Sedimentology and paleohydrology of Late Quaternary lake deposits in the northern Namib Sand Sea, Namibia

Quaternary Sczence Rewews, Vol 9, pp. 343-364, 1990 Planted m Great Bntam All rights reserved

0277-3791/90 $0 00 + 50 (~) 1990 Pergamon Press plc

SEDIMENTOLOGY AND PALEOHYDROLOGY OF LATE QUATERNARY LAKE DEPOSITS IN THE NORTHERN NAMIB SAND SEA, NAMIBIA James T. Teller,* Nat Rutteri" and N. Lancaster:~

*Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada t Department of Geology, University of Alberta, Edmonton, Alberta T6G 2E3, Canada ~tDepartment of Geology, Arizona State University, Tempe, Arizona 85287, U.S.A.

The Namlb Sand Sea is the largest active desert dunefleld m southern Africa, and is comprised mainly of large north-south hnear dunes. In the mterdune areas of the northern Sand Sea eleven small areas of calcareous lacustnne sediment have been studied. These beds are typically less than a metre thick and are dominantly comprised of calcareous sandstones to mudstones and sandy hmestones The carbonates are mainly magnesian calcites (1-14% MgCO3) with some protodolomlte and aragomte. Calofied reed casts and fresh to brackish water gastropods, diatoms, and ostracods are present in some beds b~sO values indicate a hot and dry climate. A number of ennched bt3C values may reflect high salimty, low organic populations, or carbonate recrystalhzation These carbonate-rich lacustrine deposits are indicative of increased periods of moisture availabdity in this normally hyperand region during the Late Quaternary. The origin of the water responsible for deposmng these sediments may be: (1) pondmg at the end point of the Tsondab River, which at one ttme extended farther west mto the Sand Sea; (2) flooding into interdune corndors when water levels rose m rivers such as the Kuiseb; (3) groundwater seepage into depressions either through dunes that border rivers or from the underlying Tsondab Sandstone: and (4) increased rainfall We do not believe that there is evidence to support a major increase in preclpitauon over the region. However, even a small increase in preopltation in the headwaters of valleys that dram toward the Sand Sea might: (1) generate enough additional runoff to extend the terminal point of rivers such as the Tsondab farther into the dunes; (2) cause lateral flooding from major valleys into tnterdune corridors, and (3) recharge aquifers The sedimentary records at Narabeb, Ancient Tracks, and West Pan, whtch lie along the old course of the Tsondab River, favor a ponded river origin for them, whereas groundwater seepage is favored at other sites. The chronology of deposmon, based on radiocarbon dates, suggests that pondmg and recharge occurred earlier m the lower, western part of the area, and later m the east This Js m harmony with the view that the end point of the Tsondab Rtver progressively retreated eastward between about 30 and 14 ka BP, as dunes blocked its route.

INTRODUCTION Sand seas provide important data on the Quaternary history of many desert regions, and their sedimentary records tend to accentuate the effects of both dry and wet phases (Rognon, 1982). Most palaeoclimatic information from sand seas has been derived from sediments deposited in areas between the dunes during periods of increased moisture availability (Lancaster, 1990). Moisture in these normally arid to hyperarid regions may have been contributed by increased rainfall and runoff within the desert itself, or by increased precipitation in areas that drain toward the sand sea. In this paper, we describe the interdune sediments in the hyperarid to arid Namib Sand Sea of southwestern Africa, and discuss the sedimentology and paleohydrological conditions under which they formed, along with their implications for Late Quaternary paleoclimate of the region. R E G I O N A L SETTING

The Namib Sand Sea covers an area of about 34,000 km 2 along the coast of southwestern Africa between latitudes 27° and 23°S (Fig. 1). It extends inland for 100-150 km to approximately the 1200 m contour near the base of the Great Escarpment. West of this escarpment the regional bedrock surface on which the 343

Sand Sea lies slopes at an average gradient of 1° toward the Atlantic Ocean. Rocks of Precambrian age are exposed on this pediplained surface, and are overlain in places by Tertiary sandstones, conglomerates, and calcretes, in addition to the younger dunes of the Sand Sea. A number of ephemeral watercourses drain toward the coast from the Great Escarpment. One of these, the Kuiseb River, forms the northern boundary of the Sand Sea. To the south, the Tsondab and Tsauchab Rivers (Fig. 1) extend for 40-80 km into the Sand Sea in well-defined valleys, and terminate amongst the dunes in extensive playas. Relict fluvial deposits exposed in some interdune areas indicate that these rivers formerly extended farther west into the Sand Sea before their courses were blocked by the advance of dunes from the south (Seely and Sandelowsky, 1974; Lancaster, 1984). Several smaller ephemeral streams terminate against the eastern margin of the Sand Sea in small playas. There are three main dune types in the Namib Sand Sea, the distribution of which is largely controlled by regional variations in the directional variability of the wind regime (Lancaster, 1983). Along the coast is a belt of crescentic dunes up to 20 km wide. Inland, 50-150 m high linear dunes with N-S to NW-SE alignments cover some 75% of the area of the Sand Sea. In eastern areas of the Sand Sea, where rainfall is higher, there are areas of low, partly vegetated linear dunes. Groups of

J T Telleret al

344

in a number of interdune depressions m the northern part of the Namib Sand Sea, between 50-100 m high GOLA N-S trending linear dunes. At most sites these deposits NAMIBIA are <1 m thick, cover no more than a few thousand square metres, and form shghtly elevated and eroded areas on the floors of the interdune corridors. TSUeME \ These exposures of calcareous sediment occur in the ~ ~ 20°~ area within 34 km south of the Kuiseb River valley, and )i most lie within 10 km of the river (F~gs 2 and 3). Similar loo 2oo 300 I ',, calcareous beds lie to the east, but have been interAM I ! preted by Ward (1984, 1987, 1988) as Tertiary lacu--22"strine deposits, rather than Quaternary. Only along the "~" , ~-~ eastern side of the Namib Sand Sea, in modern pla 3 ' basins such as Tsondab Vlei and Sossus Vie1, have other fine-grained calcareous lacustrine beds been discovered within this hyperarid area, which is so dominated by sand and by eolian processes (Figs 2, 3 and 4). The areal extent of calcareous sediments at the eleven sites discussed m this paper do not exceed a few thousand square metres, and they have been exteno .UC, E.,'r~ I ~" , I sively dissected by erosion. At Bone Pan, Namib IV. and Salty Pan these water-laid sediments are today completely surrounded by high dunes (Figs 5 and 6) At '"~ @A o ~ n~. other sites such as Gobabeb South, Sobeb South, Ancient Tracks, and Khommabes, deposits lie m the interdune corridor but are unconfined by dunes that FIG 1. l.~atmn of Narmb Desert (west of dashed hne) and seaward-draining valleys along the southwestern coast of Africa cross the floor of the corridor today (Fig. 7). Some Namlb Sand Sea stippled. Major channels that terminate m playas calcareous sediments form only local outcrops, either along the eastern side of the Sand Sea indicated by numbers along the toe of dune plinths (Narabeb) or within Tsondab (1), Tsauchab (2), Kolchab (3) megapolygonal depressions (macrofractures of Ward, 1987) in Tertiary sandstone in the interdune corridor (Obab South, West Pan A and B) (Figs 8 and 9). In all star dunes occur around Sossus Vlei and Tsondab Vlei, situations, the original areal extent of these thin and located at the terminus of the Tsauchab and Tsondab partly eroded lacustrine beds is unknown, and their Rivers, respectively (Fig. 1), and along the eastern locations seem to bear only a general relationship to margins of the southern part of the Sand Sea, where modern dunes. In no case does a deposit in one topographically controlled funneling of winds results m interdune corridor extend into an adjacent corridor. a seasonally reversing wind regime. In general, these Quaternary-age playa sediments The Namlb Sand Sea has accumulated by the transport of sand inland and north by the dominant consist of interbedded sandy limestones and calcareous S-SW winds from southern and western coastal source sandstones, with a few units of nearly pure limestone zones, which are fed by sand derived from the Orange and uncemented sand. At one site, Narabeb, there is a thick sequence of alternating calcareous mudstones and River (Lancaster and Oilier, 1983) The Namib Desert forms the core of the arid zone of sands. All deposits are typically well bedded to laminsouthern Africa, which covers most of the western half ated. Tertiary sandstone (Tsondab Fm) or Precambrian of the subcontinent. The climate of the central Namib granite (Salem granite) normally underlie the calcarDesert is hyperand to arid. Mean annual rainfall eous sequences at shallow depths, or crop out nearby increases from less than 15 mm on the coast to 80 mm (Fig. 10). In places, a fragmented (and frequently dispersed) on the eastern edge of the Sand Sea (J. Lancaster et al., 1984). Rainfall occurs mostly in light to moderate case-hardened sandy calcite crust, generally only a few showers generated by convectional storms during late centimeters thick, lies at the top of the interdune playa summer months (March-April). Runoff is rare except sequence. This crust commonly has a nodular to in extreme events. Annual evaporation is high and vesicular, tufa-like nature and contains the calcified totals more than 3000 mm per year (J. Lancaster et al., stem casts of Phragmites. Gastropods, diatoms, and stem casts are present in some beds below the crust, 1984). and the columnar sections in Fig. 10 note their stratigraphic position. Ostracods have also been identiLOCATION AND GENERAL DESCRIPTION OF fied in several units. Root or animal tubules are present SEDIMENTS in many sandstones immediately below the more Calcareous fine-grained sediment is locally exposed calcareous (sandy limestone) beds.

