Evolution and Controlling Factors of the Reef and Carbonate Platform in Wan’an Basin, South China Sea

Abstract

In this study, high-resolution seismic profiles and well data provided a good opportunity for better understanding the reefs and carbonate platforms in the Wan’an Basin, southwest of the South China Sea, and also provided valuable information for the oil–gas exploration in the reef reservoirs. Four evolutional phases, including the initial phase, the prosperous phase, the recession phase and the submerged phase, of the reefs and carbonate platforms are proposed according to our data. In the Early Miocene, a few small, isolated carbonate platforms initiated in the center of the basin. In the Middle Miocene, they flourished and mainly formed around the Northern Uplift and Central Uplift, with two belts of carbonate platforms in the western area and eastern area that were mainly platform-edge reefs, massive reefs and a few point reefs. In the Late Miocene, the carbonate platforms began to retreat towards the high topographic position because of the rising of sea level. Meanwhile, the numbers and styles of reefs increased to include platform-edge reefs, massive reefs, atoll reefs and point reefs. Since the Pliocene, most of the carbonate platforms have been covered by detrital materials from terrestrial sources. Crustal tectonic activity provides favorable topography for reef growth and the distribution of platforms, and eustasy controlled the vertical growth and lateral migration of reefs. Since the Late Miocene, the rapidly crustal tectonic subsidence and the rising of relative sea level may lead to the drowning of the carbonate platform.

1. Introduction

Carbonate platforms are mainly distributed in tropical and subtropical low-latitude ocean shallow water areas, and their growth and development are very sensitive to environmental changes, such as water depth, temperature, waves, salinity, dissolved oxygen, sediment and nutrients [1,2,3,4]. Changes in any of these controlling factors may alter the sedimentary structures of carbonate rocks and even cause their submerged death. Since the Cenozoic, the South China Sea has benefited from its advantageous geographical location (0~23° N) and suitable climate conditions; because of these, abundant carbonate platforms formed [1,2,3]. Though most of these carbonate platforms have been submerged, they still recorded a great deal of important information about the evolution of the South China Sea and also are the main producing reservoirs of hydrocarbon exploration in this area. More than 50 carbonate-type oil/gas fields have been uncovered so far [1,2,3,4]. Previous studies found that the carbonate platforms in the southern part of the South China Sea initially developed in the Middle Oligocene, earlier than in the north [5], and their scales were much larger than in the north.

The Miocene carbonate platforms in the southern South China Sea were mainly found in the Liyue Basin, the Zengmu Basin and the Wan’an Basin [6,7,8,9,10]. Among them, the Wan’an Basin is close to the Indochina Peninsula, as shown in Figure 1, and its characteristics and evolution have certain particularities. Fournier et al. [7] used drilling and seismic data to reveal that the Northern Palawan Platform on the periphery of the Liyue Basin is an isolated platform developed on the shoulder of the oblique fault block. The Luconia carbonate platform in the Zengmu Basin began to develop in the Early Miocene. After the Miocene, the carbonate platform in the northern part of the Nankang Platform continued to develop, while most of the carbonate platform in the south was covered by progressive delta siliceous clastic rocks. Three-dimensional seismic and well data in the central area of Luconia show that the cause of the inundation of the northern platform may be the rapid rise of relative sea level or sea eutrophication, which is similar to the inundation of the Liuhua Platform (Miocene) in the northern part of the South China Sea [11,12].

Based on some seismic data, previous researchers determined that the carbonate platforms of Wan’an Basin have the characteristics of east and west zoning, initially developed in the Early Miocene period and experienced uplifting and denudation in the Late Middle Miocene period [9,10,13,14,15].

However, due to the limited seismic and drilling data, the structure and development process of the carbonate platforms in Wan’an Basin are still unclear. Recently, using nine drilling wells, well log data and reprocessed high-quality seismic data, the attempt was made to achieve three goals: (1) finely characterize the seismic reflection structures of reefs and carbonate platforms (hereinafter referred to as RCPs); (2) determine the distribution and evolution of RCPs; (3) clarify the main controlling factors including tectonic activities, terrestrial clastic input processes and relative sea level changes to investigate the development and evolution of RCPs.

