地学雑誌 Journal of Geography 102 (7) 836-848 1993

The Marine Middle Jurassic Karajigu Formation in Northern Karakorum Mountains, Western China, and its Tectonic Implications

Chunfa JIANG*, Atsuyuki MIZUNO**, Min ZHAO*, Zhizhi ZHU * and Jingshan YU *

Abstract The Karajigu Formation is proposed to be for the marine Middle Jurassic strata in the Ka- rajigu area on the northern Karakorum Mountains, western China. The formation, with a total thickness of about 2,200 m, comprises three lithologic members ; the clastic rock member, the massive limestone member and the thin-bedded limestone member, in the ascending order. The lithology corresponds to a shallow neritic facies and the Middle Jurassic age is determined by the contained bivalve, brachiopod and ammonite fossils. Its stratigraphic relationships with the underlying Lower Permian and Upper Triassic strata and the overlying Upper Cretaceous strata in Karajigu, together with regional stratigraphies and geologic structures throughout the northern Karakorum Mountains to the northern-central Tibetan Plateau, elucidate a major profile of the East Tethys evolution during the late Paleozoic through Mesozoic. The Karajigu Formation was formed on the northern continental margin of the East Tethys in Middle Jurassic age, sub- sequent to the closure of the Late Triassic Karakorum-Hoh Xil Ocean (newly proposed) through the Indosinian movement. Its unconformable relation to the overlying Upper Cretaceous red beds demonstrates the influences of end-Jurassic Yanshanian movement to the pre-Cretaceous construction of principal parts of the northern Karakorum Mountains. The Tertiary Himalayan movement was the final marking in the structural history of the Karajigu area.

southeastward into northern-central Tibetan I. Introduction Plateau including the Tanggula Mountains The marine Middle Jurassic strata here (Fig. 1). referred to as the Karajigu Formation are Although no geologic information has been distributed near Karajigu at the altitudes of published except for Jiang et al. (1992)'s brief 3.5-3.8 km, 40-50 km north of Kunjirap Daban description, the Karajigu Formation and its along the China-Pakistan Highway, northern stratigraphic relations to the older and younger Karakorum Mountains. The strata extend strata appear to occupy a significant situation northwestward into Tajikistan Pamir and east- through an evolution of the East Tethys (Hu-

* Institute of Geology , Chinese Academy of Geological Sciences (Baiwanzhuang Road 26, Fuchengmenwai, Beijing 100037 China) ** Department of Earth Sciences , Faculty of Science, Ehime University (Bunkyo-cho 2-5, Matsuyama 790 Japan)

836 Fig. 1 Distributions of the marine Middle Jurassic strata and the other main geologic units in thenorthern Karakorum Mountains to the Tibetan Plateau (modified from CIGMR, CAGS, 1988)

ang and Chen, 1987) during the Late Paleozoic northwest of Karajigu, almost inside the NW- to Mesozoic. trending Karajigu Fault Depression, accompa- This article describes the detailed features of nied by the marine Lower Permian, marine the formation, together with some aspects of Upper Triassic and non-marine Upper Creta- the older, Lower Permian and Upper Triassic ceous strata (Fig. 2). Its exposure condition is strata and the younger, Upper Cretaceous strata excellent in general, and its detailed stratigra- in the Karajigu area, and discusses their im- phy, as well as its stratigraphic relations with plications on the tectonic evolution of the East the older and younger strata can be well observ- Tethys over an extensive region of northern ed at the cliffs located on the western side Karakorum Mountains to north-central Tibe- of the N-S-trending upper Taxkorgan River tan Plateau. valley, in particular (Fig. 3). Utilizing the cliffs as the standard section of Karajigu For- II. The Karajigu Formation mation, we measured its detailed stratigraphic 1) Generals sequence, and due to its folding structure and The Karajigu Formation is distributed in topographic condition, this section was complet- rather small mountainous area immediately ed after the incorporation of the observation

837 Fig. 2 Geologic map of the Karajigu area

Fig. 3 The measured type section of the Karajigu Formation, viewed from the east (photographed on Aug. 27, 1992)

on three auxiliary sub-sections. of this area and is unconformaly covered by Throughout the distributed area, the Kara- the Upper Cretaceous strata, whereas the for- jigu Formation is unconformably underlain by mation is thrusted by the Upper Triassic strata the Lower Permian strata at the northern end at the southern to southwestern part.

