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Tectonic evolution of the western margin of the Ordos Basin (Central China)
Russian Geology and Geophysics 49 (2008) 23–27 www.elsevier.com/locate/rgg
Tectonic evolution of the western margin of the Ordos Basin (Central China) Li Rongxi a, b, *, Li Youzhu a, b a
Key Laboratory of West Mineral Resource and Geology Engineering, Education Ministry of China, Chang’an University, 126 Yanta Road, Xian, 710054, China b School of Earth Science and Resource, Chang’an University,126 Yanta Road, Xian, 710054, China Received 20 January 2006
Abstract The western half of the Ordos Basin lies in the north of Central China, on the western margin of the North Chinese Platform. In the southwest and south it borders the Qilian and Qinling folded systems. Their frontal northeastern flank is thrust over the Paleozoic nappe of the Ordos Basin and consists of a series of large parallel linear folds separated by thrusts. The basement of the basin is made up of deformed metamorphic rocks of tentatively Late Proterozoic age. Carbonate and terrigenous intracratonic facies accumulated on the Ordos block from Early Cambrian through Middle Ordovician. From Middle Ordovician through Middle Carboniferous, the western edge of the block was uplifted as a land as a result of Caledonian tectonic movement and underwent denudation for about 150 Myr. In the Late Paleozoic, a series of coal-bearing formations was deposited. In the Triassic, the Ordos Basin became filled with fluviolacustrine delta facies. Thrusts and folds on the southwestern margin of the basin formed in the Mesozoic and were intensified by Himalayan tectonic movements. © 2008, IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: Tectonic evolution; petroleum potential; stratigraphy; Ordos Basin; China
Introduction The large petroliferous Ordos Basin lies in the center of North China, on the western margin of the North Chinese Platform (Fig. 1). Gas fields in the north of the basin are the largest source of natural gas, which is transported to both western and eastern regions of the country. Natural gas supplied for municipal and commercial use to large cities (Beijing, Tianjin, Xian, Zhengzhou, Taiyuan, Yinchuan, etc.) is now mined in the Ordos Basin. Three petroleum complexes were discovered in the basin: (1) Yanchang Series (Upper Triassic) and Yan’an Formation (Middle Jurassic), which are commercially important oil-producing beds, with lacustrine pelite with δ13C = –46.04 to –49.46‰ being their oil source rocks (Feng Fukai and Zhang Yafei, 1995). (2) Carboniferous-Permian deposits abounding in parental carbonaceous rocks. Some small gas reservoirs were discovered in fluvial sandstones of the Upper Permian Shiqianfeng Formation on the western margin and in the north of the Ordos Basin. Gas occurs in Carboniferous-Permian carbonaceous rocks and is
characterized by δ13C1 = –35.3 to –29.3‰ and δ13C2 = –27.4 to –22.3‰ (Xia Xinyu and Zhao Lin, 1998). (3) Ordovician thin crust of weathering containing the most commercially important gas pools of the basin. Dolomites of the Middle Ordovician Majiagou Formation are gas reservoirs here. They are characterized by secondary porosity and are overlain by pelites. Ordovician carbonates and Permian-Carboniferous carbonaceous strata are considered to be gas source rocks (Xia Xinyu and Zhao Lin, 1998). Oil and gas formation proceeded mainly from Late Jurassic through Cretaceous, and their accumulation, in the NeogeneEarly Quaternary (Dai Jinxing, 1995; Li Jingming et al., 2004). Uplifting of the Tibet-Qinghai plateau located south of the Ordos Basin and its movement northeastward followed by the formation of Qilian and Qinling folded systems resulted from the collision of the Indian and Asian-European Plates, which governed the evolution of the basin and generation and accumulation of oil and gas in it (An Zuoxiang and Ma Ji, 2002; Li Songshen, 1985). In this paper we will dwell only on the tectonic history of the western margin of the Ordos Basin.
* Corresponding author. E-mail address: [email protected] (Li Rongxi) 1068-7971/$ - see front matter D 2008, IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.rgg.2007.12.002
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Li Rongxi and Li Youzhu / Russian Geology and Geophysics 49 (2008) 23–27
Geologic structure The Ordos Basin is of intricate geologic structure. Its western margin borders the Qinling folded system in the south and the Qilian one in the southwest. In the northwest, the basin was separated from the Alashan block by a Cenozoic depression (Fig. 1). There is a series of parallel N-S-striking folds, thrusts, depressions, and uplifts on the western margin of the basin. Large-scale faults in the basement and on the western margin of the basin had a long evolution. The western periphery of the basin is part of the N-S-striking giant Asian structural zone separating West and East China. This separation is supported by a drastic change in geophysical fields, first of all, gravitational as well as stratigraphic and structural characteristics different for East and West China.
