Tectonic sequence boundaries are regional unconformities or corresponding conformities, which were generated by the dual effects of tectonic movements and sudden base level drop ^[18]. They are also the basis of sequence boundary identification and stratigraphic subdivision using outcrop, well and seismic data.

The Triassic outcrops in the Kuqa foreland basin have good quality. We used outcrop data and well data, together with 2D seismic surveys across the basin and 3D seismic surveys in major prospects, to perform stratigraphic boundary identification as well as sequence subdivision and correlation. Bed boundaries, e.g. sedimentary transition surface, basal conglomerate bed or erosional surface, truncation and overlap, were taken as the constraints. The Triassic in the Kuqa foreland basin was identified with 5 secondary tectonic sequence boundaries: Ehuobulake Formation bottom, Ehuobulake Formation top, Karamay Formation top, Huangshanjie Formation top and Taliqike Formation top, from the bottom up.

Ehuobulake bottom is a regional unconformity between the Triassic and underlying formations (Fig. 3a, 3b). At the end of the Permian, the Proterozoic-Permian formations cropped out extensively in central and southern Kuqa foreland basin due to tectonic uplift. For example, the Proterozoic metamorphic rocks emerged in Tugalming area and Uqturpan sag. The metamorphic pelitic siltstones, with a lot of sericite, were revealed to underlie the bottom boundary of the Ehuobulake Formation and be in unconformable contact with the Triassic strata in the Taliqike and Kapushaliang River sections (Fig. 4a-4c). Truncation could be observed on seismic section between the Triassic and underlying formations. However, the Triassic is in conformable contact with underlying formations in the area from Kelasu River to Kuqa River.

Ehuobulake Formation top to Huangshanjie Formation top are the top boundaries of the Ehuobulake, Karamay and Huangshanjie formations, respectively. In the outcrop sections in the northern depression, the boundaries are the erosional unconformable surfaces produced by intensive erosion of underlying fine-grained clastic rocks by coarse-grained clastic rocks (Fig. 4d-4e). Truncation-related unconformities also occur at the south boundary of the basin.

Taliqike top is a regional unconformity between the Triassic and overlying formations. In the Uqturpan sag in western Kuqa foreland basin, interbedded sandstones and mudstones in the Taliqike Formation and thick Cretaceous sandy conglomerates turn up respectively below and above Taliqike Formation top, which was shaped as an extensive erosional unconformity with abrupt lithologic change (Fig. 3c). The stratigraphic gap of the Upper Triassic and the angularly unconformable contact between the Triassic and overlying formations in the southern basin indicate intense tectonic activities at the end of the Triassic, which gave rise to the regional unconformity. Truncation could be clearly observed between the overlying and underlying formations above and below Taliqike Formation top on seismic section.

Based on secondary tectonic sequence boundary identification and the analysis of exposure, weathering and erosion, scouring, 7 tertiary sequence boundaries, SB1-SB7, were recognized within the Triassic (Fig. 5). It is noted that the interfaces with abrupt lithologic change were considered in determining tertiary sequence boundaries, above which there are thick sandy conglomerates and below which mudstones or silty mudstones. This agrees with the lithologic change characterized using log data.

The episodic thrusting activities in the foreland basin tended to be coupled with sedimentary cycles ^{[11,13 -14]}. The episodic thrusting activities in 4 stages, i.e. strong, weakened activities, relaxation and inactive, in the Triassic of the Kuqa foreland basin gave birth to 4 secondary sequences from the bottom up (Fig. 6), corresponding to the Ehuobulake, Karamay, Huangshanjie and Taliqike formations, respectively. Each secondary sequence also experienced the similar secondary thrusting process from the strong stage to the relaxed stage. According to the features of episodic thrusting activities, 4 secondary sequences were further divided into 11 tertiary sequences: SQ1-SQ11.

The depositional stage of the Ehuobulake Formation has experienced strong thrusting activities, at the bottom of which there are extensive alluvial fan and fan delta deposits. The Kuqa River-Xiaotailan River outcrops were observed with fine-grained to medium-grained conglomerates of 20 m thick at the bottom. This agrees with the conclusion made by Wang et al., who stated that low- grade metamorphic rocks cropped out due to southwest Tianshan orogenesis and uplift in the Early Triassic in accordance with the extremely high spessartine content ^[18]. Based on the interfaces with abrupt lithologic change and cycle features, this sequence was divided into 3 tertiary sequences (SQ1-SQ3). Regionally, formation thickness reaches the peak in the Kuqa River section and decreases toward the east and west (Fig. 6).

