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PALEOSOLS AND WIND-BLOWN SEDIMENTS | Weathering Profiles
Weathering Profiles E A Bettis III, University of Iowa, Iowa City, IA, USA ã 2013 Elsevier B.V. All rights reserved. This article is reproduced from the previous edition, volume 3, pp. 2114–2125, ã 2007, Elsevier B.V.
Introduction Weathering is a process of conversion of rock-forming minerals toward an equilibrium state. Rocks and sediments at or near the Earth’s surface are subjected to physical, chemical, and biological environments that differ from those present where constituent minerals formed, and by various processes they change toward equilibrium with Earth-surface conditions (Taylor and Eggleton, 2001). Weathering processes are grouped into two broad categories: mechanical (physical) weathering and chemical weathering. The former involves changes that do not directly result in chemical or mineralogical changes to the affected geological materials, whereas the latter result in a variety of mineralogical and chemical changes to the original materials. Mechanical and chemical weathering can and often do occur together, and both can be mediated by biota. These changes to rocks and sediments are organized into weathering profiles that reflect the interplay of weathering, erosion, and redistribution of material at and below the land surface over time. Alterations of geologic materials that take place in weathering profiles produce physical, chemical, and hydrologic characteristics that influence urban and agricultural land use, water resources, the occurrence of some economically important deposits, and landscape evolution. Weathering profiles range from centimeters to tens of meters in thickness. They begin developing upon exposure at the land surface and may form over tens of millions of years in some stable continental interiors. Fossil-weathering profiles occur throughout the geologic record, becoming more common with the spread of land plants in the Carboniferous. Because weathering profiles form from the land surface, some of their properties may reflect climatic conditions during their formation and thus provide information useful for paleoenvironmental reconstructions. Weathering profiles are the ‘roots’ of surface soils and reflect the cumulative effects of near-surface alterations of geologic materials over extended periods of time. The literature on weathering profiles is replete with terms for weathered geologic material. The most all-encompassing term is regolith – the mantle of fragmented and usually unlithified geologic material that overlies bedrock and that includes residual and transported materials. Some definitions include soil as the upper zone of the regolith, whereas engineering geologists often call all unconsolidated material above bedrock ‘soil.’ The regolith consists of several distinct materials that may be intimately related (Ollier and Pain, 1996): 1. Weathered rock. Weathered rock in place is saprolite, a soft, earthy, typically clay-rich, thoroughly decomposed rock formed in place by chemical weathering of igneous, metamorphic, or sedimentary rocks (Figure 1). Structures present in the original rock, such as veins and fractures, are preserved in saprolite.
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2. Residuum. Material originally formed by weathering of rock that has subsequently moved a short distance, either by creep or through removal of finer weathering products. 3. Transported sediments and chemical deposits. Materials transported by various agencies from a place of origin, where they were weathered to become available for transport, to a place where they have been deposited. 4. Soil. Physical, chemical, and biological alterations of geologic materials that occur immediately below a land surface and that grade downward into a weathering profile. Weathering profiles are formed in regolith. They may be defined as the vertical extent of a weathered sequence from the initiating land surface or the originating surface to the unweathered parent rock. Weathering profiles form by chemical and mechanical weathering and vary considerably on several spatial scales because of variations in materials they are formed in, topography, hydrology, surficial processes, and climate.
Weathering Processes Both mechanical and chemical weathering processes act to form weathering profiles. The boundary between the weathered and unweathered material in a weathering profile is called the weathering front (Mabbutt, 1961). The weathering front marks important differences in permeability, porosity, strength, and behavior, and it is the locus of initial changes in the unweathered geologic material that begin the weathering process. Advance of the weathering front occurs along and is dependent on the presence of fractures that allow the penetration of meteoric fluids into the unweathered material (Figure 2). Thus, rock types, tectonic and climatic settings, and landscape conditions that favor the formation of extensive interconnected fracture networks facilitate the development of weathering profiles (Figure 3). Some of the more important mechanical weathering processes include expansion from release of confining pressure (unloading), exfoliation, frost shattering, pressure induced by root growth, and pressure resulting from the growth of salt crystals. Water enters unweathered material on the macroscale by penetration along interconnected pores, primarily fractures in rocks, lithified sediments, and unlithified fine-grained sediments. Also of great importance is the presence of intra- and transgranular microcracks formed by a variety of forces, including crystallization, tectonics, and unloading. The microcrack systems are the loci of water–mineral interactions that are the initial stages of chemical weathering (Bisdom, 1967). The effects of rock or sediment texture on penetration of the weathering front are fundamental (Thomas, 1974). Disaggregation of coarse-grained rocks such as granite into constituent
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Figure 1 Clay-rich saprolite formed by in-place chemical weathering of biotite granite–gneiss. Protolith texture and structures are preserved in core stones such as the one in the lower part of this profile. Photograph by M.J. Pavich.
Fractures
Infiltrating water
Water penetration accelerated; increase in weathering surface area
Hydraulic gradient
Chemical disequilibrium maintained
Rapid mineral weathering
Advance of weathering front
Figure 3 Flowchart showing the primary factors acting to advance the weathering front in geologic materials.
Figure 2 Oxidation, marked by brown and reddish brown colors, advances into gray, unweathered middle Pleistocene glacial diamicton along subvertical fractures. The fractures are avenues along which meteoric water and the solutes it carries penetrate the deposit, thereby advancing the weathering front. Photograph by E.A. Bettis.
mineral grains by oxidation of biotite and hydration of other minerals often takes place before much fundamental chemical change has occurred. Fine-grained rocks of similar mineral composition weather more slowly, most likely because a system of microcracks and larger, interconnected fractures is usually not as extensively developed in these rocks. In arid, polar, and some coastal environments, crystallization from supersaturated solutions of adventitious salts (salts derived from sea spray or saline water bodies) can produce expansive stresses
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PALEOSOLS AND WIND-BLOWN SEDIMENTS | Weathering Profiles
leading to granular disintegration or scaling. The various physical and chemical processes acting at the weathering front act to make this a zone of relatively high hydraulic conductivity in a weathering profile. This promotes downward progression of the weathering front by transporting weathering products away from the loci of weathering (Figure 3). In the early stages of weathering, hydration (the addition of water to a mineral) produces an increase in volume. As weathering progresses, this gives way to a volume loss as solutes are transported away from the locus of weathering by soil water and/or groundwater movement (leaching). Other chemical weathering processes, including oxidation, reduction, hydrolysis, carbonation, and chelation, act to bring about significant changes to weathering materials. In weathering, oxidation simply means a reaction of materials with oxygen to form oxides or, if water is incorporated, hydroxides. The oxygen can be derived directly from the atmosphere, from oxygen dissolved in water, or from bacterial activity. Reduction is the opposite of oxidation and usually takes place in anaerobic (oxygendeficient), water-logged sites. Carbonation is the reaction of carbonate and bicarbonate ions with minerals. Carbonate and bicarbonate ions are common intermediate products in certain types of weathering, particularly the breakdown of feldspars. In one of the most common carbonation reactions, carbon dioxide (CO2) reacts with water to form carbonic acid (H2CO3), a potent solvent in weathering environments. In the process of chelation, an ion, usually a metal, is held within an organic ring structure. Chelating agents allow the removal of ions from otherwise insoluble solids and enable the movement and precipitation of metallic ions in weathering profiles. Hydrolysis is a very effective chemical weathering process in which ions of minerals and the Hþ or OH ions of water react. The concentration of hydrogen ions is of fundamental importance in all weathering reactions (Ollier and Pain, 1996). In weathering profiles, the roots of living plants and acid hydrogen clay (clay with a high proportion of Hþ ions on its cation exchange sites) are primary sources of Hþ ions, which create an acid environment that promotes mineral weathering. As a consequence of increasing efficacy of mechanical and chemical weathering toward the land surface, most weathering profiles exhibit trends of decreasing particle size, increasing clay content, and increasing voids ratio toward the surface (Gerrard, 1988). Chemical weathering of common rockforming minerals produces only a few abundant groups of secondary minerals. Because the secondary minerals produced during weathering develop preferentially after specific primary minerals, the secondary mineralogy of most weathering profiles is dependent on the primary mineralogy and its distribution in the source rock (Nesbitt and Young, 1989). The application of weathering indices to weathering profile evaluation emphasizes a continuum of change from fresh to highly altered materials and provides numerical expression to the chemical or mineralogical changes imparted by weathering. The basis of chemical weathering indices is that some elements (e.g., Ca, Na, and K) found in weatherable minerals (e.g., feldspars, pyroxenes, amphiboles, and micas) are relatively mobile, whereas others (e.g., Al, Fe, and Ti) are not. As weathering progresses, the proportion of mobile elements remaining in the weathering materials decreases while the proportion of immobile elements increases. The chemical index of alteration
