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Journal of African Earth Sciences 47 (2007) 39–48

Palaeoenvironmental significance of the clay mineral compositionof Olduvai basin deposits, northern Tanzania

Florias Mees *, Stijn Segers, Eric Van Ranst

Department of Geology and Soil Science, Ghent University, Krijgslaan 281 S8, B-9000 Ghent, Belgium

Received 7 March 2006; received in revised form 20 September 2006; accepted 2 November 2006

Abstract

Quaternary deposits in the southeastern part of the Olduvai basin, northern Tanzania, consist of lake margin deposits, followed by aseries of fluvial sediments. The clay mineral fraction of the lake margin deposits (Bed I and lower part of Bed II) is composed of smectiteand subordinate illite. All smectite is largely dioctahedral and shows indications for a limited degree of irregular interstratification byillite. In the overlying fluvial deposits (Beds II–IV), illite is the most abundant clay mineral. Smectite only occurs in lower parts ofthe fluvial deposits (up to the middle of Bed III), where it generally shows a high degree of irregular interstratification. Differences inclay mineral composition between the lake margin deposits and the fluvial deposits record differences in sediment source area and degreeof alteration. Dioctahedral smectite in the lake margin deposits and the oldest fluvial deposits is derived from a region with volcanicmaterial extending to the east and south of the basin, which also supplies a certain amount of illite. Illite in the fluvial deposits ofBed IV originates from an area with a metamorphic bedrock to the west and north. Alteration of detrital clay minerals resulted inMg-enrichment of dioctahedral smectite in part of the lake margin deposits and partial illitization of smectite in the older fluvial deposits.Formation of clay minerals during diagenesis or soil development is not documented by analysis of the total clay fraction.� 2006 Elsevier Ltd. All rights reserved.

Keywords: Olduvai Gorge; Clay minerals; Zeolite minerals; Alteration

1. Introduction

The Olduvai basin in northern Tanzania contains a suc-cession of Quaternary lacustrine and fluvial deposits,formed in a semi-arid environment. The study of verticaland lateral variations in clay mineral composition of thesezeolite-rich deposits is important for palaeoenvironmentalresearch, providing information about variations in sedi-ment source areas and changes in conditions affecting theformation and diagenesis of silicate minerals. Studiesreporting clay mineral data for this basin include few inves-tigations that cover all main stratigraphical units recogni-sed at Olduvai (Beds I–IV; Reck, 1951) (Hay, 1970,1976). All other studies concentrate on a narrow strati-graphical interval that is most relevant for palaeoanthropo-

1464-343X/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.jafrearsci.2006.11.003

* Corresponding author. Tel.: +32 9 264 4569; fax: +32 9 264 4984.E-mail address: florias.mees@ugent.be (F. Mees).

logical research, i.e. the top of Bed I and the base of Bed II(Hay and Kyser, 2001; Deocampo et al., 2002; Hover andAshley, 2003; Deocampo, 2004). In this paper, we reportclay mineral data for all main units, sampled in the south-eastern part of the basin (Fig. 1). This study supplementsan earlier investigation of deposits in the same exposures,focussed on zeolite formation (Mees et al., 2005).

2. Setting

The Olduvai basin is located west of the eastern branchof the East African Rift, in northern Tanzania. The oldestdeposits, belonging to Bed I and the lower part of Bed II,formed predominantly in lake and lake margin environ-ments (2.0–1.7 Ma; Hay, 1976). They are covered by asuccession of mainly fluvial deposits (1.7–0.6 Ma), formedduring and after a period when the surface area of thelake decreased (1.7–1.2 Ma). This major change in

Fig. 1. Location of the study area and the sampling sites. (I) Bed I site (Z),(II) Bed II site (RS), (III) Bed III site (RB), (IV) Bed IV site (MC);corresponding locality numbers of Hay (1976) between brackets.

40 F. Mees et al. / Journal of African Earth Sciences 47 (2007) 39–48

sedimentation patterns was the result of tectonic move-ments (Hay, 1976). The palaeoclimate was semi-arid atall times during the Bed I to IV stages, with indicationsfor somewhat drier conditions from the upper part ofBed I onwards. Possible drier stages have been inferredfor the upper part of Bed II (Hay, 1976), and an effectof climate change on palaeohydrology has been demon-strated for the basal part of Bed II (Deocampo, 2004).

