Jurassic sedimentary evolution of southern Junggar Basin: Implication for palaeoclimate changes in northern Xinjiang Uygur Autonomous Region, China

Shun-Li Li , Xing-He Yu , Cheng-Peng Tan Ronald Steel

1.School of Energy, China University of Geosciences (Beijing), Beijing 100083, China

2.Jackson School of Geosciences, University of Texas at Austin, Austin, Texas 78712, USA

1 lntroduction ＊

Tianshan Range is one of the major tectonic elements in Central Asia which consists of discrete microcontinents, collapsed ocean basins, subduction-related volcano-plutonic arcs and anorogenic intrusive igneous rocks (Hendrixet al., 1992).At the end of the Paleozoic,the Palaeo-Asian Ocean was closed as the Kazakhstan Plate, Tarim Plate, Siberia Plate and North China Plate assembled into a large block.Hence, Central Asia area had been developed into a uniform terrestrial system(Denget al., 2010).Junggar Basin, located at the north of Tianshan Range, northwestern China, was a foreland basin during the Late Permian to the Cenozoic (Carrollet al., 1990).Organic-rich Jurassic sedimentary successions occur widely throughout southern margin of Junggar Basin (Figure 1), where their thicknesses exceed more than 3800 m.There are widespread outcrops, wells and seismic sections around the basin revealing an extensive development of the Jurassic system.

Junggar Basin is a compressional superimposed basin with the Late Paleozoic, Mesozoic, and Cenozoic deposits, which experienced the effects of Hercynian orogeny,Indosinian, Yanshannian and Himalayan orogenisms, and was resulted in a complex tectonic framework (Chenetal., 2002).The history of the Mesozoic tectonic movements and continuously exposed Jurassic strata in Junggar Basin has been studied systemically in the past few decades, and the features of stratigraphy are understood well (Coleman, 1989; Carrollet al., 1990; McKnightet al., 1990; Hendrixet al., 1992, 1995; Eberthet al., 2001;Shaoet al., 2003; Xiaoet al., 2004; Denget al., 2010).Break-up of supercontinent started in the Middle Jurassic,which caused collision of the India Plate and the Eurasia Plate (Ali and Aitchison, 2008).Tectonic syntheses of the Mesozoic and Cenozoic assembly of southern Asia have provided few details of the palaeotectonic and palaeogeographic impacts of the Meso-Cenozoic amalgamation on northwestern China and, in particular, on the Tianshan Range (Cowardet al., 1986; Searleet al., 1987).Stratigraphic frameworks were built on the basis of well and seismic data calibration.Sequence boundaries, sequences and mega-sequences within the Jurassic were developed at different scales in Junggar Basin (Wanget al., 2001; Baoet al., 2002; Shaoet al., 2009).The Jurassic was subdivided into seven third-order sequences and fifteen systems tracts in Junggar Basin (Baoet al., 2002).Coal-bearing strata of the Lower to Middle Jurassic were studied for interpretation of sequence stratigraphy and palaeogeography in adjacent basins (Shaoet al., 2009).At a global scale,the palaeotemperature which was reconstructed by oxygen isotopes increased during the Middle Jurassic (Deraet al.,2011).The Late Jurassic became warmer on the basis of climate-sensitive sediments, plants, and dinosaurs (Reeset al., 2004).Palaeoclimate conditions in central Asia area were also constructed based on fossils and stratigraphic characteristics (Chen,et al., 1991; Russell, 1993; Eberthet al., 2001; Badarchet al., 2002; Shaoet al., 2003; Xiaoet al., 2004; Averianovet al., 2005; Denget al., 2010; Rabiet al., 2010).

Despite the existence of this Mesozoic basin in northwestern China, previous sedimentary and stratigraphic studies mainly focused on regional depositional systems or palaeontology of the Jurassic.This study explored the signature of the Jurassic nonmarine sedimentation, stratigraphy and palaeontology in the context of continental drift and tectonic movements.In particular, we emphasized thedepositional and palaeogeographic evolution in terms of facies, fossils and cycles of the Jurassic, and discussed the implication of palaeoclimate changes in central Asia area.This paper (1)presents main aspects of the Jurassic depositional systems from detailed observation on eight transects at southern margin of Junggar Basin, northern Tianshan Range; (2)describes characteristics of fossils within each formation and interprets their implications for palaeoclimate; and (3)discusses tectonic movements controlling on palaeogeography and palaeoclimate evolution in Junggar Basin.