(CIA) developed by Nesbitt and Young (1982, 1989) is based on the molecular proportions of major element oxides: CIA ¼ ½Al2 O3 =ðAl2 O3 þ CaO þ Na2 O þ K2 OÞ 100 The CaO in the equation is attributable only to silicates, necessitating the subtraction of nonsilicate CaO in materials in which they occur in significant quantities. Values of CIA increase from 50 to 55 for primary minerals, such as biotite and feldspar, to 100 for secondary minerals, such as kaolinite and gibbsite, with intermediate values (70–85) for smectites. Sueoka (1988) proposed the chemical weathering index (CWI), also based on molecular weight proportions in which CWI ¼ ½Al2 O3 þ Fe2 O3 þ TiO2 þ H2 OðÞ=all chemical components 100% Several studies have shown progressive increase in both CWI and CIA toward the land surface in weathering profiles developed on granitic rocks. The character of weathering products within a weathering profile and among weathering profiles depends on the weathering environment because the mobility of the cations released during weathering of silicate rocks varies widely and is affected by the pH and, in the case of iron, the electron potential (Eh) of groundwater (Thomas, 1974). The most mobile elements are usually lost to the site of weathering in solution; elements with intermediate mobility remain in solution for short periods and often are precipitated as new minerals close to the site of weathering. Aluminum and iron in the ferric state, the least mobile of the common elements, generally remain as residual products of weathering, usually as sesquioxides. Water is a prerequisite to chemical weathering because it provides and carries away reactants, thus playing a significant role in the direction and progression of weathering (Figure 3). In general, weathering progresses to deeper levels and at faster rates where groundwater gradients are steepest. Organic matter and biological processes also play critical roles in the development of weathering profiles. Plants and burrowing animals mix and cycle materials through weathering profiles. Organic acids produced during organic matter degradation form organic acids that lower pH, and they also form complexes (chelates) with metal cations that increase the solubility of important elements bringing some elements such as iron into solution in weathering profiles. Microbes, primarily bacteria, also reduce iron(III) and Mn(III) and Mn(IV) in saturated to near-saturated parts of weathering profiles to reduced ions that move through or out of the weathering profile in solution. Oxidation, another important process in weathering profiles, occurs when geologic materials exist in an environment in which the oxygen supply is high or exceeds the biogeochemical oxygen demand (BOD). Most oxidation in weathering profiles occurs when oxygenated waters come in contact with mineral grains. Iron is the most commonly oxidized element. The oxidation of iron, from the ferrous (Fe(II)) to the ferric (Fe(III)) state, disrupts the electrical neutrality of the crystal lattice, promoting the formation of an oxide, hematite (Fe2 O3), or a hydrous oxide such as goethite. These iron minerals impart brown, red, or yellow color to the weathered material (Figure 4). Reduction (sometimes referred to as ‘deoxidation’) occurs in an environment in which the oxygen supply is limited or the
PALEOSOLS AND WIND-BLOWN SEDIMENTS | Weathering Profiles
Figure 4 Yellowish brown colors in the upper, oxidized part of this exposure of loess give way to gray matrix colors and reddish brown mottles in the lower reduced (deoxidized) part of the exposure. Iron ions in the matrix of the oxidized zone are dominantly ferrous (Fe(III)), whereas in the reduced zone the matrix contains abundant ferric iron (Fe(II)). Iron oxides and hydrous oxides are abundant in the mottles where ferric iron, mobilized from the reduced matrix, moved to pores where higher Eh promoted the formation of iron oxides. Photograph by E.A. Bettis.
BOD is high. Saturation or near saturation and the presence of organic matter and microbes are prerequisites for this process. In this environment, iron is reduced to its mobile ferrous form. Once in this form, iron may be lost through net movement of the groundwater; it may remain in the sediment matrix and react with sulfides; or it may move into fractures, pores, or other voids and be oxidized. Matrix colors in a reduced zone usually range from gray to greenish gray, a condition reflecting relatively low free iron oxide content (Figure 4). In contrast to the grayish matrix, fractures and pores in reduced zones, where iron has migrated and become oxidized, have brown and reddish brown stains, streaks, and mottles. If geologic material has not been exposed to atmospheric oxygen or oxygenated water, it may be unoxidized. In this state, most iron is in the ferrous form, and the matrix is a uniform dark gray color without stains, streaks, and mottles (Figure 5). Weathering processes lead to the development of a weathering profile if the rate of weathering exceeds the rate of removal of weathered material by surface erosion. The upper part of the weathering profile is dominated by processes that remove soluble weathering products in solution and that lead to the concentration of more inert minerals and secondary products, such as iron oxides, that accumulate as residues. The soil profile is located at the top of this part of the weathering profile. The materials lost in solution may be removed entirely by groundwater flow or reprecipitated, either lower in the weathering profile or downslope elsewhere on the landscape.
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Figure 5 Unoxidized glacial diamicton with uniform dark gray matrix color and lack of brown or red mottles, streaks, and stains. Unoxidized materials have not been exposed to the atmosphere or to oxygenated water for a period of time sufficient for oxidation of their matrix iron to begin. Photograph by T.J. Kemmis.
Weathering Zones Weathering profiles can be subdivided into zones on the basis of physical, chemical, and mineralogical properties of the regolith. The simplest subdivision of the weathering profile includes two zones: an upper mobile or ‘active’ zone, which has been affected by biological and abiotic disturbance of weathered material, and a lower zone of in situ weathered material recognized by little or no volume change from the unweathered geologic material, the presence of joint planes, veins of resistant minerals, or other rock structures. The lower zone is usually referred to as ‘saprolite.’ Walther (1916) is credited as the first to formalize the sequence of zones found from the surface downward in deepweathering profiles: 1. 2. 3. 4.
soil and duricrust, mottled zone, pallid zone, and fresh bedrock.
This sequence has subsequently come to be called a ‘laterite’ profile. The pallid zone has preserved rock structure, is depleted in metallic cations, and usually contains kaolinite. The mottled zone contains patches of iron oxide in a pallid zone matrix and may extend up into the active zone. Several variations on Walther’s original scheme have been presented, and the reader is referred to Gerrard (1988) and Ollier and Pain (1996) for examples. Ruxton and Berry (1957) developed a physical and chemomineralogical classification scheme for weathering zones developed in granite: 1. residual debris composed of structureless sandy clay or clayey sand, 1–25 m thick with up to 30% clay, composed
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2.
3. 4. 5.
PALEOSOLS AND WIND-BLOWN SEDIMENTS | Weathering Profiles
predominantly of quartz and kaolin, reddish brown when clayey and light brown or orange when less clayey; residual debris with less than 10% free, rounded corestones (spheroidal boulders of fresh rock), less than 5% clay but common clay-forming minerals, generally light color; similar to (2) but with much of the original rock structure preserved and 10–50% corestones; rectangular and locked corestones (50–90% solid rock) set in a matrix of residual debris; and partially weathered rock with minor amounts of residual debris along major fracture planes, more than 90% solid rock, significant iron staining and biotite decomposition may be present.
Figure 6 illustrates some of the more commonly used classification schemes for weathering profiles in crystalline rocks. The specific features of weathering profiles and the zones they comprise vary, depending on the physical and chemical nature of the materials in which they form, as well as across landscapes. The weathering zone schemes of Walther (1916) and Ruxton and Berry (1957) are characteristic of those formed in crystalline rocks. Weathering profiles in sedimentary rocks seldom exhibit the distinct zoning, corestones, and welldefined basal surface of weathering associated with crystalline rocks. Relatively pure carbonates such as chalks typically have very thin weathering profiles. The residuum of carbonate rock weathering profiles contains large amounts of insoluble rock
Depth
constituents, such as quartz, chert, iron, and manganese oxides and clay minerals. Large fractures, bedding planes, and solutional openings in carbonates foster extreme lateral variability of these weathering profiles (Figure 7). Weathering profiles formed in siltstones and mudstones are characterized by a
Figure 7 Weathering profile formed in limestone. Note variations in the weathering profile associated with the occurrence of fractures and bedding planes in the rock. Also note the presence of residual chert clasts in the upper part of the profile. Photograph by E.A. Bettis.
Ruxton and Berry (1957)
Norbury et al. (1995)*
Sueoka (1998) CWI
Soil
Residual soil (VI)
40–60%
Ollier and Galloway (1990)
McFarlane (1992)
Nahon and Tardy (1992)
Ferricrete
Residuum
Iron crust
Mottled zone
Mottled zone
Mottled zone
Saprolite
Fine saprolite
Surface s Completely weathered (V)
20–40%
a p
Residual debris (I) Highly weathered (IV)
r 20–40%
o Residual debris, corestones (II) Corestones, residual debris (III)
l Pallid zone Moderately weathered (III)
t
Partially weathered (IV) Slightly weathered (II)
Bedrock (V)
i
15–20%
e
Saprock
Coarse saprolite
Fresh (I)
Unaltered bedrock *Weathering classification for heterogenous masses
Figure 6 Six weathering zone classifications in common use for weathering profiles formed in crystalline rock. Modified from Taylor RG and Howard KWF (1999) Lithological evidence for the evolution of weathered mantles in Uganda by tectonically controlled cycles of deep weathering and stripping. Catena 35: 65–94; Figure 4.
