In the process of studying the dynamic evolution of the eastern Nanling basin, the author noticed that the formation and evolution of the sedimentary basin in this area are closely related to the stress environment and tectonic background of the basin. Tectonic background is the foundation of basin formation, and stress environment is the external condition of basin formation. They are controlled by deep geodynamic factors, and their interaction is the key factor to determine the formation mechanism and type division of basins. Therefore, based on geodynamics and plate tectonic theory, combined with basin geometry, kinematics, sedimentology, boundary properties and basin formation mechanism, the basins in the study area are divided into three main types: foreland-like basins, fault basins and rift basins.
1. foreland basin (T3-J 1)
Classical foreland basin refers to the sedimentary basin formed on the continental side of the trailing edge of magmatic arc due to the flattening of the leading edge of thrust plate during the subduction of ocean plate. It is characterized by continental crust basement and thick continental coarse clastic deposits, including syngenetic folds, stepped reverse faults, fault-related folds and detachment folds. This foreland basin caused by thrust rocks is usually parallel to the orogenic belt, and their ages are similar. Although the Late Triassic-Early Jurassic basin in this area is compressional, its basin edge is limited by thrust faults, but its location is far from the plate subduction zone, belonging to the inland area, and the basin area and sedimentary thickness are small, which is different from the foreland-like basin, so it is called foreland-like basin. Its basin contour can be long, round, oval or irregular. Foreland-like basins are mostly distributed at the basement edge, and the molasses at the basin edge is covered on the basement with angular unconformity, and the basin edge is in reverse fault contact (Figure 5- 1). The strata in the basin are annular traps in the horizontal profile and generally symmetrical synclines in the profile. The latest stratum is located in the sedimentary center, which is formed by uniform overall subsidence in the deep part of the basin. The sedimentary topography of the basin is gentle and simple, with small subsidence amplitude, slow speed, thin thickness, coarse grain size at the edge of the basin and fine center of the basin. The basin is "disk-shaped" with symmetrical edges, mostly symmetrical basins, and the sedimentary thickness is basically consistent with the depth of the basin. This kind of basin has a long evolution time, and the strata in each stage are mostly continuous transition. The overall trend is close to EW, but the occurrence of individual basins is not consistent. For example, the early Jurassic or late Triassic basins in Lei Hu basin, Yongding basin, Baidu basin, Luofu basin and northern Anyuan county strike nearly east-west, while Dingnan basin strikes northeast. This kind of basins are widely distributed in the study area, most of them are well preserved, and a few basins are round or irregular due to late uplift, weathering and erosion.
Fig. 5- 1 Early Jurassic compression foreland basin profile of Shaguangping fold-thrust belt in Nan 'an, western Fujian (according to Hou Quanlin et al., 1995)
2. Faulted basin (K2 -E)
A faulted basin is a structural basin formed by extensional rifting after the caprock of the basin is folded, which often has some characteristics of rift formation (Ren Jishun et al., 1990). Most of the faulted basins in this area have undergone large-scale subsidence along the upper wall of a main fault, forming a semi-graben basin (Figure 5-2). Its prominent feature is that it extends in a narrow strip shape on the plane and is asymmetric syncline or monocline on the section. The latter basin is narrow, but the subsidence range is large. For example, the transverse width of Huichang basin is less than 15km, but its sedimentary thickness exceeds 3000 m; The width of Suichuan basin is only over 30 kilometers, and the sedimentary thickness can exceed 6000m (special structural group of Nanling project of the Ministry of Geology and Mineral Resources, 1988). The late Cretaceous-Paleogene basins in this area belong to this type, which are often developed on early foreland-like basins. The basement of the basin is mostly pre-Devonian, and sometimes developed on the basis of Jurassic basins. The long axis and stratum distribution of this kind of basin are mostly NE-SW direction, which is controlled by fault structure and generally distributed along the main fault. One side of the basin is a normal fault boundary, and the other side is an angular unconformity boundary. The basin has large subsidence range, high speed, thick deposition and coarse grain size, which is obviously affected by faults. The asymmetry of the basin is often characterized by "east fault and west overload" (for example, the eastern margin of K2 basin in longnan county is normal fault and the western margin is unconformity) or "west fault and east overload" (for example, K2-E basin in Nanxiong). Most basins migrate from east to west, old in the east and new in the west, with growth faults and marginal facies deposits. The shape of the basin is semi-circular or long, and part of it is divided into equilateral triangles. Because of the long-term activity of basin-controlling faults, the sedimentary environment is unstable, and the sedimentary center is constantly migrating to the main structural belt, which often causes the inner layer of the basin to tilt, forming a monoclinal half-graben half-graben fault basin with a sedimentary thickness greater than the depth of the basin. For example, the late Cretaceous basin in longnan county, Jiangxi Province was elongated in NE direction, the SE boundary was a normal fault inclined to NW, and the NW boundary was sedimentary unconformity; The Cretaceous Nanxiong Basin in Guangdong has a long northeast direction, with normal faults dipping south (east) in the north and sedimentary unconformity in the south (Figure 5-3). The Cretaceous basin in Ji Tai County, Jiangxi Province is irregular rhombus, which is distributed in the northeast, with a normal fault inclined to the northwest in the southeast and sedimentary unconformity on the other three sides.
