The canyon of Rud-Mojan waterfall in Torbat-e Heydarieh city is one of the significant Quaternary geological features of the Noth eastern zone of central Iran. This canyon was formed on an area of Lower Cretaceous Rudist limestones, containing Orbitolinid (Barremian-Albian) foraminifera, and from the beginning of the Quaternary. A major deformation and severe erosion have occurred in its limestone beds. The source of its waterfall is related to the quaternary karst water reserves of the northern heights of the Mojan River. In this research, using Corbel's equation (1959) and field studies, the erosion rate of the Mojan waterfall canyon has been calculated. Considering the tectonic and faulting conditions of the region, the creation of this canyon can be considered affected by tectonic-faulting factors and at the same time, water erosion. The results of the calculations indicate that the erosion rate of limestone dissolution in the canyon of Rood Mojan waterfall is about 188.14 mm/thousand years, which shows the relative effectiveness of this model.
Introduction
Dissolution erosion in carbonate rocks and masses depends on several factors. Among these factors are the turbulence or linearity of the system, the flow velocity, the separation of irons from mineral surfaces, the amount of carbonic acid (H2CO3), transport processes, partial pressure of CO2, PH and other factors (Dreybrodt, 1998).
One of the most important studies on karst erosion to date was conducted by the French geographer Corbel (1959). He examined the waters drained by rivers in cold regions and compared them with those in tropical regions in terms of the amount of dissolved calcium carbonate, and concluded that the rate of karst erosion in cold regions was greater than in tropical regions, and this can be justified by the amount of carbon dioxide dissolved in cold and warm water (Corbel, 1959). The main goal of this research is to calculate the karst erosion rate of the Rud-Majan Waterfall Canyon using the Corbel equation.
Material and Method
The canyon (deep valley) of Rud-Majan Waterfall is one of the prominent geological features in the east of the Central Iranian Zone, formed on a Cretaceous (Barremian-Aptian) Rudist limestone area, and especially since the early Quaternary, major deformation and severe erosion have occurred in its limestone bed. Rud-Majan Waterfall is located in Torbat-e Heydarieh province. This waterfall is located in Central Iranian and originates from the Chehel Tan mountain range located 54 km west of Torbat-e Heydarieh city and has an altitude of about 28 meters (Modaresi, 2019). Its geographical coordinates are between latitudes 35°18'50"N to 35°20'55"N and longitudes 58°49'40"E to 58°52'45"E. Among the important tectonic structures, the east-west trending Daruneh fault can be mentioned, and the lithology of the northern part of this fault mainly consists of Paleogene volcanic rocks with inclusions of Nummulitic limestones.
Among the limestones that this waterfall has eroded are those of Lower Cretaceous age. These limestones include Rudist and limestones with benthic foraminifers (Orbitolinides) and are of Barremian and Aptian age and equivalent to the Tirgan Formation in the Kopeh-Dagh Basin and are ridge-forming. These deposits include very thick, strong, rock-forming marly limestone, marl, and calcareous shale, rich in Orbitolinides and Miliolids.
Results
By definition, the incorporation and transport of materials by a fluid agent such as water, ice, and wind are called erosion. Fairbridge (1968) also refers to chemical erosion as opposed to corrosion, and some have considered it to be the same as chemical weathering.
Corbel (1959) proposed an equation to estimate the rate of karst erosion :
(1) X=4ET/100
(2) X=4ETn/100
where X is the erosion rate in millimeters per thousand years (mm/ky) or cubic meters per square kilometer per year, E is the depth of runoff in decimeters (dm), and T is the average concentration of dissolved solids (dissolved calcium carbonate) in water in milligrams per liter (Corbel, 1959) (mg/L).
To use the formula, it should be noted that the density of carbonate rocks should be between 1.5 and 2.9, and for dolomites, the water hardness and temperature must be measured to obtain the dissolution rate through precipitation, and to determine sulfate rocks, the calcium ion content and the hardness of carbonate rocks must be measured. Using the available data and the Corbel equation, the karst erosion rate in the study area was obtained as 188.14 mm/thousand years.
Conclusions
The Rud-Majan Waterfall Canyon in Torbat-e Heydarieh County is one of the most significant geological features of the northeastern part of the Central Iranian Zone, formed on Lower Cretaceous limestones containing Orbitolinid foraminifera (Barremian-Albian). Since the early Quaternary, major deformation and severe erosion have occurred in limestone beds. In this study, using the Corbel equation (1959) the erosion rate of this canyon can be estimated to be equal to 188.14 mm/thousand years. Considering the tectonic and fault conditions of the region, the formation of this canyon can be regarded as being affected by tectonic-fault factors and at the same time, water erosion.
References
Corbel, J., (1959). Erosion en terrain calcaire (vitesse d’érosion et morphologie). Annales de géographie, 68(366), 97-120. https://doi.org/10.3406/geo.1959.16541
Dreybrodt, W., (1998). Limestone dissolution rates in karst environments. Limestone Dissolution Rates in Karst Environments, (16), 167–183.
Fairbridge, R., (1968). Corrosion, etching, Springer Link. Retrieved February 21, 2024, from https://link.springer.com/referenceworkentry/10.1007/3-540-31060-6_69