Age of volcanism and rifting in the Burji-Soyoma area, Amaro Horst, southern Main Ethiopian Rift: geo- and biochronologic data

Late Eocene basalts capping late Mesozoic or early Tertiary basal sandstone are present in the Amaro Horst within southern sector of the Main Ethiopian Rift. This basaltic unit is time correlative to flows already reported from southwestern Ethiopia. The discovery of these lavas within the rift system proves that the oldest volcanic units of the Ethiopian volcanic province are not confined to the NW Plateau, west of the Main Ethiopian Rift as was generally believed. At the same time, the widely held notion of rift-ward migration of volcanism from the plateau is not supported by the occurrence of this Late Eocene basalt within the uplifted Amaro Horst. The Late Eocene basalt is blanketed by Early Oligocene basalt and silicic flows. Geochemical characteristics indicate that the volcanic succession consists of Late Eocene transitional tholeiitic basalt, Oligocene and Mid-Miocene hawaiites, and Mid-Miocene mildly alkaline basalt flows. The lower (Late Eocene and Early Oligocene) and upper (Mid-Miocene) basaltic sequences are separated by sedimentary strata deposited in a lacustrine environment that contains Early to Mid-Miocene fossil fauna and flora. Although the botanical study of the flora was not attempted, the Burji faunal remains appear to represent a primitive species of Choerolophodont mastodont. Biochronological evidence suggests that the proboscidean would be in the time range of 15-17 Ma. The presence of the fossil-bearing sedimentary strata suggests that a rift-related basin developed in Early to Mid-Miocene contemporaneous with the volcanic eruptions. The confinement of the Mid-Miocene basaltic units to the Amaro Horst suggests that rift-related subsidence in the southern sector of the Main Ethiopian Rift probably started in Early Miocene or Late Oligocene time, coincident with development of the sedimentary basin. The correlation of lavas of tholeiitic composition to intense tectonic activity in northcentral Ethiopia and northern Kenya implies that the Early Eocene transitional tholeiitic lava of the Amaro Horst erupted during intense tensional deformation. The amaro Horst, which exposes the volcanic pile on top of crystalline basement and pre-volcanic sandstone beds, was uplifted in post-Mid-Miocene time.