Provenance techniques are widely applied in resolving the origin of ancient sediment successions. Petrographic signatures, however, are controlled by a complex interplay of rock type, weathering mode as well as sediment transport, thus making the interpretation of data often ambiguous. One way to improve our understanding on these mechanisms is to study modern drainage systems. We selected the Kunene River in northern Namibia as a case study. This river system is well suited for such an approach because of contrasting lithologies in the drainage basin and the semiarid climatic conditions, from which a combination of physical and chemical weathering can be expected. We focus specifically on the interpretation of heavy mineral spectra.
The Kunene is one of the few perennial rivers in the Angolan-Namibian border area. The modern production of sediments is controlled by the interaction of varying climates along the river course, a variable topography and diverse rock units in the catchment area. We sampled four tributaries draining distinct rock suites as well as two sites at the main river trunk and analysed their petrography and heavy mineralogy. The working area is mainly characterised by Mesoproterozoic gneisses and amphibolites of the Epupa Metamorphic Complex (EMC), and mafic and felsic plutonites of the Kunene Anorthosite Complex (KAC) as well as clastic dominated sedimentary rocks of the Neoproterozoic Damara Supergroup. High amounts of epidote group minerals and amphiboles point to a distinctive contribution of EMC-rocks, while the presence of clinopyroxene, hypersthene and olivine can be interpreted as KAC-sourced. High amounts of zircon, tourmaline, and rutile as well as low heavy mineral concentrations can be interpreted as indicators for the influence of recycling of the Damara-rocks. Petrographically, the EMC-gneisses shed higher ratios of metamorphic quartz. A GIS analysis showed that for all sampled sites the heavy mineral spectra are primarily controlled by the rock type, in particular when transport length is short and relief is high. Chemical weathering does not significantly affect both petrography and heavy mineral spectra of the sand fraction. In conclusion, our results show that primary heavy mineral signatures are well conserved in semi-arid climates even in a relatively low-relief landscape.
Jonas Kraus, Ali Abbas Wajid, Laura Stutenbecker, Matthias Hinderer
Institute of Applied Geosciences, Technical University of Darmstadt, Darmstadt, Germany