Sedimentary cyclicity and dolomitization of the Raikküla Formation in the Nurme drill core (Silurian, Estonia)

edium-rank sedimentary cyclicity of the Raikküla Formation (Raikküla Regional Stage, middle Llandovery) is first described in the Nurme drill core (mid-western Estonia). Four shallowing up submesocycles are distinguished, which group in pairs into two mesocycles, treated as the Lower and Upper Raikküla subformations. In the Nurme drill section, totally dolomitized carbonate rocks of the Raikküla Formation are sandwiched between the unaltered limestones of the Juuru and Adavere regional stages. It suggests early dolomitization during the Raikküla Age soon after deposition. X-ray diffraction, X-ray fluorescence, and titration analyses were applied to study the composition of rocks and lattice parameters of dolomite. Geochemistry of rocks is consistent with dolomitization by normal marine water. No evidence of hypersalinity of the sedimentary environment or inflow of external fluids has been observed. The results of d104 measurements showed a trend of growing dolomite stoichiometry towards the top of the regressive, shallowing up submesocycles. The most completely ordered dolomite (< 51 mol% CaCO3) occurs in the intervals of primarily bioclastic, winnowed sediments, which indicates the promoting role of the agitated-water environment in the dolomitization process. Extensive pervasive dolomitization associated with regressive phases of the evolution of the Baltic Palaeobasin in the Silurian and was related to a zone of shallow normal-saline inner shelf facies, migrating in space in accordance with sea level changes.