2. Geological Setting

As a special marginal sea in the western Pacific, the South China Sea is located at the intersection of the Pacific plate, the Eurasian plate and the India-Australia plate. The special tectonic position determines the complexity of its Cenozoic tectonic evolution. From the Early Paleocene to the Early Oligocene, processes of continental margin extension, lithospheric thinning, and mantle stripping, and a series of NE-directed structures formed in the northern part of the South China Sea [16,17]. Subsequently, the sea-floor spreading of the South China Sea occurred from the Late Oligocene to the end of the Early Miocene. The expansion ridge first formed in the northern part of the South China Sea, and at the end of the Oligocene, the expansion ridge migrated to the central basin and the southwestern sub-sea basin [18,19,20]. During the expansion of the South China Sea, the Nansha-North Palawan Block, originally located in the northern part of the South China Sea, gradually moved southward and collided with the Northwest Borneo-Sulu Block at the end of the Early Miocene; then the expansion of the South China Sea stopped. The latest IODP349 voyage made an important contribution to the precise determination of the age of the expansion of the South China Sea. The age of the seafloor magnetic anomaly band reveals that the expansion of the South China Sea began at 33 Ma, the ridge transition occurred at 23.6 Ma, and the expansion of the central ocean basin ended at 15 Ma, slightly later than that of the southwest sub-basin, which ended at 16 Ma [21,22]. During the expansion of the South China Sea, a series of Cenozoic sedimentary basins were formed.

The main body of the Wan’an Basin is located in the continental shelf area in the southwest of the South China Sea, as shown in Figure 1. The whole basin is nearly NS distributed in a spindle shape with an area of about 6.6 × 10⁴ km² in Figure 2. It is a fault-depression compound basin formed by the extension and fracture of the crust and modified by strike–slip. Under the influence of the sea floor expansion in the South China Sea and the Wan’an strike–slip fault in the east, the faults in the basin are widely distributed in the Oligocene–Miocene strata. Most of these normal faults, mainly in the NE direction, cut the basement, resulting in the deformation and rupture of the basement in the basin. The Cenozoic tectonic evolution of the basin is broadly divided into three evolutionary phases based on the Xiwei Movement and Guangya Movement, which are the continental uplifting and local rift stretching stage in the Paleocene–Middle Eocene, the fault–depression and retransformation phase in the Late Eocene–Late Miocene and the regional subsidence phase since the Pliocene in Figure 3. The carbonate platform in this area began to develop in the Early Miocene, was active in the Middle Miocene and began to decline and submerge in the Late Miocene. There has been previous work in this area, and it is believed that the carbonate rocks in this area are high-quality hydrocarbon reservoirs.

3. Data and Methods

This study used high-resolution 2D seismic data of about 13,000 km collected from 1989 to 1993 by the Guangzhou Marine Geological Survey and reprocessed in 2015. Seismic acquisition parameters: the shot spacing is 25 m, the track spacing is 12.5 m, the sampling rate is 2 ms and the maximum record length is 9 s. All seismic data are mainly distributed in the Wan’an Basin and the adjacent area. The seismic acquisition grid reached 2 × 4 km in the main development areas of carbonate platforms in the central and southern parts of the basin and 4×8 km in the adjacent areas. According to the layer velocity analysis of the strata, the vertical resolution of seismic data reached 30–50 m. The information on nine wells distributed in the development area of the carbonate platforms provides good support for the interpretation of formation age by well-seismic calibration. In addition, well logging, lithology, mineral composition and biofossils provide valuable information for the sequence division and growth of carbonate rocks.

All seismic interpretations were completed in the Geoframe4.5 software launched by Schlumberger. Seven sequence interfaces were interpreted from bottom to top, T_g (~56.6 Ma), T₆ (35.4 Ma), T₅ (23.3 Ma), T₄ (16.3 Ma), T₃ (10.4 Ma), T₂ (5.2 Ma), T₁ (1.6 Ma) in Figure 3 [23]. The identification and spatial and temporal distribution of the RCPs were mainly completed by interpreting seismic data and analyzing drilling data. In the main tectonic units, the pseudo-wells were extracted through the common central point trace set of seismic data, and the tectonic subsidence history of the carbonate platform development area was analyzed. The estimation of water depth is mainly based on sedimentary facies and micro-body paleontology information in drilling and on drawing tectonic subsidence curves.

