Catena 78 (2009) 250–259

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Stratigraphy and sedimentology at Bir Sahara, Egypt: Environments, climate changeand the Middle Paleolithic

Christopher L. HillDepartment of Anthropology and Environmental Studies Program, Boise State University, Boise, Idaho, 83725-1950, United States

E-mail address: chill2@boisestate.edu.

0341-8162/$ – see front matter © 2009 Elsevier B.V. Adoi:10.1016/j.catena.2009.02.003

a b s t r a c t

a r t i c l e i n f o

Keywords:

Bir Sahara, situated in north GeoarchaeologyPleistoceneClimatePaleolithicAfrica

east Africa, contains a set of sedimentary sequences that imply episodic changesin climate and environment during the Middle and Late Pleistocene. Some of the stratigraphic contexts areassociated with Middle Paleolithic artifact assemblages. The artifact assemblages are typically in sands thatunderlie deposits composed of high amounts of carbonate or fine clastics (muds) indicative of expandinglakes and wetter climates. The wetter climates may have provided landscapes that were periodically inhab-

itable by Pleistocene hominids.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

The eastern Sahara Desert, in northeastern Africa, is presently oneof the most arid regions on Earth (Alaily and Pohlmann, 1995). How-ever, it contains evidence of landscape evolution and changing envi-ronmental conditions that can be related to Quaternary climate andhominids.Geologic, paleontologic, andarchaeologic studieshavedocu-mented that the arid conditions of today are the result of changes inHolocene climate (Wendorf and Schild, 2001; Foley et al., 2003; Nicoll,2004; Smith et al., 2004a; Kuper and Kropelin, 2006; Bubenzer andRiemer, 2007). In Egypt, Pleistocene archaeological occurrences(Acheulian and Middle Paleolithic artifacts) are also known to existand in some instances these assemblages are embedded in sedi-mentary deposits (Wendorf and Schild,1980;Wendorf et al.,1993;Hill,2001; Smith et al., 2004a). The sediments can be used to evaluateenvironmental settings and site formational processes associatedwith the presence of Paleolithic artifacts. In addition, the stratigraphicsequences can be used to examine broader scale patterns of climatechange and hydrologic models linked to arid region landscapeevolution (Said, 1975; Said, 1983; Embabi, 1999; Maxwell and Haynes,2001; Ghoneim and El-Baz, 2007). These patterns may have implica-tions for the presence of suitable landscapes for human habitation inthe eastern Sahara and human migration out of Africa during thePleistocene (Vermeersh, 2001; Clark et al., 2003; Smith, 2004b;Derricourt, 2005; Vaks et al., 2007).

2. Area description, methods, and material studied

The Bir Sahara region is situated in the eastern Saharan portionof northeast Africa (Fig. 1). The study area is at about 22o56′ north

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latitude and 28o45′ east longitude (about 300 km west–northwest ofAbu Simbel in the Nile Valley, about 400 km east of the Egyptian/Libyan border, and 100 km north of the Egyptian/Sudanese border).Within the area known as the Atmur Peneplain (Issawi, 1978) and theSelima Sand Sheet (Haynes, 1982) is a large patch of Quaternary rocks(Klitzsch et al., 1987). On the east side of this patch is the El TawilaMass (or TarfawiMass) composed of Precambrian granites and gneisses.Overlying the igneous rocks in the Bir Sahara region are Upper Creta-ceous quartzitic sandstones of the Nubia Formation. The Quaternarydeposits overlie a structural basin formed of Nubia Sandstone.

The northeast corner of the Selima Sand Sheet contains a set ofdeflational basins. The largest of these have been designated as BirTarfawi and Bir Sahara (Wendorf et al., 1993; Embabi, 1999). Comyn(1908) recorded the location of Bir Tarfawi (“Terfaui”) based on histraverse in 1906. The 1925 el Din expedition passed through “Tarfaoui”while traveling in southwest Egypt (Bovier-Lapierre, 1929). Ball(1927) noted the presence of sandhills covered with tamarisk bushesand surface water about 13 km west of Bir Tarfawi. Beadnell (1931)dug a well in 1927 that was designated as “Bir el Sahara,” about 35 kmfrom the largest depression at Bir Tarfawi. Beadnell's map shows thepresence of tamarisk in a north–south elongated depression about8 km from the locality designated as “Bir Terfawi” along the route tothe well. Bagnold (1935, p. 121) also reported the presence of“tamarisk cones” west of “Bir Terfawi”.

Beadnell's “Bir el Sahara” was situated about 15 km southwest ofan elongated deflational basin that contains some vegetation and a setof sedimentary remnants, designated as either Bir Sahara, Bir SaharaEast, or Tarfawi West (Schild and Wendorf, 1975; Issawi, 1978; Schildand Wendorf, 1981; Klitzsch et al., 1987). The Egyptian Bedouin referto this area as Bir Sahara. The lower part of the depression containsacacia and tamarisk associated with phytogenic dunes and surfacewater. This regionwas first studied in 1973 and 1974 by the CombinedPrehistoric Expedition. The Expedition documented the presence of

Fig. 1. Map showing location of Bir Sahara in southern Egypt, northeast Africa.

