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