Figure 1 Geologic map showing structural framework and locations of the Jurassic outcrop sections in southern Junggar Basin,northern Tianshan Range.

2 Methodology

Eight outcrop sections across the Jurassic were measured at a resolution of meters in southern Junggar Basin(Figure 1).Lithology, grain size and sedimentary structures were identified to determine types and characteristics of depositional systems of the Jurassic in the study area.Sandstone samples and fossils were collected from various facies at each outcrop section along southern Junggar Basin.Palaeoclimate was inferred from these sedimentary characters and fossils of the Jurassic.Combined with the tectonic movements (Hendrixet al., 1992)and the latest global plate reconstruction (Lawveret al.,2009), controlling factors of palaeogeography were discussed in the context of global plate drift and tectonic movements.

3 Stratigraphy

The distinct names of each Jurassic formation are given for identification within Junggar Basin due to their certain variations in lithological composition in different parts of this basin.In southern Junggar Basin, the Lower Jurassic is subdivided into the Badaowan and Sangonghe Formations; the Middle Jurassic consists of the Xishanyao and Toutunhe Formations; the Upper Jurassic comprises of the Qigu and Kalazha Formations (Table 1).The Shuixigou Group is used to represent the Lower to Middle Jurassic coal-bearing strata in parts of Junggar Basin, such as northwestern margin and central area.It was also employed at the initial stage of geological investigation when the coal-bearing strata were difficult to be subdivided into Badaowan, Sangonghe or even Xishanyao Formations(Zhanget al., 2003).A set of the Middle and Upper Jurassic is generally called Shishugou Group at eastern outcrop area in Junggar Basin (Table 1).The most upper part of the Jurassic, the Kalazha Formation, develops locally in Jun-ggar Basin, mainly occurrs in the north of Tianshan Range(southern margin of Junggar Basin)due to tectonic uplift and erosion in the Late Jurassic.

Table 1 Stratigraphic division of the Jurassic in Junggar Basin, northwestern China (modified from Deng et al., 2010).

The lowermost Jurassic Badaowan Formation (J1b)is composed of a set of greyish, greyish green thick conglomerate, gravelly coarse sandstone and dark grey mudstone.The moderately well-sorted, clast-supported conglomerate is characterized by massive structures, trough cross-beddings and abundant wood fragments.The Badaowan Formation contacts conformably with the underlying Haojiagou Formation (T3h)in southern margin of Junggar Basin.Coal beds occur at the lower and upper part of Badaowan Formation which extends 297 m at Sikeshu section in southern Junggar Basin.The Sangonghe Formation (J1s)consists of 4 to 5 sets of greyish coarse sandstone with typical trough and planar cross-beddings, greyish green fine sandstone interbedded with dark greyish mudstone, and coal seams (Figure 2).Wave ripples are common within fine sandstone.

The Xishanyao Formation of the Middle Jurassic consists of greyish sandstone, pebble-bearing sandstone interbedded with greyish siltstone, dark grey mudstone and coal beds.The Xishanyao Formation is the major coalbearing strata of the Jurassic in Junggar Basin.The coal beds which extend up to 100 m totally are primarily located in the lower and middle part of the formation.The Toutunhe Formation which extends more than 1000 m in the southern Junggar Basin generally presents a set of thick, grey gravelly coarse sandstone with a honeycomb structure and dark grey, multicolored mudstone.

The Qigu Formation in Junggar Basin is comprised of reddish mudstone with thin-bedded sandstone.It is in conformity with the underlying Toutunhe Formation and overlying Kalazha Formation (Figure 2).The thickness of Qigu Formation is up to 650 m.The Kalazha Formation is mainly characterized by thick, greyish or brownish massive conglomerate with thin-bedded reddish sandstone.Mud cracks occur at the top of mudstone and siltstone.This formation is in conformity with the underlying Qigu Formation, and in unconformity with the overlying Lower Cretaceous Tugulu Group.The Kalazha Formation presents a thickness of 290 m at Hutubi section.

