تحلیل عوامل تاثیر گذار در شکل گیری پهنه های بدبوم منطقه لارستان در جنوب استان فارس

 در نواحی جنوبی استان فارس بویژه مناطق مجاور لارستان پهنه‎های پراکنده‎ای مشاهده می‎شود که مشخصه‌های ژئومورفولوژی عنوان شده در خصوص بدبوم‎هامانند فقدان کف‎پوش رگولیتی، شبکه متراکم آب‌گذرها، خندق‎ها و تپه‌های مجزا و پرشیب را نشان می‎دهند. در پژوهش حاضر با توجه به فقدان مطالعات علمی در خصوص پهنه‌های بدبوم جنوب استان فارس و اهمیت این مناطق در بحث آمایش سرزمینی، عوامل موثر در شکل‌گیری آنها را بر اساس مجموعه‎ای از داده‎های زمین‎شناسی و رقومی مورد تحلیل قرار داده‎ایم. مشاهدات زمین‎شناسی حاصل بررسی‎های میدانی و نقشه زمین‎شناسی تهیه شده بوده و داده‎های رقومی از ‎مختصات مکانی محدوده مورد مطالعه نیز با استفاده از تصویر ماهواره‎ای این محدوده فراهم آمده است. از داده‎های زمین‎شناسی در تعیین جنس طبقات سنگی و وضعیت ساختاری زمین در بدبوم‎ها استفاده شده و از داده‎های رقومی هم پس از پردازش برای تهیه نقشه دم و دیگر نقشه‎های مورفومتریک منطقه در نرم‌افزار آرک جی آی اس استفاده گردیده است. بر پایه مشاهدات زمین‎شناسی می‎توان دریافت بدبوم‎ها در بخش‎هایی بوجود آمده‎اند که سازندهای زمین‌شناسی نرم با مقاومت کم در برابر فرسایش، بویژه طبقات مارنی و گچی سازندهای گچساران و میشان، به سبب شیب کم طبقات یا چین‌خوردگی فشرده با ضخامت زیاد در سطحی وسیع رخنمون گردیده‎اند. در این بخش‎ها شکل‎گیری پهنه‎های بدبوم تحت تاثیر فرسایش شدید ناشی از بارش‎های نادر اما شدید که از مشخصه‎های شرایط آب و هوایی گرم و خشک است صورت پذیرفته است. در تصاویر ماهواره‌ای پهنه‌های بدبوم از رنگ و تنالیته متمایز در قیاس با دشت‎ها و ارتفاعات برخوردار هستند و بر این اساس موقعیت تقریبی آنها را می‎توان مشخص نمود. با این حال برای دقت بیشتر و همچنین دست‌یابی به ابزاری برای مطالعه چگونگی تاثیر فرسایش بر لندفرم‎ها و میزان تولید رسوب طی بازه‎های زمانی مختلف تهیه نقشه دم و دیگر نقشه‎های مورفومتریک ضروری می‎باشد. بررسی انواع نقشه‎های مورفومتریک منطقه نشان می‎دهد که با استفاده از نقشه جهت‎های جریان به خوبی می‎توان موقعیت مکانی پهنه‎های بدبوم را مشخص نمود.  

analysis of influential factors in the formation of badlands in the larestan region, southern fars province

