Paleogeografia e paleoambiente de depósitos siliciclásticos da transição Mississipiano-Pensilvaniano da Bacia do Parnaíba.

Abstract

The formation of the supercontinent Pangea in the passage from the Pennsylvanian period to the Mississipian led to profound changes in the sedimentation and behavior of Paleozoic sedimentary basins in the Gondwana. Orogenies resulted to expressive uplifts. One of the orogenies that affected the northern portion of Gondwana is related to the collision of Gondwana rith Laurásia that generated the Eo-Hercynian event in 320 Ma. Currently, this orogeny is related as the main responsible for the discordance present in several Paleozoic basins throughout the Gondwana. In order to test the influence of the Hercynian Orogeny in the paleogeographic and paleoenvironmental configuration, seven columnar profiles were studied on the southern, western and eastern edges of the Parnaiba Basin, where 19 facies were divided into 3 facies associations: delta front and plataform influenced by storm waves from the Poti Formation and braided fluvial from the Piaui formation. This study used facies modeling, petrography for classification of sandstones and provenance, quartz cathodoluminescence and paleocurrent analysis. The Poti Formation deposits are constituted of delta front bars with lobes dominated by sandstone facies with sigmoidal cross stratification and at the top, deltaic abandonment deposits and marine reworking. The braided fluvial deposits were divided into three architectural elements for a better understanding of their evolution. The laterally and vertically amalgamated channel-fill elements occur in the most proximal part of this system, where they have high energy and bedload transport. The non-amalgamated channel filling elements and overbanks deposits occur in the medial parts of the Piaui system, where the energy is lower when compared to proximal regions, and sediment transport is carried out by mixed load. In the medial regions there is the preservation of deposits of overbank. Paleocurrent and provenance data point to a fluvial system that migrates to NE-NW, where the main vector is north. During the Pensilvanian, the Piauí fluvial system dominated the northesat of the Gondwana migrating towards the epicontinental seas of this period, with source areas to the south od the Parnaiba Basin. It is accepted that the Carboniferous discordance would be related mainly to glacial-eustatic movements, related to the first peak of accumulation of Late Paleozoic Ice Age (LPIA) and the development of the fluvial system would be being supplied by melting waters, explaining thus the development of a perennial fluvial system in the midst of a desert system.