ASSEMBLING THE PALAEOENVIRONMENTAL PUZZLE OF THE BAURU GROUP IN THE TRIÂNGULO MINEIRO REGION BASED ON STABLE ISOTOPE ANALYSIS

The palaeoenvironment of the Upper Cretaceous aged Bauru Group is mostly referred to as hot and dry, arid to semi-arid. Previous work classifies it based on depositional structures, presence of calcretes and the nature of palaeosols. The palaeoflora is not well known due to the lack of fossil preservation. However, the palaeofauna shows a diverse record, especially in the Triângulo Mineiro region. Regarding the fossil-bearing strata of this particular area, there is evidence of an environment that would support a fauna that includes large-sized freshwater fishes and turtles, together with large herbivorous titanosaurian sauropods. Therefore, during some time intervals, the conditions that allowed this fauna to exist and grow were less severe than those inferred for non-fossiliferous beds of the Bauru Group. In order to understand how these shifting palaeoenvironments succeeded through time, the relationships between the formations of the Bauru Group must be defined. Thus, also in this contribution, we attempted to define the type of contact between Uberaba and Marília formations, which is debated in the literature. We used stable carbon and oxygen isotopes on dinosaur fossils to evaluate several aspects of these formations; scintillometry during the field work; and petrography and X-ray diffraction on rock samples. Stable isotope results suggested that (1) herbivorous animals had a C3 plant-based diet and these plants under considerable environmental stress, (2) the palaeoclimate of this region was, indeed, hot and sunny, (3) environmental conditions were least severe in the boarders of the basin (Condomínio Brisas and Ponto 1 do Price sites) compared to the center of the basin (Fazenda Três Antas site) due to altitute. Volcaniclastic origin of the Uberaba Formation was confirmed with petrography, and the contact with Marília Formation, leastwise in the mapped area, defined as gradational. This implies that the age of both units is more similar than initially thought. Compiling all results and previous studies, we interpret that the Bauru Group deposits in the Triângulo Mineiro presented a seasonal climate, with drought periods, but which could still bear perennial water bodies.