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DESCRIPTION OF SEDIMENTS

analyses for 613C and 6180 and amino acid ratios being determined on selected specimens. Table 1 is a summary of sediment data from the eleven locations. The expanded columnar sections (Fig. 10) show the stratigraphic position from which each analysis was made. Table 2 lists the fossil types identified from these sections. Figure 11 and Tables 3 and 4 present isotopic and amino acid data. The radiocarbon ages on carbonate in the playas of this study, and from other calcareous deposits in the region, are shown in Table 5 and Fig. 12.

Introduction Eleven playa sites were studied in detail (Figs 2, 3 and 10). The data from two of these, Narabeb and Khommabes, have already been published (Teller and Lancaster, 1986a,b, 1987), and are included in this paper so that all known deposits can be evaluated together. Representative samples from the sequences at each site were analysed in the laboratory for mineralogy, grain size, water-soluble content, and acidsoluble content. Fossils were identified, with isotopic

Mineralogy and Grain Size The mineralogy of the playa sediments was mainly determined by x-ray diffractometry, supplemented by polarizing microscope work and acid dissolution. Determination of MgCO3 in calcite was done by x-ray diffraction according to the procedures of Goldsmith and Graf (1958). Siliciclastic size analysis was done by dry and wet sieving, following removal of carbonate cement by HCI and more-soluble evaporitic cement by distilled water.

Ward (1987) proposed the name Khommabes Carbonate Member for those carbonate deposits situated in, or associated with, present-day topographic depressions in the Namib Desert. The carbonate was defined as being part of the Sossus Sand Formation, which comprises the bulk of deposits in the Namib Sand Sea; the type locality was designated as the Khommabes site (Fig. 7), which has been described by Ward (1984, 1987) and Teller and Lancaster (1986a).

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FIG 30uthne of major hnear dunes of the study area and the location and names of the sites stud~ed Modern terminus of the Tsondab Raver is at Tsondab Vlel, with the previous extension of this rwer represented by fluvial sdts, sands, and gravels (stippled pattern) Spot elevations indicated m meters (after Lancaster, 1984, Teller and Lancaster, 1986b)

Beds range from 100% siliclclastic sand to > 9 0 % carbonate. In most sections there is a w~de range between units that are high in carbonates, such as calcite (high and low magnesian types) and protodolomite, and those which are low in carbonate. Most stratigraphic sequences and even many indw~dual calcareous units contain more than one carbonate mineral. Halite is commonly present in small percentages as a finely-dispersed precipitate or as a filling of fractures in calcareous mudstones (e.g. Narabeb, West Pan A and B) (Table 1). Most beds can be regarded as calcareous sandstones or mudstones - - grain-supported siliciclastic rocks cemented by varying proportions of carbonate minerals or sandy limestones, where the percentage of elastic grains falls below 50% (Fig. 13). At least one bed of calcareous elastics occurs in all sections, and sandy limestone or dolomite is found in most sequences. Some of the highly calcareous umts form a case-

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hardened crust over less well-cemented sediment (see Table 1). At some sites the crust is comprised of > 6 0 % carbonate (West Pan A [4, 5]; Khommabes [ld]; Gobabeb South [3]; Obab South [3]; Salty Pan [5b]) whereas at others the nodular to vesicular casehardening is > 6 0 % siliclclastics (West Pan B [6, 7]; Sobeb South [2, 3]; Khommabes [2f]). The carbonate of the crust is calcite (West Pan B [6, 7]; Sobeb South [2, 4]; Gobabeb South [3]; Obab South [3]), aragonite and calcite (Salty Pan [5b]), or protodolomite and calcite (West Pan A [4, 5], Khommabes [ld]). Below the case-hardened crust, the dominant carbonate mineral is calcite, although protodolomite - - a poorly ordered form of dolomite with an excess of calcium (cf. Graf and Goldsmith, 1956; Gaines, 1977) is also common, both with calcite and by itself (Table 1). True stoichiometric dolomite (Ca0 5Mg0 5CO3) is present in small proportions m only three samples (Narabeb [VIII, X]; Ancient Tracks [32]). Aragonite (A in Table 1) occurs in seven -

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Laki~ Deposits in the Northerfi Namib Sand Sea

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FIG. 4. Modernend point of the ephemeralTsauchab River (SossusVlei), central NamibDesert, whichlies alongthe eastern side of the Sand Sea (Fig. 1). Thisplayalies amongststar dunes, and periodicallyreceivesan influxof calcareoussilts fromthe higher and wetter uplands to the east (photo J.D. Ward).

HG. 5. Bone Pan (photo J.D. Ward). samples, mainly with calcite and mainly at the Namib IV site. Calcite in the calcareous sediment contains a variable amount of magnesium. Using the definition of Kelts and Hsu (1978), where calcite with 7-30 mole percent MgCO3 is considered high magnesian calcite (Cao93-o.7oMgo.o7-o3oCO3) and that with 0-7 mole percent MgCO3 is low magnesian calcite (Cal.o-o.93 Mgo-o.o7CO3), only a few samples in this study can be considered to contain high magnesian calcite (West Pan A [4], which also contains low Mg calcite, as well as protodolomite; Narabeb [I, III, V, VI, VIII]). Most

low Mg calcites contain 0 to 3 mole percent MgCO3 (Table 1). All playa sediments contain quartz sand, and quartz is the dominant siliciclastic mineral in all sand- and siltsized fractions. Albite and potassium feldspar are typically present in very small percentages, with occasional micas, chert, dark minerals, and rock fragments in the sand. Thin sections reveal that quartz grains are both single and multi crystalline. They vary from angular to rounded but are mostly subangular to subrounded with the smaller grains usually more angular. A high proportion of the grains show 'crack-

348

J T Teller et al

FIG 6. Namlb IV Location of stratlgraphlc section and Elephas reckt tooth and bones near Land Rover where person Is standing.

FIG 7 Aenal vmw looking north toward Khommabes (arrow). Dune beyond Khommabes separates this depresston from the Kuiseb valley (dark line of trees) (photo J D Ward)

ing' or fracturing. Feldspars are commonly angular to subangular and unweathered. At Narabeb, Teller and Lancaster (1986b, 1987) identified illite as the dominant clay mineral, where it comprises 38-68% of the total clay mineral fraction, with chlorite making up <155'o of the totals The percentage of kaolinite and expandable clays typically falls between 12 and 33% in the sediments at Narabeb. Size analyses of the sands and sandstones show the modal class for virtually all samples to be in the finegrained sand (2-3 q~) category, similar to that in the adjacent dunes (Lancaster, 1981); 5-15% silt and clay is present in many of these samples. Unlike the dune sands, which are very well to moderately sorted, the calcareous clastics are poorly sorted. At Narabeb, the only site where there are true mudstones, silt and clay

make up 45-68% of the clastic fraction, and sand varies from 1 to 26% of the msoluble fraction.

