was probably deposited on the slope of the accretionary prism. Sedimentation patterns in such an environment and their implications for melange formation are discussed by UNDERWOOD (1984). Shallow marine beds (eg. Uwajima Group) were deposited above angular unconformities on top of these deformed zones. Acidic tuff beds were periodically deposited over the prism as a result of fallout from ash clouds generated by subduction related acidic volcanism. Continuing subduction of oceanic plate material and associated tectonic offscraping resulted in the development of large scale melange zones. Material incorporated into Cretaceous Shimanto melange does not include Permian limestone such as that found in SCB rocks and this possibly indicates that the axis of the spl-eading ridge, associated with the generation of oceanic crust which was being subducted, was gradually moving closer to the subduction zone. Zones of melange are developed in close association with faults and some melange may have intruded up along fault zones. The particularly extensive development of melange in the late Cretaceous may be significant but is not yet fully understood.

Younger strata with less extensive melange may have developed during a period of high sedimentation rates or slower subduction or, perhaps, a major, and as yet, unrecognised tectonic event was responsible for this extensive melange zone.

Further duplexing within the subduction wedge is represented by a regional thrust fault which marks a decollment between Cretaceous and Paleogene strata. For historical reasons most authors do not recognise the thrust as a terrane boundary (d, BTL). This thrust is refered to the Nobeoka Thrust on Kyushu, Nakasuji Tectonic Line on Hata Peninsula and AId Tectonic Line on Muroto Peninsula of Shikoku, Gobo-Hagi Tectonic Line on Kii Peninsula, and is unnamed in the Akaishi

mountains. Subduction continued through the Paleogene and

simil81· lithologies developed. A notable, regionally extensive, zone of allolistostromal strata was also developed. These rocks (Shimizu, Muroto and Shimotsuyu formations, Nichinan Belt and Mikura Group) are clearly of sedimentary origin. If tectonised then these rocks could possibly look similar to Shimanto melange lithologies although basalt and chert are extremely rare. These slumped zones include interesting lithologies which have continental affinities including small boulders of orthoquartzite and granite. Slumped beds are dominated by fine grained matrix and where there are no exotic clasts they appear to be thin bedded 'distal' turbidite beds which have suffered from downslope mass movement. These beds probably developed at the toe of the accretionary prism as a result of slumping of material on the surface of the prism. Some melange developed in the Paleogene and is seen on Shikoku in close association with these allolistostromal beds. Radiolarian ages do not show a wide age range suggesting that the basaltic lithologies formed only slightly prior to their incorporation into melange. This possibly suggests that a spreading ridge was now quite near the subduction zone. Other more typical zones of extensive thick sandy turbidites were also

developed in the Paleogene. Shimanto Supergroup beds are overlain with

angular unconformity by middle Miocene beds (eg. Tanabe and Kumano groups on Kii Peninsula). Some Shimanto workers, myself included, consider that Shimanto terrane development was and is ongoing up until the present. Presently exposed middle Miocene strata represent slope basin deposits developed on mature parts of the subduction complex with accretion and development of deformed zones of flysch continuing further offshore.

A major event seems to have occurred in the Miocene with granitic rocks intruding the Shimanto

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\ I

I I !

I ! I I I

I I


in many areas. In eastern Shikoku and Kii Peninsula a variety of acidic and basic igneous rocks have intruded the Shimanto terrane (MIYAKE, 1985; MIYAKE and HISATOMI, 1986). This could be explained by suggesting the subduction of a spreading ridge which appears to have been moving gradually closer to the Shimanto subduction complex. Heat flow would possibly have been sufficient to melt sedimentary rocks and form granitoid rocks (MARSI-IAK and KARIG, 1977).

