Growth of rigid high-relief patch reefs, Mid-Silurian, Gotland, Sweden

Patch reefs up to 35 m thick and generally 100-150 m wide, separated by bed ded inter-reef sediment, dominate the Hogklint Formation (Lower Wenlock) of north-west Gotland. The spacing between adjacent patch reefs is variable, but is commonly 150-350 m. The Hogklint is a shallowing sequence, and the p atch reefs exhibit a well-developed vertical succession: (1) Axelsro-type p atch reefs developed in the underlying Visby Formation; (2) halysitid tabul ates capped by laminar stromatoporoids; (3) domical and bulbous stromatopor oids and red algae; (4) cyanobacterial-algal reef crest. The patch reefs ex pand upwards from an initial bioherm phase with a small base to a laterally extensive biostrome phase. This gives them a thumb-tack appearance. In sta ge 2 of the bioherm phase, rigid framework development and high reef relief resulted in breakage of angular blocks up to 15 m long, which were incorpo rated into the reefs or fell into adjacent sediments. Poorly sorted talus h aloes (Millingsklint Member) also developed adjacent to stage 2 of the bioh erm phase. These include angular blocks and exhibit depositional slopes up to 40 degrees away from the reefs. Stage 3 biostrome development was mainly nonrigid cluster reef, which shed skeletal debris (Domkyrka Member) but fe w lithified blocks. Stage 4 biostrome development was a reef crest with ope n to closed frame structure. Storm breakage and overturning produced large blocks with complex cavity fill sequences including double geopetals. Relie f during the bioherm phase, indicated by fallen blocks and talus slopes, wa s up to at least 15 m; during the biostrome phase, it was up to 10 m.