Potential sediment source areas are the NgorongoroVolcanic Highlands to the east and south (with basalticand trachytic materials), and an area with Precambrianmetamorphic rocks to the west and north (mainly gneissand quartzite). The basin has always been characterisedby saline alkaline lake water and groundwater composi-tions, due to weathering of volcanic rocks and semi-aridclimatic conditions.

3. Materials and methods

A total of 43 samples, covering all main stratigraphicalunits, was analysed (Fig. 2). All samples were collected in

four exposures located at a short distance from each otherin the southeastern part of the basin (around localities 44a,45 and 85; Hay, 1976) (Fig. 1). The clay fraction (<2 lm)was separated by sieving and settling, after decalcificationwith an acetic acid buffer (pH 4.9). The effect of this treat-ment on silicate minerals was tested using samples fromanalcime-, chabazite- and phillipsite-dominated intervals(Z19, RB2, MC19), demonstrating that zeolite mineralsand phyllosilicates are entirely unaffected. X-ray diffraction(XRD) analysis of the clay fraction was performed usingoriented slides, after K-saturation, Mg-saturation, K-satu-ration followed by heating (350 �C, 525 �C), and Mg-satu-ration followed by glycol treatment. Non-oriented powdersamples of the clay fraction were analysed for selectedintervals (d060 value determination). All XRD analyseswere done with a Phillips X’pert System (Cu Ka radiation,scan time 1 s per 0.02� 2h). The chemical composition ofthe clay fraction was determined for 12 samples, by atomicabsorption spectroscopy (Varian SpectrAA-600) followingstandard acid digestion procedures. Oxalate-extractable Al,Fe and Si were also determined (USDA-NRCS, 2004).

4. General features of the studied deposits

In the study area, the boundary between the lake margindeposits and the overlying fluvial deposits corresponds to alevel in the upper part of Bed II (see Table 1). Tuff layers arecommon in Beds I and II, and considerable amounts of vol-canic debris occur as part of the sand fraction up to the topof Bed III. Indications for soil development are recognisedfor many intervals throughout Beds I–IV (Hay, 1976;Kafumu, 2000). They include some prominent reddish inter-vals in Beds II and III, interpreted as palaeosol levels (RS35–RS39, RB4–RB6b, RS15–RS22) (Hay, 1976). Chabazite((Ca0.5,K,Na)AlSi2O6 Æ 3H2O) is the most common zeolitemineral in the lake margin deposits. In the fluvial deposits,analcime (Na2Al2Si4O12 Æ 16H2O) is the dominant zeolitemineral, with significant amounts of phillipsite(K2(Na,Ca0.5)3Al5Si11O32 Æ 12H2O) (RB2–RB3) or chabaz-ite (RB15–RB21) in some intervals (Mees et al., 2005).

5. Results

5.1. Lake margin deposits

The clay fraction of the lake margin deposits mainlyconsists of smectite and includes only minor amounts ofillite (Table 1; Fig. 3). XRD analysis of non-orientedsamples indicate that all smectite is largely dioctahedral(Al-rich) (d0 6 0 values of 1.510–1.514 A). A greater triocta-hedral tendency is only inferred for the interval with thehighest Mg content of the clay fraction (2Z, d0 6 0

1.518 A) (see below). The slightly irrational spacing ofthe d0 0 l reflections indicate a certain degree of interstratifi-cation. Chemical data (see further) strongly suggest inter-stratification by illite, in line with earlier reports aboutOlduvai Basin clays (e.g. Hay and Kyser, 2001; Deocampo

Fig. 2. Simplified logs of the sampled sections, with indication of samples used for clay analysis (sample codes underlined if chemical data are available).Intervals indicated as ‘soil’ are those showing field indications of pedogenesis or subaerial exposure.

