PALEOENVIRONMENTS OF UPPER OLIGOCENE STRATA, NW ETHIOPIAN PLATEAU

Stratigraphy, geochronology, sedimentology, and geochemistry are presented from Upper Oligocene strata of Chilga Woreda, northwest Ethiopian Plateau. The stratigraphy consists of three lithologic units: (1) a lower sequence of ~2000 m of basalt flows emplaced between ~32-31 Mya, (2) an intermediate 0 – 75 m thick series of intercalated basalts and weathered basaltic horizons, and (3) an upper ~120 m thick sequence of volcaniclastic strata deposited between 28.15±0.14 and 26.7±0.01 Mya. There is little to no recognizable Oligocene soil weathering of the lower calcareous basaltic sequence. The intermediate series of weathered basalts preserve intercalated basalts and massive, noncalcareous, kaolinite and Fe-oxide-rich mudstones up to 25 m thick. These weathered basalt sequences were likely Oxisols. The upper volcaniclastic sequence, the so-called “Chilga Beds”, is divided into two separate stratigraphic packages: (A) a lower ~80 m thick sequence of drab colored fluviogenic mudstones with abundant coals and underclays (i.e., 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. Sphaerosiderite is abundant within these paleosols. Collectively, these data suggest the lower Chilga Beds were deposited in a poorly drained, swampy landscape dissected by well-drained Oxisol-forming uplands of the intermediate stratigraphic unit. Paleosol profiles in the upper Chilga Beds include Protosols, Histosols, Gleysols, Vertisols and Argillisols. Sphaerosiderite is much less common in these strata, and stratiform calcite deposits occur near the top of the stratigraphy. The upper Chilga Beds are interpreted to represent an open landscape dominated by braided and meandering fluvial stream systems with variable, but generally better, soil drainage compared to the lower Chilga Beds environments. Carbon, oxygen and hydrogen stable isotope values of pedogenic kaolinite, smectite, sphaerosiderite and calcite within units (2) and (3) indicate warmer soil temperatures (29±3°C), higher rainfall, and different atmospheric circulation patterns than modern-day Ethiopia.