Recognition of paleosols

Paleosols are often distinctive and strikingbands of red clay (Bt horizon), calcareous nodules (Bk horizon), or coal (O horizon) in sedimentary and volcanic sequences (O Figure 13.2). Three general classes of observations are especially helpful in paleosol recognition: root traces, soil horizons, and soil texture (Retallack 1997).

O Figure 13.2

Kenyan Miocene paleosols have been given field names using local Luo and Turkana languages. These pedotypes are objective field mapping units for paleosols: their interpretation and classification requires laboratory study

Sentation Bodenarten

Root traces are the most diagnostic evidence of paleosols and sometimes are obvious because cemented and erosion-resistant (Kabisa pedotype of O Figure 13.2). Difficulties arise in recognition of root traces because they are often replaced by other minerals and ramify in three dimensions in such a way that one rock face reveals little of the overall pattern. Few fossil roots are carbonaceous or reveal histological structures like fossil wood (Retallack 1997). The original root has commonly rotted out, and the hole it occupied is filled with claystone or siltstone, or encrusted with iron oxide or calcium carbonate. Drab haloed root traces are very distinct, green gray mottles, in reddish paleosol matrix, formed during early burial chemical reduction by microbes fueled by consumption of root organic matter (Retallack 1991b). In all these cases, root traces are truncated at the surface of the paleosol, and branch and taper downward. These features distinguish root traces from most kinds of burrows in soils, although the relationship between burrows and roots can be complex. Roots may preferentially follow soft fill of burrows rather than hard soil matrix, and burrows may congregate around roots on which the burrowing animals fed (Retallack 1991a).

Soil horizons develop through thousands of years whereas sedimentary beds are deposited in days. Unlike sedimentary beds, which have sharp bottoms and usually sharp tops as well, paleosols have a sharp top, representing the ancient land surface, but gradational lower contacts (Retallack 2001a). Sedimentary beds also include a variety of sedimentary structures, such as lamination, cross bedding, and ripple marks (as in Tek pedotype of O Figure 13.2), whereas soil horizons develop with obliteration of these original features (Tut pedotype of O Figure 13.2). Similarly, soil formation progressively destroys the original crystalline structure of volcanic or granitic parent materials (Retallack 1991a). In dry climate soils (Aridisols), primary sedimentary or volcanic structures are obscured at first by filaments and soft, small carbonate masses, then large, hard, carbonate nodules (calcic or Bk horizon of Chogo pedotype in O Figure 13.2), and finally thick carbonate layers (petrocalcic or K horizon of Soil Survey Staff 2000). In sod-grassland soils (Mollisols), primary lamination and crystalline structure is broken up by fine roots and replaced by dark, fecal pellets of earthworms to create a crumb-textured, organic surface horizon (mollic epipedon of Dite, Chogo, Yom, and Onuria pedotypes of O Figure 13.2). A variety of other kinds of soil horizons are recognized and important to soil classification (Retallack 1997, 2001a; Soil Survey Staff 2000).

Soil structure also develops within soil horizons and is very distinct from sedimentary bedding and igneous crystalline texture. The fundamental elements of soil structure are modified cracks and other surfaces (cutans), and the clods they define (peds). Cutans include clay skins (argillans) lining cracks in the soil and rusty weathering rinds (sesquans) around clods and pebbles in soil (Retallack 2001a). Peds have a variety of shapes: lenticular in swelling-clay soils (Vertisols: Aberegaiya pedotype of O Figure 13.2), blocky subangular in fertile forest soils (Alfisols: Tut pedotype of O Figure 13.2), and crumb shaped (small and ellipsoidal) in grassland soils (Mollisols: Dite, Chogo, Yom, and Onuria pedotypes of O Figure 13.2). Although cracks and other voids are not preserved in paleosols due to compaction by overburden, peds, and cutans are common and conspicuous (Retallack 1991a). Other soil structures less diagnostic of soils include concretions, nodules, and crystals (Retallack 2001a).

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