4. Result

4.1. Sedimentary Characteristics of Drilling

Based on nine wells drilled in the basin, the sedimentary characteristics of carbonate rocks in each time slice of Wan’an Basin were analyzed in detail through parallel well comparison, and the lithologic assemblage characteristics in different regions in combination with logging information were identified. The findings show that during the Early Miocene, the western part of the Wan’an Basin was dominated by delta deposits in in Figure 4 and Figure 5, with interbedded sand–shale deposits in lithology. In the middle and east of the basin, it was the sand and mudstone deposits in the shore–shallow sea. Well Mia-1 in the north of the basin reveals that carbonate deposits existed in this period but on a small scale and scattered on the periphery of the delta.

Three of the four wells drilled through the Lizhun Formation in the north encountered carbonate rocks indicate that the Middle Miocene was also a main period of carbonate deposition in the Lizhun Formation, as shown in Figure 4 and Figure 5. The 12B-1, DUA-1 and AM-1 wells in the south have encountered carbonate rocks. However, there are some differences in the three wells. There are only two thin layers of carbonate deposits in well 12B-1, interspersed like lenses with sand and mudstone. In well DUA-1, the carbonate and mudstones are interbedded with mudstones, and the thickness of each layer of carbonate is about 100 m. In addition, a large set of carbonate rocks was deposited in Well AM-1. During this period, the content of carbonate rocks in the Wan’an Basin gradually increased from west to east, while the GR values of the carbonate formations in wells DH-1, 4A-1 and DUA-1 were generally low. Moreover, the zigzag change in the logging curve indicates that the lithology of the carbonate rocks in the western part of the basin is impure, which may be caused by the input of some terrigenous clastics from the Indo-China Peninsula.

The GR value of well AM-1 in the eastern part of the basin during the same period is also relatively low, but the curve is box-shaped, reflecting that the carbonate rocks were relatively pure in lithology and had low impurity content. Moreover, the difference in the acoustic curve between the east and the west of the basin reflects that the layer velocity of the eastern carbonate rock is stable compared with that of the western carbonate rock. This velocity difference may have been caused by lithology difference, which is consistent with the GR curve. During the Late Miocene, a small amount of thin reef deposits existed in the northern part of the basin with the form of interlayers in the sand and mudstone, while in the south part, only a nearly 100 m thick reef was identified in well 12B-1; the other three wells revealed that much sand and mudstone had occured in the shallow sea environment. Specifically, the lithology is relatively single and dominated by mudstone, and the corresponding GR is relatively stable, which is very different from the GR curves of wells DH-1 and DH-2, which show zigzagged fluctuation in the Early Miocene. The lithology is relatively simple, mainly mudstone, and GR is relatively stable, which is quite different from the Early Miocene DH-1 and DH-2 GR curves with zigzag fluctuations. On the whole, the reefs were the main form of carbonate deposition in the Kunlun Formation, but the deposition thickness is thin and scattered. From the Pliocene to the Quaternary, the sedimentary environment was stable shallow sea shelf slope deposition, and the content of lithologic sandstone gradually increased from bottom to top, which may have been caused by the increasing input of terrigenous detritus that gradually pushed the shelf eastward.

4.2. Seismic Structures of RCPs

The seismic identification of organic RCPs is determined according to its internal reflection structure, external geometry and contact relationship with the surrounding rocks [25,26,27,28]. In this paper, organic reefs are classified according to their development position and growth morphology. Based on the interpretation of high-resolution seismic profiles, combined with drilling and logging data, there are mainly four types of reefs in Wan’an Basin, point reefs, massive reefs, platform margin reefs and tower reefs. According to the types of slopes around carbonate platforms, three identification marks for carbonate platform boundaries were established, fault interfaces, lithological interfaces and tidal channels. The carbonate platform in Wan’an Basin has good imaging and clear seismic reflection structure. Platform boundaries such as fault interface and lithological interface are more common in the interior of the basin.

4.2.1. Seismic Structure of Reefs

Point reef: The shape is dome and has two nearly symmetrical wings. The top shows continuous strong reflection with disordered internal reflection. Weak bottom reflection and the interface with the lower carbonate platform are little reflected in Figure 6. It mostly develops on the top of the platform and is distributed in the northern part of the basin. Due to its small size (about 2 km in diameter), there is no landslide in front of the reef.

