Essaouira Basin Research Articles

Recharge Conceptual Model and Mineralization of Groundwater in a Semi-arid Region; Essaouira Basin (Morocco)

The geological setting of PQ and T aquifers system of Wadi Ouazi basin plays an important role in the geochemical and hydrodynamic characteristics of the groundwater systems, the most important source of water supply in Essaouira basin. The combination of hydrogeochemical and isotopic methodology applied in this investigation allowed a better understanding of the conceptual model of the aquifer response under present climatic regime. The chemical data indicate that dissolution of evaporate minerals (mainly halite and anhydrite), cation exchange process and sea spray, as the main processes controlling groundwater mineralization. The isotopic data show that the recharge is assured by precipitation of oceanic origin without notable evaporation; stable isotopes content allowed the identification of different recharge areas at different altitudes between the two aquifer systems.

Geochemical and Isotopic Tools to Deciphering the Origin of Mineralization of the Coastal Aquifer of Essaouira Basin, Morocco

The determination of the origin of salinity and mineralization of groundwater from the Plio-Quaternary aquifer of Essaouira Basin was the main goal of this research, investigated by the combined study of chemical (particularly Br and Cl) and isotopic (18O and 2H) compositions. The study of Br / Cl ratio vs Cl showed that mineralized waters of the shallow aquifer in the Essaouira Basin is the result of: (i) the marine influence (precipitation and aerosols directly from the ocean), (ii) dissolution of evaporite minerals (halite), (iii) seawater intrusion and iv) ion exchange mechanisms. The 18O-2H diagram showed that the groundwater recharge is maintained by direct precipitation without evaporation and runoff. The Cl/18O ratio showed the existence of a mixture trend of seawater-groundwater.

Geochemical and isotopic investigation of the aquifer system under semi-arid climate: case of Essaouira basin (Southwestern Morocco)

Detailed hydrochemical and stable isotopic data of groundwater were used to understand the geochemical processes occurring in the Wadi Ouazzi basin in Southwestern Morocco. Hydrogeochemical investigations show that the evaporite dissolution (halite and gypsum), followed by cation-exchange reactions with reservoir matrix and by sea spray, constitute the main processes that control groundwater mineralization. Data inferred from 18-oxygen and deuterium isotopes in groundwater samples indicated recharge with modern rainfall. Water characterized by lower 18O and 2H values is interpreted as recharged by non-evaporated rainfall originating from Atlantic Ocean. Tritium contents, ranging between 0.18 and 3.43 TU, indicate that groundwater in both aquifers derive from post- and pre-nuclear recharge. The study of the carbon isotopes made it possible to estimate the water residence time of the two aquifers studied. Moreover, the calculated ages range from the modern to the late Pleistocene with the dominance of the modern age waters.

Neotectonic fault detection and lithosphere structure beneath SW of High Atlas (Morocco)