251C.L. Hill / Catena 78 (2009) 250–259

Acheulian and Middle Paleolithic artifacts associated with lake andspring deposits (Wendorf and Schild, 1980; Schild and Wendorf,1981). This region was reinvestigated in 1986–1988, leading todetailed geological, paleontological and archaeological studies (Wen-dorf et al., 1993). At the south end of the Bir Sahara depression thereare a set of sedimentary remnants associated with Acheulian artifacts(Schild and Wendorf, 1981; Hill, 2001). To the north, a series ofsedimentary remnants are associatedwithMiddle Paleolithic artifacts.In this paper, the lithostratigraphic record from the northern sectionof the Bir Sahara depression is described and interpreted in terms of itsimplications for site formational processes ofMiddle Paleolithic archae-ological occurrences, landscape evolution, and paleoclimates.

Information obtained from field and laboratory methods form thebasis of the interpretations of the stratigraphic sequences at Bir Sahara.Geologic field studies included the examination and description ofnatural exposures as well as trenches and bore-holes. The descriptionand sampling procedure consisted of cleaning the profile, making ascale drawing of the stratigraphy (almost always at 1 mm=1 cm, or1:10 scale), noting the characteristics of the sedimentary deposits, andsampling (starting from the base of the sequence). The basic charac-teristics noted for each stratigraphic sequence were thickness ofdeposit, dominant color, state of coherence or cementation, prominentbedding and structures, nature of boundaries, a field assessment oftexture and composition, aswell as fossil and artifactual content. Colorswere determined on dry samples using color charts (Munsell, 1975).

Textural and compositional analyses were undertaken using lab-oratory techniques that estimate particle size and relative amountsof siliciclastics, organics, and carbonates of the deposits (Folk, 1980;Singer and Janitsky, 1986; Lindholm, 1987; McManus, 1988; Gale andHoare, 1991; Burt, 2004). For particle size analysis, sediments werepretreated to remove carbonates and organics, and a dispersal solu-tion was added. Carbonates were removed using hydrochloric acidand organic matter was removed using hydrogen peroxide or sodiumhypochlorite. The dispersal solution contained both sodium hexam-etaphosphate and sodium carbonate. Separation of coarse (gravel andsand) and fine (mud) fractions used wet sieving with a vacuum–

aspirator system, and size separation of the fine fraction used thepipette technique. Organic and carbonate content was determinedusing a loss on ignition procedure (Dean, 1974). Classifications andinterpretations of these data are based on principles outlined or

reviewed in Folk (1980), Lindholm (1987), Nichols (1999), Selley(2000), Cojan and Renard (2002), and Scholle and Ulmer-Scholle(2003); sediment size fraction designations are based on the Went-worth scale.

3. Results and analyses

Three sets of stratigraphic sequences were studied. The resultsof laboratory studies are presented in Table 1. One sequence isrepresented by the sediments at Trench 14/88 (T-14/88). This trenchis part of a transect in the northwestern section of the study area(Fig. 2). Another sequence is related to the archaeological assemblageat Bir Sahara site 11 (BS-11), situated in the south-central part of thestudy area. The third set of deposits is associated with archaeologicaloccurrences at Egypt site 1988-11 (E-88-11), Bir Sahara site 1 (BS-1)and Bir Sahara site 12 (BS-12) on the southeast side of the study area.This study set also includes samples from Trench 4/88 and Trench 10/73 located on the southeast remnant (Table 1). Age estimates for someof these deposits are presented in Table 2.

The sedimentary sequence in the northwest part of the study areais known from 22 stratigraphic trenches and 10 bore-holes placedover an 800 m transect. The sedimentology at T-14/88 provides arecord of deposition near the center of a Pleistocene basin (Fig. 3). Thelowest unit, documented only in boreholes, is a white (ca. N9 and10YR 8/2) poorly sorted to very poorly sorted slightly gravelly quartzsand (samples 1b and 2b). This is overlain by a weakly cemented,yellow (2.5Y 8/6) very poorly sorted, mostly fine sand with somegravel and some carbonate concretions (sample 3b). The sand fractionis about 90% of the clastic component. The overlying unit consists ofa weakly cemented, grayish brown (10YR 5/2) very poorly sortedmuddy sand (sample 4b) that grades into a light brownish grey (10YR6/2) calcic, slightly gravelly muddy sands (sample 5b). The uppersection of this unit is composed of a white (2.5Y 8/0-2) limestone(sample 6) as well as lenses of carbonates, sands, and silts. The lime-stone is a carbonate mud; carbonate content is about 96% while theclastic component consists of 38% silt and 44% clay.

