Calcareous bioconstructions formation during the last interglacial (MIS 5) in the central Mediterranean: A consortium of algal, metazoan, and microbial framebuilders (Capo Colonna - Crotone basin - South Italy)

The last interglacial (MIS 5) transgressive-regressive deposits of the Capo Colonna marine terrace provide a good fossil example of a Central Mediterranean infra/circa-littoral setting, characterized by both calcareous coralline algae-dominated low-relief bioherms and biostromes, equivalent respectively to the modern coralligenous and mäerl habitats. The skeletal primary framework of the bioherms, consists of laminar to massive encrusting coralline red algae acting as main bioconstructors, with minor bryozoans, encrusting foraminifera, and serpulids as secondary frame-builders. Whereas, the autochthonous mäerl tabular beds are mainly composed of free-branched coralline red algae rudstone. A variable amount of sandy bioclastic sediment is laterally interbedded with the bioconstructions and tends to be entrapped in their cavities and pockets. All sedimentary sub-facies of the bioconstructions and associated sediment are rich in autochthonous syn-sedimentary microbial-mediated micrite, forming aphanitic, peloidal, clotted peloidal, and filamentous fabrics. Microbial micrite can also trap and bind a variable amount of grains, or be a secondary component of the sandy sediment as micritic rims surrounding the clasts. All these early-lithified micrites show the typical nanostructure of the primary microbial-mediated carbonates, rather than a detrital mud particles accumulation, as they consist of nanospheres coalescing into subhedral microcrystals that replace both microbial cells (present with several morphological types) and extracellular substances. This in turn implies the original widespread presence of benthic lithifying microbial biofilms which colonized both the cavities of the skeletal framework of the bioconstructions, and the intergranular space of the associated sediment. These microbial communities, thanks to the metabolic processes of the microorganisms that induced the carbonate precipitation, significantly contributed to the early cementation of all the studied deposits.