INTRODUCTION

The Paleogene fossil record is sparse compared with the Neogene record for continental tropical Africa, despite the much longer duration of the Paleogene (43 vs. 23 Ma). The abundance of Neogene fossil localities is related to development of the topographically complex East African Rift, which generated sediment accommodation space and created ample opportunities for fossilization. However, limited exploration for older sites has also contributed to the shortage of Paleogene localities. Continuing investigations of pre-rift sedimentary deposits of eastern tropical Africa aim to fill this gap and have produced significant fossil sites from the Eocene of Tanzania and the Oligocene of Ethiopia (Herendeen and Jacobs 2000; Harrison et al. 2001; Gunnell et al. 2003; Kappelman et al. 2003; Murray 2003; Jacobs and Herendeen 2004; Sanders et al. 2004). These new localities provide unique opportunities to document Paleogene plant and vertebrate evolution, paleoecology, and paleoclimate in interior tropical Afro-Arabia, when the biota was evolving in isolation from those of other continents, and experiencing significant global climate changes.

In this paper we assess the paleoecological implications of late Oligocene fossils and sediments located in Chilga Woreda (henceforth “Chilga”), west of Gondar, on Ethiopia’s northwestern plateau (Figure 1). These deposits are remarkably rich in fossil plants and have produced unique associations of endemic archaic and derived mammals. Abundant paleosols document paleoenvironments independently of the fossils. This wealth of paleontological and palaeoenvironmental data provides information that makes Chilga unique for the African Paleogene.

Geological Background

The study site is located in an area of approximately 100 km2, 60 km west of Gondar in Chilga Woreda, Amhara Region, northwestern Ethiopia (Figure 1). Geologically, the area is characterized by massive flood basalts as much as 2000 m thick, emplaced approximately 30 million years ago, prior to Miocene rifting (Hofmann et al. 1997). Clastic and volcaniclastic sediments occur interbedded with volcanic deposits in a basin formed by faulting of the basalts in the middle to late Oligocene. Approximately 150 m of fossiliferous sedimentary strata are exposed in outcrops across the basin, but the most complete and best-documented section occurs along the Guang River. In this section, radiometric dates and paleomagnetic reversal stratigraphy constrain the age of about 100 m of sediment to between 27 and 28 Ma, the limits of Chron C9n, or earliest late Oligocene (Feseha 2002; Kappelman et al. 2003; Cande and Kent 1995; Figure 2).

Plant and vertebrate fossils occur throughout the Chilga deposits. Faunal assemblages, dominated by the weathering-resistant teeth of larger mammals, show little change during the time interval represented at Chilga. The plant assemblages, some of which occur in direct association with vertebrate fossils, provide an excellent record of the environment in which the animals lived and, with sedimentological data, provide a fine-scale assessment of spatial and temporal environmental variation. Below, we document and integrate sedimentological and paleontological data in order to better understand the landscape and climate at Chilga.