Yufei Gao1
Yanshu Yin
Mei Chen- 1CNOOC Research Institute LTD., Beijing, China
- 2School of Geosciences, Yangtze University, Jingzhou, Hubei, China
Gravity flow deposits record a variety of sedimentary processes preserved within depositional systems. However, comprehensive studies on the evolution changes of gravity flow depositional architectures from fault-proximal sources to rift-distal areas remain limited. In particular, large-scale cases studies that systematically integrate different types of gravity flow deposits are still lacking. Using data from cores, well logs, and three-dimensional seismic reflection, this study focuses on the lithofacies evolution and associated sedimentary architectures of the upper of the upper sequence of the Third Member of the upper Liushagang Formation in the Weixinan Sag, Beibu Gulf Basin, South China Sea. The results indicate that: 1) Lithofacies in the study area can be classified into coarse-grained and fine-grained types. Coarse-grained lithofacies dominate the fault-proximal zone, where conglomerate and sandstone account for 79% of the deposits, whereas fine-grained lithofacies prevail in rift-distal areas, with mudstone comprising 53%. 2) Coarse-grained lithofacies are mainly massive, poorly sorted, and subangular to angular, commonly containing floating gravels. In contrast, fine-grained lithofacies exhibit decreased gravel content, increased sandy material, and well-developed bedding structures such as graded bedding, wavy bedding, convolute bedding, ripple lamination, and lenticular bedding. 3) Lithofacies characteristics and proportional analysis indicate that debris flow deposits dominate the source-proximal region, while turbidite deposits represent the main depositional environment. 4) Debris flows develop in a divergent fan-shaped geometry, whereas turbidity currents form tongue-shaped lobate bodies. 5) These lobes are laterally assembled and vertically stacked, with individual lobes extending 1.2 km–1.5 km in width. They form the widely distributed thin-bedded interbedded sandstone-mudstone deposits within the Weixinan Sag. Overall, the depositional evolution reflects a shift from topographically elevated, fault-proximal debris flows to lower-relief, rift-distal turbidity currents systems. The underlying paleotopography exerted strong control on the distribution of deposits. These findings provide important insights into sedimentary processes in gravity flow systems, especially the development of thin interbedded mudstone and sandstone units, and have significant implications for predicting sediment distribution and improving hydrocarbon exploration and production efficiency.
1 Introduction
Gravity flows are the principal processes that transport sediments from the continental shelf to the deep water (Bouma, 1962). These flows form the building blocks of deep-water depositional systems and are of great economic importance as they constitute some of the world’s most significant hydrocarbon reservoirs (Weimer and Slatt, 2004). The study of these gravity flows relies on a classification framework established by Middleton and Hampton (1973). Based on rheology and current processes, gravity flows are often recognized as cohesive debris flows supported by matrix strength (Shanmugam, 1996; Wang et al., 2024), turbulent turbidity currents (Reece et al., 2024), grain flows supported by grain collisions, and fluidized flows by pore-fluid escape (Middleton and Hampton, 1973). Understanding these mechanisms is crucial for interpreting the resulting deposit characteristics, which range from chaotic, poorly sorted debris flow deposits to the classically graded beds of the Bouma sequence (Bouma, 1964).
The evolution of a gravity flow from initiation to deposition is a dynamic and transformative process (Yang et al., 2022). Initiation can be triggered by various mechanisms, including seismic activity, storm-wave loading, or the direct plunging of hyperpycnal flows (Mulder and Syvitski, 1995). A single event is rarely static in character; it can evolve spatially and temporally, undergoing significant flow transformation. For instance, an initially cohesive debris flow may dilate and mix with ambient water, transforming into a turbidity current, a process detailed by Fisher (1983). This evolution is preserved in the sedimentary record as hybrid event beds, which contain complex alternations of debris and turbidite layers, reflecting changes in rheology and energy during deposition (Haughton et al., 2009).
Gravity flow deposits form a significant component of the stratigraphic record in lacustrine basins. These deposits, which include debrites (Liu et al., 2025), hybrid (Zavala et al., 2006) and turbidites event beds (Zavala and Arcuri, 2016), are generated by slope failure triggered by mechanisms such as river floods, sediment-laden hyperpycnal flows, seismic activity, or deltaic collapse (Geng et al., 2025). The resulting deposits are characterized by distinct sedimentary structures, including graded bedding, sole marks, and soft-sediment deformation, which provide a high-resolution archive of paleoenvironmental conditions (Yang et al., 2020; Zavala, 2020). Previous research emphasizes that the final depositional units, such as feeder channel, channel-levee systems, and terminal lobes, are direct products of flow transformation (Talling et al., 2012; Guo et al., 2021). The spatial distribution of these elements is further influenced by the nature of the sediment source, whether it originates from a single river mouth or canyon that delivers sediment offshore, or from a collapsing shelf edge (Reading and Richards, 1994; Peng, 2021). Therefore, a complete understanding of gravity flows requires integrating their static classification with a dynamic model of their evolution. This integrated approach is essential for accurately reconstructing paleoenvironments, predicting the distribution and connectivity of subsurface reservoir sands, and reducing risk in deep-water exploration.
