Abstract

Moderately deep-marine Toarcian sedimentary rocks of the Médianes Nappe (Alpine Front Ranges, Switzerland) comprise calcarenites and lime-mudstones. They were deposited under storm and fairweather conditions, in the dysaerobic zone, near the storm wave base. These remnants of the northern margin of the Tethyan Ocean show facies where primary sediment ('deep water' stromatolitic micrite) was reworked during early diagenesis. Stromatactis-like cavities were formed by carbonate dissolution near the sediment-water interface. Suboxic early diagenetic conditions are indicated by corroded pyrite and newly formed glauconitic oncoids. Carbon and oxygen stable isotope data indicate cavity stabilization and cementation above the sulphate-reducing zone. Oceanographic conditions during the time of cavity development are reconstructed from the carbon and oxygen isotope composition of cavity cements, of bulk sediment and of associated ammonites.δ 13C values of up to +4‰ are related to the Oceanic Anoxic Event of the Early Toarcian and oxygen-isotope values from cements and ammonites define a shallow Toarcian thermocline (<100m) and surface-water temperatures up to 25°C.

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