Introduction
The dynamic system formed by erosion products in eroded landform areas, transported to drainage basins through base load, suspended load, and dissolved load, and eventually deposited, is called the "source-to-sink" (S2S) system, which is one of the forefronts and hotspots in current global geosciences [1⇓⇓⇓-5]. The core of S2S system research lies in integrating the erosion of the source area, transportation in the basin area, and deposition in the drainage area into one system, establishing the inherent relationship of erosion-transportation-deposition throughout the process, and reconstructing the evolutionary process of sediments from source to sink, thereby deeply revealing the genetic mechanism of sediments [6].
Since the proposal of the S2S system theory, after nearly 20 years of development, a large number of innovative achievements have been made in the research methods, gradation, classification, and main controlling factors of multi-scale S2S systems in different tectonic backgrounds [7⇓⇓⇓-11], gradually transitioning from academia to industry, with significant implications for global oil and gas exploration. Currently, deep-water domain is the main area of growth in oil and gas reserves. Among them, the prediction of sand-rich deep-water fans is one of the key aspects of deep-water oil and gas exploration and evaluation. However, the time-space span of sediment transportation from the erosion zone to the deep-water area is larger, influenced by factors such as provenance, seafloor topography, climate, and sea level fluctuations. Deep-water fan types are diverse, and development patterns are numerous, making it difficult to predict favorable sand-rich deep-water fans [12-13]. To address this, scholars have subdivided the S2S system from land to ocean into two secondary S2S systems: "source area-inner continental shelf" and "outer continental shelf-deep-water basin" using the outer continental shelf as a boundary [14-15]. They pointed out that the study of the S2S relationship of shelf-edge deltas to deep-water fans (hereinafter referred to as "SEDDF") at the shelf edge in the "outer continental shelf-deep-sea basin" subsystem can more intuitively reflect the coupling relationship between sedimentary evolution in shelf edge areas and deep-water sedimentary responses, helping to more accurately reveal the dynamic dispersal process of sand bodies, which is of great significance for predicting sand-rich deep-water fans.
The south subsag of the Baiyun Sag, located in the Pearl River Mouth Basin (PRMB), China, lies primarily in ultra-deep water area, with current water depths ranging from 1 200 m to 2 600 m. Previous studies have indicated the development of SEDDF depositional system in the south subsag of Baiyun Sag during the depositional period of the Paleogene Zhuhai Formation. However, these studies mainly focused on aspects such as shelf-edge delta sequence stratigraphy, sedimentary characteristics and evolution, reservoir physical properties, and main controlling factors from the classical perspective of sequence stratigraphy [16⇓-18]. The coupling relationship between the shelf-edge delta and deep-water fan in the study area has not been established from the perspective of S2S system, resulting in an unclear understanding of the distribution patterns and genesis mechanisms of sand-rich deep-water fans in the study area. Therefore, based on a large amount of core, drilling, and 3D seismic data, this paper systematically elucidates the coupling relationship and genetic mechanisms of SEDDF in the study area. This research not only holds significant theoretical significance for deepening the study of shelf-edge S2S systems but also bears great practical significance for predicting favorable exploration zones for deep-water fans in this area.
1. Geological setting
The south subsag of Baiyun Sag is situated between the Baiyun Sag and the Liwan Sag in the PRMB (Fig. 1a ). Structurally, it is classified as a secondary sag of the Baiyun Sag, surrounded by the Shunhe Uplift and the Yunli Low Uplift (Fig. 1b ). During the Cenozoic era, the south subsag of Baiyun Sag underwent three stages of tectonic evolution: the Paleogene-Eocene rifting, the Oligocene fault-depression, and subsequent depression since the Miocene. Overall, it represents a process of marine transgression, with the Paleogene Wenchang Formation to the Quaternary System. Among these formations, the Paleogene Wenchang Formation to the Enping Formation are predominantly terrestrial deposits, while the Zhuhai Formation is divided into six members from top to bottom, namely the first to sixth members of the Zhuhai Formation (Zhu-1 Member to Zhu-6 Member for short) (Fig. 1c ). The Zhuhai Formation develops transition-marine deposition systems. The Zhujiang Formation is marine deposition [17].