F. Mees et al. / Journal of African Earth Sciences 47 (2007) 39–48 41

et al., 2002). Based on differences between 2h values of d0 0 2

and d0 0 3 reflections for glycol-treated Mg-saturated sam-ples (Moore and Reynolds, 1989), smectite in the lake mar-gin deposits that were sampled for the present study isestimated to contain up to 30% randomly interstratifiedillite (10–30% in Bed I, 10–20% in Bed II).

In deposits of Bed I (samples ZB–2Z), the smectite con-tent is high in all samples. It is slightly lower in two samples(Z7, Z11), which represent intervals with a high sand con-tent. In the lower part of Bed II (RS5–RS19), the smectitecontent is highest in intervals that were identified as palae-osol levels by Kafumu (2000). These intervals have a lowersand content than the intervening sections without indica-tions for soil development. In higher parts of Bed II(RS22–RS31), the smectite content is lower than in lowerparts of the same unit.

Other components of the clay fraction of the lake margindeposits are minor amounts of feldspar and zeolite minerals.The dominant zeolite mineral is phillipsite, although chabaz-ite is the most abundant in bulk samples (Mees et al., 2005).

Chemical data for the clay fraction (Table 2) show thatsmectite in the lake margin deposits is a Mg-rich montmo-rillonite. The magnesium content is highest in the upperpart of Bed I (Z13, 2Z). MgVI values are too low to becompatible with a largely trioctahedral phase, and AlIV

values are low for saponitic clays. The amount of inter-layer potassium, occurring as non-exchangeable cations

following extensive contact with Na-rich solutions duringdecalcification of the samples, is exceptionally high forpure montmorillonite clays, in agreement with a highdegree of interstratification by illite.

5.2. Fluvial deposits

The clay fraction of the fluvial deposits consists of illiteor illite and smectite (Table 1). Smectite only occurs in theolder deposits (RS35–RB8), whose relative smectite con-tent is generally lower than that of the lake margin depositsof Beds I and II. Except in one interval (RB2, RB3; see fur-ther), smectite in the fluvial deposits is characterised by ahigh degree of random interstratification by illite, resultingin broad weak asymmetrical d0 0 1 reflections in XRD pat-terns, with a shoulder towards higher d-values and with a10 A lower limit of the reflections for Mg- and K-saturatedsamples (Fig. 4) (Brown and Brindley, 1980).

In the upper part of Bed II in the exposure where Bed IIwas extensively sampled (Bed II site; Fig. 1), smectite ischaracterised by random interstratification that is typicalfor the fluvial deposits (RS35–RS39). For the uppermostpart of Bed II, sampled at a different locality (Bed III site;Fig. 1), the XRD patterns for the clay fraction are moresimilar to those obtained for the lake margin deposits(RB2, RB3). In Bed III, smectite is only present in thelower part, below an interval that developed along an

Table 1Mineralogical composition of the clay fraction of lake margin and fluvial deposits of the Olduvai basin

Unit Sample Clay minerals Zeolite minerals Other

Sm Il Ch An Ph O F

Fluvial depositsBed IV MC24 � + � � · � �

MC23 � + � � · � �MC19 � ++ � · � � �MC12 � + � � · � ·MC8 � ++ � � · � ·

Bed III MC4 � + � � (·) � ·RB21 � + � � (·) � ·RB15 � + � � � � ·RB14 � + � (·) � � �RB12 (+) + � · � � ·RB10 � + · � · � ·RB8 ++ + � � · � ·RB6b ++ + � � � � ·RB4 (+) ++ � � · � ·

Bed II RB3 +++ ++ � � · � ·RB2 +++ ++ � � · � ·RS39 + + � � · � ·RS38 + + � � � � ·RS35 + + � (·) � � ·

Lake margin depositsBed II RS31 + + · (·) · C ·

RS26 ++ + � � (·) � (·)RS22 ++ � � � � E (·)RS21 +++ + � � · C (·)RS19 +++ (+) � � � C �RS18 ++ (+) � � � � (·)RS13 +++ (+) � � · � �RS12 ++ � � � � � (·)RS9 +++ + � � · � �RS8 ++ + · � · � (·)RS7 +++ � � � � � ·RS6 ++ + � � � � ·RS5 +++ + � � (·) � �

Bed I 2Z +++ (+) � � � � �ZL +++ (+) · � � � �ZK +++ (+) � � � � �Z19 +++ + � � (·) � �Z13 +++ + � � (·) � �Z11 ++ (+) � � · � ·Z7 ++ + � � · � ·Z2 +++ + � � · � (·)Z1 +++ + � � (·) � �ZJ +++ + � � � � ·ZB +++ + � � · � ·

Sm, smectite; Il, illite; An, analcime; Ch, chabazite; Ph, phillipsite; O, other zeolite minerals (C, clinoptilolite; E, erionite); F, feldspar; (�/+/++/+++)absence/presence and relative abundance; (�/·) absence/presence.