Another set of sands overlie these deposits at T-14/88. The sedi-ments are cemented (weakly coherent to strongly cemented), andeither pale yellow (2.5Y 8/4) fine sand and mud (samples 7 and 8) orwhite to light gray (5Y 8/2 to 10YR 7/2) calcareous sand (sample 1t).

Table 1Sedimentological data from northern Bir Sahara.

Samplenumber

%carbonate

% clastics

Gravel Verycoarsesand

Coarsesand

Mediumsand

Finesand

Veryfinesand

Silt Clay

T-14/88012t 24.5 1.39 1.33 8.36 17.91 26.06 13.94 26.13 4.88011t 36 0 0.56 5.57 14.13 20.34 17.09 32.48 9.89010t 74 0 1.58 4.74 13.44 19.77 15.02 33.59 11.86009t 25.5 0 0.71 10.7 28.88 30.3 11.31 13.25 4.86008t 2 0 0.44 9.49 38.26 29.82 12.19 9.63 0.17007t 63.5 0 0 2.28 12.53 10.26 7.98 64.1 2.85006t 49.5 0 0.02 1.11 2.17 3.09 6.4 80.56 6.62005t 27 0 0 0.87 2.33 2.04 5.24 78.6 10.92004t 31 0 0.15 1.35 4.52 5.11 6.92 71.42 10.53003t 92.5 0 1.79 5.35 14.29 16.07 17.86 5.93 35.71002t 84.5 0 1.27 8.28 26.76 17.19 14.65 25.48 6.37001t 27 1.2 13.21 37.3 28.49 12.12 3 4.36 3.28006b 96.5 0 0.89 0.22 3.05 5.98 7.25 38.17 44.53005b 6.5 0.02 9.02 6.58 14.52 19.19 22.86 21.71 6.1004b 1 0 13.37 7.78 16.25 21.83 25.39 11.53 3.85003b 0.5 1.01 28.16 13.3 12.73 18.21 18.13 6.32 2.11002b 0.5 0.99 22.45 11.9 15 20.92 21.58 3.83 3.29001b 0.2 4.88 0.84 11 34.93 29.63 14.73 3.79 0.25

BS-111 0.2 0.87 10.65 8.33 29.05 30.61 16.73 1.73 2.032 0.5 3.52 11.95 6.82 18.47 24.78 20.9 11.36 2.23 0.5 2.25 13.16 11.3 25.51 25.48 15.97 4.54 0.785 0.2 1 7.43 11.4 23.04 33.56 18.03 5.42 0.176 0.2 0.39 6.27 11.3 25.07 33.8 17.29 4.43 1.457 0.2 0.68 7.56 12.6 30.65 29.79 15.61 1.35 1.768 0.2 2.75 17.84 17.1 28.23 23.94 10 0.44 0.49 0.2 1.35 16.25 17.1 31.68 20.7 9.32 2.25 1.3110 2 0.44 3.14 9.77 18.59 18.94 16.75 29.67 2.6211 1 1.27 9.16 14.6 30.16 24.81 13.53 5.99 0.4912 2 0 2.95 5.98 13.93 13.21 18.39 37.06 8.48

BS-121 0.1 0 1.47 14.7 33.57 31.05 15.45 2.45 1.332 0.5 0 2.89 20.3 36.83 21.36 13.27 3.49 1.93 0.5 0 2.97 27.5 35.4 18.29 11.8 1.35 1.714 1 0 1.78 21.6 31.67 19.44 14.02 6.7 4.84

T-10/731 0.5 0 0.51 9.84 37.66 26.97 17.13 7.64 0.252 0.5 0 0.43 13 40.66 28.04 14.87 3.37 0.213 0.5 0 1.05 10.7 25.93 43.35 10.87 2.62 5.444 2.5 0 0.15 2.23 5.94 4.46 9.66 66.9 10.75 2.5 0 0 0.15 0.87 2.18 9.45 82.8 4.516 2 0 0.04 1 1.57 3.84 12.1 70.49 117 78.5 0 0 1.62 3.24 4.87 14.59 48.65 278 86.5 0 0.86 1.54 1.36 5.99 20.53 42.77 26.99 4.5 6.42 2.31 2.5 2.67 3.03 7.85 67.73 7.4910 5.5 0 0.25 0.99 3.11 4.23 13.56 67.79 10.0711 70.5 0 0 0.68 1.03 5.48 20.55 50.06 22.612 85 0 0 1.08 1.19 4.32 16.2 53.99 23.22

