3.2 Geology and geomorphology

3.2.1 General overview

The geology of the northern part of the study area is described in a detailed way in the work of Destombes and Jeannette (1966) "Mémoire explicatif de la Carte geotechnique de la meseta cotiere a l’Est de Casablanca. Régions de Mohammedia, Bouznika et Ben Slimane"; a 1:50,000 scale map is also included. Some information is subsequently reprocessed in the work of Michard (1976) that offers a very exhaustive picture of moroccan geology on a smaller scale.

The study area is mainly located in the Casablanca-Mohammedia sheet of the geological 1:100,000 scale map realised by the Ministere de l’énergie et des mines-Direction de la géologie (1987); a very small eastern part lies in the Rommani sheet (1987). Unfortunately no information has been found about the southern part of the study area and moreover the 1:100,000 geologic map doesn’t exist.

Tectonic

The structural evolution of the region of Mohammedia-Bouznika-Ben Slimane led to the building-up of a series of tectonic structures during the Palaeozoic and the Mesozoic. Proceeding from West to East it is possible to distinguish: the anticlinorium of Casablanca, the synclinal of Mediouna-Sidi Larbi, the Oued Rhebar horst, the Berrechid-El Gara synclinal and the Oued Cherrat horst (Figure 13). The compressional structural style of this region is also characterised by the development of faults with trends, mainly NE-SW and N-S.

The beginning of the tectonic activity occurred with the Hercynian orogeny (408-248 million years, MY) that folded up and faulted the Palaeozoic substratum; the formation of the Oued Cherrat horst dates back to this period. This horst has a very narrow structure and layers with a sub-vertical position that primarily dip to East. The structure is strongly faulted with a prevailing N-S direction; along these faults, the Devonian (408-360MY) and the Carboniferous (360-286MY) are in tectonic contact.

The structure of the Casablanca Anticlinorium dates back earlier than the Hercynian orogeny probably as well as the first phase of lifting that was subsequently completed during the Hercynian orogeny of the Oued Rhebar horst. This oued is delimited by two important faults with very marked throws, up to 1,500m, along which the littoral formation of Cambrian (590-505MY) is in contact with the formations of the Ordovician (505-438MY).

The Palaeozoic basement between Oued Rhebar and Oued Cherrat horsts develops in intensely fractured isoclinal folds with a prevailing NE-SW direction but also along perpendicular directions (NW-SE) as it occurs near the relief El Hajiba, where some carboniferous terrains are located.

Another very important fault proceeds from Ben Slimane to Bouznika so linking the Ordovician-Silurian formations with the Carboniferous successions.

After the tectonic hercynian phase, at the beginning of the Mesozoic (248-65MY), the area was reduced by denudational processes to a peneplain, a land surface with a very slight relief, so as not allow the direct observation of the principal discontinuities of the Paleozoic. The Paleozoic formations are in angular discordance with the overlapped grounds of the Permo-Trias and the conglomerates that constitute the base of the Permo-Trias formation suggest that the area has suffered a long phase of continental evolution.

Two important stratigraphic gaps are recorded in the Mesozoic: one during the Jurassic (213-144MY) and another one between the Cretaceous sup. (88.5-65MY) and the beginning of the Miocene (24.6MY); these gaps further testify some long phases of emergence that have interested the whole area.

Two other important structures were originated in the Mesozoic: the Mediouna-Sidi Larbi synclinal, East of Casablanca, that overlaps to the isoclinal structure of Ordovician formations and the Berrechid-El Gara synclinal that overlaps to the successions of Ordovician, Silurian, Devonian and Carboniferous. The faults produced during this period, contrarily to the ones produced in the Paleozoic, have small throws of about some decimetres.

Since the Cenozoic up to date, the area has progressively reduced its tectonic activity. During the Pliocene (5.1-2MY) a phase of levelling and a following transgression (Moghrebien), that has interested the greater part of the study area, are recorded.

