Filling the temporal gap in Plio-Pleistocene sedimentological records from the southern Afar Depression, Ethiopia

The structural development of the Afar Depression controls sedimentary basin architecture and thus directly influences the spatial and temporal pattern of depositional environments. Over the past 5 Myr these processes, in addition to climate variations and volcanic eruptions, influenced the composition and variation of early human habitats and their record is preserved in the sediment record of synchronous rift basins. Here we present results from field investigations at Geraru located within the Ledi-Geraru field site, part of the greater Hadar sedimentary basin in the Afar region of Ethiopia. We aim to characterize local basin structure and expand and refine interpretations of the complex Plio-Pleistocene history of local and regional-scale landscape change during a time of critical importance for understanding hominin evolution. We mapped Geraru geology (1:7000), measured stratigraphic sections, and sampled 20 tephra deposits for absolute age dating (40Ar/39Ar) and for compositional studies and correlation using glass shard chemistry. We mapped over 16 km2 and identified lacustrine to fluvial sediments that are well-exposed along NW-SE to NE-SW trending faulted basalt hills that offset sediment blocks up to ~30 m. Although stratigraphic sections are not continuous, faulting relationships and marker beds suggest that at least 65m of strata are accessible. The lower ~40m of sediments are lacustrine deposits indicated by finely laminated silts and clays, two <2m thick diatomite layers, and gastropod shells. Conformably overlying them, we found ~25m of coarse fluvial sands and gravels that are locally cross-bedded and contain fossils of terrestrial fauna. Compositional results of glass shards from 4 tephra samples indicate that they not chemically similar to tephras from the Hadar (ca. 3.8-2.9 Ma) or Busidima (ca. 2.7-0.16 Ma) Formations. One tephra sample collected from the central portion of the stratigraphic section yielded a plateau age of 2.814 ± 0.017 Ma (more dates are being processed). Based on our preliminary stratigraphic interpretation and mapping, and review of notes from early workers in the region, Geraru strata likely represent depositional environments ca. 2.9-2.7 Ma. Other observations that support deposition at Geraru at that time include glass chemistry correlation and sedimentation patterns. Elsewhere in the Awash region, sediments spanning ca. 2.9-2.7 Ma are scarcely documented due to a period of non-deposition and erosion. Therefore, we are re-assessing structural models of basin geometry that indicate basin-wide reorganization near 2.9 Ma because models infer that the paleolandscape was erosional at that time (evidenced by an unconformity surface) rather than depositional (as suggested by strata at Geraru) - implying that Geraru may have been in a separate basin. Our work demonstrates that the sedimentary sequences at Geraru provide access to an important depositional record with good geochronological controls. It corresponds to important changes in regional tectonics and local basin configuration, climate patterns, and diversification and adaptive shifts in hominins.