The role of salt weathering in the origin of the Qattara Depression, Western Desert, Egypt M.A.M. Aref a, * , E. El-Khoriby b , M.A. Hamdan a a Geology Department, Faculty of Science, Cairo University, Giza, Egypt b Geology Department, Faculty of Science, Mansoura University, Mansoura, Egypt Received 2 January 2001; received in revised form 20 August 2001; accepted 28 September 2001 Abstract Field studies and petrographic examinations of core samples and of the bedrock of the floor of the Qattara Depression, Egypt, indicate that salt weathering predominates in its western part in marked contrast to its eastern part. The eastern part of the depression is covered with more than 120-cm-thick, moist sands with sporadic occurrence of halite and gypsum due to the low salinity of the groundwater table. At the western part of the depression, the strongly saline, sodium chloride nature of the groundwater table favors crystallization of halite (and sometimes gypsum) at or near the surface of the outcropping bedrock of the Moghra clastics and/or Dabaa shale. Crystallization of halite and/or gypsum generates increased pressure that leads to mechanical disintegration of the bedrock into fine-grained debris. Features related to disintegration include blistering of the rock surface, splitting, spalling and/or granular disintegration. Alternation of dry and wet cycles favor halite crystallization, mechanical disintegration of the outcropping bedrock and dissolution of the halite cement, which exposes fine-grained debris to wind deflation. Removal of the debris from the floor of the depression leads to the accumulation of lunettes and other dunes in the downwind direction. Therefore, salt weathering provides fine-grained debris that are easily removed by deflation, which accounts for the topographically lower level of the western part of the depression (134 m below sea level (b.s.l.)). In contrast, the presence of moist sediments at the eastern part of the depression inhibits deflation and encourages sedimentation by adhesion of wind-blown sand to the damp surface of the sabkha at an elevation of 45 m below sea level. The disintegration of the bedrock of the Qattara Depression by salt weathering has been in effect since the onset of aridity in northern Egypt in Quaternary time. Whereas the initial excavation of the depression started in Late Miocene or Pliocene time by fluvial erosion, karstic process, mass-wasting and wind deflation. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Halite; Crystallization; Disintegration; Excavation; Qattara Depression; Egypt 1. Introduction Salt weathering is a potent agent of rock disinte- gration by salts that have accumulated at or near the rock surface in coastal, urban, polar and arid environ- ments. Important indications of the power of salt weathering come from field observations (Chapman, 1980); field monitoring (Goudie and Watson, 1984; Goudie et al., 1997) and laboratory simulations (Gou- die and Viles, 1995; Goudie and Parker, 1998), and from studies of the decay of ancient structures (Bromblet and Bocquier, 1985) and of modern engi- neering structures (Doornkamp et al., 1980). 0169-555X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII:S0169-555X(01)00152-0 * Corresponding author. E-mail address: m1aref@yahoo.com (M.A.M. Aref ). www.elsevier.com/locate/geomorph Geomorphology 45 (2002) 181 – 195