تغيير و تحول خاك هاي تشكيل شده از مواد مادري رسوبي مختلف در ناوديس شيخ، شمال شرق بجنورد

Evolution of the soils derived from different sedimentary parent rocks in Sheikh Syncline, northeastern Bojnourd

به منظور مطالعه تاثير ويژگي هاي سنگ مادر بر ويژگي هاي خاك، تنوعي از سنگ هاي رسوبي از شش سازند در ناوديس شيخ (شمال شرق بجنورد) انتخاب گرديدند كه شامل سنگانه (شيل)، آب دراز و كلات (سنگ هاي آهكي) پسته ليق (سنگ رس)، خانگيران (ماسه سنگ) و نئوژن (مارن گچ دار) مي باشند و مكان حفر خاكرخ ها بر روي پايدارترين سطوح ژئومورفيك موجود بر روي سازندها و به گونه اي انتخاب گرديد كه خاك ها بيشترين وابستگي را به سنگ مادر زيرين خود داشته باشند. بجز خاكرخ هاي مطالعه شده بر روي سازندهاي كلات و آب دراز كه بر روي شيب كناري حفر شده اند، ساير خاكرخ ها بر روي قله شيب حفر شدند. مقايسه تكامل خاك هاي حاصل از سنگ هاي آهكي نشان مي دهد كه درجه تكامل آنها متاثر از تفاوت نوع سنگ هاي آهكي مي باشد؛ همچنين وجود ذرات درشت كوارتزي در ماسه سنگ و نبود كاني هاي قابل هواديدگي محدوديت اصلي براي تحول خاك از ماسه سنگ است. نتايج نشان مي دهد كه تكامل خاك از سنگ هاي مادري ريز بافت شامل شيل، سنگ رس و مارن گچي متاثراز مقدار و نوع كاني هاي رسي و نيز مقدار گچ مي باشد به طوري كه فراواني اسمكتيت به همراه درصد زياد رس موجب تشكيل خاك هاي ورتي سول بر روي سنگ رس شده است در حالي كه با وجود اسمكتيت قابل توجه، حضور مقدار زياد گچ در خاك هاي حاصل از مارن هاي گچي موجب تعديل انبساط و انقباض و مانع تشكيل ورتي سول مي شود. نتايج نشان مي دهد كه پايداري زمين نما، استعداد خوب هواديدگي و تركيب كاني هاي رسي موجود در شيل از مهم ترين عوامل تشكيل خاك هاي عميق تر و با تكامل بيشتر بر روي شيل هاي سازند سنگانه) مي باشند.

Introduction Dependence of pedogenic processes on parent materials plays a key role in soilscape diversity in arid and semiarid areas. In order to study the effects of parent rock types on soil evolution in arid areas, a sequence of soils, comprising six soil profiles, derived from different lithologies of six formations in Sheikh syncline, Kopet Dagh basin (Northeastern Iran), were characterized based on micromorphology, clay mineralogy and physico-chemical analysis. Materials and Methods Sheikh syncline is a unique area for having a good variety of formations forming Kopet Dagh depositional basin. This site has rock outcrops of different formations from cretaceous to neogen. Six soil profiles formed on selected lithologies of six formations were characterized using physicochemical, micromorphological and clay mineralogical analyses. The selected parent rocks are shale (sanganeh formation), limestones (Kalat and Abderaz formations), sandstone (Khangiran formation), claystone (Pestehligh formation) and gypsiferous marl (Neogen). All soil profiles developing residually from underlying parent rocks and occurring on the summit position of the landscape except for the profiles were developed from limestones of Kalat and Abderaz which are in side slope. The soil moisture and temperature regimes are aridic and mesic, respectively. Results and Discussion Soil development studies of soils derived from sandstones revealed that parent material is a main factor responsible for development of beginning A- horizon over C- horizon as indicated by weak single grain to crumb microstructure in micromorphological observations. Clay mineralogical studies of these soils show that although smectite is a dominant clay mineral in their clay fraction, the abundance of quartz and absence of weatherable mineral in silt and sand fraction controls their characteristics. Comparisons of the soils formed on limestones of Kalat and Abderaz formations revealed that although calcareous nature of their parent material and pedon position in landscape are the main factors impeding profile development, the more stable landform and more intensive vegetation cause calcic horizon development and calcite redistribution in Kalat soils. Biological calcite redistribution in the form of sparitic crystals of cytomorphic calcite and acicular calcite coating are the main features observed in thin section studies. Soil genesis studies on the soils derived from clayey parent materials (claystone, marl and shale from Pestehligh, Neogen and Sanganeh formations, respectively) show that the presence and proportion of smectite and gypsum play a key role in creating vertic properties and Vertisols formation. A High amount of smectitic clay in the soils associated with claystone resulting in development of features resulting from argillipedoturbation such as slickenside, diffused horizon boundary and cracks in dry season. Decalcified matrix and planes as a dominant void type are the main features in the thin section studies. In comparison with these soils, substantial amount of gypsum in the soils developed from marl can reduce soil extensibility. Dissolving of gypsum crystals in wet seasons can modify soil swelling while its crystallization in dry season leads to modification of soil shrinkage. More landscape stability and weatherability of shales of Sanganeh formation cause the formation of the deepest and the most developed soils in the study area. Calcite illuviation is the main pedogenic process as revealed by calcite pedofeatures and calcite depleted zones in thin section studies of the soil solum. Illuviation processes completely remove gypsum from upper horizons of the soil. In spite of the other studied soils, these clayey soils do not have smectite in their clay mineral composition and there is no sign of vertic properties. Clay mineralogical variation studies show that there is no difference in the type of clay minerals between soils and their parent materials but in some profiles the relative amount of clay mineral is different due to weathering conditions. While there is no considerable change in relative distribution of clay minerals between parent materials and soils formed on marl and limestone of Abderaz formation, XRD patterns of more developed soils formed on claystone, shale and limestone of Kalat formation revealed a decreasing amount of chlorite in soils relative to parent material as indicated by variation of chlorite / mica and chlorite/ smectite ratios between soils with underlying parent materials. In comparison with mica and smectite, chlorite is more weatherable and can transform to smectite even in arid and semiarid conditions. Finally, it could be concluded that the presence of smectite is necessary for creating vertic properties because there is no vertic property in the absence of smectite in clayey soils developed on shale. In addition, vertic properties are dependent not only on type and clay percentage but also on gypsum content of the soils.