838 Fig. 4 Composite section along the western side of the Taxkorgan River valley , including the type section of Karajigu Formation (Line I in Fig. 2 see Fig. 3)

The Karajigu Formation, about 2,200m thick, of stratigraphy including fossil occurrence, and comprises clastic and dominantly carbonate then its correlation with the marine Middle rocks and is lithologically divided into three Jurassic of Karajigu is still uncertain. Hence, members ; the clastic rock member (J2a) , the we propose the name of the Karajigu Forma- massive limestone member (J2b) and the thin- tion for the latter, designating the outcrop, bedded limestone member U2c), in the ascend- shown in Fig. 3 as its stratotype to avoid possi- ing order (Fig. 4). In the type section, the ble future confusion. formation yields eleven fossiliferous beds conta- 2) Detailed stratigraphy of the measured ining neritic bivalves, gastropods, brachiopods , section corals, asteroids and so on. Among these fos- The Karajigu Formation, 2,156 m thick at sils, the bivalve and brachiopod fossils indicate the measured section, is divided into three the Middle Jurassic age. Beside the type sec- lithologic members which are in conformable tion we can find similar fossils including ammo- relation to each other ; the clastic rock mem- nites also indicating the Middle Jurassic age. ber, massive limestone member and thin-bed- The marine Middle Jurassic strata in the ded limestone member, in the ascending order. Karajigu area was once cited as the Longshan The lower, clastic rock member, 588m thick, Formation with brief description of lithology starting from basal coarse-grained elastic (Jiang et al., 1992). The Longshan Forma- rocks, consists mainly of yellowish green cal- tion, itself, has informally been used for possi- careous siltstones with thin-bedded limestones ble Middle Jurassic strata scattered in some which increase upward within this member. areas of the eastern Karakorum Mountains, The middle, massive limestone member, 1,256 200 to 300 kilometers southeast of Karajigu m thick, is dominantly greyish purple massive (unpublished report of Team No. 4, the First limestones and dark grey thick-bedded lime- Regional Geological Survey Party of Xinji- stones, rarely containing fossils. The upper, ang). However, there has been neither desig- thin-bedded limestone member, 312 m thick, nation of its stratotype nor detailed description consists mainly of dark grey thin-bedded sandy

839 limestones and argillaceous limestones often grading to marl, showing upward increase of terrigenous muddy and sandy materials. This member contains very abundant fossils. According to lithologic features, the entire Karajigu Formation is subdivided into sixteen Beds in the type section as follows, in the descending order (Fig. 5).

＜ The thin-bedded limestone member ＞ Bed 16. Dark grey medium-to thin- 164.7 m bedded limestones, with coquina. Yielding bivalves (P3H5), Arcomy- tilus cf. laitmairensis (Agassiz), Paramussium sp., Myopholas mul- ticostata xinjiangensis Yu et Mizu- no (MS.) and Eopecten (Scutulo- mopecten) taxkorganensis Yu et Mizuno (MS.). Bed 15. Dark grey medium-to thick- 80 m bedded sandy limestones, with coquina, yellowish brown when weathered. Bed 14. Dark grey medium- to thin- 22 m bedded limestones, greyish yellow when weathered. Yielding corals (P3H4). Bed 13. Dark grey thin-bedded lime- 9.6 m stones, with calcareous clastics. Yielding brachiopods (P3H3), Kut- chithyris tibetica Chin, Sun et Ye ; Bivalves, Falcimytilus jurensis Fig. 5 Integrated stratigraphic columnar section (Romer) and Pinna lanceolata of the Karajigu Formation in Karajigu Sowerby. Bed 12. Dark grey thin-bedded lime- 1.6 m stones, yellowish brown and frag- mentary when weathered. Yielding bivalves (P3H2), Plagiostoma cf. laevisculum Sowerby, Honomya gibbosa Sowerby, Pseudotrapezium cardijorme (Deshayes), Radulopec-

