Chapter 8: Formation of a cone karst after erosion of a flysch cover

Résumé - Abstract - Zusammenfassung: Polygonal karsts, especially cone karsts, are typical landforms in tropical carbonate rocks. We describe here a functional model of evolution, linked with the progressive erosion of a flysch cover. The chosen example is in the Wufeng county (Hubei), on the Changleping anticline whose structure is made of Cambrian and Ordovician limestones and dolomites. Along the road going to East / West shafts, you can see three stages of evolution in less than 10 km : 1) a fluviatile stage on the impervious Silurian flysch, with small V valleys ; 2) a fluvio-karstic stage on flysch and limestone (it consists in limestone cones with a flysch head) ; 3) a typical cone karst on pure limestone and dolomites. In the middle of the synclinorium, you can find a fourth stage, older : a cone karst with poljes. This genetic model explains the shape of some Chinese cone karsts. First, it attributes a wide influence to lithology and tectonics. Meanwhile, bio-climatic conditions (discontinuous forest cover, strong seasonal contrasts ) facilitate also the linear erosion in the flysch and the fluvial-karstic digging of the valleys. Moreover, some cone karsts, in China and elsewhere, are not related to an impervious cover, but to a hot and continuously humid climate with a rainforest cover like in Papua New Guinea karsts.

Keys-words: cone karst, impervious cover, flysch, carbonated rock, river erosion, fluvio-karst, lithology, tectonics, climate, soil, vegetation.