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controversial. Haq et al. (
1987
) suggest a sea level fall since
Early Eocene of ca. 210-260 m. By contrast, Miller et al.
(
2005
), and more recently Rowley (
2013
), suggest much
lower values of between 40 and 120 m, and no more than
40 m since the Late Eocene (Miller et al.
2005
), and no more
than 20 m since 40 Ma (Rowley
2013
). These recent data
may indicate that the maximum amplitude of sea level fall
since the end of the Eocene « Gr`s polymorphes » deposition
is less than 50 m, and then of low importance.
The cornerstone of our study is a new map (Plate 2) of
planation surfaces (and scarps) and incised rivers, in chro-
nologic order. Dating of these landforms is based on the
geometrical relationships between planation surfaces and
dated rocks (e.g. magmatic rocks such as younger
This kaolinite results from a post-depositional weathering
as indicated (Le Mar
´
chal
1966
) by (i)
in situ
weathered
feldspars and micas and (ii) illuviation of kaolinite in the
pores. In the Kwango region, the
“
Gr
`
s polymorphes
”
Fm
overlie an iron-rich level (duricrust?) (Cahen
1954
; Linol
2013
). In Katanga (Alexandre
2002
), this contact has not
been observed. This suggests that a period of weathering in
warm humid conditions preceded the deposition of the
“
Gr´s
polymorphes
Fm.
Three main types of facies were defined by Le Mar´chal
(
1966
) and here validated further by field studies (Table
14.1
,
Fig.
14.4
). The primary environment of sand deposition is a
reg covered by aeolian dunes as suggested by the occurrence
of (1) ventifacts (facetted aeolian pebbles), (2) round-frosted
grains, (3) high oblique laminations (1 to 10 m) migration of
large 2D to 3D dunes and (4) alternations of massive,
”
granites
and volcanics), sediments and weathering profiles. Two
areas are critical for the accuracy of this analysis
(Fig.
14.1
): the Cameroon Volcanic Lines (including the
Adamawa Plateau) and the Virunga-Bukavu (
”
1 cm-thick, medium-grained sands laminations and
mm-thick fine-grained sands laminations, characteristic of
grainfall on the lee-side of aeolian dunes (e.g. Brookfield
and Silvestro
2010
for a review). These aeolian deposits are
episodically reworked by probable alluvial processes as
suggested by the flat truncations of the aeolian dunes (defla-
tion surfaces) sometimes overlaid by sands with low preser-
vation 2D to 3D current megaripples cross-beddings
(bedload stream flows?). Based on the types of gastropods,
the limestones are lacustrine in origin. These sediments were
deposited in a hot desert interrupted by recurrent more
humid periods (possibly driven by Milankovitch cycles)
as witnessed by the interbedded alluvial and lacustrine
sediments.
Subsequently, the upper quartz-rich sands and limestones of
the top half of the formation were silicified. The silicification of
the quartz-rich sands results from euhedral quartz overgrowths
(Fig.
14.4
.2), while the limestones are microquartzified
(Fig.
14.4
.4). Those mineralogic modifications are characteris-
tic of groundwater silicifications (Thiry
1999
;ThiryandRibet
1999
;ThiryandMar´chal
2001
). Both the poorly consolidated
and silicified sediments are dissolved. The desilicification of
the silicified limestones and sandstones is recorded by vugs,
filled by palissadic and euhedral silica. Desilicification can
occur during groundwater circulation (Thiry and Mar´chal
2001
). But the occurrence of dissolved quartz and kaolinite
precipitation in the poorly consolidated sands, the weathering
of the rare feldspars, as well as illuviation structures
(Mouyoungou
1990
) suggest dissolution along a weathering
profile superimposed on quartz-rich sands.
Kivu) Vol-
canic Province of the East African Dome in Eastern Congo,
Rwanda, and Burundi.
The analysis is based on a combination of Digital Eleva-
tion Model (SRTM 90 m processed with ArcGIS 10) and
published geological maps, research papers and local
reports. To characterize local base level falls and to evaluate
possible different uplift at regional scale, a number of topo-
graphical sections are presented (Fig.
14.9
) to illustrate (i)
planation surface types and their extents, (ii) the amplitude
of steps between surfaces and (iii) the change of slope
gradients between surfaces.
¼
14.4
Sedimentary Rocks
Following the international
lithostratigraphic code,
the
“
Gr
`
s polymorphes
”
“
”
“
”
“
”
,
Sables ocres
,
Yangambi
,
Lodja
and
units are hereafter ranked as formations
(Fm) and the Bat
´
k
´
sands as a group (Gp). The
“
Salonga
”
“
Gr
`
s
polymorphes
”
and
“
Sables ocres
”
Fms are parts of the
Kalahari Gp.
Gr
`
s Polymorphes
14.4.1 Outcrops of
“
”
and
“
Sables Ocres
”
Sediments
14.4.1.1 “Gr
`
s Polymorphes” Formation (Fm)
(Figs.
14.3
and
14.4
; Table
14.1
)
The base of the
“
Gr
`
s polymorphes
”
14.4.1.2
Formation (Fm) (Figs.
14.3
and
14.4
; Table
14.1
)
The base of the
“
Sables ocres
”
Fm is a sharp contact with
underlying white and brown colour sands. It corresponds to
the Mid-Tertiary surface of Cahen (
1954
). In some places,
this surface is aligned with iron concretions (the
“
Sables Ocres
”
Fm is a sharp contact,
corresponding to the Late Cretaceous surface of Cahen
(
1954
). In Congo-Brazzaville (Le Mar´chal
1966
; Giresse
1990
), the
Gr
`
s polymorphes
Fm overlies kaolinite-rich
aeolian sandstones of the Stanley-Pool Fm, dated as Late
Jurassic to Early Cretaceous based on the fauna of the basal
lacustine sediments (Egoroff and Lombard
1961-1962
).
“
”
“
limo-
nitic
”
crusts of Cahen and Lepersonne), and iron crusted
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