Platform edge reef: The shape varies with the differences in the platform edge, with strong top reflection and good continuity, while the interior has sub-parallel strong and weak reflections, and the stratum at the periphery of the reef has obvious overshoot in Figure 6. They are mainly formed on the steep slopes around the edge of the platform and are mostly composed of carbonate debris in front of the reef, with strong sedimentary but relatively disorderly reflection; they are mainly distributed in the northern part of the basin.

Massive reefs: The shape of the reef is blocky, with continuous strong reflection at the top, one or more continuous in-phase axes visible inside and weak reflection at the bottom, as shown in Figure 7, Figure 8 and Figure 9. They mainly developed on the top of a relatively narrow platform and distributed in the central and southern parts of the basin, and the thickness of the reef has certain differences. Drilling well 12B-1 reveals that the thickness of the massive reef in the south is about 100 m in Figure 5 and Figure 9, and the thickness of the massive reef in the central part of the basin is more than 500 m. The landslide reflection of reef edge is strong, but the continuity is poor.

Tower reefs: Vertical growth is its main characteristic, which is in the shape of a tower as a whole. Inside, multiple continuous and parallel axes of the same phase are common. Continuous strong reflection on the top helps distinguish the weak reflection formation at the top from the weak reflection formation at the bottom, which is connected to the platform in Figure 9. Most of these reefs are developed on the top of isolated platforms with a growth height of over 300 m, indicating that the growth rate of tower reef is consistent with the rise rate of sea level, mainly distributed on the eastern edge of the southern platform in the Wan’an Basin.

4.2.2. Seismic Structure of Carbonate Platform

The carbonate platform in Wan’an Basin is mostly isolated platform, developed on structural fault block, and the edge of the platform is mostly steep slope fault. The overall reflectance of the platform is strong, and the interface between the continuous strong reflection on the top and the weak reflection on the overlying strata is obvious. The internal reflection in the platform is also strong, while the interface between the bottom and the underlying strata is not obvious. A large number of bioclasts were deposited in the platform, showing an obvious medium-continuous and strong reflection in the north, as shown in Figure 6.

At the foot of the platform slope, clastic landslides with strong reflections and cluttered seismic structures are deposited, and there is a clear overshoot at the edge of the platform. The deposition thickness decreased with distance away from the edge of the platform in Figure 7, indicating that the transport capacity of carbonate debris decreased with distance. The carbonate platform in the southern part of the basin has obvious east-west zoning, and these platforms on both sides are developed on the fault block in Figure 8. Due to the growth of reefs on the east side of the platform, both sides of the platform are higher than the topography of the platform, and it is easy to form a lagoon inside the platform. The continuous reflection of strong and weak phases is typical of lagoon deposition, but the lagoon is small in size, as shown in Figure 9. There is an obvious unconformity interface between the strong reflection carbonate platform and the weak reflection overlying layer (T₃), which indicates that the development of the carbonate platform above this interface has entered another phase. The spatial extension of these isolated platforms is limited by faults and the width is only about 5–20 km. The thickness of the platform increases gradually from west to east, and the maximum thickness in the east is more than 1000 m in Figure 5 and Figure 7.

5. Discussion

5.1. Temporal and Spatial Distribution of RCPs

For the sedimentary facies of carbonate platforms in Wan’an Basin, some preliminary analysis was made. However, due to limited seismic data and without wells, the early analyses of sedimentary facies were not convincing. Based on the detailed interpretation of seismic data, combined with regional drilling and logging data and tectonic evolution, this paper makes a detailed identification of RCPs in Wan’an Basin to analyze the sedimentary facies of each phase in this area and to illustrate the spatial and temporal distribution of RCPs.

5.1.1. Early Miocene

During this period, there were four distinct sedimentary facies areas in Figure 10A. The western part was typical delta sedimentary facies. The northwest and southwestern parts of the basin were relatively shallow and in a coastal environment. The middle and east of the basin are shallow sea and semi-pelagic sedimentary facies areas. These four sedimentary facies areas are distributed in strips, and the water body gradually deepens from west to east. In addition, there is also local depositional denudation in the basin. The drilling of well 4A-1 also confirms this feature. During this period, a few isolated platforms were developed in the upper part of the central delta front. Well MIA-1 confirmed isolated platform in the north and smaller isolated platforms in the south. In this period, small platform area and sporadic distribution are the main characteristics of the central basin.