Fig. 1. Regional location (a), division of tectonic units (b), and comprehensive stratigraphic column (c) of the study area. |
During the depositional period of the Zhuhai Formation, under the influence of the movement of the South China Sea, the PRMB changed from rifting to depression, with a continuous rise in relative sea level [19]. An extensive shelf break belt was formed in the southeast of the Baiyun Sag, the Changchang Sag, and the Heshan Sag, with large-scale SEDDF depositional systems (Fig. 1a ). Simultaneously, with the strengthening of the ancient Pearl River provenance in the northwest, the sediment flux increased [20]. The shelf break belt continued to migrate southeastward, while the SEDDF depositional systems continuously prograded into the basin [21]. The overall shelf break belt of the south subsag of Baiyun Sag exhibits similar characteristics, with a NE-SW distribution on the plane. Under the combined control of differential sediment supply, sea level changes, and ancient uplifts, the shelf break belt began to develop from the Zhu-4 Member, showing a typical shelf-slope-deepwater basin structure. It experienced three stages: initial formation stage (depositional period of the Zhu-4 Member), development stage (depositional period of the Zhu-3 Member), and stable stage (depositional periods of the Zhu-2 Member and Zhu-1 Member), continuously developing SEDDF depositional systems [17] (Figs. 2 and 3 ). In the late depositional period of the Zhuhai Formation, the Baiyun Movement caused the shelf break belt to migrate northward to the northern slope of the Baiyun Sag and the Panyu Low Uplift [21], overlaying the deep-sea thick mudstones of the Zhujiang Formation. Therefore, this paper focuses on the development characteristics and S2S coupling relationship of the SEDDF depositional system from the Zhu-4 Member to the Zhu-1 Member, and explores its genesis mechanism, aiming to provide reference for exploring favorable zones in this area.
2. Data and methods
The study primarily utilized high-resolution 3D seismic data newly acquired and processed by the China National Offshore Oil Corporation (CNOOC) in the south subsag of Baiyun Sag (Fig. 1b ). The seismic data covers an area of approximately 4 500 km², with a dominant frequency range of 15 Hz to 50 Hz, and the target layer Zhuhai Formation has a dominant frequency of 40 Hz. The seismic data was acquired with a bin size of 12.5 m × 25.0 m and a sampling interval of 2 ms, and seismic profiles were displayed using negative polarity according to the Society of Exploration Geophysicists (SEG) standard. Well-seismic correlation revealed that the variations in seismic amplitudes roughly reflect changes in sediment lithologies. Strong seismic amplitudes indicate larger impedance differences between upper and lower layers, corresponding to relatively sand-rich deposits, while weak seismic amplitudes indicate smaller impedance differences between upper and lower layers, corresponding to relatively mud-rich deposits.
Based on the subdivision of third-order sequences by previous studies [17,21], seven third-order sequence boundaries (SB33.90, SB29.50, SB28.40, SB27.20, SB26.00, SB24.80, and SB23.03) were identified in the study area through well-seismic correlation (Fig. 2 ), and the strata were accordingly divided into six members. Then, using the isochronous stratigraphic slicing technique in Paleoscan, a suite of software for professional sedimentary stratigraphy analysis, constrained by the isochronous stratigraphic framework, combined with lithofacies and seismic facies analysis, the planar distribution characteristics of the depositional systems during the deposition period of the Zhu-4 Member to the Zhu-1 Member were finely delineated. Furthermore, through the mutual verification of planar and profile seismic facies, five representative root mean square (RMS) amplitude isochronous stratigraphic slices were overlaid with paleogeomorphology within the Zhu-4 Member to the Zhu-1 Member (Fig. 3 ), ultimately determining the distribution and evolutionary patterns of depositional systems during different periods and revealing the S2S coupling relationships of different types of SEDDF systems.
Fig. 2. Typical seismic sections and corresponding sequence stratigraphic interpretation of the Zhuhai Formation in south subsag of Baiyun Sag (see section location in |
Fig. 3. Overlay map of paleogeomorphology and RMS amplitude during deposition period of Zhu-4 Member to Zhu-1 Member in south subsag of Baiyun Sag. |
3. Coupling relationships of source-to-sink in SEDDF
3.1. Characteristics and evolution of SEDDF depositional systems
3.1.1. Sedimentary microfacies analysis of cores from shelf areas
As mentioned previously, large-scale shelf-edge deltas were developed in the shelf area of the southeast of the Baiyun Sag, the Changchang Sag, and the Heshan Sag [21]. Although there is a lack of core data in the study area, a comprehensive analysis of the cores and well logging data from the Zhuhai Formation within the Baiyun Sag enables a detailed understanding of the sedimentary characteristics and filling structures of the shelf-edge deltas. Therefore, through a systematic analysis of lithofacies types and combinations of the coring intervals in the shelf area, four microfacies types were identified, including underwater distributary channels, mouth bars, front slumps, and interdistributary bays (Fig. 4 ).
