The Ordovician-Silurian transition beds in the seafloor between Gotland and Hiiumaa Islands, Baltic Proper

New seismic profiles have been used to revise earlier interpretations of the Ordovician-Silurian boundary beds between Saaremaa and Gotland. A trans-Baltic reflector with erosional features (S2) above the erosional Ordovician-Silurian boundary reflector (S1) correlates with the boundary between the Raikküla and Adavere stages. The sporadic reflector or2 below the S1 reflector offshore from Gotland represents the erosional boundary between the Pirgu and Porkuni stages. Three stratigraphic gaps occur in the Ordovician-Silurian boundary beds offshore from Gotland. The amount of eroded rocks between the Pirgu and Porkuni stages, the Ordovician and Silurian systems and the Raikküla and Adavere stages can change rapidly. Consequently, the thickness and stratigraphy of the Ordovician-Silurian boundary beds around Gotland can change considerably across short distances. The O4-5-S1 unit offshore from Gotland, including carbonate build-ups and erosional incisions infilled with Porkuni strata, belongs facially to the transitional belt between the Estonian Shelf and the Livonian Tongue. The thickness changes in the S1-S2 unit (Junru and Raikküla stages) indicate an extensive submarine erosional channel, streching from north of Estonia across the Baltic Sea and central Gotland, which developed in the Baltic Basin along a shelf to deep-basin transect during Llandovery time.