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

Importance of Water‐Clay Interactions for Fault Slip in Clay‐Rich Rocks

“…(2022) showed, based on laboratory experiments, that the permeability of OPA increases during fault slip, which agrees with slickenfibres on fault planes described by Akker et al. (2023). Swelling strain reduces the permeability by closing of cracks (e.g., Bock et al., 2010; Fang et al., 2017; Voltolini & Ajo‐Franklin, 2020; Wenning, Madonna, Kurotori, et al., 2021; C. L. Zhang, 2011), but on the other hand, our data show that swelling stress may contribute to the reactivation of critically stressed faults.…”

“…From an environmental perspective, the permeability of faults in clay-rich formations, especially in rocks such as OPA, is of paramount importance for the disposal of nuclear waste and CO 2 repositories. Wenning, Madonna, Kurotori, et al (2021) and Cappa et al (2022) showed, based on laboratory experiments, that the permeability of OPA increases during fault slip, which agrees with slickenfibres on fault planes described by Akker et al (2023). Swelling strain reduces the permeability by closing of cracks (e.g., Bock et al, 2010;Fang et al, 2017;Voltolini & Ajo-Franklin, 2020;Wenning, Madonna, Kurotori, et al, 2021;C.…”

“…While the permeability is enhanced by fault slip as shown by fault reactivation experiments (Cappa et al., 2022; Guglielmi et al., 2015) and microstructural analysis (Akker et al., 2023), swelling strain is responsible for the closure of cracks, and thus keeps the permeability of clay‐rich rock formations low (Giot et al., 2019; Wenning, Madonna, Kurotori, et al., 2021). However, the contribution of swelling stress to slip initiation of critically‐stressed faults remains elusive and the effect of polar fluids interacting with electrostatically charged clay particles on the frictional properties of clay‐rich rocks is unclear.…”