The structure and relief of the bedrock sequence in the Gotland-Hiiumaa area, northern Baltic Sea

Based on high resolution seismic reflection profiling, structural and relief maps of the sedimentary bedrock between Gotland and Hiiumaa in the Baltic Sea have been composed and analysed. The general structure and relief of the submarine Lower Palaeozoic succession reveal a westward extension of the homoclinal structure distinguished in the Estonian mainland. The main bedrock structures offshore are 1-4 km wide, and several tens of kilometres long, linear zones of disturbances. On the structural map, these disturbances appear as submeridional zones of contour changes, up to several tens of metres in offset. The seismic profiles usually reveal a faint flexure-like bending of the layers through the zone. Locally, this flexure can be intersected by small faults. These bedrock structures are ascribed to fault movements in the crystalline basement. Two different bedrock relief systems were superposed on the region during the Cenozoic uplift and the Pleistocene glaciations. The first event resulted in the formation of a subparallel cuesta-like system of alternating erosional scarps and plains. Glacial erosion created submeridional valleys and troughs. Today three large bedrock forms, namely the Baltic and the Silurian clints and the Ordovician plateau, characterize the area. The outlines of the cuesta relief, and the amount of eroded sediments, advocate a regional increase in erosional activity from the St. Petersburg district to the area of the Baltic-Bothnian mobile zone northeast of Gotland. This zone existed as a subsided meridional lower ground during the Cenozoic, accommodating a main river that collected water both from the craton margins and the inner platform areas.