Downloaded from gsabulletin.gsapubs.org on December 30, 2009 Geological Society of America Bulletin Ground-water sapping processes, Western Desert, Egypt Wei Luo, Raymond E. Arvidson, Mohamed Sultan, Richard Becker, Mary Katherine Crombie, Neil Sturchio and Zeinhom El Alfy Geological Society of America Bulletin 1997;109;43-62 doi: 10.1130/0016-7606(1997)109<0043:GWSPWD>2.3.CO;2 Email alerting services click www.gsapubs.org/cgi/alerts to receive free e-mail alerts when new articles cite this article Subscribe click www.gsapubs.org/subscriptions/ to subscribe to Geological Society of America Bulletin Permission request click http://www.geosociety.org/pubs/copyrt.htm#gsa to contact GSA Copyright not claimed on content prepared wholly by U.S. government employees within scope of their employment. 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Arvidson McDonnell Center for the Space Sciences, Department of Earth and Planetary Sciences, Mohamed Sultan Washington University, St. Louis, Missouri 63130 Richard Becker Mary Katherine Crombie} Neil Sturchio Argonne National Laboratory, Argonne, Illinois 60439 Zeinhom El Alfy Egyptian Geological Survey and Mining Authority, Cairo, Egypt ABSTRACT landforms and deposits. Using episodic wet day. For example, McCauley et al. (1982, 1986) pulses, keyed by δ18O deep-sea core record, were able to identify the presence of a relict, Depressions of the Western Desert of Egypt the model produced tufa ages that are statisti- buried drainage network in the eastern Sahara by (specifically, Kharga, Farafra, and Kurkur re- cally consistent with the observed U/Th tufa using Shuttle Imaging Radar (SIR) images. gions) are mainly occupied by shales that are ages. This result supports the hypothesis that Ground-water sapping is the process that impermeable, but easily erodible by rainfall northeastern African wet periods occurred causes entrainment of soil or rock when ground- and runoff, whereas the surrounding plateaus during interglacial maxima. The δ18O-forced water flows through and emerges from a porous are composed of limestones that are perme- model also replicates the decrease in fluvial medium at a free slope surface. This leads to able and more resistant to fluvial erosion un- and sapping activity over the past million shear strength loss of the basal support and Cou- der semiarid to arid conditions. Scallop- years, as northeastern Africa became hyper- lomb failure of the overlying rock (Dunne, 1990; shaped escarpment edges and stubby-looking arid. The model thus provides a promising Uchupi and Oldale, 1994; Schumm et al., 1995). channels that cut into the plateau units are predictive tool for studying long-term land- Characteristic requirements for ground-water suggestive of slumping of limestones by form evolution that involves surface and sub- sapping include a permeable aquifer, a recharge- ground-water sapping at the limestone-shale surface processes and climatic change. able ground-water system, a free face at which interfaces, removal of slump blocks by weath- subsurface water can emerge, and a means of ering and fluvial erosion, and consequent INTRODUCTION transporting material released from the scarp scarp retreat. Spring-derived tufa deposits face (Laity, 1988). Structural control through found near the limestone escarpments provide Among the most prominent geomorphic fea- high hydraulic conductivity faults is also typical additional evidence for possible ground-water tures of the current hyperarid Western Desert of (Laity, 1988). Sapping processes produce chan- sapping during previous wet periods. A com- Egypt are a series of depressions surrounding nels that migrate headward into escarpments, puter simulation model was developed to plateaus and escarpments. The escarpments ex- with steep sides, flat floors, and theater-like quantify the ground-water sapping processes, tend for hundreds of kilometers (Fig. 1). The de- heads that lack well-developed tributaries (Laity using a cellular automata algorithm with cou- pressions are largely floored by shales, whereas and Malin, 1985; Kochel and Piper, 1986; pled surface runoff and ground-water flow for the surrounding plateaus are composed mainly of Uchupi and Oldale, 1994). a permeable, resistant layer over an imperme- limestones (Hermina, 1990). Previous models