Les dépôts de sédiments fins dans le sud de la dépression de Kharga (désert libyque, Égypte) : indices d'une morphogenèse éolienne et anthropique

Abstract

International audienceThe Western Desert of Egypt is one of the most arid areas of the world. Because of the extreme scarcity of the rainfall (less than 1 mm of annual mean rainfall in some places) and because of the amount of available sediment, aeolian processes are the major morphogenetic factors. The studies of the palaeoenvironments in this area (Wendorf et Schild, 1980; Sanlaville, 1997; Brookes, 2003; Bubenzer et Riemer, 2007) show that the climate has gradually become hyperarid between 5800 BP and 4500 BP. So, the ascendancy of aeolian processes isn't new. During the Holocene, the aeolian processes and landforms has acted as a resource (moisture of the deposits, arable lands) and as a stress (sanding and silting up, erosion) at the same time. The geomorphological and geoarchaeological survey of the Dush area, located in the South of Kharga basin has highlighted the existence of fine-grained sediment which are neither spring deposits (as described by Bousquet, 1996) nor playa deposits (as described by Embabi, 1972, 1999, 2004). The archaeological data produced since 1976 by the IFAO (Institut Français d'Archéologie Orientale) mission and the sedimentological analyses indicates that the sediment of these deposits had been windblown and trapped by vegetation (fields of the ancient oases). The cultivation of the fields lasted from 2500 BP to 1500 BP, allowing the height of the deposits to reach several meters. The drop of the water table and the uplift of the level of the fields forced the farmers to adapt themselves, up to the abandonment of most of the sites (around 1500 BP). After 1500 BP, the deposits have been eroded by wind, involving the formation of yardangs. The current landscape of the studied hills is the result of the succession of a period characterized by the deposition of windblown sediment linked to the cultivated fields and a period of aeolian erosion linked to the end of agricultural activitiesLe désert libyque est l’une des régions les plus arides du monde. En raison de l’extrême rareté des précipitations (moins de1mm de précipitations annuelles moyennes par endroit) et de la quantité de sédiment mobilisable, les processus éoliens constituent le principal facteur de morphogénèse. Des études paléoenvironnementales (Wendorf et Schild, 1980 ; Sanlaville, 1997; Brookes, 2003 ; Bubenzer et Riemer, 2007) ont montré que l’hyperaridité s’est installée progressivement entre 5800 BP (fin de la dernière pulsation humide) et 4500 BP. La prépondérance des phénomènes éoliens est donc ancienne. Durant l’Holocène, les dynamiques et formations éoliennes ont fonctionné pour les hommes à la fois comme contrainte (ensablement, érosion) et comme ressource (maintien de l’humidité, dépôts cultivables). Les prospections géomorphologiques et géoarchéologiques de la région de Douch, située dans le sud de la dépression de Kharga, ont mis en évidence la présence de dépôts de sédiments fins sur des versants ne correspondant ni aux dépôts de sources artésiennes signalés par Bousquet (1996) ni aux dépôts de playas abonda...

Exploitation of groundwater in the oasis of Masāfī (UAE): A diachronic perspective

“…They can grow even more when wells are dug in the springs to improve their discharge. As long as the artesian pressure is sufficient for the water to reach the topographic surface, the mounds can continue to grow (Crépy & Callot, 2016: 335–338).…”

The End of the “Green Oasis”: Chronological Bayesian Modeling of Human and Environmental Dynamics in the Bahariya Area (Egyptian Sahara) from Pharaonic Third Intermediate Period to Medieval Times

“…In Kharga oasis (over 300 km southeast of Bahariya), remnants of dwellings and irrigation devices were also observed on the top of some yardangs, in Douch (Crépy 2016:46; Crépy and Callot 2016:327; 331; 333) and El-Deir (Tallet et al 2011: 184; Garcier and Bravard 2015:33), respectively at the southern and northern edges of this oasis. Depending on the site topography, the wind-sculpted layers of fine sediments accumulated during the Holocene either because the surface roughness conferred by the vegetation cover and the ambient humidity of the crops trapped the particles transported by the wind (Crépy 2016; Bravard et al 2016:480; Bravard et al 2016b:320), or because runoff carried colluviums down a closed depression, which formed a playa (Tallet et al 2011; 2012). Artifacts were again used to date the human occupations that left traces at the top of the relict formations: they range from the Persian (5th century BCE) to the Roman period.…”

“…It is therefore likely that overexploitation of fossil groundwater played a role in the aridification of the oases in Roman times (as demonstrated in southern Kharga oasis e.g. by Bousquet 1996; Wuttmann et al 2000; Crépy 2016). However, fluctuations of rainfall rates occurred simultaneously in several of these regions (North Africa, Bahariya, draining basin of the Dead Sea and Nile springs): such climate phenomena are independent of anthropogenic dynamics.…”