Gradation and geochemistry of the Fenced Lagoon sediments (Bandar Anzali) with regard to source rock and tectonic location

Received: 19.06.2019 Received in revised form: 14.11.2019 Accepted: 06.01.2020 Abstract. The location of the Fenced Lagoon in the urban basin of Bandar Anzali, which should be considered due to its impact on the lives of the people of the region in terms of the conservation of the wetland and its environmental issues, and, on the other hand, the potential of this area to be introduced as a geotourism center determine the need to investigate the area’s sedimentology and geochemistry. Therefore, to study sedimentary and geochemical properties of the Fenced Lagoon sediments located in Bandar Anzali, 33 samples were taken in the form of 6 cores and 12 grabs, and basic sedimentation tests and heavy metals measurement were carried out on them .Studies show that the sediments range from sand to clay in terms of gradation and have mainly coastal-river origin. The sediments of this lake are classified into four sedimentary types: Muddy Sand, Slightly Gravelly Muddy Sand, Sand and Slightly Gravelly Sand, and sand is the main component of all of these sediments. The most abundant sedimentary types belong to Muddy Sand and Sand and the least abundant sedimentary types belong to Slightly Gravelly Muddy Sand and Slightly Gravely Sand. The nature of the source rock is derived from acidic to intermediate combination and in general, sedimentary rocks of the area under study are within the continental arch islands and, to a lesser extent, the active continental margin.

the wetlands (Abdollahi,2010). The southern Caspian basin also has several wetlands that require studies from different perspectives.
The Caspian Sea and its rapid fluctuations at sea level during the Holocene have been the subject of many studies over the past two decades (Lahijani et al., 2009, Leroy et al., 2011Kakroodi et al., 2012b, NaderiBeni et al., 2013a, NaderiBeni et al., 2013b. The Caspian Basin, after separating from the open sea, became a lake at the time of Pliocene, and from then on, a series of tectonic activities, fluctuations in water levels, wave effects and their flows, and rivers, have shaped the present state of its morphology (NaderiBeni et al., 2014). Climate is the most important reason for changes in the water level of the Caspian Sea (Kroonenberg et al., 2007, NaderiBeni et al., 2013a, Leroy et al., 2013. These fluctuations at sea level have different impacts on costal evolution and relatively on coastal deposits (NaderiBeni et al., 2013b).
Changes in the level of Caspian Sea waters may result from a factor or a combination of factors, such as climate change, tectonic processes, and human activity. Obviously, any change on the Caspian Sea has a direct impact on its marginal wetlands and the performance of river systems. Changes in such habitats will have a direct impact on the morphology and animal life and even the coastal economy. Moreover, considering that wetlands are usually influenced by sea and river forces, and changes in the dynamics of seas and rivers affect them, recognizing these effects and various anomalies in heavy elements when they enter the wetland and are deposited there are very important both ecologically and economically (Ranjbar, 2012). The sediments of wetlands are the main components of our environment and an important source of cadmium, organic and chemical substances that can either result from natural processes and erosion or be created by human intervention. In addition, coastal wetlands play an important role in trapping river sediments and nutrients and reducing their transmission to seas. Of course, the pattern of human activities in the use of land, such as agriculture and urban planning and the way of land use have made a lot of changes in this important issue (Bruland et al. 2010). Wetlands also play an effective role in preventing flood occurrence and act as a sedimentary slab (Kazanci et al., 2004). Since climate change and human interventions have caused many of the country's wetlands to dry out, if wetlands are studied comprehensively and properly, this environmental catastrophe can be controlled largely and the destructive effects of this process can be minimized.
