Figure 4-36 Balanced Profile of HF99-700 Line in Hefei Basin
(1) Formation and development stage of foreland basin (J)
During the Indosinian movement, the Yangtze plate collided with the North China plate, stitched along the Qinling-Dabie line, and pushed and acted on the North China platform from south to north, resulting in stratum thrust, uplift and erosion, forming the basement of Hefei Basin.
The early Yanshanian movement turned into intracontinental subduction. With the crust thickening and the uplift of Qinling-Dabie orogenic belt moving to both sides, under the adjustment of lithospheric balance, stress relaxation or structural regression occurred in Dabie Mountain, which caused the northern edge of Dabie Mountain (the southern edge of North China Platform) to flex and sink, forming a pre-Dabie Mountain depression basin-Hefei Basin, which accepted Jurassic deposition.
At present, there are different views on the nature of Jurassic basin, which can be summarized as piedmont depression (Petroleum Geology of China, Volume 8, 1992), foreland and regenerative foreland basin (Li Rijun,1997; Luo et al; High, 200 1), back-land basin and its inherited basin (Xu Shuping, 1992) and lithospheric disturbance basin (,et al., 200 1).
Hefei basin is located at the northern foot of East Qinling-Dabie orogenic belt, which is in the structural position of foreland basin. The author calls it depression foreland basin because Hefei basin is obviously different from foreland basin in western China. First of all, Hefei basin was not formed in the same orogenic period, but on the basis of Dabie Mountain pre-depression in the later stress relaxation stage. The second is the lack of marine flysch deposits; Thirdly, at present, a typical sign of foreland basin-syngenetic thrust fault with compressive origin has not been found in the basin caprock. However, from the development position, filling sequence and structural style of the basin, it is very similar to the foreland basin, so it is more reasonable to call Hefei basin in Jurassic sedimentary period a foreland basin.
The Jurassic deposits in Hefei Basin are asymmetrical wedge-shaped, with the piedmont thickness of 4000m in the south and less than1000m in the north. Especially in the Lower Jurassic, it was pointed out at Heshen 3- Huoqiu line.
The earliest deposits in the basin are Lower-Middle Jurassic Sanjianpu Formation (J 1+2) and Fang Hushan Formation (J 1f). The Lower Jurassic is mainly distributed in the south-central part of the basin, superimposed to the north and west, forming the piedmont diluvial facies of Dabie Mountain, which is deposited by brick-red and gray-purple glutenite of Sanjianpu Formation (J 1+2) with a thickness greater than1700 m; In the anti-Hushan area in the south-central part of the basin, the grain size of sediments becomes finer, but it is still dominated by coarse-grained sediments, with a thickness of about 1000m, with thin coal seams or coal lines and carbonaceous mudstone; Lakeside facies and fluvial facies deposits were formed in the northern part of the basin, and shallow lake facies deposits were encountered in Well Ansan 1 with a thickness of 900 m; At this time, the western and northern parts of Huoqiu fault uplift and the northern part of Well Heshen 3 were not affected by sedimentation. The Lower Jurassic is usually characterized by an upward tapering sedimentary sequence. According to the statistics of clastic skeleton composition (Jong Li,1999; Zhou (200 1) and Formation (J 1f) are mainly derived from the cyclic orogenic belt or between the cyclic orogenic belt and the stable block. The source rocks are mainly quartz schist, granite and sandstone, which have a good genetic relationship with Foziling Group.
After the Middle Jurassic, the whole basin was depressed, and the Middle and Upper Jurassic covered the whole basin, and some areas may cross the northern boundary of Hefei Basin. At this time, the development of the basin was in its heyday, and the sedimentary center was located in the southern foreshore area of the basin, which was mainly a set of rapid accumulation in front of the mountain, and the upper part of Sanjianbao Formation (J 1+2) and Fenghuangtai Formation (J3f) were deposited. Fenghuangtai Formation is a set of red thick conglomerate, glutenite mixed with sandstone or sandstone lens with a thickness greater than1800 m; The thickness gradually decreases to the north, which is mainly caused by river deposition. According to the north-south seismic profiles such as HF2000-684 and 667, the middle Jurassic overtook the Huoqiu fault uplift and gradually covered the whole Huoqiu fault uplift, with the thinnest part only about 500 meters, indicating that the Huoqiu fault uplift is an ancient uplift of Jurassic.