Currently the region can substantially be considered as stable and signs of activity can be deduced only locally by the phenomenon of filling from red silts in the depressed zones originated from the Mediouna-Sidi Larbi synclinal.

Figure 13 - Structural sketch map of Mohammedia-Bouznika-Ben Slimane region (modified from Destombes and Jeannette, 1966)

Stratigraphy

The wider part area of the study area (Figure 14 and Table 19) is characterised by the Paleozoic substratum that lasts from the Cambrian to the Carboniferous. On these terrains, deposits of the Permo-Trias are present and can be observed along the Chaba El Hamira, on the right shore of the Oued Nefifikh; these deposits represent the passage between the Paleozoic and the Mesozoic. The Mesozoic is represented only by two formations because of the two above-mentioned big stratigraphic gaps. Also the Cenozoic is represented only by the Pliocene formation that should be present in the whole study area but that mainly outcrops near the Oued Sefrou. The quaternary deposits are found along the principal oueds, in the depressions of the plateau, at the foots of slopes and as constituents of the thick cover on the plateau.

The outcropping terrains are described starting from the older, the hercynian substratum rocks, to the younger. In the following description, the different formations are named as per the geological map.

Paleozoic formation

- Trachyandesites and volcanic complex of the Oued Rhebar (Tk): Cambrian

They are present as a small outcropping interbedded in the schists of Middle Cambrian. In the study area this formation constitutes a small elongated area near the Oued Rhebar, in the northern part. These rocks were abundantly utilised to build the track of the Ben Slimane airport.

- Schists of Bouznika (km1): Cambrian

It is mainly composed by graywackes and siltstones rich of paradoxides and can be found only in very little areas along the Oued Rhebar on the North of Ben Slimane.

- Formation of Sidi-Khriali and Oulad-Bahloul (o3-4): Lower Ordovician

It is constituted by a succession of schists, psammites, and micaceous quartzites with a layer of iron ooliths. It outcrops in the North of Ben Slimane near the relief Ait Ali and in the lower-medium part of Oued Nefifikh.

- Formation of Feddan-Taba (o5): Upper Ordovician

The upper Ordovician is mainly composed by sandstones, coarse-grained quartzites, and microconglomerates. It differentiates from the previous formation by the greater presence of coarse elements that could be correlated to the transgression movement. In the study area it is found in association with the Lower Ordovician formation.

- Schists with graptolithic limestone intercalations (s): Silurian

It is a very heterogeneous formation in which five different lithological groups can be recognised. It is mainly clayey with graptolithic limestone intercalations. At the base of the formations some coarse sandstones are present, in light discordance with the underlying grounds of the Ordovician and marking the Silurian transgression. Small internal stratigraphic gaps are present. It mainly outcrops along the boundaries of the study area, along Oued Cherrat at the North East, but also in the South-West area of Ben Slimane.

- Schists, flysh and graptolithic limestones (di1-s): Devonian-Silurian

It is very similar to the Silurian formation, in fact it is composed by schists, flysh and graptolithic limestones. It outcrops along the northern part of Oued Cherrat and is more or less in stratigraphical continuity with the Silurian formation.

- Formation d’Ain-ad-Dekhla (di2): Devonian

- Formation Dhar-es-Smene(di3): Devonian

- Formation Mohammed-Ben-Brahim (dm1): Devonian

- Formation Ain-Kheneg-an-Nemer (dm2): Devonian

- Formation Çakhrat ach-Chleh (dm3): Devonian

The Devonian formations constitute the central part of the Oued Cherrat horst. These terrains appear in the geological map but are not described in a detailed way by Destombe and Jeannett (1966). The Formation d’Ain-ad-Dekhla is mainly formed by silt and claystones, the Formation Dhar-es-Smene by limestones with interbedded clay, the Formation Mohammed-Ben-Brahim by calcareous schists, the Formation Ain-Kheneg-an-Nemer by black bedded limestones and the Formation Çakhrat ach-Chleh by organic limestones.

