Hardgrounds of the Ordovician Baltic Paleobasin as a Distinct Type of Sedimentation Induced by Cyanobacterial Mats

The cold-water Volkhov and warm-water Keila hardgrounds of the Ordovician Baltic Basin represent a specialized product of cyanobacterial communities of the seafloor, another member in a group containing stromatolites and microbially induced sedimentary structures. The leading role of cyanobacterial communities in the development of hardgrounds is suggested by their macrofeatures, structure, and elemental composition revealed by SEM equipped with a microprobe microanalyzer. During the formation of the hardgrounds, the cyanobacterial films were of considerable thickness (up to 4–5 mm), but lacked vertical zonation of bacterial communities, thereby being at an intermediate stage between the mature biofilms and true mats. The soft substrate underlying hardgrounds was inhabited by an abundant infauna. Thanks to bioturbation, the cyanobacterial film was in places destroyed, which prevented the development of hardgrounds. In the warm Late Ordovician basin of northern Estonia, cyanobacterial films formed hardgrounds on the surface of ripple marks. Abundant peloids and warm pore waters facilitated further cementation and hardening of the substrate underneath the hardground before the burial of the ripple marks and the hardground. Cyanobacterial films appeared simultaneously over large areas of the seafloor due to settling of planktonic bacteria. The extracellular polymeric substances (EPSs) released by these bacteria were rapidly mineralized within a maximum of one season, apparently at a depth of 15–25 m, below the fair weather wave base. The formation of hardgrounds in bioherms occurred in a shallower environment and was patchy due to the alternation of living and dead cyanobacterial mats, mineralizing mats and resulting hardgrounds. Inhabitants of biohermal hardgrounds were producers of carbonate detritus, including of micritic size, over vast areas of the seafloor surrounding the bioherm.