840 ten karakorumensis Yu et Mizu- pecten vagans Sowerby ; gastropo- no (MS.) and Radulopecten giga- ds (Pith), Ampulospira sp. At nteus Yu et Mizuno (MS). Loc. 1020 H near the type section Bed 11. Dark grey medium- to thin- 6.3 m Bed 4 yields Eopecten (Scutulo- bedded limestones, yellowish brown mopecten) taxkorganensis Yu et and fragile when weathered. Yie- Mizuno (MS.), Spondylopecten lding bivalves (P3H1), Plagiosto- stoliczkai Cox and Plagiostoma sp. ma wynnei Cox ; gastropods (P3H1), Bed 3. Dark grey sandy limestones 18.7 m Natica sp. interbedded with thin-bedded lime- Bed 10. Dark grey medium- to 28.1 m stones. Yielding brachiopods (P1 thick-bedded limestones, greyish H2), Burmirhynchia sp. and Avo- brown when weathered, fragile and nothyris distorta Chin, Sun et Ye. microcrystalline. Bed 2. Yellowish green calcareous 520 m siltstones, with interbeds of thin- ＜ The massive limestone member ＞ bedded limestones, greyish purple Bed 9. Greyish purple limestones, 913.5 m siltstones and blackish grey sand- containing siliceous nodules. Yield- stones. Yielding asteroids MHO, ing bivalves (P2H3), Falcimytilus Hetrastridium verrucosum (Dun- sp., Modiolus imbricatus Sowerby ker). and Ceratomya sp. gastropods Bed 1. Brick-red conglomerates. 10.3 m (P2H2), Globularia cf. subimbricata 3) Fossil fauna and correlation (d' Archiac). We discovered a large amount of fossils from Bed 8. Grey and gryish purple 75.1 m the Karajigu Formation out of the measured thick-bedded limestones, greyish section within the Karajigu area ; genera and brown and fragile when weathered, species therefrom are quite identical with those distinctly stratified. Yielding biva- from the measured section. Beside, some lves (P2H1), Ceratomya cf. ba- ammonite fossils were found in the formation jocina d'Orbigny and Cecatomya to the west of the measured section, which are excentrica (Roemer). represented by Proplanulites sp. or Cutchisp- Bed 7. Dark grey thick-bedded lime- 89.9 m hinctes sp. of Proplanulitinae, Preisphinctidae stones, microcrystalline. indicating the Bathonian-Early Callovian age. Bed 6. Dark grey medium- to thick- 177.4 m The Karajigu fauna is characterized by the bedded limestones, brown when Bathonian bivalves such as Arcomytilus cf. weathered, fragile. laitmairensis, Modiolus imbricatus, Spondylo- pecten stoliczkai and Plagiostoma wynnei, the ＜ The clastic rock member ＞ Bathonian brachiopods of Kutchithyris tibetica Bed 5. Dark grey thin-bedded lime- 12.7 m and Avonothyris distorta and the Bathonian stones, fragile. or Early Callovian ammonites (Cutchisphinctes Bed 4. Dark grey argillaceous lime- 26.5 m or Proplanulites), which occur in Macroce- stones, brown when weathered . phalus Beds (Patcham Formation) and its under- Yielding bivalves (P1 H1), Radulo- lying Kuar Bet Beds in Cutch, India (Cox,