Due to the environmental importance of this wetland and the Anzali area in general, various studies have been carried out in this area in recent years. For instance, Kowsari carried out semi-detailed explorations in the area in relation to titanium in the region in 1991. Shahrabi published a book on the seas and lakes of Iran in 1994 in which the significance of this wetland is also mentioned. A compilation report of the studies on water resources of the SefidRoud basin was prepared in 2001 by Gilan Regional Water Company. Other studies in this area include the geological map of Bandar-e-Anzali, prepared by Mousavi (2001) and Khabaznia et al. (2005) to better understand this water basin. Then, to complete them, Kheiri (2005) presented a report on remote sensing of the Bandar Anzali plate with special attention to the marine geology and morphology of the Caspian region. Karim Khani (2007), prepared a report on the sedimentology and sedimentary geochemistry of the sea plate of Bandar Anzali, and the latest research in this area was carried out by Ms. Hazer Meshar (2015), as a doctorate dissertation, which has not been published yet.
Despite the presence of the Fenced Lagoon inside the city of Anzali and its impact on the lives of residents around it, no comprehensive study has been carried out so far on the sedimentology of this area. Therefore, this study seeks to achieve this goal. Geographical location and sampling points. The Fenced Lagoon (fenced lagoon) is about 240 meters in length and in the broadest place 40 meters wide at latitude 41 47 855 N and longitude 36 51 52 E and is located in the southeast of Anzali wetland and in the north of the fire department of Ghazian district and in the city of Anzali. The elevation of the lake is 24 m from the sea level, the area of the lake is 29,353 m 2 and its perimeter is 1,137 m. This area is located in the 1:100,000 Anzali rectangular maps. In the past, this wetland was part of the Caspian Sea drainage basin, which was running to the Anzali wetland through a water slab. Surface waters and seasonal and winter rainwaters flowed into the pond after crossing various areas. At present and in the summer, its water content decreases, which somewhat plays the role of the drainage of the area. The wetland is surrounded by residential houses and private estates.
For this research, after collecting the data and office studies, the wetland area was first determined using aerial photos and satellite imagery using ArcGIS software. Then the sampling points were determined according to the extent of the wetland (Fig. 1). Materials and methods. Sampling at the surface of the wetland was carried out based on previous findings and data, which consisted of 33 specimens in the form of six cores, the cores were taken by polyethylene pipes, and 12 samples of the deposits in the wetland floor taken by the grab (Fig. 2). Samples preparation. After transferring the specimens to the laboratory and freezing them, the sedimentary cores were separated from the polyethylene tubes by a stone milling machine, so that half of the core thickness was still in the tube. At this point, care must be taken to prevent the contact of metal blade of the milling machine to the cores (Fig. 3).
The specimens inside each core were separated according to colour change and sedimentation and were accumulated and numbered in individual containers. Experiments performed on the samples included gradation, hydrometry, XRF and ICP. There are several methods for measuring the diameter of the grains (Harami,2006). Dry sieve method was used in this study. After drying, the samples were weighed and dried through sieving and were graded by the shaker (Anderson,2004). The results of gradation were used to obtain the statistical factors and the comprehensive Folk (1980) drawing method was used for the particles' sedimentology such as sorting, tilting and stretching, and the population of suspension, mutation and deflection of sediments, as well as their turning point were calculated using a cumulative diagram. Then, the frequency percentage chart of the particles was plotted using GRADISTAT software (Blott&Pye, 2001). In the following, using Folk's (1954) method, four facial types were identified and using Rockwork software, facies columns were depicted based on the depth and this information was used to interpret and comment the sedimentary environment and their vertical and lateral changes. For gradation of clay and salt particles (below 63 microns) the hydrometric method is the best method in which the particles' diameter is obtained according to Stokes's law (Faiznia, 2008). Hydrometric experiments were carried out on 3 cores and 2 crop samples, then parameters such as creep rate, time, particle diameter and ultimately transition and cumulative percentages were calculated and diagrams were plotted based on the cumulative percentage and particle diameter in F and the results were analyzed.
