Late Silurian reef development in the Baltic Sea

In the Baltic Sea, reef structures are common in the Ordovician and Siluria n sedimentary bedrock. Palaeozoic reef development culminated in the Siluri an when several successive reef barriers developed. The present investigati on has revealed new reef structures in the upper Silurian sedimentary bedro ck. Two new biostromes, E1 and E2, have been found in the Ludlovian Eke Bed s, east of Gotland. The biostromes trend in a more or less east-west direct ion and can be traced across the northern part of the Baltic Sea. In the Pr idolian, two reef-like barriers, named B5 and B6, occur at the boundary to the Devonian. The lower barrier, B5, is found to the south of the younger B 6 barrier. This indicates that a transgression occurred between the formati on of the two barriers. A tentative reconstruction of the Pridolian bay sug gests that the coast shifted from a more east--west direction in the Ludlov ian to a more north-easterly to south-westerly direction in the Pridolian. Bioherms are commonly associated with the Eke biostromes and the upper Prid olian reef-like barriers. The bioherms occur on the seaward side of the lar ger reef structures, on the biohermal slope. Patch-reefs also occur on the biohermal slope but they are more common on the landward (lagoonal) side of the barriers.