Microstructural characterization of natural fractures and faults in the Opalinus Clay: insights from a deep drilling campaign across central northern Switzerland

Abstract The Middle-Jurassic Opalinus Clay is the foreseen host rock for radioactive waste disposal in central northern Switzerland. An extensive drilling campaign aiming to characterize argillaceous formation resulted a comprehensive drill core data set. rheologically weak only mildly deformed compared over- and underlying units but shows variety of natural fractures. While these structures are hydraulically indistinguishable from macroscopically non-deformed today, their analysis allows better understanding deformation behaviour geological past. Here, we present an overview different fracture fault types recorded detailed microstructural characterization veins—natural dilational fractures healed by secondary calcite celestite mineralizations. Macroscopic revealed five that encompass tension gashes various orientations with respect bedding small-scale faults displacements typically not exceeding diameter. occurrence generally fits well local tectonic setting sites neighbouring regional zones. investigations vein often polyphase character. Fibrous bedding-parallel veins presumable early age were found be overprinted slickenfibres. nature fibrous parallel slickenfibres supported differing elemental compositions, pointing towards repeated fracturing mineralization events. Direct dating calcites U–Pb was unsuccessful. Nevertheless, constraints can inferred structural slip kinematics. Accordingly, some already formed during sediment compaction Mesozoic times, others possibly relate Early Cenozoic foreland uplift. youngest most likely related Late events, such as Jura fold-and-thrust belt south Hegau-Lake Constance Graben northeast study area. During latest previously acted stiff discontinuities otherwise comparably along which re-localized.