After the sedimentation steps, a geochemical method was used for to analyze the samples and determine the forming oxides and to identify the minerals in the sediments. Many geochemical methods, despite the high resolution, are very time consuming and cause the waste of the sediment. Therefore, in this research, the non-destructive XRF method was used (Jansen et al., 1998;Ro¨hl and Abrams, 2000). This method is widely used in the Dutch Texel Research Institute for measuring the intensity of heavy elements (Bahr et al., 2005).
For this purpose, all 45 samples were sent to the laboratory of the Geological Survey and Mineral Exploration of Iran and the elements in the samples were analyzed. Then, using the multivariate graphs, the origin of the wetland sediments was obtained. Discussion. Gradation studies. Grain size is one of the most important characteristics of sedimentary particles that is affected by their transfer and sedimentation (Folk & Ward, 1957). Therefore, analyzing particle size and shape gives us important clues about the origin of sediment, transport history and sedimentation conditions. Since the obtained results may not be directly comparable using different methods, and the analysis of the obtained data by using more than one method may be difficult (Pye, 1994), the obtained data and the results of 6 cores and 12 samples of grab were investigated in this research using GRADISTAT software. By determining the percentage of particles with different diameters in the described method, the sediment was named according to the Folk method (1954). Despite the low percentage of gravel in samples, since the abundance of coarse grains in sediments, even to a negligible amount, is valuable for interpreting the energy and the type of the environment (Harami, 2004), it was considered in the sediments' classification and environment determination. According to the Folk method (1954), the sediments of this lake were divided into four sedimentary types including Muddy Sand, Slightly Gravelly Muddy Sand, Sand, Slightly Gravelly Sand, and sand is the main component of all these sediments. The most abundant sedimentary types belong to Muddy Sand and Sand and the least abundant sedimentary types belong to Slightly Gravelly Muddy Sand and Slightly Gravelly Sand. The frequency of Muddy Sand is mainly seen in surface sediments up to the depth of 100 cm, while the facies type of Slightly Gravelly Muddy Sand is related to greater depths and both facies are more abundant in the north and northeast parts of the lake. By moving to the south and southwest regions of the lake, the two facies of Sand and Muddy Sand become more abundant (Fig. 4). The size of sediment particles increases as the depth decreases from northeast to southwest. Alternate layers of sand -mud and sand horizons containing shellfish represent the periods of  (Ghazban, 2010).
The range of elongation in the sediments of the studied basin is between 0.63 and 1. In other words, the elongation of sediments in different cores varies from very broad to broad, medium and stretched, which is more toward broader grains indicating that the curve trail (silt, clay and gravel) has a better alignment than the middle part of the curve (Faiz Nia, 2008). In most of the rivers with bed load, it is shown that with better alignment downwards, the cumulative curve elongation increases (Harami, 2004). The majority of the samples in the basin have bad to moderate alignment (between 0.5 to 1.7). This situation usually represents the flow turbulence that is probably due to the arrival of seasonal flow of rivers into the basin (Paseban & Mahbubi, 2012). The range of tilting variations in the studied sediments varies between -0.6 and 0.3. Of these, only six samples taken with the grab have positive tilting and the rest have negative tilting. Positive tilting indicates a greater frequency of fine-grained sediments, possibly due to the presence of these particles between coarser-grained sediments (Paseban & Mahbubi, 2012). Of course, the turbulence of the feeding currents of the area, erosion and crushing of coarse-grained sediments, and unstable grains that lead to the production of smaller sediments, are also the causes of positive tilting (Rice, 1999). Negative tilting also means an increase in coarse-grained particles, which can be seen on shores where the reciprocating waves are active and fine particles are removed (Faiz Nia, 2008).
Gradation changes in this basin, including changes in grain size and changes in statistical parameters such as sorting and rounding and flattening can indicate the intensity of changes in the feeding currents of the area and lithological changes in the region. Non-continuous changes in the particles' size indicate the role of subordinate feeding branches in supplying sediment in this basin.
