Jurassic continental coal accumulation linked to changes in palaeoclimate and tectonics in a fault‐depression superimposed basin, Qaidam Basin, NW China

The Middle Jurassic Aalenian–Bajocian stages represent the tectonic transition period for the group of Jurassic sedimentary basins in northwestern China and include accumulation of the most important recoverable coal seams. Geological data from borehole cores, well loggings and rock‐mineral determination in the Aalenian to Bajocian‐aged Dameigou Formation in the Yuqia coalfield, Northern Qaidam Basin were employed to unveil the relationship between coal accumulation and changes in palaeoclimates and provenance tectonic activity. The results identify six kinds of lithofacies from fluvial and lacustrine sedimentary systems, and allow the Dameigou Formation to be divided into two third‐order sequences (SIII1–SIII2) and their corresponding system tracts by regional unconformity surfaces, forced regression surfaces and their correlative conformity surfaces, and the abrupt shift surfaces of depositional facies. The main coals, the #7 and #6 seams, were deposited in a lake and swamp environments during lake transgression system tracts of SIII1 and SIII2, respectively. The varying trend of the Tectonic Index values indicate that the source area experienced strong tectonic activity from the SIII1 to the lowstand system tract of SIII2, and then gradually stabilized. The Mineral Alteration Index reveals that the climate during SIII1 was from dry‐hot to humid‐warm, and there is a climate cycle from dry‐hot to humid‐warm then a return to dry‐hot in SIII2. During the faulting stage, the occurrence and termination of coal accumulation was in response to weakening and strengthening periods of tectonic activity respectively, while during the depression stage it was in response to the humid‐warm and dry‐hot climate respectively.

The Middle Jurassic Aalenian-Bajocian stages represent the tectonic transition period for the group of Jurassic sedimentary basins in northwestern China and include accumulation of the most important recoverable coal seams. Geological data from borehole cores, well loggings and rock-mineral determination in the Aalenian to Bajocian-aged Dameigou Formation in the Yuqia coalfield, Northern Qaidam Basin were employed to unveil the relationship between coal accumulation and changes in palaeoclimates and provenance tectonic activity. The results identify six kinds of lithofacies from fluvial and lacustrine sedimentary systems, and allow the Dameigou Formation to be divided into two third-order sequences (SIII1-SIII2) and their corresponding system tracts by regional unconformity surfaces, forced regression surfaces and their correlative conformity surfaces, and the abrupt shift surfaces of depositional facies. The main coals, the #7 and #6 seams, were deposited in a lake and swamp environments during lake transgression system tracts of SIII1 and SIII2, respectively. The varying trend of the Tectonic Index values indicate that the source area experienced strong tectonic activity from the SIII1 to the lowstand system tract of SIII2, and then gradually stabilized. The Mineral Alteration Index reveals that the climate during SIII1 was from dry-hot to humid-warm, and there is a climate cycle from dry-hot to humid-warm then a return to dry-hot in SIII2. During the faulting stage, the occurrence and termination of coal accumulation was in response to weakening and strengthening periods of tectonic activity respectively, while during the depression stage it was in response to the humid-warm and dry-hot climate respectively.

K E Y W O R D S
coal accumulation, fault-depression superimposed continental basin, Middle Jurassic, palaeoclimates, provenance tectonic activity, Qaidam Basin

