PALEOCLIMATE AND PALEOECOLOGY OF OLIGOCENE STRATA AT CHILGA, NORTHWESTERN ETHIOPIAN PLATEAU

The Chilga region of northwestern Ethiopia (60 km WSW of Gondar) provides a rare view of life, landscape, and climate in the interior of tropical Africa during the Paleogene, which is otherwise known from only a few well-documented sites. Sedimentary strata interbedded with flood basalts are well constrained at 27 to 28 Ma based upon radiometric dates on volcanics, and corresponding paleomagnetic stratigraphy. Fossil leaves, fruits, seeds, stems, and flowers represent forest communities preserved in overbank, pond, volcaniclastic alluvial, and airfall tuff facies throughout the 150m sedimentary section. In situ forests of silicified trees occur in at least two localities where height estimates derived from allometric relationship to diameter indicate these forests were from 20-35 m tall.

A leaf and flower compression assemblage, with remarkable preservation of cuticular morphology, consists of at least 35 dicot species, isolated palm and other flowers, a palm inflorescence, and charcoalified trees lying prone near the base of an overbank deposit. Taxonomic composition of this flora shows affinities with modern relatives whose center of distribution is in Central and West Africa; however, palms appear to have a greater presence in the Oligocene forest when compared with today. The distribution of leaves at four sublocalities along 60 m of a single stratigraphic level documents local heterogeneity of species composition, typical of tropical forest communities. Regression equations that estimate rainfall amounts from the proportions of species in leaf size classes indicate mean rainfall exceeding 1200 mm/yr.

Five main paleosol types occur among the Chilga strata, and are interpreted as representing Protosols, Histosols, Gleysols, Vertisols, and Argillisols. These are indicative of poorly drained settings, and stratigraphic and lateral variation document intrabasinal temporal and spatial fluctuations. The sedimentological data are consistent with floral data, indicating a nearly flat landscape, meandering stream, and lush vegetation, all subjected to intermittent ashfall. These lines of evidence provide the setting for Chilga large mammals, which co-occur with plant fossils, and represent an archaic endemic African fauna.