GSA Annual Meeting in Denver, Colorado, USA - 2016

Paper No. 253-3
Presentation Time: 9:00 AM-6:30 PM


DAVIS, David, Geosciences, Georgia State University, 33 Gilmer St SE, Atlanta, GA 30303, DEOCAMPO, Daniel M., Geosciences, Georgia State University, 24 Peachtree Center Avenue Northeast, Atlanta, GA 30303, RABIDEAUX, Nathan, Department of Geosciences, Georgia State University, Atlanta, GA 30302 and CAMPISANO, Christopher J., Institute of Human Origins, School of Human Evolution and Social Change, Arizona State University, Tempe, AZ 85287,

The Northern Awash Valley is located in the southwestern portion of the Afar Depression in Ethiopia. During the 2014 field season, two core sites were drilled as part of the Hominin Sites and Paleolakes Drilling Project, recovering a total of ~600 m of sediment from both localities (NAO and NAW). Here we present preliminary mineralogical results of core material, for later integration with biological and other records of environmental and climate change in the Afar region during the Pliocene. Drilling targeted the Hadar Formation, known to be ~3.8-2.9 Ma. Samples were collected at ~16 cm intervals throughout each of two cores, NAW14-1A and NAO14-1B (~ 3 km apart).

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.