Paleoambiente da Formação Prosperança, embasamento neoproterozóico da Bacia do Amazonas

Abstract

The Neoproterozoic Prosperança Formation represents a sedimentary cover of southern Guyana Shield and is poorly exposed when contrasted with Paleozoic record of Amazonas and Solimões basins. Conglomerates, arkosic sandstones and mudstones are the characteristics deposits filling basement grabens. This unit is overlain unconformably by neoproterozoic carbonate rocks of Acarí Formation (observed only in cores). These formations represent the sedimentary basement of oil producing Paleozoic basins of the Amazon region. The precise characterization and paleoenvironmental reconstruction of Prosperança Formation are important for their distinction from Paleozoic units. Stratigraphic analysis was carried out at lower Negro River, State of Amazonas. Four facies associations were interpreted as products of a fluvial-deltaic system: prodelta/lacustrine, delta front, foreshore/shoreface and distal braided plain. Mudstones distributed along several kilometers suggest a sedimentary basin probably of lacustrine/restricted sea origins. Complex structured deltaic lobes were fed by braided distributaries that migrated mainly to SE. Sandstones generated under oscillatory/combined flux are conformable with shoreline deposits. Planar to trough cross-bedded sandstones are related to migration of subaqueous dunes, associated to fluvial-braided processes. Lenses of conglomerate, planar to trough cross-bedded sandstones, possibly of paleozoic age, which overlies unconformably the Prosperança Formation migrated to NW as product of proximal braided plain system and is in contrast to the S-directed cross-stratification of the Prosperança sandstones.