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The Sedimentary Evolution from Sinian to Cretaceous in Yangtze Gorges Area, China
Gondionnn Resmruh, V. 2, No. 4, p p . 61 7-621. 01999 Intc~rrzntionnlAssocintion for Gondzonnn Resenrcli, Jnpnn
GR
ISSN: 1342-93713
Goizdwaiza Research
The Sedimentary Evolution from Sinian to Cretaceous in Yangtze Gorges Area, China Mao Xiaodong and Wang Xiaofeng Yiclzang lnstitiite of Geology and Mineral Resources, CAGS, P.O. Box 502, Yichang, Hiibei, 443003, Chimi
General Geology of Yangtze Gorges Area Yangtze Gorges region is situated along the Yangtze River starting at Feiigjie County of east Chongqin City and ending at Nanjingguan of Yichang City of west Hubei Province with a total length of about 190km, which includes Qutang gorge, Wu gorge, Xiling gorge and is therefore sometimes called ”three gorges”. The stratigraphic sequence from the Sinian to Cretaceous is well recorded with a great deal of animal and plant fossils. Therefore, national and international interest was focused on this area since the pioneer works of Grabau (1922) and Lee and Chao (1924).The item ”Sinian” was named in the Gorges area to represent the strata overlain by Cambrian. It is equated to the Sturtian and Vendian period between 800 to 540 million years (Ma et al., 1984), which has now been accepted throughout China and all over the world.
formation of the Xianniushan fault is considered to be apparently after the Cretaceous, and in the Tertiary. For the Tianyangpin-Jianlifault, its development is regarded to have Formation
1
Shamao Formation
-.............. _ - _
149
--__ T..: .=.T..623 -_- _- __ _
Longmaxi Formation
-__
_ _ _ _ 5.6~
Linxiang Formation Baota Formation Miaopo Formation
-.............. -__
665
77 -I
Luoreping Formation
Wufeng Foumation
Rocks
Section
.................... ..............
’
’1 ’/
‘I
/
‘
/’I
‘
I
7 -
.’
Guniutan Formation
/
’ I
‘5 12
1
shale sandstone, siltstone, siltstone and shale with minor limestone ac s ale intercalated :itfsitstone with lots of priptoliles mudrtone with lots of maptdites and hydromica rmicrlte limestone micrite limestone
2-3
rddstone Y rormca with
23
micrite limestone
99
Idolomitc, limestone
1
Fault System The area is located in the central south part of the Yangtze Platform of the South China block since the Late Proterozoic Jinningian Stage. Its sub-tectonic units are described as Zigui synclinorium, the Huangling dome and the Changyang fold system. The principal faults of the Gorges area are distributed along two directions: northwest and southeast, containing the Wuduhe fault (F,), the Xinghua fault (FJ, the Xianniushan fault (FJ, the Tianyangping-Jianli fault (FJ, and the Qiyaoshan fault (F,) (see Fig.1 of Wang et al., this issue). The Huangling granite batholith and mafic-ultramafic magmatic body in Yangtze Gorges area of the Yangtze Platform are distributed along the Wuduhe fault. It could indicate that this fault had a powerful influence during preSinian time; the compounded phenomena is notable during the Mesozoic to Cenozoic because of its influence on its caprock succession. Carboniferous, Permian and Triassic obliquely striking beneath the Cretaceous could be observed at Huangko arid Xiayangquan, and therefore the time of
Xinping Formation
ianzhushan Mem.1
’
II
I
p
y
,, ,
1
’
I0.7-51dolomite with P, Si
II
I
--____ - Datangpo - _ _ Formation __ Gucheng Formation
Pre-Sinian
Fig. 1. The Sinian-Early Paleozoic stratigraphic section in Yangtze Gorges area.
618
undergone a long history. The most important aspect is that this fault forms the boundary, which distinguishes the two distinct types, the Yangtze Platform type and the Jiangnan (southernarea of the River) transitional type. The Qiyaoshan fault was probably a strong active one during the Mesozoic to Cenozoic above the supra-crustal level of the basement, which was possibly the result of the high magnitude stressfield in the tectonically active region of the Pacific Ocean.
Sedimentary S tratigraphy Sedimentary stratigraphic units are well exposed systematically and continuously in the area. The record starts from Sinian, completely includes Early Paleozoic (Cambrian, Ordovician, Silurian), the Late Paleozoic (Devonian,Carboniferous, Permian), and also the Mesozoic (Triassic, Jurassic, Cretaceous). This series of sedimentary sequence reflects the nucleation, development and final disappearance of the central south part of the Yang tze basin after the formation of Yangtze paleo-continental crust. A great deal of scientific work has been achieved, spread
Fig. 2. The Late Paleozoic-Cenozoic stratigraphic section in Yangtze Gorges area.
through many publications of well-known geologists (such as Wang Y., 1938; Wang X.F., et al., 1983; Ma et al., 1989, among others). Here, a brief synthesized section is presented in Fig. 1and Fig. 2.
