Lower Cretaceous lithofacies of the black shales rich Audia Formation, Tarcău Nappe, Eastern Carpathians: Genetic significance and sedimentary palaeoenvironments

Abstract

Lower Cretaceous deposits of the Tarcău Nappe, central part of the Eastern Carpathians, were studied, aiming to point out their lithofacies and to reconstruct the changes in depositional palaeoenvironment of those times. The investigated deposits extend within the NC5-NC10 calcareous nannoplankton biozones, covering the Late Barremian–Late Albian. Based on sedimentological, petrographical and geochemical features, ten lithofacies were identified, grouped in three categories, such as shaly, siliciclastic and carbonate. The shaly lithofacies consist of black and grey shales, as well as carbonate shales yielding an average of organic matter content around 3%. The organic matter and pigments of iron, such as hydrotroillite are responsible for the occurrence of the black colour. Preservation of the organic matter is due to anoxic environments induced by reduced current flow and intensified water density stratification. The siliciclastic lithofacies are mainly composed of quartz arenites, quartzwackes, and subordinately sublitharenites and lithic graywackes. The carbonate lithofacies include predominantly marls and siderites, originating by diagenetical processes of the shaly lithofacies, and subordinately calcarenites with terrigenous material and sponge spicules. The petrography and geochemistry indicate that the main sources of the sandy detrital material are the basement and sedimentary cover of the Central (Scythian) and East European platforms. The black and grey shale deposition, in the Barremian–Late Aptian interval, reflects suspension settling of the hemipelagic and pelagic material. Thin sandy beds, with parallel lamination and current ripples, suggest low-density turbidity currents, while the Albian thick sandy beds with massive or normal grading and parallel lamination are interpreted as high-density turbidity currents or even sandy debris flows. The shaly depositional intervals are linked to the basinal plain, while the sandy dominated sequences are associated with turbiditic lobes.