Field recognition ofpaleosols

Three main features of paleosols are useful for distinguishing them from enclosing rocks: root traces, soil horizons, and soil structures. Fossil root traces are best preserved in formerly waterlogged paleosols. In oxidized paleosols their organic matter may not be preserved, but root traces can be recognized by their irregular, tubular shape, and by their downward tapering and branching. Often root traces are crushed like a concertina, because of compaction of the surrounding paleosol during burial. The top of a paleosol may be recognized where root traces and other trace fossils are truncated by an erosional surface. Root and other trace fossils are not useful for recognizing paleosols of middle Ordovician and older age, since large land organisms of such antiquity are currently unknown. Soil horizons usually have more gradational boundaries than seen in sedimentary layering. Commonly these gradational changes are parallel to the truncated upper surface of the paleosol. Some kinds of paleosol horizons are so lithologically distinct that they have been given special names; for example, cornstone (Bk) and ganister (E); the letter symbols are equivalent horizon symbols of soil science. Compared to sedimentary layering, metamorphic foliation, and igneous crystalline textures, soil structure appears massive, hackly, and jointed. The basic units of soil structure (peds) are defined by a variety of modified (for example, iron-stained or clayey) surfaces (cutans). Peds may be granular, blocky, prismatic, columnar, or platy in shape. Concretions, nodules, nodular layers, and crystals are also part of the original soil structure of some paleosols. Complications to be considered during field recognition of paleosols include erosion of parts of the profile, overlap of horizons of different paleosols, development of paleosols on materials eroded from preexisting paleosols, and the development of paleosols under successive and different regimes of weathering.