841 1952). On the other hand, the fauna also con- that the Middle Jurassic strata paleontologically tains some bivalves resembling those of W est evidenced occur extensively throughout north- Asia (Sinai and South Islael) and W est Europe, ern to central Tibetan Plateau, eastward up represented by Plagiostoma cf. laevisculum, to the Tanggula Mountains, with essentially Pinna cf. lanceolata, Ceratomya excentrica similar stratigraphy and lithology to the Kara- etc. These evidences show a strong connection jigu Formation (The Compiling Group for with the Middle Jurassic faunas on the south- Stratigraphic Scheme of the Xinjiang-Uygur ern margin of Tethys, the Ethiopian faunas Autonomous Region, 1981). For example, the proposed by Hallam (1977), and it is obvious Yanshiping Group on the northern slope of the that the Karakorum region was a part of the Tanggula Mountains (Yang and Yin, 1988) and Tethys at that time. the Mali and Liuwan Formations in Lhorong The Upper Jurassic strata have not been area of eastern Tibet (Tong, 1987) are char- known in the Karajigu area, but occur in the acterized by lithologic sequence dominantly of northeastern Karakorum Mountains as the Kun- marl and limestone with arenaceous-pelitic jirap Formation which yields many marine elastic rocks in their lower part. Thus, it fossils of ammonites, corals and others (Jiang appears that there occur the Middle Jurassic et al . , 1992). The Kunjirap fauna has not been shallow or neritic shelf facies very extensively definitely characterized, but its general aspect through northern Karakorum to eastern Tibet. including some ammonites differs from those III. The older and younger strata, and of the Karajigu fauna. Among the ammonite species, Spiticeras aff scripta (Strachey), S. their stratigraphic relationships with the Karajigu Formation spitiense (Blanford) and Virgatosphictes den- seplicatus aagen), and Belemnites spp. show The older, Lower Permian and Upper Trias- its younger age than the Karajigu fauna, i. e., sic strata and the younger, Upper Cretaceous the Calovian, and the formation can be corre- strata in the Karajigu area are as follows. lated with the Belemnites sulcatus Bed in In- 1) The Lower Permian strata dia (Yang and Nie, 1990) The Upper Juras- The Lower Perminan strata consist of alter- sic Kunjirap Formation extends its distribu- nating beds of dark green and black siltstone tion to the Tajikistan Pamir area northwest of and slate, with distinct rhythms and rare fos- Karajigu along its general distribution trend sils, originated from turbidity current. Their (CIGMR, CAGS, 1988). In the Karajigu area thickness could not be measured, due to very the Kunjirap Formation could have been depos- complicated geologic structure by deformation. ited subsequent to the Karajigu Formation There is no fossil evidence indicating geologic but eroded away before deposition of the Up- age in this area. Their equivalent, however, per Cretaceous continental beds. regionally widespreads and contains Early The lithologic sequence and fossil fauna of Permian fusulinids and less commonly corals the Karajigu Formation evidently suggest that and brachiopods which indicate the Early Per- its depositional site was under a shallow or mian in age (Jiang et al., 1992). neritic marine environment, influenced by sup- 2) The Upper Triassic strata ply of terrigenous materials in its early and The Upper Triassic strata consist of meta- late stage of deposition. It has been known morphosed carbonate rocks, clastic rocks and

842 basic lavas (on the southern end of the section) in the Karajigu area. The strata occur mostly as strongly folded and faulted blocks (Figs. 3 and 7), so their stratigraphic sequence and thick- ness can not be known, The carbonate rocks yield many fossils, Among them, brachiopods include the Late Triassic species such as Oxy- colpella elongata Ching et Fang, Aulacothyris aff. angustaeformis Yang et Xu, Spiriferina Fig. 6 Sketch of the unconformable relation between the Karajigu Formation (J2) cf. shalshalensis Bittner, Zeilleria lingulata and the Lower Permian strata (Pi) at II Chin, Sun et Ye and Ninglangothyris subcircu- in Fig. 2 laria Jin et Fang. The Upper Triassic strata gradually expand their distributional area from Karajigu eastward, accompanied by lithologic change. In the Longmu Co area they contain flysch facies of 4,000 to 5,000 meters thick (un- published data of Team No. 4, the First Region- al Geological Survey Party of Xinjiang), and further eastward to Hoh Xil grade entirely to flysch facies (unpublished data of Y. F. Zhang and others, 1991). 3) The Upper Cretaceous strata Entirely represented by red beds, the Upper Cretaceous strata consist of purple red conglom- erates, pebbly sandstones, grits and sandstones observed between the Karajigu Formation and with a minor amount of siltstone, unconform- Lower Permian and Upper Cretaceous strata, ably covering the Karajigu Formation. The whereas the Upper Triassic strata are in fault lower part is mostly occupied by conglomerates, contact with the Karajigu Formation and the and the clasts therein are rounded in shape and Upper Cretaceous strata have no direct contact variegated in composition and size. The sand- with the Lower Permian strata. stones exhibit large-scale cross-bedding of sup- At the northern end of the measured section posed continental fluvial origin. Toward the (II in Fig. 2), the base of Karajigu Formation top there appear siltstones. We have no fos- displays unconformable relation to the Lower sil evidence therein, but their age can be in Permian strata with an angle over 50 ° (Fig. ferred on the basis of correlation with similar 6). The boundary is rather unsmooth, and we strata in the neibouring areas to be Late Cre- can recognize a large difference not only in li- raceous (Bian et al., 1991 ; unpublished data of thology but also in metamorphic grade and style Teams No. 4 and No. 10, the First Regional of deformation between the both strata. The Geological Survey Party of Xinjiang). Lower Permian strata, though not highly meta- 4) The stratigraphic relationships morphosed, are obviously slaty and phyllitic In the Karajigu area an unconformity is and strongly folded. On the other hand, the

