Basic research on laterites in tropical countries

Part IV -- Short Papers We believe that detailed study of sedimentary columns is necessary before any system of 'mechanical infiltration' other than the clay illuviation process that occurs in recent soils is considered.

CONCLUSIONS We think that the existence of a process of mechanical infiltration of clays that is not related to soil formation has not been proven. We suggest that sedimentologists and pedologists join in an effort to try to solve this question, because it is an important issue in the interpretation of fossil environments.

REFERENCES Bartelli, L.J. and Odell, R.T. (1960). Field studies of clay-enriched horizons in the lowest part of the solum of some Brunizem and Gray-Brown Podzolic soils in Illinois. Soil Science Society of America Proceedings, 24, 388-390. Bullock, P., Fedoroff, N., Jongerius, A., Stoops, G., Tursina, T. and Babel, B. (1985). Handbook For Soil Thin Section Description. Waine Research Publications, Wolverhampton, U.K. Buurman, P., Jongmans, A.G. (1987). Amorphous clay coatings in a lowland oxisol and other andesitic soils of West Java, Indonesia. Pemberitaan Penelitian Tanah dan Pupk, 7, 31-40. Ducloux, J. (1970). L'horizon bSta des sols lessivds sur substratum calcaire de la plaine poitevine. Bulletin de l'Association Fran¢aise pour l'Eude du Sol, 3, 15-25.

BASIC RESEARCH

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Dunn, T.L. (1992). Infiltrated materials in Cretaceous volcanogenic sandstones, San Jorge Basin, Argentina. In: Origin, Diagenesis, and Petrophysics of Clay Minerals in Sandstones. Society for Sedimentary Geology, SEPM 47, 159-174. Jongmans, A.G., Van Oort, F., Buurman, P. and Jaunet, A.M. (1993). Micromorphology and submicroscopy of isotropic A1/Si coatings in a Quaternary AUier terrace (France). In: Ringrose-Voase A.R. and Humphrys, G. (eds), Proceedings of the 9th International Working Meeting on Soil Micromorphol'ogy. 1992, TownsviUe, Australia. Matlack, K.S., Houseknecht D.W. and Applin, K.R. (1989). Emplacement of clay into sand by infiltration. Journal of Sedimentary Petrology, 59, 77-87. Miedema, R. (1987). Soil formation; microstructure and physical behavior of Late Weichselian and Holocene Rhine deposits in the Netherlands. Ph.D. Thesis, University of Wageningen, 339 pp. Molenaar, N. (1986). The Interrelation between clay infiltration, quartz cementation and compaction in Lower Givetian terrestrial sandstones, Northern Ardennes, Belgium. Journal of Sedimentary Petrology, 56, 359-369. Moraes, M.A.S. and De Ros, LF. (1990). Infiltrated clays in fluvial Jurassic sandstones of Rec6ncavo Basin, northeastern Brazil. Journal of Sedimentary Petrology, 60, 809-819. Moraes, M.A.S. and De Ros, L.F. (1992). Depositional, infiltrated and authigenic clays in fluvial sandstones of the Jurassic Sergi formation, Rec6ncavo Basin, northeastern Brazil. In: Origin, Diagenesis and Petrophysics of Clay Minerals in Sandstone. Society for Sedimentary Geology, SEPM 47, 197-208. Van Reeuwijk, L.P. and De Villiers, B.M. (1985). The origin of textural lamellae in Quaternary coast sands of Natal. South African Journal of Plant and Soil, 2, 38-44. Walker, T.R. (1976). Diagenetic origin of continental red beds. In: Falke, H. (ed.), The Continental Permian in Central, West, and South Europe. Dordrecht, D. Reidel. Publ. Co., pp. 240-282.

