Carbonate distribution in the East Baltic deep shelf in the late Ordovician-early Silurian

Calcite and dolomite diminished and disappeared offshore in the East Baltic shelf in the Ordovician-Silurian. The distribution was influenced by transgressions-regressions, transportation processes, and dissolution effects. Calcite dissolved due to the relatively low pH of deep seawater and pore waters. Only small amounts of dolomite were present in the deep shelf. Low pH was also evidenced by clay minerals of the Llandovery bentonite layers. The bentonite matrix varied in composition from illite-smectite to kaolinite-containing. Kaolinite formation was promoted by the decrease of seawater pH with depth. Three time intervals can be distinguished when different prevailing factors governed the carbonate distribution in the East Baltic shelf: the late Ordovician global regression, the Aeronian increased primary productivity, and the Telychian high atmospheric CO2.