Desert Pavement Development and Landscape Stability on the Eastern Libyan Plateau, Egypt

Geomorphology 107 (2009) 178–194 Contents lists available at ScienceDirect Geomorphology journal homepage: www.elsevier.com/locate/geomorph Desert pavement development and landscape stability on the Eastern Libyan Plateau, Egypt Katherine A. Adelsberger ⁎, Jennifer R. Smith Washington University in St. Louis, Department of Earth and Planetary Sciences, One Brookings Dr., St. Louis, MO 63130, USA article info abstract Article history: Desert pavement surfaces of the eastern Libyan Plateau in central Egypt represent a stable landscape Received 20 May 2008 preserving Middle and Upper Paleolithic artifacts. Detailed measurements of pavement clasts indicate Received in revised form 4 December 2008 significant variability in clast size, density, lithology and orientation between pavements, but no spatial Accepted 5 December 2008 relationship among any of these pavement variables over the study area. Pavement characteristics are Available online 16 December 2008 unrelated to local geomorphic features including slope gradient and aspect, suggesting a desert pavement fi fl Keywords: surface that has developed without signi cant in uence from transporting mechanisms such as overland fl Desert pavement ow and slope failure. Meridional vertical cracks in surface clasts implicate thermal stresses due to diurnal Arid geomorphology solar variation as a mechanical weathering process, whereas the presence of a clast-free silty layer within all Middle Paleolithic soil profiles indicates that these are accretionary pavement surfaces that have grown upward over time. The Upper Paleolithic desert pavement in this region has likely developed in situ through mechanical breakdown of surface clasts Egypt and desert pedogenesis, indicating long-term stability for this region and minimal taphonomic effects on Libyan Plateau artifacts N2 cm in diameter deposited on this surface over the last ca. 100 ka. © 2008 Elsevier B.V. All rights reserved. 1. Introduction studies. Desert pavements can also be useful relative age indicators, as the comparison of desert pavement morphology (Al-Farraj and Harvey, The Abydos Survey for Paleolithic Sites (ASPS) has been conduct- 2000) or desert varnish development (Dorn and Oberlander, 1982) ing a systematic surface survey of the eastern Libyan Plateau near may offer one of the only feasible in situ methods for determining the Abydos, Egypt since 2000 (Fig. 1). The unexpectedly rich Middle and relative ages of desert surfaces such as river terraces and alluvial fans. Upper Paleolithic record preserved in this high-desert environment, Other potential taphonomic information can be obtained from desert representing periods of hominin activity during the last 140,000 years pavement surfaces, including the potential for lateral transport of (Chiotti et al., 2007), has the potential to inform models of desert surface clasts as well as downward movement of artifactual materials. occupation that could serve as a comparison to landscape use in the Once the transport processes acting on any given desert pavement nearby Nile Valley (Olszewski et al., 2005). However, the almost clast are known, more reliable behavioral interpretations can be made exclusively surface context of these artifacts necessitates a thorough using artifacts obtained from these desert pavement surfaces. Under- understanding of desert pavement formation and developmental standing archaeological context begins by understanding the pave- processes in order to recognize possible depositional and taphonomic ment formation and landscape stability of the region in question. effects on archaeological materials. Site-scale examination of desert The Libyan Plateau of Central Egypt is currently a hyperarid pavements on the Libyan Plateau performed as part of the ASPS survey environment, as even the nearest Nile Valley cities receive less than a focused on both surface and subsurface characteristics of these millimeter of rainfall per year on average (Shahin, 1985). This area has pavements in order to clarify the relationship between archaeological probably been an arid region since the early Holocene, when pluvial materials and their depositional setting. conditions were last present in North Africa (Szabo et al., 1995; Both archaeological and geomorphological work in arid zones often Hoelzmann et al., 2001). A more significant humid period also involves a primary focus on surface contexts, in which the dating and occurred across the Sahara ca. 120–140 ka (e.g., Gaven et al., 1981; correlation of different desert surfaces can be difficult (e.g., McFadden McKenzie, 1993; Smith et al., 2004); these wetter conditions may have et al.,1989). Because they are common arid zone landforms (Cooke and encouraged the use of the Libyan Plateau