ON LATERITES IN TROPICAL

COUNTRIES

P.K. Banerjee

School of Oceanography, Jadavpur University, Calcutta, India In spite of extensive occurrence of laterites in the tropical belt of the continents and descriptive-exploratory efforts spanning more than a century, basic questions on their genesis and systematics are still unresolved. Major areas of ignorance include: delineation and classification of biotic versus abiotic processes and textures, precise chronology of onset of lateritization, identification of diagnostic signals of active lateritization and systematics of minor and trace element dispersion in laterites. Both well-focused research and development programs using state-of-the-art technology and concepts, as well as assured funding in the medium to long term, are required if the past cycles of sub-critical efforts in laterite research are to be broken. © 1998 INQUA/Elsevier Science Ltd. All rights reserved.

INTRODUCTION Laterite was recognized as a product of surficial weathering over different rock types in the Tropical Climatic Belt of Asia, Africa, Australia, South America, etc. nearly a century ago. With the growth of commercial interest in this weathering product for economic mineral deposits and awareness of agronomic problems, we have witnessed a semantic anarchy between the geologists and the pedologists, and between the Anglophile and the Francophile schools of scientists. A plethora of terms - - laterite, ferricrete, duricrust, regolith, plinthite, petroplinthite - - are strewn around for the same or similar materials! For example, what we call latosol in India would qualify for the term laterite among Brazilian geologists. In the course of UNESCO-

IGCP Project No. 129 on Lateritization Processes (1978-83), we attempted to resolve this semantic confusion as a first step towards defining common programs of research. Aleva (1986) and Schellmann (1986) summarized these efforts in the form of consensus definitions and classifications of laterite. The project was, however, terminated before many fundamental scientific problems, related to these paleopedological products, could be studied rigorously and systematically. Since then, nothing scientifically exciting has come about - - partly because of inadequate research facilities available in the lateritic terrains of many Asian, African, and South American countries, coupled with a curious obsession among third world geologists to take up fashionable and high brow research, and partly because the mining

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Part IV -- Short Papers

geologists from Europe, North America, etc. who come into contact with laterites are essentially satisfied as long as the commercial interests are served. From aluminum to precious metals, the history of research into laterites by the mining companies has been a saga of empirical exercises with driblets of research and development. In consequence, many fundamental questions on the genesis of laterites remain in the realm of speculation.

SOME MAJOR UNRESOLVED PROBLEMS

Delineation and classification of biotic textures Among the various textures of laterite, listed by Aleva (1986), one is uncertain what individual textures are fingerprints of biotic versus abiotic processes. For example, Chukrov (1981) had reported 'hematitic disc shaped small bodies rather similar to the relic discs of iron bacteria Galionella' viewed under an electron microscope, and Machado (1983) had argued that the tubulo-alveolar cavities in laterites and the associated pisoliths are biotic products. Heydeman et al. (1983) had successfully isolated micro-organisms from laterites of Australia, India and Africa and demonstrated significant leaching of alumina from saprolite by one of the bacterial isolates - - a very important find whose quantitative role on lateritization is yet to be evaluated. Similarly, Schwartzman and Volk (1989) demonstrated biotic enhancement of weathering processes. Given the suggestive indications of biotic forcing on lateritization, it is necessary to define: (a) whether typical textures of biotic versus abiotic transformations are distinguishable under the electron microscope; and (b) the distinction between biotic and abiotic processes in the end products at different facies levels of the laterite catena. Target areas for long-term intensive studies on this subject, where complete profiles from unaltered bedrock to the top iron crust are exposed and are easily accessible by modern means of transport, include: (a) Laterite hill 0.56 near Mangalore, India, described by Banerjee (1990) [Basement: granite gneiss]. (b) Bauxite mines in hill top Pocos de Caldas, Brazil [Basement: alkaline igneous rocks]. (c) Bauxite profile, Ivory Coast, described by Boulange (1984) [Basement: two mica granite]. (d) Laterite profile in the deep railway cutting at Jarrahdale near Perth, Australia [Basement: acid and basic igneous rocks].