Paleontology Fossil remains are present at all sites in this study (Table 2), and Teller et al. (1988) have discussed these biological constituents in detail. The stratigraphic positions of these fossils are shown by symbols in the stratigraphic columns of each playa in Fig. 10. The calcified stems of the reed Phragmites sp. occur in seven of the eleven pans, mainly in the casehardened tufaceous sandy hmestones and calcareous sandstones that cap the sections. In some places, such as at Khommabes, they occur as loose calcified stem fragments on the surface (Fig. 14). Calcified root nodes

Lake Deposlts in the Nortl~ern l~hmib Sand Sea

349

FIG. 8. Part of polygonal fracture near West Pan B in which calcareous lacustnne sediment is preserved (photo J.D. Ward) Floor of fracture lies several meters below that of interdune floor and is largely covered by modern eolian sand. Width of fracture about 70 m.

of these reeds are occasionally found within the calcareous unit (Fig. 14). The calcified root casts of the Nara plant (Acanthosicyos horrida) have also been found at Khommabes and possibly at Narabeb (Vogel and Visser, 1981, pp. 73, 76; Ward, 1987). Infilled 'pedotubules', related either to ancient root systems or burrowing organisms such as termites, are present in all sections except Ancient Tracks, and typically occur in the top of the sandstone unit immediately below a highly calcareous bed (Fig. 15). Gastropods have been found at five sites (Table 2). Their stratigraphic position is indicated in Fig. 10. At West Pan B snails in a calcareous sandstone near the base of the section were identified by C. Appleton (University of Natal) as fresh to brackish water types,

Biomphalaria pfeifferi, Melanoides tubercula, Bulinus

FIG. 9. Straugraphic section at West Pan B, showing pit that exposes sandy limestone overlain by tufaceous crust of calcareous sandstone

sp. (Fig. 16), and Tomichia (cf. T. ventricosa). Bulinus sp. is also present in the surface sandy limestone at West Pan B. A tentative identification of Succinia sp. from the Gobabeb South site was made by Ward (1987). The freshwater snail Lymnaea natalensis and Biomphalaria pfeifferi were identified and dated at the Ancient Tracks site (Vogel and Visser, 1981, p. 73). In a silt below the bed containing the snails at Ancient Tracks, Sandelowsky et al. (1976) identified 'eleven large impressions', with oval shapes that are 60 to 80 cm long, 20 to 25 cm wide, and up to 10 cm deep, that she believed to be footprints of a large animal. Tracks of birds and animals of a smaller size were also observed on this buried surface. Mineralized bones and tooth fragments of the extinct elephant Elephas recki were discovered by Shackley

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(1980) at the Namlb IV site (Fig. 6), in the calcareous silty sandstone below the surface case-hardening (d of columnar section in Fig. 10). These were found together with bifacial artefacts, tooth fragments of a small horse (Equus sp.) and buffalo (Synceras sp.), and the mineralized bone fragments of an indeterminate aicephaline antelope (possibly red hartebeeste or black wildebeest) and an indeterminate bovid (Shackley, 1985). Additional, non-mineralized, remains were found 'in a white calcrete' by Shackley (1985) 250 m north of the above site, and were identified as a small horse (Equus sp.), rhinoceros (Rhmocerondae sp.), steenbok (Raphiceras campestris), blue wildebeest (Connochaete taurinus), and gemsbok (Oryx gazella) (Klein, 1984). Diatoms have been identified at five sites, West Pan B, Khommabes, Gobabeb South, Narabeb, and Namib IV. Only 15 of 61 samples investigated from the playas in this study contained any diatom frustules, and the abundance in eight of these was too low to allow a quantitative analysis (Teller et al., 1988). Campylodtscus clypeus or Synedra ulna are dominant in all samples except one at Narabeb, where Teller and

Lancaster (1986b) reported Tabellaria fenestrata to be the most abundant. All diatom species indicate fresh to brackish water conditions. In a preliminary study by P. DeDeckker (Australian National University), ostracods were identified in three calcareous sandy to silty units (West Pan B [2]: Narabeb [V]; Namib IV [c]), but were absent in four similar units (West Pan B [7]: Gobabeb South [2]; Bone Pan [1]: Namib IV [d]) (Fig. 10). Fresh and permanent waters are indicated by these ostracods (P. DeDeckker, 1989, pers. commun. )

Isotopic Measurements Both 6180 and 613C isotope raaos were determined on a number of carbonate samples from various horizons at nine sites (Table 3), including three gastropod specimens from West Pan A and B. The analyses were performed by the method outlined by McCrea (1950). The stratigraphic position of each 6180 and 6BC measurement is shown on the columnar sections of Fig. 10. Figure 11 is a plot of the 6180 vs. 6~3C ratios In addition, Table 5 presents the 613C values for the dated carbonates of this study.

351

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+

3 and 9 4

+ --r -* ---

. .

pDo

+

.

_ * -+ -_k

Calote Mg% c

3 3 -.

--

m

0.77

2 6

1 1

1 7

0 51

2 6

0.53

2 5

mineral x-ray and microscope anal

dBulk

.

50 71 32 29 22

0 1 0

mudstone

St d e v

O 6 40

Gobabeb South 3 s a n d y Is crust 2 calc s a n d s t o n e 1 sand

Narabeb Xl sand X calc I X sand fcalc VIII I.calc VII sand V I calc

Mean

25 11

22 38 0 3 (5) (10)

CClasttc stats.

0 56

2 5

0 73

3.2

0.82

3.1 2.1 2.2

13 13 13

+ +

+ +

+ + + + + + + + + + + + + +

+ + + + + + -+ + + + + + +

+

--

--

--

3 and A

--

+ + +

+ ---

~_ ---

_ + +

9 ---

__ + +

-+ -+ -. + -+ -+ ---4-

.

-4 3 4 14 . . 8 8 8 -9 10 -8

----+

+ +

------------

wont.)

353

Lake Deposits in the Northern Namib Sand Sea TABLE 1 "Soluble in

H20

HCI

Bone Pan 2 sdy dolomite 1 sand

6 1

49 3

Namib IV h sandy limestone g caic sandstone f sandy limestone e sandy limestone d talc silty ss c calc silty ss b silty sand a sand

0 1 0 1 0 0 3 1

51 11 65 90 29 29 2 1

Anoent 32 30 29 31

6 6 3 1

15 2 37 3

a b c d (or

Tracks calc siltstone sand calc silty ss sand

bInsol, resid,

cl+si

(cont.)

sd

Mean

St.dev.

91

2.5

0.83

79

2.8

0.81

11 13 15 6

60 58 80 92

2.9 2.9 3.0 2.8

0.92 1.05 1.0 0.62

79 0 14 4

0 92 46 92

2.1 3.1 2.0

0 73 1.5 16

45 5 49 9

dBulk mineral x-ray and mtcroscope anal.

cciastic Stats.

35 9

Qtz

Alb

Ksp

Hal

Calcite Mg %=

pDo

Doi

+

+

+

+

--

+

--

+ + +

+ + +

-+ +

----

+

+

+

--

+ + + +

+ + + +

+ + + +

--. +

3 and A 3 and A 3 and A 3 A 3 and A . . --

+ + + +

+ + + +

-+ -.

-. -.

.

. 7 .

--

------

--

--

--

+

.

2 .

--

--+ ---

. -.

= Percent of sample soluble m water and 5 : 1 dilute hydrochlonc acad. = Percent of sample not soluble; cl+si ffi clay and silt; sd = sand, values between columns are clay + silt + sand. = ~ mean and ~b standard deviation of insoluble residue calculated by method of moments (of. Folk, 1980). = Qtz = quartz; Alb = albite; Ksp = potassium feldspar; Hal -- halite; p D o = protodoiomlte; Dol = dolomite; + = present; - - = absent not observed). = Numbers represent mole % MgCO3 in calcite (after GoldsmRh and Graf, 1958, Fig 4); A = aragonite.

T A B L E 2. Organisms and archeological remains identified from pans in thts study (after Teller et al., 1988) Site Bone Pan G o b a b e b South Khommabes Namib I V Narabeb Obab South Salty Pan Sobeb South West Pan A West Pan B Ancient Tracks

Diatoms

Gastropods

X X X X"

Xb

Phragmues X

X

X X X X

X X X X X X X X

Artefacts ×

Xc Xe X~

X X

"Teller and Lancaster (1986b). bWard (1984, p. 268). cWard (1987, p. 37); Shackley (1985). dShackley (1980, 1985). eSeely and Sandeiowsky (1974); Shackley (1985).

