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Volume 37, Issue 2 (2022)
GeoRes 2022, 37(2): 285-293 |
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Received: 2021/11/17 | Accepted: 2022/02/5 | Published: 2022/04/8
Received: 2021/11/17 | Accepted: 2022/02/5 | Published: 2022/04/8
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Beiramali Kivi F, Sabokkhiz F, Amirahmadi A, Ghahroudi Tali M, Jamalabadi J. Investigation of Sedimentary Environment Changes in Quaternary Deposits of Mahdasht Alluvial Fan. GeoRes 2022; 37 (2) :285-293
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1- Department of Geomorphology and Climatology, Faculty of Geography and Environmental Management, Hakim Sabzevari University, Sabzevar, Iran
2- Department of Physical Geography, Faculty of Geographical Sciences and Planning, University of Isfahan, Isfahan, Iran
3- Department of Physical Geography, Faculty of Earth Sciences, Shahid Beheshti University, Tehran, Iran
4- Department of Physical Geography, Faculty of Geography and Environmental Management, Hakim Sabzevari University, Sabzevar, Iran
2- Department of Physical Geography, Faculty of Geographical Sciences and Planning, University of Isfahan, Isfahan, Iran
3- Department of Physical Geography, Faculty of Earth Sciences, Shahid Beheshti University, Tehran, Iran
4- Department of Physical Geography, Faculty of Geography and Environmental Management, Hakim Sabzevari University, Sabzevar, Iran
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Introduction
Facies landscapes and their physicochemical characteristics play a major role in the formation or modification of the Earth’s surface morphology [Böse, 2014]. Many facies landscapes are currently located in regions where Quaternary climatic changes have created environmental conditions markedly different from those prevailing at the time of their formation. The impact of sediments associated with these facies, typically derived from highly erodible formations, on the existing erosional system and, ultimately, on regional development is considerable [Derruau, 2000].
The Quaternary represents the most recent period of the Cenozoic Era and is characterized by pronounced climatic and biotic transformations. This period, which is also regarded as the time of human emergence, experienced repeated and extensive climatic fluctuations, giving rise to multiple glacial and interglacial phases worldwide [Kavyani & Alijani, 2019]. Numerous studies have addressed the Quaternary and its remaining geomorphic and environmental legacies [Motamed, 2003]. In Iran, several researchers have examined Quaternary geomorphic transformations. Some have argued that colder and drier climatic conditions than those of today prevailed over Iran, estimating that during glacial periods the snowline in the Alborz and Zagros Mountains was approximately 600–800 m lower than the present snowline, with mean annual temperatures about 4–5°C lower than current values. In contrast, others have suggested that during the Pleistocene (the Quaternary glacial age), Iran experienced colder and more humid conditions than at present, thereby favoring water accumulation in low-lying depressions [Huckriede et al., 1962; Blanford, 1973; Van Zeist & Wright, 1963]. In other words, a slight increase in precipitation combined with a marked decrease in evaporation due to lower and more persistent temperatures during these periods is considered responsible for a positive moisture balance and, consequently, for rising lake levels or increased water coverage in Iran’s deserts [Ganji, 1978]. Some Iranologists have also maintained that today’s saline depressions were formerly occupied by brackish waters, attributing this to substantial differences in the hydrological balance compared with present conditions [Trikar, 1990]. In 1982, a map of the permanent snowline of Iran’s mountain ranges was produced based on glacial evidence, biological indicators, and lake and desert records [Pedrami, 1982]. Regarding the permanent snowline and Quaternary glacial evidence in Iran, numerous valuable studies have been conducted by domestic researchers, including, for example, Ramesht [2001], Yamani [2002], Ghohroudi [2011], Amirahmadi [2011], and Sabokkhiz et al. [2019].
At present, both Iranian and international researchers generally agree on the existence of mountain glaciers during the glacial periods and on the more humid climatic conditions that prevailed in Iran at that time. The density of Quaternary glacial landforms is greatest in northern Iran and decreases in importance from north to south and from west to east [Mahmoudi, 1988]. Based on geomorphic evidence, it can be argued that meltwaters from upstream glaciers occasionally formed lakes or large alluvial fans downstream of mountain fronts. For instance, as a result of meltwater from glaciers in the western Alborz within the Karaj Basin, a cold lake with a depth of less than 15 m formed on the Eshtehard Plain, which ultimately overflowed and disappeared [Ramesht & Beiramali, 2014].
Among Quaternary facies landscapes of particular importance to humans are alluvial fans, whose primary formative agents are surface runoff processes and whose typical locations are at mountain fronts. During the Quaternary, intense rainfall events and rapid warming played a significant role in the development of alluvial fans. After traversing steep mountainous valleys and entering the gently sloping piedmont plains, runoff gradually loses energy and velocity, inevitably depositing a substantial portion of its sediment load [Hosein Khan Nazer, 2015]. Geologists, emphasizing crustal movements, have attributed the primary driving mechanism of such landforms to regional tectonic activity and have considered abrupt changes in base level as a prerequisite for alluvial fan formation [Harvey et al., 1999]. In some cases, alluvial fans have also been regarded as indicators of fault activity and cited as evidence of tectonic movements [Hooke & Rohrer, 1979]. Sweeney and Loope [2001] have interpreted alluvial fans as products of sheet floods and heterogeneous sediment flows under humid climatic conditions. Similarly, Scally and Owens [2005] have identified debris-rich, heterogeneous flows as the main agents responsible for the surface deposits of alluvial fans. The first study to seriously address alluvial fans on the southern slopes of the Alborz Mountains was conducted by Beaumont [1972], who, through morphometric analysis, have concluded that these fans were formed under climatic conditions that have not existed in Iran for at least the past 750 years and therefore represent fossil landforms. However, in some studies, such as that by Ritter et al. [2000], climatic changes are considered the dominant factor in alluvial fan formation, while the effects of tectonics, base-level changes, and basin lithology are regarded as negligible [Maghsoudi, 2009]. Numerous researchers within Iran have also investigated the formation and evolution of alluvial fans. Abedini and Rajayi [2006] have examined the role of sediments and topography in the development and evolution of the Darreh-Diz alluvial fans in East Azerbaijan. Pashazadeh et al. [2019] have used sedimentological and geophysical data to model a Quaternary alluvial fan southeast of the city of Yazd.