- Formation of Ain Aliliga (ds-h): Carboniferous-Devonian

The passage from the Devonian to the Carboniferous corresponds to the Strunien stage and was discovered in Morocco in 1929. This formation is mainly constituted by strongly jointed quartzites, often with a fine structure and with inclusions of iron oxide. It represents an important structural element of the study area and it mainly outcrops in the Forest of Ben Slimane near the Oued Bouznika and in the neighbourhood of the town where it constitutes the isolated reliefs (skhour) of Sokhra Nemra and Sokhra El Hajiba-Sokhra bou Chouitina respectively on the North and on the South. Also some sandstones outcrop in the northern part of the Oued Cherrat, on the right shore. This formation represents a regressive facies that, after the gap of the early Carboniferous (Viséen inf.), is followed by the formation of upper Carbonifeours (Viséen sup.).

Figure 14 - Geological map of the study area (modified from Ministere de l’énergie et des mines-Direction de la géologie, 1987)

- Sandstones, limestones and schists (hv): Carboniferous

This formation was deposited during the stage Viséen sup. of the Carboniferous and it is connected by the transgression facies with which the cycle of the Paleozoic deposits finished. All Paleozoic formations constitute a 4,000m high sedimentation environment half of which could be represented only by the Carboniferous. A small stratigraphical gap, corresponding to the Viséen inf., divides this formation from the previous one of the Strunien. Banks of sandstones, limestones, schists and all the possible transitions among these deposits made up the formation. It constitutes the most representative series of the substratum of Ben Slimane forest where it is observed on the East of the quartzite of the Strunien. In particular a big outcropping is along the eastern border of Oued Cherrat. Because the sandstones are more erodible than the quartzites of the Sturien (hs), they rarely form just some isolated reliefs.

- Basal conglomerates, often ferruginous (r): Permian?

At the end of the Paleozoic a great unconformity is found, along which the post Paleozoic deposits are in contact with the Paleozoic ones, previously eroded and exposed to a peneplanation. The Paleozoic ends with a basal conglomerate, whose dating is uncertain but could be in the Permian. It is formed by deriving debris from the Paleozoic schists and quartzites cemented in a matrix rich in limonite often ferruginous that determines a reddish colour. This conglomeration is well developed (about 15m) on the South of Ben Slimane along the Chaba El Hamira.

Mesozoic formation

- Lower red siltstones (t1): Trias

This formation is constituted by arenaceous claystones characterised by small intercalations of gray sands with a very variable texture and pebbles of quartz. In the upper part, just under the basalts, layers of gypsum, anhydrite, and salt are present. This formation is well developed in the study area, near the Oued Dir and the Oued Sefrou.

- Doleritic basalts (Bt): Trias

It is made up by several vesicular and compact structured basalt flows with celadonite, a phyllosilicate belonging to the mica group, quartz, amethyst, jasper (a multicoloured quartz aggregate), and chalcedony.

From the petrographic point of view it is a labradoritic basalt with interstitial structure. It is weathered and it appears on the study area along the right side of the Oued Dir and the left side of Oued Sefrou above the red clays.

Cenozoic formations

- Calcarenites (pM): Pliocene

After the Miocene, the region was affected by a very advanced flatness; related traces are found on the summit structural surface of the plateau on the West of Oued Cherrat and on some of the reliefs surrounding Ben Slimane (El Hajiba and Nemra).

The pliocenic sea advanced along this flat area during the transgression of the Moghrebien (Villafranchian inf. stage in the Mediterranean Quaternary) of Atlantic origin. During this phase an organic calcarenite was deposited, 7-8m thick and very evident along the Oued Sefrou in the southern part of the Ben Slimane plateau.

