Development of a clay-rich interval above a limestone substrate in the Condroz region of southern Belgium

Abstract A profile comprising a limestone substrate overlain by a clay-rich interval, which represents a common sequence, was studied with the objective of determining the relationship between substrate and cover. The profile was characterized by various types of mineralogical and chemical analyses, and by a micromorphological study. The mineralogical and chemical data demonstrate that the cover is unrelated to the parent rock, including all horizons below the loess-derived Ap horizon. Thin section observations show that the basal C3 horizon is largely composed of illuvial clay, deposited around claystone fragments, resulting in a fabric that, together with profile position, relates the interval to so-called beta horizons. In higher parts, up to a level recognized as a lithological discontinuity based on mineralogical composition, the profile largely consists of fragments of clay coatings and disintegrated claystone fragments, with an overall upward increase in degree of pedoturbation. The source of the clay is assumed to be Cenozoic deposits that are known to occur in the region. The main conclusion of this study is that clay intervals overlying limestone can have a complex mode of development, to be studied by a combination of methods, and with implications for the valorization of existing soil maps.