Genesis and morphology of argillic horizons in some semiarid soils in the west of Urmia Lake

In this study argillic horizons in calcareous soils of a semiarid region, with xeric and mesic soil moisture and temperature regimes, in the west of Urmia Lake were studied. Thus, a toposequence in different physiographic positions including piedmont plains, plateau, hills and mountains was investigated. In each land type, three soil profiles were dug and described and their morphological properties were determined. Disturbed samples for physical and chemical studies and undisturbed and oriented samples for micromorphological studies were obtained from characteristic horizons. Physical and chemical properties of soils were determined using standard methods. In order to micromorphic studies, thin sections with the thickness of 25 - 30 microns were prepared from undisturbed and oriented samples and were studied using polarizing microscope under cross polarized (XPL) and plain polarized (PPL) lights. Field observations revealed the presence of argillic horizons only in piedmont plains and plateau land types and the physico-chemical and micromorphological results confirmed their presence as well. According to the results, argillic horizons had redder Hue, more clay content and more ratio of fine clay to total clay than the overlying horizons. Thin section observations showed that clay skins are present as coatings with a thickness of 0.08 - 0.34 mm on the faces of voids and skeletal grains and comprise 2 – 7 % of thin sections of argillic horizons and in some cases, they have filled all of pore spaces. Regarding to semiarid climate of this region and presence of high quantities of calcium carbonate equivalent in these soils, the genesis of argillic horizons in this region was probably related to the wetter past climate and thus, argillic horizons in these soils are relict features from a wetter climate in lower Quaternary. In some of these studied soils clay coatings have been engulfed by calcium carbonate and in some cases, they have been disappeared completely. The engulfment of clay coatings by calcium carbonate shows that the climate of this region has been changed from wet to arid which caused to calcium carbonate accumulation in argillic horizons and changing them to the complex argillic- calcic horizons and thus, these soils are polygenetic. Moreover, the presence of argillic horizons in these soils shows that this region was geomorphologically stable and the stability of this area along with other suitable condition for translocation of clay particles in profile (such as wetter climate) have been lead to formation argillic horizons in this area.