Stratigraphy The more than 5000 m thick Helan complex of metamorphic rocks of pre-Sinian age is the most ancient in the Ordos Basin (Fig. 2). Recognized in the inner part of the basin, it is composed of deep-seated thin-banded gneisses, quartz schists, and biotite granulites and is correlated with the Jining complex of the North Chinese Platform. On the western margin of the basin, Cambrian-Middle Ordovician sedimentary strata are formed by persistent layered carbonate deposits with marine fauna. The Cambrian carbonate rocks, more than 600 m thick, are mainly granular and pelitomorphic limestones with calcareous mud cement. Also, chert intercalates are present. The Lower Ordovician strata are composed of granular-pelitomorphic sediments with mud impurity as well as biogenic and dark-colored dolomitized pelitomorphic limestones. The Middle Ordovician complex of detrital deposits, quartz sandstones, and siltstones with schist and limestone intercalates is regarded as a flysch formation. The western margin of the Ordos Basin, like the entire western margin of the North Chinese Platform, lacks Upper Ordovician deposits. Silurian-Devonian deposits are also absent from the western margin of the basin, but they are widespread in its north, overlapping drastically inconsistent Middle Ordovician layered strata. Silurian rocks exposed on the northern margin of the basin are quartz-feldspathic sandstones with basal conglomerates in the base. Devonian sediments, mainly of fluviolacustrine facies, consist of red-colored conglomerates, sandstones, and siltstones. In the Middle and Late Carboniferous, there was a new stage of sedimentation on the western margin of the Ordos Basin. Middle Carboniferous deposits are mainly gray-colored calc-quartz sandstones and sandy limestones with intercalates of bioclastic limestone and thin carbon-bearing horizons. The Upper Carboniferous Taiyuan Formation is a continuous stratigraphic sequence of gray-colored quartz sandstones, finely layered limestones, and schists. The formation includes several coal-bearing beds. The Permian unit here concordantly overlies the Taiyuan Formation. The Lower Permian strata are
Fig. 1. Tectonic scheme of the western margin of the Ordos Basin. 1 — Ordos Basin, 2 — study area, 3 — folded systems, 4 — Cenozoic grabens, 5 — faults, 6 — thrusts, 7 — anticlines, 8 — faults in the basement. Folded systems: QLFS — Qilian, QFS — Qinling.
subdivided into the Shanxi and Xiashihezi Formations. The former consists mainly of gray-yellow basal bed of sandstone and finely layered siltstones and schists and contains two or three coal-bearing beds. The latter formation continues the first one and is composed of quartz sandstones with intercalates of siltstones and black shales but lacks coal-bearing beds. The Upper Permian strata include the Shangshihezi and Shiqianfeng Formations, which continue the Lower Permian strata. Both formations are an association of fluvial red-colored sandstones with siltstone intercalates. The Shiqianfeng Formation contains two beds of red tuff. In the Early Triassic there was a gap in sedimentation. Middle and Upper Triassic deposits are widespread on the western margin of the Ordos Basin. The Middle Triassic Zhifang Formation is composed of rocks of fluviolacustrine deltaic facies, mainly gray sandstones with members of red-colored mudstones, and the Upper Triassic Yanchang Series, also belonging to deposits of fluviolacustrine facies, is formed by yellow sandstones and schists with thin coal beds in the upper part of the section. Jurassic deposits were mapped only on the northwestern margin of the basin. Lower Jurassic sediments are absent, but Middle and Upper Jurassic rocks are widespread. The Middle Jurassic unit is subdivided into the Yan’an and Zhiluo Formations. The former, discordantly overlying the Upper Triassic Yanchang Series, consists of sandstones, conglomerates, and schists with thin coal beds; the latter, continuing the section of the Yan’an Formation, is composed of sandstones, mudstones, and schists. The Upper Jurassic unit is formed
Li Rongxi and Li Youzhu / Russian Geology and Geophysics 49 (2008) 23–27
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Tectonic evolution
Fig. 2. Stratigraphic column of the western margin of the Ordos Basin. For explanations, see the text.
mainly by violet-red mudstones alternating with sandstones and siltstones. Lower Cretaceous sedimentary strata are ubiquitous on the western margin of the Ordos Basin. They are composed of layered continental red-colored conglomerates, sandstones with siltstone and mudstone intercalates, and fluviolacustrine deposits more than 1700 m in thickness. Upper Cretaceous deposits are lacking. Neogene deposits were recognized throughout the western margin of the basin. They occur as an association of red-colored clastics of fluvial and lacustrine facies. The Quaternary unit is formed by different types of deposits of different thicknesses. Along river valleys, alluvial deposits are more than 2000 m thick, and on uplifts they form thin beds. Most of the study area is covered with Quaternary loess.
