GOMEZ, FRANCISCO and MUAWIA BARAZANGI, Cornell University; WELDON BEAUCHAMP, ARCO International; AHMED DEMNATI, Office National de Recherche et d'Expliotation Petrolieres

Abstract: Evolution of the Atlas Mountains (Morocco) and Adjacent Sedimentary Basins: Implications for Hydrocarbons

Regional Setting

The High and Middle Atlas Mountains of Morocco represent Early Mesozoic continental rifts associated with the opening of the central Atlantic Ocean. In response to convergence between the African and Eurasian plates, these rift basins subsequently contracted and uplifted during the Cenozoic (i.e., "inversion tectonics") to create the present topography. Cenozoic sedimentary basins adjacent to the Atlas Mountains record these tectonic processes. These sedimentary basins include flanking "foreland" basins, intermontane basins, and hybrid basins reflecting structural interactions with the collisional Rif thrust belt. Understanding the context of these Cenozoic basins and their structural relationships with the Atlas ranges has implications for hydrocarbon exploration.

The Atlas Mountains

The Atlas Mountains are divided into the ENE-WSW trending High Atlas and the NE-SW oriented Middle Atlas (Figure 1). Recent studies have shown that the Atlas Mountains are Neogene mountain belts - the main episode of uplift and deformation initiated during the Middle Miocene and Quaternary. As a result of their various orientations, the High and Middle Atlas exhibit two different tectonic styles: The High Atlas displays pure contraction, whereas the Middle Atlas is a transpressional system involving strain partitioning of strike-slip and thrust-related deformation. Cenozoic tectonics involve both contractionally reactivated rift-related faults and younger moderately dipping thrust faults. The fold belts are bounded by moderately dipping thrust faults.

Types of Cenozoic Basins

The bivergent High Atlas Mountains are flanked by basins akin to flexural foreland basins. Two examples bordering the central High Atlas, the Ouarzazate and Tadla basins (Figure 1), contain up to I km of Cenozoic strata. However, Neogene contraction in the adjacent mountain belt has created elevations in excess of 4,000 meters. The shallow depth of the basins suggests that either (1) loading of the lithosphere due to thickening of the Atlas Mountains is minimal, or (2) the lithosphere beneath the Atlas is very strong. Seismic reflection data document significant overthrusting of the basin margins by the frontal thrust faults of the High Atlas. The Saiss Basin bordering the western High Atlas (Figure 1) is another example of a foreland basin flanking the Atlas.

The intermontane Missour Basin (Figure 1), located between the High and Middle Atlas ranges, contains only a thin veneer of Cenozoic strata. Along with the extension of the two adjacent Atlas rift basins, the Missour Basin experienced minor Mesozoic extension, as well. However, the Missour Basin experienced very little tectonic inversion, relative to the neighboring Atlas, and therefore, the Missour Basin contains buried Mesozoic rift structures. Tertiary strata in the Missour Basin, as well as the two other examples above, are comprised of conti-.nental sediments.Another example of an intermontane basin of the Atlas is the Haouz Basin in the western High Atlas (Figure 1).

The Guercif Basin developed abruptly along strike of the Middle Atlas (Figure 1). Despite the transpressional setting of the Middle Atlas, the Guercif Basin evolved as an extensional basin. The timing and basin architecture suggest that the basin reflects the competing tectonic influences of the Middle Atlas and the Rif thrust belt farther to the north. In the Middle Miocene, the Guercif Basin experienced contraction and uplift, along with the rest of the Middle Atlas. However, the Late Miocene saw a brief extensional episode localized in a narrow graben. Extension was accompanied by deep marine deposition. Subsequently, in the latest Miocene, the Guercif Basin once again experienced contraction. In total, more than 2.5 km of Neogene and Quaternary strata were deposited in the Guercif Basin.

Implications for Hydrocarbon Exploration

The main source rocks in the "foreland" basins of the High Atlas are Paleozoic (Silurian) shales.Additional sources may be found in the Cretaceous strata, similar to those in the Middle Atlas containing high (~18%) total organic carbon (TOC). Although Neogene depositional burial of these source rocks is insufficient to produce maturity, the basin margins may be sufficiently buried beneath overthrusts from the Atlas Mountains to place them in the oil window. In addition, these regions also contain old Paleozoic structures such as thrust faults and folds of the Hercynian fold belt, and these may also provide suitable structural traps.

The Missour Basin contains Carboniferous, Jurassic, and Cretaceous shales, all of which are viable source rocks in the adjacent Middle and High Atlas.The main structural traps in the Missour Basin relate to Mesozoic rifting. Mild Cenozoic tectonic inversion may further augment the development of structural traps, creating fault propagation folds from contractionally reactivated normal faults.As with the flanking basins discussed above, Cenozoic structures at the boundaries of the High and Middle Atlas may provide structural traps in a sub-thrust context.

The Guercif Basin was part of the same depositional system as the Mesozoic Middle Atlas rift and therefore should contain similar organic-rich Jurassic shales as source rocks. In the Guercif Basin, Neogene extension buried potential Paleozoic and Mesozoic source rocks beneath up to 2.5 km of basin strata, and thus within the "oil window". Middle Miocene contraction created structural traps in the form of fault propagation folds above reactivated Mesozoic rift faults prior to the Late Miocene burial of viable source rocks and possible hydrocarbon maturation. Jurassic carbonates and Neogene clastics serve as reservoir rocks.

Conclusions

The Cenozoic sedimentary basins bordering the ranges of the Atlas Mountains evolved in different tectonic contexts with respect to the adjacent Cenozoic mountain chains. These basins contain important records of the tectonic events responsible for the Atlas, including aspects of the Mesozoic rifting, as well as the Cenozoic uplift of the Atlas. To date, petroleum exploration in the Atlas system has found limited success, but a better understanding of these basins and how they relate to the Atlas should improve the prospects for hydrocarbon exploration.

AAPG Search and Discovery Article #90923@1999 International Conference and Exhibition, Birmingham, England