Coupling Field Investigations with the Contribution of Remote Sensing and GIS for the Moroccan Middle Atlas Karst: A Case Study of the El Menzel Causse

Abstract


Introduction
Karst is a type of natural phenomenon that usually arises within carbonate rocks (limestone, dolomite) or evaporates (gypsum, anhydrite, rock salt); any terrain where change removable rocks are soluble above and below the ground by water soluble carrying distinctive characteristics of relief and drainage (Al-Hetty et al., 2021). The rate of solubility depends particularly on the local geology, hydrogeological and tectonic features. Also, climatic features are a major contributing factor (Habiballah et al., 2022). It is estimated that natural phenomena karst occupies up to 10% of the Earth's surface and that up to a quarter of the world's population is provided with water karst (Jennings, 1971).
In Morocco, Karstic outcrops cover a large part of the Moroccan territory this makes the country one of the most favorable terrains for major karstological research in North Africa. The major karst areas are divided into structural domains of varying importance: The Rif area, the Atlaso-mesetian area, and the anti-atlasic-saharian area.

Geological Framework
The Moroccan Atlas (High Atlas and the Middle Atlas) are intracontinental ranges of alpine age orogeny resulting from the inversion of the Atlasic rift of triasico-liasic age (Frizon de Lamotte et al., 2000;Domènech et al., 2015).
The Moroccan Middle Atlas is an elongated belt of highlands oriented to the NE. It divides obliquely in the Central High Atlas at the level of the Beni-Mellal region in the direction of Taza. This chain separates the Western Meseta from the Eastern Meseta. It offers a mountainous barrier that influences both the climate, hydrology, ecology, and human distribution in Morocco. The Middle Atlas chain is subdivided longitudinally into two structural domains separated by a major tectonic line called the "North Middle Atlas Accident" (Colo, 1961). These are, on the one hand, the sub-tabular Causse to the NW and on the other hand the folded Middle Atlas to the SE (Sabaoui, 1998) (Fig.2).
The Causse of El Menzel is part of the central northeastern section of the Central Tabular Middle Atlas (Martin,1981). This Causse with the Causse de Tahla which plunges gently under the Miocene formations of the southern rifain constitutes the tabular unit of the Northern Middle Atlas. It creates a carbonate dale that is approximately 15 km wide and 25 km long (Charrière et al., 2011). It corresponds to the Tazzeka massif's Jurassic carbonate sedimentary envelope (Martin, 1981).

Stratigraphy
The Causse of El Menzel constitutes a set of formations that begins with schists of Ordovician age at the level of the buttonhole of El Menzel, located at the intersection of the N 45 longitudinal accident.
The Paleozoic outcrop has an almond shape aligned along transverse faults directed N 140 (Charrière, 1990).
The Triassic formations begin with a thick homogeneous series (10 to 25m) of argillaceous red siltstones (Charrière, 1990), accompanied by evaporites and highly altered doleritic basalts. The first red series was always overcome in major unconformities on the underlying Paleozoic terrains (Charrière, 1990). It outcrops essentially at the level of valleys and canyons as well as along the faults, for example, in the case of « El-Aukba Elhamra » in the west of the city of El Menzel. Hydrogeologically, the impermeable Triassic formations play a significant role as they constitute the phreatic zone wall, which is surmounted by reservoir formations, such as the fissured and karstified Lias (Obda, 2004). Structurally, the top-Triassic clays constitute a level of the detachment of the cover (Sabaoui, 1987;Sabaoui and Villard, 1987) at the infra-and supra-basalt surfaces. Insisting that pre-Miocene compressive tectonics is responsible for these detachments, Charrière (1990) showed their extension to the whole panel of Oued Zra-El Menzel.
The Jurassic series have marine facies represented by layers of carbonate nature that cover the entire landscape of the Tabular Middle Atlas. The Lias is represented by carbonates and lies in concordance with the red Triassic formations. Several facies can be distinguished in the Jurassic outcrops, the main ones being: massive dolomites, sandy dolomites, and brecciated dolomites of the Lower Lias age. The Middle Lias are always well-stratified, which makes it possible to recognize it from afar, the limestones have a lighter color, and the rocks have a smooth surface, more uniform than the dolomites, thus one can notice the presence of some whitish marly beds within the limestones (Martin, 1981).