The Proterozoic (Fig. 3, A). The N-S-striking tectonic system, which originated before the Middle Proterozoic, determined the basis of not only the Ordos Basin but also the entire western margin of the North Chinese Platform (Huang Jiqing et al., 1996). The Qilian sea and a pericontinental margin formed on the western periphery of the Ordos block. Within the margin, a complex of marine inequigranular clastic sediments and basic volcanics accumulated. Later on, Proterozoic deposits were subjected to regional metamorphism, accompanied by the intrusion of biotite granites, which led to the cratonization and uplifting of the regional basement. In the Middle Proterozoic (Changcheng time), the Ordos block again subsided. The Qilian sea transgressed from south to north, thus leading to the accumulation of intracratonic carbonate rocks with clastic intercalates throughout the block. In the late Middle Proterozoic, the region underwent tectonic movements and, as a result, uplifted and transformed into land. Later tectonic movements transformed the western margin of the Ordos block into a narrow N-S-striking trench zone, where glacial deposits then accumulated. The Paleozoic (Fig. 3, B, C). In the Early Cambrian, the Qinling-Qilian sea basin, which was localized at the place of the modern Qinling and Qilian folded systems, transgressed to the north and east across the Ordos block. This led to its considerable expansion and, finally, merging with the North Chinese sea basin in the Middle Cambrian. In the Early Paleozoic (from Early Cambrian to Early Ordovician), carbonate deposits of open platform facies accumulated on a very vast area on the western margin of the North Chinese plate. In the Middle Ordovician, the subduction of the bottom of the Qinling-Qilian basin resulted in the termination of marine sedimentation in the Ordos block area. At the end of this period, Caledonian tectonic movement led to the rise of the block and its transformation into land. Its western margin underwent long (about 150 Myr, up to the late Middle Carboniferous) denudation. In the Carboniferous, the QinlingQilian basin again overlapped the Ordos block, transgressing into it from the south. There was a long gap in sedimentation between the Middle Ordovician and the Middle Carboniferous, during which the strata of these ages came into a structurally discordant contact. In this period, the entire western margin of the Ordos block was subjected to intense faulting and volcanism. In the Late Carboniferous, the Qinling-Qilian sea again merged with the North Chinese basin. In this time, carboniferous strata accumulated in the tide-ebb zone and lagoons along the periphery of the North Chinese plate with warm humid climate. In the Early Permian, the sea regressed, and, as a result, carboniferous strata of the Shanxi Formation migrated throughout the region. In the Late Permian, the climate again became hot and humid, which favored the formation of variegated clastic rocks of continental facies. In the Permian, sedimentation and N-S-striking synsedimentary faults were developed on the western margin of the block.
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Fig. 3. Tectonic evolution of the western margin of the Ordos Basin.
