Architecture and management of a geological repository - Andra

2 – General Descriptionfollowing the direction of the dip. Its thickness also varies gradually from 130 metres in the South to160 metres in the North of the zone.Vertically, the proportions of the main mineralogical phases vary and are structured into threesedimentary sequences 12 . The upper sequence is characterised by higher carbonate content.The Callovo-Oxfordian argillites were laid down between 158 and 152 million years ago. Thedeposited sediments were gradually compacted over a period of some fifteen million years under theweight of subsequent deposits (at least 500 metres of sediment); water was expelled during thisprocess. At that time, the argillites had acquired properties similar to those observed today, withmovements of fluids and ions becoming very slow. The large-scale tectonic events which subsequentlyaffected the whole of the Paris Baris, in the Cretaceous and Tertiary, did not cause any significantdisturbance to the geological medium.The Callovo-Oxfordian is bound by two large limestone formations, the underlying Dogger formationand the overlying calcareous Oxfordian formation, both containing porous sedimentary horizonswhere water flows. However, these horizons have low permeability and are aquifers in thehydrogeological sense (they do not represent a water resource).Figure 2.3.3Geological cross-section at the site of the underground research laboratory12These sequences express slight cyclic variations in the relative marine level at the time when the layer was deposited.DOSSIER 2005 ARGILE -ARCHITECTURE AND MANAGEMENT OF A GEOLOGICAL DISPOSAL SYSTEM46/495