E-88-111 0.5 0 1.44 16.7 38.45 24.66 13.63 2.95 2.225 0.5 0.11 2.59 18.3 38.79 23.74 12.24 2.58 1.676 1 1.56 11.12 23.6 30.25 17.26 10.3 3.66 2.247 2 0 0.93 15 34.84 22.38 10.9 15.36 0.588 20.5 0 1.68 10.5 30.67 21.31 13.56 15.59 6.749 10.5 0 2.06 9.42 22.84 28.26 14.19 14.19 7.1

E-88-1 (T-4/88)1 0 0 1.82 20.1 38.98 24.09 14.55 0 0.452 0.5 0 1.75 21 32.25 25.36 12.09 7.5 03 1.5 0 0.84 7.55 13.41 9.33 11.22 37.73 19.924 4.5 0 0.15 3.41 5.54 5.68 13.2 67.85 4.175 4 0 0.61 1.92 3.27 3.97 12.94 44.81 32.656 3.5 0 0.21 2.08 4.31 4.69 12.83 59.96 16.167 14.5 0 1.57 1.57 16.53 14.78 18 35.71 2.948 61 0 1.67 11.2 15.03 10.5 18.62 42.96 09 35 0 0 0.2 11.24 63.71 24.85 0 010 3.5 0 5.08 24.4 21.38 22.54 15.73 8.46 2.45

252 C.L. Hill / Catena 78 (2009) 250–259

The detrital component is 92% quartz sand and 8% mud, whilecarbonate is about 27%. There are two luminescence ages from thisdeposit at T-14/88 (Wendorf et al., 1994). These age estimates arecentered on 291,000 and 71,000 years ago. The sands underlie asequence of white (ca. N9 to 10YR 7/2) sandy limestone or marl(samples 2t and 3t). The carbonate content is 84–91%. The clasticcomponent is very poorly sorted muddy sand (mostly medium to veryfine sand). These are overlain by marl or calcareous mud depositscontaining 27–30% carbonate and 70–80% silt in the clastic compo-nent (samples 4t and 5t). The lower section is strongly cemented,white to light gray (10YR 8-7/2) while the top is weakly cementedand white (5Y 8/2). An erosional surface separates these depositsfrom a weakly cemented, white (5Y 8/2) to pale olive (5Y 6/4)marlstone or calcareous silt. Clastic and carbonate proportions areabout equal, with silt composing over 80% of the clastic component(sample 6t). A slightly cemented to very strongly cemented, cellular,white (10YR 8/2) muddy limestone or marl (sample 7t) lies above. Itcontains abut 63% carbonate and the clastic component is mostly silt.

Sand forms over 90% of the overlying unit (sample 8t). It is anuncemented, very pale brown to light gray (10YR 7/3-2) calcic, poorlysorted sand. At T-14/88, two luminescence ages centered on 84,000and 114,000 years ago are available for this sand (Wendorf et al., 1994).An erosional surface separates this from the overlying deposit. Abovethis unconformity, sediments contain Melanoides tuberculata, Gyrauluscostlatus, and Biomphalaria alexandria. Carbonate content is as high as74% in the overlying pinkish light grey (5YR 7/2) sandy limestone(sample 10t). Melanoides is also present in white (5YR 8/1) to grayishbrown (10YR 5/2) marl or calcareous sand (sample 11t). Carbonateis about 36% and nearly 70% of the clastic component is fine sand to silt.The top of the sequence consists of a dark reddish gray (5YR 4/2)calcareous sand (sample 12t); carbonate content is about 24%. Depositsabove these sediments at an adjacent trench (Trench 15/88, Fig. 2) havebeen dated to around 56,000 years ago (Wendorf et al., 1994).

BS-11 is situated in the south-central section of the study area(Fig. 2). The lowest deposits consist of white to yellow (10YR 8/1 and2.5Y 8/6), slightly gravelly sand or slightly gravelly, muddy sand(Fig. 4, samples 1–3). The upper surface of this sand is an erosionalsurface. These sands are overlain by a cross-bedded, uncemented toweakly cemented, light grey (10YR 7/1) slightly gravelly sand(samples 5–8). The sand contains Middle Paleolithic artifacts. Thebedding consists of concave laminations which dip towards thenorthwest. These deposits are interpreted as reflecting a lake marginsetting consisting of either beach or dune sands. The slight increase incoarse sand and mean particle size may indicate that these depositswere formed by an encroaching beach or regressing lake margin.

A darker colored deposit, consisting of slightly gravelly sand orslightly gravelly muddy sand (samples 9, 10), also contains some arti-facts in its lower section. The sand is variable in color, ranging fromgray, grayish brown to yellow brown (10YR 5/1-3) to dark grayishbrown (10YR 4/3). The detrital component is composed of 67–95%sand. Vertical and horizontal carbonate cemented tubes might bebiogenic in origin (rhizoconcretions). The dark color of this depositappears to reflect the presence of secondary manganese.