The objective of this research is to understand the complex diversity of gravity flow deposits and to examine their behavior across proximal fault environments and distal rift turbidite fields, and to investigate the distribution and evolutionary patterns of gravity flow sedimentary features in the Weixinan Sag based on core, logging and seismic data.
2 Regional setting
The Beibu Gulf Basin is located in the northern part of the South China Sea, with the Weixinan Sag forming a sub-basin in its northern region (Figures 1a,b). Structurally, the Weixinan Sag lies within the Northern Depression Zone of the Beibu Gulf Basin and represents a secondary tectonic unit. It is bounded to the northwest by the Weixinan Fault, to the southeast by the Qixi Uplift, and connects southwestward to the Haizhong Sag, from which it is separated by the Weixinan Low Uplift. The sag covers an exploration area of approximately 2,300 km2 (Figure 1b). It is bounded by the downthrown wall of fault F1 and the upthrown wall of fault F2, with numerous minor faults developed throughout the structure (Figure 1c).
Figure 1. Regional geological setting. (a–c) Geographic location of the study area, showing the South China Sea, the Beibu Gulf Basin, and the Weixinan Sag. The base map in (a–c) was downloaded by GEBCO (General Bathymetric Chart of the Oceans) gridded bathymetric data, 2025. (d) Sedimentary setting of the Weixinan Sag (modified from Liu et al. (2023)). (e) Stratigraphic characteristics of the Liushagang Formation. (f) Tectonic setting of the Liushagang Formation.
The structural framework trends NW-SE, showing an overall elevation higher in the north and west, and lower in the south and east, with a relief of nearly 1,000 m (Figure 1c). The NEE-trending segment of the fault F1 underwent primarily low-angle extension during the Paleogene, controlling the subsidence and filling of the Paleogene rift. In contrast, its NE-trending segment began to experience high-angle differential uplift and subsidence in the early Neogene, resulting in varying degrees of modification to Paleogene strata. The central segment of the F2 fault was mainly active during the Eocene, while its western segment developed primarily during the deposition of the Oligocene Weizhou Formation and was reactivated in the early Miocene. The study area is divided into two main parts by Fs1: the proximal-fault zone and the distal-rift area (Figures 1b,c,f).
The stratigraphic sequence of the study area includes the Paleogene, Neogene, and Quaternary of the Cenozoic Era (Figure 1d). The Paleogene Liushagang Formation is primary hydrocarbon-generating and oil-gas-bearing formation, and can be subdivided into the First, Second, and Third Members, from oldest to the youngest. The Third Member was deposited during the initial rifting stage under strong NW-SE extensional stress and rapid subsidence (Figures 1e,f), and mainly comprises proximal coarse clastic deposits along the lake margin. From bottom to top, it consists of sedimentary bodies including alluvial fan-floodplain, humid alluvial fan, fan-delta, and shallow-lake beach-bar deposits. The lower part of the Third Member is characterized by thick-bedded massive conglomerate and sandstone that form high-quality reservoirs, while the upper part consists of interbedded mudstone, siltstone, and fine sandstone, which is the primary interval of interest. This interval, buried at depths of 2,100–3,700 m, is widely distributed as thin interbeds, with individual sand layers typically 1–3 m thick. Previous study indicate that these thin interbedded sandstone deposits are mainly associated with sedimentary facies such as distributary channels and sheet sands in the outer front of a fan delta (Figure 1e).
Recent drilling core data shows that near the provenance area, Well A5 within the F1 fault zone contains abundant debris-flow sedimentary structures. In contrast, Well B2 in the central section exhibits mixed sedimentary and tectonic characteristics. In the distal rift area, Well B13 is dominated by well-developed Bouma sequence. However, the flow mechanism and distribution of the sedimentary units remain unclear.
3 Materials and methods
This study uses three-dimensional (3-D) seismic data and boreholes provided by China National Offshore Oil Corporation (CNOOC). The Wells are mainly located in the northwest part and southeast part of the Weixinan Sag. The dataset includes 30 wells and their corresponding well logs (Gamma, Resistivity, and Neutron). Well B13 and Well B17 are highly deviated, while all other wells are vertical. The locations of wells with cores are shown in purple, and cores from Member 3 of the Liushagang formation were observed.