The Sedimentary Evolution of the Gorges Area Through the study of the sedimentary rocks and structures, section sequence, as well as the correlation of these sections, a three-stage sedimentary evolution can be recognized in Yangtze Gorges area as summarised below.
Sinian to Silurian Sinian is the first caprock deposit after the consolidation of the continental crust during the Jingling Movement, which is the most important conversion period in the organism and tectonic evolutionary history in this area (even in south China).At the beginning of the Sinian times, the eastern region of the Gorges (near Yichang area) was above sea level, and the environment was only of river and gulf, indicated by the Early Sinian Liantzio Formation of feldspar-quartz sandstone and minor volcanic clastic materials. The west part of the Gorges area was supposed tobe the glacial near-shore deposit of sandstone with minor volcanic tuff materials. In the north, nearby Shenglongjia area of the Yangtze Gorges, the middle Hubei uplift had already existed above the sea level (Liu and Xiu, 1994).The following period is the well-known lower latitude continental glacial or glacial marine tillite named as Nantuo and Gucheng Formation which is very widely distributed in South China such as in Hubei, Hunan, and JiangxiProvince. Meanwhile, the Datangpo Formation was deposited as integrate deposit of the glacial event. The tillite was first studied by Professor Lee J. S. (Lee and Chao, 1924). With the remelting of the early Sinian glacier, and the weather and temperature raised, seawater transgressed and covered the whole Gorges area (even most of the south China). The seawater became further more deep since early Late Sinian (except in upper Later Sinian) to Middle Cambrian. The tectonic setting here was regarded as Yangtze epicontinental alternated basin with the sediments of clastic siltstones, black shales, limestones and dolostones containing a lot of chert band and nodules, manganese and phosphorite. The original life forms which thrived in this period, such as Miaohe biota (Zhu and Chen, 1984; Steiner, 1994)have been found in the Upper Doushantuo Formation of Sinian, which included Enteromophites Zhu et Chen genmov., Enteromophites siniansis Zhu et Chen sp.nov., Chuaria sp., Beltanellformis brunsae, Longifuniculum dissoltittim,Glomulus filamenturn n.gen.et sp.,Cucullusfvaudiilentus n. gen. et sp., Liulingjitaenia alloplecta, Gesinella gracitis, Miaohephyton Gondwann Research, V.2, No. 4,1999
619
bifiircatiim, Dozishantriophyton lineare, Konglingiphyton erecta, Sinospoongia tubulata nsp. The earliest trilobites have been found in the ShzieijingttioFormation black shale of lower most Cambrian, such as Tszinyidisczis brevicus, T.sanxiaensis, T.xiadongensis, T.yicliangensis,T.zigiiiensis,Sinodisciis sirnilis, S.clmngyangensis, Warrgzishia wangzishiensis,W.(Wnnganlentis) sp. etc.. Also some significant small shelly fossils from different phyla, such as Hyolitlzellus, monoplacophorans, gastropods and chancellorids, have been found in the member of Tiamhushan of the upper most Sinianjust under the Cambrian. As the Cambrian started, the so-called ”big explosion in evolution of life forms” occurred in the Gorges area, as in many places all over the world. Consequently, the environment was changed in the Gorges area from restricted platform to the open carbonate platform and, a series of limestones, dolomites, shale and siltstoneswere deposited with a total thickness of more than 2000 meters from Late Cambrian to Silurian (Fig.3).As the carbonate platform deposit of Late Sinian (Denying Formation) began, the sedimentary environment changed frequently. The carbonate ramp of early Cambrian, carbonate restricted platform of middle to late Cambrian, and open carbonate platform of Ordovician were the dominant facies during the interval of Late Sinian to Ordovician. Because of the sea level change, the sub-facies included some tide-flat or coastline and shallow shelf deposit into where the continental clasts were transported. Several layers of black shale also exist (such as Miaopo and Wzifeng Formation of Ordovician) which have been considered as restricted platform deposit with plenty of fossils. The Guangxi movement {equivalent to the Caledonian movement) affected the Gorges region resulting in the compaction, folding and orogeny of the sediments from Sinian to Silurian approximately at 410 Ma (Yang and Yang, 1982).Thus, the primitive shape of the Huangling dome was defined. After the Guangxi movement, the whole Gorges area was raised. This resulted in the weathering and denudation of the Upper Silurian sediments and the continental sources
Northeast Sichuan
West
West
Hubei
Hunan
North Guangxi
Cambrian-Silurian deposit
/
Fig. 3. Schematic deposit profile from Sinian to Silurian in Yangtze palaeo-continental edge.
