SPRING MOUND FORMATION AND PRESERVATION: CONTRIBUTIONS TO THE QUATERNARY DEPOSITIONAL HISTORY OF DAKHLEH OASIS, WESTERN DESERT, EGYPT

The currently hyperarid Western Desert of Egypt retains evidence of Quaternary pluvial (humid) climatic phases in the form of lacustrine, spring and fluvial deposits found primarily in oasis depressions. The Quaternary sediments of Dakhleh Oasis consist of iron-bearing spring mounds produced by localized outflow of iron-rich Nubian Aquifer groundwater, as well as carbonate lacustrine marls and tufas recording surface runoff from the limestone-capped Libyan Plateau to the north. Clarifying the temporal relationship between these chemically distinct sediments allows for a more detailed depositional history of the oases while identifying periods of water resource availability in Dakhleh, as previous work in the oasis revealed Acheulian artifacts in spring mound sediments as well as Middle Paleolithic materials preserved in lacustrine marls. Spring mounds occur in southern Dakhleh as isolated hills of Cretaceous-age shales capped by iron-rich silts and sands and iron precipitates. Basal units are composed of goethite- and jarosite-stained silts, which variably underlie goethite-cemented sands and ferricrete deposits. Sedimentary units are laterally discontinuous and topographically variable on even small scales (50-100 m), indicating isolated deposition of similar units at spring vents and localized vegetated ponds. The extensive bioturbation and evidence for thick vegetation in these sediments suggest a wetland environment in which ironstone formation was mediated by microbial activity, ponding of surface water and acidic conditions. Iron precipitates formed resistant caps on spring-deposited sands and silts, leading to the preservation of spring mound remnants in the modern oasis. In some cases these iron-rich spring sediments can be found underlying carbonate-rich lacustrine sediments, suggesting that spring mound formation occurred in a wetland environment that predated maximum lacustrine development in the oasis. This wetland may have formed during the initial stages of pluvial activity in the region, perhaps coexisting with a smaller lake. Reconstruction of these features provides a more accurate paleohydrologic model of Dakhleh Oasis while furthering our understanding of the habitability of the Sahara during the Pleistocene.