Types and Evolution of the Miocene Reefs Based on Seismic Data in the Beikang Basin, South China Sea

Abstract

During the Miocene, several reefs formed in the Beikang Basin, South China Sea, which may be potential targets for hydrocarbon exploration. This is due to the environment that developed as a result of the collision, splitting, and splicing of the Nansha Block, which was influenced by the Neogene expansion of the area. However, studies on the types, distribution, controlling factors, and evolution stages of these reefs are scarce. In this study, we used high-resolution seismic data and extensive well-drilling records to gain insights into the evolution of reefs in this particular area. Six distinct types of reefs, namely, the point reef, the platform-edge reef, the block reef, the bedded reef, the pinnacle reef, and the atoll reef, were identified based on our data. These reefs underwent four stages of development. During the initial stage, a few small-sized point reefs emerged in the basin and experienced significant growth during the early Middle Miocene. In the flourishing stage, the reefs predominantly thrived around the Central Uplift and Eastern Uplift areas. In the recession stage, the reefs began to deteriorate during the late Middle Miocene period as a result of the rapid increase in relative sea level caused by tectonic subsidence. In the submerged stage, since the Late Miocene, as the relative sea level continued to rise steadily over time, many reefs that had previously flourished surrounding the Central Uplift and Eastern Uplift areas became submerged underwater, with only a handful of atoll reefs surviving near islands located on the Eastern Uplift. This study indicated the presence of a significant number of well-preserved reefs in the Beikang Basin that have experienced minimal subsequent diagenesis and therefore exhibit high potential as reservoirs for oil and gas exploration.

1. Introduction

A biological reef generally refers to a special carbonate structure formed by sessile benthic organisms growing in situ [1]. The development of a reef is highly influenced by variations in the environment, including fluctuations in the water depth, temperature, waves, and salinity levels. Any change in these factors, such as dissolved oxygen, substrate, and nutrients, may change the growth form or internal structure of reefs and even lead to their death while submerged [2,3,4]. At the same time, because of the characteristics of good physical properties, high production capacity, high recovery rate, and low exploration and development costs, reefs are significant for the exploration of oil and gas [5,6,7,8,9,10,11,12]. Due to their sensitivity to environmental changes and capacity for hydrocarbon accumulation, the reefs have garnered significant attention and research both domestically and internationally.

A significant proliferation of organic reefs has occurred in the tropics and subtropics, particularly following the Late Oligocene since the formation of the South China Sea [3,5,8]. The occurrence of this phenomenon can be attributed to the influence of tectonic evolution in the South China Sea; the reefs in the southern region developed earlier than those in the northern part, but they also surpassed them significantly in terms of scale and number [5,13,14,15]. The results of hydrocarbon exploration show that the reefs in the southern South China Sea have higher exploration potential than those in the northern part. At present, countries around the South China Sea have discovered more than 30 hydrocarbon fields in reefs, including the Lanlong Oilfield and Honglanhua Gas Field in the Wan’an Basin, the L, F6, and F23 Gas Fields in the Zengmu Basin, and the Nido Oil Field in the Palawan Basin [16,17,18,19]. Studies have shown that these reefs initially developed in the Oligocene, flourished in the Early Miocene, and started to decrease in the Late Miocene, all with similar evolutionary processes [14,18,20]. Based on the structural properties of the basins where they develop, reefs can be divided into two types. The first type is represented by reefs on isolated carbonate platforms, mainly in the Liyue Basin and the North Palawan Basin. These two basins drifted from the northern continental margin of the South China Sea with the expansion of the sea floor to their present positions, and organic reef towers of this type dominate [15]. The second type is represented by reefs in continental margin faulted basins, mainly in the Wan’an Basin and the western region of the Zengmu Basin. The fault basins were formed due to many NE-oriented faults caused by crustal tension and thinning from the Eocene to the Oligocene [16]. These reefs, mainly platform marginal reefs, are mainly developed on structural fault blocks [10]. Many studies have been carried out on the above two types of reefs to clarify the controlling effects of the types, development modes, and structures, such as relative sea level, terrigenous debris, nutrient salts, and other factors, and the reefs are also important reservoirs for oil and gas exploration and development in countries around the South China Sea [17,18].

