The main geomorphological unit in the studied area is Al-Batin Alluvial Fan. Al-Batin Alluvial Fan, is one of the ideal cases of alluvial fans of a desert plateau. It comprises essentially of ill-sorted gravely sand, with gypcrete top cover. The size of the gravels varies from coarse gravels (5 - 20 cm), around the apex of the fan, to fine gravels and pebbles (2 - 5 cm), in the peripheral parts. The thickness of the gypcrete varies from (0.5 - 1.5) m [15] [16] . The present day fan was formed in four stages. The stages are concentric with parallel iso-transport energy lines forming concentric arcs about the discharge point, having typical fan shape for all the stages; almost with flat top and very gentle gradients, small break separates between the four stages. It is very long fan; attaining110.192 Km. The length of the fan during the four stages were measured by ArcGIS technique. They were: 36.916 Km, 12.860 Km, 33.438 Km and 26.978 Km for the four stages respectively, whereas the coverage areas is about 10842 Km². The feeder channel is one of the largest valleys in the Iraqi Southern Desert called Al-Batin valley. It is still active, but had changed the location its course, being now in the extreme eastern part of the fan body, and almost without feeding the fan with new sediments, due to climatic changes, especially the annual precipitation, as confirmed by given [18] .

Other geomorphological units and forms are: Estuarine sabkha, Inland sabkha, gypcrete, flood plain sediments, sand sheets and sand dunes, depression fill sediments, dry marsh, active marsh (Figure 2). Because all these units have no significant importance, which concern this study; therefore they are not described. The details can be seen on the aforementioned references. However, Jabal Sanam is an outstanding geomorphological feature; located east of the alluvial fan (Figure 2).

The studied area is located within two different tectonic zones. The lower half part is within the Inner Platform of the Arabian Plate, whereas the upper half parts is located within the Outer Platform; represented by the Mesopotamia Foredeep, of the Arabian Plate [4] . The contact between the two tectonic zones is represented on the surface by a wide shallow depression, which is formed due to the active Abu Jir-Euphrates Fault Zone [2] - [4] . Moreover, Fouad [2] [3] pointed out that Abu Jir Fault Zone exhibits some geomorphologic features (pressure ridges and sag ponds) that are related to the lateral movement of the active fault zone. The studied area shows no any structural evidence for presence of structural element. However, some lineaments (Figure 3) within the alluvial sediments may indicate active structural forms; otherwise they shouldn’t appear in the sediments.

The pre-Quaternary rocks are presented only by the Dibdibba Formation. Many other Quaternary types of sediment are present too in the studied area, but only the sediments of the alluvial fan besides the Dibdibba Formation are described briefly based on Al-Sharbati and Ma’ala [7] , Diekran [9] , Jassim and Al-Jiburi [13] and Yacoub [16] ; hereinafter.

-Dibdibba Formation: The Dibdibba Formation is exposed in the studied area (Figure 2). Although it covers extensive areas, the thickness ranges from 3 to 8 m [7] . The Dibdibba Formation comprises of poorly sorted sand and sandstone; together with gravels. The sand and sandstone are mostly quartz; they consist of about 84.2% quartz grains of mono-crystalline and crystalline types, with around 8.5% of rock fragments and 7.3% of feldspar. The Dibdibba gravels consist mainly of acidic and intermediate igneous rocks, with smaller amount of metamorphic rocks, limestone and chert (Sadik, 1977 in [19] ).

The age of the Dibdibba Formation is a matter of argument; it is assumed to be Late Miocene-Pliocene to Early Pleistocene [20] . However, Jassim and Buday in [21] claimed its age to be post Middle Miocene and Late Miocene-Pliocene, whereas Sissakian and Mohammed and Sissakian and Fouad [17] [22] suggested Pliocene- Pleistocene age, which is adopted by the current authors.

It is worth mentioning that the differentiation between the Dibdibba Formation from Al-Batin Alluvial Fan sediments depends on the type of the cementing material of the sandstone beds. The sandstones of Dibdibba Formation are commonly cemented by calcareous material, whereas gypsiferous cementing materials are prevailing in Al-Batin Alluvial Fan sediments.

