The case study is situated in Lali area, Khuzestan province in Iran and 200 km far from the north of Ahwaz province. From geological point of view, it is placed in the sedimentary structural zone of the folded Zagros. It is expected that this zone mainly holds formations of Bakhtiary conglomerate, Aghajari sand stone, Marl of Mishan, Gachsaran gypsum and Asmari lime. Furthermore, there are outcrops like Pabdeh- Gurpi, etc.  . Various blocks of these layers are separated and transported into the downstream of river in Lali area because of weathering and erosion factors on the area’s geological formations especially on the upstream of the water catchment based on the lithology type of the rock layers. The blocks range from angular pebbles to angular to semi-an- gular and rounded to semi-rounded fine grains  . These coarse grains of alluvia sediments mainly originate from Asmari lime rock formations of Aghajari and Bakhtiary conglomerate. This research profoundly analyzes the types of such geological formations. This area has been remarkably affected by Dezful embayment (DE = Dezful Embayment) which its main folded structure has been Valachin orogeny. Figure 1 displays the geographical situation and the accessible ways to Lali area.
Since sediment and sedimentation are the most essential factors in civil projects and establishments especially in dams’ reservoirs, lots of studies have been done in this regard. However, some important geological and sedimentary points have been ignored such as affective factors on sedimentation, lithological type and the geological formations of the intended area producing and increasing the amount of sediments  . Furthermore, identifying the specifications of rock outcrops and sedimentary type have been considered.
2. Review of Literature
Several base studies have been done in this regard, for example, various investigations have revealed the relationship between sediments with the type of the geological formations of the area and the amount of their erodability. Norlan (1986) assessed the ratio of erodible formations as the origin of floating load in California  . Kasimir, et al. (1990) demonstrated that small basins with the ratio of looser formations affect the floating sedimentary load  . Khojeini and Nezhadha Shami (1998) indicated the effect of erodible formations on sediment production in Taleghan, Iran water catchment  .
Figure 1. The geographical situation and the accessible ways to Lali area, from Google earth.
Aminsobhani (1998) proved the impact of sedimentation on marl and shale erodible formations in Namak Lake of Iran  . These studies demonstrate the amount of sensitivity to erosion in geological formations and sedimentations in the basins. Furthermore, erodible geological formations highly affect sedimentation in water catchments. Moreover, the amount of sedimentation relies on the surface area of the water catchment basin and the surface ratio of the erodible formations out of the whole basin. In case there are formations with looser lithology, the amount of sedimentation will be enhanced.
Field work studies on the geological formations of the area specify both the lithology type of the area and the resistance of this type of lithology due their physico-mechanical properties of the rock layers. The type of the geological formations and the lithology of these formations have been specified by analyzing the geology and topography maps of the water catchment upstream of the sedimentary basin and field work in Lali area. The method of this research is based on analyzing the topography and geology maps and field work in order to identify the lithology of the geological formations. In fact, the amount of its erodability has been characterized by the type of the geological formations
According to various geological formations, changes in the types of layers, different physico-chemical especially mechanical properties and the erodability and strength of the soil and rock masses, it is demonstrated that these geological formations are able to produce sediment. In this research, these specifications are first identified and then the area’s geological formations are scrutinized from the viewpoint of the potential of producing less or more sediments  . Holistically, it is specified that formations with low erosion and more strength are less probable to produce sediment. In contrast, formations with high erosion and less strength can produce sediment  .
4. Geological Setting
The erosion amount of the lithological units and sediments transportation to sedimentary basins are recognized as one of the key factors determining the geomorphological properties, the accumulation specifications and the replacement of the sedimentary masses  . From morphological point of view, this situation is placed approximately 250 m away from the crossing point of Harkesh and Taraz Rivers and it has a strait shape made by the floods from Harkesh River. Figure 2 indicates a view of Taraz Dam situated in the dam axel in which the morphological extension and river bed of Harkesh River (HR). In addition, Figure 3 features out a view of the porous lime stone (I). there inter-beds inside gypsums of Gachsaran formation (Mg) which are seen as small and big blocks (up to 2 m diameter) on the ground.
The conditions of its extensions both at the right and left sides are as below:
・ Right side extension: The slope of the extensions from top to bottom include: 70˚ above the heights as long walls and declining from the bottom of the wall to the alluvia terraces of the lower level (17˚). It is scored that the slopes become less than 5˚.
