A MINERALOGICAL ANALYSIS OF HSPDP CORE SAMPLES FROM THE NORTHERN AWASH: PLIOCENE HADAR FORMATION, ETHIOPIA
The NAO and NAW cores have similar mineral assemblages. Cores at both locations contain diatomaceous silt and clay and are intermittent throughout the top and lower end of the NAO and NAW cores. In general, the appearance of diatom fragments at both localities suggests at least some lacustrine intervals. Anorthite and albite are present in both cores, along with minimal K-spar. Pyroxenes, Mg-calcite, and calcite are common. Most of the feldspars and pyroxenes are likely derived from basalt. Both gypsum and pyrite are present in NAW, but NAO has abundant gypsum with almost no pyrite. Whereas the pyrite must have formed under reducing conditions (e.g.) beneath an anoxic lake bottom, it is likely that the gypsum represents subsequent oxidation. Therefore, these two core sites may have undergone different post-depositional histories, with greater oxidation occurring at NAO. This could provide some indication of paleo-bathymetry of the paleolake, as shallower areas may have been more frequently oxidized.
The clay mineralogy of NAO and NAW are also very similar. The clays are a mix of illite, smectite, and interstratified illite/smectite. 060 analysis indicates that most of these clays are dioctahedral clays and possibly aluminum rich, and weathering products of feldspars and micas. To date, no trioctahedral phases have been identified, suggesting that the paleolake waters were not evaporated to highly saline, alkaline conditions for any significant time. Future work will provide a higher resolution mineralogical and geochemical record for the core and help us to understand climate change and evolution in East Africa during the Pliocene.