Baltic-Bothnian mobile zone, its geological evidences and role in formation of the sedimentary bedrock succession in the northern Baltic

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 profiling. They are shown to decrease in size and number downslope, towards the deeper part of the Palaeobaltic Basin. The seismic signature variations, reflecting the structural and compositional changes of the carbonate buildups point 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 ramp of the Late Ordovician sea, closely east of the Isle of Gotska Sandön. This depth development promoted the formation of a major linear reef of fringing 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 the offshore area northeast of northern Gotland, the Late Ordovician sedimentary environment was entirely different from that farther east, offshore the Island of Hiiumaa, Estonia. The great diversity of the sediments, and the rapidly changing thicknesses of the layers, laid down on the fore-reef slope, were to a large extent governed by the numerous mud mounds and reef structures. Syn-and post-depositional compaction effects induced relative movements of the masses inside the deep-water carbonate buildups which led to their 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 intertwined with reflectors of erosional origin. Channel-like depressions, which probably form a unique set of ancient riverbeds, appear in the Late Ordovician sequence about midway between northeastern Gotland and Hiiumaa. They were formed before the formation of the Vormsi-Fjäcka layers, i.e., before the establishment of the Upper Ordovician carbonate buildup environment. The latest Ordovician erosional surface, most likely related to the Gondwana glaciation, is distinctly imprinted in the bedrock of the area.