| INTRODUCTION
As an important part of the global carbon cycle, coal accumulation represents a global geological event (Lu et al., 2016) sequestering atmospheric carbon dioxide from photosynthesis in terrestrial higher plants into coal seams by peatification and coalification. Large-scale coal accumulation in geological history was not limited to a single stratigraphical time interval, but consisted of a series of sub-level coal-accumulating events controlled by internals mechanism (Yang & Han, 1979). To help further understand the global carbon cycle through geological history, it is important to consider the causes of coal accumulation occurrence and termination. Coal accumulation is influenced by the relationship between rates of accommodation creation and peat accumulation (e.g., Diessel, Boyd, Wadsworth, Leckie, & Chalmers, 2000;Petersen & Ratanasthien, 2011). Bohacs and Suter (1997) and Holz, Kalkreuth, and Banerjee (2002) considered that peat can be preserved to form coal only when the overall rate in peat generation of accommodation approximately equals the accumulation rate of peat. Among them, the tectonic subsidence and base-level change are the main controlling factors of accommodation variation; the supply of plant availability provides material for the accumulation of peat (Xu et al., 2020). In continental lacustrine basins, the primary driving factors of base-level fluctuation include tectonic and climatic controls (Shanley & McCabe, 1994). However, the explanation for the occurrence and termination of coal accumulation is extensively at the qualitative descriptive level, in general expressed as 'coal accumulation was an integrated product of palaeostructures, palaeoclimates, palaeoenvironments, and palaeo-plants' (Liu, 1990;Phillips & Peppers, 1984). There are few reports of the occurrence and termination of coal accumulation and its response to the changes in palaeoclimate and provenance tectonic activity. Moreover, during practical analysis of coal accumulation, researchers tend to concentrate in a single fault basin or a depression basin, and coal accumulation models for faultdepression superimposed basin are frequently neglected.
During the Yanshanian movement, the Jurassic basins in northwestern China, for example, Qaidam Basin and Turpan-Hami Basin, generally underwent a tectonic evolution history from the fault to depression stage (Zhao, Qi, Xue, Meng, & Zhao, 2000) in the Aalenian-Bajocian, when the most important recoverable coal seams accumulated (Zhang et al., 1998). A series of NW-SE oriented faultdepression superimposed basins were pulled apart along the margin of the Qaidam Block by NE-trending extensional stress during the Early Jurassic (Wan, 2012). On this basis the Qaidam Basin represents an ideal target location to unveil the relationship between coal accumulation and the changes in palaeoclimates and provenance tectonic activity.

| GEOLOGICAL SETTING
The Qaidam Basin is one of the most important Jurassic coal-bearing basins in China. It covers an area of approximately 120,000 km 2 and is   Figure 1c; Zhao et al., 2000). The basin then entered the transitional period from fault to depression basin, which lasted until the end of the Bajocian (Liu et al., 2013;Yang et al., 2001). At this time, a regional lake transgression occurred and was initially centred on the Dameigou area, but subsequently, extended to the surrounding areas, including the Yuqia coalfield ( Figure 1d). The Yuqia coalfield, located in the centre of the northern Qaidam Basin (Dai et al., 2003), is a narrow NE-trending coalfield with southern Qilian Mountain to the west and Lvliang Mountain to the south ( Figure 1d). The basement comprises Proterozoic metamorphic rocks, Ordovician flysch, Carboniferous clastic rocks and limestone, and Caledonian intrusive rocks and volcanic rocks (Lu et al., 2007).
The coal-bearing strata of the Yuqia coalfield are Middle Jurassic in age and comprise the Dameigou Formation and the overlying Shimengou Formation (Figure 2). Several different stratigraphic divisions and correlation schemes for the Qaidam Basin have been proposed previously (Geology and Mineral Bureau of the Qinghai Province, 1991;Ritts & Biffi, 2001;Wu et al., 2011;Zhang et al., 1998).
In this article, the updated stratigraphic scheme proposed by Zhang et al. (1998) is adopted. This scheme proves to be the most detailed and reliable one, as it is based on multi-disciplinary approaches, with refined subdivisions at stage-level, based on various evidence including lithostratigraphy, biostratigraphy (bivalves, plants, and spore-pollen) and chronostratigraphy (Wang, Mosbrugger, & Zhang, 2005).
According to this scheme, the Dameigou Formation was dated to the Aalenian-Bajocian Stage, the Shimengou Formation was dated to Bathonian Stage (Figure 2).

| MATERIALS AND METHODS
The database for this study consists of 30 borehole cores and well logs undertaken during coal and shale gas exploration, 76 sandstone Sequences within the Dameigou Formation were identified and correlated using borehole cores, well logs of gamma-ray and apparent resistivity. A series of analytical contour maps have been drawn in the third-order sequences using single-factor analysis and multifactor comprehensive mapping as proposed by Feng (2004). These lithofacies, sequence stratigraphic, and palaeogeographical characteristics were further related to the basin evolution and coal accumulation.
Sandstone samples were cut into slices and identified by the point-counting method under a microscope according to China national standards (SY/T5368-2003; SY/T 5368-2016), with more than 300 effective points of each sample. The classification of clastic rock components is in accordance with that of Dickinson (1985).
The mineral index of alteration (MIA) was applied to recover the evolution history of palaeoclimate during the Aalenian-Bajocian, which is a measure of unstable feldspars. It can be used to quantify the degree of weathering in the source area, and is not affected by sorting or abrasion (Nesbitt, Young, McLennan, & Keays, 1996;Rieu, Allen, Plötze, & Pettke, 2007). The Mineral Alteration Index is calcu-  (Roy & Roser, 2013). The TI (L/Q) reflects the tectonic activity of the sandstone detrital deposits (Cao, Li, & Wang, 2008).
High lithic fragments content indicates strong tectonic activity, while low lithic fragments content indicates relatively stable tectonic activity (Cao, 2007).

