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

Journal of Palaeogeography 2014, 3(2): 145-161 DOI: 10.3724/SP.J.1261.2014.00049

Lithofacies palaeogeography and Palaeogeography sedimentology

Jurassic sedimentary evolution of southern Junggar Basin: Implication for palaeoclimate changes in northern Xinjiang Uygur Autonomous Region, China Shun-Li Li1, 2, Xing-He Yu1, *, Cheng-Peng Tan1, Ronald Steel2 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

Abstract Junggar Basin, located in northern Xinjiang, presents continuous and multikilometer-thick strata of the Jurassic deposits. The Jurassic was entirely terrestrial fluvial and lacustrine deltaic sedimentation. Eight outcrop sections across the Jurassic strata were measured at a resolution of meters in southern Junggar Basin. Controlling factors of sedimentary evolution and palaeoclimate changes in Junggar Basin during the Jurassic were discussed based on lithology, fossils and tectonic setting. In the Early to Middle Jurassic, the warm and wide Tethys Sea generated a strong monsoonal circulation over the central Asian continent, and provided adequate moisture for Junggar Basin. Coal-bearing strata of the Badaowan, Sangonghe, and Xishanyao Formations were developed under warm and humid palaeoclimate in Junggar Basin. In the late Middle Jurassic, Junggar Basin was in a semi-humid and semi-arid environment due to global warming event. Stratigraphy in the upper part of the Middle Jurassic with less plant fossils became multicolor or reddish from dark color sediments. During the Late Jurassic, collision of Lhasa and Qiangtang Block obstructed monsoon from the Tethys Sea. A major change in climate from semi-humid and semi-arid to arid conditions took place, and reddish strata of the Upper Jurassic were developed across Junggar Basin. Key words sedimentary evolution, palaeoclimate change, Jurassic, Junggar Basin, China

1

Introduction *

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 (Hendrix et 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 * Corresponding author. E-mail: [email protected]. Received: 2013-08-18 Accepted: 2014-01-04

(Deng et al., 2010). Junggar Basin, located at the north of Tianshan Range, northwestern China, was a foreland basin during the Late Permian to the Cenozoic (Carroll et 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 (Chen et

146

JOURNAL OF PALAEOGEOGRAPHY

al., 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; Carroll et al., 1990; McKnight et al., 1990; Hendrix et al., 1992, 1995; Eberth et al., 2001; Shao et al., 2003; Xiao et al., 2004; Deng et 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 (Coward et al., 1986; Searle et 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 (Wang et al., 2001; Bao et al., 2002; Shao et al., 2009). The Jurassic was subdivided into seven third-order sequences and fifteen systems

Apr. 2014

tracts in Junggar Basin (Bao et al., 2002). Coal-bearing strata of the Lower to Middle Jurassic were studied for interpretation of sequence stratigraphy and palaeogeography in adjacent basins (Shao et al., 2009). At a global scale, the palaeotemperature which was reconstructed by oxygen isotopes increased during the Middle Jurassic (Dera et al., 2011). The Late Jurassic became warmer on the basis of climate-sensitive sediments, plants, and dinosaurs (Rees et al., 2004). Palaeoclimate conditions in central Asia area were also constructed based on fossils and stratigraphic characteristics (Chen, et al., 1991; Russell, 1993; Eberth et al., 2001; Badarch et al., 2002; Shao et al., 2003; Xiao et al., 2004; Averianov et al., 2005; Deng et al., 2010; Rabi et 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 the

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

Vol. 3 No. 2

Shun-Li Li et al.: Jurassic sedimentary evolution of southern Junggar Basin: Implication for palaeoclimate changes in northern Xinjiang Uygur Autonomous Region, China

depositional 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.

