Development and distribution characteristics of karst caves in the Xiangxi river basin

Xiangxi river, a tributary of the Yangtze River, is situated at Xingshan county, Hubei Province ,South China. The river is 97.3 km long and has a catchment of 3,190 km2. Tectonic uplift and subsequent erosion have produced a steep terrain of middle-low mountains and deep ravines that are characterized by complex karst landforms and great topographic relief. The region is located in the southwestern margin of warm and humid subtropical karst, adjacent to the karst demarcation line between North and South-Qinling, and belongs to the transitional zone of north-south karst in China.There are many types of karst caves in China, and many have been investigated and studied in the areas of north and south China. However, there are few reports about karst caves which are distributed in the transitional zone of north-south karst in China. Besides, from the perspective of the watershed, the summaries of the development and distribution of karst caves are still lacking.Through the 1∶50,000 hydrogeological survey in the Xiangxi river karst basin, we investigated and detected the karst caves in this area, and analyzed the size, shape, orientation and height of karst caves and associated rock formations. The results show that the scale of caves is medium. The development of karst caves shows an obvious selectivity to the type of carbonate rocks, with highest cave occurrence in the Cambrian-Ordovician strata. By trace test, the measurement of fissure and the comparison of surface karst morphology, it is found that the orientation of the karst caves is mostly NNE-and NNW-trending, consistent with the regional tectonic line. Affected by the intermittent uplift of tectonic movement, the karst caves are concentrated in four elevation zones, corresponding to the four periods of karst development in the Three Gorges area, of which the most developed karst caves are in the height range of 800-1,200 m. In relation to tectonic locality, the distribution of karst caves is mainly in the southeast wing and axe of syncline, showing a feature of horizontal zoning. Affected by geo-environmental conditions, development of these carves in the region prematurely ended at their early stages, which is different in terms of cave size, type and extension from those of the southwest of Hubei Province. This is a typical feature of karst caves developed in the geographically transitional zone in China.