Gloeocapsomorpha prisca Zalessky, 1917: a new study. Part II: Origin of kukersite, a new interpretation

Kukersite is the name given to exceptionally carbon-rich Middle Ordovician oil shales found in the Baltic Shale Basin of Estonia. The organic component of these sediments consists mostly of accumulations of the microfossil Gloeocapsomorpha prisca which, we propose, was an intertidal, marine, mat- forming benthonic cyanobacterium similar to, but not identical with, the extant cyanobacterium Entophysalis major. Kukersite beds are formed in situ or occur as allochthonous deposits resulting from transport of G. prisca from large intertidal areas by water or wind currents into anoxic offshore environments. It is proposed that the cyclical nature of sediments containing kukersite and marine carbonate facies in the Baltic Shale Basin results from changes in relative sea level, probably caused by both local tectonism and eustatic sea level changes.