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2008
; Beauvais and Chardon
2013
). Based on
39
Ar-
40
Ar
cryptomelane ages of weathering profiles (Fig.
14.12
), the
sequence of successive landforms there is as follows: (1) the
Bauxitic Surface (59-45 Ma), (2) the Intermediate Surface
(29-24 Ma), (3) the High Glacis (18-11 Ma), (4) the Middle
Glacis (7-6 Ma) and (5) The Low Glacis (3 Ma).
Correlations were tested in two areas, viz. CAR and
Katanga. In the CAR (Ubangian Rise, Table
14.3
), Tardy
and Roquin (
1998
), following the interpretations of Boulvert
(
1985
,
1996
), suggest that (1) the Lima Bocaranga Surface
(here equivalent to the Upper planation surface 1) could be
equated to the Bauxitic Surface, (2) the Bouar-Baboua Sur-
face (here equivalent to the Upper planation surface 2) the
Intermediate Surface, (2) the Centrafrican Surface (mainly
equivalent to the Lower planation surface) the High Glacis
and (4) the Piedmonts the Middle Glacis. In Katanga (fol-
lowing Alexandre
2002
), the Iron duricrust 1 (possible
equivalent to the Upper Surface 1) corresponds to the
Fantofa Level (
could be time equivalent of V and W pediments. As stated
above, because of the geomorphological isolation of the
Angola Plateau, the equivalence of the Kwango—Cuenza
V
kc
and W
kc
pediment with the other V and W pediments
is uncertain.
Pediments X (X
a
X
c
X
kc
), Y (Y
a
Y
c
Y
kc
) and Z (Z
a
Z
c
)
enhance the trend initiated at time of pediments V and
W. The base levels are well differentiated (South Atlantic
Ocean, CB, Chad-Sudan) and the pediments are narrower
(mainly pedi-valleys, few true pediplains). The labelling
is the same than for pediments V and W. No X or Y
pediments were identified along the eastern flank of the
Central African Atlantic Swell. Again, the equivalence of
the Kwango—Cuenza
X
kc
and
Y
kc
pediment with the
other X and Y pediments is uncertain.
The only
dates
available for these landforms are from the
oldest (
u1
,
C
) and the youngest ones (X, Y). Most of their
ages are based on their geometrical relationships with volca-
nic rocks dated by K-Ar on whole rock (most of the dates) or
Ar-Ar on minerals. Dating of the same sample by the two
techniques provides an age difference between 1 to 4 Ma.
Upper planation surfaces 1 (u1)
are dated (
14.5.2.1
)
between
Bauxitic surface), Iron duricrust 2 (possible
equivalent to the Upper Surface 2) to the Intermediate Sur-
face, Iron duricrust 3 (Pediment X) to the High Glacis and
Iron duricrust 4 (Pediment Y) to the Middle Glacis.
Summarizing the correlations proposed by Tardy, Roquin
and Alexandre, we can deduce than (1) the Bauxitic Surface
weathered until 45 Ma corresponds to the Upper planation
surface 1 (Katanga), (2) the Intermediate Surface formed
between 45 and 24 Ma corresponds to the Upper planation
surface 2 (both Katanga and Ubangian Rise), (3) the High
Glacis eroded between 24 and 11 Ma is an equivalent of the
Lower planation surface (Ubangian Rise) and Pediment X
(Katanga).
Our results are consistent with the age of Upper planation
surface 1 (here dated between 67
¼
1.1 Ma, i.e.
Maastrichtian-Danian and Middle Lutetian, with an
upper
67
7 Ma
and
45.5
range that could be at 51.8
1.2 Ma (see
14.5.2.1
for discussions).
The
Congolese Surface
(
C
) is younger than the
“
Gr`s
polymorphes
”
Fm, dated as Paleogene, and probably
Eocene.
Pediments X (X
a
X
c
X
kc
)
and
Y(Y
a
Y
c
Y
kc
)
have different
(apparently contradictory) ages.
- In the Cameroon Highlands (Rumpi Hills,
14.5.2.1
)—
Atlantic base level—
Y
as
and probably
X
as
are younger
than 9.9
1.1),
but we disagree with the age of Pediment X, which must be
younger (Late Miocene). The 45-29 Ma (Upper Eocene-
Early Oligocene) age for the mechanical erosion of the
Upper planation surface 2 (if equivalent to the Intermediate
Surface) and the 24-18 Ma (Late Miocene) age for the
Lower planation surface (if equivalent to the high glacis)
have to be tested by other geological means.
One test is to evaluate the balance between the products
of the erosion of the planation surfaces against the volume
and sedimentation rate of the siliciclastic sediments depos-
ited along the Atlantic passive margin. These data are avail-
able for the Congo Delta at low resolution from the
Cretaceous (Anka et al.
2010
) and at a higher resolution
from the uppermost Eocene (Lavier et al.
2001
, Fig.
14.12
).
Both are based on regional seismic lines provided (with ages
constraints) by TOTAL (Zaiango Project). Anka et al.
(
2010
) characterized three periods of silicicliclastic sedi-
ment
7 Ma and 45.5
0.2 Ma.
- Along the East African Dome (Mitumba Mounts,
14.5.2.3
)—Congo base level—the upstream incised
rivers of the pediment system X
c
are younger than
5.9 Ma.
- In Katanga (Lubumbashi-Kolwezi,
14.5.2.4
), the
upstream incised rivers of the pediment system X
c
may be older than 10.3 Ma and the ones of
Y
c
(and
may be
Z
c
) between 10.3 and 3.66 Ma.
The apparent differences in age may be due to either a
different relative base level or (and) the delayed time
response of the upstream incised rivers of the pediment
system to the local base level constituted by the pedi-valleys.
For these reasons, we suggest a large time range for these
pediments, from Late Miocene to Present-Day.
Indirect dating relies on apparent similarities between
Congolese landforms and weathering sequences, and those
dated in West Africa (Chardon et al.
2006
; Beauvais et al.
0.4 Ma, and may be as young as 5.6
supply: Albian-Turonian (200,000 km
3
between
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