CROSS FORMATIONAL FLOW IN THE PARIS BASIN

Cross formational flow of water has occurred across several hundred me tres of Liassic mudstone from the Triassic Chaunoy Formation sandstone to the Middle Jurassic Dogger Formation carbonate in the Paris Basin, France. This has been demonstrated by chemical and isotopic data from rocks and formation waters, sampled on a basin scale, from the Dogger and Chaunoy formations. Present-day and palaeoformation waters in the Dogger record input of exotic water in terms of salinity and carbon a nd strontium isotopes. The exotic water was highly saline and containe d isotopically light carbon and Sr-87-enriched strontium in comparison to the indigenous Dogger Formation water. The source of the exotic wa ter can only have been the Chaunoy Formation. Salinity and isotope dat a show that present-day Dogger Formation water contains 10-15% invasio n Chaunoy water. Modelling strontium isotope ratio and concentration d ata from mineral cements shows that over the duration of Dogger cement ation, Dogger palaeoformation waters were replaced by 1-5% Chaunoy wat er. Cross formational flow of water must have occurred by convective m ass flow, rather than diffusion, in order to preserve the characterist ics of water input from the Chaunoy. Flow probably occurred via the ma jor Hercynian fault and fracture systems that run through the centre o f the Paris Basin. Upward flow in this setting is most likely in a com pressional tectonic regime, the situation during much of the Tertiary in the Paris Basin. Thermodynamic modelling indicates that cross forma tional flow from the Chaunoy has probably caused carbonate cementation in the Dogger.