This geodynamic setting evidences that a N-S-striking tectonic system gradually formed west of the Ordos block. The Mesozoic (Fig. 3, D). In the early Mesozoic, the Tibet-Qinghai lithospheric plate collided with the Asian-European plate, which resulted in orogeny and the uplifting of the Qinling-Qilian folded system in the Triassic. Nappe-folded systems with abundant N-S-striking thrusts formed on the western margin of the Ordos block. The block rapidly subsided, thus leading to the rapid accumulation of fluviolacustrine clastic rocks in arid climate. The Ordos Basin formed on the nappe-folded basement at the same time. By the end of the Triassic, Indo-Sinian tectonic movement gave rise to new uplifts and led to fold and thrust formation on the western margin of the basin and discordance between Triassic and Jurassic deposits. In the Middle Jurassic, the climate became warmer and more humid, which favored the formation of carboniferous strata in fluviolacustrine facies. The Yanshan tectonic movements resulted in large S-shaped thrusts in N-S-striking nappe-folded systems on the western margin of the Ordos Basin. The basin was partly uplifted and dried. In the Late Jurassic, sedimentation took place only in the north of its central part owing to the second pulse of the Yanshan tectogenesis. In the late Jurassic, the continuing tectonic movements led to the uplifting of the entire northern part of the Ordos block and downwarping of the southern margin of the basin. In the Cretaceous, red-colored fluviolacustrine clastic sediments more than 1000 m in thickness accumulated here. In the Middle Cretaceous, the thrust zone was intensely
developed, leading to the uplifting of the entire region, which was accompanied by fold and fault formation in the Lower Cretaceous deposits (Sun Shu et al., 1989; Zhang Hong et al., 2000). The Late Cretaceous was the epoch of uplifting and denudation. The Cenozoic (Fig. 3, E). The continuing pulse Himalayan tectonic movements in the Cenozoic led to the uplifting of the western margin of the Ordos Basin. The N-S-striking large fold and thrusts that formed during the Yanshan tectonic activity in the region became more complicated. This might be due to the collision of the Indian and Tibet-Qinghai lithospheric plates. Many faults that formed in this time are thrusts, whose shift surfaces are mainly tilted to the southwest, with their dip becoming more gentle with depth. Modern seismic activity on the western margin of the basin evidences that tectonic processes are still active there. Formation of natural-gas reservoirs in the Ordos Basin might have been related to the Himalayan tectonic movements (Yang Hua et al., 2005).
Conclusions The Ordos Basin, the largest petroleum basin in China, lies in the north of Central China. Its modern western margin is a series of N-S-striking subparallel folds, thrusts, narrow depressions, and uplifts. The N-S-striking narrow downwarping zone on the western margin of the Ordos tectonic block formed as early as the Late
Li Rongxi and Li Youzhu / Russian Geology and Geophysics 49 (2008) 23–27
Proterozoic. Carbonate sedimentary strata of platform facies accumulated throughout the block from Early Cambrian throughout Middle Ordovician as a result of periodical transgressions of the Qinling-Qilian sea to the north along the narrow downwarped zone. From Middle Ordovician to Middle Carboniferous, the western margin of the block was a land uplifted by Caledonian tectonic movements and was subjected to a long (about 150 Myr) denudation. In the Late Carboniferous and Early Permian, carboniferous strata accumulated within the basin. The Ordos Basin formed on the basement of the Ordos block in the Triassic, when the Tethys ocean became closed and the Tibet-Qinghai lithospheric plate collided with the Asian-European one, thus leading to the formation and uplifting of the Qinling and Qilian folded systems. The Indo-Sinian tectonic movements caused the rise of the western margin of the basin and the formation of folds and faults on it in the late Triassic. In the Mesozoic, the western margin of the basin was subjected to intense thrust formation. In the Middle Jurassic, the Yanshan tectonic movements resulted in large N-S-striking S-shaped thrusts there. The Himalayan tectonic movements, resulted from the collision of the Indian and Tibet-Qinghai lithospheric plates, led to the complication of folds and faults on the western margin. Many faults that formed in the Cenozoic are large thrusts, whose shift surface dips become more gentle with depth. We thank our colleagues from the Northwestern University for discussion of the studied problem and support of field works. Special gratitude is to Professor Zh.G. Zholtaev, Kazakhstan Satpaev National Technical University, and Professor S.A. Khusainov, Kazakhstan Al’-Farabi National Uni-
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versity, who participated in the research carried out on the western margin of the Ordos Basin. We thank Professor A.K. Basharin and Doctor S.Yu. Belyaev, Trofimuk Institute of Petroleum Geology and Geophysics of SB RAS, for help in preparing the manuscript for publication. This work was supported by grant 973 (No. 2003SV214606) from the Chinese State Basic Research Program.
