Diagenesis and Geochemistry of Upper Muschelkalk (Triassic) buildups and associated facies in Catalonia (NE Spain): a paper dedicated to Francesc Calvet

Abstract

Carbonate buildups are well developed in the Triassic Upper Muschelkalk of eastern Spain in the La Riba Unit, but they are completely dolomitised. These mud-mounds with reefal caps have well-developed fibrous and botryoidal marine cements which were probably high-Mg calcite and aragonite originally. The dolomite is fabric retentive indicating an early origin, but the d18O values are quite negative (average -3.‰), interpreted as indicating recrystallisation during shallow burial, but without fabric destruction. Low Sr and Na contents support this. The d13C signature is quite uniform (~ +1‰) and this is probably the inherited, original marine CaCO3 value. The Alcover Unit, deposited between and above the La Riba buildups after a sea-level fall terminated mound growth, is an organic-rich laminated dolomicrite with exquisitely-preserved fossils. The d18O signature (average –3.4‰) is similar to the La Riba dolomites, also interpreted as suggesting recrystallisation. The d13C values, however, show a stratigraphic trend of increasingly negative (to –5.5‰) and then more positive to marine values (~0‰), over a thickness of 10 metres. This is interpreted as a reflection of increasing stratification and developing anoxia, which would have led to the preservation of the special fossils, and then a return to conditions of more open-marine circulation. The dolomicrites of the Alcover unit may well have been formed on or close to the sea-floor. Recrystallisation of the dolomites took place during shallow to moderate burial, with the resetting of the d18O signatures and loss of Sr. The dolomitisation of the La Riba Unit is attributed to circulating seawater, probably driven by sea-level change, during deposition of the Alcover Unit or shortly thereafter.