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Clay-rich paleosols are common in the rock record in upper delta-plain, fluvial floodplain, marginal lacustrine, and playa deposits. In environments characterized by a marked seasonality of precipitation, these soils are generally Vertisols. Modern Vertisols contain over 35% clay, predominantly smectite, have a distinctive slickensided ped structure, and locally develop surface microrelief and carbonate nodules.
The optimum development of Vertisols is in subtropical to tropical monsoonal climates. Surface microrelief (gilgai) is common in subhumid and semiarid, but not arid, climatic regimes. Carbonate nodules are common in both drab and pigmented Vertisols in semiarid climates. In the transition to subhumid regimes, carbonate nodules are likely to be either absent or restricted to drab Vertisols. Vertisol pigmentation appears to be a function of inherited color or the development of organic complexes, and thus only indirectly controlled by climate.
Changing precipitation patterns can cause the production or destruction of evaporites or carbonates in a Vertisol profile. In contrast, the slickensided ped structure has a high preservation potential and is probably modified little during changes in climate and subsequent burial. Unlike marine and profundal lacustrine clays, Vertisols have a high bulk density and appear to be affected little by burial compaction. Syndepositional cementation, in the form of veins and nodules of low-Mg calcite (i.e., calcrete), is common. However, major changes in the clay mineralogy of these paleosols may occur during diagenesis, including the transformation of smectite into mixed-layer clays, illite, or kaolinite.
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