The overlying deposits seem to be linked to an increase inmoisture.They consist of a light grey to brownish yellow (10YR 7/1–6/8),slightly gravelly sand to poorly sorted muddy sand (samples 11 and12). These deposits appear to reflect short-term intervals of laketransgression and regression. Lithified limestones (platey and spongyevaporates of Schild and Wendorf, 1981) containing fossilized reedstems overlie this sequence. Uranium series ages on these carbonateshave large standard deviations but are centered on 381,000–307,000 years ago (Wendorf et al., 1994).

Pleistocene sediments are also exposed to the east of BS-11. Theeastern sedimentary remnant has been extensively studied primarilyin conjunction with the excavation of Middle Paleolithic archaeolo-gical occurrences at sites E-88-11, BS-1, and BS-12 (Fig. 2).

Fig. 2.Map of study area, northern Bir Sahara (data source: Wendorf et al., 1993). There are three areas that contain surface exposures of Pleistocene lake-related sediments. Trench14/88 (T-14/88) and Trench 15/88 (T-15/88) are situated in the northwest part of the study area. The sequence at BS-11 is covered by limestones in the central part of the area. Thesedimentary remnant on the east side of the study area contains strata associated with Sites BS-12, BS-1, and E-88-11. The excavation and trench locations in the E-88-11 area are alsodepicted.

253C.L. Hill / Catena 78 (2009) 250–259

Sedimentological studies of some of these deposits were previouslyconducted by Kossakowaska-Such and Wesolowsk (Schild andWendorf, 1981). Fig. 5 provides a cross-section for the north part ofthe remnant in the vicinity of E-88-11. At Site E-88-11 (Fig. 6) most ofthe sediments are composed of coarse clastics (samples 1–7). Forexample, the artifact-bearing zone is mostly sand with some gravel(sample 6). The sands are laminated, uncemented, very pale brown(10YR 7/4), and contain trace fossils. These seem to be lake margindeposits. Trace fossils at the artifact zone may represent an interval ofstabilization by vegetation. The sands above this horizon have anoptical luminescence age centered on 104,000 years ago (Wendorf et al.,

Table 2Chronometric measurements from the north area of Bir Sahara.

Location Age Plus

Tr. 14/88 (T-14/88), Unit 4 291 65Tr. 14/88 (T-14/88), Unit 4 71.1 2.8Tr. 14/88 (T-14/88), Unit 6 114 9.8Tr. 14/88 (T-14/88), Unit 6 84 91Tr. 15/88 (T-15/88), above Unit 7 at 14/88 55.9 17.3BS-11, limestone overlying sequence 381 96BS-11, limestone overlying sequence 339 156E-88-11, Unit 1, above main artifact zone 103.9 9.5E-88-11, Unit 2 203 71E-88-11, Unit 3 257 171E-88-11, Unit 3 148 60E-88-11, Unit 3 307 110BS-12, Tr. 5/73 (T-5/73), Unit 1 84 10BS-1, Unit 2 105.4 10.5BS-1, Unit 2 108.6 10.6BS-1, Tr. 7/73 (T-7/73), Unit 4 105 15

Data source: Wendorf et al. (1994).

1994). The upper deposits contain increased amounts of carbonate andmud, which appear to reflect the onset of a wet interval.

To the east, at Trench 4-88 (T-4/88), Middle Paleolithic artifactsare also found within sands (Fig. 7, samples 1 and 2). The lowersection of these sands is a massive, weakly cemented, white (10YR 8/2), poorly sorted sand (sample 1). Loose to weakly coherent, mottledwhite (10YR 8/2) to yellow (10YR 7/6) sediments above this arecomposed of poorly sorted sands (sample 2).

These deposits are overlain at T-4/88 by a sequence that is initiallycharacterized by an increase in silt and clay (fine siliciclastics, samples3–7) and then by high amounts of carbonate (sample 8). The lowest

Minus Sample no. Material Method

65 GdTL-200 Sand TL6 OXOD748(S)-3a Sand Optical11.2 OXOD748(S)-3 Sand Optical91 GdTL-201 Sand TL14.1 OXOD748(S)-4 Sand Optical96 87BSH-11 Carbonate U-Series

156 87BSH-12 Carbonate U-Series13.2 OXOD748(S)-2 Sand Optical71 87BHS-13 Carbonate U-Series171 87BSH-10:2 Marl U-Series60 87BSH-10:1 Marl U-Series110 87BSH-7 Limestone U-Series10 GdTL-164 Sand TL10.5 OXTL506a Burned Silt TL10.6 OXTL506b Burned Silt TL15 GdTL-166 Sand TL

Fig. 3. Trench 14/88 (T-14/88) sedimentology. Sample numbers designate location of samples from trench and borehole. Three pluvial events dating to OIS 5 or earlier appear to berepresented by the stratigraphic sequence.

