## 4 Results
### Morphological Characteristics from Seismic Data  
The seismic identification of carbonate platforms is mainly based on their external geometry, internal reflection structure, and contact relationship with the surrounding rock formations [5; 24; 25; 26]. Carbonate rocks are more seismically pronounced than the overlying mudstone or siltstone, and the impedance difference is noticeable. Hence, the top surface of carbonate rock presents a noticeable strong reflection and a good continuity. Most carbonate platforms are directly developed on early sandstone and other rigid basements. The difference in wave impedance is not significant, and the reflection energy of the bottom interface is not vital. Therefore, the seismic response characteristics of the bottom interface are of medium-weak amplitude and medium-low continuity, as shown in Figures 3 and 6. Based on the seismic profiles, the drilling data, the slope type, and the seismic characteristics of the carbonate platform boundary in the adjacent area [27], three identification marks of the platform boundary in the Xisha sea area have been established: abrupt lithological interfaces, fault interfaces, and tidal channels, as shown in Figure 6. Among them, a lithological interface is a sudden change in lithology between the carbonate rock and the mudstone. Its seismic response is characterized by a sudden change from the carbonate area's strong amplitude to the mudstone area's medium-weak amplitude, as shown in Figure 6A. This kind of interface is common in the northern Guangle Uplift and the western periphery of the Xisha Uplift. The sedimentary environment gradually transitioned from a shallow-water continental slope to a bathyal continental shelf. Fault interfaces are mostly located at faults, steep cliffs, or platform margin slopes. The existence of platform margin faults caused a large number of carbonate rock fragments to collapse, and the strata overstepped along the fault plane--as shown in Figure 6B--and later developed into bordered platforms. Such platform boundaries are mainly distributed in the southern Guangle Uplift and the southern Xisha Uplift. In addition, eastern tidal channels are mostly located on the gentle slopes of carbonate platforms. The characteristics of the channels are apparent. The beginning of the channel is the outer boundary of the carbonate platform. Seawater enters and exits the platform along the channel, forming a channel at the platform's edge that is consistent with the descent direction of the gentle slope of the platform. The changes in the platform range and the migration of the channels to land or to the sea form a vertical superposition of multiple channels. The seismic profile shows an abrupt phase transition between the strong continuous reflection of the carbonate platform and the cluttered reflection of the tidal channel, as shown in Figure 6C. The cross-section shows that these tidal channels appear as \"V\"-shaped undercuts with widths of 1.5-3 km, as shown in Figure 6D. These features are similar to the current tidal channels around the tongue bay of Grand Bahama Beach [28], and they are also channels for transporting debris from the platform shallows to the deep-water areas. In this work, using a combination of regional drilling data, sedimentary and tectonic environmental data, detailed interpretation of seismic data, and the three identification marks of the platform boundary, the carbonate platform in the Xisha sea area was systematically identified. Drilling data indicate that the development of carbonate platforms in the Xisha sea area began in the Early Miocene as the Sanya Formation. These isolated platforms mainly developed at the structural high points of ridge-shaped fault blocks but were partially modified by late diagenesis, volcanic activity, and tectonic processes [29]. In the early Middle Miocene (lower Meishan Formation), a stable tectonic environment, a relatively slow-rising relative sea level, and very few terrigenous clastics promoted the full development of the platform [10; 19].