42 F. Mees et al. / Journal of African Earth Sciences 47 (2007) 39–48

important palaeosurface (RB14) (Mees et al., 2005). It isalways characterised by random interstratification by illite(see Fig. 4). In Bed IV, which contains a significantly loweramount of volcanic debris than Beds I–III, illite is the onlyclay mineral present (Fig. 5).

In most of the fluvial deposits, feldspar is a minor con-stituent of the clay fraction. Phillipsite is the most commonzeolite mineral in this size fraction, whereas analcime, themost abundant zeolite mineral in bulk samples, is quite rare.

The chemical composition of the purely illitic samples,representing the upper part of the studied deposits(RB15–MC19), is quite typical for illite (e.g. Newmanand Brown, 1987), although the Mg content is relativelyhigh and the K content is rather low (Table 2). In lowerparts of Bed III and the upper part of Bed II (RS35–RB8), the composition of the total clay fraction is inter-mediate between that of smectite (ZJ–RS21) and illite(RB15–MC19). The interval with the highest smectite

Fig. 3. Representative XRD patterns for the clay fraction of the lake margin deposits of Bed I and the lower part of Bed II (sample Z13) (Sm smectite, Ilillite, Ph phillipsite).

F. Mees et al. / Journal of African Earth Sciences 47 (2007) 39–48 43

content (RB3) has the most smectitic chemical composi-tion (layer charge, AlIV content, K content), with arelatively low Mg content of the smectite in the smec-tite-illite mixture that this sample represents. As in thelake margin deposits, the total amount of oxalate-extract-able Fe, Al and Si is low in all intervals (RS35–MC19).

6. Discussion

6.1. Lake margin deposits

Dioctahedral smectite is the dominant clay mineral inthe lake margin deposits of Beds I and II, in agreementwith earlier reports (Hay, 1970). It shows indications for

the presence of minor to moderate amounts of randomlyinterstratified illite, but an interstratified mineral thatoccurs together with discrete smectite (Hay and Kyser,2001) is not present.

Dioctahedral smectite is derived from source areas in theeast and south, which also supply a certain amount of illite(Hay, 1976). In the source area, smectite was formed byweathering of volcanic material (Hay, 1976; Hover andAshley, 2003). This mineral can be transformed to Mg-richsmectite in a saline alkaline lake margin environment,which is recognised for Olduvai Basin deposits by TEManalysis (Hover and Ashley, 2003) and by studies of thefine clay fraction (Deocampo et al., 2002). The presenceof Mg-rich smectite is confirmed by chemical analysis of

Table 2Chemical composition of the clay fraction of selected samples (wt%), number of cations on the basis of O20(OH)4, total layer charge and percentage ofoctahedral charge, and oxalate-extractable Fe, Al and Si (wt%)

