Facies Analyses, Chronostratigraphy and Paleo- Environmental Reconstructions of Jurassic to Cretaceous Sequences of the Congo Basin - Geology and Resource Potential of the Congo Basin - page 148

185 km 360 km

SOUTH

Gilson (well log)

Ba se-lev el
Rise

C S fS mS cS Gr P

Fall

0 m

G1b

Dekese (core)

100 m

Major
sequences

Ba se-lev el
Rise

C SfSmS cS Gr P

Fall

0 m

200 m

G1a

C la y stones

TERRESTRIAL

D1

D2c

G2c

100 m

300 m

D2b

FLUVIAL

S a n dstone
p o or l y sorted
c a l c areous

G2b

200 m

400 m

D2a

Cl a y s tone
c a l ca r eous

300 m

500 m

G2a

PALEOSOLS

FRONT
DELTA

D3

400 m

600 m

D4c

G3b

500 m 700 m

G3a

AEOLIAN
DUNES

D4b

600 m 800 m

Sandstone
well sorted,
ferruginous

G4

D4a

700 m 900 m

D5

G5

KAROO
SUPERGROUP

Sea /
Lake

Soil /
Dunes

Delta River

1000 m

Sea /
Lake

Soil /
Dunes

Delta River

Fig. 8.14 (continued)

truncated by an erosion surface at depth -297 m
(Fig. 8.8b ), and covered by fluvial conglomerates.
• An upper sequence is mainly transgressive, comprising
cross-bedded red sandstones fining upward into flood
plain mudstones (sub-Unit S3c).
These three main depositional sequences are also
identified within the Mbandaka, the Gilson, and the Dekese
sections (Fig. 8.14 ). In the Mbandaka section, the Unit M2
(445 m thick) comprises in the lower part 212 m thick
upward-shallowing carbonated mudstones (sub-Unit M2a),
followed by two distinct sandstone members (sub-Units
M2b and M2c, in total 110 m thick), and in turn overlain
by 122 m thick siltstones coarsening upward into sandstones
(sub-Unit M2d). In the Gilson section, the Unit G2 (332 m
thick) comprises 151 m thick carbonated sandstones and
mudstones (sub-Unit G2a), sharply overlain by 138 m thick

poorly sorted sandstones and mudstones (sub-Unit G2b), and
then 43 m thick siltstones (sub-Unit G2a). In the Dekese
section, the Unit D2 (288 m thick) is mainly regressive,
comprising superimposed sequences of delta plain and
swamp deposits with numerous paleosols and calcretes (e.
g. Fig. 8.13 ).
In the lower part (sub-Unit D2a), fossiliferous mudstones
can record the first main transgression (MFS). This sequence
shallows upward with alternations of sandstones and mud-
stones with calcretized conglomerates. It is sharply overlain
by two successive fining upward intervals of channel
infillings (sub-Units D2b and D2c). The top of these fluvial-
lacustrine sequences, in all four boreholes, is an erosion
surface overlain by unconsolidated conglomeratic sandstones
and mudstones attributed to the (Cenozoic) Kalahari Group,
in total between 37 m and 242 m thick (Fig. 8.14 ).