for The objective of this paper is to gather and in- able, friable unit. Erosion, deposition, slump- the origin of these landforms include wind ero- tegrate evidence to explore quantitatively the hy- ing, and generation of spring-derived tufas sion (Ball, 1927), fluvial erosion (Said, 1983; pothesis that escarpments and depressions of the were parametrically modeled. Simulations us- McCauley et al., 1982), and ground-water sap- Western Desert were caused by fluvial and sap- ing geologically reasonable parameters dem- ping (e.g., Maxwell, 1982; Higgins, 1990). Wind ping processes. The procedure followed is to onstrate that relatively rapid erosion of the alone cannot erode the resistant limestone cap- first establish a qualitative conceptual model for shales by surface runoff, ground-water sap- rock. Fluvial erosion alone cannot explain some the processes involved, using published work, ping, and slumping of the limestones, and de- peculiar geomorphic features, such as scalloped remote sensing, and field observations to pro- tailed control by hydraulic conductivity inho- escarpments and large depressions. For the de- vide constraints. The next step is to quantify the mogeneities associated with structures explain pressions and escarpments of the Western Desert, primary processes with a numerical computer the depressions, escarpments, and associated we believe that fluvial erosion and ground-water model that integrates the evidence into an over- sapping working together offer the best explana- all landform evolution system that includes cou- tion (e.g., Maxwell, 1982; Issar, 1982; Higgins, pled fluvial and sapping processes. The model- *Present address: Department of Geography, Northern Illinois University, DeKalb, Illinois 60115. 1990). Furthermore, these processes were much ing approach provides insights into the processes E-mail address: [email protected]. more important in the past as compared with to- that are no longer operating in the study area. Data Repository item 9636 contains additional material related to this article. GSA Bulletin; January 1997; v. 109; no. 1; p. 43–62; 21 figures; 2 tables. 43 Downloaded from gsabulletin.gsapubs.org on December 30, 2009 LUO ET AL. Figure 2. Flow diagram of the modeling ap- proach of this study. The parameter values are selected based on independent measure- Figure 1. Location map of Western Desert, Egypt, showing major depressions (dark stipple ments documented in the literature if possi- areas), plateaus, and aeolian sand deposits. Escarpments bordering the depressions extend for ble. The model was used to estimate the few hundreds of kilometers. Box a denotes the study area shown in Figures 3 and 6. Boxes A and B parameters that have no independent con- denote footprints of Thematic Mapper data and Système Probatoire d’Observation de la Terre straint by comparing model outputs with the (SPOT) data shown in Figures 5 and 11, respectively. Study areas visited in the field include the observations. The result is a predictive model Kharga, Farafra, and Kurkur regions (adapted from Hermina, 1990). that can provide insights into processes that are no longer operating. The approach is summarized in a flow diagram Rufuf Pass, which we examined in 1994. Fig- shown in Figure 2. The paper focuses on the ure 4 shows representative cross sections. El Ru- base of the escarpment and consists of primarily Kharga and Farafra regions, because these two fuf Pass is the site of a major fault system striking marl and green shales, enclosing minor carbon- areas were visited by us in 1994 and 1995. An northeast-southwest. Spring- and wadi-related ate intercalations. The thickness reaches ≈160 m additional site, the Kurkur Oasis, was also vis- tufa deposits crop out along and close to the es- and decreases toward the south (Hermina, 1990; ited by us in 1995 and results were reported in carpments, providing direct evidence for emer- Handley et al., 1987). Overlying the Esna For- Crombie et al. (1995, 1997). The Kurkur Oasis gent ground-water in the past (Caton-Thompson mation and forming the large plateau is a se- setting is similar to the ones found at the Kharga and Gardner, 1932; Stringfield et al., 1974). quence of predominantly carbonate rocks called and Farafra areas. Thus, a detailed discussion of The Kharga depression