Discontinuity networks in mudstones: a geological approach

Although usually masked by a veneer of weathered products, a discontinuity network has been observed in mudstones dated from the Lower Cambrian to the Miocene, principally in pits and quarries. Its origin is attributed to an early hydraulic fracturing of the initial muds with their drainage and progressive compaction. In contrast no natural discontinuities have been observed in the galleries of underground nuclear waste laboratories in Boom Clays at Mol, Belgium, and in Opalinus Clays at Mont Terri, Switzerland, or in deep reconnaissance borings. However, the observations were made immediately after the works. At depth, natural discontinuities are closed by the stress field. As there is no cement and with the temperatures of diagenesis being too low to allow any re-crystallization, they are sealed but not healed. They could reopen with the release of stress and/or desiccation.