The Present Day Drainage Patterns of the Congo River System and their Neogene Evolution - Geology and Resource Potential of the Congo Basin - page 319

15.3.8.2 Lake Tanganyika/East African Rift System
(C18 in Fig. 15.3 )
The eastern margin of the CB is dominated by a contorted
drainage pattern. The area of contorted pattern corresponds
to the Eastern Highlands and thus forms the headwater zone
of many of the westerly flowing rivers. The streams and
rivers of this zone lack a consistent drainage pattern, with
rivers flowing in several different directions and near-
straight channelled rivers flow adjacent to rivers. This lack
of consistent pattern indicates a dominance of fault and
lithology controls in the region. The fact that much of this
drainage system corresponds to the Western Branch of the
EARS, it is likely that much of the drainage has been
influenced by phases of tectonic activity (Fig. 15.7 ). Indeed
several phases of tectonic activity have been identified,
resulting in the overprinting of faults by succeeding faults
and the formation of valleys and regional blocks (Cahen and
Snelling 1966 ; Ring 2008 ; Bauer et al. 2010 ). In the
Rwenzori Mountains (Fig. 15.2 ), the Precambrian basement
rocks widely resist fluvial erosion, causing the rivers to
predominantly exploit predetermined structures (Bauer
et al. 2012 ). It is thus likely that the contorted drainage
pattern is strongly linked to the ongoing tectonic activity
associated with development of the Western Branch. For
example, the uplift of the Ufipa Plateau severed the drainage
linkages to the Tanzanian Plateau in the east, notably those
of the greater Rufiji drainage system (Fig. 15.1A ). This uplift
appears to be Pliocene in age (e.g. Cotterill and de Wit 2011 ;
Goodier et al. 2011 ).
Therefore a large portion of Lake Tanganyika

15.4 Timing of Emplacement of the Modern

River Systems of the Congo Basin

15.4.1 Cenozoic Events

The marine sedimentary evidence lead to the suggestion of
several scenarios of CB evolution. The three models pro-
posed to account for this marine sedimentary record are :
1) at the end of the Cretaceous there was a progressive
change of the depocentre from the Ogou ´ and Cuanza
(Kwanza) Rivers to the Congo as a result of capture of
the endorehic Congo Basin (Babonneau et al. 2002 ; Anka
et al. 2010 ). Today these southern and northern systems
exhibit multiple incised valleys and gorges along the
coastal areas, often occupied by under fit river channels.
2) There was a net southward migration of the outlet of the
CRS from the Cretaceous to the Cenozoic (Karner and
Driscoll 1999 ; Nibbelink and Budihardjo 2002 ). During
the late Cretaceous, the entire CB was drained by an outlet
at or near the present day Ogou´ River (northern Gabon)
(Karner and Driscoll 1999 ; Nibbelink and Budihardjo
2002 ). Through the mid-Cenozoic the outlet migrated
south, forming a connection near/at the present day
Kouilou River, reachingthe present day CR outlet in the
Oligocene (Karner and Driscoll 1999 ; Nibbelink and
Budihardjo 2002 ). Karner and Driscoll ( 1999 ) suggest
that feedbacks between sediment loading and flexural
uplift of the hinterland mayhaveplayedaroleinthe
migration of the outlet, although the mechanism of migra-
tion is unclear. This migration may be evidenced in the
misfit of the present Ogou´ and Kouilou Rivers (see
Fig. 15.1a, c , the Kouilou River is near Point-Noire
Fig. 15.1c ) that occupy abandoned channels and their
valleys, relic drainages and the large estuary near
LibervilleandthelargesizeoftheLateCretaceous—
early Cenozoic delta along the Gabon coast relative to
present-day coastal river systems (Karner and Driscoll
1999 ). This tectonic activity may have lead to the capture
of the interior drainage of the Congo Basin by a coastal
valley (Cahen 1954 ).The lower CR crosses an area of
lower modelled flexural uplift (Anka et al. 2010 )
suggesting that the lower Congo River comprises of
inherited drainage. This antecedent lower CR is evidenced
by the deeply incised gorge and waterfalls downstream of
Kinshasa (Anka et al. 2010 ).

s drainage
is geological young and fault controlled, with the contorted
pattern being related to a combination of multiple phases of
uplift, subsidence, tectonic activity and exposure of Precam-
brian rock. This is distinctly seen in the lower Malagarasi
(Fig. 15.2 ) and Lukuga systems that drain into and out of
Lake Tanganyika respectively. The Malagarasi systems
changes from a dendritic pattern to a contorted pattern in
the zone where the river begins to flow off craton and
becomes influence by rifting. Previous to the formation of
the Western branch there was greater linkage between the
drainage of Eastern Africa and the CB. This palaeo-channel
is represented in the topological unity of modern Malagarasi
River, draining into Lake Tanganyika. These preserved
landforms attest to the profound impact of the EARS, in
particular its Western branch, on the evolution of the eastern
rivers of the CRS (Fig. 15.3 ).

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