Scholarly article on topic 'Lacustrine basin evolution and coal accumulation of the Middle Jurassic in the Saishiteng coalfield, northern Qaidam Basin, China'

Lacustrine basin evolution and coal accumulation of the Middle Jurassic in the Saishiteng coalfield, northern Qaidam Basin, China Academic research paper on "Earth and related environmental sciences"

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{"Sedimentary facies" / "Middle Jurassic" / "Northern Qaidam Basin" / "Lacustrine basin evolution" / "Coal accumulation"}

Abstract of research paper on Earth and related environmental sciences, author of scientific article — Meng Li, Long-Yi Shao, Lei Liu, Jing Lu, Baruch Spiro, et al.

Abstract Based on an extensive borehole survey of the Middle Jurassic coal-bearing sequences in the Saishiteng coalfield, northern Qaidam Basin (NQB), a total of 20 rock types and 5 sedimentary facies were identified, including braided river, meandering river, braided delta, meandering river delta, and lacustrine facies. The distribution of rock types and sedimentary facies contributed to the reconstruction of three periods' sedimentary facies maps of the Middle Jurassic in the Saishiteng coalfield, namely, the Dameigou age, the early Shimengou age and the late Shimengou age. That also provided the basis for the development of a three-stage depositional model of the Middle Jurassic in the NQB, indicating the lacustrine basin of the NQB in the Dameigou age and early Shimengou age were corresponding to an overfill basin, and that in the late Shimengou age was related to a balanced-fill basin. The analysis of the stability and structure of coal seams based on sedimentary facies maps showed that the preferred coal-forming facies in the Saishiteng coalfield were inter-delta bay and interdistributary bay of lower delta plain in the Dameigou age. In particular, the swamps that developed on the subaqueous palaeohigh favored the development of thick coal seams. Thus, minable coal seams may also be found along the Pingtai palaeohigh in the western part of the Saishiteng coalfield.

Academic research paper on topic "Lacustrine basin evolution and coal accumulation of the Middle Jurassic in the Saishiteng coalfield, northern Qaidam Basin, China"

Accepted Manuscript

Lacustrine basin evolution and coal accumulation of the Middle Jurassic in the Saishiteng coalfield, northern Qaidam Basin, China

Meng Li, Long-Yi Shao, Lei Liu, Jing Lu, Baruch Spiro, Huai-Jun Wen, Yong-Hong Li

PII: S2095-3836(15)31068-3

DOI: 10.1016/j.jop.2016.03.001

Reference: JOP 23

To appear in: Journal of Palaeogeography

Received Date: 15 December 2015

Accepted Date: 2 March 2016

Please cite this article as: Li, M., Shao, L.-Y., Liu, L., Lu, J., Spiro, B., Wen, H.-J., Li, Y.-H., Lacustrine basin evolution and coal accumulation of the Middle Jurassic in the Saishiteng coalfield, northern Qaidam Basin, China, Journal of Palaeogeography (2016), doi: 10.1016/j.jop.2016.03.001.

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Lacustrine basin evolution and coal accumulation of the Middle Jurassic in the Saishiteng coalfield, northern

Qaidam Basin, China

Meng Li, Long-Yi Shao, Lei Liu, Jing Lu, Baruch Spiro, Huai-Jun Wen, Yong-Hong Li

Meng Li College of Geosciences and Survey Engineering, China University of Mining and Technology, Beijing 100083, People's Republic of China; present address: Institute of Resources and Environment, Henan Polytechnic University, Jiaozuo 454000, People's Republic of China; lmgeology@126.com

Long-Yi Shao College of Geosciences and Survey Engineering, China University of Mining and Technology, Beijing 100083, People's Republic of China; ShaoL@cumtb. edu.cn

Jing Lu College of Geosciences and Survey Engineering, China University of Mining and Technology, Beijing 100083, People's Republic of China; Lujing@cumtb. edu.cn

Lei Liu College of Geosciences and Survey Engineering, China University of Mining and Technology, Beijing 100083, People's Republic of China; Lujing@cumtb. edu.cn

Baruch Spiro Department of Mineralogy, Natural History Museum, London, United Kingdom; baruch.spiro@gmail.com

Huai-Jun Wen Qinghai Bureau of Coal Geological Exploration, Xining 810000, People's Republic of China; qh 105whj @126. com

Yong-Hong Li ~ Qinghai Bureau of Coal Geological Exploration, Xining 810000, People's Republic of China; lyh6807@163.com

Lacustrine basin evolution and coal accumulation of the Middle Jurassic in the Saishiteng coalfield, northern Qaidam Basin, China

Meng Li1,2, Long-Yi Shao1*, Lei Liu1, Jing Lu1, Baruch Spiro3, Huai-Jun Wen4, Yong-Hong Li4

Abstract: Based on an extensive borehole survey of the Middle Jurassic coal-bearing sequences in the Saishiteng coalfield, northern Qaidam Basin (NQB), a total of 20 rock types and 5 sedimentary fades were identified, including braided river, Meandering river, braided delta, meandering river delta, and lacustrine. The distribution of rock types and sedimentary facies contributed to the reconstruction of three sedimentary facies maps of the Middle Jurassic in the Saishiteng coalfield, namely, the Dameigou age, the early Shimengou age and the late Shimengou age. That also provided the basis for the development of a three-stage depositional model of the Middle Jurassic in the NQB, which was classified as an overfill and a balanced-fill lacustrine basin. The analysis of the stability and structure of coal seams based on the sedimentary maps showed that the preferred coal-forming facies in the Saishiteng coalfield were the inter-delta bay and the interdistributary bay of the lower delta plains in the Dameigou age. In particular, the swamps that developed on the subaqueous paleohigh favored the development of thick coal seams. Thus, minable coal seams may also be found along the Pingtai paleohigh in the western part of the Saishiteng coalfield. Keywords: sedimentary facies, Middle Jurassic, northern Qaidam Basin, lacustrine basin evolution, coal accumulation

