Neoproterozoic to Lower Paleozoic Sequences of the Congo Shield: Comparisons Between the Congo and Its Peripheral Basins - Geology and Resource Potential of the Congo Basin - page 103

observed at the bottom of the Mbandaka-1 and Gilson-1
wells. The modelled low susceptibility values of some
layers suggest that they could contain a significant pro-
portion of salt.
￿ The poorly defined seismic facies that led to the previous
interpretations of crystalline basement uplift is shown to
be also consistent with paleo-salt tectonics.

carbonate formation with evaporite showings (the “ carbon-

ate-evaporite sequence of ECL 1988 ; Lawrence and

Makazu 1988 ; Daly et al. 1992 ), was reported from the
lower Mbandaka-1 well and defined as Sequence 1
(670-1,200 thick). The carbonates are considered as late
Neoproterozoic, Ediacaran-Cryogenian (e.g. Daly et al.
1992 ; Kadima et al. 2011a ; Linol 2013 ; Delpomdor, et al.,
Chap. 4 , this Topic) by lithostratigraphic correlation with and
between the outcropping sequences of carbonates in the
peripheral basins. Beneath this carbonate unit, the basal
siliclastic sequence (Sequence 0) tends to thicken towards
the centre of the CB (to up to ca. 1,100 m). Overlying the
carbonate unit is a predominance of siliciclastic sequences
with intercalation of thin carbonate layers, defined in the two
deep exploration wells and in the seismic profiles as
Sequences 2 and 3 (1,700 to 4,100 m), and here attributed
to the late Neoproterozoic-early Paleozoic.
In the marginal West Congo, Lindian and Katangan
basins, recent observations suggest that the uppermost clas-
tic sequences (the Inkisi, Biano and Banalia Groups) are not
affected by Pan-African deformations and therefore may be
early- to mid-Paleozoic in age (Alvarez et al. 1995 ; Master
et al. 2005 ; Batumike et al. 2006 ; Tack et al. 2008 ;
Kampunzu and Cailteux 1999 ; Kampunzu et al. 2009 ;
Cailteux et al. 2007 ; Tait et al. 2011 ).
In the CB, seismic Sequences 1 to 4 have been locally
affected by a compressional tectonism event responsible for
inversion structures such as folds and thrusts (Daly et al. 1992
and Kadima et al. 2011a ). An erosional unconformity
separates Sequence 4 from Sequence 5. We consider that the
first four sequences form the late Neoproterozoic-early Paleo-
zoic sedimentary record of the CB, with the lower major
erosional surface represented by a Pan African unconformity.

”

6.4 Discussion

The Neoproterozoic period in Central Africa was marked by
crustal extension with the opening of rift basins during the
initial development of the CB near the centre of the Congo
Shield. This was accompanied by the formation and preser-
vation of Neoproterozoic basins surrounding the CB. Litho-
stratigraphic, structural, and paleo-environmental similarities
have been established between all these basins (Tait et al.
2011 ; Delpomdor 2013 ; Delpomdor et al., Chap. 4 , this
Topic). The sediments were subsequently affected by com-
pressional tectonics related to the Pan African and late
Paleozoic-early Mesozoic orogens.
The data discussed here suggest to us rather that the
Neoproterozoic CB was initiated during an intracratonic
rifting process, possibly related to the break-up of the
Rodinia Supercontinent. Such extensional setting may also
have initiated the development of the other Neoproterozoic
basins now preserved along the periphery of the Congo
Shield. Mafic magmatism associated with this regional
rifting event is observed in the West Congo, Semb´-Ouesso,
Bangui, Fouroumbala-Bakouma, Itombwe, Malagarazi-
Bukoban, Roan, Kundelungu and Mbuji-Mayi basins (Tack
et al. 2001 , 2010 ; Vicat et al. 1989 ; Hanson et al. 1994 ;
Johnson et al. 2005 ; Key et al. 2001 ; Kampunzu and
Cailteux 1999 ; Batumike et al. 2009 ; Delpomdor et al.
2013b ). Within the CB, there is no evidence to confirm or
exclude similar magmatic activity, but 2-D modelling
suggests that a linear NW-SE positive residual gravity
anomaly in the centre of the basin may be associated with
mafic intrusions into the crustal basement. Both Evrard
( 1957 ) and Kadima et al. ( 2011a , b ) attributed the short
wavelength gravity anomalies observed near Dekese and
Mbandaka-1 to such intrusions into the upper crust.
The lithostratigraphic successions in all the Neoprotero-
zoic basins developed across the Congo Shield show first-
order similarities, with in their lower parts carbonate-rich
sediments overlain by siliciclastic sequences that include
tillites (for more details see Delpomdor et al., Chap. 4 , and
de Wit and Linol, Chap. 2 , this Topic). In most of the
peripheral basins, the carbonate units overlie a basal con-
glomeratic sequence. As shown by the deep well logs (Esso
Zaire 1981a , b ) and previous seismic analysis, the deepest
part of the CB may contain similar units. A massive

Conclusion
A series of basins formed in Central Africa at the periph-
ery of the Congo Shield and the large CB formed near its
centre during the early Neoproterozoic by rifting and
mafic magmatism related to the break-up of Rodinia.
The early phase of the CB developed during this time
along a NW-SE elongated structure corresponding to the
Mbandaka-Dekese axis, intruded by mafic material that
locally densified the crust; and was filled subsequently by
up to 9 km of Neoproterozoic to Cenozoic sediments
following long term (ca. 700 million years) thermal- and
density-driven subsidence. Indirect observations suggest
that the first seismic-stratigraphic sequence in the CB
(Sequence 0) may consist of syn-rift clastics, with slightly
divergent reflectors, followed by the deposition of
carbonates, with an undetermined amount of evaporites
(Sequence 1), and then two siliciclastic sequences with
thin carbonate layers and tillites interbedded (Sequences
2 and 3). These four sequences are grouped in the seismic