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 (Hendrix et al., 1992) and the latest Table 1

global plate reconstruction (Lawver et 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 (Zhang et 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-

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

Area

Strata Overlying strata

Southern margin

Eastern margin

Western margin

Tugulu Gr. (K1t)

Tugulu Gr. (K1t)

Tugulu Gr. (K1t)

Tithonian Upper Jurassic

Kimmeridgian

Kalazha Fm. (J3k)

Oxfordian

Qigu Fm. (J3q)

Callovian Middle Jurassic

Bathonian Bajocian Aalenian Toarcian

Lower Jurassic

Pliensbachian Sinemurian Hettangian

Underlying strata Note: Gr. = Group; Fm. = Formation;

Toutunhe Fm. (J2t)

Shishugou Gr. (J2+3 sh)

Qigu Fm. (J3q) Toutunhe Fm. (J2t)

Xishanyao Fm. (J2x)

Xishanyao Fm. (J2x)

Xishanyao Fm. (J2x)

Sangonghe Fm. (J1s)

Sangonghe Fm. (J1s)

Sangonghe Fm. (J1s)

Badaowan Fm. (J1b)

Badaowan Fm. (J1b)

Badaowan Fm. (J1b)

Haojiagou Fm. (T3h)

Shangcangfanggou Gr. (T3sh)

Xiaoquangou Gr. (T3xq)

= unconformity;

147

= parallel unconformity.

148

JOURNAL OF PALAEOGEOGRAPHY

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. 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

Apr. 2014

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

Vol. 3 No. 2

Shun-Li Li et al.: Jurassic sedimentary evolution of southern Junggar Basin: Implication for palaeoclimate changes in northern Xinjiang Uygur Autonomous Region, China

149

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).

150

JOURNAL OF PALAEOGEOGRAPHY

(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. 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). Thin-

Apr. 2014

bedded 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-

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).

Vol. 3 No. 2

Shun-Li Li et al.: Jurassic sedimentary evolution of southern Junggar Basin: Implication for palaeoclimate changes in northern Xinjiang Uygur Autonomous Region, China

Depth (m) Lithofacies Sr 125

b

a

0

151

50 m

100

Sl Sr Sl

75

20 m E

c 0

50 m

50

25

Fl

0 M VF F M C G

1m d

Sr

f

e

500 µm

g

10 m

Sandstone

Mudstone

Quaternary cover

Coarsening-upward sequence

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.

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.

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,

152

JOURNAL OF PALAEOGEOGRAPHY

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

b 0

Apr. 2014

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, the

Depth (m) 10 m 100

Lithofacies Fl

a

Sp St St

75 C

E 0

C Sr

50

c

10 m

Sp St 25

Fl Sr

0 M VF F M C G

f

d

e

500 µm

Grayish, yellowish Grayish, yellowish sandstone mudstone

Grayish green mudstone

Coal bed

Quaternary cover

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.

Vol. 3 No. 2

Shun-Li Li et al.: Jurassic sedimentary evolution of southern Junggar Basin: Implication for palaeoclimate changes in northern Xinjiang Uygur Autonomous Region, China

b

a

0

153

Depth (m) Lithofacies 50 St

5m

Fl St

40

St

0.5 m 30

Gt

c 0

E

5m

Fl Sl St Gt

20

St Gt Fl

10 2000 µm

d

e

g

f

0 M VF F M C G

2m

Grayish sandy conglomerate

Grayish gravelly sandstone

Honeycomb sandstone

Yellowish mudstone Quaternary cover

Figure 6 Sedimentary characteristics of the Toutunhe Formation, Middle Jurassic at Liuhuanggou section in Junggar Basin. aMassive 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.

relatively 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. 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 sand-

stone 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, Xishanyao

154

JOURNAL OF PALAEOGEOGRAPHY

a

b

Apr. 2014 Depth (m) Lithofacies St 125

0

5 10 m

Fl

100 Sl

10 m c

75

0

E

5 10 m

50

Sl

5m d

25 Fl 0 M VF F M CG e

f

2m

Yellowish coarse sandstone

g

500 µm

Grayish fine sandstone

Reddish fine sandstone

Reddish mudstone

Quaternary cover

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.

and 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; and Phlebopteris-Marattiopsis (Deng et al., 2010). In the Badaowan Formation, there are two plant assemblages of Todites princeps-Clathropteris elegans and Coniopteris 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 flouring Coniopteris-Phoenicopsis flora. The flouring degree was consistent with other contempo-

rary flora around the world. The predominant emergence of Coniopteris hymenophylloides is one of the significant marks in the Middle Jurassic across the world (Brick, 1953; Harris, 1961; Chen et al., 1984). 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 were

Vol. 3 No. 2

Shun-Li Li et al.: Jurassic sedimentary evolution of southern Junggar Basin: Implication for palaeoclimate changes in northern Xinjiang Uygur Autonomous Region, China

155

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; gMud cracks in vertical view.

less 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 the Bellusaurus-Monolophosaaurus Fauna, 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 Jurassic Mamenchisaurus fauna (Deng et 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. 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

156

JOURNAL OF PALAEOGEOGRAPHY

Apr. 2014

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, hCone-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).