In the Middle Miocene, the Beikang Basin in the South China Sea also experienced the formation of multiple reefs, which provided a good opportunity for exploration. Limited by the quality of seismic data and the complexity of regional tectonic evolution, research on reefs in this area is relatively backward and has not made substantial progress. In this study, based on nearly 8000 km of seismic lines and regional drilling data, we systematically identify the types of reefs through their corresponding seismic reflection characteristics and analyze the development model and control factors of reefs in the Beikang Basin. The aforementioned study contributes to further analysis of the developmental mechanisms of reefs and holds significance for the prediction of carbonate reservoirs in the Beikang Basin, South China Sea.

2. Regional Geological Background

The Beikang Basin is located in the central area of the Nansha Sea (Figure 1), with a large range of water depths, gradually transitioning from shallow water in the southern part of the basin to deep water in the northern part of the basin, with a water depth of more than 2000 m. Situated within the Nansha Block, the southern section of the basin and the Zengmu Basin are demarcated by the Tingjia Fault [21]. During the Middle Eocene, the Indian Ocean expanded for the third time and collided with the Eurasian plate, subducting obliquely toward Southeast Asia at a large angle. This accelerated the southeast extrusion of the Southeast Asian continent and pushed the southward movement of the Nansha Block [22], leading to the formation of regional unconformities. This movement, called the Xiwei Movement, resulted in the development of numerous faults toward the north within the Beikang Basin, cutting the Eocene and Paleocene lacustrine strata. In the Middle Oligocene (32 Ma), the sea floor of the South China Sea began to expand, and the Nansha Block began to drift southward overall. During the drift, the northeast-trending faults in the earlier period continued to be active, thus forming the Nanhai Movement. The southernmost part of the Nansha Block subducted under the Borneo Block. The subduction process gradually closed along the Borneo plate from southwest to northeast, and the duration of this event extended until the conclusion of the Early Miocene epoch [22,23,24,25]. The end of subduction could be attributed to the depletion of the ancient South China Sea oceanic crust when the Nansha Block reached its present position and collided with the Borneo Plate. This collision event led to a large-scale uplift and erosion of the initial sedimentary layers in the Nansha Sea region, thus forming the Nansha Movement [22,26]. The drilling results of the IODP349 voyage revealed that the Nansha Movement may also serve as the primary reason for the cessation of expansion in the southwestern subbasin of the South China Sea [27,28]. This tectonic movement is one of the most important tectonic movements in the South China Sea. Since the Middle Miocene, a significant increase in the subsidence rate has been observed in the Beikang Basin, and the main driving force of its subsidence may be sedimentary loads [22,29]. During this process, numerous faults with NE and NW orientations emerged within the Beikang Basin [30], the formation of six secondary tectonic units, namely the Northwest Depression, Western Depression, Central Uplift, Northern Uplift, Southeast Depression, and Eastern Uplift occurred (Figure 1B).

The pre-Cenozoic metamorphic rocks and acidic–basic igneous rocks are the predominant geological formations in the basement of the Beikang Basin, with a major concentration of igneous rocks observed in the eastern region of the basin. The sedimentary cover of the basin is dominated by Cenozoic strata, and the gravity and magnetic inversion results show that the western depression is the thickest, exceeding 12,000 m [20]. The well-seismic calibration results show that six seismic reflection interfaces (T_g, T₅, T₄, T₃, T₂, and T₁) can be identified from bottom to top in the sedimentary strata of the whole area (Figure 2), where T_g, T₅, T₄, and T₃ correspond to the Liyue Movement, Xiwei Movement, Nanhai Movement, and Nansha Movement, respectively [21,22,26]. T_g is generally the top boundary of the acoustic base and is mainly composed of medium–low frequency, medium–strong seismic amplitude, and low continuous reflection waves. T_g shows the characteristics of a weathering denudation surface and is the initial unconformity surface separating the rocks in the pre-rift period and the co-rift period. T₅ is a regional unconformity surface, which is generally a medium-frequency, medium–strong-amplitude, low continuous reflection wave. T₅ represents a ruptured unconformity that delineated the transition between the syn-rift stage and post-rift stage. Above the interface, there are common overpass and underpass characteristics, while below the interface, there are relatively minor erosion phenomena. The strata below the interface show a set of filling and sedimentary characteristics in the early stage of basin development [22,29]. The T₃ interface represents a regional unconformity caused by a large regional tectonic movement. Above the interface, there is an obvious overpass phenomenon, weak or no deformation of the strata, and good continuity of the reflected waves. Below the interface, there is strong deformation of the strata and poor continuity of the reflected waves. The age of formation of this interface can be compared with the low sea levels occurring from the late Middle Miocene to the early Late Miocene [20].