The High Atlas is a 100 km wide zone defined by E-W to NE-SW trending folds nearly orthogonal to the Atlantic coastline. The major compressional structures in the High Atlas consist of large-scale fold systems which affect Mesozoic and Cainozoic formations. The extreme West of the High Atlas including the region of Agadir is defined as an earthquake Zone. Historical seismicity data shows that the Agadir region was hit by two destructive earthquakes in 1731 and 1960 with magnitude 6.4 and 6.0, respectively. The present study has two main goals: 1) to use remote sensing techniques to detect and map the surface geological structures including faults; 2) to use the local earthquake tomography for imaging the lithosphere (subsurface) and detect deep structures. For the remote sensing techniques we used ETM + Landsat7 images and the SRTM 90 m image as a Digital Terrane Elevation Model. This study focuses on the computerized identification, feature extraction and quantitative interpretation of lineaments over the SW High Atlas. The analysis developed here is based on the numerical enhancement of a Landsat image and on the statistical processing of data generated through enhancement. The results generated by the numerical enhancement and statistical analysis are presented on fault maps, lineament maps, polar diagrams and lineament density maps. The lineaments have a high concentration of orientations around the directions N40E, N80W and N-S. For the subsurface study, seismic data sets were used to define the 3-D velocity structures. We also used local earthquake tomography to obtain the velocity map and crustal structure of the SW High Atlas region. The tomography results show a new and detailed lithosphere structure defined by a high velocity body in the northern of SW High Atlas from 15 to 45 Km depth, dipping to the north beneath the Essaouira basin in the western Meseta with P velocity variations from 6.5 to 7.8 km/s. This anomaly can be interpreted as an old subduction zone marking the limit between SW of High Atlas and the western Meseta. A second high velocity body, dipping north beneath the Hercynian Tichka Massif is detected. This positive velocity anomaly can be interpreted as an old subduction zone marking the limit between Meseta Domain and West African Craton. The occurrence of tholeiitic and alkaline magmatism respectively in the Essaouira basin zone and in the Hercynian Ticka Massif zone leads us to conclude that remains of subduction exist in these zones. The negative lithospheric anomalies are interpreted as heat or hot asthenospheric material upwelling from depth crossing the main crustal structures in SW High Atlas. The negative lithospheric anomalies are interpreted as hot asthenospheric material or heat coming from depth to replace the detached crust. Based on our analysis of local P wave velocities and profiles, the crustal thickness beneath SW of High Atlas region varies from 30 km to 45 km in central and western part.

Climate Change Impact on Aquifer System of Essaouira Basin during the Past 4 Decades (Morocco)

Since the 50s, many changes are observed in the climate of the globe, the warming of the atmosphere and the ocean, the decrease in the extent of snow and ice, and rising sea levels. In addition to the demographic pressure, the Maghreb countries are suffering from the drought, which accentuates the phenomenon of desertification and soil erosion, as well as their salinity, which leads to the salinization and depletion of water resources. In Morocco, the climate change leads to the erosion of the soil under the effect of desertification resulting in siltation of reservoirs and the increased salinization of soil and water resources. The Essaouira basin, which is the subject of this study, is located at the Western extremity of the High Atlas to the Southeast of the Essaouira city; between 31°20' to 31°30'N and between 9°00' to 9°46'W. The basin is characterized by a semi-arid climate with average annual rainfall of around 300 mm·year-1 and an average temperature of 20°C. Considering the importance of the aquifer in Essaouira in the supply of the region's groundwater, a study was conducted to assess quantitatively and qualitatively the impact of climate change on water resources in this area. The aquifers present in this region are one of cenomano-turonian upstream and plio-quaternary downstream, separated by the diaper of Tidzi. A piezometric map of the basin was made (1990-2009) and the hydrochemical method of the waters shows that they present a facies Na-Cl with an electrical conductivity ranging from 700 to 7000 μs/cm. The isotopic tracers like 18O, 2H, 3H and 14C were analyzed to determine the age of groundwater, and a right local meteoric close to the right meteoric world characterizing the precipitation of oceanic origin has been determined. The Essaouira basin turns out to be very vulnerable and sensitive to climate change because its recharge is entirely dependent on the rainwater. These results should be taken in consideration for the future water management in the region.

Lithospheric structure in NW of Africa: Case of the Moroccan Atlas Mountains

This study presents the outcomes of the local earthquake tomography applied in the Moroccan Atlas domains. A seismic data collected by 36 seismic and a linearized inversion technics are used for determination of local velocity structure.The interpretation of tomography images results emphasizes a new and detailed lithosphere structure: a remaining subducted zone beneath the Souss Basin located from 20- to 45-km depth dipping to the North is detected and interpreted as a body that marks the border between the Moroccan Anti-Atlas and the Meseta-Atlas domains.A subduction zones is detected in the SW of the High Atlas, beneath the Hercynian Tichka massif from 10 to 50-km inclined away from Anti Atlas and in the eastern part of Anti Atlas, dipping northward from Jbel Ougnat at 15–40 km.The junction of the western and middle High Atlas is depicted by two high velocity blocks subducting from 10 to 50 km depth. The first is dipping SW beneath the High Atlas and the second is dipping SE beneath the Ouarzazate Basin.In the northern part of the southwestern High Atlas, a high velocity body dipping towards the north beneath the Essaouira Basin from 15 to 45 km depth.In northeastern part of the High Atlas in the Mougeur zone, a high velocity body is detected from 10 to 45 km depth, dipping to the S–E beneath the eastern High Atlas.The negative lithospheric anomalies found in the upper and in the lower crust are interpreted as a hot asthenospheric material upwelling from deep and gradually replacing the part of crust detached in the High Atlas. The occurrence magmatic activities in these regions testify the existence of a remaining subduction process. This paper argues the implication of these deep structures in the evolution of the Moroccan Atlas Mountain.