The depositional stage of the Karamay Formation has experienced weakened thrusting activities, with the change of sporopollen assemblage from fern spores to coniferales pollen and a change from arid climate to semiarid-semihumid climate ^[19]. Granites occurred more extensively in the Tianshan orogenic belt, and more high-grade metamorphic rocks cropped out. Compared with the Ehuobulake Formation, the Karamay Formation was deposited with finer-grained material, and the subsiding center moved from the Kelasu River toward the Kuqa River. The formation thickness was estimated to reach 550 m in the Kuqa River section ^[20]. Controlled by secondary episodic thrusting activities, the Karamay Formation was further divided into 3 tertiary sequences: SQ4-SQ6 (Fig. 5). Laterally, the Karamay Formation covers more area than the Ehuobulake Formation and, toward the east, overlaps the area on the west of the Yangxia section. Therefore, the formation thickness decreases toward the east (Fig. 6).

At the depositional stage of the Huangshanjie Formation with relaxed thrusting activities and insufficient sedimentary sources, the basin was mainly filled with fine-grained sediments with more extensive distribution than in other periods. In view of the second-episode secondary thrusting activities, the Huangshanjie Formation was subdivided into 2 tertiary sequences: SQ7 and SQ8 with two upward-fining normal cycles, which consist of two-package hard sediments and two-package soft sediments ^[21] (Fig. 5). The formation thickness reaches the peak in the Kuqa River section and decreases toward the east and west (Fig. 6).

The depositional stage of the Taliqike Formation has experienced inactive tectonic compression featured by small formation thickness and coarse and fine sandstones.Flourishing pinaceae, podocarpus and osmundaceae, combined with warm humid subtropical climate made a great contribution to well-developed coal-bearing formations in this period ^[19]. The formation thickness reaches the peak of 350 m in the Kuqa River section and decreases toward the east and west (Fig. 6). Due to the stratigraphic break caused by the tectonic uplift at the end of the Jurassic, the Taliqike Formation is in unconformable contact with the overlying Cretaceous in the Uqturpan sag (Fig. 3). In view of secondary episodic thrusting activities, the Taliqike Formation was subdivided into 3 tertiary sequences: SQ9-SQ11 (Fig. 5).

Episodic thrusting activities had a unified impact on the pattern of stratigraphic superposition ^[22], and accommodation space also showed episodic change in the evolutionary process of the Kuqa foreland basin in the Triassic during stages of strong to weakened episodic thrusting activities. In the stage of strong tectonic compression (the depositional stage of the Ehuobulake Formation), the basin was greatly flexed and subsided. Owing to sufficient sediment supply and the short time of duration, the basin with small accommodation space was filled with coarse-grained sediments. In the stage of weakened tectonic compression (the depositional stage of the Karamay Formation), decreased sediment supply and slow lake level rise in a long period of time led to an increase in accommodation space with medium- to coarse-grained sediments. In the stage of relaxed tectonic compression (the depositional stage of the Huangshanjie Formation), a further decrease in sediment supply and rapid lake level rise in a long period of time gave rise to the largest accommodation space, in which fine-grained sediments were deposited. In the stage of inactive episodic thrusting (the depositional stage of the Taliqike Formation) with small surface relief, accommodation space was shrunk due to fast lake level drop in a short period of time. The formation thickness is small or nearly zero, and rocks mainly involve sandstones alternating with mudstones ^[11] (Fig. 7).

On the whole, owing to the episodic thrusting activities, the lithologic association in each secondary sequence exhibits a distinct binary structure, which is characterized by a depositional pattern with coarse-grained sediments in the lower part and fine-grained sediments in the upper part. Coarse-grained clastic rocks in the lower part occurred at the early stage of thrusting activities, when sufficient supply of coarse-grained clastics led to thick deposition of sandy conglomerates. Fine-grained clastic rocks in the upper part occurred in the stage of relaxed thrusting activities, when the basin was in a transgressive system in view of deepened lake water ^{[11,13 -14]}. The next stage of sedimentary evolution launched at the end of each cycle due to rapid lake level drop.

