The Bauru Group is a significant geological group of Late Cretaceous continental sedimentary rocks found primarily in southeastern Brazil, extending into parts of Paraguay. It represents a vast depositional system that covered a large portion of the Paraná Basin during the final stages of the Mesozoic Era. Renowned for its rich paleontological record, the Bauru Group provides crucial insights into the paleobiota and paleoenvironments of South America during the Late Cretaceous.
Geographical Extent
The Bauru Group outcrops over an extensive area of approximately 250,000 square kilometers, covering parts of the Brazilian states of São Paulo, Minas Gerais, Goiás, Mato Grosso do Sul, and Paraná. It also extends into eastern Paraguay. The group derives its name from the city of Bauru in São Paulo, where its characteristic lithologies were first described.
Stratigraphy and Subdivisions
The Bauru Group is characterized by a complex stratigraphy, typically composed of various formations that often interdigitate or succeed one another. These formations mainly consist of sandstones, siltstones, mudstones, and conglomerates, often with calciferous components. The age of the group spans roughly from the Turonian to the Maastrichtian stages of the Late Cretaceous (approximately 94 to 66 million years ago).
Key recognized formations within the Bauru Group include:
- Uberaba Formation: Generally the oldest unit, characterized by reddish sandstones and conglomerates, often with volcanic rock fragments.
- Adamantina Formation: Composed of fine to medium-grained reddish sandstones and mudstones, frequently calcareous. It is particularly rich in fossils.
- Vale do Rio do Peixe Formation: Consists of fine-grained sandstones and mudstones, often interbedded, representing fluvial and lacustrine deposits.
- Marília Formation: The uppermost unit of the Bauru Group, characterized by coarse-grained sandstones, conglomerates, and calcretes, reflecting a more arid environment and often containing significant fossil concentrations.
- São José do Rio Preto Formation: A less extensive unit, consisting of sandstones and conglomerates, sometimes interpreted as a lateral equivalent or partial facies of other formations.
It is important to note that the precise correlation and nomenclature of these units can vary across different regions of the basin and among researchers.
Depositional Environment
The sediments of the Bauru Group were deposited in a large intracratonic basin under predominantly semi-arid to arid climatic conditions. The depositional environments were varied, including:
- Fluvial systems: Braided and meandering rivers, responsible for depositing most of the sandstones and conglomerates.
- Lacustrine environments: Temporary or perennial lakes, where fine-grained silts and muds accumulated.
- Eolian systems: Wind-blown sands, indicating periods of significant aridity.
The presence of calcretes and evaporite minerals in some units further supports an arid to semi-arid climate with seasonal rainfall.
Paleontology
The Bauru Group is internationally renowned for its exceptional fossil record, making it one of the most important Late Cretaceous continental fossil localities in South America. Its paleofauna provides crucial evidence for the understanding of Gondwanan biodiversity just before the K-Pg extinction event. Notable fossil finds include:
- Dinosaurs: A diverse array of dinosaurs, including numerous titanosaurian sauropods (e.g., Maxakalisaurus, Uberabatitan, Baurutitan), abelisaurid theropods (e.g., Thanos, Pycnonemosaurus), and possibly other theropods. Dinosaur eggs and nests are also common.
- Crocodylomorphs: An exceptionally rich assemblage of notosuchian crocodylomorphs, often with unusual adaptations (e.g., Baurusuchus, Mariliasuchus, Sphagesaurus).
- Turtles: Numerous podocnemidid turtles, including some large forms.
- Fishes: Freshwater fish remains, including lungfish and teleosts.
- Other vertebrates: Remains of anurans (frogs), squamates (lizards and snakes), birds, and small mammals.
- Invertebrates: Gastropods, ostracods, and insect trace fossils.
- Flora: Fossil plant remains, including silicified wood and palynomorphs, which help reconstruct the paleovegetation.
The paleontological evidence from the Bauru Group sheds light on the faunal interchange between continents and the evolution of unique Gondwanan lineages during the Late Cretaceous.