Karst sinkhole

As mentioned above, tiankeng is a special karst landform. According to the research results of Zhu et al. in recent years and their scientific understanding of karst, the existing karst academic terms are no longer suitable to represent their scientific connotation. In the field of karst science abroad, it is generally called collapse funnel. In different parts of the world and China, local residents have special names for them. For example, it is called "Kukave" or "Kolisevye" in Slovenia, and sometimes it is also called "Vigledi". In Mexico, it is called "Cenote" in Khendan, and some are called "Pit". The place names in China are also different. In Xingwen County, Sichuan Province, it is called "Yanwan", such as the first large sinkhole "Xiaoyanwan". Chongqing Fengjie is called "Tiankeng"; Leye county in Guangxi is called "Shiwei"; In Yunyang, Chongqing, it is called "Dragon Tank"; In Wulong, it is called "Stone Academy". In order to avoid confusion in the use of nouns, it is more appropriate for academic circles to use "karst tiankeng".

Figure 3-4 1 Schematic Diagram of Geological Structure of Shihuadong Formation (according to Dong Demao et al., 2004)

The scientific meaning or definition of "karst sinkhole" put forward by Zhu et al. is that it is a large pit-like negative terrain developed in carbonate rock layer with huge continuous sedimentary thickness and aquifer vadose zone thickness (deep underground water level), which leads from underground to the ground, surrounded by steep rock walls, with depth and plane width ranging from 100 meters to hundreds of meters, and its bottom is connected with underground river during its development period. In addition, whether it is rare, steep, majestic, spectacular and unique in ecological environment in tourism appreciation should also be regarded as one of the important basis to distinguish sinkholes from general funnels or shafts.

The development of tiankeng is closely related to the important karst hydrodynamic system, which is a sign of the existence of important pipeline water flow (underground river) and strong karst intensity in aquifer. Except for a few older ones, most of them are the latest members in the chronological order of karst morphology in their karst areas. In southern China, karst sinkholes are mainly distributed in karst peak areas (funnels, depressions and dry valleys), especially in the Three Gorges of the Yangtze River and its tributaries on both sides (especially on the right bank), Hongshui River (including Beipanjiang River and Nanpanjiang River), Youjiang River and Zuojiang River basins, which are greatly affected by the deep drainage datum. It can be predicted that the super-large karst sinkholes in China should be mainly developed in this area, which is also the most extensive, typical and spectacular karst peak cluster landform area in the world and the only area where super-large karst sinkholes are developed. The distribution of important sinkholes discovered in China is shown in Figure 3-42.

Figure 3-42 Distribution Map of Important Tiankeng in China (According to Zhu, 2003)

The karst sinkholes in China, except Dayan Bay and Xiaoyan Bay in Xingwen, Sichuan, were known earlier, and have been continuously discovered in the joint cave exploration activities and tourism resources investigation between China and foreign countries in recent ten years. There are fengjie county Xiaozhai Tiankeng Group, Wulong County Qingkou Tiankeng Group, Yunyang County Zhongshiyuan and Xiaoshiyuan Tiankeng Group, Yunyang County Longgang Group, Guangxi Leye County Dashiwei Tiankeng Group, Bama County Long Hao and Jiaoletian Pit. The special karst areas, such as the upper reaches of the Pearl River basin, the banks of Wujiang River and the deep valleys on both sides of the Three Gorges of the Yangtze River, are all potential distribution areas of large tiankeng.

