Detection of Lithologic Discontinuities in Soils: A Case Study of Arid and Semi-arid Regions of Iran

— Knowledge of lithologic discontinuities (LDs) provides important information about sedimentology processes and soil development. The main objective of this research was to evaluate different criteria from the Soil Taxonomy (ST) system to detect LDs in soils of arid and semi-arid regions. For that, five sites in Iran were selected, and several parameters were measured to identify LDs in studied pedons, these were: profile morphology, depth functions of clay-free particle size distribution, clay-free sand to silt ratio, uniformity value index, particle size parameters (mean, sorting, skewness and kurtosis) and particle size mode, morphology of quartz grains and chronological data. Results confirmed that, in some occasions, field evidences (differences in texture, color and parent material that are not the result of pedogenic processes) failed to reveal LDs, laboratory data could show this diagnostic soil characteristic. Also, the results showed that quartz grains morphology revealed a lithologic discontinuity (LD) between the Btk and Cry1 horizons within the pedon at site 1. Skewness and kurtosis parameters and particle size mode of differnt horizons were useful parameters to distinguish and confirm the presence of LD in the soil. Moreover, results showed that the clay-free depth functions of particle size distribution were useful to ascertain the presence of LDs so long, as far as sufficient amounts of the silt and sand fractions were available. Since field evidences were not always clear, the revision of some qualitative rules of the ST system and defining quantitative criteria for an abrupt change in particle size distribution seems necessary to improve its ability for LDs detection.