The pre-Oligocene strata are dominated by lacustrine sediments, but the drilling of well Mulu-1 revealed that the Beikang Basin hosted mostly semiabyssal–abyssal deposits during the Oligocene. After the Early Miocene period, coinciding with the general elevation of the area, and after being eroded by the seabed, the Beikang Basin was in a coastal shallow sea environment overall, creating conditions for the development of this biological reef. Since the Late Miocene, as subsidence has accelerated, the whole area has gradually evolved from a littoral–shallow sea environment to a semiabyssal–abyssal environment, which is not conducive to the growth of reefs [31,32]. Therefore, the strata related to the development of reefs mainly range from the Early Miocene to the Quaternary, and from bottom to top are the Riji Formation, Haining Formation, Nankang Formation, and Beikang Formation (Figure 3).

3. Data and Methods

This research was conducted by analyzing high-resolution two-dimensional seismic data covering nearly the entire study area, approximately 8000 km, obtained from the Guangzhou Marine Geological Survey (GMGS) in both the 1990s and 2019. The majority of seismic profiles were spaced at intervals of 4 × 8 km, with some being spaced at intervals of 1 × 2 km. The primary data utilized in this study were predominantly acquired by Marine Geology 12 through the utilization of a generator/injector (GI) gun with a capacity of 2250 scf and operating at a pressure of 2000 PSI. The source facilitated an acquisition time ranging from 0 ms to 9.0 s, with an average line distance spanning between 26 m and 50 m. All interactive interpretations pertaining to poststack time offset data (postSTM) were successfully conducted within the Geoframe 4.5 software platform developed by Schlumberger. Various reefs were identified and described based on seismic profiles. The structural and distribution characteristics of the reefs and carbonate platforms were determined and analyzed based on seismic interpretation. A geological model related to the characteristics of the evolutionary development was established.

The primary method for seismic identification of reefs involves analyzing the reef’s morphology, the reflection properties of the top-bottom interface, the internal reflection structure, and the contact relationship with the adjacent strata [10,19,33]. The growth forms of different reefs vary, leading to distinct reflection structures. Meanwhile, the diversity in reef classification stems from varying bases and research objectives [1,3].

4. Results

Through careful interpretation of the seismic data from the Beikang Basin and comparisons with neighboring regions, such as the Zengmu Basin and the Wan’an Basin, various reefs were identified within the study area. Six distinct reef types were discerned based on geographical location and growth structure.

4.1. Types of the Reefs

4.1.1. Point Reefs

The form of this reef is characterized by a mound-like shape, with both wings exhibiting almost symmetrical features. The upper part displays continuous intense reflection, while the internal reflection appears weak or lacks any reflective signal. Additionally, there is no distinct demarcation between the lower section and the underlying formation (Figure 4 and Figure 5). This type of reef is small, with a spatial distribution of 1–2.5 km, and is mainly present on the platform. The numbers of this type of reef are relatively small in the Beikang Basin; they are concentrated in the Central Uplift and the western part of the Eastern Uplift.

4.1.2. Platform Edge Reefs

This type of reef is mound-shaped overall, with continuous strong reflections at the top and chaotic reflections inside, and sometimes one or two concentric axes are visible. The strong reflection shape is easily distinguished from the strata, with weak reflections at the periphery (Figure 4, Figure 5 and Figure 6). The platform edge reef in the Beikang Basin has seismic facies similar to the platform edge reef developed in the western slope area of the Zengmu Basin [34]. The platform edge reef is mainly present at the platform edge; its scale varies according to the difference in the terrain characteristics of the platform edge, and the transverse length varies from 2 to 5 km. In the process of its growth, part of the reef debris collapsed under the action of gravity and was deposited at the foot of the slope, forming the front slide of the reef, which has a chaotic reflection. These reefs are extensively found in the western region of the Eastern Uplift.

