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3.2.2 Description of Land Systems

In the study area four land systems were identified (Figure 15) essentially basing on geographic location and geomorphological patterns. The plateau of Ben Slimane is in the northern-central part, delimited by an eastern and a western scarp. In the South the landscape is made up by reliefs. Due to the final working scale (1:100,000) and also for the fairly homogeneous landscape, not many landforms were identified and in two cases the landform even corresponded to the landscape.

Plateau of Ben Slimane

The plateau of Ben Slimane is almost flat and tilted toward West. To the North it degrades with regularity toward the Atlantic Ocean. In this land system six different landforms were recognised. This plateau is what remains of an ancient erosional surface that has influenced the formation of the Palaeozoic substratum.

Flat planation surface

This landform is characterised by a very weak inclination variable between 1 and 4% and it is located in the eastern part of the plateau, near the Oued Nefifikh. It corresponds to the flatter part of the land system and it is completely occupied by the so-called tirs (vertisols). They are very dark soils, almost black, rich in organic matter and clay minerals; they almost always overlap with a calcarenitic layer corresponding to the Moghrebien transgression deposits. In some case they also overlap with the triassic basaltic rocks even if these are found in an another system: the "western scarp".

Because this landform is substantially a fat surface, no present or potential erosion phenomena are evident.

The origin of such soils is not clear and the existing bibliography is not able to give suitable explanations. After the field work a possible theory could be hypothesised in relation to the possible climatic conditions that, subsequently to the Moghrebien and during the quaternary interglacial phases, favoured the formation of a very rich vegetation cover. During the regressions, the sea withdrawal could have favoured local conditions for the creation of small closed basins connected to the presence of very big quartzitic outcrops that could have formed a barrier to the withdrawal of the sea waters. The almost total absence of water turn-over favoured the lacustrine or lagoonal conditions that allowed the decomposition of organic matter and the deposition of clays and argillaceous silts. The combination of such factors could have determined the formation of the vertisoils (Photo 1).

Closed depressions (dayas)

In this class both the dolines-dayas and the barrages-dayas are present. At present the morphodynamic processes that created these depressions are inactive. In particular the circular and large size ones are connected to dissolution processes that, in the last period, did not evolve in depth but laterally. The inactivity of these landforms is confirmed by the presence of the eluvio-colluvial deposits inside them (Photo 2).

Undulating planation surface

This landform occupies most of the plateau; from a morphodynamic point of view it is similar to the above-mentioned "flat planation surface" but nevertheless it presents some characteristics for which it constitutes a different landform. At small scale, as previously stated, it is substantially flat, while at greater scale it has some slopes between 3 and 15% with sometimes values of 20-27% too. These variations are connected both to the structural setting of the substratum and to the thickness of the quaternary deposits that made up the substratum of the plateau. An important characteristic is also represented by small rises that give it an undulating feature. These ones are constituted by weathered quartzites that outcrop in the higher part (at about 300m) and that are covered by very thick soils in the lower and more flatter parts of the plateau.

Figure 15 - Land systems and landforms

Photo 1 - Plateau of Ben Slimane: flat planation surface characterized by the dark colour of the "tir"; the bright materials dug from the soil profile are composed by weathered calcarenite

Photo 2 - Closed depression:a typical doline-daya of the plateau

The principal lithology is constituted by grey silts with pisolithes (q1-6lr) with locally red silts with pisolithes (q1-6lb) and small outcropping of moghrebien calcarenite (pM). Along the paleo-rivers, deposits of grey silts (A1b) are present too.

Although relevant values of slope are locally recorded, the whole area is not prone to erosion phenomena; only in few cases a sheet erosion was identified as connected to the normal overland flow due to the superficial runoff (Photo 3).

Low residual crest

This landform is formed by the ordovician formations made up of sandstones, quartzites, and microconglomerates of the Feddan-Taba formation (o5) and schists, psammites, and micaceous quartzite of the formation of Sidi-Khriali and Oulad-Bahloul (o3-4). This landform is present in the northern-eastern part of the plateau and it is represented by isolated reliefs that sometimes reach 2m height. With time, these formations were affected by an important alteration and were subject to selective erosion on the most erodible rocks. This process is confirmed by the presence of these residual crests.

Strongly fractured, these rocks seem to have a chaotic position but in some cases it is possible to recognise clear layers of varying thicknesses from 30 to 60cm. Their genesis is to be dated to the folding that interested the Palaeozoic substratum during the Hercynian orogeny. Subsequently the area was subject to a series of transgressions that deposited various quaternary formations isolating the summit surfaces of the substratum (Photo 4)

High residual crest (shkour)

It corresponds to the ordovician fine-grained quartzites (Hv) that outcrop in strongly fractured very thick layers. They also reach relevant heights, as in the case of Sokhra El Hajiba, 366m high. They developed along their longitudinal axis in the prevailing NW-SE direction. On the South-East and on the North-East of Ben Slimane they are quite sharply folded. The intense diaclase and the presence of these folds confirm the strong tectonic activity to which the whole area was prone during its evolution.

The long exposure to weathering agents, also in very different climatic periods, determined a strongly altered characteristic in the deposit. The surface presents a reddish colour, so implying that in the past the quartzites were covered by soil, which was responsible for ferrous mineral leacking whose oxidation thus determined the coloration of the deposit. These weathering phenomena also affected the same quartzites where, along the finest fractures, water infiltration caused oxidation. In this case the ferruginous crusts, inside the quartzites, are present too (Photo 5).

Accumulation footslope

This landform corresponds to moderately sloping surfaces with inclinations between 8 and 20%; they are located at the feet of the "high residual crest" and they form a typical slope deposit. It is lithologically composed by the weathered materials of the quartzite of Ben Slimane with angular pebbles of several dimensions, coming from the quartzite in an abundant sandy-silty matrix of the same nature. In some cases the presence of the clastic fraction is so abundant so as not to allow soil formation and development, as in the area around Sokhra Nemra.