The study area of the present research is the Mahdasht alluvial fan, located south of the Alborz mountain range and west of the modern Karaj alluvial fan, effectively situated between the old and new Karaj fans. Additionally, hummocky hills occur to the north of the fan, developed on Tertiary and Quaternary formations, indicating tectonic movements and thereby providing evidence for the evolutionary history of the Mahdasht alluvial fan. Influenced by environmental conditions in upstream basins and by Quaternary thermal and moisture fluctuations, the study area has experienced distinct geomorphological transformations that have exerted undeniable effects on the plain [Ramesht & Beiramali, 2014]. Based on geoelectrical data, Sadough et al. [2017] have attributed the formation of the Mahdasht alluvial fan to a late Quaternary shift of the Karaj River toward the east and the Shur River toward the south.
Given the limited application of quantitative laboratory-based methods in geomorphological studies of the Mahdasht region, the present research seeks to investigate the late Quaternary facies-environmental changes of the Mahdasht alluvial fan through analysis of instantaneous and logarithmic grain-size distributions, sediment color, and sediment acidity–alkalinity (pH). The ultimate aim is to provide a more precise framework for environmental management and the preservation of landscape stability.
Methodology
This empirical study was conducted in 2020 within the geographical setting of the Mahdasht alluvial fan. The study area is located between 50°43′29″ and 50°52′42″ E longitude and 35°38′39″ to 35°46′06″ N latitude, encompassing parts of Karaj County (Alborz Province) and Malard County (Tehran Province). Geographical location of the study area on Google Earth and Landsat 8 satellite imagery was downloaded from the USGS website [USGS.com, 2018]. The area lied within the temperate belt of the Earth and is characterized by a humid steppe climate influenced by a Mediterranean regime. According to Iran’s hydrological zoning, it belongs to the Central Drainage Basin. The Mahdasht alluvial fan is bounded by the Halqeh-Dar hills to the north, the Karaj alluvial fan to the east, the Eshtehard Plain to the west, and the Safadasht Plain to the south. Geomorphologically, the Mahdasht alluvial fan is situated within a convex surface landscape and has developed primarily through fluvial processes.
The seasonal Mahdasht River originates from the southern slopes of the Alborz Mountains west of Karaj City and drains the area between the Karaj and Kordan basins. After passing west of the Mahdasht alluvial fan, it joins the Shur River at the entrance of the Mahdasht–Eshtehard road and ultimately flows into the Hoz-e Soltan Lake, located approximately 40 km north of Qom County. Based on calculations derived from the 1:50,000 topographic map produced by the National Cartographic Center of Iran, the area of the studied alluvial fan is about 131 km², with a mean elevation of approximately 1,150 m above sea level and a general slope toward the southwest. From a geological perspective, the alluvial fan lies within the Central Iran structural zone. According to the 1:100,000 Hashtgerd geological map, the deposits forming the area consist mainly of young Quaternary alluvial and alluvial-fan sediments [National Cartographic Center, 1975]. Neogene evaporitic deposits, including unconsolidated sandstones, conglomerates, gypsum, and marl of the Halqeh-Dar hills, surround the northern margin of the fan. Andesitic to trachytic lavas and rhyodacitic tuffs form the southern part of the fan. The bedrock of the area is composed of Plio-Quaternary sedimentary deposits consisting of loose conglomerates with interlayers of sand, silt, and clay, unconformably overlying easily erodible Neogene sedimentary formations (red marl, gypsum and salt interbeds, sandstone, siltstone, and limestone) [Geological and Mineral Exploration Organization, 2000].
In this study, following a review of library resources, maps, satellite images, and previous research, a field reconnaissance survey was conducted in July 2020, during which sampling locations were determined. The principal sampling site was located in the middle part of the alluvial fan, where urban development activities had exposed a sedimentary section. Samples were collected from a trench with a depth of 5 m. The sedimentary layers were coded from bottom to top, photographed, and described in the field. Subsequently, 19 samples were collected from the exposed sedimentary horizons, placed in plastic bags, and labeled. Laboratory analyses were then performed on the collected samples at the Soil and Water Laboratory of the University of Isfahan. These analyses included various physical and chemical tests, notably grain-size analysis to determine sediment texture and chemical analyses to infer depositional environmental conditions.
In general, the texture of clastic sediments reflects the processes acting from the stage of weathering and detachment of particles from the source rock to their final deposition [Goudie & Middleton, 2001]. To determine sediment texture and type, dry sieving was performed in accordance with ASTM standards. During grain-size analysis, the samples were first dried and weighed, then placed on a series of sieves arranged from coarse to fine mesh sizes (top to bottom) and mounted on an electric shaker with a 0.37 kW electromotor (Arvin Instruments, Iran). After 30 minutes, the material retained on each sieve was weighed. The fraction passing the final sieve (below mesh 230, corresponding to silt and clay) was dried and weighed, and subsequently analyzed using a laser particle size analyzer (LA950, Horiba, Japan) based on the ISO 13320:2009(E) standard at the Sedimentology Laboratory of the National Institute of Oceanography, Tehran. Statistical analyses were then performed using Gradistat 4 software.
Subsequently, instantaneous sediment analysis [Friedman & Johnson, 1982] and logarithmic graphical analysis [Folk & Ward, 1957] were applied to determine sediment type and texture. The parameters used to describe grain-size distribution in both methods fall into four main categories: (a) mean grain size, (b) sorting or dispersion around the mean, (c) skewness, and (d) kurtosis (degree of concentration around the mean). The logarithmic graphical method is considered one of the most effective approaches for tracing environmental changes and identifying sediment sorting using the standard deviation indicator, as approximately 90% of the curve is calculated using this formula. Skewness and kurtosis are additional empirical indices that allow assessment of distribution asymmetry from the curve. In the logarithmic method, the inclusive graphic skewness (Ski) is used; positive values indicate a dominance of fine-grained particles with better sorting of coarse fractions, whereas negative values reflect a dominance of coarse particles with better sorting of fine fractions. Moreover, if sediment particles are predominantly fine-grained (e.g., clay and silt), the curve tail extends to the right, indicating deposition under low-energy conditions. Conversely, a leftward tail reflects an abundance of coarse-grained particles and deposition in high-energy environments [Mousavi Hormi, 2014].
