Paleoenvironmental reconstruction of Late Oligocene terrestrial volcaniclastic deposits from the northwestern Ethiopian Plateau.

GARCÍA MASSINI, J.L.

Bonn, Alemania

Conferencia; VIII International Organization of Paleobotany Conference; 2008

Abstract This paper describes the sedimentology and paleobotany of a Late Oligocene (27.36 ± 0.11 Ma) succession of tuffaceous strata from the Magargaria River region of the northwestern Ethiopian Plateau. Sedimentological data indicate fluvial deposition of clay- and silt-rich strata during the early phases of sediment accumulation, 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 the presence of three types of paleosols in the basal and middle part of the succession. Paleobotanical data represented by megafossils indicate an exclusive flora of angiosperms, 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. Preliminary palynological results are consistent with megafossil data, where abundant fern spores dominate the organic-rich deposits in the middle part of the sequence, while these and dicot pollen taxa appear similarly common in the basalmost part of the succession. Ashier strata are comparatively less rich and diverse than the remaining part of the sequence and show a combination of fern and angiosperm taxa. The combined analyses of sediments and plants indicate that the lower part of the succession represents a riparian environment inhabited by angiosperm taxa where rather calm deposition was carried out by means of a meandering river. The middle and upper parts of the succession represent deposition by small-scale crevasse-like channels or in-situ in either ephemeral ponds or on ash-covered settings colonized by pioneer vegetation. The physiographic changes that are inferred from the succession are interpreted to result directly from the influence of volcanism on the surface environment. This study is among the few that have used a combined approach, including fine-scale sedimentological and paleobotanical data to infer the evolution of the environment during a short time interval, and it is unique among those already carried out since its focus is the poorly known Paleogene of tropical Africa.