The seismic data in the Liushagang Formation, with a dominant frequency of 32 Hz, provide a vertical resolution capable of resolving strata approximately 19 m thick. This resolution corresponds to the scale of individual reservoir units, which in our study area are the Upper L3I, Lower L3I, and L3II members of the upper Liushagang Formation. Consequently, these seismic data were used to map the distribution of sand bodies within these key units.
Cores were used to identify gravity flow types and to establish high-resolution lithofacies and sedimentary facies distribution for reservoir characterization. This allows interpretation of depositional processes based on diagnostic sedimentary features. For instance, turbidites are identified by graded bedding, whereas debris flows are recognized by their muddy, clast-rich matrix.
To accurately map the 3-D distribution and connectivity of these deposits, an integrated methodology combining core, well log, and seismic data was employed. Core data calibrate well log responses to build a facies model, enabling high-resolution vertical extrapolation along the wellbore. Seismic data are then used to delineate large-scale architectural elements, such as fans and lobes, through attribute analysis, collectively enabling the construction of a predictive reservoir pattern.
4 Results
4.1 Lithofacies
4.1.1 Observation
Based on core data from 12 wells, sedimentary features and lithologies were examined to identify flow characteristics and sedimentary environment. The lithologies in the study area include medium-to fine-grained conglomerate, coarse-grained sandstone, medium-grained sandstone, fine-grained sandstone, siltstone, and mudstone. Core proportions indicate that medium-fine grained conglomerate and coarse-grained sandstone are mainly concentrated in the proximal fault zone (11%), whereas they account for 2% in the distal rift area, where no medium-grained conglomerate in present. Medium-grained sandstone, fine-grained sandstone, siltstone, and mudstone (39%) dominate the distal rift area (98%), while they comprise 88% of the proximal fault zone, including 21% mudstone.
Regarding sedimentary structures, four major types are observed in the proximal fault zone (Figure 2).
1. Massive structure containing poorly sorted floating clasts, characterized by a chaotic arrangement of gravels. Typical core photographs show that Well A7 contains floating, brightly colored gravel particles in multiple sections, with poor sorting and larger gravel particle sizes at the bottom. Well A6 similarly displays large, poor rounded gravel particles.
2. Mudstone rip-up clasts embedded within sandstones, forming torn structures. Typical examples include flaky clasts (Well B12), banded clasts (Wel lA5), and abundant dark gray muddy debris irregularly dispersed within a light gray medium sandstone matrix (Wel l A8).
3. Bedding structures, including graded bedding (coarse-to fine-grained sediments corresponding to the Bouma sequence) and plane parallel laminations (Bouma, 1962). Typical core photographs show graded bedding and parallel stratification in coarse-to medium-grained sandstone (Well A5), representing the characteristic Ta and Tb intervals of the Bouma sequence. Parallel stratification within fine silt to sand layers also contains abundant muddy streaks (Well A8 and B12).
4. Bioturbation structures and cross-bedding, characterized by burrows, reddish-brown bioturbation, and minor cross-bedding. Well B12 exhibits lenticular sand-filled bioturbation structures; Well A9 shows irregular reddish-brown disturbed fills; and Well A3 displays cross-bedding with dark siltstone interbeds within fine sandstone.
Figure 2. Examples of typical sedimentary structures in the proximal fault zone. (a) well A7, 2047.5 m, coarse sandstone containing floating gravel. (b) well A6, 2076.2 m, fine conglomerate containing floating pebbles. (c) coarse sandstone containing floating gravel. (d) mudstone embedded in coarse sandstone in torn shapes. (e) mudstone embedded in coarse sandstone in torn shapes. (f) well A5, 2041.5 m, Basal graded bedding interval. (g) well A8, 2013.3 m, wavy bedding interval of Bouma Sequence. (h) well A8, 2053.8 m, wavy bedding interval of Bouma Sequence. (i) well A9, 2027.6 m, reddish brown bioturbation in medium sandstone. (j) bioturbation in minor cross-bedding fine sandstone.
In the distal rift area, sedimentary structures are dominated by four types (Figure 3): Bouma sequence (45%), cross-bedding (4%), deformation structures (35%), and bioturbation structures (16%).
1. The bedding structures within the Bouma sequence (Bouma, 1962; Bouma, 1964) include upper parallel laminae, vortex-shaped (convolute) laminae, and lower parallel laminae. In Well B7 at 2614.59 m, basal graded bedding and mid-section convolute bedding are observed; at 2610.19 m, sandstone cross-bedding occurs; Well B17 shows basal parallel bedding with upper wavy bedding, indicating an upward increase in clay content.
2. Deformation structures primarily refer to sandstones intruding into mudstones and undergoing deformation. In Well B7 at 2616.19 m, deformation structures characterized by medium-fine sandstone intruding into siltstone, while at 2973 m, banded deformation is observed where mudstone intrudes into gray-black mudstone.