Gondzvana Research, V. 2, No. 4,1999
supplying sediments into west Hubei and Hman marginal basin. At that time the transgression from the South China palaeo-sea did not approach the Gorges area since the early Devonian to early Middle Devonian, but, the transgressing sea water reached only the Yichang area of north Hunan Province (near lOOkm northwest Changsha). Hence, the Upper Silurian sediment has worn away and was transported into the South China basin. So the Upper Silurian sediment could not be seen. Also, the lower to lower middle Devonian sediments did not exist; the disconformable contact between lower Silurian and upper Middle Devonian is quite clearly documented in the Yangtze Gorges area.
Late Middle Devonian to Triassic Marine transgression occurred in the Gorges area during late Middle Devonian from east and south, therefore, the very pure sedimentaryquartzite of upper Middle Devonian, the so-called Yiintaigzian Formation, was deposited, which correlates the sediment to open coastline deposit. But its environment changed into restricted lagoon-tide or near shore clastic sediments during the Late Devonian to Early Carboniferous. It formed red color fine-grained sandstone, shale or dark-gray carbonate rocks, intercalated with some oolitic iron deposits in the Hziangjiaden Formation of Upper Devonian, and a minor thin-bedded coal layer in Lower Carboniferous. The thickness of the sediment is about a hundred meters. During the interval from Late Carboniferous to Early Triassic, most of the sediments became carbonate of open platform or lagoon-tide intercalated with a little shale, which has a tremendous thickness. The carbonate ramp existed in the Early Permian, which was probably carbonate deposits without any high energy shallow beach or bio-reef deposit generally with a great number of chert concretions or chert bands. Geographically, these carbonate rocks literally constructed the Qutang gorge and the Wu gorge. Nevertheless, the last gorge called Xiling gorge was constructed by the carbonate rocks of Sinian to Ordovician. Apparently, the disconformable contact between Upper Permian and Lower Triassic in several places of the Gorges area could be observed, in which the boundary clay is easy to be checked out. This biostratigraphicboundary (Wanget al., 1996) coincides basically with the border of the lithostratigraphic units. The boundary is situated just between the thin-bedded micrite of the Lower Triassic Daye Formation and the underlying massive micrite containing siliceous concretions of uppermost Permian Changxing Formation or the black chert and cherty shale of the Dalong Formation. Of particular importance is the large virtual biological gap of the Permian-Triassic boundary representing an
620
important event of mass extinction between the Paleozoic and Mesozoic in the Gorges area. Unfortunately,no isotopic ages are available for the P-T boundary in Yangtze gorges area. But the analysis of 40Ar/39Ar data from the plagioclase of tuffs (GS-1)sampled in P-Tboundary of Yangtze Platform in Shangsi of North Sichuan Province (not far from the Gorges area) in southwest China have yielded a combined weighted mean plateaux date of ca. 250.04f0.36Ma (Renne et al., 1995) for the P-T boundary (internal error) with an external error of k1.13 Ma. This age could be considered as strong proof to indicate the accurate mass extinction time, as well as the time of the Dongwu movement (approximately 230+10 Ma) (Yang and Yang, 1982)in South China, including Yangtze Gorges area, which is equivalent to the time of the Hercynian orogeny. The red color clastic rocks of Upper Triassic intercalated with several layers of coal have abundance of non-marine biota represented by Pleitromeria and Annalepis (Meng et al., 1995). Its sedimentary environment is thus suggestive of the continental flood plain and lake delta to back shore lake (Liu and Xiu, 1994).Therefore, the depositional environment of the late Triassic is the last regression stage since the early Triassic transgression. The setting has already changed into continental environment in the Gorges area. In fact, at ca. 195+5Ma (Yang and Yang, 1982), the IndoChina plate movement had made the Yangtze Gorges area and even most of south China area compact and has caused the uplift of the continent above the sea level. The seawater retreated from this region, and never approached here again. The end of the Triassic deposition is the symbol of the closure of ocean deposit history in the Yangtze Gorges area. Thus, the important Indo-China plate movement had determined the coastline of South China as defined now.
Jurassic to Cretnceoits Since the Indo-China plate movement occurred in the late Triassic, the sediment in the Gorges area and most of the Central Sichuan Province was dominantly occupied by the inner continental lake and back shore coal-bearing clastic rocks. Early Jurassic sediments also kept this feature, for the tectonic activity was more powerful, and the sedimentary basin was raised further and overwhelmingly transformed to inner continental down-faulted basin with only river and lake sediments, represented by sandy and muddy materials. Because of the warm weather and abundance of plants, this stage was the most valuable incoalation time in Yangtze Gorges area and in Central to East Sichuan province. During the early Yanshanian movement (ca. 130 Ma) which occurred in the late Jurassic, the gorges area was uplifted, and mountain type folds were formed. The sedimentary basins were limited as small inner-continental downfaulted basins in Cretaceous.Nevertheless, Cretaceous
sediments were deposited only in the depression area of east edge of the Gorges area, such as Xiaotin area (just east of Yichang), Yichang and north Changyang area, which contains conglomerate, sandy conglomerate and mudstone with red color. The red conglomerate is unconformably overlain by the Ordovician carbonate in the entrance of the Xiling gorge and is best observed in the locality of Nanjingguan. Importantly, the tectonic movement of Yanshanian affected the Gorges area intensively in the late Cretaceous and caused the disappearance of the downfaulted basin. The sedimentation retreated towards east and south directions. Therefore, the Yanshanian movement is considered to reflect the end of the sedimentary basin evolution in the geological history of the Gorges region and, geographically, this movement also defined the geographic features of the Gorges area as seen today.