If the Shimanto terrane did develop as a subduction complex then one should be able to predict the existence of other lithologies related to the subduction process. TAIRA (1981) describes some of these rocks in an earlier summary paper on Shimanto Belt tectonics. A magmatic arc including granitic rocks with a major cluster of radiometric ages between 70-95Ma. (gradually younging eastwards) is found on the inner side of southwest Japan. In the late Cretaceous major forearc basins were developed, between the magmatic arc and the Shimanto accretionary prism, associated with tensional breakdown of crust due to large scale strike slip movement along that is now called the Median Tectonic Line. Thick marine sequences (eg. Izumi Group) accumulated rapidly in these basins and were synclinally folded. Sambagawa high pressure/low temperature metamorphism, which has radiometric ages which closely parallel the period of early Shimanto development in the Cretaceous, can be inferred to have been associated with Shimanto subduction. Many of the elements of a Cretaceous to Recent subduction related margin can be readily seen in various strata of SW Japan and all of the available evidence points to the development of the Shimanto terrane rocks in an accretionary prism which evolved on the outer side of southwest Japan since latest Jurassic time.

Acknowledgements

I would like to thank all those Japanese geologists who kindly assisted in my study of Shimanto rocks by proving reprints, pre-prints and numerous discussions. I would especially like to thank Y. Teraoka for help with Kyushu and Uwajima, S. Bada for introducing me to and enlightening me about Shikoku geology, M. Tateishi for help with Kii Peninsula, Akaishi Mountains and numerous other matters and F. Kumon and H. Suzuki who also helped in many ways. I am most grateful to Prof. T. Uemura who arranged my study in Japan and for the financial assistance of Japanese Ministry of Education, Science and Culture (Monbusho).

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______________ , ELLIS, M.J., BREEN, N.A., and SHIPLEY, T.H. (1985) Comments on the growth of accretionary wedges. ibid., 13, 6-9.

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_____ (1978) Neogene volcaniclastic sediments from Atka Basin, Aleutian Ridge. ibid., 62, 87-97.

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______________ and (1983) Accretionary Melange of Cretaceous Age in the Shimanto Belt in Japan. in HASI-IIlvIOTO, M. and UYEDA, S. (eds.) Accretion Tectonics in the Circum-Pacific Regions. Terra Sci. Pub. Co., Tokyo.

_______________ , , UEMURA, K., KADO, K., SAKA-MOTO, Y. and NAKAGAWA, H. (1978) Genetic consideration of the green rocks in the Shimanto Belt-with special reference to the mode of occurrence-. Earth Science (Chikyu Kagaku), 32, 321-330. (J, E).

_______________ - and YAMAGUCHI, Y.' (1981) Genetic consideration of ultramafic and basic rocks in the southern part of the Hidakagawa Group, Wakayama Prefecture, Call. Abs. 88th meeting Geol. Soc. Japan, no. 368, (J).

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I

II

.,--

362 Jonathan C, AITCIJ[SON

----, OKADA, H., WHITAKER, lII. McD. and

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---, OKAMURA, M., KA'1'1'O, J, TASHIRO, M., 31\ITO, y" KanA1VIA, K., HASHIMOTO, M., CIllBA,

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----, TASHIRO, M., OKA;\IlURA, M. and KATTO, J. (1980b) The Geology of the Shimanto Belt in Kochi Prefecture, Shikoku, Japan. ibid. (J, E).

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TANAKA, T., (1975) Geological significance of rare earth elements in Japanese geosynclinal basalts. Contrib. Min. & Petrol. 52, 233-246.

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---, BESSHO, T., HISATOMI, K., INoucHI, Y., ISHIGAMI, T., KmvloN, F., NAKAYA, S. SAKAMOTO, T" SUZUKI, I-I., TormoKA, T. (1979) Geology of the Esumi district. Quadrangle Series, scale 1,'

50,000, Geo1. Surv. Japan, 6Sp. (J, E). TERAOKA, Y. (1979) Provenance of the Shimanto

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----, IKEDA, Y., and KASlllMA, N. (1986) Geology of the Uwajima District. Quadrangle Series, Scale 1.' 50,000, Geo1. Surv. Japan.

and OBATA, 1. (1975) Stratigraphy of the Upper Cretaceous Uwajima Group. Mem. Nat. Sci. NIuseum, Tokyo, 8, 5-20. (J, E).

