MODELO DE PREENCHIMENTO DE VALES INCISOS POR ASSOCIAÇÕES DE FÁCIES ESTUARINAS, FORMAÇÃO RIO BONITO NO NORTE DO ESTADO DO PARANÁ

Abstract

The Rio Bonito Formation is the lowermost unit of the Guata Group, Early Permian of the Parana Basin. It comprises from bottom to top, the Triunfo, Paraguacu and Sideropolis Members. In the State of Parana, the thickness of the Rio Bonito Formation decreases northwards, mainly because of the thinning and wedging out of the Triunfo Member, which disappears in the south of the State of Sao Paulo. The thickness and facies variations in the Rio Bonito Formation in the northeast of the State of Parana was the main focus in the present work because of good outcrops, and the existence of important coal beds. Vertical stratigraphic profiles, stratigraphic sections, paleocurrent data and sedimentary facies were analysed, and five facies associations were characterized in the study area. The thickness variation of the Rio Bonito Formation in the northeast of the State of Parana resulted from sedimentation on an irregular depositional surface. Due to the existence of paleovalleys, bipolar paleocurrents towards the north and south and sedimentary structures produced by tide currents, the lower portion of the Rio Bonito Formation was interpreted as a product of incised-valleys filled-up by estuarine facies associations. The lower alluvial and tidal channels deposits (facies associations 1 and 2, Triunfo Member) are overlain by mudstones, coal beds and sandstones of tidal-channels from the innermost portion of the estuary (facies association 3), and by coastal to marine systems facies (facies association 4). Facies associations 3 and 4 correspond to the Paraguacu Member. This vertical stratigraphic succession is interpreted as an estuary dominated by tides and waves. A valley fill facies model was proposed to explain this transgressive depositional systems tract. Regressive coastal sandstones of the facies association 5 (Sideropolis Member) occur discontinuously at the upper portion of the Rio Bonito Formation.