Localised Deformation of Weakly Cemented Sands: A Case Study

Deformation band occurrence in cemented granular materials is intimately related to grain and cement properties. In this work we focus on a Cretaceous sand deposit (France), in which numerous deformation bands have been locally observed. Material from this deposit is used to fabricate artificially cemented samples at the laboratory scale. We principally use two experimental techniques to visualise the inner structure of the natural and artificial samples and comment on the deformation processes: the Scanning Electron Microscopy (SEM) and the X-Ray Computed Tomography (XRCT). So far, microstructural observations on the material far from the deformation bands have revealed that the medium-sized sand grains are held together by weak menisci bonds made of clays. Inside one deformation band we have discovered, instead, sand grains cemented with clays and syntaxial quartz overgrowths. We argue that grain fragmentation in the deformation band is the main silica source for quartz precipitation. The syntaxial overgrowths formation, however, is still weakly justified by the geological reconstructed pressure and temperature conditions within the sand depositional area. Furthermore, in the clay cement we have identified quartz micro-fragments, derived probably from the mentioned grain breakage, and micro-quartz, which may have generated from the interaction between free silica and clay in the pore space.