كليدواژه :
جنب يخچالي , ژلي فلوكسيون , فراسايي دامنه , كرايوپديمنت , كرايوپلانيشن
چكيده لاتين :
Introduction
Cryopediment and cryoplanation are created in arid and semi-arid regions. According to the studies performed around the world, they are usually formed in the preglacial climatic conditions. The cryopediments are very gently concave erosional bench-like landforms that are usually cut into the base of valley sides or marginal slopes of highlands and mountains. The cryopediments developed by cryogenic processes in cold climates of the periglacial climate-morphogenetic zone are mostly created with the presence of permafrost. Cryopediments are developing due to the activity of a range of cryogenic processes whereby frost action, nivation and other processes cause to retreat back scarps. Cryoplanation terraces are bedrock steps or terraces on ridge crests and hilltops. The tread or “flat” area is 10 to several hundred meters wide and long and the slopes from 1 to 5 degrees parallel to the ridge crests. Cryoplanation terraces exist in many parts of the world in present or past periglacial environments. They occur chiefly in non-glaciated regions and near the general altitude of snowline. Cryoplanation terraces are formed by scarp retreat as the result of nivation. Surficial debris is removed across the terrace by mass-wasting. Terrace morphology depends mainly upon climate, bedrock type, and terrace orientation. The aim of this research is to investigate periglacier landforms especially cryopediments and cryoplanation in Tange-Chenar Catchment, located in the south part of Mehriz city, Yazd province. The study region is located in the west slopes of Shirkuh mountain, more than 4000 meters high, from 31° 18´ to 31° 26´ of northern latitudes and from 54° 18´ to 54°29´ of eastern longitudes.
Material and methods
This research has been performed on basis of field studies and direct observation of the basin landforms. Therefore, we visited the area in two time periods involving 13/11/2015 and 30/9/2014. However, we studied topographical and geological maps and DEM for the study area at first. Therefore, the resulted information of the maps is controlled with GPS system in the field. For knowing morphology of landforms and the role of lithology in their formation, we drew geological and geomorphological maps. In addition, to studythe effects of climate on the formation and development of the landforms, we evaluated climate data, especially temperature parameters in the past and present time. Thus, we drew isotherm map for cold season using temperatures data of 8 stations for 14 years. To explore the the forming components of the landforms created in the Pleistocene, we reconstructed temperature data for that time using snowline and equilibrium of ice-water line and drew isotherm map for that time.
Results and discussion
The study area in terms of ground morphology and facies is divided into four categories including mountain, hill, slope terraces and pediments. The results showed that the pediments in the region are different from those in warm arid and semiarid region in the world. Although, both of them have similar morphology with the same slope about less than 12 degrees, they have some significant differences. For example, the cryopediments in the study area have been created and developed as a result of the side valleys. Besides, deposits that cover the surface of cryopediments are not alluvial, while, pediments are covered by alluvial sediment and formed by surface water and floods. Also, one or two and sometimes three terraces can be observed on some slope with relatively flat surfaces. The slope values of their surfaces are often less than 5 degrees. Thick sediments and sometimes suitable soil have covered their surfaces. Thus, gardens and farms had been created on some of them. The width of the terraces varies from 10 to 100 meters. Some of them are still in developing state. Most of the surfaces are located above 2000 meters. Thus, there are alluvial deposits on any of these landforms.
The investigation of climatic parameters shows that these landforms must be currently developed. We also drew an isotherms map for cold season of the year. The map indicates altitudes above 2000 meters where have periglacier condition especially in the cold season. Reconstructed temperature of Pleistocene period also showed that periglacial morphoclimatic system has been dominated on most of the study area. The system has provided favorable context for physical weathering of granites and conglomerate which had formed most of the outcrop stones. The periglacial processes such as creep frost, nivation, congelifluction, rill erosion, dell formation and etc. can form landforms and retreat the slopes.
Conclusion
Cryoplanations and Cryopediments are Quaternary landforms developed in the cold periods of the Pleistocene. The processes responsible for carving cryopediments were frost creep, frost heaving, nivation, congelifluction (solifluction over the permafrost table), rill erosion, piping, dell formation (dells acted as main lines of material removal) and backwearing of the back slope. Although, the morphology of cryopediment is similar to those of the pediments in arid and semiarid region, aridity in both environments can influences weathering and slope processes, which results in similar landforms. But they are created in different condition and different processes. Pediments are created by surface water processes especially sheet washes; while, cryopediments and cryoplanations are formed with permafrost processes and, therefore, their sediments are not alluvial. However, cryoplanations are the landforms created in periglacier morphogenetic system with the context of stones of special outcrop. These stones in the region have been granites and conglomerates.
