The Carboniferous of Svoge Coalfield (Bulgaria)

Abstract

The Svoge Coalfield Carboniferous is subdivided into Tsarichina Formation with Dalbochitsa Member (250 m – Namurian A), Svidnya Formation (220 m – Namurian C-Westphalian A), Dramsha Formation (180 m), Svoge Formation (500 m – Westphalian A), Berovdol Formation (280 m – Westphalian A-B) and Chibaovtsi Formation (420 m – Westphalian B-C). Except Dramsha Formation, that is built of claystones, the other formations consist mainly of sandstones, with few levels of claystones with or without coal seams. Conglomerates occur at the base of Tsarichina, Svidnya and Chibaovtsi Formation and are intercalated in the sequences of Svidnya, Svoge and Chibaovtsi Formation. The coal seams are superantracites (95—98% volatile) mainly thin, some are 0.6 to 2.0 m, but in place reach even 12 m most frequently in result of tectonic accumulation. Breaks in sedimentation are between Tsarichina and Svidnya Formation and at the base of Berovdol Formation. The basement is built of a continuous Ordovician, Silurian and Devonian sequence covered with erosive contact and angular (up to 30°) unconformity by the Carboniferous. The folded (in normal and reversed folds) Palaeozoic section is unconformably covered in the western part of the coalfield by Lower Triassic. Tectonically the Coalfield is a complex graben superimposed on the core of the Alpine Svoge anticline. The south parts are built of symmetric folds. The north parts are of recumbent folds which in the east half are north vergent, whereas in the west part are south vergent, being separated by a prominent wrench fault. The south part is folded in pre-Permian time, possibly during the Leonian Phase, when the sedimentation stopped, and the graben has been transformed in a horst structure, maybe during the Asturian Phase. Both parts are refolded during an Alpine (Late-Cretaceous) north-vergent folding. The western south vergent folds are separated from the Triassic cover by decollement whereas most probably the Svidnya Pluton has resisted to the north directed pressure.