Geology Reference
In-Depth Information
Table 14.3
Relative chronology, characteristics and synonymy of the landforms of the Ubangian Rise
ELEVATION LANDFORM—WEATHERING (W.)
SYNONYMY
Upper Planation
Surface 1
(
u1u
)
900-1,200 m Planation surface
degraded
Highest reliefs of the Bongo Mounts and Dar Challa Massif
w.
: occurence of bauxites in the Bangali Massif (N8
39
0
-E20
07
0
, Boulvert
1996
)
Tentatively correlated with u1 of the Cameroon Highlands on the basis of:
- A difference of elevation between this surface and the underlying one (u2) and
- The occurrence of bauxites
Lim-Bocaranga
Surface (Boulvert
1996
)
Upper Planation
Surface 2
(
u2, u2d
)
700-1,000 m Pediplain
degraded by incised rivers, then called
degraded Upper Surface 2
(
u2d
)
Bouar-Baboua
Surface (Boulvert
1996
)
Ouadda(
Carnot-Berberati) Plateau both made up of Lower
Cretaceous sandstones southward tilted
w.
: thin weathering mantle—some iron duricrusts (Mainguet
1975
; Boulvert
1996
)
¼
Mouka) and Gadzi (
¼
Lower Planation
Surface
(
l, lp, ld
)
500-800 m
3 types:
-
Nearly flat, weathered duricrusted pediplain (
l
)
most of the Ubangian Rise
-
Pediplain with few duricrusts (
lp
)
south U ´l ´ Plateau - NE Congo
-
Degraded pediplain (
ld
) dissected by incised rivers and younger small pediments
between the gadzi and Ouadda Plateaus
w.
: highly weathered and duricrusted by laterites (Boulvert
1996
; Beauvais and
Mazaltarim
1988
; Beauvais and Roquin
1996
; Beauvais
1999
)—the bauxites
described by Sluys (
1946
) along the Ituri River (Congo) are aluminium-rich kaolinites
of « classical» laterite profile (Vanderstappen and Cornil
1955
)
Central-African
Surface (Boulvert
1996
)
Pediments V - W
(
VW
c
)
Large pedi-valleys/pediplains similar to the Cameroon Highland ones
eastern side of the Gadzi Plateau
50 to 200 km large,
400 km long
Pediments X
(
X
c
)
Pedi-valleys—Congo Cuvette local base level 2 directions: WNW-ESE (dominant)
and NNE-SSW (between the Bianga and Kembe Plateau)
10 km large (20-80 km)
controlled by the lithology: cutting through Neoproterozoic Lindian sediments and
bounded upslope by Archean and Paleoproteoroic crystalline rocks
Pediment Z
(
Z
c
)
Large pedi-valleys pediment Y - uncharacterized—merges with Z
Z
c
is the local basel level of the Central Congo Cuvette
across surface
u1a
) dated at 16.3
0.34 Ma (Marzoli
surface 2 (
u2, u2d
), (3) Lower planation surface (
l, lp, ld
),
(4) Pediments V and W (
VW
c
) (5) Pediments X (
X
c
) and (6)
Pediments Z (
Z
c
). The stepping between those different
landforms is quite low. Planation surfaces can (1) merge
upslope or (2) be bounded by steepest slopes rather than by
scarps.
Along the central CB, the Upper planation surface 2 (
u2
)
merges with the erosional Congolese Surface (
C
), whereas
the Lower planation surfaces (
l, lp, ld
) are located below,
with a marked scarp between these two planation surfaces.
No age constraints are available.
et al.
1999
—
39
Ar-
40
Ar on feldspars).
14.5.2.2 North: Ubangian Rise (Central African
Republic and NW DRC)
This relatively low elevated rise (500-1,000 m) extends
from the Cameroon Highlands to the Blue Mountains (NE
Congo—northwestern limit of the East African Dome, Plate
1). To the west, this rise is the divide between the Congo
(Ubangi) and the Logon´—Chari Rivers feeding the
endoreic Chad Lake drainage, and to the east, the divide
between the Congo and Nile Rivers. The highest relief of the
Ubangi Rise (Plate 1), the Ouadda Plateau and the Bongo
Mounts (1,300 m), are located at the intersection of these
two divides. On both the Chadian and Sudanese areas, the
northern side of the Ubangian Rise corresponds to
piedmonts, the Ouham-Bamingi Piedmont on the Chadian
side and the Nilotic Piedmont on the Sudanese side, with the
Ouangjia-Vakaga Piedmont in between. The area was exten-
sively studied by Boulvert (
1985
,
1996
and numerous
reports available on the IRD website (
http://horizon.docu
Six landforms
are defined (Plate 2, Fig.
14.9
UR, see
description on Table
14.3
), from the oldest to the youngest,
(1) Upper planation surface 1 (
u1u
), (2) Upper planation
14.5.2.3 East: Western Limb of the East African
Dome (E DRC, Uganda, Rwanda, Burundi)
This domain (Plate 1) corresponds to the western flank of the
East African Dome, and mainly the western border of the
western branch of the East African Rift. This domain
extends from the border between the DRC, Uganda and
South Sudan (Blue Mountains) to northeastern Zambia
(Mporokoso Plateau), crossing through Kivu and northeast-
ern Katanga (former Shaba).
Six landforms
are defined (Plate 2, Fig.
14.9
EAD, see
description on Table
14.4
), from the oldest to the youngest, (1)
Upper planation surface 1 (
u1u
), (2) Upper planation surface
2(
u2, u2d
), (3) Lower planation surface (
l, lp, ld
), (4) Pediments
X(
X
c
), (5) Pediments Y (
Y
c
) and (6) incised channels.
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