Palaeogeography and geodynamic evolution of the Balkanides and Moesian ‘microplate’ (Bulgaria) during the earliest Cretaceous

In Bulgaria, the palaeogeography of Balkanides and the Moesian ‘microplate’ (or Moesia), at the beginning of the Cretaceous Period, was marked by the presence of a deep NE–SW to E–W trough (Axial Basin, or Nish-Trojan Trough), inherited from Late Jurassic times and situated in the southern part of the future Alpine tectorogen (Fore-Balkan). This trough was filled with terrigenous, more or less turbiditic, basinal sediments, particularly during the Berriasian (Cherni Osam flysch), which prograded towards the axis of this palaeostructure. Laterally, the two passive margins of this trough are asymmetrical: (1) to the south, a mobile and narrow margin, restricted to some fault-blocks (more or less tilted) against the Rhodopes (‘Thracian Island’) and covered by coarse siliciclastics (Kostel Formation); (2) to the north (Moesia), a more stable and considerably wider shallow-water carbonate platform (Kaspichan Formation). Towards the north-west, the transition between the trough and the northern margin is characterized by a NW–SE framework of sub-basins (Glozhene and Salash formations) and small carbonate Bahamian-type ‘islands’ (Brestnitsa Formation, Slivnitsa Formation) whereas, to the south, the transition is sharper. The geodynamic evolution of this system is particularly marked by the centrifugal backstepping of the terrigenous facies on the two margins. The end of flysch-type/turbiditic sedimentation during the Valanginian coincided with the spectacular narrowing of the northern carbonate internal/proximal platform during the Hauterivian. In this synthesis, three successive palaeogeographic maps complemented by NE–SW-orientated cross-sectional (palinspastic) profiles are presented which demonstrate the evolution of this part of the Peritethyan area of Bulgaria, situated to the north of the ‘Vardar Ocean’, part of the Tethyan oceanic realm.