Fluvial-aeolian interactions and sand provenance in large African Sand Seas (Sahara and Kalahari)

Deserts are virtually the largest exposed sand depository on earth and their interaction with the river network may unveil information of present and past routing systems. Provenance studies of Sahara, Kalahari Deserts intend to apply common petrographic, geochemical and isotopic analysis to propose suggestive sedimentological information of dryland areas.

The mineralogical composition of aeolian dunes and its variability across a sand sea reflect the relative importance of fluvial and aeolian processes and the degree of their interplay. Sand seas largely fed by river systems are typically characterized by partly first-cycle detritus including various amounts of diverse types of rock fragments, feldspars and heavy minerals, generally allowing identification of a single dominant source. The opposite end member is represented by dunefields where sand is dominantly generated in situ from disaggregation of locally exposed rocks with high sand-generation potential (e.g., quartz-rich sandstones) and next reworked and homogenized by winds during several sedimentary cycles. In these cases, sand typically bears a distilled homogenous composition consisting almost exclusively of mostly rounded monocrystalline quartz associated with an extremely poor tHM suite dominated by durable ZTR minerals, as for the Sahara Desert (Pastore et al., 2021).

The Kalahari Basin, which extends over twenty degrees of latitude, is characterized by a pronounced increase in precipitation from the southwest to the subequatorial north and has seen repeated changes in climatic conditions through the recent and less recent past, provides both end-member examples, as well as a series of intermediate situations. Sand mineralogy is rather homogeneously pure quartzose in the north, closer to humid equatorial regions, but presents peculiar feldspar-rich or even lithic-rich compositions at both western and eastern margins of the erg, where detrital modes with more abudant and varied tHM suites indicate largely first-cycle supply from local rivers (Garzanti et al., 2022) . The evidence shows that a better developed fluvial network can interrupt the “recycling factory” of the desert, introducing first cycle eroded sediment deflated from river flanks into the dunes.

The geographic distribution of such contrasting desert types is mainly controlled by precipitation in adjacent highlands fueling fluvial discharge. In hyper-arid tropical deserts dominated by aeolian dynamics, such as the Sahara, river action may be weakened to the point that fluvial supply to the aeolian system becomes insignificant. Fluvial sources are instead readily identified for dunefields accumulated in drylands at the foot of high mountain areas, as in Kalahari flanks or central Asia and Argentina (e.g., Rittner et al., 2016, Garzanti 2020, 2021).

Pastore, G., et al., 2021. Provenance and recycling of Sahara Desert sand. Earth-Science Reviews, 216.

Garzanti, E., et al., 2022. Provenance of Kalahari Sand: Paleoweathering and recycling in a linked fluvial-aeolian system. Earth-Science Reviews, 224.

Rittner, M., et al., 2016. The provenance of Taklamakan desert sand. Earth and Planetary Science Letters, 437.

Garzanti, E.,  et al., 2020. Provenance of Thal Desert sand: Focused erosion in the western Himalayan syntaxis and foreland-basin deposition driven by latest Quaternary climate change. Earth-Science Reviews, 207.

Garzanti, E. et al., 2021. Transcontinental retroarc sediment routing controlled by subduction geometry and climate change (Central and Southern Andes, Argentina). Basin Research, 33.