(2) foreland basin superimposed strike-slip flexural basin stage (K 1-K2x)
At the end of Late Jurassic, the tectonic stress field in the whole eastern China changed due to the subduction of the Pacific plate and the Indian plate, and entered an active period of tectonic movement, with extremely strong volcanic activity. At this time, a large-scale left-lateral strike-slip translation movement occurred in the Tan-Lu fault zone, which constituted the eastern boundary fault of Hefei Basin. The seventh section of this chapter introduces the relationship between the Tan-Lu fault zone and Hefei basin, and points out that the eastern part of Hefei basin belongs to strike-slip flexural basin at this time. The electrodeless fault on the western boundary also started right strike-slip activity, and a strike-slip flexural basin was also formed in the western part of the basin. Therefore, Hefei basin in this period was superimposed with the characteristics of strike-slip flexural basin on the background of foreland basin.
Figure 4-37A Basin-forming Evolution Model of Hefei Basin
Figure 4-37B Basin-forming Evolution Model of Hefei Basin
In recent years, eclogite gravel has been discovered (Wang Daoxuan et al. ) formed below 80 kilometers underground and existed in Fanghushan Formation of Lower Jurassic. It can be seen that due to the subduction of the Yangtze plate, the Dabie Mountain in Jurassic is dominated by orogeny, with obvious vertical movement, and the vertical rising speed is 0. 12cm/ year (Xu Shudeng, 1994). In the late Jurassic, due to the strengthening of oblique subduction of the Pacific plate, an extensional tectonic environment was created in the southern margin of the basin, which led to a large number of volcanic eruptions and magma emplacement. At this time, the Xiaotian-Mozitan fault may also be in an extensional environment, leading to volcanic eruption and gravity detachment (also known as strike-slip) of the strata carried by Xinyang thrust belt, forming the Xiaotian crater basin. However, this extensional tectonic environment is a kind of tension crack formed under the condition of deep compression and shallow arch, which has little influence on the sedimentation of the basin.
According to the seismic profile, the upper Jurassic and the lower Cretaceous are basically continuous deposits, and the lower Cretaceous and the upper Jurassic are in overlapping unconformity contact. So, how is this overlapping unconformity contact formed? From the seismic profile, no denudation was found in the Upper Jurassic, that is to say, there was no large-scale compression movement in the Hefei Basin in the late Late Jurassic, so it is inferred that the overlap unconformity between the Lower Cretaceous and the Upper Jurassic was caused by the large-scale sinistral strike-slip of the Tanlu fault zone, so the Hefei Basin at this time was greatly affected by the sinistral strike-slip of the Tanlu fault zone.
(3) Late Yanshanian compressional thrust fault (K2x/K2z) and faulted basin stage (K2z-E)
At the end of late Cretaceous, Dabie orogeny occurred again, and the late Yanshan orogeny was strongly reflected in the basin. Seen from the basin, the uplift of arch and thrust existed at the same time in this orogeny, which showed a strong compression thrust from south to north. It is found in the ancient city of Feidong County that the upper Cretaceous Zhang Qiao Formation (K2z) and the underlying Lingdaopu Formation (K2x) are in obvious angular unconformity contact. Near-EW faults, such as Feixi-Hanbaidu fault, Fei Zhong fault and Shouxian fault, are mainly characterized by re-extrusion, thrust, uplift and erosion of thrust surfaces formed along Indosinian period. Today's Fei Zhong fault zone arched and uplifted under the compressive stress at that time, forming a large anticline structure and suffering from strong erosion. Under the later tensile stress, structural exhumation occurred along these faults, and the rising plate in the compression period fell to accept deposition, which controlled the formation of a large fault that declined and expanded in the east-west direction. At the same time, the weak zone along the axis of Fei Zhong anticline was fractured, which caused the Shushan fault to split Fei Zhong anticline in two, forming the present Fei Zhong fault zone. Dashushan and Xiaoshushan in the western suburbs of Hefei are the products of extensional fault activity of Shushan fault in this period. The Tan-Lu fault has obvious extensional activity, and the west side of the fault receives belt-shaped dustpan deposits in the northeast direction. At this time, Hefei basin is a semi-graben-type fault basin with faults in the north and south, which was formed under the extension background. Strictly speaking, the late Late Cretaceous and the Early Tertiary should belong to two rift basins.