Biosedimentary and palaeoecological characterization of late Triassic bioconstructions (Western Tethys, North Calabria, Italy)

The Monte Cocuzzo tectonic window, located in the central part of the Catena Costiera Calabra (Calabria - South Italy), hosts a thick Norian-Rhaetian carbonate succession characterized by three different depositional units representing three successive carbonate platform systems of the western Tethys realm: early-middle Norian Lower Complex, middle-late Norian Intermediate Complex and late Norian-Rhaetian Upper Complex. The Lower Complex platform system is characterized by spread and isolated bio-constructed bodies showing an oligotypic community with a primary framework composed by sponges, red algae and corals; and a secondary framework by microbialites, foraminifera and serpulids. Globose clusters of sponges (Olangocoeliidae), encrusting red algae (Solenopora) and microbial crusts, represent the start-phase of the reef construction. In the successive frame-building phase, sponges and red algae progressively decrease in favor of pennulate isolate (Distichophyllia sp.) and colonial (Retiophyllia sp.) corals, the latter forming dendroid fun-shaped tufts (50-60 cm thick). Coral skeletons are encrusted by microbial laminae, serpulids and red algae and are immersed into bioclastic detritus composed by echinoid fragments, bivalves and gastropods tests and rare bryozoans. The primary framework is composed by 41% sponges, 35% red algae and 24% corals, whereas the secondary framework by 41% foraminifera, 35% microbialites and 24% serpulids. The accessory organisms are 19% echinoids, 8% bivalves and 6% gastropods and the rest of the volume is represented by non-bioclastic sediments (39%) and primary cements (28%). The bioconstructions are commonly onlapped by bioclastic packstone grainstones enriched into dasycladacean algae and foraminifera indicating a relatively shallow water environment, probably characterized by disphotic and eutrophic waters as testified by the presence of pennulate corals. These bioconstructions can represent patch reefs located along the carbonate platform margins directly facing an internal restricted basin generally characterized by stressed environmental conditions. In fact, during the whole Norian, syn-depositional down-faulting pulses, accompanied by gradual eustatic sea-level drops, induced repeated episodes of basin restrictions with a consequent limited water circulation and stressed conditions affecting the late Triassic Western Tethyan general carbonate platform domain.