湘中灰山港—煤炭坝地区岩溶发育特征及其构造控制

湘中灰山港—煤炭坝地区岩溶塌陷发育，但对该地区岩溶发育总体特征及地质背景尚缺乏全面、深入地研究。文章依托1∶5万区域地质调查项目，结合相关资料，对该地区岩溶发育特征及其构造控制条件进行研究与总结。研究区可溶岩主要属于上古生界，少量发育于白垩系—古近系底部。地层潜在岩溶发育强度依次为马平组＞大埔组＞栖霞组和茅口组＞百花亭组（仅指其中的灰岩质砾岩）＞大隆组。平面上可划分为裸露型、覆盖型和埋藏型3种类型岩溶区，其中覆盖型岩溶区可进一步划分为强、中、弱3类。覆盖型和埋藏型岩溶区垂向上可分为浅部岩溶发育带、中部溶洞裂隙发育带和深部岩溶弱发育带。二叠统东吴抬升，中三叠世以来多次挤压和白垩纪—古近纪区域伸展等构造事件形成的褶皱、不同方向断裂裂隙系统、盆地基底构造，以及古近纪以来的差异升降运动等控制了岩溶发育特征。岩溶作用具有三个演化阶段:第一演化阶段形成龙潭组之下的溶孔、溶洞等古岩溶形态；第二演化阶段形成了岩溶低丘地貌；第三演化阶段形成覆盖型岩溶地貌。 

Characteristics of karst development and its structural control in the Huishangang-Meitanba area of central Hunan

Carbonate rocks of the upper Paleozoicare are widely distributed in central Hunan， and karst and karst subsidence are well developed in the Huishangang-Meitanba area of central Hunan， which leads to the karst collapse in mining areas such as Loudi and Ningxiang. Among them， the karst collapse in Ningxiang Meitanba area is particularly serious， and predecessors have carried out much exploring and monitoring work. However， there is still a lack of comprehensive and in-depth study on the overall characteristics and geological conditions of karst development in this area， which affects， to a certain extent， the scientific understanding and prevention of geological disasters caused by karst collapse. Based on the 1:50，000 regional geological survey project and the previous environmental geological survey data， this paper studies the characteristics of karst development and its tectonic control in this area. The soluble rocks in the study area mainly occur in the upper Palaeozoic， with a small amount of development at the bottom of Cretaceous-Paleogene. The average content of CaO in the main soluble rock bearing strata in the study area is 47.71% in Dapu formation， 54.09% in Maping formation， 50.00% in Qixia formation， 50.62% in Maokou formation， 53.09% in Dalong formation and 47.17% in Baihuating formation. Combined with the actual CaO content， the potential karst development intensity of the strata is in the following order， Maping formation＞Dapu formation＞Qixia formation and Maokou formation＞Baihuating formation （only referring to the limestone conglomerate） ＞ Dalong formation. The karst area can be horizontally divided into three types， namely， bare， overburden and burial， among which overburden karst areas can be further divided into three types （strong， medium and weak）. The overlying and buried karst areas can be vertically divided into shallow karst development zone， middle karst cave fissure development zone and deep karst weak development zone. The characteristics of karst development are controlled by the folds formed by tectonic events such as Dongwu uplift of Permian， multiple compressions since middle Triassic and regional extension from Cretaceous to Paleogene， together with fracture systems in different directions， basement structures of basin， and differential vertical movements since Paleogenethe. According to the regional tectonic evolution， the karst development characteristics of different horizons and ages， the analysis of Cretaceous Paleogene underlying basement surface and quaternary system， the karst process and karst landform in the study area have experienced a multi-stage evolution. On the whole， there are three stages of karstification. The first stage is that after the rise of Dongwu at the end of the middle Permian， the sea water retreated， resulting in the exposure of the surface of Maokou formation and the formation of ancient karst forms such as karst pores and caves under Longtan formation. The second stage is from the late middle Triassic to the early Cretaceous. The regional folds were uplifted and subjected to weathering and dissolution， and then the fault subsidence was deposited， forming the karst low hill landform from the middle south to the middle east of the study area. The third stage is the development of modern karst. In the middle and late Paleogene， the rift basins contracted and uplifted， mostly forming covered karst landform， and if partially covered by red beds， forming buried karst. All kinds of structures in the study area have obvious control over the development characteristics of karst. Among them， folds determine the spatial distribution of strata， so as to control the distribution of soluble rocks and karst zones. The water content and water permeability of the fault zone are usually significantly higher than those of the normal stratum； the joint fissures of the surrounding rock near the fault are generally more developed； the fault fracture zone can effectively connect the groundwater in different soluble rock layers separated by non-soluble rock or water resisting layer. Therefore， the karstification intensity of the fault zone and the surrounding rock near the fault zone is higher， and the karst depth is greater. Generally， the larger the scale of the fault zone is， the stronger the karstification is. The karst development in the fault zone and its affected area is strong， and the karst development form is mainly karst collapse pit， karst cave and karst fissure. The long axis direction of collapse pit and karst fissure and the distribution direction of karst development are basically consistent with the fault trend. The joint fissure is a water holding and permeable structure， which is conducive to the dissolution of carbonate rock， so as to form karst forms such as karst ditch， karst tooth， karst fissure and karst pore with developed permeability. Under certain conditions， the joint fissure can control the direction of groundwater runoff.