Cyanobacterial origin and morphology of the volkhov hardgrounds (Dapingian, Middle Ordovician) of the St. Petersburg region (Russia)

Hardgrounds (substrate produced by synsedimentary lithification of marine sediments exposed on the sea floor) of the Volkhov Regional Stage (Middle Ordovician, Dapingian) in the east of the St. Petersburg region (Russia) are studied. Surface hardgrounds are formed due to accumulation of micrite by cyanobacterial biofilms, inferred from the presence of small irregularly round-shaped structures (diameter 1-3 cm) produced within the hardground by bubbles of gas accumulated below the seafloor, covered by microbial films or mats. SEM study of the hardgrounds supports this hypothesis and shows numerous mineralized structures reflecting biological activity of microbes. These are mainly represented by a mineralized extracellular polymeric substance coating numerous calcite grains, usually 5-15 µm thick. The composition of the calcite grains lacks significant admixture of additional elements. In the mineralized extracellular polymeric substance, the composition is considerably more complex. The predominantly cyanobacterial composition of the microbial community shows the presence of characteristic sheaths and filaments coated with an extracellular polymeric substance and forming an entangled network of filaments. The initial thin biofilms (a few mm) for hardgrounds could have been intermediate in form between cyanobacterial films and mats, thick enough to be classified as mats, despite its characteristic vertical zonation is lacking. They can be interpreted as “immature” or “incipient” mats.