The 6180 ratios are high for all samples, which suggests a high rate of evaporation in a dry, hot climate (Zimmerman et al., 1967; Merlivat and Jouzel, 1979). There appears to be no relationship to age and, although there is less variation in carbonate 6~So at each site than between sites, no geographic trend is identifiable. There appears to be a positive correlation between the two isotopic measurements, with higher 6~So correlated with higher 613C values (Fig. 11). This is interpreted to mean that there is less biological activity as evaporation increases (i.e. 613C values are inversely proportional to biological activity).

Amino Acid Analysis Amino acid analyses were carried out on three different species of gastropods at four sites in order to determine relative ages of the specimens. Alloisoleucine/isoleucine ratios were determined by ion exchange chromatography as outlined in Hare (1975). Results clearly show three distinct age groupings (Table 4). There is good amino acid ratio separation between Ancient Tracks Site and West Pan A, where their radiocarbon ages are 13-14 ka BP and 27.5 ka BP, respectively (Table 5). The low ratio from Biota-

J.T. Teller et al

354

TABLE 3 6~sO (PDB) and 6~3C values for carbonate sediments and gastropods. Strattgraphic poSltion at each site shown on columnar sections of Fig 10

TABLE 5 Radiocarbon dates and 6~3C values from pans m th,s study All dates on carbonate Slte/14C date/lab no

Location

Sample number

6~so

613C

West Pan A

4 (Gastropod)

+5 2

-6 7

West Pan B

6 5 4 3 2 2 (Gastropod) 2 (Gastropod) 7 (Gastropod)

+8 +8 +9 +8 +9 +0 +2 +1

5 9 7 5 3 3 6 4

-1.6 -2 2 -2 4 -2 5 -2 1 -8.2 -7 9 -3.6

Sobeb South

4

+2.8

-1 8

Gobabeb South

3 A

+3 8 -1.9

-0.8 -4 1

Obab South

3

+11.2 +10 8

+0.3 -0 1

Salty Pan

5b

+6 5

-0 6

Bone Pan

2 Ab ,~a

+8 3 +10.3 +102 +8 7

+2 4 +1 4 +12 +1 4

h g f e d c

+3 +7 +6 +2 +4 +5

7 6 3 7 4 7

+0 1 +2 2 +1 0 +0 6 +0 7 +0.8

32 30 29 31

-3 -3 +1 -1

4 6 6 2

-3 -3 -4 -0

1

Namtb IV

Ancient Tracks

West Pan A (new) 27,500 + 1000 (BGS-1128)

613C (o/oo)

+0.4

Material dated

limestone

Gobabeb South (Vogel and Vlsser, 1981, p 76) 21,300 + 260 (Pta-2651) +1.3 reed cast 21,500 + 260 (Pta-2652) + 1.3 reed cast Khommabes (Vogel and Vlsser, 1981, pp 76-77) 20,900 + 230 (Pta-1091) -5.2 root cast 21,500 + 190 (Pta-2604) -3.6 termite nest 22,400 + 210 (Pta-2584) -2.8 worm channel 27,400 + 310 (Pta-2590) -4.5 reed cast 28,500 + 370 (Pta-2591) -4.4 reed cast 31,600 + 430 (Pta-2589) -8.6 reed cast 31,900 + 460 (Pta-2588) -8 5 reed cast Narabeb (Teller and Lancaster, 1986b; Vogel and Vlsser, 1981, p 73) 20,320 + 300 (Beta-9115) +6.6 mudstone 22,330 + 600 (Beta-9116) +6 4 mudstone 22,500 + 280 (Pta-3704) +0 7 mudstone 26,400 +_ 340 (Pta-3759) -0 5 slit 28,500 + 500 (Pta-1197) -3 2 root cast 39,800 +_ 1700 (Pta-3770) - 11 mudstone Salty Pan (new) 11,130 + 125 (BGS-1129)

-4 6

calcite crust

Anoent Tracks (Vogel and Vtsser, 1981, p 73) 13,300 + 90 (Pta-1043) -7 1 snail shells 14,300 _+ 130 (Pta-1502) -2 8 silt

3 4 2 6

Namlb IV (new) 17,500 + 170 (Pta-4704)

+13

silty sandstone

Bone Pan (new) 36,600 _+ 1100 (Pta-4701)

+1 4

sandy dolomite

phalana sp. f r o m the surface o f T s o n d a b Vlel, 10 k m

Radiocarbon Dates

east o f A n c i e n t T r a c k s , ~s r e a s o n a b l e b e c a u s e r a d i o c a r b o n d at es f r o m this a r e a (e.g. V o g e l a n d Visser, 1981) indicate that d e p o s i t i o n t h e r e is y o u n g e r and, in fact, still g o i n g o n t o d a y . D a t a f r o m W e s t P a n B is n o t e n t i r e l y consistent. If the r a d i o c a r b o n d a t e s w e r e not a v a i l a b l e , we w o u l d h a v e b e e n t e m p t e d to i n t e r p r e t th e fossils as o l d e r b e c a u s e o f the high ratios o b t a i n e d . F o r ratios to be this high, historic t e m p e r a t u r e v a l u e s must h a v e b e e n high. T h i s is c o m p a t i b l e , o f c o u r s e , with o t h e r d a t a o b t a i n e d f r o m the r e g i o n .

T h e c a r b o n a t e o f 21 s a m p l e s f r o m ei g h t o f t h e pl a ya s e q u e n c e s has b e e n r a d i o c a r b o n d a t e d . T h e d a t e s r a n g e f r o m 11,130 + 125 B P ( B G S - 1 1 2 9 ) at Salty Pan to 39,800 _+ 1700 B P (Pta-3770) at the base o f t h e section at N a r a b e b ( T a b l e 5). 13C was u s e d to c o r r e c t all da t e s for i s o t o p i c f r a c t l o n i z a t i o n . N e a r l y all r a d i o c a r b o n d at es f r o m c a l c a r e o u s d e p o s i t s e l s e w h e r e in the n o r t h e r n part o f t h e N a m i b S a n d Sea span the s a m e t i m e p e r i o d , 10-40 ka B P , an d can be g r o u p e d into t h r e e age c a t e g o r i e s , 10-15 ka B P , 20-23 ka BP , a nd 2 7 - 3 6 ka B P (Fig. 12). T h e r e a p p e a r s to be a similar

TABLE 4 AllOlsoleucmehsoleucme amino aod ratios determined from three genera of gastropods Stratlgraphlc positions mdtcated on columnar sections of Fig. 10, except for Tsondab Vlel sample Location 10 km east of Ancient Tracks Site on land surface of modern Tsondab Vie, A n o e n t Tracks A n o e n t Tracks West Pan B West Pan B West Pan B West Pan A West Pan A

Sample number

-28 28 2 2 7 4 4

Fossd name

Blomphalana sp Bmmphalana sp Buhnus sp Bmmphalarta sp Buhnus sp Melanoides sp Buhnus sp Buhnus sp

Alle/lle

0 138 0.479 0 443, 0.493 0 755 0 535 0 747 0 860 0 919

355

Lake Deposits in the Northern Namib Sand Sea 45

• •

40

3"5

(:3

Bone Pan Salty Pan • 0bah South [] Oobabeb S • Anckmt Tracks • ~/est Pan A x West Pan B + SobebSouth

•

n

[]

tO

•

0

[]

•ml

[] 30

Naml~ N A Narn~b N C

4.

rt At

n 25 - 10

,

,

,

,

,

,

-8

-6

-4

-2

0

2

Gastropod

4

61~C (%,) FIG. 11 Plot of b~so

ratios determined from carbonate sediment and gastropods. Stratigraphic position at each site shown on columnar section of Fig. 10

vs. 613C

0'" ZO 10,000

20,000

30,000

RADIOCARBON

YEARS

40,000

B.P.

FIG. 12. Dmributlon of radiocarbon dates from sediment and shells in the northern part of the Namib Sand Sea. Letters m box refer to sites m tb_ts study (A = Ancient Tracks; B = Bone Pan; E = Narnib IV; G = Gobabeb South; K = Khommabes; N = Narabeb; S = Salty Pan; W = West Pan A); black boxes are other dated sites m the northern Sand Sea. All dates are on carbonate. (From Vogel and Visser, 1981; Vogel, 1989; Teller and Lancaster, 1986a; and unpublished new dates.)

