Mineralogical Composition and Geographical Distribution of African and Brazilian Periatlantic Laterites

. E 4 J

Joumal ofA/rcan EarrhSciences. VOL 12. No. 1/72, pp. 283-295,lWl. 0899-5362/91$3.00+ 0.00 PrintedinTZhailaIxi Q 1991 pergamoclpressplc

Mineralogical composition and geographical distribution of African and Brazilian periatlantic laterites. The influence of continental drift and tropical paleoclimates during the past 150 million years and implications for India and Australia

Y. TARDY,B. KOBILSEXand H. PAQUET

Centre de Géochimie de la Surface (CNRS), Institut de Ghlogie (ULP), 1 rue Blessig, 67084 Strasbourg Cedex, France Institut Français de Recherche pour le Développement en Coopération (ORSTOM) B.P. 2528, Bamako, Mali

Abstract - Following thebreak-upof Pangea at the end of the Jurassic time, the African andSouth American umtinemts and thenIndia and Australia drifted into the fringes of the equatorial or tropical climatic zone with the humidity varying according to theepochs.hdia,Brazil andthesouthempart ofEast Africawhichwerepreviouslymore arid andprobably hotter during the Jurassic, became progressively more humid and cooler. By contrast, West AfÌica, Central AfÌica and Australia, formerly subject to very humid climates became subject to warmer and drier climatic conditions. In the fist case, the femcretes were rarely preserved intact but bauxites, generally massive and non pisolitic and essentially formed of gibbsite (AI(OH),) are abundant. In the second case, hematite (Fez03occurring as nodular femcretes are extensively developed togetherwithgibbsite-bearing pisoliticbauxites andboehmite(A100H). Gibbsite and goethite are hydrated minerals related to humid and rather cool climates, whereas hematite and boehmite are dehydrated minerals related to less humid and warmer climatic conditions. Thus temperature,relative humidity of the atmosphere and the activity of water are the major climatic and thermodynamicparameters controlling the mechanisms of formation, the processes of evolution and the geographical distribution of bauxites and fenicretes. Furthermore, the geographical distribution and the mineralogical composition of fenicretes and bauxites are considered tobecontrollednot onlyby thedifferentpresentday climates but also and perhaps above all by thesuccession of paleoclimates during the past 150 million years.

Key words: Climates, Paleoclimates. Bauxites, Femcretes, Africa, Brazil, India, Australia

Résume -A la suite de ladérive continentalequi commence avec la fragmentationde la Pangée, àla fin du Jurassique, les continents africain et sud-américain tout d'abord, I'lnde et l'Australie plus tardivement, se sont déplacks dans des franges climatiques Equatoriales ou tropicales plus ou moins humides ou plus ou moins sèches, selon les époques. L'Inde, leBrésil etlapartieméridionale del'Afrique del'ht, au Jurassique plus arides,sontdevenus progressivement plus humides et moins chauds. Fa revanche, l'Afrique de l'Ouest, l'Afrique centrale et l'Australie, autrefois soumises àdes climats trks humides, ont évolué vers des climats plus contrastés, plus chauds et plus secs. Dans lepremiercas,les cuirasses ferrugineuses subsistant intactes sontraresetlescuirasses bauxitiques, généralement massives et sans nodule contiennent essentiellement de la gibbsite (Al(0H)J. Dans le second cas. les cuirasses ferrugineuses, nodulaires à hématite (FezOJ sont t2s abondantes, tandis que les bauxites à gibbsite montrent trks fréquemment, en surface, des pisolites de boehmite (MOOH). La gibbsite et la goethite (FeOOH) sont des minéraux hydratésqui apparaissentlib aux climats humides ouplutot frais, tandis quel'hématiteetlaboehmitesontdesminéraux déshydrat6s qui apparaissent comme plutôt liés àdes climats moins humides et plus chauds. La distributiongéographique, de même que la composition minéralogiquedes bauxites et des cuirasses fermgineuses apparaissent ainsi non seulement réglées par la diversité des climats actuels, mais encore et surtout comme le reflet de I'évolution des climats anciens qui se sont succédé au cours des 150 derniers millions d'années.