3. Bioturbation consists mainly of trace fossils (e.g., burrows) and extensive reddish-brown bioturbation structures. Well B17 exhibits bioturbation structures with worm burrows (2882.32 m) and scattered reddish-brown bioturbation features (2979.16 m).
4. Dropstone development refers to poorly sorted gravels arranged chaotically on sediment surface. Well B13 shows distinct gravel particles dispersed within sandstone. At 3,129 m, floating gravel is observed within siltstone, while at 3136.59 m, parallel-aligned sandstone lenses occur within coarse sandstone.
Figure 3. Examples of typical sedimentary structures in the distal rift area. (a) well B7, 2614.59 m, massive bedding developed in the Ta division. (b) well B7, 2610.19 m, cross-bedding developed in the Ta division. (c) well B17, 2049.6 m, Parallel bedding developed in the Ta division. (d) well B7, 2616.19 m, deformation structure. (e) well B17, 2973.89 m, deformation structure. (f) well B17 2882.32 m, bioturbation in sandy-muddy deposits. (g) well B17, 2979.16 m, reddish bioturbation in silty deposits. (h) well B13, 3129.0 m, fine sandstone with floating gravel, indicating debris flow deposits. (i) well B13, 3136.59 m, coarse sandstone with floating gravel, indicating debris flow deposits.
4.1.2 Interpretation
Descriptions of lithology and sedimentary structures from wells (A3, A5, A6, A7, A8, A9 in the near fault zone, and B5, B7, B9, B12, B13, B17 in the distal rift area) suggest six types of lithofacies (Figure 4).
1. Clast-supported, pebbly sandstone/conglomerate facies (G):Composed mainly of coarse-grained gravel (2–10 mm in diameter) with a minor fraction of particles larger than 10 mm. The primary sedimentary structure is massive. Gravel is generally poorly sorted and subrounded to subangular. A few large clasts float within the matrix, forming a chaotic fabric. Scour surfaces are observable. These characteristics collectively indicate debris flow deposits within gravity flow systems (Yang et al., 2014; Fan et al., 2018; Geng et al., 2025).
2. Graded-bedded, coarse-grained sandstone facies (Ta):Composed mainly of coarse sand, often containing gravel (2–20 mm) at the base. Characterized by rhythmic, fining-upward units comprising massive and normally graded bedding. Interpreted as the Ta interval, with its coarse-to-fine graded bedding deposited by high-density turbidity currents in a deep-water environment (Postma et al., 2009). Represents rapid grain setting from a waning flow, forming the erosive base of a turbidite sequence on a lobe (Shanmugam, 1996; 1997; Pohl et al., 2023).
3. Planar-laminated, medium-grained sandstone facies (Tb):Composed mainly of medium sand, mixed with coarse sand and gravel at the base. Primary sedimentary structures include massive bedding, scour surfaces, and ripple cross-lamination. Interpreted as the Tb interval, characterized by parallel lamination in finer sand/silt. Represents deposition from the waning, upper-flow regime of a turbidity current (Shanmugam, 1997). Signifies sustained, planar traction transport immediately following high-energy Ta deposition, typically found in medial sections of deep-water lobes (Postma et al., 2014).
4. Ripple-cross laminated, fine-grained sandstone facies (Tc):Composed mainly of fine sand. Primary sedimentary structures include cross-bedding, ripple lamination, and graded bedding, with local deformation structures. Interpreted as the Tc interval, characterized by cross-lamination or ripple lamination in very fine sand and silt. Forms during the further waning stages of a turbidity current (Bouma, 1962; Shanmugam, 1997). Represents deposition from lower flow regime traction, often as ripples in the distal parts of the turbidite system (Geng et al., 2025).
5. Planar-laminated, siltstone facies (Td):Composed mainly of fine sand and mud. Primary sedimentary structures include convolute bedding, bioturbation, and ripple cross-lamination. Interpreted as the Td interval, consisting of fine parallel-laminated silt and mud. Formed from the final suspension settling of the dilute tail of a turbidity current, marking the quiet, distal conclusion of the turbidite event in the basin plain before a return to background pelagic sedimentation (Stow and Shanmugam, 1980; Yang et al., 2022).
6. Mudstone facies (Te):Includes variegated mudstone (mainly mud, ferruginous cement, and silt; massive with deformation structures), massive mudstone (mainly mud and carbonaceous fragments; massive with floating carbonaceous fragments); carbonaceous mudstone (mainly mud, carbonaceous fragments, and high organic matter content; massive); lenticular mudstone (mainly mud, silt, and carbonaceous fragments; characterized by floating carbonaceous fragments). Interpreted as the Td interval, comprising parallel-laminated silt and mud, representing the final stage of turbidite deposition (Stow and Shanmugam, 1980). Formed from gradual suspension settling of the waning turbidity current’s dilute tail in a quiet, deep-water basin plain, immediately preceding a return to background hemipelagic or pelagic sedimentation (Zou et al., 2022; Zou et al., 2023).