Following grain-size analysis, sediment colors and their corresponding codes were determined using the Munsell Soil Color Charts. Despite the limitations of the present study, efforts were made to minimize potential errors by establishing optimal lighting conditions during laboratory measurements and increasing the number of observers. Soil color is an easily measurable indicator of soil composition and is of particular importance because it indirectly reflects soil temperature and moisture conditions, thereby providing insight into the climatic environment under which the soil developed. Soil color also serves as an indicator of drainage conditions [Evans & Franzmeier, 1986]. For instance, the presence of yellowish goethite is indicative of poor drainage conditions [Bigham & Ciolkosz, 1993; Madeira et al., 1997]. Additionally, dry soils tend to appear lighter in color than moist soils due to changes in the refractive index of light [Escadafal et al., 1989].
Finally, the acidity and alkalinity (pH) of the samples were determined. For this purpose, a specific amount of each sample was mixed with distilled water (pH=7) at a fixed ratio, and after equilibrium was reached, the pH was measured using a pH meter (model p514, Consort, Belgium). Soil pH is a fundamental parameter in soil analysis, indicating the degree of acidity or alkalinity. Positively charged hydrogen ions create acidic conditions, whereas higher concentrations of negatively charged hydroxyl ions result in environments with lower acidity and higher alkalinity.
Findings
The results obtained from granulometric analyses, pH measurements, and soil color determination for each sample, following statistical processing, indicated that all facies associated with the alluvial fan environment of the study area were deposited under varying environmental conditions, ranging from coarse-grained to fine-grained sediments accompanied by muddy deposits. The textures of these sediments included sandy gravel, muddy sand and gravel, sand, gravelly mud, sandy mud, and mud, reflecting mono- to polygenetic sedimentary origins. Samples A2, A5, A11, A12, A13, and A14, with logarithmic sorting values between 1.1 and 2.0, fall within the poorly sorted category, whereas the remaining samples, with logarithmic values ranging from 2.1 to 4.3, were classified as very poorly sorted.
Skewness, another parameter evaluated in the samples, showed values between 1.5 and 2.4 for sample A2, indicating that in the grain-size distribution curve of this sample, coarse-grained particles are better sorted than fine-grained particles. This condition shifted the sediment texture toward a gravelly–sandy mud. The other samples exhibit negative skewness values, which indicate a predominance of fine-grained textures.
The color of the samples collected from the sedimentary layers, as determined using the Munsell Soil Color Charts, corresponds to 2.5Y 7/3, representing a light yellow color with relatively high chroma. Chemical variations in iron content and the formation of organic materials at different stages have resulted in darker gray, red, and similar soil colors, which are present only in very minor amounts in some layers and were not reflected in the assigned color codes. It should be noted that due to the large number of samples, the calculated graphs and tables derived from the granulometric analyses are beyond the scope of this article and have therefore been omitted.
The pH values of the muddy samples range from 7.3 to 7.5, indicating neutral to very weakly alkaline conditions in the depositional environment at the time of sediment accumulation. For horizon A2, owing to its coarse-grained texture and the absence of muddy material, pH measurement and color determination were not feasible; therefore, no specific pH value or color code was assigned to this horizon.
Discussion
Based on the findings of the present study, the environmental conditions prevailing during the deposition of the investigated horizons of the Mahdasht alluvial fan can be reconstructed from bottom to top as follows:
Horizon A0: This sedimentary horizon, with a thickness of 3 cm, w:as char:acterized by a muddy gravel–sand texture. The sediments are monogenetic and very poorly sorted, indicating high-energy turbulent flow conditions associated with intense rainfall and strong water discharge. Given the monogenetic nature, the considerable proportion of bedload sediments accompanied by silts, and the thinness of the layer, the depositional event is interpreted as a short-duration flood flow. Overall, during the deposition of this layer, a channelized flood flow with moderate to relatively high energy prevailed.
Horizon A1: This 20 cm thick layer consists of polygenetic gravelly mud with very poor sorting. The increase in fine-grained sediments suggests a reduction in environmental energy during deposition, such that, in addition to transporting limited coarse-grained material, a substantial amount of mud was conveyed to the depositional environment.
Horizon A2: This horizon is 159 cm thick and composed of sandy gravel with a tri-genetic origin and poor sorting, representing an improvement in sorting relative to the underlying horizon (A1). As observed, channel deposits formed under moderate-energy flow conditions. Given the considerable thickness of this layer, these conditions persisted for a relatively long period. This horizon constitutes one of the most important layers within the studied trench. Stable depositional conditions and relatively uniform, significant precipitation led to the absence of flood deposits, suggesting a cold and humid climatic phase.
Horizons A3 and A4: These two layers, with a combined thickness of 45 cm, display a gravelly muddy sand texture with very poor sorting and a tri-genetic origin. The increase in mud content and decrease in gravel indicate flow instability and torrential rainfall events.
Horizon A5: This horizon is 37 cm thick and consists of monogenetic gravelly mud with poor sorting. Compared with the two underlying layers, it reflects a more stable environment. The presence of coarse silt together with medium-sized gravel indicates high-energy conditions and a stable channelized flow.
Horizon A6: This 40 cm thick layer exhibits a gravelly sandy mud texture, comprising silt and medium-grained sand with fine gravel, and is tri-genetic with very poor sorting. During deposition, environmental energy decreased, suspended load increased significantly, and non-muddy sediments became finer, which may indicate persistent but low-intensity precipitation.
