Journal of African EarthSciences,Vol. 13, No. 3•4, pp. 437-447, 1991. 0899-5362/91 $3.00+ 0.00 Printed in Thailand © 1991 Pergamon Pressplc Age of volcanism and rifting in the Burji-Soy0ma area, Amaro Horst, southern Main Ethiopian Rift: geo- and biochronologic data G. WOLDEGABRmL*, T. YEMAr~**, G. Suw^***, T. WHrrE****, and B. ASFAW***** *Los Alamos National Laboratory, EES-1/D462, Los Alamos, NM 87545, USA **Ethiopian Institute of Geological Surveys, P.O. Box 2302, Addis Ababa, Ethiopia ***Primate Research Institute, Kyoto University, Japan ****Department of Anthropology, University of California, Berkeley, CA 94720, USA *****Ethiopian Ministry of Culture and Sports Affairs, Addis Ababa, Ethiopia (Accepted for publication 7 November, 1990) Abstract- Late Eocene basalts capping late Mesozoic or early Tertiary basal sandstone are present in the Amaro Horst within the southern sector of the Main Ethiopian Rill 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 northcenlxal Ethiopia and northern Kenya implies that the Early Eocene transitional tholeiitie 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-volcanie sandstone beds, was uplifted in post-Mid-Miocene time. INTRODUCTION The Tertiary volcanic province of Ethiopia is asymmetrically distributed on either side of the Main Ethiopian Rift (MER). The volcanic succession in the NW Plateau is voluminous, thicker (-2000 m), and covers a larger area than that of the SE Plateau on the opposite side of the rift system (Juch, 1980; Zanettin etal., 1980a, b; Mohr, 19831. Various studies of volcanic sequences in SE, NW, and central Ethiopia, west of the Ethiopian Rift System suggest that the earliest volcanism (Eocene) was restricted to these regions (Merla, 1979; Davidson and Rex, 1980; ZanetUn et a/., 1980a; Berhe et al., 1987). These and several other studies led to the suggestion that volcanism and rifting migrated from the NW and SW volcanic province toward the Afar Rift and MER with time (Zanettin et al., 1974a, 1980a; Kazmin, 1980; Berhe et aL, 1987). However, Quaternary basaltic fields occur outside the present-day rifts in SW (Davidson and Rex, 1980), SE (Berhe etal., 1987), and central Ethiopia (WoldeGabriel eta/., 1990). It is generally reported that the major rift structures in southern and southwestern Ethiopia formed in the Middle to Late Miocene (,t 14 m.y.) period (Moore and Davidson, 1978; Davidson and Rex, 1980; Berhe et al., 1987), however, Zanettin et at (1978) proposed Late Oligocene or Early Miocene proto-rift development in the southern rift. The recent discovery of Paleogene and Neogene volcanic rocks in the Amaro Horst as reported here within the southern sector of the MER and new evidences for Mid-Miocene or older rifting in this region are discussed in light of the published volcanotectonic history of the southern sector of the rift and the adjacent area. The MER at its southern terminus is bifurcated 437