Geology Reference
In-Depth Information
stromatolite-bearing carbonate and minor
evaporitic strata (Goodman & Kah, 2004) of the Atar
Group represent craton-wide fl ooding and depos-
ition in predominantly shallow marine environ-
ments (Bertrand-Sarfati & Moussine-Pouchkine,
1988). The overlying Assabet el Hassiane Group is
composed of deep to shallow marine, fi ne-grained
siliciclastic strata (Trompette & Carozzi, 1994;
Moussine-Pouchkine & Bertrand-Sarfati, 1997).
The most lithologically variable of these
successions, the ~750 m thick Atar Group, consists
of ten formations that record alternating deposition
of stromatolite-bearing carbonate and siliciclastic
material (Fig. 3; Bertrand-Sarfati & Moussine-
Pouchkine, 1988, 1999). The basal Foum Chor
(unit I-3) Formation is markedly more coarse-
grained than the remainder of the Atar Group,
and consists primarily of fi ning-upward, coarse-
to medium-grained fl uvial to fl uvial-marine
sandstones that onlap both the Reguibat Shield and
deeply incised strata of the Char Group. Remaining
formations (Ksar Torchane, unit I-4; Atar, unit I-5;
Oued Tariofet, unit I-6; Tawaz, unit I-7; Oued
Terrarit, unit I-8; Aouinet Ould Bou Derbala, unit
I-9; Aouleigate, unit I-10; Touiderguilt, unit I-11;
and Tifounke, unit I-12) preserve regionally
extensive depositional packages of siltstone and
shale alternating with stromatolite-bearing car-
bonate and subordinate evaporites. Spectacular
stromatolite biostromes of the Atar Group (R1-R3
of the Atar Formation, unit I-5) are the focus of
this study. The presence of hummocky cross-
stratifi cation, wave and interference ripples, and
lenses of fl at-pebble conglomerates in siliciclastic
strata suggests predominantly shallow subtidal
to intertidal depositional environments. Subtidal
to intertidal depositional environments are also
inferred from stromatolite morphologies in the
Atar Group (Bertrand-Sarfati, 1972; Bertrand-
Sarfati & Moussine-Pouchkine, 1985), suggesting
that the alternation of siliciclastic and carbon-
ate strata may result primarily from regional cli-
matic or tectonic activity and the episodic infl ux
of siliciclastic material from source areas outside
the Taoudeni basin (Bertrand-Sarfati & Moussine-
Pouchkine,
Atar Formation
110
100
90
R3
80
70
60
50
*
R2
40
30
20
*
10
R1
0
Legend of symbols
Conophyton/Jacutophyton
Limestone
Baicalia/Tilemsina
Tungussia
Siltstone/shale
Sandstone
Molar-tooth structure
Fig. 3.
Stratigraphy of the Atar Group. The ~750 m thick
Atar Goup consists of ten formations (I-3, Foum Chor; I-4,
Ksar Torchane; I-5, Atar; I-6, Oued Tariofet; I-7, Tawaz;
I-8, Oued Terrarit; I-9, Aouinet Ould Bou Derbala; I-10,
Aouleigate; I-11, Touiderguilt; I-12, Tifounke) that record
deposition of fi ne-grained siliciclastic and carbonate sedi-
ment. The Atar Formation (I-5) is marked by deposition of
three prominent
Conophyton-Jacutophyton
reefs (R1-R3).
Each reef interval initiates at a marine transgression and
represents transgressive to highstand systems tract depos-
ition. Deposition of the lower two reef intervals terminates
at an abrupt fall in relative sea level (marked by asterisks)
and is followed by deposition of a predominantly silici-
clastic lowstand systems tract. The uppermost reef inter-
val is marked by a gradual fall in relative sea level, and is
overlain by lowstand siliciclastic, carbonate, and evaporite
deposition of the Oued Tariofet Formation (I-6).
1988;
Moussine-Pouchkine
&
Bertrand-Sarfati, 1997).
The age of the Atar Group is poorly constrained.
Rb-Sr geochronology (Clauer, 1976, 1981; Clauer
et al.
, 1982) performed on glauconite and illite
in shaly intervals provide ages from 998
fi ne-grained siliciclastic strata and subordinate
carbonates of the Char Group are preserved as
erosional remnants in cratonal depressions and
represent initial mantling of the craton by fl u-
vial, coastal aeolian and shallow-marine depos-
its (Benan & Deynoux, 1988). In contrast to the
Char Group, fi ne-grained siliciclastic strata,
32 Ma
(Azougi Formation, unit I-2, underlying Char
Group) to >694 Ma (Assabet el Hassiane Group,
unit I-15), with most formations in the Atar
