Geology Reference
In-Depth Information
Table 11.1a
Stratigraphic data of the Samba 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
158
1.167
1.167
0.270
0.490
2,650
0.000
0.130
1
0.330
S5 lower
153
1.167
0.995
0.354
0.539
2,685
0.020
0.160
1,030
0.000
S5 upper
149
0.995
0.845
0.318
0.518
2,670
0.010
0.160
1,000
0.000
Overburden 143
0.845
0.845
0.282
0.497
2,655
0.000
0.120
1
0.040
S4
100
0.845
0.565
0.450
0.595
2,725
0.100
0.260
1,000
0.040
S3
93.6
0.565
0.192
0.474
0.609
2,735
0.020
0.280
1,000
0.020
Overburden 65.5
0.192
0.192
0.342
0.532
2,680
0.010
0.230
1,000
0.070
S1 + S2
33.9
0.192
0.001
0.282
0.497
2,655
0.000
0.220
1
0.150
Hiatus
0
0.001
0.001
0.270
0.490
2,650
0.000
0.000
1
0.370
years; (3) thickness—in kilometers; and (4) petro-physical
characteristics—porosity-depth coefficient, surface porosity
and dry sediment density—for each stratigraphic unit; (5)
paleo-bathymetry—
Table 11.2
Eroded units in the Samba section
Overburden unit age
(Ma)
Erosion age
(Ma)
Eroded thickness
(km)
Eustasy
(km)
0if
fluvial and aeolian; (6) eustasy—deduced from the long-
term sea-level curves of Haq et al. (
1987
); Haq and Schutter
(
2008
); (7) density of depositional fluid (fresh vs. salt water
or air); and (8) paleo-elevation—in kilometers above paleo-
eustatic sea-level.
>
0 if marine and lacustrine or
¼
143
112
0.300
0.040
0.070
65.5
55.8
0.300
IsosSed
¼
EpS
ˁ
ð
M
‐
ˁ
PS
Þ = ˁ
ð
M
‐
ˁ
Mil
Þ
ð
11
:
3
Þ
¼
ˁ
ð
Þ = ˁ
ð
‐
ˁ
Þ
ð
:
Þ
IsosWat
PaleoBat
Mil
M
Mil
11
4
11.2.2.1 Paleo-Elevations of the Congo Basin
The paleo-elevations of the CB are estimated from paleo-
environmental interpretations, as described below
(Fig.
11.4
), and calibrated to the eustatic sea-level curve of
Haq et al. (
1987
) and Haq and Schutter (
2008
). Although
there is debate about the global amplitude of this curve (e.g.
M¨ ller et al.
2008
,
2011
; Rowley
2013
), it best describes the
relative sea level variations observed in and around Africa
over the last 500 million years.
At the base of the CB, the well-recognized, thick
Carboniferous-Permian sequences of diamictites and black
shales (the Lukuga Group) are considered to have been
sourced through large west-facing paleo-glacial valleys and
fjords flanking the eastern and southern margins of the basin
Topic), and possibly intermittently connected with the
Paran´ Basin of southeastern Brazil, where there is evidence
for marine turbidites (the Itarar
´
Group; Vesely
2007
; see
low-lying paleo-surface of the CB is estimated to have been
at approximately 100 m above the paleo-eustatic sea level
(Fig.
11.4
). This paleo-elevation is then elevated by about
200 m during deposition of the overlying, more proximal,
thick Triassic sequences of conglomerates, red sandstones
and siltstones of the Haute Lueki Group (Fig.
11.4
). An
emergence of the CB and increased regional relief of
south-central Africa at the end of the Paleozoic is also
consistent with the stratigraphic record of the Karoo Basins
of southern Africa, where an abrupt change from turbidites
(the Ecca Group) to fluvial sediments (the Beaufort Group)
where IsosSed and IsosWat are the sediment and water
loadings, respectively, and PaleoBat is the paleo-bathymetry;
ˁ
Mil: the density of the deposit fluid (1,000 kg/m
3
for fresh
water, 1,030 kg/m
3
for marine water, 1 kg/m
3
for air);
ˁ
M: the
density of the mantle (3,300 kg/m
3
); and
ˁ
PS: the density of
the considered sedimentary section.
11.2.1.3 Overburden
Because not all the stratigraphic units are preserved in all of
the four studied borehole-sections, overburden units with
estimated thickness and petrological properties are incorpo-
rated in this modeling. This assumes continuous deposition
of the modeled units across the entire central CB. These
units are then removed (eroded) resulting in a decrease of
the thickness of the considered section. The erosion and the
associated uplift do not change the compaction of the under-
lying units because the compaction is an irreversible pro-
cess. Only once the stratigraphic sequences are buried to a
greater depth than before the erosion, are the units
compacted further.
11.2.2 Input Data
The input data required for this computation is summarized
and listed in Figs.
11.3
and
11.4
and Tables
11.1
-
11.8
,
respectively. This includes: (1) sediment-type—percentage
of clay and silt compared to sand; (2) age— in millions of
Search WWH ::
Custom Search