References Grabau, Y. (1922) The Sinian System, Bull. Geol. SOC.China, v. 1, pp. 1-4. Lee, J.S. and Chao, Y.T. (1924) Geology of Gorges district of Yangtze (Ichang to Tzekuei) with special reference to development of the Gorges, Bull. Geol. SOC.China, v. 3, pp. 351-391. Liu, B.J. and Xiu, X.S. (1994) Lithofacies and paleogeographic map of the late early Sinian, south China. Atlas of the lithofacies and paleogeography of south China. Science Press, Beijing, pp. 26-27. Ma, G.G., Li, H.Q., and Zhang, Z.C. (1984)An investigation of the age limits of the Sinian System in south China, Bull. Yichang Inst.Geol.Miner.Resour., v. 8, pp. 1-29 (in Chinese with English abstract). Ma,G.G.,Zhang,Z.C., Li, H.Q.,Chen, P. andHuang,Z.X. (1989) A geochronostratigraphical study of the Sinian system in Yangtze platform, Bull. Yichang Inst. Geol. Miner. Resour., v. 14, pp. 83-124 (in Chinese with English abstract). Meng, F.S., Xu, A.W. and Zhang, Z.L. (1995) Nonmarine biota and sedimentary facies of the Badong Formation in the Yangtze and its neighbouring area. China Univ. Geosci. Press, Wuhan, pp.62-64 (in Chinese with English abstract). Renne, P.R., Zhang, Z.C., Richard, M.A., Michael, T.B. and Asish, R.B. (1995) Synchrony and causal relations between Permian-Triassic boundary crises and Siberian flood volcanism, Science, v. 269, pp. 1413-1416. Steiner, M. (1994) Die neoproterozoischen Megaalgen Sudchnas, Berliner Geowissen. Abh., E 15, pp.1-146. Berlin (in German). Wang, X.F., Zhen, Q.L. and Zhou, T.M. (1983) The issue of biostratigraphy of the boundary between the Ordovician and Silurian of east Yangtze Gorges, China, Scientia Sinica, v.12, pp. 1123-1132 (in Chinese). Wang, X.F., Erdtmann, B.-D. and Mao, X.D. (1996) Geology of the Yangtze Gorges Area, Field Trip Guide for 30th International Geological Congress, T106/ T340, Geological Publishing House, Beijing, pp.16-18. Gondwana Research, V. 2, No. 4, 1999
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Wang Yu (1938)The time of the Yichang (Ichang) Limestone in east Yangtze Gorges, Hubei Province, Geological Review, pp.131-142 (in Chinese). Yang, S.N. and Yang, W.R. (1982) Regional Tectonics of China. Geol. Publ. House, Beijing, p. 60 & 99 (in Chinese).
Zhu, W.Q. and Chen, M.E. (1984) On the discovery of macrofossil algae from the Late Sinian in eastern Yangtze Gorges, south China. Acta Botanica Sinica, v. 26, pp. 558560 (in Chinese) .
Gondrvann Research, V: 2, No. 4, p p . 621-626. 01999 International Association for Gondiuann Resenrck, Japan.
GR
I S S N : 1342-937X
Goidwarin Research
Precambrian Geological Evolution in the Yangtze Gorges Area, China Wang Xiaofeng, Ma Daquan and Chen Xiaohong Yichang Institute of Geology and Mineral Resources, CAGS, PO Box 502, Yichang, Htibei 443003, China
Introduction The well-known Yangtze Gorges, including the Qitang Gorge, the Wu Gorge and the Xiling Gorge, begin at Baidingcheng, in eastern Sichuan and end at Nanjingguan in Yichang, western Hubei.
The Sinian and Palaeozoic strata, consisting - mainly of marine deposits, are well exposed in the eastern Yangtze Gorges area and are distributed "onion-skin-like" around the Huangling Arch (Fig.1). The pre-Sinian crystalline metamorphic rock series, exposed in the core
Fig.1. General geological map of the Yangtze Gorges area. 1.Tertiary-Quaternary 2. Cretaceous 3. Jurassic 4. Triassic 5. Devonian-Permian 6. Ordovician-Silurian 7. Cambrian 8. Sinian 9. Neoproterozoic-Mesoproterozoic 10. Palaeoproterozoic 11. Neoarchaean-Mesoarchaean 12.Dalaoling Suite 13.Huangling Suite 14. Granite 15. Fault.