PALEOSOLS AND WIND-BLOWN SEDIMENTS | Weathering Profiles
Birkeland (1999)
Tandarich et al. (1994) O A E EB BE B
s o l u m
s o l u m
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Hallberg et al. (1978)*
s o l u m
BC Cox CB JOL JOU
C
JOU2 CD
MJOU-UJU Cu
RJU UJU
DC
D
UU
*All possible weathering zones or weathering zone combinations not indicated Figure 8 Weathering zone schemes commonly applied to unlithified deposits in the United States.
relatively thin low-strength and low-permeability zone near the surface that grades downward into a jointed and fissured zone of higher permeability (Deere and Patton, 1971). Fissures are extremely important features in the weathering of shale, siltstone, and mudstone. They often develop along bedding planes, from structural modifications, and as a response to unloading and land surface processes. Weathering profiles in unlithified sediments have generally received less attention than those formed in rock. Glacial sediments, loess, eolian sand, alluvium, and exposed marine sediments are widespread in many areas of the world. Weathering zones in these materials are typically identified by color changes, mottling patterns, the presence/absence of fractures, and sometimes chemical changes such as carbonate leaching or accumulation. Three weathering zone schemes for unlithified sediments are in general use (Figure 8):
A conceptual problem common to all these classification schemes is that they imply a genesis beyond their simple description – an unfortunate circumstance given the great uncertainty surrounding the mechanisms by which weathering mantles develop. Taylor and Howard (1999) proposed the use of strictly descriptive graphic logs, similar to those employed to describe lithostratigraphy, as a way to objectively describe weathering profiles. Practical problems with applying weathering zone schemes to unlithified sediments usually center around two issues: (1) determination of the physical and chemical properties of the initial material that has undergone weathering and (2) complications with using color as a proxy for oxidation state in materials that contain significant amounts of detrital carbon.
1. Birkeland (1999) used the designation ‘Cu’ for unoxidized material and ‘Cox’ for oxidized material. 2. Hallberg et al. (1978) used a more elaborate scheme that can be applied to materials whose unaltered state is oxidized or unoxidized (Table 1). 3. Tandarich et al. (1994) provided a classification that unifies unaltered deposits and the overlying soil profile into a single ‘pedo-weathering profile.’
Spatial Patterns of Weathering Profiles Much discussion has focused on the distribution of thick weathering profiles, their age, and conditions fostering their development. Common assumptions are that deep weathering is associated with warm, wet climates and that the development of thick weathering profiles implies great age of the associated geomorphic surface. Although greater amounts of
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PALEOSOLS AND WIND-BLOWN SEDIMENTS | Weathering Profiles
Table 1 Weathering zone terminology proposed by Hallberg et al. (1978) for Quaternary materials First symbol O (oxidized)
Munsell color reference 60% of matrix with hues redder than 2.5Y; hues of 2.5Y with values of 5 or higher; may have segregation of secondary Fe and Mn compounds into mottles, nodules, etc. R (reduced) 60% of matrix with hues of 2.5Y with values of 3 or less; hues of 2.5Y with value of 4 and chroma of 2 or less; hues of 5Y, N, 5GY, 5G, 5BG, 10Y, and 10GY with values of 4 or higher; colors are usually mixed as mottles, bands, or diffuse blends of colors; may have considerable segregation of secondary Fe and Mn compounds into mottles, nodules, etc. D (deoxidized – applied 60% of matrix with hues of 10YR, 2.5Y, only to loess) and/or 5Y, values of 5 and 6, and chromas of 1 or 2; segregation of Fe compounds into tubules and/or nodules U (unoxidized) Uniform matrix color; hues of 5Y and N with values of 5 or less; 5GY, 5G, 5BG, 10Y, and 10GY with values of 6 or less; no secondary segregation of Fe and Mn compounds Second symbol Fractures (if present) J (jointed – fractured) Well-defined vertical to subvertical fractures present; often identified by colors that contrast with the matrix and coatings and/or rinds of secondary minerals Second or third symbol Leached or unleached state – used only with materials having primary matrix carbonate U (unleached) Reacts with dilute HCl L (leached) Does not react with dilute HCl L2 Leached of primary carbonates but secondary carbonates present as coatings, nodules, or other accumulations Modifier symbol When used precedes first symbol M (mottled) Has zones with 20–50% contrasting mottles; when used with the unoxidized zone it infers 20% or less mottles of reduced colors Examples of descriptions using weathering zone terminology OL Oxidized and leached; yellowish brown (10YR5/6) matrix color, does not react with weak HCl MOU Mottled, oxidized, and unleached; strong brown (7.5YR4/4) matrix color with common gray (5Y5/1) and light olive brown (2.5Y5/4) mottles, reacts with weak HCl UJU Unoxidized, jointed, and unleached; uniform dark greenish gray (5GY4/1) matrix with few thin vertical fractures which have mottled light olive brown (2.5Y5/6) and olive gray (5Y5/2) faces, reacts with weak HCl
environmental and geomorphic history on weathering profiles. Strakhov (1967) proposed five global-scale zones of weathering related to latitudinal variations in climate and vegetation (Figure 9). He also recognized three major disturbances to the latitudinal pattern: glacial sedimentation, arid sedimentation, and active tectonics. Weathering profiles on intrusive igneous rocks, such as granite, have been described and studied in many areas of the world and have provided a ‘standard’ for comparing global patterns. The development of thick weathering profiles has traditionally been associated with humid tropical and subtropical environments, and their occurrence in areas with other climatic regimes has been cited as evidence of relict weathering under past wetter and/or warmer climates. With several caveats, the assemblage of clay minerals formed in weathering profiles in granitic rocks generally reflects the climatic regime or annual rainfall average of the area (Gerrard, 1994). In arid regions, where leaching is not intense, smectite is the dominant alteration product. Illite (mica) is probably a poor indicator of macroclimatic conditions because it rarely forms in weathering environments. Kaolinite can be produced under a variety of climatic conditions. It can dominate very wet temperate regions, and it is the most commonly reported secondary clay mineral in wet tropical environments. In addition to kaolinite, smectite can occur in drier tropical regions, and gibbsite and boehmite can occur in the wettest regions. Seasonality of precipitation is also important, and weathering profiles in areas with monsoons often contain illite and montmorillonite. Other characteristics of weathering profiles, such as particlesize distribution, have also been suggested as indicators of climate-driven weathering. Two end member types of profiles formed on granitic rocks, ‘arenaceous’ or sandy profiles and ‘argillaceous’ or clayey, have been defined. Sandy regoliths are widely distributed in temperate regions, contain common to abundant weatherable minerals, and form primarily by the alteration of plagioclase feldspars to clay, accompanied by mechanical fracture of quartz and orthoclase to sand-size material. In contrast, argillaceous regoliths, most commonly found in wet tropical and subtropical regions, contain few weatherable minerals and have high clay and low silt content due to the rapid transformation of all feldspars directly to clay minerals. Weathering profile characteristics also vary across landscapes. Although the primary control on weathering profile thickness appears to be the intensity and direction of jointing in rocks, landscape-related patterns are also apparent. Thomas (1974) summarized evidence relating deep-weathering profiles to topography:
• •
water passing through a material and higher temperatures will speed chemical weathering, the depth of weathering is a function of the rate of weathering profile advance and the rate of erosion. Thus, weathering profile thickness is related to rock/ sediment properties, environmental history, and geomorphic history. Since the effects of rock and sediment properties have already been discussed, this section focuses on the role of
• • •
The weathering front is usually highly irregular and usually exhibits a pattern of discrete basins and domes. Weathering patterns are aligned with the fracture system developed in the rock. Weathering depth is usually shallow beneath streams. Where marked inequalities in the resistance of rock do not occur, the depth of weathering usually increases irregularly away from streams toward interfluves in the humid tropics. In drier savannas and semiarid regions, deep-weathering profiles are usually restricted to depressions in the landscape.
Precipitation Litter fall from vegetation Temperature Evaporation
Semi-desert and desert
Lateritic crust (Fe2O3 and Al2O3)
Iron and aluminium oxides
Kaolinite
Smectite
Fragmented rock with little chemical alteration
Tropical forest
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Savannas
Taiga zone
Savannas
Tundra
Steppes
PALEOSOLS AND WIND-BLOWN SEDIMENTS | Weathering Profiles
Fresh rock
Figure 9 Pole-to-equator variation of weathering profiles as proposed by Strakhov (1967). Adapted from Figure 9.10 in Schaetzl RJ and Anderson S (2005) Soils: Genesis and Geomorphology. New York: Cambridge.
