Reef biostromes and related facies from the Middle Silurian of Gotland, Sweden

Abstract: Reef biostromes and related facies from the upper part of Wenlock and from Ludlow were investigated in order to account for their formation, their palaeoecology and biology. In the Hemse Group of Ludlow age, all studied biostromes comprise a low diversity fauna where one or few species dominates. These biostromes are divided into Kuppen type biostromes, Grogarnshuvud type biostromes and Sjaustrehammarn type biostromes, based on their internal texture. Kuppen type biostromes display a wide range of stromatoporoid morphologies from laminar to columnar and bulbous. This type is an autoparabiostrome with tall forms mostly overturned and low profile forms displayed in situ. Low profile forms dominate Grogarnshuvud type biostromes. It is an autoparabiostrome with the taller morphologies tilted and low profiled stromatoporoids in situ. The Sjaustrehammarn type biostromes are allo/parabiostromes with most organisms tilted, and also some abraded. The morphotype range is similar to the Kuppen type biostromes. All biostromes initiated on hard substrates, either crinoid lenses or detrital reef material. The biostromes at the easternmost Hemse Group belong to complexes of reef biostromes and coarse ruditic material, where the biostromes are vertically stacked and separated by erosion surfaces. Palaeobiolgical investigations of these biostromes suggest that they were formed by the same fauna. Also, a further look into biostromes and diversity in general suggest that reef biostromes are generally of low diversity. Sedimentation dynamics and degree of water turbulence seem to be the two most important external controls on the growth form of stromatoporoids. The main control, however, is genetical, and a strong relation of morphotype to species is seen in all investigated stromatoporoid reef biostromes. One deep setting biostrome from the late Wenlock Halla Formation was investigated in order to clarify the biology and development. It contains densely packed heliolitids, and can be separated into several successive stages, that reflect the forming and development of the biostrome: a pre-biostrome stage of clays and mudstones with occasional intercalations of distal packstone tempestites; a pioneering stage where halysitids initiated growth on a firmground; a climax stage of numerous packed mostly branching heliolitid corals; a post-biostrome stage, with a gradual decrease in reef forming organisms and a backstepping to pre-biostrome conditions. The use of Kershaw’s (1994) biostrome classification scheme seem to work well in facies analysis. Also, incorporation of data on stromatoporoid morphologies in both facies analysis and interpretations of the depositional history of the investigated areas seem to work well, and do complement sedimentological methods.