GR
ISSN: 1342-93713
Goizdwaiza Research
The Sedimentary Evolution from Sinian to Cretaceous in Yangtze Gorges Area, China Mao Xiaodong and Wang Xiaofeng Yiclzang lnstitiite of Geology and Mineral Resources, CAGS, P.O. Box 502, Yichang, Hiibei, 443003, Chimi
General Geology of Yangtze Gorges Area Yangtze Gorges region is situated along the Yangtze River starting at Feiigjie County of east Chongqin City and ending at Nanjingguan of Yichang City of west Hubei Province with a total length of about 190km, which includes Qutang gorge, Wu gorge, Xiling gorge and is therefore sometimes called ”three gorges”. The stratigraphic sequence from the Sinian to Cretaceous is well recorded with a great deal of animal and plant fossils. Therefore, national and international interest was focused on this area since the pioneer works of Grabau (1922) and Lee and Chao (1924).The item ”Sinian” was named in the Gorges area to represent the strata overlain by Cambrian. It is equated to the Sturtian and Vendian period between 800 to 540 million years (Ma et al., 1984), which has now been accepted throughout China and all over the world.
formation of the Xianniushan fault is considered to be apparently after the Cretaceous, and in the Tertiary. For the Tianyangpin-Jianlifault, its development is regarded to have Formation
1
Shamao Formation
-.............. _ - _
149
--__ T..: .=.T..623 -_- _- __ _
Longmaxi Formation
-__
_ _ _ _ 5.6~
Linxiang Formation Baota Formation Miaopo Formation
-.............. -__
665
77 -I
Luoreping Formation
Wufeng Foumation
Rocks
Section
.................... ..............
’
’1 ’/
‘I
/
‘
/’I
‘
I
7 -
.’
Guniutan Formation
/
’ I
‘5 12
1
shale sandstone, siltstone, siltstone and shale with minor limestone ac s ale intercalated :itfsitstone with lots of priptoliles mudrtone with lots of maptdites and hydromica rmicrlte limestone micrite limestone
2-3
rddstone Y rormca with
23
micrite limestone
99
Idolomitc, limestone
1
Fault System The area is located in the central south part of the Yangtze Platform of the South China block since the Late Proterozoic Jinningian Stage. Its sub-tectonic units are described as Zigui synclinorium, the Huangling dome and the Changyang fold system. The principal faults of the Gorges area are distributed along two directions: northwest and southeast, containing the Wuduhe fault (F,), the Xinghua fault (FJ, the Xianniushan fault (FJ, the Tianyangping-Jianli fault (FJ, and the Qiyaoshan fault (F,) (see Fig.1 of Wang et al., this issue). The Huangling granite batholith and mafic-ultramafic magmatic body in Yangtze Gorges area of the Yangtze Platform are distributed along the Wuduhe fault. It could indicate that this fault had a powerful influence during preSinian time; the compounded phenomena is notable during the Mesozoic to Cenozoic because of its influence on its caprock succession. Carboniferous, Permian and Triassic obliquely striking beneath the Cretaceous could be observed at Huangko arid Xiayangquan, and therefore the time of
Xinping Formation
ianzhushan Mem.1
’
II
I
p
y
,, ,
1
’
I0.7-51dolomite with P, Si
II
I
--____ - Datangpo - _ _ Formation __ Gucheng Formation
Pre-Sinian
Fig. 1. The Sinian-Early Paleozoic stratigraphic section in Yangtze Gorges area.
618
undergone a long history. The most important aspect is that this fault forms the boundary, which distinguishes the two distinct types, the Yangtze Platform type and the Jiangnan (southernarea of the River) transitional type. The Qiyaoshan fault was probably a strong active one during the Mesozoic to Cenozoic above the supra-crustal level of the basement, which was possibly the result of the high magnitude stressfield in the tectonically active region of the Pacific Ocean.
Sedimentary S tratigraphy Sedimentary stratigraphic units are well exposed systematically and continuously in the area. The record starts from Sinian, completely includes Early Paleozoic (Cambrian, Ordovician, Silurian), the Late Paleozoic (Devonian,Carboniferous, Permian), and also the Mesozoic (Triassic, Jurassic, Cretaceous). This series of sedimentary sequence reflects the nucleation, development and final disappearance of the central south part of the Yang tze basin after the formation of Yangtze paleo-continental crust. A great deal of scientific work has been achieved, spread
Fig. 2. The Late Paleozoic-Cenozoic stratigraphic section in Yangtze Gorges area.
through many publications of well-known geologists (such as Wang Y., 1938; Wang X.F., et al., 1983; Ma et al., 1989, among others). Here, a brief synthesized section is presented in Fig. 1and Fig. 2.
