Formation and Collapse of the Kalahari Duricrust [‘African Surface’] Across the Congo Basin, with Implications for Changes in Rates of Cenozoic Off-Shore Sedimentation - Geology and Resource Potential of the Congo Basin - page 197

Fig. 10.10 Lupemban stone tools typically made out of silcrete rock fragments ( ' Polymorph Sandstones ' ), found at artisanal diggings within the

Kwango river terrace deposits

erosional gullies and ravines, indicating fragmentation and
erosion of an overlying unit of Kalahari duricrusts, like the
'

Old seismic refraction surveys (Evrard 1960 ) mapped the
sediment thicknesses at 111 stations across the entire central
CB (Fig. 10.11a ). On this refraction data the Kalahari
sequence is imaged as a thin distinctive upper unit with a
low velocity range of ca. 1,800-2,200 m/s (Linol 2013 ; see
also Chap. 7 , this Topic). The reconstructed surface elevation
of this uppermost main reflector shows the thickest accumu-
lations (ca. 180-200 m) to be located beneath the Middle
Congo River and Lake Ndombe, in the western part of the
central CB (Fig. 10.2 ). It is bounded to the south by an E-W
unconformity that coincides at surface with the Lukenie
River (Fig. 10.11a ).
In the four deep boreholes drilled in the center of the CB
(Fig. 10.11b ), this Kalahari succession corresponds to
unconsolidated sandstones with pebbles, mainly of quartz,
flint (e.g. silcretes) and sandstones, and varicolored clay-
stones (Cahen et al. 1959 , 1960 ; Esso-Zaire 1981a , b ). Its
thickness ranges from 37 m at Dekese to 242 m thick in the
Gilson-1 well. In the Samba section, only the uppermost
80-110 m ( ' Couche ' [Bed] 1 of Cahen et al. 1959 )was
originally attributed to the Kalahari Group. However, new
logging of the cores (Linol 2013 ) has identified a marked
erosional surface etched across red mudstones at a depth of
-192 m (Fig. 10.11c ), which more likely corresponds to the
base of the Kalahari Group, consistent with the seismic data
and the depth of the first Cretaceous fossils found in this
section, at -195 m (Cahen et al. 1959 ). Biostratigraphically,
the Kalahari Group is dated from the Eocene to the Oligo-
Miocene in the lower parts (Units G1 and M1) of the

.
Lower down, along the large meandering Kwango River,
river terraces comprise basal conglomerates (1 to 5 m thick)
supporting blocks and boulders of silcrete and calcrete,
ranging between 1 to 10 m in length (Fig. 10.9c ). This type
of deposit also appears to have originated from the fragmen-
tation of

Polymorph Sandstones '

duricrusts and re-
deposited by groundwater along the river terraces (e.g. the
'

the Kalahari ' polymorph '

of Goudie 1983 ). These giant boulders of
silcrete and calcrete form good trap for alluvial diamonds
(Chap. 16 , this Topic), and also contain some human arte-
facts (e.g. Fig. 10.10 ). The latter are stone tools mainly with
elongated bifacial points constructed out of

detrital model '

' Polymorph

'

Sandstones , and characteristic of the Lupemban industry

in central Africa (e.g. Clark and Brown 2001 ), which is as
old as middle Pleistocene in age (300 Ka). These indicate
that deposition of river sands (and underlying calcrete
boulders?) onto the Kwango terraces is a relatively recent
event.

10.4.2 The Kalahari Group Across the Central
Congo Basin

Based on seismic, litho- and bio-stratigraphy, the Kalahari
Group has also been identified across the CB (Fig. 10.11 ).