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Karst networks can be considered as zones of alteration, storage volumes of eroded material and also as zones of transit. Study of sedimentary fillings in karst allows to determine past hydrodynamic conditions as well as sediment sources. In order to better understand chalk karst genesis differents sedimentary fillings from Northwestern Paris Basin have been studied: Petites Dales, Villequier II and Orival caves. Geochemical (major elements) and grain size analysis explain the sedimentary variability both within and between sites. This can be explained by the variation of two main parameters: (i) the origin of karst sediments and (ii), the hydrodynamic conditions in the karst system. Karstic sediments can origininated (i) from allochtonous materials associated with surperficial deposits (quaternary loess, tertiary sand deposits, or a mixture of them), or (ii) from autochtonous material related to chalky residual deposit (in situ weathering). Three main deposit conditions are distinguished according to the sedimentological characteristics: flood, low flow and medium hydrodynamic conditions. A conceptual model is proposed (i) to explain hydrosedimentary conditions in undercovered chalk karst and (ii), to relate surface (plateaus) and underground (karst) geomorphological comparments in term of sediment reworking, transport and sedimentation.

Abstract: Breakthrough tailing has been observed during dye-tracing recovery tests in the Norville aquifer system (chalk), France. Karst-conduit flow and transport parameters were assessed using two different interpretative methods: the linear graphical method and the Chatwin method (implemented in the Qtracer2 program). The linear graphical method was used to model the observed tailing effects, which was explained by a second smaller delayed breakthrough curve. By comparing the results of tracer-test interpretation for the two methods, it was possible to relate the area of this second curve to the importance of turbulent flow in spring discharge. The more turbulent the flow, the less important the contribution of the second breakthrough curve and the tailing effect. The observed tailing could possibly be controlled by hydrodynamics to a greater extent than usually expected, the tailing effects being mostly attributed to diffusion phenomena. Tailing effects were expected to increase with discharge and the piezometric level, which would have resulted in overpressure in conduits, fissure flooding, etc. Instead, breakthrough tailing tended to disappear with increasing aquifer discharge, which would support the hypothesis of there being mostly hydrodynamic-controlled tailing effects instead of matrix- or fissure-diffusion.

Im Kreidekarst des westlichen Pariser Beckens liegt die „Petites Dales“ Höhle die beispielhaft Material zum Studium der Sedimentationsfüllungen des Pariser Beckens liefert. Untersucht wurde die Lithologie der Karstsedimente sowie deren chemische Zusammensetzung. Ebenfalls untersucht wurden die möglichen Sedimentquellen: Kreidegesteine, erodiertes Material aus Ton und Kieselsteinen, Löß. Die Ergebnisse zeigen das Vorhandensein dreier Hauptsedimentationsfamilien im “Petit Dales” Karst: braune, tonige Schluffe, beige Schluffe, helle Schluffe. Der Großteil dieser Sedimente lässt sich auf Lösserosion zurückführen. Einzig die braunen, tonigen Schluffe stellen mit einem geringen Anteil Überreste der Kreideverwitterung dar. Aufgrund der Ergebnisse wurden drei Modelle zum hydrologischen Sedimentationsverhalten des “Petit Dales” Karstsystem konzipiert.