Evaluating the Effects of Sedimentary Cycles (Aeolian and Fluvial) on Chemical Weathering Indices in Rafsanjan Region, Southeast of Iran

Abstract

Chemical indices are widely used for characterizing the degree of weathering. In this study, we focused on evaluating the capability of chemical weathering indices to distinguish the sequence of aeolian-fluvial sediments in an arid region of Iran. Seven dominant geoforms were selected in Rafsanjan region, southeast of Iran, namely pediment, alluvial fan, margin of pediment and sand sheet, desert pavement, margin of fan and cultivated clay flat, active drainage, and margin of fan and uncultivated clay flat. One representative pedon was selected, described, and sampled for each geoform. The soil physicochemical properties of different horizons of each pedon were determined. We calculated various weathering indices, including weathering index of Parker (WIP), product index (PI), chemical index of alteration (CIA), silica-sesquioxide ratio (Kr), and CIA/WIP ratio (IR), and elucidated their depth distributions. The heterogeneity of the parent material within a given pedon was confirmed with field evidence, depth functions of clay-free sand fractions, and the uniformity value (UV) index. The horizon sequence with lithologic discontinuities (LDs) indicated that the studied pedons were formed by cyclic deposition of aeolian and fluvial sediments. The vertical variations of the weathering indices as well as the vertical trend of the Al2O3/SiO2 ratio (as a grain size index) were entirely consistent with the presence of the LDs. The results suggested that different factors, such as grain size, sedimentation cycling, and their interactive effects, should be considered in order to accurately assess the vertical trend of chemical weathering indices.

Keywords

• geochemical composition
• grain size
• lithologic discontinuity
• sedimentation cycling