Shun-Li Li et al.: Jurassic sedimentary evolution of southern Junggar Basin: Implication for palaeoclimate changes in northern Xinjiang Uygur Autonomous Region, China

Vol. 3 No. 2

157

tion 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 (Zhong et 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.

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.

6.1 Tectonic setting The Mesozoic deformation and tectonic context in the Tianshan Range has been studied for decades (Watson et al., 1987; Graham et al., 1990; McKnight et al., 1990; Badarch et al., 2002; Xiao et 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 (Chen et 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 (Deng et 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 subduc-

6.2

Early Jurassic

The climate during the sedimentary period of the Badaowan Formation (J1b) in the Lower Jurassic and Xis-

70 Ma Kohistan–Dras collision Kohistan–Dras

Lhasa

Qiangtang

Tarim

Junggar

S

N 125 Ma Lhasa collision

Jurassic

Qiangtang

Tarim

Junggar

S

Age (Ma)

Lhasa

140 Ma

150

N 200 Ma

200

Stratigraphy

100

Cretaceous

80 Ma

230 Ma S

Tarim

Qiangtang

Oceanic crust

Continental crust

Amphibolite

Junggar

Granite

Sedimentary rock

N

250

Triassic

Qiangtang collision

Study area

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

158

JOURNAL OF PALAEOGEOGRAPHY

Apr. 2014

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).

Vol. 3 No. 2

Shun-Li Li et al.: Jurassic sedimentary evolution of southern Junggar Basin: Implication for palaeoclimate changes in northern Xinjiang Uygur Autonomous Region, China

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. 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 of Coniopteris-Phoenicopsis flora 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; Ab-

159

bink et 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.

160

JOURNAL OF PALAEOGEOGRAPHY

Apr. 2014

of Northern Xinjiang, China. Hefei: University of Science and

References Abbink, O., Targarona, J., Brinkhuis, H., Visscher, H., 2001. Late Jurassic to earliest Cretaceous palaeoclimatic evolution of the southern North Sea. Global and Planetary Change, 30(3-4): 231-256. Ali, J. R., Aitchison, J. C., 2008. Gondwana to Asia: Plate tectonics, paleogeography and the biological connectivity of the Indian sub-continent from the Middle Jurassic through latest Eocene (166-35 Ma). Earth-Science Reviews, 88(3-4): 145-166. Averianov, A. O., Martin, T., Bakirov, A. A., 2005. Pterosaur and dinosaur remains from the Middle Jurassic Balabansai Svita in the northern Fergana Depression, Kyrgyzstan (Central Asia). Palaeontology, 48(1): 135-155. Badarch, G., Cunningham, W. D., Windley, B. F., 2002. A new terrane subdivision for Mongolia: Implications for the Phanerozoic crustal growth of Central Asia. Journal of Asian Earth Sciences, 21(1): 87-110. Bao, Z. D., Guan, S. R., Li, R. F., Wang, Y. M., Liu, L., Zhao, X. Q., Qi, X. F., 2002. Sequence stratigraphy of the Jurassic in Junggar basin. Petroleum Exploration and Development, 29(1): 48-51 (in Chinese with English abstract). Brick, M. I., 1953. Mezozoiskaya Flora Vostochno-Ferganskogo Kamennougol'nogo Basseina Paporotniki (Mesozoic Flora of the East Fergana Coal Basin. Ferns). Moscow: Gosgeolizdat Publishing House, 1-112 (in Russian). Carroll, A. R., Liang, Y. H., Graham, S. A., Xiao, X. C., Hendrix, M. S., Chu, J. C., McKnight, C. L., 1990. Junggar basin, northwest China: Trapped Late Paleozoic ocean. Tectonophysics, 181(14): 1-14. Chen, F., Dou, Y. W., Huang, Q. S., 1984. The Jurassic Flora of West Hills, Beijing. Beijing: Geological Publishing House, 1-136 (in Chinese). Chen, Y., Cogne, J. P., Courtillot, V., Avouac, J. P., Tapponnier, P., Wang, G. Q., Bai, M. X., You, H. Z., Li, M., Wei, C. S., Buffetaut, E., 1991. Paleomagnetic study of Mesozoic continental sediments along the northern Tien Shan (China) and heterogeneous strain in central Asia. Journal of Geophysical Research: Solid Earth, 96(B3): 4065-4082. Chen, X., Lu, H. F., Shu, L. S., Wang, H. M., Zhang, G. Q., 2002. Study on tectonic evolution of Junggar Basin. Geological Journal

