Weathering horizon

There is no doubt of the presence of roll fronts in the Horton Group sandstones, however the source of the uranium within the system may be related to the weathered horizons beneath the Horton Group rocks and not exclusively the result of diagenetic change within the sandstones (Ryan O Beirne-Ryan 2007). 

Chamberlain et al. (1999) measured the oxygen isotope composition of authigenic kaolinite, smectite and illite in weathered horizons from intermontane basins on the east side of the Southern Alps. Neither smectite nor illite produced useful results, as reflected in the... 

Gjems, O., 1963. A swelling dioctahedral clay mineral of a vermiculite-smectite type in the weathering horizons of podzols. Clay Miner., 5 183-193. 

Residual accumulation of oxides due to long-term chemical weathering Horizon altered by human-related activities Accumulation of silica (as opal)... 

Figure 9 Early Paleozoic changes in (a) soil differentiation as indicated by clay content (volume percent) and alumina/bases (molar ratio) of the most weathered horizon of calcareous red paleosols (b) soil bioturbation as indicated by proportion of transect in paleosols occupied by roots or burrows (percent) and by measured rooting depth (m) (c) atmospheric CO2 levels (PAL) calculated from a sedimentary mass balance model (d) maximum coal seam thickness and average thickness of at least 10 consecutive seams (m) (e) diameter of fossil plant stems and roots (m) (f) diversity of fossil land plants (number of species) (g) diversity of soil animals (number of families) (Retallack, 1997c) (reproduced from Dinofest, 1997, pp. 345-359).

The development of wells for groundwater supplies in rural areas, especially in developing countries, frequently Is of major Importance. In such areas, fractured zones and weathered horizons of granitic or gneissic masses may provide sufficient water for small communities. 

The amount of overburden that has to be removed is an important factor in quarrying operations, for if this increases and is not useable, then a time comes when quarrying operations become uneconomic. The removal of weak overburden is usually undertaken by scrapers and bulldozers, the material being disposed of in spoil dumps on site. Unfortunately, in the case of weathered overburden, weathered profiles are frequently not a simple function of depth below the surface and can be highly variable. Furthermore, in humid tropical areas, in particular, weathered horizons may extend to appreciable depths. Consequently, assessment of the amount of overburden that has to be removed can be complicated. Indurated overburden... 

A swelling clay mineral from the weathering horizon of podzols. Clay Min, Bull, 5 159. 

As an example, the migration of clay from the surface of a soil to a lower horizon results from several processes occurring when certain soil and environmental properties exist. First, claysized minerals must form, usually requiring weathering to have occurred. Clay minerals formed in the surface soil can then go into suspension when salt concentrations in solution are low. Seasonal rains can move the clay down... 

Movement of carbonates and salts can also occur in a similar fashion. As these minerals are weathered in the upper soil profile, their component ions go into solution and are moved down through the soil by rainfall entering the soil. As the water moves down the soil there may not be enough water to move the ions out of the soil, so they precipitate in a lower horizon where they accumulate. Such accumulations are common in arid environments with limited rainfall. In high rainfall areas, carbonates and salts are usually completely removed from the soil through leaching. 

Distribution of U-Th-Ra in weathering profiles. The first U-Th studies (Pliler and Adams 1962 Rosholt et al. 1966 Hansen and Stout 1968) generally showed a U loss relative to Th at the base of the profiles, and an enrichment in the uppermost horizons and/or in some accumulation layers. The development of weathering studies, however, point out that this situation is not to be generalized and that reverse trends can be observed even at the scale of a single toposequence (Fig. 11). 

Figure 11. Distribution with depth of U/Th normahzed to the bedroek in two lateritic profiles of the Kaya toposeqnenee, about 300m apart (Burkina Faso) (Deqnineey et al. 2002 submitted). One profile is located downhill (Kaya 5) and the other one at the top of a residual hill (Kaya 1). The laterite consists of an uppermost fermginous hardtop, an intermediate pink clay nnit and a lowest pistachio unit. For Kaya 5 profile, U/Th distribntion shows a relative enrichment of U in the nppermost horizon and depletion in the lower part of the profile. This kind of distribution is quite conunon in weathering profiles bnt is not systematic as illnstrated by the Kaya 1 profile. In the latter, a relative depletion of U is observed in the npper part and a U-enriched level in the intermediate horizon. This lateral difference in U distribution is explained by vertical redistribntion of U from the ferruginons top to the nnderlying horizons, whose intensity is controlled by the evolntion of the iron oxides from the nppermost horizons (Dequincey et al. snbmitted).

Coarse-sized particles dominate the particle size distribution of arid soils. Some soils are also quite gravelly. The subsurface horizons commonly exhibit accumulation zones of carbonates, gypsum and more soluble salts. Many arid zone soils are shallow and gravelly, some are alkaline. Their structure is weak. From most soils, clay accumulation horizons (argillic horizons) are absent, or are only weakly developed, and so are minerals that indicate an advanced degree of weathering. 

B Subsoil horizon that shows evidence of weathering or illuviation... 

In terms of soil development and the development of soil horizons, the smectites and fine-grained micas are found in younger, less weathered soils. Kaolinite and amorphous clays are found in highly weathered soils. Considering a time sequence, at the beginning of formation, soil will contain more complex clays that weather to simpler forms over time. However, it is convenient to start with a description of the simpler layer silicate clays and then describe the more complex clays. 

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