14C age distribution for calcareous deposits throughout the Namib Desert (cf. Vogel and Visser, 1981; Vogel, 1989).

CALCITE

o

DISCUSSION

Sedimentology / ,,E,,~ v.,

o,, o

\~

\

\'~ o\

\%*o\

~ ~--~I o SANDY \~"4~. SILT/MUD/CLAY

/ . ~k~/SILTY/MUDDY/CLAYEY SAND SAND

10

50

SILT ÷ CLAY

FIG. 13 Tnangular diagram of calcite, sand, and silt + clay, showing names used in this paper. Specific names for silt + clay regoon are based on relative percentage o f these two components as defined in Blatt et ai. (1980), where >2/3 silt = silt(stone), 1/3--2/3 silt = mud(stone), and <1/3 silt = day(stone). Dolomite, halite, etc. are substituted for limestone where they are the dominant chemical precipitate.

Ponding of water rarely occurs in the hyperarid Namib Sand Sea today, and there is no evidence for recent precipitation of carbonates nor the influx of siltor clay-sized material into the depressions between the large linear dunes in the region. If these sediments represent ponded water events, hydrological conditions during the Late Pleistocene must have differed significantly, at least periodically, from those in the region today. Those beds in which carbonate dominates probably were deposited as a primary precipitate within a standing body of water. The elastic component in these limestones may have been washed or blown into the accumulating sequence. The characteristics of the sandsized quartz in the calcareous mudstones at Narabeb (Teller and Lancaster, 1986b, 1988a), and in most beds examined in this study, are similar to the quartz grains found in adjacent dunes and interbedded sandstones.

356

J T. Teller

et al

FIG 14 Calcified stem and root attachments (carcles) of the reed

Phragmltes

m the calcareous crust at Khommabes

FIG 15 Cemented "pedotubules' or termite tubes that have weathered out of matrix at Khommabes (Teller and Lancaster, 1986a) The consistent mean grain size of 2-3 tp (fine sand) for the clasUc portion in the mterdune deposits suggests that these sands were derived from the dunes of the Sand Sea, which have a similar mean size. However, the comparatively poorly sorted nature of the sand fraction is in contrast with the better sorting of both dune and interdune eohan sediments (Lancaster, 1981) and even with the modern fluvial sands in the Kmseb River valley (Ward and von Brunn, 1985). The presence of large proportions of silt- and clay-sized siliciclastic grains at many sites (Narabeb, West Pan A and B, Khommabes, and Ancient Tracks) and even

amounts of > 1 0 % of this fine grain size in other playa deposits (Salty Pan, Namib IV) suggests either a source outside of the Sand Sea and/or derivation from the scattered siltstones within the underlying Tsondab Sandstone Formation. Silty deposits are present in modern playas at the end points of the Tsondab, Tsauchab, and Koichab rivers along the eastern side of the Sand Sea (see Fig. 4), and in terraces along the Kuiseb valley (e.g. at H o m e b and Rooibank). The presence of mica in many ~:alcareous sediments, which is largely absent in the dunes of the region (Lancaster and Oilier, 1983), further suggests that some of the

Lake Deposits in the N'6ttiteri~ Narttib Sand Sea

357

H G . 16. Gastropods from silty calcareous sandstone at West Pan B (#2 m columnar secuon of Fig. 10, near base of section in Fig. 9). Left to nght: Melanoides sp., Melanotdes sp., Buhnus sp., Bmmphalana sp.

clastics were transported by water from outside of the Sand Sea. In contrast, the interbedded noncalcareous sands at Narabeb contain no micas, which led Teller and Lancaster (1986b) to conclude that they were derived from the adjacent dunes. Calcareous sandstones - - those beds in which clastic grains form the framework of the rock and carbonate only fills the interstices - - may also have been deposited in a standing body of water, when the influx of eolian or fluvial elastics exceeded the rate of carbonate precipitation. Alternatively the carbonate may have been precipitated at the capillary fringe of the water table within sands already deposited on the interdune floor. This groundwater process is most likely to have occurred at the lowest points in the Sand Sea - - in the interdune depressions that may also have allowed water to be ponded. A third possibility is that the calcareous sandstones formed by the downward translocation of surface or near-surface carbonates into underlying permeable sands. This process, like the previous one, is analogous to the formation of a calcrete and produces a carbonate that is younger than the sand in which it has precipitated, albeit not necessarily much younger. Calcretization may continue over a long period of time, and can result in a massive bed of calcareous sandstone or even a massive calcareous bed containing 'floating' elastic grains (e.g. Goudie, 1983). Interdune closed depressions are likely sites for this to occur, and carbonates that were previously precipitated from standing water could provide a source for the remobilized ions needed to cement the playa elastics. The presence of lamination and bedding in most units in the calcareous sequence argues for a primary origin from evaporating lake water for most units.

Furthermore, the carbonate content typically changes abruptly at bed contacts, something that rarely will occur during the formation of a secondary cement by downward translocation or by evaporation at the capillary groundwater fringe. Indeed there is evidence for the remobilization of calcite at the top of most sequences, where a case-hardened limestone crust with a vesicular surface has formed, but similar units are only present below the surface at Khommabes (20 and Sobeb South (2). The carbonate minerals in both the siliciclastics and in the limestones vary. Although low magnesian calcites with <3 mole percent MgCO3 are dominant, protodolomite is also common, and some samples contain high magnesian calcite, aragonite, and dolomite (Table 1). There appears to be no special geographic, stratigraphic, or sediment-related association with any of these minerals, except for the presence of aragonite at sites most likely to have received waters from groundwater discharge (see later sections). Some single samples contain two or more different carbonate minerals.

Geochemica7 Considerations The geochemistry of carbonate precipitation and diagenesis is complex (e.g. Scoffin, 1987), and we have not established with certainty the formative chemical environment(s) of the carbonate mineral complex in the Namib Desert sediments. In a few instances, recrystallization is known to have occurred, forming the case-hardened crusts that commonly contain calcified Phragmites stems. We feel this must represent a wet phase during which ions were remobilized downward in the sediment by ponded water and/or upward in the sediment by groundwater discharge. The

358

J T

Teller et a/

presence of halite m mud-cracks at Narabeb and in other sediments may also reflect the translocation or introduction of salts after the sediment was deposited. Except for these, no other secondary precipitates (e.g. calcretes) have been identified in the interdune sediments of this study. At Khommabes, a calcareous dolocrete has formed in the granite that undedies the calcite and dolomite-rich sands (sample la, Table 1) (Teller and Lancaster, 1986a). Because protodolomite is metastable (e.g. Morrow, 1982), and probably cannot survive for more than a few tens of thousands of years without converting to stoichiometric dolomite, it is unlikely that protodolomite in the Namib playas (Table 1) has been derived as clast~c material from older dolomitic rocks of the region. The only two sites where true dolomite occurs, Narabeb and Ancient Tracks, appear to lie along a former extension of the Tsondab River (see Fig. 3), which once carried water from the Naukluft Mountains where Precambrian dolomite outcrops. Moiler et al. (1972), in a classic study of fresh to hypersaline lakes, concluded that at Mg/Ca ratios of <2, low magnesian calcite is precipitated; at ratios of 2-12, high magnesian calcite is precipitated; and at ratios of > 12, aragonite is precipitated. Dolomite may also precipitate at high Mg/Ca ratios (e.g. Muller et al., 1972; Folk and Land, 1975). Thus a high Mg/Ca ratio of the precipitating or replacing brine seems to be required at West Pan A, Khommabes, Narabeb, Salty Pan, Bone Pan, and Namib IV, where most beds contain aragonite, protodolomite, and/or calcite with a high content of MgCO3 (Table 1). Modern groundwater in sands of the Kmseb valley downstream from Gobabeb is low m sahnity (typically <600 mg/l TDS), with ionic concentrations of all major ~ons low, normally falling in the following ranges: Na (40-100 mg/l), K (10-30 rag/l), Ca (20-150 mg/l), Mg (20-150 mg/l), CO3 (100-240 mg/l), SO4 (20-80 mg/l), NO3 (0-3 mg/l), CI (40-150 mg/l), and SiO2 (3-20 mg/l) (Department of Water Affairs, 1981). The pH of these waters ranges from 7.1 to 8.6. Although the ionic values for groundwater within the Namib Sand Sea are unknown, their relative proportions probably are similar to those along the Kuiseb valley, because the origin for both lies upslope to the east in similar bedrock. For the same reason, ancient groundwater or surface runoff from the east into the interdune depressions of the Sand Sea probably would also have been of similar composition The isotope geochemistry of the carbonates (Tables 3 and 5) may indicate that some recrystallization has occurred. Specifically, the relatively high ('enriched') 8~3C values at many sites are not typical of most lacustrine carbonates, which are normally negative. Many (e.g. Nissenbaum et al., 1972; Irwin et al., 1977) have attributed high 8~3C values in organic-bearing sediments to fermentation of organic matter (methanogenesis). Turner and Fritz (1983) suggested that metabolism of methanogenic bacteria may have resulted in the progressive enrichment in 8~3C with depth in a

