Ooid-stromatolite association as a precursor of bioevents (Silurian, Timan–northern Ural Region)

This study reports the frequent occurrence of ooid-stromatolite association levels at the starting of bioevents at various Silurian stratigraphical boundaries in the Timan–northern Ural region. Five genetic types of ooids show various depositional settings and environments of their formation. The important feature of studied ooids is the fact that calcite crystals of their cortex have the distinct traces of dissolution formed by organic acids of embedded microorganisms. A honeycomb-like pattern of subpolygonal to subspherical pits and walls is interpreted as calcified extracellular polymeric substance (EPS) within stromatolite fabrics and some microscopic ooids (less than 0.1 mm in size) in the beginning of their microbial cortex formation are locally visible. This is important observation because well-preserved different fabrics seen in ancient ooids and stromatolites have previously been interpreted to represent paleoenvironmental conditions brought about by different scale changes in benthic assemblages. Results from this study suggest that it is possible that the connection of ooids with stromatolites at these time-levels depends on their genetic relationship to abundance microbial habitats in the Silurian shallow water marine basin of the region.