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.
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