Donghe 92 Summary

Chapter 6 : Karst Infillings and Environment Destabilisation


Résumé - Abstract - Zusammenfassung  : Karst Infillings and Environment Destabilisation. In the mountainous subtropical karsts of Hubei and Hunan, detritic fillings are abundant in almost active and fossil caves. These sediments are formed from allogenic elements like soils. They are related to climatic breaking. We studied two kinds of fillings : old and thick fillings showing an ancient sharp climatic breaking and more recent fillings linked with human impact on environment. In the giant Dadong cave (Wufeng county), two sections were studied. The older one is 25 m thick and is located in an old fossil gallery, 100 m above the sinkhole. The sedimentologic and stratigraphic study of this deposit shows an important hydroclimatic change which could be older than Quaternary. The other deposit is younger : a fossil wood (C14 : 18 590 ± 160 y BP) lies on the bottom of 20 to 30 m of brown laminated deposits. In this section an important change of seasonary rythm of laminated loams was observed. It indicates the appearence of a more active monsoon season towards the summit.
These deposits shows that these subtropical karsts, especially at medium altitude, are very good recorders of the climatic evolution since the end of Tertiary. In-situ neoformations certify the role of sulfo-bacteries and algae in the biosedimentation and registration of paleoclimate under the soil surface. Moreover, some cave fillings contain charcoal (Yanziping syncline, Wufeng county ). These pieces of coal, like the polje fillings, are the proof of the destruction of the forests by man during the last millenaries. These deposits are subactual (beginning of the 20th century and before 1950 by C14). Some are corresponding to a long period of destabilisation due to forest exploitation for metallurgy of steel. Others are witnesses of extensive cultivation of the Mao era.

Keywords: Endokarst, infillings, varves, datings, environment, paleoclimate, deforestation, soil erosion.


Karstologia Mémoires N° 6 Année 1995 DONGHE 92 - ISBN : 2-7417-0162-8