References An Zuoxiang, Ma Ji, 2002. On separation of North China and gas bearing of Upper Paleozoic of Ordos Basin. Xinjiang Petroleum Geology 23 (4), 279–282. Dai Jinxing, 1995. Formation of the Central-Asian coal-formed gas accumulation domain and its source rocks — series study on the Central-Asian coal-formed gas accumulation domain. Petroleum Exploration and Development 22 (3), 1–6. Feng Fukai, Zhang Yafei, 1995. Natural Gas Geology of China [in Chinese, with English abstract]. Geological Publishing House of China, Beijing. Huang Jiqing, Ren Jishun, Jiang Chunfa, 1996. The Geotectonic Evolution of China. Science Press, Beijing. Li Jingming, Wei Guoqi, Li Dongxu, 2004. Natural gas exploration prospect in superposition basins, Western China. Natural Gas Geoscience 15 (1), 1–6. Li Songshen, 1985. Yanshan movement in North China. Geosciences 6 (4), 20–33. Sun Shu, Li Jilinag, Chen Haihong, 1989. Mesozoic and Cenozoic sedimentary history of South China. AAPG Bulletin 73 (10), 1247–1269. Xia Xinyu, Zhao Lin, 1998. Gas sources and mixing ratio of Ordovician weathering crust reservoir of central gasfield in Ordos basin. ACTA Sedimentologica Sinica 16 (3), 75–79. Yang Hua, Fu Jinhua, Wei Xinshan, 2005. Characteristic of natural gas reservoir formation in Ordos Basin. Natural Gas Industry 25 (4), 5–8. Zhang Hong, Meng Zhaoping, He Zonglan, 2000. Study on the tectonic stress fields in the Ordos Basin. J. Chinese Coal Science 25, 2–5.
Tectonic evolution of the western margin of the Ordos Basin (Central China) Li Rongxi a, b, *, Li Youzhu a, b a
Key Laboratory of West Mineral Resource and Geology Engineering, Education Ministry of China, Chang’an University, 126 Yanta Road, Xian, 710054, China b School of Earth Science and Resource, Chang’an University,126 Yanta Road, Xian, 710054, China Received 20 January 2006
Abstract The western half of the Ordos Basin lies in the north of Central China, on the western margin of the North Chinese Platform. In the southwest and south it borders the Qilian and Qinling folded systems. Their frontal northeastern flank is thrust over the Paleozoic nappe of the Ordos Basin and consists of a series of large parallel linear folds separated by thrusts. The basement of the basin is made up of deformed metamorphic rocks of tentatively Late Proterozoic age. Carbonate and terrigenous intracratonic facies accumulated on the Ordos block from Early Cambrian through Middle Ordovician. From Middle Ordovician through Middle Carboniferous, the western edge of the block was uplifted as a land as a result of Caledonian tectonic movement and underwent denudation for about 150 Myr. In the Late Paleozoic, a series of coal-bearing formations was deposited. In the Triassic, the Ordos Basin became filled with fluviolacustrine delta facies. Thrusts and folds on the southwestern margin of the basin formed in the Mesozoic and were intensified by Himalayan tectonic movements. © 2008, IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: Tectonic evolution; petroleum potential; stratigraphy; Ordos Basin; China
Introduction The large petroliferous Ordos Basin lies in the center of North China, on the western margin of the North Chinese Platform (Fig. 1). Gas fields in the north of the basin are the largest source of natural gas, which is transported to both western and eastern regions of the country. Natural gas supplied for municipal and commercial use to large cities (Beijing, Tianjin, Xian, Zhengzhou, Taiyuan, Yinchuan, etc.) is now mined in the Ordos Basin. Three petroleum complexes were discovered in the basin: (1) Yanchang Series (Upper Triassic) and Yan’an Formation (Middle Jurassic), which are commercially important oil-producing beds, with lacustrine pelite with δ13C = –46.04 to –49.46‰ being their oil source rocks (Feng Fukai and Zhang Yafei, 1995). (2) Carboniferous-Permian deposits abounding in parental carbonaceous rocks. Some small gas reservoirs were discovered in fluvial sandstones of the Upper Permian Shiqianfeng Formation on the western margin and in the north of the Ordos Basin. Gas occurs in Carboniferous-Permian carbonaceous rocks and is
characterized by δ13C1 = –35.3 to –29.3‰ and δ13C2 = –27.4 to –22.3‰ (Xia Xinyu and Zhao Lin, 1998). (3) Ordovician thin crust of weathering containing the most commercially important gas pools of the basin. Dolomites of the Middle Ordovician Majiagou Formation are gas reservoirs here. They are characterized by secondary porosity and are overlain by pelites. Ordovician carbonates and Permian-Carboniferous carbonaceous strata are considered to be gas source rocks (Xia Xinyu and Zhao Lin, 1998). Oil and gas formation proceeded mainly from Late Jurassic through Cretaceous, and their accumulation, in the NeogeneEarly Quaternary (Dai Jinxing, 1995; Li Jingming et al., 2004). Uplifting of the Tibet-Qinghai plateau located south of the Ordos Basin and its movement northeastward followed by the formation of Qilian and Qinling folded systems resulted from the collision of the Indian and Asian-European Plates, which governed the evolution of the basin and generation and accumulation of oil and gas in it (An Zuoxiang and Ma Ji, 2002; Li Songshen, 1985). In this paper we will dwell only on the tectonic history of the western margin of the Ordos Basin.