Isotope dating of onset of lateritization Laterites occur almost everywhere on uplifted planation surfaces at different elevations. Some morphological indications suggest that these surfaces might range in age from Cretaceous to Quaternary, and that lateritization on these surfaces might have started pari passu or kept pace with uplift. This is however yet to be

proved. Conventional dating tools e.g. radioisotopes, thermoluminiscence, etc. have not been successful due to the open system behavior of the laterite catena and the inherent limitations of analyzing a polycyclic profile, which evolved through many phases of warm and humid versus less warm but arid climate. Erosion from the upper parts and growth at the lower parts of catenas would have proceeded at varying rates during the lowering of the landscape. At many places, it has been demonstrated in consequence of this variable rate of net accumulation (growth minus erosion), that all parts of a continuous laterite sheet do not necessarily have the same age of formation. Polygenetic laterite profiles record repeated episodes of progressive and regressive pedogenesis (Johnson et al., 1990). Irrespective of whether the laterite is pedogenetic or ground water induced, the development of the residuum would have involved both lateral and vertical transport of material by variable combinations of mechanical and chemical processes. During the onset of lateritization of the uplifted planation surface, cosmogenic radionuclides impinging on the surface, like 3He, 1°Be, etc. might remain dispersed in the upper few cms; but unless the top layer is hardened quickly, such cosmogenic isotopes, housed in the top layers, would rarely have been preserved in the early fragments. Some of these might accumulate as talus/colluvium in paleo-depressions of the planation surface during the next climatic reversal, although in a majority of occurrences, such signals might have been lost in the course of prolonged transport. It is therefore recognized that the probability of dating the onset of lateritization by cosmogenic radioisotopes is very low; but given the geomorphologic stability of parts of the laterite, an extensive search for long life cosmogenic radionuclides in such fragments of old laterite, which potentially lie embedded in some profiles, might just succeed. With the recent advances in cosmogenic isotope systematics and instrumental methods using minute quantities of sample, this is a field worthy of attention. In India alone, possible target areas for such a program are the Kansas laterite in the Sukinda ultramafic region, and the Panchpatmali bauxite mine in Koraput, Orissa.

Dignostics of climatic and landscape zones The precise roles of biotic and abiotic forcing in areas with seasonal rainfall and moderately warm climate in the formation of laterites, including the underlying saprolites, which are generally depleted in elements like Au but enriched in other elements like Ni, are still not clear. Without questioning the validity of the general observation that laterites are most abundant in humid and warm climatic belt of the tropics, one can state that it has not been possible to determine precise routes of alteration from the parent rock to the duricrust end product in a framework that follows laws of inorganic geochemistry. In each region, the transformation sequence apparently follows different routes!

Part IV -- Short Papers On the basis of micro-textural analysis of a number of laterite regions in India, Banerjee (1990) concluded that the duricrust carries indications of alternating humid and arid cycles, possibly corresponding to the Pleistocene climatic perturbations. Gonzalez et al. (1991) recorded paleoclimatic fluctuations during the Quaternary from the ferricretes of Cuba and Cameroon. In order to introduce further refinements, we have to apply potential climate indicators like oxygen isotope ratios in the clays and iron oxides of a laterite catena. But for this one needs hard data on (a) modern precipitation and meteoric systems in tropical areas; and (b) demonstrable high-resolution correlation between climatic parameters and active lateritization. We do not have the systematics of stable isotope behavior at many rainfall stations within the tropical belt, so the application of oxygen isotope ratios for paleoclimatic reconstruction will be premature at this time. It is also a curious fact that no clear descriptions have been published about areas where lateritization has been proved to be active. From India, South Africa, Gabon, Guinea, Burkina Fasso to Cameroon, the active present day surface process is one of breakdown of the duricrust and podsolization of the top layers. In fact, in a recent workshop, Zeegers (in Smith et al., 1991) emphasized that 'laterite profiles are generally undergoing degradation, not formation' in humid, tropical climates and rainforest terrains. In contrast, some (e.g. Wilhelm and Essono Biyogo, 1992) believe that 'duricrust layers are placed in a situation of thermodynamic instability by the advent of an equatorial climate' (my emphasis) where it alternates with a tropical climate, such as in south central Gabon, Equatorial Africa. The underlying unity in the transformation milieu between the near surface duricrust layer where podsolization may be in progress and the lowest part of the weathered zone, where the removal of alkali and alkaline earth elements as a part of lateritization might be active, is still to be related in a coherent and general framework of dynamic pedogenesis. In the absence of such an integrated pedogenetic model, wrong conclusions and speculations are flooding the literature on laterites; and instead of developing a paradigm, laterite studies are wallowing in contradictory empiricisms. It is therefore urgent that experiments on time frames of at least a decade are initiated for collection of data on mineralogical, micro-organic, chemical and physical parameters for measuring changes induced by biotic and inorganic forcings in tropical and equatorial forested areas. For critical conclusions on paleopedology, such data are essential. Possible target areas for such long term observations are: (a) the Amazon rainforest; (b) in Guinea, West Africa; (c) at Hill 0.56 near Mangalore, India. Systematics of minor and trace element dispersion

