Geology Reference
In-Depth Information
14.5.1.4 The Largest Planation Surface of the CB:
The Congolese Surface
The CB and the Katanga, Kamina, Kasai, Lunde, Kwango,
Bat
´
k
´
Plateaus (Plate 1) are characterized by flat planation
surfaces (Cahen and Lepersonne
1948
; Cahen
1954
;
Lepersonne
1949
,
1956
,
1978
). In the Central CB, the plana-
tion surface is purely erosional, while across the surrounding
plateaus, it corresponds to a remnant of the low erosional
weathered surface across the top of the
(1) Upper planation surfaces 1 (
u1u
), subdivided along the
Western Cameroon Highlands into three surfaces, surface 1a
(
u1a
), surface 1b (
u1b
) and surface 1p (
u1p
), (2) Upper
planation surface 2 (
u2
), (3) Lower planation surface (
l
),
(4) Pediments V and W (
V
a
,V
c
,W
a
,W
c
) (5) Pediments X,
Y(
X
a
,Y
a
) and (6) Pediments Z (
Z
a
). Along the Ngaound´r´
Plateau, planation surfaces
u1a
,
u1b
and
u1p
merge into a
complex composite relief with numerous lava flows, filling
old landforms (Humbel
1966
). This composite irregular
surface was mapped as undifferentiated Upper planation
surface 1 (
u1u
). Pediments were labelled according to their
base level, e.g. V
a
for the Atlantic Ocean one and V
c
for the
CB local base level.
The chronological relationships between the magmatic
rocks of the Cameroon Volcanic Line and the different
landforms are summarized in Fig.
14.10
. The Cameroon
Volcanic Line (D´ruelle et al.
1991
,
2007
;Njonfangetal.
2011
)
Gr
`
s polymorphes
”
Fm. Mapping of these planation surfaces (Plate 2) indicates
that they represent the same planation surface, the largest of
which, here called Congolese Surface to avoid confusions
with the previous surface names of King, Cahen or
Lepersonne. The diachronic
“
Fm are surficial
deposits, covering this surface during Neogene times.
In the Central Cuvette, incised rivers connected to the
local base level of the Likouala and Ubangi Swamps, dis-
sected and degraded this Congolese Surface.
“
Sables ocres
”
granites,
syenites, gabbros) and volcanic rocks (basalts, trachytes
and rhyolites). Some magmatic rocks are highly weath-
ered (first
is composed of plutonic (
“
younger
”
14.5.1.5 The Pediment System: An Assembly
of the Previous Landforms
At the scale of
“
old
”
basalts) or duricrusted by bauxites
10
3
to 10
4
km
2
, pediplains, pediments and
incised valleys form a unique geomorphic system fitting to
the same regional base level, here called
pediment system
(Fig.
14.8
), comprising from downstream to upstream:
1. a pediplain,
2. piedmont pediments or numerous pedi-valleys, both
upstream bounded by scarps and
3. a network of incised rivers connected to the flattest part of
the downslope pediments that constitutes a local base
level for these rivers.
This erosional river network degrades, or dissects, the
oldest upstream stepped pediments, making them sometimes
difficult to differentiate. Most of the incised valleys of the
studied area, mainly located along the Cameroon Highlands,
the Ubangian Rise and the western flank of the East African
dome, are upstream parts of such pediment systems.
(trachytes).
The magmatic rocks provide a maximum age for the
landforms that erode them. In contrast, basalt flows or aerial
trachytic and rhyolithic domes covering planation surfaces
provide a minimum age for these landforms. Only Upper
planation surfaces 1 and Pediments V
a,
X
a
and Z-Y can be
dated in this manner using the age of magmatic rocks (see
age arguments on Table
14.2
). Upper planation surfaces 1
(
u1
) were shaped between 67
7 Ma and 45.5
1.1 Ma
and probably between 67
1.2 Ma,
according to the confidence granted to the datings of Moundi
et al. (
2007
). Pediment V
a
is younger than 46.7
7 Ma and 51,8
1.1 Ma.
Pediment Y
a
is younger than 9.9
0.4 Ma and possibly as
0.2 Ma (whole rock
39
K-
40
Ar dates). Pedi-
ment Z-Y is younger than 9.3 Ma.
The first volcanic rocks of the Cameroon Volcanic
Lines—the widespread, highly weathered
young as 5.6
basalts,
extending from the southern Bamil´k´ Plateau (Mount
Bangou, Fosso et al.
2005
) to the Adamawa Plateau
(Lasserre
1961
)—sealed pre-existing landforms. On the
Adamawa Plateau, west of Ngaoud
´
r
´
(Humbel
1966
),
basalt flows filled flat bottom large valleys (5-10 km in
width, 100 to 400 m deep—possible pediments).
The South Cameroon Plateau has acted as a major water
divide since the onset of incision of pediments V. Unlike
pediments V and W, pediments X and Y are found only
along the Atlantic side of the South Cameroon Plateau
and north of the Cameroon Highlands, as a relatively
“
narrow
”
band (100-200 km in width).
One of the famous occurence of bauxites (Fongo-
Tongo—Bamil´k´ Plateau) can be dated to about Middle
Miocene, because the bauxites rest on trachytes (flowing
“
old
”
14.5.2 Relative Chronology—Relationships
Between Landforms, Sediments and
Magmatism (Plate 2)
The following analysis is based on the CB landforms map
(Plate 2) and several geomorphologic cross-sections
(Fig.
14.9
). All the geographical names are provided on
Plate 1.
14.5.2.1 Northwest: Cameroon Highlands
(Cameroon Volcanic Line—E Nigeria,
Cameroon, W Central African Republic)
Eight landforms
are defined (Plate 2, Fig.
14.9
CH, see
description on Table
14.2
), from the oldest to the youngest,
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