1 Introduction

A classification of lake types, including overfilled, balanced-fill, and underfilled,

was proposed by Carroll and Bohacs (1999, 2001), and allowed us to differentiate the

1 continental basins according to their tectonic and paleoclimatic settings (Nichols and

2 Fisher, 2007; Türkmen et al., 2007; Diessel, 2007; Wadsworth et al., 2010; li et al,

3 2014b). However, the tectonic environment, lacustrine basin evolution, and energy

4 resources associated with different sedimentary facies in continental coal-bearing

5 basins are not yet fully understood.

6 The Jurassic non-marine basin in the northern Qaidam Basin (NQB), northwestern

7 China, formed on top of the pre-Jurassic basement and is geologically significant

8 because of its abundant oil, gas, and coal resources (Yang et al., 2000; Zhao et al.,

9 2000; Liu et al., 2013; li et al., 2014a). The Saishiteng coalfield which is located in

10 the western part of the northern Qaidam Basin, represents a new coal exploration area

11 with high-quality coal resources beared in the Middle Jurassic. In the Saishiteng

12 coalfield, a large number of coal and oil boreholes were drilled (Figure 1) in recent

13 years, facilitating a more detailed analysis of rock types, sedimentary facies,

14 paleogeography, paleocurrents, provenance, subsidence, and coal accumulation.

15 These detailed primary data and subsequent in-depth analyses allow us to address the

16 sedimentary facies distribution, basin evolution and coal-forming characteristics of

17 the continental lacustrine basin, thus a three-stage depositional model was

18 conceptualized and the lacustrine basin evolution and coal accumulation during the

19 Middle Jurassic in the Saishiteng coalfield were discussed.

21 2 Geological setting

22 The Saishiteng coalfield is situated along the northern edge of the Saishiteng

23 depression. At the end of the Late Triassic, mountains around the Qaidam microplate

24 were uplifted because of the Indosinian movement which dated from the late Permian,

25 and a series of sub-depressions associated with interior uplifts were developed in the

26 NQB (Yang et al., 1992; Tang et al., 2000; Zhao et al., 2000; Dai et al, 2003; Chen

27 et al., 2012; Withjack et al., 2013). The Saishiteng depression was one of these

28 depressions, where the Middle Jurassic non-marine coal-bearing sequences were

29 deposited. The basement of the Saishiteng depression was mainly composed of the

Upper Ordovician gneiss, phyllite, slates, tuff and quartzose sandstone in the northwestern and eastern part, and the Upper Ordovician granite and gabbro in the center part (Wang et al., 2006; Liu et al., 2013).

Coal-bearing sequences of the Middle Jurassic in the Saishiteng coalfield consist of the Dameigou Formation and the Shimengou Formation (e.g., Zhang et al., 1958; Zhang, 1998; Dang et al., 2003; Huang et al., 2007; Lu et al., 2014) (Figure 2). The Dameigou Formation is typically composed of coarse- to fine-grained siliciclastic rocks and two medium to thick coal seams. The regional dichotomy of the Dameigou Formation (the lower conglomerate and the lower coal-bearing members) is not clear in the Saishiteng coalfield (Hu et al., 2004; Lu et al., 2009; Dang et al., 2003; Liu et al., 2013; Pang et al., 2014). The Shimengou Formation can be subdivided into the lower member (also known as the coal-bearing member) and the upper member (also known as the shale member). The lower member of the Shimengou Formation mainly consists of fine-to medium-grained siliciclastic rocks intercalated with thin to moderately thick coal seams, while the upper member is characteristic of mudstones and shales.

3 Methods

The database for this study consists of 56 full-hole coring boreholes which cover most part of the Saishiteng coalfield (Figure 1). Rock types were described in core and included lithology (hand specimen and thin section), primary and secondary sedimentary structures, biological features, and sediment body geometries. Rock types were arranged into assemblages representing different sedimentary facies, which were described and correlated in an east-west cross section (Figure 3). The analytical contour maps included the strata isopachs, the sandstone and conglomerate isopachs, the sandstone and conglomerate percentages, the conglomerate percentages, the coarse/fine thickness ratio (sandstone and conglomerate/siltstone and mudstone), the coal and carbonaceous mudstone isopachs, and the coal seam isopachs (Feng et al., 1992, 2004, 2014; Zhang et al., 1993; Shao et al., 2003, 2008, 2009; li et al,

1 2014a). The sedimentary facies maps were mainly based on the coarse/fine ratio

2 contours and modified by the other parameters (e.g. the sandstone and conglomerate

3 isopachs and the mudstone isopachs). The ages used for the reconstruction of these

4 sedimentary facies maps include the Dameigou age, the early Shimengou age, and the

5 late Shimengou age, because the interfaces between the Dameigou Formation, the

6 lower Shimengou Formation, and the upper Shimengou Formation were

7 corresponding to the third-order sequence boundaries of the Jurassic successions

8 (Figure 3; Li et al., 2014a).