4 Lithology

The terrestrial Jurassic strata in southern Junggar Basin is one of the best occurrences across China, which is widespread and exposes almost complete stratigraphic sequence along northern Tianshan Range.Depositional systems were identified by grain sizes, sedimentary structures, fossils and sedimentary cycles at each outcrop sections in the study area.The Lower and Middle Jurassic(J1b, J1s, J2x)which mainly consist of fan or braided delta systems and lacustrine facies are major coal-bearing strata in Junggar Basin.The upper part of the Middle Jurassic(J2t)to lower part of the Upper Jurassic (J3q)were fluvial systems with less sediment supply.The upper most part of the Jurassic (J3k)was alluvial fan system developed locally at southern margin of Junggar Basin.

Lithofacies and their assemblages which are fundamental units of deposits represent hydro-mechanisms of depositional systems and even environments (Yu, 2008).The distinctive nine lithofacies in the Jurassic were recognized at outcrops based on grain sizes and sedimentary structures.The Badaowan Formation of the Lower Jurassic mainly presents trough cross-bedding gravels (Gt), trough cross-bedding coarse sandstone (St), laminated mudstone(Fl)and coal bed (C)(Figure 3).The Sangonghe Formation of the Lower Jurassic primarily consists of trough crossbedding medium sandstone (St), planar cross-bedding medium sandstone (Sp), wave ripple cross-bedding fine sandstone (Sr)and laminated mudstone (Fl)(Figure 4).The Xishanyao Formation of the Middle Jurassic shows trough cross-bedding gravelly coarse sandstone (St), planar crossbedding coarse sandstone (Sp), laminated mudstone (Fl)and coal bed (C)(Figure 5).The Toutunhe Formation of the Middle Jurassic is mainly comprised of tough crossbedding gravelly coarse sandstone (St), planar crossbedding coarse sandstone (Sp), and laminated mudstone(Fl)(Figure 6).The Qigu Formation of the Upper Jurassic principally exhibits laminated fine sandstone (Sl)and laminated mudstone (Fl)(Figure 7).The Kalazha Formation of the Upper Jurassic mainly represents massive gravels(Gm)and massive coarse sandstone (Sm)(Figure 8).

4.1 Early Jurassic

The Badaowan Formation of the Lower Jurassic presents thick, stacked grey conglomerate and sandstone with a fining-upward cycle (Figure 3a).Amalgamated, massive,coarse-grained (gravel, pebble)channel deposits which show weak grading of grain size represent gravelly channel deposition (Figure 3b).Petrified wood segments within the conglomerate (Figure 3c)suggest proximal deposition with steep slope gradients.At the upper part of the Badaowan Formation, conglomerate and coarse sandstone(Figure 3d)interbedded with grey mudstone (Figure 3e)indicate a transgression in southern Junggar Basin during the late period of the Badaowan deposition.Three-dimension

(3-D)wave ripples occur within coarse sandstone (Figure 3f)in the Upper Badaowan Formation at Shichang section.Coal is abundant in the Badaowan Formation (Figure 3g)throughout southern Junggar Basin.

Figure 2 Lithological column with fossils showing the sedimentary characteristics of the Jurassic strata in southern and western margins of Junggar Basin.Note that M, VF, F, M, C, and G in the lithological column refer to mud, very-fine sandstone, fine sandstone,medium sandstone, coarse sandstone, and gravel, respectively (the same below).

The entire coarse-grained, amalgamated, fining-upward succession represents a fan delta system which was developed under transgression.Braided channel and bar units are common on fan delta plain, where coal beds occur frequently.The discontinuous coal beds which were partly eroded by braided channels suggest that they were generally formed in relatively small depressions on fan delta plains.Grey mudstone beds and 3-D ripples in the upper part of the Badaowan Formation indicate that fan delta front was subaqueous and influenced by waves.