XNTRODUCl'ION some different chemical. mineralogical and structural characteristics. On both sides of the Atlantic Ocean. areas sub- The purpose of this paper is to demonstrate that ject to present-day tropical and equatorial climatic such features result as much from latitudinal conditions. on the South American and African differences in present-day climates as from continents, are covered by a thicklateritic mantle. separate paleoclimatic histories since the break- Those laterites including bauxites. ferricretes and up of Pangea and the opening of Atlantic ocean nodular soils which have similar features but also during the Mesozoic. Such an interpretative

283 O. R. S.F. O. M. Fonds Documentaire 284 Y. TARDY,B. KOBUEKand H. PAQUET approach may also explain the differences observ- A similar relation can be written for the ed in the laterites of India and Australia. equilibrium between gibbsite (AUOH),) and boehmite (Al00I-X). as follows: REUTIONS BETWEEN CLIMATIC AND THERMODYNAMIC FACTORS

Temperature and water activity Nomal chemical potentials are a function of The two principal climatic factors governing the temperature so that. at equilibrium. water activlty mineraloglcal composition of lateritic covers are is itself a function of temperature for both re- temperature and atmospheric relative humidity. actions. The latter is closely related to what chemists call Thus, the relations between climatic and water activity. In fact, by definition (Garrels and thermodynamic factors can be explained. A very Christ, 1965)water activity written as a, or [H,O] humid climate correspondsto a permanentlywatm- is a dimensionless number equal to the f,,/f,, ratio saturated atmosphere (p/p" = 1, HR = 1Wo)and of the fugacity values of water vapor in air. f, is the a water activity equal of 1 ( [%O1 = 1).An arid or les fugacity value of water vapor in air, in equilibrium humid chtecorresponds to a drier atmosphere with an assemblage of minerals in equilibrium in (p/po c 1. HR < 10%) and a water activity lower turn with capillq water. f, is the fugacity vdue than 1. such as (H,O] c 1, is equal to the capillw of water vapor at saturation with respect to liquid water activity which governs the equilibrfa water at the metemperature. In air, f, is not between iron and aluminium oxides or hydmddes equal to the partial pressure of water vapor p,,. in laterites (Tardy and Nahon. 1985; Tardy and However. f,/P,, ratio may be considered as equal to Novikoff, 1988; Tardy etal., 1988b). the p/p" ratio of the partial pressures of water vapor and consequently water activity may also be Thermodynamic and climatic equilibria considered as equal to the atmospheric relative between goethite, hematite, gibbsite and humidity (p/p" = HR[O/o)/ 100) whichcharacterizes boehmite in lateritic profiles a given climate (Tardy md Novikoff. 1988): ln lateritic mantles, besides gibbsite and boehmite which are always Fe-depleted minerals, a,, [%O]= f,/P,, = p/p" = HR/ 100 goethites and hematites are Al-bearing minerals and are thus considered as solid solutionsbetween The chemical potential K, of bound water in goethite and diaspore ((AlxFe,.JOOH)on one hand soil caplillaries is calculated from the normal or between hematite and corindon chemical potlential powof liquid and free water on the other hand (Didier et d, and from bound water activity by the following equilibrium conditions then become more relation: complicated and the stability fields of minerals depend on water activity (Fig. la), on temperature H, = pow+ WT Lna, (Fig. lb) and on the chemical composition of the material (Trolard and Tardy, 1987).The effect of a Chemical equilibria between hydrated and de- water activity decrease at fixed temperature hydrated minerals as well as reactions of mineral [Fig. 1 a) is equivalent to a temperature increase at hydration and dehydration are ruled by the fixed water activity, in stimulating dehydration fluctuations ofwater activity. Equilibrium between reactions and thus in favouring the formation goethite (FeOOH) and hematite (Fe,O,): of boehmite (A100H) with respect to gibbsite (Al(OH),) and of hematite (Fe,OJ with respect to 2 FeOOH = Fe,O, + H,O goethite (FeOOH). When goethite and hematite associated to gibbsite, the content of substituted is thus reached if: Al increases as water activity or temperature decreases. This behavior fits perfectly with the observatfom made on laterites in West Africa. In ferricrets of As hematite and goethite are pure minerals and Senegal and Mauritania. the hematite-beam thus in ;a standard state, it follaws so that: horizons develop over the goethitic ones, located close tothewater table (Nahon. 1976).ThemoStfi substituted hematite is found in the uppeast part of profiles (Tardy and Nahon. 1985). MOre' so that: over, the goethite contents decrease from East Senegal to Mauritania (Nahon. 1987). In the Case of bauxites. the boehmite content increases fbm 1 %I