Figure 4. Lithofacies types, main components, sedimentary structures, indicative sedimentary facies, and typical core photos in the study area.
4.2 Sedimentary facies
4.2.1 Main types of sedimentary facies
In the proximal fault zone,the lithofacies are mainly formed by G (3.6% in 5.5% based on all cores), Ta and Tb. These facies are interpreted as fan deposits at the front of fan-delta. Poor sorted and floating gravels indicate debris-flow deposit (Dou et al., 2021). In the main depositional area on the basin-floor, the lithofacies are mainly formed by Ta-Te, with interbedded thin sandstones and mudstones. These are interpreted as lobe deposits at the front of fan-delta. Well-developed Bouma sequences indicate deposition by turbidity currents.
Combined with the logging features, an interpretation template for sediments was established (Figure 5), including distributary channel, lobe main body, lobe margin and lacustrine mudstone. 1) Channel is composed of coarse sand with cross-bedding and scour structures, exhibiting normal grading and bell-shaped logging feature. 2) Lobe main body mainly consists of medium sand with graded bedding and parallel bedding, showing normal grading and box-shaped logging feature. 3) Lobe margin is mainly composed by fine sandstone and silt with parallel bedding and ripple lamination, exhibiting normal grading and bell-shaped logging feature. 4) Lacustrine mudstone is formed by mudstone and shale with horizontal lamination; The grains are unstructured, and the logging feature is characterized by low-amplitude or nearly straight curves.
Figure 5. Depositional microfacies types and their characteristics: lithology, sedimentary structures, rhythm patterns, typical well-log responses, and representative core examples.
4.2.2 Distribution of sedimentary facies
Well-connected facies were mapped using a combination of single-well facies interpretation and sand body distribution characteristics. Along the provenance direction (Figure 6), stratigraphic thickness gradually increases with water depth. Sedimentary facies evolve from fan-delta front lobes (Well A3) to turbidite lobes (Wells B2, B7and B11), a trend consistent with seismic interpretations of stratigraphy and sand bodies. In the cross-provenance direction (Figure 7), stratigraphic thickness is relatively uniform. Turbidite lobes mainly exhibit vertical isolation, while different lobes are laterally connected. Overall, the upward-thickness trend of individual turbidite lobes is consistent with the lacustrine transgressive background.
Figure 6. Connected-well facies profiles along the provenance direction (location is shown in Figure 1c).
Figure 7. Connected-well facies profiles in the cross-provenance direction (location is shown in Figure 1c).
Based on seismic attribute analysis, a good negative relationship (R2 = 0.7) exists between minimum amplitude and sand-shale ratio. Minimum amplitude of seismic property was selected to predict the distribution of sand and sedimentary facies (Figure 8). Using well-log patterns and sedimentary characteristics shown in Figure 5, four sedimentary environments were identified. Correlations in Figures 6, 7 further support variations of these sedimentary environments across different intervals. On this basis, sedimentary facies maps were constructed for L3I-1up, L3I-1dn and L3I-2. Within the L3I reservoir unit, from sub-unit L3I-2 upward to the upper sand group of L3I-1, the succession reflects a sedimentary system evolving from fan-delta front debris-flow lobes to turbidite lobes. The depocenter of the fan-delta to lobe system migrated progressively landward, displaying a retrogradational pattern that indicates a lacustrine transgressive background. In the upper sand group of L3I-1 (Figure 8a), lobate bodies in the eastern part are more dispersed than those in the west, where lateral splicing and vertical stacking of multi-stage lobe axes and lobe fringe deposits are observed. In the lower sand group of L3I-1 (Figure 8b), turbidite lobes were extensively developed, with frequent stacking in the western region. In the L3I-2 sand group (Figure 8c), sand-body distribution expanded further, with lobe axes extending over considerable distances and multi-stage lobate bodies stacked vertically.
Figure 8. Seismic attribute analyzing. (a–c) Minimum amplitude maps for the Upper L3I, Lower L3I, and L3II members. (d) Relationship between minimum amplitude and sand-shale ratio.
4.3 Architecture of sedimentary facies
4.3.1 Morphology of sediment units
In the upper sequence of the Liushagang Formation, reservoir characterization of the study area reveals sand bodies developed over an average of 24 cycles. Each sublayer comprises, on average, 4 lobe systems, defining a total of 7 lobe complexes and 37 discrete lobe elements. Lobe systems have a maximum width ranging from 3,000 to 9,000 m. Lobe complexes average 2,500 to 3,000 m in width, while individual lobes (single sand bodies) range from 1,000 to 1,500 m wide. Individual sand bodies developed over 4 to 13 distinct stages, with an average thickness of 0.77–1.39 m.