Using geochemical investigations for determining the interaction between groundwater and saline water in arid areas: case of the Wadi Ouazzi basin (Morocco)

Abstract. The characteristics of the Essaouira basin water resources are a semi-arid climate, which is severely impacted by the climate (quantity and quality). Considering the importance of the Essaouira aquifer in the groundwater supply of the region, a study was conducted in order to understand groundwater evolution in this aquifer. The Essaouira aquifer is a coastal aquifer located on the Atlantic coastline of southern Morocco, corresponding to a sedimentary basin with an area of nearly 200 km2. The control of the fluid exchange and the influence of mixing zones between the groundwater and saline water was investigated by sampling from 20 wells, drillings and sources belonging to the Plio-Quaternary and Turonian aquifers. It is hypothesized that groundwater major ions chemistry can be employed to determine the interaction between the groundwater and saline water (coastal aquifers). Groundwater samples examined for electric conductivity and temperature showed that waters belonging to the Plio-Quaternary and Turonian aquifers present very variable electric conductivities, from 900 μs/cm to 3880 μs/cm. Despite this variability, they are from the same family and are characterized by sodium-chloride facies. However, a good correlation exists between the electrical conductivity and chloride and sodium contents. The lower electrical conductivities are situated in the North quarter immediately to the south of the Wadi Ouazzi.

Management and protection of groundwater resources in semi-arid zones: application of hydrochemical methodologies to Essaouira Synclinal Basin, Morocco

The water resources of the Essaouira basin are characteristic of a semi-arid climate. It is a coastal aquifer located on the Atlantic coastline, corresponding to a sedimentary basin with an area of about 200 km2. The geological structures delineate a syncline bordered by the Tidzi diapir of Triassic age which outcrops to the E and S. In the Essaouira basin, a multi-aquifer composed of detrital deposits of the Plioquaternary and Turonian dolomitic limestones was identified. The main flow direction is from SE to NW towards the Atlantic Ocean, the recharge area being located near the Tidzi diapir. In the Essouaira basin, groundwater is of Na-Cl-type. The chemical signature of these waters should be the result of the preferential recharge area that is located in the Tidzi diapir. It was possible to notice that there exists an important contribution of water-rock interaction in groundwater mineralisation, corroborating the influence of Tidzi diaper.

