Petrographic evidence of major changes in the course of the Nile tributary Atbara River in eastern Sudan during the Pleistocene

Abstract

Pleistocene alluvial sediments (∼230 – < 17 ka) along the middle Atbara River in eastern Sudan were investigated using sedimentary petrography to reconstruct the river’s evolution, changes in source areas and course, and the climatic and tectonic factors controlling its development. Sands from Pleistocene channel fills and modern river sands were analyzed for framework petrography and heavy mineral composition, and basalt samples were dated to constrain the age of local volcanism. The results reveal a clear compositional difference between the lower units (> ∼27 ka), dominated by quartz with an amphibole–epidote–clinopyroxene assemblage, and the upper unit (< ∼27 ka) and modern sands, which are richer in basaltic lithics and clinopyroxene. Ar–Ar ages indicate that volcanic activity persisted beyond the Oligocene into the late Miocene–Pliocene. The mineralogical change indicates a shift in provenance from Neoproterozoic metamorphic rocks, Cenozoic basalts, and granitoids, transported by precursors of the Gash and Tekeze rivers, to predominantly Cenozoic basalts eroded by the < ∼27 ka Paleoatbara and the modern Atbara River. Heavy mineral surface textures indicate a consistently arid to semi-arid climate during deposition, similar to today’s conditions. The marked mineralogical contrasts between the lower units and both the upper unit and modern sands indicate a fundamental shift in source area and a change in river course. The changes in the river's course indicate that the river system underwent substantial modifications in its recent geological past, raising questions regarding the age and stability of the Nile system.