IRON-RICH DEPOSITS FROM EXTINCT SPRINGS: FORMATION, MODERN ANALOGS AND ARCHAEOLOGICAL SIGNIFICANCE

Paleolithic archaeological materials in Dakhleh Oasis, in the Western Desert of Egypt, are sometimes found in association with iron-rich Pleistocene spring deposits, suggesting that prehistoric occupants of the region made use of the water sources represented by active springs. The morphology of these spring deposits supports the hypothesis that the ironstones formed in a shallow wetland, where the interaction between iron-rich Nubian groundwater and wetland vegetation led to the deposition of iron oxides. Iron precipitates in Dakhleh preserve evidence of vegetation in the form of casts, whereas microlaminations and lenticular voids may indicate microbial mediation of ironstone formation in the oasis. Iron-rich units are composed of jarosite-stained quartz silts, goethite-stained sands, and indurated ironstones in the form of ferricretes and iron precipitates. Mineralogical and geochemical analyses of the ironstones of Dakhleh indicate the presence of goethite, jarosite and quartz in varying amounts, often coexisting within individual samples. Elemental analyses indicate that goethites are largely homogenous, containing very little inclusive material. These goethite deposits are elementally similar to modern iron-rich groundwater deposits in Dakhleh, suggesting that Nubian groundwater obtained by modern peoples in the oasis is similar to that discharged through natural springs during the Pleistocene. Spring discharge and ironstone formation in Dakhleh during the Pleistocene occurred in an area of shallow water with pockets of localized acidity, as indicated by limited jarosite formation in spring mound environments. The waters discharging into the oasis at this time were likely potable and available as a resource for early human groups utilizing the area despite localized acidity.