The red conglomerates are part of the Buffelskloof Formation. This formation has also been called Enon in the past, so some people still refer to the red rocks as the Enon Conglomerates. The Buffelskloof Formation also contains breccias . The conglomerates and breccias were deposited in a large, land-locked basin in the early Cretaceous (approximately 144 to 65 million years ago). The red coloration is due to iron oxide being supplied during deposition of the sediments, and more importantly, the rapidity of the deposition of the conglomerates, so that there was no time for the iron to be worn down. The heavy grains of iron oxide pigmented the sediment during oxidation.The diagram below explain the different geological periods as they occurred.
The conglomerates were deposited by rivers and streams while the breccias represent scree deposits. The precise age of the Buffelskloof Formation is not known very well because of poor fossil preservation, most likely due to the sediments being emplaced in an oxidizing environment. Oxidation is also responsible for the reddish color of the rocks. The red color results from oxidation of iron in the matrix of the conglomerate/breccia rocks. Soil itself is very complex. It would be very wrong to think of soil as just a collection of fine mineral particles. Soil also contains air, water, dead organic matter, and various types of living organisms (Figure 1). The formation of a soil is influenced by organisms, climate, topography, parent material, and time. The following items describe some important features of a soil that help to distinguish it from mineral sediments.
Colour is a physical property of soils that allows us to know some of its most important characteristics, such as mineral composition, age and soil processes (chemical alteration, carbonate accumulation, the presence of humified organic matter, etc.). The presence of water in the soil profile during long periods of time also affects soil colour as a result of changes in the oxidation rate. Similarly, together with other physical properties, colour helps us to differentiate between types of horizons of the same profile or different soil profiles.
The substances resulting from decomposition of the organic matter are oxidized and acquire a dark coloration. On the other hand, some of these substances can be combined with mineral substances present in the soil as a consequence of mineralization of organic materials or chemical alteration of clays (such as iron oxides). After physical and chemical alterations, organic and inorganic materials contribute to soil colour in different ways and proportions and, as a result, colour is a product of co-working soil processes and environmental conditions. Soil colour is related to soil processes, and is used as a diagnostic criteria for soil classification.
Among the most frequent soil colours, we can find the following :
Clear or white. Soluble salts such as NaCl, may form a surface white crust or be present in the soil matrix in a diffuse form. Carbonates and soluble salts may be present in the parent material or accumulate in soil due to the arid climate or other processes. In other cases, light colour is due to a relatively high proportion of sand (quartz crystals), either in the whole soil or in profile horizons that have suffered extensive washing under very humid climate.
Dark or black. Dark colours are usually due to the presence of organic matter, so that the darker the surface horizon more organic matter content is assumed. At other times, the dark colour is due to the presence of compounds of reduced iron and manganese (as in black poorly drained soils), the presence of charcoal in soils from the Brazilian Amazon and some areas Africa (these soils are known as “Terra preta”, dark soil in Portuguese).
Red. The red colour is usually a result of alteration of clay minerals, so it usually occurs in the argillic horizons. Weathered clay minerals release aluminium and iron oxides such as hematite (Fe2O3). This process is favoured in hot climates with a long and intense dry season, as the Mediterranean climate. Reddish colours indicate good drainage and aeration of the soil, allowing the existence of oxidizing conditions to form oxides. In the Mediterranean basin, red soils are known as “Terra rossa” (Italian expression for red soil).
Yellowish brown/orange. Yellowish or orange colours may be due to the presence of goethite, FeO(OH), and bound to the clay and organic matter. Therefore, although composition is similar to some red soils, we know that these soils were formed under conditions of increased moisture availability.
QUESTIONS
1. What role does water play in the soil profile?
2. In which era does the Cretaceous period fall?
3. In your observation what is the grain size of the soil in the cuttings? (fine, medium, coarse)
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