843 Karajigu Formation is not metamorphosed, There is no direct contact between the Lower showing gentle and open folding. A bed of Permian and Upper Triassic strata. It has been basal conglomerate, 5-10 m thick, commonly known that throughout the extensive region occurs above the boundary. of northern Karakorum the Lower Permian At the southern end of the section (III in and Triassic strata display a disconformity with Fig. 2), the thin-bedded limestones of the Ka- each other, both being characterized by very rajigu Formation with a NW-plunging syncli- strong folding and low metamorphic grade of a nal structure are covered with an angular un- uniform degree (Jiang et al., 1992). This sug- conformity by the overlying Upper Cretaceous gests that the strong folding and the low meta- red beds consisting of conglomerates and sand- morphic grade of both the strata in Karajigu stones (Fig. 7). The red beds form a small formed through some tectonic movement before steeply overturned syncline, thrusted by the the deposition of Karajigu Formation. This Upper Triassic limestones which are heavily movement should be assigned to a certain epi- disturbed with tight-folding structure (Fig. 8). sodic event in the Indosinian movement which prevailed in China during the Middle Triassic through Early Jurassic (Ren et al., 1980 ; Hu- ang and Chen, 1987). The unconformity between the Karajigu For- mation and the Upper Cretaceous is the evi- dence that the Yanshanian movement (Ren et al., 1980 ; Huang and Chen, 1987) occurred the northern slope of the Karakorum Mountains. The Tertiary Himalayan movement due to the collision of Indian and Eurasian Plates (Ren Fig. 7 View of the southern end of the type et al., 1980) is well demonstrated by the section of the Karajigu Formation (pho- NW-trending fault between the Upper Triassic tographed on Aug. 27, 1992) (see Fig. 8) limestones and the Upper Cretaceous red beds with an overturned syncline in the Karajigu

Fig. 8 Sketch at III in Fig. 2, showing the unconformity between the Karajigu Formation (J2C)and the Upper Cretaceousstrata (K2), the latter thrusted by the Upper Triassic strata (T3)

