Karst Water. Resources (Proceedings of the Ankara - Antalya Symposium, July 1985). IAHSPubl.no. 161 - THOUGHTS ON KARST WATER RESOURCES IN THE MIDDLE ATLAS MOUNTAINS ^MOROCCO Abdel kader Latati Directorate of Water Research and Planification Hydraulic Administration, Rabat, Morocco Abstract The Central Middle Atlas is a chain of mountains stretching from the Southwest to the Northeast between the Central Massif at the West, the Eastern Middle Atlas at the Northeast and the plains of upper Moulouya at the Southeast. Plateaus of less than 2000 m in altitude called Middle Atlas causses dominate the West and North. An anticlinal structure is situated SE and oriented SW—NE enclosing large synclinal structures. The dolomitic and calcareous formations of the lower and middle Lias landscape outcrop on the causses. Series of layers from Dogger to Pliocene cover these formations that are found in the folded chain. Up North, the calcaro - dolomitic layers of the Lias sink under Neogene basin of Fes - Meknes. The Central Middle Atlas is drained by the two largest rivers of Morocco : The Sebou to the North and Northeast and the Oum Er Rbia to the Southwest. The causses offer an abundant and varied sampling of karstic dissolution hill - shading ranging from minute forms to poljes and large dry valleys. But it is mostly just a superficial karst where lateral corrosion is no more active. The classical procedures for karstological studies are very limited for the following reasons : Lack of accessible subterranean galleries; springs are generally located far away from the karstic areas. The sires and discharges variabilities of the springs are very diverse. The subterranean flow occurs through a multitude of elemantary channels rather than through wide subterranean galleries. In addition to systematic observations of springs and rivers discharges, the emphasis is placed on the evaluation of the regularized flow of water by karstic aquifers, especially by the use of simulation models. Water resources mobilization undertaken at earlier time on the Oum Er Rbia basin will be reinforced by multiple purpose project dams for the upper Sebou basin. 633 634 A.Latati, Introduction to the domain The Middle Atlas is a mass of high grounds stretching along 400 kms from the Upper Atlas at its Southwest to the Guercif plain at its Northeast, forming.an orographic barrier between the Atlantic: and oriental parts of Morocco. The Middle Atlas is subdivided into three structural domains. : The Meridianal Middle Atlas which overhangs the Tadla plain at the South up to the gap of Srou river, from which point a sub - tabular structure of layered plateaus of 800 to 2000 m normally called "The causses of the Middle Atlas" is found to the North. At the East and Southeast, we find the "folded Middle Atlas". The Central Middle Atlas takes place between the Srou valley at the South, the Central Plateau at the West and the Fes - Meknes plain up North. Its limit with the Oriental Middle Atlas is a hollow limit NE-SW passing by the East of Ahermoumou causse up to the antic­ linal of Tichkout mountain, at the East of which, the folded chain that started at the Bekrite - Timahdite synclinal as small basins and thin valleys, presents large folded sets. The Central Middle Atlas covers about 8000 km2. The Middle Atlas causses that show the most important karstie forms are the area where the dolomitic and calcareous formations of the lower and middle Lias outcrop. Layers of the Trias rock salt separate these formations from the Paleozoic substratum. In the folded Middle Atlas, these formations sink under thick layers of Dogger to Pliocene, but outcrop in almost all over the anticlinal ripples. At the Northern limit of the causses, the calcareous and dolomitic formations sink toward the North, under the Meknes - Fes Neogene basin and rest on the Southern Rrf Substratum. Under the Meknes - Fes basin, the structure of the Lias is very upportioned by faults and flexures some of which appear at the surface. The superficial layer is marly Miocene series (Om South to many hundred m thick up North)keeping the Lias nappe under pressure. Resting over these series, we find a complex of lacustrine limestone, sandstone and conglo­ merates that hold the supperfical nappe. The two nappes communicate through the faults and flexures or through the semi - permeable marly layers. The Middle Atlas is considered as the "water tower of Morocco". This expression applies better to the Central Middle Atlas that represents the origin of the two major rivers of the country. The most important one is the Sebou and its tributaries : Beht, R'Dom and Mikkes that drain 2/3 of the area. The Oum Er Rbia river and its tributaries of the left side, particularly the Srou river, drain about 1600 km2. However, surface and subsurface basins are different. The dividing line of the subterranean water basins goes approximately through the Azrou plateau at Central West, around de Tizi - Ntratten accident and separates the Amekla and Guigou causses. Large amount of infiltrated water in the Southern causses converges to Oum Er Rbia springs (Fig. 2). Up North, the infiltrated water returnes directly or indirectly to the Sebou river at the East and NE, or supplies either the Ribaa - Bittit springs located between El Hajeb causse and Meknes - Fes basin or the aquifers of this basin. Thoughts on karst water resources 635 ^~J MEDITERRANEAN Tanget® A SEA X » / ^ Ûuîd^C C Rabat/ Mok^jahata I O / Khonifra"^» Sa ,./^ „QV'MJd8lt u 7' Marrakech ^ffi° Er.Rachidi. a •s. ***** Agadir] 8 fk' «» • 1 a * / v*» * / o^n D +y •v / o /Laayoun / .