-Al-Batin Alluvial Fan: It is one of the ideal cases of alluvial fans of a desert plateau. It comprises essentially of ill-sorted gravely sand, sandy gravel and gypcrete, with subordinate layers and lenses of silty and sandy clay. The sands and gravels are composed mainly of quartz and feldspars, with less limestone pebbles and occasional acidic igneous and volcanic rock fragments. Aqrawi et al. (2006) in [13] also mentioned the presence of heavy mineral assemblages, mostly of rutile and zircon; in these sediments. These heavy minerals prove the origin of the sediments to be derived from the Arabian Shield. The sands are commonly medium to coarse grained, occasionally pebbly, yellowish pink in color, which might be derived from erosion of the Fluvial Plain (Dibdibba Formation). The erosional materials are transported by Al-Batin stream during the Pleistocene. The apex of the fan is characterized by gravelly sand, whereas its toe, near Hor Al-Hammar, has fine and friable sands [7] . The gravels form thin lenses, lenticular beds, reach up to 2 m in thickness, or randomly scattered within the sand beds. The size of the gravels varies from coarse gravels (5 - 20 cm in size), around the apex of the fan in the main channel of Wadi Al-Batin, to fine gravels and pebbles (2 - 5 cm in size), in the peripheral parts of the fans. Planar bedding, cross-bedding and gravel lag are common fluvial sedimentary structures in this fan system. The gypcrete and gypsiferous cementing material are fairly common in surface beds. The thickness of the gypcrete beds varies in average between (0.5 - 1.5) m [16] .

Al-Batin Alluvial Fan has delta shape (Figures 2-4), with length of 110.192 Km and very wide periphery, since its maximum width is 119.1 Km. The fan was deposited by single feeder channel; Wadi Al-Batin, which is one of the largest valleys not only in Iraq, but in near surroundings too (Figure 3 and Figure 4). During the wet phases of the Pleistocene, four stages of the fan were deposited [23] .

Figure 5, Figure 7 and Figure 8 show clearly the four stages of the alluvial fan. It is clear that the oldest stage is No. 1 and the youngest is No. 4. Moreover, in the northeastern part of the oldest stage, clear concentric forms occur, which are parallel to the periphery of the oldest stage; indicating continuous retreatment of the fan. However, it also may indicate continuous shifting of the course of the Euphrates River northeastwards due to the continuous growth of the first stage of the alluvial fan. It is believed that the second assumption is more likely.

The two topographic cross sections (Figure 9) show the shape of the alluvial fan. The topography along the slope ((A) and (B) in Figure 9) shows the break in the gradient of the four stages. The clearest break in the gradient is between stages 2 and 3. Whereas, the topography across the fan ((C) and (D) in Figure 9) shows that the eastern side of the fan is higher and steeper, as compared to the western side. This is attributed to regional slope; during deposition, which was towards northeast. Therefore, the coverage area of the fan towards east is more than in the western side of the main feeder channel.

The measured slopes of the four stages (Table 2) and that of the whole fan (about 0.7˚) contradicts with the assumption of [24] , which states that “These findings contrast with a previous notion that depositional landforms rarely have slopes of 0.5˚ (0.009) to 1.5˚ (0.026) and hence only semi-conical landforms steeper than 1.5˚ should be called alluvial fans”. However, it is believed that the original slope of the fan body was higher; otherwise the fan wouldn’t cover such vast area, but it was flattened due to tectonic activity of the depression.

Figure 7 also shows clearly in the extreme eastern part of the alluvial fan an ancient course (Point AC in Figure 7), which may be a distributary course of the Euphrates River that was draining directly in Khor Al-Zu- bair. The course was abandoned due to its continuous shifting eastwards, besides the shifting of the Euphrates River northeast wards. The presence of the ancient course is also evidenced by the presence of tens of small valleys draining the alluvial fan to the course. Some of those valleys have deposited small alluvial fans (Point AF in Figure 7).

The Dibdibba Formation is known to be fresh water sediments [19] [20] . Sadik (1977) in [19] suggested the environment of the Dibdibba Formation to be fluvial based on the grain size analysis, shape and size of the gravels and the sedimentary structure. According to [19] , the Dibdibba Formation represents fluvial sedimentation in extensive sheet probably as large old alluvial fans. From thickness studies, it seems that at least two such fans may be considered. Salman (1984) in [13] Jassim and Al-Jiburi (2009), assumed a fresh water environment to the formation according to its fossil contents. Jassim and Buday in [21] Jassim and Goff (2006) suggested that the formation was deposited in a fresh water environment, becoming deltaic some times.

From the regional geology of Iraq, the Dibdibba Formation is deposited mainly as alluvial fans; like Karbala- Najaf and Al-Batin alluvial fans [7] [17] [25] - [27] . Figure 7 shows the distribution of the exposures of the Dibdibba Formation around Wadi Al-Batin, which gives indication for an ancient fan; before the deposition of Al- Batin Alluvial Fan. The “Dibdibba Fan” (it will be called so in the current study) was deposited most probably during Pliocene-lower Early Pleistocene, as the age of the formation is proved to be Pliocene-Pleistocene and as most of the alluvial fans in Iraq have Pleistocene age [1] [13] [15] -[17] [22] [28] [29] . The deposition of the fan was terminated either due to climatic changes (successive wet and dry phases), which have happened during Pleistocene [18] [30] and no more sediments supply was reaching the fan; or due to the uplift of the area, which is less likely. Then Wadi Al-Batin started to dissect the fan body, which is very normal case [31] [32] and formed the main course of the valley, which is still present (Figure 4 and Figure 5 and Figure 7), but not the curved part towards the east; as it is in nowadays course. The wide extension of the Dibdibba Fan towards east is also evidenced by the presence of outcrops of the Dibdibba Formation surrounding the spectacular Sanam Mountain in a circular form (Figure 2), covered by the sediments of the first stage of Al-Batin Alluvial Fan. Otherwise, there is no explanation for the presence of outcrops of the Dibdibba Formation surrounding Sanam Mountain; in a uniform circular form (Figure 2). The presence of radial valleys on the surface of the Dibdibba Fan, especially in the extreme eastern and western sides (Figure 3 and Figure 10) is also evidence that the sediments were laid down as an alluvial fan

