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 145

Suspended-load

E

F

sub-section E

x10 or
x100 m

Anoxic

Depth Facies Grain-size

C SfSmS cS Gr P

Bioturbated
heterolithics

sub-section

Depth Facies Grain-size

S mc-b

C S fS mS cS Gr P

-835 m

F luvia l

St

Smc

Flood
deposits

Fb

-740 m

Heterolithics

Fl

-840 m

Smc

-745 m

Smc

Flood
deposits

Sm

Gtc

-845 m

Fig. 8.11 Perennial lake facies succession from the middle part of the
Samba core-section (Unit S4; Fig. 8.8b ). Sub-section E: bioturbated
heterolithic sandstones (Smc-b facies) alternating with green shales (Fl
facies). Sub-section F: heterolithic sandstones with clasts of carbonate

and bed intercalations of black shales (Fb facies) and cross-bedded
sandstones (St facies), indicating a persistent and episodically anoxic
lake (e.g. Talbot and Allen 1996 )

aeolian succession comprises two main depositional Formation, as it is best described from the Dekese cores

(

sequences:
•

[Beds] Ca, Cb and Ba of Cahen et al. 1960 ).
This formation is largely equivalent to the aeolian red
sandstones of the Lower Kwango Group mapped from
outcrops along the southwestern margin of the CB (see
Fig. 8.4 ). Across the correlation profile in the center of the
basin (Fig. 8.14 ), it is constrained in age (at Gilson) by
biostratigraphy between the Upper Jurassic (Kimmeridgian)
Stanleyville Group and the middle Cretaceous (Albian) Loia
Group.
In the Samba section, the Units S4 and S3 are the
stratotypes of the Loia and Bokungu Groups, respectively
(Cahen 1983a ), both dated biostratigraphically to the
Albian-Cenomanian (Colin 1994 ). These two distinct lacus-
trine successions, first periodically anoxic with several beds
of black shales, and then shallower with more carbonate-rich
and fluvial sediments are separated by a fractured horizon of

' Couches '

In the lower part, mixed aeolian and flood deposits (sub-
Unit D4a) record the main maximum transgression
(MFS). This sequence terminates with a thin (1 cm
thick) carbonate layer at depth

622 m that can represent
a desiccation surface and a hiatus (Fig. 8.10 ).
• An overlying sequence includes numerous superimposed
aeolian dunes (sub-Unit D4b) and intercalates near the top
with two carbonate breccias (sub-Unit D4c), which could
also attest of prolonged episodes of desiccation and hiatus.
This aeolian succession is also recognized within the Unit
G4 of the Gilson section (Fig. 8.14 ), where it corresponds to
163 m thick, well-sorted, and ferruginous coarse sandstones,
and can correlates with distinctive (very coarse), 36 m thick
arkoses forming the Unit M4 of the Mbandaka section. We
propose to name this aeolian succession: the Dekese