Mineralogical composition and geographical distribution of African and Brazilian periatlantic laterites. 285 T = 125°C 0-

20 - ~ I 1 1 ‘OeAl 40 -

60 -

O. 8

0.7 100 c \ I 0.0 0.5 Moles of %O3 for 1 mole of F,O, Fig. lb. Stability diagram in the system Fe,O,-Al,O,-QO as + Boe a function of the temperature and the aluminum content for a water activity [H,O] = 1. at a total pressure of 1 bar (after I Trolard andTardy, 1987). 0.5 Flg. lb. Magramme de steblllté dans le systhe Fe,O,-Al,O,-I-I,O en fonctlor. de la temperature et de la teneur en aluminium pour une -Moles of +O, for 1 mole of F,O, actlvlté hydrique [H,Ol- 1. Q une pression totale de I bar (d‘a@s Fig. la. StabflIty diagram in the system Fe,O,-Al,O,-H,O as Trolanl et Tardy. 1987). a function of the aluminum content and the water actlvíty at a temperature of 25°C and at a total pressure of 1 bar F’ERIATFTIC CLIMATES (after Trolard and Tardy, 1987). AND PALEOCLIMATES Fig. la. Magramme de stabilité dans le systhe Fe,O,-Al,O,-~O en fonctlon de la teneur en aluminíum et de I’activlté hydrique B une tempkature de 25’C et B une pression totale de 1 bar (daPr& Main features of present-day periatlantic Trolard et Tardy, 1987). climates equatorial or tropical climates with contrasted seasons prevail in West Africa, Central South to North in the Côte d’Ivoire region Africa and East Africa. (Boulangé. 1984). They also increase in the In equatorlal areas, climate is characterized by northern border of Fouta-Djalon mountains, mean annual temperature rangingbetween 25 and towards the warmer and more arid conditions of 28OC. mean annual rainfall higher than 1700mm. , the Sahara (Balkay and Bardossy, 1967). atmospheric relative humidity close to saturation, Clearly, gibbsitic bauxites continuously form and a short or non-existent dry season. For higher under permanently humid equatorial climates while latitudes in areas situated between the equator hematitic ferricretes at the present times and on and the tropics, climates are characterized by the same parent rocks, continue to develop under contrasted seasons. higher temperatures, ranging contrasted tropical climates. between 28°C and 35°C. annual rainfall lower than Goethite and gibbsite are hydrated minerals 1700 mm and long dry seasons during which which are to be related to constantly humid atmospheric relative humidity decreases, some- climates. Hematite and boehmite are both de- times below 50%.

hydrated minerals compared to goethite and However, striking differences are evident I gibbsite. Their occurrence in laterites is related to between Africa and South America, as well as tropical climates with a marked dry season and between the equatorial areas and the tropical relatively high temperatures (mean annual zones (RatiSbona. 1976; GriffithS. 1972; Leroux. temperature 28°C). In such clímates, humidity is 1983).First, thepredominantlylow-altitude areas, usually sufficient to induce a strong weathering Amazonia, Central Africa (zaire, Congo, Gabon). during rahfall but the temporary aridity of the dry and a part of West Africa (Sierra Leone, Guinea, season results in dehydration of gibbsite and South Côte d‘Ivoire) are subject to a humid tropical goethite into boehmite and hematite respectively. climate close to equatorial. Second, the equatorial Accompanyingthese mineralogical changes is also and tropical areas of South-East Africa (Kenya, the formation of nodular or pisolitic structures and Uganda, Tanzania) and the tropical zones of induration of ferricretes and bauxites (Fig. 2). Central Brazil (Goias, Mato Grosso) are character- 286 Y. TARDY,B. KOBIISEKand H. PAQUET T 1 L: TI H R B N

