NON-CARBONATE COMPONENTS OF FOSSIL SPRING TUFAS AS A CONSTRAINT ON GROUNDWATER DISCHARGE CHEMISTRY DURING PLEISTOCENE PLUVIAL PHASES IN KHARGA OASIS, WESTERN DESERT, EGYPT

Extensive Pleistocene-aged carbonate (tufa) and ironstone spring sediments in Egypt’s Western Desert provide a record of enhanced regional groundwater discharge during past pluvial periods. These sediments, often associated with Paleolithic artifacts, shed light on the role of environmental change on hominid movement and occupation throughout the region, but their interpretation as archives of local rainfall is complicated by uncertainties in the extent of the recharge area which contributed to spring flow. Some previous studies assume carbonate-forming springs along the Libyan Escarpment were sourced predominantly from a perched limestone aquifer, without significant contributions from the Nubian Aquifer, which supported iron-rich springs along the oasis floor. Iron-bearing minerals found within tufa carbonates, however, may indicate non-negligible contributions of iron rich Nubian Aquifer water to tufa-forming springs. In order to understand the nature of the non-carbonate fraction of Egyptian tufas, insoluble residues were characterized using optical petrography, XRD, and electron probe microanalysis. Insoluble minerals consistently comprised less than 6% of tufa samples by weight, with quartz and aluminum phyllosilicates the predominant minerals identified through XRD. Iron oxide minerals were not detected in significant amounts by XRD. Accumulations of silicates primarily occur in pore spaces and are associated with enrichment in Al, Na, Fe, and Mg. Correlation matrices created using electron microprobe data show strong positive relationships between Fe concentration and both Si and Al. These relationships suggest Fe is closely associated with concentrations of clay minerals. Element and mineral distributions are consistent with the non-carbonate phases accumulating through secondary processes (e.g., eolian transport and deposition) rather than as authigenic spring-derived precipitates. The absence of authigenic iron oxides associated with carbonate formation is consistent with a relatively insignificant contribution of iron saturated Nubian Aquifer water to carbonate forming spring systems in the Kharga Oasis. Geochemical models were also developed to further evaluate the results of variable mixing between the two possible source aquifers.