The sedimentary directions of the fan delta and lobes at various scales are consistently NW-SE, approximately perpendicular to the major NW boundary fault F1 and the SE fault segment F2, and subparallel to minor internal faults (Figure 8). Delta facies are primarily distributed near fault zones, whereas lobe deposits occurs in distal faulted depression belts.
A clear gradient in lobe size across different hierarchical levels is observed from stratigraphic perspectives (Table 1). The lobe system in the L3I-1up layer has a maximum width of 3,675 m, an average lobe complex width of 2,500 m, and an average individual lobe width of 1,200 m. In the L3I-1down layer, the lobe system attains a maximum width of 3,975 m, with average widths of 2,557 m for lobe complexes and 1,300 m for individual lobe. In the L3I-2 interval, the lobe system reaches a maximum width of 5,833 m, with average lobe complex and individual lobe widths of 3,000 m and 1,480 m, respectively.
Table 1. Scales of lobes at different hierarchies in the Upper L3I, Lower L3I, and L3II members.
4.3.2 Stacking pattern
The sedimentary units in our study area can be divided into two types: debris fans and turbidite lobes (Figures 6, 7, 9, 10). Plan-view observations show that frontal lobes are laterally overlapping. Turbidite lobes are hierarchically organized, with systems comprised of multiple lobe complexes, each formed by several individual lobes. Laterally, at the same stratigraphic level, these lobes show mutual incision and stacking. Cross-sectional profiles (Figures 6, 7) further reveal that lobe elements stack in various patterns, including vertical incision and stacking, lateral amalgamation, and isolated development.
Figure 9. Sedimentary facies map of the Upper L3I (a), Lower L3I (b), and L3II (c) members. (d) a lobe system from L3I-1; (e) a lobe complex from (d); (f) an individual lobe from (e).
Figure 10. Planar and vertical stacking patterns of fan-delta front lobes and turbidite lobes.
5 Discussion
5.1 Evolution of gravity flows
In the proximal-fault zone, fan-delta front lobes are predominantly developed, while the distal rift area is primarily characterized by lobate turbidite lobes. Within the L3I reservoir group, from the L3I-2 to the upper sand sub-unit of L3I-1, the interval overall exhibits a sedimentary succession from fan-delta front debris flow lobes to turbidite lobes (Figure 11). The depocenter of the fan-delta to lobe system migrated progressively landward, displaying a retrogradational stacking pattern indicative of a lacustrine transgressive background. Along a dip-oriented section (B6-B7-B8, parallel to sediment transport), stratigraphic thickness gradually increases, water depth deepens, and depositional facies evolves from fan-delta front lobes to turbidite lobes, consistent with seismic interpretation of strata and sand bodies. In a section perpendicular to the sediment transport direction, stratigraphic layers show relatively uniform thickness distribution. Lobate turbidite sands stack both vertically and laterally. Plan view observations show the number of distal lobes developed decreases with increasing distance from the provenance area. Vertically, from L3I-2 to L3I-1, the lake level gradually rose, resulting in a progressive reduction in the scale of sand body development and a decrease in their stacked width (Figure 11). The average lobes width decreases from 1480 m in L3I-2–1,200 m in the L3I-1up, reflecting a weakening of gravity-flow energy.
Figure 11. Variations in architectural-element stacking patterns with distance from the provenance area in the Upper L3I, Lower L3I, and L3II members.
5.2 Controls and pattern of gravity flow deposits
The distribution and character of gravity flow deposits are governed by a complex interplay of allocyclic and autocyclic controls (Feng et al., 2021; Yang et al., 2022). Tectonic activity represents a primary allocyclic factor, as it shapes basin topography and can triggers slope failure (e.g., through earthquakes), which serves as the fundamental trigger of sediment gravity flows (Mulder and Alexander, 2001; Tillmans et al., 2021; Geng et al., 2025). Simultaneously, lake-level fluctuations exert a dominant influence on sediment supply and delivery points. For instance, a broad transgressive evolution from L3I-2 to L31up is expressed by increasing isolation of the sand bodies due to rising lake level (Figures 1d, 8, 9, 11). Wells B11 and B6-8 attest that these sand-rich deposits decreased in thicknesses from an average of 1.39 m in L3I-2 to 1.0 m in L3I-1up.
