Dolomitization by hypersaline reflux into dense groundwaters as revealed by vertical trends in strontium and oxygen isotopes: Upper Muschelkalk, Switzerland

Abstract

The Trigonodus Dolomit is the dolomitized portion of the homoclinal ramp sediments of the Middle Triassic Upper Muschelkalk in the south‐east Central European Basin. Various dolomitizing mechanisms, followed by recrystallization, have been previously invoked to explain the low δ18O, high 87Sr/86Sr, extensive spatial distribution and early nature of the replacive matrix dolomites. This study re‐evaluates the origin, timing and characteristics of the dolomitizing fluids by examining petrographic and isotopic trends in the Trigonodus Dolomit at 11 boreholes in northern Switzerland. In each borehole the ca 30 m thick unit displays the same vertical trends with increasing depth: crystal size increase, change from anhedral to euhedral textures, ultraviolet‐fluorescence decrease, δ18OVPDB decrease from −1·0‰ at the top to −6·7‰ at the base and an 87Sr/86Sr increase from 0·7080 at the top to 0·7117 at the base. Thus, dolomites at the top of the unit record isotopic values similar to Middle Triassic seawater (δ18OVSMOW = 0‰; 87Sr/86Sr = 0·70775) while dolomites at the base record values similar to meteoric groundwaters from the nearby Vindelician High (δ18OVSMOW = −4·0‰; 87Sr/86Sr = >0·712). According to water–rock interaction modelling, a single dolomitizing or recrystallizing fluid cannot have produced the observed isotopic trends. Instead, the combined isotopic, geochemical and petrographic data can be explained by dolomitization via seepage‐reflux of hypersaline brines into dense, horizontally‐advecting groundwaters that already had negative δ18O and high 87Sr/86Sr values. Evidence for the early groundwaters is found in meteoric calcite cements that preceded dolomitization and in fully recrystallized dolomites with isotopic characteristics identical to the groundwaters following matrix dolomitization. This study demonstrates that early groundwaters can play a decisive role in the formation and recrystallization of massive dolomites and that the isotopic and textural signatures of pre‐existing groundwaters can be preserved during seepage‐reflux dolomitization in low‐angle carbonate ramps.

Facies and depositional environments of the Upper Muschelkalk (Schinznach Formation, Middle Triassic) in northern Switzerland

“…Considering the underlying supratidal facies and basin morphology, the shoal must have retrograded eastwards towards the Vindelician High during a seawater transgression. As the facies overlying the Kaistener Schichten have been eroded, the final environments of the Upper Muschelkalk are unknown; however, microfacies of the Kaistener Schichten show anhydrite cementation, which indicates the formation of an environment capable of producing anhydrite-supersaturated and dolomitizing brines following the deposition of the oolite shoal, in agreement with Adams et al (2019).…”

“…These shoals separated the tempestite-rich mid-ramp from a sheltered backshoal lagoon (Alesi 1984). Following the deposition of low-energy backshoal sediments, a basin-wide sea-level fall formed lenticular, nodular and chicken-wire sulphates, and desiccation cracks at the top of the Upper Muschelkalk (Schauer and Aigner 1997;Adams et al 2019). In many regions in the south-eastern CEB, the final sulphate-rich facies of the Upper Muschelkalk have been eroded away during the deposition of the overlying brackish/terrestrial Lettenkohle unit (Franz et al 2015).…”

“…Chertified microbial-laminates are uncommon in modern supratidal settings; however, they are common features of ancient tidal flats (Shinn 1983). The upwards increase in evaporite textures and microbial laminated sediments results from increasing evaporite cementation during dolomitization (Adams and Diamond 2017;Adams et al 2019) and an upward transition from peritidal to supratidal environments. Based on the sedimentary evidence, LFA 1 is interpreted to represent a (Alesi 1984;Schauer and Aigner 1997;Braun 2003;Koehrer et al 2010).…”

“…Despite the recognized connection between Upper Muschelkalk facies and reservoir properties, and in contrast to the many investigations of facies in the German Upper Muschelkalk, the Upper Muschelkalk of Switzerland has been the focus of only one descriptive facies study (Merki 1961). In light of the extensive deep-drilling campaigns by Nagra (National Cooperative for the Disposal of Radioactive Waste) and SEAG (Schweizerische Erdöl AG) since the publication of Merki (1961), the recent revelation of significant diagenetic differences between Switzerland and Germany (Adams and Diamond 2017;Adams et al 2019), and the current interest in geo-energy applications of sedimentary reservoirs (Chevalier et al 2010), a revaluation of the facies of the Swiss Upper Muschelkalk is warranted.…”

Integrated bio- and cyclostratigraphy of Middle Triassic (Anisian) ramp deposits, NW Bulgaria

“…Desiccation cracks and tepee structures mark episodes of subaerial exposure, indicating the establishment of tidal flats under a semi-arid climate. These climatic conditions are favorable for microbially-mediated dolomitizaton of microbial mats (e.g., Petrash et al 2017) and/or early-diagenetic reflux dolomitization (e.g., Adams et al 2018). However, no detailed geochemical studies were preformed and the possibility of late-diagenetic dolomitization cannot be excluded either (e.g., Lukoczki et al 2019).…”