Palaeoenvironments of the latest Cretaceous (Maastrichtian) dinosaurs of Romania: insights from fluvial deposits and paleosols of the Transylvanian and Haţeg basins

Abstract A diverse Maastrichtian dinosaur-dominated fauna is known from the Red Continental Strata of the Transylvanian Basin and the Densus-Ciula Formation and Pui beds of the Haţeg Basin, western-central Romania. Sedimentological and palaeopedological studies of these deposits provide insights into the prevailing palaeoenvironmental and palaeoclimatic conditions at the end of the Cretaceous. The Red Continental Strata and Pui beds are dominated by red mudstone sequences and exhibit highly variable palaeocurrent direction. Inclined heterolithic strata with palaeocurrent indicators orientated perpendicular to the dipping strata are also present in channel deposits of the Red Continental Strata. All these features suggest deposition by meandering rivers. Although red mudstones are abundant in the Densus-Ciula Formation, this formation differs from the aforementioned formations in containing thick sandstone beds displaying sedimentary structures typical of braided stream deposits and characterized by low variability of palaeocurrent direction. These features indicate deposition in a low-sinuosity fluvial system. Paleosols of the three studied formations are characterized by the presence of carbonate nodules closely associated with iron oxides, rhizocretions, and slickensides; mottles and indurated calcareous horizons are occasionally present in paleosols of the Red Continental Strata and Pui beds. These features are indicative of a climate characterized by seasonal precipitation in which evapotranspiration exceeded precipitation, and where the watertable fluctuated during the year. Dark red, noncalcareous paleosols also occur occasionally in the Pui beds, representing leached profiles that developed on sandier parent material. The geochemical composition of all paleosols studied indicates that palaeoprecipitation was less than 1000 mm/year, which is significantly lower than estimates inferred from the tropical palaeoflora of the region (1300–2500 mm/year). This apparent incongruence in paleoprecipitation estimates can be explained by the fact that tropical plants can live in warm, monsoonal climates if they have access to sufficient quantity of water during the dry seasons to satisfy their metabolic needs. Comparison of the studied formations to the famous contemporaneous Sânpetru Formation of the Haţeg Basin confirms similarity of palaeoclimatic conditions. However, palaeoenvironmental and taphonomic conditions differ. Whereas the Sânpetru Formation comprises a mosaic of wetlands and moderately-drained floodplains in which areas of impeded drainage were predominant, the Red Continental Strata, Densus-Ciula Formation, and Pui beds represent moderately- to well-drained floodplain settings. Although the Pui beds have long been assumed to pertain to the Sânpetru Formation on the basis of palaeontological and superficial lithological similarities (i.e., red colour), comparison of the results of the detailed palaeoenvironmental studies suggests that the Pui beds are distinct from the Sânpetru Formation and may warrant their own formational recognition (Barbat Formation). Finally, fossil preservation conditions in the Sânpetru Formation indicate concentration of remains by hydraulic processes on wet floodplains (lenticular bonebeds or “fossiliferous pockets”) while those of the other three formations suggest attritional mortality assemblages preserved in dry paleosol profiles. Such taphonomic differences must be considered carefully in palaeoecological studies of the Maastrichtian dinosaurian fauna of Romania.