湖南怀化盆地岩溶发育特征与分布规律

怀化盆地位于沅麻盆地和雪峰山冲断带2个构造单元接触带上，区内地质构造条件复杂，岩溶强烈发育。文章以区内碳酸盐岩为研究对象，通过资料收集、野外调查、钻探以及测试等工作手段分析空间上的地层、岩性组合特征，从岩溶形态、地层与岩性、构造条件、侵蚀基准面、地貌与水动力条件、可溶岩与非可溶岩接触带六个方面，系统总结怀化盆地岩溶发育特征。结果发现：研究区以覆盖型岩溶区为主，区内各类岩溶形态发育；区内自高向低存在三个溶蚀最发育段，其标高分别为500~600 m、210~270 m以及190~210 m，分别受历史侵蚀基准面、泸阳－怀化－中方谷地局部侵蚀基准面与舞水河侵蚀基准面控制；区内各地层钻孔遇洞率一般为57.1%~61.5%，最高为87.5%；谷地内各地层岩溶发育强度依次为厚层灰岩>白云岩>红层灰砾岩>薄层灰岩，但受地貌、水动力条件影响，溶蚀构造峰脊洼区的白云岩岩溶发育明显较谷地内更为强烈；舞水河沿岸、断层带附近、可溶岩与非可溶岩接触带以及山前冲洪积区等地下水交替循环强烈的地带，岩溶发育相对更为剧烈；区内岩溶强发育区主要分布于泸阳－城东新区、白沙－菜门溪一带。

Karst development characteristics and distribution law in Huaihua basin,Hunan Province

Huaihua basin is located at the center of the marginal areas of Hunan, Hubei, Chongqing, Guizhou and Guangxi. Shanghai-Kunming high-speed railway, Zhangjiajie-Jishou-Huaihua high-speed railway, Huaihua-Guilin high-speed railway (under planning), Chongqing-Huaihua high-speed railway and Huaihua-Shaoyang-Hengyang high-speed railway converge in Huaihua basin in the shape of Chinese character "mi", which makes Huaihua City a new national comprehensive transportation hub.Located in the zone of temperate subtropical monsoon climate, the study area is humid with abundant rainfall. The average rainfall over the year is 1,326.3 mm, with the maximum of 1,663 mm. Rainfall is mainly concentrated in April to August, accounting for more than 70% of the average annual precipitation. The landforms of the study area are mainly middle and low mountains and karst valleys. In the west is Zhongpo mountain-Shiziyan mountain range, and in the east is Liangshan mountain range. The overall topography is high in the northwest and southeast, and low in the middle, showing a basin form spreading in the NNE to NNE. The pure carbonate strata in the study area from old to new sequence are middle Dapu formation of Carboniferous (C₂d), upper Maping formation (CPm), middle Qixia formation of Permian (P₂q), Maokou formation (P₂m), upper Wujiaping formation (P₃w), second section of lower Daye formation of Triassic (T₁d2), and lower Shimen formation of Cretaceous (K₁s). The regional structure in the area belongs to the Neocathaysian tectonic system, which is located in the contact zone of thrust-fold belt between the Yuanma basin and the Xuefengshan. The main structure in the area is the north (north) east compressive structure, and the others are associated structures.The geological structure of Huaihua basin is very complex and karst is strongly developed. However, the previous studies mainly focus on the basic introduction of karst development characteristics in a small area. In addition, the 1∶50,000 regional geological-and-hydrogeological survey has not yet covered the study area. Therefore, the geological research in this area is generally not profound, and the existing geological data is difficult to meet the needs of urban planning and construction. Taking carbonate rocks in the area as the research object, a large number of previous geological data has been collected and analyzed. New geological investigation and drilling have been used to explore the space characteristics of strata and lithology combination. The characteristics of karst development in Huaihua basin have been systematically summarized from six aspects, namely, karst morphology, strata and lithology, tectonic conditions, base level of erosion, landform and hydrodynamic conditions, the contact zones of soluble and non-soluble rocks. Furthermore, Huaihua basin has been zoned according to the development degree of karst.The research findings show as follows,(1) The spatial distribution of soluble rocks in the study area is not uniform, mainly in covering karst areas and partially in buried-karst areas. Solution grooves, stone teeth and sinkholes are developed in the whole area, while the underground river is mainly developed in the tectonically dissolved peak ridge depression in the Baisha-Caimenxi area in the eastern part of the study area.(2) Karst development in the area is mainly controlled by the stratigraphic group, and the next controlling factor is lithology. The karst development intensity of each layer in the valley is ranked as thick-layer limestone (CPm, P₂m, P₂q, P₃w)>dolomite (C₂d)>red-bed gray conglomerate (K₁s )>thin-layer limestone (T₁d 2). However, influenced by geomorphology and hydrodynamic conditions, the karst development of dolomite (C₂d ) in tectonically dissolved peak ridge depression is more intense than that in valley.(3) From high to low, there are three most developed segments of dissolution in the study area, with elevations of 500-600 m, 210-270 m and 190-210 m respectively. These three segments are respectively controlled by base level of historical erosion, base level of partial erosion in Luyang-Huaihua-Zhongfang valley and base level of partial erosion of Wushui river. Among them, the main karst forms in the segments at the depth of 210-270 m and of 190-210 m are karst springs, underground rivers, dissolved pores, solution cracks and caves under the overburden of valley.(4) The karst development is more intense in areas with strong alternating groundwater circulation, such as the banks of Wushui river, the fault zone, the contact zone between soluble and non-soluble rocks, and the piedmont alluvial area. The karst area is mainly distributed in Luyang-Chengdong new area and Baisha-Caimenxi area in the southeast of Luyang valley. If engineering activities such as groundwater exploitation, drainage of foundation pit, pile foundation construction, tunnel construction, etc. are planned to carry out in these areas, special risk assessment of geological disasters caused by karst collapse in construction site is needed.