Palaeoenvironmental reconstruction of the Late Cretaceous Sânpetru Formation (Haţeg Basin, Romania) using paleosols and implications for the “disappearance” of dinosaurs

Abstract

Paleosols of the Late Cretaceous Sânpetru Formation (Haţeg Basin, Romania) are the subject of macroscopic, microscopic, and geochemical investigation in order to reconstruct the palaeoenvironmental and palaeoclimatic conditions in which dinosaurs lived. Study of overbank deposits present in an 860-m-thick stratigraphic interval of the Sânpetru Formation exposed along the Sibişel Valley reveals the presence of one facies and five different pedotypes: 1) finely laminated gray-green mudstones and siltstones interpreted as pond deposits; 2) bioturbated gray-green mudstones lacking macroscopic pedogenic features, interpreted as very poorly-developed hydromorphic paleosols; 3) bioturbated gray-green mudstones containing carbonate concretions, interpreted as poorly-developed hydromorphic paleosols; 4) mottled gray-green mudstones containing carbonate concretions and limited illuvial clay accumulation, interpreted as moderately-developed paleosols characterized by a high but fluctuating watertable; 5) bioturbated brown-red mudstones lacking macroscopic pedogenic features other than the occasional mottles, interpreted as poorly-developed paleosols that formed on the higher and better-drained floodplains; and 6) bioturbated brown-red mudstones containing carbonate nodules, interpreted as moderately-developed calcareous paleosols that formed on distal floodplain settings. Although limited exposure prevents the documentation of lateral relationships of pedotypes, the presence of paleosol profiles that display characteristics of two different pedotypes suggests that they represent a lateral continuum on the landscape related to local differences in hydrology and relief (i.e., catena). The pedotype assemblage reveals that the Sânpetru landscape was a mosaic of wetlands, seasonal wetlands, and better-drained floodplain habitats, similar to the present-day Indogangetic Plains. Pedogenic features and paleosol geochemistry indicate that the Maastrichtian climate of Romania was subhumid (< 1000 mm rain/year) and characterized by strongly seasonal precipitation. The distribution of pedotypes through the Sânpetru Formation reveals small- and large-scale palaeoenvironmental changes associated with the shifting of a complex mosaic of wet and dry habitats in response to shifts in river position on the alluvial plain. However, a major palaeoenvironmental change occurs in the upper Sânpetru Formation, where the region was transformed into extensive wetlands. Such dramatic changes coincide with river competence increase, change in palaeocurrent directions, and dearth of macrovertebrate remains, which had been previously misinterpreted as evidence for the disappearance of dinosaurs at the Cretaceous-Tertiary boundary. These palaeoenvironmental changes, induced by tectonism, are responsible for the shift from preservation of macrofossils concentrated by hydraulic processes into conspicuous lenticular bonebeds in the lower Sânpetru Formation to preservation of microfossil, and more rarely macrofossil, remains in hydromorphic calcareous paleosols in the upper Sânpetru Formation. The consequences of palaeoenvironmental changes on vertebrate preservation mode must be considered in the search for fossils and interpretation of the fossil record.