Probably the nature of the materials that made up this landform prevented erosion. Only moderate sheet erosion processes were recognised; these processes determined a greater concentration of coarse material immediately at the foot of the quartzites and an increase of the finest components far from the isolated relief and close to the base of the accumulation deposit (Photo 5).

Western scarp of plateau

The western scarp of the plateau is a surface connecting the summit surface of the plateau and the incisions of the main oueds, in particular along the Oued Nefifikh and Oued Sefrou. In this system two different landforms "accumulation slopes" and "alluvial valley bottom" were identified; they are related by a topographic sequence, controlled by lithological and structural characteristics.

Accumulation slopes

This landform represents most of the system and it is present at the foot of the above plateau. In general it is characterised by gently to moderately inclined slopes with a steepness range between 10 and 35%. In general the deposit is lithologically heterogeneous because it is due to on the rocky outcrops in the upper part. The latter outcrops vary among the Moghrebien calcarenites and the triassic deposits, mainly doleritic basalts. Actually this unit is mainly affected by slight sheet erosion (more than 50%) and locally rill to gully erosion. A very severe phenomenon of gully erosion with incisions that reaches a depth of 200cm with a spacing of 100cm is located at the confluence of the Oued Chabat El Hamira and Oued Sefrou with the Oued Nefifikh. In this area some slight mass movement (5%) is visible too. These geomorphological processes are both caused by the steep slopes and by overgrazing (Photo 6).

Alluvial valley bottom

This unit is the least present in the system, in fact it represents only a narrow strip along the Oued Dir and Daliya. It is mainly composed by coarse and fine sandy silt with gravel. In this unit also fluvial terraces and slope debris were included. The erosional processes are negligible (Photo 7).

Photo 3 - Undulating planation surface: the smooth of the undulating surface of the plateau

Photo 4 - Low residual crest: the low residual crest that outcrops in the plateau; the strong diaclase of the quartzite materials is evident

Photo 5 - High residual crest (shkour): the typical shape of the elongated, narrow and high residual crest; it is possible to recognise the accumulation footslope at the base of a shkour

Photo 6 - Accumulation slope: The gully erosion affecting the accumulation slope of the western scarp of the plateau

Photo 7 - Alluvial valley bottom: the riverbed of the Oued Dir already quite dry in April; in evidence the flat surface of the alluvial terraces

Eastern scarp of plateau

This system delimits the whole oriental sector of the plateau and crosses the study area in an almost longitudinal direction. From a tectonic point of view, this area corresponds to the left side of the horst of Oued Cherrat that morphologically appears as a narrow gorge. From the external to the internal part of the Oued Cherrat, different lithology outcrops. Mainly sandstones, limestone and schists (Hv) are present but also outcrops of silty claystones (di2) and Silurian schists outcrop. In the southern part this system is in contact with the land of the "southern reliefs" whose limit is not clear but was identified solely on the basis of their different elevation. In general the less steep slopes, with respect to the western ones, determine the different morphology of this scarp with respect to the western scarp of the plateau. These different features are also related to the different outcropping lithologies.

dissected sloping area

Only this landform was identified. The relative relief is high enough, in fact in the northern part the altitude varies between about 100m in the bottom of Oued Cherrat and 350m at the top of the scarp. This scarp is characterised by several incisions that transversally cut the system and that give it its main characteristic. In fact this area results very dissected and the incisions delimit narrow crests, called with the arabic name of dhar. These crests are characterised by slopes from moderately inclined to very steep. The erosional process is not very important because of a dense and natural vegetation, mainly Tetraclinis articulata, that characterises this scarp in comparison with the western one. Nevertheless, in some parts, slight sheet (5-10 %) and rill erosions are present. Moreover, sometimes rills become gullies in the lower part of the scarp. In the summit part of the sloping area rocky outcrops are evident while the lower parts are locally covered by colluvial deposits. These ones have different characteristics in the northern and in the southern part on the basis of the different parent materials, respectively mainly limestones and schists (Photo 8).

Southern reliefs

flat topped hills

In this land system just this landform is present. At small scale it is characterised by different geomorphological features but in detail it presents different patterns. The absence of a geological map causes a fragmentary knowledge of the lithology at the basis of the fieldwork. In general the landform presents a gentle morphology with low limestone and schist hills characterised by a flat summit connected to the Pliocene erosional surface. The energy relief increases from the western to the eastern part and due to the slope steepness two different patterns can be identified: steep and moderate-gentle slope. The first type is located in the narrow area along the Oued Cherrat where the steepness is between 56 and 32%. The relative relief ranges from 250 to 500m and the principal rock outcropping is mainly schists. Broad interfluves separate the hills. Because these areas are characterised by a natural vegetation cover, even if an inclination of 40% is present, no strong erosional processes are evident except for a widespread but moderate sheet erosion. In all this area a dense drainage network has developped. The moderate slopes were identified in the western area where the morphology can be described as undulating rises. The slopes have steepness between 32 and 10% and in the least sloping sector they are covered by heterogeneous colluvial deposits. Sometimes in the summit part of the relief, the substratum outcrops are composed of grey limestone vertically bedded. The schist outcrops are evident along the incisions caused by gully erosion. Except for these last processes, the area is characterised by slight erosion but also severe gully erosion can occur. As for the steepness, also the relative relief decreases reaching a difference in level of about 100m (Photo 9).

Photo 8 - Dissected sloping area: the ravine or gully erosion in the very steep eastern slopes

Photo 9 - Flat topped hills: the steepness of the slope and the flat top surfaces are evident


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