* Corresponding author. E-mail address: [email protected] (Li Rongxi) 1068-7971/$ - see front matter D 2008, IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.rgg.2007.12.002
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Li Rongxi and Li Youzhu / Russian Geology and Geophysics 49 (2008) 23–27
Geologic structure The Ordos Basin is of intricate geologic structure. Its western margin borders the Qinling folded system in the south and the Qilian one in the southwest. In the northwest, the basin was separated from the Alashan block by a Cenozoic depression (Fig. 1). There is a series of parallel N-S-striking folds, thrusts, depressions, and uplifts on the western margin of the basin. Large-scale faults in the basement and on the western margin of the basin had a long evolution. The western periphery of the basin is part of the N-S-striking giant Asian structural zone separating West and East China. This separation is supported by a drastic change in geophysical fields, first of all, gravitational as well as stratigraphic and structural characteristics different for East and West China.
Stratigraphy The more than 5000 m thick Helan complex of metamorphic rocks of pre-Sinian age is the most ancient in the Ordos Basin (Fig. 2). Recognized in the inner part of the basin, it is composed of deep-seated thin-banded gneisses, quartz schists, and biotite granulites and is correlated with the Jining complex of the North Chinese Platform. On the western margin of the basin, Cambrian-Middle Ordovician sedimentary strata are formed by persistent layered carbonate deposits with marine fauna. The Cambrian carbonate rocks, more than 600 m thick, are mainly granular and pelitomorphic limestones with calcareous mud cement. Also, chert intercalates are present. The Lower Ordovician strata are composed of granular-pelitomorphic sediments with mud impurity as well as biogenic and dark-colored dolomitized pelitomorphic limestones. The Middle Ordovician complex of detrital deposits, quartz sandstones, and siltstones with schist and limestone intercalates is regarded as a flysch formation. The western margin of the Ordos Basin, like the entire western margin of the North Chinese Platform, lacks Upper Ordovician deposits. Silurian-Devonian deposits are also absent from the western margin of the basin, but they are widespread in its north, overlapping drastically inconsistent Middle Ordovician layered strata. Silurian rocks exposed on the northern margin of the basin are quartz-feldspathic sandstones with basal conglomerates in the base. Devonian sediments, mainly of fluviolacustrine facies, consist of red-colored conglomerates, sandstones, and siltstones. In the Middle and Late Carboniferous, there was a new stage of sedimentation on the western margin of the Ordos Basin. Middle Carboniferous deposits are mainly gray-colored calc-quartz sandstones and sandy limestones with intercalates of bioclastic limestone and thin carbon-bearing horizons. The Upper Carboniferous Taiyuan Formation is a continuous stratigraphic sequence of gray-colored quartz sandstones, finely layered limestones, and schists. The formation includes several coal-bearing beds. The Permian unit here concordantly overlies the Taiyuan Formation. The Lower Permian strata are
Fig. 1. Tectonic scheme of the western margin of the Ordos Basin. 1 — Ordos Basin, 2 — study area, 3 — folded systems, 4 — Cenozoic grabens, 5 — faults, 6 — thrusts, 7 — anticlines, 8 — faults in the basement. Folded systems: QLFS — Qilian, QFS — Qinling.
subdivided into the Shanxi and Xiashihezi Formations. The former consists mainly of gray-yellow basal bed of sandstone and finely layered siltstones and schists and contains two or three coal-bearing beds. The latter formation continues the first one and is composed of quartz sandstones with intercalates of siltstones and black shales but lacks coal-bearing beds. The Upper Permian strata include the Shangshihezi and Shiqianfeng Formations, which continue the Lower Permian strata. Both formations are an association of fluvial red-colored sandstones with siltstone intercalates. The Shiqianfeng Formation contains two beds of red tuff. In the Early Triassic there was a gap in sedimentation. Middle and Upper Triassic deposits are widespread on the western margin of the Ordos Basin. The Middle Triassic Zhifang Formation is composed of rocks of fluviolacustrine deltaic facies, mainly gray sandstones with members of red-colored mudstones, and the Upper Triassic Yanchang Series, also belonging to deposits of fluviolacustrine facies, is formed by yellow sandstones and schists with thin coal beds in the upper part of the section. Jurassic deposits were mapped only on the northwestern margin of the basin. Lower Jurassic sediments are absent, but Middle and Upper Jurassic rocks are widespread. The Middle Jurassic unit is subdivided into the Yan’an and Zhiluo Formations. The former, discordantly overlying the Upper Triassic Yanchang Series, consists of sandstones, conglomerates, and schists with thin coal beds; the latter, continuing the section of the Yan’an Formation, is composed of sandstones, mudstones, and schists. The Upper Jurassic unit is formed
Li Rongxi and Li Youzhu / Russian Geology and Geophysics 49 (2008) 23–27
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Tectonic evolution
Fig. 2. Stratigraphic column of the western margin of the Ordos Basin. For explanations, see the text.