The mio-pliocenic surfaces of levelling, previous to the marine deposition of the Moghrebien, were eroded and an Appalachian style relief was created where the carboniferous quartzites remained as prominent hills or isolated mountains (skhour); conversely the more friable schistose rocks were eroded. On such morphology the calcarenites are deposited in a discontinuous way, mainly in the low parts of the reliefs. The Ben Slimane plateau, at the feet of the skhour, corresponds to the structural surface of the Moghrebien deposit.

Neozoic formations

- Brown silts with pisolithes (q1-6lb): Pleistocene

After the Moghrebien stage, a series of marine movement (regressions and transgressions) occurred in the study area resulting in a very large variety of deposits. Brown silts with pisolithes of the Upper Villafranchian are present, generally on the top of Moghrebien calcarenites. This deposit outcrops in the central part of the Ben Slimane plateau.

- Red silts with pisolithes (q1-6lr): Pleistocene

The red silts with pisolithes represent the Soltanien stage (Wurm stage in the alpine Quaternary) and, as the previous deposits, are located on the top of Moghrebien calcarenites. They are characteristic of the northern part of the study area where, with their elongated outcropping, they testify the old phases of the coastal line.

- Black soils, tirs (A2): Holocene

These types of soils are located in the southern part of Ben Slimane plateau. They are related to the red silts and, similarly, are very often present on calcareous substrata, with various depths; sometimes they are also present on a basaltic substratum. In this last case they could constitute the alteration of basaltic materials of the Permo-Trias successions.

- Gray black soils of dayas (A1a): Holocene

These deposits are composed by grey black clays that mainly cover the bottom of the depressions (dayas). These soils evolve under the temporary action of the water that clogs the deeper horizons.

- Gray silts of oueds and lower fluvial terraces (A1b): Holocene

The grey clays constitute the grounds of the Ouljien (Neotyrrhenian, stage of the Mediterranean Quaternary). These deposits are located along the Oued Nefifikh and the Oued Cherrat.

- Slope deposits (D): Holocene

These deposits are located in all the valleys of the principal oueds where they lay over a substratum of essentially triassic dominant clay. In the study area they are along the Oued Dir, localised on the South-East of Ben Slimane. These deposits contain few rocky elements and are mainly constituted by reworked triassic clays. Also some talus cones are present, formed by block accumulations; some of them are probably of the salétien (Gunz stage in the alpine Quaternary, probably) and are well developed around some quarzites around Ben Slimane.

Development of quaternary deposits

The region of coastal meseta was affected by a particularly complex quaternary evolution in which numerous sedimentary cycles (transgression and regression) determined the sedimentation in the coastal area and conditioned the continental deposition in the more internal one. The former area can be considered as a zone of sea level variations on the continental shelf while the latter is a link between the continental reliefs and summit surfaces of the Central plateau.

According to the two previous geomorphologic features, the quaternary deposits can be described considering the coastal and the continental deposits.

Coastal deposits

After the Moghrebien transgression and during the Pleistocene (2MY-800.000Y), the area has been influenced by five sedimentary cycles whose effects are visible in the dunes and sandy deposits or are represented by abrasive surfaces with scattered marine deposits. The different transgression phases during the Pliocene-Pleistocene period are briefly described and resumed, with the corresponding probable quaternary Mediterranean stages, in Table 20.

The first transgression, the most ancient and important, is called Messaoudien and has reached a maximum height of 90-100m above the actual sea level. This sea movement, in certain cases, has flattened the Paleozoic substratum and the triassic cover. In other cases the Messaoudien transgression has covered the Moghrebien marine deposits.

Successively a new transgression, called Maarifien, reached 50-60m has eroded in some cases the formations of the substratum and in other cases the dune limestones connected to the previous transgressions. During this sea movement coarse conglomerates are deposed, instead of eroded terrains, and they are surmounted by dune gray limestones deposited during the regressive phase.

The Rabatien transgression is inserted in the deposits of the previous sea and it reached around 20-22m depositing small beach sands.

Table 20 - Relationship between moroccan and mediterranean stages

During the Ouljien transgression the sea level reached up to 7m forming a cliff in the deposits of the Maarifien transgression. Before the regression, the sea of the Ouljien suffered a new small raising that has determined the deposit of red clays on the new surfaces.

