Geology Reference
In-Depth Information
Most of the geomorphological studies since 1935 have
focussed on planation surfaces. The different interpretations
of land-surfaces of the CB and surrounding areas are
summarized in Fig.
14.2
, against recent radiometric dates
of weatherings surfaces in West Africa (Beauvais et al.
2008
). Only few geomorphologic maps are available (e.g.
King
1962
at the scale of Africa; S
´
galen
1967
for
Cameroon; and Boulvert
1996
for the Central African
Republic). In the central CB, the chronology of land surfaces
is based on their relationships with sediments (Veatch
1935
;
Cahen
1954
; Alexandre
2002
). Along the surrounding areas
(Cameroon—S´galen
1967
; Central African Republic—
Boulvert
1985
,
1996
; Katanga—Alexandre-Pyre
1971
;
Alexandre and Alexandre-Peyre
1987
; Alexandre
2002
),
the chronology of land surfaces is based on comparisons
with the sequence of weathering profiles and planation
surfaces of West Africa (Ivory Coast, Burkina Faso, Mali)
studied by the soil geologists and geomorphologists of the
Strasbourg University and of the French overseas scientific
research agency (O.R.S.T.O.M, now I.R.D.—
thin (
10 m) accumulations on the Ubangian Rise (Bigotte
and Bonifas
1968
; Miauton
1980
).
For
subsurface
data, only four deep wells are available in
the CB (see Plate 2 for location): (1) two old ones (Samba
and Dekese) with cores and cuttings, drilled by the
“
Syndicat
pour
l
'
Etude g´ologique
et mini`re de
la Cuvette
congolaise
(Professional association for the geological
and mining Study of the Congolese Cuvette) and described
by Cahen et al. (
1959
,
1960
) and now stored in the Royal
Museum for Central Africa (RMCA) in Tervuren (Belgium)
and (2) two petroleum wells (Mbandaka 1 and Gilson 1)
drilled by ESSO ZAIRE (EXXON). We acquired access to
the cuttings of Dekese and Samba wells. A petrological
study including grain shape analysis using Scanning Elec-
tron Microprobe (exoscopy) was completed on cuttings. The
study of Mbandaka 1 and Gilson 1 was based on well-logs
(gamma-ray, resistivity and sonic) and cutting descriptions
available on the masterlogs (Plate 2).
For
surface
data, published data of outcrops of
”
“
Gr`s
“
Institut de
polymorphes
were studied (grain-size,
grain shape and clay mineralogy analysis: Claeys
1947
;
Beugnies
1950
; Le Mar
´
chal
1966
; de Ploey et al.
1968
;
Alexandre-Pyre
1971
) supplemented by observations during
a short field trip managed by the BRGM (French geological
survey), north of Brazzaville (facies sedimentology). For the
Kisangani outcrops (see Plate 2 for location), we used the
excellent descriptions of de Heinzelin (
1952
).
”
and
“
Sables ocres
”
Recherche pour le D´veloppement
) (Michel
1973
; Grandin
1976
; Tardy and Roquin
1998
; see Chardon et al.
2006
for a
recent review).
This compilation (Fig.
14.2
) shows numerous
inconsistancies, including different ages for the same
surfaces. This is likely due to different methods of age
determinations. (1) Along the coast, surfaces are dated
where they merge with the coastal basins (albeit with a lot
of uncertainties abouth the accuracy of correlations). (2) The
age of the surface is function of their elevation. Another
cause of error is to confuse an exhumed basal sediments
surface with a younger land-surface cross cutting and locally
merging with this old exhumed surface.
Several other investigations were carried out in northwest
Congo (Ituri), with a controversy about the importance of
faulting along the western highs flanking Lake Albert Rift
(Ruhe
1954
,
1956
,
1958
,
1959
; Lepersonne
1956
,
1958a
,
b
;
Moeyersons
1975
; Mbuluyo
1991
), in Lower Congo (van
Kerschaver
1975
,
1983
,
1984a
,
b
; Kaseba et al.
1997
) and in
Gabon-Congo (Congo-Brazzaville or Congo Republic—
Dresch
1946
; Vogt
1962
; Novikoff
1974
; Petit
1990
).
”
14.3.2 Geomorphology: Planation Surfaces
Analysis
This study is mainly focussed on the planation surfaces
defined as nearly flat erosional surfaces cutting across the
underlying heterogenous rocks (Migon
2004a
). The two
characteristic planation surfaces of the CB and the
surrounding reliefs are etchplains and pediplains to
pediments (see definitions in
14.5.1.1
and
14.5.1.2
). Such
planation surfaces can be deformed (tilted and/or domed) by
long to very long wavelength deformations (
100 km and
1,000 km), with possible major basement faults
reactivations. These uplift induced a local base level fall
and the growth of a new planation surface below the previ-
ous one. The consequences of such deformations are forma-
tion of stepped planation surfaces, with the oldest landform
at higher elevation and the youngest one at lower elevation.
The boundaries between such surfaces can be marked by
sharp scarps, or may be gradational.
Other processes that can generate base level fall are
eustasy with the relief connected to the sea (exoreic system)
or climate changes for the endoreic systems through control
by the watershed hydrological budget. The amplitude of the
sea level variations since Eocene times continues to be
14.3 Methodology
14.3.1 Sedimentary Record
Most of the Cenozoic sediments are preserved in the central
part of the CB and its southern extension into the Kalahari
Basin. The only Cenozoic sediments preserved in the
surrounding areas (and not reported on here) are
paleovalleys dammed by lavas flows (e.g. on the Adamawa
Plateau, Anloua area, Letterman
1984
; Oustri
`
re
1984
)or
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