Geochemical studies. One of the most reliable methods to identify the source rocks and separate sediments is the use of geochemical data in the separation of the main and secondary elements. Many researchers have carried out geochemical studies to determine the origin of sediments in the wetlands, their water quality, as well as the pollutants contained in them (Amini, 2012). In the meantime, the volume of studies carried out on Anzali wetlands and its surrounding wetlands is remarkable because of its importance as an international wetland (Khzaiei, 2012).
Swamps and bays of the Caspian margin are formed under three main processes of longitudinal transmission of coastal sediment, elevation of the Caspian Sea and anticline-syncline structures (Leontiev et al., 1977), and are considered the environment of the land-to-sea transition. These environments, which are semi-closed with sandy slabs or structural complications, provide a relatively lowenergy environment for sedimentation (Leeder, 1982), thus in addition to being influenced by the marine environment, they are affected by the catchment area and surrounding shores as well.
In general, geochemical data of the main and secondary elements can be used for chemical classification of rocks, determination of the prevailing weathering conditions of sedimentary rocks, separation of adult sediments from immature, determination of the source rock and the main tectonic position of igneous rocks and some sedimentary rocks (Rollinson, 1993). The study of the main elements is often limited to 10 elements, Ca, Na, K, P, Ti, Al, Fe, Mn, Mg and Si, which are conventionally expressed as oxidation in XRF chemical decomposition. Controlling processes of the elements in sedimentary rocks can be studied using the normalization diagrams similar to the spider diagrams (Rollinson, 1993).
The main part of the Caspian coastline is covered with delta, river and coastal sediments of the present day (Agha Nabati, 2006). The present sediments of the study area include the granitoid complex remains between the hardened sediments attributed to continental -sea climates of the Quaternary (Alavi, 1996). Based on geochemical similarities and previous studies ( (Nazari, et al, 1995), this granitoid is comparable to Lahijan granites. This unit itself is located on older sediments, including sedimentarydeltaic facies of the sub-intertidal zone of the upper Triassic, lower Jurassic to early Cretaceous, deposited in various tectonic climates. The most ancient rocks in the area under study are also a row of destructive rocks, sandy shale and thin to middle layer sandy argilfaceae of olive green, which can be considered equal to the upper part of the Mila Formation or the middle section of the Lalun Formation (Alavi,1996). Before examining the results of the geochemistry on the commonly used charts and their interpretation, it is necessary to analyze the statistical processing of the decomposition of the main and secondary elements of the cores. As shown in the table, the mean SiO 2 content of the Fenced Lagoon sediments (61.37%) is approximately equal to the mean SiO 2 content of the continental crust (Talor&McLenan, 1985; SiO 2 = 64/8). Moreover, the mean CaO content of the wetland sediment (1.4%) is about twice as much as the CaO content of the upper crust (4.19), which indicates a high amount of fine stones and carbonate cement, causing a relative reduction in SiO 2 and Al 2 O 3 percentage. The amount of K 2 O and Na 2 O (1.55 on average) is higher than that of the continental crust, which can indicate the presence of feldspar in the samples. A higher percentage of Fe 2 O 3 and MgO oxides, as compared to the upper continental crust, may also represent mafic minerals.
