Basaltic sources but quartzose sand: sediment provenance, weathering, and recycling in the Uruguay River catchment

ABSTRACT The Uruguay River, a classic example of dome-flank drainage, traces a wide arc across the Lower Cretaceous Paraná large igneous province, thus presenting an unexcelled opportunity for studying the generation and fate of basaltic detritus in a subtropical climate. In this study we integrate new petrographic, heavy-mineral, clay-mineral, detrital-zircon geochronology, and geochemical data to monitor the compositional evolution of clay, silt, and sand throughout the Uruguay catchment, from southern Brazilian sources to the Río de la Plata mouth. Pure basalticlastic sediment in Pelotas headwaters is progressively diluted by quartzose detritus recycled from upper Paleozoic to Cenozoic sandstones and almost no trace of volcanic-derived sand is left at the mouth. Even sand of Uruguay tributaries flowing entirely within the lava field is never purely basalticlastic but invariably contains significant or even dominant quartz recycled from underlying or locally intercalated quartzarenite layers. This testifies to the extremely low sand-generation potential of basaltic rocks in a subtropical climate. In humid southern Brazil, strong weathering intensity is attested by kaolinite-rich clay-mineral assemblages. In drier Uruguay, instead, either Fe-rich smectitic mud derived from basaltic lavas in the north or Fe-poor smectitic mud generated in non-volcanic terranes in the south is invariably dominant. The decrease in weathering intensity with increasing southern latitude is documented by the geochemistry of clay and fine silt, whereas the chemical composition of sand is overwhelmingly provenance-controlled although optical observations show much stronger weathering of basaltic glass in Brazilian headwaters than in Uruguay. Because of very low sand-generation potential and rare zircon content, Fe-rich smectite and rare chalcedony grains remain as the only testimony of basaltic provenance at the Uruguay River mouth. The worrying conclusion is that, in provenance studies of ancient analogues, even a huge basaltic lava field spanning most of the catchment and occupying the core of an intracratonic basin is quite likely to go undetected, even if weathering intensity was not extreme and even under the implausible condition of negligible postdepositional selective breakdown of mafic detritus.