Structural Framework
The El Menzel unit forms a carbonate slab about 15 km wide by 25 km long (Charrière et al., 2011). The detachment of this unit from the Triassic is visible at its SE edge where, at the level of the Middle-Atlasic North Fault, other thrusts appear, directed towards the south on the Zloul syncline. In fact, the Causse of El Menzel is entirely detached from its Paleozoic substratum, as well as the different tabular panels included between the "Middle-Atlasic lineament" and the North Middle-Atlasic lineament (Charrière et al, 2011). In this sector, there is a fault system directed N45. It follows from the locality of El Menzel, where it is intersected by an important reverse fault, to the E edge of the buttonhole of Bsabis.
Among this cluster of faults, we have the reverse and overlapping lineament of Ben Yazraa which constitutes the main branch. It was first defined by Charrière (1984) and was later included in the A.M.M.A. network "L'accident Median Moyen-Atlasique" by Sabaoui and Viallard (1987).
The various carbonate blocks in this area have not remained stable, but have instead been detached and shortened by a system of faults in benches and ramps. These overlaps imply Paleozoic schists scales, suggesting a tightening at the socle level (Charrière et al, 2011). The analysis of the drillings carried out in the block of Oued Zra-El Menzel confirmed the existence of several surfaces of detachment as well as unidentifiable overlaps on surfaces (Sabaoui, 1998). The pre-Miocene tectonics is responsible for these detachments. Charrière in 1990 showed their extension to the whole block of Oued Zra-El Menzel.

Bio-Climatic Frame
Morocco has a Mediterranean climate that is predominantly arid. It is distinguished by two seasons of roughly equal duration. The first is humid and somewhat cold, extending from November to April and the second is dry and warm, ranging from May to October (Ait Houssa et al., 2017). However, in mountain karst areas, orography favors rainfall on the peaks, and the average annual precipitation is between 800 and 1000 mm (Akdim, 2015). Precipitation decreases from north to south with exposure, and vice versa for temperatures (Sebbar et al., 2020). A general tendency for the Middle Atlas to dry out and its large internal water disparities have been confirmed by the results of recent hydro-climatic studies (Akdim andSary, 2001, Nejjari, 2004;Obda, 2004;AHBS -EC 2007).
The Mediterranean climate favors a Mediterranean basin that is the third richest hotspot on the planet in terms of endemic plants and vegetal biodiversity (Mittermeier et al., 2004). The Middle Atlas Forest resources in Morocco are characterized by their unique biodiversity and dynamics. They demonstrate a fragile natural heritage with astounding potential for forests and biodiversity whose preservation is sometimes challenged by increasing anthropogenic pressure (Labhar et al., 2007). The Amazighian cedar (Tetraclinis articulata) is the dominant plant, accompanied with holm oak, carob tree, filaria, and Juniperus Phoenicia growing along its edges (Martin, 1981).

Materials and Methods
The structural framework of the Causse of El Menzel represents a key point in our attempt to explain the tectogenetic origin of the exokarstic and endokarstic forms observed in the field. To meet this objective, our methodology consists of combining fieldwork and remote sensing to correlate, establish and validate the relationship between tectonic structures and the development of karsts in the area.
The fieldwork consists of describing the karstic geomorphology of the Causse of El Menzel in the light of its structural framework in different areas: Qaçbat Beni Yazgha, Kasbat Sebou, Tamzirt, El Menzel, Boudarham, Taghit. Remote sensing techniques allow us to accurately locate major and insurface visible geological structures, such as faults, fractures, and abnormal contacts at the Causse of El Menzel. The remote sensing approach is used for the following purposes: • Establishing a digital fracture mapping that covers the Causse of El Menzel; • Show the tectogenetic origin of karsts by the projection of geomorphic field data on digital lineament maps. We choose a Sentinel-2 image as input, taking advantage of its high spatial resolution for the visible bands (10m). This Sentinel-2 image provides three times the detail of a Landsat image, making it ideal for lineament extraction over a large area. Our image dates from 08/31/2021 and it's identified by its tile ID: T30SUC; The entire image measures 100 x 100 km and covers the entire study area. The input image is cloud-free with no noise recorded on all bands, which makes it perfect for our study. The vegetation cover is evident on the input original image. In order to highlight the vegetation, we display our image with an RGB 832 false-color composite, this is essential to identify and eliminate lineaments caused by vegetation (agricultural land). To choose a single band on which we will extract the lineaments, we proceeded with Principal Component Analysis (PCA) on the 10 m bands (Bands 2, 3, 4, and 8), as it is the best spatial resolution freely available. In this resolution, plurimetric lineaments can be highlighted. However, smaller scale structures such are micro faults and micro-fractures cannot be detected, hence the importance of fieldwork. PCA reduces the redundancy present in the image by eliminating bands correlations; the result of this processing is an image whose new bands (called "principal components -CP") are classified in descending order concerning their information content. The first component CP1 contains the maximum amount of information, while the CP4 contains the least information.CP1 is therefore chosen to be used in the following treatment. Two methods are proposed for the lineament's extraction.
• The first consists of the application of 4 directional filters on the CP1 image: N00, N45, N90, and N135, then applying the "LINE" algorithm of PCI Geomatica for the automatic extraction of lineaments; • The second method applies the LINE algorithm directly to the CP1 image without going through the directional filters. The results of these two treatments are two files containing the extracted lineaments in Shapefile format, ready to be integrated into a geographic information system (GIS). The last step consists in filtering the results obtained; the LINE algorithm highlights all the linear structures present on the image without considering their nature, hence the need to eliminate undesirable structures, in particular the lineaments resulting from roads, tracks, or boundaries of agricultural land. The complete process of lineament extraction from the satellite image is illustrated in Fig. 4.