Horizon A7: With a thickness of 25 cm, this horizon consists of sandy mud with very poor sorting and a bi-genetic origin, containing medium silt and fine sand. It reflects reduced flow energy with a relatively short duration.
Horizon A8: This 15 cm thick gravelly mud layer contains medium silt accompanied by medium-sized gravel, indicating an increase in bedload transport energy and the introduction of medium-grained suspended load under more continuous flow conditions and higher energy compared with the preceding facies.
Horizons A9 and A10: These horizons, with a combined thickness of 56 cm, consisted of sandy mud and reflected conditions similar to those of horizon A7, indicating a return to weak channelized flow conditions.
Horizons A11 and A12: With thicknesses of 30 cm and 24 cm, respectively, these horizons display muddy textures with poor sorting. Horizon A11 contained medium silt, whereas A12 was dominated by fine silt, indicating a continued decrease in flow energy relative to the underlying horizons and a transition from monogenetic to bi-genetic sediments. In these layers, reduced flow energy, the absence of bedload, and an increase in coarse suspended load suggest persistent, low-intensity rainfall.
Horizon A13: This 11 cm thick layer consisted of monogenetic gravelly mud with medium silt to medium gravel and poor sorting, indicating an increase in persistent rainfall and the reintroduction of bedload into the flow, reflecting higher-energy conditions.
Horizon A14: This horizon, 25 cm thick, w:as char:acterized by sandy mud containing medium silt and fine sand, with a tri-genetic origin and poor sorting. Compared with the preceding facies, a reduction in precipitation volume led to decreased flow energy and, consequently, a reduction in the grain size of the bedload (fine sand).
The color of samples collected from all muddy layers indicates dark yellow hues with relatively high chroma. The pH values of the muddy samples range from 7.3 to 7.5, reflecting neutral to very weakly alkaline depositional conditions at the time of sediment accumulation. These characteristics suggested variable but relatively warm environmental temperatures and somewhat poor soil aeration and drainage during deposition. Consequently, the presence of calcium carbonate, saline and sodic salts, small amounts of organic matter (owing to the alluvial fan setting and soil salinity), and reduced iron within the soil can be inferred.
Previous studies conducted in the Mahdasht region have yielded varying results. At the University of Isfahan, Ramesht and Beiramali [2014], using paleogeomorphological evidence and satellite imagery in reconstructing Quaternary climate conditions in the Karaj River Basin, identified remnants of an ancient, cold, and shallow lake (with a maximum depth of approximately 15 m) west of the Mahdasht alluvial fan, extending toward Bouin-Zahra. This lake reportedly drained and disappeared as a result of megafloods generated by glacial meltwater in the Karaj Basin, overflowing in the Deh-Shesh area. Similarly, researchers at Shahid Beheshti University [Sadough et al., 2017], based on geophysical, geomorphological, and geological data, concluded that in the past the Karaj and Shur rivers were connected and flowed toward the Qom–Masileh region from the southwest of the Mahdasht alluvial fan. Subsequent southward migration of the Shur River and eastward shift of the Karaj River led to their separation and parallel southward flow. The presence of deep depressions at the distal end of the Mahdasht alluvial fan and the considerable sediment thickness were cited as evidence supporting this interpretation. Furthermore, researchers at the University of Mohaghegh Ardabili [Nayebzadeh et al., 2018], using granulometric, morphoscopic, and hydrometric data, confirmed the existence of an ancient shallow lake (approximately 10 m deep) in the Eshtehard area west of Mahdasht, attributing it to the accumulation of overflow waters from a lake north of the Halqeh-Dar hills.
Comparison of the results of the present study with those of the aforementioned research supports the conclusion that the fine-grained lacustrine sediments of the Eshtehard Plain, reported in previous studies with thicknesses of 10–15 m, correspond to the channel-flow facies A2, which exhibits a thickness of 159 cm in the studied trench. The Mahdasht trench, excavated on the alluvial fan, reveals a succession of alluvial fan facies in which sediment textures vary from silt and clay to coarse gravel, reflecting fluctuating climatic conditions. As no clear tectonic discontinuities were observed within the examined layers, the abrupt and marked change in sediment texture in horizon A2 is interpreted as the result of a sudden climatic transition from cold–dry to more humid conditions. This facies, which appears upstream as evidence of snowmelt-induced channel flows (long-term channelized flows with tri-genetic sediment sources), preserves signatures of a cold and humid climatic period. The relatively long persistence of this climatic phase likely led to lake formation and the transport of substantial amounts of fine-grained sediments to the Eshtehard Plain, resulting in the accumulation of thick lacustrine deposits consistent with the findings of previous studies.
Since none of the existing studies, including the present research, have employed absolute dating methods, it is recommended that future investigations apply thermoluminescence dating to determine the timing of depositional events and to achieve more precise correlations with late Quaternary climatic changes in Iran.
Conclusion
The Mahdasht region, located south of Karaj and east of the Eshtehard Plain, has undergone significant environmental changes over time. Numerous geomorphic landforms in the area represent valuable evidence of these transformations. Analyses conducted on 19 sediment samples collected from a 5 m–deep sedimentary trench indicate the occurrence of distinct climatic and environmental changes during the late Quaternary.
Among the analyzed horizons, horizon A2 represents the most significant sedimentary unit in terms of the magnitude of change. With a thickness of 159 cm and a sandy gravel texture, this horizon reflects sustained, high-energy precipitation events characteristic of a cold and humid climatic phase in the region. In contrast, during the deposition of the other analyzed horizons, environmental conditions were relatively warmer, drier, and characterized by lower precipitation with high variability, accompanied by the dominance of silty and muddy sediments. These conditions indicate the concurrent influence of aeolian and fluvial processes in the study area.
Acknowledgements: The authors gratefully acknowledge the Soil and Water Laboratory of the University of Isfahan and the Sedimentology Laboratory of the National Institute of Oceanography, Tehran, for providing appropriate facilities and support that greatly facilitated the advancement of this research.
Ethical Permission: There is nothing to report.