Autocyclic processes within lobate or channelized systems are controlled by the dynamics of gravity flows, interacting with the pre-existing basin floor to shape depositional patterns and architectures (Guo et al., 2021; Bouchakour et al., 2025a). Flow properties, including sediment concentration, grain size, and flow rheology, determine whether a deposit manifest as a chaotic debris flow, a well-organized turbidite, or a hybrid event (Shanmugam, 2006). These flows are often channelized, with coarse-grained materials deposited within channels, while finer sediments spread overbank to form lobes and sheet-like deposits (Deptuck et al., 2003; Hansen et al., 2015; Bouchakour et al., 2023; Bouchakour et al., 2025b). Confinement by lake-basin-floor topography, such as structural slopes or salt diapirs, can force flow deflection, ponding, or reflection, significantly altering depositional patterns (Fonnesu et al., 2020). Integrations of these controls reveals that the most significant accumulations of gravity flow deposits often occur in specific architectural elements, such as channel-lobe systems and frontal splays.
The depositional patterns in the Beibu Gulf were controlled by the interplay of fault-controlled paleo-topography and lake level fluctuations, establishing a broad transgressive evolution during deposition of the studied gravity flow deposits. Given the observed interaction between lobes and fault systems, a potential scenario can be suggested: if the older member (L3I-2) was constrained by fault, then subsequent lake transgression allowed lobe deposits to spread laterally and become isolated. Early lobe deposition and architecture were fundamentally controlled by fault-induced paleo-topography, as evidenced by amalgamated stacking patterns and consistent NW-SE lobe orientations subparallel to internal faults, indicating sediment gravity flows were channeled through fault-defined conduits. However, the later widespread transgression marked a significant shift in depositional regime. Rising base level promoted the development of more aggradational lobe complexes, as increased accommodation space reduced the topographic influence of underlying faults. Consequently, this transgressive phase led to the deposition of more isolated, vertically stacked sand bodies, effectively decoupling later sedimentation from direct structural control. Debris flow deposits predominantly occur in proximal, high-relief areas near faults, whereas turbidity current deposits are located in distal, low-relief rift regions (Figure 12). A gradual rise in lake level from L3I-2 to L3I-1 intervals (Figure 11) drove a vertical reduction in both the scale and lateral stacking width of sand bodies. Overall, the depositional model evolves from debris-flow-dominated lobes on the fan-delta front to turbidite lobes in the shallow-to deep-lake environments (Figure 13).
Figure 12. (a) Relative paleogeomorphology before the deposition of Liushagang Formation; (b) Well-connected seismic profile from proximal to distal.
Figure 13. Sedimentary model of the fan-delta lobe to turbidite lobe system.
5.3 Implications of gravity flow deposits consisting of interbedded thin sandstones and mudstones
Gravity flow deposits characterized by thin-bedded, intercalated sandstones and mudstones represent a complex deep-water facies association that goes beyond the simplified model of massive, amalgamated sandstones. These heterolithic sequences provide a rich, high-resolution record of depositional conditions, posing both challenges and opportunities for paleoenvironmental reconstruction and hydrocarbon exploration. Their interpretation is critical for accurate basin analysis and effective resource evaluation (Cao et al., 2021; Niu et al., 2023).
The stratigraphic architecture of thin-bedded turbidites serves as a key archive for interpreting ancient deep-water environments. In terms of paleogeography, widespread sheets of thin-bedded turbidites intercalated with hemipelagic muds typically indicate distal, unconfined settings such as basin plains or the fringes of depositianal fans (Mutti and Lucchi, 1978). These deposits mark the farthest extent of turbidity current energy and often act as correlation markers for defining genetic stratigraphic sequences (Zou et al., 2022), which are bounded by the maximum flooding surface and the conformity surface, and consist of progradational, aggradational, and retrogradational depositional systems recording the spatiotemporal distribution of sedimentary episodes at basin margins. When these thin beds show thickening and coarsening patterns, they help delineate the boundaries of individual lobes within a larger fan system, mapping the progradation and retraction of the depositional system through time. Regarding depositional process, the repetitive alternation of thin sandstones and mudstones reflects deposition from waning, low-density turbidity currents (Zou et al., 2023). Each sand-mud couplet often represents a single, sporadic surge-type flow event, with the mudstone cap recording the final suspension settlement. The frequent occurrence of “linked debris-turbidite” beds, in which a thin turbidite is sharply overlain by chaotic debris, provides key evidence of flow transformation, indicating that the original flow was hybrid, containing both turbulent and cohesive components that segregated during deposition (Haughton et al., 2003). This is fundamental for understanding the complexity of subaqueous sediment gravity flows. Thin-bedded sequences also carry significant paleotopography implications. Their geometry and stacking patterns are sensitive to lake basin floor relief. Laterally continuous, sheet-like beds suggest deposition on a flat, unconfined basin plain, while basin-filling deposition, in which successive sand layers infill subtle lows rather than stacking directly, indicates deposition on a rugose surface. This pattern develops as flows seek to create a smoother depositional topography, profoundly influencing the lateral and vertical connectivity of sand bodies (PrÉLat et al., 2009).
