Differentiating Lacustrine from Paludal Environments in Dakhleh Oasis, Western Desert, Egypt

Lacustrine carbonates and iron-rich paludal sediments in Dakhleh Oasis, Western Desert of Egypt, provide a record of Mid-Pleistocene freshwater wetlands in what is currently a hyper-arid environment. Previously continuous lacustrine sediments typically occur as erosional remnants above the modern land surface. Paludal spring-mound sediments, originally deposited as isolated sediment packages, are found in almost identical remnant settings. The similarity in modern expression of these two sediment types makes it difficult to differentiate lacustrine from paludal sediments using modern geomorphology. Although limited stratigraphic relationships indicate that the primary episode of spring mound formation and iron deposition occurred prior to the onset of fully lacustrine conditions, the presence of discontinuous lenses of ferruginous spring-sourced sediment within the basal portions of lake sediments and the presence of spring mounds in geographically discrete localities make it difficult to determine the precise temporal relationship between spring activity and fully lacustrine conditions. Additional questions also arise, such as whether the spring mounds were active during lacustrine conditions or whether the slightly acidic, iron-oxide rich Nubian aquifer groundwater had any effect upon paleolake water chemistry. In order to address these issues the paleotopography, geochemistry, and paleoenvironmental setting of spring and lacustrine sediments have been reconstructed using differential GPS, geochemical modeling and stratigraphic analysis, respectively. The occurrence of spring mounds and their associated sediments was likely controlled by Nubian aquifer discharge along preexisting faults and structural features in the southern Dakhleh basin. This contrasts with later palustrine and lacustrine environments, which were likely widespread across the oasis and formed primarily through surface runoff, as indicated by water balance modeling and high carbonate content in lacustrine sediments. Differentiating these paludal and lacustrine environments in Dakhleh Oasis has implications for the occupation of this area and usage of the landscape and available water resources by early human groups.