Fig. 4. Typical core facies characteristics of the shelf-edge delta in Zhuhai Formation of Baiyun Sag. (a) Well F, 3 231.97 m, grayish-green fine sandstone with glauconite, scouring and filling structure, underwater distributary channel in delta front; (b) Well F, 3 236.5 m, Grayish-white medium sandstone with wedge cross-bedding, underwater distributary channel in delta front; (c) Well G, 4 103.50 m, brownish-gray argillaceous siltstone with bioturbation, mouth bar; (d) Well H, 3 150.6 m, Grayish-green fine sandstone with Parallel bedding, mouth bar; (e) Well H, 3 527.82 m, dark gray fine-silt sandstone with syngenetic deformation structure, front slump; (f) Well H, 3 157.82 m, grayish-green fine sandstone, thin interbedding of grayish-black mudstone and gray siltstone with lenticular bedding, wavy bedding, vein bedding and occasional bioturbation, interdistributary bay. |
Underwater distributary channels are mainly composed of medium-thick shallow gray coarse to medium-grained sandstones, often with scouring-filling structures at the base (Fig. 4a ). The grain size gradually becomes finer upwards, developing tabular or trough cross-beddings (Fig. 4b ), with occasional bioturbation at the top. Mouth bar microfacies consists mainly of medium-thick medium to fine-grained sandstones, exhibiting an overall inverted rhythm that becomes coarser upwards, with well-developed bioturbation in the lower part (Fig. 4c ), transitioning upwards to cross-bedding and parallel bedding (Fig. 4d ). Front slumps are predominantly composed of thick massive muddy sandstone or silty mudstone, interbedded with syngenetic deformed siltstone or sandstone, which are formed at the distal end of the delta front as a result of gravity sliding (Fig. 4e ). Interdistributary bay microfacies mainly comprise gray-black mudstones interbedded with silty bands or laminae, exhibiting horizontal bedding, wavy bedding, or lenticular bedding (Fig. 4f ). Bioturbation structure and biogenic boring are much developed, with occasional small-scale cross-beddings.
3.1.2. Typical seismic facies and major depositional units in shelf-slope area
Through well-seismic correlation and based on seismic reflection structures and planar RMS amplitude attribute, five main depositional units were identified in the study area, including shelf-edge deltas, wave-dominated sand ridges, continental slope channels, lobes, and mass transport deposits (MTDs) (Fig. 5 ).
Fig. 5. Seismic response characteristics of major sedimentary units in Zhuhai Formation of south subsag of Baiyun Sag (the location of the plane attribute on the left is shown in |
(1) Shelf-edge deltas: Widely distributed in various periods in the study area, these deltas are developed near the shelf break belt, appearing as arc-shaped or crescent-shaped on the plane, with predominantly medium-strong seismic reflections, good continuity, lengths ranging from 7 km to 35 km, and widths ranging from 150 km to 180 km. Seismic sections show distinct S-shaped and oblique prograding reflection structures (Fig. 5a ). The prograding layers are thicker, with each member exceeding 200 m and reaching a maximum thickness of about 380 m. The regions near the shelf break belt are the largest sedimentation and subsidence centers, with rapid thinning of strata towards both landward and seaward directions.
(2) Wave-dominated sand ridges: Relatively developed from the late stage of the Zhu-4 Member to the early stage of the Zhu-1 Member, these ridges were formed in the front of the shelf-edge deltas. Individual sand ridges have a potato-like shape on the plane, with areas ranging from 10 km² to 30 km². The seismic response shows short-axis lenticular strong reflections vertically (Fig. 5b ), with thicknesses ranging from 18 m to 35 m. Multiple sand ridges collectively form a band shape, mainly distributed along or parallel to the shelf break belt, representing typical signs of shelf-edge deltas modified by fluid under strong hydrodynamic action.
