Introduction

Volcanogenic deposits are valuable for the creation and preservation of in situ sequential stages of biotic change not commonly represented in the fossil record (e.g., Spicer et al. 1985; Taggart and Cross 1991; Hoffman et al. 1997; Falcon-Lang and Cantrill 2002; Johnson 2007). These might otherwise be represented by fluvial or lacustrine sediments, which are more likely to preserve transported and time-averaged fossil plant assemblages (e.g., see Spicer 1991; Georgieff et al. 2004). Additionally, volcanogenic sediments offer the possibility of age control using radiometric analysis, biochronology, and paleomagnetic reversal stratigraphy (e.g., Kappelman et al. 2003). This paper describes autochthonous and parautochthonous plant assemblages preserved within Oligocene volcaniclastic strata from Chilga Woreda (Chilga), northwestern Ethiopia, that show increasing, although intermittent, activity in terms of ash supply.

The Paleogene of Africa is represented by a limited number of fossil localities, most of which are coastal (e.g., Late Eocene - Early Oligocene Fayum, Egypt [Bown et al. 1982]). Late Oligocene (27 Ma) terrestrial fossiliferous deposits from Chilga discussed in this paper are among the few known Paleogene examples from the inland African tropics (Kappelman et al. 2003; Feseha 2005; Jacobs et al. 2005; Jacobs 2006). In addition, Chilga contains the only known Late Oligocene strata in Africa that preserve both plant macrofossils and vertebrates (Kappelman et al. 2003; Jacobs et al. 2005; Pan et al. 2006). Fossil assemblages from Chilga consist of fruit, seed, and leaf taxa with Guineo-Congolian and East African affinities and an Afro-Arabian endemic fauna (Kappelman et al. 2003; Sanders et al. 2004; Jacobs et al. 2005; García Massini et al. 2006; Pan et al. 2006). Among these plant remains are fern and angiosperm macrofossils. Ferns in particular are poorly represented elements of Cenozoic plant communities and contribute relatively little to environmental reconstructions in Africa based upon macrofossils (Jacobs 2004, 2006; García Massini and Jacobs 2009). In fact, only a limited number of African fern macrofossils are known (e.g., García Massini et al. 2006;). However, based on their Cenozoic palynological record, ferns were diverse, although generally not as diverse as angiosperms (see e.g., Sah 1967; Yemane et al. 1987a, 1987b). The depaupurate African fern macrofossil record relative to angiosperms may be a reflection of lower fossilization rates, lesser abundance and diversity, greater challenges to identification, or a combination of these factors. Ferns, in general, do not shed their leaves or other macroscopic plant parts. Instead, they weaken, rot, or fragment while still attached to the parent plant and are most likely to enter the fossil record when buried in situ (Spicer 1991). This suggests that even if ferns were as abundant and diverse as angiosperms during the Cenozoic, they may not have been fossilized without rapid in situ burial, such as in instances of volcanogenic ash deposition.

Deposits from Chilga include a number of tuffaceous strata deposited in a moist tropical climate (Feseha 2005; Jacobs et al. 2005), which accumulated during early development of the East African Rift in association with flood basalt eruption (Hoffman et al. 1997). Sedimentological and paleobotanical data presented herein from a coherent set of tuffs, called Ash-IV, document plant assemblages preserved within an unstable landscape characterized by rapid, and perhaps catastrophic, deposition of volcanic ash-derived sediments. The objective of this paper is to reconstruct the evolution of the paleoenvironment represented by the Ash-IV sequence by high-resolution (cm-dm) analysis of sediments, lithologies, paleosols, and plant macrofossils. A thin set of volcaniclastic strata immediately below Ash-IV is also described and used as a point of reference for paleoenvironmental conditions prior to deposition of the more tuffaceous strata.