Late Ordovician carbonate buildups and erosional features northeast of Gotland, northern Baltic Sea

The dimensions and form, structure and distribution of the Upper Ordovician carbonate buildups in the Baltic Sea, northeast of the Island of Gotland, have been determined by means of shallow-marine seismic reflection profilin g. They are shown to decrease in size and number downslope, towards the dee per part of the Palaeobaltic Basin. The seismic signature variations, refle cting the structural and compositional changes of the carbonate buildups po int to an increase in reef skeleton forming organisms and enhanced rigidity and stability of the buildups upslope. Supported by the tectonic setting, a local shallow-water break was formed on the slightly sloping carbonate ra mp of the Late Ordovician sea, closely east of the Isle of Gotska Sandon. T his depth development promoted the formation of a major linear reef of frin ging or barrier character, separating a patch reef complex and reefs in the offshore belt adjacent to deep shelf muds from a beach or tidal flat. In t he offshore area northeast of northern Gotland, the Late Ordovician sedimen tary environment was entirely different from that farther east, offshore th e Island of Hiiumaa, Estonia. The great diversity of the sediments, and the rapidly changing thicknesses of the layers, laid down on the fore-reef slo pe, were to a large extent governed by the numerous mud mounds and reef str uctures. Syn- and post-depositional compaction effects induced relative mov ements of the masses inside the deepwater carbonate buildups which led to t heir lateral spalling and collapse. The locally discordant reflectors from these rapidly changeable lithological units and strongly anomalous, chaotic reflector configuration around the collapsed carbonate buildups are intert wined with reflectors of erosional origin. Channel-like depressions, which probably form a unique set of ancient riverbeds, appear in the Late Ordovic ian sequence about midway between northeastern Gotland and Hiiumaa. They we re formed before the formation of the Vormsi-Fjacka layers, i.e., before th e establishment of the Upper Ordovician carbonate buildup environment. The latest Ordovician erosional surface, most likely related to the Gondwana gl aciation, is distinctly imprinted in the bedrock of the area.