The Rukwa Rift Basin: a stratigraphic window into the last 150 million years of continental sedimentation and tectonics in the East African Rift System

Résumé fourni par la source

The Rukwa Rift Basin, a segment of the western branch of the East African Rift System (EARS) in southwestern Tanzania, yields one of the thickest and most extensive onshore, Cretaceous–Holocene continental sedimentary records in sub-equatorial Africa. In contrast to the Malawi and Tanganyika rift segments, covered by deep lakes; late Quaternary inversion has exposed the Rukwa stratigraphy along the rift axis. Here we present an updated synthesis of the tectonics, stratigraphy, sedimentology and age of the Rukwa Rift Basin succession spanning the last 150 million years. This work is based on extensive fieldwork in tandem with paleontological exploration, and in collaboration with hydrocarbon and helium exploration industry partners. Six discrete depositional phases, divided into two mega-sequences, are now recognized and correlated across the basin. Basin formation is linked to tectonically distinct Mesozoic and Cenozoic rifting episodes and reactivation of long-lived Proterozoic basement structures. New U–Pb dating results provide details on the timing of rifting and deposition, while extensive paleosol analysis, provenance analysis, and paleontological investigations yield new insights into the evolution of paleoenvironments and paleoclimate throughout the rift. Initial Cretaceous deposition is linked to rift reactivation associated with break-up of Africa and South America and is characterized by thick fluvial sandstones deposited within a major northwest flowing river system during the Aptian–Albian ‘cool’ greenhouse period. A second Cretaceous depositional cycle is linked to rift reactivation during the Santonian–Coniacian ‘Compressional’ event, and represents a warmer, humid fluvial system associated with continued northwest transport directions. Late Oligocene initiation of the EARS led to a major drainage reversal, initiation of rift lakes and internally draining rivers, coupled with explosive alkaline volcanism. This sequence was short-lived between 26–24 Ma. The final three Neogene depositional packages are associated with continued EARS tectonics within a highly-seasonal, volcanically and tectonically modulated period of rifting. These sedimentary packages are subdivided into a Mio–Pliocene (~10–3 Ma) alluvial fan to fluvial succession; followed by a structurally complex, poorly exposed early to Middle Pleistocene (~2 Ma–250 ka) mixed fluvial-lacustrine package; and capped by an extensive, well-exposed Late Pleistocene to Holocene (~60–5 ka) fluctuating lake system.

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