Lake margin deposits Fluvial deposits

Bed I Bed II Bed III Bed IV

ZJ Z13 2Z RS9 RS21 RS35 RB3 RB6b RB8 RB15 MC8 MC19

SiO2 51.02 51.32 50.43 51.27 51.40 46.98 49.36 46.14 47.70 43.84 47.41 49.58AI2O3 9.58 6.15 5.30 7.60 8.44 12.29 12.42 10.68 10.62 14.89 13.98 12.30Fe2O3 9.12 9.77 6.61 9.23 7.44 14.68 11.11 15.86 14.00 13.75 12.88 11.88TiO2 0.59 0.89 0.69 1.75 1.27 2.37 1.74 2.31 2.17 2.21 1.63 1.60MnO 0.06 0.16 0.12 0.08 0.15 0.23 0.06 0.10 0.13 0.15 0.12 0.07CaO 0.20 0.25 0.21 0.19 0.17 0.71 0.30 0.74 0.67 0.57 0.80 0.34MgO 9.83 10.79 13.74 8.96 7.67 5.01 4.39 3.40 4.31 3.21 2.94 2.99Na2O 2.69 2.92 5.20 2.96 2.84 2.41 2.31 2.68 2.73 2.50 1.09 2.05K2O 3.11 0.96 1.20 0.85 2.08 4.02 2.57 3.62 3.57 4.71 4.73 5.73P2O5 0.10 0.11 0.06 0.10 0.06 0.18 0.09 0.18 0.22 0.51 0.36 0.09H2O� 6.13 10.12 8.83 9.94 11.01 5.61 9.72 7.34 7.50 6.53 6.84 6.73H2O+ 7.00 6.38 7.09 6.13 6.72 5.15 5.78 6.81 6.89 6.53 6.29 6.43

Total 99.43 99.82 99.48 99.06 99.25 99.64 99.85 99.86 100.51 99.40 99.07 99.79

SiT 7.49 7.73 7.63 7.71 7.85 6.91 7.40 7.06 7.19 6.66 7.11 7.40AlT 0.51 0.27 0.37 0.29 0.15 1.09 0.60 0.94 0.81 1.34 0.89 0.60RT 8.00 8.00 8.00 8.00 8.00 8.00 8.00 8.00 8.00 8.00 8.00 8.00

Alo 1.15 0.82 0.57 1.06 1.37 1.04 1.59 0.99 1.08 1.32 1.58 1.56Feo 1.01 1.11 0.75 1.04 0.86 1.63 1.25 1.83 1.59 1.57 1.45 1.33Tio 0.07 0.10 0.08 0.20 0.15 0.26 0.20 0.27 0.25 0.25 0.18 0.18Mno 0.01 0.02 0.02 0.01 0.02 0.03 0.01 0.01 0.02 0.02 0.02 0.01Mgo 2.15 2.42 3.10 2.01 1.75 1.10 0.98 0.78 0.97 0.73 0.66 0.67Ro 4.38 4.47 4.52 4.32 4.14 4.06 4.03 3.87 3.90 3.89 3.89 3.75

Cai 0.03 0.04 0.03 0.03 0.03 0.11 0.05 0.12 0.11 0.09 0.13 0.05Nai 0.77 0.85 1.52 0.86 0.84 0.69 0.67 0.80 0.80 0.74 0.32 0.59Ki 0.58 0.18 0.23 0.16 0.41 0.75 0.49 0.71 0.69 0.91 0.90 1.09Toti 1.38 1.08 1.79 1.06 1.27 1.55 1.21 1.62 1.59 1.74 1.35 1.74

CH �1.46 �1.15 �1.83 �1.13 �1.30 �1.72 �1.30 �1.84 �1.81 �2.12 �1.68 �1.83CHo(%) 64.95 76.54 79.62 74.44 88.78 36.78 53.58 48.80 55.41 36.75 46.92 67.15

Feox 0.43 0.26 0.51 0.28 0.22 0.56 0.24 0.44 0.30 0.36 0.11 0.26Alox 0.36 0.26 0.39 0.18 0.13 0.47 0.26 0.42 0.65 0.68 0.24 0.23Siox 0.88 nd 1.93 0.66 0.47 nd nd nd nd nd nd 0.19

44 F. Mees et al. / Journal of African Earth Sciences 47 (2007) 39–48

the total clay fraction. The d0 6 0 values, which are similar tothose reported elsewhere (Deocampo, 2004), indicate thatno purely trioctahedral phase developed. The relativelyhigh K values are in agreement with the presence ofadmixed and interstratified illite.

The development of Mg-rich smectite in the study areais most pronounced in the upper part of Bed I, the intervalfor which this process is most extensively documented(Hover and Ashley, 2003; Deocampo et al., 2002; Deo-campo, 2004). In higher strata, lower Mg values are com-patible with lower salinities and alkalinities recorded byan interval with largely zeolite-free freshwater deposits(Mees et al., 2005).

