Geology Reference
In-Depth Information
14.6.3 Middle Eocene-Late Oligocene (?)
(45-23? Ma): Upper Planation Surface 2
300 m of uplift recorded by the pediments of the Lower
planation surface (see above) can probably account for the
uplift occurring at time of Pediments V and W.
This is the first occurrence of Cenozoic uplift in the studied
area, initiated in the north (Cameroon Highlands and
Ubangian Rise) and in the east (western flank of the East
African dome). The amplitude of this Middle Eocene-Late
Oligocene uplift (taking the height of the scarps as a proxy
for the uplift) is greatest along the Cameroon Highlands and
Mitumba Mounts (East African Dome), at a magnitude of
100 to more than 500 m, and lower along the Ubangian Rise
and the Blue Mountains (East African Dome), between 0 to
200 m.
The restoration of the E-W section (Fig.
14.14
) shows
that the CB was uplifted along its northern and southern
margins (incision of Upper planation surface 2). Newly
formed pediments connected to a local base level located
in the western and southern parts of the Central CB. The rest
of the CB was either affected by limited erosion or was by-
passed, creating a long lasting, polygenic surface, known as
the Congolese Surface.
A third period of weathering occurred at the end of this
period. Upper planation surface 2 stripped most of the later-
itic profiles of Upper planation surface 1, leaving a stripped
etchplain. We assumed that these weathering processes were
contemporaneous with those of the
14.7
Late Miocene (11.5-5 Ma): Pediment X
The entire studied region, including the CB was now further
uplifted. Major uplift was located along the Atlantic side,
elevating both the Central African Atlantic Swell and the
eastern CB, as indicated by the absence of Pediments X on
the Congolese side of the Swell. The mean order of uplift is
around 100-300 m with higher values (500 m) along the
Atlantic side of the Cameroon Highlands. During this period
the present-day Ogoou
´
River system crossed the Swell
(probably inherited from an alluvial system coming from
the CB).
The western part of the CB was the local base level of
the pediments of the Ubangian Rise and of the East Afri-
can Dome, probably initiated at time of Pediments V and
W. The base level of these reliefs is not the Atlantic
Ocean.
With reference to the CB local base level, the Ubangian
Rise, the East African Dome and the Kasai-Lunde-Kwango
Plateau were all uplifted between 100-200 m (Ubangian
Rise) and 200-500 m (East African Dome). This uplift is
difficult to quantify for the Kasai-Lunde-Kwango Plateau,
because of the absence of pediments along the incised
valleys. But, based on the Katanga and Kwango pediment
data, the magnitude of uplift was at least 300 to 500 m. This
period records a change in the wavelength of uplift, from
”
(Beauvais et al.
2008
; Beauvais and Chardon
2013
) that
formed during Late Oligocene.
“
Intermediate Surface
14.6.4 Early Miocene (?) (23? to 16? Ma): Lower
Planation Surface
100-1,000 km (Cameroon Highlands, Ubangian Rise) to
>
1,000 km of the CB along the Atlantic coast and along the
western flank of the East African Dome (e.g. regional curvi-
linear bending parallel to the Atlantic margin).
Uplift continued along the northern and western flanks of the
CB and is more homogeneous in term of amplitude (ca.
0-200 m in the north and 100-300 m in the east).
A fourth period of weathering occurred at the end of this
period, best recorded along the Cameroon Highlands and the
Ubangian Rise to Ituri. The Lower planation surface is a
duricrusted pediplain passing upstream to piedmont
pediments (dominant) and pedi-valleys, stripping Upper pla-
nation surfaces 1 and 2.
14.8
Late Miocene-Present-day (5-0 Ma):
Pediments Y and Z
From end of the Miocene onward, uplift mainly affects the
Atlantic side of the Central African Atlantic Swell, with
higher magnitudes along the Cameroon Highlands (ca.
500 m). This increasing uplift of the Cameroon Highland is
recorded by a southward migration of the Ogoou´ Pediment
Y compared to the earlier Pediment X.
In the CB, the uplift was less pronounced. Pediments Y
and Z only deepen the previous pediments X, except in the
southern plateaus and along large pedi-valleys the the
Kwango, where the difference of elevation between X and
Y is high, and rivers contemporaneous with deelpment of Y
pediments, are well incised.
14.6.5 Middle Miocene (?) (16? to 11.5 Ma):
Pediments V and W
This is the first clear topographic evidence of the onset of the
Central African Atlantic Swell, with maximum uplift of
100-200 m along the Gabon-Congo Swell and up to 650 m
along the Kwango large pedi-valley (uplift of the Angolese
Plateau). Unfortunately, Pediments V and W were not
characterized along the East African dome, and the 100 to
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