Diamond-Bearing Gravels Along the Lower Kwango River DRC - Geology and Resource Potential of the Congo Basin - page 334

Fig. 16.4 Simplified stratigraphic section

Planar and cross-bedded conglomerates, with red siltstones
and dropstones, were mapped from the base of the Francois-
Joseph Falls up to the Massemba rapids just upstream of the
falls. Linol ( 2012 ) has interpreted these sediments as being
part of the Permo-Carboniferous Lut˜e Series.
The basement gneisses and Proterozoic sediments are
overlain by Mesozoic (Upper Jurassic Lualaba Series, and
Cretaceous Kwango Group) and Cenozoic sediments as
illustrated by Fig. 16.4 .
The Lower Cretaceous sediments are represented by the
Loia and Bokungu Groups (lacustrine), which pass into
Middle Cretaceous Kwango Group sediments (fluvial/
deltaic) and Upper Cretaceous Kamina Series cross-bedded
sandstones (Linol 2012 ). In Angola these units are contem-
poraneous with the intrusion of kimberlites which have been
dated between 146 and 111 Ma (Eley et al. 2008 ). Hence
Kwango Group (Calonda in Angola) sediments are locally
diamond bearing. Mineralised kimberlites, such as those for
instance at Alto Cuilo where more than 73 kimberlites have
been identified with grades up to 23 cpht (Pettit 2009 ) and
Cucumbi in the Luangue cluster (Robles-Crus et al. 2009 )in
the upper Cuango basin, provide diamonds into the system
(Fig. 16.1 ).
The basal unit of the Kwango Group consists of irregular
lenses of conglomerate with abundant agates and heavy
minerals. These conglomerates have, in places like Tshikapa
and along the upper Cuango and Lui Rivers in Angola, been
mined for diamonds, but are absent along the ' international '
Kwango.
The age of the Kwango Group is believed to be
Cenomanian to Turonian for the lower part and Campanian

to Maastrichian for the upper part (Gobbo-Rodrigues et al.
2003 ). This provides a maximum age for the Late Creta-
ceous Kalamba planation surface that is marked by silcretes
and, in turn, by younger Kalahari Group sediments (Cahen
and Lepersonne 1952 ). The latter forms low rolling hills
which terminate in a plateau that lies about 150 m above
the present river.
The Kwango Group sediments that occur downstream
from the Francois-Joseph Falls are composed of aeolian
sandstones (Linol 2012 ), and comprises a lower unit of
cross-stratified white, coarse-grained sandstones containing
ventifacts referred to as the Kiongo Formation (Linol 2012 ),
and an upper fine- to medium-grained red sandstones with
large planar cross-beds (Fig. 16.4 ) (Linol 2012 ). Linol
( 2012 ) has demonstrated a southerly to south-westerly
paleao-wind direction for these Kwango Group sediments.
Silcretes or ' gr`s polymorphes ' , which occupy a large
section of the southern DRC and Angola, and which contain
freshwater gastropod faunas (De Ploey et al. 1968 ), and
sands of the Kalahari Group overly the Kwango Group.
Clasts eroded from these silcretes play a significant role in
trapping diamonds within the river terraces and recent allu-
vium that have essentially a Kwango Group sandstone
footwall.
Sands of the Kalahari Group, or the serie des sables ocre

'
(ochre sandstones), are homogeneous, reddish to yellowish
in colour and between 100 and 300 m thick. Originally
thought to have an aeolian origin, De Ploey et al. ( 1968 )
suggest that the sands could have been deposited in a fluvial
environment, with slow currents, low gradient and constant
arid climate and tectonic stability. The grains do indicate

'