The Sedimentary Evolution of the Gorges Area Through the study of the sedimentary rocks and structures, section sequence, as well as the correlation of these sections, a three-stage sedimentary evolution can be recognized in Yangtze Gorges area as summarised below.
Sinian to Silurian Sinian is the first caprock deposit after the consolidation of the continental crust during the Jingling Movement, which is the most important conversion period in the organism and tectonic evolutionary history in this area (even in south China).At the beginning of the Sinian times, the eastern region of the Gorges (near Yichang area) was above sea level, and the environment was only of river and gulf, indicated by the Early Sinian Liantzio Formation of feldspar-quartz sandstone and minor volcanic clastic materials. The west part of the Gorges area was supposed tobe the glacial near-shore deposit of sandstone with minor volcanic tuff materials. In the north, nearby Shenglongjia area of the Yangtze Gorges, the middle Hubei uplift had already existed above the sea level (Liu and Xiu, 1994).The following period is the well-known lower latitude continental glacial or glacial marine tillite named as Nantuo and Gucheng Formation which is very widely distributed in South China such as in Hubei, Hunan, and JiangxiProvince. Meanwhile, the Datangpo Formation was deposited as integrate deposit of the glacial event. The tillite was first studied by Professor Lee J. S. (Lee and Chao, 1924). With the remelting of the early Sinian glacier, and the weather and temperature raised, seawater transgressed and covered the whole Gorges area (even most of the south China). The seawater became further more deep since early Late Sinian (except in upper Later Sinian) to Middle Cambrian. The tectonic setting here was regarded as Yangtze epicontinental alternated basin with the sediments of clastic siltstones, black shales, limestones and dolostones containing a lot of chert band and nodules, manganese and phosphorite. The original life forms which thrived in this period, such as Miaohe biota (Zhu and Chen, 1984; Steiner, 1994)have been found in the Upper Doushantuo Formation of Sinian, which included Enteromophites Zhu et Chen genmov., Enteromophites siniansis Zhu et Chen sp.nov., Chuaria sp., Beltanellformis brunsae, Longifuniculum dissoltittim,Glomulus filamenturn n.gen.et sp.,Cucullusfvaudiilentus n. gen. et sp., Liulingjitaenia alloplecta, Gesinella gracitis, Miaohephyton Gondwann Research, V.2, No. 4,1999
619
bifiircatiim, Dozishantriophyton lineare, Konglingiphyton erecta, Sinospoongia tubulata nsp. The earliest trilobites have been found in the ShzieijingttioFormation black shale of lower most Cambrian, such as Tszinyidisczis brevicus, T.sanxiaensis, T.xiadongensis, T.yicliangensis,T.zigiiiensis,Sinodisciis sirnilis, S.clmngyangensis, Warrgzishia wangzishiensis,W.(Wnnganlentis) sp. etc.. Also some significant small shelly fossils from different phyla, such as Hyolitlzellus, monoplacophorans, gastropods and chancellorids, have been found in the member of Tiamhushan of the upper most Sinianjust under the Cambrian. As the Cambrian started, the so-called ”big explosion in evolution of life forms” occurred in the Gorges area, as in many places all over the world. Consequently, the environment was changed in the Gorges area from restricted platform to the open carbonate platform and, a series of limestones, dolomites, shale and siltstoneswere deposited with a total thickness of more than 2000 meters from Late Cambrian to Silurian (Fig.3).As the carbonate platform deposit of Late Sinian (Denying Formation) began, the sedimentary environment changed frequently. The carbonate ramp of early Cambrian, carbonate restricted platform of middle to late Cambrian, and open carbonate platform of Ordovician were the dominant facies during the interval of Late Sinian to Ordovician. Because of the sea level change, the sub-facies included some tide-flat or coastline and shallow shelf deposit into where the continental clasts were transported. Several layers of black shale also exist (such as Miaopo and Wzifeng Formation of Ordovician) which have been considered as restricted platform deposit with plenty of fossils. The Guangxi movement {equivalent to the Caledonian movement) affected the Gorges region resulting in the compaction, folding and orogeny of the sediments from Sinian to Silurian approximately at 410 Ma (Yang and Yang, 1982).Thus, the primitive shape of the Huangling dome was defined. After the Guangxi movement, the whole Gorges area was raised. This resulted in the weathering and denudation of the Upper Silurian sediments and the continental sources
Northeast Sichuan
West
West
Hubei
Hunan
North Guangxi
Cambrian-Silurian deposit
/
Fig. 3. Schematic deposit profile from Sinian to Silurian in Yangtze palaeo-continental edge.