TormoKA, T. and KUMON, F. (1979) The Shimanto Terrain in the Akaishi Mountainland and the Kii Peninsula-A Consideration on Mineral Composition of Sandstones. Bull. Nat. l11useum, Tokyo, 12, 41-5'1. (J, E).

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in the Mimi River Area, Kyushu. Jour. Geol. 80e. Iapan .• 85. 4,15-454. (J, E).

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YAMADA, T., WATANABE, T., KA"YACHI, Y., YUN.,A, M" SEK1NE, M" MATSU'URA, K., OGAWA, K., YOKOTA, Y., KWANKE, E" KINOSlHTA, F. and DE1vIAClII, M. (1983) Geology of the Shimanto Belt of the Northern Akaishi Mountains, Central Japan. Earth Science (Chikylt Kagaku), 37, 329-348. (J, E).

Y ANAI, S. (1983) Paleogeographic development of the Shimanto geosyncline in the case of the eastern Shikoku district, southwest Japan. lour. Geol. Soc. Japan, 89, 575-593. (J, E).

--- (1984a) Paleogeography of the Cretaceous Shimanto Geosyncline, in respect of Forearc Tectogenesis in Active Continental Margin. Jour. Fac. Sci. Uttiv, Tokyo, Sec. II, 21/1, 1-37,

--- (1984b) Tectonic Development of the Shi, manto Geosyncline in the western Kii Peninsula, southwest Japan. Jour. Geol. Soc. Japan, 90, 223-243.

YERINO, L.N. and MEYNARD, J.B. (198'1) Petrography of modern marine sands from the Peru.Chile Trench and adjacent areas. Sedimento/" 31, 83-89.

(J) Japanese only (J, E) Japanese with English Abstract.

(Received May 7, 1986; accepted July 8, 1986)

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Plate I 0. AITCHISON) EartiJ Sci., Vol. 40, No.5 (September 1986)

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Plate II (J. AITCHISON) F..arth Sci., Vol. 40, No.5 (September 1986) Stratigraphy, sedimentology and tectonic evolution of the Shimanto Terrane, 363

ii!;n' (, J;. C, i"l71-t-"il'/J:j?t£ < i: <b Gc!P."Cn, (, cp!i:ti IIl;IiJJ JtmCiJ>jr-J--c, !J '7 7" V - r lO~t""O)-lT 7' f !J v ~ '/ ~c.. 1:

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Pbte: I

a: Cretaceous melange in Shimanto terrane rocks at Ryoji, Koehl Prefecture, Shikoku.

b: Valley cut through Shirane Group melange, Akaishi Mts. Photo was taken looking

south from near the summit of Mt. Shiomi

c: Well preserved basalt pillows in Cretaceous Shimanto melange at Tei, east of Koehl

city, Shikoku.

c1: Interbedded turbidites and tuff of late Cretaceous slope basin beds of the Sumata

gawa Group, Akaishi Mts.

Plate II

a: Uridani Formation, thin turbidite beds form part of the Otonashigawa Group out·

cropping on the west coast of Kii Peninsula.

b: Ohyamamisaki Formation resedimented conglomerate beds of the Murotohanto

Group, Muroto peninsula. Shikoku. Conglomerate beds contain a wide variety of

lithologies including crystalline schists and granite possibly derived from erosion of

the Kurosegawa terrane.

c; Soft sediment slumped turbidite beds of the Mitogawa Formation, southern Muro

Group, Kii Peninsula.

d: Oligocene olistostromal beds of the Muroto Formation at Kuromi, Muroto Peninsula,

Shikoku.

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Aitchison 1986 Shimanto terrane

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