(3) Continental slope channels: Extensively distributed in the continental slope area, these channels exhibit distinct seismic responses, appearing as bands of strong reflections with varying widths ranging from several hundred meters to several kilometers, large differences in curvature, and lengths extending up to tens of kilometers. Seismic sections show overall V-shaped to U-shaped reflections, predominantly consisting of weakly continuous strong reflections internally (Fig. 5c ). These channels reflect the lateral swinging and vertical superposition characteristics of multiphase channels, serving as the main conduits for sediment transport from the shelf edge to deep water.
(4) Lobes: Commonly found in the middle-lower slope areas of the study area, lobes are primarily developed at the terminus of continental slope channels. Due to the decrease in terrain slope and the weakening of channel confinement, sediment disperses and unloads, forming fan- shaped strong reflections on the plane, with areas ranging from 15 km2 to 200 km². On seismic sections, they are characterized by seismic reflections with low frequency, strong amplitude and moderate continuity, appearing as "thick in the middle and thin on both sides" lenticular reflections in the vertical direction relative to the sediment source (Fig. 5d ). Lobes exhibit no significant erosional undercutting of the underlying strata, with thicknesses ranging from 20 m to 50 m and an average thickness of approximately 35 m, making them favorable reservoirs.
(5) Mass Transport Deposits (MTDs): Mainly distributed in the Zhu-3 Member, MTDs are formed by the collapse of sediments on the shelf edge and upper slope. In planar attributes, they appear as large areas of weak-amplitude transparent reflections, with widths ranging from 9 km to 21 km and extending downward for approximately 34 km. On seismic sections, the top and bottom interface reflections are discontinuous, and internal strata exhibit severe deformation, appearing as weak-amplitude, chaotic, and imbricated reflections (Fig. 5e ). They sharply contrast with the overlying and underlying strata and are typical products of gravitational collapse, making it difficult to form effective reservoirs.
3.1.3. Types and characteristics of fluid processes
In 1975, Galloway proposed a classification scheme for deltaic systems based on the relative strength of fluid processes such as river, wave, and tidal action, which are known as river-dominated, wave-dominated, and tide-dominated deltas [22]. This classification scheme has been widely applied in the study of deltaic sedimentary systems. In recent years, with the deepening understanding of marine deltaic sedimentation, scholars have gradually realized that a single sedimentary dynamic mechanism cannot accurately characterize the complex sand body structures and spatial-temporal distribution in sedimentary records. Moreover, it is observed that a mixture of river, wave, and tidal dynamics is common in modern continental shelves, with stronger wave action [23]. Therefore, the analysis of mixed processes of river, wave, and tidal action has gradually gained attention in the study of marine deltaic sediment characteristics [24⇓-26]. Based on detailed core descriptions of the Zhuhai Formation in the Baiyun Sag, it is revealed that a mixture of river, wave, and tidal dynamics influences the shelf area of the Baiyun Sag. Different hydrodynamic conditions exhibit significant differences in core characteristics. Among these, river- dominated dynamics is the most common type, showing an overall upward-fining grain size rhythm, with developed cross-bedding, parallel bedding, and visible scouring-filling structures (Fig. 6a ). Sedimentary structure sizes decrease upwards, reflecting a transition from strong to weak hydrodynamic conditions. Wave action modifies and disrupts sand bodies supplied by previous river inputs, resulting in medium-thick sandy deposits with wedge-shaped cross-bedding and wave-formed cross-bedding, along with oriented mud pebbles (Fig. 6b ). Bioturbation and bioclasts are common, and sandstone sorting is relatively good, with sub-rounded to rounded grains. Tidal action is characterized by rhythmic bedding of sandstone and mudstone, muddy draping, and double clay layers. Thickness variations of sandstone and mudstone layers show obvious periodicity, reflecting changes in tidal sedimentation related to tidal magnitude (Fig. 6c ). In terms of planar distribution, river-dominated and wave-dominated processes dominate the inner shelf, while wave-dominated and tide-dominated processes dominate the outer shelf [24]. Consequently, the shelf-edge deltas in the study area are susceptible to modification by wave and tidal processes.
