AR RADIOMETRIC AGES OF RARE PLIO-PLEISTOCENE STRATA FOR INTERPRETING BASIN EVOLUTION AND HOMININ PALEOENVIRONMENTS IN SOUTHERN AFAR, ETHIOPIA

The time period between 2.9 and 2.6 Ma is of particular interest in east Africa because a shift towards a cooler and drier climate coincided with a clustering of first appearance and extinction events in the faunal record (including hominins) and significant changes in African landscapes. Yet 2.9 to 2.6 million year old sediments are sparse in eastern Africa, and are especially rare at paleoanthropological sites such as Hadar, Gona, and Dikika in southern Afar, Ethiopia where tectonic processes altered the paleogeography that resulted in a disconformity spanning ~2.9-2.7 Ma. The paucity of disconformity aged sediments significantly impedes our ability to resolve important paleoenvironmental, evolutionary, and geological changes in the southern Afar record, such as the environmental context for the last known appearance of Australopithecus afarensis.

Here we present new ⁴⁰Ar/³⁹Ar radiometric ages of tephra deposits from the eastern Ledi-Geraru (ELG) project area in the lower Awash Valley, (adjacent to Hadar and Dikika). Results confirm that sediments exposed at ELG span 3.0 to 2.8 Ma (likely to 2.7Ma), thus providing the first glimpse of depositional landscapes and associated sediments that existed at that time. Our mapping and stratigraphic analysis at ELG show at least 65 vertical meters of lacustrine to fluvial sediments are well exposed west of the Awash River. These sediments lie near the northern- and eastern-most exposures of the Hadar and Busidima Formations (mapped 10-12 km southwest) suggesting they are contemporaneous. The results of our work provide an important opportunity to test proposed links between biotic events, global/regional climate change, and local tectonic events during what is a particularly interesting period of evolutionary and structural change in the Afar.