The Sangonghe Formation consists of relative finegrained sediments with minor coal seams (Figure 4a).Thinbedded sandstone (less than 1 m)which shows large laterally-extending distance presents thickening-upward feature at Haojiagou section (Figure 4b).However, except the major portion of thin-bedded sandstone, this succession also represents relatively thick (2 m to 5 m), medium- to coarse-grained sandbodies which show concave up and erosional base (Figure 4c).Sediments within the Sangonghe Formation even present poorly-rounded but relatively well-sorted feature comparing to the underlying interval(Figure 4d).Wave ripples are very common throughout this succession which indicate that the depositional system was strongly influenced by wave (Figure 4f).The greenish grey mudstone at the upper part of the Sangonghe Formation (Figure 4g)suggests subaqueous deposition with increasing water depth.

This formation which consists of three coarsening-upward sequences represents multiple delta progradations along basin margins.The thin-bedded, fine-grained and well-sorted sandstone with wide lateral extension indicates wave-dominated delta systems along southern margin of Junggar Basin.In the middle part of the Sangonghe Formation, the thick, amalgamated coarse sandstone with large trough cross-beddings and planar cross-beddings represents the main feeder channels of delta system.Therefore, the Sangonghe Formation collectively represents a wave-dominated braided delta system.

Figure 3 Sedimentary characteristics of the Badaowan Formation, Lower Jurassic at Sikeshu section in Junggar Basin.Lithological column indicates braided channel unit of fan delta system.a-Amalgamated conglomerates of amalgamated braided channels at base of the Badaowan Fm.; b-Braided channel with erosional surface; c-Massive conglomerate with petrified woods; d-Conglomerate of channel unit; e-Lacustrine deposit of dark grey mudstone with lenticular sandstone, upper part of Badaowan Fm.; f-Wave ripples;g-Coal bed within channel deposition.Note that abbreviations of lithofacies are listed as follows: G: conglomerate; S: sandstone; F:fine-grained sandstone to mudstone; C: coal; t: trough cross-bedding; p: planar cross-bedding; l: lamination; r: ripples (the same below).

Figure 4 Sedimentary characteristics of the Sangonghe Formation, Lower Jurassic at Haojiagou section in Junggar Basin.Lithological column of coarsening- and thickening-upward sequences with plenty of wave ripples, low angle cross-beddings, and laterally well extended sandbodies represent a wave-dominated delta system.a-Mudstone interbedded with thin beds, laterally well extended sandstone of delta front, base of Sangonghe Formation; b-Delta front progradation showing stacked coarsening-upward sequences; c-Low angle cross-beddings within laterally well extended sandbody; d-Medium-coarse sandstone in distributary channel; e-Distributary mouth bar; f-Wave ripples; g-Grey mudstone of delta front deposition.

4.2 Middle Jurassic

The Xishanyao Formation of the Middle Jurassic is mainly comprised of two coarsening-upward sequences(Figure 2)with the largest portion of coal beds throughout the Jurassic in Junggar Basin.The lower coarsening-upward cycle is comprised of grey mudstone, fine sandstone with ripples and medium sandstone (Figure 5a).However,the upper cycle is coarser than the lower one, which is primarily composed of greyish coarse sandstone, mudstone and coal beds with plenty of plant fossils (Figure 5b).The widespread coal beds in the Xishanyao Formation can even be ignited on ground surface due to their high quality (Figure 5c, 5d).Sandstone with fierce calcite cementation in this succession presents poorly sorted and rounded feature (Figure 5e).Gravelly coarse sandstone with large trough cross-beddings was common at the upper part of this formation (Figure 5f).

The Xishanyao Formation which consists of relatively coarser sediments and less wave ripples than the Sangonghe Formation was river-dominated braided delta system at southern margin of Junggar Basin.Distributary channel system of braided delta brought adequate sediments and organic matters.A large portion of inter-distributary channels at delta plain provided well condition for formation of coal beds.The thick greenish grey mudstone at the middle part indicates a maximum transgression in Junggar Basin during the Jurassic.

