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Fig.
14.3
) except near the contact with the overlying unit,
where kaolinite occurs along horizontal fractures (level 2/
29, Cahen et al.
1959
). These features would be typical of
the base of a laterite profile (fissured level). That suggests
a truncation of most of the weathering profile before the
deposition of the above coarse-grained unit.
A
coarse-grained sandstones unit
(85 m), composed of
subangular grains (Fig.
14.5
.3 to 5.5) with local evidences
of dissolution (Fig.
14.5
.3). These grains are very irregu-
lar and only the crests and angles are blunted, indicating
(limited?) fluvial transport. Two thin conglomeratic
layers (0.5-1 cm angular to subangular pebbles of quartz,
cherts) mark the top of this unit.
A
sandy claystones unit
(105 m) comprising silty
claystones with numerous coarse-grained sands similar
to those of the underlying coarse-grained unit but with
more evidences of dissolution at the surface of the grain
(Fig.
14.5
.2). The claystones are mainly made up of
smectites and illites, kaolinite is only dominant toward
the top (Fig.
14.3
). These silty clays (by comparison with
Mbandaka 1) could be of lacustrine origin (see discussion
below), and were later weathered as suggested by the
kaolinite increase toward the top.
No fossils were found in any of the cuttings.
The
Dekese
borehole (35 m of Cenozoic deposits,
Fig.
14.3
) consists of three units:
A
basal truncated weathering profile
, comprising faded
claystones along cracks (level B/3, Cahen et al.
1960
).
This is interpreted as a truncation of the profile before the
deposition of the above fine-grained unit.
A
fine-grained sandstones unit
(
The
Gilson 1
well (225 m, Fig.
14.3
) is made up of clayey
silstones with very fine to fine-grained sandstones (10 m)
overlaid by silty claystones similar to those of Mbandaka
1 (60 m) and then 10 to 30 m thick alternations of silty
claystones and sandstones (coarsening upward). The
sandstones are very fine to coarse-grained. Near the top,
they are moderate- to well-sorted, with rounded-frosted
quartz grains. This suggests an overall change from lacus-
trine at the base to aeolian environments at the top.
For both Mbandaka 1 and Gilson 1 (fossils lists in
Fig.
14.3
), the ostracodes and particularly the characeans
(
Grambastichara
sp.) of the clayey lower part indicate (1)
a similar age than the
Gr
`
s polymorphes
Fm and (2) a
Paleogene (and probably Eocene) age (Colin and Jan du
Ch
ˆ
ne
1981
).
“
”
14.4.3 Eastern Outcrops: Yangambi
and Salonga Formations
In the eastern part of the Cuvette (Kisangani area—location
on Plate 2), the Cenozoic sediments are subdivided into two
units (Fig.
14.6
):
the
“
Yangambi
”
Fm overlaid by the
“
”
Fm (de Heinzelin
1952
). These unfossiliferous
sediments were first considered as Plio-Pleistocene
(de Heinzelin
1952
) and later time equivalent of the
Salonga
“
Gr`s
polymorphes
Fms (de Heinzelin
1962
).
These formations fill up a network of incised channels
(Fig.
14.6a
from de Heinzelin
1952
) with around 80 m of
incision.
The
Yangambi Fm
(Fig.
14.6b
) is a medium to coarse-
grained poorly sorted sandstone with some granules
layers and few claystones beds. The sandstone shows
clear evidences of current megaripple cross-beddings.
The study of the quartz grain shape and texture
(Fig.
14.6b
) by de Heinzelin (
1952
) shown the dominance
of aeolian grains (round and frosted) on fluvial ones (dull
and shinning). The description of the facies is precise
enough to identify (i) bioturbated clayey sands that are
characteristic, in continental environments, of lacustrine
deposits and (ii) mud cracks and soils, records of emer-
sion. In agreement with de Heinzelin (
1952
), we interpret
this as an alternation between aeolian sediments
reworked by rivers (floods deposits) and lake deposits
(Fig.
14.6b
). The Yangambi Fm evolves from alternating
aeolian—lacustrine conditions to more lacustrine and
then more aeolian.
”
and
“
Sables ocres
”
10 m thick), composed
of fine to very fine-grained sands with a clayey level. The
occurrence of medium grained sandstones with very well
rounded grains at the base suggests aeolian influences.
The
coarse-grained unit
(
25 m thick), bounded at its
base by a conglomeratic layer (pebbles less than 10 cm in
size) made up of quartzites,
“
Gr
`
s polymorphes
”
and
cherts. This unit is composed of claystones, coarse-
grained sandstones (subangular quartz grains) and fine-
grained conglomerate (granules and pebbles less than
3 cm, quartz, chert). This suggests fluvial transport. The
amount of smectite and illite is around 40 % (Fig.
14.3
).
No fossils were found in any of the cuttings.
14.4.2.2 Gilson 1 and Mbandaka 1 Petroleum
Wells (See Plate 2 for Location)
The
Mbandaka 1
well (160 m, Fig.
14.3
) is mainly made up
of red-brown to grey silty claystones with one bed of
medium to coarse grained-sandstones at the base (rounded
quartz grains). Based on the occurrence of numerous
ostracodes and characeans, these claystones are interpreted
as lake deposits.
Iron duricrust and weathering
at the top of the Yangambi
Fm has preserved truncated laterite profile. The iron
duricrust of the top of the laterite is preserved in only
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