Cl a O1 n u2 a P 1

[H,O]= 1.0 Fig. 2. Distribution of ferricretes in Africa (after Petit, 1985). Discontinuous indurations (1); continuous ferricrete (2). F [H,O)= 1.0 Fig. 2. Distribution des ferrlcrètes en Afrique (d'après Petit, 1985). Indurations discontinues (1); U ferricrète continu (2). F ized by usually lower temperatures, related to basin to the Tropic of Capricorn. Important dlffe- rt higher altitudes. Last the contrasted season rences in the mean annual rainfalls are also evi- domain of Westem and Central Africa (Senegal, dent. At a same latitude, Brazil and East Africa are Mali, Burkha Faso, Central Africa) are hotter and ingeneral cooler and more humid, whilst West and drier. and desert areas are better developed and Central Africa are hotter and drier, Some global closer one to other desert areas in West Africa and climatic similarity thus exists between Brazil and Sudan, whereas the desert areas are reduced in East Africa, South of equator, whilst there are size In Brazil (Nordeste)or westwards decentred in important dimatdc differencesbletween these areas South Africa (Kalahari).This results in higher and the region of West and Central Africa, North Of temperatures in Central and West Mrica (5-6°C the equatolr (Table 1). higher at the equator, more than 20°C higher at the Moreover. evolving but persistent differences tropics) than for the same latitude in Brazil and in periatlantic climates have been af€Emg East Africa. Thus, when going North from the Afi-ica and Soutli America for more than 100 MSa. equator to the Tropic of Cancer, the mean annual These paleloclimatic changes have now been Over- temperature increases. whilst a temperature de- printed by the present-day climatic character' crease is observed when going South from Congo istics.

b Mineralogical composition and geographical distribution of African and Brazilian periatlantic laterites. 287 Table 1. Clhatic datas from AMcan stations (1). (2). (3) and During Jurassic times, the equator was located from BrasIlian stations (4), (5). (6) (after RaMsbona, 1976; near present-day Mauritania and Egypt Africa. Grifflths. 1972 and Leroux. 1983). in and at the northern extremity of South Amerka 1 (Figs. 3a and b). Present-day South Africa and the Lat(? TC) "1 greater part of present-day Brazilian shield were Dahu (1) 4'55" 29.0 93 then subject to arid chates. During Cretaceous Tabora (2) 4'53's 28,O 53 and Cenmoic times. a northward drift of these both Niamey(3) 13'30"N 37 58 continents took place, whilst, at the same time, a Belem(4) 1'28s 25.7 86 slow rotation of Africa occurred. An equatorial Manaus(5) 3"OB'S 26,9 82 1996 Formosa (6) 15'32's 2 1.1 72 climate was affecting Amazonia since the Maastrichtian: this climatic zone progressively Lai(*) latitude in degne extended southwards. and, at the same time. there T(*C) tempemhvein degree was a reduction in size of the dry areas of Central HR(%)refdve humidity of the atmosphere in percent Africa. the equatorial chticzone which P(mm) precipitation in millimeters per year Brazil. ln E(mm)ev~oninmillimetersperyearinmillimetefiperyear was well-developed during the Maastrichtian Nmbs number of dry months per year progressively contracted as equator moved southwards. Because it was close to the equator, South- Paleocllmates East Africa was subject to high rainfall. From the Recent paleogeographical maps provide re- Jurassic to present times, the previously arid constructions of the arrangement of continents climates of South America and South-East Africa and of the probable past distribution of the earth's became progressively more humid, whilst major climatic zones since he break-up of Pangea the formerly humid chates of West Africa and the opening of the Atlantic ocean. i.e. over the progressively became more arid. Consequently, past 100 M.a. (Parrish et al.. 1982: Tardy et al., paleoclimatic successions are similar in South- 1988a). East Africa and in Brazil: both these areas changed

TROPIC OF CANCER ......

...... ROPlC...... OF CAPRICORN

EARLIEST TRIASSIC LATEST JURASSIC LATEST CRETACEOUS MIDDLE EOCENE RECENT

Fig. 3a. Predicted distribution of rainfall through geological times. Numbers are to show relatfve values only: no units are implled. The rainfall is divided into four categories: < 50 = low rainfall, 50-100 = moderately low rainfall, 100-200 = moderate!@ high rainfall, > 200 = high &dl (after Parrfsh et aL. 1982). Example of Africa. Fig. 3a. Distribution supposéedes précipitations au cours des temps géologiques. Les nombres indiquent seulement des valeurs 2, relativessansunités.Lesprécipitatlonssont répartiesen4catégories: < 50. faibles; 50- 100: assez faibles;100-200: assezélev+ > 200: élevées (d'après Parrish et al., 1982). Exemple de l'Afrique.

