Pyroxene-rich Orange sand highway from basaltic highlands to the ocean: Modern sediment-routing system of an Early Jurassic river

Abstract

Studying the origin of rivers, the development of drainage through geological time, and the multiple processes that affect the composition of the sediment is a fascinating scientific adventure. This article probes into modern sedimentary processes in the Orange River catchment, which covers much of South Africa, and monitors changes in petrographic, mineralogical, geochemical, and geochronological signatures along the ∼4000 km-long fluvial and coastal conveyor belt that transferred pyroxene-rich and diamond-bearing sand from Lesotho to Angola.The Orange River, established as early as the Early Jurassic as a classic example of dome-flank drainage, is perhaps the oldest river on our planet. A quarter of Orange sand is supplied by the erosion of Lesotho basaltic highlands, reaching 3482 m a.s.l. and representing a remnant of one of the most extensive magmatic effusions of Phanerozoic history, the Lower Jurassic Karoo lavas. Basaltic lavas, dolerite sills and dykes shed rock fragments and clinopyroxenes that constitute the unique fingerprint of Orange River sand. Only a tenth of the sediment is supplied by the Vaal River, the longest Orange tributary that drains siliciclastic and volcanic units ranging in age from Neoarchean in the Transvaal to early Mesozoic in the Karoo Basin. In the arid middle and lower reaches, the Orange River carves its course into the Namaqua Belt and receives the Molopo River flowing only episodically across the vast Kalahari Desert and the Fish River draining sedimentary rocks of the Nama Group and Karoo Supergroup. Fragile sedimentary rock fragments do not survive high-energy wave transport, but basaltic rock fragments and pyroxenes do, allowing us to trace sand transport for ∼1800 km all along the Atlantic coast of Namibia to as far as southern Angola. Understanding sediment mass transfer has scientific as well as practical interest, being a prerequisite for effective fluvial and coastal management, with particular economic significance in the special case of diamond-bearing Orange sand.