Caves and subterranean deposits of the Charente karst groundwater (Touvre Basin, La Rochefoucauld). "Ghost-rock" speleogenesis, karst records of Pleistocene and Holocene, role of site effect
This PhD aims to study caves and subterranean karst infillings deposits from Charente (Touvre catchment) as a memory of past climate and environments. From the concept of site effect, the two studied caves (Bois du Clos and Fuie) show an original recording of the regional palaeoclimatic conditions of the Middle-Upper Pleistocene and of the Holocene. The prominent role of the ghost-rock processe during the long geological time of Lower Cretaceous (45 Ma) was demonstrated by the study of three quarries, drilling and the young cave of La Fuie. This alteration process explains the genesis of labyrinthic caves and also the complexity of the Touvre aquifer which supplies the city of Angoulême. Indeed the large water reserve is situated into the porous rock (slow drainage) while a small part is drained by the karst conduits (fast drainage coming from river losses). The geochemical, sedimentological and mineralogical studies of detrital infillings deposits into Bois du Clos and la Fuie allowed to characterize hydrosedimentary dynamics and palaeoenvironmental conditions between marine isotope stages 6 and 3, obtained by 14C et U/Th in speleothems of Bois du Clos cave. The microchemical imagery by X-ray fluorescence has highlighted the existence of some carbonated levels related to the process of speleogenesis by ghost rock and the major role of groundwater level fluctuations. The compared chronostratigraphic study of the regional speleological and archaeological sequences finally allowed reflecting the role of local and regional settings, as well as global parameters in palaeoclimate.