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

Abstract

This article presents a detailed petrographic, mineralogical and geochemical study of clay, silt, sand and fine gravel generated in the Paran a-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 lm to >2000 lm. 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, a Al E 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 Paran a 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-locallyexposed 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.

Holocene fluvial depositional regimes of the Huab River, Skeleton Coast, Namibia

“…This indicates input from the basalt lavas of the Etendeka Plateau, and the gabbro‐dolerite sills within the Huab Basin, respectively (Figure 2). Clinopyroxene in the HM dataset is likely sourced from both the Etendeka basalts and gabbro sills, yet olivine is only from the gabbro sills (Duncan et al, 1989; Garzanti et al, 2014, 2021). There is a close association between the occurrence of pyroxene and olivine, and when these heavy minerals increase, gabbro and volcanic fragments increase in the PT dataset.…”

“…Chemical weathering rates in dryland Namibia are slow (Garzanti et al, 2021; Viles, 2011), and so changes in sediment composition are more likely to have resulted from physical processes. Increased collisions and more efficient abrasion from geomorphic processes would have resulted in more effective partitioning of sand particles (Cleary & Conolly, 1972; Fontana et al, 2003; Harrell & Blatt, 1978).…”

“…The Etendeka large igneous province is composed of basalt, basaltic andesite, and quartz latites (Ewart et al, 2004;Garzanti et al, 2021;Marsh et al, 2001). Assemblages from the Koigab and Axiab rivers draining the Etendeka are dominated by mafic lathwork volcanic fragments, clinopyroxene, and plagioclase (Garzanti et al, 2014(Garzanti et al, , 2021.…”

“…The Etendeka large igneous province is composed of basalt, basaltic andesite, and quartz latites (Ewart et al, 2004;Garzanti et al, 2021;Marsh et al, 2001). Assemblages from the Koigab and Axiab rivers draining the Etendeka are dominated by mafic lathwork volcanic fragments, clinopyroxene, and plagioclase (Garzanti et al, 2014(Garzanti et al, , 2021. Alongside Lower Cretaceous fluvial and aeolian sandstones (Jerram et al, 2000;Stanistreet & Stollhofen, 1999), igneous intrusives in the form of gabbros and dolerite sills occur in the southern part of the Huab basin (Jerram et al, 1999).…”

The Segmented Zambezi Sedimentary System from Source to Sink: 1. Sand Petrology and Heavy Minerals

“…Although the sand generation potential of basalt is notably less than that of sandstone or granite (Garzanti et al 2019a(Garzanti et al , 2021a(Garzanti et al , 2021bLe Pera and Morrone 2020;Morrone et al 2020), mafic lava contains and sheds much more clinopyroxene than the few heavy minerals that quartzose sandstone contains and can thus supply. Therefore, wherever basaltic detritus mixes with recycled quartz, as in the Upper Zambezi, quartz still dominates among main framework grains but the tHM suite rapidly becomes clinopyroxene-dominated (Figs.…”