Late syn-rift sequence architecture and sedimentary evolution of a continental rift basin: A case study from Fulongquan Depression of the Songliao Basin, northeast China

Marine and Petroleum Geology 100 (2019) 341–357

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Research paper

Late syn-rift sequence architecture and sedimentary evolution of a continental rift basin: A case study from Fulongquan Depression of the Songliao Basin, northeast China

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Yunchao Houa,b, Hongyu Wanga,b,∗, Tailiang Fana,b, Runze Yanga,b, Chunlong Wua,b, Ying Tangc,d a

China University of Geosciences, Beijing, 100083, China Key Laboratory for Marine Reservoir Evolution and Hydrocarbon Accumulation Mechanism, Ministry of Education of China, China University of Geosciences, Beijing, 100083, China c College of Petroleum Engineering, Xi'an Shiyou University, Xi'an, 710065, China d Engineering Research Center of Development and Management for Low to Extra-Low Permeability Oil and Gas Reservoirs in West China, Ministry of Education, Xi'an Shiyou University, Xi'an, 710065, China b

A R T I C LE I N FO

A B S T R A C T

Keywords: Fulongquan depression Continental rift basin Late syn-rift stage Depositional systems Sequence architecture Controlling factors

As an important stage in continental rift basin evolution, late syn-rift stage is characterized by the combination of broad crustal subsidence and episodic pulses of extension. Sequence architecture and sedimentary filling process during this period present a certain particularity due to distinctive tectonic subsidence and variable influencing factors. This paper analyzes sequence stratigraphy and sedimentary facies development and their evolution in the Fulongquan Depression, a continental rift basin of the Songliao Basin, and discusses the determinants influencing the sequence architecture and depositional systems evolution. During late syn-rift stage, a major transition in the depositional systems, from predominantly lacustrine and correlated depositional systems to fluvial-floodplain settings, was recorded in the evolution of basin fill. This gradual and slow peneplanation is the final response to the decrease of differential subsidence. The spatial and temporal variability of sedimentary evolution are mainly associated with specific growth history of the major boundary faults. Sequence stratigraphic units recognized in the Fulongquan Depression are likely corresponded to different episodic pulses of extension respectively. Each sequence can be further divided into a lowstand systems tract (LST), transgressive systems tract (TST) and highstand systems tract (HST). Maximum differential subsidence and water deepening mainly occur in the LST. The expanded TST and HST of each sequence tend to be developed in a broader thermal subsidence regime. The depositional systems transition and detrital provenance changes usually occur in the HST. Throughout the late syn-rift stage, in addition to major changes of provenance systems, tectonic movement is likely to be the main factor in controlling the variations of sediment supply. With the decrease of tectonic activity, the impact of climate changes and lake level fluctuations on basin filling tends to be increased.

1. Introduction Late syn-rift stage is characterized by a decrease in fault slip and a broad crustal subsidence, which caused previously isolated rift basins to unite and form a large basinal depression (Chen et al., 1984; Sopeña and Sánchez, 1997). The response to the change in tectonic regime at this stage will be the gradual and slow peneplanation of the topography created by differential subsidence. As the late syn-rift stage is a major transition period both in the basin configuration and depositional environment (e.g. Nottvedt et al., 1995; Liu et al., 2007; Pereira and Alves, 2012; Scherer et al., 2014; Wang et al., 2015; Li et al., 2014a),

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the stratigraphic architecture, reservoir characteristics and oil/gas accumulation conditions show some specificity and complexity, which are different from early intensive rifting stage and later post-rift stage (Fan, 2005; Liu et al., 2007; Song, 2007). Numerous studies have been conducted on the recognition of distinct stages and the description of characteristic linked depositional systems in rift basins (e.g. Chen et al., 1984; Blair, 1987; Olsen, 1990; Nottvedt et al., 1995; Sopeña and Sánchez, 1997; Pereira and Alves, 2012; Scherer et al., 2014; Wang et al., 2015). However, the limited areal extent of many exposures and subsurface datasets restrict our ability to analyze the lateral and vertical variability of the facies in syn-

Corresponding author. China University of Geosciences, Beijing, 100083, China. E-mail address: [email protected] (H. Wang).

https://doi.org/10.1016/j.marpetgeo.2018.11.029 Received 19 July 2018; Received in revised form 30 October 2018; Accepted 19 November 2018 Available online 20 November 2018 0264-8172/ © 2018 Elsevier Ltd. All rights reserved.

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Upper Cretaceous Qingshankou (K2qn), Yaojia (K2y) and Nenjiang (K2n) formations (Figs. 1 and 2). Fault development features, stratigraphic architecture and thickness indicate that rifting events consisted of three sub-stages: a rift initiation phase, an intensive rifting phase and a final recession phase. The Upper Jurassic Huoshiling Formation is not confined to the extensional fault blocks and was deposited during the doming and initial rifting stage of the basin (Fig. 3). In the late Jurassic period, regional uplift caused denudation of the Houshiling Formation in most regions. A regional unconformity formed between Upper Jurassic and Lower Cretaceous units. During the Early Cretaceous, approximal E–W regional extensional stress influenced the Songliao Basin (Stepashko, 2006; Feng et al., 2010). Extensional faulting caused widening of the Fulongquan Depression and exerted primary control on basin morphology and topography. The Lower Cretaceous Shahezi and Yingcheng formations are restricted to fault blocks. The activities of three major boundary faults reached their maximum during this intensive rifting phase. During the deposition of Denglouku Formation, the decreasing movement of eastern border-faults no longer exerted a strong influence on basin configuration, especially the Funan and Gujiadian faults. The lateral extent of deposition broadened gradually and the horizontal thickness of units became more uniform across the basin (Fig. 1D). By the end of the Lower Cretaceous Denglouku Formation, the regional tectonic uplift resulted in the formation of a regional unconformity between the Denglouku and Quantou formations.

rift sedimentary sequences (Elliott et al., 2017). As a result, these studies rarely concentrated on rift basin tectono-sedimentary evolution over long temporal and spatial scales, even though a holistic study helps to improve our understanding of the coupling between tectonics and sedimentary evolution. Although a series of tectono-stratigraphic (e.g. Chen et al., 1984; Leeder and Gawthorpe, 1987; Prosser, 1993; Gawthorpe and Leeder, 2000) and sequence-stratigraphic models (e.g. Lin et al., 2000; Martins and Catuneanu, 2010; Dong et al., 2011) have been proposed to characterize rift basins stratigraphy, the danger must be borne in mind when applying these general models to describe a given stages with no constraints. On the one hand, variations from these models can occur due to so many interacting variables and controls (Sopeña and Sánchez, 1997; Carroll and Bohacs, 1999; Almeida et al., 2009; Martins and Catuneanu, 2010; Leeder, 2011; Elliott et al., 2017). On the other hand, each stage may consist of several phases of activity and variations in sequence stratigraphic style may vary in different areas (Young et al., 2002; Gawthorpe et al., 2003; Wang et al., 2015). Thus, many of those models are largely conceptual and have yet to be tested by specific examples. Fulongquan Depression is one of a series of rift basins formed from Late Jurassic to Neogene time within the Songliao Basin, China (Fig. 1). It is characterized by a half-graben which is bounded by a segmented normal fault system. Over the last few years, extensive subsurface datasets, comprising new 3D seismic reflection, wireline log and core data, have become available, and these data have permitted a more detailed analysis of the tectono-stratigraphic evolution of Fulongquan Depression over relatively large spatial scales. In this study these data are integrated to test rift sequence stratigraphic models and present a detailed case study of sequence architecture and sedimentary evolution of strata formed during late syn-rift stage. The sedimentary facies, sequence stratigraphy and changes of depositional systems over time and space are described; then, different controls on sequence architecture, stratal stacking patterns and depositional environment transformation are discussed. This study provides an improved understanding of late syn-rift sedimentology and stratigraphy.