Illite, occurring in minor amounts, represents a purelydetrital phase. The high illite/smectite ratio in sand-richintervals suggests that it mainly occurs in the coarse clayfraction. Indications for authigenic formation of illite-related minerals such as celadonite (Hover and Ashley,2003) are not recognised (e.g. low Fe content forceladonite).

There are no differences in clay mineralogy between pos-sible palaeosol intervals (Kafumu, 2000) and the interven-ing sections, as well as no significant differences in chemicalcomposition of the clay fraction. The patterns observed forBed II are determined by differences in texture rather thansoil development, with high illite contents in the morecoarse-grained layers.

The near-absence of chabazite in the clay fraction iscompatible with a relatively large crystal size, as observedin thin sections (Mees et al., 2005). Phillipsite is more com-monly recognised for the clay fraction, which is partly dueto the elongated prismatic shape of the crystals and to theenhancement of the diagnostic d0 2 0 reflection (7.16 A) bypreferential orientation during sample preparation forXRD analysis.

6.2. Fluvial deposits

In the only earlier study providing information aboutthe clay mineral composition of the fluvial deposits (Hay,

Fig. 4. Representative XRD patterns for the clay fraction of the fluvial deposits of Bed III containing smectite with irregularly interstratified illite (sampleRB8) (Sm smectite, Il illite, Ph phillipsite, F feldspar).

F. Mees et al. / Journal of African Earth Sciences 47 (2007) 39–48 45

1970), the presence of illite, smectite (montmorillonite) andmixed-layer clays (smectite-illite) is mentioned. Accordingto that study, the clay fraction of Bed II consists predom-inantly of illite, and the clay fraction of Beds III and IV iscomposed of illite and mixed-layer clays. In the presentstudy, the < 2 lm fraction is found to consist of illite andsmectite in Bed II and the lower part of Bed III, generallywith a high degree of interstratification of smectite by illite.In higher parts, illite is the only clay mineral present. Thechemical composition of the clay fraction of the smectite-and illite-bearing lower part of the fluvial deposits is influ-enced both by the presence of discrete illite, occurring ingreater relative amounts than in the lake margin deposits,and by the greater degree of illite interstratification. The

relative contributions of these factors could not bedetermined. Amorphous compounds are not an importantconstituent, as shown by the low amounts of oxalate-extractable Al and Si.

During deposition of Beds II and III in the study area,detrital material was derived from the same region as dur-ing the preceding period of lake margin sedimentation(Ngorongoro Volcanic Highlands) (Hay, 1976), which isconfirmed by the presence of volcanic detritus in the sam-pled deposits. If the catchment was entirely unchanged, dif-ferences in clay mineralogy between these fluvial depositsand the lake margin deposits of Beds I and II must berelated to differences in the degree or nature of alteration,in the source area or after deposition. However, detrital

Fig. 5. Representative XRD patterns for the clay fraction of the fluvial deposits of Bed IV (sample MC8) (Il illite, Ph phillipsite, F feldspar).

46 F. Mees et al. / Journal of African Earth Sciences 47 (2007) 39–48

material is likely to have been derived from a different areain the southeastern part of the basin following the majorchange in sedimentation patterns recorded by the transi-tion between lake margin and fluvial sedimentation.

In the smectite-bearing lower part of the fluvial deposits(upper part of Bed II, lower part of Bed III), the commonoccurrence of interstratified illite in the smectite structure iscompatible with partial illitization of dioctahedral smectitein a saline alkaline environment (e.g. Turner and Fishman,1991). A change to conditions with higher salinity andalkalinity around the transition from fluvial to lake marginsedimentation in the study area is recorded by a change inzeolite mineral composition, with analcime instead ofchabazite occurring as the dominant zeolite mineral inthe fluvial deposits.