Conflict of Interest: This article is derived from the PhD dissertation of the first author, a doctoral student at Hakim Sabzevari University of Sabzevar, conducted under the advisory role of the second author (Assistant Professor, University of Isfahan), the supervision of the third author (Professor, Hakim Sabzevari University of Sabzevar), the guidance of the fourth author (Professor, Shahid Beheshti University, Tehran), and the consultancy of the fifth author.
Author Contributions: Beiramali Kivi F (First Author), Principal Researcher (20%); Sabokkhiz F (Second Author): Principal Researcher (30%); Amirahmadi A (Third Author), Assistant Researcher (20%); Ghahroudi Tali M (Fourth Author), Contributing researcher (20%); Jamalabadi J (Fifth Author), Assistant Researcher (10%)
Funding: There is nothing to report.
Facies landscapes and their physicochemical characteristics play a major role in the formation or modification of the Earth’s surface morphology [Böse, 2014]. Many facies landscapes are currently located in regions where Quaternary climatic changes have created environmental conditions markedly different from those prevailing at the time of their formation. The impact of sediments associated with these facies, typically derived from highly erodible formations, on the existing erosional system and, ultimately, on regional development is considerable [Derruau, 2000].
The Quaternary represents the most recent period of the Cenozoic Era and is characterized by pronounced climatic and biotic transformations. This period, which is also regarded as the time of human emergence, experienced repeated and extensive climatic fluctuations, giving rise to multiple glacial and interglacial phases worldwide [Kavyani & Alijani, 2019]. Numerous studies have addressed the Quaternary and its remaining geomorphic and environmental legacies [Motamed, 2003]. In Iran, several researchers have examined Quaternary geomorphic transformations. Some have argued that colder and drier climatic conditions than those of today prevailed over Iran, estimating that during glacial periods the snowline in the Alborz and Zagros Mountains was approximately 600–800 m lower than the present snowline, with mean annual temperatures about 4–5°C lower than current values. In contrast, others have suggested that during the Pleistocene (the Quaternary glacial age), Iran experienced colder and more humid conditions than at present, thereby favoring water accumulation in low-lying depressions [Huckriede et al., 1962; Blanford, 1973; Van Zeist & Wright, 1963]. In other words, a slight increase in precipitation combined with a marked decrease in evaporation due to lower and more persistent temperatures during these periods is considered responsible for a positive moisture balance and, consequently, for rising lake levels or increased water coverage in Iran’s deserts [Ganji, 1978]. Some Iranologists have also maintained that today’s saline depressions were formerly occupied by brackish waters, attributing this to substantial differences in the hydrological balance compared with present conditions [Trikar, 1990]. In 1982, a map of the permanent snowline of Iran’s mountain ranges was produced based on glacial evidence, biological indicators, and lake and desert records [Pedrami, 1982]. Regarding the permanent snowline and Quaternary glacial evidence in Iran, numerous valuable studies have been conducted by domestic researchers, including, for example, Ramesht [2001], Yamani [2002], Ghohroudi [2011], Amirahmadi [2011], and Sabokkhiz et al. [2019].
At present, both Iranian and international researchers generally agree on the existence of mountain glaciers during the glacial periods and on the more humid climatic conditions that prevailed in Iran at that time. The density of Quaternary glacial landforms is greatest in northern Iran and decreases in importance from north to south and from west to east [Mahmoudi, 1988]. Based on geomorphic evidence, it can be argued that meltwaters from upstream glaciers occasionally formed lakes or large alluvial fans downstream of mountain fronts. For instance, as a result of meltwater from glaciers in the western Alborz within the Karaj Basin, a cold lake with a depth of less than 15 m formed on the Eshtehard Plain, which ultimately overflowed and disappeared [Ramesht & Beiramali, 2014].
Among Quaternary facies landscapes of particular importance to humans are alluvial fans, whose primary formative agents are surface runoff processes and whose typical locations are at mountain fronts. During the Quaternary, intense rainfall events and rapid warming played a significant role in the development of alluvial fans. After traversing steep mountainous valleys and entering the gently sloping piedmont plains, runoff gradually loses energy and velocity, inevitably depositing a substantial portion of its sediment load [Hosein Khan Nazer, 2015]. Geologists, emphasizing crustal movements, have attributed the primary driving mechanism of such landforms to regional tectonic activity and have considered abrupt changes in base level as a prerequisite for alluvial fan formation [Harvey et al., 1999]. In some cases, alluvial fans have also been regarded as indicators of fault activity and cited as evidence of tectonic movements [Hooke & Rohrer, 1979]. Sweeney and Loope [2001] have interpreted alluvial fans as products of sheet floods and heterogeneous sediment flows under humid climatic conditions. Similarly, Scally and Owens [2005] have identified debris-rich, heterogeneous flows as the main agents responsible for the surface deposits of alluvial fans. The first study to seriously address alluvial fans on the southern slopes of the Alborz Mountains was conducted by Beaumont [1972], who, through morphometric analysis, have concluded that these fans were formed under climatic conditions that have not existed in Iran for at least the past 750 years and therefore represent fossil landforms. However, in some studies, such as that by Ritter et al. [2000], climatic changes are considered the dominant factor in alluvial fan formation, while the effects of tectonics, base-level changes, and basin lithology are regarded as negligible [Maghsoudi, 2009]. Numerous researchers within Iran have also investigated the formation and evolution of alluvial fans. Abedini and Rajayi [2006] have examined the role of sediments and topography in the development and evolution of the Darreh-Diz alluvial fans in East Azerbaijan. Pashazadeh et al. [2019] have used sedimentological and geophysical data to model a Quaternary alluvial fan southeast of the city of Yazd.
The study area of the present research is the Mahdasht alluvial fan, located south of the Alborz mountain range and west of the modern Karaj alluvial fan, effectively situated between the old and new Karaj fans. Additionally, hummocky hills occur to the north of the fan, developed on Tertiary and Quaternary formations, indicating tectonic movements and thereby providing evidence for the evolutionary history of the Mahdasht alluvial fan. Influenced by environmental conditions in upstream basins and by Quaternary thermal and moisture fluctuations, the study area has experienced distinct geomorphological transformations that have exerted undeniable effects on the plain [Ramesht & Beiramali, 2014]. Based on geoelectrical data, Sadough et al. [2017] have attributed the formation of the Mahdasht alluvial fan to a late Quaternary shift of the Karaj River toward the east and the Shur River toward the south.