1. Xiaoyan Bay and Dayanwan Tiankeng in Xingwen, Sichuan

Xiaoyan Bay and Dayanwan Tiankeng are located in Shilin (Shihaidong Township) World Geopark, Xingwen County, Sichuan Province. According to the research of Zhu et al. (2003), it is a complete Shunhe-Donghe river basin with an area of 40km2. In the north of the basin, there are sandstone and mudstone karst areas such as Leping Fm and Silurian of Lower Permian, and Feixianguan Formation of Lower Triassic in the south, in which limestone and dolomite limestone karst of Qixia Formation and Maokou Formation of Lower Permian are distributed. Through the comprehensive exploration of the sixth Sino-British joint cave exploration in September 1992, the underground river, cave system and its hydrogeological characteristics in the basin were basically found out. There are 89 caves in the karst distribution area, with a total length exceeding 30km. Among them, Tianquan Cave and Zhucaojing are the largest cave systems, with a length of 8 100 m and 8800 m respectively. Both of them ran directly through the Xiaoyan Bay sinkhole. Groundwater flow in karst area is divided into two runoff zones: north and south. The northern line flows into the karst aquifer from the surface river-Shunhe River, comes from the non-karst (exogenous water) area in the west, and flows eastward through the projected bottom of Dayan Bay and Xiaoyan Bay sinkholes and the lowest underground passage in the whole system. This line is clean and pollution-free water. The southern line is mainly replenished by a series of Upper Permian coal measures and pyrite pit water from the west source of Zhucaojing cave system. The water quality is brownish yellow, and the pH is 2.6 ~ 3.9. After the north-south line is merged, it is discharged from Donghe, the main underground drainage outlet of this basin, and the mixed water quality is pH=4.670.

The karst area in Shunhe-Donghe river basin is shallow peak cluster landform. Due to the influence of lithology and gently inclined occurrence, its shape and landscape are not typical and pleasing to the eye, but its aquifer vadose zone thickness is greater than 150 ~ 300 m, the current underground hydraulic gradient is 30 ‰ ~ 53 ‰, and the distribution area ratio of limestone and sand shale in karst area is 1: 1.08, so it has the largest external inflow. These conditions are very favorable for the development of karst pits.

Xiaoyanwan sinkhole has a quasi-circular plane, with a length of 625 meters from east to west and a width of 475 meters from north to south. The surrounding walls are closed by steep cliffs, with a height of 60 ~ 130m, the highest point elevation of 870.0m and the bottom elevation of the sinkhole of 62 1.7m, so its maximum depth is 248.3m and its total volume is 40× 106m3. The east and west sides of the tiankeng are connected with Tianquan Cave and Zhucaojing (Xiangshui Cave) respectively, with a total length of more than 17km. They should be the same underground water flow system in the early days, which is closely related to the Xiaoyanwan sinkhole, and the water level of modern underground rivers has dropped to a depth of 73m below the bottom of the sinkhole (548.0m above sea level).

Dayan Bay sinkhole is only about 400m west of Xiaoyan Bay, with a length of 680m from east to west, a width of 280m from north to south, a depth of 1 100 m and a volume of 15.0× 106m3. Due to the later erosion and destruction, the sinkhole is not complete, the western edge has become a gentle slope, and the sinkhole has been formed under the precipice of the eastern edge, and most of the collapsed stones have been covered by alluvial soil in the later period. It is estimated that a smaller Yanwan sinkhole was formed earlier.

According to 1992 karst plan of Xingwen Stone Forest Scenic Area, it can be clearly seen that the karst development in this area can be clearly divided into two stages: early stage and recent stage. The former is marked by the overall development of Tianquan Cave and Zhucaojing (Xiangshui Cave), and at the same time forms Dayanwan (front) and Xiaoyan Bay (back) sinkholes. The latter formed a modern karst water system with two branches from west to east, namely

Genesis and global comparison of typical geological landscapes in China

From the morphological characteristics, Heping Cave in the west, the underground cave along the river mouth on the surface, the new branch of the upper source in the middle and west of the pig trough well cave system, and the middle and lower veinlets in the Tian Quan cave system are all newly developed independent or superimposed cave systems. The height difference between the two floors is about 55 ~ 75m.

