GEOLOGY AND GEOCHRONOLOGY OF THE MUSH PALEONTOLOGICAL SITE, NORTH-CENTRAL ETHIOPIA

Abstract

The Mush basin, located in north central Ethiopia, has yielded a 21.736 Ma floral and faunal assemblages, with significant potential for paleoclimate reconstruction and new data on the geological history of the Ethiopian Highlands. It lies in a volcanic province typified by thick successions of Tertiary volcanics and interbeded lacustrine sequences.  Research was conducted in the Mush basin with multiple objectives including: to document the origin, age, volcanic episodes, and overall evolution of the sedimentary basin; and to provide a physical setting for associated fossil flora, fauna, and paleoecological interpretations. Field survey, petrographic and XRD analyses, and radioisotopic age determinations were conducted to understand the geological history and environment of the sedimentary basin.  The Mush lacustrine carbonaceous shales host abundant compressions of leaves, fruits, seeds, insects, concentrations of organically preserved amphibians, pollen, woods and remains of mammals.  Fieldwork also identified nearby multiple volcanic episodes associated with volcanic bombs of up to a meter in diameter, and several in situ carbonized trees.  The origin of the Mush basin is interpreted as volcanic caldera; and the early stage of lacustrine deposition is dated at 21.736±0.015Ma. X-ray diffraction analyses (which document the presence of kaolinite) and the study of field relationships indicate that the fossiliferous shales originated in a fresh water lake, which most likely formed in a volcanic caldera, and which had anoxic or hypoxic conditions at depth. These conclusions are consistent with the presence of organically-preserved amphibians and aquatic plants, and indicate a climate having abundant precipitation.