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.