Geology Reference
In-Depth Information
Table 14.7
Relative chronology and characteristics of the landforms of the Central African Atlantic Swell-South Cameroon, Gabon, Congo
Swelly
RELATIONSHIPS WITH THE
OLDEST FORM
ELEVATION LANDFORM-WEATHERING (W.)
Lower Planation
Surface and
older?
(
l
)
600-900 m
Relict weathered planation surface
South Cameroon Plateau (600-700 m), Central Equatorial Guinea Plateau, summit of the Chaillu Massif
(800-900 m)
w.
: Laterites with iron duricrusts (Cameroon: Yongue-Fouateu et al.
2006
; Gabon: Novikoff
1974
)—some
bauxites (Makongonio area): relicts of Upper planation surfaces?
Pediments V
(
V
a
) and
Pediments W
(
W
a
)
Large pedi-valleys to pediplain 100 km-width (
South
Cameroon Plateau
)
-
Piedmont pediments (
Chaillu Massif
)
w.
: no published descriptions
Difference of elevation (
l, V
a
0
,W
a
): low
(scarps
100 m) - can merge in the same
planation surface
Pediments X
(
X
a
)
pedi-valleys—well-organized network - located on the Atlantic
flank of the swell (except Oogou
´
) narrower than Pediments
V-W (
10 km-width)
-South: Chaillu Massif—Mayombe
pedi-valleys parallel to
the coast (20-60 km-width) controlled by the NW-SE
Neoproterozoic structures
-Middle: present-day Ogoou ´ River
large pediplain cutiing-
across di_erent lithologies of the swell
-North: Equatorial Guinea, South Cameroon
pedi-valleys
oriented along a NE-SW trend (Neoproterozoic Ubangides
structures)
w.
: no published descriptions
- Scarps or steepest slopes mean height:
100 m-up to 300 m (Ngouni
´
Plain and
Niari Valley)
-Scarps
100 m along the North Gabon/
Ivindo Plateau
-Scarps
100 m
Pediments Y
(
Y
a
)
and
Pediments Z
(
Z
a
)
Pedi-valleys superimposed on Pediments X - enhance the
previous network narrower than Pediments X
- South: Ngouni ´ Plain/Niari Valley and Nyanga Plain
controlled by the dolomitic limestones of the
Neoproterozoic
Gp
-Middle: Boou ´ and Franceville low plateaus and hills
only pediment Y
a
- shifted to the south of X
a
“
Schisto-calcaires
”
Three main rivers: Congo, Ogouo
´
and Sanaga
Congo incision: Ubangi Flat to coastal plain
Ogoou
´
incision: along Pediment Y
a
Sanaga incision: Lower p. surf. (
l
) to the coastal plain
Incised rivers
Incision: 50-150 m
14.5.2.7 Regional Chronology and Dating
of Landforms
The relative chronology of the landforms of the CB and
surrounding highs (Fig.
14.11
) is summarized as follows
(from the oldest to the youngest).
Upper planation surfaces 1
(
u1
) are the oldest landforms
preserved in the studied area. They are identified: (1) in
the Cameroon Highlands by successive stepped planation
surfaces (
u1a
,
u1b
,
u1p
); (2) on top of the Ubangian Rise
(Bongo Mounts); (3) along the East African dome, from
the Blue Mountains (flanking the western margin of Lake
Albert) to the Marungu Plateau (Tanganyika Lake) and
(4) on the Angola Plateau. The absence of spatial conti-
nuity between these relict forms, hampers demonstration
of their synchronicity. In Katanga, these surfaces may be
the equivalent of the lowermost
south and the South Cameroon-North Gabon Plateau, in the
north.
Six landforms are defined (Plate 2, Fig.
14.9
AS, see
description on Table
14.7
), from the oldest to the youngest,
(1) Lower planation surface (
l
) and older?, (2) Pediment V
(
V
a
) and Pediment W (
W
a
), (3) Pediment X (
X
a
), (4) Pedi-
ment Y (
Y
a
) and Pediment Z (
Z
a
) and (5) incised rivers.
The Atlantic slope of the swell comprises planation
surfaces that merge with marine sediments of the Atlantic
passive margin. An Upper and Lower planation surfaces
undifferentiated (
ulu
) is defined along the Atlantic Ocean
(mainly Mayombe and Sierra de Cussianzumbo), with very
low stepping amplitudes and sometimes a merge of the
Upper and Lower planation surfaces. In the Lower Congo
area, Kaseba et al. (
1997
) defined three planation surfaces
with a difference of elevation of 150 m. The two younger
ones are bauxitized and the older one is stripped from
weathered materials.
No dates are available from any of these surfaces.
iron duricrust (iron
Fm.
Upper planation surface 2
(
u2
) is more widely
distributed, with numerous relicts, large enough to be
duricrust 1) located above the
“
Gr`s Polymorphes
”
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