Evolution of sedimentary basin

Crustal tectonic movement is the main controlling factor of basin formation and evolution, and episodic movement controls basin evolution from basin formation to basin extinction. The early Early Cretaceous was the first basin-forming period, forming a platform basin, which was closed during the Fujian-Zhejiang movement. The trough basin was formed in the late Early Cretaceous, which is the beginning of the second basin-forming period. The southeast uplift in the late Cretaceous did not close the basin in the west of Lishui-Yuyao fault, but transformed into a wide basin. Trough basin and wide basin are two evolutionary stages of the second basin-forming period, not two basin-forming periods. This conclusion may be controversial, especially in Zhejiang. Because many geologists in Zhejiang believe that Jinqu Basin is a "new basin", the author obtains the continuity of two basins from the inherited composite superposition of early and late Cretaceous basins in South China block and the integrated contact between Fangyan Formation and Hengshan Formation in Duntou Basin in Yangtze block. From small to large or from large to small, it does not mean that the basin has died and re-formed.

In addition to the changes of basin types caused by the episodic movements mentioned above, the periodic changes of basin types in each basin-forming period and even controlled by tectonic movements also belong to the category of basin evolution. Early Cretaceous is a volcanic basin, lacking of sedimentary records, and the evolution of the basin is still unclear. However, the evolution direction of the trough basin of the South China block in the late Early Cretaceous is similar to that of the wide basin basin in the late Cretaceous. * * * is similar in that it consists of four stages of development and evolution: initial basin filling stage → lake expansion stage → lake shrinkage stage → alluvial plain stage; The only difference is that due to the difference of crustal stability, most of the basins in the South China block in the late Early Cretaceous were fault basins, and the sedimentation rate was higher than that in the lake expansion stage, forming a semi-deep-deep lake subfacies stratum; On the Yangtze block, however, the stability is high, and the settlement speed is basically the same as the deposition speed. There is no hungry basin, and the upper part of Lower Cretaceous is dominated by red beds. To the most stable Jiangnan ancient land (southern Anhui), it only developed into an alluvial plain and did not form a lake basin. The fundamental reason is that the basin is directly formed on the crystalline basement and has high stability.

The second basin-forming stage can be divided into three evolution stages, as shown in table 1 1- 1. These three stages can be seen in the Yangtze block, but the third stage has been extended to Paleogene. The activity of South China block is high, and the uplift in the late Yanshan period is large, which is equivalent to the complete loss of Tongxiang Formation and Quxian Formation. On the surface, the contact relationship between Tongxiang Formation and Quxian Formation is not visible, and the underground data is presumed to be false integration. According to the analysis of the integration relationship between Nanxiong Formation and Shanghu Formation in Guangdong Province, it may still be an integration rather than a new basin-forming period.

Some geologists believe that due to the influence of Lanjiang Movement, Fujian-Zhejiang Movement and Lishui Movement, three tectonic basins with different structural lines were formed in the Cretaceous in Zhejiang, namely, Shou Chang Basin with NE direction in the early early Cretaceous, Yongkang Basin with NNE direction in the late early Cretaceous and Jinqu Basin with Nee-EW direction in the late Cretaceous, and the strata of the three basins each formed a structural sublayer. Although they did not clearly put forward the genetic mechanism of this structural line change, judging from the proposal of several structural sublayers and the establishment of Lishui movement, it is undoubtedly based on the compressive or strike-slip stress field produced by orogeny. Li Shenggu (1992) once suggested that the tectonic stress field in the southeast of Mesozoic and Cenozoic basically rotates counterclockwise, so that the directions of folds and faults formed in this way are NE→NNE→NW→EW. This argument is basically similar to the formation sequence of the above three basin types, except that Li's counterclockwise rotation spanned the whole Yanshan and Himalayan periods. The fatal point of this argument is that the base point is set under the control of compressive stress field in Mesozoic and Cenozoic. However, this is quite different from the modern understanding that the stress relaxation in southeastern China has been under the control of tensile stress from the early Cretaceous to Cenozoic, and there is a lack of understanding of the geodynamic mechanism that causes tectonic transfer, and its demonstration basis is insufficient.

