Characteristics of the Doushuai karst cave in Hunan and its formation and development process

The Doushuai karst cave, is located at the southern end of the Doushuai island in Dongjiang lake of Hunan Province. it is situated in warm and humid subtropical climate zone with an average annual precipitation of 1,645 mm and a mean annual temperature of 17.1 ℃. As the landform is dominated by erosive and corrosive geomorphologic types, the area is less than 500 m amsl and is well vegetated. In order to study the formation and evolution of the karst cave, the lithologies and geological structures in the vicinity of the cave were investigated in detail. The cave morphology and the characteristics of cave sediments were measured. Furthermore, 14C and U-system dating methods were used to estimate the growth rate of sediments. The results show that the Doushuai cave is an isolated karst cave with well-developed speleothems such as stalagmites. The growth rates of stalagmites are slightly faster than that of stalactites. The height and diameter of stalagmites vary from 2 to 26 m and from 0.5 to 5 m, respectively, with a growth rate ranging from 0.24 to 0.50 mm·a-1. The lengths of stalactites change from 3 to 20 m with growth rate varying between 0.20 and 0.40 mm·a-1. In the cave, the tallest stalagnate is 37 m in height; and the longest secondary sediment can reach 12 m high with a diameter of 0.6 m. The formation of the Doushuai cave is a combined result of carbonate rocks, geological structures, water flow and regional tectonic movement, in which faults and fissures provide groundwater flow paths that control the erosion and corrosion of carbonate rocks; and regional tectonic movement plays an important role in slowly uplifting the crustal block which lead to the formation of the cave. In this process, the cave have undergone three stages of development: initial formation phase of 2-0.78 million years ago, development phase of 0.78-0.14 million years ago and stable sedimentation phase from 0.14 million years ago to the present. Most of the sediments have been deposited since 0.14 million years ago and a small amount of new sediment was formed nearly 700 years ago. The secondary sediments were produced around 10 thousand years ago. At present, the cave is in the later period of the stable sedimentation stage. The non-stratification of the Doushuai cave is perhaps caused by the slow ascending of the regional crustal block and the collapse of rocks on the cave roof.