Fig. 6. Typical core facies characteristics of different hydrodynamic depositional sequences in Zhuhai Formation of south subsag of Baiyun Sag. |
3.1.4. Evolutionary patterns of depositional systems
Regional studies reveal that the sedimentation of the Zhuhai Formation in the Baiyun Sag mainly originated from the South China continent. The ancient Pearl River, flowing from northwest, served as the main transportation channel connecting the source area with the depositional area, supplying a large amount of marginal debris that radiated into the sag [16-17,21]. During the early depositional period of the Zhuhai Formation (from Zhu-6 Member to Zhu-5 Member), the Baiyun South Subsag was predominantly in a shelf sedimentary environment, with normal shelf deltaic deposition [17]. From the depositional period of Zhu-4 Member, a typical shelf-slope structure began to form, with the primary source being the ancient Pearl River flowing from the northwest, leading to the development of SEDDF depositional systems. During this period, shelf-edge deltas were mainly distributed on the southwest side, with multiple wave-dominated sand ridges along the slope break belts (Figs. 3a and 7a). However, deep-water fans did not develop beneath them. On the north side, shelf-edge deltas did not develop, and a series of ribbon-like channels were developed on the slope, terminating in small-scale deep-water fans dominated by channels and lobes. During the inheritance development of the Zhu-3 Member, in addition to the northwest source, the northern supply began to strengthen. Shelf-edge deltas increased significantly in size compared to the depositional period of Zhuhai-4 Member, with multiple deep-water fans on both the south and north (Figs. 3b and 7b). During the depositional period of Zhu-2 Member, with the continuous decrease in relative sea level, the supply from the northwest source increased, and a large amount of sediments were transported to the deep-water area, resulting in large-scale lobe complexes (Figs. 3c and 7c). During the depositional period of Zhu-1 Member, the source area of the study area remained primarily in the north and northwest, and large-scale shelf-edge deltas continued to develop (Figs. 3d and 7d). However, with the rapid rise in relative sea level during this period, the upward-fining amplitude attribute reveals a retrograding stacking pattern of the deep-water fan depositional system. The sand bodies gradually migrated towards the shelf edge, and late deep-water fans did not develop, with only isolated channels locally (Figs. 3e and 7e). On the RMS amplitude map, there are clear strip-shaped strong amplitude reflections. Therefore, during the depositional period from Zhu-4 Member to Zhu-1 Member, the scale of the SEDDF depositional systems in the study area show a characteristic of initially increasing and then decreasing. The development of deep-water fans occurred during the depositional period of Zhu-3 Member to early Zhu-1 Member, with the depositional period of Zhu-2 Member showing the largest distribution scale. Well E in the slope area reveals the development of sand bodies during the depositional period of the Zhu-3 Member, with a thickness of up to 94 m, showing multiphase reverted cycles on the well logging curve, reflecting continuous progradation (Fig. 8 ). It also reveals the development of thick medium-coarse deep-water sandstones in the Zhu-2 Member, with a thickness of up to 68 m. The main depositional units of the deep-water fans underwent an evolution process from slope channels to lobes, to lobe complexes, and finally to slope channels (Figs. 3 and 7 ).
Fig. 7. Sedimentary facies map of the fourth member to the first member of the Zhuhai Formation in south subsag of Baiyun Sag (see the boundary range in |
Fig. 8. Comprehensive stratigraphic column of Zhu-4 Member to Zhu-1 Member in Well E of continental slope area in south subsag of Baiyun Sag. |
3.2. Coupling types of source-to-sink systems in SEDDF
For the S2S systems at the ocean-land edge, Helland- Hansen et al. [27], based on the differences in topography, transport distance, and water depth of the deposition area, divided them into three types: "steep slope-near source-deep water", "gentle slope-far source-shallow water", and "gentle slope-far source-deep water". However, it is difficult to accurately reveal the dynamic dispersion process of sand bodies at the continental shelf edge. Gong et al. [28], through the study of 127 SEDDF S2S systems in 24 continental edges worldwide, further proposed a division scheme for the continental shelf edge based on three end-members: sediment supply, accommodation space, and climate change, on the basis of Allen's hierarchical thinking of ocean-land S2S systems [12]. This scheme provides a reference for the refinement and quantification of shelf-edge deltas to deep-water sedimentary systems at continental shelf edges. Guided by the concept of SEDDF S2S, based on a detailed analysis of the shelf edge depositional systems of Zhujiang Formation in the Baiyun South Subsag, and from the perspective of whether shelf-edge deltas or deep-water fans are developed and their practicality, the coupling relationship between shelf-edge deltas and deep-water fans at the shelf edge is divided into three types: "deltas that are linked downdip to fans", "deltas that lack downdip fans " and "fans that lack updip coeval deltas".