Technology of China ress, 1-279. Dera, G., Brigaud, B., Monna, F., Laffont, R., Pucéat, E., Deconinck, J., Pellenard, P., Joachimski, M. M., Durlet, C., 2011. Climatic ups and downs in a disturbed Jurassic world. Geology, 39(3): 215-218. Eberth, D. A., Brinkman, D. B., Chen, P. J., Yuan, F. T., Wu, S. Z., Li, G., Cheng, X. S., 2001. Sequence stratigraphy, palaeoclimate patterns, and vertebrate fossil preservation in Jurassic-Cretaceous strata of the Junggar Basin, Xinjiang Autonomous Region, People's Republic of China. Canadian Journal of Earth Sciences, 38(12): 1627-1644. Graham, S. A., Brassell, S., Carroll, A. R., Xiao, X., Demaison, G., McKnight, C. L., Liang, Y., Chu, J., Hendrix, M. S., 1990. Characteristics of selected petroleum source rocks, Xinjiang Uygur Autonomous Region, Northwest China. AAPG Bulletin, 74(4): 493-512. Harris, T. M., 1961. The Yorkshire Jurassic Flora, I. ThallophytaPteridophyta. London: British Museum (Natural History), 1-212. Hendrix, M. S., Graham, S. A., Carroll, A. R., Sobel, E. R., McKnight, C. L., Schulein, B. J., Wang, Z. X., 1992. Sedimentary record and climatic implications of recurrent deformation in the Tian Shan: Evidence from Mesozoic strata of the north Tarim, south Junggar, and Turpan basins, northwest China. GSA Bulletin, 104(1): 53-79. Hendrix, M. S., Brassell, S. C., Carroll, A. R., Graham, S. A., 1995. Sedimentology, organic geochemistry, and petroleum potential of Jurassic coal measures: Tarim, Junggar, and Turpan basins, Northwest China. AAPG Bulletin, 79(7): 929-959. Lawver, L. A., Dalziel, I. W. D., Norton, I. O., Gahagan, L. M., 2009. The PLATES 2009 Atlas of Plate Reconstructions (750 Ma to Present Day), PLATES Progress Report No. 325-0509. University of Texas Technical Report, 196: 57-63. Maisch, M. W., Matzke, A. T., 2003. Theropods (dinosauria, saurischia) from the Middle Jurassic Toutunhe Formation of the southern Junggar Basin, NW China. Paläontologische Zeitschrift, 77(2): 281-292. Matzke, A. T., Maisch, M. W., Sun, G., Pfretzschner, H. U., Stöhr, H., 2005. A new Middle Jurassic Xinjiangchelyid turtle (Testudines; Eucryptodira) from China (Xinjiang, Junggar Basin). Journal of Vertebrate Palaeontology, 25(1): 63-70. McKnight, C. L., Graham, S. A., Carroll, A. R., Gan, Q., Dilcher, D. L., Zhao, M., Yun, H. L., 1990. Fluvial sedimentology of an

of China Universities, 8(3): 257-267 (in Chinese with English

Upper Jurassic petrified forest assemblage, Shishu Formation,

abstract).

Junggar Basin, Xinjiang, China. Palaeogeography, Palaeoclima-

Coleman, R. G., 1989. Continental growth of northwest China. Tectonics, 8(3): 621-635.

tology, Palaeoecology, 79(1-2): 1-9. Rabi, M., Joyce, W. G., Wings, O., 2010. A review of the Mesozoic