...... TROPIC OF CANCER

TROPIC OF CAPRICORN

EARLIEST TRIASSIC LATEST JURASSIC LATEST CRETACEOUS MIDDLE EOCENE MIDDLE MIOCENE RECENT

Fig. 3b. Example of South America. Fig. 3b. Exemple de l'Amérique du Sud. 288 Y. TXRDY,B. Ko~nsmand H. PAQU@T

u ...... , ------_------__ -::::. -::::. .. .. ---_ 3Q' S

70 M.y.

LATEST CRETACEOUS

EARLIEST TRIASSIC Fig. 3c.Example of India Fig. 3c. Exemple de l'Inde.

EOUATOR

EARLIEST TRIASSIC LATEST JURASSIC LATEST CRETACEOUS MIDDLE EOCENE MIDDLE MIOCENE RECEN1 Fig. 3d. Example of Australfa. Fig. 3d. Exemple de l'Australie progressively from dry to humid clfmates. By annual rainfall ranging between 1200to 1700mm contrast. WestAfrlcakamemoreandmore subject a year, 4 months of dry season, mean atmospheric to the arid Saharan influence. lying in the belt relative humidity smaller than 80%. mean annual around the Tropic of Cancer. temperature around 28°C. In West Africa, Central Thus, Southern AfrIca and South America have Africa and Uganda where ferricretes are similar present-day climates as well as similar continuing at the present-day to form (Maignien, paleoclimatic history which are very different from 1958: Brücher, 1957: Quantin, 1965; Delvigne those of West Africa. and Grandin, 1969: McFarlane. 1976; Beaudet. . It is notable that in intertropical areas, the 1978: Fritsch. 1978; Michel, 1978: Petit. 1985).In geographical distribution and the mineralogical Africa (Fig. 2). the development of ferricrete composition of ferricretes and bauxites can be gradually decreases northwards, as humidity correlated with these climatic changes during decreases and Saharan influence increases, and geological times. eastwards, astemperature decreases and altitude increases. DISTFUl3UTION AND MINERAU)GICAL In the case of bauxites, many authors such as COMPOSITION OF LATERITES IN AFRICA Bardossy (1979, 1981), McFarlane (1983) and AND BR#ZIL (Figs.- 4, 6) Valeton (1983)have described the most favourable climatic zones asfollows: tropical to humid subtro- The distribution of the different lateritic picalzoneswithmean annual temperature greater ' formations has been correlated with climatic than 20°C and mean annual rainfall of more than zonations by many aufhors. 1200mm. approximately located in a belt between Fenicretes are found and develop principally in 30" latitude North and 30" latitude South. But, it ' humid tropical zone characte-d by: mean seems to us that these characteristics are not tl

Mineralogical composition and geographical distribution of African and Brazilian periatlantic laterites. 289 ... 1

UPPER JURASSIC JURASSIQUE SUPERIEUR

U U O- 1000 km O- 1000 km I

O. 1000 km O- 1000 km Fig. 4. Possible dlstribution of bauxites and furlcretes in Africa over the past 150 millton years (dotted = ferricretes; grey = bauxites). Fig. 4. Distribution possible des bauxites et ferricrètes en Afrique depuis 150 donsd'années (pointille ferrícrètes; gris2 bauxites).

restrictive enough. For our part, we consider that Ferri

Ma : MANAUS Be : BELIEM Re : RECIFE CRÉTACÉ SUPÉRIEUR Cu I CUIABA 70 MA 140 MA Br : BRASILIA - Bel: BELO HORIZ. 5P : SA0 PAULO o- 1000 km O 1000 km