For hydrocarbon exploration, thin-bedded heterolithic gravity flow deposits represent a challenging yet potentially lucrative “low-resistivity, low-contrast” pay. The primary challenge lies in reservoir characterization (Hansen et al., 2015), as standard petrophysical logs often fail to accurately resolve thin, hydrocarbon-bearing sandstones from encasing shales, leading to underestimation of reserves (Worthington, 2011). Permeability is highly anisotropic; vertical connectivity is limited by intervening mudstones, resulting in numerous hydraulically isolated compartments. However, successful development is possible through an integrated approach. Geologically, understanding the broader stratigraphic framework is essential for identify the variability of gravity flow deposits both vertically and laterally, which directly impacts the distribution of thin-bedded sand and sealing conditions. Determining whether thin beds represent distal lobe fringes or levee deposits adjacent to a major channel guides strategies for targeting more amalgamated sands updip (Weimer and Slatt, 2004).
Our study shows that the rift zone plays a key role in confining lobes and enhancing sand body (i.e., reservoirs) connectivity. Stepwise separations and isolation caused by the transgressive lake may provide natural seals that preserve hydrocarbon accumulations. These findings significantly advance understanding of gravity flow sedimentary environments, particularly the processes controlling thin interbedded mudstone and sandstone, improving predictions of sediment distribution and enhancing the efficiency of oil and gas exploration and development.
6 Conclusion
Based on the analysis and interpretation of core, well logging, and 3D seismic data from the Weixinan Sag in the Beibu Gulf Basin, South China Sea, this study characterizes the distribution patterns and sedimentary architectures of various lacustrine gravity-flow deposits within the Paleogene Liushagang Formation. The identified lithofacies in the study area include facies G (conglomerate facies) and the Ta, Tb, Tc, Td, and Te intervals of the Bouma sequence. The main sedimentary facies types are debris-flow fans at the fan-delta front and turbidite lobes at the distal front of the fan-delta. Turbidite lobes were further subdivided into distributary channels, lobe main bodies, and lobe margins. Debris-flow fans develop near the faulted slope, exhibiting a divergent fan-shaped geometry, while turbidite lobes mainly form near the base of the slope, slightly extending toward the basin floor, showing a tongue-shape morphology. The stacking patterns of these architectural units are characterized by vertical incision, lateral superimposition, and isolated development. A single lobe system ranges from 3,500 to 6,000 m in width, a lobe complex spans 2,500 to 3,000 m, and individual lobes are 1,200 to 1,500 m wide. Overall, the gravity-flow deposits evolved under a broad transgressive lake-basin context, forming isolated bodies and laterally distributed, isolated lobes. The study concludes that the sedimentary paleo-topography and lake-level fluctuations exert a primary control on the morphology and stacking patterns of lobe units in this area.
Data availability statement
The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding authors.
Author contributions
YG: Writing – original draft, Data curation. XK: Writing – original draft, Formal Analysis, Data curation, Conceptualization. HW: Conceptualization, Validation, Investigation, Writing – review and editing, Project administration. XW: Writing – review and editing, Methodology, Resources. YY: Funding acquisition, Writing – review and editing, Project administration, Supervision. MC: Data curation, Methodology, Writing – review and editing, Investigation, Conceptualization, Writing – original draft.
Funding
The author(s) declared that financial support was received for this work and/or its publication. NSFC NO:42372137.
Conflict of interest
Authors YG, XK, HW, and XW were employed by CNOOC Research Institute Ltd.
The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Correction note
A correction has been made to this article. Details can be found at: 10.3389/feart.2026.1804351.
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Keywords: beibu gulf basin, debris, fan-delta, gravity flow, south China sea, turbidite
Citation: Gao Y, Kong X, Wang H, Wang X, Yin Y and Chen M (2025) Sedimentary architecture of gravity flow deposits in the liushagang formation of the paleogene, weixinan sag, beibu gulf basin, South China sea. Front. Earth Sci. 13:1736562. doi: 10.3389/feart.2025.1736562
Received: 31 October 2025; Accepted: 28 November 2025;
Published: 19 December 2025; Corrected: 11 February 2026.
Edited by:
Xiaoming Zhao, Southwest Petroleum University, ChinaReviewed by:
Wei Wu, Henan Polytechnic University, ChinaMassine Bouchakour, Southwest Petroleum University, China
Dali Yue, China University of Petroleum, China
Copyright © 2025 Gao, Kong, Wang, Wang, Yin and Chen. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Mei Chen, bWNoZW5AeWFuZ3R6ZXUuZWR1LmNu Xingxing Kong, MTIxNDc5MTgyNUBncS5jb20=