Gebihe 89 Contents

Chapter 9: THE KARST SEDIMENTS

Résumé -Abstract - Zusammenfassung -:The karst deposits studied are concretions (speleothems and travertines), lithified detritic deposits (conglomerates) and the large underground detritic series of Dadong (Wufeng/Hubei). The speleothems of the subtropical karsts of China are very important in the stepped fossil systems. They are the environment recorders. Morphologies (e.g.  columnar, meshed saccharoid, mosaic calcites) and crystalline structures (e.g. microstromatolitic, balled, oncolitic) especially provide information  about the infiltration conditions. The detritic contaminations (grains of soil, hydroxides, eolian dusts) provide information  about the soil withdrawings, i.e. the unsteadying of the pedologic and vegetal cover ("rhexistasie"). The distribution diagram of 58 U/Th isotopic datations shows an important concretion phase during the upper Pleistocene, especially during the Eemian. However, it seems that the speleothems formation has been relatively unceasing.
 Some cave sediments are preserved by the consolidation of the detritic fillings (breccia, pudding stone). The petrographic study of the skeleton parts provides information about the origin of the deposits and about the original watershed. The calcitic cements are varied, e.g. travertinous (tuffs of acicular crystals), meshed, micritic saccharoid, etc. The fluviatile conglomerates, located on several levels (e.g. Daxiaocaokou/Zhijin-Guizhou), provide information about the karstification hollow  during the Tertiary uplift.
 The rhythmic series of Dadong (Wufeng county), 20 to 25 m thick, is located in the fossil level of the Dadong sinkhole-cave (+ 100 m). It shows an alternation of  gravel beds and  layers of varved loams. This sedimentary site is a record of the regional hydroclimatic regime and testifies to  climatic alterations lasting several centuries or millenias. The important red colouring of the gravels and varved seams (hydroxides resulting from the pedologic alteration of the schistous Ordovician) testifies to soil erosion and withdrawing. This process must have been triggered off by a climate with very contrasted dry and wet seasons.

Keywords : Karst fillings, speleothem, travertine, U/Th dating, detrital deposit, conglomerate, breccia, loam, clay, rhythmic sedimentation, recorder, indicator, soil, alteration, erosion, climatic change, paleoclimate.