Early diagenesis driven by widespread meteoric infiltration of a Central European carbonate ramp: A reinterpretation of the Upper Muschelkalk

Abstract

Meteoric diagenesis of carbonate ramps is often difficult to interpret and can commonly be confused with other coinciding diagenetic processes. The Middle Triassic Upper Muschelkalk of Switzerland provides an insightful case in which the effects of several overprinting diagenetic environments, including matrix dolomitization, can be clearly unravelled. Previous studies suggested that diagenesis took place in connate marine waters, with later meteoric waters being invoked to explain recrystallization of dolomite. In this study, diagenetic analyses (C-O stable isotope ratios, thin-section point counting, cathodoluminescence and UV-fluorescence microscopy) of calcitic bioclastic samples have revealed that early diagenesis (pre-stylolitization) and the accompanying porosity evolution did not occur exclusively in the presence of marine fluids. Five sequential stages of diagenesis have been identified: marine, shallow burial, mixing-zone, meteoric and dolomitization. Marine diagenesis induced precipitation of bladed and inclusion-rich syntaxial cements that fluoresce strongly under UV-light. Both cements account for a mean 7.5 vol% reduction in the porosity of bioclastic beds. Shallow burial diagenesis likely induced mouldic porosity and associated fluorescent dog-tooth cementation. Based on light oxygen isotope and elevated strontium isotope ratios, matrix aragonitecalcite neomorphism is interpreted to have occurred in a mixture of marine and meteoric fluids. The combination of shallow burial and mixing-zone processes reduced porosity on average by 4.8 vol%. Evidence for subsequent meteoric diagenesis is found in abundant dog-tooth and blocky calcite cements that have mean δ 18 O VPDB of −9.36‰ and no signs of recrystallization. These meteoric

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

“…No facies, structures or allochems escaped dolomitization. The TMD are finely to coarsely crystalline (on average 2 to 40 μm), matrix‐replacive, fabric destructive, anhedral to euhedral dolomites (Adams & Diamond, ). At the top of the Trigonodus Dolomit the TMD are anhedral with crystal sizes around 2 μm (Fig.…”

“…In some boreholes TMD and Hauptmuschelkalk Dolomites are associated with, and enclosed by, poikilotopic and replacive anhydrite cements, along with replacive anhydrite nodules or their late‐dissolution vugs (Adams & Diamond, ). All anhydrite textures decrease in abundance with depth into the Hauptmuschelkalk.…”

“…Hauptmuschelkalk dolomite (HMD) consists of fine‐grained to medium‐grained (60 to 250 μm), sucrosic, planar‐e, replacive rhombs disseminated in the Hauptmuschelkalk and in the base of the Trigonodus Dolomit (Adams & Diamond, ). The HMD generally cross‐cuts facies boundaries but it can also form centimetre to decimetre‐thick beds in bioturbated mudstones.…”

“…The cores of HMD crystals show dull, speckled cathodoluminescence with up to two additional zones in some boreholes (Fig. E and F; Adams & Diamond, ). Under UV illumination, HMD crystals show dull fluorescence, although some rhombs are zoned similarly to TMD crystals at the base of the Trigonodus Dolomit (Fig.…”

“…Stratigraphy of Middle Triassic rocks of the Upper Muschelkalk (modified from Adams & Diamond, ) in Switzerland (Disler, ; Jordan, ; Pietsch et al ., ) and Germany (DSK, ; where DSK denotes Deutsche Stratigraphische Kommission, ). Note the erosional contact between the Lettenkohle and Trigonodus Dolomit.…”