Cenozoic Landscape Evolution in and Around the Congo Basin: Constraints from Sediments and Planation Surfaces - Geology and Resource Potential of the Congo Basin - page 291

14.6.1 Late Cretaceous-Early Paleocene (72-66
to 66-56 Ma): Top Cretaceous Sediments
Weathering Surface

115-90 Ma giving a sedimentation rate of 75 m/Ma);
Coniacian-Eocene (around 10,000 km 3 between
90-34 Ma—effectively suggesting a period of basin starva-
tion); and Oligocene-Present-day (700,000 km 3

between

34-0 Ma, at a rate of 80 m/Ma).
The sharp increase of the siliciclastic input at the Eocene-
Oligocene boundary confirms the occurrence of a major
denudation event onshore that may be the mechanical ero-
sion of the Upper planation surface 2, enhanced by the
climate change occurring at the Eocene-Oligocene transition
(S ´ ranne 1999 ). Sedimentation rates also increased from
Oligocene to Middle Miocene (Lavier et al. 2001 —
Fig. 14.12 ), followed by decreases in the north and further
increases in the south (related to delta shifting, Anka et al.
2010 ). We relate this increase in two steps (Early and Middle
Miocene) tentatively to the erosion of the Lower planation
surface and of Pediments V and W (for more detail see Linol
et al., Chap. 10 , this Topic).

The crust of the CB was flexured at large wavelengths,
uplifted (Fig. 14.13 —NS section B) and eroded to be uncon-
formably overlapped by Cenozoic sediments. This is consis-
tent with: (1) the absence of sediments younger than Late
Albian in the central part of the CB (e.g. Linol 2013 ; Linol
et al., Chap. 8 , this Topic) while Santonian to Maastrichtian
sediments are preserved along the western CB (Colin 1994 );
and (2) the occurrence of incised valleys and channels pre-
served below the of the Adamawa Plateau

(Humbel 1966 — 14.5.2.1 ) and below the possible Paleogene
(Eocene?) sediments of the Yangambi area (Fig. 14.6 ).
We cannot as yet quantify the amplitude of the Late
Cretaceous topography since it was levelled by erosion
during uppermost Cretaceous-lowermost Cenozoic (?) and
weathered (Top Cretaceous sediments laterites) along the
eastern and across the central part of the CB.

“ old basalts ”

14.6 Model of Landforms Evolution: Relief
Growth and Main Uplifts

14.6.2 Paleocene(?)-Middle Eocene (66-56
to 47 Ma): low subsidence (

Gr ` s “

The evolution of the topography and the characterisation of
the uplifts of the CB and surrounding highs are based on
restored cross-sections (Fig. 14.14 ). The topographic resto-
ration is based on two observations: (1) All pediments and
pediplains that frmed subsequent to the deposition of the
“

polymorphes Fm) and planation

(Upper planation surface 1 and
Congolese Surface)

”

Gr ` s polymorphes Fm, merge together at two base levels,

viz. the Atlantic Ocean and the Likouala and Ubangi
Swamps for the CB and surrounding reliefs; (2) The slope
of pediments increase from the youngest to the oldest in
response to uplift.
The principle of topographic restoration is to start from
the youngest pediments and to assume that successive
pediments have the same profile than the youngest one, i.e.
Pediments Z or Y. In the case of pediments dowslope merg-
ing to the same base level, the height of the scarp is a good
proxy of the vertical displacement. All the restorations
shown in Fig. 14.14 are drawn with reference to the
present-day sea level; eustasy was not taken into account
(see for discussions 14.3.2 ) compare to the order of magni-
tude of the uplifts.
The successive restoration of the pediments or the river
incisions (rivers of the degraded Congolese Surface), all indi-
cate that: the Central Atlantic African Swell, the Ubangian
Rise, the western East African Dome and the Kasai-Lunde-
Kwango and Angolese Plateaus are younger than the Upper
Surface of Paleocene to Early Eocene age. We therefore
propose the following (seven) episodes that shaped the Ceno-
zoic evolution of landscapes in and around the CB:

” Sedimentation occurred across a long wavelength low subsid-

ing flexure covered by a large lake (Yangambi Fm and subsur-
face equivalents), passing eastward and southward ( Gr ` s “

polymorphes Fm) into an aeolian desert (Fig. 14.7 ). This

environments distribution implies that fresh water derived
from the northern very humid equatorial domain (Fig. 14.7 )
to keep a positive hydrological budget for this perennial lake.
A remaining question relates to the timing of the uplift of
the Central African Atlantic Swell. The thickness of the
“

”

Fm is greater along the Bat´k´
Plateaus, east of the present-day Gabon Swell. This suggests
that this formation extended across the Gabon Swell. Resto-
ration of the E-W section (Fig. 14.14 ) shows that the CB
elevation was near the sea level, but without connections to
the ocean as indicated by the absence of marine fossils.
The large lake was first filled during Middle Eocene,
followed by a base level fall that induced a second period of
weathering (Top

Gr`s polymorphes ”

“

Gr ` spolymorphes

”

Fm laterite). Limited
erosion shaped the Congolese Surface ( C )thatpasses
upstream across the basement to Upper Surface 1 (a mantled
etchplain). These lateritic profiles record chemical weathering
at an Africa-scale known as the Bauxitic or African Surfaces
(Burke and Gunnell 2008 ; Beauvais and Chardon 2013 ).