| Lithofacies types and characteristics
According to the colour, lithology and sedimentary structure of the sediments in the Dameigou Formation, four major lithological types and seven kinds of lithofacies are identified ( Table 1). The characteristics, distribution, and interpretation of each lithofacies are described as follows.

| Conglomerate
Conglomerate comprises lithofacies Cg and is mainly distributed in the eastern part of the research area. This lithofacies is characterized by a cylindrical-shaped logging curve ( Figure 4a) and composed of greyish-white, thick-bedded conglomerate, with gravelly clastic texture, upward-fining sequence, graded bedding, and scoured surface developed (Figure 5b,c). The gravel is more than 85% in content with a grain size range from 0.5 to 2 cm, sub-rounded to rounded, moderately sorted, and arranged with imbrication. The conglomerate comprises the gravel and the matrix with the composition of quartz, flint, and the sandy detritus, respectively, which is porous cementation with the calcareous cement. The Cg lithofacies represents most likely a  (Table 1).

| Sandstone
Sandstone includes lithofacies Sp and Sw, which are widely distributed throughout the coalfield. Lithofacies Sp, with sandy clastic texture, middle/thick-bedded, upward-fining sequence, parallel bedding and scoured surface developed (Figure 5e,g), is greyish-white, medium-coarse grained sandstone. The detritus from lithofacies Sp, with a grain size range from 0.5 to 2 mm, is sub-rounded to rounded, well-sorted, whose composition is mainly quartz, followed by feldspar, lithic fragment, and a small amount of charcoal ( Figure 5i). This lithofacies represents medium energy and weak oxidizing environment, such as the riverbank and shore-shallow lake (Table 1).  (Table 1).

| Organic rock
Organic rock mainly comprises lithofacies C. This lithofacies is composed of black coal seam and is characterized by a cylindrical-shaped logging curve (Figure 4b-e) and thin-bedded, with plenty of plant fossils on the plane of formation. The lithofacies C is widely distributed throughout the study area and represents the low-energy and reducing peat swamp environment (Table 1).

| Depositional systems and sedimentary characteristics
Nine samples (sample number: M1-M9 from the bottom to the top) were selected to draw the probability graph of sandstones (S1-S3) in the Dameigou Formation based on the grain-size analysis ( Figure 6).
T A B L E 1 Lithofacies of the Dameigou Formation in the Yuqia coalfield (the lithofacies designations are modified by Miall, 1977)  The sedimentary grain-size characteristics of the sandstones are analysed as follows: The probability graph of sample M1-M6 is characterized by 'high slope-saltation-suspension', which mainly consists of two parts: the saltation population and the suspension population (Figure 6). The saltation population is the main part of the sediment, with a content of more than 80%. Linear segment slopes of the saltation population are high and the detrital particles are wellsorted. However, the suspension population is rarely developed, and linear segment slopes are relatively low. Therefore, sample M1-M6 can be interpreted as point bar or channel bar sediments (Li, Zheng, Gong, Zhou, & Cheng, 2013;Yang, 1987).  Another three points fall within the turbidity sedimentary range, which may be attributed to the rapid flow of rivers during the flood period, causing the resuspension of sediments. Overall, sandstones in the study area have a famous distribution pattern of the fluvial depositional system in the C-M diagrams (Passega, 1977;Passega & Byramjee, 1969).
Based on the comprehensive analyses of lithofacies, the logging phase, and the sandstone grain-size probability graph, it is believed that the Dameigou Formation mainly developed as a fluvial-lacustrine depositional system (Table 2). each sequence would last for 2.9 Ma, which is within the range of 0.5-5 million years for a third-order sequence duration (Vail, Mitchum, & Thompson III, 1977). Systems tracts are interpreted based on stratal stacking patterns (Catuneanu, 2006) (Figures 8-10). The sequence-stratigraphic characteristics are described as outlined below.