by Middle Paleolithic groups, Warren, 1973), desert pavements are often the focus of arid landscape and would explain the presence of earlier artifact assemblages in this currently uninviting environment. Pluvial events likely served as the primary control on the timing of human occupation in the high desert ⁎ Corresponding author. Current address: Knox College, Department of Environ- environments of Egypt (Chiotti et al., 2007). mental Studies, Two East South St., Galesburg, IL 61401, USA. Tel.: +1 309 341 7274; fax: +1 309 341 7718. In most places the Libyan Plateau is capped by the Eocene Thebes E-mail address: [email protected] (K.A. Adelsberger). Group limestones (Said,1990); in the study area examined here this cap is 0169-555X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2008.12.005 K.A. Adelsberger, J.R. Smith / Geomorphology 107 (2009) 178–194 179 Fig. 1. A. Abydos Survey for Paleolithic Sites field area illustrated over MODIS imagery of Egypt, image credit Jacques Descloitres, MODIS Land Science, courtesy of Team Visible Earth (http://visibleearth.nasa.gov/). B. Pavement and archaeological sampling locations shown against satellite imagery of the eastern Libyan Plateau. The Nile Valley is the dark low area to the east; plateau edge is outlined in white. composed of the Drunka and Serai Formations of the upper Thebes Group overall grain size and sorting, and a measure of overall clast coverage (Klitzsch et al., 1987). Given that rates of granite weathering in the on a given surface (e.g., Poesen et al., 1998; Al-Farraj and Harvey, climatically similar Central Namib Desert exceed 30–80 cm per 100 ka 2000). These measurements may provide an indication of the relative (Cockburn et al., 1999), surface lowering in areas of the Libyan Plateau ages of desert surfaces, where decreasing clast sizes and increased where limestone bedrock is exposed has probably averaged more than a coverage by smaller clasts indicate older pavement surfaces (Al-Farraj meter per 100 ka. However, plateau surfaces here are almost uniformly and Harvey, 2000). Clast coverage may also vary with slope gradient covered by desert pavement, with very little post-Eocene sedimentary due to spatial variation in the depositional and erosional history of deposition other than limited soil formation and localized deposition of particular desert slopes. In these cases, both coverage and size of “old” (undifferentiated Oligocene to Pleistocene (Klitzsch et al., 1987)) fragments increase as slope gradient increases (Poesen et al., 1998), gravels. The uniform nature of the pavement surface present here and the resulting in larger clasts on steeper slopes. lack of significant post-Eocene sedimentation or erosion indicate wide- Clast lithology can affect pavement formation, since lithologies spread stability of the soil horizons present. Surface lowering has probably more susceptible to weathering processes will break down more been limited to unprotected (pavement-free) surfaces, whereas sediment quickly into smaller pieces (Poesen et al., 1998; Al-Farraj and Harvey, accumulation occurs only where the surface gravels produce sufficient 2000; Allison et al., 2000). Clast coverage, while related to landform surface roughness to serve as a dust trap (e.g., Dong et al., 2002). These development itself, is probably not directly influenced by clast processes limit sediment removal and accumulation, as they both require lithology (e.g., Poesen et al., 1998). However, coverage may control significant geologic timescales to effect significant change on a landscape; infiltration, underlying soil development and the location of vegeta- the plateau surface has probably remained relatively unchanged since tion within an arid landscape. Pavement surfaces will prevent Middle Paleolithic times (140–40 ka (Chiotti et al., 2007)). infiltration, leading vegetation to prefer (and disturb) more open The majority of Quaternary erosion in this region would have areas in a pavement landscape (Wood et al., 2005). The presence of occurred along the edge of the escarpment and as headward erosion vegetation may in turn prevent erosion on hillslopes, leading to within drainages, as is apparent in the modern landscape. Wadi lowered rates of pavement formation on those slopes (Poesen et al., downcutting and headward erosion has resulted in isolated plateau 1998). The measurement of pavement clasts and the percent coverage surfaces connected by narrow ridges separating opposing wadi heads. of desert pavement surfaces may elucidate the primary controls on The result of these erosional patterns is a modern plateau surface desert landscape development by revealing the relationships between incorporating significant topographic variation, where wadi-facing lithology, clast size, slope

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