Correlation and stratigraphy of the BKT-2 volcanic complex in west-central Afar, Ethiopia

Located adjacent to the paleoanthropological site of Hadar in Afar, Ethiopia, the Ledi-Geraru project area preserves multiple tephra deposits within the Pliocene sediments of the hominin-bearing Hadar Formation. Tephra deposits of the Bouroukie Tuff 2 volcanic complex (BKT-2) are important regional markers, and here we provide correlations between the Hadar and Ledi-Geraru project areas using major-element glass chemistry, stratigraphic relationships, outcrop characteristics, and (super 40) Ar/ (super 39) Ar dates. These correlations greatly expand existing temporal and spatial resolution, aid in interpretations of regional depositional environments, and increase the documented extent of BKT-2 to approximately 600 km (super 2) . BKT-2 exposures at Ledi-Geraru are the thickest and most complete yet observed. There, the BKT-2 complex is preserved as two air-fall lapilli layers, BKT-2U (<97 cm thick) and BKT-2L (<9 cm thick), separated by <2.5 m of silts and clays or diatomite that overlie the Green Marker Bed (GMB), a laminated ash tuff. Measured sections were evaluated to create a stratigraphy-based model of paleo-landscape variations using BKT-2 tephra as laterally extensive isochronous surfaces. BKT-2 was mainly erupted into lacustrine and nearshore environments. The eastern Ledi-Geraru region was likely located at the depocenter of an expansive fluviolacustrine network. Representing the last major lacustrine phase of the Hadar Formation, lateral facies variations show the westward expansion of a lacustrine setting ca. 2.96 Ma followed by eastward regression initiated sometime prior to the eruption of BKT-2U ca. 2.94 Ma. High-resolution, well-correlated, and temporally constrained stratigraphic records are key to the interpretation of paleoenvironmental variation in East Africa.