STABLE HYDROGEN AND OXYGEN ISOTOPE COMPOSITIONS OF PEDOGENIC MINERALS FROM OLIGOCENE (~32-27 MA) STRATA OF THE NORTHWESTERN ETHIOPIAN PLATEAU: IMPLICATIONS FOR PALEOTEMPERATURE, PALEOENVIRONMENT AND PALEOELEVATION

The stable hydrogen (δD) and oxygen (δ¹⁸O) isotope compositions of pedogenically-formed phyllosilicate minerals (1) appears to form in equilibrium with meteoric water, (2) offer estimates (±3° C) of crystallization temperature, and in the case of paleosol phyllosilicates offers (3) estimates of the δD and δ¹⁸O of paleo-rainfall and paleotemperatures of soil formation. We present here δD and δ¹⁸O values of phyllosilicate minerals taken from paleosol profiles in Oligocene-age (32.4 to 26.8 Ma) strata of the Chilga basin, northwestern Ethiopian Plateau, in order to provide insights to Oligocene-age paleotemperatures, meteoric rainfall compositions and patterns, and the Oligocene paleo-elevation of this region that now occupies an elevation between 1800-2000 m above sea level in tropical Africa.

The mineralogy of the phyllosilicate fine-clay fraction samples (n=22) within these strata is dominated by kaolinite, with δD and δ¹⁸O values ranging from -83 to -67‰ and 16.0 to 18.3‰, respectively. These δD and δ¹⁸O values, in conjunction with the hydrogen and oxygen isotope fractionation equations for kaolinite, correspond to (1) a range of crystallization temperatures from 26 to 32° C, (2) rainfall δD and δ¹⁸O values ranging from- 38 to -63‰ and -6.0 to -9.1‰, respectively. Modern sites with a corresponding range of mean annual temperatures and δD and δ¹⁸O values of precipitation to those estimated from the Oligocene Chilga strata are few. Nevertheless, all modern analogs are limited to tropical lowland sites in southeast Asia characterized by rather intense monsoonal circulation systems with a long-duration (>10 month) wet season. In contrast, the modern Chilga region has a substantially lower range of mean annual temperatures (16-18° C), and a much more positive range of δD and δ¹⁸O values of precipitation (~-6 to -14‰ and -2.0 to -3.0‰, respectively) due to its high-elevation and paratropical position of first-rain in the African monsoonal atmospheric circulation system. Collectively, these data from Oligocene-age Chilga strata point toward a much lower paleo-elevation (<300 m above sea level) during weathering and emplacement of Chilga sedimentary strata, suggesting a substantial (>1000 m) vertical uplift of the Ethiopian basaltic plateau since that time.