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Cretaceous-Lower Miocene Active-Margin Sedimentation

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Cretaceous-Lower Miocene Active-Margin Sedimentation Downloaded from http://sp.lyellcollection.org/ by guest on October 6, 2021 The Shimanto Belt of Japan: Cretaceous-lower Miocene active-margin sedimentation A. Taira, H. Okada, J. H. McD. Whitaker & A. J. Smith SUMMARY: South-east of the two paired metamorphic belts of Kyushu, Shikoku and Honshu, and separated from them by the Chichibu and Sambosan belts and the Butsuzo Tectonic Line, lies a belt of mildly metamorphosed sedimentary rocks called the Shimanto Supergroup. Their area, extending from the Nansei Islands through Kyushu, Shikoku, Kii Peninsula, Akaishi and Kanto Mountains to the Boso Peninsula, is comparable in size with the combined Franciscan Formation and the Great Valley sequence in California. A Cretaceous lower group is flanked on the Pacific side by an upper group of Palaeogene to early Miocene age. The Shimanto sediments, mostly sandstones and mudstones, were deformed by cyclic subduction into open and isoclinal folds which close south-eastwards, and thrust slices which in places incorporate basaltic pillow lavas and radiolarian cherts. Sandstones from Kyushu and Shikoku show marked changes of composition with stratigraphic position and the feldspar content can be used to distinguish Cretaceous from Tertiary sandstones. Data on sandstone petrography and palaeocurrents indicate that sediments of the Shimanto Supergroup were probably derived from the NW. Precambrian gneisses and older Mesozoic granitic rocks of the Korean Peninsula may have contributed sediments in Early Cretaceous times, when the Shimanto Terrane was located to the east of southern Korea. The unroofing of granites in the Inner Zone of SW Japan may also have contributed to the Cretaceous and later sedimentation. Quartz-rich, well-sorted sands in younger Shimanto sediments may have been recycled from older Shimanto formations. The thick Shimanto sedimentary sequences were laid down in a variety of environments within a forearc basin on accretionary complex, trench-slope break, trench inner slope and trench settings; first in the Cretaceous and again, in a more southerly position, in the Palaeogene and early Miocene. In the shallower waters on the inner (arc) side of the Palaeogene forearc basin, coarsening- and thickening-upward deltaic sequences were deposited. In deeper water further offshore, submarine channel and fan complexes, base-of-slope slump deposits and red shales with cherts accumulated. M~langes with basalts and slump olistostromes occupied the trench-slope break, while inner trench wall perched basins on accretionary basement were filled with coarsening-upward flysch, slump-olistostrome facies and m61anges with metabasalts: similar lithologies probably occupied a trench fill. Sedimentation and deformation were controlled by intermittent cyclic subduction of the Kula Plate towards the north. Phases of tension and down-faulting, forming long, narrow intra-arc basins, alternated with those of compression. Comparable sedimentary environ- ments are found today off SE Japan on the submarine terraces, trench-slope break, inner slope (including perched basins) and fill of the Nankai Trough. To Western geologists, Japan is best known for tion plus the Great Valley sequence of California. its two pairs of metamorphic belts and for its The purpose of this paper is to summarize the igneous rocks, well summarized by Miyashiro structural and sedimentological features ot the (1973), and more recently by various authors in Shimanto Belt and to reconstruct the conditions Tanaka & Nozawa (1977). Less well known, of deposition of Shimanto rocks. We believe because the literature is mainly in Japanese, is they accumulated through Cretaceous and the mildly metamorphosed Shimanto Belt lower Tertiary times in an active margin setting lying to the SE of the Sambagawa metamor- consisting of forearc basin on accretionary phic and Chichibu and Sambosan Belts and complex--trench-slope break--trench inner separated from them by the Butsuzo Tectonic slope--trench environments (terminology of Line (Fig. 1). Extending some 1800 km along Dickinson & Seely 1979). Thrusting during strike from the South Nansei Islands to the accretion, with the production of m61anges Boso Peninsula and up to 70 km wide across the bearing blocks of ocean-floor pillow basalts, strike in the Kii Peninsula, the Shimanto Belt is hyaloclastites, cherts and sandstones was a fea- comparable in area with the Franciscan Forma- ture of this active margin. Downloaded from http://sp.lyellcollection.org/ by guest on October 6, 2021 6 A. Taira et al. cut" ~Lower ShimantoGroup (N.Belt) 2.~~~°''~ ~wA#-OSHIM~~.~ .rb~ "S. ~ -i~~- UpperLATERALAXIAL Shimanto CURRENT CURRENT Group (S.BeIt) .