4.1.3. Massive Reefs

The shape of this type of reef exhibits a block-like structure, predominantly concentrated in the upper section of the steep slope bordering the platform; it displays consistent and intense reflections at both the top and bottom, which are clearly demarcated from the adjacent medium–weak reflection layers above or below (Figure 6). In the interior, one or two events showing continuous strong reflections are visible, and the overall thickness of the reef is mostly between 50 m and 150 m. The scale of this type of reef is mostly limited by the geomorphology of early carbonate platforms [35], and the scale of these reefs varies in different regions. In the southern region of the Eastern Uplift, the lateral spread of these reefs can reach more than 10 km, and they migrate to higher terrain with the retreat of the platform. On the Central Uplift, such reefs are relatively narrow, and there is a scarcity of reef debris on the fore-platform slope, resembling the massive reefs that formed on the relatively slender platform during the Middle Miocene in the Wan’an Basin [19]. The limitation of the bottom of such a reef is not solely determined by the landform at the top of the platform but also has a relatively short development period. With the rise of relative sea level, biological reefs are rapidly submerged, and the lateral distribution of massive reefs in the Dongsha Sea area in the northern part of the South China Sea is extremely spectacular, but their thicknesses generally do not exceed 200 m, showing this feature well [7].

4.1.4. Layered Reefs

Layered reefs are formed primarily through the vertical growth of massive reefs in the same location. This type of reef is characterized by a cohesive and continuous strong reflection overall. The interface between the upper layer and the overlying strata is clearly visible, while the lower section develops directly on the earlier massive reefs, resulting in less apparent interfaces. Additionally, multiple axes of continuous reflection can be observed within these layered formations. The thicknesses of the reefs are mostly between 100 m and 200 m, and some can reach 300 m (Figure 7). This type of reef is mainly distributed in the Eastern Uplift, and the number and scale are relatively small, but the scale can reach approximately 10 km.

4.1.5. Tower Reefs

This reef type is in the shape of a tower overall, and in the seismic section it appears as a vertical superposition of continuous mound-shaped strong reflections. The interior of a single mound is a messy weak reflection, the bottom is a nearly blank reflection, the upper reflection is strong, and the interface with the overlying layer is clearly discernible. The horizontal distance of the tower reef is 1–3 km, and the maximum altitude achievable is 1000 m, which shows that it has the characteristics of multiphase continuous growth and reflects that its growth rate is consistent with the rate of relative sea level rise (Figure 7, Figure 8 and Figure 9). In general, there is a strong hydrodynamic environment around the tower reef, which usually leads to the development of waterways on its flanks. This phenomenon is more common in the southeastern Beikang Basin, which is uplifted in the east near the Beikang shoal area, which is similar to the Xisha Sea area. A channel with a “U” incision developed around the tower reef and is similar to that in the other area [11]. At the same time, tower reefs often migrate to high-lying parts during their development, indicating that the sea level continued to rise during the development period. Tower reefs are the most common, numerous, and important type of reef in the Beikang Basin. Their extensive development is observed in the Eastern Uplift and Central Uplift regions. This particular reef type bears a resemblance to the tower reef found on the Nankang Platform within the Zengmu Basin. Seismic facies and developmental ages can be compared effectively [32].

4.1.6. Atolls

Atolls are the main type of reef in the submerged stage of carbonate platforms, and most of them have the characteristics of rapid vertical growth. There are certain differences in geological form, which is mound-shaped overall, featuring continuous intense reflections occurring atop the reef and chaotic weak reflections internally. Their horizontal scale ranges from 2 km to 4 km (Figure 9). Some atolls have a growth height of more than 1000 m, are primarily constructed on the elevated landscape of the platform in the high-terrain area, and are mainly distributed in the southeastern region of the Eastern Uplift.

In summary, the reefs in the Beikang Basin are numerous and complete; they include platform edge reefs and tower reefs. These reef formations are extensively distributed across the Central Uplift, as well as the western and southeastern regions of the Eastern Uplift. These reefs mainly developed in the Middle Miocene. Although the duration of development was short, only some tower reefs and atolls developed into the Late Miocene, but the reef body imaging was good and made reef bodies easy to identify. These reefs are easily compared with the reefs in the Zengmu Basin and Wan’an Basin in neighboring areas as well as in the Dongsha Sea area in the northern South China Sea; this distribution is also a feature of the reefs in the Beikang Basin in the South China Sea.

4.2. Spatiotemporal Distribution of the Reefs

The interpretation of seismic sections reveals that the carbonate rock platforms in the Beikang Basin predominantly occur on elevated areas or fault blocks, most of which are isolated platforms, and faults are often developed at the edges of the platforms. In the seismic section, the top of the carbonate formation exhibits strong amplitude characteristics that are parallel or subparallel owing to a significant impedance difference between this formation and the mudstone layer above it. Point reefs, platform edge reefs, layered reefs, tower reefs, etc. often develop on the edge or inside the carbonate platform, usually in the shape of hills or strips, showing strong reflections on the top and chaotic reflections in the interior. Carbonate debris and slump deposits are visible on the edge slope of the platform, showing a chaotic reflection formed by water denudation or gravity collapse.