At the lower and middle part of the Toutunhe Formation, ten to eleven sets of sandbodies consist of thick,amalgamated, and massive gravelly coarse sandstone.The honeycomb weathering structures of sandstone which indicate high sedimentary rate of massive sandstone are significant identification features at outcrops (Figure 6a).These sandbodies which were interbedded with grey mudstone and siltstone commonly show symmetric concaveup shape and erosional base (Figure 6b).Thickness of the amalgamated sandbodies which were composed of coarsegrained sandstone with trough cross-beddings and planar cross-beddings (Figure 6c, 6d, 6e)ranges from 10 m to 35 m.Laminated sandstone with current ripples also occurred at the upper part of the Toutunhe Formation (Figure 6f).The proportion of mudstone increased upwards with the color changing from greyish to mottled or reddish (Figure 6g).

The grain size features, internal structures and external configuration of these massive amalgamated sandstones indicate braided fluvial systems were developed in the late Middle Jurassic.The channel complexes with coarse sediments and plant trunk fossils stacked vertically at the lower part of the Toutunhe Formation.However, the upper part of this formation which presents thin-bedded, laminated sandstone with mottled mudstone and less plant fossils indicates that energy of fluvial system decreased with less sediment supply during this period.The changes of grain sizes, fossils and colors within the succession suggest evolution from braided fluvial environment to meandering fluvial environment.

4.3 Late Jurassic

The Qigu Formation mainly shows thick reddish mudstone with lenticular or thin-layered sandstone (Figure 7a).The thin sandstone (less than 1 m)generally presents laminated or structureless feature (Figure 7b).However, therelatively thick sandbodies (2 m to 5 m)show asymmetric concave-up shape, erosional base and fining-upward sequence (Figure 7c, 7d).Trough cross-beddings, planar cross-beddings and parallel beddings were the major sedimentary structures developed within thick sandstone (Figure 7e).The channel and point bar of sandstone within the significant thick reddish mudstone indicate that meandering fluvial system was developed in flood plain.The thick muddier and finer deposition implies a starved basin margin plain with a low rate of sediments supply during the early Late Jurassic.

Figure 5 Sedimentary characteristics of the Xishanyao Formation, Middle Jurassic at Anjihai section in Junggar Basin.a-Distributary channels interbedded with coal beds of delta plain deposition; b-Delta front deposition at base of the Xishanyao Fm.; c-Reddish surrounding rock of ignited coal; d-Coal bed with roof and floor of fine sandstone; e-Sub-angular, calcite-cemented, medium-coarse sandstone of distributary channel; f-Trough cross-bedding of gravelly coarse sandstone in distributary channel.

Figure 6 Sedimentary characteristics of the Toutunhe Formation, Middle Jurassic at Liuhuanggou section in Junggar Basin.a-Massive coarse sandstone with honeycomb structures at base of braided channel; b-A large set of massive or trough cross-bedding gravelly coarse sandstone with weak fining-upward sequence of braided fluvial system; c-Coarse sandstone of braided channel deposits; d-Trough cross-bedding at lower part of braided channel deposits; e-Multiple sets of planar cross-bedding at upper part of braided channel deposits; f-Current ripples of overbank deposits; g-Multicolor mudstone of overbank deposits.

Large sets of reddish, stacked massive conglomerate of the Kalazha Formation (Figure 8a, 8b, 8c)imply that alluvial fan system was developed at the end of the Jurassic.The matrix-supported, moderately-sorted pebbles without orientation at Shichang section represent middle fan facies(Figure 8b).The conglomerate with variable particle components (Figure 8d)presents minor coarse-grained sandstone with trough cross-beddings (Figure 8e).Mud cracks occurred within fine sediments among conglomerate beds indicate a subaerial exposed environment (Figure 8f, 8g).The alluvial fan system was only discovered partially at southern margin of the Junggar Basin.These large alluvial fan systems could be penetrated subsurface at tens of kilometers from outcrops.The thickness of the Kalazha Formation is up to 300 m.The thick alluvial fan aprons of the Kalazha Formation indicate high relief and adequate sediments supply in local areas of the Junggar Basin during the most end of the Late Jurassic.