5.1.2. Middle Miocene

The central and southern parts of the basin during this period are characterized by large sedimentary carbonate platforms in Figure 10B. The central and south-central parts of the basin are the main development areas of carbonate platform. The platforms in both areas are distributed along the northern uplift and the central uplift. The slope around the platform is narrow and dominated by steep slopes.

Laterally, the distribution of carbonate platform has obvious east–west zonation, roughly 108°50′ E as the boundary, which is basically consistent with the previous understanding [9,14]. The reef group distributed sporadically at the edge of the platform and was characterized by small quantity and different sizes. In addition, the delta withdrew from the Wan’an Basin during this period, and the western part of the basin as a whole was in shallow sea, while only a few shore-shore deposits remained in the northwest.

5.1.3. Late Miocene

Compared with the Middle Miocene, the dimension of the carbonate platform was greatly diminished, and there was more carbonate and mudstone mixed deposition in the southern part of the basin in Figure 10C. Carbonate platforms in this period were mainly distributed in the central and southern part of the basin and along the middle uplift. The sedimentary range of the peripheral slope of the platform has increased, resulting in the connection of two isolated platform slopes. The southernmost platform of the basin is located on the southeastern and southern slopes of the southern depression and is connected to the “L” reef in the western slope of the Zengmu Basin [9], and there are only two isolated small platforms in the north of the basin. On the whole, although the scale of the platform in the basin is small, it still has the characteristics of east and west zonal distribution. The number and scale of reefs exceeded that of the Middle Miocene, and these reefs were mainly scattered on the steep slopes on the edge of platform.

5.1.4. Pliocene–Quaternary

During this period, under the influence of continental shelf progradation in Figure 6, Figure 7 and Figure 8 the water body in the Wan’an Basin gradually deepened from west to east. The sedimentary facies consisted of shallow-sea shelf sand and mudstone deposits, shallow-semi-deep ocean shelf margin mudstones and semi-deep ocean slopes [29] in Figure 10D. The central and western regions of the basin are almost under the control of continental shelf progradation, and the input of a large amount of terrigenous clasts is not beneficial to the growth and development of RCPs, while the abyssal and semi-abyssal environment in the eastern part of the basin is also not conducive to the development of carbonate platforms. In short, since the Pliocene, there are no more RCPs in the Wan’an Basin. Therefore, this work no longer describes the sedimentary facies of this period.

5.2. Evolution of RCPs

According to the temporal and spatial distribution characteristics, combined with the sequence stratigraphic framework and relative sea level changes in the area, as shown in Figure 3, the evolution of RCPs in the Wan’an Basin was divided into four phases as shown below in Figure 11.

5.2.1. Initial Developmental Phase

During the Early Miocene, the Wan’an Basin was in a coastal and shallow sea environment, which was conducive to the initial formation of carbonate platforms. While the western part of the basin was under the control of estuarine deltas, it was not conducive to the extensive development of platforms. Only a few isolated platforms developed in the high position of the shallow sea in the basin, and there were only a few reefs in this period, as shown in Figure 11A. Although the size and number of the platforms are small in Figure 10A, they are crucial for the development and evolution of the carbonate platform, reflecting the characteristics of the initial development phase of the carbonate platform.

5.2.2. Prosperous Phase

Since the early Middle Miocene, the development of RCPs has entered a flourishing phase with the rising of sea level in Figure 11B. The carbonate platform is widely developed in the shallow sea environment in the middle of the basin, with large scale and large quantity as its main characteristics in Figure 10B. The extremely thick carbonate deposits in Figure 5, Figure 7 and Figure 9 reflect that the growth rate of carbonate rocks is consistent with the rate of rise of relative sea level. The reefs of this period that have appeared are massive reefs, platform edge reefs and point reefs, but the number is small, and they mainly formed on the steep slopes around the platform edges. At the end of the Middle Miocene, the rapid decline of relative sea level resulted in the exposure of organic RCPs in Figure 6 and Figure 7, and the development duration was relatively short.

5.2.3. Recession Phase

In the early Late Miocene, the relative sea level rose rapidly again, and the evolution of RCPs entered a decline phase in Figure 11C. During this period, the main carbonate platform stopped growing, and a few isolated platforms developed in the southern region of the basin in Figure 10C. In the early phase, the high terrain at the edge of the platform created conditions for the organic reef to erupt. The types of reefs are point reefs, massive reefs, tower reefs, etc., and the numerous organic reefs are also the main forms of carbonate rock deposition at this phase. In the middle and late of Late Miocene, the reefs began to shrink on a large scale. By the end of the Late Miocene, the numbers of reefs were relatively small, and only part of the vertical growth atolls remained.