The Karstic Landscapes of the Causse of El Menzel: Field Contribution
The Causse of El Menzel is characterized by a slightly accentuated relief with wooded carbonate karstic plateaus which rarely exceed 1000 m asl, affected by fairly intense fracturing. It is dissected by a large river network like that of Oued Sebou which crosses the Causse in a spectacular 250-300 m deep canyon. This is entrenched in the lower Lias and the Triassic; its layout is extremely sinuous, forming large steep-sided meanders (Martin, 1981).
The Causse plateau represents an area with an almost flat topography, with various surface and underground karstic phenomena. Several factors have influenced the morphology, including inherited tectonic deformations, Atlas uplift, fluvial dynamics, eustatic fluctuations, and bio-climatic conditions. The result is a complex geomorphology of the fragmented Causse, dissected by tectonic features, gorges of fluvio-karstic origin, and a multitude of exokarstic and endokarstic forms of structural origin. The karst features include clints, grikes, kamenitzas, ruiniformes landforms, rillenkarrens, dry valleys, crypto-karsts, uvalas, caves, and karst spring.

The epikarst of the Causse of El Menzel • Clints and Grikes
This type of Karren represents limestone blocks called «clints» separated by cracks called «grikes», particularly well expressed near tectonic lines found in the field such as diaclases, joints, and the faults of directions N 90, N 60, and N 130 in the sector of Qaçbat Beni Yazgha (Fig.5-a). The clints and grikes observed in the field result from the widening of faults and fractures that guide the meteoric water, shaping carbonate slabs into «limestone pavement». These karst expressions characterize areas marked by active tectogenesis or a strong omnipresence of fracturing. They are often observed in tectonic crushing zones or fault escarpments where the faults branch out in a way anastomosed into breaks more or less parallel to the main direction, NE-SW. Grikes tend to be longer, wider, and deeper near escarpment edges (Ford and Williams, 2007). In the field, grikes are the main conduits of surface waters to dry valleys and deep to karst aquifers

• Small solution basins (Kamenitzas)
Kamenitzas represent small depressions or pools a few centimeters wide and deep, that develop on gently sloping bare limestone where trapped organic matter accumulates. These types of karren or lapies are generated by bio-chemical processes such as the action of bacterial communities and associated CO2 production and dissolution ( Fig.5-b).

• Rillenkarren
This type of karren is the direct result of the action of rain on steeply dipping carbonate masses. The result will be vertical flutes called rillenkarrens. NE of the village of El Menzel, Rillenkarren were observed on particular geomorphological sites characterized by a steep slope of naked limestone, most often on the upper edges of dry valleys where the rainwater can run out by fast flow, forming grooves or rills which drain sometimes the water towards dolines or dry valleys.

• Ruiniform reliefs
The ruiniform reliefs represent an abundant morphological aspect of the Causse of El Menzel which attracts the eye. It is a field of Jurassic carbonate blocks that takes the appearance of ruins, observed on several areas such as El Menzel, Quaçbat Ben Yazgha, and Taghit. This type of morphology results from the action of erosion on the heterogeneously fractured liassic carbonate outcrops. The fractures, joints, and faults in the limestone or dolomites encourage the action of meteoric agents to form reliefs that take irregular and ruiniform aspects (Fig.5-d-e). Most of these ruiniform landscapes are preconditioned by large cracks widened into giant lapies (Martin, 1981). These appear systematically on the steepest slopes, on slope breaks, or on the axis of very wide dry valleys (Martin, 1981).