freshwater marl in Ontano, as pore waters highly enriched in 13C (up to +13 o/oo) reacted with the originally-precipitated carbonate. Thus, the positive 813C values in sediments of the Namib playas may indicate recrystallization of the carbonates, although not necessarily a long time after deposition. Teller and Lancaster (1986b) attributed the high 13C values in several calcareous mudstones at Narabeb to methanogenesis. Cerling (pers. c o m m u n . , 1985), in conjunction with a discussion of the reliability of using these carbonates for ~4C dating, suggested that 8~3C values as high as 6-7 at Narabeb were unlikely if recrystallizing had been accompanied by a re-equilibration with atmospheric carbon. Several alternatives to recrystallization exist to explain the enriched 8 x3C values of some carbonates in the northern Namib Sand Sea. For example, evaporating brines in the Dead Sea were shown by Stiller et al. (1985) to have a very high enrichment of ~)13C (up to 16.5 o/oo under natural conditions). They interpret this as being related to the loss of CO2 from water evaporation. According to Talma and Netterberg (1983), precipitating carbonates will show an increase in 813C by 4--6 o/oo when 80% of the calcium in solution is precipitated. Anderson and Arthur (1983) note that more positive 8~3C values at a given salinity may result by extensive photosynthetic activity that preferentially removes 12C. A second alternative relates to the origin of the carbon (and ~3C) in the water precipitating the carbonate. In most lakes, a major portion of the carbon in surface waters is derived from biological processes, and is generally deficient in 13C (Stuiver, 1975). Where biological activity was minimal, as it may have been in the lnterdune playas, 813C values probably would have approached isotopic equilibrium with atmospheric CO2, which would yield carbonate with a 813C value of about +4 o/oo (Emrich et al., 1970). Thus, the positive 613C values in lacustrine carbonates may be partly or entirely related to: (1) early or late post-depositional recrystallization related to methanogenesis; (2) precipitation from an evaporating brine, possibly supplemented by extensive photosynthetic withdrawal of 12C; or (3) waters where biological activity was very low and the ~3C approached equilibrium with atmospheric CO2. Negative 813C values are probably related to environments where biological activity was normal during carbonate precipitation, and little post-depositional alteration has occurred. It seems likely that most calcareous beds that contain more than one carbonate type represent multi-stage crystallization from a single brine and/or diagenetic evolution from an original carbonate mineral. In the latter case, the various mineral types may be nearly contemporaneous, or they may represent a series or progression of events spanning thousands of years, where one carbonate is partially converted to another in response to changing groundwater or surface water chemistry. Similarly, beds with a single carbonate mineral may have undergone complete conversion

Lake Deposits in the Northern Namib Sand Sea from one type to another. Waters with a high Mg/Ca ratio may have resulted in the formation of, or conversion to, dolomite, protodolomite, high magnesian calcite, or aragonite. Conversely, when the Mg/Ca ratio falls below about 2, low magnesian calcite is likely to form. In a playa environment in a region where calcium, magnesium, and carbonate are available in the bedrock and groundwater, it is not difficult to develop a variety of chemical environments during the Late Quaternary evolution of an interdune depression.

Age of Sediments Estimates of the age of the water laid sediments between dunes in the northern Namib Sand Sea is based on radiocarbon dating of carbonate. The potential for age inaccuracy due to the 'old' carbon effect is well known, but, in our view, poses less of a concern than does the potential for post-depositional recrystallization of the carbonate; this has been previously discussed in conjunction with methanogenesis and 613C values of the carbonates. However, we believe the radiocarbon dates accurately reflect the age of sediment deposition in the playas of this study. RecrystaUization and resetting of the radiocarbon clock seems unlikely because: (1) few dates have anomalously high 613C values (Table 5); (2) there are reasonable alternatives to explain high 613C values; (3) microscopic examination of the carbonates does not indicate that the carbonates have been recrystallized; and (4) there is a close association of carbonate content with specific stratigraphic units, not the more diffuse relationship that commonly occurs when ions are mobilized and reprecipitated at a later date. There are several problems, however, if the radiocarbon dates are accepted as the true age of deposition of the thin playa sequences. For example, there are two 234U/23°Th dates from the basal unit (I) at Narabeb, which gave an age of 210 ka and 260 ka BP (Selby et al., 1979). These dates, however, are from calcareous mudstones, and Teller and Lancaster (1986b) reject them, partly because of the high potential for contamination by detrital clays which carry with them uranium/ thorium ratios that reflect the age of the bedrock from which they were derived. Another dating problem relates to the age of the bones and teeth of Elephas recki found at Namib IV. Shackley (1980) and others consider Elephas recki to have become extinct within the mid-Pleistocene; Cooke (1984, Fig. 2) shows its extinction at about 250 ka BP. Although the precise age of extinction is not known, Klein (1988, pers. commun.) says that it almost certainly occurred sometime between the Brunhes/ Matuyama boundary (about 700 ka BP) and the beginning of global Isotope Stage 5 (about 120 ka BP); this elephant is not known from any sediments above Isotope Stage 5. At the Namib IV site the mineralized remains of this elephant were collected from the surface unit (see Fig. 10), and were 'lightly embedded in deflation surfaces of lacustrine carbonate' (Shackley, 1985, p. 44). Shackley (1980, p. 340) notes that

359

some associated bones bear 'residual traces of red calcrete'. It seems possible that these faunal remains were derived from beds of the Tertiary age Tsondab Fm, which are red in color and are exposed within a few hundred meters of this site at higher elevations. A wide variety of stone implements, including handaxes, cleavers, choppers, flakes, and cores, commonly in considerable abundance, have been found in close proximity to six of the eleven playa sites of this study (Table 2). In all cases these artefacts were found lying on the surface of the interdune floor or adjacent dune plinth. At Namib IV some contained residual patches of red carbonate on their surfaces (Shackley, 1985). At Namib IV the artefacts are interpreted to be related to an Early Stone Age (ESA) industry 700 ka to 400 ka BP (Shackley, 1980, 1985). At Narabeb the age of the assemblage is dominantly ESA (Seely and Sandelowsky, 1974), although Shackley (1985) concludes there are also younger materials present. A wide range of artefact types have been found at Khommabes, including ESA, Middle Stone Age (MSA), and Late Stone Age (LSA) types (Ward, 1987). Scattered artefacts at West Pan A and B are interpreted to be MSA (J. Lancaster, 1988, pers. commun.), which probably spans the period 300 ka to 25 ka BP (Shackley, 1985, p. 79). Elsewhere in the central Namib, artefacts range from ESA to LSA, with a mixture at many sites (Shackley, 1985). This fact indicates that the same sites were occupied on more than one occasion during the Quaternary, and casts serious doubt on the use of any specific assemblage of stone implements to date the age of the calcareous sediments at a site.

