Microbial-dominated carbonate depositional systems: a biosedimentary and stratigraphic reconstruction in the Late Triassic of Western Tethys (northern Calabria, Italy)

The northern Calabria Norian-Rhaetian carbonate platform system is characterized by three depositional units: early-middle Norian Corvino Unit (CU), middle-late Norian Vaccuta Unit (VU) and late Norian-Rhaetian Grisolia Unit (GU). The CU presents a deep barrier reef consisting of an oligotypic framebuilder community of sponges, serpulids, algae, and stromatolitic microbialites. The intra-supratidal facies mainly comprise flat to low-relief, frequently subaerial-exposed, stromatolites and thrombolites, whereas subtidal facies include high-relief stromatolites and skeletal packstones. In the outer shelf, debrites, turbidites, mudstones, and slumped layers are present. In the VU, bioclastic sand-barriers, inhabited by monospecific bivalve clusters (Isognomon exilis) and low- to high-relief small-scale stromatolites, protected a peritidal domain characterized by flat to high-relief thrombolites and stromatolites. The outer-ramp facies include laminated mudstones and tempestites interbedded with sporadic flat stromatolites. Finally, tempestites and marlstones, characterizing the GU shelf, overlie all previous shallow- to deep-water facies, testifying a terrigenous input and the end of the previous carbonate factory. The Norian oligotypic fossil associations of the CU and VU imply anoxia and eutrophy, likely due to limited water circulation, related to basin isolation from the open ocean. During the whole Norian-Rhaetian interval syn-depositional down-faulting pulses was accompanied by gradual eustatic sea-level drop, inducing episodes of isolation and opening of the basin. Tectonic pulses influenced the sequence stratigraphic architecture creating an atypical system-tracts succession characterized by retrogradation of the CU and progradation followed by retrogradation for the VU and GU.