Fig. 5. Possible distrlbution of bawdtes and fc”h in South America over the past 150 muion y- (dotted = ferricretes; grey = bauxites). Fig. 5. Distribution possible des bauxites et ferricrètes en Amérique du Sud depuis 150 millions d‘années (pointillé:ferric* tes: gris& bauxites). Novikoff, 1974;Martinetclt, 1981;Boulvert, 1983) ferrlcretes have been observed on an Eocene lad- where they are more or less eroded remnants of surface (called South-American surface) (King- ferricretes associated to ferruginous nodular layers 1956) in areas subject to a contrasted tropicd and overlaid by a loose ferrallitic layer (Muller etd. chatein Nordeste, Goias and Mato Grosso @On* 1981 Boaquier et d,1984). 1964; Journaux, 1978: Melfi et ,d,197% Some femicretes also mur inthe Sahara (Nahon. Trescases and Melfi, 1985; Weggen,, 1986). In 1976;Lepxun. 1979).buttheyarefossilferricretes addition. nodular ferruginous accumulations evidencing more humid ancient climates. formed under a loose surficial cower in the FerIlCretles are rare in South-East Africa Amazonfa equatorial zone (Chauve] et al., 1982; (Tanzania, Rhodesia) (Grubb, 1981; Valeton and Lucas et d,1986) can be compared to those Of Mutakyawa, 1987) and in Brazil Ivolkoff. 1986 Central kfrica. According to Nahon et al. (1989b Tardy et aL, 1988a). where climate is cooler and the scientists working on the southern border of more humid and where non-indurated lateritic the Amazonia basin frequently find fedCrete ~ soils prevail. However, some relatively thick currently being degraded with the rain i ~

