The Silurian in the West Balkan Mts. (Svoge Unit, Srednogorie Zone) – 110 years later

Earliest studies on the Silurian System in Bulgaria are associated with the names of Dimitar Allahverdzhiev and Stefan Bonchev. Allahverdzhiev was the first to find Silurian graptolites in this country; his specimens are housed in the Laboratory of Geocollections of the Geological Institute (Bulgarian Academy of Sciences). Bonchev laid the foundations of the lithostratigraphic subdivision of Palaeozoic sediments in the West Balkan Mts. by distinguishing all their lithologic variations. In terms of regional tectonic zonation, the study area is a part of a first-rank Late Alpine unit, the Srednogorie Zone, represented by the Svoge Unit. The lithostratigraphy of the Palaeozoic marine successions in the Svoge Unit consists of nine formal units (Grohoten, Tseretsel, Sirman, Saltar, Mala Reka, Yabukov Dol, Ogradishte, Romcha and Katina formations) and one informal unit (silty-clayey metaformation). Their distribution is illustrated in a geologic map. The biostratigraphic subdivision of the Silurian System in Bulgaria is based on graptolites. Herein, the latest two “standard” zonal schemes are shown: the Generalized graptolite zonation and the Graptolite zonation for the peri-Gondwana (Europe). About 25% of these graptolite zones have not yet been recognized in Bulgaria; the extent of the others is plotted against a generalized section of the Silurian System for the region.