3.2.1. Deltas that are linked downdip to fans
The main characteristic of deltas that are linked downdip to fans is the development of shelf-edge deltas in the shelf edge area, with deep-water fans in the downslope direction in the slope area. This type of SEDDF S2S system is the most common in the study area, widely distributed during the depositional periods of the Zhu-3 Member to Zhu-1 Member (Figs. 3 and 7 ). Both the shelf-edge deltas and the deep-water fans in the continental slope area are larger, facilitating the formation of large reservoirs. Exploration practices in regions such as both sides of the Atlantic Ocean, the Gulf of Mexico, and the northwest shelf of Australia have confirmed the enormous exploration potentials of this type of SEDDF S2S system [29⇓-31], greatly advancing the theoretical framework of deep-water sedimentation and the development of oil and gas exploration.
3.2.2. Deltas that lack downdip fans
The main characteristic of the deltas that lack downdip fans is the development of shelf-edge deltas in the shelf area, while deep-water fans are not developed in the downslope direction of the continental slope area. This type of SEDDF combination pattern can be observed in the southwest of the Zhu-4 Member, the central part of the Zhu-3 Member, and the upper part of the Zhu-1 Member (Figs. 3 and 7 ). Additionally, the scale of continental shelf-edge deltas is larger in each period, manifested as distinct strong amplitude reflections in seismic data, mainly characterized by progradation-aggradation vertically. The widespread distribution of this type of SEDDF S2S system in the study area at various stages further reveals the complexity of sediment transport from "source" to the terminus of the deep-water fan "sink" at the continental shelf edge. For example, during the depositional period of the Zhu-3 Member, deep-water fans were not developed in the deep-water area even under strong sediment supply and relative sea-level fall conditions. Its causes will be discussed in detail later.
3.2.3. Fans that lack updip coeval deltas
The main characteristic of the fans that lack updip coeval deltas is the development of deep-water fans in the continental slope area, without updip coeval shelf-edge deltas. This type of SEDDF S2S system is not common in the study area, mainly developed during the depositional period of the Zhu-4 Member, distributed in the northern segment of the study area (Figs. 3 and 7a ). During this period, the shelf-slope structure was initially formed in the south subsag of Baiyun Sag, with a lack of terrestrial material supply on the northern side, resulting in the absence of shelf-edge deltas. However, the overlay map of paleogeography and RMS amplitude displays a series of stripe-like strong amplitude-filled channels in the continental slope area, forming a lobe-shaped deep-water fan at the terminus with smaller scale.
4. Genetic mechanisms
From the perspective of S2S system, shelf-edge deltas serve as a "transfer station" for the transportation of terrestrial materials into deep-water areas. The existence of shelf-edge deltas allows a large amount of sediments to be transported through the shelf to deep-water areas, forming deep-water fans. Earlier studies emphasized the role of sea-level changes in the dispersal of sand bodies in shelf-edge deltas, pointing out that sand bodies in shelf- edge deltas during sea-level decline are prone to downward transportation, forming sand-rich deep-water fans[32-33]. However, increasing studies have shown that, apart from relative sea level, factors such as sediment supply (strong/weak), climatic conditions (greenhouse/icehouse), fluid action (river, wave, tide), and sediment grain size (coarse/fine) can all affect the transportation of sediments from shelf-edge deltas to deep-water areas [34⇓-36]. Therefore, the coupling relationship between the "source" of shelf-edge delta and the "sink" of deep-water fan deposition is highly complex. There are differences in the developmental characteristics and formation mechanisms among different types of SEDDF systems, as well as among the same type of SEDDF system. This is also the case for the three different types of SEDDF S2S systems in south subsag of Baiyun Sag.
