Estudo sedimentológico da Formação Pimenteira (Devoniano) na borda sudoeste da bacia do Parnaíba (TO)

Abstract

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.