Geology Reference
In-Depth Information
3.3.2.5 The Semb
´
-Ouesso Basin, Cameroon,
CAR, RC
The Dja Group (Alvarez
1995
) records the evolution of
multiple graben-like basins in the northeastern RC, the
southeastern Cameroon and the southwestern CAR
(Fig.
3.3A
; Poidevin
1985
). This group rests, by an angular
unconformity on the Archean basement of the Chaillu block
in the northeastern RC and on the Ntem Complex in
Cameroon (Lasserre and Soba
1976
; Vicat et al.
1997
).
The Dja Group is divided into two series: (i) the Lower
Dja series, presumably of Paleoproterozoic (Poidevin and
Pin
1986
; Censier
1989
; Vicat et al.
1997
) or Tonian ages;
and (ii) the Upper Dja series, presumably of Ediacaran age
(Alvarez
1995
; Vicat et al.
1997
; Poidevin
2007
). In the
northeastern RC, the Lower Dja series, previously Semb
´
-
Ouesso series, was divided into three sub-series (Chochine
1950
; Sonet
1958
; Gr`s and Legras
1967
; Jaunatre
1969
),
and grouped into two sub-series as follows, from youngest to
oldest (Vicat et al.
1997
): (i) the
1,500 m-thick Schisto-
Quartzitic sub-series (Bol
´
series in southwestern CAR and
southeastern Cameroon; Poidevin
1985
); and (ii) the uncon-
formable
3.3.2.2 Local Isolated Formations Reported to the
Lindi Supergroup
Isolated sedimentary series located in the northeastern DRC
(Fig.
3.3A
) attributed to the Lindi Supergroup has been
separated by erosion or faults (Cahen
1954
). Stratigraphic-
equivalent Ituri carbonates are observed along (i) the
Niangara and Gada rivers (Meyer
1953
); (ii) near Yambuya
on the Aruwimi river (Mathieu
1922
); and (iii) Buta, Poko,
Gwane and Amadi to the base of the Lindi Supergroup
(Cahen
1954
). Two important diamictitic sequences are
observed in the Haute Ibina (Biena) and south of Irumu, in
the Homas Mount (Cahen
1954
).
3.3.2.3 The Fouroumbala-Bakouma Basin, CAR
The Fouroumbala-Bakouma Basin is located in the south
central region of the CAR. Three main sedimentary groups
(Fig.
3.3C
), with two diamictite horizons, i.e. the Lower
Tillite in the extreme SW and the Bondo Tillite at the base
of the Bakouma Subgroup, have been recognized (Bigotte
and Bonifas
1968
; Poidevin et al.
1981
; Poidevin
1985
) and
are from base to top: (i) the siliciclastic Bougboulou Sub-
group, with a cap carbonate unit with locally silicified stro-
matolitic carbonates in the south of the Fouroumbala sub-
basin (Mestraud
1952
,
1953
), overlies an angular unconfor-
mity on the Archean Bangui-Kett´ basement; (ii) the uncon-
formable deltaic and neritic Kemb
´
-Nakando Subgroup; and
(iii) the post-glacial fluvio-marine Bakouma Subgroup.
Gr
´
so-P
´
litique
700 m-thick 1167-850 Ma
'
'
sub-series (
'
Gr`s de Douma
'
in the Semb´ sub-basin and
Gr
`
sdeS
´
ka-Nzoulabout in the Ouesso sub-basin;
Vicat and Vellutini
1987
; Vicat et al.
1997
) including the
the
'
100 m-thick Dja Tillitic Complex in the northeastern RC
and the Bandjia Tillitic Complex in southwestern CAR and
Cameroon (Cahen
1982
; Poidevin and Pin
1986
; Vicat and
Vellutini
1987
; Vicat and Pouclet
1995
; Alvarez
1995
). The
Upper Dja series was formerly renamed as the Mintom
Formation (Alvarez
1995
) and subdivided into four
members; i.e. Kol, M´tou, Momibol´ and Atog Adjap; on
the basis of field, sedimentological and geochemical
evidences, by Caron et al. (
2010
).
3.3.2.4 The Ubangui Basin, CAR
Lithostratigraphically, the Ubangui Basin has been
described by Babet (
1935
), Legoux and Hourcq (
1943
),
G´rard and G´rard (
1952
), Bessoles and Trompette (
1980
)
and Poidevin (
1976
,
1979a
,
b
,
1985
,
2007
). Neoproterozoic
sedimentary rocks exposed in the SW of the CAR, around
Bangui extend southwards to the Congo River and overlie
metasedimentary rocks of the Nola Group that are thought to
be Paleoproterozoic in age. The Nola Group is intruded by
undated dolerite dykes, but yielded Pan-African resetting
40
Ar-
39
Ar age of 571 Ma (Moloto-A-Kenguemba et al.
2008
). Lithostratigraphically, the Neoproterozoic of the
Ubangui Basin is divided as follows, from the youngest to
the oldest, (Fig.
3.3D
): (i) up to 200 m-thick tectonized
'
3.3.2.6 The Likki-Bemb ´ Basin, DRC
The Likki-Bemb
´
Group, formerly Bembe-Ugangi Group,
renamed by Aderca (
1950
), is exposed between Mobayi-
Mbongo and Libenge in the Equateur Province of the DRC
(Fig.
3.3A
). The weakly to moderately metamorphized
Likki-Bemb´ Group is divided into two systems or
subgroups (Aderca
1950
): (i) the dominantly siliciclastic
lower systems; and (ii) the quartzites, schists, phyllads
locally with calcschists and limestones upper systems.
Kembe conglomerates
'
; (ii) 130 m-thick fluvial
'
Bimbo
sandstones
passing eastwards into the fluvio-lacustrine
Kemb´-Nakando sandstones of the Fouroumbala-Bakouma
Basin in the eastern CAR (Alvarez
1995
,
2000
); and (iii)
'
'
3.3.3 The Eastern Margin of CS
, including a cap carbonate series,
rhythmites and microbial limestones, considered as late
Neoproterozoic in age and coeval with the Schisto-Calcaire
Subgroup of the West Congo Supergroup (Alvarez
1995
).
Bangui carbonates
'
Neoproterozoic sedimentary rocks exposed on the eastern
CS, are recognized in the Itombwe and Malagarazi-Bukoban
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