Microgranulometric approach to a chalk karst, western Paris Basin, France

By definition, karst is the result of dissolution, and if the rock is not completely soluble, residues will remain ("acquired" particles). These insoluble residues provide a history of karstic activity and can be found in the outflows after a possible storage period in the endokarst. A direct connection, even if temporary, between the endokarst and the surface is reflected by the contribution of particles, which are referred to as "inherited". We have studied the chalk karsts of Haut Normandie by comparing microgranulometric spectra of suspended matter (SM), in subterranean waters and in solutions of the main surface formations (Clay-with-Flints Complex (CFC) and loess) and the Chalk dissolution products of all local stratigraphic levels. Based on these microgranulometric spectra, we propose a conceptual model for processes occurring in chalk karsts and a classification scheme for karstified systems according to how such systems deal with particles. In a "closed" karst, the suspended matter tends to come from the Chalk itself, while in the case of an "open" karst, the majority of suspended particles comes from surface formations. This notion of "openness" differs from the currently used categorization into allogenic and autogenic systems, which depends upon an impermeable cover concentrating the infiltration. (C) 2002 Elsevier Science B.V All rights reserved.