Abstract : Very low-permeable argillaceous rocks like Callovo-Oxfordian claystones or Vraconian siltstones were
chosen to host a research laboratory built to determine the physico-chemical properties of the host
formations for a potential underground disposal of radioactive waste. Knowledge and understanding of
post-sedimentary modifications are of prime importance for definition of these properties; evaluation and quantification of the post-sedimentary changes represent the aim of this study, focused
specifically on the clay material of the sequences.
Samples were taken from two drillings (HTM102 and MAR501). In the HTM102 core samples, illite
and mixed-layers illite/smectite are the dominant clay components of most clay fractions. Systematic
SEM and TEM observations and isotopic K-Ar and Rb-Sr analyses pointed to diagenetic neoformations of carbonates (calcite, dolomite) and clays. For instance, veils and laths of authigenic clay particles around old detrital ones can distinctly be observed. The epoch, duration and extent of the diagenetic activity(ies) are difficult to evaluate because of an overall detrital contribution even in the finest granulometric fractions. However, analysis of a bentonite layer in the sequence provides a diagenetic reference for the authigenic clay material. Correlation between relative sea level and authigenesis of smectite-rich mineral has been outlined. Chemistry of diagenetic fluids also seems to be reliable with sea level variations. These observations argue in favour of diagenetic activities limited in restricted rock volumes.
The case study of MAR501 is close to the HTM102 one: smectite-rich illite/smectite mixed-layers
represent the major component of the clay fraction and K-Ar values argue in the sense of a mixing
between detrital and younger clay populations. Diagenetic glauconites in the sequence yield an age
close to 93,7±0,3Ma for Vraconian level, in agreement with stratigraphical data. The case study of a
clay-filled fault within the sequence favors a confined-sequence behaviour. Indeed, no migrations of
elements and especially of REE took place during fluid flows, the clay minerals preserving their
All the results favor reliable confinment properties of the low-permeable argillaceous sequences.