Clay illuviation and mechanical clay infiltration — Is there a difference?

Part IV -- Short Papers

66 CLAY ILLUVIATION --

AND MECHANICAL CLAY INFILTRATION IS T H E R E A D I F F E R E N C E ?

P. B u u r m a n , * A . G . Jongmans* and M.D. PiPujolt *Department of Soil Science and Geology, Agricultural University, 6700 AA Wageningen, The Netherlands tDepartment of Geology, Institute of Earth Sciences, 3508 TA Utrecht, The Netherlands

Soil scientists attribute the presence of coatings of oriented clay in soils and sediments to the process of clay illuviation, which is a process that occurs at the Earth's surface and may penetrate to a depth of several metres. Geologists attribute coatings of oriented clay in coarse sediments to a process of mechanical infiltration of muddy river water or muddy overland flow. Clay illuviation is a climate-dependent process (seasonal climate) which is restricted to the vadose zone. The source of the translocated material is internal, i.e., material from upper soil layers is transported to deeper layers. The resulting accumulation horizons are up to 2 m thick and continuous in clayey deposits, but in sandy deposits they are discontinuous and may reach a depth of several metres. The process of mechanical infiltration is not climate-related, and its effect is not restricted to the vadose zone but may reach far below the ground-water table. The source of the transported material is external: it is provided by muddy water entering the soil. The accumulations are supposed to be deep and continuous. The environment of muddy waters, with higher electrolyte concentrations than water that percolates the soil upon first rain, and a gradient of particles coarser than clay, should, however, preclude mechanical infiltration because: (lj clays will tend to flocculate; (2) both coarser suspended material and flocculated clay will tend to clog the pore system; and (3) below-ground-water transport of suspended material will be virtually impossible because of low percolation velocity. In addition, flocculated clay will not produce oriented coatings upon desiccation. The model of mechanical infiltration is not supported by theory or observations. We suggest that the process of mechanical infiltration of clay needs further study to prove whether it is different from clay illuviation in soils. The distinction between the two is of utmost importance for the interpretation of paleosols. © 1998 INQUA/Elsevier Science Ltd. All rights reserved.

INTRODUCTION Paleosols are mainly studied by two widely different groups of scientists: sedimentologists and pedologists. Younger paleosols and those that are found near to the present surface are usually studied by soil scientists, while sedimentologists have a virtual monopoly on paleosols that are found in thick sedimentary sequences, such as oil- or gas-bearing deposits. The two groups of scientists have a different terminology. Sedimentologists sometimes borrow terms from soil science, but pedologists, however, do not usually borrow terms from sedimentology. Borrowing each other's terms has a distinct advantage of increased mutual understanding. However, sedimentologists sometimes borrow pedologists' terms without properly understanding the features and the environments for which such terms are used. In addition, they create new terms for phenomena that may or may not have a relation to soil forming processes. The terms for clay redistributions in sedimentary deposits are an example of such terms. It is not clear whether the clay redistribution processes that are described by sedimentologists are the same as those agreed upon by pedologists, whether there are phenomena of clay redistribution in sediments that do not occur in soils, and whethe]" the phenomena of clay redistribution as we know them for soils are significantly changed upon burial.

CLAY TRANSLOCATION AND NEW FORMATION IN SOILS Soil scientists recognize translocation of clay and other fine particles in soils. Such translocations occur in all soils where clay can disperse in the topsoil and resediment in a deeper layer. For dispersion, special

circumstances are required: a moderate pH (4.5-6.5), low cation activity (unsaturated soils), or high Na activity. Sudden wetting of dry soils favors dispersion. In recent soils, clay transport is usually a downward movement, over up to several meters depth, perpendicular to the soil surface. In landscapes with a strong lateral groundwater movement, the clay transport may be oblique. The resedimented clay usually orients itself parallel to the surface on which it is sedimented. This results in strongly oriented, microlaminated, birefringent coatings, especially if the clays are of 2:1 type (illites, smectites, vermiculites, chlorites). In soils that have easily dispersable topsoils and sufficiently large pores, fine silt and organic matter may move together with the clay fraction. As a result, there is a considerable variety of accumulations, each with its own composition, texture, and orientation pattern. Mobilization of clays in the phreatic zone is unknown from recent soils. New formation of clays is common to many subaerial environments. Such new formation does not usually lead to coatings, unless it is restricted to larger pores. New formation of smectite and palygorskite are well known from aridic, magnesium-rich environments, but such new formations do not form birefringent coatings. Birefringent clays do form by hydrothermal alteration of primary mineral grains, such as hypersthenes, amphiboles, and feldspars, but these neogenic clays are usually found as clay frameworks in primary grains or as clay pseudomorphs. They do not form coatings in the sediment. Birefringent coatings may also form by crystallization of allophane coatings in environments that are not perhumid (Buurman and Jongmans, 1987). Jongmans et al. (1993) found such coatings filling intergranular voids between sand grains in Quaternary alluvial deposits. They may be associated with clay illuviation coatings and are then difficult to tell apart.

