Oligotypic Microbial-Metazoan dominated carbonate platform evolution during upper Triassic rifting (northern Calabria-Italy)

The Norian-Rhaetian of Northern Calabria consists of three depositional sequences: Corvino (Cds) (Early-Middle Norian), Vaccuta (Vds) (Middle-Upper Norian), and Grisolia (Gds) (Rhaetian), representing a carbonate platform system successively drowned (Perri et al., 2003; 2017). In the first two sequences, two different oligotypic fauna/flora associations have been distinguished: microbialites-sponges-serpulids dominate the Cds, whereas bivalves-microbialites dominate the Vds. In particular Cds, interpreted as a deep rimmed platform, is characterized by a deep bioconstructed margin in which a genus of sphinctozoan sponges results the main framebuilders and is associated to serpulids, one species of udotacean algae and microbialites. Inter-supratidal environments in the inner-platform of the Cds are dominated by flat to domal stromatolites and thrombolites, affected by frequent subaerial expositions; in subtidal environments microbialites form high relief bodies sometimes associated with megalodontid bivalves. The outer-platform is mainly characterized by a slope constituted by debrites, mainly turbidites and slumps, associated with pelagic sedimentation. The Vds is interpreted as a ramp-type platform, in which sand barriers protect inner ramp lagoonal to supratidal environments. In particular, a species of isognomonid bivalves (Isognomon exilis) colonized the sand bodies associated with encrusting forams, rare gastropods and low relief microbialites. The latter form relatively high relief buildups in the subtidal settings where serpulids are also present. The outer ramp is characterized by mudstones and storm beds. Lastly, the Gds is characterized by carbonate storm beds, fine-grained turbidites and rare slumps, intercalated to thin bedded mudstones, marls and shales. The fauna/flora assemblage characterizing the Cds and Vds implies general environmental stressed conditions through the whole Norian, characterized by limited water circulation, anoxia, high salinity and eutrophy. These environmental conditions are most probably due to the interplay between climate factors and multiple syn-depositional trans-tensional tectonic pulses, and less to eustatic sea level variations, which induced the opening and the progressive deepening of intra-platform basins (to which these platforms faced) with limited connections with the open ocean. In fact, the three studied sequences can be defined as Tectonically Controlled Depositional Sequences (TCDS) in which sequence boundaries are absent: Cds and Gds represent high stand system tracts, whereas Vds a transgressive system tract.