5 Floral analysis

Plant fossils are pretty abundant within the Lower and Middle Jurassic in Junggar Basin.The plant fossils were preserved very well as compression types.Numerous petrified woods were preserved in the Badaowan, Xishanyaoand Toutunhe Formations in the southern Junggar Basin.In addition, some of the fossils were destroyed due to surface weathering and auto-ignition of coal beds at outcrops.The Early Jurassic flora in northern Tianshan Range can be subdivided into three assemblages (Figure 9a-9f):Todites princeps-Clathropteris elegans;Coniopteris gaojiatianensis-Cladophlebis; andPhlebopteris-Marattiopsis(Denget al., 2010).In the Badaowan Formation, there are two plant assemblages ofTodites princeps-Clathropteris elegansandConiopteris gaojiatianensis-Cladophlebis.A number of thermophilous genera and species were found in the Sangonghe Formation.Coniopteris increases its diversity with various fossils and shows strong rachis.The plant fossils from the Xishanyao Formation demonstrate the features of flouringConiopteris-Phoenicopsisflora.The flouring degree was consistent with other contemporary flora around the world.The predominant emergence ofConiopteris hymenophylloidesis one of the significant marks in the Middle Jurassic across the world (Brick,1953; Harris, 1961; Chenet al., 1984).

Figure 7 Sedimentary characteristics of the Qigu Formation, Upper Jurassic at Hutubi section in Junggar Basin.a-Thick, reddish mudstone with thin bedded fine sandstone of flood plain deposits; b-A large set of reddish mudstone with thin bedded or lenticular sandbodies indicating that the strata were developed in an arid, starved basin; c-Lenticular sandstone with typical fining-upward sequence (dual structure; trough, planar, and parallel cross-beddings)of meandering channel; d-Meandering channel unit; e-Trough cross-bedding at the base of a channel; f-Medium sandstone of channel deposits; g-Laminated mudstone of flood plain deposits.

The various plant fossil assemblages which occurred in the Lower and Middle Jurassic successions indicated a very warm, humid climate during the Early to Middle Jurassic.Zone of vegetation was widespread throughout southern margin of Junggar Basin.The characterization of depositional system also suggested high volume of rainfall which resulted in adequate sediment supply, and provided well conditions for prosperity of plants.These plants were developed on fan delta or braided delta plain along the southern margin of Junggar Basin where plenty of coal beds were yielded.

In the Middle and Upper Jurassic, plant fossils wereless than in the underlying strata.Cone-in-cone calcites which represent shallow water carbonate deposition were discovered at the upper part of the Xishanyao Formation(Figure 9g-9h).Petrified woods and leaves only occurred at the Xishanyao Formation and the lower part of the Toutunhe Formation (Figure 9i-9k).However, bivalves, conchostracans and vertebrates were commonly discovered at outcrops.The Toutunhe Formation also yields theBellusaurus-MonolophosaaurusFauna, whose general characteristics are rather primitive (Zhao and Currie, 1993).Although the genera and species are less than those in other areas around the world, they are characterized by the transitional types that are outstandingly different from the Late JurassicMamenchisaurusfauna (Denget al., 2010).The Qigu Formation is characterized by reddish, muddier deposition with relatively fewer fossils.Some fossils of vertebrate (Figure 9l-9m), charophyte and gastropod have been found within this formation.

Figure 8 Sedimentary characteristics of the Kalazha Formation, Upper Jurassic at Shichang section in Junggar Basin.Note that:a-A large set of reddish, greyish massive or structureless conglomerate with mud cracks representing relatively proximal alluvial fan deposits at southern margin of the basin; b-Poorly sorted conglomerate; c-Erosional base of gravelly braided channel unit; d-Finegrained sediments filled pores of conglomerate; e-Lenticular sandstone with trough cross-bedding; f-Mud cracks in plane view; g-Mud cracks in vertical view.