mainly by violet-red mudstones alternating with sandstones and siltstones. Lower Cretaceous sedimentary strata are ubiquitous on the western margin of the Ordos Basin. They are composed of layered continental red-colored conglomerates, sandstones with siltstone and mudstone intercalates, and fluviolacustrine deposits more than 1700 m in thickness. Upper Cretaceous deposits are lacking. Neogene deposits were recognized throughout the western margin of the basin. They occur as an association of red-colored clastics of fluvial and lacustrine facies. The Quaternary unit is formed by different types of deposits of different thicknesses. Along river valleys, alluvial deposits are more than 2000 m thick, and on uplifts they form thin beds. Most of the study area is covered with Quaternary loess.
The Proterozoic (Fig. 3, A). The N-S-striking tectonic system, which originated before the Middle Proterozoic, determined the basis of not only the Ordos Basin but also the entire western margin of the North Chinese Platform (Huang Jiqing et al., 1996). The Qilian sea and a pericontinental margin formed on the western periphery of the Ordos block. Within the margin, a complex of marine inequigranular clastic sediments and basic volcanics accumulated. Later on, Proterozoic deposits were subjected to regional metamorphism, accompanied by the intrusion of biotite granites, which led to the cratonization and uplifting of the regional basement. In the Middle Proterozoic (Changcheng time), the Ordos block again subsided. The Qilian sea transgressed from south to north, thus leading to the accumulation of intracratonic carbonate rocks with clastic intercalates throughout the block. In the late Middle Proterozoic, the region underwent tectonic movements and, as a result, uplifted and transformed into land. Later tectonic movements transformed the western margin of the Ordos block into a narrow N-S-striking trench zone, where glacial deposits then accumulated. The Paleozoic (Fig. 3, B, C). In the Early Cambrian, the Qinling-Qilian sea basin, which was localized at the place of the modern Qinling and Qilian folded systems, transgressed to the north and east across the Ordos block. This led to its considerable expansion and, finally, merging with the North Chinese sea basin in the Middle Cambrian. In the Early Paleozoic (from Early Cambrian to Early Ordovician), carbonate deposits of open platform facies accumulated on a very vast area on the western margin of the North Chinese plate. In the Middle Ordovician, the subduction of the bottom of the Qinling-Qilian basin resulted in the termination of marine sedimentation in the Ordos block area. At the end of this period, Caledonian tectonic movement led to the rise of the block and its transformation into land. Its western margin underwent long (about 150 Myr, up to the late Middle Carboniferous) denudation. In the Carboniferous, the QinlingQilian basin again overlapped the Ordos block, transgressing into it from the south. There was a long gap in sedimentation between the Middle Ordovician and the Middle Carboniferous, during which the strata of these ages came into a structurally discordant contact. In this period, the entire western margin of the Ordos block was subjected to intense faulting and volcanism. In the Late Carboniferous, the Qinling-Qilian sea again merged with the North Chinese basin. In this time, carboniferous strata accumulated in the tide-ebb zone and lagoons along the periphery of the North Chinese plate with warm humid climate. In the Early Permian, the sea regressed, and, as a result, carboniferous strata of the Shanxi Formation migrated throughout the region. In the Late Permian, the climate again became hot and humid, which favored the formation of variegated clastic rocks of continental facies. In the Permian, sedimentation and N-S-striking synsedimentary faults were developed on the western margin of the block.
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Fig. 3. Tectonic evolution of the western margin of the Ordos Basin.