1- introductionbadlands are unique geological formations characterized by their dry, eroded landscapes, which often feature steep slopes, minimal vegetation, and intricate drainage systems. this terrain arises primarily in arid to semi-arid regions where softer sedimentary rocks and clay-rich soils have been extensively eroded by water and wind. the result is a landscape filled with canyons, gullies, buttes, hoodoos, and other striking geological features. badlands are formed through a combination of geological processes primarily involving erosion and deposition. the key factors contributing to the formation of badlands are presence of soft sedimentary rocks such as clay, shale, gypsum and anhydrite which are highly susceptible to erosion, arid climate that results in sparse vegetation cover and heavy rainfall. in a large part of the southern regions of fars province, especially in the larestan areas, there are regions that match the geomorphological characteristics described for badlands (figure 1). in general, scientific papers on the badlands subject in iran are little, and in particular there is not any research work on larestan’s badlands. in figures 2 and 3 two examples of landscapes seen in the larestan’s badlands are displayed. the lack of any scientific reports on badlands of the larestan area raises an important question, how it could be done the water resource management and agricultural activities while the necessary data such as annual sediment yield and its impact on the quality of surface and groundwater is not available. 2- methodologydata and methods which employed in the current research to identify the natural characteristics and effective factors contributing to the formation of badlands in the larestan region are as follows:2-1 field observation: during field studies, the surface pavement type, slope, aspect, lithology, vegetation cover, and geomorphological features such as landform shape and drainage patterns were investigated (figures 2 and 3). furthermore, the accuracy of the badlands areas identified in the satellite image and the prepared morphometric maps was assessed.2-2 satellite image: using satellite imagery of the study area, the approximate extent of the badlands regions was determined based on the variations in color, tone, and texture of these areas compared to the plains and highlands. additionally, using the same imagery, gpx data containing the geographic coordinates and elevations of points in the study area was obtained by uploading the kmz image file to the gps visualizer website. this data was utilized in arcgis software to create a digital elevation model (dem) and other morphometric maps of the study area, including flow directions, slope aspect, and additional morphology-related maps such as slope change, flow accumulation, drainage density, and drainage systems maps.2-3 geology map: the geology map of the studied area (figure 4) was created by combining five 1:100,000 scale maps of the lar, mahleka, bastak, chauhar berke, and gav bast regions using arcgis software to examine the exposed rock layers of geological formations and associated geological structures. since the resistance of rock strata to erosion is influenced by their lithology, and ongoing rapid erosion necessitates a significant thickness of soft rock strata, our data on the lithostratigraphic sequence of the larestan region is particularly crucial. according to geological studies, the fars group, which encompasses the mishan, gachsaran, and aghajari formations (figure 5), is significantly represented in the study area, with a thickness of approximately 3,000 meters. it is situated above the resistant carbonate strata of the asmari-jahrom formations and below the quaternary conglomerate strata of the bakhtiari formation. an examination of this map indicates that in the badlands, the extensively exposed rock strata are primarily from the gachsaran and mishan formations, consisting of marl and gypsum layers.2-4 morphometric maps:a) dem: this map illustrates regions with comparable elevations using a consistent color scheme (figure 6). it serves as foundational data for generating other morphometric maps. additionally, because the elevation of badlands is greater than that of plains but lower than that of mountain ranges, they typically exhibit a distinct local elevation range and are depicted in a uniform color on the dem. in the study area, the majority of badlands correspond to yellow and pink hues. b) flow directions map: this map categorizes the area according to the number of surface water flow paths (figure 7). in other words, it illustrates the density of water channels in various regions. given that a key characteristic of badlands is the abundance of drainage channels, this map serves to pinpoint their locations. based on field observations, the flow direction map is most effective in identifying badlands areas. c) aspect map: this map categorizes the study area based on the orientation of slopes in relation to the north. in other words, it illustrates the aspect or azimuth of slopes in each section using specific colors. in badlands regions, the presence of numerous drainage channels in various directions leads to a wide range of slope orientations. consequently, the areas with the most significant color diversity on this map correspond to the locations of badlands (figure 8). the gachsaran and mishan formations, composed of marl and gypsum, are prominently exposed.3-  resultsbased on investigations into geology, various morphometric maps, and field observations, it can be concluded that a combination of several factors—including geological, tectonic, and erosional processes under arid climatic conditions—has played a fundamental role in the formation of the badlands in the larestan region. in the larestan region, three types of geomorphologies—mountains, plains, and badlands—can be observed, depending on the prevailing geological and tectonic conditions (figures 1 and 6). the mountains are formed at anticlines and faults, showcasing resistant rock layers from formations such as aghajari, asmari-jahrom, and daryan. in the gently sloping areas of folds and synclines, where sediment gathers, plains can be found. badlands arise in regions where soft rock layers, prone to erosion, are prominently exposed. the extensive outcrop of these layers results either from their gentle dip or, as seen in the badlands northwest of evaz city, from the dense folding of the layers (figure 9). the rapid and severe erosion of these rocks, caused by infrequent yet intense rainfall in arid climates, has created the necessary conditions for the formation of the distinctive geomorphology of the badlands in these areas. investigation of various morphometric maps, along with the assessment of field observations, revealed that the maps of flow directions, aspect, and curvature exhibit the highest accuracy in detection of the spatial location of the badlands. in contrast, the maps depicting drainage density, flow accumulation, stream order, and stream network are not particularly effective, likely due to the loose nature of alluvial deposits and the presence of the exceedingly high number of surface drainage channels.4- discussion & conclusions although this research focuses on a specific geographical area, larestan, it is important to note that, due to similar geological, tectonic, and climatic conditions across a much larger region in southern iran, the results and findings can be generalized to areas beyond larestan. the formation of anticlines, synclines, and faults—fundamental elements in the geomorphology of various regions, including the badlands—is primarily influenced by tectonic forces and stratigraphic characteristics. in the study area, badlands landscapes develop in regions where the fars group formations are visible. these formations can be observed in many southern parts of the country, extending to the shores of the persian gulf. based on the lithology of the exposed formations in the larestan region, it is clear that soft, low-resistance layers are located between three series of hard, erosion-resistant layers. the soft gachsaran layers are situated between the resistant limestone layers of the asmari-jahrom formation and the gouri member, while the soft mishan formation is located between the resistant layers of the gouri member and the aghajari formation (figure 5). the stratigraphy of the fars group and its lithological diversity are crucial elements that contribute to the varied landscapes observed in the badlands of the larestan region (figures 2 and 3). continuous erosion necessitates the ongoing vertical movement of the earth; thus, the formation of badlands requires a balance between erosion and uplift rates. the badlands of the larestan regions have evolved into areas where extensive outcrops of soft marly and gypsiferous strata are present, and the rate of uplift has not been so rapid as to cause the complete erosion of the younger mishan and gachsaran formations. comparing the elevation above sea level in regions with differing geomorphology shows that the elevation of the badlands is 200 to 400 meters higher than the surrounding plains, and roughly the same amount lower than the nearby mountain ranges (figure 6). in the mountains of larestan, where the uplift rate has been more rapid, we observe increased erosion of the soft and young layers of the gachsaran and mishan formations. as a result, older rock layers are now exposed in these regions. while it is possible to identify the locations of large badland areas using satellite imagery, morphometric maps seem essential for pinpointing smaller and more dispersed areas. furthermore, to assess the extent of erosion and the volume of sediment produced, it is crucial to utilize dem maps prepared at various time intervals, and their changes should be thoroughly examined and analyzed.