Since Al 2 O 3 is almost unchanged during weathering, diagenesis and transformation, it can be used as an appropriate factor for comparison with other major elements. Therefore, in order to investigate the geochemical position of the main and rare elements in the sand and mud sediments of the Fenced Lagoon, the frequency diagrams of the oxides of the main elements have been used against Al 2 O 3 (Afarin, 2012). The changes trends of the oxides of the main and trace elements against Al 2 O 3 indicate that the sediments under study can be classified into two general groups of sand and mud. In Figure 5, the relationship between Al 2 O 3 and K 2 O, Fe 2 O 3 , MgO, TiO 2 and SiO 2 is positive and it does not show a significant relationship with MnO, CaO, Na 2 O, K 2 O and P 2 O 5 . The positive relationship between Al 2 O 3 and Fe 2 O 3 and K 2 O can be due to the presence of these elements in clay and mica minerals (Mclennan et al., 1993;Jin et al., 2006). Furthermore, K 2 O can represent a rich aluminum phase, especially illite, or concentration of potassium minerals in samples (Lee, 1999;Das et al, 2006). Since the amount of clay in stone background is low in sand samples, the K 2 O content is probably related to clay minerals found in shale specimens. The positive association of Al 2 O 3 and SiO 2 indicates the presence of feldspar and mica and clay minerals. Increasing TiO 2 with Al 2 O 3 indicates the association of TiO 2 or phyllosilicates, especially illite (Dabard, 1990). High percentage of CaO indicates that the samples can contain carbonate, and calcite, in particular. A relatively high proportion of K 2 O / Na 2 O (especially in shale) can be attributed to the presence of albicagmatic plagioclase, feldspar potassium of mica and illite (Pettijohn et al., 1963, Mclennan et al., 1993, Nath et al., 2000. The main elements during weathering, transport and diagenesis can be used to determine the degree of maturation of sediments (Maclennen, 1993). A low amount of Na 2 O in the studied sandstone can be attributed to their high maturity. In addition, the SiO 2 / Al 2 O 3 ratio is also an indicator used to determine maturity. This amount increases during weathering, transport and re-cycling as a result of quartz increase over unstable parts such as feldspar and rocky parts. A greater proportion of 5 to 6 in sediments and sedimentary rocks represents the high maturity of sediment or rock. (Roser et al., 1996) The average of this fraction in the studied samples is less than one, which indicates the immaturity of these sediments; this is also characterized by high Na 2 O content.
Source Rock. Geochemistry of sediments can play an important role in determining the source and mother rock and climatic conditions of sediments. The analysis of secondary elements, as well as major oxides such as Al 2 O 3 and TiO 2 , is very important for Fig. 5. Chart of changes of some of the oxides of the main and secondary elements against the weight percent of Al2O3 in order to study the geochemical status of the main and rare elements in the Fenced Lagoon sediments (Harker, 1909) the interpretation of sediments (Katemaunzanga and Gunter, 2009;Maslov et al., 2011). For example, trace elements such as barium, lanthanum, vanadium, zirconium in sediments provide researchers with important information about mother rock of the sediments, and using different techniques such as the use of triangular and dual diagrams can provide useful information on wetlands (Lak, 2015). In this regard, many classifications can reveal the origin of deposits. For example, Maynard et al., 1982;Bhatia, 1983;Bhatia & Crook, 1986;Roser&Korsch, 1986 ; the methodology used for this study is presented by Roser&Korsch (1988), which helps us recognize four kinds of sediments origins in four different zones including mafic, intermediate, felsic and depositional quartz. These diagrams are obtained from the total oxides of Ti, Al, Fe, Mg, Ca, Na and K. The advantage of using this method is that one can ignore the percentage of biogenic SiO 2 and CaO and consider the percentage of the oxide of other elements against Al 2 O 3 while using these diagrams to determine the source (Roser&Korsch, 1986).
With respect to (Fig. 6), the obtained samples show a gradual transition from mafic to intermediate igneous origin. Samples containing shale are well plotted in mafic zone (Roser&Korsch 1988). It seems that the ratio of Al 2 O 3 / TiO 2 in shale is similar to that of source rocks and can therefore be used as an indicator of origin. (Hayashi, 1997). Hayashi et al insist that a ratio of above 21 could indicate the felsic origin. This average in our research was between 16.93 and 20.67, which could indicate the origin of the combination of mafic to intermediate.
