Abstract: The uppermost Cretaceous marine red beds from the Romanian Carpathian bend area were investigated in detail from the lithostratigraphic and biostratigraphic points of view. Geochemical and mineralogical investigations were performed along the same profile. The investigated sequence spans the Upper Campanian and the whole Maastrichtian stages, including the K-T boundary, as evidenced by the nannofloral mass extinction and the well known carbon isotope excursion. Both qualitative and semiquantitative calcareous nannofossil studies were conducted. The semiquantitative investigations focused on six taxonomic groups, such as Watznaueria barnesae, Micula spp., Boreal nannofossils, Tethyan nannofossils, Braarudosphaera bigelowii, and the calcareous dinoflagellate genus Thoracosphaera. The new data indicate constant δ 13 C values in the Upper Campanian and Lower Maastrichtian red marls of the Gura Beliei Formation. In the Upper Maastrichtian deposits of the Ialomica section, lithological and mineralogical changes, together with several negative δ 13 C and δ 18 O excursions, suggest instability of the ecosystems, fluctuating sea level, and/or detrital input as well as climatic changes during the Late Maastrichtian interval. The age and the lithology of the Gura Beliei red beds are similar to other red beds deposited in the Tethys Realm (i.e., the Nierental Formation of Austrian Northern Calcareous Alps, the Puchov Marl of the Slovakian and Polish Western Carpathians, the Vojvodina region of Serbia, and the Scaglia Rossa of Apennines).