Correlation of lower-middle Llandovery sections in central and southern Estonia and sedimentation cycles of lime muds

Stratigraphical units of the Juuru and Raikküla regional stages (lower to middle Llandovery) are described and correlated in the Põltsamaa, Heimtali, and Ikla drill cores. A new unit, the Heinaste Member, is established at the base of the Saarde Formation. The Distomodus kentuckyensis, Aspelundia expansa, and A. fluegeli conodont biozones, as well as the Ancyrochitina laevaensis, Belonechitina postrobusta, Euconochitina electa, Ancyrochitina convexa, Conochitina alargada, and C. malleus chitinozoan biozones are recognized, the last two zones in both groups for the first time in Estonian sections. Biostratigraphical correlation by means of chitinozoans and conodonts proves synchroneity of extensive strata of micritic limestones recurring three times in the sequence of the Raikküla Stage. The pure, micritic limestones, almost barren of benthic shelly fossils, alternate with more argillaceous, nodular, biomicritic limestones (skeletal packstones and wackestones) in the Nurmekund Formation, and with marlstones or argillaceous limestones in the Saarde Formation. Alternation of micritic deposits, very poor in benthic fauna, with argillaceous, biomicritic deposits rich in shelly fauna, suggests that such cyclicity was not directly controlled by changes in water depth, but more likely by cyclic changes in certain biogeochemical conditions. Alternation of arid and humid climate states supposedly induced lime mud accumulation cycles: During humid periods terrigenous input and freshwater runoff from the adjacent continent produced more argillaceous sediments, whereas during the arid periods terrigenous input decreased but salinity increased, causing accumulation of pure lime muds. Cyclic accumulation of lime muds in low-latitude shallow seas was characteristic of the supposed latest Ordovician-earliest Silurian ice-house period.