عنوان مقاله [English]
The word "Loess" has been derived from the German word "Lӧs", meaning loess, which was used for the first time by Leonard in 1824. Regarding the definition of loess presented by many authors, aeolian origin and silt fraction dominance have been accepted as the two main attributes of loess deposits. Wind- blown deposits of loess are characterized by a lack of stratification and uniform sorting and occur as blanket deposits (Bell, 2000). The common mineralogical composition of loess deposits is as follows: quartz, feldspars, carbonates, mica and heavy minerals in the silt fraction. According to Pye (1995), four fundamental requirements are necessary for its formation; a dust source, adequate wind energy to transport the dust, a suitable accumulation area, and a sufficient amount of time.
The study area is situated in southeastern coast of the Caspian Sea. Most of this area is covered by Quaternary deposits including loess. The loess deposits, in this area have questionable distributions, and are as the bed of specific landforms. Moreover, they often have sufficient void space in their natural state, high porosity, high permeability, and low compaction. This may be representative of collapse, subsidence, landslide, and liquefaction. The aim of this research is answer the questions: what is Loess? How are their distributions in Iran? How are distributions of loess deposits in the study area? What is the role of loess deposits on morphogenesis systems? And what is their role in environmental management?
Materials and methods
In this research, field works and document data such as satellite images (LANDSAT of Mars 19, 2011, July 18, 2011, and July 27, 2011), and NIOC geologic maps at 1:100,000 and 1:250000 scales have been used. Therewith, the Digital Elevation Model (DEM), acquired from Digital Topography Maps of National Cartographic Center of Iran (NCCI), at scale 1:25,000, and its derivatives and also land use maps were used for spatial analysis. Field observations were carried out in May, 2012, afterwards the spatial distribution of loess was mapped and the spatial analysis based on GIS functions were performed to show the role of loess deposits on morphogenesis systems.
Results and discussion
Most of this area is covered by Quaternary deposits including Quaternary marine deposits, alluvial deposits, debris and landslides, and wind deposits. Loess as wind deposit covered a considerable part of the research area. The loess distribution analysis indicates that loess is found in two regions. One of them is situated between Gorgan and Atrak Rivers in Golestan province and the other one in foothills of northern slope of Alborz mount, which totally covers an area of about 1011 square kilometers.
Neka, Gorgan and Atrak River basins all have different amount of loess deposits. Loess elevation distribution analysis shows that about 77% are located in elevations of 100 to 400 meters. Loess slope distribution analysis expresses that 88.29% are located in 1 to 15 degrees of slope. So it can be concluded that Loess deposits in north of Iran cover evenly the area, but over time and due to highly erosion they remained just on the eroded slops, behind anticlines, hills and low- slope mount sides. The analysis of loess distributions according to landuse map indicates that 13% percent of loess sediments are used for agriculture and in Garden they are covered about 15% by forest. While more than 63% of loess areas is covered by pastures. It is recommended to use these sediments for pasture and forests for management of erosion. The analysis of spatial loess distributions and villages show that within 13 to 28 percent of villages including Tamran, Roshan Abad, Atrak, Anjirab, Yanghagh, and Agh Abad are located inloess depositions. Therefore, to prevent environment hazards any environment development plan must be associated with exact assessment and usage of different ways of loess compacting.
Presence of high percent of loess deposits in low slope and low elevation areas is the consequence of high erodibility of the deposits. Regarding the high erodibility of loess deposits, it seems that the existence of loess deposits in low slope and low elevations is the consequence of fluvial processes over time. The presence of cutting hills and different erosion types such as rill erosion, gully erosion, piping, and badland erosion in the research area confirm this fact.
The loess distribution analysis considering landuse map indicates that a remarkable parts of loess areas is pasturage. It seems very important to prevent intensifying erosion in this area, prevention of pasturage destruction and their improvement.
Existence of considerable thickness of loess, that contains silt and clay which can absorb and retain high moisture, and their highly coincidence with low-slope area have provided suitable conditions for agricultural activities. Consequently, gully erosion has been intensified. Different erosion types, such as rill, gully, piping, and badland are found in the study area because of loess deposits. The loess distributions in upstream of basins result in an increase in sediment load of rivers especially in Gorganroud. Therefore, the viscosity of floods and their destructive effects are increased. The results of this study indicate that morphogenesis systems in this region are strongly affected by loess deposits distribution and their land use type. Ultimately, this can be said that southern and eastern Caspian Sea loess deposits have effective role in landform and landscape genesis and also their specific attributes are reason of various environment hazards. Hence, their spatial analysis can be useful in environmental management and hazard prevention.