Navegando por Assunto "Devoniano"

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Acesso aberto (Open Access)
Estudo petrográfico e geoquímico dos ironstones da base da Formação Pimenteiras, Borda Oeste da Bacia do Parnaíba, Tocantins
(2012-06) AMARO, Gabriel de Jesus Lavareda; VILLAS, Raimundo Netuno Nobre; KOTSCHOUBEY, Basile
Ironstones that occur at the base of the Devonian Pimenteiras Formation (northwestern border of the Parnaíba basin) were investigated along the Xambioá-Vanderlândia and Colinas do Tocantins-Couto Magalhães sections. The ironstones consist of discontinuous decimeter-thick layers intercalated in sandstones and shales that underlie rocks of the Neoproterozoic Araguaia belt. The Xambioá-Vanderlândia ironstones are texturally and chemically quite distinct from those of the Colinas do Tocantins-Couto Magalhães area. Besides the presence of oolites, the former shows smaller amounts of terrigenous material (especially quartz) and higher iron oxide-hydroxides proportions. They also are more enriched in V, Sr, Zr and ΣREE, and poorer in Al2O3 and Rb. The North American Shale Composite (NASC) normalized REE distribution pattern is also different, especially concerning the IREEN values, which, being higher in the oolithic ironstones, display convex curves and, being lower in the non-oolithic ironstones, display concave ones. In the field, spatial relationships could not be defined between both ironstone types. It is then suggested that they represent different lithological facies of the same iron formation. Probably, the deposition of the non-oolithic facies occurred in deeper and quieter waters away from the continental border, where larger amounts of detrital sediments were discharged, whereas the deposition of the oolithic facies took place in shallower and more agitated waters, with less supply of terrigenous material. Iron was largely derived from the erosion of continental areas where reduced environments favored its mobilization and transport by rivers as suspended particles, colloids, and organic complexes.
Acesso aberto (Open Access)
Estudo sedimentológico da Formação Pimenteira (Devoniano) na borda sudoeste da bacia do Parnaíba (TO)
(Universidade Federal do Pará, 1998-04-15) SILVA, Valter Fernandes; CAPUTO, Mário Vicente; http://lattes.cnpq.br/1028384858323270
The part of the Pimenteira Formation studied occurs in the southwest portion of the Parnaíba Basin, in the region between the cities of Paraíso, Miranorte, Miracema, Tocantinea, Pedro Afonso and Itacajás in the State of Tocantins. This formation is considered to be deposited in a transgressive sea system tract of Middle and Late Devonian age (Givetian to Famenian). In the area on stage, the studies of the facies made it possible to individualize three associations of facies named here as A, B and C that were deposited on a shallow marine platform during transgressive sea level, regressive sea level and high sea level, respectively. These facies associations were interpreted as a product of the following depositional environments: 1) LAMMA MARINE PLATFORM (association of facies A) represented by F1 (laminated shale facies), Alp (sandstone with plane-parallel lamination and Aab (sandstone intercalated to claystones with sand pockets) where the deposition took place mainly from fine suspended sediments (pelites) intercalated with fine to very fine sandstones (psamites) deposited under the influence of oscillatory and tractive flow originated by waves; 2) PROXIMAL GLACIOMARINHO with SUBGLATIAL CHANNEL, associated (association of facies B), represented by facies Dmm (massive diamictite), Acf (sandstone with fluidized clast), F1 (laminated shale), Pgm (massive coarse paraconglomerate), Pfm (massive fine paraconglomerate) and Agm ( massive coarse sandstone), were deposited from glaciers, with associated subglacial channels, ice rafts and/or icebergs that displaced from the continent, floated in the sea, released generating melt water bringing a large amount of fine and coarse sediments, forming a plume loaded with suspended sediments in front of the gallery. With the decrease in energy, extensive layers of mud are deposited, with pebbles and gravels, dispersed, being released from ice rafts and/or icebergs, as they melt. The facies F1, Pgm, Pfm and Agm represent a bar deposit remaining from a subglacial channel. Subglacial channels discharge coarse sediments in front of the glacier that can be transported further by turbidity currents forming lenticular or layered deposits interspersed with diamictites; 3) SHALLOW MARINE PLATFORM UNDER THE ACTION OF STORMWAVES (association of facies C) represented by facies F-A1 (shale-laminated sandstone), F1-S (laminated shale interleaved with siltstones), Aco (sandstone with wavy cross bedding truncated by wave), Ap (sandstone with plane-parallel bedding), Apt (sandstone with plane-parallel and tabular cross bedding), Acot (sandstone with wavy truncated wave and tangential cross bedding), Ach (sandstone with hamocky cross bedding), Aptb (sandstone with plane-parallel and cross-tabular bioturbated bedding), Apmo (sandstone with plane-parallel bedding and wavy marks), Amg (massive sandstone with dispersed granules and pebbles) and Pm (massive paraconglomerate). The deposits are characteristic of platform bars with dominant hummocky structures, attesting to the action of storm waves, covered by extensive layers of marine shales deposited during the good weather phase. This facies association predominates in the upper portion of the Pimenteira Formation gradually passing to the deformed sandstones and diamictites of the Cabeças Formation east of the town of Pedro Afonso.