Evolução da costa nordeste do Brasil na região de Parnaíba durante o Pleistoceno Superior e Holoceno

Abstract

Previous studies in the Parnaíba region were focused on the Pirapemas lineament, the stabilization of a dune field, the evolution of the Parnaíba Delta and changes in the coastline. However, further studies are still needed to develop an evolutionary model for the Upper Pleistocene and Holocene. The study area is located in the Ceará Basin, the Quaternary deposits are on the basement. These deposits correspond to wind sands, river deposits and tidal deposits. This study aims to propose an evolutionary model, based on the following techniques: a) facies description; b) dating by optically stimulated luminescence (OSL); and c) remote sensing. Samples were collected for OSL granulometric and textural analysis, quartz cathodoluminescence (CL) and heavy minerals. The oldest Quaternary deposits are related to dune fields cut by braided rivers. The current scenario presents a new phase of dune formation with NE-SW wind direction, interdune areas are developed close to the water table, while mangroves are present in the northwest, the Parnaíba River is one of the most important features due to the supply of sediments. 17 facies were described, organized into 9 facies associations assisted by the morphostratigraphic units. FA1- briaded rivers are characterized by coarse materials with a high content of quartz clasts and iron cementation; FA2 - dune field - occupies the southern area and is associated with parabolic dunes stabilized by vegetation and with the current wind dunes in the north. FA3 – deflation área - interdune - is associated with FA2, formed in depressed áreas with subsequent growth of vegetation. FA4 - tidal channels, AF5 - mangroves and FA6 - tidal flat, are associated to the main tidal channels with preferential direction NW-SE. FA7 - shoreline - corresponds to deposits related to wave action. The meandering river FA8 is cutting all deposits and is characterized by a large floodplain, the FA9 lake is associated with fine sediment deposition formed in dammed channels. The collected samples contain quartz with high sensitivity to luminescence. The dose rate ranges from 0.585 ± 0.06 to 1.55 ± 0.07 Gy / ka. The ages obtained range from 2.1 ± 0.41 to 86.1 ± 6.4 ka and represent the evolution of a wind system. The sediments presented subangular to sub-rounded shapes with medium sphericity; the quartz catodoluminescence indicated a source associated with intrusive metamorphic and igneous rocks, which is consistent with the assembly of heavy minerals where ZTR and kyanite predominate. An evolutionary model was made for the last 86 ka, starting with the coalescence of river and wind processes (> 86 ka), damming of the main channels by wind sedimentation (86-20 ka), the stabilization of the dunes by climate changes associated with migration of the Intertropical Convergence Zone (20 to 10 ka), and the implementation of extensive mangrove areas from 10 ka to the present. This setting reveals a complete model for the development of transgressive dunes.