AUTHIGENIC MINERALS IN VOLCANICLASTICS FROM AN EASTERN AFRICAN PALEOLAKE: A PROXY FOR PALEOENVIRONMENT

Abstract

Olduvai Gorge exposes the stratigraphy of Paleolake Olduvai, an often saline-alkaline rift lake that records volcanism from the Ngorongoro Volcanic Highlands. In 2014, cores retrieved from the study area revealed new stratigraphy, including the lacustrine Naibor Soit Formation (Fm) and the volcaniclastic Ngorongoro Fm. In the Ngorongoro Fm part of the cores, mineralogy and geochemistry are the best paleoenvironmental proxies, since under saline-alkaline conditions, volcanic glass alters into specific authigenic minerals (e.g., zeolite, feldspar). This study employs X-Ray Diffraction, X-Ray Fluorescence, and Scanning Electron Microscopy to analyze authigenic mineralogy and geochemistry. The lower Ngorongoro Fm experienced the most alteration, with less preserved glass and more zeolites. The Upper Ngorongoro Fm experienced less alteration, preserving more glass and less zeolites. The uppermost units contained abundant glass and no alteration minerals, indicating very fresh conditions. Overall, there is a large-scale freshening trend throughout the duration of the Ngorongoro Fm.