OL JOU UU
• MJOU
• • •
Figure 10 Weathering profile developed beneath a slope formed in late Pleistocene glacial diamicton. The oxidized zone is thick beneath the summit, thins beneath the sideslope, then thickens in the lower part of the slope where sandy zones promote deep penetration of oxygenated water. Weathering zone terminology of Hallberg et al. (1978): OL, oxidized and leached; JOU, jointed oxidized and unleached; UU, unoxidized and unleached; MJOU, mottled jointed oxidized and unleached. Photograph by T.J. Kemmis.
• •
High relief and steep slopes foster a pattern of rocky hills and weathered valley floors in both humid and arid areas. Patterns of surface relief may offer few clues to deepweathering patterns.
Weathering profiles formed in Quaternary sediments that have been dated provide insights into spatial patterns of weathering profiles during the early stages of their development (Figure 10). General trends include:
• • •
advance of the weathering front along fractures and other pores in the sediment matrix; increasing thickness of weathering profiles with increasing coarseness of the sediment matrix; deeper penetration of the weathering front on uplands near valley margins than beneath either uplands distant from
valleys or beneath valley floors except in arid and semiarid regions, where the thickest weathering profiles are found in valleys; weathering profiles that are thicker in well-drained than in poorly drained landscape positions, again with the exception of arid and semiarid regions; increasing thickness of weathering profiles in well-drained landscape positions with increasing mean annual infiltration; correspondence between oxidation state of dominant ferromanganese oxides (and therefore sediment matrix color) and the position of the water table; great complexity in the details related primarily to variations in sediment properties (bedding, structure, porosity, density, and mineralogy), water movement through the sediment, biotic effects, and the nature of surface erosion.
Dating Weathering Profiles There are many problems surrounding generalizations about the relationship among weathering profile thickness and morphology and the age and stability of geomorphic surfaces; however, as Thomas (1974) suggested, deep lateritic profiles with thick pallid zones, as well as distinctly zoned weathering profiles, are probably reasonable indicators of surface stability and prolonged development. Deep-weathering profiles are not the product of a single weathering event in a well-defined time in the past but are the combined result of long-lasting superimposed weathering processes during extended periods of time. Changes in climate, vegetation, and landscapes over the length of time during which weathering has taken place complicate the interpretation of weathering profile histories. Most deep-weathering profiles occur in relatively stable cratonic regions that were not directly affected by glaciation during the Quaternary, or are in stratigraphic positions indicating development prior to the Quaternary.
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PALEOSOLS AND WIND-BLOWN SEDIMENTS | Weathering Profiles
Three methods have been used to determine the age of deep-weathering profiles: K–Ar, 40Ar/39Ar, and cosmogenic isotope dating. The age of neoformed alunite-group sulfates (alunite and jarosite) and hollandite-group manganese oxides (cryptomellane and hollandite) precipitated through weathering reactions can be determined using K–Ar and 40Ar/39Ar geochronology (Vasconcelos, 1999). Supergene alunite and jarosite, because of the relative simplicity of their mineral structures, can be dated equally well using either method, provided that large enough quantities of pure mineral separates can be obtained. On the other hand, the complex nature of supergene Mn oxides and the large number of mineral structures encompassed by the Mn-oxide group make application of the 40Ar/39Ar incremental-heating methodology better suited to dating these phases (Vasconcelos, 1999). The K–Ar and 40Ar/39Ar methods can be applied across the full sweep of geologic time, but age measurements at the young end of the spectrum are more likely to be compromised by contamination with atmospheric Ar or primordial 40Ar. The 40Ar/39Ar method allows for assessment of such contamination and therefore performs better in the younger (0.25–1.0 Ma) age range. Cosmogenic 10Be inventories have been used to estimate the length of accumulation time and hence the minimum age of weathering profiles formed during the past 5 Myr (Pavich et al., 1985). Complexities such as surface erosion and shielding effects of glacial ice limit the application of this technique in some areas, but several studies have successfully applied this and other cosmogenic isotope dating techniques to Quaternary weathering profiles (Pavich, 1986; Phillips et al., 1998).
Applications Knowledge of the distribution, properties, origin, and age of weathering profiles has a wide variety of geological, engineering, hydrological, and land-use applications. Initially, interest in weathering profiles was fueled by geologists seeking to understand the origin and distribution of economically important supergene minerals and ores associated with some deepweathering profiles. Subsequently, geologists also began to realize that information on the nature and distribution of past climates could also be gleaned from weathering profiles. In many cases, truncated weathering profiles are the only surviving evidence of ancient geomorphic surfaces (Bettis, 1998; Figure 11). Geochemical, isotopic, and geochronological studies of weathering profiles provide important insights into longterm erosion rates and landscape evolution. Engineers and engineering geologists have long appreciated the variations in physical properties of the regolith that occur through weathering profiles. Mapping the thickness and properties of weathering profiles has added a new dimension to land-use planning, construction engineering, and landslide hazard assessment (Hencher and McNicholl, 1995). Infiltrating water moves through weathering profiles on its way to both discharge areas and aquifers. Variations in hydrologic and hydrochemical properties of weathering profiles control many important aspects of water movement and quality in the vadose (unsaturated) and shallow phreatic (water-saturated) zones that have critical impacts on water supplies in many areas of the world.
(a)
(b)
Figure 11 (a) A weathering profile formed in late Pleistocene glacial diamicton overlying another weathering profile formed in older glacial diamicton. The contact between the gray unoxidized and unleached (UU) diamicton and the underlying brown oxidized and leached (OL) diamicton marks a significant stratigraphic break during which the lower weathering profile developed. Photograph by T.J. Kemmis. (b) Abrupt contact between unoxidized and unleached and oxidized and leached glacial diamictons. The presence of an unoxidized zone overlying an oxidized zone marks a significant stratigraphic break. Upper parts of the oxidized unit’s weathering profile were removed by glacial erosion during advance of the glacier that deposited the upper diamicton. Photograph by E.A. Bettis.
Conclusions Weathering profiles form over extended periods of time as geologic materials are altered in response to physical and chemical changes in the near-surface environment. Weathering profiles are the long-lasting ‘roots’ of surface soils, and their properties reflect the cumulative effects of weathering under changing climatic, biotic, and geomorphic conditions at a given location. Variations in the factors that affect weathering from place to place on a landscape, around the globe, and through time, have produced patterns in the properties of weathering profiles. Deciphering the history preserved in these patterns can provide information crucial to understanding long-term processes that shape the Earth’s surface and that affect the environment in which we live.
See also: K/Ar and 40Ar/39Ar Dating; Loess Deposits: Origins and Properties. Paleosols and Wind-Blown Sediments: Nature of Paleosols.