Mineralogical composition and geographical distribution of African and Brazilian periahtic latentes. 291 The nodular ferruginous soils observed under (Boulangé, 1984; Patterson et af., 1986).They are. forest covers could appear to be relics of ancient generally considered to have formed durfng ferricreteswhich developed when these areas were Jurassic, Cretaceous or Eocene times (Michel, subject to a tropical climate with a well defined dry 19731, under more humid climates than those season but which are now being destroyed under of the present time. In West Africa, bauxites the very humid tropical conditions that prevail are characterized by high gibbsite (Al/OH),) to-day. The ferricretes of the Goias are similarly contents, but they also contain boehmite (AIOOH) destined for future destruction, if present-day in the surficial horizon. This dehydrated mineral climate continues or if the equator conttnues to forms like hematite in ferricretes, as pisolites move southwards. develop in the uppermost horizons (Belinga. 1968: In summary, two hypotheses were proposed by Hieronymus. 1973: Boulangé, 1984: Valeton and Martin and Volkoff ( 1987)to explab nodular fem- Beissner, 1986). The boehmite content is higher ginous accumulations. because of the proximity to the Saharan zone with (1) An old ferricrete which was destroyed its hot and dry climate (Balkay and Baidossy, generating nodular ferruginous layers. When only 1967). "'his mineral is also common in Egyptian the layers are preserved, they correspond to a bauxites (Germann eld., 1987). pedological horizon. However in some cases this It is interesting to note that the bauxites of horizon was destroyed and then regenerated by Southeast Africa, Rhodesia. Mozambic and geochemical deepening of the weathering cover. Malawi (Grubb. 1973a. b), the bauxites of the This resulted in the formation of a "'stone-line" different landsurfaces of Tanzania (Valeton and whose ferruginous gravel clasts are older than the Mutakyawa, 1987) and of numerous Brazilian soil in which they are found. deposits such as those of Amazonia or Minas (2) The occurrence of nodular ferruginous ac- Gerais (Weber. 1959; Valeton, 1973: Dennen and cumulations alone does not necessarily indicate a Norton, 1977; Grubb. 1979: Aleva. 1981: pre-existing ferricrete. Ferruginous accumulation Kotschoubey and Truckenbrodt, 1981; Groke et can develop without ever reachhg the "ironcrust" al., 1982: Kronberg etd,1982; Lemos and Villas. state and can instead be considered as "aborted" 1983: Melfi and Carvalho, 1983: Trescases and ferricretes. But in every case, these ferruginous Melfi. 1985) are usually massive and essentially accumulations appear to have been herited from composed of gibbsite. No pisolite is found and ancient landscapes found under less humid boehmite is rare. conditions than prevail at the present-day. Consequently, we can assume that lateritic Our preference is far the first hypothesis. bauxites only develop underveryhumid equatorial The induration of ferruginous accumulations or tropical climates; in that case, gibbiste is the leading to ferricrete formation results from an ac- prevailing mineral. Boehmite is considered to be a cumulation of hematite in the small pores in a pre- probable secondary mineral whose formation took &sting accumulation of kaolinite (Tardy and place at a later stage in the development of tropical Nahon, 1985) resulting from a decrease of water climates with long dry season. i.e. low atmopheric activity and of atmospheric relative humidity and relative humidity and high temperature. During from a temperature increase. as shownpreviously. the paleoclimatic history of West Africa, for Ferricretes are thus typical of hot, humid tropical example, boehmite formation could be considered chnates but with long dry season. Under more arid as secondary and contemporaneous to hematite climates (Leprun, 1979) or under permanently and ferricrete formation in the ferruginous ac- humid conditions in equatorial forests (Martinand cumulations located downslope to have developed Volkoff, 19871, ferricretes do not develop or are .later i.e. from the Miocene to the present-day destroyed. Whengoing fromwet-dry tropical zones times. to arid ones (Sahara), ferricretes are marked by decreasing goethite contents and by increasing EXAMPLES OF INDIA AND AUSTRALIA hematite contents and by nodule formation (Nahon. 1987). In contrast, a decrease in Present-day climates induration together with a goethite development is India is subject to a tropical climate with evident when going from contrasted tropical zones contrasted seasons. The arrangement of reliefs to equatorial ones. controls high rainfall on the western coast and the eastem foothills ofthe Himalayawhich are covered Bauxites with dense forest. Additionally aridity in the inner Bauxites are wide-spread in West Africa. They part of Deccan which is covered with savannah is formed in various latitudes, in humid zones like in part a consequence of the relief. Guinea, Nigeria, Cameroon or Côte d'Ivoire or in The principal chatic features of Australia are drier areas such as Burkina Faso, Mali etc. its generally low relief, its insular character and its 292 Y. TARDY,B. KOBUEKand H. PAQUET location around the Tropic of Capricorn; 40 Yo of central and southern India, however, bawac the area is subject to a tropical climate. The formations show some boehmitic pisolitic layers Australian conment is also relatively arid with due to present-day semi-arid conditions (Ghosh mean annual rainfàll of only 600 mm or less afTect- and Dutta, 1978: PatterSon et d,1986) and ing 80 % of the area. Deserts are widely developed associated with low landsurfaces ferriCretes (mean annual temperature higher than 30°C. (Demangeot, 1978). In the northern and north- atmospheric relative humidity of 20 %). The humid eastern part of India, bauxites contain boehmite tropical zone is restricted to the northeastern coast and diaspore but this mineralogical content could whilst temperate to meditemanean type climatic be the result of tectonic movements caused by fie conditions prevail on the south-eastem, southern collision of India against Eurasia rather than of and south-westem margins of the continent. climatic varfation~(Hasan Ziad, 1966: Caillère and Singh, 