It should be added that the distribution of metal elements in sediments around the Caspian Sea region suggests that, in some cases, such as barium, their concentration is more than the amount in the ground, which indicates their entry through human activities, although most of them are supplied through destructive deposits of the watershed (Lahijani, 2002). Com-  parison of metal composition of surface sediments with deep sediments of the same area in terms of chemical composition can indicate the geochemical background of the region (Karageorgis et al., 2006. Buccolieri et al., 2006. Muller, 1979. Adams et al., 1992. Tectonic Location. Determination of tectonic location is influenced by factors such as sedimentation, diagenesis and initial composition of sediment (Pettijohn et al., 1987;Bhatia, 1983;Chamley, 1990). Therefore, the main elements of the sand can be used to determine their tectonic position (Bhatia, 1983;Von Eynatten, 2003;Armstrong-Altrin &Verma 2005;Al-Juboury et al., 2009;Sahraeyan & Bahrami, 2012 As shown in Fig. 7, based on changes in the values of the main elements, the cranial rocks of oceanic arch islands,, continental arch islands, active continental margin, and passive margin can be separated from each other. In the Roser & Korsch (1986) diagram, major oxides of SiO 2 , Al 2 O 3 , Na 2 O and K 2 O are used to determine the long-standing tectonic location of sediments. With regard to Fig. 7, it appears that the Fenced Lagoon sands are more oriented to the continental arch islands. The reason for the displacement of the samples in relation to the determined limits can be the presence of Fe 2 O 3 and MgO oxides. The presence of some Al 2 O 3 can be due to the presence of particles containing iron and magnesium phyllosilicates.
In general, the summary of the graphs used to study the tectonic position of sedimentary rocks suggests that the sediments of the studied area are located in the continental arch islands and, to a lesser extent, in the active continental margin. The existence of a stable tectonics in the region has resulted in relatively regular geochemical data accumulation (Adabi, 2011). Therefore, with regard to petrographic and tectonicstratigraphic evidence of the sediments in the region it can be concluded that these sediments have been deposited near the source. Geochemical study of surface sediments of the region indicates that there is a good correlation between frequency of elements and gradation and mineralogy in some elements. In fact, the destructive origin of sediments is the main contributor to the distribution of elements, especially heavy metals in the bed, and the anomaly of the concentration of the elements is mainly dependent on the mud blasters and the penetration of groundwater from the bed (Glazovski et al,. 1991, Brusilovskii andTuruchkina 1974). Conclusion. Sedimentary and geochemical studies of destructive deposits of the Fenced Lagoon were carried out to identify the source rock and tectonic location and the following results were obtained. Sediments of this lake were divided into four sedimentary types including Muddy Sand, Slightly Gravelly Muddy Sand, Sand, and Slightly Gravelly Sand, and sand is the main component of all these sediments. the tectonic location of the deposits. Bhatia, 1983;Roser&Korsch, 1986 The most abundant sedimentary types belong to Muddy Sand and Sand and the least abundant sedimentary types belong to Slightly Gravelly Muddy Sand and Slightly Gravelly Sand. The frequency of Muddy Sand is mainly seen in surface sediments up to the depth of 100 cm, while the facies type of Slightly Gravelly Muddy Sand is related to grater depths and both facies are more abundant in the north and northeast parts of the lake. By moving to the south and southwest regions of the lake, the two facies of Sand and Muddy Sand become more abundant. The size of sediment particles increases as the depth decreases from northeast to southwest. Alternate layers of sand -mud and sand horizons containing shellfish represent the periods of relaxation of the lagoon and its relationship with the sea (Ghazban, 2010). The changes in trends of the oxides of the main and trace elements against Al 2 O 3 indicate that the sediments under study can be classified into two general groups of sand and mud. A review of the charts to determine the nature of the origin rock suggests that the sediment samples of the Fenced Lagoon fall into different ranges of mafic and felsic origin rock, that is, they have acidic to intermediate combination. The survey of the total graphs used to study the tectonic position of the sediments indicates that the sediments of the studied area are located in the continental arch islands, and to a lesser extent, the active continental margin.