Geographic and Geological
settings
Geographic and geological aspects
The Atlas system extends over some 2000 km between the Moroccan Atlantic
margin in the West and Tunisia in the East. This system is bound by
major lineaments, called South and North Atlas Faults, with adjacent
subatlasic depressions and, is subdivided within Morocco in Middle and
High Atlas (figure 1).
The Atlas Mountains represent an intracratonic belt and consist of
Mesozoic and Cenozoic formations that accumulated in the foreland of the
Tell-Rif Thrust Belt during the Alpine/Atlasic Orogeny.
The Middle Atlas is limited to the North by the Saïs plain and the Rif
front-thrust, to the NE by the Guercif Basin, to the E and the SE by the
Moulouya depressions, and to the W by the Paleozoic massif of Central
Morocco (belonging to the Western Moroccan Meseta). It is composed of
the juxtaposition of two structural units: the tabular (or the ”Middle
Atlas Causse” ; Termier, 1936) and the folded Middle Atlas. These two
units are separated by the North-Middle Atlas Fault as a major
lineament. The tabular Middle Atlas is a sub-horizontal structure,
composed mainly of neritic carbonates of the lower and middle Liassic.
It is organized into a tiered plateau reflecting tilted block structures
and consists of two sections (northern and southern), separated by the
Tizi n’Tretten Fault. The folded Middle Atlas, oriented NE-SW, is
composed of large syncline depressions delimited by narrow anticlinal
ridges. the syncline depressions are occupied, essentially, by Toarcian
and Dogger material. The Cretaceous and the Paleogene are confined to
depressions situated to the W of the Boulemane meridian, while Neogene
outcrops occur largely to the NE. Anticline ridges form. The hinge of
narrow anticlines, generally represented by Liassic carbonates, is often
affected by faults and injected with Triassic - Liassic shale and/or
intruded by Jurassic/Cretaceous magmatites (Fedan & El Hassani, 2018).
Geological description
Reading the geological map of Sefrou 1: 100.000th(Charrière 1989 : figure 2) and relative documents (i.e. Termier &
Dubar, 1940 ; Martin 1981 ; Benshili, 1987 ; Cirac, 1987 ; Charrière,
1984 and 1992 ; Fedan, 1988; Ahmamou 1987) allows us to subdivide the
stratigraphic series of this part of the tabular Middle Atlas into four
groups (figure 3):
A Paleozoic basement, with Ordovician to the Carboniferous series, in
particular a complete Devonian sequences of conglomerates, limestones,
reef limestones and carbonate marls (see Aboussalam et al., 2020).
Post-Paleozoic cover were sedimentation generally starts with the upper
Triassic clay-evaporite series (gypsiferous argillites and mixed
basalts);
- followed by Kandar Dolomitic Formation of Sebkha’s environment
(Charrière, 1992) of lower Liasic age dolomitic deposits. Then
brecciated dolomites, dolomitic sands and bedded dolomites;
- The Liasic series continues with massive limestones with Oncolites,
Gastropods, Terebratula and imperforate foraminifera, locally
dolomitized;
- Flint-bedded limestones with ammonites (Tropidoceras) at the base,
which is overcome by reef constructions (lumachel limestones and
epi-reef levels);
- Ammonite limestone (protogrammoceras celebratum and Fuciniceras);
- And finally, oolite limestones with reefs which shows frequent
dolomitization.
After a major unconformity, above the Jurassic series, outcrop north of
Sefrou (in the Sais plain and its borders) marly formations that filling
the basin (fig. 3) that shows from base to top:
- Red formations with volcano-sedimentary levels and volcanic flows (in
the area of Zra wadi), then a formation composed of clays, marls and
lacustrine limestones; channeled sandstones and conglomerates
(fluvio-deltaic facies); finally rose marl showing silt-sandstone
intercalations with microfauna.
- The three Messinian formations:
- Bhalil Formation (biocalcarenites, reef constructions and
conglomerates);
- Sefrou Formation of ocher silts with Globorotalia dutertrei;
- and finally, the Fez “blue marls” Formation which is rich in
planktonic foraminifera.
The stratigraphic series close, after an unconformity, by lacustrine
limestones of the Sais (middle to upper Pliocene) and finally the
Quaternary Formations (travertines, crusts, alluviums, glacis and
terraces).