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Middle Miocene to Pleistocene sedimentary record of rift evolution in the southern Albert Rift (Uganda)

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Abstract

This study presents an almost complete Middle Miocene to Pleistocene sequence of synrift sediments in the western branch of the East African Rift. The studied succession is exposed in several patches on an eastward tilted block between the northern tip of the Rwenzori Block and the eastern shoulder of the Albert Rift. In this position, it reaches a maximum thickness of 600 m of which 350 m have been logged systematically by analysing lithofacies and sediment architecture. Stratigraphic subdivision of the succession relies on published biostratigraphic data of endemic mollusc associations and their correlation across East Africa. The synrift sediments encountered are siliciclastics ranging from clay to coarse gravel with gypsum and ferrugineous interlayers or impregnations. Lithofacies and architectural analysis indicate alluvial plain, delta plain, nearshore, delta front, or lacustrine depositional environments. Based on the vertical stacking pattern, prograding and retrograding trends of the depositional environments, and climatic indicators (e.g. conservation of feldspar, gypsum, and/or iron hydroxide precipitation), four evolutionary phases can be distinguished: (i) a first phase between ca. 14.5 and 10.0 Ma is characterised by bedload-dominated fluvial environment with massive sandy to gravelly bedforms, feldspar-rich sands, rare iron impregnations and relatively low accommodation space. This phase is interpreted as pre- and early synrift sedimentation under a semiarid climate. (ii) From ca. 10.0 to 4.5 Ma predominantly fine-grained siliciclastics were deposited in a distal fluvial plain to lacustrine setting characterised by limited accommodation space. Fluctuation of thin beds, dominance of clay and frequent iron impregnations point to a more humid climate with seasonality and weak tectonic activity. (iii) During the third phase between 4.5 and 2.0 Ma delta plain and nearshore deposits with frequent ferrugineous impregnations and rich mollusc associations occurred, indicating a humid period with lake-level highstands and accelerated subsidence. (iv) During the final sedimentary interval between 2.0 and 1.5 Ma gravel units reoccurred with less iron- but more carbonate and gypsum impregnations, and arkosic sandstones. This phase recorded a general aridisation trend most probably caused by the upcoming rain barrier of the Rwenzori Mountains together with accelerated rift-flank uplift and strong subsidence of the rift floor. The results of this study are of particular importance for delineating key controls on sedimentation in the Albert Rift.

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Acknowledgments

The project “B3–Linking Source and Sink in the Rwenzori Mountains and adjacent rift basins, Uganda” is funded by DFG grant HI643/7-1 within RIFTLINK research group. We acknowledge the extremely helpful reviews made by S. Back and H. v. Eynatten which considerably improved the manuscript. We thank the Uganda National Council for Science and Technology and the Uganda Wildlife Authority for research permissions, and our Ugandan research partners from Makerere University for close cooperation. Many thanks go to the staff of the Toro-Semliki Wildlife Reserve for unpayable and dedicated support during many weeks of fieldwork in the bushes.

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Roller, S., Hornung, J., Hinderer, M. et al. Middle Miocene to Pleistocene sedimentary record of rift evolution in the southern Albert Rift (Uganda). Int J Earth Sci (Geol Rundsch) 99, 1643–1661 (2010). https://doi.org/10.1007/s00531-010-0560-z

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