1967). Paleoclimates In Australia, lateritic formations are found At aboutthe time when SouthAmerica and Africa throughout and cover 20 % of the whole continent, separated, India also separated from Gondwana, Ferruginous laterites are nodular or massive. and rotating and moving northwards (Fig. 3c). &om they contain goetkdte and hematite. In the arid the end of the Triassic to the Cretaceous the Indian zone, they are preserved in the form of Rlictual continent's climate evolved from hot and dry to hot plateaus and reliefs, resulting from the destruct- and humid. Climatk conditions were favourable ion of former peneplains (Hubble el d,1983). for lateritization from Cretaceous to Paleocene Bauxitic deposits are restricted to the northern, times for during that period. the Indian continent southeastern and southwestem coastal zones. They crossed the zone between 30" latitude South and are Tertiary in age and contain predominantly O" latitude North (Kumar, 1986).After the collision gibbsite (Grubb, 1970: Grubb, 1971a and b; of India against Eurasia at the end of the Miocene. Valeton. 1972; Ball and Gilkes, 1987). Pisolitic climate became more temperate or more tropical structures related to boehmite formation are but remained humid with no important arid observed near the surface (Schellmann and episode. Jepsen. 1973: Grubb, 1973a; White, 1976 The present-day configuration of Australia Patterson et d,1986). developed during the Tertiary. From a tectonic Thus, the distribution of ferricretes and bauxites point of view, Australia has been relatively stable as well as of boehmite in Australia and India might since the terminal Jurassic. During the be controlled by the same factors as those operat- Cretaceous, Australia together with Antarctic inginAfrica andBrazil. namelythe evolution ofthe separated from the African continent and moved prevailing climate through time and the result- eastwards (Fig. 3d). The separation of Antarctic ing formation and destruction of bauxites and from the Australian contlment was completed by laterites. the end of the Paleocene and during the Tertiary, Australia started moving 25" latitude northwards CONCLUSIONS and was afTected by an about 70" dextral rotation at the same time. Important climatic variations The areas where ferricretes are numerous and were generated by the fragmentation of Gondwana well-developed, are also the areas with high due to a shift of oceanic circulations southwards boehmite contents in the bauxites. This is the case and the development of a circum-polar current in West Africa and Australia which were formerly system. A warm to hot and humid climate (until the Eocene) subject to very humid climates, prevailed from the Paleocene to the Miocene which resulted in the wide-spread development Of times, favouring lateritkation (Beckmann,1983) gibbsite-bearing bauxites but was probably not but during the QuatemaIy. the climate of the favourable for ferricrete formation. As the clmate Australian continent became predominantly arid. became characterized by marked wet and dry seasons, as well as hotter and drier, hematite- Lateritric formatiolns bearing ferricretes developed and secondm In India. laterites are very wide-spread. Their transformation ofgibbsite into boehmite tookplace development began at thle end of the Cretaceous in the bauxites. and continued during the Tertiary. On the western ParalEeling this trend in the areas of Braza, coast (Balasubramania. 1978: Valeton, 1983: South-East Africa and India, ferricretes are rare, Patterson et al., 1986). bauxites have similar the bauxites do not generally contain boehmite. In features to those of Brazil due to the similar humid India, however, boehmite is almost always present tropical climatic eonlditPons and to a plateau in small amounts (0.3-3%) but locally. fn the location. The Indian deposits are essentially Kutch Peninsula, boehmite reaches 10 96 gibbsitic and contain little or no boehmite. In (Bardossy. 1982).Unlike themore marked wet-dv Mineralogical composition and geographical distribution of African and Brazilian pexiatlantic latentes. 293 hotter and drier climate of West Africa and Beckmann, G. G. 1983. Evolution of ausWan land- Australia. those of South-West Africa. Brazil and scapes and soils. Chapter 4: Development of old land- India are cooler and more humid. Unlike the scapes and soils. In: SoaS. an Australian vteuJpht, paleoclimates of West Africa and Australia which CSIRO, Academic Press. '51-72. evolved from humid to arid leading to the pro- Belinga, S. 1968. Composition minéralogique des bauxites de l'Adamaoua (Cameroun).Ann. gressive extension of Saharan and Australian de k Fac. des Scien~esdu Cameroun Y&, 2,59-76. deserts, the climates of Brazil. East Africa and Bocquier. G., Muller. J. P. and Boulangé. B. 1984. Les India evolved from arid to humid together with the latérites. COM~~SS~~C~Set perspectives actuelles sur progressive enlargment of the equatorial or humid les mécanismes de leur différenciation. In: &e tropical zone on continents. Jubilaire du Cinquantenaire, Paris, 123-138. Thevariation in distribution of ferricretes as well Boulangé, B.. Delvigne, J. and Eschenbrenner. V. 1973. as the dmerent mineralogical compositions of Descriptions morphoscopiques, géochimiques et bauxites in Africa, Brazil. Australia and India are minéralogiques des faciès cuirassés des principaux the consequence of paleoclimatic evolutions wer niveaux géomorphologiques de Côte d'Ivoire. Cah the past 150 million years rather than of present- ORSTOM. s&. G&d,Paris, V(1), 59-81. day climatic differences. Boulangé, B. 1984. Les formations bauxitiques latéritiques de Côte d'Ivoire. Trav. Doc. ORSTOM, In conclusion, the differentiation of the lateritic Paris, 341 p. covers are mainly due to continental drift and Boulvert, Y. 1983. Carte pédologique de la République movement of continents relattve to the equator and Centrafricaine à 1/1 O00 000. Notice explicative to the tropics. no 100, ORSTOM. Paris, 126 p. Brückner. W. D. 1957. Laterite and bauxite profiles of Acknowledgements - This study was supported West Africa as an index of rhytmical climatic variations by the "Action Thématique Programm& (ATP) in the tropical belt. Eclogue geoL Helu.. 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