3.2.2. 1 Central Guangxi Depression Belt
The thickness of sedimentary caprock in late Paleozoic is several thousand meters to ten thousand meters, and the lithology and lithofacies of strata change greatly. In addition to carbonate formation, there are single continental detritus and siliceous rocks containing iron, coal and manganese. Triassic sandstone or carbonate strata are scattered and hundreds to thousands of meters thick; The Cretaceous is mainly composed of red continental debris, which is tens to hundreds of meters thick. In Yishan-Luzhai and Yongfu-Wuxuan areas, submarine faults occurred in late Devonian and early Carboniferous, which not only controlled the changes of lithofacies and thickness at that time, but also affected the distribution of later structures and the development of karst. The folds formed by Indosinian movement are mostly short-axis anticlines, and a few are long-axis inversion folds. The structural direction is mainly north-south, and the rest are locally developed. For example, in the area from Huanjiang to Yongfu, caprock folds and faults are mainly distributed in the northeast, and some of them bend to the west, slightly arc-shaped. The area near Yishan is an east-west arc fault fold belt. The folds seen are mostly destroyed by faults, and the dip angle of strata changes 10 ~ 60. All these have different degrees of influence on karstification. Control the occurrence and development of various karst forms. At the same time, it is also common to produce karst transformation along faults on the basis of tectonic rocks to form karst rock series.
3. 2. 2. 2- Northeast Guangxi Depression Belt
On the basis of early Paleozoic geosyncline flysch and flysch formation, after Guangxi movement, a late Paleozoic depression area was formed, but the subsidence amplitude was not large, and the carbonate rocks of middle-upper Devonian and Carboniferous were 2000 ~ 3000 m thick. The regional structure is dominated by the north-south arc structure protruding westward, with long axis backs and synclines appearing alternately, and normal faults and reverse faults parallel to them are quite developed. Duzhongling and Haiyang Mountain in the east, Dayaoshan area in the south and Qiaoling in the southwest are all uplift in the early Paleozoic. Although they are terrigenous material supply areas in geological history, it is difficult to find their material composition from the late Cretaceous karst construction, but mainly the residual of local surrounding rock dissolution and erosion. Therefore, its position and role in the late Cretaceous karst stage remains to be explored.
3.2.2.3 Yunkai Tailong District
Yunkaidashan area, located in the southeast of Guangxi, has gradually uplifted since the middle and late Caledonian cycle and has continued to this day. Among them, metamorphic rocks, migmatites and granites in different periods of Lower Paleozoic are widely distributed and have complex structures. Late Paleozoic Devonian and Carboniferous platform caprock were found in the northwestern margin of the uplift. The lower part was composed of single continental clastic rocks, and the upper part was composed of carbonate rocks and a small amount of coal-bearing formations, with a thickness of more than 3,000 meters. Due to the Indosinian movement, folds and faults appeared, most of which were long-axis, inclined or reversed, and were mostly destroyed by NE-trending, NW-trending and N-S-trending faults, and the dip angle of strata was generally between 20 and 60. Since Mesozoic, Shuiwen, Liu Ma, Hepu and other fault basins have been formed under the influence of fault block movements with alternating compression and extension. Sedimentary Jurassic, Cretaceous and Paleogene red complex continental debris, molasses-like rocks and basic-acid volcanic rocks, in some areas there are coal-bearing and gypsum-bearing salt layers, with a thickness of hundreds to 3000 meters. The structure is relatively simple, and most of them are widely distributed short-axis oblique or equiaxial oblique, and the dip angle is 10 ~ 35.
In view of the above structural characteristics, the uplift area is not karst, but is analyzed from the related factors of geological and historical development. What is the direct or indirect relationship between it and karst development in different periods in adjacent areas is still a question worth discussing.