2.2. Regional stratigraphy The Fulongquan Depression includes an independent hydrocarbon generation center covering an area of approximately 1000 km2. The distribution of rift strata, with a maximum thickness of 3000 m, is largely controlled by faulting in the eastern part of the depression and thins to the west, resulting in a cross-sectional half-graben geometry (Fig. 3). During its rifting stage, the Fulongquan Depression contained sedimentary fill of interbedded clastic and volcanic rocks. Huoshiling Formation, observed in the western part, was mainly composed of coarse conglomerate and volcanic rocks (e.g. andesitic). During intensive rifting stage, correspondent to the Shahezi and Yingcheng formations, a deep lake was developed and filled mainly by a thick succession of dark-colored, organic-rich mudstones, coarse graineddominated sandstones and sandy conglomerates. These mudstone units are highly mature through deep burial and are the most important source rocks in the Fulongquan Depression. In the margin of the basin several discontinuous coal beds can be found in the Shahezi Formation. Both sandstone grain size and mudstone color show broad variations within Denglouku strata which was deposited during late rifting stage. Strata consist of grey to purple mudstone and siltstone interbedded with grey sandstone and conglomerate. Coarse-grained sandstone and conglomerate mainly occurred in the lower Denglouku Formation. Overlying post-rift (subsidence) deposits unconformably overlie Denglouku Formation and exceed the distribution of the earlier rift-related units. Strata from this stage, with a thickness of 1000–1700 m (maximum 2000 m), are comprised mainly of fine-grained sandstone and siltstone interbedded with grey mudstone.

2. Geologic setting 2.1. Tectonic setting The Songliao Basin, a large continental sedimentary basin in northeast China, formed in the Late Jurassic to Neogene. The basin was filled in three major tectonic stages: rifting, post-rift thermal subsidence and structural inversion. It is divided into six structural units: the northern plunge, the central downwarp, the northeastern uplift, the southeastern uplift, the southwestern uplift and the western slope (Hu et al., 2005; Ge et al., 2010; Feng et al., 2010). During the Late Jurassic to Early Cretaceous rift phase, a series of NNE-to NS-oriented faultbound depressions were formed in the Songliao Basin. During the postrift thermal subsidence, these initial fault-bound depressions connected and became a much larger basin (Hu et al., 2005; Ge et al., 2010). Strata from this stage cover a wider distribution than that of older units. The Fulongquan Depression is a relatively small rift basin which is located in the southeastern uplift zone of the Songliao Basin (Fig. 1-A, D). It borders upon the Shuangtuozi uplift to the southwest, Nonganxi uplift to the west and Yangdachengzi uplift to the east. The three major border-faults in the eastern part controlled the development of three smaller sags: Fubei sag in the north, Funan sag and Gujiadian sag in the south (Fig. 1C). In response to strong structural inversion of Songliao Basin, including folding and uplift, only rift and subsidence-related deposition were preserved in the Fulongquan Depression (Hou et al., 2004; Liu et al., 2015b). The rift-related sequence in Fulongquan Depression consists of the Upper Jurassic Huoshiling Formation (J3h) overlain by the Lower Cretaceous Shahezi (K1sh), Yingcheng (K1y) and Denglouku (K1d) formations. The depression-related sequence consists of the Lower Cretaceous Quantou Formation (K1q) overlain by the

3. Data and methods 3.1. Data The data set for this study includes 400 km2 of recently acquired 3-D seismic data within the Fulongquan Depression. Until now, the 3-D seismic data coverage area has reached more than 1100 km2 with an inline and cross-line spacing of 25 m, covering almost the entire region of Fulongquan Depression. Furthermore, in this study we utilize approximately 90 exploratory wells which had been drilled throughout the depression. Most of these wells yielded gamma-ray, sonic, 342

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Fig. 1. Location and geological setting of the study area. (A) Index map of the Songliao Basin showing the major structural divisions referred to in the text and the location of the study area. (B) Tectonic setting of the Fulongquan Depression. During the Late Jurassic to Early Cretaceous rift phase, a series of NNE-to NS-oriented fault-bound depressions were formed in the Songliao Basin. (C) Surface structural features, seismic and well data coverage of the study area, as well as the transect location of the cross-sections and seismic profiles cited in the text. For the sake of clarity, only a subset of wells is shown. (D) Structural cross section across the central part of the Sangliao basin (transect a-b in Fig. 1A). Note: ①Rift related sequence; ②Depression related sequence and ③Structural inversion sequence.

1987; Posamentier et al., 1988). Identification of sequence boundaries in the Fulongquan Depression is based on analysis of 3-D seismic profiles complemented by well logs. The variation of lithofacies, electrofacies and strata stacking patterns were used to identify sequence boundaries and flooding surfaces. Meanwhile, sequence boundaries and systems tracts were mapped from seismic surveys based on seismic reflector characteristics and relationships such as downlap, onlap, toplap and truncation. Then, well interpretations were calibrated against the 3-D seismic data. Finally, sequence architecture was reconstructed in detail based on this stratigraphic framework.

spontaneous potential, resistivity and density log data. Cores or sidewall materials have been recovered from about 11 wells within Denglouku Formation. 3.2. Methods Detailed analysis of the depositional systems is based mainly on the integration of well logs, drilling cores and seismic data. Gamma ray, resistivity logs, and drilling cores provided specific constraints on lithofacies and electrofacies, which are the basis of our interpretation of sedimentary facies. The seismic reflection and multiwell contrast analysis of stratigraphic and sedimentary facies characteristics defined the sedimentary system's spatial distribution. Generally, unconformities and their correlative conformities are used as sequence-bounding surfaces because they represent a hiatus in deposition, or a time gap (Vail et al., 1977; Mitchum, 1977; Haq et al.,

4. Sequence stratigraphy Sequence stratigraphy is a methodology that provides a framework within which to interpret the evolution of depositional systems through space and time (Catuneanu, 2002; Catuneanu et al., 2009, 2011). The 343

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Fig. 2. Stratigraphy, tectonic setting and seismic features of the Fulongquan Depression. Paleoenvironment data from Huang et al. (1999).

regional uplift occurring throughout the Songliao Basin (Ren et al., 2002; Liu et al., 2015a,b). Active stretching and rotation of fault blocks during the rift stage is terminated by the development of the T3 unconformity. The baseline of LLD well log shows abrupt change in several wells, which is likely attributed to lithological variation and intense erosion of depositional stratal successions (e.g. Deng and Wang, 1996; Mao et al., 2007; Deng et al., 2018) (Fig. 5, see well SL5 and F17).

