Soil formation in a Quaternary terrace sequence of the Allier, Limagne, France. Macro- and micromorphology, particle size distribution, chemistry

Abstract

Soil formation in a terrace chronosequence of nine gravelly Quaternary terraces was studied, based on macro- and micromorphological, mineralogical, chemical, and physical characteristics. Based on textural characteristics, the C horizon of the youngest soil appears to be representative for the parent material of lower B and C horizons in the youngest soils, and of all horizons in older soils. The parent materials contain a volcanic and a granitic component. With increasing age of soils, the following stages of soil formation were observed: Holocene soils on lowest terraces show evidence for mixing of sedimentary layers by biological activity, resulting in texturally homogeneous A and B horizons. In the absence of periglacial processes, soil formation in Holocene and in Late Pleistocene terraces took place under continuously well-drained conditions. Late Pleistocene soils on higher terraces show effects of clay illuviation, with prominent weathering features in A horizons. Middle and Early Pleistocene soils on the highest terraces were influenced by periglacial conditions and display effects of various cycles of soil forming processes alternated with periglacial processes, increasing in complexity with age. Weathering of all minerals except quartz, neoformation of clay, and clay illuviation has resulted in distinct differentiation between sandy A and E horizons and a clayey B horizon. Periglacial processes during the Quaternary, weathering, clay formation and clay illuviation have caused a considerable compaction and decreased water permeability. This has resulted in imperfect drainage, now reflected by abundant, locally cemented, iron and manganese oxide segregations in the surface horizon, typical for surface water gley with evidence of ferrolysis. Ferrolysis, the increased weathering of clay under influence of alternating oxidation-reduction in the seasonally waterlogged surface soil, may have further contributed to the marked textural change between A and E versus B horizons. Easily weatherable minerals, observed in strongly leached surface layers of older end-members of the terrace sequence might be attributed to aeolian volcanic inputs. The Holocene and Late Pleistocene soils are intensively used as arable land, as a result of the continuously well-drained conditions. On account of the poor drainage, the Middle and Early Pleistocene soils are used mainly for grassland and forest.