Comparative grain size analysis of fine-grained sediments: A case study of the Opalinus Clay, northern Switzerland

Grain size variability in fine-grained sediments is a critical factor influencing the physical, geomechanical, geochemical, and hydrological properties of sedimentary rocks. Even small changes in grain size can significantly alter rock behavior. Despite many decades of research on fine-grained sediments in different depositional settings, studies focusing on the comparative and quantitative calibrated analysis of grain size variations in fine-grained Mesozoic sediments are scarce. This study provides the first integrative, quantitative, comparative, and comprehensive analysis of grain size variability in the detrital fraction of the Opalinus Clay from northern Switzerland (Stadel-2 core), which serves as the designated host rock for radioactive waste disposal. The analyses include 2D image analysis, X-ray micro-computed tomography, laser particle size analysis, and sieving/decantation analysis, alongside mineralogical (X-ray diffraction, XRD) and geochemical (X-ray fluorescence, XRF) analyses. Strong correlations between grain size and elemental ratios (Si/Al, Ti/Al, Zr/Al, Zr/Rb) suggest that these ratios can serve as effective proxies for grain size variations in the Opalinus Clay, primarily controlled by the presence of quartz and phyllosilicates. Results evidence the dominance of silt-sized particles in the Opalinus Clay. Distinct trends of grain size coarsening and fining are observed based on XRF-derived grain size proxies, providing the base for further studies on sediment transport processes, including sediment provenance, flow strength, and current dynamics associated with the Opalinus Clay. Results highlight the importance of calibrating grain size variations in fine-grained sediments before their application in the understanding of rock properties and sedimentary processes.