meta argumentativas

The Ouarzazate Basin and the Southern Atlas Marginal Zone lie in between the intra-continental High Atlas range to the North and the Anti-Atlas to the South. A detailed overview of the regions geological setting is provided by Michard et al. (2008), Piqué et al.

(2002) and Stets & Wurster (1981). Table 3-2 provides a chronological overview.

Table 3-2: Summary of the geodynamic evolution of the Upper Drâa region (Michard et al. 2008);

Time scale according to the International Commission on Stratigraphy (Walker & Geissman 2009)

Million years before present

( MYBP)

Era Period Epoch Dominant processes

0.01

Cenozoic

Quaternary Holocene

Alpidian orogeny: Inversion and uplift of the Atlas rift domain

↑

359 Carboniferous metamorphosis of basement

416 Devonian

444 Silurian

488 Ordovician

570 Cambrian

… Precambrian ~685 Pan-African orogeny

~2000 West African Craton consolidates

The Anti-Atlas represents the northern margin of the West African Craton, which consists of widely uncovered Precambrian rocks (~2000 MYBP, million years before present). The Anti-Atlas folded up when a Precambrian allochthonous terrane (Northern Morocco) collided with

the West African Craton during the Pan-African orogeny (~685 MYBP, Villeneuve & Cornée 1994) and has been only slightly reshaped since then.

The Meseta block, consisting mainly of the Western Meseta and the Western High Atlas, has been uplifted as result of the Hercyan orogeny (also referred to as Variscan orogeny), which resulted in the formation of Pangaea. During this orogeny a weak rift system developed, as the Meseta block sheared along a NE-SW axis. Small basins subsided in Permian and have been filled predominantly with red beds. During the Late Triassic another rifting event occurred in the Atlas domain, along with the breakup of Pangaea (Laville et al. 2004). The rift led to the development of half-grabens and volcanism; most basaltic sills or dykes in the region originate from this volcanic activity. Fault activity and volcanic activity gradually declined from Early Liassic times on. The first major marine incursion from the Tethys Sea occurred at approximately 200 MYBP and gave way to flooding of the rifted area. A progressive westward onlap of marine deposits took place onto the rift and onto its margins.

Throughout the Jurassic and Cretaceous the trough formed by the failed rift was filled with sediments. They attest to two major episodes of sedimentation: late Cretaceous to middle Eocene, where marine deposition was controlled primarily by eustatic changes and late Eocene to Quaternary, during which the Basin of Ouarzazate formed and filled with thick continental series largely through the uplift of the Central High Atlas.

The southern slopes of the western part of the Central High Atlas consist of the following structural units (El Harfi et al. 2001), see in Figure 3-5:

 Axial zone: This is the mountainous zone of the High Atlas, composed of a Hercyan basement and Mesozoic, mainly Liassic, limestone cover. Neogene deposits crop out in intra-montane basins.

 Subatlas Zone or Southern Atlas Marginal Zone: Compressional deformation of this zone, which is folded over a width of approximately 10 km. It includes very narrow anticlines and synclines. Deposits in this folded and thrusted zone are mostly Cretaceous and Tertiary.

 Ouarzazate Basin: An asymmetrical synclinorium of structural origin (~150*40 km) with Cenozoic continental formations locally up to 1200 m thick.

 Anti-Atlas: Stable domain of Precambrian basement, partly with Paleozoic cover.

This setting describes the actual geological framework that has to be accounted for in a hydrological simulation. In terms of porosity and hydraulic conductivity, the lithology and hydrogeological properties of the different zones can be summarized as follows (Klose 2008b; Cappy 2006):

Fractured and partly karstified Liassic formations have locally varying permeabilities. In general, these rocks act as aquifers. Due to the aquifer structure and morphology, many groundwater outcrops are contact springs emerging from Liassic aquifers overlying Triassic aquitards or aquicludes (e.g. basalts). Tritium ages younger than 10 years indicate an efficient recharge of the Liassic aquifer system in the High Atlas by recent precipitation. Recharge rates of 4% (Assif-n'-Ait Ahmed catchment: 100 km²) and 11%

(Ifre catchment: 1,240 km²) have been computed in water balance studies (Cappy 2006).

The isotopic signatures of most of the groundwater samples from the alluvial aquifers in the Basin of Ouarzazate reflect mean altitudes of the recharge area from 2,400 to 2,900 masl, locating these areas in the High Atlas Mountains.

Figure 3-5: Geological setting of the Upper Drâa valley (El Harfi et al. 2001)

In the South Atlas Marginal Zone strong compressional deformations lead to narrow anticlines and synclines of Cretaceous and Tertiary rocks of different hydrogeological properties thus effectively blocking groundwater transfer from the mountains to the basin.

Therefore Cappy (2006) assumes groundwater flow occurs only in the alluvial aquifers as

their porous quaternary deposits are highly permeable. These assumptions have been affirmed by MODFLOW simulations that resulted in a riverbed infiltration to total recharge ratio of 85% (Cappy 2006). Therefore direct recharge by precipitation within the basin is considered not significant. Older alluvial terraces, located mainly in the basin, are of rather variable permeability due to different degrees of consolidation and cementation.

The uncovered fractured basement is exposed in the Anti-Atlas and central parts of the High Atlas. These rocks are characterized by low to ultralow hydraulic conductivities and low water content.

Summarizing, the Anti-Atlas features poor groundwater storage capacity and renewability.

The carbonate rocks of the High Atlas represent the main aquifer system in regard to its volume and its recharge. The recharge of this aquifer system by recent precipitation is efficient. A connection of this Aquifer and the basin via mountain-front discharge has to be neglected, as the Southern Atlas Marginal Zone acts as a decisive hydrogeological barrier between the aquifers of the High Atlas Mountains and the Basin of Ouarzazate. Therefore groundwater from the High Atlas is mainly transported via the wadis and the corresponding alluvial aquifers. The alluvial aquifers in the Basin of Ouarzazate are mainly fed by these wadis, as recharge by precipitation within the basin can be considered subordinate.