Seismic stratigraphy, architecture and outcrop pattern of the Wenlock–Přidoli sequence offshore Saaremaa, Baltic Sea

Abstract

The Wenlock–Přidoli sequence has been subdivided and mapped offshore Saaremaa in the Baltic Sea by means of high resolution seismic reflection profiling. The most striking and consistent seismic reflectors correlate with contrasting lithological boundaries of the Silurian regional stages, the Jaani/Jaagarahu (S 8), the Jaagarahu/Rootsiküla (S 10), the Rootsiküla/Paadla (S 11), the Paadla/Kuressaare (S 12), the Kuressaare/Kaugatuma (S 13) and the Kaugatuma/Ohesaare (S 14) stages, which often coincide with erosional disconformities on Saaremaa. The shallow-shelf Silurian sequence that crops out on Saaremaa is also exposed in the closest offshore area of the island. Due to the westerly to south-westerly increasing erosional cut below the Baltic, the deeper facies varieties that are concealed in southern Saaremaa, are revealed and exposed at the seafloor towards Gotland. This is particularly well expressed in the Jaagarahu Stage, which undergoes a facies change with significant increase in thickness on the basinal slope of the Baltic Silurian Basin, both onshore and offshore Saaremaa. The Silurian sequence overlying the Jaagarahu Stage does not show any considerable thickness or facies changes neither below nor offshore Saaremaa. Offshore Saaremaa, seismic reflectors limiting new deeper facies subunits in the Jaagarahu Stage become either successively or simultaneously exposed inside the Vilsandi (S 8–S 9) respectively the Maasi–Tagavere (S 9–S 10) seismic units towards Gotland. These outcrop patterns were largely formed by two different erosional agents that during different time periods have played a considerable role in the formation of the present day Baltic Sea depression. Hence, the gradually south-westerly deepening cuesta landscape formed by the Cenozoic erosion, namely the Silurian Klint and the plateaus north and south of it, discloses progressively deeper part of the Baltic Silurian Basin. The deeper facies subunits are exposed rapidly by the submeridional erosional incisions of the Pleistocene glaciers. Except for the erosional component, there appears also a distinct tectonic factor in the genesis of the present Baltic Sea depression. Today, the coeval Silurian rocks that once were formed in separate facies belts, i.e. in different depth intervals of the same sedimentary basin on Saaremaa and on Gotland, are presently located at about equal altitude.