Given the limited application of quantitative laboratory-based methods in geomorphological studies of the Mahdasht region, the present research seeks to investigate the late Quaternary facies-environmental changes of the Mahdasht alluvial fan through analysis of instantaneous and logarithmic grain-size distributions, sediment color, and sediment acidity–alkalinity (pH). The ultimate aim is to provide a more precise framework for environmental management and the preservation of landscape stability.
Methodology
This empirical study was conducted in 2020 within the geographical setting of the Mahdasht alluvial fan. The study area is located between 50°43′29″ and 50°52′42″ E longitude and 35°38′39″ to 35°46′06″ N latitude, encompassing parts of Karaj County (Alborz Province) and Malard County (Tehran Province). Geographical location of the study area on Google Earth and Landsat 8 satellite imagery was downloaded from the USGS website [USGS.com, 2018]. The area lied within the temperate belt of the Earth and is characterized by a humid steppe climate influenced by a Mediterranean regime. According to Iran’s hydrological zoning, it belongs to the Central Drainage Basin. The Mahdasht alluvial fan is bounded by the Halqeh-Dar hills to the north, the Karaj alluvial fan to the east, the Eshtehard Plain to the west, and the Safadasht Plain to the south. Geomorphologically, the Mahdasht alluvial fan is situated within a convex surface landscape and has developed primarily through fluvial processes.
The seasonal Mahdasht River originates from the southern slopes of the Alborz Mountains west of Karaj City and drains the area between the Karaj and Kordan basins. After passing west of the Mahdasht alluvial fan, it joins the Shur River at the entrance of the Mahdasht–Eshtehard road and ultimately flows into the Hoz-e Soltan Lake, located approximately 40 km north of Qom County. Based on calculations derived from the 1:50,000 topographic map produced by the National Cartographic Center of Iran, the area of the studied alluvial fan is about 131 km², with a mean elevation of approximately 1,150 m above sea level and a general slope toward the southwest. From a geological perspective, the alluvial fan lies within the Central Iran structural zone. According to the 1:100,000 Hashtgerd geological map, the deposits forming the area consist mainly of young Quaternary alluvial and alluvial-fan sediments [National Cartographic Center, 1975]. Neogene evaporitic deposits, including unconsolidated sandstones, conglomerates, gypsum, and marl of the Halqeh-Dar hills, surround the northern margin of the fan. Andesitic to trachytic lavas and rhyodacitic tuffs form the southern part of the fan. The bedrock of the area is composed of Plio-Quaternary sedimentary deposits consisting of loose conglomerates with interlayers of sand, silt, and clay, unconformably overlying easily erodible Neogene sedimentary formations (red marl, gypsum and salt interbeds, sandstone, siltstone, and limestone) [Geological and Mineral Exploration Organization, 2000].
In this study, following a review of library resources, maps, satellite images, and previous research, a field reconnaissance survey was conducted in July 2020, during which sampling locations were determined. The principal sampling site was located in the middle part of the alluvial fan, where urban development activities had exposed a sedimentary section. Samples were collected from a trench with a depth of 5 m. The sedimentary layers were coded from bottom to top, photographed, and described in the field. Subsequently, 19 samples were collected from the exposed sedimentary horizons, placed in plastic bags, and labeled. Laboratory analyses were then performed on the collected samples at the Soil and Water Laboratory of the University of Isfahan. These analyses included various physical and chemical tests, notably grain-size analysis to determine sediment texture and chemical analyses to infer depositional environmental conditions.
In general, the texture of clastic sediments reflects the processes acting from the stage of weathering and detachment of particles from the source rock to their final deposition [Goudie & Middleton, 2001]. To determine sediment texture and type, dry sieving was performed in accordance with ASTM standards. During grain-size analysis, the samples were first dried and weighed, then placed on a series of sieves arranged from coarse to fine mesh sizes (top to bottom) and mounted on an electric shaker with a 0.37 kW electromotor (Arvin Instruments, Iran). After 30 minutes, the material retained on each sieve was weighed. The fraction passing the final sieve (below mesh 230, corresponding to silt and clay) was dried and weighed, and subsequently analyzed using a laser particle size analyzer (LA950, Horiba, Japan) based on the ISO 13320:2009(E) standard at the Sedimentology Laboratory of the National Institute of Oceanography, Tehran. Statistical analyses were then performed using Gradistat 4 software.
Subsequently, instantaneous sediment analysis [Friedman & Johnson, 1982] and logarithmic graphical analysis [Folk & Ward, 1957] were applied to determine sediment type and texture. The parameters used to describe grain-size distribution in both methods fall into four main categories: (a) mean grain size, (b) sorting or dispersion around the mean, (c) skewness, and (d) kurtosis (degree of concentration around the mean). The logarithmic graphical method is considered one of the most effective approaches for tracing environmental changes and identifying sediment sorting using the standard deviation indicator, as approximately 90% of the curve is calculated using this formula. Skewness and kurtosis are additional empirical indices that allow assessment of distribution asymmetry from the curve. In the logarithmic method, the inclusive graphic skewness (Ski) is used; positive values indicate a dominance of fine-grained particles with better sorting of coarse fractions, whereas negative values reflect a dominance of coarse particles with better sorting of fine fractions. Moreover, if sediment particles are predominantly fine-grained (e.g., clay and silt), the curve tail extends to the right, indicating deposition under low-energy conditions. Conversely, a leftward tail reflects an abundance of coarse-grained particles and deposition in high-energy environments [Mousavi Hormi, 2014].
