TOWARD A CHRONOSTRATIGRAPHIC FRAMEWORK FOR THE LAST MILLION YEARS OF ENVIRONMENTAL AND HUMAN EVOLUTION IN THE MIDDLE AWASH, ETHIOPIA: 40AR/39AR GEOCHRONOLOGY AND TEPHRA CHEMISTRY

The Middle Awash study area of Ethiopia within the Afar Depression of the Eastern African Rift System contains fossiliferous terrestrial strata spanning the Late Miocene to the Late Pleistocene. ⁴⁰Ar/³⁹Ar dating of tephras from volcanic centers associated with rifting has provided the framework for constraining the timing and tempo of biological and cultural evolution in the study area during the last c. 1 Ma. However, in this time range of the basinal succession, many tuffs lack potassium-rich minerals, resulting in less precise ages from plagioclase or low-K alkali feldspars and thus requiring many more analyses to yield weighted mean ages with relative 1σ < 5%. Tephrochronology has previously assisted in the temporal characterization of intercalated fossiliferous deposits, as aphyric or sparsely phyric tephras that are challenging to date directly are nonetheless yielding a growing chemically-correlated framework for which ⁴⁰Ar/³⁹Ar geochronology is providing crucial anchor points. Recent work from c. 500 single-grain feldspar ⁴⁰Ar/³⁹Ar analyses has resulted in new or refined ages of six crucial tuffs frequently found interbedded with Mid- to Late- Pleistocene sediments with paleoanthropological and/or archeological material. Because these tephras are chemically correlated to additional tephra deposits in other Afar Rift study areas and in marine sequences, these ages additionally inform chronostratigraphic sequences beyond the Middle Awash, where most fossiliferous localities previously lacked absolute ages. Here we present preliminary age results from ongoing studies of tephra, description of intercalated fossiliferous sediments and associated depositional facies, and we discuss the significance of these results for understanding geological and evolutionary change during the past ~1 Ma.