Geology Reference
In-Depth Information
Fig. 5.1
General geological map
of the studied area and location of
Mouila city where the old Mouila
quarry is situated, exposing the
carbonate Neoproterozoic
(
) sampled for
the study. Bz is for Bouenza
'
'
Nsc3
'
or
'
SCIII
'
Group
'
, SCIa-b, SCIc and SCII
for
NSc1a-b, Nsc1c and Nsc2
formations of the Schisto-
Calcaire Subgroup (modified
from Thi
´
blemont et al.
2009
;
Pr
´
at et al.
2010
)
'
in Namibia, has a proposed maximum age of 564 Ma (Saylor
et al.
1998
), and therefore likely correlates with the Gaskiers,
or possibly another Ediacaran glaciation. However, the
precise age of the formations of the West Congo Supergroup
is still unknown.
The
Clays are not abundant and consist of subcentimetre-scale
thicknesses. The stromatolites are hemispherical (height of
10-20 cm, lenghts up to 50 cm) and typically laterally linked
(respectively LLH and SH, Logan et al.
1964
, Fig.
5.3c,d
).
The facies are fine grained, mud-supported, greyish to
slightly whitish and particularly homogeneous on the field
(Fig.
5.3a,b
).
Sixty samples were collected to ascertain the lithology
and sedimentary features of formation Nsc1. Detailed imag-
ing of the samples was performed using a JEOL Model JSM-
6400 Scanning Electron Microscope (SEM) with a Pioneer
Si-Li crystal X-ray detector (EDX) operated at a specific
resolution of 138 eV. The samples were gold or carbon
coated and mounted on metal stubs. The software used for
spectral interpretation was a Voyager Version 3.5 from
Thermo-Noran. High resolution images (1.2-3.0 nm at 15
and 1 kV, respectively) were further obtained with a JEOL
JSM-7000F Field Emission Scanning Electron Microscope.
Thin sections and rock slabs were sputter coated with gold-
palladium and mounted on metal stubs.
The analyses enabled the recognition of 5 major
is predominantly a
carbonate sequence with four formations (
'
Schisto-Calcaire Subgroup
'
),
and an overall maximum thickness of about 650 m. The
lowermost three formations are calcareous to dolomitic
shales with an uppermost sandy shale-siltstone unit with
interbedded limestones. For our study, we collected fresh
samples (standard thin sections and slabs were prepared
from freshly cut samples after removing 2-3 cm from the
outer surfaces) from formation Nsc3, which is exposed in the
old Mouila quarry in South Gabon (Fig.
5.1
).
'
Nsc1 to Nsc4
'
5.2.1 Microfacies of the
'
Nsc3 Formation
'
In the old Mouila quarry, 20 m of thin to medium-bedded
homogeneous dolo-mudstones interstratified with massive
laminar peloidal and stromatolitic greyish dolomites are
exposed (Figs.
5.2
and
5.3a,b
). The rocks are partly silicified
and contain stratiform black chert nodules, a few centimeters
thick, between the beds and thin siliceous impregnations
perfectly moulding the stromatolite laminae (Fig.
5.3c,d
).
diage-
netic
'
dolomitic microfacies (MF1-5), whose succession
(1-5) constitutes a standard sequence of shallowing-upward
sedimentation and a corresponding increase in post-
depositional diagenetic events related to the influence of
hypersaline groundwaters in a sabkha-like environment
'
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