DEPOSITIONAL ENVIRONMENT RECONSTRUCTION FROM STRATIGRAPHIC ANALYSIS OF THE MARKAY-TOLI SEDIMENTARY RIFT BASIN, SOUTHERN AFAR DEPRESSION, ETHIOPIA

The strata and fauna preserved within rift basins are excellent archives of both paleoenvironment and climate. This preliminary study provides a local scale paleo-depositional environment reconstruction of the Markay-toli sedimentary rift basin during a critical interval in Pleistocene human evolution when hominins were migrating out of Africa and inhabiting Europe and Asia. Understanding the paleoenvironment allows us to see how habitat changes impacted paleohumans and potentially aided the onset of key human evolutionary events. The Markay-toli rift basin, located in the Ledi-Geraru Research Project area in the southern Afar Depression of Ethiopia (11°18’N, 40°50’E), is a 4km², north-south striking half-graben that preserves both Oldowan and Achulean style stone tools (none yet in situ) and a diverse faunal assemblage including several small colobine and cercopithecine monkeys and a possible hominin tooth fragment. Preliminary biostratigraphic analyses of the well-preserved vertebrate fossils within the stratigraphy suggest an approximate age within the Early to Middle Pleistocene. We measured a series of ten, short (5-20 meters) stratigraphic sections at different locations within the basin to characterize facies variations among the northern and southern portions. The northern basin is dominated by a massive gravel of varying thickness (1-5 meters or more) covering approximately a 1 km² area which transitions into alternating beds of silts and clays to the south. Varying degrees of paleosol development and presence of root casts in the clays indicate subaerial exposure suggesting floodplain overbank depositional environment. Lying unconformably above the silt and clay unit in the mid basin are interbedded sands and conglomerates of a moderately high energy alluvial river system. A vitric tephra discontinuously exposed in the mid basin just below the conglomerates may correlate to tephras further to the north and potentially provide a correlated age for the sediments. A northerly and possibly a westerly paleo-fluvial source are indicated by clast imbrications, clast compositions, and cross bedding of sandstones. The generally undeformed basin sediments transition from very coarse to fine to coarse deposits suggesting a dynamic depositional paleolandscape dominated by fluvial systems.