844 area. mir southeastward to eastward through Kara- jigu to the Xijir-Yushu region. This elongat- IV. Tectonic implications ed ocean may be comparable to the "Palaeo- The lithologies and stratigraphic relations Tethys lying on the south of Laurasia" (Sea- of the Karajigu Formation and associated strata Or, 1979). This oceanic basin was tectonically of different ages reveal some significant aspect active, located geographically in harmony with of tectonic evolution of the northern Karako- the Longmu Co-Yushu Suture Zone, the rum to the Tanggula Mountains under a com- boundary between Gondwanaland and Eurasia in the Permian (Huang et al., 1984 ; Huang parison with their tectonic equivalents. Strongly deformed Upper Triassic strata con- and Chen, 1987), and extended largely along taining flysch facies occur througout the region the zone from the northern foot of the from the northern Karakorum Mountains to Karakorum Mountains to the southern foot of the Hoh Xil Range, as noted in the preceding the Hoh Xil Range. We tentatively call it chapter. Recently Bian et al. (1991) showed the Karakorum-Hoh Xil Ocean. that ophiolites are exposed from Xijir Ulan to The Karakorum-Hoh Xil Ocean might have Yushu along the Hoh Xil Fault (Fig. 1), but developed on the Early Permian continental their age remains still unsolved. According to crust which started breakup in the Late Per- Y. F. Zhang and others' unpublished data at mian age (Jiang et al., 1992). The Indosinian about 200 km east of Xijir Ulan Lake, clastic movement led to the closure of the oceanic rocks of the Upper Triassic Batang Group are basin, forming the Indosinian fold belt (Jiang associated with the Bayan Chawuma gabbro et al., 1992). Meanwhile, the continental crust (diabase) together with ultramafic rocks and along the Bangong Co-Nujiang River line, are intercalated with intermediate-basic vol- located south of the oceanic basin, the sector canic rocks. The gabbro (diabase) occurs as from Bangong Co to Denggen in particular, strongly altered small stocks or dikes in the was disrupted and evolved to a small oceanic Batang Group having sharp boundaries with basin gradually (Huang and Chen, 1987). By their surrounding unaltered clastic rocks. This the Middle Jurassic an oceanic basin with a implies that the association of the rocks must certain extent thus had been formed (Pan et al., have formed as a tectonic melange and, further- 1983). more, the ophiolites must be the Late Triassic In Jurassic time, a new tectonic environ- in age as well as the Batang Group. The tec- ment was thus generated with deposition of new tonic melange may have been resulted from type sediments in the wide areas of Karajigu the Indosinian movement. to central Tibetan Plateau, showing a change The occurrence of Late Triassic basic lavas from the Early Triassic oceanic basin facies to in Karajigu and also Tajikistan Pamir (Pashkov Middle Jurassic shelf facies. The equivalents and Shuvolyman, 1979) and the distribution of marine Middle Jurassic of Karajigu are of very thick Late Triassic flysch on the north distributed extensively from Karajigu south- ern slope of the Karakorum Mountains (un- eastward along the northern slope of the pudlished data of Y. F. Zhang and others) sug- Karakorum Mountains to the Tanggula Moun- gest that an elongated oceanic basin existed tains, constituting an important part of the during the Late Triassic from the Central Pa- strata of the northern Tibetan Plateau (The