The last transgression, called Mellahien determined a 2m raising of the sea level through the deposit of grey argillaceous sands.

Continental deposits

Among the continental deposits the ones along the Oued Nefifikh and the ones in the depression of the plateau of Ben Slimane are the most important and developed.

The Oued Nefifikh flows on the West of Ben Slimane. In the southern part it forms meanders in a valley 1-2km wide and about 120m high; the valley slopes are often concave and covered by a black and slightly compact soil. The oued, flowing from Feddan-Taba, South-West of Ben Slimane, to the sea, flows deeper and deeper in gorges and narrow valleys with very steep slopes. The Nefifikh mainly flows SSE-NNW, conforming to the general inclination of the meseta. In the detail, the course is very sinuous and the various meanders correspond to the interruptions in the beach ridges that transversally cut the oued. The development of a secondary drainage pattern is also associated to the distribution of the beach ridges. In fact the various tributaries of the Nefifikh prevalently flow transversal to its valley and parallel to each other because they are developed in small valleys alternated to the dunes.

The Nefifikh has deposited a great deal of grey silt along its course that can be found in correspondence with the meanders, where the lower energy of the waters creates the conditions for their deposit. Moreover these deposits build up the alluvial terraces, at different elevations, that witness the intense action of erosion and deposition Nefifikh completed during the time.

The dayas constitute a feature that characterises the central part of the region. This Arabic term stands for the depressions diffused on the greater part of the plateau neighbouring Ben Slimane, also known as the daya plateau. According to their genesis the dayas can be distinguished in two different types: dolines-dayas and barrages-dayas.

The first ones are located on the calcarenites of the Moghrebien and can be defined as karst depressions like dolines. They have a sub-circular form with very clean edges and a flat bottom. Their dimensions increase proceeding from the northern part of the plateau to the quartzite crests in the southern part. The smallest dayas are essentially originated by dissolution processes while the greater ones were affected by the groundwater oscillation between the calcarenites and the impermeable substratum. The greatest dayas are situated at the quartzite crests feet (dayas El Habchi, El Tine, El Jmamera and ben-Haoula) and represent the last stadium of the karst evolution. In particular the daya El Jmamera has a flat bottom where the Paleozoic schists outcrop. Its North-East side is constituted by the quartzite outcropping of the Sokhra El Hajiba, while the other side is also covered by a calcareous layer about 6m thick. In the dayas located at the edges of the quartzite residual reliefs, the karst dissolution can be considered completed because the limestone deposit resulted less thick and so more erodible and because the quartzites developed a preferential drainage way, carrying great quantities of water in the depressions.

The barrages-dayas are mainly located directly in contact with the substratum terrains and where the calcarenitic layer is thin. Their shape is less regular with more blurred edges than the dolines-dayas. Frequently on their bottom a more or less thick layer of pisolithes can be identified, locally known as kercha. Many of these dayas are aligned along paleo-river or can be found in correspondence to their headings. Examples of these dayas are the daya El Kalaa, the greatest one located on the West of Ben Slimane, and the smallest ones near the right bank of the Oued Nefifikh.

The origin of barrages-dayas could date back to the period in which the Moghrebien surface was incised by small streams that eroded it with a retrogressive action. Immediately later, in the Salétien (Lower Pleistocene), morphoclimatic variations determined flow mass-movements of slope deposits that were at the base of the residual reliefs. These flows have blocked the small valleys of the streams isolating some little course and causing the formation of such dayas. After the Salétien no rejuvenation of the hydrographic network took place and only the sub-aerial erosive processes developed the previous morphology. In particular the oscillation of the groundwater and the rainfalls caused the consequent development of the pisolitic crust. An example of the barrage deposit, formed by angular quartzite pebbles in a grey silt matrix, is found in the south-east boundary of the daya El Kalaa.