The number of point reefs was relatively large in the Early Miocene and Late Miocene. They were mainly located in the western regions of both the Central Uplift and the Eastern Uplift (Figure 10). Platform marginal reefs flourished at the end of the Early Miocene and were widely distributed in the west of the Eastern Uplift, with a horizontal spread of up to 10 km and a vertical growth of more than 200 m. Blocky reefs and layered reefs were mainly developed in the Middle Miocene, mostly distributed on the flat and open tops of the Central Uplift and Eastern Uplift (Figure 10A). The tower reefs began to develop widely at the end of the Early Miocene, were mainly distributed on the western slopes of the Central Uplift and the western slopes of the Eastern Uplift, and showed inherited development characteristics. Some tower reefs continued to develop until the Late Miocene (Figure 10B). Atolls mostly developed on top of the southeastern high-lying platform that was uplifted in the east during the Late Miocene.

5. Discussion

5.1. Controlling Factors

5.1.1. Tectonics

The structures play a significant role in shaping the development and evolution of carbonate platforms in the Beikang Basin, and these structures are intricately linked to the history of the expansion of the South China Sea and the evolution of block collisions. In the Late Eocene–Early Oligocene (34–32 Ma), the South China Sea began to expand, the Zengmu Block and the Borneo (Kalimantan) Block collided, and the water depth began to decrease, while the Beikang Basin in the Nansha Block was stripped from the ancient South China continent. During the Early Miocene (approximately 23 Ma), the South China Sea spreading ridge moved southward, resulting in the development of numerous carbonate platforms within the Nankang Platform located in the Zengmu Basin, and the Nansha Block and the Philippine Island arc began to collide. The Beikang Basin was uplifted overall due to compression, and the development of carbonate platforms was also initiated. In the Middle Miocene (15.5 Ma), the expansion of the South China Sea gradually stopped and the collision of blocks slowed. The Beikang Basin maintained a stable structural environment. During this period, carbonate platforms flourished [22,25,28,36]. Since the Nansha Block and the Zengmu Block were both subducted and extruded in a southeasterly direction, the collision of the blocks produced a NE-oriented structural belt that formed on the uplift and fault block. Most carbonate platforms in China are also NE-trending. Some uplift structures are controlled by NW-trending strike-slip faults and trend NW, and some carbonate platforms with NW-trending faults are also present [25,37].

After the Late Miocene, the Tinja Fault became active again, changing from right-handed to left-handed; the basin was further stretched and differentially subsided and was obviously thickened from northeast to southwest, forming a “seeker” with the pre-Miocene sedimentary layers. The basin’s subsidence and deposition center underwent a shift from the northeast to the southwest [25,29,38,39]. After the Late Miocene (10.5 Ma), due to the rapid northwestward progradation of the Borneo provenance, the basin suffered a large sedimentary load and quickly subsided. The sedimentation rates in the eastern part of the basin were 140–300 m/My, while the subsidence rates in the central and western parts reached 300–460 m/My, and the rapid increase in water depth caused many carbonate platforms to be submerged (Figure 11) [11,22,29].

The distribution of carbonate platforms in the Beikang Basin is primarily affected by structural uplift, as well as by fault distribution. The Miocene carbonate platform in the Beikang Basin exhibits similarities to the Nankang platform located in the neighboring Zengmu Basin, both of which are dominated by isolated platforms; the trend is mainly NE, and some platforms are oriented NW. Notably, the trend of carbonate platforms in the Beikang Basin is also relatively consistent with that of today’s carbonate platforms, such as the Beikang Ansha, Nanwei Shoal, and Polang Reef platforms. Many carbonate platforms in the Beikang Basin are asymmetrically developed due to the control of faults, which may be the result of fault development, subsidence of the thrown side, and rotation of fault blocks.

5.1.2. Sea Level Change

The development and evolution of carbonate platforms in the southern South China Sea were controlled by relative sea level changes and the growth rates of these platforms. The rate of relative sea level change is affected by both tectonic movement and fluctuations in sea levels; among them, tectonic movement includes tectonic subsidence and tectonic rise; sea level change also includes rising and falling changes [41].

