Armando Márcio Coimbra

Silicificação hidrotermal neocretácea na Porção Meridional da Bacia Bauru

Many occurrences of silicified sandstones were found in areas of exposition of the Caiua and Bauru groups (Bauru Basin, Late Cretaceous), in the northwest part of the states of Parana and extreme west of Sao Paulo, Brasil. Silicification occurs sometimes as pore lining, sometimes as partially or totally filling the intergranular space. Four different but often coexistent types of silicification were identified: (a) microcrystal growth in palisades, (b) sintaxial overgrowth (megaquartz), (c) pore filling by silica in mosaic polygonal texture, and (d) pore filling by silica in microcrystalline texture. Silicification developed in two different settings: (a) in a framework without compaction and (b) in mechanical compaction framework. Geological features of the occurrences and petrographic analyses indicate a hydrothermal origin for the silicification. It is very likely that such a hydrothermal event could have been associated with the Late Cretaceous alkaline magmatism. The silicification developed at the end of sedimentation and closing of the Bauru Basin

Palinologia de sedimentos da Bacia de São Paulo, Terciário do Estado de São Paulo, Brasil

The Tertiary sediments of the Sao Paulo Basin, which include the Sao Paulo and Itaquaquecetuba Formations, have been recently interpreted as a Paleocene sedimentary ensemble comprising: a basal alluvial fan to braided-stream system; a partially contemporaneous lacustrine system; a meandering fluvial system at the top of the sequence. The origin of the Itaquaquecetuba Formation (braided-channel system) is controversial, and it has been considered by different authors as having been associated with the basal deposits of the basin and as due to tectonic reactivation and sediment reworking during the Neogene or Quaternary. This study comprises the palynological analysis of sediments from the alluvial fan, laustrine and braided-stream systems (this latter attributed to the Itaquaquecetuba Formation); the respective palynofloras are homogeneous and include several taxa of stratigraphic and paleoclimatic value, suggesting very close ages and climates. The dorninance of Oligocene taxa together with only a few others considered as Eocene or older suggest two hypothesis: 1) possible reworking of Eocene sediments in the Oligocene; 2) the taxa previously considered as Eocene in age may have, in fact, lived until Oligocene, and so should have their distribution interval suitably corrected. The relative abundance of taxa indicating colder climates (conifers) and the scarcity of others more typical of hot and wet climate suggests that sedimentation took place under cold conditions. However, it was not possible to establish precisely the influence of altitudinal floristic differentiations in the preserved palynological assemblage

Sedimentologia dos depósitos cenozóicos continentais do Baixo Vale do Rio Ribeira do Iguape, SP

Recent studies on continental deposits that occur at the lower Ribeira do Iguape river valley (southern State of Sao Paulo, Brasil), led to the recognition of five different stratigraphic units, namely: the Sete Barras Formation, the Pariquera-Acu Formation, two levels of fluvial terraces, and colluvial-alluvial deposits. All these sediments were formerly attributed to the Pariquera-Acu Formation (with stages I,IIand III), considered to be Pleistocene. This paper presents the stratigraphic and sedimentological data which support the re-definition of the Pariquera-Acu Formation. Field work and grainsize analysis allowed to determine the main sedimentary processes for each unit. Maturity of heavy-minerals and nature of clays support the interpretation of paleoclimates. Both processes and paleoclimates pointed toward probable depositional systems and Cenozoic evolution of the area