Cenozoic Landscape Evolution in and Around the Congo Basin: Constraints from Sediments and Planation Surfaces - Geology and Resource Potential of the Congo Basin - page 269

Gr ` s polymorphes Table 14.1 Main sedimentological characteristics of the Formations, based on published data and

samples provided by the Royal Museum of Central Africa (Tervuren, Belgium) coming from the Mount Mougengenge (or Mense Peak), located
10 km far SE from the Kinshasa International Airport (DRC)
“

“ ” and “ Sables ocres ”

GR ` S POLYMORPHES ” FORMATION

White fine to medium-
grained well-sorted sandstones

Mean grain-size
Maximum
Clay fnraction

150-350 μ m
1mm
3-8 %

Quartz (rare feldpars) and kaolinites subrounded dominant—well
rounded to subangular (finest grains)—Fig. 4.1 - 4.5 round-frosted grains
of the French-speaking sedimentary petrologists due to coating by a
silicieous pellicule

More or less consolidated Moderately to well-sorted

SEM : Dissolution on all grains (Fig. 4.6 a ), later coated by automorphic quartz and kaolinite (Fig. 4.6 b )

Le Mar ´ chal ( 1966 ), de Ploey et al.,
( 1968 )

Sedimentary structures: Large oblique laminasets ( 1 m, up to 10 m-thick), simple or compound, resulting
from the migration of large 2D-3D dunes with grainfall laminations, truncated by flat surfaces sometimes
overlain by low preservation 2D to 3D current megaripple cross-beddings near the base, facetted pebbles
(ventifacts) in Katanga (Mortelmans 1946 )

Silicified sandstones to quartzites
(Tshidibi 1986 ; Mouyoungou 1990 )

Same grain-size distribution and grain shape than above—two main types of silicification:
1. Cemented quartz grains by suhedral and sutured silica overgrowth (crystal > 100 μ m)—Fig. 4.2
2. More or less cemented quartz grains with small vugs (200-300 μ m) with palissadic and euhedral quartz
growth—Fig. 4.3
Mainly located in the second half of the formation

Silicified limestones (Le Mar´chal
1966 ; Tshidibi 1986

Marls to fine-grained limestones with continental subaqueous gastropods ( Limnea, Planorbis, Physia ) and
some characeans later silicified by microcrystalline quartz with vugs filled, with different generations of
euhedral quartz with different shapes (palisadic, radiolitic..) and different types of silica (chalcedony,
lutecite, quartzine)—Fig. 4.4
Mainly located in the second half of the formation

“SABLES OCRES” FORMATION
Brown fine-grained homogenous
sandstones

Mean grain-size
Maximum
Clay fraction
Moderately sorted
less than G.p. Fm

100-250 μ m
500-700 μ m
5-30 %

Quartz (no feldpars) and kaolinites - four types of quartz grains:
1. Subrounded (shinning—coarser grains—Fig. 4.7 ) to subangular
(frosted-finer < 300 μ m) some with impact structures - few rounded
(coarsest grains > 500 μ m)
2. Polyhedral non-worn shinning
3. Corroded and fractured
4. Automorphic (bipyramidal)

(Le Mar ´ chal 1966 )
(de Ploey et al. 1968 )
(Alexandre-Pyre 1971 )

SEM: Evidences of dissolution (Fig. 4.8 a , b), grains later coated by different types of iron oxydes/
hydroxydes
Sedimentary Structures: Homogenous, with no evidence of sedimentary structures

Fm (see Table 14.1 for facies descrip-
tion) onlaps Proterozoic basement to the south (in Katanga;
Cahen and Lepersonne 1952 ; Cahen 1954 ), and to the west
(the Bat´k´ Plateaus, Le Mar´chal 1966 ).
The age and environmental setting of the

The “ Sables ocres ” expected for aeolian sands), and grains are partly corroded

by water dissolution; (2) No sedimentary structures indica-
tive of aeolian sediments (e.g. horizontal sand sheets) are
preserved.
The quartz grain dissolution is consistent with a model of
intense weathering, as already mentioned above with respect
to the occurrence of iron crusts and silicified levels in the
“

“ ”

Fm
are both unresolved. In the absence of dating, there is no other
data that indicates that this formation not diachronous is across
the CB. In Congo-Brazzaville, the same facies extends across
both over the

Sables ocres

Fm of the Bat ´ k ´ Plateau (Le Mar ´ chal 1966 ;
Giresse and K'Vadec 1971 ). The sand deposition may thus
be a combined effect of aeolian and alluvial processes (e.g.
de Ploey et al. 1968 ; interpreted as ephemeral rivers with
sheet-wash by Alexandre-Pyre 1971 ), and subsequent
weathering.
Because in Katanga (Alexandre 2002 ) the

Sables ocres ”

“ Gr`spolymorphes ” Fmand the basement where

these sands overlie (beds of angular quartz

pebbles). In Gabon, charcoals interbedded within these sandy
facies were 14 C dated to the Holocene (c. 3000-2000 BP) and
an aeolian origin has been proposed (Thi´blemont et al. 2013 ;
Thi ´ blemont 2013 ), consistent with a very recent Pleistocene
age for the

“ Stone-Lines ”

“ Sables ocres ”

Fm in this area.
Several observations question the generalization of this
recent age to the

“ Sables ocres ” Fm is older than a major (iron duricrust 3) of

possible Miocene age, we suggest that the

' cuirasse '

“ ”

is
a convergent facies, shaped by the same processes, at differ-
ent times during the Neogene. Thus, the “ Sables ocres ” Fm
is not a time-marker horizon.

Sables ocres

Fm all around the Central
Cuvette: (1) Based on SEM work, the “ Sables ocres ” grains
appear more subrounded to subangular than rounded (as

“ Sables ocres ”