• Dry valleys
Dry valleys carrying intermittent flow are very common forms of the Middle-Atlasic Causses (Martin, 1981), particularly the Causse of El Menzel. These valleys are not arranged randomly; they are closely related to the dolomitic and limestone lithology. These valleys are permeable in large, because of joints, cracks, crushing, or even occasional porosity (Martin, 1981). They are organized in more or less rectilinear dendritic geometric forms with a sometimes-vertical hollowing which can reach 200 m depth in the area of Qaçbat Beni Yazgha (Fig. 5-c).
Several valleys of the Causse of El Menzel follow the NE-SW and NW-SE tectonic lineaments, this implies the tectogenetic nature of these karst forms that can be confirmed by a simple observation in the field which testifies a tecto-morphogenetic evolution of these karst forms: • A landscape in disharmony with ruinous and chaotic rocks that testify an activity neotectonics and a replay of faults (Fig.5-e); • Crushing zones accompanied by significant cracking in the dolomites of the Lower Lias; these zones of weakness appear where the valleys are incised by exceptional flow following a long period of rain (Fig.5-e); • A stony valley floor due to the detachment of carbonate rocks along the slopes of the valleys following an accentuation of the gravity processes (Fig.5-d and 6-b). The faults-oriented NE-SW and NW-SE have played an important geomorphic role during the Atlasic uplift in the Causse of El Menzel by a negative inversion of tectonic structures (Hinaje et al., 2015). This occurred on one hand because of Pleistocene desiccation and the individualization of carbonate blocks (Martin, 1981), and on the other hand, it facilitated the formation of a large network of valleys observed today.

• Karst springs
From a historical point of view, the distribution of the tribes of Ben Yazra depends on the presence of water. Several villages such as Qaçbat Beni Yazgha, Oulad Sahnoun, Oulad Berrada, Sraghna, Alga 'da... in the sector of El Menzel were built near several karst springs such as Ain Kbir, Ain Batout, Ain Charij, Ain Sahnoun, Ain Berrada, Ain Sraghna, Ain Alga' da, Ain Mellah. Generally, the karstic springs in the Causse of El Menzel emerge at the surface in low topographic areas such as dry valleys. According to the field observations conducted in Qaçbat Beni Yazgha, Mternagha, Sraghna, and El Menzel, we can notice the presence of several morphological features that can lead us to deduce the tectogenetic nature of these karst springs which can be defined as "Fault Controlled Spring" (Fig.6-d). The occur in crushed faulted zones marked by an intense cracking of the liasic carbonate series with brecciated dolomite facies, such as Ain Kbir of Qaçbat Beni Yazgha " x: 33.831366; y: -4.593211; z: 781 m ".

• Speleogenesis, or Cave Formation in the Causse of El Menzel
The term cave implies a natural underground void or opening in soluble rocks like carbonates where a man can move through this underground network, from a genetic perspective a cave is an opening enlarged by dissolution to a diameter sufficient for human entry (Ford and Williams, 2007). Caves are important archives of paleoclimate and past tectonic activity (Stock, 2013).
In Morocco, several cities like Sefrou, Taza, and Casablanca ... were built near caves that functioned as the primitive habitat of man. The caves have great scientific, historical, and cultural value. The Causse of El Menzel caves represent a characteristic fundamental element of the area; it is a land of caves. Several caves are still used today as homes and temporary shelters for some villagers of Qaçbat Beni Yazgha, Kasbat Sebou, Mellaha. From a genetic standpoint, the El Menzel caves are essentially tectonokarstic in nature as they are located in fault escarpments ( Fig. 6-a-b-c). Fracturing has good karstification potential in the area; The presence of tectonic structures is considered essential for cavity development (Glazek, 1989). The NE-SW and NW-SE tectonic lineaments guide the action of external and internal processes for millions of years in the Causse. The result is widening by the dissolution of the soluble elements of the fractured carbonate rock where a network of valleys and cavities is established. This is why on the field the caves are particularly well expressed at the level of the dry valleys and in the escarpments of the faults.
The caves located in the Qaçbat Beni Yazgha area and in the Oued Sebou valley, appear at several levels ( Fig. 6-c). These show a polyphase evolution with a reorganization of the drainage system linked to a subsidence of the base level, driven by the energy of the Atlasic uplift. Each phase of base-level lowering correlates to fluvial incision of the liasic series corresponding to old cave levels. Climatic conditions are also important in the genesis of caves; in many watersheds, river incision occurs mainly during climatic transitions from cold to hot or from hot to cold, in accordance with regional or global climatic fluctuations (Antoine, 1997).