One interval in the upper part of Bed II (RB2–RB3, BedIII site) has a clay mineral composition that is similar tothat of the lake margin deposits in some respects (natureand abundance of smectite), suggesting a certain similarityin chemical environment and source area. However, thedifference in chemical composition of the smectite compo-nent and the higher illite content, as well as the high totalphillipsite content, imply that important differences in set-ting may have existed. Note that a considerable strati-graphical hiatus separates this interval from the reddishBed II deposits at the top of the Bed II exposure (RS35–RS39) (Mees et al., 2005). The presence of this interval doespreclude a basin-wide absence of well-developed smectitewithin the catchment from the onset of the period with flu-vial sedimentation onwards, which would be expected if a

F. Mees et al. / Journal of African Earth Sciences 47 (2007) 39–48 47

climate-controlled change in degree of alteration wasinvolved.

Around the top of the smectite-bearing part of Bed III,no clear relationship between clay mineralogy and the pres-ence of a reddish colour is recognized. If the reddish inter-vals are palaeosol levels, pedogenesis after deposition hadno apparent effect on the composition of the clay fraction.

In the upper part of Bed III, the absence of smectite canbe related to a high degree of alteration, resulting in a com-plete breakdown of smectite, or to a further change indrainage patterns. The former is unlikely to occur in anenvironment with illitization and zeolite formation, imply-ing that smectite would need to be altered in the sedimentsource area. If alteration of smectite already became moreimportant during an earlier part of the Bed III stage, thestrong development of irregular interstratification in thefluvial deposits could be partly related to an increased sus-ceptibility of the detrital clays to illitization. The lowerboundary of the smectite-free deposits, around the middleof Bed III, roughly coincides with the base of a thick(chabazite-bearing) reddish interval (RB15–RB21), abovean important calcareous palaeosol interval (RB14). Thislevel marks a major change in the study area, following aperiod with soil development along a stable land surface.Drainage patterns and degree of alteration can both havechanged during this period.

The clay mineral composition of the Bed IV deposits,which contain illite as the only clay mineral and alwayshave an admixture of quartz in the silt and sand fractions(Mees et al., 2005), was largely determined by the natureof the sediment supplied to the basin. The transitionbetween Beds III and IV in the study area marks a changein sediment source, following the northward displacementof a central E-W oriented drainageway (Hay, 1976). Dur-ing the Bed IV stage, illite-rich detrital material was derivedfrom the north and west, where the pre-Quaternary sub-strate is composed of Precambrian metamorphic rocks,dominated by gneiss and quartzite (Hay, 1970, 1976).The detrital origin of illite in Bed IV and the upper partof Bed III is confirmed by its chemical composition, whichis different from that of Mg-rich illite and celadonitic claysrecognised for Beds I and II (Deocampo et al., 2002; Deo-campo, 2004).

The significance of the nature of zeolite occurrences inthe clay fraction of the fluvial deposits is similar to thatof zeolites in the lake margin deposits, with analcimeinstead of chabazite as the relatively coarse-grained phasethat is largely absent from the clay fraction.

7. Conclusions

The clay mineral composition of the Olduvai basindeposits is partly determined by the composition of thedetrital fraction, which is related to the nature of the bed-rock in the sediment source area. Transformation of clayminerals is another important factor, resulting in the devel-opment of high degrees of interstratification of smectite in

part of the fluvial deposits. Smectite is absent in a higherinterval, although the general direction of sediment supplyremained unchanged, reflecting a change in drainage pat-terns or in degree of alteration within the catchment. Thealteration of detrital clay minerals, besides the alterationof volcanic detritus, will have contributed to zeolite forma-tion. However, this is not recorded by a simple relationshipbetween mineral composition of the clay fraction and zeo-lite content. Extensive neoformation of silicates hasoccurred in the form of zeolite formation, but no clear indi-cations for the formation of authigenic clays are recognisedby analysis of the total clay fraction. Although an earlierstudy has demonstrated an effect of palaeoclimate changeon clay mineral chemistry, for a relatively narrow intervalwith a uniform mineralogical composition (Deocampo,2004), the present study shows that climate effects are notclearly recorded by changes in composition of the clay frac-tion at a larger scale in the study area.

This study provides a first record of vertical changes inclay mineralogy for a succession that covers all main strati-graphical units at Olduvai. It allows the recognition of fac-tors that influence these changes and indicates that certainexpected factors had no effect. For part of the fluvialdeposits, a better knowledge of changes in the sedimentsource area with time is needed, to be investigated by abasin-wide study of lateral variations.

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