Denglouku Formation in the study area can be divided into two sequence stratigraphic units (SQ1 and SQ2) based on three local and/or regional unconformities identified from well logs and seismic data.

4.1. Sequence boundaries The bottom interface of SQ1 is a non-conformable contact with the underlying Yingcheng Formation along the margins of Fulongquan Depression. In southwest regions of the depression, T4 seismic reflector (also corresponding to SB1) shows features typical of an angular unconformity, including truncation below the interface and overlap above the interface and grade toward the central basin into conformable contacts shown as strong parallel reflection zones (Fig. 4). SB1 can also be recognized by abrupt changes in lithofacies and log curve responses. It is marked by a basal conglomerate in many wells and the mud color changes from gray or dark gray to brown, which indicate a variation of redox conditions (Fig. 5, see well SL2). The sequence boundary SB2 is an erosional surface along the basin margin with truncation and onlap characteristics on the seismic profile (Fig. 4). The configurations of seismic reflectors reveal an obvious transition from divergent forms or wedge-shaped geometry to a simple parallel infill with sub-parallel seismic reflection patterns. It also appears in many drilling wells as a lithologic change from thin sand bodies interbedded with mudstones below the unconformity to thick sandstones interbedded with mudstones above it (Fig. 5). The T3 seismic reflector (SB3) marks the contact between Denglouku (older) and Quantou (younger) formations, which appears as a high amplitude and continuity horizon on the seismic profile (Fig. 4). It is a disconformable surface in most areas and an angular unconformity along the basin margin, which corresponds to a large

4.2. Slope-break and systems tracts Slope-break zone is an important topography in sedimentary basins and it controls the development of the sedimentary systems, sedimentary environments and the distribution of lowstand systems tracts within a sequence (Feng, 1999; Lin et al., 2000; Li et al., 2003; Feng et al., 2013, 2016; Zhang et al., 2016). Two different kinds of slopebreak zones were identified in Flongquan Depression: the syndepositional fault slope-break zone and the flexural slope-break zone. The syndepositional fault slope-break zones occur where fault movement leads to the obvious differential uplift and subsidence relief in a lake bed. They were developed along the eastern border faults and the west part of the Fubei sag (Fig. 5). Syndepositional flexural slope-break zones usually form at flexures of growth anticlines associated with paleohighs and tilted fault blocks. They were mainly developed along the west margin of Gujiadian sag (Fig. 4). Each sequence in Denglouku Formation can be divided into a lowstand systems tract (LST), transgressive systems tract (TST) and highstand systems tract (HST) by integrated analysis of major flooding surfaces and stacking patterns of the stratal units. Flooding surfaces are represented by lacustrine mudstones or floodplain fine-grained beds (Fig. 4). The most common stacking patterns of transgressive and 344

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Fig. 3. Schematic structural cross sections of the Fulongquan Depression in the Songliao Basin, showing tectonostratigraphic characteristics and basin geometry. Locations of cross sections are shown in Fig. 1.

Fig. 4. Interpreted seismic profile calibrated with borehole (F23) showing sequence boundaries (SB) and systems tracts (see Fig. 1 for location). Sequence boundaries (SB) are local subaerial unconformities at the basin margin (SB1, SB2), slope onlap also can be recognized above SB1 and SB2. SB3 (seismic reflection T3) shows a parallel unconformity in this seismic profile. Arrows demarcate onlap, or truncation; LST = lowstand systems tract; TST = transgressove systems tract; HST = highstand systems tract. 345

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Fig. 5. Well-log cross section (see Fig. 1 for locations of wells) showing sequence stratigraphy of the Denglouku Formation. SB1 is characterized by abrupt changes in lithology, sedimentary facies or mudstone color (see F14, SL2, SL5). SB2 is best defined based on obvious changes in lithology, electrofacies and is typically overlain by thick sandstones represented by high amplitude box motifs (see F14, SL2, SL1 and SL5). SB3 typically is overlain by fine-grained siltstone or mudstone. The baselines of LLD well logs show abrupt change responses in well F17 and SL5. Wireline logs are gamma ray (GR) and resistivity deep-lateral well log curves (LLD). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

silty mudstone and mudstone suggest the region tend to be exposed and the sediment oxidized. The thickness of this facies is variable and the deposits are mainly formed by vertical aggradation. This facies is interpreted as a result of deposition in interdistributary bays of braided delta plain (e.g. Jiang et al., 2011; Liu et al., 2015a; Wang et al., 2017). Trough and planar cross stratification found in sandstones suggest a relatively lower flow regime condition and result from the migration of dunes (e.g., Collinson, 1970; Walker and James, 1992). The conglomerate and fine-to coarse-grained sandstone with fining-upward successions (fluvial cycles) can be interpreted as braided distributary channelfill deposits (e.g., Miall, 1977; Wang et al., 2017). The thickness of single distributary channel primarily presents a variation on the order of 1.5 m–3.5 m, but these channels may stacked vertically and show a broadly consistent thickness (even up to 10 m). Their gamma-ray (GR) and resistivity deep-lateral (LLD) well log curves commonly show box motifs with relatively high amplitude (Fig. 7A). This facies association was mainly found in the basin margin of the Fulongquan Depression. Observation of drilling cores indicates that pebble-sized, gray to dark gray broken mud clasts are more common in the west of the Fulongquan Depression (Fig. 6-B, C). By contrast, the composition of gravels deposited near the eastern border faults is more complex, which comprises metamorphic rock fragments, volcanic rock fragments and a small amount of compacted, deformed muddy debris (Fig. 6A). This variation might be attributed to the different nature of source rock. In the early depositional period of Denglouku Formation, the basement of Yangdachengzi uplift tend to be exposed and eroded. Drilling wells indicate that the basement lithology of Yangdachengzi uplift comprises metamorphic rock (slate and quartz schist) and volcanic rock (andesite and tuff). Consequently, rivers carried metamorphic and volcanic gravels westward into Fulongquan Depression and deposited near the eastern boundary faults. However, in the western slope of Fulongquan Depression, dark-colored mud clasts are likely eroded from underlying

highstand systems tracts are composed of retrogradational sets (finingupward) and progradational (coarsening-upward) to aggradational sets respectively. The vertical thickness of lowstand systems tract varies considerably and may not be deposited over the highest parts of the basin margin (Fig. 5, see well F17 and SL5). By contrast, transgressive and highstand systems tract are usually marked by widespread and stable stratal successions in study area. From SQ1 to SQ2, the thickness of LST gets smaller and LST features become difficult to recognize in seismic profiles in the southern part of the basin. With the basin surface and slope-break features become more and more gentle, the distribution of TST tend to occupy a larger area. Highstand systems tract of the SQ2 was eroded prior to sedimentation of overlying Quantou Formation, especially in the basin margin.

