Boundaries of the Silurian System in Bulgaria defined by graptolites

Two sections in the area of Sofijska Stara Planina Mountain are described. These are the exposures along the Saltarski Dol River and west of the village of Cerecel at which new graptolite finds are used to place the lower and upper boundaries of the Silurian System respectively. Different concepts on de fining the Ordovician-Silurian boundary on graptolites are reviewed. The present author proposes to define it by the appearance of Glyptograptus persculptus (Salter), i.e. at a basal horizon of the Saltar Formation. Thus, the Ordovician-Silurian boundary is considered to coincide with global facies changes which reflected the late Ordovician glacio-eustatic sea level rise. The author has adopted also the concept of thе Ordovician-Silurian Boundary Working Group to place the boundary at the appearance of Akidograptus ascensus Davies, 1 m above the base of the Saltar Formation in the Saltarski Dol section. The Silurian-Devonian boundary is placed within the Gradište Formation and defined by the appearance of Monograptus uniformis Přibyl. The sections described are internationally correlated with the Silurian System boundaries stratotypes in West Scotland and Czecho-Slovakia.