・ Left side extension: 75˚ of the higher levels are like long walls extending to the downstream, trenches and road way, the extension slope is 35˚ - 40˚.
The sedimentary process influences the erosion index of the area’s units and the determination of the sediment abundance percentage of each unit in the area.
Figure 2. A view of Taraz Dam situated in the dam axel in which the morphological extension and river bed of Harkesh River (HR) clearly shown. The alluvium of the river bed-Qt-the old alluvium as alluvia terrace, S-foundation outcrop (Sarsen), Bcc, Bci, Bcs and Bct are the conglomerate units of Bakhtiary formation. The layers are somehow horizontal.
Figure 3. A view of the porous lime stone (I). there inter-beds inside gypsums of Gachsaran formation (Mg) which are seen as small and big blocks (up to 2 m diameter) on the ground.
The general appearance of the morphological specifications is as the following.
Lime stone pebbles have surrounded parts of the conglomerate bed rock at the river bed and the foundation outcrop is observed and the high level of the bed rock of the alluvia is indicated.
The right side extension is more even than the left one for the sake of streams specifically in the upper part of the left extension  .
Both extensions are bedded in the conglomerate layers of Bakhtiary formation and are somehow horizontal.
The old left alluvia of Harkesh River are observed as a long strip on the right side of the river, but they have become like alluvial terraces.
Above 388 m, there are thick conglomerates on both extensions which are long and their height is scored more than 20 m. The extension of macro joints can be measured in the conglomerate layers within distance more than 1 - 2 m.
Soil and Lithological Masses at the Geological Situation
The lithological masses observed as thick layers of Bakhtiary conglomerate outcrops make more than 95% of the conglomerate aggregates of Asmari and/or older lime stones. Generally, they are rounded to semi-rounded. More precise analyses on the folded conglomerate Bakhtiary rocks indicate the following points: Coarse grain layers (the average diameter of 0.7 cm) are made of lime stones and wall making layers. The grains are paired from bad to moderate and some of them have been displaced. Some layers hold finer grains (the average diameter of 5 cm) and their matrix is lime sand. As sand stone and silt are more than the coarse ones in these layers, some of them hold sand stones with cross-bedding  . There are several red inter-layers of silt and sometimes sand stone in the inter-beds of conglomerate which their width isn’t more than 1m, they are exposed to more erosion than conglomerate layers and they have outcrops in the extensions of topography.
In addition, there are coarse grains of alluvia as big as pebbles, sand, gravel and stone blocks at the river bed. It seems that its width doesn’t reach to more than 1m. Holistically, more than 90% of them are lime stone grains mainly belonging to the separated blocks of Asmari lime formation on the upstream of this water catchment.
Hence, the type of blocks forming these pebbles affects erosion and sedimentation. So, erosion, sediments transportation in sedimentary basins and the sedimentation of these deposits in dams’ reservoirs decrease the life span of dams  . It is essential to study the effective processes on sedimentation. Figure 4 shows the lithology, structure and geology map of the lithological-soil units of this position.
Table 1 summarizes the general specifications of lithological units in Lali area.
5. The Specifications of the Structural Geology of Lali Area
The water catchment expansion of rivers flowing through the area specially Talug and Shur rivers are situated in the sedimentary- structural zone of the folded Zagros (see Figure 5). Therefore, the general characteristics of this zone are as below.
There are asymmetric simple folds with different slope sides. In another word, their axial surface is different. However, some of the folds less than 10 - 20 km might be vertically close to one another. As this folding style gets closer to the high Zagros, it is changed from the south-east to the north-east  . As expected, more irregularities are observed. Since the intended expansion is far from the high Zagros, the folds are very simple and the sedimentary layers aren’t remarkably extended.
There are few faults and the length of the main ones are more than 10 km which all show the same trend of Zagros. They direct a deep slope toward north-east (more than 65˚ - 70˚). Figure 6, Figure 7, Figure 8 and Figure 9 represent the distribution pattern and dispersion of the amount of fractures and the joints in the right and left sides of Harkesh River in Lali area, Khuzestan caused by tectonic activities and the effects of the erodability ratio of the formations in the area on th Schmidt network (STERIONET) or Rose diagram. As observed, most of the fractures and sediment production have been mostly concentrated at the NW-SE of the STRERIONET. It is concluded they are along with the folds of Zagros chain of mountains in Iran, as expected.