9 4 Rock types and sedimentary facies

10 The rock types and Sedimentary facies of the Middle Jurassic Saishiteng coalfield

11 were analyzed based on borehole cores (Figure 1, 2) and on an east-west section

12 (Figure 3). Using lithological and paleontological criteria, as well as geometry and

13 lateral relationships, a total of 20 types of distinct rock types were identified. Their

14 salient features and probable sedimentary interpretations are summarized in Table 1.

15 Based on field observations and geological mapping, the formation of these rock

16 types was related with five sedimentary facies: (1) braided river, (2) meandering

17 river, (3) braided delta, (4) meandering river delta, and (5) lacustrine (Table 2).

19 4.1 Braided river

20 The braided river facies was clearly observed in the lower member of the

21 Dameigou and Shimengou Formations, where the braided channel and flood plain

22 subfacies were identified (Figures 2 and Figure 3). This sedimentary facies consisted

23 primarily of imbricated clast-supported conglomerates, commonly exhibiting trough

24 and tabular cross bedding (No. 1 in Table 1). These clasts were mainly composed of

25 quartzite chlorite and schist, with grain sizes ranging from 10 to 50 mm.

26 Matrix-supported conglomerates were subtle or absent. Thick-bedded, well-rounded,

27 and moderately sorted conglomerates with erosive bases and small tree trunks

28 characterized the channel lag deposits. Lenticular sand bodies exhibiting trough cross

29 bedding were assumed to represent longitudinal bar deposits, while wide sheet-like

1 sand bodies with a typically planar cross bedding were interpreted as transverse bar

2 deposits. Laterally continuous, thin-bedded sandstones and siltstones represented

3 flood-plain deposits (No. 10 and 13 in Table 1). No significant coal deposits formed

4 within this facies.

6 4.2 Meandering river

7 Compared with the braided river facies, the Meandering river facies was clearly

8 characterized by upward fining successions, consisting of meandering channel and

9 well-developed overbank and flood basin deposits associated with swamps. This

10 sedimentary facies was found in the lower member of the Shimengou Formation in

11 the Tuanyushan exploration area.

12 The meandering channel was dominated by a channel lag and point bar deposits

13 (Tables 1 and 2). Lenticular pebbly sandstones, conglomerates containing mudstone

14 clasts, transported tree trunks, parallel bedding, tabular cross bedding, imbrications,

15 and erosional bases characterized the meandering channel log deposits (No. 5 in

16 Table 1). The point bar deposits were composed of moderately to well-sorted fine to

17 medium sandstones (No. 7 in Table 1). The overbank subfacies was composed of

18 levees and crevasse splays and typically characteristic of fine-grained sandstones and

19 siltstones, intercalated with mudstone layers with ripple laminations and typically

20 climbing ripple laminations (No. 12 and 13 in Table 1). The occurrence of root traces

21 was common in these deposits. Flood plain, back bank lake, and swamp were also

22 found in the flood basin facies, which were dominated by siltstones, mudstones, coal

23 seams with ripple marks, ripple laminations, horizontal laminations, rootlets, and

24 common sideritic concretions (No. 17, 18 and 19 in Table 1).

27 4.3 Braided delta

28 The braided delta facies was mainly observed in the Dameigou Formation, in the

29 western part of the Hongliuquan and Tuanyushan exploration areas, and in the lower

1 member of the Shimengou Formation in the Beilutian exploration area. In contrast

2 with the braided river, the braided delta deposits contained grossly upward

3 coarsening successions, with funnel-shaped well logs in the upper part and

4 bell-shaped well logs in the lower part. The rock types of the braided delta was

5 characterized by pebbly sandstones and conglomerates, intercalated with a

6 thin-bedded subaqueous facies of fine-grained siltstones and coals. The variation

7 observed within these sediments allowed the recognition of braided deltaic plain,

8 delta front, and prodelta (Figure 3 and Table 1). The braided deltaic plain was

9 dominated by the gravelly and sandy braided distributary channel. The

10 fine-to-medium-grained sandstones with large cross bedding in the Dameigou

11 Formation were interpreted as the sandy braided distributary channel (No. 6 and 8 in

12 Table 1). In the lower member of the Shimengou Formation in the Beilutian

13 exploration area, thick-bedded conglomerates with a large trough cross bedding,

14 erosional bases, and local imbrications characterized the gravelly braided distributary

15 channel (No. 2 in Table 1). The funnel-shaped successions in the well logs

16 resembling gray-black fine sandstone and siltstone with rhythmic bedding, small

17 cross bedding, and horizontal bedding were interpreted as the delta front and prodelta

18 (No. 4, 8, 9,11, 12, 14 and 18 in Table 1).

20 4.4 Meandering river delta

21 The meandering river delta facies was represented by broadly distributed sandy

22 deltaic deposits with overall upward coarsening successions (Table 1, 2). These

23 sedimentary strata showed many characteristics similar to classically described

24 deltaic sequences, such as the Allegheny deltas in the Appalachian Basin (Horne et

25 al., 1978) and the Westphalian deltas in the Pennine Basin in northern England

26 (Fielding, 1984). The variation observed within these sediments allowed the

27 recognition of fluvial-dominated upper delta plain, lacustrine-dominated lower delta

28 plain, subaqueous delta and interdelta bay facies, both in the vertical and cross

29 sections of the basin (Figures 2 and 3).

