The Maastrichtian in The Netherlands is composed of a series of calcarenites and calcisiltites with layers of flints. At some locations, boreholes have revealed the presence of carbonate sands rather than rock. Several scenarios are proposed to explain the observed lack of cohesion and better predict the spatial distribution of the carbonate sands in the Maastrichtian. A 3D Leica microscope, an Environmental Electronic Microscope (ESEM) and a desktop micro-Computerized Tomography scanner (micro-CT scan) have been used to characterize differences in the micro-structures of limestone and carbonate sands samples. Grain crushing, the presence of a mud matrix binding up grains and the lack of a well developed calcite fringe cementation are observed in the carbonate sands and not in the calcarenites. Degradation during core recovery cannot be excluded. Cone penetration and pressuremeter tests in between rigid layers are recommended to detect any layer with an extremely weak cementation and high deformability.

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