Sand and mud generation from continental flood basalts in contrasting landscapes and climatic conditions (Paraná–Etendeka conjugate igneous provinces, Uruguay and Namibia)

This article presents a detailed petrographic, mineralogical and geochemical study of clay, silt, sand and fine gravel generated in the Paraná–Etendeka large igneous province, once united and today exposed in contrasting climatic and orographic conditions on conjugate sides of the Atlantic Ocean. The comparison between low-relief wetter Uruguay and steeper hyper-dry Namibia allows to investigate the influence of weathering on basalt-derived sediments spanning in grain size from <2 μm to >2000 μm. In Namibia, as in Uruguay, X-ray diffraction analyses of mud samples from streams draining basaltic lavas show only smectitic clay minerals. Macroscopic and microscopic observations indicate that sand is composed in variable proportions of glassy to lathwork volcanic rock fragments, plagioclase and augitic clinopyroxene with minor pigeonite, and gravel dominantly consists of volcanic clasts with chalcedony or quartz geodes occurring locally. Geochemical data highlight the strong grain-size dependence of weathering intensity. Whereas fine gravel closely reflects source-rock composition in both Namibia and Uruguay, and mobile elements in sand are moderately depleted, αAlE indices increase notably in silt and reach maximum in clay, with mobility sequence Na >> Ca ≥ Mg > Sr > K. Differences in sand versus mud generation potential in the two regions are prominent. Namibian rivers draining the dry Etendeka Plateau carry purely volcaniclastic sand, whereas streams draining even exclusively the Paraná lava field carry sediments containing invariably significant and even overwhelming amounts of recycled quartz, testifying to the much greater sand-generation potential of the very-locally-exposed underlying, interbedded or overlying quartz-rich sandstones and loess deposits. Smectite is dominant throughout the Uruguay River catchment, even in the lower course where sand is pure quartzose and pyroxene negligible. A comparison with basaltic provinces worldwide indicates that, especially in low-relief wet regions where weathering is intense, basalt generates gravel, very minor sand and abundant smectite. Kaolinite forms where chemical leaching is extreme.