The behavior of minor and trace elements within a laterite catena does not appear to follow any general

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rule; the dispersion patterns are erratic even on the scale of hand specimens (Banerjee, 1990). Some have built up elaborate calculations of mass transfer on the questionable assumption of immobility of elements like Ti of Zr (Nesbitt and Young, 1989) and a uniform dispersion in the parent rock! Others have assumed that the structured portion of the saprolite underlying a duricrust had not changed in volume during the formation of the whole catena (Gardner et al., 1978). Fallacies abound in publications. The situation will not improve as long as the systematics of the controlling factors are not clearly formulated and respected. Until then the subject of solution and colloid geochemistry of minor and trace elements in the course of transformation of rock into a laterite catena is open for wild speculations. To illustrate from a recent example, Au was found to concentrate in pisolites at one field (Le Comte, 1988) and in the matrix at another (Wilhelm and Essono Biyogo, 1992). One has just to pull up the appropriate reference in support of the assumed processes. There will be no end to such surprises in our understanding of laterites! What McFarlane (1986) called the familial progression of weathering products in space-time continuum is largely shrouded in ignorance.

CONCLUSION In most of the Third World countries, the lack of funds, state-of-the-art instruments and trained manpower is not going to disappear in the foreseeable future. The mysteries of laterites will continue to be exploited, but not understood for the foreseeable future. Old opinions and myths still continue to be reworked and churned out for publications, seminars and general public approval. To address this problem, long term bilateral agreements between scientists of advanced countries and those of the Third World are needed for establishing research stations in key regions for state-of-the-art research on laterite genesis and related natural resources. A rigorous, more holistic approach is needed for the study of laterites.

ACKNOWLEDGEMENTS I am indebted to the Director General of the CSIR of India for continued funding for my studies on laterites.

REFERENCES Aleva, G.J.J. (1986). Memoir, Geological Survey of India, 120, 8-26. Banerjee, P.K. (1990). Indian Minerals, 44(4), 243-286. Boulange, B. (1984). Tray. documents de L'Orstom, 175, 341 pp. Chukrov, F.V. (1981). Proceedinos of the 1st International Seminar on Lateritization Processes, Trivandrum, India, pp. 11-14. Gardner, L.R., Kheoruenromne, T. and Chen, H.S. (1978). Geochim. Cosmochim Acta, 42, 417-424. Gonzales, J.E., Onguene, M. and Federoff, N. (1991). XIII INQUA Conoress Symposium VII-3, p. 115. International Union for Quaternary Research, Beijing.

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Johnson, D.L., Keller, E.A. and Rockwell, T.K. (1990). Quaternary Research, 33, 306-319. Heydeman, M.T., Button, A.M. and Williams, H.D. (1983). Proceedings of the 2nd International Seminar on Lateritization Processes, Sao Paolo, Brazil, pp. 225-236. Le Comte, P. (1988). Journal of Geochemistry Exploration, 30, 35-61. Machando, A. de Barros (1983). Proceedings of the 2nd International Seminar on Lateritization Processes, Sao Paolo, Brazil, pp. 251-256.

