The Present Day Drainage Patterns of the Congo River System and their Neogene Evolution - Geology and Resource Potential of the Congo Basin - page 324

margin is likely providing a large sediment supply that is
transported westward by the eroding eastern rivers, and
along with sediments transported by the northerly flowing
rivers of the south CRS, accumulates in the central basin
(Fig. 15.6a, b ), where it undergoes reworking before being
deposited offshore. This accumulation of poorly
consolidated sediments has allowed the central drainages
( Cuvette central and Western Congolain wetlands) to
accommodate fluvial changes through horizontal re-working
of the sediments. Thus the rivers of the central basin are
more likely to have accommodated rejuvenation and adjust-
ment than those on the Precambrian basement, as shown by
their sub-dendritic to dendritic pattern. This development of
a highly dynamic river network in the region is probably
younger than the drainages of the eastern basin; serving as
the regional interface between changes in the eastern and
western drainages and changes in the CRS base level at the
CR mouth. Thus the eastern and central drainages of the CB
are likely to be Pliocene in age, with elements being
inherited from Middle to Late Miocene.
The peripheral drainage patterns to the south, west and
north of the central basin, are dominantly structurally con-
trolled (the trellis drainages of southern Bat´k´, Sembe-
Ouseso, Kwango Valley and mid-Congo; Table 15.2 and
Fig. 15.3 ) and along with parallel drainage in the south
(Lunagwe, Table 15.2 and Fig. 15.3 ) pre-date the central
basin drainage (i.e. mid-Cenozoic to Mid-Miocene). This is
not to say there have been no changes in these drainages
systems but rather that the dominant drainage was already
established by the Mid-Miocene.
Biotic evolutionary events provide important lines of
evidence to constrain timing of major changes in location
of river channels of the Congo drainage net. These include
timing of formation of the present day CR and expansion and
development of the drainage network. Comparison of river
topology at regional scales reveals strong controls of the
local geology, as indicated by the juxtaposed drainage
patterns.
The south-to-north flowing rivers of the central basin
region are likely to precede the east-to-west flowing rivers
of the region; with the northerly flowing rivers being
emplaced ca . 2 Ma. This is evidenced by genetic differences
of the guenons and genetic variance of bonobos to the east
and west of the Lomami River (Hart et al. 2012 ; Kawamoto
et al. 2013 ; Table 15.3 and Fig. 15.8a, b ). None of the east-
to-west flowing rivers (apart from the CR) appear to have
been a barrier to bonbos (Kawamoto et al. 2013 ), these rivers
are therefore likely younger and/or of have not been contin-
uously flowing for a sufficient time to form a true barrier.
Similarly, it appears that only the north-to-south lower
Oubangui River is a barrier dividing central and eastern
chimpanzees, with easern chimpanzees moving across the
east-to-west flowing rivers on the northern central basin

15.5 The Evolution of the Congo Basin

Drainage

Owing to the sensitivity of rivers to vertical tectonic
displacements, and their ability to adjust to surface warping
through changes in their channel, and subsequently their
drainage pattern (Howard 1967 ; Holbrook and Schumm
1999 ), the CB rivers patterns provide insight into the evolu-
tion of the broader landsurface. Although the time response
to change remains difficult to estimate, as it depends on rock
strength, stream characteristics and the magnitude of tec-
tonic activity (Holbrook and Schumm 1999 ; Leturmy et al.
2003; Lucazeau et al. 2003 ).
Major changes in the fluvial geomorphology of the CB
occurred during the late Cenozoic (Table 15.3 ). Throughout
the Neogene, the major rivers of the CRS began to develop
their modern day forms. This change was driven by several
factors, dominantly structural controls, tectonics and related
autogenic fluvial processes. However this ongoing develop-
ment was not uniform, as can be seen from 18 of the present
day drainage patterns, indicating that the development of the
CRS has been multi-phase and has several dominant
controls.
The rivers of the eastern CB have been highly influenced
by the geological young, and ongoing tectonic activity of the
EARS, in particular its Western Branch (Fig. 15.7 ). While the
geodynamic effects of the Western Branch have a limited
spatial extent in the north-east of the CB, it is extensive in
the south-east being and has resulted in the present contorted,
trellis and modified rectangular-angulate and sub-dendritic
drainages (Table 15.2 ;Figs. 15.3 and 15.7 ). These drainages
are probably the second youngest of the CB drainages, having
undergone significant re-organisation since the development
of the EARS. The affects of the EARS on drainage in the
region likely began in the Late Miocene with the formation of
horst and graben structures, with increasing tectonic activity,
including uplift, until the Pliocene-Pleistocene (Ring 2008 ;
Bauer et al. 2010 , 2012 ;Decr´eetal. 2010 ;Rolleretal.
2010 ). This multiple tectonic phases resulted in the contorted
pattern of over the majority of the region (Figs. 15.3 and
15.7 ). This is evident in Fig. 15.6c , where vertical incision is
dominant and the river channels in the region have eroded to
similar elevations.
According to Pinet and Souriau ( 1988 ) there are two
phases that characterise regional uplift: the initial 2.5 Myr
phase involving constant denudation focused on the uplifted
region, resulting in high sediment production and infilling of
basin areas. The second phase occurs once the tectonic
activity has ceased, with a 25 Myr period of weaker erosion
occurring across a greater spatial extent leading to sediment
movement throughout the greater basin (Pinet and Souriau
1988 ). Therefore, in the CB, the uplifting eastern basin