254 C.L. Hill / Catena 78 (2009) 250–259

part of this sequence is composed of pale yellow (2.5Y 7/4) fine andvery fine sand and mud (sample 3). This is overlain by a 2 cm thick,very weakly cemented, reddish yellow (5YR 6/6) sandy silt (sample4). A weakly cemented, light gray (5Y 7/2) sandy mudstone (sample5) above this contains high amounts of silt and clay. An uncemented toweakly cemented deposit containing over 60% silt (sample 6) is alsopresent. The overlying sediment shows an increase in carbonate toaround 15% with almost twice as much sand as mud (sample 7). Car-bonates from this set of deposits have provided a U-series measure-ment with a large standard deviation centered on 203,000 years ago(Wendorf et al., 1994).

A white (10YR 8/2) cemented carbonate interbedded with thinlenses of sands and silts (sample 8) appears to signal a major lakeexpansion, and perhaps seasonal expansion and contraction of a pe-

rennial lake. Three U-series measurements suggest that these car-bonates may range in age from about 307,000 to 148,000 years ago,although they are associated with large standard deviations (Wendorfet al., 1994). The top of the section shows an increase in coarse clastics(70% sand), reflecting a lake regression interval.

A similar pattern is reflected at BS-1 and BS-12 (Fig. 8). At both ofthese localities sandy deposits containing artifacts are overlain bydeposits composed of high amounts of carbonates or fine siliciclastics,apparently formed as a result of a wet climate and the presence ofa lake. Luminescence ages for the sand deposits range from about109,000 to 84,000 years ago (Wendorf et al., 1994). The sedimentsassociated with the paleolake can be subdivided into five major litho-facies. One facies coincides with the initial presence of moisture,perhaps related to rising ground water and the development

Fig. 4. Bir Sahara site 11 (BS-11) sedimentology. Dark circles with white numbers show location of sediment samples. Paleolithic artifacts are found under strata that reflect atransgressive lake event, as indicated by fine siliciclastics. These strata are elsewhere overlain by limestones.

255C.L. Hill / Catena 78 (2009) 250–259

of hydromorphic conditions within sands. The presence of runningwater during the beginning of a wet climate interval is suggested by asecond facies consisting of redeposited sands, basin wash sands, andforedune and lake margin deposits, perhaps before the presence ofsubstantial vegetation around the periphery of the basin could reduceerosion. This lithofacies is characterized by high amounts of coarseclastics. A third lithofacies is characterized by episodic deposition ofhigh frequencies ofmuds. This lithofacies appears to reflect lowenergysedimentationwithin an overall expanding playa-lake setting. A larger,

Fig. 5. Stratigraphic cross-section in the vicinity of Egypt site 1988-11 (E-88-11). Paleolithicepisode of wetter climate (compare detailed sedimentology depicted in Figs. 6 and 7).

possibly perennial lake is reflected bya fourth lithofacies characterizedby high amounts of carbonates. Finally, a fifth lithofacies consisting ofclastics interbedded with carbonates and duricrusts appears to reflectdrier conditions associated with fluctuating water levels, perhaps on aseasonal basis.

In the BS-12 area (Fig. 9), the hydromorphic deposit contains highamounts of very fine sand and higher amounts of silt and clay (Trench10/73, sample 1). A slight transgressive phase is reflected in theincrease in clay content in overlying deposits (samples 2 and 3). Above

artifacts are found in sands (unit 1). These sands are overlain by strata formed by an

Fig. 6. Egypt site 1988-11 (E-88-11) sedimentology. Dark circles with white numbers show location of sediment samples.

256 C.L. Hill / Catena 78 (2009) 250–259

this, sediments contain 4–5 times as much carbonate, and also show achange in the ratio of coarse to fine clastics (samples 4 and 5).Throughout the sequence there is a trend towards increased mud andcarbonate, indicating an expanding lake. Clay peaks within thesequence may hint at the presence of several transgressive episodes.

Fig. 7. Trench 4/88 (T-4/88) sed

4. Discussion and conclusions

The sedimentology of Pleistocene stratigraphic sequences at BirSahara provides information useful in examining the environmentalcontexts and site formational processes associatedwith severalMiddle

imentology, east of E-88-11.

Fig. 8. Bir Sahara site 1 (BS-1) and Bir Sahara site 12 (BS-12) stratigraphic cross-section (data source: Wendorf and Schild, 1980). Luminescence measurements from these strataimply the sequence was formed during OIS 5.

257C.L. Hill / Catena 78 (2009) 250–259

Paleolithic localities. In addition, the stratigraphic sequences appear todocument changes that can be examined in the context of broaderclimate events and landscape evolution.

