DEPOSITIONAL ENVIRONMENTS AND VOLCANIC CHARACTERIZATION OF THE BKT-2 VOLCANIC COMPLEX IN AFAR, ETHIOPIA

The Bouroukie Tuff 2 (BKT-2) complex (ca. 2.96-2.94 Ma) is a series of three airfall tephra units preserved throughout more than 600 km² of the hominin-bearing Hadar Formation of west-central Afar. The identification of BKT-2 in the Ledi-Geraru region has permitted the characterization of thicker and better preserved deposits that offer insight into the style, intensity, and size of the eruptions. The upper bed of the BKT-2 complex, BKT-2U, records an explosive bimodal eruption likely deposited via fallout from a Plinian style umbrella cloud. Electron microprobe analysis of glass from BKT-2U reveals the presence of ferrobasalts, subalkaline rhyolites, and rare intermediate compositions. Field observations of BKT-2U show a unique stratigraphic gradation in the proportion of rhyolitic pumice and basaltic scoria pyroclasts, suggesting that the magmas coexisted and were erupted contemporaneously. Most pumice and scoria occur as individual clasts, while some are joined along sharp contacts that may have formed during hot, high-velocity interactions in the eruption column. Additionally, the absence of mantled glass shards or banded pumice suggests that negligible mingling occurred prior to the eruption. While the rhyolitic glass chemistry of BKT-2L (approximately 2 meters below BKT-2U) is indistinguishable from BKT-2U, the difference in feldspar geochemistry can be used to differentiate the two units, and may be related to differences in physical conditions between the BKT-2L and -2U eruptive events.

Located near the top of the Hadar Formation, the BKT-2 complex is concurrent with the last major lacustrine phase of the formation and is stratigraphically above fauna associated with and including Australopithecus afarensis. A stratigraphic-based model of paleolandscape variations using BKT-2 as laterally extensive isochronous surfaces indicates that it was primarily erupted into lacustrine and near-shore environments, with the eastern Ledi-Geraru region likely located at the depocenter of an expansive fluvial-lacustrine network. Lateral facies variations show the transgression of lacustrine conditions ca. 2.96 Ma, shortly before the eruption of BKT-2L, followed by a regression initiated sometime prior to the eruption of BKT-2U ca. 2.94 Ma.