Multi-proxy facies analysis of the Opalinus Clay and depositional implications (Mont Terri rock laboratory, Switzerland)

Abstract

Located in NW Switzerland, the Mont Terri rock laboratory is a research facility primarily investigating the Opalinus Clay as potential host rock for deep geological disposal of radioactive waste. In the Mont Terri area, this Jurassic shale formation is characterized by three distinctive lithofacies: a shaly facies, a carbonate-rich sandy facies and a sandy facies. However, the lithological variability at dm-to cm-scale is not yet fully understood and a detailed lithofacies characterization is currently lacking. Within the present study, petrographic descriptions at micro-and macro-scale, geophysical core logging (P-wave velocity and gamma-ray density), geochemical core logging (X-ray fluorescence) and organic matter quantification (Rock-Eval pyrolysis) were combined on a 27.6 m long Opalinus Clay drillcore comprising the three major lithofacies. The high-resolution investigation of the core resulted into a refinement of the threefold lithofacies classification, and revealed high intra-facies heterogeneity. Five subfacies were defined and linked to distinctive depositional regimes. The studied succession is interpreted as a shallowing-upward trend within a storm-wave-dominated epicontinental sea characterized by relative shallow water depths.

Geochemical fingerprinting of key lithologies and depositional processes across the upper boundary of the Opalinus Clay (Aalenian, Middle Jurassic, northern Switzerland)

“…It might also explain parts of the diachronous, biostratigraphic framework of the OPA/UBU transition (Bläsi et al ., 2013; Hostettler et al ., 2020; Wohlwend et al ., 2019). Hence, the Mont Terri Opalinus Clay succession exhibits two major coarsening‐upwards sequences that can be interpreted as two fifth‐order parasequences during the Opalinum Subzone (Lauper et al ., 2018). Further to the east, the Opalinus Clay is characterised by one overall shallowing‐upwards sequence (third order and fourth order), with higher order, 'Dachbank' cycles occurring within the upper part of the succession, and so, probably younger subsidence pulses provided the new accommodation space (Bläsi, 1987; Burkhalter et al ., 1997; Wetzel and Allia, 2003).…”

“…The petrography and mineralogy of the Opalinus Clay and its upper bounding unit is documented in several reports and publications (Bläsi, 1987; Burkhalter, 1995; 1996; Franz and Nitsch, 2009; Kneuker et al ., 2020; Lauper et al ., 2018; Lerouge et al ., 2014; Matter et al ., 1987; 1988; Mazurek, 2011; Mazurek and Aschwanden, 2020; Nagra, 2001; 2002; Wetzel and Allia, 2003; Wohlwend et al ., 2019). Additionally, new petrographic analyses of selected drill core sections were performed at macro‐scale and micro‐scale.…”

“…The Opalinus Clay lithology varies regionally. In the Mont Terri area, the formation is divided into five lithostratigraphic sub‐units, characterised by three lithofacies: the shaly facies, the carbonate‐rich sandy facies and the sandy facies (Bossart and Thury, 2008; Hostettler et al ., 2017; Lauper et al ., 2018). Further east, the Opalinus Clay is typically divided into five to six sub‐units, tentatively correlatable across the area (Bläsi, 1987; Matter et al ., 1987; 1988; Nagra, 2001).…”

“…Detailed lithofacies and subfacies descriptions and interpretations are provided by Wetzel and Allia (2003) and Lauper et al . (2018).…”

“…The rifting was associated with the formation of fault bounded tilted blocks, which led to a series of local basins by differential subsidence. The resulting depositional area was morphologically differentiated into swells and depressions, contributing significantly to the lateral facies variability (Burkhalter, 1996; Lauper et al ., 2018; Wetzel and Allia, 2000; 2003; Wetzel and Meyer, 2006).…”