The deposits at BS-11 contain a set of artifacts assigned to theMiddle Paleolithic or Mousterian (Wendorf and Schild, 1980). Alongwith retouched flakes (mostly denticulates and notches), the collec-tion includes Levallois flakes, cores, and debitage. The sands contain-ing artifacts show a slight coarsening upward trend; these appear tobe eolian or beach-margin sediments associated with a lake regres-sion. They are overlain by sediments interpreted associated with asubsequent lake event. The horizontal spatial distribution of artifactsfrom BS-11 (Wendorf et al., 1993) does not appear to show any sub-stantial redistribution based on the size of the artifacts. The strati-graphic sequence at BS-11 is overlain by limestones associated with

Fig. 9. Bir Sahara site 12 (BS-12) area sedimentology. Dark circles with white numbers show lMiddle Paleolithic artifacts. At Trench 10/73 (T-7/73) the sediments dominated by sands areclays, and carbonates).

a lake transgression and associated with U-series age estimates ofaround 381,000 to 339,000 years ago (Table 2).

At the eastern remnant, Middle Paleolithic artifacts at E-88-11,BS-1, and BS-12 are found in sands that underlie evidence of a majorlake event. At BS-12, Middle Paleolithic artifacts were recovered em-bedded within dark, manganese-enriched sands. Trace fossil horizonsoccur at BS-1, E-88-11, and BS-12. Some of these trace fossil horizonsmay be the result of aquatic macrophytes and other biologic activity(such as burrowing animals) within the deposits of shallow lakemargins (Klappa, 1980; Cohen, 1982; Mount and Cohen, 1984).Some may also be the result of vegetation on the edges of waterbasins and nearby eolian deposits (Glennie and Evamy, 1968; Plaziatand Mahmondi, 1990). Artifacts were found embedded in thesedeposits at E-88-11 and BS-1. From a paleogeographic perspective,

ocation of sediment samples. The sediments dominated by siliciclastics at BS-12 containoverlain by a stratigraphic sequence indicating a transgressive lake event (higher silts,

Fig. 10. Agemeasurements from Bir Sahara compared to broader scale patterns of globalclimate change. The episodes of lake transgression and regression indicated by the sedi-mentary sequences appear to coincide with OIS 5 or older climate events (see Table 2).The sediments indicate the episodic presence of landscapes inhabitable by hominidspossibly as early as OIS 11.

258 C.L. Hill / Catena 78 (2009) 250–259

the trace fossil horizons imply stabilized land surfaces. Artifacts em-bedded in these horizons may have been vertically displaced by biotur-bation. At trenches east of E-88-11 artifacts are found in redepositedsands underlying lake muds and limestones.

If the trace fossil horizon at E-88-11 represents the stabilization ofthe sand substrate by vegetation cover, perhaps around the margin ofan emergent groundwater marsh setting, then the interval of MiddlePaleolithic presence at this locality may be related to this environ-mental context. The possibility of a short erosion and deposition inter-val is indicated by a subtle unconformity at the top of the artifact zone.This truncated surface is marked by a peak in the frequency of verycoarse sand within the site's stratigraphic sequence (sample 6). Thepresence of artifacts within this boundary argues for the possibilitythat the horizontal spatial pattern may be partially a function ofgeologic structuring. The artifact zone at the site is 10 to 30 cm thick;artifacts are embedded in sands below the trace fossil horizon andwithin the horizon. The vertical distribution of these artifacts mayhave been rearranged by bioturbation.

Sediments composed of high amounts of mud may have formedas a result of deposition of suspended particles away from the marginof the basin and nearer the center of the playa or lake basin. Therelationship between the ratios of coarse to fine clastics and silt toclay can be used to evaluate the relative energy regime at the localityof deposition. The relationship is also reflected in the graphic meanstatistic, where the smaller mean grain size (or a larger mean phiindex) implies a lower depositional regime and more distance fromthe basin margin. The depositional setting associated with the bio-chemical precipitation of carbonate within a lake basin is similar tothe setting associated with fine clastics. Limestones and marls aregenerally deposited in quiet, low energy settings except for sometypes of traverties and tufas.

The T-14/88 depositional sequence provides direct evidence ofthree major lake events within the same basin (Fig. 3). The depo-sitional cycles are broadly similar.Within anyepisode there is a decreasein coarse siliciclastics relative to fine clastics. Increasing proportionsof mud and carbonate content reflect lake expansions. A similar patternoccurs at the other localities thatwere examined. The sedimentary rem-

nants in the south part of the study area appear to reflect sedimento-logical variations associatedwith a single dry–wet–dry hydrologic cycle.

The timing of the wetter hydrologic conditions at Bir Sahara can beestimated based on luminescence and U-series ages. Because of thelarge standard deviations associated with the chronometric measure-ments (Table 2), these events cannot be assigned precise ages or cor-related with a high degree of confidence. These chronometric analysessuggest that the wet episodes could date to the late Middle and earlyLate Pleistocene (Fig. 10).

