O Pensilvaniano da Bacia do Parnaíba, norte do Brasil: implicações paleoambientais, paleogeográficas e evolutivas para o Gondwana Ocidental

Abstract

The end of the Paleozoic Age was marked by landmasses tectonic movements that were forming the supercontinent Gondwana, which stretched mainly in the southern hemisphere, surrounded by the Panthalassa oceans to the west and Tetis to the east. During the Carboniferous period, there were several cycles of climatic variations in the earth's history, evidenced in glaciogenic icehouse and greenhouse carbonate deposits, both in the paleocontinent Gondwana and in Laurasia. These global cycles of eustatic variation were recorded in the epicontinental seas, as they represent the thawing phase and eustatic level increase in a greenhouse period, which together with a favorable paleogeography form extensive marine transgressions over the continental blocks, with cyclic stacking patterns, called cyclothems (eg., terrestrial; terrestrialmarine mixed; marine and restricted marine / evaporitic pattern). Records of these events in northern Brazil are found in the intracratonic basins, particularly in the Balsas Group of Parnaíba Basin, where the exhibits allow us to assess the Pennsylvanian sedimentary history. The sedimentary succession studied belongs to the Piauí Formation Upper Member, described among the cities of José de Freitas, União, Miguel Alves and Lagoa Alegre, and exhibits richly fossiliferous carbonate deposits overlapped by thick pelitic packages and progradant clinoforms. Seventeen sedimentary facies were grouped into four facies associations (FA), representative of a shallow carbonate platform, adjacent to a coastal dune field, later replaced by lacustrine-delta deposits. The FA 1- coastal dune field/interdune comprises well selected, intensely bioturbed, fine to medium sandstones with plane-parallel stratification, tabular crossbedding and climbing translatent lamination. The shallow-sea deposits FA 2 consists of a succession of fossiliferous carbonate rocks, laterally continuous for hundreds of meters, interspersed with bituminous shale. These carbonates were dolomitized and have negative δ13Ccarb values covariate with positive values of δ18Ocarb, suggesting that the supersaturated fluid volume was sufficient to change not only δ18O but also δ13C. The FA 3- suspension lobes/mouth bar and FA 4- lacustre prodelta consist respectively of sigmoidal cross-stratification and planeparallel stratification sandstones and intercalated fine sandstones and pelites. The thick politic layers of prodelta in contact with FA 2 feature quartz grains with morphology texture of windorigin sediments, with textures such as bulbous and smooth edges, upturned plates, irregular depressions, and percussion marks. Subarea exposure surfaces in carbonate marked by shrinkage cracks and dissolution features indicate the end of carbonate sedimentation (eg., Marine Sequence - Highstand System Tract) with the Pennsylvanian Sea Retreat and Confinement in an extensive lacustrine system. (eg., Continental Sequence – High Accommodation System Tract) in the central portion of Gondwana. The FA 4 mineral clay assembly confirms the aridest climatic pattern to the top of the studied succession, presenting mainly smectites and illite. This marine retraction was concomitant with the Appalachian orogeny (300 Ma) that caused the uplift in western Gondwana and definitively disconnected the Itaituba-Piaui epeiric sea from the Panthalassa ocean to the west. Restricted seas or lakes were progressively beset by prograding hipopycnal flows with the establishment of the most extreme arid conditions triggered during Pennsylvanian.