1 The upper delta plain subfacies was dominated by a meandering fluvial system

2 and was observed in the lower member of the Shimengou Formation, in the western

3 part of the Hongliuquan and Tuanyushan exploration areas. The meandering channel

4 cross-bedded thick sandstones (No. 7 in Table 1), levee and crevasse splay ripple

5 laminated siltstones (No. 10 in Table 1), and interdistributary flood basin gray

6 mudstones and carbonaceous mudstones (No. 10, 15 and 17 in Table 1) characterized

7 this facies. Meanwhile, fining-upward sandstones with an erosional base (No. 14 in

8 Table 1), trough and tabular cross bedding overlain by thick-bedded siltstones (No.

9 16 in Table 1), and coal and carbonaceous mudstones (No. 18 and 19 in Table 1)

10 were interpreted as the distributary channel, the interdistributary bay, and the

11 crevasse splay deposits of the lower delta plain subfacies, respectively. Coal-forming

12 swamps were clearly observed in the lower delta plain and were suggested to be

13 responsible for the widespread coal accumulation in the Dameigou Formation of the

14 Saishiteng coalfield.

15 The subaqueous delta subfacies was used here to represent the subaqueous parts of

16 the delta, including the delta front and the prodelta deposits. The delta front

17 comprised the massive or cross-bedded calcite-cemented sandstones of the

18 subaqueous distributary channel, the mouth bar sandstones and siltstones with

19 wedge-shaped cross-bedding and convolute bedding (No. 8 and 9 in Table 1), and the

20 distal bar sheet-like ripple laminated siltstones (No. 10 in Table 1). The prodelta

21 deposits were mainly composed of silty mudstones and thin-bedded siderite layers

22 with ripple laminations or horizontal laminations (No. 14 in Table 1).

23 The interdelta bay subfacies was found between the two delta lobes, where the

24 water energy was restricted by their proximity. These deposits were characterized by

25 a sequence of thick-bedded coal seams with few sandstone intercalations (No. 18 and

26 19 in Table 1). This facies represented a preferred coal-forming environment and was

27 responsible for the major coal seams in the Dameigou Formation.

29 4.5 Lacustrine

1 The lacustrine facies was widely found in the upper member of the Shimengou

2 Formation throughout the Saishiteng coalfield. These deposits were characterized by

3 dark gray fine sandstone, siltstone, mudstone, and thick-bedded shale or oil shale.

4 The longshore bar deposits of the shallow lacustrine environment were characterized

5 by fine sandstones with low-angle cross bedding, grayish bioturbated siltstones with

6 burrows, intercalated with black mudstones, and horizontal laminations (No. 15 in

7 Table 1). Storm deposits, consisting of argillaceous conglomerates with irregularly

8 arranged muddy gravels and sharp erosional bases, associated with deformed

9 bedding, were found in the lacustrine facies. The shallow lacustrine mudstones often

10 contained freshwater bivalve communities and vitrinized tree trunks (Figure 2).

11 The deep lacustrine subfacies was represented by black shales, black/brown oil

12 shales, and dark mudstones that developed in the upper part of the upper member of

13 the Shimengou Formation (No. 14 and 20 in Table 1). In the Saishiteng coalfield, the

14 bitumen content of these oil shales was relatively low, generally ranging between

15 0.5% and 3.5%.

17 5 Sedimentary facies maps

18 To compile the Middle Jurassic rock types and sedimentary facies, several

19 lithological contour maps were drawn based on a total of 56 boreholes drilled for coal

20 and oil exploration. Space limitation precluded the reproduction of all maps in this

21 study, but all these maps are available upon request to the corresponding author. The

22 sedimentary facies maps were mainly based on the coarse/fine ratio contours, and

23 modified by the other parameters (Figures 4, 5, and 6).

25 5.1 The Dameigou age

26 During the Dameigou age, sedimentary stratum were found in the entire

27 Beiloutian exploration area and in the Laogaoquan coal mine, and the southern part

28 of the Hongliuquan and Tuanyushan exploration areas (Figure 4). In the Saishiteng

29 coalfield, the thickness of the Dameigou Formation was commonly between 90 and

30 200 m, even though it ranged between 60 and 240 m, being thicker around borehole

13-11. The major sedimentary facies developed in the Dameigou age included the meandering river, meandering upper delta plain, meandering lower delta plain, the braided delta plain, the subaqueous delta (delta front and prodelta), and shallow lacustrine (Figure 4).

The meandering river is characteristic of parallel bedding sandstone and large tabular cross-bedded sandstone around the borehole No. 17, and the corresponding coarse/fine ratio is greater than 2.0 (Table 1, 2). The meandering upper delta plain could be identified by the siltstones with convolute bedding, ripple-laminated siltstone, interlaminations of fine sand, silt and mudstone and coal, having a coarse/fine ratio of 1.0-2.0. The meandering lower delta plain consists of dark gray mudstones of distributary channel, sideritic mudstone and coal, having a coarse/fine ratio of 0.5-1.0. The braided delta plain is composed of the medium-small sized conglomerates of gravelly braided distributary channel, Interbedded fine sandstone and siltstones of distributary bay, having a coarse/fine ratio of 0.5-2.0. The subaqueous delta (delta front and prodelta) could be identified by sandstone having wedge-shaped cross-bedding and siltstones with convolute bedding, and its coarse/fine ratio ranged from 0.3 to 0.5. The shallow lacustrine consists of siltstones containing burrows and bioturbation structures, having a coarse/fine ratio <0.3 (Figure 4).

