Fácies sedimentares da Formação Prosperança, Neoproterozóico do Sul do Escudo das Guianas, estado do Amazonas

Abstract

The Neoproterozoic Prosperança Formation represents a sedimentary cover of southern Guyanas shield, poorly exposed when compared with Paleozoic sedimentary record of Western Amazonian basins. The Prosperança Formation consists of conglomerates, arkosic sandstones and mudstones that fill NW-SE oriented-grabens on crystalline rocks of Guyanas shield basement. These unit and the carbonate rocks of Acarí Formation (observed only in cores), constitutes the Purus Group, sedimentary basement of oil producer Paleozoic basins of the Amazon region. Based-outcrop facies and stratigraphic analysis were carried out at lower Negro River, State of Amazonas, indicated a fluvial-deltaic system to Prosperança Formation composed by facies associations of 1) prodelta/lacustrine, 2) delta front, 3) foreshore/shoreface and 4) distal braided plain. Prodelta mudstones laterally continuous for hundreds of kilometers suggest a wide lacustrine or restricted sea sedimentary basin. Complex deltaic lobes were fed by braided distributaries formed by subaqueous dunes that migrated mainly to NW of Amazon block during the Neoproterozoic times. Sandstones generated under oscillatory and combined flow representative of foreshore and shoreface zones were adjacent to active delta front. The Prosperança basin was weakly inverted during the Cambrian-Ordovician interval. Afterwards, these deposits were unconfomably overlaid by sandstone and conglomerate with planar to trough cross-bedded sandstones related to SE-oriented braid plain of Eo-Paleozoic Trombetas Group of Amazon basin.