Description of the type section, cyclicity, and correlation of the Riksu Formation (Wenlock, Estonia)

The lower Wenlock to middle Ludlow section of the Riksu drill core from western Saaremaa Island (Estonia), including the stratotype interval of the Riksu Formation (Jaagarahu Regional Stage, middle Wenlock) is described. The lower boundary of the formation is redefined in the type section and is placed on the top of the Paramaja Member of the Jaani Formation. Three subunits, the Lower, Middle, and Upper Riksu beds, are established. An intricate multistage sedimentary cyclicity of the Riksu Formation is described; the meso-, submeso-, mini-, and subminicycles are provisionally equated with 400, 100, 41, and 23 Ka Milankovitch cycles, respectively. The Riksu Formation is roughly correlated with its lateral equivalents - the Jaagarahu, Muhu, and Jamaja formations in Estonia, and with the successive upper Högklint, Tofta (Kopparsvik), and Slite beds on Gotland.