~ / % o _~o ~oo ~oo~ FIG. 1. Map of Japan to show the distribution of the Lower and Upper Shimanto Groups, with generalized palaeocurrents. K = Kochi; N = Nichinan; S -= Shizuoka: U = Uwajima. BTL = Butsuzo Tectonic Line. MTL = Median Tectonic Line. General geology of the Shimanto Belt the Southern Belt this facies contains gabbro, diabase and serpentinite. Recently, 'umber' The Shimanto Supergroup is now exposed in deposits comparable to modern oceanic sedi- the Nansei Islands, South Kyushu, South Shi- ments have been found between pillows of koku, Kii Peninsula, Akaishi and Kanto Moun- basalt in the Mineoka Group in the Boso tains and on the Boso Peninsula (Fig. 1). This Peninsula (Tazaki et al. 1980). Large-scale long, narrow belt is divisible along its length olistostromes are common in both belts, but are into two major tectonostratigraphic units, a better developed in the Southern Belt. Northern Belt (Lower Shimanto Group) and The overlying flysch facies in both belts is Southern Belt (Upper Shimanto Group) characterized by terrigenous turbidites alternat- (Teraoka 1979). The former is characterized by ing with mudstones, infrequently intercalated uppermost Jurassic to Cretaceous strata, the with acidic tuff layers. In the Northern Belt, latter by Palaeogene and Lower Miocene rocks. ammonites and inoceramids have been found Both these groups are characterized by meta- sporadically (Matsumoto & Okada 1978), while basalt and mudstone facies generally overlain bivalves and gastropods occur rather commonly by flysch facies, forming successions many in some limited sequences in the Southern Belt. kilometres in thickness (Kanmera 1976). These Trace fossils are prolific locally, e.g. in Shikoku rocks are highly deformed, often into isoclinal (Katto 1960, 1964). Generally, however, fossils folds, and there is important repetition by are rare and subdivision of the Shimanto Super- imbricate thrusting (Sakai 1978). There is group must use sandstone petrography. general low-temperature regional metamorph- Sandstones of the Northern Belt (Lower Shi- ism of the prehnite-pumpellyite and greenschist manto Group) tend to be highly feldspathic in facies. composition, less sorted and coarser grained In the cross-sections of the Shimanto Belt, than the quartz-rich, better sorted Upper Shi- sediments tend to become younger oceanwards manto sandstones of the Southern Belt. In the (Fig. 2) as does the unconformity between the mature arenites of the latter group, extremely highly deformed Shimanto strata and the over- well-rounded quartz grains abound. These lying mildly deformed post-Shimanto sedi- sandstones are further characterized by the ments. A similar trend continues out of the stable heavy mineral suite of zircon-tourmaline- Shimanto Belt into the present trench inner rutile, and by a higher muscovite content. In slope (Fig. 2, columns 5 & 6). both groups, microcline is a major constituent, The lower metabasalts in both the Northern but it decreases upwards. The total amounts of and Southern Belts are characterized by pil- feldspars also decrease upwards: the Cre- lowed basalt and hyaloclastite (Kanmera 1976; taceous sandstones of Kyushu, for example, Suzuki & Hada 1979; Tsuchiya et al. 1979). In have more than 35% feldspar, Tertiary ones Downloaded from http://sp.lyellcollection.org/ by guest on October 6, 2021 The Shimanto Belt of Japan 7 Shimanto Belt in Kyushu Off SW Japan N,Belt., BeltS. " Tanega- Upper Lower Shikoku gii Akaishi Ma INorthlSouth] shima Inner Trench Slope Peninsula Mtns, O. Tanabe & ,-. :Op.'.'.'-lnag.a Gp: K ~" Ace ~ [ Kumano Mioc. " " • '''" [..'.... ,,'..,: -- Misaki ,"Groups,',. • . -- .~_f~ ,'"• ..Group , .' ' ' -- ~'"" " ...... F __ 20. , iNichinan ? .... ;_GroupF / f- S """ "": &uro/ ,~ Group gawa5 ~°~ Oligoc c/Hyuga / -- ~r-- r 7t 7 f /if p Gr°u~r Muroto- GrouPs Kumage ~ ~ ~ 40: f Group Group # shigawa ~ e c Group £ f ~ J Mikado $ )~ f D Fm Pa]aeoc, ef # i F F ? Owa3ima .idaka- ~7'Shi- y Late ¢J- ; f if' J & Nano- gawa 3 manto gawa f Groupf Group" "~ s ~o,o- ix/ Fms. etc Cret. tsuka o ~ o roupJ i//i P,// ~ Acc = Accretionary prism / ? ~ F 0 ~ ~100-r j~/; P = Piei ............... aro & / / Hayama f / early Pleistocene Fms. etc 120- T = Early to Mid-Miocene ;/ // Cret. = Unconformity / i -- F -- = Fault _ 2-/f_ .,>/ i I:/ ]40 ~ = Deformed strata = Post-orogenic strata Fro. 2. Simplified stratigraphy of the Shimanto Supergroup and immediately overlying strata in four areas. Many formation names are omitted. The four Kyushu columns show the southward younging of the Shimanto rocks, which is continued offshore (next two columns, from Okuda et al. 1979). Acc, P, K and T are based on acoustic stratigraphy. less than 35%. Moreover, the ratios between the Southern Uplands of Scotland by Floyd orthoclase and plagioclase and those between (1975): these zones can be traced along strike orthoclase and microcline
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