关键词: 喀斯特;盘县大洞

Abstract: The Panxian Dadong , with a ma pped passage of 1660m long is a large karst cave system complicated by multi-genesis of morphology. The main passage, 250m× 40m× 26m in volume, contains much relics o f human activities in the Paleolithic Age. Based o n analysis o f some dynamic geomorphical and speleo genetic processes in the cave area, including karstification and polje structure, fossil drainage and speleogenesis, this paper deals with hydraulic-genetic relations on development and evolution between karst and caves. It suggested a evolutional model of the karst landforms and caves in the area, supplemented by paleo-geographic environment data and U-series dating.
Early Pleistocene: The limestone areas around the peak-cluster depression on the Dado ng Hill anticline w ere still covered by basalt, sandstone, shale and co alformations with an active erosion-transportation process. The Shiliping Riv er, as a surface stream at an elevation of 1710-1750m above sea-level, flowed along the fault in the northern part o f the Dadong Hill via the Dadong Col to the w est. With the strong uplift of neotectonics and the active downcutting of rivers, limestione on the river bed was ex posed by erosion and the riv er disappeared underground, through joints in the fault zone, and captured by underground water of the Mabie River System to the south, which gives rise to a sinking stream and hence underground riv er o f the Guanniudong Cave.
Beginning of the Middle Pleistocene: As upstream riv er bed o f the Shiliping further seeped and water of the underground riv er then decreased, the underground riv er became a dry cave, the Guanniudong Cave at present. At the same time, the riv er water found a new way to disappear underground on the eastern slope of the Dadong Hill. One of stream sinks along bedding-plan joints, as a former foot-cave at 1685m above sea-level, collected much upstream water from allogenic area o f classic rocks. The water drained into the Xiaodong via Yinhepo passage and into the Loushuidong via Shuidong passage. The flow - path gradually became a main underground riv er with phreatic features, the early Dadong Cave.
Mid of the Middle Pleistocene: With the further uplift of neotectoincs and a lowering o f base level, the water passage w as enlarged by downcutting in dry-season and undercut- ting a t the level o f the Shiliping polje lake during wet season. Later, water in the passage w as captured by low er leveled rivers and disappeared underground again, a seasonal flooded passage o f the Dadong Cave w as then formed. With alternating changes of cold-warm, dry-humid climate before 300ka, a set of debris by frost weathering and loess by wind process and breakdowns by collapse w ere acumulated at the passage entrance to form a natural dam. The water in the polje lake had to drained along the Huashiban Fault to the southe which gives rise to a lowering o f the polje bottom, about 30cm.
Late of the Middle Pleistocene: With frequent changes o f surface water and under-ground water, caves in the Dadong Hill developed w ell and the Shiliping River in the polje disappeared underground again a t the hill-base to form the low est water passage 20m below the polje floor. Since then onward, the passage was dry , stable and large. The Dadong Cave underwent along time alternation of speleo them deposition and classic accumulation up to the Holocence.

Key words: Karst, Panxian Dadong