5. Depositional systems 5.1. Braided-delta 5.1.1. Braided-delta plain This facies association is characterized by massive, poorly to moderately sorted conglomerates (Fig. 6A), trough/planar cross-stratified, fine-to coarse-grained sandstones and brown or sepia silty mudstones and mudstones. A few randomly dispersed granule-to pebble-size mud intraclasts may occur within the sandstone (Fig. 6-B, C). Granule-to pebble-dominated conglomerates occur commonly as thin layers defining the base of trough/planar cross-stratified sandstones. These cross-stratified sandstones primarily present a variation on the order of 1.5 m–3.5 m vertical scale and most exhibit fining-upward profiles which begin with erosional contact at their base. Previous studies suggest that braided deltas are likely to be formed in the late stages of rift and widely developed in rift basins of East China (McPherson et al., 1987; Liu et al., 2015a). The brown- or sepia-colored 346

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Fig. 6. Core photographs of typical sedimentary structures of K1d, the well location is shown in Fig. 1 (A) Scour surface and pebble conglomerate. The conglomerate is mixed with metamorphic and volcanic rock fragments, and a small amount of muddy debris. The medium-to coarse-grained sandstone contains some mudstone debris with good orientation (2147.6 m in well SL503). (B) Scour surface and structureless coarse-grained sandstone which contains lots of muddy debris (2630.4 m in well SL2). (C) Fine-to medium-grained sandstone which contains some dark-grey muddy debris (2628.1 m in well SL2). (D) Scour surface and conglomerate. The conglomerate is mainly composed of subrounded to rounded, gray to dark gray mud pebbles which appear to be imbricated. Cross-bedding occurs in the upper part in medium-grained sandstone (1686.0 m in well SN70). (E) Sandstone with subrounded to rounded, gray to dark gray mud pebbles (2672.0 m in well SL201). (F) Dwelling burrows in dark gray mudstones (1829.1 m in well SN70). (G) Wavy bedding and ripple cross bedding in gray siltstones (2669.3 m in well SL201). (H) Crossbedded, fine-to medium-grained sandstone which show abrupt contact with the underlying dark grey siltstone (1888.25 m in well SN70). (I) Wavy bedding in light grey fine-grained sandstone (2672.26 m in well SL201). (J) Soft sediment deformation in dark grey siltstone (1683.3 m in well SN70). (K) Trough cross-bedding (1659.38 m in well SL501). (L) Parallel bedding in fine-grained sandstone (1363.5 m in well SL5). (M) Wedge-shaped or planar cross-bedding (upper part of core) and parallel bedding (lower part of core). Brown muddy pebbles occur in the lower part of sandstone (1361.1 m in well SL5). (N) Scour surface and conglomerate. The conglomerate contains lots of poorly sorted, brown mudstone debris (1363.3 m in well SL5). (O) Poorly sorted conglomerates mainly composed of muddy debris (1364.1 m in well SL5). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

G, I) and gray to dark gray, laminated and massive mudstone. In general, cross-stratified sandstones show fining-upward character and are bounded by sharp-erosive surfaces. Above these surfaces, some crudely bedded gravel deposits, ranging in thickness from 5 to 20 cm, can be observed and are composed mostly of sub-rounded to rounded, gray to dark gray mud pebbles (Fig. 6-D, E). Low-gradient cross-stratified or ripple cross-stratified fine sandstones can exhibit coarsening-upward

sedimentary strata (e.g. strata of Yingcheng Formation) (see Fig. 5-well SL2 or Fig. 7-A). 5.1.2. Braided-delta front This facies association largely includes fine-to medium-grained sandstone with trough/planar cross stratification (Fig. 6-D, H), lowgradient cross-stratified or ripple cross-stratified fine sandstone (Fig. 6347

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Fig. 7. Main sedimentary facies and their lithologic and well log response features. (A) Braided-delta plain; (B) Braided-delta front; (C) Fluvial-floodplain; (D) Sublacustrine fan; (E) shallow lake.

lower flow regime (Miall, 1977). The fine-grained sandstone presented in cores with coarsening-upward reverse grading seems compatible with deposits found in mouth bar (e.g. Jiang et al., 2011; Zhu et al., 2017; Wang et al., 2017). The dominance of gray to dark gray mudstones point to a relatively calm and more or less anoxic environment (e.g. Jiang et al., 2011; Zhu et al., 2013). Mudstones with massive/ horizontally laminated beddings indicate a low current energy regime and are deposited by suspension settling. These dark-colored mudstones with soft sediment deformation structures, bioturbation and plant fragments are likely related to distal depositional environment to the braided delta or subaqueous distributary bay deposits. The lithology and well log responses of braided delta front show several typically coarsening upward, progradational stacking patterns (Fig. 7B), which are likely formed by the progradation of the braided fluvial system into a standing body of water. Subaqueous distributary channel sandstone can be identified by medium to high amplitude box or bell motifs in GR and LLD well log curves (Fig. 7B). Additionally, this facies association is commonly characterized by relatively high-amplitude, medium-to high-continuous sub-parallel sheet-like or tabular reflections on seismic profiles (Fig. 8).

reverse grading and may show gradational contact with underlying gray to dark gray mudstone. GR and LLD well log curves from these sandstones typically show medium amplitude funnel shaped motifs (Fig. 7B). Massive or laminated, gray to dark gray mudstones and argillaceous siltstones may range in thickness from a few centimeters to several meters. Soft sediment deformation structures, bioturbation and plant fragments also can be observed in these gray silty mudstones (Fig. 6-J, F). Trough/planar cross stratification result from the migration of subaqueous dunes, under unidirectional, bedload dominated currents (Miall, 1977; Ashley, 1990). Ripple cross-stratified fine sandstones are attributed to ripple migration and deposition (Miall, 1977). Asymmetrical and symmetrical ripple marks the unidirectional and bidirectional currents, which are likely suggest deposition in shallow water. Fine-to medium-grained sandstones with these cross stratifications are attributed to sedimentation in the subaqueous distributary channel (e.g. Liu et al., 2015a; Zhu et al., 2017). This is also supported by the presence of fining-upward successions and scoured bases. Sub-rounded to rounded mud pebbles reveal a certain distance of depositional transportation and are likely derived either via erosion from the base of the channel or from bank collapse at the channel margin. These mud pebbles are interpreted as coarse lag deposits within a distributary channel. Lowgradient cross stratification and ripple cross stratification indicate a

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Fig. 8. Interpreted seismic profile (A and B in different direction) calibrated with borehole showing the deposits of the Denglouku Formation (see Fig. 1 for location of the profile). Sublacustrine fan are characteristic of hummocky reflections on 3-D seismic profiles (see A). Braided deltas front mainly shows high-amplitude, medium-to high-continuous reflections. Lacustrine fine-grained deposits are associated with low-amplitude, tabular to lenticular reflections. Fluvial system shows parallel or subparallel, variable amplitude reflections on seismic profiles.

responses of those shallow lake deposits show characteristic shale GR baselines, low resistivity and occasional low-amplitude finger and funnel motifs (Fig. 7E). Generally, mudstone dominated lacustrine deposits can be identified on seismic profiles by low-amplitude, tabular to lenticular reflections and less commonly continuous parallel reflections (Fig. 8-A, B).

