Abstract
The Opalinus Clay is notable in Switzerland as being the selected host rock for deep geological disposal of radioactive waste. Since the early 1990’s, this argillaceous mudstone formation of Jurassic age has been intensively studied within the framework of national and international projects to characterize its geological, hydrological, mechanical, thermal, chemical, and biological properties. While there is no formal stratigraphic subdivision, the Opalinus Clay lithology is classically divided into several, dam- to m-scale sub-units (or facies), depending on location. Recent multi-proxy studies (combining petrographic, petrophysical, geochemical, and mineralogical analyses) have however demonstrated that high, intra-facies, lithological heterogeneity occurs at the dm- to cm-scale. To constrain this small-scale heterogeneity into distinct lithological units (subfacies), the present study aims at defining and presenting a convenient subfacies classification scheme covering the overall Opalinus Clay lithology across northern Switzerland. Petrographic (macro- and microfacies), mineralogical (X-ray diffraction) and textural (image analysis, machine learning and 3D X-ray computed tomography) analyses are performed on diverse drill cores from the Mont Terri rock laboratory (northwestern Switzerland), and results are extended further to the east (Riniken, Weiach, and Benken). Most of the investigated Opalinus Clay can be described by the use of five distinctive subfacies types (SF1 to SF5), which are visually and quantitatively distinguishable by texture (grain size, bedding, fabric, and color) and composition (nature and mineralogy of components). The five subfacies types can be further refined by additional attributes and sedimentary characteristics (biogenic, diagenetic, and structural). Eventually, the widespread and consistent use of standardized Opalinus Clay subfacies types provides the means to harmonize petrographic descriptions within multidisciplinary research projects, enhance reproducibility of in situ experiments, and further evidence the tight relations between lithology and various rock properties.
Highlights
In Switzerland, the Opalinus Clay (OPA) has been proposed as the potential host rock for deep geological disposal of radioactive waste (Nagra, 2014)
The characterization of smaller, identifiable, lithological units increases the accuracy of correlation studies, which is essential for the elaboration of predictive lithological models
The goal is to define and present a uniform subfacies classification scheme covering most of the OPA lithology across northern Switzerland, from the Mont Terri rock laboratory to the east (i.e., Nagra deep drillings at Riniken, Weiach, and Benken)
Summary
In Switzerland, the Opalinus Clay (OPA) has been proposed as the potential host rock for deep geological disposal of radioactive waste (Nagra, 2014). No formal stratigraphic subdivision has been established to date, the OPA is classically divided (informal) into several, dam- to m-scale sub-units (or facies), which vary regionally and depending on the believes and methods of investigation (e.g., Bläsi, 1987; Matter et al, 1987, 1988; Bläsi et al, 1991, 1996; Wetzel and Allia, 2003; Hostettler et al, 2017; Mazurek and Aschwanden, 2020). The goal is to define and present a uniform subfacies classification scheme covering most of the OPA lithology across northern Switzerland, from the Mont Terri rock laboratory to the east (i.e., Nagra deep drillings at Riniken, Weiach, and Benken). Gypsum commonly forms as an alteration product at the surface of exposed or drilled OPA sections due to the evaporation of pore water at the interface to atmosphere (Mäder and Mazurek, 1998)
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