AN INVESTIGATION, USING THE CHALK KARST O F HAUTE-NORMANDIE (FRANCE),AS AN EXAMPLE OF THE RELATIONSHIPS BETWEEN THE SURFACE AND ENDOKARST USING A GRANULOMETRIC METHOD

Karst, by definition, is the result of rock dissolution. Ii the rock i s not completely soluble, residues will remain (''acquired'' particles ). This insoluble material, present in the springs issuing From the ka rst body after some time lag, provides information regarding karst pro cesses taking place within the rock body. The presence of pathways bet ween the surface and the endokarst is reflected by an increase in the suspended particulate material (SPM) that may be considered to be ''in herited'' from outside of the karst system, By the study of microgranu lometric spectra the origins of the particles are differentiated and, on this basis, a classification of karst systems is proposed. The tech nique was applied to the chalk karat of Haute Normandie (France) by ob taining characterisations of the microgranulometric fraction of the ma in surface formations (clay-with-flints and loess) and that produced b y dissolution of the chalk. By the comparison of these spectra with th ose of the SPM contained in ten karst springs, it was possible to defi ne two types of karsts (''open'' and ''closed'') and their intermediat es. In ''closed'' karst a majority of the particles originated from th e dissolution of the chalk itself, while in the ''open'' karst, the ma jority of the particles are derived from the surface formations. This notion of ''aperture'' is quite different from the conventional alloge nic/authigenic karst classification which implies the formation of an impermeable residual soil that focuses surface water inputs.