3. Rift basin (J 1 Late -J2)
Rift basin is a basin formed by the thinning, fracture and subsidence of the crust or lithosphere under tension. Both sides of the basin are normal faults, which extend to the center of the basin to form a graben basin. Rift basin was formed in the geodynamic background of post-orogenic continental cracking, and its formation mechanism is related to the tectonic-magmatic activity after Wilson orogenic cycle, which is in the intraplate geodynamic background, marking the end of Wilson and the beginning of continental cracking. Some late Early Jurassic-Middle Jurassic basins in this area belong to this type, such as Dongkeng in longnan county and Late Early Jurassic-Middle Jurassic basins in Linjiang, Jiangxi Province, which strike near SN, with large subsidence range, slow speed, large thickness, fine grain size and stable sedimentary environment. The sedimentary center is the subsidence center, and the sedimentary thickness is basically consistent with the depth of the basin, and the center of the basin is the thickest. The shape is mainly elongated, and some of them are asymmetric rhomboids, which are mostly caused by rifting. The main difference between rift basin and fault basin is that the basin extends deeply and can penetrate the crust, and the basin has high heat flux and mantle uplift. Basalt, bimodal volcanic rocks and alkaline intrusive rocks often developed in the basin, which represented the cracking and stretching environment at that time. The sedimentation of the basin is controlled by this extensional activity. The bimodal volcanic rocks in the early Jurassic volcanic-sedimentary basins in Rudongkeng and Linjiang represent the cracking and extensional environment when the basins were formed (Chen Peirong et al.,1999); In the Jurassic pyroclastic sedimentary basin of Renju Basin, Guangdong Province, basaltic andesite was developed in the late middle Jurassic, and purple rhyolite and lithic tuff were mainly deposited in the late Jurassic, with a large thickness. In the late stage (upper stage), the K2-E faulted sedimentary basin is mainly composed of brick-red glutenite, siltstone and mudstone, which is in normal fault contact with the sedimentary rocks in the early basin (Figure 5-4), indicating that the basin has entered a cracking and stretching environment in the late Middle Jurassic.
Figure 5-2 Geological Schematic Diagram of Heyuan Basin in Guangdong Province
Figure 5-3 Nanxiong Basin Profile (A-A-A')
Figure 5-4 Geological Schematic Diagram of Rencha Basin in Guangdong Province
In addition to the above three main types, there are other basin types in this area, such as composite basin and incomplete basin, which are the result of late reconstruction or the joint action of various genetic mechanisms.
(1) composite basin: This kind of basin, also known as inherited sedimentary basin, was formed on the basis of early basins and constantly accepted new sediments. It is widely distributed in the study area, and the Mesozoic and Cenozoic basins in the east of Nanling and its adjacent areas are mostly inherited basins. The shape of this basin is changeable. In the later evolution of the basin, it can inherit the characteristics of the early basin, or it may expand or shrink, and the size and shape of the basin have no certain law. According to the different fillings in the basin, it can be divided into two types: volcanic fillings (JBOY3 -K 1) and sedimentary fillings (K2-E). For example, Meixian basin in Guangdong and volcanic-sedimentary basins along the coast of Zhejiang and Fujian inherited the late continental red deposits on the basis of early volcanic fault basins. Generally speaking, the range of Cretaceous red beds in the northwest of volcanic belt is often much larger than that of Jurassic variegated sediments, indicating that Cretaceous expansion is strengthened.
(2) Incomplete basin: The prototype of the incomplete basin in this area is a late Cretaceous faulted basin, which was transformed by late tectonic action after its formation, mainly characterized by strong compressive deformation and structural overthrust at the basin boundary, and the lack of marginal facies deposits at the two edges, which led to the reduction of the basin area. This kind of basin can be divided into three sub-categories: (1) Basin boundaries are mostly reverse faults and normal faults. These faults may have controlled the basin in the early stage, but the activities after the formation of the basin will destroy the integrity of the basin, make it fragmented, or make the original basin smaller, so the shape of the basin is irregular, strip, triangle and island, such as Liu Che Basin in Xunwu County, Jiangxi Province. (2) One side is a reverse fault and the other side is an unconformity interface. The basin is semicircular (such as Xingning Basin in Guangdong Province)-irregular, and the strata tend to tilt to the fault side, so it is also called semi-basin. More common in Jurassic basin, developed in western Guangdong and Jiangxi. Due to the late thrusting and squeezing from southeast to northwest, the basement of the basin was also thrust napped in Jurassic basin. After weathering, Feilaifeng (basement lithology floating on the red bed) and structural window (basement lithology covered with red bed exposed after weathering) can appear, and the scale is extremely small. (3) Weathered residual basin, which is not obviously reformed in the later stage, is mainly weathered and denuded. In the case of thin sediments, the basement lithology is sometimes stripped; If the later uplift is obvious, there is still a small piece left on the hillside. Such basins often appear in areas where volcanic rocks are scarce.
It can be seen that the types of Mesozoic and Cenozoic sedimentary basins in the study area are complex, and their formation and evolution are controlled by deep geodynamic factors. Most basins were formed in tensile stress environment, and the genetic mechanism and types of basins were different in different periods. Foreland-like basins developed in the Late Triassic-Early Jurassic, rift basins developed in the Middle Jurassic, volcanic-sedimentary fault basins developed in the Early Cretaceous, and almost all of them have been dustpan fault basins since the Late Cretaceous. Foreland-like basin, rift basin and fault basin are three basic types of sedimentary basins in this area.