McFarlane, M.J. (1986). Memoir, Geological Survey of India, 120, 29 40. Nesbitt, H.W. and Young, Gm.M. (1989). Journal of Geology, 97(2), 129- 147. Schellmann, W. (1986). Memoir Geological Survey oflndia, 120, pp. 1-7. Schwartzman, D.W. and Volk, T. (1989). Nature, 340, 457-460. Smith, R.E., Zeegers, H. and Oliveira, S.M.B. (1991). Journal of Geochemical Exploration, 41,233-244. Wilhelm, E. and Essono Biyogo, J.P. (1992). Journal of Geochemical Exploration, 43, 167 186.

S U B S O L U M W E A T H E R I N G P R O F I L E CHARACTERISTICS AS I N D I C A T O R S O F THE RELATIVE RANK O F S T R A T I G R A P H I C BREAKS IN TILL S E Q U E N C E S E. Arthur Bettis, III Iowa Department qf Natural Resources - - Geological Survey Bureau, Iowa City, IA 52242, U.S.A. INTRODUCTION Reconstructing paleoenvironmental conditions and the duration of weathering associated with stratigraphic breaks in stacked sequences of glacial tills is not an easy task. Traditional interpretations derived from examination of buried soil profiles at the stratigraphic breaks are fraught with problems related to burial diagenesis, pedogenic (and ecological) overprinting, and glacial erosion and deformation. A more trustworthy approach in interpreting and comparing stratigraphic breaks in till sequences is afforded by the examination of subsolum weathering profiles.

WEATHERING ZONE TERMINOLOGY Weathering profiles, like soil profiles, consist of logical sequences of zones (horizons) containing secondary characteristics produced by physical and chemical alterations related to a land surface. Hallberg et al. (1978) present an easy to use terminology for describing the interpreted oxidation state and distribution of free-iron oxides (as inferred from color), matrix carbonate status, and presence or absence of secondary fractures in Quaternary deposits of the Mideontinent, U.S.A. The following is a brief summary of their weathering zone terminology applicable to glacial diamictons (see Hallberg et al., 1978 for the complete discussion): First symbol: color reference O oxidized R reduced U unoxidized Second symbol: leached or unleached state U unleached L leached; no carbonates detectable (with dilute HC1) L2 leached; primary carbonates absent, secondary carbonates present U2 matrix carbonates unleached, secondary carbonates present

Modifier symbols: M mottled; zones containing 20-50% contrasting mottles; when used with unoxidized zone designation it infers 20% or less mottles of reduced colors; precedes first symbol when used. J jointed; describes the presence of well defined vertical joints; joints often show oxidized and reduced colors and often have coatings or rinds of secondary iron-oxides and occasionally other secondary minerals such as calcite or gypsum; second symbol when used.

Examples of descriptions using weathering zone terminology: OL oxidized, leached yellowish-brown (10YR5/6) or strong brown (7.5YR5/6) matrix color; leached. MOU mottled, oxidized, and unleached yellowish brown (10YR5/4) matrix color with strong brown (7.5YR5/8) and grayish brown (2.5Y5/2) mottles, unleached. RJU reduced, jointed, and unleached mixed olive (5Y4/4 and 5Y4/3) and very dark grayish brown (2.5Y3/2) matrix color with common gray (5Y5/1) and light olive brown 2.5Y5/4) mottles; prominent vertical joints with 1-cm thick strong brown (7.5YR5/8) segregations along joints; unleached. U U unoxidized, unleached --- uniform dark gray (2.5Y4/0); unleached. With slight modifications to adjust for regional variations in primary matrix color and carbonate status, this terminology is applicable to most Quaternary continental glacial sequences.

INTERPRETATION OF STRATIGRAPHIC BREAKS Weathering profiles have predictable vertical sequences progressing downward from a land surface.