Seismic correlation of the Cambrian sequence between Gotland and Hiiumaa in the Baltic Sea

Seismic reflection profiling in the Baltic Sea between Gotland and Hiiumaa reveals that the lower boundary of the Cambrian mainly coincides with the crystalline sub-Cambrian peneplain here. In the Gotska Sandön area, the Cambrian sequence rests partially on Proterozoic sedimentary bedrock, whereas Vendian rocks may underlie the Cambrian at the Estonian coast. The sandy-argillaceous Cambrian sequence is covered by Lower Ordovician limestones in the western part, and by an up to 10 m thick layer of Lower Ordovician siliciclastic rocks in the eastern part. At the end of Lontova time, the early Cambrian depocenter shifted westwards from the Moscow Basin to the Baltic Basin, prohibiting any detailed Trans-Baltic seismic correlation. Two Cambrian reflectors extend westwards from Hiiumaa, and seven reflectors eastwards from Gotska Sandön. Only one reflector extends across the area. It is located near the base of the Cambrian sequence within the När Member shale unit in the Gotska Sandön drill core, and at the boundary between the Irbeni and Soela Formations near the top of the Cambrian sequence on Hiiumaa. Some 55 km NE of Gotska Sandön, an area of "stone-pillar" structures divides the investigated area. These hardrock structures cross vertically through the Cambrian softrock sequence. The "stone-pillars" are interpreted as due to secondary cementation of the Cambrian sandstone by gases and fluids percolating through the sequence along tectonic faults. A similar formation and age is proposed for the so-called "Precambrian sandstone" of the Gotska Sandön drill core.