Groundwater Chemical Evolution in the Essaouira Aquifer Basin—NW Morocco

The sustainability of groundwater resources for drinking water supplies, agriculture, and industry a prime concern in countries dominated by arid and semi-arid climates such as Morocco. The growing demand for groundwater coupled with impacts from land use and climate change make sustainability an even more important water management goal. In order to make sound decisions about water use and protection of water quality, managers and policy makers must have a sound understanding of such factors as the location and amount of groundwater recharge and groundwater ages. Due to the population growth and climate change (causing long periods of drought) in the world, many countries have intensively increased their use of water sources for supplying potable water to population and for their agricultural (irrigation) and industrial developments. Due to the lack of surface waters, people exploit mainly underground water reservoirs. So, it is necessary to study and characterize these water reservoirs to avoid any excess of exploitation. The water resources of the Essaouira basin are characteristic of a semi-arid climate, and are severely impacted by the climate (quantity and quality). Considering the importance of the Essaouira aquifer in the groundwater supply of the region, a study was conducted in order to comprehend this aquifer groundwater evolution. It is an aquifer located on the Atlantic coastline, southern (Morocco), corresponding to a sedimentary basin with an area of near200 km2. Covering the Palaeozoic bedrock, the sedimentary series range from the Triassic to the Quaternary. The geological structures delineate a syncline bordered by the Tidzi diapir of Triassic age which outcrops to the East and South. In the Essaouira basin a multi-aquifer was identified constituted by detrital deposits of the Plioquaternary and dolomitic limestones of the Turonian. The Plioquaternary is unconfined below the Senonian marls. However, in some places it can be in direct contact with the other Cretaceous and Triassic units. The Plioquaternary is generally up to60 mthick. The Turonian is confined by the Senonian marls and in direct contact with the Plioquaternary on the edges of the syncline structure. The main flow direction is from SE to NW towards theAtlantic Ocean, being the recharge area located near the Tidzi diapir. In the Essouaira basin, in spite of the occurrence of calcareous and dolomitic levels, all waters are of Na-Cl-type. The chemical signature of these waters should be the result of the preferential recharge area that is located in the Tidzi diapir. Using a simple mass balance model through the PHREEQC program this scenario was tested. The reaction path was assumed to be such that waters observed at shallow depths evolved to more mineralized waters. It was possible to notice that these waters have an important contribution of water-rock interaction in groundwater mineralization, corroborating the influence of the preferential recharge area located in the Tidzi diaper in the waters signature.

Post-rift vertical movements and horizontal deformations in the eastern margin of the Central Atlantic: Middle Jurassic to Early Cretaceous evolution of Morocco

Wide regions of Morocco, from the Meseta to the High Atlas, have experienced km-scale upward vertical movements during Middle Jurassic to Early Cretaceous times following the appearance of oceanic crust in the Central Atlantic. The area experiencing exhumation was flanked to the W by a domain of continuous subsidence, part of which is named the Essaouira-Agadir basin. Comparison with vertical movement curves predicted by lithospheric thinning models shows that only 50–60 % of the subsidence documented in the Essaouira basin can be explained by post-rift thermal relaxation and that <30–40 % of the observed exhumation can be explained by processes (in)directly related to the evolution of the Central Atlantic rifted margin. Syn-sedimentary structures in Middle Jurassic to Lower Cretaceous formations of the Eassouira-Agadir basin are common and range from m-scale folds and thrusts to km-scale sedimentary wedges. These structures systematically document coeval shortening generally oriented at high angle to the present margin. As a working hypothesis, it is suggested that regional shortening can explain the structural observations and the enigmatic vertical movements.

Hydro-geochemical behaviour of two coastal aquifers under severe climatic and human constraints: comparative study between Essaouira basin in Morocco and Jeffara basin in Tunisia

Since surface waters are mostly irregular and rare, groundwater in arid and semi-arid regions are submitted to high human constraints enhanced by severe climatic conditions. Multiple isotope analyses and chemical tracing of groundwater from the Essaouira basin (South-Western Morocco) and from the Jeffara basin (South-Eastern Tunisia) reveal that salinisation processes are mainly related to natural conditions such as dissolution of evaporate rocks (gypsum and halite minerals) along the recharging outcrops. However, since these basins are coastal, seawater intrusion is one of multiple salinity sources that could affect the quality of groundwater if intense exploitation goes on. The comparison of hydro-geochemical and isotopic data available for the two basins provide a framework for a comprehensive diagnostic in which different states of risks could be defined according to climatic and human constraints. We argued that these heavily exploited coastal aquifers have exacerbated vulnerability given relatively low current recharge and salinisation processes.

