Mesozoic Geology of Japan

2 P 勿 Shin .i r o M izutani and Akira Yao R a d io la ria n s a n d terran es： M esozoic geology of Japan Nobodv can understand the geology, of Japan R a diolarian biostratigraphy of the w itn o u t tn e Icn o w te a t e o f r a tl io ia r ta ii m o s tr a tt u a 一 Paleozoic to M esozoic group p n v ana th e co ncep t of terra ties. Iti th e P a st aeca tle , considerable research has been carried out oii ra- R adjolarian biostratigraphic research and age assignment by mean diolarian fossils, pai-ticulailv those in the basem ent of rads w ere not done successfully until the 1970s. O ur know ledge (A rads has im proved greatly , how ever, in the past decade, w hich i } rocKs of r ateozoic m ul M esol-oic ages. I tie rcsults show n by several leading papers such as those w ritten by N akasek indicate that Jurassic f})rm atiotis are w idespread and N ishim ura (1979) on L ate T riassic rads, Y ao and others （19801‘ and that they constitute at? accrettonarv com plex. on M iddle Triassic to M iddle Jurassic rads, M izutam and other- (198 1) on M iddle to Late Jurassic rads, and Ishiga and Im oto 门98U ， A fter estab lish itu , a n ew series of ra d io la ria n lo ssit on P erm ian rad s. ,,ones, w e have reexam ined the stratigraph), i)t the M esozoic rad i o la rian assem blage zones w ere set up tirst by Y a, basem ew rocks and have recognized inanY tectono- and others (1980) in chert-clastic sequences that w ere assigned to th, M id dle T riassic to M id d le Jurassic on the basis o f co no do nts aric stratigraphic terranes iii介Japanese Islands. Dis- stratigraphic relationships. Subsequently. m any assem blage zone-, cusson on the tectonc nistory, of the Mesozoic ter- have been proposed, though locally, that range in age from Perm iaii to C retaceous. Integrating these data w ith their ow n observations ranes s exten e尹 to that o easternAsa，reaton M atsuoka and Y ao (1986) established the Jurassic rad zones a, to accret on a na d sp ersio n p rocesses . defi ned by biohorizons. A t present, the rad zones in Japanese late C arboniferous to Jurassic tim e are com piled as show n in figure I ．0 11 these zones, the Jurassic ones are correlated broadly w ith the rad zones of the A tlantic O cean, M editerranean 'Iethys. and N orth A m er ican regions (Y ao , 1990). A ccording to the latest report of the O ceaii D rilling Program (O D P), the rad zones of late M iddle Jurassic tu earliest C retaceou s tim e in the cen tral w estern P acifi c O cean fl oo f correspond w ell to those of the Japanese Islands. In tr o d u c tio n System atic investigations of "Paleozoic" st ra t ig raphy and the age assignm ents based on our rad scale indicate that chert or siliceou} shale in the "Paleozoic" group com m only contains rads ot Perm ian T he Paleozoic：。Mesozoic group of rocks underlies。major part of T riassic, or Jurassic ages. T he occurrences of these rads suggest that the Japanese Islands, and it has been investigated stratigraphically the group is a m 6lange com plex that is com posed principally ot a since the 19th century, chiefl y on the basis of fusulinaceans and C arboniferous to Perm ian greenstone and lim estone (seam ount) fa- m acrofossils. In contrast to this school of biostratigraphy, studies of cies; a C arboniferous to Perm ian shale and chert (pelagic) facies; and radiolarians (rads), particularly using the scanning electron m icro- a Triassic bedded chert, Jurassic chert and siliceous shale (som etim e· scope, w ere undertaken in the 1970s and becam e w idely popular carrying m anganese carbonate nodules), and Jurassic elastic facies in thereafter. In fact, of m ore than 400 papers published on rads in w hich granite-bearing conglom erate is intercalated sporadically. Japan since 1926, m ore than 80 percent appeared in the 1980s. T hese papers, w ritten m ainly by enthusiastic young geologists, have offered startling new evidence for the geologic age of the "P aleozoic" group . T he m ost noticeable papers present the findings of Jurassic rads from M esozoic terranes of the Japanese "Paleozoic" provinces, and it has been dem onstrated that the "Pale- ozoic" group