Stratigraphy of the Arai Group: record of paleoproterozoic rifting in Central Brazil

The Arai Group is characterized by a succession of low-grade metassedimentary and metavolcanic rocks that overlaps the Aurumina Suit and the Ticunzal Formation. The related basin fill-succession of the Arai Group was probably deposited in an intracontinental rift, which evolution initiated before 1,77 My in the Paleoproterozoic. Originally this unit was subdivided in the Arraias Formation (continental sequence) and Trairas Formation (marine sequence). However, the analysis, compilation and integration of the geologic data, mainly related to the sedimentary geology and the tectono-stratigraphy, had shown the necessity to formalize four unit subdivisions, including the Agua Morna, Arraias, Caldas and Trairas formations. The Agua Morna Formation represents a braided river fluvial deposicional system, evolved in an intracontinental SAG basin type, during a pre-rift phase, and representing the Agua Morna Tectonosequence submitted to thermal subsidence processes. The Arraias Formation represents a thick package of continental sediments, subdivided in four members: Cubiculo (aluvial fans), Prata (braided river), Mutum (aeolian), Ventura (lacustrine) and Buracao (volcanic and pyroclastic), representing the Arraias Tectonosequence, which constitute the infilling of the main rift phase, under mechanic subsidence. The Caldas Formation represents the transitional depositional sequence of the Arai Group, subdivided in two informal members related to beach system: Lower Member (backshore and foreshore) and Upper Member (shoreface). The Trairas Formation represents the marine sequence, subdivided in the Boqueirao Member (mixed platform dominated by tides), Rio Preto Member (siliciclastic platform dominated by tides) and Rosirio Member (mixed platform dominated by tides), integrating the Trairas Tectonosequence and developed during flexural subsidence (post rift phase). Here the rewied stratigraphy has shown well functional for mapping and regional correlations in different areas of occurrence of the Arai Group in Central Brazil.