Contributions of climate change on water resources in semi-arid areas; example of the Essaouira Basin (Morocco)

The ground waters in semi-arid regions are subject to the impact of climatic and anthropogenic pressures. In the case of coastal aquifers, the vulnerability that arises is compounded by the risk of seawater intrusion. The joint application of several methodologies hydrodynamic and isotopic to the aquifer's coastal Essaouira basin in semi-arid region of Morocco has established a diagnosis of the current state of the aquifer and the risks it is submitted. This area includes two main superimposed aquifers. The Plio-Quaternary, which provides most of the water supply, is a reservoir is mostly sand, sandstone and conglomerates. Turonian calcareous dolomite is seeking recently by deep wells to supply drinking water to the city of Essaouira. The study area to form a syncline at heart Senonian is between two structures has diapiric Triassic material: the diapir Tidzi outcropping east and south and the cache of Essaouira diapir WEST, who was detected by the geophysics. In the Essaouira Basin, water resources are less important and are seriously affected by the climate in terms of quantity and quality, rainfall in the area does not exceed 300 mm year -1 on average, the average temperature hovers around 20 ° C, potential evapotranspiration is very high, about 920 mm year -1. The main direction of groundwater flow is from southeast to northwest toward the Atlantic Ocean. Essaouira basin is most vulnerable to climate drought since it is recharge is entirely dependent on rainwater. This finding should be incorporated into a management strategy directed towards safeguarding resources.

Characterization of the Cretaceous aquifer structure of the Meskala region of the Essaouira Basin, Morocco

The aquifer of early Cretaceous age in the Meskala region of the Essaouira Basin is defined by interpretation of geological drilling data of oil and hydrogeological wells, field measurement and analysis of in situ fracture orientations, and the application of a morphostructural method to identify lineaments. These analyzes are used to develop a stratigraphic–structural model of the aquifer delimited by fault zones of two principal orientations: NNE and WNW. These fault zones define fault blocks that range in area from 4 to 150 km 2. These blocks correspond either to elevated zones (horsts) or depressed zones (grabens). This structural setting with faults blocks of Meskala region is in accordance with the structure of the whole Essaouira Basin. Fault zones disrupt the continuity of the aquifer throughout the study area, create recharge and discharge zones, and create dip to the units from approximately 10° to near vertical in various orientations. Fracture measurements and morphometric-lineament analyzes help to identify unmapped faults, and represent features important to groundwater hydraulics and water quality within fault blocks. The above geologic features will enable a better understanding of the behaviour and hydro-geo-chemical and hydrodynamics of groundwater in the Meskala aquifer.

Mass balance simulation and principal components analysis applied to groundwater resources: Essaouira basin (Morocco)

The water resources of the Essaouira coastal basin (west of Marrakesh) are characteristic of a semi-arid climate and are severely impacted by the climate in terms of quantity and quality. Considering the importance of the Essaouira aquifer in the groundwater supply of a vast region (nearly 1,200 km2), a research study was conducted in order to better understand groundwater evolution in this aquifer system. It is a coastal aquifer located on the Atlantic coastline, southern Morocco, and salinization problems have been reported. Covering the Palaeozoic bedrock, the sedimentary series range from the Triassic to the Quaternary. Besides the possibility of seawater intrusion problems, the geological structures delineate a syncline bordered by the Tidzi diapir (of Triassic age) outcropping to the east and south. This is a recharge area for the aquifer, whereas the main groundwater flow direction is from SE to NW towards the Atlantic Ocean. In spite of the occurrence of calcareous and dolomitic levels, all waters in the Essaouira basin are of the Na–Cl-type. Based on a range of experimental methodologies, combined with PCA and geochemical modelling, it was possible to identify the mineralization processes occurring in the groundwater system, and the importance of the water–rock interaction in the water chemistry. Scenarios were tested using a simple mass balance model through the PHREEQC programme. The reaction path was assumed to be such that waters observed at shallow depths evolved to more mineralized waters. An important contribution of water–rock interaction in groundwater mineralization was found, corroborating the influence of preferential recharge from the Tidzi diapir in the water’s signature. Anthropogenic contamination was also identified and could lead to serious problems with groundwater degradation in the near future, in a country with scarce water resources.