Decreasing of diversity of plants within the upper part of the Jurassic reflected a transition of palaeoclimate from humid to semi-arid in the Late Jurassic.Water area in the Junggar Basin shrank dramatically due to less rainfall which was caused by semi-arid climate.Hence, large amounts of hygrophyte extincted in the Junggar Basin.However, vertebrate like dinosaur and turtle which were fed by droughtenduring plants survived in the Late Jurassic.The relatively arid environment with less degree of biodegradation provided well conditions for fossil preservation.

6 Evolution of sedimentation and palaeoclimate

Sediments and fossils are significant information carriers of palaeoclimate.The grain size, color, lithology and

sedimentary sequence of depositional systems are indicators for palaeoclimate.Coal beds generally suggest a warm, humid climate period during which depositional systems developed.However, reddish muddier sediments represent a relatively arid environment to some extent.Most of the plant fossils within strata play important roles in indicating palaeoclimate.

Figure 9 Representatives of flora in the Jurassic of southern Junggar Basin.Note that: The Early Jurassic flora assemblages (Deng et al., 2010)include a-Clathropteris elegans Oishi; b-Coniopteris gaojiatianensis Zhang; c-Sphenobaiera huangi Florin; d-Cladophlebis haibuanensis Brongniart; e-Todites princeps Gothan; f-Equisetites lateralis Phillips.The Middle Jurassic consists of g, h-Cone-in-cone calcites, Xishanyao Formation; i-Equisetites lateralis Phillips; j-Sphenobaiera huangi Florin; k-Petrified wood, Toutunhe Formation.The Late Jurassic is comprised of l-Carnosaur teeth, Toutunhe Formation (Maisch and Matzke, 2003); m-Xinjiang chelys, Qigu Formation (Matzke et al., 2005).

6.1 Tectonic setting

The Mesozoic deformation and tectonic context in the Tianshan Range has been studied for decades (Watsonet al., 1987; Grahamet al., 1990; McKnightet al., 1990; Badarchet al., 2002; Xiaoet al., 2004, 2009).The Junggar Basin, located in the eastern wedge of the Kazakhstan Plate and part of the Mongolia-Xing’anling accretion fold belts, developed on the basis of Junggar terrain (Chenet al., 2002).At the end of the Paleozoic, the Palaeo-Asian Ocean closed when the Kazakhstan Plate, Tarim Plate,Siberia Plate and North China Plate jointed into a large block.The Junggar Basin developed into uniform terrestrial systems subsequently (Denget al., 2010).From the Early to Middle Jurassic, the significant Yanshan Tectonic Event occurred in the east of China due to the southward collision of the Siberia Plate and the westward subduction of the Pacific Plate.However, this tectonic event had relatively weak impact on the west of Tianshan Range.The significant influence on the study area occurred in the Late Jurassic when the northward movement of the Lhasa Block collided with the Qiangtang Block (Figure 10)that had integrated with Eurasia during the end of the Triassic(Zhonget al., 2003).This event led to uplift of mountain systems with reddish coarse-grained deposition in northwestern China.The absence of the upper most part of the Jurassic formed the regional unconformity between the Jurassic and Cretaceous.

In the Early Jurassic, the Junggar Basin was under a relatively steady tectonic setting.Thick delta-lacustrine units deposited due to high-rate of sediment supply and tectonic subsidence.However, in the late period of the Middle Jurassic, a slight unconformity between the Xishanyao and Toutunhe Formations was developed by collision of the Lhasa Block and Qiangtang Block.In the Late Jurassic,the Tianshan Range kept uplifting which formed high relief of basin margin and alluvial deposits.

6.2 Early Jurassic

The climate during the sedimentary period of the Badaowan Formation (J1b)in the Lower Jurassic and Xis-hanyao Formation (J2x)in the Middle Jurassic was similar to that of the Late Triassic and was dominated by a warmhumid environment.The strong monsoon from the Tethys Sea provided adequate moisture to the Junggar Basin (Figure 11).Hence, the dark color deposits characterized by coal beds and dark grey mudstone indicate that plenty of organic matter was preserved under the humid climate.

Figure 10 Schematic cross sections across the South Asia continental margin to Junggar Basin (modified from Hendrix et al., 1992).