The fluvial channel sandstones and conglomerates mainly originated from the Saishiteng paleohigh, located at north and northeast, indicating a dominating northern source area. The lacustrine deposits developed in the center of the study area and were not clearly revealed by the existing boreholes. The swamps were characteristically developed in the interdelta bay, lower delta plain, and along the subaqueous Pingtai paleohigh in the west of the study area (Figure 4 and 7).

5.2 The early Shimengou age

At the early Shimengouage, the thickness of this stratigraphic unit ranged between 50 and 150 m, with an average thickness of 97 m. The depocenters were situated in

1 the central area of the Tuanyushan exploration area, with thicknesses of 110-140 m.

2 The meandering river developed in the northwestern part of the Saishiteng coalfield

3 with a coarse/fine ratio >2.0, while the braided river was distributed in the

4 southeastern part of the Saishiteng coalfield. The braided delta plain, meandering

5 upper delta plain and lower delta plain occupied the northern part of the Saishiteng

6 coalfield with a overall coarse/fine ratios between 0.5-2.0. The subaqueous delta (0.3

7 <coarse/fine ratios <0.5) and shallow lacustrine facies(coarse/fine ratios <0.3)were

8 mainly developed in the southern part of the study area. (Figure 5). The thick-bedded

9 conglomerates and coarse sandstones found in the eastern and northern parts mainly

10 originated from the Saishiteng paleohigh. In addition, up to four layers of coarse sand

11 bodies occurred in these depositional intervals mostly formed in the channel lag and

12 distributary channel.

14 5.3 The late Shimengou age

15 The stratigraphic unit of the late Shimengou age had an overall thickness of

16 30-170 m, with a southward increase in thickness, suggesting that the depocenter was

17 located in the southernmost region of the Saishiteng coalfield. In this coalfield, the

18 most common stacking pattern in this stage was progradational (Figures 2 and 3). The

19 overall sedimentary facies of this stage was a deeper lacustrine with a coarse/fine

20 ratio <0.3 and the thickness of shales and mudstones more than 25m. Thick-bedded

21 and oil shales characterized the deposits of these facies. The areas with a coarse/fine

22 ratio more than 0.3 and the thickness of shales and mudstones less than 25m were

23 inferred to be a shallow lacustine. The fine sandstones and siltstones of the shallow

24 lacustrine were only found along the northeastern residual boundary of the Middle

25 Jurassic in the early phase of this stage, showing a predominantly northeastern origin.

26 Marginal facies were not found in this area and the sediments probably derived from

27 the paleohigh bounding the Jurassic, NQB (Figure 6).

29 6 Lacustrine basin evolution

1 The Lower and Middle Jurassic coal-bearing successions developed during a

2 period of continuous migratory and extensional rift (Di and Wang, 1991; Hu et al.,

3 1999; Cao et al., 2005; Lu et al., 2014). In the Saishiteng coalfield, this rifting

4 emerges from the Middle Jurassic, coinciding with other depression along the NQB.

5 The sediments from this interval record an evolution in the basin fill from a

6 predominantly shallow-water fluvial delta to a predominantly deep lacustrine

7 environment (Table 2; Figures 4, 5, and 6). These different depositional facies of the

8 Middle Jurassic coal sequences corresponded to a three-stage depositional evolution,

9 specifically including the Dameigou, the early Shimengou, and the late Shimengou

10 ages (Figure 7).

11 The Dameigou age corresponded to the initial rifting stage of the Middle Jurassic

12 occurring along the NQB (Tang et al., 2000; Yang et al., 2000; Pang et al., 2014).

13 During this age, the relatively continuous and slow tectonic subsidence mainly

14 provided the space for the basin infill. The sedimentary facies of this stage in the

15 Saishiteng coalfield were dominated by a braided delta and a low-sinuosity

16 meandering delta associated with swamps in the paleohigh area. The early

17 Shimengou age was correlated with the rapid tectonic rifting stage of the Middle

18 Jurassic, which provided sufficient clastic materials for the development of the

19 braided and meandering river with a well-developed flood plain and overbank

20 deposits occupying the major part of the Saishiteng coalfield. The late Shimengou

21 age was dominated by a major lake flooding during the period of increasing

22 deepening, possibly related with a rapid subsidence of the basement (Figures 3 and

23 7).

24 The development and distribution of these different sedimentary facies were

25 strongly influenced by tectonics, sediment supply, and climate changes. The type of

26 lacustrine basin depended on the balance between the rate of potential

27 accommodation, mostly influenced by tectonics, and the sediment and water supply,

28 mostly influenced by the climate (Carroll and Bohacs, 1999; Lin et al., 2001). Based

29 on the rock types, sedimentary facies, and facies associations, basin types of the

1 Middle Jurassic in the Saishiteng coalfield can be classified into two kinds of

2 lacustrine basin evolution (Figure 7, Tables 1 and 2). The rock types of the Dameigou

3 Formation and lower member of the Shimengou Formation were dominated by

4 sandstones, mudstones, coal, and locally by conglomerates with a progradational

5 stacking pattern, while rooted mudstones were commonly found in the roof and floor

6 of the coal seams, clearly suggesting an overfilled lacustrine basin type of Carroll and

7 Bohacs (1999, 2001). The late Shimengou Formation stage included siltstones,

8 mudstones, and thick-bedded shales with a fine gypsum lamination, indicative of a

9 fresh to saline lake, and suggesting a balanced-fill lacustrine basin.