Caractérisation hydrodynamique, hydrochimique et isotopique du système aquifère de Kourimat (Bassin d’Essaouira, Maroc)

The synclinal basin of Kourimat, subdivided in two sub-basins (Igounzar in the North, and Zeltene in the South), is situated in the oriental part of Essaouira basin shelters a carbonate aquifer system contained in the limestones and dolomitic limestones of the Cenomanian and the Turonian. These ground waters represent the only drinking water and irrigation resource for the region. The geological formations outcropping in the studied area, start with the clay and marly formations of the terminal Jurassic in the south of Zelten watershed, and end with the sandy and loamy Quaternary formations in coastal areas in the Low Qsob watershed. The Middle and Upper Cretaceous (marl, fractured limestone, and dolomites) represent the rest of the stratigraphic series especially in the areas of the Igrounzar watershed (up to 85% of the outcrops) and in Zelten (up to 50%). The field occupation is not very significant. It basically consists of three types: 1) cultures (essentially cereals) where it is possible (on the marly soils of the Cretaceous, the Quaternary and the Eocene); 2) uncovered areas corresponding to the fissured and karstic limestone outcrops of the Cretaceous and the Jurassic formations, and 3) forests (coniferous trees) in the downstream part of the catchments area (Plio-quaternary soils and mountainous reinforcements of the Western High Atlas). Like for other undeveloped areas of Morocco, the Meskala basin is also subjected to an intensive deforestation (wood for heating and cooking). The study undertaken on the Cenomano-turonian aquifer has allowed us to characterize it from the hydrodynamical, hydrogeological and hydrogeochemical point of view. The piezometric map of the two regrouped aquifer levels shows water flow senses from the SE to the NW. The hydraulic gradient varies from upstream to downstream, being weak n the central zone relative to the best hydrodynamic characteristics. The time evolution of the piezometry shows annual and seasonal fluctuations in connection with the variations of precipitations. Hydrodynamic characteristics have been calculated by means of the interpretation of data of pumping tests, performed in the region by the General Hydraulic Administration of Morocco. On the hydrochemical point of view, results of chemical analyses have allowed us to determine the chemical facies of waters of the two aquifers, their interaction and their quality for the human consumption and their use in agriculture. To better understand the functioning modalities of the Meskala-Kourimat aquifer system, an isotopic study has been performed that showed the variations relative to the altitude of the recharge zones of the different channels. The altitudinal gradient in O 18 , estimated to 0,25‰ every 100 m confirms the validity of this result at a regional scale. The oxygene-18/deuterium relationship (δ 2 H = 7,74*(δ 18 O) + 10,83) indicates an Atlantic system of the precipitation without evaporation. This suggests a quick infiltration, also confirmed by the importance of the fracturation developed in limestones and dolomitic limestones of the Cenomano-turonian aquifer of Meskala-Kourimat.

Une approche multicritères pour l'implantation de forages dans les zones productives des systèmes fissurés / A multicriteria approach for well drilling in productive zones of fissured systems

The multicriteria approach used in this paper aims to examine the aquifer of the Essaouira basin (Morocco). Investigation methods such as interpretation of hydrogeological wells and seismic sections, as well as the analysis of in situ fissuration measurements and field observations are used together in this study. The established hydrostructural models reveal the groundwater flow and the structure of the coastal aquifer system, whose heterogeneous hydrogeological formations contain very important water resources for the Moroccan coastal area. The partition of the Cretaceous Essaouira aquifer, and of its impermeable basement (Cenomanian marls) corresponds to a tectonic structure (horsts and grabens) resulting from reactivation of Hercynian structures. The identification of the normal faulting network demonstrates an influence of the structural geology on the specificity of coastal aquifers, on the groundwater flow, on the piezometric variation and the productivity of wells, depending on their location.

Découverte de Thyrsoporella pseudoperplexa Granier et Braik, 2002 (algue dasycladale) dans le Kimméridgien inférieur du bassin d'Essaouira (marge atlantique marocaine)

A dasycladacean alga recently attributed to the species Thyrsoporella pseudoperplexa Granier and Braik 2002, has been discovered within the Jurassic sediments of the Atlantic domain. This alga, only found in the Tethysian domain, was identified in the Early Kimmeridgian sediments of the Essaouira basin (Morocco).