PALEOSOLS AND WIND-BLOWN SEDIMENTS | Weathering Profiles
References Bettis EA III (1998) Subsolum weathering profile characteristics as indicators of the relative rank of stratigraphic breaks in till sequences. Quaternary International 51–52, 72–73. Birkeland PW (1999) Soils and Geomorphology, 3rd edn. New York: Oxford University Press. Bisdom EBA (1967) The role of micro-crack systems in the spheroidal weathering of an intrusive granite in Galicia (N.W. Spain). Geologie en Mijnbouw – Netherlands Journal of Geosciences 46: 333–340. Deere DU and Patton ED (1971) Slope stability in residual soils. In: Proceedings of the 4th Panamanian Conference on Soil Mechanics and Foundation Engineering, vol. 1, pp. 87–170. San Juan, Puerto Rico. New York: American Society of Civil Engineers. Gerrard AJ (1988) Rocks and Landforms. London: Unwin Hyman. Gerrard AJ (1994) Weathering of granitic rocks: Environment and clay mineral formation. In: Robinson DA and Williams RBG (eds.) Rock Weathering and Landform Evolution, pp. 3–20. New York: Wiley. Hallberg GR, Fenton TE, and Miller GA (1978) Standard weathering zone terminology for the description of Quaternary sediments in Iowa. In: Hallberg GR (ed.) Standard Procedures for the Evaluation of Quaternary Materials in Iowa, pp. 75–109. Iowa City: Iowa Geological Survey. Technical Information Series No. 8. Hencher SR and McNicholl DP (1995) Engineering in weathered rock. Quarterly Journal of Engineering Geology 28: 253–266. Mabbutt JA (1961) Basal surface or weathering front? Proceedings of the Geological Association 72: 357–358. McFarlane MJ (1992) Groundwater movement and water chemistry associated with weathering profiles of the African Surface in Malawi. In: Wright EP and Burgess WG (eds.) Hydrogeology of Crystalline Basement Aquifers in Africa, pp. 101–129. London: Geological Society of London. Special Publication 66. Nahon D and Tardy Y (1992) The ferruginous laterites. In: Butt CRM and Zeegers H (eds.) Regolith Exploration Geochemistry in Tropical and Sub-tropical Terrains. Handbook of Exploration Geochemistry, vol.4, pp. 41–55. New York: Elsevier. Nesbitt HW and Young GM (1982) Early Proterozoic climates and plate motions inferred from major element chemistry of lutites. Nature 299: 715–717. Nesbitt HW and Young GM (1989) Formation and diagenesis of weathering profiles. Journal of Geology 97: 129–147. Norbury DR, Hencher SP, Cripps JC, and Lumsden A (1995) The description and classification of weathered rocks for engineering purposes. Geological Society
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Engineering Group Working Party Report. Quaterly Journal of Engineering Geology 28: 207–242. Ollier CD and Galloway RW (1990) The laterite profile, ferricrete and unconformity. Catena 17: 97–109. Ollier CD and Pain C (1996) Regolith, Soils and Landforms. Chichester: Wiley. Pavich MJ (1986) Processes and rates of saprolite production and erosion on a foliated granitic rock of the Virginia Piedmont. In: Colman SM and Dethier DP (eds.) Rates of Chemical Weathering of Rocks and Minerals, pp. 551–590. Orlando: Academic Press. Pavich MJ, Brown L, Valette-Silver JN, Klein J, and Middleton R (1985) 10Be analysis of a Quaternary weathering profile in the Virginia Piedmont. Geology 13: 39–41. Phillips WM, McDonald EV, Reneau SL, and Poths J (1998) Dating soils and alluvium with cosmogenic 21Ne depth profiles: Case studies from the Pajarito Plateau, New Mexico, USA. Earth and Planetary Science Letters 160: 209–223. Ruxton PB and Berry L (1957) Weathering of granite and associated features in Hong Kong. Bulletin of the Geological Society of America 68: 1263–1292. Strakhov NM (1967) Principles of Lithogenesis, vol. 1. Edinburgh: Oliver and Boyd. Fitzimmons P (trans.). Sueoka T (1988) Identification and classification of granitic residual soils using chemical weathering index. In: Proceedings of the 2nd International Conference on Geomechanics in Tropical Soils. Geomechanics in Tropical Soils, vol. 1, pp. 55–61. Tandarich JP, Darmody RG, and Follmer LR (1994) The pedo-weathering profile: A paradigm for whole-regolith pedology from the glaciated midcontinental United States of America. In: Cremeens DL, Brown RB, and Huddelson JH (eds.) Whole Regolith Pedology, pp. 97–117. Madison: Soil Science Society of America. Special Publication 34. Taylor G and Eggleton RA (2001) Regolith Geology and Geomorphology. Chichester: Wiley. Taylor RG and Howard KWF (1999) Lithological evidence for the evolution of weathered mantles in Uganda by tectonically controlled cycles of deep weathering and stripping. Catena 35: 65–94. Thomas M (1974) Tropical Geomorphology: A Study of Weathering and Landform Development in Warm Climates. New York: Wiley. Vasconcelos PM (1999) K-Ar and 40Ar/39Ar geochronology of weathering processes. Annual Review of Earth and Planetary Sciences 27: 183–229. Walther J (1916) Das geologische Alter und die Bildung des Laterits. Petermanns Geographische Mitteilungen 62(1–7): 46–53.
Introduction Weathering is a process of conversion of rock-forming minerals toward an equilibrium state. Rocks and sediments at or near the Earth’s surface are subjected to physical, chemical, and biological environments that differ from those present where constituent minerals formed, and by various processes they change toward equilibrium with Earth-surface conditions (Taylor and Eggleton, 2001). Weathering processes are grouped into two broad categories: mechanical (physical) weathering and chemical weathering. The former involves changes that do not directly result in chemical or mineralogical changes to the affected geological materials, whereas the latter result in a variety of mineralogical and chemical changes to the original materials. Mechanical and chemical weathering can and often do occur together, and both can be mediated by biota. These changes to rocks and sediments are organized into weathering profiles that reflect the interplay of weathering, erosion, and redistribution of material at and below the land surface over time. Alterations of geologic materials that take place in weathering profiles produce physical, chemical, and hydrologic characteristics that influence urban and agricultural land use, water resources, the occurrence of some economically important deposits, and landscape evolution. Weathering profiles range from centimeters to tens of meters in thickness. They begin developing upon exposure at the land surface and may form over tens of millions of years in some stable continental interiors. Fossil-weathering profiles occur throughout the geologic record, becoming more common with the spread of land plants in the Carboniferous. Because weathering profiles form from the land surface, some of their properties may reflect climatic conditions during their formation and thus provide information useful for paleoenvironmental reconstructions. Weathering profiles are the ‘roots’ of surface soils and reflect the cumulative effects of near-surface alterations of geologic materials over extended periods of time. The literature on weathering profiles is replete with terms for weathered geologic material. The most all-encompassing term is regolith – the mantle of fragmented and usually unlithified geologic material that overlies bedrock and that includes residual and transported materials. Some definitions include soil as the upper zone of the regolith, whereas engineering geologists often call all unconsolidated material above bedrock ‘soil.’ The regolith consists of several distinct materials that may be intimately related (Ollier and Pain, 1996): 1. Weathered rock. Weathered rock in place is saprolite, a soft, earthy, typically clay-rich, thoroughly decomposed rock formed in place by chemical weathering of igneous, metamorphic, or sedimentary rocks (Figure 1). Structures present in the original rock, such as veins and fractures, are preserved in saprolite.
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2. Residuum. Material originally formed by weathering of rock that has subsequently moved a short distance, either by creep or through removal of finer weathering products. 3. Transported sediments and chemical deposits. Materials transported by various agencies from a place of origin, where they were weathered to become available for transport, to a place where they have been deposited. 4. Soil. Physical, chemical, and biological alterations of geologic materials that occur immediately below a land surface and that grade downward into a weathering profile. Weathering profiles are formed in regolith. They may be defined as the vertical extent of a weathered sequence from the initiating land surface or the originating surface to the unweathered parent rock. Weathering profiles form by chemical and mechanical weathering and vary considerably on several spatial scales because of variations in materials they are formed in, topography, hydrology, surficial processes, and climate.
Weathering Processes Both mechanical and chemical weathering processes act to form weathering profiles. The boundary between the weathered and unweathered material in a weathering profile is called the weathering front (Mabbutt, 1961). The weathering front marks important differences in permeability, porosity, strength, and behavior, and it is the locus of initial changes in the unweathered geologic material that begin the weathering process. Advance of the weathering front occurs along and is dependent on the presence of fractures that allow the penetration of meteoric fluids into the unweathered material (Figure 2). Thus, rock types, tectonic and climatic settings, and landscape conditions that favor the formation of extensive interconnected fracture networks facilitate the development of weathering profiles (Figure 3). Some of the more important mechanical weathering processes include expansion from release of confining pressure (unloading), exfoliation, frost shattering, pressure induced by root growth, and pressure resulting from the growth of salt crystals. Water enters unweathered material on the macroscale by penetration along interconnected pores, primarily fractures in rocks, lithified sediments, and unlithified fine-grained sediments. Also of great importance is the presence of intra- and transgranular microcracks formed by a variety of forces, including crystallization, tectonics, and unloading. The microcrack systems are the loci of water–mineral interactions that are the initial stages of chemical weathering (Bisdom, 1967). The effects of rock or sediment texture on penetration of the weathering front are fundamental (Thomas, 1974). Disaggregation of coarse-grained rocks such as granite into constituent
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Figure 1 Clay-rich saprolite formed by in-place chemical weathering of biotite granite–gneiss. Protolith texture and structures are preserved in core stones such as the one in the lower part of this profile. Photograph by M.J. Pavich.
Fractures
Infiltrating water
Water penetration accelerated; increase in weathering surface area
Hydraulic gradient
Chemical disequilibrium maintained
Rapid mineral weathering
Advance of weathering front
Figure 3 Flowchart showing the primary factors acting to advance the weathering front in geologic materials.
Figure 2 Oxidation, marked by brown and reddish brown colors, advances into gray, unweathered middle Pleistocene glacial diamicton along subvertical fractures. The fractures are avenues along which meteoric water and the solutes it carries penetrate the deposit, thereby advancing the weathering front. Photograph by E.A. Bettis.
