The making of pure quartzose sand in continental interiors: Paraná River (Brazil and Argentina)

ABSTRACT As part of a comprehensive project on sedimentary processes in South America, this study focuses on sediment generation in the intracratonic Paraná Basin and monitors the evolution of sand composition along the Paraná River from central Brazil to the Río de la Plata estuary in Argentina. Several techniques (petrography, heavy minerals, detrital‐zircon geochronology, elemental geochemistry and mineral‐chemical analyses) are combined to evaluate the relative importance of the various factors that contribute to the production of pure quartzose sand. The huge Paraná River catchment consists of 79% siliciclastic sediments and sedimentary rocks, with basaltic lavas and Precambrian basement in subequal subordinate proportions. Pure quartzose sand supplied to the Río de la Plata is largely produced by extensive recycling in intracratonic settings, but even Andean tributaries carry quartz‐rich litho‐quartzose sand recycled from Palaeozoic to Cenozoic siliciclastic rocks exposed along the retro‐side of the cordillera. First‐cycle basement‐derived detritus is generated only in the headwaters of major Brazilian tributaries, where feldspatho‐quartzose sand rapidly evolves into plagioclase‐depleted quartzose sand as rivers incise across upper Palaeozoic siliciclastic rocks of the Paraná Basin. Basement‐derived and volcanic‐derived detritus is negligible in the lower Paraná course in Argentina because of the very low sand‐generation potential of basaltic lavas and the impoundment of basement‐derived and basalt‐derived detritus into numerous artificial gigantic reservoirs as well as natural wetlands (e.g. Pantanal). Chemical weathering contributes to the increased durability of intracratonic sand in wetter subequatorial latitudes, but only the full integration of petrographic and geochemical parameters can help distinguish whether selective depletion of labile detrital components is the result of current weathering or is inherited from previous sedimentary cycles. The Paraná River offers a precious lesson to learn how the typical quartz‐rich ‘cratonic interior’ signature of sediments is produced and evolves in continental intraplate settings to be finally transmitted to passive‐margin beaches, continental embankments or deep‐sea fans, with notable implications on reservoir quality and hydrocarbon exploration.