Pedogenesis in lutitic Cr horizons of gypsiferous soils

The weathering of lutites in aridic or near-aridic environments leading to soil horizons is not well known. Lutites are a common soil parent rock composed of silt and clay, that may be massive or may have their sedimentary origin marked by layers of alternating color or granulometry, or by fissility along planes. Lutites underlying soils are considered Cr horizons and show different alteration stages recognizable in the field by the degree of the layering disturbance or by the loss of coherence. In the study area, neither the dry climate nor the kind of clay minerals, mainly illite, can explain the weathering of lutites. This study was conducted to investigate the initial weathering reactions as encountered in Cr horizons, and to depict the processes responsible for pedogenesis from lutites in gypsiferous soils of the Ebro Valley (Spain). We described 12 soils in the field under two different soil moisture regimes, with further characterization through chemical analyses and micromorphology by thin section and scanning electron microscope (SEM). The microscopic study revealed two distinctive pedofeatures in the Cr horizons: (i) lenticular gypsum crystals; and (ii) "queras," sub-millimetric biotic features produced by calcification-decalcification processes. Growth of gypsum crystals in these horizons resulted in an "isles fabric," that is, isles of fine materials in a mass of gypsum crystals, with much greater porosity than the parent lutite. The growth of gypsum crystals and the development of queras result in a particle-size distribution change and an increased porosity of the Cr horizons.The first author was funded by the Government of Aragón from October 1991 to September 1995. We gratefully acknowledge the help of Dr. J. Wierzchos and Prof. R. Rodríguez with the electron microscope of the University of Lleida. Some preliminary results of this work were presented at the 6th International Meeting on Soils with Mediterranean Type of Climate held in 1999 in Barcelona.Peer reviewe