A distinct pedogenetic path under a Mediterranean climate: the case of soils on Areny sandstone formation (Tremp basin, NE Iberian Peninsula).

ABSTRACT:

The Areny Formation is an Upper Cretaceous sandstone outcropping in the South Pre-Pyrenean area. It is composed of well-sorted quartz sand and gravel, cemented by calcite. It outcrops at a wide range of altitudes (400 to 1700 m). Soils developed on this formation in the Tremp basin (NE Iberian Peninsula) have xeric/ustic and mesic soil climate regimes. They display several soil characteristics corresponding to advanced stages of pedogenesis such as decarbonation, clay formation, and illuviation and rubefaction, ranging from Cambisols and Luvisols to Lixisols. These pedofeatures are absent in adjacent soil units of similar age, developed on finer materials, such as marls and calcareous conglomerates, where the dominant soil formation processes are carbonate translocation and weak cementation. These neighbouring soils are mainly Calcisols, some of them with a petrocalcic horizon. Six profiles formed on the Areny sandstone were selected for an in-depth study of their soil formation processes. A multi-scale approach, from geomorphological to micromorphological analyses, was employed. The soils have a neutral to slightly acidic reaction in the Bt horizons, with loamy sand textures and a clay content of 10 % that appears completely as illuviated clay in the thin section; reddish hues (2.5YR) and high chromas, the absence of calcite in the upper horizons or in the whole profile, and the presence of iron pans in some locations. Amorphous iron is found in low amounts compared to Fe in silicates and as finely crystalline forms. The latter increases with depth in the decarbonated profiles. The high weathering degree of the oldest profiles is shown by kaolinite being the dominant clay, which was probably inherited from the pre-Quaternary period. The clay fraction also contains remarkable amounts of mixed layers chlorite/smectite, chlorite/vermiculite or illite/smectite, which may be considered products of present-day pedogenesis. The geomorphological analyses allowed us to determine the ages of the surface formations of four of the profiles (50 to more than 350 ky), which indicated much faster soil formation rates than those reported in similar Mediterranean environments. Thus, the proposed stages of soil evolution in these sediments imply a fast decarbonation, followed by clay formation, and illuviation. These processes have strong implications in establishing the soil-landscape relationships in the area.