Part IV - - Short Papers In sandy materials, translocated clay occurs as coatings on sand grains and pebbles, and as bridges between grains, and may completely fill intergranular space. In loamy or clayey soils, translocated clays form coatings on vertical and horizontal pores and planes, especially at the bottom of pores. They may completely fill even larger pores. In sandy materials, clay accumulation zones are not usually continuous, but are concentrated in thin bands of up to 10 cm thickness, separated by zones without illuviated clay (see e.g. Van Reeuwijk and De Villiers, 1985; Miedema, 1987). Optically, features of clay translocation are easily distinguished from those of clay orientation by pressure. Pressure orientation in sandy sediments leads to clay 'rinds' that have a strong orientation directly adjacent to the sand grain, but orientation decreases with increasing distance to the grain surface (Miedema, 1987). In illuviation coatings, orientation is more or less homogeneous throughout the coating (see Bullock et al., 1985).

CLAY REDISTRIBUTION IN SEDIMENTS Apart from features of clay redistribution that are ascribed to soil formation, some sedimentologists distinguish redistributions that they do not ascribe to soil formation. The fact that such clay redistributions are not ascribed to soil formation is, according to the present authors, mainly due to the paper by Walker (1976), which describes the formation of clay coatings in sediments without any reference to literature on soil formation, and consequently without any understanding of clay translocation in modern soils. In recent papers, Moraes and De Ros (1990, 1992; Jurassic, fluvial), and Dunn (1992; Cretaceous, fluvial-volcanogenic) distinguish the following interstitial clays in sandstones: (1) depositional clays; (2) authigenic clays; and (3) mechanically infiltrated clays. The depositional and authigenic clays pose no specific problem in the present discussion. The 'mechanically infiltrated' clays occur in the sediments as grain coatings, bridges, and pore-infillings that are completely similar to those observed in the illuvial horizons of modern sandy soils with clay transport. Therefore, the question arises whether the mechanically infiltrated clays' identified by these authors are the same as the illuviated clays that are recognized by soil scientists. Moraes and De Ros (1990, 1992), following Walker (1976), recognize three different mechanisms/conditions of accumulation of 'mechanically infiltrated' clays: (1) accumulation in the vadose zone; (2) accumulation near the phreatic level; and (3) concentration above impermeable barriers. (1) Accumulation in the vadose zone. The mechanism proposed for this process is virtually identical to that for clay illuviation: penetration of clay with seepage water, and sedimentation of the clay where the seepage stops. Moraes and De Ros (1990)

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mention that 'reworking of upper layers in alluvial settings eliminates most of these types of accumulation'. This suggests that these authors noted the absence of clay cutans in the upper part of their 'profiles', but did not stop to consider the upper layers as a possible source of illuviated clay in the subsoil. (2) and (3) Accumulation near the phreatic level and concentration above impermeable barriers. Moraes and De Ros suggest that clay may accumulate at the phreatic level because percolation velocity of seepage water decreases at this level. It is very unlikely, however, that mere accumulation under the permanent groundwater level, without periodic desiccation, would lead to coatings of paralleloriented clay. Walker (1976) states that 'fluctuation of the phreatic level increases the thickness of the infiltration zone, which can locally reach tens of meters'. This remark suggests that drying-out is an effective agent in the placement of the clays. Concentration of dispersed material above an impermeable contact is a phenomenon frequently observed in soils. In contrast to the suggestions of Moraes and De Ros (1990, 1992) and Walker (1976), such contacts are not necessarily those of highly permeable material overlying materials with much lower pore space. Concentration of dispersed material, be it clay, silt, or organic matter, occurs just as frequently at contacts where fine material overlies coarse. Also at such contacts, the water flow is effectively halted and can only proceed after a certain pressure is built up. The dispersed material usually stays behind at the contact and further enhances the textural contrast and the stagnation of seepage water. Dispersed particles that pass deep into the soil are effectively stopped by both finer and coarser layers, frequently leading to a second horizon of illuviated clay (e.g. the beta horizons of loess-over-calcareous gravel profiles; Bartelli and Odell, 1960; Ducloux, 1970). Neither the accumulation close to the ground-water table nor the stagnation at sedimentary contacts is at variance with the pedogenic process of clay illuviation as described for recent soils. The difference between Walker's (1976) opinion and the common theories in soil genesis is that in Walker's opinion, all translocated clay is due to infiltration of muddy surface run-off, while soil scientists generally agree that most of the translocated clay is mobilized at the soil surface and transported downwards without significant overland movement.

