ABSTRACT

Pedological processes occur during subaerial diagenesis, in the rhizosphere, the zone in which the root system of plants is active. Some plant roots may penetrate deep into the sediment. As a result of extensive evaporation and calcium ion concentration, CaCO₃ is concentrated at the sediment/root interface in column formations that surround the roots.

With soil evolution calcareous columns disintegrate progressively into lens-shaped or concretionary structures. At the same time the sparry fabric of the columns changes to a fabric consisting of micron-sized grains. In the transitional stages numerous perforations appear that suggest a leaching process is taking place. Quartz grains incorporated in the concretions are peripherally replaced by calcite spar composed of needle-like crystals of the size of thousands of angstroms. Hence in the concretions the CaCO₃ concentration exceeds that in the columns. This reaction between dissolution of quartz and precipitation of calcite is pH-dependent and takes place under alkaline conditions. Increase in concentration of CaCO₃ is paralleled by an increase in magnesium ions and iron oxides; conversely the strontium concentration falls. Soil processes tend to concentrate iron and magnesium and to deplete strontium.

These changes under pedological conditions account for the advanced stages of subaerial lithification.