RECONSTRUCTING PALEOLANDSCAPE, PALEOCLIMATE, AND PALEOECOLOGY OF THE LATE OLIGOCENE CHILGA BASIN, NORTHWESTERN ETHIOPIA

Late Oligocene (28 – 27 Ma) strata from the Chilga Basin of northwestern Ethiopia provide a unique opportunity to reconstruct a tropical Paleogene ecosystem using evidence from multiple proxies. These fossiliferous deposits occur at a critical time in the geologic history of Africa: after flood basalt eruption and global climate changes of the Early and Middle Paleogene, but before influx of Eurasian fauna or development of the East African Rift. Geochemical, sedimentological, and paleontological studies of these strata have yielded data that provide a detailed understanding of ancient landscape, paleoclimate, and biotic communities. Here, we summarize the techniques and results of our multi-proxy paleoecosystem analysis.

Abiotic parts of the paleoecosystem were documented by geologic mapping and sedimentological studies. The Chilga Beds are divided into two units: (A) a lower ~80 m thick sequence of drab colored fluviogenic mudstones with abundant coals and paleosols and (B) an upper ~40 m thick sequence of buff to dusky-red interbedded fluviogenic mudstones and cross-bedded sandstones. Paleosol profiles in the lower Chilga Beds include Protosols, Gleysols, and Histosols, and in the upper Chilga Beds include Protosols, Histosols, Gleysols, Vertisols, and Argillisols. These data suggest that the lower Chilga Beds were deposited in a poorly drained, swampy landscape, whereas the upper Chilga Beds represent an open landscape dominated by braided and meandering fluvial stream systems with variable, but generally better, soil drainage.

Quantitative paleotemperature and paleoprecipitation estimates are derived independently from geochemical and paleobotanical techniques. Carbon, oxygen, and hydrogen stable isotope values of pedogenic kaolinite, smectite, sphaerosiderite, and calcite indicate warmer soil temperatures (29±3°C), higher rainfall, and different atmospheric circulation patterns than modern-day Ethiopia. Paleobotanical climate estimates were obtained using overlapping distribution analysis and foliar physiognomy, and give a mean annual temperature of 24±3^oC and mean annual precipitation of 1300-1600 mm/yr. Thus, all these methods provide statistically overlapping estimates and indicate warmer temperatures and more equable rainfall distribution than today.

Fossil plant remains are abundant in the Chilga strata. Plant fossils preserved in ash beds document transient pioneer communities dominated by ferns, legumes, and palms. Plant species preserved in overbank and pond deposits are characteristic of diverse, heterogeneous lowland or submontane forests. In situ silicified trees indicate a canopy height of 24-30 m. The forests are dominated by trees with affinities to relatives found today in Central and West Africa or the Eastern Arc and coastal forests of East Africa, and they differ from their modern counterparts by the common occurrence of palms, which today are rare or absent. Fossil insect herbivore damage on the Chilga leaves is dominated by specialized feeding types, particulary galls. These results suggest that although the Chilga plants were well-defended against generalist herbivores, they were susceptible to specialized herbivores.