Gondzvana Research, V. 2, No. 4,1999
supplying sediments into west Hubei and Hman marginal basin. At that time the transgression from the South China palaeo-sea did not approach the Gorges area since the early Devonian to early Middle Devonian, but, the transgressing sea water reached only the Yichang area of north Hunan Province (near lOOkm northwest Changsha). Hence, the Upper Silurian sediment has worn away and was transported into the South China basin. So the Upper Silurian sediment could not be seen. Also, the lower to lower middle Devonian sediments did not exist; the disconformable contact between lower Silurian and upper Middle Devonian is quite clearly documented in the Yangtze Gorges area.
Late Middle Devonian to Triassic Marine transgression occurred in the Gorges area during late Middle Devonian from east and south, therefore, the very pure sedimentaryquartzite of upper Middle Devonian, the so-called Yiintaigzian Formation, was deposited, which correlates the sediment to open coastline deposit. But its environment changed into restricted lagoon-tide or near shore clastic sediments during the Late Devonian to Early Carboniferous. It formed red color fine-grained sandstone, shale or dark-gray carbonate rocks, intercalated with some oolitic iron deposits in the Hziangjiaden Formation of Upper Devonian, and a minor thin-bedded coal layer in Lower Carboniferous. The thickness of the sediment is about a hundred meters. During the interval from Late Carboniferous to Early Triassic, most of the sediments became carbonate of open platform or lagoon-tide intercalated with a little shale, which has a tremendous thickness. The carbonate ramp existed in the Early Permian, which was probably carbonate deposits without any high energy shallow beach or bio-reef deposit generally with a great number of chert concretions or chert bands. Geographically, these carbonate rocks literally constructed the Qutang gorge and the Wu gorge. Nevertheless, the last gorge called Xiling gorge was constructed by the carbonate rocks of Sinian to Ordovician. Apparently, the disconformable contact between Upper Permian and Lower Triassic in several places of the Gorges area could be observed, in which the boundary clay is easy to be checked out. This biostratigraphicboundary (Wanget al., 1996) coincides basically with the border of the lithostratigraphic units. The boundary is situated just between the thin-bedded micrite of the Lower Triassic Daye Formation and the underlying massive micrite containing siliceous concretions of uppermost Permian Changxing Formation or the black chert and cherty shale of the Dalong Formation. Of particular importance is the large virtual biological gap of the Permian-Triassic boundary representing an
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important event of mass extinction between the Paleozoic and Mesozoic in the Gorges area. Unfortunately,no isotopic ages are available for the P-T boundary in Yangtze gorges area. But the analysis of 40Ar/39Ar data from the plagioclase of tuffs (GS-1)sampled in P-Tboundary of Yangtze Platform in Shangsi of North Sichuan Province (not far from the Gorges area) in southwest China have yielded a combined weighted mean plateaux date of ca. 250.04f0.36Ma (Renne et al., 1995) for the P-T boundary (internal error) with an external error of k1.13 Ma. This age could be considered as strong proof to indicate the accurate mass extinction time, as well as the time of the Dongwu movement (approximately 230+10 Ma) (Yang and Yang, 1982)in South China, including Yangtze Gorges area, which is equivalent to the time of the Hercynian orogeny. The red color clastic rocks of Upper Triassic intercalated with several layers of coal have abundance of non-marine biota represented by Pleitromeria and Annalepis (Meng et al., 1995). Its sedimentary environment is thus suggestive of the continental flood plain and lake delta to back shore lake (Liu and Xiu, 1994).Therefore, the depositional environment of the late Triassic is the last regression stage since the early Triassic transgression. The setting has already changed into continental environment in the Gorges area. In fact, at ca. 195+5Ma (Yang and Yang, 1982), the IndoChina plate movement had made the Yangtze Gorges area and even most of south China area compact and has caused the uplift of the continent above the sea level. The seawater retreated from this region, and never approached here again. The end of the Triassic deposition is the symbol of the closure of ocean deposit history in the Yangtze Gorges area. Thus, the important Indo-China plate movement had determined the coastline of South China as defined now.
Jurassic to Cretnceoits Since the Indo-China plate movement occurred in the late Triassic, the sediment in the Gorges area and most of the Central Sichuan Province was dominantly occupied by the inner continental lake and back shore coal-bearing clastic rocks. Early Jurassic sediments also kept this feature, for the tectonic activity was more powerful, and the sedimentary basin was raised further and overwhelmingly transformed to inner continental down-faulted basin with only river and lake sediments, represented by sandy and muddy materials. Because of the warm weather and abundance of plants, this stage was the most valuable incoalation time in Yangtze Gorges area and in Central to East Sichuan province. During the early Yanshanian movement (ca. 130 Ma) which occurred in the late Jurassic, the gorges area was uplifted, and mountain type folds were formed. The sedimentary basins were limited as small inner-continental downfaulted basins in Cretaceous.Nevertheless, Cretaceous
sediments were deposited only in the depression area of east edge of the Gorges area, such as Xiaotin area (just east of Yichang), Yichang and north Changyang area, which contains conglomerate, sandy conglomerate and mudstone with red color. The red conglomerate is unconformably overlain by the Ordovician carbonate in the entrance of the Xiling gorge and is best observed in the locality of Nanjingguan. Importantly, the tectonic movement of Yanshanian affected the Gorges area intensively in the late Cretaceous and caused the disappearance of the downfaulted basin. The sedimentation retreated towards east and south directions. Therefore, the Yanshanian movement is considered to reflect the end of the sedimentary basin evolution in the geological history of the Gorges region and, geographically, this movement also defined the geographic features of the Gorges area as seen today.