11 7 Coal accumulation

12 The balance between the rate of potential accommodation generated by tectonic

13 subsidence and sediment (clastic material and peat) supply generally controls the

14 accumulation of coal (Han and Yang, 1980; Nemec, 1988; Bohacs and Suter, 1997;

15 Diessel et al., 2000). Moderate rates of subsidence ensure a gradually rising base

16 level that maintains the necessary accommodation for peat and coal accumulation and

17 the exclusion of siliciclastic input by fluvial and distributary channels. The tectonic

18 subsidence and the sediment supply are influenced by the different sedimentary

19 facies.

20 Based on the sedimentary facies maps and the evolution of the lacustrine basin

21 (Figures 4, 5, 6, and 7), we analyzed the stability and structure of the coal seams

22 formed in the different subfacies/microfacies. The analyses indicated that the

23 preferred coal-forming environments were interdelta bays, interdistributary bays on

24 lower delta plains, and swamps on subaqueous paleohighs. In these

25 subfacies/microfacies, moderate levels of subsidence were maintained, and with little

26 input of coarse-grained sediments (mostly less than 50%), the water level remained

27 relatively stable. Coal seams formed in these areas were thicker and continuous both

28 horizontally and vertically, like Coal Nos. M7 and M4 (Figure 3 and Table 3).

29 However, in the flood plains/overbank of braided(meandering) river and distributary

bays of braided delta facies, the subsidence rates were either too low or too high and not ideal for coal accumulation. Coal seams formed in these areas were commonly thinner with intercalations of siltstone and mudstone, and their stability was relatively poor. Coal Nos. M1, M2, M3, and M6 generally developed in such facies. This suggests that the ideal sites for coal accumulation were those with moderate rates of subsidence that favor the development of interdelta bays, interdistributary bays on lower delta plains, and swamps on subaqueous paleohighs.

It is thus inferred that thick-bedded coal seams can also be found in the western part of the study area along the Pingtai paleohigh, where no sufficient deep drilling boreholes and seismic data are available at present. Thus, another potential exploration area is proposed, suggesting that the economic potential of the Saishiteng coalfield has yet to be fully realized (Figures 1, 4, and 5, and Table 3).

8 Conclusions

1) Twenty different rock types and five sedimentary facies were identified in the Middle Jurassic coal-bearing sequences in the Saishiteng coalfield. These five sedimentary facies include braided river, meandering river, braided delta, meandering river delta, and lacustrine. These sedimentary facies could be further subdivided into 14 subfacies and several more detailed microfacies.

2) Three sedimentary facies maps of the Middle Jurassic in the Saishiteng coalfield were reconstructed based on the thickness and facies variation across the coalfield. The depositional units developed during the Middle Jurassic contained braided (meandering) river, delta plains, subaqueous deltas, and lacustrine. In summary, during the Middle Jurassic, the basin experienced cyclic changes from swamp conditions to a lacustrine flooding facies, which was responsible for the formation of coal-bearing sedimentary sequences. The sediments supply to the basin during the Middle Jurassic included two dominant sources: the Saishiteng paleohigh to the north and east, and the Pingtai paleohigh to the west.

1 3) The Saishiteng coalfield experienced three stages of lacustrine basin evolution

2 during the Middle Jurassic, for which three corresponding evolutionary schemes were

3 proposed. During the Dameigou age, an approximately continuous and slow tectonic

4 subsidence provided the accommodation for the basin infill. The early Shimengou

5 age correlated with the sharp tectonic rifting stage of the Middle Jurassic, providing

6 sufficient clastic materials for the development of the braided and meandering rivers,

7 which occupied most of the Saishiteng coalfield. Then, the late Shimengou age was

8 dominated by a major lake flooding during a period of increasing deepening, possibly

9 related with rapid subsidence of the basement. In the Saishiteng coalfield, the

10 Dameigou and early Shimengou ages were related with an overfilled lacustrine basin

11 type, while the late Shimengou age correlated with a balanced-fill lacustrine basin.

12 4) The stability and structure of the coal seams formed in the different

13 subfacies/microfacies in the Middle Jurassic were analyzed. The results showed that

14 the preferred coal-forming subfacies/microfacies were interdelta bays,

15 interdistributary bays on lower delta plains, and, especially, swamps on subaqueous

16 paleohighs. The existence of thick coal seams in the western part of the study area

17 along the Pingtai paleohigh was suggested, potentially corresponding to another

18 future exploration area.

20 Acknowledgements

21 This research was supported by the National Natural Science Foundation of China

22 (41030213, 41502108) and the Major National Science and Technology Program of

23 China (2011ZX05033-002).

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■Laogaoquan Coal Mine

«J/ -/wy S KT-4 - <- 0 21-22 „p,/ /»22-21 oF

2 ° 23-23 ■ (

' 25-25 oBeiloutian I Exploration Arec' \__'

21-19 KT-23^"^ 29-26 "25-4

Tuanyushan ~ "—ï——0 24-28 Exploration Area 21-20 21-22

Coal exploration borehole (2Z3 Strata boundary of erosion a Oil exploration borehole East-west borehole section in Figure 3

The Saishiteng Coalfield

Golmud

90° " -J i \ 35°

100°, -

Figure 1 Location of the Saishiteng coalfield, northern Qaidam basin. The location of borehole data used in Figure 3, 4,5 and 6 are shown in the upper part.

Stratigraphy

GR/ (API),,

Lithology

cn Lithology Description

Sedimentary Facies

Black shale and black brown oil shale with interlays of black -grey mudstone

Dark grey fine sandstone, siltstone and mudstone

Dark grey mudstone, siltstone with interbeds of medium sandstone, coal and siltstone.

