Chemical and mineralogical characteristics of Pleistocene caliche deposits from the Central Ebro Basin, NE Spain

Chemical and mineralogical analyses of fossil caliche profiles developed on top of Lower Pleistocene alluvial formations in the East-central sector of the Ebro basin indicate that they have high content of carbonate (average 84%) and high Ca/Mg ratios (average 136). The clay mineral assemblages vary slightly depending on the host material, and, therefore, on the alluvial formation in the source area. Inherited detrital minerals (illite, kaolinite and chlorite), transformed components (from chlorite to mixed-layers of chlorite-vermiculite, and from smectite to palygorskite) and a neoformed phase (palygorskite) have been observed. The contents of carbonate and magnesian clay minerals (smectite and palygorskite) increase from the bottom to the top of the profiles, in relation to hardpan laminated caliche facies. The amount of palygorskite is controlled by the concentration of Mg 2+ which in turn depends on the absolute content of Mg 2+ in the host material and on its relative concentration by processes of evaporation linked to decreasing permeability in the profile during the biogenic-pedogenetic carbonate accumulation stages. These processes form part of the development of caliche profiles in a semi-arid environment.