Search Results

The late Miocene Adu-Asa Formation comprises major sedimentary and volcanic units that crop out along the densely faulted and tilted frontal blocks of the western rift margin of the transition zone between the northern Main Ethiopian Rift (MER) and the southern Afar Rift. This chapter discusses the geological and tectonic processes that led to the origin and evolution of the rift basin of the transition zone. It highlights a decade-long geological and paleontological reconnaissance and focuses investigation along the strike of the frontal fault blocks of the western rift margin of the Middle Awash study area.

This chapter examines the Mio-Pliocene volcanic materials exposed along the western margin of the Middle Awash area, with a primary focus on the late Miocene deposits of the Adu-Asa Formation. The primary goals of this chapter are to: define the different types of volcanic material preserved in the Adu-Asa Formation; present a working composite stratigraphic view of the Adu-Asa Formation volcanic horizons; present the existing geochemical data for western margin volcanic samples, including some with uncertain chronostratigraphic context; illustrate the data and observations that provided the basis for previous correlations and naming of key volcanic marker horizons; and illustrate the data and observations that provide new regional correlations and hence the establishment of newly named volcanic horizons.

This chapter describes the cercopithecid fossils recovered from the late Miocene sediments of the western margin and Central Awash Complex (CAC) of the Middle Awash. Cercopithecids in the Adu-Asa Formation and the Kuseralee Member of the Sagantole Formation are rare compared to other taxa, with total specimens collected from late Miocene Middle Awash deposits constituting only 3.8 percent of the total number of collected vertebrate specimens identifiable below the family level.

40Ar/39Ar dating has been applied extensively to volcanic units of late Miocene to Pleistocene age in the Middle Awash study area, and is the most important dating method applied to these deposits. This chapter discusses all available 40Ar/39Ar data for the Adu-Asa Formation, including some data not previously reported, plus some previously unpublished results from the nearby Central Awash Complex (CAC). The methods and facilities employed for 40Ar/39Ar dating is divided into two different approaches: incremental heating and single-crystal total fusion.

This chapter describes the Middle Awash carnivore fauna, which represents one of the most diverse in the latest Miocene of eastern Africa. At least 15 genera in seven families are recognized from the Asa Koma Member of the Adu-Asa Formation and the Kuseralee Member of the Sagantole Formation. The carnivore assemblage from the late Miocene of the Middle Awash has important implications for regional and global biogeography, and it also documents the first local appearances of some taxa that established the modern African carnivore fauna.

The first known representatives of the family Hippopotamidae left a limited fossil record during the Miocene. In this chapter, two assemblages of hippopotamus remains are identified in the late Miocene of the Middle Awash. The first one was recovered from the Asa Koma Member of the Adu-Asa Formation, dated to 5.5 to 5.8 Ma. The second set came from the Kuseralee Member of the Sagantole Formation, from sandstones just below the 92-15 basalt (dated to 5.2 Ma). The recent discoveries of new late Miocene and early Pliocene hippopotamid material in the Middle Awash study area constitute an important step in understanding this crucial period for Hippopotamidae and, more generally, their evolution in the Afar basin.

This chapter discusses the systematics and paleoecology of the small mammals recovered in fossiliferous localities of the Adu-Asa Formation. The micromammalian fauna from the late Miocene of the Middle Awash is diverse and documents at least 16 taxa in eight families. The chapter also includes important new fossils from the Haradaso Member of the Sagantole Formation belonging to Alilepus, Tachyoryctes, and Thryonomys. These remains, dated to approximately 4.85 Ma, are important additions to earliest Pliocene small mammal faunas.