mineral grains by oxidation of biotite and hydration of other minerals often takes place before much fundamental chemical change has occurred. Fine-grained rocks of similar mineral composition weather more slowly, most likely because a system of microcracks and larger, interconnected fractures is usually not as extensively developed in these rocks. In arid, polar, and some coastal environments, crystallization from supersaturated solutions of adventitious salts (salts derived from sea spray or saline water bodies) can produce expansive stresses
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PALEOSOLS AND WIND-BLOWN SEDIMENTS | Weathering Profiles
leading to granular disintegration or scaling. The various physical and chemical processes acting at the weathering front act to make this a zone of relatively high hydraulic conductivity in a weathering profile. This promotes downward progression of the weathering front by transporting weathering products away from the loci of weathering (Figure 3). In the early stages of weathering, hydration (the addition of water to a mineral) produces an increase in volume. As weathering progresses, this gives way to a volume loss as solutes are transported away from the locus of weathering by soil water and/or groundwater movement (leaching). Other chemical weathering processes, including oxidation, reduction, hydrolysis, carbonation, and chelation, act to bring about significant changes to weathering materials. In weathering, oxidation simply means a reaction of materials with oxygen to form oxides or, if water is incorporated, hydroxides. The oxygen can be derived directly from the atmosphere, from oxygen dissolved in water, or from bacterial activity. Reduction is the opposite of oxidation and usually takes place in anaerobic (oxygendeficient), water-logged sites. Carbonation is the reaction of carbonate and bicarbonate ions with minerals. Carbonate and bicarbonate ions are common intermediate products in certain types of weathering, particularly the breakdown of feldspars. In one of the most common carbonation reactions, carbon dioxide (CO2) reacts with water to form carbonic acid (H2CO3), a potent solvent in weathering environments. In the process of chelation, an ion, usually a metal, is held within an organic ring structure. Chelating agents allow the removal of ions from otherwise insoluble solids and enable the movement and precipitation of metallic ions in weathering profiles. Hydrolysis is a very effective chemical weathering process in which ions of minerals and the Hþ or OH ions of water react. The concentration of hydrogen ions is of fundamental importance in all weathering reactions (Ollier and Pain, 1996). In weathering profiles, the roots of living plants and acid hydrogen clay (clay with a high proportion of Hþ ions on its cation exchange sites) are primary sources of Hþ ions, which create an acid environment that promotes mineral weathering. As a consequence of increasing efficacy of mechanical and chemical weathering toward the land surface, most weathering profiles exhibit trends of decreasing particle size, increasing clay content, and increasing voids ratio toward the surface (Gerrard, 1988). Chemical weathering of common rockforming minerals produces only a few abundant groups of secondary minerals. Because the secondary minerals produced during weathering develop preferentially after specific primary minerals, the secondary mineralogy of most weathering profiles is dependent on the primary mineralogy and its distribution in the source rock (Nesbitt and Young, 1989). The application of weathering indices to weathering profile evaluation emphasizes a continuum of change from fresh to highly altered materials and provides numerical expression to the chemical or mineralogical changes imparted by weathering. The basis of chemical weathering indices is that some elements (e.g., Ca, Na, and K) found in weatherable minerals (e.g., feldspars, pyroxenes, amphiboles, and micas) are relatively mobile, whereas others (e.g., Al, Fe, and Ti) are not. As weathering progresses, the proportion of mobile elements remaining in the weathering materials decreases while the proportion of immobile elements increases. The chemical index of alteration
(CIA) developed by Nesbitt and Young (1982, 1989) is based on the molecular proportions of major element oxides: CIA ¼ ½Al2 O3 =ðAl2 O3 þ CaO þ Na2 O þ K2 OÞ 100 The CaO in the equation is attributable only to silicates, necessitating the subtraction of nonsilicate CaO in materials in which they occur in significant quantities. Values of CIA increase from 50 to 55 for primary minerals, such as biotite and feldspar, to 100 for secondary minerals, such as kaolinite and gibbsite, with intermediate values (70–85) for smectites. Sueoka (1988) proposed the chemical weathering index (CWI), also based on molecular weight proportions in which CWI ¼ ½Al2 O3 þ Fe2 O3 þ TiO2 þ H2 OðÞ=all chemical components 100% Several studies have shown progressive increase in both CWI and CIA toward the land surface in weathering profiles developed on granitic rocks. The character of weathering products within a weathering profile and among weathering profiles depends on the weathering environment because the mobility of the cations released during weathering of silicate rocks varies widely and is affected by the pH and, in the case of iron, the electron potential (Eh) of groundwater (Thomas, 1974). The most mobile elements are usually lost to the site of weathering in solution; elements with intermediate mobility remain in solution for short periods and often are precipitated as new minerals close to the site of weathering. Aluminum and iron in the ferric state, the least mobile of the common elements, generally remain as residual products of weathering, usually as sesquioxides. Water is a prerequisite to chemical weathering because it provides and carries away reactants, thus playing a significant role in the direction and progression of weathering (Figure 3). In general, weathering progresses to deeper levels and at faster rates where groundwater gradients are steepest. Organic matter and biological processes also play critical roles in the development of weathering profiles. Plants and burrowing animals mix and cycle materials through weathering profiles. Organic acids produced during organic matter degradation form organic acids that lower pH, and they also form complexes (chelates) with metal cations that increase the solubility of important elements bringing some elements such as iron into solution in weathering profiles. Microbes, primarily bacteria, also reduce iron(III) and Mn(III) and Mn(IV) in saturated to near-saturated parts of weathering profiles to reduced ions that move through or out of the weathering profile in solution. Oxidation, another important process in weathering profiles, occurs when geologic materials exist in an environment in which the oxygen supply is high or exceeds the biogeochemical oxygen demand (BOD). Most oxidation in weathering profiles occurs when oxygenated waters come in contact with mineral grains. Iron is the most commonly oxidized element. The oxidation of iron, from the ferrous (Fe(II)) to the ferric (Fe(III)) state, disrupts the electrical neutrality of the crystal lattice, promoting the formation of an oxide, hematite (Fe2 O3), or a hydrous oxide such as goethite. These iron minerals impart brown, red, or yellow color to the weathered material (Figure 4). Reduction (sometimes referred to as ‘deoxidation’) occurs in an environment in which the oxygen supply is limited or the
PALEOSOLS AND WIND-BLOWN SEDIMENTS | Weathering Profiles
Figure 4 Yellowish brown colors in the upper, oxidized part of this exposure of loess give way to gray matrix colors and reddish brown mottles in the lower reduced (deoxidized) part of the exposure. Iron ions in the matrix of the oxidized zone are dominantly ferrous (Fe(III)), whereas in the reduced zone the matrix contains abundant ferric iron (Fe(II)). Iron oxides and hydrous oxides are abundant in the mottles where ferric iron, mobilized from the reduced matrix, moved to pores where higher Eh promoted the formation of iron oxides. Photograph by E.A. Bettis.
BOD is high. Saturation or near saturation and the presence of organic matter and microbes are prerequisites for this process. In this environment, iron is reduced to its mobile ferrous form. Once in this form, iron may be lost through net movement of the groundwater; it may remain in the sediment matrix and react with sulfides; or it may move into fractures, pores, or other voids and be oxidized. Matrix colors in a reduced zone usually range from gray to greenish gray, a condition reflecting relatively low free iron oxide content (Figure 4). In contrast to the grayish matrix, fractures and pores in reduced zones, where iron has migrated and become oxidized, have brown and reddish brown stains, streaks, and mottles. If geologic material has not been exposed to atmospheric oxygen or oxygenated water, it may be unoxidized. In this state, most iron is in the ferrous form, and the matrix is a uniform dark gray color without stains, streaks, and mottles (Figure 5). Weathering processes lead to the development of a weathering profile if the rate of weathering exceeds the rate of removal of weathered material by surface erosion. The upper part of the weathering profile is dominated by processes that remove soluble weathering products in solution and that lead to the concentration of more inert minerals and secondary products, such as iron oxides, that accumulate as residues. The soil profile is located at the top of this part of the weathering profile. The materials lost in solution may be removed entirely by groundwater flow or reprecipitated, either lower in the weathering profile or downslope elsewhere on the landscape.
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Figure 5 Unoxidized glacial diamicton with uniform dark gray matrix color and lack of brown or red mottles, streaks, and stains. Unoxidized materials have not been exposed to the atmosphere or to oxygenated water for a period of time sufficient for oxidation of their matrix iron to begin. Photograph by T.J. Kemmis.
Weathering Zones Weathering profiles can be subdivided into zones on the basis of physical, chemical, and mineralogical properties of the regolith. The simplest subdivision of the weathering profile includes two zones: an upper mobile or ‘active’ zone, which has been affected by biological and abiotic disturbance of weathered material, and a lower zone of in situ weathered material recognized by little or no volume change from the unweathered geologic material, the presence of joint planes, veins of resistant minerals, or other rock structures. The lower zone is usually referred to as ‘saprolite.’ Walther (1916) is credited as the first to formalize the sequence of zones found from the surface downward in deepweathering profiles: 1. 2. 3. 4.
soil and duricrust, mottled zone, pallid zone, and fresh bedrock.
This sequence has subsequently come to be called a ‘laterite’ profile. The pallid zone has preserved rock structure, is depleted in metallic cations, and usually contains kaolinite. The mottled zone contains patches of iron oxide in a pallid zone matrix and may extend up into the active zone. Several variations on Walther’s original scheme have been presented, and the reader is referred to Gerrard (1988) and Ollier and Pain (1996) for examples. Ruxton and Berry (1957) developed a physical and chemomineralogical classification scheme for weathering zones developed in granite: 1. residual debris composed of structureless sandy clay or clayey sand, 1–25 m thick with up to 30% clay, composed
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2.
3. 4. 5.