| Sequence-stratigraphic analysis
The lower sequence boundary of SIII1 is a regional unconformity The basin is in the equilibrium compensation stage during this period, while the HST is locally developed in the study area and comprises mudstones developed in the shallow lake. The coal seam #6, with a thickness of 1.37-11.7 m, mainly developed in the TST of SIII2 (Figures 8-10).

| Lithofacies palaeogeography and distribution characteristics of coal seams
To summarize the lithofacies and palaeogeography of the  The sedimentary boundary of LST (SIII2) is shown in Figure 11c.
Controlled by the F1 fault, the sedimentary interface is still high in the west and low in the east in the study area, with the meandering river palaeogeographic unit developed from west to east. Since the sedimentary interface is flatter than that in the LST of SIII1, the curvature of the river significantly increased. The terrigenous clastic sediments are sufficient at this time, and the supply rate is higher than the accommodation generation rate, so that the basin is in the over-com-

| Palaeoclimate and provenance tectonic activities
The identification results of sandstone thin sections from the borehole YQ-1 in the study area are shown in Figure 12.  (Figure 13), and the content varies from 61 to 84% (x = 73%). In the stratigraphic direction, the quartz content is characterized by rapidly increasing from bottom to top in the S1, while tends to be stable in the S2, and increasing first and then decreasing from bottom to top in the S3, with the average value of 65, 71, and 77%, respectively ( Figure 12). The feldspar fragments mainly comprise potassium feldspar and plagioclase (Figure 13), and the content varies from 3 to 12% (x = 7%). The feldspar content is characterized by rapidly decreasing in the S1, then increasing from the bottom to top in the S2 and S3, with the average value of 6, 7, and 8%, respectively ( Figure 12). Lithic fragments include magmatic lithics (the interaction between quartz and feldspar), metamorphic quartz lithics, mica schist lithics, and slate lithic (Figure 13), and the content varies from 8 to 18% (x = 13%). From bottom to top the lithic fragment content presents a gradually decreasing trend in the S1, a gradually decreasing trend in the S2, and tends to be stable in the S3, with average values of 16, 15, and 10%, respectively ( Figure 12). values of the S1-S3 are 0.24, 0.20, and 0.13, respectively. There is a gradual decrease from 0.30 to 0.17 in the S1 while a significant fluctuation arises from the upper part of S2 to the lower part of S3, which decreases from 0.22 to 0.14. After that, the TI value is basically stable range from 0.10 to 0.15 ( Figure 12).

| Sequence stratigraphy and palaeogrography
Based on the analysis of the key bounding surfaces and sequence boundaries, the Dameigou Formation in the Yuqia coalfield was subdivided into one third-order sequence by Li et al. (2014).
Howerver, in this study, we divide the Dameigou Formation into two third-order sequences. According to the Stratigraphic Chart of China (2018) (Cohen et al., 2013), the Dameigou Formation lasts for 5.8 Ma, which exceeds the range of 0.5.-5 million years for a third-order sequence duration (Vail et al., 1977). This is in agreement with our finding of dividing the Dameigou Formation into two third-order sequences. In the palaeogeographic map of the Dameigou Formation, five palaeogeographic units (including alluvial fan-braided fluvial plain, upper delta plain, lower delta plain, subaqueous delta, and shore-shallow lake) and three source areas (southeastern side, northwestern side, and eastern side) were recognized by Li et al. (2014). However, the sandstone grain-size probability graph in this study shows that the Dameigou Formation mainly developed as a fluvial-lacustrine depositional system. And suggest that the major source area is more likely located in the Northwestern side of the basin.  (Feng et al., 2017). High TI valves may indicate that tectonic activity in the provenance area is strong, which corresponds to the faulting stage of the basin. In contrast, low TI values may represent that the provenance tectonic activity tend to be stable, which corresponds to the depression stage of the basin. Furthermore, Yu et al. (2017) also shows that the northern Qaidam Basin was in the rifting phase during the earliest part of the Middle Jurassic, and then developed into an extensional basins since the middle stage of the Middle Jurassic that was sustained until the end of the Jurassic.

| Provenance tectonic activity analysis under the sequence stratigraphy framework
Therefore, a curve showing the intensity of tectonic activity is recovered inferred from TI values (Figures 15 and 16).