845 Compiling Group for Stratigraphic Scheme of Formation with many neritic marine fossils Qinghai Province, 1980). Their lithologies are indicate a stable shallow depositional environ- quite similar from place to place except for ment during Middle Jurassic time. It was form- some minor difference, all represented by nerit- ed on the northern continental margin of ic deposits as described in the preceding chap- the East Tethys at that time, subsequent to the ter. However, the Middle Jurassic strata south closure of the Late Triassic Karakorum-Hoh of the Bangong Co-Dengqen Fault is quite Xil Ocean through the Indosinian movement. different in lithofacies, mostly represented by The unconformable relation between the Kara- turbidity current deposits with siliceous rocks jigu Formation and the overlying Upper Cre- (Yu and W ang, 1990), which in some places taceous red beds shows the influences of end- form ophiolitic melanges together with ophio- Jurassic Yanshanian movement to the pre-Cre- lites (Yang and Zheng, 1990 ; Liu et al., 1990). taceous construction of principal parts of the These evidences suggest that a stable neritic northern Karakorum Mountains. Thus, the continental margin environment prevailed in Karajigu Formation and associated strata of the zone from Karajigu to Tanggula in Middle different ages in Karajigu record a major pro- Jurassic time, whereas the Bangong Co-Den- file of Tethys evolution in the northern Kara- gqen area on its south was occupied by a tec- korum Mountains to northern Tibetan Plateau tonically active oceanic basin, and this shows during the late Paleozoic through Mesozoic. a peculiar situation of the Karajigu Formation Acknowledgements in the tectonic and paleogeographic evolution of the East Tethys. We are grateful to Prof. Y. Z. Wang, Prof. X. G. Zhang and other geologists of the Second Geological The Yanshanian movement at the end of the Survey Party of the Xinjiang Bureau of Geology and Jurassic caused the Karajigu Formation to have Mineral Resources in Kashi for their kind guiding us to select the section during our field investigation. been folded and exposed on the surface to- Our sincere thanks are due to Prof. Z. S. Li for his gether with the possible Upper Jurassic strata. useful constructive discussions on the manuscript and During the Late Cretaceous a transgression identifying bivalves, to Prof. L. Li for identifying brachiopods and to Prof. G. F. Liu for identifying took place in most areas of southeastern Xin- ammonites. Profs. X. O. Lao, Y. C. Chai, B. G. jiang and northern Tibet (Jiang et al., 1992). Feng and J. S. Yang also cooperated with us in our In the Karajigu area, however, the Upper Cre- field investigation in the Karajigu area. Prof. Y. F. Zhang kindly permitted citing part of his unpublish- taceous strata are characterized by terrestrial ed data in this paper. deposits, although their missing original upper Main part of the field work was achieved by C. part might have been accompanied by marine F. Jiang, M. Zhao and Z. Z. Zhu in 1981, funded by MGMR. Part of our paleontological study was facies, from distribution of the Upper Creta- carried out by J. S. Yu and A. Mizuno during her ceous marine deposits in the other areas of the stay in Ehime University sponsored by the Japan So- northeastern Karakorum Mountains (Jiang et ciety for the Promotion of Science in the 1991 Japa- nese fiscal year. A. Mizuno's field observation in the al., 1992). The Tertiary Himalayan move- Karajigu area in 1992 was partly supported by the ment completed the geologic structure of the Grant-in-aid, 1992 of the Tokyo Geographical Socie- Karajigu area. ty of Japan.

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847 中 国 西 部, カ ラ コル ム 山 脈 北 部 の 海 成 中部 ジ ユラ 系 カ ラ チ グ層 と, そ の構 造 発 達 史 上 の意 義

姜 春 発* 水野 篤行** 〓 民* 朱 志 直* 于 菁 珊*

中国西部,カ ラコルム山脈北部 のカ ラチ グ地域 討 した結果,後 期 古生代 か ら中生代 にか けての東 に分布す る海成 中部 ジュラ系 を カラチ グ 層 と呼 部 テチ スの構造 発達 史 の主 要な様相 と,そ のなか ぶ。 カラチ グ層 は,厚 さ約2,200mを 持 ち,次 の で のカラチ グ層の位 置づ けが明 らかに され た。 カ 3部 層 に区分 され る。すなわ ち,下 位 か ら上位 に ラチ グ層は,後 期三 畳紀のカ ラコルム― ホーシル 向 かって,砕 層岩部層,塊 状石灰岩部層,薄 層理 海(新 しく提 唱)が イ ン ドシナ運動 によ って閉 じ 石灰岩部層で ある。多数 の二枚貝類,腕 足類 な ど た後 に,中 期 ジ ュラ紀 に東部 テチスの北縁浅海部 のほか,ア ンモナイ ト類 は,本 層が中期 ジュラ紀 に広 く堆積 した主 として石灰岩相 の一部 にあたる にお ける浅海相で あることを示 してい る。 カラチ ものである。カ ラチグ層 と上部 白亜系 との間 の不 グ地域 にお ける本 層 と下位 の下部二畳系 ・上部三 整合 関係 は後期 ジ ュラ紀 の燕 山運動 がカラコル ム 畳系,上 位 の上部 白亜系 との層位 学的関係 に加 え 山脈北部 に及 び,そ れ によって同地域 の先 白亜紀 て,カ ラコルム山脈 か らチベ ッ ト高原 中北 部にか の基本構造 ができ上 ったことを示す。 カラチグ地 けての諸地質系統 の層序 ・地 質構 造 を広域 的 に検 域 の全地質構造 は第三紀 の ヒマラヤ運動 によって * 中国地質科学院地質研究所 完成 した もので ある。 ** 愛媛大学理学部地球科学教室

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