4.1. Deltas that are linked downdip to fans
Due to their significant academic and industrial value, the types of S2S system with deltas that are linked downdip to fans have been widely reported globally. This type of SEDDF S2S system is mainly developed on the shelf-edge fed by deltas [37]. Therefore, sufficient sediment supply, resulting in the accumulation of shelf-edge deltas, is crucial for the formation of such systems. Given a certain sediment supply condition, the formation of deep- water fans is closely related to the migration trajectory of the shelf break. Large-scale deep-water fans are usually formed in descending or gently migrating trajectories[38-39]. The SEDDF S2S systems in the study area during the depositional periods of the Zhu-3 Member and Zhu-2 Member had above two conditions. They were both formed during periods of relative sea-level decline with strong sediment supply, mainly featuring descending shelf break migration trajectories and developing large-scale channel-lobe complexes. Additionally, the largest planar distribution of deep-water fans occurred in the late stage of the Zhu-2 Member (Figs. 3 and 7 ). It is noteworthy that during the early depositional period of the Zhu-1 Member, when the relative sea level rose and sediment supply weakened, a certain scale of deep-water fans were still developed in the study area. The main reason, as revealed by RGB attribute (Red Green Blue) fusion, was the development of a series of channels along the continental slope that cut through the shelf break and connected with the shelf-edge deltas (Fig. 9 ). This allowed the shelf-edge delta sand bodies to continue to be transported downward through the channels, forming sand-rich deep-water fans. A similar feature is also observed in the shelf-edge of the Eocene of the Porcupine Basin in western Ireland [40]. During the early Eocene, when the sediment supply was relatively weak, the development of channelized shelf edges formed sand-rich deep-water fans within the basin. However, during the late Eocene, when shelf-edge channels were less developed, the deep-water area was dominated by mud-rich mass transport complexes and pelagic deposits.
Fig. 9. RGB attribute map of lower shelf-edge of the Zhu-1 Member in south subsag of Baiyun Sag (see the location in |
4.2. Deltas that lack downdip fans
As previously mentioned, shelf-edge deltas that lack downdip fans are widely distributed in the study area, but their genetic mechanisms vary, mainly due to three reasons: (1) Lack of "source": During the late depositional stage of the Zhu-1 Member, as sea level rapidly rose, sediment supply in south subsag of Baiyun Sag was insufficient. Sediments mainly accumulated in shelf-edge areas, where the shelf break migration trajectory exhibited a high-angle ascending pattern, resulting in the absence of deep-water fans. This feature has also been reported in the Quaternary shelf-edge of the Qiongdongnan Basin [39]. (2) Insufficient "channels": During the depositional period of the Zhu-3 Member, under the conditions of strong sediment supply and relative sea-level decline, deep-water fans were developed on the north and south sides of the study area. However, MTDs were developed in the central region due to a steeper terrain gradient in the middle and gentler gradients on both sides. The main channels in the continental slope area were developed on the north and south sides, while those in the central region were underdeveloped, making it difficult for sediments to be transported stably into deep-water areas. Both the Spitsbergen continental edge and the Paleogene northwestern continental edge in Norway [41] have revealed that without the development of channels connecting the shelf-edge in the continental slope area, even during periods of relative sea-level decline, sand-rich shelf-edge deltas have difficulty transporting shelf-edge sands into deep-water basins. (3) Fluid modification: During the depositional period of the Zhuhai Formation, different fluid actions such as river, wave, and tide were widespread in the shelf-edge area of the Baiyun Sag. Specifically, during the depositional period of the Zhu-4 Member, the shelf-edge of the south subsag of Baiyun Sag was initially formed with a relatively gentle terrain gradient (0.4° to 1.5°), resulting in intense wave action. Wave-dominated sand ridges were formed along the shelf break, and the shelf-edge deltas, after being modified by waves, were unable to continue transporting downwards to form deep-water fans (Fig. 10a ). This is similar to the characteristics of the shelf-edge in the North Carnarvon Basin in Australia during the Early Cretaceous [42] (Fig. 10b ). Therefore, the lack of "source", insufficient "channels", and fluid modification are the three main causes of the formation of shelf-edge delta that lack downdip fans. This is supported by numerical simulations and seismic sedimentological analysis of case studies by Gong et al. [9]
Fig. 10. Comparison of typical RGB attribute characteristics of the deltas that lack downdip fans of delta-fan source-sink system under fluid reworking (see the location of |
4.3. Fans that lack updip coeval deltas
