Chemical weathering rates of clastic sedimentary rocks from the Paraná Basin in the Paulista Peripheral Depression, Brazil
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2019-12-01
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The chemical weathering plays an important role in the processes of surface geochemistry, which is fundamental to understand the landscape evolution. The Paraná Basin is an intracratonic sedimentary basin developed on the South American platform from Paleozoic to Cenozoic, with area of 1.5 × 106 km2 and 5000 km3 of clastic sediments and igneous rocks. The main clastic sedimentary rocks in the Paraná Basin are sandstones and mudstones. Surprisingly, there are no studies assessing the chemical weathering rates of clastic sedimentary rocks from the Paraná Basin under tropical climate conditions in Brazil or in South America. Thus, two small watersheds were chosen to assess the chemical weathering rates of clastic sedimentary rocks from the Paraná Basin in the Paulista Peripheral Depression (São Paulo State, Brazil), i.e. the Monjolo Grande and Jacutinga stream basins. The chemical composition of rain and surface waters, as well as the chemical and mineralogical composition of the fresh rocks (sandstones and mudstones) and soil (horizon C) were characterized. The instantaneous daily flux indicates that ca. 60% of the dissolved solids loads are carried from October to March in both watersheds. The natural fluxes due to chemical weathering (Fw) were ca. 4.5 t km−2 yr−1, with a mixed of local rainwater and chemical weathering of sandstones and mudstones controlling the surface waters composition. The possible chemical weathering processes of sandstones and mudstones occur by quartz dissolution and partial hydrolysis (monossialitization) of albite, microcline and illite, respectively. The chemical weathering rates were of 0.3 and 0.8 m Myr−1 for the Monjolo Grande and Jacutinga Streams basin, respectively, confirming the lower chemical weathering rates for clastic sedimentary rocks in relation to basalts (Paraná CFB) of the Paraná Basin and for the igneous and metamorphic rocks of the Ribeira Belt, other important geological units in the São Paulo State. In addition, these chemical weathering rates are also lower than the suggested for other rock types elsewhere, i.e., carbonates, basalts, granites, schist, gneisses or alkaline rocks. Finally, the effect of climate on [SiO2] was demonstrated, also described in basaltic and granitic watersheds, suggesting that the chemical weathering of sandstones and mudstones are more effective in the wet season.
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Brazil, Geomorphological provinces, Sandstones and mudstones, Water-rock interactions
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Journal of South American Earth Sciences, v. 96.