(J a>®Smara v- - + / 1 / -*" 1 * • / Cj + Ad-Oakhlcr/ f * 1 '///• <v +1 //<?^ Studied area + • / '• 1 + /*—+:—••- + - +-+-+-,' 0 250 500km LLagwira Fig. 1 - Situation of the studied area , 636 A.Latati Karstic Forms On the causses, karstic forms are very noticeable from tiny holes to very wide depressions that give the landscape its identity. But it is mostly just a superficial karst. The thin layer of the limestone and the porous dolomite do not allow the development of caves, natural wells and subterranean galleries. The areas with the most karst area totaly dry while springs emerge on borders where different conditions exist. Possibilities for research on karst in these areas are then very limited. Specific Conditions and Karstification Process Lithological conditions — Karstic rocks are limestone, dolomite, gypsum or rock salt: — In spite of the importance of outcropping Carixien and Domerian limestones, they are usually thin layers (dm to m) alternating with clay and marly layers unfavorable for their karstification. In areas where the marl and the clay are absent, the limestone offers however an excellent field of minute dissolution forms as grikes. That is the case of the Carixien calcaro -dolomitic "metrical banks" which present a good fissure permeability and offer the best field of rock - shelters and dolines. — The lower Lias dolomitic series are less karstified because of their porosity. Even though the large space they occupy, the density of the dolines is relatively low. — The outcropping of the gypsum and the rock salt is usually rare, the water that washes out these formations is briny (Oum Er Rbia springs). When outcropping, the clays of the Trias are impermeable and the dissolution forms are limited. On the other hand, high density of dolines is found in places where clay is covered with dolomitic thin layers or an old solifluxion flow. In the karstification process, the covering porous formations seem to be a compress that undertakes the dissolution of the underlying clay. But in such case, it is often difficult to identify the karst of the dolomitic formations from the repercussion of a deep karstification of the Trias that appears on the surface. Tectonic conditions — The layer dips in the Middle Atlas causses are seldom very high. The density of dolines is higher on the low dip layers, beyond certain limit (15—17°) we find less dolines. The karstification process is also developed on synclinal dispositions and on the accident lines that may be the origin of a "karst barre" (Afriroua in El Hammam causse). The lateral corrosion is more active where water table is maintained near the surface. Karstic forms The Middle Atlas causses present a very rich and varied samples of karstic forms : minute carving, dolines, poljes... In addition to the lithological and the tectonic conditions, the distribution of karstic forms depends on the altitude and the continentality. Minute forms — The minute karstic forms and nivokarstic shelfs ar located at high altitude or occupy the croups summits of the causses. Thoughts on karst water resources 537 "ahta causse J ( J bel } Mountain 0 ( Oued) River Lacustrine limestone of the Pliocene Marl and sandstone of the upper Miocene j j Post lias formations fciri."-) Limestone of middle lias Dolomite of tower lias Trias and Paleozoic Fig. 2 - Central Middle Atlas structural sketch . 638 A.Latati Blind, dry valleys- They are very developed in the porous and alterated dolomite when outcropping in absence of collapse bowels. The larges form is locally called "I ssiane" and means a deep shakehole cultivated especially in the Imouzzer causse^ The dolines - Their density and size are variable with karstified rocks. According to their size, we consider two "types: - Collapse dolines are developed by the squeezing of subterranean galleries following the attitude of the calcaro - dolomitic layers or by drawing off the terra - rossa. - Giant Dolines and Uvalas are developed on the dolomite less soluble by superficial dissolution but are sometime occupied by collapse dolines resulting probably from the underlying Trias karstified formations. Poljes — They are found in certain areas of Amekla, El Hammam and Ajdir causses at mean altitude of 1450—1650 m and are always digged out by superficial dissolution in a hollow structure (faults, synclinal dispositions..).