JWSS - Journal of Water and Soil Science

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Gypsiferous soils, as the most characteristic soil in arid and semi-arid regions, are widespread in Isfahan Province. The north-west region of Isfahan is a closed basin and is surrounded by geologic formations. This area is a part of Zayandehrood watershed. The origin of gypsum in this area was investigated in this study. Considering the geologic distribution of gypsum, twenty-two locations, suspected to have been affected by gypsum, were selected for sampling. Natural and artificial excavations were used to discover their relationships. To distinguish the differences between fresh and weathered shales, some fresh samples were taken from the depth of Shemshak formations (a lead mine).

Field observations showed that in Shemshak and Nayband formations, gypsic crystals were present within the layers of weathered shales, and pyrite was observed only in the mine shales. In weathered red conglomerates (Lower Cretaceous), considerable amounts of gypsum had accumulated. In the hydrothermal veins of Shemshak and Nayband formations, gypsic crystals were found in matrix or between rock layers. In some parts, the Qom formation (Evaporate sediments) contained plenty of gypsum.

From field observations and analytical data, such as mineralogy of untreated and floated rock powders, determination of residual sulfides in shales, and the examination of thin and polished sections of rocks, we conclude that the origin and main resources of gypsum in our study area are as follows:

1. Gypsum being released from Cretaceous limes particularly from red conglomerates

2. Oxidation of pyrite and any sulfidic minerals existing in shales

3. Gypsum being released from evaporates (Qom formation and Oligomiocenic Marls)

4. Gypsum formed through hydrothermal reactions during late Tertiary.

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Geopedology is a systematic approach of geomorphic analysis for soil mapping that construct field operation mainly upon the work in sample area. The main goal of this research is to determine the effect of location of sample area and expert knowledge on credibility of generalization of the results obtained from geopedological approach for similar landforms in south-east of Borujen area. Upon preparation of primary interpretation map of the study area on air photos (1:20,000 scale), and considering different locations of Pi111 unit that encompasses the maximum surface of the study area, the sample area was planned in three different locations. Then, a second-order soil survey was conducted and final soil map was prepared. Also, two different experts were considered to determine the amount of credibility of generalization of the results obtained through geopedological approach for the mentioned unit. Results showed that changing the location of sample area has taxonomic levels (order, subgroup and/or family) and map unit type (complex and consociation) differences in Pi111 unit. In spite of similarity of the profiles selected by the two experts, soil taxonomies of these profiles were different in comparison with representative pedons (at family level). Therefore, the use of landform phases is recommended to increase the accuracy of geopedological results.