Groundwater silicification in Paris Basin limestones: Fabrics, mechanisms,and modeling

Paris Basin Tertiary formations contain large deposits of lacustrine limest ones, The limestones have a low elastic content and many secondary dewateri ng and pedogenic-paludine fabrics indicating deposition in shallow environm ents. These lacustrine limestones commonly contain cherts that crosscut sed imentary structures. The silicified zones may be pervasive and retain the s tructure and dull aspect of the limestone or form irregularly shaped transl ucent nodules, Quartz is almost the only silica phase present in the cherts , Two main types of silicification occur together: (1) voids partly or enti rely filled with quartz, and (2) limestone matrix that has been replaced by microcrystalline quartz with preservation of most of the primary lime ston e fabric. There is a systematic relationship between silicification and hig h-porosity zones. The replacement of the limestone matrix by quartz is dire ctly connected to voids infilled with quartz. Because the Limestones are pure, without clayey layers, the silica must hav e come from other formations (overlying sands and soils) and been introduce d by groundwater how, In view of the weak solubility of silica in surficial waters, substantial groundwater flow is needed to supply the silica precip itated from the solution, This explains the observed relationships between voids and silicification. A coupled mathematical model (reaction-transport) of this type of silicific ation was used to characterize the physicochemical conditions and to attemp t a quantitative treatment of the phenomenon, Kinetics seem to be the limit ing factor of quartz precipitation in the voids. However, the modeling show s that the kinetics of quartz precipitation limit the development of the si lica replacement, whereas the diffusion of the dissolved species, from the replacement front towards the voids, seems to limit calcite dissolution. Li mestone is replaced by silica, without any increase in the porosity, if the groundwater is close to equilibrium with calcite. The precipitation rate o f the quartz depends on the number of quartz nuclei. The model predicts tha t silica deposition and calcite replacement can be completed in about 10,00 0 to 100,000 years.