The lineament maps of the El Menzel Causse
Several digital lineament maps were elaborated on the El Menzel Causse by remote sensing at regional and local ( Fig.7-8-9) scales. Fracturing and many tectonic structures that generally correspond to faults have been identified from satellite images (PC1, DF); these are not observed in the large-scale geological maps that cover the areas of El Menzel and Sefrou.The rose diagrams that correspond to the "PC1, DF, and GeoMap" clearly show that the main fault families have a NE-SW directional distribution, this implies a good similarity between the faults measured in the field and the lineaments that refer to the faults as well as the general deformation trend in the Middle Atlas.
We projected karst features such as dry valleys, caves, and karst springs on digital maps of the area, in order to establish and deduce the genetic relationship between the structural alignment and the fluviokarst morphology.

Genetic Relationship between Fluvio-Karst and Structural Alignment Observed in the Field and Remote Sensing
Clarifying the relationships between regional tectonics, local geologic structures, and speleogenesis is important for predicting the development of unconventional karst systems. We define unconventional karst as the case where speleogenesis is strongly controlled by geologic context rather than chemical dissolution of carbonate. Examples of unconventional karst systems develop at the contact between insoluble rocks and carbonates and/or in the presence of major tectonic discontinuities (Antonellini et al.,2019).
The El Menzel Causse offers a good example of an unconventional karst system as it is strongly influenced by major tectonic faults of the Causse; this NE-SW oriented tectonic heritage plays a decisive role in the emplacement of a large network of generally dry, flat-bottomed, linear valleys with a general orientation (NE -SW) that deeply incise the Liassic carbonate series and converge towards the major river system of Oued Sebou (Fig. 10-11). The latter crosses the Causse in a very spectacular canyon 250 to 300 m deep, entrenched essentially in the Lower Lias and Triassic; its course is extremely sinuous and describes large incised meanders (Martin, 1981).
The analysis of the fractures extracted by remote sensing (Fig.7-a, 8-a, 9-a) confirms the tectogenetic origin of the valleys, karstic springs, and caves of the Causse. The NE-SW and NW-SE trending tectonic lineaments inherited from the Hercynian orogeny control the spatial distribution of karst forms by playing an inhibiting role during the different karst phases in the Middle Atlas: on the one hand, by attenuating the resistance of massive carbonate rocks to chemical attack by meteoric agents and on the other hand, its tectonic structures will trigger and guide the action of meteoric waters on the surface by the fluvial dynamics of valleys and canyons, and at depth in cavities and subterranean rivers.
In summary, similar to the river system, the caves and karst springs are located in the same preferential direction as the NE-SW structural system.

Factors that Influence the Karst Processes in the El Menzel Causse
The Causse of El Menzel was the object of several karstic phases which are demonstrated in the field by several karstic levels in the liasic series of the Causse, observed in the valley of Oued Sebou. Our results confirm that the karstification in the El Menzel Causse is of tectogenetic nature. On the other hand, the concept of karstification should never be confined to a single agent because karst phenomena are complex, involving the interaction of several factors both external and internal that contribute to the potential of karstification, for example: • The changes in base level; each decrease in base level of either tectonic or climatic origin implies a fluvial downcutting in the valley of Oued Sebou which would be correlative to karst stages. Consequently, some karst features store paleohydrologic levels (Johnson et al., 1977;Kober et al., 2007).

•
Climatic conditions also govern karst development by controlling the hydrological system and fluid flows (Bruthans et al., 2017); During each pluvial period, karst processes can develop in the Causse thanks to the presence of water. This meteoric water is an essential primary agent of karstification combined with CO2 and fractured carbonate rocks. Water intervenes as an agent of erosion, either directly as a mechanical agent, or indirectly as a solvent combining with carbon dioxide. These two components control the rate of karstification, as well as its intensity (Delannoy, 1997).

Conclusions
Little information is available on the origin of karst phenomena of the Causse of El Menzel. This work aims to decipher the factors involved during the karst processes. According to the field study and based on satellite images collected by different methods (PC, DF, and Geologic map), it appears clear that the El Menzel Causse is a particularly tectonized karstic area, as observed by significant fracturing. This fracturing, generally oriented NE-SW play a decisive role in the emplacement of the large fluvial network that dissects the Causse into different carbonate blocks. This fluvial network of canyons or valleys offers a good place for the observation of various karstic forms observed in the El Menzel Causse. This emplacement of these karst features forms a coherent network that is not arranged randomly. They are in good agreement with both the fluvial and the structural network. This superposition implies a tectogenetic nature of the fluvio-karst expressions along the NE-SW trending regional faults inherited from the Hercynian history.
To make sense of our results we believe that the polyphase uplift of the Middle Atlas associated with a lowering of the base level provided the potential energy necessary for karstification during the fluvial pleo-quaternary periods. The result will be a fluvial incision by corrosion along the faulted zones correlative to the multiple karstic phases observed in the Jurassic series of the El Menzel Causse.