Geology Reference
In-Depth Information
An 850 km long, N-S reference profile across the Samba
and Dekese sections is shown in Fig. 7.6 . Along this refrac-
tion profile five distinct seismic units are identified:
1. Basement rocks with very high velocities (
5,200 m/s)
rapidly rise along the southern part of the profile, from a
depth of
>
3,000 m at Dekese (station 72) to a depth of
400 m (station 82) over a distance of 250 km (Fig. 7.6 ).
Along the southern margin of the CB, this bedrock
corresponds to the Kasai and NE Angola Cratons
(Cahen et al. 1984 ), and here named the Kasai and
Kwango Highs (Fig. 7.7 ).
2. A marked seismic contrast with a high velocity range
(4,200-4,600 m/s) defines a tectonic high in the center
of the profile (between stations 39A and 49), and progres-
sively rises northward to the surface (station 115) where it
links with outcropping upper Neoproterozoic quartzitic
sandstones of the Aruwimi Group (Lepersonne 1974 ). In
the four deep boreholes in the center of the basin, this
seismic discontinuity also corresponds to the tops of
quartzitic sandstones, which have distinct high velocities
(
4,200 m/s). Although these lithologies (e.g. quartzites)
occur at different stratigraphic levels (see Fig. 7.4 ).
3. A distinct seismic unit with intermediate velocities (3,600
to 3,900 m/s) occurs in a limited zone, 170 km wide, of
the southern part of the profile (Fig. 7.6 ). In the Dekese
section (station 72), this unit corresponds to 694 m thick
black shales with diamictites (Unit D8), dated by
palynostratigraphy to the Permian and attributed to the
Lukuga Group (Cahen et al. 1960 ; Boulouard and
Calandra 1963 ).
4. An overlying unit of lower velocities (2,400 to 3,600 m/s)
stretches continuously across the profile, between 300 m
(station 73) to maximum 1,000 m thick at Samba (station
23A). In the four deep boreholes, it is dated biostrati-
graphically to the Upper Jurassic and middle Cretaceous
(Cahen et al. 1959 , 1960 ; Grekoff 1960 ; Defretin-Lefranc
1967 ; Maheshwari et al. 1977 ; Colin 1981 ; Colin and Jan
du Chˆne 1981 ). This main seismic unit displays major
lateral variation along the profile (Fig. 7.6 ). In the south
the seismic velocities are relatively low (~3,000 m/s), and
in the Dekese section (station 72) these correspond to
predominantly cross-bedded red coarse sandstones
(Units D2 to D5). By contrast, in the north their lateral
equivalents have higher velocities (~3,600 m/s), which in
the Samba section (station 23A) correspond to alternating
green mudstones and black shales with lesser sandstones
(Units S3 to S5). This discernible change in seismic
velocity reflects a major N-S lateral change in the late
Mesozoic lithostratigraphy (see also Chap. 8 , this Topic).
5. A thin uppermost seismic unit of low velocity range
(1,800-2,200 m/s), with a maximum thickness of 200 m
>
Fig. 7.5 Photo of drill core (No. 8) from the depth interval
4,139 m
to
4,147 m in the Mbandaka-1 borehole. The cut face of the core
shows thinly laminated (stromatolithic?) dolomitic fine sands and dark
shales with large nodules of anhydrite. The core is 90 mm in diameter
terminates at a depth of
4,645 m in the dolomites
(Fig. 7.4 ).
7.3.2.2 Seismic Refraction Data from the 1950s
The old (1952-1956) seismic refraction surveys essentially
determined the sediment thicknesses at different locations
(e.g. 111 stations) across the entire central CB (Fig. 7.2A ).
The depths of the different seismic discontinuities
(reflectors) were calculated at each seismic station, and
because the sedimentary packages were considered largely
tabular and the horizontal velocities much higher than the
vertical velocities, the calculated depths were corrected
using an arbitrary reduction of 10 % on the observed
velocities (Evrard 1960 ). Comparison of results with the
well-data shows that all the main geological contacts were
systematically determined (Fig. 7.4 ).
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