| Palaeoclimates analysis under the sequence stratigraphy framework
In Cyatheaceae and Dicksoniaceae, and Cycadophyte-Ginkgophyte gymnosperms, indicating a humid and warm climate (Wang et al., 2005). Considering the greenhouse climate in the Jurassic (Donnadieu et al., 2011) (Lu et al., 2016). Humid and warm climates are generally correlated with the rising of sea-level while the dry and hot climate corresponding to the sea-level falls of JBj2 and JBj4 in the short-term sea-level curve revised by Haq (2018).

| Coal accumulation linked to the changes in palaeoclimate and provenance tectonic activity
Coal accumulation based on sequence-stratigraphic analysis has been broadly used (Bohacs & Suter, 1997;Diessel et al., 2000;Holz et al., 2002;Li et al., 2018;Xu et al., 2020). The peat-forming environments were also discussed in sequence-stratigraphic intervals (Petersen, Lindström, Therkelsen, & Pedersen, 2013). Previous works can be summarized as 'a coal seam would develop well when the increasing rate of accommodation kept balance with the production rate of peat, and the destruction of the balance will terminate the coal accumulation' (e.g., Li et al., 2014Li et al., , 2018Xu et al., 2020  entered on the normal LST of SIII2 when the baselevel starts to rise again. As the rising rate of the baselevel was less than the sediment supply rate, the fluvial channels began to fill the incised valleys, forming relatively continuous sand bodies, and the accommodation was primarily occupied by terrigenous clastics. The relatively flat depositional interface formed in this period was an important prerequisite for the basin swamping. As a whole, during the faulting stage, the occurrence and termination of coal seams were mainly controlled by tectonic activity. In the fault-depression area (Y6, Y10, and Y13), tectonic activity was strong and the basement subsidence rate the fastest, where it was difficult to reach a balance between the rate of accommodation creation and the peat accumulation for a long time because of the small catchment area and the fast-rising baselevel. Therefore, coal seam #7 that developed in this area was multi-gangue and bifurcates in its lateral extent.
However, the basement subsidence rate was slow in the palaeo-uplift (Y3 and Y8), and the area of the sedimentary water gradually expanded as the lake level passed through the palaeo-uplift. This resulted in a decrease in the rate of base-level rise and accommodation creation in the palaeo-uplift. Thus, the balance between the rate of peat production and accommodation creation can be sustained for a long time. Therefore, coal seam #7 developed in the palaeo-uplift was large in thickness and less in gangue.
During the depression stage with stable tectonic activity, the coal accumulation under the sequence-stratigraphic framework can be summarized into the following four periods. forming a large area of exposed and weak-overlying water environment conducive to peat accumulation at the flat sedimentary interface formed previously. Therefore, the balance between the rate of peat production and accommodation creation can be reached which led to the formation of continuous coal seam #6. (i) During the late TST, the climate also experienced a short dry and hot process at the roof of the coal seam #6. As the climate became drier and hotter, the vegetation in the source area decreased and river erosion was enhanced, which result in the resurrection of the terrigenous system and termination of coal accumulation. A fine-grained succession was deposited overlying coal seams. (j) The HST started when the climate became warm and humid again and the baselevel reached the MFS. The rate of base-level rise was higher than the rate of peat accumulation, depositing lacustrine siltstones and mudstones.
Generally, during the depression stage, the difference in subsidence rate of the basement was reduced, and the change of climate affected the change of the baselevel and the growth of coal-forming plants, thus controlling the development of the coal seam. Due to the relatively flat depositional interface formed in this period, coal seam #6 in the study area was less in gangue and good in continuity.
6 | CONCLUSIONS 1. We recognized six kinds of lithofacies and two sedimentary systems that divide the Dameigou Formation into two third-order sequences by regional unconformity surface, forced regression surface and their correlative conformity surface, and the abrupt shift surface of depositional facies. Results demonstrate that the main #7 and #6 coal seams were deposited in lake swamp environment during periods of lake transgression.
2. The TI indicates that the source area was in strong tectonic activity stage from the SIII1 to the LST of SIII2 and then gradually stabilized. MIA results reveals that the climate evolution of SIII1 was from dry-hot to humid-warm and there is a climate cycle from dryhot to humid-warm then a return to dry-hot in the SIII2.
3. During the faulting stage, the occurrence and termination of coal accumulation was in response to the weakening and strengthening period of tectonic activity respectively, while it response to the humid-warm and dry-hot climate during the depression stage.