Grey white coarse sandstone with interbeds of siltstone.

Dark greymudstone, siltstone with interbeds of siltstone and coal.

Distributary bay

Sandy braidec

distributary

channel

Distributary bay

Coal with interlays of siltstone and mudstone.

Grey white coarse sandstone with interbeds of siltstone mudstone.

Sandy braided

distributary

channel

Dark grey mudstone and coal with interlays of siltstone and mudstone

Grey white coarse sandstone with interbeds of siltstone mudstone.

Meandering channel

Dark grey mudstone, siltstone with interlays of medium sandstone.

Grey white coarse sandstone with interbeds of siltstone mudstone.

Dark grey mudstone and coal with interlays of carbonaceous mudstone.

Sandy braidec

distributary

channel

Distributary bay

Sandy braidec

distributary

channel

Grey conglomerate.

Braided channe

Erosion surface 7777 Tabular bedding >- Plant fossil

^ Shale f=r-

Mudstone e^;

^ Parallel bedding ^ Small cross bedding 5) Plant leaves

Muddy siltstone Siltstone

i• • • • i Fine s • • • Me diu I * * I Coars'

sandstone

dstp: indst

!..,»,! Graded bedding

3 Deformed bedding - —-------

_ d.j:.-,.. .. .—, , ,. mudstone

XT Kaaicites □ Massive bedding <£> Siderite nodules

= Horizontal bedding ^ Wavy bedding

)bly s; [H Conglomerate

■■ Coal

lc - -I Carbonaceous-

1 Figure 2 Columnar section showing sedimentary features of the Middle Jurassic coal

2 measures in the borehole 24-28.

Figure 3 East-west Sedimentary facies fence diagram through the Saishiteng area, showing facies variation in the Middle Jurassic.

67.5 X \ 05.0

65.0 /\ 07.5

38 30V\94 15' 10. 0\ X 62. 5

Laogaoquan Coal Mine

0 1.0 km

Pingtai-P ale o high

62¡ 5/

Meandering river

Meandering-upper delta plain

Subaqueous delta\-'m-'m- Lower delta plain >---_--; Shallow lacustrine

Interdelta bay swamp Provenance

Contours of the coarse/fine ratio

Residual stratum or sedimentary boundary

Swamp on-subaqueous paleohigh Basin boundary

Meandering-upper delta plain

Lower delta plain Subaqueous delta Shallow lacustrine

Provenance

Contours of the coarse/fine ratio

Residual stratum or sedimentary boundary

Basin boundary

^ Provenance

Contours of the-coarse/fine ratio

Residual stratum or-sedimentary boundary

Basin boundary x^ Shallow lacustrine

Deeper lacustrine

Dameigou stage

Alluvial fan Erosional area —

Swamp on subaqueous palaeohigh -Braided and meandering river

Delta plain and swamp

Early Shimengou stage

Aquatic palaeohigh-.

Erosional area — Alluvial fan —

-Braided and meandering river

- Flood fan — Fluvial plain and swamp

Delta plain and swamp

P/E= ?

Late Shimengou stage

-Fan delta (not found)

Deep lake

Shallow

P/E=1.0

POTENTIAL ACCOMMODATION RATE (proportional to basin subsidence)

Figure 7 Depositional models showing the three-stage evolution and corresponding lacustrine basin types of the Middle Jurassic in the Saishiteng coalfield (lower left is modified after Carroll and Bohacs, 1999).

Table 1 Rock types of the Middle Jurassic coal-bearing sequences in the Saishiteng coalfield.

No. Rock types Lithology Sedimentary stuctures Geometry Fossils Sedimentary facies

1 Massive Clast supported conglomerates, Trough and tabular cross bedding. Multistoreyed Braided channel

coarse-sized Gravel: 10-50mm, medium Erosional bases. Imbrications. lenticular,

conglomerates rounded, poorly sorted, mainly one to several

rock debris from quartzite meters thick

chlorite and schist. Matrix: fine

sands with siliceous and

calcareous cements.

2 Medium-small sized Grey-white clast supported Graded bedding. Tabular cross Multistoreyed Plant debris, Braided channel,

conglomerates. conglomerates, Gravel: 3-8mm, bedding. Erosional bases. lenticular, tree trunks meandering channel

well-rounded, medium-to-well 1-10 m thick

sorted, mainly quartz with some

feldspar having kaolinization.

Matrix: clean, well-rounded sand

with calcareous cements.

3 Massive gravelly Massive coarse-to -very -coarse Erosional base Multistoreyed Plant debris, Channel lag

sandstone sand, in combination with poorly layers to thick tree trunks

sorted and rounded gravels. bedded, 0.5-2m.

4 Massive medium to White-grey, moderately to well Massive bedding. Occasionally Sheet or wide In-situ tree Mouthe bar

coarse sandstone sorted, 0.5 to several meters having graded bedding. lenticular trunks

thick. calcareous cementation.

Loosely under the influence of

water.

No. Rock types Lithology Sedimentary stuctures Geometry Fossils Sedimentary facies

5 Parallel bedding Well sorted and rounded Parallel bedding with 0.5-1mm thick Sheet. Transported Meandering

sandstone medium to coarse sandstone, cosets. Parting lineation on bedding Multistoreyed. tree trunks and channel.

mainly siliceous or argillaceous plane. „1-10 m thick. stems.

cementation.

