|
The glaciogenic succession at the eastern and southern margins of the Paraná Basin was accumulated under the influence of glaciers incoming from southwestern Africa during the passage form Late Carboniferous to Permian, with the ice advancing and retreating several times, resulting in a complex stratigraphic and facies framework. The goal of the present poster is the integration of lithostratigraphy, biostratigraphy and third order sequence stratigraphy from new data and interpretations on the geological evolution of these deposits, offering an updated overview of the glacial succession in the Paraná Basin. The glaciogenic succession can be divided into twelve distinct facies types mainly related to subaqueous glacial outwash fans. These facies can be grouped into four main facies associations: rhythmic and massive mudstones from distal turbidity currents in basinal settings, massive and laminated sandstones and conglomerates, diamictites and conglomerates, and wave-laminated sandstones. Sequence stratigraphic analysis has been applied to the glaciogenic succession using the concept that valleys, tillites and striated pavements mark the base of a glacial depositional sequence, representing the time of maximum glacier advance, while deep valley incisions would reflect faster rate of sea-level fall than the rate of isostatic loading and fine-grained facies would mark the "maximum flooding surface" during epochs of maximum glacier retreat. Using such guidelines, at least nine third-order boundaries can be recognized within the glacial succession The most complete and thick outcropping portion of the glaciogenic succession occurs in the northeastern part of the Paraná Basin, where several (at least nine) cycles of ice advance and retreat can be recognized. The succession is lithostratigraphically labeled the Itararé Group, with a thickness of up to 1,500 m. Palynomorph data indicate that the glaciogenic sedimentation began during the Bashkirian (Late Carboniferous). By this time, the southern part of the Paraná Basin was structurally high and probably covered by ice. Only during the Asselian and earliest Sakmarian (earliest Early Permian) sediments onlaped the southernmost margin of the basin. A major erosional feature filled by the so-called Lapa/Vila Velha Sandstone seems to be indicative of a major base-level fall, hence, marking a major sequence boundary within the glaciogenic succession. Biostratigraphically, two well defined stratigraphic gaps within the Permo-Carboniferous sequence are recognized, one within the Itararé Subgroup, and another at the Itararé/Rio Bonito boundary. The boundary between palynozones Ahrensisporites cristatus Zone and Crucisaccites monoletus Zone is associated with the oldest stratigraphic gap within the glaciogenic succession, while the boundary between Crucisaccites monoletus Zone and Vittatina costabilis Zone is associated with the maximum flooding surface represented by a shale bed labeled the Lontras Shale.
|