The Bir Sahara strata can be compared to other regional evidencefor climate and hydrologic change. Using Shuttle Radar TopographyMission (SRTM) data, Ghoneim and El-Baz (2007) have proposed thepresence of a large drainage system in southwest Egypt activated bywet climate episodes. This drainage system flowed eastward from thehighlands in western Egypt and northern Sudan and is considered tohave formed the largest paleo-lake in Egypt at Bir Tarfawi and BirSahara. This model may provide a mechanism for the presence for thehydrologic conditions documented at Bir Sahara.

The age determinations from Bir Sahara (Table 2) can be comparedto other chronometric analyses on sediments from this region ofsouthern Egypt and Sudan as well as deposits studied to the northsuch as at Kurkur Oasis and Kharga Oasis. In the vicinity of Bir Saharaand Bir Tarfawi, limestones and marls have been dated to N350,000,and around 277,000, 233,000, 155,000, and 45,000 years ago (Szaboet al., 1989; Szabo et al., 1995). Other U-series ages from localities insouthern Egypt and northern Sudan indicate paleolake episodesdating from about 320,000 to 250,000, 240,000 to 190,000, 155,000 to120,000, and 90,000 to65,000 years ago (Szabo et al., 1995). The mostprecise ages from Bir Tarfawi, based on thermal ionization mass spec-trometry (TIMS) U-series analyses of ostrich egg-shell fragments,have a range from about 137,000 to 126,000 years ago (Schwartz,1992). These imply the presence of pluvial conditions during marineoxygen isotope stage (OIS) 6 and OIS 5.

At Kurkur Oasis, U-series dating of travertines suggest the presenceof humid intervals at greater than 260,000, 220,000 to 191,000, and160,000 to 70,000 years ago which have been linked to increasedprecipitation as a consequence of insolation forcing and enhancedAtlantic and Indian monsoons (Crombie et al., 1997). From KhargaOasis, U-series determinations on tufa of greater than 450,000, andcentered on 286,000 to 272,000,185,000, and 45,000 years ago (Sultanet al., 1997; Hamdan et al., 1999) have been linked to local rainfallunder cool, wet climates. Precise U-series ages of spring-depositedcarbonates at Kharga ranging from about 137,000 to 114,00 years agohave been related toMiddle Paleolithic artifacts and humid conditionsrelated to late OIS 6 andOIS 5 (Smith et al., 2007). Osmand andDabous(2003) suggested that U-series measurements indicated pluvial con-ditions occurred during OIS 6, OIS 5 and OIS 4.

The episodes of lake expansion and regression at Bir Saharadescribed in this paper suggest the presence of habitable landscapesfrom around 400,000 years ago until after 100,000 years ago, corre-sponding to fluctuating global climates between OIS 11-OIS 3 (Fig. 10).The sequence of three wet-episodes at T-14/88 indicate habitableconditions probably during OIS 5, although a TL age from unit 4(Table 2) would imply that the underling pluvial deposits (unit 3,samples 5b and 6b) are related to OIS 9 or an older climate event. Thetwo U-series measurements of limestone stratigraphically overlyingthe BS-11 sequence (Table 2, Fig. 10) also would suggest the presenceof pluvial conditions before or during OIS 9. The luminescence mea-surements from the strata at the sedimentary remnant containing BS-1, BS-12, and E-88-11 are associated with OIS 5. If this is so, they mayreflect environmental conditions contemporaneous with the pluvialepisodes at T-14/88. The U-series measurements from this sedimen-tary remnant suggest the occurrence of older pluvial events, from OIS9-6 (Fig. 10).

Regardless of the exact timing of the wet climate events, strati-graphic and sedimentologic evidence from Bir Sahara provides

259C.L. Hill / Catena 78 (2009) 250–259

documentation of environmental change and geohydrologic eventsthat could help explain human migration out of Africa during theMiddle and Late Pleistocene (Van Peer, 1998; Clark et al., 2003;Derricourt, 2005; Vaks et al., 2007). Wet climate intervals may beconnected with habitable landscapes and migrations into the Sahara,while arid conditions may have motivated human groups to move outof the region. Thus, the changing environments and landscapesdocumented at Bir Sahara may help to explain Pleistocene humanmigrations into and out of North Africa.

Acknowledgments

Field research was conducted as a member of the CombinedPrehistoric Expedition, under the leadership of Fred Wendorf andRomuald Schild. I am especially grateful to Roman Schild for theopportunity to conduct sedimentological studies of the strata from BirSahara; this research builds on his geologic studies in the region.Laboratory studies were conducted at the University of Minnesota'sArchaeometry Laboratory, directed by Rip Rapp. This research wassupported by National Science Foundation grants BNS-8415650, BNS-8518574, and BNS-8718908 to Fred Wendorf as well as funds fromSigma Xi, the Scientific Research Society. Thanks to Cheryl LynnWofford Hill for the preparation of figures and tables.

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