PALEOSOLS AND WIND-BLOWN SEDIMENTS | Weathering Profiles
predominantly of quartz and kaolin, reddish brown when clayey and light brown or orange when less clayey; residual debris with less than 10% free, rounded corestones (spheroidal boulders of fresh rock), less than 5% clay but common clay-forming minerals, generally light color; similar to (2) but with much of the original rock structure preserved and 10–50% corestones; rectangular and locked corestones (50–90% solid rock) set in a matrix of residual debris; and partially weathered rock with minor amounts of residual debris along major fracture planes, more than 90% solid rock, significant iron staining and biotite decomposition may be present.
Figure 6 illustrates some of the more commonly used classification schemes for weathering profiles in crystalline rocks. The specific features of weathering profiles and the zones they comprise vary, depending on the physical and chemical nature of the materials in which they form, as well as across landscapes. The weathering zone schemes of Walther (1916) and Ruxton and Berry (1957) are characteristic of those formed in crystalline rocks. Weathering profiles in sedimentary rocks seldom exhibit the distinct zoning, corestones, and welldefined basal surface of weathering associated with crystalline rocks. Relatively pure carbonates such as chalks typically have very thin weathering profiles. The residuum of carbonate rock weathering profiles contains large amounts of insoluble rock
Depth
constituents, such as quartz, chert, iron, and manganese oxides and clay minerals. Large fractures, bedding planes, and solutional openings in carbonates foster extreme lateral variability of these weathering profiles (Figure 7). Weathering profiles formed in siltstones and mudstones are characterized by a
Figure 7 Weathering profile formed in limestone. Note variations in the weathering profile associated with the occurrence of fractures and bedding planes in the rock. Also note the presence of residual chert clasts in the upper part of the profile. Photograph by E.A. Bettis.
Ruxton and Berry (1957)
Norbury et al. (1995)*
Sueoka (1998) CWI
Soil
Residual soil (VI)
40–60%
Ollier and Galloway (1990)
McFarlane (1992)
Nahon and Tardy (1992)
Ferricrete
Residuum
Iron crust
Mottled zone
Mottled zone
Mottled zone
Saprolite
Fine saprolite
Surface s Completely weathered (V)
20–40%
a p
Residual debris (I) Highly weathered (IV)
r 20–40%
o Residual debris, corestones (II) Corestones, residual debris (III)
l Pallid zone Moderately weathered (III)
t
Partially weathered (IV) Slightly weathered (II)
Bedrock (V)
i
15–20%
e
Saprock
Coarse saprolite
Fresh (I)
Unaltered bedrock *Weathering classification for heterogenous masses
Figure 6 Six weathering zone classifications in common use for weathering profiles formed in crystalline rock. Modified from Taylor RG and Howard KWF (1999) Lithological evidence for the evolution of weathered mantles in Uganda by tectonically controlled cycles of deep weathering and stripping. Catena 35: 65–94; Figure 4.
PALEOSOLS AND WIND-BLOWN SEDIMENTS | Weathering Profiles
Birkeland (1999)
Tandarich et al. (1994) O A E EB BE B
s o l u m
s o l u m
397
Hallberg et al. (1978)*
s o l u m
BC Cox CB JOL JOU
C
JOU2 CD
MJOU-UJU Cu
RJU UJU
DC
D
UU
*All possible weathering zones or weathering zone combinations not indicated Figure 8 Weathering zone schemes commonly applied to unlithified deposits in the United States.
relatively thin low-strength and low-permeability zone near the surface that grades downward into a jointed and fissured zone of higher permeability (Deere and Patton, 1971). Fissures are extremely important features in the weathering of shale, siltstone, and mudstone. They often develop along bedding planes, from structural modifications, and as a response to unloading and land surface processes. Weathering profiles in unlithified sediments have generally received less attention than those formed in rock. Glacial sediments, loess, eolian sand, alluvium, and exposed marine sediments are widespread in many areas of the world. Weathering zones in these materials are typically identified by color changes, mottling patterns, the presence/absence of fractures, and sometimes chemical changes such as carbonate leaching or accumulation. Three weathering zone schemes for unlithified sediments are in general use (Figure 8):
A conceptual problem common to all these classification schemes is that they imply a genesis beyond their simple description – an unfortunate circumstance given the great uncertainty surrounding the mechanisms by which weathering mantles develop. Taylor and Howard (1999) proposed the use of strictly descriptive graphic logs, similar to those employed to describe lithostratigraphy, as a way to objectively describe weathering profiles. Practical problems with applying weathering zone schemes to unlithified sediments usually center around two issues: (1) determination of the physical and chemical properties of the initial material that has undergone weathering and (2) complications with using color as a proxy for oxidation state in materials that contain significant amounts of detrital carbon.
1. Birkeland (1999) used the designation ‘Cu’ for unoxidized material and ‘Cox’ for oxidized material. 2. Hallberg et al. (1978) used a more elaborate scheme that can be applied to materials whose unaltered state is oxidized or unoxidized (Table 1). 3. Tandarich et al. (1994) provided a classification that unifies unaltered deposits and the overlying soil profile into a single ‘pedo-weathering profile.’
Spatial Patterns of Weathering Profiles Much discussion has focused on the distribution of thick weathering profiles, their age, and conditions fostering their development. Common assumptions are that deep weathering is associated with warm, wet climates and that the development of thick weathering profiles implies great age of the associated geomorphic surface. Although greater amounts of
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PALEOSOLS AND WIND-BLOWN SEDIMENTS | Weathering Profiles
Table 1 Weathering zone terminology proposed by Hallberg et al. (1978) for Quaternary materials First symbol O (oxidized)
Munsell color reference 60% of matrix with hues redder than 2.5Y; hues of 2.5Y with values of 5 or higher; may have segregation of secondary Fe and Mn compounds into mottles, nodules, etc. R (reduced) 60% of matrix with hues of 2.5Y with values of 3 or less; hues of 2.5Y with value of 4 and chroma of 2 or less; hues of 5Y, N, 5GY, 5G, 5BG, 10Y, and 10GY with values of 4 or higher; colors are usually mixed as mottles, bands, or diffuse blends of colors; may have considerable segregation of secondary Fe and Mn compounds into mottles, nodules, etc. D (deoxidized – applied 60% of matrix with hues of 10YR, 2.5Y, only to loess) and/or 5Y, values of 5 and 6, and chromas of 1 or 2; segregation of Fe compounds into tubules and/or nodules U (unoxidized) Uniform matrix color; hues of 5Y and N with values of 5 or less; 5GY, 5G, 5BG, 10Y, and 10GY with values of 6 or less; no secondary segregation of Fe and Mn compounds Second symbol Fractures (if present) J (jointed – fractured) Well-defined vertical to subvertical fractures present; often identified by colors that contrast with the matrix and coatings and/or rinds of secondary minerals Second or third symbol Leached or unleached state – used only with materials having primary matrix carbonate U (unleached) Reacts with dilute HCl L (leached) Does not react with dilute HCl L2 Leached of primary carbonates but secondary carbonates present as coatings, nodules, or other accumulations Modifier symbol When used precedes first symbol M (mottled) Has zones with 20–50% contrasting mottles; when used with the unoxidized zone it infers 20% or less mottles of reduced colors Examples of descriptions using weathering zone terminology OL Oxidized and leached; yellowish brown (10YR5/6) matrix color, does not react with weak HCl MOU Mottled, oxidized, and unleached; strong brown (7.5YR4/4) matrix color with common gray (5Y5/1) and light olive brown (2.5Y5/4) mottles, reacts with weak HCl UJU Unoxidized, jointed, and unleached; uniform dark greenish gray (5GY4/1) matrix with few thin vertical fractures which have mottled light olive brown (2.5Y5/6) and olive gray (5Y5/2) faces, reacts with weak HCl
environmental and geomorphic history on weathering profiles. Strakhov (1967) proposed five global-scale zones of weathering related to latitudinal variations in climate and vegetation (Figure 9). He also recognized three major disturbances to the latitudinal pattern: glacial sedimentation, arid sedimentation, and active tectonics. Weathering profiles on intrusive igneous rocks, such as granite, have been described and studied in many areas of the world and have provided a ‘standard’ for comparing global patterns. The development of thick weathering profiles has traditionally been associated with humid tropical and subtropical environments, and their occurrence in areas with other climatic regimes has been cited as evidence of relict weathering under past wetter and/or warmer climates. With several caveats, the assemblage of clay minerals formed in weathering profiles in granitic rocks generally reflects the climatic regime or annual rainfall average of the area (Gerrard, 1994). In arid regions, where leaching is not intense, smectite is the dominant alteration product. Illite (mica) is probably a poor indicator of macroclimatic conditions because it rarely forms in weathering environments. Kaolinite can be produced under a variety of climatic conditions. It can dominate very wet temperate regions, and it is the most commonly reported secondary clay mineral in wet tropical environments. In addition to kaolinite, smectite can occur in drier tropical regions, and gibbsite and boehmite can occur in the wettest regions. Seasonality of precipitation is also important, and weathering profiles in areas with monsoons often contain illite and montmorillonite. Other characteristics of weathering profiles, such as particlesize distribution, have also been suggested as indicators of climate-driven weathering. Two end member types of profiles formed on granitic rocks, ‘arenaceous’ or sandy profiles and ‘argillaceous’ or clayey, have been defined. Sandy regoliths are widely distributed in temperate regions, contain common to abundant weatherable minerals, and form primarily by the alteration of plagioclase feldspars to clay, accompanied by mechanical fracture of quartz and orthoclase to sand-size material. In contrast, argillaceous regoliths, most commonly found in wet tropical and subtropical regions, contain few weatherable minerals and have high clay and low silt content due to the rapid transformation of all feldspars directly to clay minerals. Weathering profile characteristics also vary across landscapes. Although the primary control on weathering profile thickness appears to be the intensity and direction of jointing in rocks, landscape-related patterns are also apparent. Thomas (1974) summarized evidence relating deep-weathering profiles to topography:
• •
water passing through a material and higher temperatures will speed chemical weathering, the depth of weathering is a function of the rate of weathering profile advance and the rate of erosion. Thus, weathering profile thickness is related to rock/ sediment properties, environmental history, and geomorphic history. Since the effects of rock and sediment properties have already been discussed, this section focuses on the role of
• • •
The weathering front is usually highly irregular and usually exhibits a pattern of discrete basins and domes. Weathering patterns are aligned with the fracture system developed in the rock. Weathering depth is usually shallow beneath streams. Where marked inequalities in the resistance of rock do not occur, the depth of weathering usually increases irregularly away from streams toward interfluves in the humid tropics. In drier savannas and semiarid regions, deep-weathering profiles are usually restricted to depressions in the landscape.
