A Study Of Karsts Evolution Around Changjiang Three Gorges

Three gorges lies in the edge of the second hierarchy of the topography, where karsts are quite developing but inadequately researched. Wulong county of Chongqing municipality belongs to the lower reaches of Wujiang and is also in the southeast of Sichuan basin, where carbonate rock bed is widely Dispersed. Wulong County has typical gorges karsts geomorphology. The construction of the karsts geomorphology in Wulong embodies abjectly the rise of the new structure and validly proves the formation of the Three Gorges. However, because of the rise of the new structure in this region, the topography is very broken; in addition that the subtropical climate makes abundant precipitation and the ground surface is therefore is terribly erosed; besides the lack of measure materials, theevolution of the karsts geomorphology in this region never comes to a conclusion.According to the field investigation by route and the analysis of the topographic surface feature by studying the distribution of the karsts geomorphology, topographic maps, and the geologic maps, as well as implementing the GIS, conclusions as the following are drawn:1. Wulong County has a large scale of karsts geomorphology and possesses a unique subtropical mid-mountain karsts gorge characteristic, enriched abundant surface karst represented by cone fengcong-depression, giant doline, gorge, natural bridge, and underground karst represented by giant caves and secondary speleothems.2. Karst geomorphology represents a characteristic of periodically—enlivened. In the dividing ridge far away from modern water-draining ground, grounding integration is still favorable because the geomorphology is not largely affected under the periodically-enlivened, and the karsts geomorphology here mainly represents cone fengcong-depression. However, most region, which is affected by the periodically—enlivened, releases a scenery of core-shaped karsts topography and reflects a youth period of karsts development period. Some places, but in minority, are still in the early-middle period. Therefore, The variety of distribution and combination karst geomorphic features can reflect the evolution history of karstification processes.3. Because of the regional particularity of the rock character, structure, neotectonic movement and the evolution, karsts geomorphology in this region finds its unique characteristics in geomorphologic feature, developing degree and developing regularity. It obviously ranges along the structure line horizontally and distributes with layers vertically.4. The submerging rock bed never has a integrating formation under the reasons that the ground surface is rised for many times by the neotectonic movement; the time of karsts development is very short both in horizontal and vertical directions; and the shale in carbonate rock bed separates groundwater.5.There exists two planation surfaces, three horizontal caves and four steps of terrace in the study area. The higher planation surface, approximate 1700m to 2000m above sea leval distributed in the dividing ridge ,was fomed by residue which peneplain upheaved at the latest period of Early Teriary and named of Exi Surface. The lower planztion surface , named of Shanyuan Surface ,about 1200m- 1500m above sea level ,was fomed in a peaceful period during the Late Teriary when phase II of the Himalayan movement and phase III of the Himalayan movement alternated at the Late Pliocene ; The horizontal caves and terrace may make a contrast and after that moden river system of Wujiang was stably and the channels began incision to form terrace systems. Up to four terrace have been found in the Wujiang low Reaches, dated back to 0.47Ma (T₄), 0.15Ma(T₃), and9-31Ka(T,), respectively.