5.2. Sublacustrine fan Sublacustrine fan deposits discussed in this article refer to subaqueous gravity-flow deposits formed in a sublacustrine environment. Such deposits are widely developed in the lower part of Denglouku Formation and typically display discontinuous hummocky reflections on seismic profile (Fig. 8A). This hummocky reflection character is indicative of coarse-clastic material delivered down a steep lake-floor gradient by mass-transport processes (Scholz et al., 1990). However, this typical seismic facies is not always recognized on seismic profile because of relatively low resolution seismic data. In most cases, sublacustrine fan deposits can be recognized in well logs by distinctive high-resistivity signals and chaotic package of high and low gamma-ray within background high gamma ray and low resistivity sections (Fig. 7D). Mud-logging lithologic data indicates that conglomeratic sandstone and fine-to medium-grained sandstone are the main lithology. Sublacustrine fan deposits range in thickness from 20 to 90 m, and those relatively thicker fans mainly developed near basin-bounding faults, especially in Fubei sag.

5.4. Fluvial-floodplain This facies association is dominated by brown- and purple-colored, interbedded mudstones and siltstones, medium-to fine-grained sandstones, and subordinate conglomerates and conglomeratic sandstones. The structures of these sandstones include massive bedding, trough/ planar cross bedding and parallel bedding (Fig. 6-K, L, M). A few brown and purple, well-rounded mud pebbles with a maximum size of 2.5 cm may occur randomly dispersed within these sandstones (Fig. 6M). Muddy debris or mudstone shivers, accompanied by scoured bases, commonly occur at the bottom of the fining-upward succession (Fig. 6N). Sometimes poorly sorted, muddy debris dominated conglomerates (up to 1 m) can be observed (Fig. 6O). The basal surfaces of these sets are erosional and they are most commonly occur in packages of cross-stratified sandstones. The brown- and purple-colored, interbedded mudstone and siltstone (up to several meters thick) likely represent deposition from suspension during overbank events in the floodplain. Cross-bedded sandstones with fining-upward successions are interpreted as a result of deposition in fluvial streams. Laminated to massive sandstones may occur during flood stage when the channel floor becomes a traction carpet (Miall, 1977). Trough and planar cross-stratification are attributed to the migration of large bedforms, under bedload dominated currents. The structureless, poorly sorted, muddy debris dominated conglomerate, up to 1 m thick, suggest the deposition by debris flow and related sediment gravity mechanisms, which probably occurs during stream flood stage (Miall, 1985). The group of evidence presented here seems compatible with deposits found in ancient braided-fluvial systems (e.g. Miall, 1977,

5.3. Shallow lake This facies association is characterized by taupe and gray to dark gray mudstones, argillaceous mudstones and thin-bedded siltstones or very fine sandstones. The mudstone is a few meters to tens of meters thick (Fig. 7E). Siltstones and very fine sandstones exist as relatively thin layers (mostly < 2.5 m) and are commonly intercalated with the surrounding thick mudstone. The dominance of mudstone in this facies points to deposition in a low-energy environment. Those thick, dark-colored mudstones are likely attributed to shallow lake deposits (sometimes may be referred to as pro-delta deposits). The thin-bedded siltstone and very fine sandstone are possibly transported longshore from proximal deltas and result from wave processes (Wang et al., 2008). They are interpreted as a result of deposition in shallow lake bars (e.g. Wang et al., 2017). Log 349

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Fig. 9. Multi-well sedimentary facies profile showing prominent features of depositional cycles, sedimentary facies type and lateral variation (see well location in Fig. 1C). Correlation has been flattened on SB3 (T3) unconformity surface. Facies associations have been extrapolated from lithologies and wireline log characteristics. The peneplanation first occurred in the southern part of the Fulongquan Depression (see F23, F14 and F12 which located in the Gujiadian sag). Sublacustrine fan mainly developed above SB1 during early lowstand of the SQ1. Fluvial systems widely spread over study area not until late highstand of the SQ2. Noting the thick volcanic rocks within SL501, which indicate the active nature of the basin-bounding faults.

According to core observations, well log motifs and seismic reflection characters, the four major sedimentary facies described above are strongly transitional and, in some instances, are likely interfingered by lateral and longitudinal. These facies associations shift from one to another, but two fundamental depositional patterns can be recognized. The first is lacustrine and correlated depositional systems such as sublacustrine fan and braided delta. This one is dominated in the lower part of Denglouku Formation (Fig. 9). The second one is fluvial, formed in a hydrologically open basin and characterized by basinwide floodplain and channel system. The paleocurrent patterns are dominated by one direction (from southeast to northwest) across the basin (Li et al., 2014a). This pattern was mainly developed during the late period of Denglouku Formation (Fig. 9). In addition, these sedimentary facies and the process of peneplanation exerted a significant difference over time and space in the Fulongquan Depression.

with the northern part. The displacement of border faults and syndepositional structural movement usually form the zone of abrupt change in the topography of a lake basin, which plays a crucial role in controlling the water depth and the distribution of depositional systems (Fig. 10). Drilling wells and seismic profiles indicate that the stratal distribution of the lowstand systems tract is smaller than in the underlying Yingcheng Formation. Lowstand systems tract composed of sublacustrine fan and braided-delta front sand bodies is mainly present in the downdip areas of the syndepositional structural slope-break zones and border faults. The region above the slope break, at the same time, was mainly exposed and the sediment oxidized. Along the eastern part of the Fubei sag a series of footwall-sourced, coarse-grained braided-deltas were deposited in the hanging wall. High rates of accommodation caused retrogradational stacking of these footwall-derived braideddeltas that pinched-out rapidly basinward into fine-grained lacustrine deposits. Sublacustrine fans were usually deposited near the central part of the basin (Fig. 11A). The stratal unit of transgressive systems tract spans larger areas than the lowstand systems tract. This systems tract is dominated by braideddelta front and shallow lake deposits. Thin layers and fine-grained shallow lake bars are usually deposited over the proximal, shallow part of the depression (Fig. 11B). During late highstand of the SQ1, drill core, lithology and well logs indicate that Fubei sag was still dominated by braided-delta front and shallow lake deposits. However, with a relatively decline in lake level the southern part of the Fulongquan Depression gradually transformed into delta plain and fluvial settings (Fig. 9).