actually form s a M esozoic m elange com plex. Isla n d s It used to be believed that palcogeographic reconstructi o n c ould be m ade by collecting paleoenvironm ental data and by com piling W e have reviewed and reexamined all the classic data in order t( them according to their ages and sim ilarities. H ow ever, the new ly present the w hole tectonic history of the Japanese Islands, w here w e obtained age data have revealed that adjoining geologic units have recognized m any disparate terranes as show n in fi gure 2. TheN bounded by a significant discontinuity have different geologic histo- are m etam orphic terranes (H icla, "Sangun," S anbagaw a, and A bu- ries. T he terrane concept has been accepted, and the term is used now kum a), Paleozoic terranes com posed m ainly of Paleozoic folded especially by the geologists w ho have been w orking in the "Paleozo- form ations (A kiyoshi and South K itakam i), an early M esozoic ter- ic" provinces. rane (M aizuru), m 6lange and disrupted terranes (M inot C hichibu. T hus, the g e o logy of the Japanese Islands prior to the opening of and N orth K itakam i), a m uch younger terrane of an accretionar\ the Sea of Japan is discussed here in term s of collage tectonics on the com plex (Shim anto), and the C enozoic terrane of Izu. O ur discu eastern continental m argin of A sia in M esozoic tim e. sions extend to intraterrane features and to interteff ane relationships Episodes, Vol. 14, no. 3 R a diolaria n 4 F ig ure L - L ate C arboniferous to J urassic radiolarian fossils Assem blag e zone and their assem blage zones in the Japanese Islands. (1) P prim itiva Pseudodietyom itra prim itiva M atsuoka and Yao. (2) P arvicin- gula m ashitaensis M izutani. (3) G ongylothorax sakaw aeusis T yaoi M atsuoka. (4) D ictyom itrella(?) kam oensis M izutani and K ido. G . s a k a w a e n s is - (5) U num a echinatus Ichikaw a and Y ao. (6) H suum hisuiky- 0 S . n a ra d a n ie n s is 1}1 oense Isozaki and M atsuda. (7) M esosaturnalis hexogonus (Yao). (8) Parashutim sim plum Yao. (9) Canoptum triassicum 一 G u e x e lla n u d a ta E -2 5 乏 Yao. (10) T riassocam pe nova Yao. (11) Y eharaia elegans N a- 琳加，‘，．l，公．’么 U n u m a e c h in a tu s kaseko and N ishim ura. (12) T riassocam pe dew everi N akaseko S ‘ ⋯ and N ishim ura. (13) N coalbaillella ornithoform is Takem ura V H. hisuikyoense 淤﹃斌︸〔J 睡零舆哭滚赴比琳内 and N akaseko. (14) F ollicucullus scholasticus O rm iston and ,m a 甸kt’ P （？）grande 针介公︸ B abcock. (15) Pseudoalbaillella u 4orm a H oldsworth and n 游蘸幼 W 0 瞬以鱿． J ones. Scale bar equals 0.1 m m . A bbreviations: C , C arbonifer- ，从 hexag onus 粉引俗色‘ 汗 ous; E , E arly; M , M id dle; L , L ate. In Jurassic and Triassic 册豁行定卜 e lk h o rn e n s is 怡书吹少 ages: T . yaoi, T ricolocapsa yaoi; S. naradaniensis, Sti- Lij n 洪盆 dl E卜 (?) SP. C chocapsa naradaniensis; P .(刀grande, Parahsuum (即grande, 住粉护口卜 T . elkhornensis, T rillus elkhornensis; E ., E ucyrtidiellum ; K , B～ k u ru s u e n s is '.泛﹂ K atrom a; A . com p actum , A rchaeospongoprunum com pac- o} jParahsuumaff tum . In P erm ian and C arboniferous ages: N ., N eoalbaillella; 1 Io n g Ic o n Ic u m F ., F ollicucullus; P ., Pseudoalbailiella; A ., A lbaillella. C a nop tum 蕊 0 ﹄ tria s s ic u m sliced and interleaved together. Som e of the fragm entary units in this ｜一乏专 suture zone can be correlated w ith those of the A kiyoshi and "San- 5 ｜ Tria sso cam p e 5 ﹄ n o v a ,-}7 ' ｛ gun" terranes. ＜｜嘿叶弓盆咬‘ Th e A kiy o sh i terrane is com posed principally of C arboniferous － Tria sso cam p e 匡比 to Perm ian lim estone and underlying greenstone plus Perm ian chert ｜ d e w e v e ri 二娘命暴！卜 and shale; all of these are deform ed severely and are overlain uncon- ：候斌释 A . co m p actu m 耀 form ably by slightly deform ed, Triassic, coal-bearing form ations. 乏 6心只 6 田份 T he Paleozoic group in the A kiyoshi terrane represents an older 诵兮攀啼壑 N 吟沟街舞缪一 accretionary com plex that was related to the collision and disruption o rn ith o fo rm is 心、一 of ancient seam ounts, w hich w ere associated w ith the surrounding N o p tim a 一 pelagic chert and shale during latest Perm ian tim e.

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