Middle Triassic mud-mound limestones from Mahmudia, Tulcea Unit – North Dobrogean Orogen, Romania

For nearly three decades, mud-mounds were thought to be essentially a Paleozoic phenomenon. Buildups composed of a mosaic of facies, like for instance the widespread Carboniferous Waulsortians and the Belgian Devonian récifs rouges were virtually considered as mud-mound archetypes. It is only since the middle of nineties that the term mud-mound is widely applied to Mesozoic sponge mounds.
   In the north-eastern part of Tulcea tectonic Unit from North Dobrogean Orogen, around the Mahmudia village are cropping out Middle Triassic limestones, described in the literature as the first occurrence of Middle Triassic mud-mound deposits in Romania. Associated with zebra and stromatactis typical mud-mound structures, there are some carbonate crusts whose origin seemed to be microbial, but are clues to assume that are similar with cemented grainstone crusts.
   The aim of the study is to separate the carbonate microfacies and it will include as methods, optical microscopy, staining and UV fluorescence as keys for sedimentary structures and frequency of allochems, cathodoluminescence (CL), scanning electron microscopy (SEM) and stable isotopes (C, O) analysis for microstructures and diagenetic features and petrography of fluid inclusions for paleoenvironmental conditions. In addition to the optical methods we have also investigated the geochemical composition of selected particles or lamina directly from polished slabs using a microXRF device (Horiba XGT 7000).
   Among the carbonate microfacies separated so far, we can include Tubiphytes boundstone, radiolarian and sponge spicule wakestone with stromatactis, bioclastic grainstone to packestone, dolomitised bioclastic grainstone and laminitic mudstones.
   The microbial nature of the mud-mound is sustained by the abundance of Tubiphytes, which dominate some areas resulting true boundstones, and the presence of Baccinella, a real microbial product. Metasomatism, dolomitization and cementation which affected the primary fabrics given by early marine diagenesis are proved also by the stable isotope analyses. A deep water environment (internal shelf – 70-100 m water depth) for the mud-mound is suggested by the typical wackestone with sponge spicules and calcified radiolarians or calcisferes.