Anthropogenic features in the Sines (Portugal) and Essaouira (Morocco) coastal aquifers: a comparative study of their hydrochemical evolution by a principal component analysis

Considering the effects of climatic conditions on groundwater resources salinization and quality, a comparative study was conducted on the coastal aquifers of Sines (Portugal) and Essaouira (Morocco). Under the climatic and environmental conditions these two basins present different vulnerabilities to anthropogenic activities. Both aquifers correspond to sedimentary basins with similar structures and lithologies. From the available physical, chemical and piezometric data, two series of results of each area were selected corresponding to two different years that were analysed by Principal Component Analysis (PCA). Sines basin is characterised by a temperate climate. In the Sines aquifer the waterrock interaction process is the major mechanism responsible for the groundwater evolution, conferring a calcium-bicarbonate facies. Applying the PCA, punctual anthropogenic contamination was identified and linked to agricultural activities. The water resources of the Essaouira basin are characteristic of a semi-arid climate, and are severely impacted by the climate (quantity and quality). PCA allowed the evaluation of the contribution of the Tidzi diapir in the water recharge that confers to the groundwater a sodium-chloride facies. Although this statistical method did not shown a nitrate contamination input in the Essaouira multi-aquifer, this polluent presents locally high values. Also the very high evaporation and scarce precipitation activate the processes of salinization and contamination.

Thin-skin tectonics in the Essaouira basin (western High Atlas, Morocco): evidence from seismic interpretation and modelling

In the Essaouira basin, detailed re-examination of seismic profiles and the use of modelling computer programs based on widely accepted concepts in fold kinematics has allowed us to set additional kinematic constraints on the geometry of the structures in this area. Forward modelling, conducted with the help of computer programs, was carried out in order to find the best kinematic pathway accounting for the geometry of the folds. We have observed two main geometrical types: fault-bend folds and a fault-propagation fold. Simple fault-bend folds were observed at the Tabelaouart, Oued Tahria, Bouzergoun and N’dark anticlines. All the ramps are located within the Triassic formations and have variable height. However, the ramps in the western structures have a northerly dip, whereas the eastern anticlines have grown upon southerly-dipping ramps. The amount of displacement is variable. Complex fault-bend folds occur at the Amsitten anticline, developed upon three major fault planes. Finally, an example of fault-propagation folding was observed at the Jeer anticline. In conclusion, the cover in the Essaouira basin appears decoupled from its basement in most structures, and its kinematics follows a ramp-flat geometry. From the oil exploration point of view, we believe that the whole tectonics of the Essaouira basin has to be reviewed on the basis of geometrical analysis, in order to improve the knowledge on the potential targets.

Groundwater nitrate pollution in the Essaouira Basin (Morocco)

The Essaouira Basin (Morocco) contains a multi-layered aquifer situated in fractured and karstic materials from the Middle and Upper Cretaceous (the Cenomanian, Turonian and Senonian). Water percolates through the limestone and dolomite formations of the Turonian stage either through the marls and calcareous marls of the Cenomanian or through the calcareous marly materials of the Senonian. The aquifer system may be interconnected since the marl layer separating the Turonian, Cenomanian and Senonian aquifers is thin or intensively fractured. In that case, the water is transported through a network of fractures and stratification joints. This paper describes the extent of the nitrate pollution in the area and its origin. Most of the wells and drillholes located in the Kourimat perimeter are contaminated by nitrates with some concentrations over 400 mg 1(-1). Nitrate contamination is also observed in the surface water of the Qsob River, which constitutes the natural outlet of the multi-layered complex aquifer system. In this area, agriculture is more developed than in the rest of the Essaouira Basin. Diffuse pollution of the karstic groundwater body by agricultural fertiliser residues may therefore partially explain the observed nitrate pollution. However, point pollution around the wells, springs and drillholes from human wastewater, livestock faeces and the mineralisation of organic debris close to the Muslim cemeteries cannot be excluded. (C) 2003 Academie des sciences. Published by Editions scientifiques et medicales Elsevier SAS. All rights reserved.