After Pliocene and during Early Pleistocene, the sediments supply restarted again and the first stage was laid down (Figure 7). This stage was the largest one; as it can be seen and it was continuously shifting the course of the Euphrates River, as evidenced by successive circular forms (point C in Figure 7). The other three stages were deposited continuously, but with shorter distances. This is attributed to the climatic changes and/ or the activity of the Abu Jir-Euphrates Fault Zone, which is represented by a depression in which the fan was deposited. The traces of the depression below the sediments of the fan (D1 - D2 in Figure 7) can be seen, which has dark tone as compared with the dry surrounding sediments. The retreating of the fan stages towards the apex may indicate tectonic activity in the depositional basin [33] . The activity of the depositional basin is related to the activity of Abu Ji-Euphrates Fault Zone, which is proved to be still active [2] - [4] .

The Southern Desert, during Pliocene-Pleistocene was still uplifted area, due to the collision of the Arabian Plate with the Neo-Tethys terrains (Sanandaj-Serjan zone). Therefore, continental, fluvial deposits prevailed over the area in different forms, among them are alluvial fans. And because the existing valleys were very large, mainly draining from Saudi Arabia; therefore, the deposited alluvial fans are very large, like Al-Batin and Karbala-Najaf Fans [1] .

However, medium sized alluvial fans are also present in different parts of the Iraqi deserts, like the Hab’bariyah Fans [29] . All these fans are deposited in depressions, which were active during deposition phases and some of them are still active. The presence of the four stages in Al-Batin Alluvial Fan is a clear indication for the activity of the depression, besides the NNW-SSE lineament within the fan sediments (Figure 3, Figure 6 and Figure 11). This lineament, which is clearly visible in Figure 11 is a good evidence for the activity of the depression, after the deposition of the fan (Late Pleistocene and younger), in which the fan sediments were laid down. It may represent oblique fault; within the depression, such faults are confirmed by [2] - [4] . However, the lineament may represent the ancient course of Wadi Al-Batin during deposition of the “Dibdibba Fan”.

When comparing the trend and approximate width of the depression in both images (Figure 11), it can be seen obviously that both of them have the same trend and width. Moreover, it is very clear evidence that the mentioned fault zone is wider, especially west of Al-Batin valley and extends even in the Arabian Gulf, which is not mentioned by [2] - [4] when he defined the boundaries of the tectonic zones in Iraq.

Figure 11 (right) also shows probable ancient irrigational straight and uniform channel, which has white tone; it is dissected by Khor Al-Zubair channels system, indicating tectonic activity that had caused dissection of the irrigation channel (after being abandoned due to tectonic activity and/or climatic changes) and erosion of its banks. The point C (Figure 11, right), which is along the southwestern limit of the depression shows widening in the channel embankments along the southwestern limits of the depression (C and D in Figure 11); indicating tectonic activity within the depression and its near surroundings.

The surface of Al-Batin Alluvial Fan was dissected by water erosion during early Holocene (rainy stage), which produced highly dissected surface due to tens of beheaded valleys (Figure 3, Figure 4 and Figure 7). During the late Holocene (dry stage), the fan surface was influenced by wind erosion, which produced deflation depressions. In these depressions, coarse gravels were led in site to form pavement landform. Most of the deflation depressions are covered by sand drift. However, some of them, which are connected to ephemeral dry valleys, are modified to playas and their muddy sediments cover the residual gravels. Moreover, Al-Batin River was modified to dry valley, which is partly covered by sand drift accumulations and Nebkha [15] .

The surface of Al-Batin Alluvial Fan shows also a clear lineament in NNW-SSE trend (Figure 3 and Figure 7), this lineament is within the depression of the active Abu Jir-Euphrates Fault Zone and it may represent one of the pressure ridges, which are present in the depression along the fault zone, besides the sag ponds [3] [4] .

The presence of the outcrops of the Dibdibba Formation (Dibdibba Fan) west of Al-Batin Alluvial Fan in higher elevation than the latter fan indicates that the latter fan was deposited in a basin, which was lower than the top surface of the former fan. This is another indication for the activity of the area. The presence of large groves and irregular depressions on the surface of Dibdibba Fan (Figure 7) indicate that the area had suffered from intense erosion by the flowing valleys during wet phases of Pleistocene and Holocene.