Givetian-Frasnian arid palaeoenvironments from the northern Gondwana: a case study from the Moesian platform (Bulgaria)

Abstract

Palaeozoic deposits in the Moesian platform (northern Bulgaria) are known only from deep wells drilled for oil and gas prospecting. Sixteen microfacies (MFT 1–16) have been distinguished and described in the Givetian to Frasnian carbonate and evaporite sequences, represented in thirteen wells from the eastern parts of the platform. They are respectively interpreted as arid peritidal deposits, formed in a variety of carbonate/evaporite environments (from shallow subtidal zones, adjacent to tidal flat, to supratidal sabkha). The shallow subtidal zone is characterised by wave-dominated (possible carbonate sand shoals, MFT 1–3) and protected lagoon (MFT 4–8) settings. The intertidal zone is distinguished by the development of microbial mats with typical lamination (MFT 9 and MFT 10) and “bird’s eyes” type fenestrae (MFT 11). Unfossiliferous laminated mudstones (MFT 12), lag deposits rich in intraclasts (MFT 13), brackish ponds with charophytes (MFT 14), unfossiliferous homogeneous mudstones/dolomudstones (MFT 15) and sabkha evaporites (MFT 16) were formed within the range of the upper intertidal and supratidal zones. These deposits presumably reflect the transition of the platform from carbonate ramp to rimmed shelf. Most of the studied Givetian–Frasnian microfacies are comparable with Wilson’s Standard Microfacies Types and/or with other microfacies described from shallow-marine Middle–Upper Devonian carbonate and evaporite successions in Europe. It seems likely that during the Givetian–Frasnian time the Moesian carbonate platform was located in the arid climate zone on the northern shallow shelf of the Gondwana.