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

Abstract

The Opalinus Clay is an argillaceous to silty mudstone formation, notable in Switzerland as the selected host rock for deep geological disposal of radioactive waste. Its upper bounding unit (Passwang Formation and eastern equivalents) is composed of successions of mudstone, sandy bioclastic marl and limestone separated by ooidal ironstone beds. The lithostratigraphic transition is diachronous across northern Switzerland and shows high vertical and lateral lithological variability. To constrain this variability into predictive models, and to identify horizons with properties that could potentially influence radionuclide mobility, the sedimentological and diagenetic processes involved in the genesis of this transition have to be investigated. The present study aims at testing the applicability of X‐ray fluorescence chemostratigraphy to characterise the mixed carbonate–siliciclastic units and understand the complex genesis of the lithostratigraphic transition from the Opalinus Clay towards its upper bounding unit. Sediment drill cores from four locations across northern Switzerland (Mont Terri, Riniken, Weiach and Benken) are analysed using high‐resolution X‐ray fluorescence core scanning. Data are compared to petrographic and additional geochemical data sets (inductively coupled plasma mass spectrometry, scanning electron microscopy with energy dispersive X‐ray analysis, micro‐X‐ray fluorescence mapping) obtained from powdered samples, thin section analyses and drill core slabs. The results demonstrate that the combination of these rapid and non‐destructive measurements along with multivariate data analysis allows the fast and objective classification of lithofacies along complex sedimentary successions. Moreover, it provides quantitative means for differentiating between prominent depositional and post‐depositional processes. The lithostratigraphic transition has been traced by the use of specific elemental proxies as a discontinuity, and its genesis linked to sediment bypassing.

Quantification of Lithological Heterogeneity Within Opalinus Clay: Toward a Uniform Subfacies Classification Scheme Using a Novel Automated Core Image Recognition Tool

“…Source: Mont Terri Project. bioturbation, and diagenesis (Bläsi, 1987;Etter, 1990;Allia, 2000, 2003;Lerouge et al, 2014;Lauper et al, 2018Lauper et al, , 2021. Based on (storm-)wave oscillation structures found in OPA successions, interpreted the average water depth at the time of deposition to be about 20 to 50 m. To provide sufficient accommodation space for the 80-to 130-m-thick OPA (estimated as 2.5 times thicker before compaction; Wetzel and Allia, 2000), differential subsidence has been considered Allia, 2000, 2003;.…”

“…Based on (storm-)wave oscillation structures found in OPA successions, interpreted the average water depth at the time of deposition to be about 20 to 50 m. To provide sufficient accommodation space for the 80-to 130-m-thick OPA (estimated as 2.5 times thicker before compaction; Wetzel and Allia, 2000), differential subsidence has been considered Allia, 2000, 2003;. During the Early Jurassic, the opening of the Tethys and the Atlantic Ocean induced the reactivation of pre-existing basement structures (Permo-Carboniferous troughs: Wildi et al, 1989;Reisdorf and Wetzel, 2018), which in turn led to a depositional area morphologically differentiated into swells and depressions, contributing highly to the OPA facies diversity and thickness variations Allia, 2000, 2003;Nagra, 2002;Wetzel and Meyer, 2006;Lauper et al, 2018Lauper et al, , 2021. The hypothesis of synsedimentary differential subsidence during sediment deposition is strengthened by paleoflow directions (inferred from sedimentary structures) pointing toward OPA thickness maxima, and which are spatially related to the Late Paleozoic basins (Allia, 1996; see also isopach maps in Allia, 2000, 2003).…”

“…It comprises the lithologies residing between the marly section of the Early Jurassic Staffelegg Formation (Reisdorf et al, 2011) and the muddy, calcareous and Feoolitic basal lithology of the Middle Jurassic Passwang Formation (Burkhalter, 1996) and its eastern equivalents («Murchisonae-Oolith Formation»; Bläsi et al, 2013). While the lower lithostratigraphic boundary shows in most cases a clear facies change (e.g., Bläsi, 1987;Reisdorf et al, 2011), the upper limit of the OPA is more transitional in several locations and can be subject to discussion (Wohlwend et al, 2019b;Lauper et al, 2021).…”

“…In addition to the main lithological characteristics defining the above-presented subfacies types, further notable features, such as early or late diagenetic concretions and veins, may locally modify the textural aspect of the rock. The most common features include cm-sized pyritic concretions, sideritic patches, layers and concretions, calcite veins (occasionally mixed with Ba-Sr-sulfates), layers with cone-in-cone structures, and larger carbonate concretions and nodules (Figure 4; see further examples in e.g., Lerouge et al, 2014;Lauper et al, 2018Lauper et al, , 2021. These features occur punctually or repeatedly, typically enriched…”

“…For the sake of this study, these carbonate-rich units are assigned to either (i) silty, limestone beds, or (ii) concretionary/intraclastic calcareous horizons. Detailed analysis, classification and interpretation of some of these units can be found in Wetzel and Allia (2000); Lauper et al (2021) and Lauper (in press).…”