Important faults affecting the geological structure of Lali area include: Dali fault, Sarvand-Meyboran fault (SMF), Durak fault (DRF), Jagiri fault (JF), Lali Fault (LF), Qale Khaju fault (QKF), Cheshmeh Shirin fault (CSF) fault and Chiti fault (CF).
Table 2 illustrates the specifications of all faults in the expansion of 300km in Lali area. As observed, most of the faults closer to the position are the normal ones which are gradually converted into inverse and thrust- compressive faults  .
Figure 4. The geological map (T/GE/1), Lali area, Khuzestan, Iran quoted from the geology report of Ab-Niru Consulting Engineering Company, 2005.
Table 1. The general specifications of Lithological units in Lali area.
Figure 5. Apart of Talug River (TR) has outcrop between Sartang and Dom Baba Ahmad (BTBA) in which marl lime stones have been emerged.
Figure 6. The situation of the fractures and joints concentration on the left abutment in Lali area.
Figure 7. The equal curves of joints poles and fractures in the geological formations in Lali area.
Figure 8. The equal curves of joints poles and fractures in the geological formations in Lali area.
Figure 9. The situation of the fractures and joints concentration on the right abutment in Lali area.
Table 2. The specifications of faults in the expansion of 300 km in the Lali area.
From geological point of view, Lali area is situated in the structural zone of the folded Zagros and it affects other structures. It is expected that this zone mainly holds formations of Bakhtiary conglomerate, Aghajari sand stone, Marl of Mishan, Gachsaran gypsum and Asmari lime. Furthermore, there are outcrops like Pabdeh-Gurpi. Various blocks of these layers are separated and transported into the river downstream in Lali area because of weathering and erosion factors on the area’s geological formations. Kasimir, et al. (1990) demonstrated that small basins with the ratio of looser formations affect the floating sedimentary load. Khojeini and Nezhadha Shami (1998) indicated the effect of erodible formations on sediment production in Taleghan, Iran water catchment. Ghami Arusmahalleh and Aminsobhani (1998) proved the impact of sedimentation on marl and Shale erodible formations in Namak Lake of Iran  . The morphological specifications of the intended area reveal that the north part of the basin is totally in the ridge-valley mountainous unit and the southern parts are in Dezful embayment. In another word, two morphological zones can be marked in the water catchment: the northern mountainous zone and Dezful embayment zone .Parallel valleys and uprising parts along with Zagros characterize the northern mountainous zone. There are narrow valleys and other vast alluvia plains. The mountains are usually anticline. There are many blocks affecting the erosion of these walls and their fractures make soil fine grains and conglomerate blocks and increase sedimentation  . The outcrops of Bakhtiary conglomerate are found as long walls and very deep macro joints (more than 50 - 60 m). There are dissolving pores in gypsum stones in the water catchment of Shur River at the depth of more than 15 - 20 m. The stratigraphic units identified in the northern morphological zone are chronological as the following: Formations of Darian, Kazhdomi, Soruk, Gurpi, Pabdeh, Shahbazan or Taleh Zang, Amiran and Asmari have scored a widths of 3500 m. Lime, shale and marl rock units have been alternatively made in these sedimentary units and ridge-valley structure is observed for the sake of their very different stratigraphic rock foundation  . The lime stones of the Asmari formation occupy almost more than half of the ridge-valley of Bakhtiary zone. The formation thickness reaches to 350 - 400 m. The highest part of Asmari formation holds an alternation of lime stone and marl layers specifically in the north east side of Bala Ahmad anticline. The karst process is considered as one of the important remarks on Asmari formation. At the axial part of Baba Ahmad anticline, the Asmari lime stone layers are somehow horizontal and they have made 10 meter walls. Notably, the width of layers and looser marl or marl lime stone layers feature outa step like landmark. According to the lithology type and its looseness or hardness, some lithological units and formations are harder and some of them are looser. As a result, looser units affect sediment production and sedimentation the most  . Moreover, the type of the lithological units should be also considered. Marl limes or silica lime produce different sediments. Consequently, looser marl materials produce more sediments and harder silica materials produce less sediments  . Hence, fine grain sediments as size as silt and clay are made as the result of erosion, materials transportation and mal and silt marl layers of Aghajari and settled all along the river direction. Holistically, particles have been deposited as floating sedimentary load (clay-silt fine grains) and the rolling sedimentary load (as size of pebbles and sand).Land sliding phenomenon accumulates sediments at the bottom of mountains extension as deposits transported towards the lower parts because of erosion and weathering phenomenon in lithological formations and tectonic factors making fracture in lithological layers like joints and faults  . Table 3 displays the amount of erodability and sedimentation of the geological formations based on the lithology type in Lali Area, Khuzestan, Iran.