Despite the comprehensive effects of paleosources, paleoclimates and paleostructures, tertiary sequences against the structural setting with episodic thrusting activities also show binary structures. Regional tectonic activities in the South Tianshan Mountains generated similar sequence patterns, which could be correlated between the Kuqa foreland basin and the Tabei uplift. We compared outcrops from Kuqa depression with well JL104 from Tabei uplift lithologic sections at well sites and found that SQ1 basal conglomerates were lost in the Tabei uplift, which is the consequence of intense uplift and denudation of the foreland basin front due to the tectonic compression caused by thrusting activities at the initial stage. Therefore, no coarse-grained clastic rocks were settled in this period (Fig. 7). The sedimentary formations in Kuqa and Tabei feature similar binary structures in the phase of SQ4-SQ6, but the grain size and scale of coarse-grained clastic rocks of SQ4 are greater than those of SQ5-SQ6. The Karamay Formation shows a normal cycle on the whole. SQ7 and SQ8 are tertiary sequences with normal cycles, and the Huangshanjie Formation shows a normal cycle on the whole. Three tertiary sequences of SQ9-SQ11, which all present normal cycles, are relatively thin due to tectonic stress relief, leading to the rebound of the flexed deformed basement in a short period of geologic time. In summary, binary structures produced by the episodic thrusting activities in the foreland basin could be used as reference for the classification and correlation of secondary and tertiary sequences.

The Kuqa foreland basin is similar in the mechanisms of tectonic subsidence and evolution to other foreland basins in China and abroad ^[2,23]. Previous studies showed that with enhanced tectonic activities in the foreland basin, sediments from overthrust and foredeep uplifted zones to depression zones manifest the change in grain size from coarse-grained alluvial fan deposits to lacustrine deposits. The latest studies have indicated that owing to the joint influences of basin properties, orogenesis related to thrusting activities and sedimentary sources, together with multi-phase variable-order tectonic activities, both the secondary and the tertiary sequences in the Triassic of the Kuqa foreland basin demonstrate lithologic associations with binary structures.

(1) In the stage of strong episodic thrusting activities, corresponding to the depositional stage of the Ehuobulake Formation, ample sediment supply and small accommodation space led to a sedimentary system mainly composed of alluvial fans and fan deltas. For example, the Kuqa River section in the northern Kuqa Basin was deposited with thick reddish brown medium- to fine-grained conglomerates of fan delta facies, with moderately to perfectly rounded, oriented pebbles and massive bedding. (2) In the stage of weakened episodic thrusting activities, corresponding to the depositional stage of the Karamay Formation, decreasing sediment supply and increasing accommodation space owing to lake level rise resulted in a sedimentary system mainly composed of braided river deltas and lacustrine deposits. Braided distributary channels were filled with reddish brown pebbled coarse sandstones and coarse sandstones, with moderately to perfectly rounded pebbles, poorly rounded and oriented clastic particles, and trough and wedge-like cross-beddings. In outcrop sections, braided distributary channels could be observed with multi-package fine-grained conglomerates with multi-phase normal grading in the vertical direction, indicating the vertical stacking of multi-channel sands. Lacustrine deposits dominated by shore-shallow lacustrine deposits are distributed locally. (3) In the stage of relaxed episodic thrusting activities, corresponding to the depositional stage of the Huangshanjie Formation, obviously insufficient sediment supply and the largest accommodation space gave rise to a fine-grained sedimentary system composed of braided river delta fronts and lacustrine deposits (Fig. 8). Braided river delta fronts were deposited with gray medium to fine sandstones with a few pebbles only appearing at the bottom. Clastic particles in sandstones were moderately rounded, most of which were transported by saltation and some of which by rolling. Sedimentary structures include trough and tabular cross-beddings, and massive bedding. The lake area was settled with thick black mudstones of shallow to semi-deep lacustrine facies, in which there are horizontally laminae. In general, the stage of relaxed episodic thrusting activities features fine-grained sedimentation, during which dominant source rocks developed in the foreland basin. In the stage of inactive episodic thrusting, corresponding to the depositional stage of the Taliqike Formation, the basin was mainly filled with meandering river and limnetic sediments due to rapidly decreasing accommodation space. Distributary channels in the meandering river delta plain were deposited with gray siltstones and fine sandstones with smaller grain size than in braided distributary channels. Clastic particles were poorly rounded, most of which were transported by saltation and suspension. Trough and tabular cross-beddings were well developed. The limnetic area was mainly settled with grayish black mudstones with carbonaceous fragments, pelitic siltstones, and coals of peat swamp microfacies. Coal seams are generally smaller than 1 m thick.

Sedimentary facies in tertiary sequences also correlate with episodic thrusting activities. Owing to the secondary episodic thrusting activities, the lithologic association in each tertiary sequence features an upward-fining binary structure, with coarse-grained sediments dominating in the stage of relatively strong thrusting activities and fine-grained sediments in the stage of relatively relaxed thrusting activities. On the whole, SQ1 was mainly deposited with fan deltas, and SQ2-SQ3 with braided river delta plain-front deposits. As the lake level rose, SQ4 to SQ6 underwent a transition from braided river delta plain deposition to front deposition, SQ7-SQ8 from braided river delta-front deposition to shore-shallow lacustrine and semi-deep lacustrine deposition, and SQ9-SQ11 from braided river delta-front deposition to meandering river delta flood-plain deposition.