Precipitation Litter fall from vegetation Temperature Evaporation
Semi-desert and desert
Lateritic crust (Fe2O3 and Al2O3)
Iron and aluminium oxides
Kaolinite
Smectite
Fragmented rock with little chemical alteration
Tropical forest
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Savannas
Taiga zone
Savannas
Tundra
Steppes
PALEOSOLS AND WIND-BLOWN SEDIMENTS | Weathering Profiles
Fresh rock
Figure 9 Pole-to-equator variation of weathering profiles as proposed by Strakhov (1967). Adapted from Figure 9.10 in Schaetzl RJ and Anderson S (2005) Soils: Genesis and Geomorphology. New York: Cambridge.
OL JOU UU
• MJOU
• • •
Figure 10 Weathering profile developed beneath a slope formed in late Pleistocene glacial diamicton. The oxidized zone is thick beneath the summit, thins beneath the sideslope, then thickens in the lower part of the slope where sandy zones promote deep penetration of oxygenated water. Weathering zone terminology of Hallberg et al. (1978): OL, oxidized and leached; JOU, jointed oxidized and unleached; UU, unoxidized and unleached; MJOU, mottled jointed oxidized and unleached. Photograph by T.J. Kemmis.
• •
High relief and steep slopes foster a pattern of rocky hills and weathered valley floors in both humid and arid areas. Patterns of surface relief may offer few clues to deepweathering patterns.
Weathering profiles formed in Quaternary sediments that have been dated provide insights into spatial patterns of weathering profiles during the early stages of their development (Figure 10). General trends include:
• • •
advance of the weathering front along fractures and other pores in the sediment matrix; increasing thickness of weathering profiles with increasing coarseness of the sediment matrix; deeper penetration of the weathering front on uplands near valley margins than beneath either uplands distant from
valleys or beneath valley floors except in arid and semiarid regions, where the thickest weathering profiles are found in valleys; weathering profiles that are thicker in well-drained than in poorly drained landscape positions, again with the exception of arid and semiarid regions; increasing thickness of weathering profiles in well-drained landscape positions with increasing mean annual infiltration; correspondence between oxidation state of dominant ferromanganese oxides (and therefore sediment matrix color) and the position of the water table; great complexity in the details related primarily to variations in sediment properties (bedding, structure, porosity, density, and mineralogy), water movement through the sediment, biotic effects, and the nature of surface erosion.
Dating Weathering Profiles There are many problems surrounding generalizations about the relationship among weathering profile thickness and morphology and the age and stability of geomorphic surfaces; however, as Thomas (1974) suggested, deep lateritic profiles with thick pallid zones, as well as distinctly zoned weathering profiles, are probably reasonable indicators of surface stability and prolonged development. Deep-weathering profiles are not the product of a single weathering event in a well-defined time in the past but are the combined result of long-lasting superimposed weathering processes during extended periods of time. Changes in climate, vegetation, and landscapes over the length of time during which weathering has taken place complicate the interpretation of weathering profile histories. Most deep-weathering profiles occur in relatively stable cratonic regions that were not directly affected by glaciation during the Quaternary, or are in stratigraphic positions indicating development prior to the Quaternary.
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PALEOSOLS AND WIND-BLOWN SEDIMENTS | Weathering Profiles
Three methods have been used to determine the age of deep-weathering profiles: K–Ar, 40Ar/39Ar, and cosmogenic isotope dating. The age of neoformed alunite-group sulfates (alunite and jarosite) and hollandite-group manganese oxides (cryptomellane and hollandite) precipitated through weathering reactions can be determined using K–Ar and 40Ar/39Ar geochronology (Vasconcelos, 1999). Supergene alunite and jarosite, because of the relative simplicity of their mineral structures, can be dated equally well using either method, provided that large enough quantities of pure mineral separates can be obtained. On the other hand, the complex nature of supergene Mn oxides and the large number of mineral structures encompassed by the Mn-oxide group make application of the 40Ar/39Ar incremental-heating methodology better suited to dating these phases (Vasconcelos, 1999). The K–Ar and 40Ar/39Ar methods can be applied across the full sweep of geologic time, but age measurements at the young end of the spectrum are more likely to be compromised by contamination with atmospheric Ar or primordial 40Ar. The 40Ar/39Ar method allows for assessment of such contamination and therefore performs better in the younger (0.25–1.0 Ma) age range. Cosmogenic 10Be inventories have been used to estimate the length of accumulation time and hence the minimum age of weathering profiles formed during the past 5 Myr (Pavich et al., 1985). Complexities such as surface erosion and shielding effects of glacial ice limit the application of this technique in some areas, but several studies have successfully applied this and other cosmogenic isotope dating techniques to Quaternary weathering profiles (Pavich, 1986; Phillips et al., 1998).
Applications Knowledge of the distribution, properties, origin, and age of weathering profiles has a wide variety of geological, engineering, hydrological, and land-use applications. Initially, interest in weathering profiles was fueled by geologists seeking to understand the origin and distribution of economically important supergene minerals and ores associated with some deepweathering profiles. Subsequently, geologists also began to realize that information on the nature and distribution of past climates could also be gleaned from weathering profiles. In many cases, truncated weathering profiles are the only surviving evidence of ancient geomorphic surfaces (Bettis, 1998; Figure 11). Geochemical, isotopic, and geochronological studies of weathering profiles provide important insights into longterm erosion rates and landscape evolution. Engineers and engineering geologists have long appreciated the variations in physical properties of the regolith that occur through weathering profiles. Mapping the thickness and properties of weathering profiles has added a new dimension to land-use planning, construction engineering, and landslide hazard assessment (Hencher and McNicholl, 1995). Infiltrating water moves through weathering profiles on its way to both discharge areas and aquifers. Variations in hydrologic and hydrochemical properties of weathering profiles control many important aspects of water movement and quality in the vadose (unsaturated) and shallow phreatic (water-saturated) zones that have critical impacts on water supplies in many areas of the world.
(a)
(b)
Figure 11 (a) A weathering profile formed in late Pleistocene glacial diamicton overlying another weathering profile formed in older glacial diamicton. The contact between the gray unoxidized and unleached (UU) diamicton and the underlying brown oxidized and leached (OL) diamicton marks a significant stratigraphic break during which the lower weathering profile developed. Photograph by T.J. Kemmis. (b) Abrupt contact between unoxidized and unleached and oxidized and leached glacial diamictons. The presence of an unoxidized zone overlying an oxidized zone marks a significant stratigraphic break. Upper parts of the oxidized unit’s weathering profile were removed by glacial erosion during advance of the glacier that deposited the upper diamicton. Photograph by E.A. Bettis.
Conclusions Weathering profiles form over extended periods of time as geologic materials are altered in response to physical and chemical changes in the near-surface environment. Weathering profiles are the long-lasting ‘roots’ of surface soils, and their properties reflect the cumulative effects of weathering under changing climatic, biotic, and geomorphic conditions at a given location. Variations in the factors that affect weathering from place to place on a landscape, around the globe, and through time, have produced patterns in the properties of weathering profiles. Deciphering the history preserved in these patterns can provide information crucial to understanding long-term processes that shape the Earth’s surface and that affect the environment in which we live.
See also: K/Ar and 40Ar/39Ar Dating; Loess Deposits: Origins and Properties. Paleosols and Wind-Blown Sediments: Nature of Paleosols.
PALEOSOLS AND WIND-BLOWN SEDIMENTS | Weathering Profiles
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