PALEOHYDROLOGY

Introduction Calcareous water-laid sediments in the interdune corridors of the northern Namib Sand Sea have been dated at less than 40,000 radiocarbon years, and their orion has been related to various hydrological factors (Seely and Sandelowsky, 1974; Teller and Lancaster, 1986a, b, c; Ward et al., 1987; Ward, 1987; Teller et al., 1988). Today there is no intrusion of water into these interdune sites from outside of the Sand Sea, and no seepage of groundwater into these depressions is known to have occurred in recent times. Rainfall gradually increases inland, from <25 mm year -l near the coast to as much as 100 mm along the eastern side of the Sand Sea and 200 mm in the headwaters of the rivers that drain toward the west (J. Lancaster et al., 1984). Runoff in the upper to middle reaches of the major valleys of the central Namib Desert occurs in most years in response to seasonal rains to the east, but reaches the terminus of these valleys less frequently. The last times flow in the Kuiseb valley reached the Atlantic Ocean were in 1934 and 1965 (Huntley, 1985), although groundwater is always present in the alluvium, and there was flow as far down-valley as

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Gobabeb in 18 of 22 years between 1962 and 1984 (Ward and von Brunn, 1985). We believe that there are several possible hydrological scenarios that can account for the formation of calcareous water-laid sediments in the northern Namib Sand Sea. These are based on the assumptions that modern precipitation is not able to produce runoff or ponding in the interdune corridors where the calcareous beds are found and that the headwaters of valleys draining toward the Sand Sea are, and always have been, in areas of higher rainfall. Thus, water reaching the topographically isolated sites discussed in this paper may have resulted from: (1) extensions of rivers such as the Tsondab beyond their present end points, (2) lateral intrusions of floodwaters into interdune corridors from rivers that border or enter the dunes, (3) groundwater seepage, either through permeable aquifers such as the Tsondab Sandstone or through modern dunes of the Sand Sea, and (4) increased rainfall, either within or to the east of the Sand Sea.

Former End Points of Rtvers Playas occur today at the end points of valleys such as the Tsondab and Tsauchab (Fig. 4) that drain into the Sand Sea from the higher and wetter regions to the east (Fig. 1). In years of high rainfall and runoff, water reaches its terminal playa, where it remains for weeks or months (Stengel, 1970; Ward, 1988). These playas contain dominantly calcareous silts and muds, 4-12 m thick (Lancaster, 1984; van Zinderen Bakker, 1984), and commonly support an ephemeral population of gastropods, ostracods, copepods, cladocera, and algae (Grobbelaar, 1976) that survive until moisture conditions become unfavorable. Seely and Sandelowsky (1974) proposed that the Tsondab River once extended westward as far as Narabeb, and Besler and Marker (1979), Marker (1979), Besler (1980), and Lancaster (1984) present additional data to support this former extension. Figure 3 shows the fluvial silts, sands, and gravels along the now-abandoned route west of Tsondab Vlel, and the disturbed dunes in this area can be clearly seen on Landsat photos (Fig. 2). Seely and Sandelowsky (1974) and Wienecke and Rust (1972) suggest that the Tsondab River may have at one time flowed all the way to the Atlantic Ocean. Teller and Lancaster (1986b; 1987) describe the deposits at Narabeb in detail, and conclude on the basis of grain size and grain properties, mica content, sedimentary structures, and stratigraphy that the calcareous sandy mudstones were deposited at the former end point of the Tsondab River before linear dunes blocked the water course. Although the known sequence at Ancient Tracks is much thinner than at Narabeb, sediment characteristics are similar, and the site lies along the former Tsondab route (Fig. 3). Micaceous calcareous silts dominate. Both contain the only stoichiometric dolomite, which

we interpret as being clastic and derived from the Naukluft Mountains, and both contain the highest percent MgCO3 in calcite of all samples (Table 1). Northwest of Narabeb, along the probable former route of the ancient Tsondab River as indicated by fluvial sediments (Fig. 3) and disturbed dunes (Fig. 2), lies West Pan. Although there is some similarity of the sediment at West Pan A and B to that at Narabeb and Ancient Tracks, such as presence of micaceous silts, it is mainly its geographic location that suggests that these deposits were also left by ponded water at the former end point of the Tsondab River. Preservation of these isolated calcareous deposits is probably related to deposition within large macrofractures in the Tsondab Sandstone Formation (Fig. 8).

Flooding into Interdune Corridors Flooding along rivers such as the Kuiseb and Tsondab may pond water laterally into depressions between linear dunes. Such events may also be responsible for depositing fluvial sediment in the main valley, such as the 19,000-23,000 year-old Homeb Silts near Homeb (Fig. 3) (Ward, 1987). Such floods may carry coarse sediment from the main valley into the dune corridor, or may form a cul-de-sac lake where only fine sediment is deposited along with minerals that precipitate as waters trapped in local depressions evaporate. Teller and Lancaster (1986a) suggested that Khommabes, which lies at about the same elevation as the Kuiseb valley, and is today separated from this valley by only a low transverse dune (Fig. 7), experienced an initial phase of micaceous sandy alluvial sedimentation, followed by a period of calcrete formation, and then by eolian deposition. Subsequently, water was reponded at the site and a Phragmites-rich calcareous sand was deposited. This later stage may represent lateral flooding from the Kuiseb valley or, if the dune barrier that now separates the site from the valley had already been deposited (as the underlying eolian unit suggests), seepage through that dune barrier during high water stages. Thus, several wet periods are represented at Khommabes: fluvial flood sedimentation, calcaretization, and lacustrine Phragmites-rich sedimentation. Calcareous deposits in nearby mterdune corridors at Sobeb South, Gobabeb South, and Obab South (Fig. 3) are similar to the upper sediments at Khommabes, consisting of Phragmites-bearing calcareous sandstone to sandy limestone that is case hardened and dominated by low magnesian calcite (Table 1); mica is rare to absent. Although lateral flooding from the Kuiseb valley may have caused ponding at these three sites, their elevations lie 30-70 m above the Kuiseb. Alternatively, flooding from the formerly more extensive Tsondab valley to the south, which lies at or above the elevations of these three playas (Fig. 3) may have provided the waters. Lancaster (1984) notes that the Tsondab system probably has behaved much like an alluvial fan, with waters encroaching into low areas in

Lake Deposits in the Northern Namib Sand Sea distal regions outside of the main valley during highest rainfall and runoff.

Groundwater Seepage Groundwater seeps occur today along the walls of the Kuiseb valley and tufas have been precipitated in some localities (Ward, 1987). A number of perennial groundwater discharge sites are known in the Namib Desert, but none have been discovered within the Sand Sea. Along the Skeleton Coast, north of Walvis Bay (Fig. 1), oases have formed by river seepage through many kilometers of dunes. During floods there is seepage into small interdune depressions just south of the Kuiseb, downstream from Gobabeb, at Bubuses and Ostrich Oasis. Algae, Phragmites, and other plant types frequently colonize these sites. Calcareous deposits are not present at any of these sites, although carbonate is present in the watershed in the Tsondab Sandstone, Karpfenkliff Conglomerate, Oswater Conglomerate, Precambrian bedrock, and calcretes of the Kamberg Formation (Ward, 1987), as well as in the groundwater. At most playa sites the permeable Tsondab Sandstone Formation underlies the calcareous sediments, or crops out nearby (see Fig. 10). At Khommabes and Gobabeh South, granite underlies the sediment. Although the continuity of the Tsondab Sandstone is not known, it is widespread and extends eastward beyond the edge of the Sand Sea, into a region of higher rainfall (Besler and Marker, 1979; Ward, 1987). It is possible that all lacustrine sediments of this study are related to groundwater discharge, either from bedrock like the Tsondab Sandstone or from modern dunes. Even those sites where a fluvial origin for the water seems likely, groundwater may have contributed to the lakes by groundwater discharge. Seepage through the dune at Khommabes, for example, was previously mentioned as a possible source for waters that deposited the upper calcareous unit. The location of lacustrine sediments within macrofractures in the Tsondab Sandstone at West Pan and Obab South enhance the possibility that groundwaters were being contributed. Seepage from the eastward-receding end point of the Tsondab River may have helped sustain lakes at Narabeb and Ancient Tracks, as well as West Pan. The sites most likely to have received all their waters from seepage are Salty Pan, Bone Pan, and Namib IV. These are presently surrounded by high dunes and lie >100 m above the Kuiseb valley. The floor of Tsondab valley, at its terminal point 20 km to the south, however, lies 60-90 m above these playas. It seems likely that groundwater recharge to the Tsondab Sandstone, which underlies Tsondab Vlei, has flowed (and even today may flow) north toward the Kuiseb valley, discharging into interdune depressions whenever recharge was great. A similar situation may have existed at Gobabeb South, Sobeb South, and Obab South, which lie 30-70 m above the Kuiseb valley, but are at or below the elevation of the former Tsondab

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River valley (see Fig. 3). Again the widespread Tsondab Sandstone, or the dunes themselves, would have acted as the aquifer. With only one exception, Namib IV and Salty Pan are the only sites where aragonite occurs, typically along with low magnesian calcite (Table 1). Bone Pan is one of the few sites where protodolomite is found. As discussed previously, both aragonite and protodoiomite demand a high Mg/Ca ratio in the precipitating waters. Modern groundwaters have the potential to supply the necessary Mg. However, if the "presence of units with carbonates requiring a high Mg/Ca ratio for precipitation are used as an indication that groundwater discharge occurred at that site, other playas must be added to the list: West Pan A (protodolomite), Narabeb (high Mg calcite, some protodolomite), several beds at Khommabes (protodolomite), and scattered beds at other sites (Table 1). We do not believe that a conclusion that such minerals demand a groundwater source is warranted at this time, but we acknowledge that groundwater contributions at these sites are reasonable. Finally, as previously discussed, the present carbonate mineralogy may be either the result of primary precipitation from a standing brine or of diagenesis from groundwater solutions.