SQ7 differs from SQ8 in the Huangshanjie Formation. Due to the impact of strong to weakened secondary episodic thrusting activities, both SQ7 and SQ8 were deposited with thick sandy conglomerates and coarse sandstones at the lower part, but sandstone thickness and grain size in SQ7 are much greater than in SQ8 (Fig. 6), which indicates stronger tectonic activities in SQ7 sedimentary period than in SQ8 sedimentary period. SQ7 and SQ8 were deposited with thick mudstones in the stages of relaxed and inactive thrusting, and black mudstones in SQ7 are much thinner than in SQ8. Laterally unstable black mudstones in SQ7 turned up locally in the semi-deep lacustrine zone (Fig. 6) in a short period of time. In contrast, SQ8 black mudstones in the Taliqike section reach 140 m thick and extend steadily in the lateral direction, at the depositional stage of which a wide lake basin was shaped with the largest lake level. These differences indicate intense secondary episodic thrusting activities within a short period of time in SQ7 and relatively weak thrusting activities within a long period of time in SQ8.

The enhanced uplift of the South Tianshan Mountains from north to south dominated the vertical evolution of sedimentary sequences in the basin ^[22]. On the whole, the Kuqa Depression in the Triassic was deposited with a system changing from fan delta, braided river delta plain, braided river delta front, shore-shallow lacustrine, semi-deep lacustrine, shore-shallow lacustrine, braided river delta front, to meandering river delta plain. Such a distribution pattern corresponds to the complete evolutionary process from strong to inactive thrusting in the foreland basin.

Episodic thrusting activities dominated the development and distribution of source-reservoir-caprock assemblages. Coarse-grained clastic rocks forming in the stages of strong and weakened thrust compression may function as important reservoir rocks. According to outcrop and well data, there are extensive braided river-meandering river delta sands with good reservoir properties in the Triassic, the porosities of which are 6.51%-15.32% with the average of 9.73%. Source rocks occurred in the stages of relaxed and inactive thrusting. Lacustrine argillaceous source rocks in the Huangshanjie Formation deposited in the stage of relaxed thrusting activities are 100-500 m thick and cover an area of 18 800 km². This package of source rocks, with TOC of 1.3%-7.1%, H-C atomic ratio of 0.4-0.8, O-C atomic ratio of 0.35-1.10, and Type-II₂ kerogens in the majority, is the dominant source contributor in the Kuqa foreland basin. Oil-gas and source-rock correlation showed that natural gas recently discovered in the Jurassic Ahe Formation, Dibei area, originated from the Triassic source rocks ^[24]. In addition to coal-measure source rocks, the Triassic capping beds also formed in the stage of inactive thrusting.

The Triassic lacustrine source rocks in the Kuqa Depression were mature enough to generate hydrocarbon at the depositional stage of the Jidike Formation (23 Ma) and reached the peak of hydrocarbon generation at the stage of the Neogene Kangcun Formation (5-12 Ma) ^[24]. The episodic thrust tectonic cycles in the Triassic dominated sedimentary evolution and source-reservoir configuration, in addition to deep stratigraphic sequences. Consequently, multi-type primary hydrocarbon reservoirs could form before the Himalayan orogeny ^[25-26]. Lithologic hydrocarbon reservoirs may occur inside the Huangshanjie Formation, and structural-lithologic gas reservoirs may occur in the reverse drag structures forming in the hanging walls of early reverse faults.

Primary hydrocarbon reservoirs were adjusted by the thrusting activities taking place in the Himalayan, and episodic thrusting activities dominated the occurrence and distribution of gas reservoirs. As shown on regional 2D seismic sections, deep structures in the Triassic of the Kuqa foreland basin are relatively complete and large, which means that there may be large oil and gas fields provided that source-reservoir configuration is promising. As per the source-reservoir configuration nowadays and accumulation conditions, lacustrine source rocks and braided river delta front sands in the Huangshanjie Formation as well as coal-measure source rocks and meandering river delta sands in the Taliqike Formation may constitute self-generating and self-storing lithologic gas reservoirs. Lacustrine source rocks in the Huangshanjie Formation and sandstones in the Taliqike Formation, particularly in the Tugalming anticlinal zone, may constitute structural-lithologic gas reservoirs with an assemblage pattern of lower source rocks and upper reservoir rocks. Source rocks in the foot walls and the structural highs in the thrust of hanging walls may constitute fault block-lithologic gas reservoirs (Fig. 11).