Transformations des carbonates dans une sequence de sols developpees sur marnes dans Le Bassin Aquitain, France

Abstract

Soils of a toposequence formed chiefly in Tertiary marls in the south-eastern part of the “Bassin Aquitain” have been studied by field and micromorphological methods. Profiles were examined in detail in the field and oriented samples taken of horizons for observations of micromorphology. From the upper to the lower part of the toposequence, the four profiles also form a chronosequence, viz., a recent calcareous brown soil (sol brun calcaire), an old calcic brown soil (sol brun calcique), a still older leached brown soil (sol brun lessivé) and a hydromorphic leached soil (sol lessivé hydromorphe). The profile distribution and forms of calcite can be related to the morphology, ages and parent materials of the soils. Calcite is found as relics in the upper horizons of the calcareous brown soil and in the form of accumulations in the Bca and Cca horizons of the calcareous brown soil, the calcic brown soil and the leached brown soil. Where accumulations are limited, calcite forms pseudomycelia associated with roots, first as calcitans in tabular pores or craze planes and second as coalescing pseudomycelia. Finally, the calcite invades the S-matrix by epigenesis and results in crumbly masses. These masses have the shapes of the structural voids originally present in the parent materials. The common voids in the marl are vertical craze planes, the widths of which differ with the moisture regime of a soil. In the sandstones, the craze planes are horizontal and very small. The distribution of the calcitans and the presence of the crumbly masses indicate that the processes of accumulation of calcite are in part biodependent, i.e., related to root distribution, and in part lithodependent, i.e., related to structure of the parent rock. Finally, the presence in the calcareous brown soil of calcite as relics in the upper horizons and as successions of cutans from outsides to insides of pores (calcitan, argillan, calcitan) in the Bca horizon shows that the profile has a polycyclic origin.