The traditional sequence stratigraphy model for predicting lowstand fans assumes that during periods of relative sea-level decline, the accommodation space in the shelf area diminishes, causing coarse clastic sediments to be transported across the shelf-edge to the slope and basin, forming deep-water fans [32-33], resulting SEDDF S2S system with fans that lack updip coeval deltas. However, it's noteworthy that the fans that lack updip coeval deltas during the depositional period of the Zhu-4 Member in the study area were not formed during a period of relative sea-level decline. Additionally, there were no typical sedimentary transport signatures such as the merging of seismic events between the top and bottom strata, as previously proposed [43]. This system was due to insufficient sediment supply, resulting in the underdevelopment of shelf-edge deltas in the northern segment of the study area, despite the presence of deep-water fans below. The primary reason for this is speculated to be the modification of shelf-edge deltas developed on the southern continental edge by waves, which transported sediments northward along the shelf break belt. Meanwhile, a series of channel systems were developed in the northern slope region. On the upper slope, multiple isolated narrow channels were developed, with individual channel widths ranging from 160 m to 950 m, and the entire channel system spanning approximately 23 km (Fig. 11a ). As the channels progressed to the middle slope, they began to converge, but the channel widths increased, with individual channel widths ranging from 300 m to 2 000 m, and the entire channel system spanning approximately 10.6 km (Fig. 11b ). On the lower slope, multiple channels converged to form a composite channel system with a width of approximately 5.2 km (Fig. 11c ). Sediments modified and transported from the south continued to be transported and converged along the channels in the northern slope region, ultimately forming deep-water fans. Therefore, the formation of shelf-edge delta to fan systems with fans that lack updip coeval deltas requires the coupling of two geological conditions: (1) Fluid modification: Sediments are laterally transported by waves or ocean currents to the heads of slope channels; (2) Smooth flow of "channels": The presence of a series of "channels" connecting the shelf-edge in the slope region, allowing the sediments modified and transported by fluids to continue to be transported and dispersed into deeper waters along these "channels". Previous studies on deep-water fans in the shallow Zhujiang Formation on the northern slope of Baiyun Sag in the PRMB have also noted this point, where the ancient Zhujiang shelf-edge delta was modified by wave action and influenced by channels, resulting in the coupling relationship between shelf-fan systems with fans that lack updip coeval deltas in the deep-water area of the Baiyun slope [9].
Fig. 11. Typical seismic reflection sections of the channels in Zhu-4 Member of the Zhuhai Formation in south subsag of Baiyun Sag continental slope area (the locations of the sections are shown in |
5. Conclusions
From the perspective of the development of shelf-edge delta to deep-water fan systems (SEDDF), three distinct types of SEDDF coupling patterns were identified in the Zhuhai Formation in the south subsag of Baiyun Sag: "deltas that are linked downdip to fans", "deltas that lack downdip fans", and "fans that lack updip coeval deltas".
There are significant differences in the genetic mechanisms of these different types of SEDDF S2S systems. Strong sediment supply and relative sea-level decline are the two main factors controlling the formation of the "deltas that are linked downdip to fans" type of SEDDF S2S system. However, the development of deep-water channels at the continental shelf-edge can contribute to the development of the "deltas that are linked downdip to fans" type of SEDDF S2S system even under conditions of weak sediment supply and during periods of relative sea-level rise. The lack of "source", insufficient "channels", and fluid modification are the three main reasons for the development of the "deltas that lack downdip fans" type of SEDDF S2S system. The coupling of fluid modification at the continental shelf-edge and the smooth flow of "channels" in the continental slope controls the development of the "fans that lack updip coeval deltas" type of SEDDF S2S system.
A smooth and connected continent shelf-edge delta-fan coupling relationship is the key to the development of sand-rich deep-water fans. The coupling relationship between the SEDDF S2S was optimal in the early stage of the Zhu-1 and Zhu-2 members, leading to the largest sand-rich deep-water fans and bigger exploration potential in the middle section of middle-lower continental slope area.
Acknowledgements
We thank Professor Zhu Rui from Yangtze University for his guidance and assistance in the analysis of rock core facies. The authors also thank editor Yi Yingjie for editorial handing and comments and to two anonymous reviewers for their insightful and constructive comments, all of which significantly improved the overall quality of this research.