Geology Reference
In-Depth Information
comparison of the landforms chronology and associated
weathering with those dated in West Africa and tested:
(iii) againt the siliciclastic sediment budget on the Congo
passive margin.
The oldest preserved outcropping relief is Paleocene-Early
Eocene in age (Upper Surface 1). The only old (Late Creta-
ceous?) landforms are preserved below the first basalt flows
in the Adamawa Basin or below the Paleogene (Eocene?)
sediments of the Yangambi area. This implies a major
peneplanation of the studied are at the end of the Cretaceous,
but no Cretaceous or Jurassic surfaces are preserved (cf.
King
1962
).
Most of the preserved landforms are Cenozoic, possibly
Upper Eocene-Late Oligocene for Upper Surface 2, possibly
Early to Middle Miocene for the Lower surface and
Pediments V and W and Late Miocene to Pliocene for
Pediments X to Z.
14.9
Conclusions
14.9.1 Sediment Infill of the Central CB and
Surrounding Highs
1.
Age-dating
: Biostratigraphic data (ostracodes and
characeans) from the Central CB wells (Gilson 1,
Mbandaka 1) indicate that (i) the Central CB deposits
are Paleogene (and probably Eocene) in age and that (ii)
these sediments are time equivalent of
Gr
`
s
the
“
polymorphes
Fm, preserved in the western and southern
plateaus of the Central CB. The outcrops of the
Yangambi Fm (Kisangani area) may be lateral equivalent
of the
”
“
Gr
`
s polymorphes
”
Fm.
2.
Relative stratigraphy
: From the base to the top, the
infilling is as follows: (i) truncated laterite profiles on
top of the Cretaceous sediments eroded by the Paleogene
deposits; (ii) deposition of the
Fm
and its subsurface lateral equivalents; (iii) laterite; and
(iv) deposition of the
“
Gr`s polymorphes
”
14.9.3 Topographic Evolution and Uplifts
Fm. was diachronous
from Neogene to the present-day, corresponding to
weathered aeolian to alluvial surficial deposits, veneering
older planation surfaces.
3.
Paleogene pre- to syn-Middle Eocene environments and
palaeogeography
: A hot arid desert with aeolian dunes
(
“
Sables ocres
”
1. The CB and most of the surrounding areas are inferred to
have been close to sea level during the Paleocene-
Middle Eocene times.
2. The Central African Atlantic Swell has been uplifted
since the Middle Miocene? (16 Ma) (Pediments V and
W); and uplift
Fm) graded northward into a lake
with marshes (northern CB). The
“
polymorphic
”
texture
of these quarzitic sandstones resulted from a complex
diagenetic evolution during a period of base level fall
with (i) adjusted ground water circulations and (ii) laterite
weathering.
“
Gr`s polymorphes
”
increased since the Late Miocene
(11.5 Ma).
3. The Cameroon Highlands have been uplifted since the
Eocene (45 Ma) during two pulses: Middle Eocene-Late
Oligocene (Upper Surface 2); and post
the Late
Miocene.
4. The Ubangian Rise was moderately uplifted from Mid-
dle Eocene-Late Oligocene? (45-23 Ma; Upper Surface
2) to the Late Miocene (11.5 Ma).
5. The East African Dome has been uplifted since the
Middle Eocene-Late Oligocene (45-23 Ma) (Upper Sur-
face 2) with an increase during Late Miocene (11 Ma)
and a decrease subsequently.
6. The Angolese Plateau and the western part of the Kasai-
Lunde-Kwango Plateau were uplifted since the Middle
Miocene? (16 Ma) (Pediments V and W).
7. The CB was uplifted after the Late Miocene (11 Ma).
8. The present topography of the CB and surrounding areas
results from two wavelengths of crustal deformation:
long wavelength (100-1,000 km) and very long
(
14.9.2 Landforms
1.
Type of landforms
: Two main types of landforms are
identified: planation surfaces and incised rivers and
channels. According to their size, planation surfaces are
etchplains and pediplains (large—
10
4
10
6
km
2
)
to 2
10
4
km
2
).
2.
Relative chronology of landforms
: Nine successive pla-
nation surfaces are characterized, from the oldest to the
youngest: (i) the Upper planation surface 1 (etchplain);
(ii) the Upper planationsurface 2 (etchplain); (iii) the
Congolese Surface (long-lasting surface time equivalent
of the the two Upper surfaces); (iv) the Lower planation
surface and five pediments, labelled from V (the oldest)
to Z (the youngest).
3.
Age of formation of landforms
: An age model is proposed,
based on: (i) the intersection of the planation surfaces
with dated volcanic and plutonic rocks;
or pediments (small—10
3
to
1,000 km). From the Middle Eocene to Middle Mio-
cene, the long wavelength was dominant (Cameroon
Highlands, Ubangian Rise); since the Late Miocene,
very long wavelength deformation is expressed by the
uplift of the CB and parts of the Kalahari Plateau (Kasai-
Lunde-Kwango Plateaus).
>
(ii)
the
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