Paleobotany and Sedimentology of Late Oligocene Terrestrial Strata from the Northwestern Ethiopian Plateau

Abstract

This paper describes the sedimentology and paleobotany of a Late Oligocene (27.36 ± 0.11 Ma) succession of volcaniclastic strata from the Margargaria River region, northwestern Ethiopian Plateau. Sedimentology indicates fluvial deposition of clay- and silt-rich strata during the early sedimentary phases, whereas organic-rich deposits and massive and fluvially-reworked ash layers are increasingly more common in the middle and upper parts of the succession, respectively. Periods of interrupted deposition are indicated by three paleosol types present in the basal and middle parts of the succession. Paleobotany documents an exclusively angiosperm flora typical of riparian environments in the lower parts of the succession, and a flora dominated by ferns and a few angiosperm taxa typical of disturbed environments in the middle and upper parts. Sedimentology and paleobotany, combined, indicate a riparian environment inhabited by angiosperms for the lower part of the succession, characterized by rather calm deposition by meandering streams. The middle and upper parts of the succession represent transient environments colonized by pioneer vegetation consisting of a diverse fern community and fewer pioneer angiosperms, characterized by deposition of organic- and ash-rich strata in situ in ephemeral ponds and by small-scale crevasse-like channels and by aereal means on the landscape, respectively. Physiographic changes are interpreted to have resulted directly from the influence of volcanism on the surface environment. In summary, this study shows that the volcanic activity associated with the elevation of the northwestern Ethiopian Plateau during the Oligocene repeatedly influenced the development of plant communities and paleoenvironments, favoring the succession of heterogeneous ecosystems on short temporal and spatial scales. Finally, this study is the first that combines paleobotanical and sedimentological data for paleoenvironmental reconstruction and understanding of plant community dynamics in Paleogene deposits from Africa, and it demonstrates the advantages of a multiproxy approach for assessment of paleoecosystem dynamics on an unstable landscape.