6.1. SQ1

6.2. SQ2

During the early stage of Denglouku Formation, peneplanation first occurred in the southern part of the basin (Fig. 9). Drilling wells and seismic data indicate that overall depositional thickening is toward the northern part of Fulongquan Depression. The prominent depocenter was mainly located in the Fubei sag and its maximum sedimentary thickness of Denglouku Formation is more than 800 m. Regional difference of topography in the southern part is relatively small compared

In the early stage of the SQ2, lake water was mainly confined to the area of the Fubei Sag. The braided-delta front and shallow lake deposits continued to be deposited in the footwall of Fubei fault, while the connected fluvial and floodplain systems were widely developed in the south of the Fulongquan Depression. Because maximum subsidence still occurs in the north of the depression, a large amount of sediments from the footwall crest and western highlands were transported to the lower

1985; Wang et al., 2012; Chen et al., 2015). Moreover, several authors (Li et al., 2014a, 2014b) have pointed out that braided fluvial systems and braided deltas seems widely developed in the southeastern uplift zone of the Songliao Basin during the deposition of Denglouku Formation, which corroborate such interpretation. Fluvial-floodplain deposits are dominated in the upper Denglouku Formation and characterized on seismic profiles by sets of relatively flat and/or low relief reflections with variable amplitude (Fig. 8-A, B). Both gamma-ray and resistivity well logs from braided-channel sandstones show high amplitude responses with box or bell motifs (Fig. 7C). 6. Sedimentary evolution

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Fig. 10. Palaeogeomorphology of depositional stage of the SQ1 showing different geomorphic units. The depocenter is located in the north of the Fulongquan Depression. Slope break can be recognized along the basin margin both in the west and east of the depression.

The pulses of faults activation commonly cause episodic subsidence with short stages of rapid creation of accommodation, followed by relatively longer periods of time of thermal subsidence (Keen and Dehler, 1993). Owing to this specific subsidence pattern of rifts, Martins and Catuneanu (2010) suggests that a typical rift sequence consists of transgressive and highstand systems tracts, and the lowstand systems tracts tend to be poorly developed or absent. In addition, rift depositional sequences are usually bounded by flooding surfaces and arranged internally in dominantly coarsening-upward successions (Fig. 12). The sequence architecture observed in the Fulongquan Depression during late syn-rift stage, which consists of LST, TST and HST in the depocenter of the basin, does not conform to this model (Figs. 4 and 12). Actually, throughout a whole continental rift basin development, the tectonic subsidence history and basin structure will change

part of Fubei sag, and thus forming braided-deltas downdip of the break zones (Fig. 11C). With constant sediment infilling of the remnant basin topography inherited from the early stage of the SQ2, the lake water depth decreased gradually. Drilling wells indicate that the whole depression was not dominated by widespread fluvial and floodplain depositional environment until late highstand of the SQ2 (Fig. 9). 7. Discussion 7.1. Properties of sequence architecture The tectonic history of rift basins is complicated by multiple phases of rifting, instead of a uniform or continuous event (e.g. Chen et al., 1984; Prosser, 1993; Sopeña and Sánchez, 1997; Wang et al., 2015).

Fig. 11. Major episodes of the Fulongquan Depression sedimentary evolution during Denglouku Formation. (A) Lowstand systems tract (LST) of the SQ1; (B) Transgressive systems tract (TST) of the SQ1; (C) Transgressive systems tract (TST) of the SQ2. 351

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Fig. 12. Internal architecture of a complete late syn-rift sequence compared with ideal rift sequence. (A) Sequence stratigraphic interpretation of well SL1 and SL2 (see well location in Fig. 1), showing the stacked sediments that accumulated within each systems tract. Columns (B) are conceptual, showing aggradation to retrogradation in LST and TST, progradation to aggradation in HST. Note relatively lower sand/mud ratio of the HST in both SL1 and SL2. (C) Internal architecture of a complete (ideal) rift sequence, showing the overall coarsening-upward vertical stacking pattern, as well as the shift from underfilled to filled and overfilled conditions during the accumulation of the sequence (Martins-Neto and Catuneanu, 2010). FS: First flooding surface; MFS: Maximum flooding surface.

Wang and Sun, 1994; Magnavite and da Silva, 1995; Strecker et al., 1999; Lin et al., 2000; Wang et al., 2015). However, the color of mudstone changing from dark gray to grey or brown indicates that the lake water depth has not increased significantly from early lowstand to late transgressive systems tracts in the study area. On the contrary, this succession tends to overall shallow upwards (Figs. 5, 13 and 14). Such variation shows that the deepest water seems not always occur at the time of maximum flooding in the continental rift basin. Highstand systems tracts are usually not confined to areas of the Fulongquan Depression and they even expand to the degraded footwall crest (Figs. 5 and 9). The internal stratal stacking patterns from different wells suggest that using a simple pattern, such as underfilled, balanced or overfilled, to describe the internal architecture of highstand systems tract is difficult and insufficient. This is attributed to general change in the balance between accommodation and sediment supply across the rift basin, as well as the provenance systems.

dramatically. To clarify sequence architectures and their internal depositional systems therefore presents a difficult challenge. As different stages in basin evolution have distinct features of interplay between tectonics and sedimentary evolution, there may have some problems in using a general rift sequence stratigraphic model to describe sequence architecture developed in a certain stage with no restrict. For example during rift initiation stage numerous nearly isolated small grabens and half-grabens formed in the crust's surface, which are generally characterized by hummocky and discontinuous internal reflectors. Sedimentary facies and lithologic compositions are complex and readily to change in these subbasins. Seismic sequence and systems tracts are usually difficult to identify (e.g. Prosser, 1993; Gawthorpe and Leeder, 2000; Wang et al., 2015). As stated previously, three systems tracts in each sequence can be recognized in the Fulongquan Depression during late syn-rift stage. Although it is likely difficult to identify the lowstand and transgressive systems tracts on seismic profiles, these systems tracts still can be recognized in drilling wells (Figs. 4 and 5). The explanation why the lowstand and transgressive systems tracts cannot be effectively identified in seismic data set is mainly related to small strata thickness and limited seismic resolution. According to different depositional architecture, two key sequence types can be recognized. Type A sequence is expressed by standing bodies of water in the basin axis, which developed before the process of peneplanation (Fig. 13). Type B sequence is regarded as an overfilled basin model and represents a type of slow peneplanation (Fig. 14). These two sequence types can occur in one period but different tectonic units or in one tectonic unit but different periods. The syndepositional structural slope-breaks, such as syndepositional fault and flexural slope-break zones (Figs. 4 and 5), are common in continental rift basins and these slope-breaks play a similar role in controlling sand body distribution as shelf-breaks in passive continental margins (Howell and Flint, 1996; Lin et al., 2000; Feng et al., 2013, 2016; Zhang et al., 2016). These slope-break zones control the development and distribution of the lowstand systems tract. Coarse-grained successions, such as sublacustrine fans and lowstand braided deltas, can be observed at the downdip areas of these slope-break zones (Figs. 11 and 13). Transgressive systems tract is evidenced by retrogradational parasequence sets and progressive onlap of lacustrine fill reflections on to higher areas of the basin margins (Figs. 4 and 5). The maximum flooding surface is marked by the physical and temporal change from a retrogradational to a progradational parasequence set and dominated by fine-grained deposits (Fig. 5). In general sequence stratigraphic models of rift basins, this surface is commonly marked by an appreciable increase in water depth and deep-water sedimentary systems (e.g.

