Abstract

The intracratonic Paleozoic Parnaíba Basin, NE Brazil covers c. 600 000 km². Lithostratigraphic, mineralogical and geochemical approaches were used to deduce the changing environments of deposition of the Parnaíba Basin based on 150, mostly argillite, samples (Silurian–earliest Triassic). The results indicate that brackish conditions and mixed kerogen sources were important and that the palaeowater depth and sources of meteoric water varied from west to east. The chemical index of alteration results, together with other elemental and mineralogical data, indicate that the western margin was humid until the end of the Carboniferous and then became arid, whereas the eastern margin showed the reverse pattern. This supports observations of major stratigraphic differences between the two margins. There were also major variations in the palaeoredox and palaeosalinity conditions (oxic and fresh to brackish water systems in the east v. suboxic to dysoxic and saline water systems in the west). The clay mineral assemblages are characteristically dominated by kaolinite, which is considered to be inherited from weathering horizons developed in silicic granitic terranes. Regional palaeoclimate reconstructions were carried out in the Parnaíba Basin by linking GPlates software with the South America plate motion model (northwards drift during the Paleozoic coupled with a latest Paleozoic rotation phase) and with the regional database of information on the location, formation name and original references for each locality.