5.2.4. Submerged Phase

Since the Pliocene, the relative sea level has continued to rise, and the main part of the basin is in a deep-semi-deep-sea environment, and there is no longer the formation of RCPs in Figure 11D. Most of the RCPs developed in the Miocene period have been covered by detrital materials from terrestrial sources and entered the burial phase.

5.3. Controlling Factors of RCPs

The development and evolution of RCPs are controlled by a variety of factors [6,7,12]. Based on the spatial distribution characteristics, development and evolution process of the organic RCPs in Wan’an Basin and the lithologic changes of carbonate rocks revealed by drilling, it is believed that the development and evolution of organic RCPs in the study area are mainly constrained by crustal tectonic action and relative sea level change.

5.3.1. Tectonic Effect

Numbers of studies have shown that the influence of tectonics on the formation and evolution of RCPs is mainly reflected in the palaeotopography before platform development and the activities during the platform growth period [2,27,30]. Early intense tectonic movement (such as subduction, collision, compression, etc.) formed some structural high terrain, including fault uplift, palaeouplift and buried palaeohill, which were conducive to the initial formation of RCPs. The late stable tectonic environment was very important for the stability of RCPs [5,31]. The Liyue Movement, which occurred in the Early-Late Cretaceous period, was a relatively important tectonic movement in the early Wan’an Basin [32,33,34,35]. It caused the pre-Cenozoic stratum to suffer strong denudation and folding, and a large number of intermediate-acid igneous rocks intrusive and eruption, forming the basement of Wan’an Basin. Subsequently, under the influence of the Wan’an strike–slip fault in the eastern part of the basin, the most important Xiwei Movement occurred in the Middle and Late Eocene [36]. The fault was active from the late Eocene to the end of Oligocene, and the tectonic blocks formed by the fault created a higher topography for the initial development of RCPs in the Early Miocene. In addition, the uplift–sag pattern controlled by faults limited the spatial distribution of late RCPs in Figure 2 and Figure 10.

In addition, these high-angle normal faults also determine that the slope at the edge of the carbonate platform is dominated by steep slopes in Figure 6 and Figure 7. Since the Early Miocene, the faulting activity in the basin has basically stopped, and the subsequent relatively slow tectonic subsidence provided stable favorable conditions for the mass development of RCPs. In the early Middle Miocene, although the expansion of the South China Sea had ceased, the Nansha block continued to move southeastward and collided with the Borneo block, resulting in the compression of the southern part of the South China Sea and forming an important Wan’an Movement in the Nansha Sea area [20,21]. The compressed movement reached its maximum at the end of the Middle Miocene and caused the overall uplift of the sedimentary strata in the Nansha Sea area and suffered large-scale denudation.

For the Wan’an Basin, the early developed carbonate rocks were exposed and denuded, forming an important T₃ unconformity in Figure 3. The denuded carbonate clasts were mixed with mudstone and redeposited, forming a large mixed sedimentary zone throughout the late Middle Miocene in Figure 10C. Since the Late Miocene, the evolution of the basin has entered the phase of rapid subsidence, and the RCPs have developed again. With rapid subsidence in Figure 11, RCPs rapidly migrated to higher ground. Vertical coral reefs are identified only from the high points of the early platforms. Since the Pliocene, with the acceleration of tectonic subsidence, the whole region is in a semi-bathyal environment, which is not conducive to the growth of RCPs.

5.3.2. Relative Sea Level Change

The cyclic growth of RCPs is closely related to the third-order relative sea level fluctuation in Southeast Asia and the Maldives [27,30]. In general, RCP growth is mainly vertical with the rapid rise of relative sea level. When the relative sea level falls, the accumulation of RCPs is mainly lateral. The tectonic activity of the Wan’an Basin began to weaken in the early Miocene, and the relative sea level became stable in Figure 3, which was conducive to the initial development of RCPs.

