Significance of microbialites, calcimicrobes, and calcareous algae in reefal framework formation from the Silurian of Gotland, Sweden

The structure of stromatoporoid–tabulate coral reefs from the Mid- to Upper Silurian of Gotland includes complex microbialite and algal framework fabric.Non-skeletal microbialites show variable fabrics, structures, and morphologies ranging from largely non-laminated, peloidal, irregular masses to well-laminated stromatolite crusts, indicated by convex upwardly oriented overgrowth structures, trichome-like arrangement of peloids and intergrowth with skeletal calcimicrobes and metazoans. Non-skeletal microbialites contribute to framework formation during different stages in the Högklint reef development as well as in the marginal, or flanking beds of stromatoporoid–coral patch reefs in the Tofta Formation. In addition to non-skeletal microbialites, calcimicrobes (mainly Rothpletzella) and coralline algae (Graticula gotlandica) substantially contribute to framework formation in reefs from the Högklint and the lower Hamra Formation. Depending on the occurrence pattern and abundance of microbialites, calcimicrobes, and calcareous algae, four framework types can be differentiated based on the environmental setting. Type (1)—a tabular stromatoporoid and non-skeletal microbialite framework, characteristic for lower and intermediate parts of the Högklint reefs, developed in slightly deeper, lower energy, fully marine conditions. Type (2)—a graticulacean-microbial/calcimicrobial framework, typically developed in the upper and top parts of the Högklint reefs, marking shallow, moderate to highly agitated water. Type (3)—a stromatolite–Coenites framework, being part of the reef development in the Tofta Formation. Stromatolites formed in sheltered, partly restricted parts of stromatoporoid–coral patch reef complexes. Type (4)—a stromatoporoid–calcimicrobial framework, forming relatively small reef bodies in the lower Hamra Formation, situated in a highly agitated, very shallow, open marine environment.These examples indicate that microbial crusts, calcimicrobes, and graticulacean algae were more important contributors to framework formation in Silurian metazoan reefs than previously thought.