6 Trough Medium to coarse sand, Predominately large scale trough Lenticular shape Transported Braided channel,

cross-bedded moderately sorted cross-bedding, and occasionally, having tree trunks distributary channel

sandstone wedge cross-bedding. Brupt connection with underlying sediment. erosional base

7 Large tabular Coarse to medium; moderate to parallel bedding and tabular Branching, Transported Meandering

cross-bedded well-sorted cross-bedding, containing mudstone lenticular tree trunks channel,

sandstone clasts, Erosional bases and wide sheet distributary channel

8 Sandstone having Fine to medium sand, wedge-shaped cross-bedding, rippled Sheet, wide Transported Mouth bar

wedge-shaped well-sorted, cross-bedding lenticular tree trunks

cross-bedding

9 Siltstones with Convolute bedding. Siltstones, mudstones Convolute- and wavy bedding. Sheet. Fauna. Mouth or distal bars.

10 Ripple-laminated Silts having fine sand, ripple marks and ripple lamination, Sheet Leaves, plant Various

siltstone interbedded with mudstone layers horizontal lamination, debris, in-situ trunks, root horizons environments but common in flood plain

11 Siltstone with Siltstone and mudstone horizontal bedding, and, Sheet plant debris, Various

horizontal bedding occasionally, ripple bedding faunal environments but common in

No. Rock types Lithology Sedimentary stuctures Geometry Fossils Sedimentary facies

lacustrine

12 Interbedded fine Rhythmically stacking of fine Erosional bases, small cross bedding, Fan-shaped or Plant debris Crevasse splays.

sandstone and sands and siltstones, always ripple- and graded-bedding. lenticular. and stems.

siltstones having muddy gravels and coarse at their base of fine sandstone.

13 Interlaminations Interbedded fine, moderate to Interlaminations, horizontal bedding, Sheet. Rootlets and Levees or flood

of fine sand, silt well-sorted and ripples and bioturbation. plant fossils. plains.

and mudstone. rounded sandstones, siltstones and mudstones.

14 Dark gray Mudstones, clays, shales and Interlaminations, horizontal bedding, Sheet. Fauna and Various

mudstones. muddy limestones containing pyrite and siderite. ripples and bioturbation. plant debris. environments but common in lagoons of lacustrine and delta plain

15 siltstones containing burrows and bioturbation structures gray sandy mudstones and siltstone burrows and bioturbation structures sheet Shallow lacustrine

16 Sideritic mudstone Mudstone having sideritic massive, sometimes having Lenticular or Plant debris Interdistributary

and concretion concretions and nodules, lamination thin bedded bay, lacustrine bay

17 Seat earths. Commonly mottled seat earths. Massive. Sheet. Rootlets and plant debris. Swamps or pedogenic

No. Rock types Lithology Sedimentary stuctures Geometry Fossils Sedimentary facies

environments.

18 Carbonaceous Carbonaceous mudstone Massive or banding Sheet Plant debris Swamp, interdeltaic mudstone bay

19 Coal Coal Horizontal lamination, banding Sheet, Plant debris Swamp,

Branching,

20 Oil shale Foliated lamellar, low oil Horizontal bedding Sheet Sporopollen Deep lacustrine

content fossils

Table 2 Sedimentary facies/subfacies/microfacies of the Middle Jurassic coal bearing sequences in the Saishiteng coalfield.

Sedimentary facies

Subfacies

Microfacies

Rock types*

Braided river

Braided channel

Flood plain

channel lag, longitudinal and transvverse bars

1G, 1З

Braided delta

Meandering river

Braided delta plain

Flood basin

Gravelly braided distributary channel

Sandy braided distributary channel

Distributary bay

Flood plain, back lake, back swamp

11, 12 ,1б

14, 18, 19

Gravelly mouth bar 4, 8

Delta front Sandy mouth bar 8, 9,12

Distal bar 9

Prodelta 11, 14, 18

Meandering channel Channel lag and point bar 5, l

Overbank deposits Levee and crevasse splay 12, 1З

ll, 18,19

Meandering river delta

Upper delta plain

Lower delta plain

Subaqueous delta

Meandering distributary channel

Levee and crevasse splay

Interdistributary flood plain

Distributary channel

Interdistributary bay

Interdistributary swamp

Mouth bar

Distal bar

Ш,12,1З

1G,15,1l

1б,11,18,19

1б, 18

Sedimentary facies Subfacies Microfacies Rock types*

Prodelta 14

Interdelta bay 18, 19

Lacustrine Shallow lacustrine Swamp on subaqueous palaeohigh, longshore bar, including storm deposits 15

Deep lacustrine 14,20

*-refer to numbers of rock types in Table 1.

Table 3 Coal accumulation characteristics of different sedimentary facies in the Middle Jurassic of the Saishiteng coalfield.

Characteristics of coal seams

Sedimentary facies Facies/subfacies -

Structure of coal seams Stability Coal Nos.

Braided (Meandering) river Overbank/flood plain Thin-bedded, moderately to complicatedly stacked. Poor lateral continuity, relatively stable thickness Medium lateral continuity and stable M1, M2, M3, M6

Braided delta Distributary bay and overbank Medium beded, simply to complicatedly stacked. Thick beded on interdelta bay and interdistributary bay with thickness on distributary bay, instable thick and poor lateral continuity on overbank. M4, M5

Meandering river delta Interdistributary bay in delta plain and interdelta bay medium thickness on interdistributary flood basin, overall simply to moderately stacked. Good lateral continuity and stable thickness M4, M7

Lacustrine Swamp on subaqueous palaeohigh Perfectly thick coal seams and simply stacked Continuous locally and stable thickness M7