Figure 11 Schematic diagrams illustrating evolution of depositional systems, palaeogeography and palaeoclimate during the Jurassic in the Junggar Basin, central Asia.Plate reconstructions are modified from Lawver et al. (2009).

During the Sangonghe depositional period (J1s), the characteristics of sedimentation were different from the underlying Badaowan Formation which was dominated by finer grained sediments and less coal beds.The Sangonghe Formation which even consists of delta facies and various plants reflected that the coal-forming environment in the early stage of the Early Jurassic was changed.It might be the transgression of lake level that drowned most areas in southern margin of Junggar Basin, which formed the subaqueous well-sorted, fine-grained deposits without coal.

6.3 Middle Jurassic

The Xishanyao Formation which is the major coalbearing stratum in the Jurassic is widespread across the whole basin and indicates a humid climate in Tianshan Range area.The river-dominated braided deltas which presented large number of channel units at outcrops prograded into the Junggar Basin and developed wide delta plain.The plant fossils ofConiopteris-Phoenicopsisflora including ferns and gymnosperms were abundant in the Junggar Basin.These forest vegetation and petrified woods documented the existence of seasonal climate (Figure 11).With plenty of plant fossils and wide area of shallow water swamps in delta plain, coal beds were developed widely in the Junggar Basin.

The Toutunhe Formation was dominated by braided fluvial facies.The lower part of this formation which was mainly comprised of thick, massive greyish gravelly coarse sandstone and petrified woods reflected a braided fluvial system developed under a humid environment.However,the upper part of the Toutunhe Formation presents mottled or reddish muddier sediments, which suggests decreasing of rainfall and sedimentary rate.The proportion of red beds increases upwards, which suggests possible climate changes from humid to semi-humid and to semi-arid finally (Figure 11).

6.4 Late Jurassic

The Qigu Formation is totally characterized by reddish or brownish red sediments.Due to the uplift of the Qiangtang Block and the global warming event which began at the late Middle Jurassic (Valdes and Sellwood, 1992; Abbinket al., 2001), rainfall in the Junggar Basin decreased and most of the moisture from ancient Tethys Sea was obstructed from southwest.Therefore, fine-grained meandering fluvial system with relatively low rate of deposition was developed in the arid environment of Junggar Basin.

The thick, massive, amalgamated reddish conglomerates of Kalazha Formation represent alluvial fan facies which developed at the steep frontier of mountain in an arid environment.Collision of the Lhasa Block and Qiangtang Block formed high relief in Tianshan Range area and blocked moisture from the Tethys Sea (Figure 11).The coarse sediments which were driven by ephemeral flooding events settled at the foot of Tianshan Range, southern margin of the Junggar Basin.This coarse-grained depositional system with rare fossils only developed locally in the Junggar Basin due to lack of continuous rainfall and discharge.

7 Conclusions

1)The widespread distribution of the Lower Jurassic organic-rich, coal-bearing deltaic-lacustrine deposits demonstrates the presence of a humid climate in the Early Jurassic.The deposition with less fossils in the Middle Jurassic indicates a transition from deltaic to fluvial depositional systems and increasing of aridity.The Upper Jurassic reddish strata devoid of organic matter reflect alluvial deposition under an arid environment during the Late Jurassic.

2)Plate tectonic configuration was the key driving force in shaping palaeoclimate and palaeogeography.According to plate reconstruction, the warm and wide Tethys Sea between the proto-Laurasia and proto-Gondwana generated a strong monsoonal circulation over the central Asian continent.Due to global warming event and the Himalayan uplift during the Late Jurassic, a major change of climate from relatively humid to arid conditions took place across Xinjiang area, central Asia, which resulted in formation of red rocks devoid of organic matter but few vertebrates.

Acknowledgements

This work was supported by PetroChina Xinjiang Oilfield Company, the Chinese National Natural Science Fund Project (No.41072084; No.41272132), National Program on Key Basic Research Project (973 Program)(No.2009CB219502-3), and the Fundamental Research Funds for the central universities (2-9-2013-97).We thank Bin-Tao Chen, Jing Xie, Jian-Hua Qu and who provided valuable help in field.

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