Following grain-size analysis, sediment colors and their corresponding codes were determined using the Munsell Soil Color Charts. Despite the limitations of the present study, efforts were made to minimize potential errors by establishing optimal lighting conditions during laboratory measurements and increasing the number of observers. Soil color is an easily measurable indicator of soil composition and is of particular importance because it indirectly reflects soil temperature and moisture conditions, thereby providing insight into the climatic environment under which the soil developed. Soil color also serves as an indicator of drainage conditions [Evans & Franzmeier, 1986]. For instance, the presence of yellowish goethite is indicative of poor drainage conditions [Bigham & Ciolkosz, 1993; Madeira et al., 1997]. Additionally, dry soils tend to appear lighter in color than moist soils due to changes in the refractive index of light [Escadafal et al., 1989].
Finally, the acidity and alkalinity (pH) of the samples were determined. For this purpose, a specific amount of each sample was mixed with distilled water (pH=7) at a fixed ratio, and after equilibrium was reached, the pH was measured using a pH meter (model p514, Consort, Belgium). Soil pH is a fundamental parameter in soil analysis, indicating the degree of acidity or alkalinity. Positively charged hydrogen ions create acidic conditions, whereas higher concentrations of negatively charged hydroxyl ions result in environments with lower acidity and higher alkalinity.
Findings
The results obtained from granulometric analyses, pH measurements, and soil color determination for each sample, following statistical processing, indicated that all facies associated with the alluvial fan environment of the study area were deposited under varying environmental conditions, ranging from coarse-grained to fine-grained sediments accompanied by muddy deposits. The textures of these sediments included sandy gravel, muddy sand and gravel, sand, gravelly mud, sandy mud, and mud, reflecting mono- to polygenetic sedimentary origins. Samples A2, A5, A11, A12, A13, and A14, with logarithmic sorting values between 1.1 and 2.0, fall within the poorly sorted category, whereas the remaining samples, with logarithmic values ranging from 2.1 to 4.3, were classified as very poorly sorted.
Skewness, another parameter evaluated in the samples, showed values between 1.5 and 2.4 for sample A2, indicating that in the grain-size distribution curve of this sample, coarse-grained particles are better sorted than fine-grained particles. This condition shifted the sediment texture toward a gravelly–sandy mud. The other samples exhibit negative skewness values, which indicate a predominance of fine-grained textures.
The color of the samples collected from the sedimentary layers, as determined using the Munsell Soil Color Charts, corresponds to 2.5Y 7/3, representing a light yellow color with relatively high chroma. Chemical variations in iron content and the formation of organic materials at different stages have resulted in darker gray, red, and similar soil colors, which are present only in very minor amounts in some layers and were not reflected in the assigned color codes. It should be noted that due to the large number of samples, the calculated graphs and tables derived from the granulometric analyses are beyond the scope of this article and have therefore been omitted.
The pH values of the muddy samples range from 7.3 to 7.5, indicating neutral to very weakly alkaline conditions in the depositional environment at the time of sediment accumulation. For horizon A2, owing to its coarse-grained texture and the absence of muddy material, pH measurement and color determination were not feasible; therefore, no specific pH value or color code was assigned to this horizon.
Discussion
Based on the findings of the present study, the environmental conditions prevailing during the deposition of the investigated horizons of the Mahdasht alluvial fan can be reconstructed from bottom to top as follows:
Horizon A0: This sedimentary horizon, with a thickness of 3 cm, w:as char:acterized by a muddy gravel–sand texture. The sediments are monogenetic and very poorly sorted, indicating high-energy turbulent flow conditions associated with intense rainfall and strong water discharge. Given the monogenetic nature, the considerable proportion of bedload sediments accompanied by silts, and the thinness of the layer, the depositional event is interpreted as a short-duration flood flow. Overall, during the deposition of this layer, a channelized flood flow with moderate to relatively high energy prevailed.
Horizon A1: This 20 cm thick layer consists of polygenetic gravelly mud with very poor sorting. The increase in fine-grained sediments suggests a reduction in environmental energy during deposition, such that, in addition to transporting limited coarse-grained material, a substantial amount of mud was conveyed to the depositional environment.
Horizon A2: This horizon is 159 cm thick and composed of sandy gravel with a tri-genetic origin and poor sorting, representing an improvement in sorting relative to the underlying horizon (A1). As observed, channel deposits formed under moderate-energy flow conditions. Given the considerable thickness of this layer, these conditions persisted for a relatively long period. This horizon constitutes one of the most important layers within the studied trench. Stable depositional conditions and relatively uniform, significant precipitation led to the absence of flood deposits, suggesting a cold and humid climatic phase.
Horizons A3 and A4: These two layers, with a combined thickness of 45 cm, display a gravelly muddy sand texture with very poor sorting and a tri-genetic origin. The increase in mud content and decrease in gravel indicate flow instability and torrential rainfall events.
Horizon A5: This horizon is 37 cm thick and consists of monogenetic gravelly mud with poor sorting. Compared with the two underlying layers, it reflects a more stable environment. The presence of coarse silt together with medium-sized gravel indicates high-energy conditions and a stable channelized flow.
Horizon A6: This 40 cm thick layer exhibits a gravelly sandy mud texture, comprising silt and medium-grained sand with fine gravel, and is tri-genetic with very poor sorting. During deposition, environmental energy decreased, suspended load increased significantly, and non-muddy sediments became finer, which may indicate persistent but low-intensity precipitation.
Horizon A7: With a thickness of 25 cm, this horizon consists of sandy mud with very poor sorting and a bi-genetic origin, containing medium silt and fine sand. It reflects reduced flow energy with a relatively short duration.
Horizon A8: This 15 cm thick gravelly mud layer contains medium silt accompanied by medium-sized gravel, indicating an increase in bedload transport energy and the introduction of medium-grained suspended load under more continuous flow conditions and higher energy compared with the preceding facies.
Horizons A9 and A10: These horizons, with a combined thickness of 56 cm, consisted of sandy mud and reflected conditions similar to those of horizon A7, indicating a return to weak channelized flow conditions.
