3.6 Pedostratigraphical units
3.6.1 General properties and rules
Pedostratigraphy deals with the stratigraphical classification of ancient, buried horizons that contain soil profiles formed by pedological processes. Pedological processes comprise chemical weathering, biogenic reworking and activity, and formation of secondary minerals. A vertical soil profile is characterized by various pedological horizons. The most common terms for these, from the surface downwards, are: the A horizon, which is a mixture of organic material and weathered mineralogical material; the B horizon, which consists of redeposited material and/or concentrations of residual material; and the C horizon, which is weathered parent material. The C horizon usually shows a gradual transition downwards to unweathered parent material. There may be a layer of organic debris above the soil profile, the O horizon.
A pedostratigraphical unit is a body of rock or superficial deposits consisting of one or more pedological horizons developed on a surface made up of one or more lithostratigraphical and/or lithodemic units. It is overlain by one or more formally defined lithostratigraphical units.
The upper boundary of a pedological unit is defined by the top of the uppermost horizon formed by pedological processes. The O horizon, which is not formed by such processes, is not included in a pedostratigraphical unit. (Any organic top-layer present can be classified as a litho- or biostratigraphical unit.)
The lower boundary is placed at the lowest level that is clearly affected by pedological processes (usually the lower boundary of the B horizon).
The physical properties of a pedological unit are clearly distinguished from both the parent material below and the lithostratigraphical unit above. The physical properties of a single pedological unit, such as colour, texture, content of organic material, mineral content or concretions, can change laterally as a function of variations in, for example, parent material, drainage conditions and vegetation.
The boundaries of a pedostratigraphical unit are time-transgressive. Identification and lateral tracing of a pedostratigraphical unit are accomplished on lithological criteria and stratigraphical correlation with the help of the underlying and overlying litho- and/or biostratigraphical units, and perhaps by numerical age determinations.
The fundamental and only pedostratigraphical unit is a palaeosol (paleojord). A palaeosol is defined and named after a type locality or a type area in accordance with the "general rules for naming and defining geological units" (Chap. 2).
Pedostratigraphical classification has so far not been used on Norwegian territory, but it could perhaps find application within the succession on the continental shelf.
Key reference: NACSN (1983). (NACSN calls the pedostratigraphical unit a geosol.)