abstract: Sedimentary basins in the Afar Depression, Ethiopia archive the progression of continental breakup, record regional changes in east African climate and volcanism, and host what are arguably the most important fossiliferous strata for studying early human evolution and innovation. Significant changes in rift tectonics, climate, and faunal assemblages occur between 3-2.5 million years ago (Ma), but sediments spanning this time period are sparse. In this dissertation, I present the results of a geologic investigation targeting sediments between 3-2.5 Ma in the central and eastern Ledi Geraru (CLG and ELG) field areas in the lower Awash Valley, using a combination of geologic mapping, stratigraphy, and tephra chemistry and dating. At Gulfaytu in CLG, I mapped the northern-most outcrops of the hominin-bearing Hadar Formation (3.8-2.9 Ma), a 20 m-thick section of flat-lying lacustrine sediments containing 8 new tephras that directly overlie the widespread BKT-2 marker beds (2.95 Ma). Paleolake Hadar persisted after 2.95 Ma, and the presence and characteristics of the Busidima Formation (2.7-0.016 Ma) indicates Gulfaytu was affected by a reversal in depositional basin polarity. Combined with regional and geophysical data, I show the Hadar Formation underlying CLG is >300 m thick, supporting the hypothesis that it was the lower Awash Pliocene depocenter. At ELG, I mapped >300 m of sediments spanning 3.0-2.45 Ma. These sediments coarsen upward and show a progression from fluctuating lake conditions to fluvial landscapes and widespread soil development. This is consistent with the temporal change in depositional environments observed elsewhere in the lower Awash Valley, and suggests that these strata are correlative with the Hadar Formation. Furthermore, the strata and basalts at ELG are highly faulted, and overprinted by shifting extension directions attributed to the northern migration of the Afar triple junction. The presence of fossiliferous beds and stone tools makes ELG a high-priority target for anthropological and archaeological research. This study provides a new temporally-calibrated and high-resolution record of deposition, volcanism, and faulting patterns during a period of significant change in the Afar. / Dissertation/Thesis / Ph.D. Geological Sciences 2013