Residual geosyncline in Qinzhou, 3.2.2.4
It is distributed in the northeast Hualixi geosyncline in Qinzhou-Yulin area of southeast Guangxi. Silurian-Early Permian is mainly composed of flysch, manganese-bearing siliceous rocks and argillaceous rocks, and some of them are slump turbidite geosyncline deposits with a thickness of more than 10,000 meters. Only in the Darongshan-Cenxi Tang Bo area in the central uplift belt, the early and middle Devonian once became a coastal environment, sedimentary clastic rocks and carbonate rocks. The Wu Dong movement caused the Ordovician-Lower Permian to fold into mountains, accompanied by magmatic activity. From the Late Permian to the Early Triassic, molasses and heterogeneous continental clastic rocks with a thickness of nearly 10,000 meters were deposited in the piedmont depression zone in Xiaodong, Qin Zhou, Fangcheng. The Indosinian movement caused strong folds and fractures, accompanied by magmatic activity. Since Mesozoic, late Triassic-Paleogene continental basins such as Shiwandashan, Pingji, Dongping and Shatian have developed along the fault zone. The regional structures are generally NE-trending, and some are E-W or NE-trending. From Paleozoic to early Triassic, most strata formed close composite folds or inversion folds, and the central uplift zone was short-axis folds. Most continental basins form gentle and open synclines. Faults are characterized by multi-stage activities, which directly control the formation of Paleozoic sedimentary lithofacies, Mesozoic-Cenozoic basins and magmatic activities. Because the area is dominated by insoluble rocks, the local karst phenomenon is not typical.
3.2.2.5 Youjiang regeneration geosyncline area
It is located in the west and southwest of Guangxi, accounting for about 1/3 of Guangxi's area. The development of geological structure has experienced the evolution process from geosyncline to platform to regenerative geosyncline. That is, the early Paleozoic is the transitional area between the Caledonian geosyncline in South China and the Yangtze paraplatform. The Cambrian in Longlin, Xilin and Napo areas is dominated by carbonate rocks, with an exposed thickness of 797 1m, while most other areas are geosynclinal compound structures with a thickness of about10000m. After the Guangxi movement, from Devonian to Permian, due to mantle uplift, the crust expanded slightly, and nearly east-west and some northeast tensile fault zones developed, which were eroded when exposed to the sea during the Guizhou-Guangxi movement. Later, in Maokou period, due to seawater intrusion, limestone veins and limestone "bodies" containing breccia were deposited, as seen in the carbonate rocks of Middle and Upper Devonian in Lingyun Shendong area. At the end of the late Paleozoic, the platform was transformed into an geosyncline, and the Triassic was transformed into a regenerative geosyncline. The early and middle Triassic was dominated by composite geosynclines, and some of them were carbonate rocks, argillaceous rocks and basic-acidic volcanic rocks. The sedimentary thickness was 1500 ~ 9000 m, and the Indosinian movement strata folded up to the land, and the late Triassic-Cretaceous sediments were largely missing. In this area, folds and faults are developed, the tectonic line is mainly NW-trending, and the Triassic folds are tight, with the characteristics of compound, linearity and inversion, and strike-slip faults are developed. The folds in carbonate rock area are relatively open, including simple box-shaped, drawer-shaped, arch-shaped folds or domes, as well as complex multi-height anticlines, with faults developed in the axial and wing parts.
In short, under these structural backgrounds, the vertical karst morphology in the carbonate back (oblique) area is easy to separate, and tubular and irregular karst strata are developed. The syncline area is easy to form a combination of catchment (flow) karst forms, and horizontal tunnels and side ditches are developed. Karst structures are easily connected or distributed in a strip or line shape intermittently. Fault zones, especially regional multi-stage active faults, often form a tectonic-karst catchment, and different karst types of fault basins, residual hills, residual mountains and karst formations are distributed intermittently in this area, which has obvious directionality and companionship.
Karst is mainly developed in carbonate rock distribution area in platform depression area and Youjiang regenerated geosyncline area in central and eastern Guangxi. It reflects the structural characteristics of thick carbonate rocks formed by basin subsidence in late Paleozoic and tectonic movements in Indosinian and Yanshan periods, and is an important controlling factor of karst in late Cretaceous.