Coward, M. P., Rex, D. C., Khan, M. A., Windley, B. F., Broughton,

turtles of the Junggar Basin (Xinjiang, Northwest China) and the

R. D., Luff, I. W., Peterson, M. G., Pudsey, C. J., 1986 Collision

paleobiogeography of Jurassic to Early Cretaceous Asian testudi-

tectonics in the NW Himalayas. In: Coward, M. P., Ries, A. C.,

nates. Palaeobiodiversity and Palaeoenvironments, 90(3): 259-

(eds). Collision Tectonics. Geological Society, London, Special Publications, 19: 203-219. Deng, S. H., Lu, Y. Z., Fan, R., Pan, Y. H., 2010. The Jurassic System

273. Rees, P. M., Noto, C. R., Parrish, J. M., Parrish, J. T., 2004. Late Jurassic climates, vegetation, and dinosaur distributions. Journal

Vol. 3 No. 2

Shun-Li Li et al.: Jurassic sedimentary evolution of southern Junggar Basin: Implication for palaeoclimate changes in northern Xinjiang Uygur Autonomous Region, China

of Geology, 112(6): 643-653. Russell, D. A., 1993. The role of Central Asia in dinosaurian biogeography. Canadian Journal of Earth Sciences, 30(10): 20022012. Searle, M. P., Windley, B. F., Coward, M. P., Cooper, D. J. W., Rex, A. J., Rex, D., Li, T. D., Xiao, X. C., Jan, M. Q., Thakur, V. C., Kumar, S., 1987. The closing of the Tethys and the tectonics of the Himalaya. GSA Bulletin, 98(6): 678-701. Shao, L. Y., Zhang, P. F., Hilton, J., Gayer, R., Wang, Y. B., Zhao, C. Y., Luo, Z., 2003. Paleoenvironments and paleogeography of the

161

source rock deposition onshore China. Marine and Petroleum Geology, 4(3): 205-225. Xiao, W. J., Zhang, L. C., Qin, K. Z., Sun, S., Li, J. L., 2004. Paleozoic accretionary and collisional tectonics of the eastern Tianshan (China): Implications for the continental growth of central Asia. American Journal of Science, 304(4): 370-395. Xiao, W. J., Windley, B. F., Yuan, C., Sun, M., Han, C. M., Lin, S. F., Chen, H. L., Yan, Q. R., Liu, D. Y., Qin, K. Z., 2009. Paleozoic multiple subduction-accretion processes of the southern Altaids. American Journal of Science, 309(3): 221-270.

Lower and lower Middle Jurassic coal measures in the Turpan-

Yu, X. H., 2008. Clastic Petroleum Reservoir Sedimentology (Sec-

Hami oil-prone coal basin, northwestern China. AAPG Bulletin,

ond Edition). Beijing: Petroleum Industry Press, 1-551 (in Chi-

87(2): 335-355.

nese).

Shao, L. Y., Gao, D., Luo, Z., Zhang, P. F., 2009. Sequence stratig-

Zhang, Y. J., Qi, X. F., Cheng, X. S., 2003. Jurassic System in the

raphy and palaeogeography of the Lower and Middle Jurassic

North China, Volume III, Xinjiang Stratigraphic Region. Beijing:

coal measures in Turpan-Hami Basin. Journal of Palaeogeogra-

Petroleum Industry Press, 1-400 (in Chinese with English ab-

phy (Chinese Edition), 11(2): 215-224 (in Chinese with English

stract).

abstract). Valdes, P. J., Sellwood, B. W., 1992. A palaeoclimate model for the Kimmeridgian. Palaeogeography, Palaeoclimatology, Palaeoecology, 95(1-2): 47-72.

Zhao, X. J., Currie, P. J., 1993. A large crested theropod from the Jurassic of Xinjiang, People's Republic of China. Canadian Journal of Earth Sciences, 30(10): 2027-2036. Zhong, X. C., Zhao, C. B., Yang, S. Z., Shen, H., 2003. Jurassic Sys-

Wang, Y. L., Wang, Y. M., Qi, X. F., Guan, S. R., Zhao, X. Q., Li, R.

tem in the North China (II): Palaeoenvironment and Oil-Gas

F., 2001. Classification of stratigraphic sequences of Jurassic in

Source. Beijing: Petroleum Industry Press, 1-201 (in Chinese

Junggar Basin. Xinjiang Petroleum Geology, 22(5): 382-385 (in

with English abstract).

Chinese with English abstract). Watson, M. P., Hayward, A. B., Parkinson, D. N., Zhang, Z. M., 1987. Plate tectonic history, basin development and petroleum

(Edited by Xiu-Fang Hu)