When the author studied the Carboniferous lithofacies palaeogeography in southeastern China in 1980s, he also studied the synsedimentary faults in this period. It is found that the fault system in the northern Yangtze block is mainly NE-trending, while that in the southern China block is mainly NNE-trending. In the recently completed mapping of the fault system map of the southeastern continental margin of China, according to the comprehensive study of paleotectonic framework, fault structure types and the control of various fault zones on igneous activities and sedimentary basins, it is considered that many faults in Zhejiang were formed during the subduction of the Nanhai plate in the northwest and the Cathaysian block in the southeast in Proterozoic and the subsequent collision between the two blocks. The subduction of the Yangtze block formed a NE-trending Jiangnan fault system. There were Majin-Wuzhen fault in Bosi period, Xuechuan-Huzhou fault, Changshan-Julie fault, Qiuchuan-Xiaoshan fault and Jiangshan-Shaoxing fault in Jinning period of Zhejiang Province. The subduction of the Cathaysian block formed the NE-trending Wuyi fault system, and there are Yuyao-Lishui fault, Zhenhai-Wenzhou fault and Pucheng-Suichang fault in Zhejiang. Subsequently, the two landmasses collided with each other, forming a nearly east-west zonal fault system, including Quzhou-Tiantai fault, Changhua-Putuo fault and Huzhou-Jiaxing fault (Figure 1 1-2).

After many compressions of orogenic cycles and extensional movements after orogenic period, fault groups as dense as cobwebs have been formed in Yanshan period. Most of these fragile zones in the crust were reactivated in the later crustal movement and became long-term active faults, except a few of them were welded to death by the later magmatic activity. The crustal activity of Yanshan cycle, as mentioned above, entered the relaxation stage after entering Cretaceous; After the Fujian-Zhejiang movement, it turned to the stretching stage. Even during the Fujian-Zhejiang movement, there was still a certain intensity of compressive stress, but due to the revival of a large number of old faults, the structural stress was eliminated and buffered, and the strength of forming new faults could not be reached. Therefore, the Yanshanian fault activity is mainly manifested in the overall revival of old faults, and few new faults, mainly including the NW-trending Songyang-Pingyang fault, Chun 'an-Wenzhou fault, Xiaofeng-Sanmenwan fault and Changxing-Fenghua fault. Table 1 1-2).

Figure 1 1-2 Fault System Map of Zhejiang Province

Cretaceous is the stage when the geodynamic mechanism changed from compression relaxation to extension. The upper volcanic rock series formed after the Fujian-Zhejiang movement is a bimodal volcanic rock series, and the continental red basins at this stage are mostly half graben fault basins, all of which are the information of the extensional stress field and the evidence of the control of the extensional stress field from the eruption of Neogene basic magma to the detachment of Cenozoic Taiwan Province Province and Hainan Island. Therefore, there were three tectonic basins with different tectonic lines in the Cretaceous period in Zhejiang Province, which were established by Lishui movement. This situation of controlling compressive stress does not exist at this stage. It is obvious that sedimentary basins are controlled by faults, but the basin-controlling faults are all these old faults that have been formed. In the early Early Cretaceous, these old faults became the channels of volcanic eruption. In the late Early Cretaceous, due to the conditions of fault depression (unloading under the crust and pressurizing on the crust), when the stress field becomes tensile, these faults will be pulled apart in different degrees, which is convenient for them to collapse on the unloaded high magma chamber and form a basin. The change of geodynamic field is basically simultaneous or quasi-simultaneous in a large range. There is no reason to think that the NE-trending Shou Chang Basin was formed in the Early Cretaceous, because the NE-trending fault is the easiest to pull open, so it is the earliest. There is no reason to think that the NEE-oriented Jinqu Basin was formed in the Late Cretaceous, because the NEE-oriented fault was the most difficult to pull open, so it was formed at the latest. It is more practical to say that different types of basins were formed in different regions than in different periods. The so-called "Shou Chang-type basin", "Yongkang-type basin" and "Jinqu-type basin" are basins in which Shouchang Formation, Yongkang Group, Qujiang Group and their corresponding strata are deposited respectively. The reason why the "Shou Chang-style basin" is NE-oriented is because the Shouchang Formation is only distributed in the Yangtze block, where the basin-controlling fault system is NE-oriented, but the fault in this direction is easy to be pulled into a basin, so it is formed first; Moreover, these faults were not newly born in the early Cretaceous. The basins where Chawan Formation and Jiuliping Formation deposited in the upper part of Shou Chang period in South China Block are not Shou Chang-style NE basins. The reason why the "Jinqu basin" is in the Nee-EW direction is because it is controlled by the Jiangshao fault zone, which was originally in the Nee-EW direction, and was formed by the collision between the Proterozoic Yangtze block and the Cathaysian block, rather than a new basin-controlling fault formed by the Lishui movement as known in geological circles, not to mention the fact that the Lishui movement did not exist at all. The reason why "Yongkang Basin" is NNE is because Yongkang Group is completely distributed in South China block. At the same time as the Yongkang Group, the Hengshan Formation, Lengshuiwu Formation and Zhoujiadian Formation in the neighboring area (northeast Jiangxi) are distributed on the Yangtze landmass, and the Duntou Basin and Xinjiang Basin are not the "Yongkang Basin" in the northeast direction, but the "Jinqu Basin" in the northeast-east direction (see Figure 4- 1).

Table 1 1-2 Main Fault Zones in Zhejiang Province