7.2. Evolution of depositional systems The spatial distribution and temporal evolution of sedimentary facies and facies associations in continental rift basins are usually complicated. In a conceptual model, extensional tectonism will produce a series of half-grabens/grabens during rift initiation stage. These subbasins tend to poorly linked and are usually characterized by coarsegrained deposits, such as alluvial fan, fan delta, axial fluvial systems and gravity-driven sedimentary systems in subaqueous environment (e.g. Chen et al., 1984; Leeder and Gawthorpe, 1987; Prosser, 1993; Gawthorpe and Leeder, 2000; Wang et al., 2015). Facies and facies distributions in these subbasins are commonly influenced by surface topography associated with fault breaks and growth folds. With extension and subsidence progressed, lateral propagation and interaction between fault segments lead to enlargement and coalescence of early fault depocenters (Gawthorpe and Leeder, 2000). A larger volume and more fully extended basin formed during this rift climax stage. Although evaporate or subaerial deposits may developed (e.g. Chen et al., 1984; Almeida et al., 2009), the facies complexes in most rift basins are commonly characterized by lacustrine and associated coarse-grained facies such as sublacustrine fan and fan-delta along the basin margins due to maximal rates of extension and subsidence coupled with adequate supply (e.g. Chen et al., 1984; Scholz et al., 1990; Schlische and Olsen's, 1990; Gawthorpe and Leeder, 2000; Lin et al., 2001; Dong et al., 2011; Jia et al., 2014; Wang et al., 2015). During early period of the late syn-rift stage, the rift basin may inherit the major characteristics of the previous depositional systems. With the decrease of displacement on the basin-bounding faults, the topography created 352

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Fig. 13. Shallow-lacustrine sequence model (type A) developed in one pulse of extension during late syn-rift stage. (A) Three-dimensional block diagram, representing the proposed lacustrine and delta succession during a temporal series of lowstand systems tract (LST), the source area uplift results in the increase of erosion rate and sediment supply. (B) Transgressive systems tract (TST) and (C) highstand systems tract (HST) represent the other two different stages. The diagrams (A to C) represent successive stages in the evolution, with decreasing in differential subsidence, sediment supply and water depth. A: accommodation.

continuous event, but rather consisted of several major episodic pulses of extension. Each episodic pulse is commonly separated by key erosional unconformities along the basin margins which indicate the transition from one tectonic movement to another (Nottvedt et al., 1995; Sopeña and Sánchez, 1997). For example two sequence stratigraphic units separated by key unconformity surfaces were identified in the Fulongquan Depression, which is likely corresponded to two major episodic pulses of extension respectively (Figs. 4 and 5). As the tectonic subsidence rate tends to be lower during late syn-rift stage, sedimentation has a greater chance to outpace the potential accommodation. The basin therefore will fill with sediment, and excess sediment and water will leave the basin. The final response to the decrease of differential subsidence will be the gradual and slow peneplanation and this process may take many millions of years. The subsidence pattern of rift basins varies significantly in different evolution stage. In most conceptual models and basin analysis, general thermal subsidence was rarely taken into account during rift climax stage as episodic behavior have a primary impact on basin structures and tectonostratigraphic units. However, with the decrease of displacement on bounding faults and differential subsidence, broad crustal subsidence may become more important in controlling sedimentary filling process. Superimposed on this general tectonic subsidence,

through faulting tends to be overfilled by sediments. As a result, gradually shallowing and possibly eliminating the lacustrine environment are more likely to occur during the late syn-rift stage. In terms of the Fulongquan Depression, the process of boundary faults growth and death shows significant differences during late syn-rift stage. Based on the analysis of fault growth index obtained according to the ratios of strata thickness in hanging-wall and footwall, the activity of the Fubei fault, compared with Funan and Gujiadian faults, seems much stronger during each pulse of extension (Fig. 15). This specific growth history of the major boundary faults leads to deeper lake and long-lived lacustrine deposits in the Fubei sag. Therefore, the peneplanation in the Fulongquan Depression indicates that the spatial and temporal variations should be taken into account due to different growth history of the major boundary faults, which will be discussed in later section.

7.3. Controls on the development of sequences and depositional systems 7.3.1. Tectonic movement Different from the passive margin settings, the accommodation space in continental rift basins is significantly influenced by tectonic movement (Leeder and Gawthorpe, 1987; Blair, 1987; Olsen, 1990). Throughout the late syn-rift stage, the tectonic movement may not be a 353

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Fig. 14. Shallow-lacustrine to fluvial sequence model (type B) developed in one pulse of extension, showing a major sedimentary environmental reorganization. (A) Lowstand systems tract depositional stage, the lake level dropped and the sand bodies moved to the downdip area of slope-break zone. (B) Transgressive systems tract depositional stage, the water depth tends to be shallower and the sandbodies backstepped toward the basin margin. (C) Highstand systems tract, sedimentary environment transformed into fluvial-floodplain system. A: accommodation.

subsidence mainly occurred during lowstand of each sequence, evidence from wedge-shaped strata, volcanic rocks adjacent to boundary fault and the absence of lowstand systems tract above the footwall crest (Figs. 4 and 9). Since the tectonic subsidence in lowstand systems tracts are highly asymmetric, the accommodation increasing belt, decreasing belt and transformation belt will be formed in different zones (Jiang et al., 2005; Deng et al., 2008; Yang et al., 2009). The highly expanded highstand systems tracts with smaller changes of strata thickness indicate a broad crustal subsidence regime and obviously decreased faults activity (Fig. 9). However, combined with variable sediment supply, strata stacking patterns are usually complex across the basin. An additional important feature in continental rift basins is syndepositional structural slope-breaks. The formation of these slope-breaks is mainly associated with the slip on nonplanar bounding faults which exert a primary control on the style of secondary faulting and folding in the hanging-wall (e.g. Mcclay and Scott, 1991; Xiao and Suppe, 1992; Hardy, 1993; Withjack et al., 1995). Different types of slope-breaks are responsible for the development of systems tracts and the distribution of depositional systems.

Fig. 15. Major boundary fault activity (y-axis) within the Fulongquan Depression. Based on the interpretation of seismic data, fault growth index was used to estimate major boundary fault activity (Fubei, Funan and Gujidain faults). The growth index was obtained according to the ratios of strata thickness in hanging-wall and footwall.

episodic fault movements and basin floor rotation during late syn-rift stage usually complicate the depositional geometries in rift basins (Nottvedt et al., 1995). The maximum fault activity and differential 354

