Development of coral–sponge–microbialite reefs in a coated grain-dominated carbonate ramp (Upper Jurassic, eastern Sardinia, Italy)

The Late Jurassic is a peak time of diversification of reefs with corals, stromatoporoids, calcareous and siliceous sponges, and microbialites during the Phanerozoic. This study focuses on the Callovian–Kimmeridgian carbonate succession of eastern Sardinia, deposited at tropical latitudes on the European passive margin that recorded from the late Oxfordian the evolution from a coated grain-dominated to a reef-bearing carbonate ramp. The coated grain-dominated carbonate ramp (phase 1; Callovian–middle Oxfordian) includes inner ramp ooidal shoals and peloidal packstone in the middle-to-outer ramp. The overlying reef-bearing ramp (phase 2; late Oxfordian–late Kimmeridgian) is characterized by three types (1–3) of bioconstructions. The distribution of these build-ups along the middle-to-outer ramp depositional profile reflects bathymetric parameters, related to the interplay of water energy and light penetration. Type 1 build-ups developed in the proximal middle ramp and consist of 45 m thick, 100 m wide, coral-stromatoporoid boundstone associated with coral–stromatoporoid rudstone–grainstone. Type 2 build-ups, colonizing deeper environments in the middle ramp, are lens-shaped coral–calcareous sponge–diceratid boundstone including stromatoporoids and chaetetid sponges, 1–2 m thick and 3–4 m wide, associated with bioclastic packstone–grainstone. Type 3 lens-shaped calcareous and siliceous sponge–coral–microbialite boundstone build-ups (1 m thick and a few metres wide) formed in lower energy, distal middle-to-outer ramp settings. The evolution of the eastern Sardinian carbonate ramp reflects the Oxfordian–Kimmeridgian spread of the coral–sponge-microbialite reefs along the Tethyan European passive margin.