This geodynamic setting evidences that a N-S-striking tectonic system gradually formed west of the Ordos block. The Mesozoic (Fig. 3, D). In the early Mesozoic, the Tibet-Qinghai lithospheric plate collided with the Asian-European plate, which resulted in orogeny and the uplifting of the Qinling-Qilian folded system in the Triassic. Nappe-folded systems with abundant N-S-striking thrusts formed on the western margin of the Ordos block. The block rapidly subsided, thus leading to the rapid accumulation of fluviolacustrine clastic rocks in arid climate. The Ordos Basin formed on the nappe-folded basement at the same time. By the end of the Triassic, Indo-Sinian tectonic movement gave rise to new uplifts and led to fold and thrust formation on the western margin of the basin and discordance between Triassic and Jurassic deposits. In the Middle Jurassic, the climate became warmer and more humid, which favored the formation of carboniferous strata in fluviolacustrine facies. The Yanshan tectonic movements resulted in large S-shaped thrusts in N-S-striking nappe-folded systems on the western margin of the Ordos Basin. The basin was partly uplifted and dried. In the Late Jurassic, sedimentation took place only in the north of its central part owing to the second pulse of the Yanshan tectogenesis. In the late Jurassic, the continuing tectonic movements led to the uplifting of the entire northern part of the Ordos block and downwarping of the southern margin of the basin. In the Cretaceous, red-colored fluviolacustrine clastic sediments more than 1000 m in thickness accumulated here. In the Middle Cretaceous, the thrust zone was intensely
developed, leading to the uplifting of the entire region, which was accompanied by fold and fault formation in the Lower Cretaceous deposits (Sun Shu et al., 1989; Zhang Hong et al., 2000). The Late Cretaceous was the epoch of uplifting and denudation. The Cenozoic (Fig. 3, E). The continuing pulse Himalayan tectonic movements in the Cenozoic led to the uplifting of the western margin of the Ordos Basin. The N-S-striking large fold and thrusts that formed during the Yanshan tectonic activity in the region became more complicated. This might be due to the collision of the Indian and Tibet-Qinghai lithospheric plates. Many faults that formed in this time are thrusts, whose shift surfaces are mainly tilted to the southwest, with their dip becoming more gentle with depth. Modern seismic activity on the western margin of the basin evidences that tectonic processes are still active there. Formation of natural-gas reservoirs in the Ordos Basin might have been related to the Himalayan tectonic movements (Yang Hua et al., 2005).
Conclusions The Ordos Basin, the largest petroleum basin in China, lies in the north of Central China. Its modern western margin is a series of N-S-striking subparallel folds, thrusts, narrow depressions, and uplifts. The N-S-striking narrow downwarping zone on the western margin of the Ordos tectonic block formed as early as the Late
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Proterozoic. Carbonate sedimentary strata of platform facies accumulated throughout the block from Early Cambrian throughout Middle Ordovician as a result of periodical transgressions of the Qinling-Qilian sea to the north along the narrow downwarped zone. From Middle Ordovician to Middle Carboniferous, the western margin of the block was a land uplifted by Caledonian tectonic movements and was subjected to a long (about 150 Myr) denudation. In the Late Carboniferous and Early Permian, carboniferous strata accumulated within the basin. The Ordos Basin formed on the basement of the Ordos block in the Triassic, when the Tethys ocean became closed and the Tibet-Qinghai lithospheric plate collided with the Asian-European one, thus leading to the formation and uplifting of the Qinling and Qilian folded systems. The Indo-Sinian tectonic movements caused the rise of the western margin of the basin and the formation of folds and faults on it in the late Triassic. In the Mesozoic, the western margin of the basin was subjected to intense thrust formation. In the Middle Jurassic, the Yanshan tectonic movements resulted in large N-S-striking S-shaped thrusts there. The Himalayan tectonic movements, resulted from the collision of the Indian and Tibet-Qinghai lithospheric plates, led to the complication of folds and faults on the western margin. Many faults that formed in the Cenozoic are large thrusts, whose shift surface dips become more gentle with depth. We thank our colleagues from the Northwestern University for discussion of the studied problem and support of field works. Special gratitude is to Professor Zh.G. Zholtaev, Kazakhstan Satpaev National Technical University, and Professor S.A. Khusainov, Kazakhstan Al’-Farabi National Uni-
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versity, who participated in the research carried out on the western margin of the Ordos Basin. We thank Professor A.K. Basharin and Doctor S.Yu. Belyaev, Trofimuk Institute of Petroleum Geology and Geophysics of SB RAS, for help in preparing the manuscript for publication. This work was supported by grant 973 (No. 2003SV214606) from the Chinese State Basic Research Program.
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