Geomorphological Investigation of the Tectonic Cuesta of Negur (Tolki Mountain) and its Role in the Development of Urban and Rural Settlements

Wielding a direct effect on the local climate and water resources across different latitudes, particularly arid areas, and creating favorable living conditions, geomorphological phenomena are among the primary factors driving urban and rural settlements' development. One example of such geomorphological features is the cuesta, a single-dip, inclined structure with hard upper strata and a slope of less than 15 percent on one side, and asymmetrical, steep valleys extending normal to the loose sedimentary rock strata. Different theories have been presented regarding cuestas in Iran. Some argue based on geological grounds that cuestas do not exist in Iran, whereas proponents recognize the features as pseudo-cuesta. However, the authors of this paper believe in the existence of both cuesta and pseudo-cuesta, as geomorphological features depend on the form and not the geological properties or age. In this light, an attempt was made to prove this theory by investigating the geomorphology of the largest cuesta in southeast Iran, the cuesta of Negur, located 30 Kilometer east of Chabahar City, covering 221.66 Square kilometer and 92.69 of Perimeter at 25°19′05″ to 25°28′55″N and 60°57′42″ to 61°09′47″E coordinates on the coastal plains of the Gulf of Oman.

The field study was carried out, relying on RS and statistical analysis. Different sets of data and statistics, including topographic and geological maps, ALOSPALSAR and ASTER DEMs, satellite images from Sentinel-2 and Landsat8, radar data from Sentinel-1, daily statistics of synoptic weather stations of Area, and climatology, rain gauge, and local CRU data. The present study uses RS and GIS for analysis. Geostatistics was used to calculate the temperature and annual precipitation in the area was performed using Simple Cokriging. The stream network layer was prepared using the ArcHydro extension. Moreover, the SAVI Index was used to create the vegetation map, and radar data from the Sentinel-1 and digital elevation models 30 meter, as well as the DInSAR technique With a time difference of 27 months, were adopted to determine the reasons for the formation and evolution of the Cuesta.

The Makran mountain range is the result of the subduction of the Gulf of Oman's oceanic crust under the Iranian Plate. However, the transverse faults also make a notable contribution to the fracture and changes in the heights to which the formation and evolution of the cuesta can be attributed. The cuesta faces west extending to the east with a gentle slope until reaching the plains on the coast of the Gulf of Oman. The hard and thin caprock is composed of conglomerate and sandstone dating back to the Pleistocene and tops a loose layer of thick, fossiliferous, chalky and lime marls dating back to the Late Miocene. The cuesta stretches on a 2.99 percent slope from its forehead of cuesta (outcrop) to coastal plains. The cuesta reaches a maximum height of 545 Meter at the outcrop, and dips to a 30 Meter elevation, making an average elevation of 153 Meter from the Gulf of Oman. The elevation difference promotes climate change on a local scale. The average temperature and rainfall in the cuesta region were calculated at 27.8° Celsius and 130 millimeter, which is different from the conditions at the Chabahar Synoptic Weather Station. Given the fine-grained texture of sediments, high humidity, and low evaporation, the temperature drop and more precipitation over the heights provide abundant water resources to the downstream, particularly at the ends of cataclinal streams of the cuesta. Due to the fine-grained soil, vegetation covers 1264 hectares only at the foot of the mountain and in areas with suitable soil porosity and water resources. A remarkable feature of this area is its rainwater reservoirs, locally referred to as Hootaks, often constructed in arable areas. Accordingly, most of the local vegetation is concentrated around these Hootaks. The DInSAR (Differential SAR Interferometry) approach was used to explain the formation and evolution of the cuesta. The results are suggestive of the rise due to the subduction and active tectonics in the area. Accordingly, over a 27- month period, a maximum tectonics- induced rise of 2.8 centimeter was recorded southwest of the cuesta outcrop, as well as a maximum descent of 2.5 centimeter caused by the stream erosion in the floodplains southeast of the geomorphological feature. According to the above evidence, the cuesta of Negur is of the tectonic type and is the result of the simultaneous rise of the coastline due to the subduction of the Gulf of Oman and tectonics of the transverse fault pushing the cuesta outcrop upward.

Given that most urban and rural settlements in the area are established at the lowest part of the cuesta along the cataclinal (consequent) streams, this part of the cuesta houses most of the population. According to a 2016 consensus Negur, the highest urban area across the coastal plain of the Gulf of Oman sitting at a 40 meter elevation, is home to 5670 in 1320 households, as well as 45 small and large rural areas with an overall population of 17,118 in 3896 households. In total, the area houses a population of 22,778 who have dug large wells known as Hootaks to make use of surface water resources of the cuesta. A total of 558 Hootaks have been dug for agricultural, husbandry, and other purposes to hold water, promoting vegetation in the surrounding area. Indicating the remarkable volume of runoff in the area, 627 hectares of the entire 1264 hectares covered in vegetation are concentrated around the Hootaks downstream of the cuesta. The substantial water resources can be attributed to the temperature drop and promoted precipitation at the 545 meter height, which creates a substantial discharge given the high humidity of the Gulf of Oman region and the low permeability of the loose layers supporting the cuesta. Accordingly, before discharging into the Gulf of Oman, the surface runoff is trapped in the Hootaks to be used. In this light, the geomorphological role of the cuesta of Negur was found to be crucial in the establishment of local urban and rural settlements.