Distribution, lithology and provenance of peridesert loess deposits in northeastern Iran

Abstract

Silty deposits cover different landforms bordering the city of Mashhad in NE-Iran. Despite the potential of these deposits to provide important information regarding landscape evolution as well as land use management, little information has been published on their nature and origin. The main objective of this study was to determine the nature and origin of silty deposits in this area. Based on particle size distribution, abrupt particle size change between silty deposits and underlying materials, the shape and microtexture of quartz grains, chemical and geochemical data and geochemical homogeneity of silty deposits from different landforms and geomorphic positions, the silty deposits appear to be wind-blown in origin and are classified as loess. They show a particle size distribution similar to that of typical loess deposits, containing more than 70% silt, mostly coarse silt (35–45%) and low quantities of clay and sand. Bulbous edges and evidence of wind transportation on quartz grains confirm the aeolian nature of the quartz silt. The amount of carbonates and gypsum in loess deposits are mostly in the range of 10–32% and 4–15%, respectively. Silty deposits around the study area are geochemically homogenous. Similar geochemical signatures of gypsiferous marls and loess deposits, the existence of gypsum enriched layers in the loess deposits and the geomorphic position show that gypsiferous marls are the main source of aeolian deposits. This is further confirmed by the existence of detrital palygorskite in loess layers and the presence of a high amount of palygorskite in gypsiferous marls. Since the silt in loess deposits is not the result of weathering processes, the loess deposits of the study area bordering the large deserts of the Central Iranian plateau are defined as peridesert loess.