Increased Rainfall and the Late Quaternary Climate A large increase in rainfall across the northern Namib Sand Sea, or in the headwaters of rivers draining toward the Sand Sea, would cause additional runoff and ponding in valleys such as the Kuiseb and Tsondab, and might also produce local runoff or groundwater discharge in interdune corridors. Thus, increased precipitation would increase the chances for an influx of water to the playa sites by all of the three previously described mechanisms, and, by itself, might generate enough runoff to cause ponding at some sites. Although there is no specific evidence for overland flow into these depressions, an increase in precipitation within the hydrological watershed seems necessary in order to produce the calcareous sediments described in this study. In fact, radiocarbon dates on other precipitated carbonates in the Namih Desert are clustered into the same age groups as those in the northern Namib Sand Sea playas (Fig. 12), suggesting that increased precipitation did occur. Evidence from other parts of the arid zone of southern Africa summarized in Deacon and Lancaster (1988) indicates that the region experienced periods of significantly increased moisture availability in the period 32-20 ka BP and again between 17 and 11 ka BP. The latter period seems to have been less wet than that prior to 20 ka BP, and was apparently confined to areas in the western Kalahari. The period 40-20 ka BP appears to have been a time of generally increased rainfall throughout the summer rainfall zone of southern Africa, and it seems likely that such rainfall increases also affected the Namih Desert. In contrast, Holocene climatic fluctuations in the Kalahari and adjoining regions appear to have been of a lower order,

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and remained within the range of extremes known from the meteorological record (Deacon and Lancaster, 1988). Fossils such as diatoms, gastropods, reed casts, and vertebrate remains all indicate a sustained period of fresh to brackish water availability at all playa sites (Teller et al., 1988). Water levels and salinity appear to have fluctuated dunng deposition of the calcareous units. Values of 61So indicate a high rate of evaporation in a hot and dry climate, and enrichment in 13C may suggest low productivity in these lakes due to unfavorable conditions. It is important, however, to remember that playas such as these, like modern ones in the region, are 'oases', which deposit a record similar to that in areas with much higher precipitation. Thus, the hyperarid conditions in the Namib Sand Sea may have remained unchanged during the Quaternary, with periodic ponding in interdune depressions being produced by relatively small and episodic increases in precipitation in the headwaters of the Kuiseb and Tsondab basins that were dispersed by surface runoff and groundwater recharge.

QUATERNARYHISTORY Aridity along the southwestern coast of Africa appears to date from the Early Tertiary, and is related to establishment of the cold northward-flowing Benguela Current following the breakup of West Gondwana (Siesser, 1978, 1980; van Zinderen Bakker, 1975). The overall nature of the sedimentary record in the Namib Desert has been reviewed by Ward et al. (1983). The modern Sand Sea probably originated m the Pliocene (Ward et al., 1983), rather than the Pleistocene as some have suggested (e.g. Martin, 1973; Tankard and Rogers, 1978); older Tertiary dune sandstones (Tsondab Sandstone Formation) and scattered lacustrine beds (Zebra Pan Carbonate Member) underlie the modern eolian dunes and lake sediments (Besler and Marker, 1979; Ward, 1987; Lancaster and Teller, 1988). Most investigators believe that prevailing southerly winds have progressively pushed the edge of the Sand Sea northward, although east-west rivers such as the Tsauchab and Tsondab probably were temporary barriers to this advance, just as the Kuiseb River is today (see Fig. 1). Although there is some uncertainty about the reliability of radiocarbon dated carbonates in the Namib, we believe, for reasons previously discussed, that they represent the age of sedimentation at the playa sites. Even if these dates represent a phase when recrystallization reset the radiocarbon clock, they still reflect a major period of water availability. Archeological materials (Early Stone Age artefacts at Namib IV, Narabeb, and Khommabes) and vertebrate evidence (extinct Elephas recki at Namib IV) may be explained by reworking from older deposits, and the numerous Middle and Late Stone Age implements at some sites indicates that conditions were, in fact, wet during the

Late Quaternary as suggested by Korn and Martin (1957). We believe the region remained arid to hyperarid throughout the Late Quaternary, and ponding and sedimentation were related to small increases in runoff and groundwater recharge. Sites as far west as West Pan were flooded 25-30 ka BP when the Tsondab River extended at least 60 km northwest of its modern terminus. Some deposits at Narabeb also relate to the same period of ponding along the ancient Tsondab valley. Subsequent eastward retreat of the end point of this river, because of the growth of dunes across its valley, produced ponding and deposition at Ancient Tracks by 13-14 ka BP, and, after this, at its present terminus at Tsondab Vlei. Concurrent with deposition at Narabeb and West Pan was aUuviation at the Khommabes site by the Kulseb River. This appears to have preceded construction of the dunes that now surround that depression. During the retreat of the end point of the Tsondab River, encroaching dunes probably encouraged recharge of the underlying Tsondab Sandstone by slowing runoff. Because the steepest hydraulic gradient of the groundwater system would have been northward toward the entrenched Kulseb River valley, depressions in the interdune corridors between these two river systems would have received discharge during periods of highest runoff in the Tsondab system. Thus, the age of ponding on the interfluve may be oldest in the west (e.g. Gobabeb South and Khommabes, 21-22 ka BP; and possibly undated Sobeb South and Obab South) and youngest to the east (e.g. Salty Pan, 11,130 BP; Namib IV, 17,500 BP). This does not preclude ponding at other times (e.g. Bone Pan, 36,600 BP) or for other reasons (e.g. Khommabes seepage directly from the Kuiseb River). Deposition of the Homeb Silts in the Kuiseb valley between 19 and 23 ka BP has been interpreted as reflecting ephemeral river flooding (e.g. Ward, 1987) or ponding behind a dune dam (e.g. Goudie, 1972). In either case this may have led to (or contributed to) ponding and sedimentation at this time in playas such as Gobabeb South and Khommabes which lie within the flood height indicated by the Homeb Silts. Since establishment of the end point of the Tsondab River at its present location, perhaps 10 ka BP, it appears that river flooding and groundwater recharge have not been enough to cause ponding at any of the known playa sites. This is consistent with the lack of radiocarbon dated evidence for wetter conditions in the Namib after 11 ka BP (Vogel, 1987). ACKNOWLEDGEMENTS Thanks are owed to many for this many-facctedresearch project. John Ward, Geological Survey of Namibia, was instrumental in planning and executing the field program, and has provided sutntantial help m compiling our results. Mary Seely of the Desert Ecological Research Unit at Gobabeb and the GeologicalSurveyof SWA/Namihia provided vehicles for fieldwork. We are particularly grateful to John Vogel (CSIR, Pretona) for determining new

Lake Deposits in the Northern Namib Sand Sea radiocarbon ages for us. We thank J. Brigham-Grette and K. Muehienbachs for aiding in the determinations of amino acid and isotopic raUos, respectively, P. De Deckker for ostracod identification, P. Black for preparing thin sections, R. Lemoine for grain size and carbonate measurements, Judith Lancaster for help in the field, and E. Neethling for geochemical data. Brian Jones (University of Alberta) helped with mineral identificauun, and Bill Last (University of Manitoba) provided invaluable insight into our carbonate interpretation. Discussions with J. Ward, M. Rybak, T. Ceding, W. Last, and R. G. Klein are gratefully acknowledged. Travel assistance was provided to JTI" and NWR by the Natural Sciences and Engineering Research Council of Canada and to J'FF by the University of Manitoba.

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