There is actually a collection of lithological units in this morphological zone called Fars Group including: Gachsaran, Mishan and Aghajari. Bakhtiary conglomerate is on these units aging back to Miocene-Pliocene. The units are as below: Gachsaran formation aging back to Miocene is situated in Zagros Mountains on the lime stone of Asmari formation. In this expansion, Mishan formation embraces gray marls which are highly affected by erosion and it doesn’t have an outstanding outcrop, but some layers alternatively formed on Gachsaran formation In addition, Mishan formation or Gachsaran formation are mostly made of red clastic rocks with sand stone and conglomerate inter-beds called Aghajari aging back to Miocene-Pliocene. Bakhtiary conglomerate has two characteristics in this expansion: 1) Bakhtiary conglomerate has been placed on older rocks after orogeny. 2) Tectonic forces after forming Bakhtiary lithological unit only make some mild slopes and joints. In another word, other layers of Bakhtiary formations are almost horizontal. In fact, its outcrops are found everywhere on the highest topography parts of Dezful embayment. The geological situation of Taraz has been detected in these Bakhtiary conglomerate layers. There is an alluvial unit all through the
Table 3. The amount of erodability and sedimentation of the geological formations based on the lithology type in Lali Area, Khuzestan, Iran  .
expansion especially in Dezful embayment, and young and old alluvial sediments. Alluvia are usually in form of pebbles and coarse ones on rivers beds. The rivers’ alluvia are also separated while flooding. Old alluvia have become flood plains at rivers banks. Their widths sometimes reach to several hundred meters. In conclusion, the oldest alluvia are changed into long and short alluvia terraces. These old alluvia look like large alluvial fans which have been smaller by erosion. Sediments are observed all around the extension of the heights. Remarkably, they sometimes cover the bed rocks of the formations. Sediments are accumulated in two forms of fine ones and stone blocks at the bottom of the long walls. The stone block deposits and fine grains have been expanded everywhere.
As a whole, the area’s formations are different from one another according to the lithology type, layer material, erosion and sediment production and sedimentation. They are chronologically arranged. Pabdeh-Gurpi formation is made of shale and marl although it has low permeability and has high potential of erosion. Hence, it affects sedimentation the most.
Although lime stone is very hard, Asmari formation made of lime stone to marl lime has the characteristic of karst. Gachsaran formation made of gypsum has the ability to be solved and weathered. It can also produce sediment. Bakhtiary conglomerate is mainly hard and compacted and it doesn’t have a remarkable ability to make erosion and produce sediment.
Conclusively, Pabdeh-Gurpi and Gachsaran formations are more able to produce sediments, while Asmari and Bakhtiary conglomerate affects sedimentation the least and Aghajari formation is in between. The quaternary alluvia can be transported and settled. Results specify that it is essential to scrutinize and identify the types of formations and lithological units of the upstream and the banks of the dams’ reservoirs in civil projects and dam construction studies in order to find out better alternatives against sediments made by the erosion of these soil-lithological units. It is recommended to plant trees, establish berm, reduce topography slope and choose more suitable methods to control water in order to decline erosion and sedimentation.
Regarding the studies and observations in erosion phenomenon, sedimentation and sediment accumulation in geological formations the following items are suggested.
In situation of geological formations with the potential of sedimentation and erodability, there should be given a good consideration on investigations of the amount of sediment production in the geological formations situated in water catchments directing into identifying formations ready to be eroded. Presenting factors affecting the erosion process is led into selecting the most convenient methods to diminish the effect of these processes and control its harmful effects in the intended area before starting any civil project specially dam construction studies. Basic studies on the amount of sediment taken from the geological are directed into the increase of sedimentary load entering into reservoirs. Proper alternatives are taken to reduce its harmful effects through analyzing the data of these studies.
The authors acknowledge Dr. Ali Solgi, the head master of geology department of Islamic Azad University of Science and Research of Tehran, Eng. M.H Nabavi and Dr. Hussein Jalali, the managers of Ab-Niru Company, and all the ones who contributed us to prepare this research.