Evolution of the northern Albertine Rift reflected in the provenance of synrift sediments (Nkondo-Kaiso area, Uganda)

Abstract

Rift sediments are important archives for the evolution of the East African Rift System. We present a source-to-sink study of the Nkondo-Kaiso area in the northern Albertine Rift from a ∼200 m thick late Miocene to early Pleistocene sediment succession. The multi-proxy provenance analysis includes framework petrography, heavy mineral studies and single grain studies of garnet and rutile. Mineral textures and provenance signatures indicate two major stages. Sediment of Stage 1 (late Miocene - late Pliocene; ∼6.5–2.6 Ma) is less mature with quartz contents of 58–81%, K-feldspar of 9–15%, plagioclase of 4–15%, and rock fragments of 1–9%. Heavy mineral spectra are dominated by epidote and amphibole with minor abundances of zircon, sillimanite and garnet. Garnet is almandine-rich and can be grouped according to grossular contents (<10% and >10%). Rutile exhibits a wide range of Nb and Cr concentrations, most of them typical for a metapelitic origin. Zr-in-rutile formation temperatures range between ∼570 and 940 °C. Garnet and rutile geochemistry mainly correspond to amphibolite-to granulite-facies igneous and metasedimentary rocks, which exist in the Mesoarchean Karuma Group and the Neoarchean granitoid gneiss of the North Uganda Terrane. Both are part of the adjacent eastern rift flank of the Albertine Rift. A slight change of the heavy mineral composition at ∼4.5 Ma can be correlated with the onset of the synrift stage ∼0.5–1.0 Ma later than in the southern Albertine Rift. Sediment of Stage 2 (early Pleistocene ∼2.6–1.0 Ma) shows a higher quartz content of 82–93%, and lower contents of K-feldspar of 5–11%, plagioclase of 1–7%, and rock fragments of 1–2%. Stable heavy minerals (ZTR index: 19–63) and epidote dominate with minor amphibole and garnet. Rutile and garnet chemical compositions remain largely unchanged to Stage 1 pointing to the same primary source. Obviously, Pleistocene sediment was mainly recycled from sedimentary rocks of the Neoproterozoic Bunyoro Group, which originate from identical Archean rock assemblages. Ongoing river incision reactivated the primary Archean source in the upper part of Stage 2. In conclusion, Miocene to Pleistocene sediment of the Nkondo-Kaiso region is supplied from local sources of the rift flank and the adjacent Ugandan plateau. Tectonic pulses at ∼4.5 Ma, ∼2.6 Ma, and ∼1.5 Ma led to river incision and moderate expansion of drainage basins towards the east. These pulses are roughly coincident with other parts of the EARS but also show regional time lag and delayed response of the source-to-sink system.