In the Middle Miocene, the tectonic subsidence rate was slow, and the overall relative sea level rise rate was slow. During this period, the change of the third plane relative to sea level was multi-cyclic, which is consistent with the multi-cyclic evolution of RCPs revealed by drilling in Figure 4 and Figure 5. In the Late Middle Miocene, influenced by the Wan’an Movement, the whole region was generally uplifted, and the relative sea level dropped by more than 100 m in Figure 3, resulting in the early sedimentary RCPs being fully exposed and eroded to form obvious regional unconformity (T₃). In the early Late Miocene, the relative sea level began to rise, and the RCPs developed again. After that, the relative sea level rose rapidly, as shown in Figure 3, leading to the large-scale shrinkage of the RCPs and entering the decline phase. This was very similar to the decline process of Xisha carbonate platform during the same period.

5.3.3. Terrigenous Debris Input

Under stable conditions, the accumulation of carbonate plants exceeds the rate of relative sea level rise, which is the seemingly irreconcilable contradiction between carbonate platform submerging and relative sea level. Based on the analysis of a large number of outcrops, sedimentary microfacies, geochemistry and the seismic reflection structure of carbonate platforms, it is concluded that the inundation of carbonate platform was mainly the result of terrestrial debris input and water eutrophication [10,14,15,37,38]. Therefore, in order to elucidate the submerged mechanism of RCPs in Wan’an Basin, it is necessary to discuss the paleomarine environment of RCPs except water depth.

The main part of the Wan’an Basin in the southern South China Sea is located on the shelf of the Indo-China Peninsula, and the deposition is mainly controlled by the Mekong River. At the end of the Late Miocene, terrigenous debris began to accumulate on a large scale from west to east in Figure 6 and Figure 7. A large amount of sediment input makes the seawater turbid and reduces its transparency, causing the RCPs that require clean water to suffocate, leading to the inundation of the platforms [14]. This explanation seems reasonable. However, the information from nine wells showed that no terrigenous particles were found on the top interface of the RCPs, and there was no obvious abnormality in the GR curve from bottom to top, which also showed that the lithology of the RCPs was relatively pure in Figure 4 and Figure 5. This is obviously inconsistent with the inundation of the platform caused by the input of a large amount of terrestrial debris. In fact, a large number of studies have shown that in the process of importing terrestrial debris, the transportation speed of nutrients carried by it far exceeds the transportation speed of sediment, and the scope of its impact is also wider [10,14,37]. The results of research on indicators such as clay minerals/feldspar, kaolinite/chlorite ratio and bio-silica content accumulation rate from ODP1143 station showed that the sedimentary minerals at ODP1143 station mainly came from the input of the Mekong River, and it became obvious at 8.5 Ma; this could have been related to the increase in surface runoff caused by the uplift of the Indo-Chinese Peninsula and the strengthening of the East Asian summer monsoon over the same period. In the process of importing terrestrial debris, the nutrients such as phosphate and nitrate carried by it first reach the development areas of RCPs, causing the eutrophication of the water body, reducing the growth rate of RCPs [10,14,15,37]. The rapid subsidence of the basement and the relative stability of the global sea level further accelerated the inundation of RCPs. The intensification of the East Asian summer monsoon lasted 2.3 Ma (8.5 Ma~6.2 Ma), which was consistent with the fact that the organic RCPs began to be covered by terrigenous debris after 5.5 Ma.

6. Conclusions

(1) Miocene reefs in the Wan’an Basin were mainly point reefs, platform margin reefs, massive reefs and tower reefs, mainly developed on the steep slopes of the platform margins and in the Late and Middle Miocene period.

(2) Miocene carbonate platforms are mainly distributed in the northern uplift, central uplift and surrounding areas, and are characterized by east–west zonation horizontally. There are some differences in lithology and depositional environment of carbonate rocks in eastern and western China. The carbonate rocks in the eastern area are relatively pure in lithology and thick in deposition. The western region is close to terrigenous clastic rocks, carbonate rocks contain detrital impurities and the deposition thickness is small.

(3) The development and evolution of carbonate platforms in the Wan’an Basin underwent four evolutionary phases: the initial development phase of the Early Miocene, the prosperous phase of the Middle Miocene, the recession phase of the Late Miocene and the submerged phase since the Pliocene.

(4) During the development and evolution of RCPs, basement faults controlled their spatial position. The rapid subsidence and relative sea level changes since the Late Miocene have controlled the platform development process. The nutrient substances carried during the input of terrestrial debris caused the eutrophication of the water body, which led to the submergence of RCPs.