Starting from the Early Miocene (23 Ma), a southward migration of the spreading ridge of the South China Sea occurred, resulting in a collision between the Nansha Block and the Philippine Island Arc [25,27]. Because of the uplift of the geological structure, there was a decrease in water depth within the Beikang Basin, resulting in the formation of a littoral–neritic environment where carbonate rocks started to form [5,14]. During the Miocene period (15.5 Ma), the expansion of the South China Sea reached a halt, and there was a gradual decrease in block collisions [25]. During the Middle Miocene, consistent tectonic sedimentary conditions facilitated the development of carbonate rock reefs and carbonate platforms. In the Late Miocene (after 10.5 Ma), the Beikang Basin experienced a significant increase in water depth because of rapid subsidence, ultimately resulting in the submersion of numerous carbonate platforms [5].

5.1.3. Terrigenous Debris Input

The presence of terrigenous debris causes a rise in water turbidity, ultimately resulting in the extinction of organisms secreting carbonate skeletons and leading to the swift deterioration of the platform. Hence, terrigenous debris input plays a significant role in influencing the growth and formation of the platform. During the Miocene period, the global climate was suitable, and the Larang Delta and the Balam Delta developed and expanded (Figure 1). According to the calculations conducted by Hall and Nichols (2002), approximately 1,560,300 km³ of Neogene sediment were deposited in this region primarily through the two deltas mentioned. Their research findings led them to conclude that a minimum of 6 km of crust underwent erosion from Borneo during the Neogene period [25,42,43]. The Middle Miocene–Recent delta has been the main contributor to the thick sedimentary sequence that covers the rifted continental margin in the southern South China Sea [25,44]. In addition, a large number of platforms in the Nansha Sea area were inundated by terrigenous clastic injection, and, in some areas, carbonate sediments and clastic sediments alternately co-existed [17,18,37,45]. The carbonate platform of the Beikang Basin is primarily affected by terrigenous sediments originating from the Larang Delta and the Baram Delta on the Sunda Shelf.

During the Miocene Epoch, the southern region of the Beikang Basin was situated near the periphery of the continental shelf, near the provenance area. As a result, the formation of the carbonate platform was significantly influenced by the introduction of terrigenous clastics, which was the primary cause of the cessation of the platform [25]. In contrast, the central and northern regions of the Beikang Basin were situated at a considerable distance from the provenance area. As a result, the formation of carbonate platforms in these areas experienced minimal influence from terrigenous clastics.

In contrast to the Beikang Basin, the presence of terrigenous clastics significantly influences the growth of carbonate platforms in certain basins located in the southern South China Sea [39]. For instance, a significant portion of the carbonate platforms in the neighboring Nankang platform within the Zengmu Basin experienced submersion due to the deposition of progradational delta siliciclastics following the Miocene Epoch [37]. After the Late Miocene, the formation of the carbonate platform in the Wan’an Basin in the southwestern part of the South China Sea stopped as a result of the injection of a large amount of terrigenous clastic material from the Mekong River, and the carbonate platform entered the submerged stage [46,47].

5.2. Evolution Stage

Seismic data show that the Beikang Basin has a complete range of reef types, which mainly formed in the Middle Miocene; this was also the main period of carbonate rock deposition, which resembled the development of Late Miocene reefs in the neighboring Wan’an Basin [47,48]. At the same time, the temporal and spatial distribution characteristics of reefs in the Beikang Basin, the regional geological background, the sequence stratigraphic framework, and fluctuations in relative sea level were combined. The progress and evolution of carbonate platforms can be categorized into four distinct stages: the initial development stage during the Early Miocene, the flourishing stage in the early Middle Miocene, the recession stage in the late Middle Miocene, and the submersion stage in the Late Miocene. Based on the distribution of reef-like landforms and the geomorphic characteristics of the Beikang Basin in the late Early Miocene, a model of reef development was established (Figure 12).