Horizons A11 and A12: With thicknesses of 30 cm and 24 cm, respectively, these horizons display muddy textures with poor sorting. Horizon A11 contained medium silt, whereas A12 was dominated by fine silt, indicating a continued decrease in flow energy relative to the underlying horizons and a transition from monogenetic to bi-genetic sediments. In these layers, reduced flow energy, the absence of bedload, and an increase in coarse suspended load suggest persistent, low-intensity rainfall.
Horizon A13: This 11 cm thick layer consisted of monogenetic gravelly mud with medium silt to medium gravel and poor sorting, indicating an increase in persistent rainfall and the reintroduction of bedload into the flow, reflecting higher-energy conditions.
Horizon A14: This horizon, 25 cm thick, w:as char:acterized by sandy mud containing medium silt and fine sand, with a tri-genetic origin and poor sorting. Compared with the preceding facies, a reduction in precipitation volume led to decreased flow energy and, consequently, a reduction in the grain size of the bedload (fine sand).
The color of samples collected from all muddy layers indicates dark yellow hues with relatively high chroma. The pH values of the muddy samples range from 7.3 to 7.5, reflecting neutral to very weakly alkaline depositional conditions at the time of sediment accumulation. These characteristics suggested variable but relatively warm environmental temperatures and somewhat poor soil aeration and drainage during deposition. Consequently, the presence of calcium carbonate, saline and sodic salts, small amounts of organic matter (owing to the alluvial fan setting and soil salinity), and reduced iron within the soil can be inferred.
Previous studies conducted in the Mahdasht region have yielded varying results. At the University of Isfahan, Ramesht and Beiramali [2014], using paleogeomorphological evidence and satellite imagery in reconstructing Quaternary climate conditions in the Karaj River Basin, identified remnants of an ancient, cold, and shallow lake (with a maximum depth of approximately 15 m) west of the Mahdasht alluvial fan, extending toward Bouin-Zahra. This lake reportedly drained and disappeared as a result of megafloods generated by glacial meltwater in the Karaj Basin, overflowing in the Deh-Shesh area. Similarly, researchers at Shahid Beheshti University [Sadough et al., 2017], based on geophysical, geomorphological, and geological data, concluded that in the past the Karaj and Shur rivers were connected and flowed toward the Qom–Masileh region from the southwest of the Mahdasht alluvial fan. Subsequent southward migration of the Shur River and eastward shift of the Karaj River led to their separation and parallel southward flow. The presence of deep depressions at the distal end of the Mahdasht alluvial fan and the considerable sediment thickness were cited as evidence supporting this interpretation. Furthermore, researchers at the University of Mohaghegh Ardabili [Nayebzadeh et al., 2018], using granulometric, morphoscopic, and hydrometric data, confirmed the existence of an ancient shallow lake (approximately 10 m deep) in the Eshtehard area west of Mahdasht, attributing it to the accumulation of overflow waters from a lake north of the Halqeh-Dar hills.
Comparison of the results of the present study with those of the aforementioned research supports the conclusion that the fine-grained lacustrine sediments of the Eshtehard Plain, reported in previous studies with thicknesses of 10–15 m, correspond to the channel-flow facies A2, which exhibits a thickness of 159 cm in the studied trench. The Mahdasht trench, excavated on the alluvial fan, reveals a succession of alluvial fan facies in which sediment textures vary from silt and clay to coarse gravel, reflecting fluctuating climatic conditions. As no clear tectonic discontinuities were observed within the examined layers, the abrupt and marked change in sediment texture in horizon A2 is interpreted as the result of a sudden climatic transition from cold–dry to more humid conditions. This facies, which appears upstream as evidence of snowmelt-induced channel flows (long-term channelized flows with tri-genetic sediment sources), preserves signatures of a cold and humid climatic period. The relatively long persistence of this climatic phase likely led to lake formation and the transport of substantial amounts of fine-grained sediments to the Eshtehard Plain, resulting in the accumulation of thick lacustrine deposits consistent with the findings of previous studies.
Since none of the existing studies, including the present research, have employed absolute dating methods, it is recommended that future investigations apply thermoluminescence dating to determine the timing of depositional events and to achieve more precise correlations with late Quaternary climatic changes in Iran.
Conclusion
The Mahdasht region, located south of Karaj and east of the Eshtehard Plain, has undergone significant environmental changes over time. Numerous geomorphic landforms in the area represent valuable evidence of these transformations. Analyses conducted on 19 sediment samples collected from a 5 m–deep sedimentary trench indicate the occurrence of distinct climatic and environmental changes during the late Quaternary.
Among the analyzed horizons, horizon A2 represents the most significant sedimentary unit in terms of the magnitude of change. With a thickness of 159 cm and a sandy gravel texture, this horizon reflects sustained, high-energy precipitation events characteristic of a cold and humid climatic phase in the region. In contrast, during the deposition of the other analyzed horizons, environmental conditions were relatively warmer, drier, and characterized by lower precipitation with high variability, accompanied by the dominance of silty and muddy sediments. These conditions indicate the concurrent influence of aeolian and fluvial processes in the study area.
Acknowledgements: The authors gratefully acknowledge the Soil and Water Laboratory of the University of Isfahan and the Sedimentology Laboratory of the National Institute of Oceanography, Tehran, for providing appropriate facilities and support that greatly facilitated the advancement of this research.
Ethical Permission: There is nothing to report.
Conflict of Interest: This article is derived from the PhD dissertation of the first author, a doctoral student at Hakim Sabzevari University of Sabzevar, conducted under the advisory role of the second author (Assistant Professor, University of Isfahan), the supervision of the third author (Professor, Hakim Sabzevari University of Sabzevar), the guidance of the fourth author (Professor, Shahid Beheshti University, Tehran), and the consultancy of the fifth author.
Author Contributions: Beiramali Kivi F (First Author), Principal Researcher (20%); Sabokkhiz F (Second Author): Principal Researcher (30%); Amirahmadi A (Third Author), Assistant Researcher (20%); Ghahroudi Tali M (Fourth Author), Contributing researcher (20%); Jamalabadi J (Fifth Author), Assistant Researcher (10%)
Funding: There is nothing to report.
Keywords:
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