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restricted areas. Mostly the duricrust is reworked, leaving
pebbles (
records a lake base level rise and fall. A maximum lacus-
trine flooding surface can be defined and traced along the
six studied sections (Fig.
14.3
). The retrogradational
trend preserves most of the aeolian facies of the
“
concretions
”
) at the base of the Salonga Fm.
The
Salonga Fm
(or
—Plateaus
Sands—Fig.
14.6b
), made up of medium to coarse-
grained sands bearing aeolian grains (dominant—round-
frosted: 10-55 %), mixed ones (blunt shinning on previ-
ous frosted grains: 4-48 %) and fluvial ones (blunt shin-
ning: 4-40 %), shows a similar facies and then a similar
depositional setting than the
“
Sables des Plateaux
”
“
Gres
polymorphes
Fm, while in the central Cuvette it
corresponds to a low volume of lacustrine sediments
except in the Samba borehole (alluvial facies aggrada-
tion). The progradational trend is a coarsening-upward
evolution going from lacustrine conditions at the base to
more fluvial and aeolian environments in both southern
Dekese and Gilson 1 drillings. The occurrence of pebbles
of
”
Fm.
These formations are eroded by at least three generations
of alluvial
“
Sables ocres
”
terraces all with abundant ventifacts
(de
Heinzelin
1952
).
Fm in the progradational trend of
the Dekese borehole (Cahen et al.
1959
) suggests either
(1) intraformational deformation (exhumation of the Cre-
taceous rocks located between the Cuvette Cenozoic
sediments and the Kasai Plateau) or (2) a younger age
for these sediments.
3. An Eocene-Pliocene base level fall and a second episode
of lateritization. The
“
Gr`s polymorphes
”
Fm is capped
by a major weathering surface (informally called Top
“
“
Gr`s polymorphes
”
14.4.4 Regional Correlations and CB Evolution
Constructed from the Sedimentary
Records
Regional correlations (Fig.
14.3
) are based on the biostrati-
graphic ages as well as regional major lithology and facies
changes. Two main groups of facies associations were
characterized: (1) from the west (Bat
´
k
´
Plateau) to the
southeast (Katanga), the outcropping
laterite, equivalent to Iron cuirasses
1 and 2 of Alexandre
2002
, in Katanga).
4. Pleistocene to older: multiple episodes of deposition of
“
Gr`s polymorphes
”
”
Fm, sandstones dominated by aeolian facies, and (2) in the
central part of the CB, subsurface deposits and outcrop of the
Yangambi Fm, dominated by lacustrine facies which corre-
spond to the Mio-Pliocene on the geological map of Zaire
(Lepersonne
1974
).
Our main new constraint is based on the biostratigraphy
of ostracodes and characeans, which are of the same age
(Paleogene and probably Eocene) as well in the silicified
lacustrine deposits of the
“
Gr`s polymorphes
facies. This faci
`
s is only preserved in the
southern outcrops, from the Bat´k´ Plateaus (W) to
Katanga (SE). The Salonga Fm, overlying the Yangambi
Fm, could be a
Sables ocres
”
facies. The central Cuvette
is free of this facies as shown by the four drillings. As
mentioned before, in the absence of dating, this probably
diachronic facies (Miocene to Holocene) cannot be used
as a time marker of the CB evolution.
“
Sables ocres
”
Fm as in the
lacustrine silty claystones of the Gilson 1 and Mbandaka 1
wells. Although no fossils were found in the Yangambi Fm,
a similar age than that of the subsurface Cenozoic sediments
from the wells (Paleogene, Eocene?) is realistic because they
have similar facies, lacustrine-dominated with aeolian facies
and are both bounded by laterites.
To summarize, the sedimentary evolution of the CB can
be consolidated into four steps:
1. An uppermost Cretaceous to base Paleogene base level
fall, followed by lateritization and local river incision
(Kisangani area). This first laterite profile (informally
called Top Cretaceous sediment laterite) chemically
eroded the underlying Late Jurassic to Cretaceous
sediments before being partially removed ahead of the
deposition of the
“
Gr`s polymorphes
”
14.4.5 Eocene Paleogeography
The thickness of the
”
Fms is greatest in the western part of the CB (Bat
´
k
´
s
Plateaus,
“
Gr`s polymorphes
”
and
“
Sables ocres
Sables
ocres
”
Fm:
100 m, Fig.
14.3
) decreasing toward the
southeast (Katanga) where they pinch out (Mporokoso Pla-
teau, Plate 1). Along the Kwango River, the thickness is less
than 200 m (Cahen
1954
,
“
Gr`s polymorphes
”
Fm:
300 m,
“
Gr
`
s polymorphes
“
”
Fm: 60-80 m,
“
Fm: maximum 120 m). In the northern Cen-
tral Cuvette, the maximum thickness of the time equivalent
lacustrine sediments is 230 m (Gilson 1) thinning out south-
ward (Dekese) and eastward (Yangambi), but these
thicknesses represent residuals because of an erosional sur-
face at the top of these sediments (Congolese Surface, see
below) (Plate 1).
A paleogeographic map compiled to depict
Sables ocres
”
Fm and its subsur-
face equivalent (Dekese, Gilson 1, Mbandaka 1, Samba).
2. Paleogene (Eocene?) lake to desert deposition comprising
the
“
Gr`s polymorphes
”
—Yangambi Fms time interval
which represents a retrogradational (
“
Gr`s polymorphes
”
the
Gr
`
s
“
transgressive
”
)—
progradational
—
Yangambi Fms cycle is shown as Figure
14.7
. Two distinct
trend
of
the
“
polymorphes
”
progradational
(
“
regressive
”
) stratigraphic cycle that
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