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7.3.2. Climate changes and lake level fluctuations The climate, through its control on weathering rate, erosion rate, vegetation biomass and water balance, thus emerges an important control on environment of deposition and stratigraphy (e.g. Olsen, 1990; Carroll and Bohacs, 1999; Gawthorpe and Leeder, 2000; Ilgar and Nemec, 2005; Balázs et al., 2017). Carroll and Bohacs (1999) suggest that it is the relative balance of rates of potential accommodation (mostly tectonic) with sediment + water fill (mostly a function of climate) that controls lake occurrences, distribution, and character. Thus, with the decrease of tectonic activity, the function of climate tends to become more important during late syn-rift stage. The humid environment is likely conductive to the standing bodies of water in the basin axis and large fans that reach out into the basin. Coarse-grained sublacustrine fans developed in the lowstand of the SQ1 suggest the high sediment influx and water discharge during early wet climate periods (Figs. 9 and 11). In an arid environment the amount of water fluxes will be limited and the lake water volume tend to decrease gradually. In terms of the Fulongquan Depression, the climate has changed dramatically from humid to arid during late period of the Denglouku Formation (Huang et al., 1999). This variation may have contributed to the shrinkage of lake water in the study area. The flash floods and ephemeral, low sinuosity streams are finally developed in highstand of the SQ2 in the Fulongquan Depression (Figs. 9 and 14). High frequency lake level fluctuation commonly corresponds to changes of climate in lacustrine basins due to their limited volume and water mass. This sort of fluctuation behavior can occur on a wide range of space-scale, from a few meters to hundreds of meters (e.g. Olsen, 1990; Scholz, 1995; Cohen et al., 1997; Ilgar and Nemec, 2005; Samanta et al., 2016). As differential subsidence is much lower during late syn-rift stage, especially late period of each episodic pulse of extension, the topography becomes gentler and water depth tend to be shallower. With lower accommodation apace caused by tectonic subsidence, the effect of lake level fluctuations on the variation of potential accommodation space might become more and more important, and thus have a greater influence on depositional sequence. The higherorder transgressive-regressive cycles developed in highstand of the SQ1 are probably to be associated with higher-order climatic effects and lake level variations (Fig. 12). Actually, in addition to climatic milieu, tilting of the basin floor and changes in the elevation of spillover points also will redistribute the lake water, and therefore, change the lake water level (Strecker et al., 1999; Carroll and Bohacs, 1999). As noted above, rapid tectonic subsidence and tilting of the basin floor occur during the early period of each pulse of extension. If assuming the elevation of spillover points remains stable, this specific subsidence pattern will redistribute the water volume and therefore tends to create a deeper lake in the lowstand systems tract as long as the tectonic subsidence rate exceed the rate of lake level fall and sediment accumulation.

provenance and depositional systems. Although the climate and nature of the source rock are important factors to consider (e.g. Christie-Blick, 1991; Prosser, 1993; Carroll and Bohacs, 1999), tectonic movement is regarded as the primary one in governing regional erosion rates and constraining the supply of sediments during the late syn-rift stage. If a continental rift basin is topographically closed system, drainage between adjacent rift basins will be prevented by the development of basement highs at the segment boundaries. Such a closed basin is hydrologically distinct, water and sediment fluxes are largely conserved (Gawthorpe and Leeder, 2000). During the early phase of the episodic pulse (lowstand systems tract), asymmetrical subsidence tilts the basin floor and makes the water mass shift laterally, resulting in the uplift and erosion of the topographic highs surrounding the basin, and the increase of the gradient on existing streams (Fig. 13). Such episodic uplift of the source area results in a rejuvenation of river systems and may also be accompanied by an increase in the calibre and quantity of the sediment load of the river (Prosser, 1993). By contrast, the cessation of differential subsidence or tectonic movement (mostly highstand systems tract) results in gradual erosional reduction of adjacent source-area relief and reduction in the quantity and calibre of the sediment load, yielding regional-scale finegrained dominated deposits. Such variation can be documented by higher sandstone content in the lowstand of each sequence, while low sand/mud ratio in the highstand systems tract of the SQ1 (Fig. 12). With the decrease of tectonic subsidence and landscape gradients, the influx rate of water + sediment fill is probably exceed potential accommodation, resulting in an open rift basin. In this case, two major depositional systems can be developed mainly according to the variation of the provenance systems: (1) the accumulation of mainly finegrained sediments with the source area degraded; and (2) expanded drainage basin dominated by fluvial systems, such as the Fulongquan Depression (Fig. 14). The three dimensional linked depositional systems and the rate of sediment supply associated with this stage will vary according to climate, nature of source rock, gradient of slope and the position of the rift basin.

8. Conclusions During late syn-rift stage, the Fulongquan Depression experienced a major transition in depositional systems, which is recorded the evolution in basin fill from predominantly lacustrine and correlated depositional systems to fluvial-floodplain settings. This gradual and slow peneplanation is the final response to the decrease of differential subsidence. The specific growth history of the major boundary faults mainly controls the spatial and temporal variations of sedimentary evolution. Different sequence stratigraphic units separated by key unconformity surfaces and their correlative conformities are likely corresponded to major episodic pulses of extension respectively. Each sequence can be further divided into a lowstand systems tract (LST), transgressive systems tract (TST) and highstand systems tract (HST). Maximum differential subsidence and water deepening occur mainly in the LST. HST is commonly characterized by broader crustal subsidence and sedimentary expansion. The peneplanation and major transition of depositional systems usually occur in the HST. Both broad crustal subsidence and episodic pulses of extension are important subsidence patterns during the late syn-rift stage. Their combination has a profound impact on stratigraphic architecture. In addition to major changes of provenance systems, tectonic movement is regarded as the main factor in controlling the variations of sediment supply. With the decrease of tectonic activity, the impact of climate changes and lake level fluctuations on basin filling tends to be increased. This study shows that general rift sequence stratigraphic models have limitations to describe sequence architecture developed in late syn-rift stage.

7.3.3. Variations of sediment supply The interplay of accommodation and sediment supply determines the depositional trends (progradation, retrogradation and aggradation) (Van Wagoner et al., 1990). Rift basins have their accommodation history strongly related to tectonic subsidence. When sediment supply is assumed to be stable, a typical rift sequence characterized by progradational depositional trend (Martins and Catuneanu, 2010) may be more commonly seen in rift climax stage. This is because the episodic pulse of tectonic subsidence is commonly much stronger compare with rift initiation and late syn-rift stages. Although the episodic pulse of extension still can be observed during late syn-rift stage, the maximum subsidence rate is smaller than that during rift climax stage. The rate of sediment supply therefore has a greater chance to outpace the tectonic subsidence. Their variations will play an important role in controlling strata stacking patterns. Moreover, the basin geometry and topography of the late syn-rift stage are more likely to be changed dramatically. Correspondingly, there is also a great chance of major switch in detrital 355

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Acknowledgements

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All data used in this study were provided by Northeast Branch Corporation of SINOPEC. We wish to thank Meihua Zhang, Qiuxing Han, Guangming Xi and Hao Jiang for the assistance with data collection, core observations, and seismic interpretation. This research was supported by National Natural Science Foundation of China (Grant No. 51574208), Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDA14010201-02), Major National Sci-Tech Projects (Grant Nos. 2017ZX05009-002, 2017ZX05005-002003) and Fundamental Research Funds for the Central Universities (Grant No. 2-9-2015-141). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.marpetgeo.2018.11.029. References Almeida, R.P.D., Janikian, L., Cesar, A.R.S.F., Marconato, A., 2009. Evolution of a rift basin dominated by subaerial deposits: the Guaritas Rift, early Cambrian, Southern Brazil. Sediment. Geol. 217 (1), 30–51. Ashley, G.M., 1990. 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