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 154

Late Jurassic (Kimmeridgian) Middle Cretaceous (Albian)

~150 Ma ~100 Ma

Paleocurrents:

Winds

Paleocurrents:

Fossil fishes:

Delta-
lacustrine

Fig. 8.18 Schematic paleogeographic reconstructions ( left : Late
Jurassic; right : middle Cretaceous) showing the extension of deposi-
tional sequences ( thick black line ) and paleocurrent directions ( arrows )

in central West Gondwana, superimposed on paleoglobes from Moore
and Scotese ( 2012 ). Red dots and outline are the borehole locations and
modern Congo drainage basin, respectively

of the active proto-Andes along the southwestern margin of
Gondwana (e.g. Ramos and Aleman 2000 ).

discernible in all the four borehole-sections (Fig. 8.14 ),
and thus possibly reflect a regional, or even global, climatic
warming during the mid-Cretaceous, as during this period
average global ocean temperatures have been estimated to
have risen up to 20-30 C (e.g. Huber et al. 2002; Puc ´ at
et al. 2003 ).

8.7.2 Middle Cretaceous Sequences

The two succeeding middle Cretaceous (Albian-Cenomanian)
Loia and Bokungu Groups represent two superimposed cycles
of lacustrine sedimentation: first episodically anoxic, and then
(ephemeral) more carbonate-rich. Depocenters in the Samba
(Units S3 and S4) and Mbandaka sections (Units M2 and M3)
pass laterally southwards into a prograding delta in the
Dekese section (Fig. 8.14 ).
The first, Loia Group (Unit S4) is a green and analcime-
rich section containing several (seven) black shale inter-
calations that may overlap with global anoxic events during
the Albian, ca. 100-110 Ma (e.g. Ogg et al. 2008 ). Also, the
presence of analcime suggests volcanic contributions during
sedimentation. This could possibly characterize the period
following the eruption of the Paran´-Etendeka LIP during
the onset of opening of the South Atlantic (Fig. 8.18 ).
The second, Bokungu Group (Unit S3) is more carbo-
nate-rich, with fractured carbonated mudstones and pur-
plish marlstones, intraformational conglomerates, and
devoid of black shales. The increase of carbonated sedi-
ments between the Loia and Bokungu Groups is clearly

8.7.3 Upper Cretaceous Sequences

The uppermost Kwango Group was not recognized in the
Kwango Valley area or in any of the four boreholes. This
Upper Cretaceous sequence appears to be limited to the
southwestern part of the central CB, where the youngest
Cretaceous fossil suite was found, containing another unique
fish fauna, also possibly of marine affinities (Taverne 1976 ).
However, the abundant ostracods and phyllopods are all
non-marine, which rather suggests an inland lake or a closed
lagoonal environment during this final episode of sedimen-
tation of the CB.

Conclusion
The J-K sequences of the Congo Supergroup in the CB
overlie a major unconformity across Carboniferous to Tri-
assic sequences, linked to regional uplift and erosion dur-
ing the initial period of Gondwana break-up. Near the base,