Ancient turbidite systems can be characterised by a basin fill made up of compositionally different types of sandstone petrofacies reflecting multiple sources, with complex relationships from the hinterland and the related sedimentary basin. In such situations, modal analysis of sandstones represents an important tool in provenance studies, especially if combined with detailed facies analysis. For this purpose, modal compositions of arenites have been plotted into QFL and LmLvLs diagrams, using the Gazzi-Dickinson method. The analysed successions belong to the eastern sector of the Transcarpathian Basin (Northern Romania) and are formed by deep-water turbidite deposits consisting of alternations of sandstones and marls, Oligocene – Lower Miocene in age.
Three different petrofacies (PfA, PfB and PfC) have been identified. Deposits of PfA, essentially emplaced by low-density turbidity currents, were deposited in settings of mid fan and outer fan. QFL diagram gives an arkose composition, while diagram for lithic fragments (LmLvLs) shows the predominance of carbonatic and metamorphic grains. The calcareous nannoplankton assemblages belong to the Oligocene, with NP25 biozone. Deposits of PfB, emplaced by high-density turbidity currents, represent prograding massive sandstone lobes. QFL diagram shows an arkose-subarkose composition, while in LmLvLs diagram siliciclastic grains dominate. The calcareous nannoplankton assemblages identify the Upper Oligocene – Lower Miocene age, with NP25, NN2 and NN3 biozones. Deposits of PfC are essentially outer fan turbidites. QFL diagram shows an arkose composition, while in LmLvLs diagram, metamorphic grains dominate. The analysed samples identify the Lower Miocene age by nannoplancton assemblages belonging to NN1 and NN2 biozones.
The eastern part of Trascarpathian Basin represents a case study in which the succession of different sandstone petrofacies can be explained with active tectonics in the source area, with a consequent variation of the relative contribution of different lithotypes in sediment supply. In particular, the abrupt transition from PfA to PfB deposits, with increase of quartz and decrease of felspars and carbonatic clasts, could be explained with variations in catching areas caused by vertical displacements and not a simple unroofing trend. In the youngest arenites of PfC, the initial composition is partially regained, thus justifying the hypotesis of a regional tectonics that activated different source areas. The progradation of the depositional turbidite systems, supported by the biostratigraphic study of calcareous nannofossil assemblages, and the variations in paleocurrent directions, detected from facies analysis, can be a clue of some tectonic activity into the basin itself.
