Geology Reference
In-Depth Information
Table 11.7
Stratigraphic data of the Mbandaka section
Base
depth
(km)
Top
depth
(km)
Surface
porosity
(%)
Density of
deposit fluid
(km/m
3
)
Age
(Ma)
Porosity-depth
coeff. (km
1
)
Dry sediment
density (km/m
3
)
Bathymetry
(km)
Eustasy
(km)
Elevation
(km)
Unit
Hiatus
350
3.991
3.991
0.270
0.490
2,650
0.000
0.120
1
0.080
M9
318.1 3.991
3.408
0.318
0.518
2,670
0.020
0.000
1,000
0.200
M8
298
3.408
2.622
0.354
0.539
2,685
0.050
0.080
1,000
0.120
Hiatus
251
2.622
2.622
0.270
0.490
2,650
0.000
0.040
1
0.240
M6 + M7
199.6 2.622
1.536
0.318
0.518
2,670
0.020
0.000
1,000
0.460
M5
183
1.536
0.844
0.318
0.518
2,670
0.020
0.060
1,000
0.400
Hiatus
158
0.844
0.844
0.270
0.490
2,650
0.000
0.130
1
0.330
Hiatus
149
0.844
0.844
0.270
0.490
2,650
0.000
0.160
1
0.000
M4
143
0.844
0.808
0.270
0.490
2,650
0.000
0.120
1
0.040
Hiatus
112
0.808
0.808
0.270
0.490
2,650
0.000
0.180
1
0.120
M3
100
0.808
0.607
0.474
0.609
2,735
0.100
0.260
1,000
0.040
M2
93.6
0.607
0.161
0.354
0.539
2,685
0.020
0.280
1,000
0.020
Overburden 65.5
0.161
0.161
0.342
0.532
2,680
0.010
0.230
1,000
0.070
M1
33.9
0.161
0.001
0.498
0.623
2,745
0.000
0.220
1
0.110
Hiatus
0
0.001
0.001
0.270
0.490
2,650
0.000
0.000
1
0.330
11.2.3 Results
Table 11.8
Eroded unit in the Mbandaka section
Overburden unit age
(Ma)
Erosion age
(Ma)
Eroded thickness
(km)
Eustasy
(km)
The total subsidence curves for the Samba, Dekese, Gilson
and Mbandaka sections are analyzed in Fig.
11.5
. Variations
of both the total (Subtot) and tectonic (Subtect) subsidences
for each section are shown in Fig.
11.6
.
The curves of total subsidence for the CB demonstrate
the existence of two main episodes of rapid subsidence
(Fig.
11.5
):
65.5
55.8
0.300
0.070
Cenomanian-Turonian paleo-eustatic maximum, i.e. 280 m
above the present-day sea-level (Fig.
11.4
). This low-lying
paleo-surface of the CB is compatible with a potential Late
Cretaceous marine connection to explain paleo-spawning
sites of Kwango fish fossils found at Kipala near the center
of the basin (Casier
1965
; Taverne
1976
; Giresse
2005
;
the Cenozoic Kalahari Group are relatively poorly con-
strained by sedimentological and biostratigraphic data and
here simply considered to be deposited at the same elevation
as the present-day basin surface, ca. at the elevation of
borehole heads (e.g. 370 m above sea-level; Fig.
11.4
).
In summary, the paleo-elevation of the CB is relatively
well-constrained by sedimentological and stratigraphic
data for the Jurassic-Cretaceous, but is less precisely
known for the Carboniferous to Triassic sequences. How-
ever, in their geomorphic study of the CB, Stankiewicz and
de Wit (
2006
) identified two recurrent surface elevations: at
457 m and 610 m above the present-day sea level. The
lower (457 m) peneplain was interpreted as the base level
of the basin during the Cretaceous. The higher (610 m)
peneplain was proposed to represent an older, late Paleo-
zoic erosion surface. These two paleo-surfaces reflect a
relative depositional base level drop of 150 m between
about 300 Ma and 100 Ma, which is independently consis-
tent with the paleo-topographic model of the CB described
above (Fig.
11.4
).
Episode A
A first and most pronounced episode of subsidence starts at
ca. 350 Ma with the onset of the main Carboniferous-
Permian (Dwyka) glaciation of Gondwana (e.g. Milani and
de Wit
2008
) and terminates at about 180 Ma, and which
corresponds to the deposition of the Lukuga and Haute Lueki
Groups in the CB. This late Paleozoic to early Mesozoic
single phase of subsidence (Phase 1; Fig.
11.5
) has a magni-
tude of between 1,500 m at Dekese and 3,500 m at
Mbandaka, and a calculated rate of subsidence of 10-20 m/
Ma. In the Dekese section, an episode of uplift occurs at
around 200 Ma (Fig.
11.6
), resulting in the attenuation of the
Triassic Haute Lueki Group here to a thickness of only 40 m
(see Fig.
11.3
).
Episode B
The second main episode of subsidence starts at ca. 160 Ma
in the Samba and Mbandaka sections in the northern part of
the basin, and about 10 Ma later (ca. at 150 Ma) in the Gilson
and Dekese sections in the south-central CB (Fig.
11.5
). This
late Mesozoic subsidence corresponds to the deposition of
the Stanleyville, Loia, Bokungu and Kwango Groups, and
has a magnitude of between 800 m at Dekese and 1,500 m at
Samba, and a mean calculated rate of subsidence of 5-10 m/
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