5.2.1. Initial Stage

During the Early Miocene, with the beginning of the Nansha Movement, the Nansha Block began to uplift overall, resulting in a gradual transition from the Oligocene semideep sea to the late Early Miocene coastal sea environment in the Beikang Basin, which was dominated by mudstone and sand mudstone deposits and underwent enormous denudation, with the extent of erosion above the uplift exceeding 2600 m. The erosion extent in the depression is close to 500 m [22], forming a strong denudation unconformity (Figure 12). During the overall uplift of the basin, a small number of reefs also developed in the slopes and other areas around the uplift, mainly point reefs. The fact that the G2-1X well encountered carbonate rocks in the Lower Miocene strata provides strong confirmation of this [23], but most of the rocks were uplifted and suffered denudation in the later period, which made it difficult to identify them in the seismic profile. Although the number of reefs in this period was small and the type was singular, they played a vital role in the growth and progression of the reefs located in the Beikang Basin, showcasing the distinctive features observed during the initial development stage of the reefs within this basin.

5.2.2. Flourishing Stage

After the end of the Nansha Movement in the late Early Miocene, the Beikang Basin reentered a structurally stable environment after experiencing strong uplift and denudation; subsidence was the main manifestation. The uplift of the early Middle Miocene and its periphery were mainly coastal shallow sea sediments, the terrigenous delta progradation had not yet reached the eastern and southeastern regions, and the water quality was clean, creating conditions for the flourishing of reefs. During this period, there were many reefs, and various types of reefs appeared. For instance, tower reefs were extensively developed on the western slope of the Central Uplift as well as the Eastern Uplift (Figure 4, Figure 5, Figure 6, Figure 7, Figure 8 and Figure 9). On the other hand, platform edge reefs predominantly formed along the periphery of the uplift (Figure 5 and Figure 8), and a few point reefs also emerged in this region. These reefs exhibit remarkable abundance, diversity, and size (Figure 12).

5.2.3. Recession Stage

In the early Middle Miocene, with the conclusion of the collision between the Nansha Block and the Borneo Block, the provenance sediment from the Borneo Block began to accumulate rapidly northwestward, and this large load was also the main driving force for the basement subsidence of the Nansha Block since the Middle Miocene [22,24,47]. The Beikang Basin, situated in the southern region of the Nansha Block, is one of the areas with the strongest basement subsidence. The basement subsidence in this stage led to a significant increase in the relative sea level (Figure 3). By the late Middle Miocene, the slopes around the uplift were already in a bathyal environment, and the reefs in the Beikang Basin had begun to enter a stage of decline. In this period, reefs mainly developed on top of the Central Uplift and the Eastern Uplift, and the reef types were primarily massive and layered. The dimensions of massive reefs and layered reefs could exceed 10 kilometers due to the relatively flat and open topography of the uplift (Figure 6 and Figure 7).

5.2.4. Submerged Stage

During the Late Miocene, as the basement continued to subside [29], the relative sea level continued to increase, and the Beikang Basin was in a deep-sea–bathyal environment overall. Only the reefs above the uplift and their surrounding reefs continued to develop, which also indicated that the reefs had entered the inundation stage. During this period, vertical growth of biological reefs was more common, and a small number of tower reefs continued to form on the slopes around the reefs. Since the Pliocene, the uplift area of the Beikang Basin has been characterized by a bathyal environment, with no occurrence of reef formation in the basin.

6. Conclusions

(1)

Six types of reefs were identified in the Beikang Basin: the point reefs, the platform marginal reefs, the massive reefs, the layered reefs, the tower reefs, and the atolls. The platform marginal reefs are the most representative, with some reefs exhibiting a large scale and lateral spread exceeding 10 km. These reefs are extensively distributed on both the Central Uplift and Eastern Uplift of the Beikang Basin as well as on their surrounding slopes.

(2)

The development and evolution of reefs in the Beikang Basin can be categorized into four distinct stages: the initial development stage in the Early Miocene, the flourishing stage in the Early Middle Miocene, the recession stage in the late Middle Miocene, and the submerged stage in the Late Miocene. The landform of the Beikang Basin in the late Early Miocene, resulting from the Nansha Movement, provided favorable conditions for the initial development of reefs. Subsequently, rapid basement subsidence during the late period played a decisive role in shaping different stages of reef development since the Late Miocene.

(3)

The spatial distribution of reefs in the Beikang Basin is primarily influenced by tectonic and sea level fluctuations, intricately intertwined with the historical expansion and collision evolution of the South China Sea. Since the Early Miocene, the Beikang Basin has experienced a shallowing of the water depth as a result of block collision, leading to the initiation of platform development. During the Middle Miocene, the South China Sea expansion halted, leading to a stable tectonic environment and the flourishing of the platform. However, following the Late Miocene period, platform development experienced a decline due to rapid sedimentation and the influence of marginal clastic injection until eventual submergence.