Geology Reference
In-Depth Information
wells. Sequence 1 is terminated by deformation associated
with the early Pan-African event.
Subsequent deposition of marine clastics (Sequence 2)
over a regional unconformity was interpreted by Daly et al.
(
1992
) as a marine transgression that flooded the craton.
Acritarchs recovered from the Mbandaka well were corre-
lated with the Lokomo Group of northeastern DRC and
attributed to the Early Cambrian. Continued (late) Pan-
African syn- and post-depositional compression resulted in
the development of a regionally extensive subaerial, ero-
sional unconformity (Daly et al.
1992
).
Sequence 3 is referred to by Daly et al. (
1992
) as the
Aruwimi Group, and represents a basin-wide Ordovician-
Devonian transgressive-regressive marine cyclothem. The
Aruwimi Group is characterized by basal transgressive
sandstones, overlain by shales, and capped by progradational
sandstones (Banalia Arkose). The latter have correlatives in
the northern DRC (Bembe Group) and Angola (Bembe For-
mation, Inkisi Group), and are of Ordovician age (
the subject of most previous investigations conducted in the
Congo Basin. They also represent the most debated and
poorly understood sequences across the basin, perhaps only
because they have been subject to more work (Fig.
9.4
).
Giresse (
2005
) in a comprehensive literature review of the
Mesozoic and Cenozoic history of the basin admirably
attempted to synthesize the stratigraphic findings from
many of the key regions across the basin (see Table 1 in
Giresse
2005
). Although considerable research by numerous
authors on the paleontology of the Congo Basin has greatly
improved the age assessments for certain units, uncertainty
remains largely due to the limited nature of fossil-bearing
strata, the great size of the basin and difficulty in making
long distance correlations, and also because some aspects of
the biostratigraphy are still debated and remain somewhat
ambiguous.
The base of the Mesozoic succession is dated as Upper
Triassic, and referred to as the Haute Lueki Group. It is
exposed along the eastern margin of the basin and has been
identified in the Cuvette Centrale by Daly et al. (
1992
)in
seismic sections, but has not been unambiguously identified
in either the Samba or Dekese cores (Cahen et al.
1959
,
1960
). The Haute Lueki Group is separated from overlying
units by a regional hiatus and unconformity (Lepersonne
1977
; Daly et al.
1992
), spanning the Early Jurassic.
Correlatives of the Haute Lueki Group are exposed in the
Cassange Graben in Angola (Atunes et al.
1990
). The Haute
Lueki and Cassange Groups have been correlated with the
Beaufort Group of southern Africa, on the basis of fish
fossils (Atunes et al.
1990
).
The Stanleyville Group comprises a succession of cal-
careous sandstones and shales, some of which are bitu-
minous, that were deposited in a lacustrine basin south of
Kisangani. Paleontological work by De Saint-Seine (
1955
)
and Grekoff (
1957
) and palynological work by Colin (
1994
)
indicates that the Stanleyville Formation is Middle to Late
Jurassic in age. A recent paleoclimate investigation of the
Stanleyville Formation was conducted on samples from the
Samba Core, which suggest quite a hot and arid depositional
environment during this time (Myers et al.
2012
).
The overlying continental Loia, Bokungu and Kwango
groups are of Cretaceous age. Ostracodes (Grekoff
1957
),
fossil fish (Casier
1961
; Lepersonne
1974
) and pollen
(Boulouard and Calandara
1963
) date the Loia Group to
the Neocomian-Aptian (145-112 Ma), the Bokungu Group
to the Albian (112-100 Ma), and the Kwango Group to the
Cenomanian-Turonian (100-89 Ma).
500 Ma:
Frimmel et al.
2006
; Jelsma et al.
2012
). Following this
cyclothem, a long depositional hiatus is recorded across the
basin.
Sequence 4 comprises Upper Carboniferous-Lower
Permian glacial, peri-glacial and interglacial lacustrine
deposits of the Lukuga Group, correlated to the Dwyka-
Ecca interval of the Karoo Supergroup of southern Africa
(Cahen and Lepersonne
1978
). In the Congo Basin, deposits
of this sequence are exposed across the eastern margin, as
well as within the Dekese borehole, where it is
<
450 m thick
(Cahen et al.
1960
). Interestingly, the Lukuga Group was not
identified by Cahen et al. (
1959
) in the Samba Well. A thin
veneer of correlative late Paleozoic tillites outcrops in river
drainages along the southern portion of the basin in Angola
(Lutˆe Series; Rocha-Campos and dos Santos
1981
). In the
Dekese Well, these units have high dip angles (
>
30-90
;
Cahen et al.
1960
), indicating a major episode of contrac-
tional deformation in the basin. The top of this sequence is
characterized by a marked angular unconformity.
The final depositional sequence (Sequence 5) represents a
regionally extensive, generally flat-lying, mostly continental
series of Triassic-Cenozoic clastic units. This sequence
comprises five major units, identified as the Triassic Haute
Lueki Group, the Middle-Late Jurassic Stanleyville Group,
the Lower Cretaceous Loia and Bokungu Groups and capped
by the Kwango Group, which Daly et al. (
1992
) inexplicably
refer to as Tertiary, contrary to most published reports (e.g.,
see Giresse
2005
).
>
9.2.3.2 Mesozoic and Cenozoic Cover Sequences
(Sequence 5)
The Mesozoic and Cenozoic cover sequences are the best
exposed units across the basin in outcrop, and they have been
9.2.3.3 Kalahari and Younger Sediments
Kalahari remnants are preserved on isolated topographic
highs in the southern part of the basin, such as at Mt.
Bunzo in the Kasai region of the DRC (Giresse
2005
) and
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