References Grabau, Y. (1922) The Sinian System, Bull. Geol. SOC.China, v. 1, pp. 1-4. Lee, J.S. and Chao, Y.T. (1924) Geology of Gorges district of Yangtze (Ichang to Tzekuei) with special reference to development of the Gorges, Bull. Geol. SOC.China, v. 3, pp. 351-391. Liu, B.J. and Xiu, X.S. (1994) Lithofacies and paleogeographic map of the late early Sinian, south China. Atlas of the lithofacies and paleogeography of south China. Science Press, Beijing, pp. 26-27. Ma, G.G., Li, H.Q., and Zhang, Z.C. (1984)An investigation of the age limits of the Sinian System in south China, Bull. Yichang Inst.Geol.Miner.Resour., v. 8, pp. 1-29 (in Chinese with English abstract). Ma,G.G.,Zhang,Z.C., Li, H.Q.,Chen, P. andHuang,Z.X. (1989) A geochronostratigraphical study of the Sinian system in Yangtze platform, Bull. Yichang Inst. Geol. Miner. Resour., v. 14, pp. 83-124 (in Chinese with English abstract). Meng, F.S., Xu, A.W. and Zhang, Z.L. (1995) Nonmarine biota and sedimentary facies of the Badong Formation in the Yangtze and its neighbouring area. China Univ. Geosci. Press, Wuhan, pp.62-64 (in Chinese with English abstract). Renne, P.R., Zhang, Z.C., Richard, M.A., Michael, T.B. and Asish, R.B. (1995) Synchrony and causal relations between Permian-Triassic boundary crises and Siberian flood volcanism, Science, v. 269, pp. 1413-1416. Steiner, M. (1994) Die neoproterozoischen Megaalgen Sudchnas, Berliner Geowissen. Abh., E 15, pp.1-146. Berlin (in German). Wang, X.F., Zhen, Q.L. and Zhou, T.M. (1983) The issue of biostratigraphy of the boundary between the Ordovician and Silurian of east Yangtze Gorges, China, Scientia Sinica, v.12, pp. 1123-1132 (in Chinese). Wang, X.F., Erdtmann, B.-D. and Mao, X.D. (1996) Geology of the Yangtze Gorges Area, Field Trip Guide for 30th International Geological Congress, T106/ T340, Geological Publishing House, Beijing, pp.16-18. Gondwana Research, V. 2, No. 4, 1999
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Wang Yu (1938)The time of the Yichang (Ichang) Limestone in east Yangtze Gorges, Hubei Province, Geological Review, pp.131-142 (in Chinese). Yang, S.N. and Yang, W.R. (1982) Regional Tectonics of China. Geol. Publ. House, Beijing, p. 60 & 99 (in Chinese).
Zhu, W.Q. and Chen, M.E. (1984) On the discovery of macrofossil algae from the Late Sinian in eastern Yangtze Gorges, south China. Acta Botanica Sinica, v. 26, pp. 558560 (in Chinese) .
Gondrvann Research, V: 2, No. 4, p p . 621-626. 01999 International Association for Gondiuann Resenrck, Japan.
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I S S N : 1342-937X
Goidwarin Research
Precambrian Geological Evolution in the Yangtze Gorges Area, China Wang Xiaofeng, Ma Daquan and Chen Xiaohong Yichang Institute of Geology and Mineral Resources, CAGS, PO Box 502, Yichang, Htibei 443003, China
Introduction The well-known Yangtze Gorges, including the Qitang Gorge, the Wu Gorge and the Xiling Gorge, begin at Baidingcheng, in eastern Sichuan and end at Nanjingguan in Yichang, western Hubei.
The Sinian and Palaeozoic strata, consisting - mainly of marine deposits, are well exposed in the eastern Yangtze Gorges area and are distributed "onion-skin-like" around the Huangling Arch (Fig.1). The pre-Sinian crystalline metamorphic rock series, exposed in the core
Fig.1. General geological map of the Yangtze Gorges area. 1.Tertiary-Quaternary 2. Cretaceous 3. Jurassic 4. Triassic 5. Devonian-Permian 6. Ordovician-Silurian 7. Cambrian 8. Sinian 9. Neoproterozoic-Mesoproterozoic 10. Palaeoproterozoic 11. Neoarchaean-Mesoarchaean 12.Dalaoling Suite 13.Huangling Suite 14. Granite 15. Fault.