ABSTRACT: 

This paper presents the first detailed, outcrop-based sedimentologic investigation of the Pleistocene–Holocene upper Lake Beds succession in the Rukwa Rift Basin, located in the Western Branch of the East African Rift System, southwestern Tanzania. The goal of this investigation is to examine the sedimentary facies and reconstruct the depositional environments of this important archive of Quaternary climate and environmental change. Eleven diagnostic facies associations comprising 24 facies were identified and provide the basis for recognition of three key deposystems: 1) alluvial-to-fluvial channel system; 2) lake delta system; and 3) profundal lacustrine system. Analysis of paleocurrent indicators and sandstone provenance indicate widely dispersed source regions and drainage patterns that were strongly influenced by major border-fault systems and episodic volcanism. Six stratigraphic sequences (A–F), ranging from ∼ 2 to 17 m thick, were identified based on stratal stacking patterns and the development of sequence-bounding unconformities and lacustrine flooding surfaces. Sedimentation processes, facies architecture, and stratigraphic packaging record a complex interplay between Quaternary climate fluctuations and intense episodic volcanism in the nearby Rungwe Volcanic Province, set against large-scale tectonic controls associated with synchronous development of the East African Rift System.

Sequence stratigraphic analysis reveals that the Rukwa Rift Basin episodically shifted between a balanced-fill lake basin and an overfilled lake basin. Deep water, basin-wide lake expansion occurred at different times during the late Quaternary. The final depositional sequence preserved in the basin, a fluvial–underfilled lake basin, initiated ∼ 7.9 ka and has persisted to the present day. High-frequency climate change played the key role in sequence development in the upper Lake Beds. However, voluminous, rift-related volcanism and erosion of abundant labile volcanic materials from the Rungwe Volcanic Province, as well as syntectonic evolution of the rift, led to high sedimentation rates and transformation of flash floods and debris flows in the hinterlands (rift margin) to hyperpycnal flows towards the basin depocenter.

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