MOBILIZATION AND TRANSPORT OF MECHANICALLY INFILTRATED CLAYS Clays that 'mechanically infiltrate' into coarse sediments are supposed to originate from surface run-off or muddy river waters ("deep penetration of muddy waters"). It is here that 'mechanical infiltration' appears

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Part IV - Short Papers

to deviate from pedogenic transfer of dispersed clay. Is it possible for muddy water river water to infiltrate into coarse-grained bottom sediments? To judge this possibility, we have to consider two factors that are of utmost importance to this transport of clay: (1) the state of clay in water; and (2) the nature of the contact between coarse (bottom) sediments and 'muddy' water. Matlack et al. (1989) investigated the infiltration of fine material into sieved sand of specific grain-size classes. In their experiments with pure clay mineral suspensions, these authors found that: (1) coatings on and bridges between grains do form when clay suspensions are percolated through sieved sand; (2) a significant amount of suspended material forms a filter cake on top of the sand columns; and (3) chlorite was much more effective in forming coatings and bridges than illite or smectite; the latter formed virtually no coatings. An experiment with Missouri River water also resulted in significant infiltration of clay and formation of coatings, but filter cakes formed at the top of each column within one hour of the initiation of the experiment. This terminated flow. Although the experiments of Matlack et al. (1989) do show that clay may infiltrate into sand columns, the difference from natural circumstances induces some doubt as to the applicability of such experiments. The experiments with pure clay minerals were carried out with peptised clays. This means that the clay platelets have a minimum tendency to flocculate. This explains the different behavior of chlorite (low surface charge), illite (intermediate surface charge) and smectite (high surface charge). The higher charge of the particles, the smaller the chance that the attraction due to Van der Waals forces exceeds the repulsion of particles due to double layer charge. Therefore, chlorite does form coatings in the sand columns, and smectite does not. In water flowing through the vadose zone of a soil, clays may be peptised, but this is not usually the case in river water. In most waters clays are dispersed at high stream velocities, but they flocculate when velocity decreases. Flocculated clays contain a high amount of water and may remain suspended, but they will tend to settle as transport velocity decreases. Especially in river systems of arid regions, that tend to have a sufficiently high cation concentration due to the presence of calcium carbonate (and frequently also of gypsum and more soluble salts), circumstances for flocculation of clay are ideal. This implies that the sediment load will fall out as soon as the water transport slows down. In the columns used by Matlack et al. (1989), free gravity flow induced a fairly high percolation speed of the suspensions. This is a situation very different from that under a coarse-grained river bed. The latter will normally be water-saturated, and the flat river landscape guarantees a very slow groundwater flow. This implies that the suspended load will most probably flocculate and clog the upper pores of the sediment, thus minimizing penetration of suspended matter. Another adverse factor is the nature of the contact between 'muddy water' and the coarse-grained bottom

sediment. At most sites, this contact will probably consist of a thin fining-upwards sequence. This finingupwards hampers penetration of water, and increases the effect of flocculation. In addition, because much of the sediment load, especially the silt-size particles, will tend to get stuck in the upper pores of the bottom sediment, penetration is much reduced and deep percolation of large amounts of fine material is unlikely. The sum of the above arguments is that, under natural circumstances, penetration of suspended matter from river water into a coarse-grained bottom sediment is highly unlikely. Matlack et al. (1989) did their scanning electron microscope studies after drying the samples. This means that the observed parallel orientation in the coatings may have been the effect of drying and that it did not necessarily exist previous to the drying. In addition, the authors did not study thin sections of their coatings and bridges. It is therefore difficult to compare the coatings created in their experiments with birefringent clay coatings that are found in soils, even though the electron micrographs do show orientation parallel to grain surfaces. Because circumstances for dispersion of clays below the groundwater level are extremely unfavorable, it is unlikely that clays could be mobilized here, except by high pressure. Pressure, however, does not lead to dominant orientation of clay parallel to grain surfaces.

VERTICAL CONTINUITY AND THICKNESS OF ZONES OF 'MECHANICALLY INFILTRATED' CLAYS What then, is the cause of deep, massive zones of clay coatings in coarse-grained sediments? The supposed vertical continuity of zones with 'mechanically infiltrated' clays cannot be explained by soil forming processes. In recent sandy soils, horizons of clay illuviation consist of thin bands up to 10 cm thick, spaced by unaffected sand. The illuviation may be found as deep as 10 m below the soil surface. Continuous clay illuviation horizons tens of meters thick are unlikely in soils for three major reasons: (1) in most soils seepage water does not penetrate this far; (2) if the seepage water penetrates this far, the climate is probably perhumid and clay illuviation is uncommon; and (3) sandy soils do not contain sufficient clay to form such horizons. Certainly, deep infiltration of clay-bearing seepage water is incompatible with a (semi)arid environment. The question arises whether the accumulation zones of 'mechanically infiltrated' clays are indeed thick and continuous. Scattered samples in very thick sedimentary columns may give the impression that illuviation is universally present, while it is in fact restricted to thin bands, separated by zones without clay coatings. This question can only be solved by more detailed studies of cores and search for a succession of individual soils and soil horizons. These are very likely to exist in fluvial systems.

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