According to morphological observation, in the early stage of the development of Tianquan Cave, the east entrance of Tianquan Cave should be the entrance of surface water system (inflow entrance), not the exit of underground water flow (mainly for the following reasons: the entrance is trumpet-shaped and the scale is particularly huge; Secondly, indicate the flow direction of the flow marks on the upper hole and the branch hole, both in the hole, that is, west or south). Obviously, it is the same cave system as Zhucaojing (Xiangshui Cave), and it is the groundwater flow channel for material output during the development of Dayan Bay and Xiaoyan Bay tiankeng. At that time, the surface topography, surface and underground hydrological system were very different from the current pattern and status quo, and the deep valleys along the surface rivers and the east entrance of Tianquan Cave were not formed. Judging from several major sinkholes found in China, Dayan Bay and Xiaoyan Bay belong to earlier sinkholes, and their ages should be in the early Middle Pleistocene or the late Early Pleistocene. The shape of Dayanwan sinkhole has been seriously destroyed and rebuilt, and its formation sequence should be earlier than that of Xiaoyan Bay sinkhole (Figure 3-43 and Figure 3-44).

2. Dashiwei Tiankeng Group in leye county, Guangxi

Dashiwei Tiankeng Group is located in Bailang Underground River Basin in Leye County, and concentrated in Le Tong Town in the middle reaches of the underground river basin. So far, 26 sinkholes have been discovered, of which Dashiwei sinkhole is the largest and most magnificent (Figure 3-45, Figure 3-46).

Dashiwei sinkhole is pear-shaped, with a length of 600 meters from east to west, a width of 420 meters from north to south, a circumference of 1580 meters, and a maximum depth of 6 13 meters. It is surrounded by cliff traps, and the bottom is colluvial gravel. The accumulation thickness is estimated to be greater than 100 ~ 150 meters. The accumulation terrain is composed of The source of the underground river was closed by the recent landslide, and water flowed out of the rockfill. Six kilometers downstream, it was submerged in a deep crack by the water on the "crack point" of the river bed. Figures 3-45 and 3-46 are the plan and schematic diagram of Dashiwei sinkhole.

Figure 3-43 Xiaoyanwan Tiankeng

Figure 3-44 Xiaoyanwan sinkhole and its nearby cave system (according to Zhu 2003)

Dashiwei Tiankeng Group belongs to all kinds of collapse causes. Among the 26 sinkholes discovered so far, Dashiwei is super-large, with the large ones being Datuo, Dengjiatuo, Chuandong and Baidong, and the rest being small and medium-sized. The characteristics of the main sinkholes are shown in Table 3-7.

Dashiwei Tiankeng Group is developed under the "S" type geological structure, carbonate rock distribution and geological-hydrogeological conditions of the large Bailang underground river basin (Figure 3-47). The thickness of aquifer vadose zone in the concentrated distribution area of Tiankeng Group is 400 ~ 800 m, and the carbonate rocks are thick limestone and dolomitic limestone of Carboniferous and Permian, with gentle stratum occurrence and large karst peaks and deep depressions on the surface. This comprehensive feature of geology, structure, geomorphology and hydrogeology provides superior internal conditions and external environment for the development and formation of Tiankeng Group.

Figure 3-45 Plan of Dashiwei Tiankeng

Figure 3-46 Schematic Diagram of Dashiwei Tiankeng

Table 3-7 Large and Medium-sized Tiankeng in Dashiwei Tiankeng Group-Overview

Figure 3-47 "S" structure and soluble rock distribution are consistent with the three elements of Bailang underground river basin (according to Zhu, 2003).

The following characteristics of Dashiwei sinkhole group are of great significance to the in-depth study of collapsed sinkholes:

1) has the largest number, the most concentrated distribution and the highest density among similar tiankeng in China.

2) It is concentrated in the middle reaches of Bailang underground river, and the underground river moves most frequently (confirmed by cave mapping).

3) The bottoms of tiankeng groups are distributed at different altitudes. There seems to be a general rule that the larger the distribution elevation, the smaller the size of the sinkhole and the earlier its age.